CCNA Routing And Switching Study Guide

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CCNA®
Routing and Switching
Study Guide
Todd Lammle
CCNA®
Routing and Switching
Study Guide
Senior Acquisitions Editor: Jeff Kellum
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Technical Editors: John Swartz and Dax Mickelson
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Copyright © 2013 by John Wiley & Sons, Inc., Indianapolis, Indiana
Published by John Wiley & Sons, Inc. Indianapolis, Indiana
Published simultaneously in Canada
ISBN: 978-1-118-74961-6
ISBN: 978-1-118-74973-9 (ebk.)
ISBN: 978-1-118-74970-8 (ebk.)
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Dear Reader,
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Acknowledgments
There are many people that work to put a book together, and as an author, I dedicated an
enormous amount of time to write this book, but it would have never been published with-
out the dedicated, hard work of many other people.
Monica Worthy Lammle was crucial to the nished product this book became, going
over every word of every chapter with me to ne-tune the language and grammar. Without
Monica’s support in all areas of my life, I’d never have nished writing this book.
Next in line to thank is my new technical editor, John Swartz, who also coauthored
the CCNA Data Center study guides with me. His expertise in the Cisco technical eld,
and his history of networking in general, is second to none. His detailed analysis of my
work helped make this my best CCNA study guide ever. Thank you, John, for working
hard under pressure, with tight deadlines, and for staying the course of delivering high-
quality work in a short time frame.
Jeff Kellum, my acquisitions editor, is instrumental to my success in the world of Cisco
certication. Jeff, I look forward to our continued progress together.
Christine O’Connor, my production editor, and Judy Flynn, my copyeditor, were my
rock and foundation for formatting an intense editing of every page in this book. This
amazing team gives me the condence to help keep me moving during the difcult and very
long days, week after week. How Christine stays so organized with all my changes, as well
as making sure every gure is in the right place in the book is still a mystery to me! You’re
amazing, Christine! Thank you! Judy understands my writing style so well now, after doing
at least a dozen books with me, that she even sometimes nds a technical error that may
have slipped through as I was going through the material. Thank you Judy for doing such
a great job! I truly thank you both.
Troy McMillian really helped me on this book (on all books actually!) by working on
the review and bonus questions, ash cards, as well as a 3rd technical edit on the last stage
of editing. He did a high-quality job in a short time! Thanks, Troy! Jim Frey and Paul
Sutton really helped me put together this book’s amazing gures, and an all-around second-
look technical edit. Thank you both!
Also, thanks to Dax Mickelson and Dennis Frye for performing the technical proofread
of the book.
Finally, a big thanks to Craig Woods at Happenstance-Type-O-Rama and to the Sybex
media-development team.
About the Author
Todd Lammle is the authority on Cisco certication and internetworking and is Cisco certi-
ed in most Cisco certication categories. He is a world-renowned author, speaker, trainer,
and consultant. Todd has three decades of experience working with LANs, WANs, and large
enterprise licensed and unlicensed wireless networks, and lately he’s been implementing large
Cisco data centers worldwide. His years of real-world experience is evident in his writing; he
is not just an author but an experienced networking engineer with very practical experience
working on the largest networks in the world, at such companies as Xerox, Hughes Aircraft,
Texaco, AAA, Cisco, and Toshiba, among many others. Todd has published over 60 books,
including the very popular CCNA: Cisco Certied Network Associate Study Guide, CCNA
Wireless Study Guide, and CCNA Data Center Study Guide, all from Sybex. He runs an
international consulting and training company based in Colorado, Texas, and San Francisco.
You can reach Todd through his forum and blog at www.lammle.com.
Contents at a Glance
Introduction xxvii
ICND1 (100-101) Exam Objectives xxxviii
ICND2 (200-101) Exam Objectives xliv
CCNA Composite (200-120) Exam Objectives xlvii
Assessment Test lviii
Part I ICND1 1
Chapter 1 Internetworking 3
Chapter 2 Ethernet Networking and Data Encapsulation 41
Chapter 3 Introduction to TCP/IP 87
Chapter 4 Easy Subnetting 139
Chapter 5 VLSMs, Summarization, and Troubleshooting TCP/IP 181
Chapter 6 Cisco’s Internetworking Operating System (IOS) 213
Chapter 7 Managing a Cisco Internetwork 283
Chapter 8 IP Routing 331
Chapter 9 Open Shortest Path First (OSPF) 385
Chapter 10 Layer 2 Switching 425
Chapter 11 VLANs and InterVLAN Routing 459
Chapter 12 Security 501
Chapter 13 Network Address Translation (NAT) 541
Chapter 14 Internet Protocol Version 6 (IPv6) 569
Part II ICND2 613
Chapter 15 Enhanced Switched Technologies 615
Chapter 16 Managing Cisco Devices 661
Chapter 17 IP Services 699
Chapter 18 Troubleshooting IP, IPv6, and VLANs 741
Chapter 19 Enhanced IGRP 783
Chapter 20 Multi-Area OSPF 847
Chapter 21 Wide Area Networks 897
Appendix A Answers to Written Labs 967
Appendix B Answers to Review Questions 987
Appendix C Disabling and Configuring Network Services 1029
Appendix D About the Additional Study Tools 1041
Index 1045
Contents
Introduction xxvii
ICND1 (100-101) Exam Objectives xxxviii
ICND2 (200-101) Exam Objectives xliv
CCNA Composite (200-120) Exam Objectives xlvii
Assessment Test lviii
Part I ICND1 1
Chapter 1 Internetworking 3
Internetworking Basics 4
Internetworking Models 12
The Layered Approach 13
Advantages of Reference Models 14
The OSI Reference Model 14
The Application Layer 16
The Presentation Layer 17
The Session Layer 17
The Transport Layer 18
The Network Layer 23
The Data Link Layer 25
The Physical Layer 28
Summary 29
Exam Essentials 30
Written Labs 32
Written Lab 1.1: OSI Questions 32
Written Lab 1.2: Defining the OSI Layers and Devices 33
Written Lab 1.3: Identifying Collision and
Broadcast Domains 34
Review Questions 35
Chapter 2 Ethernet Networking and Data Encapsulation 41
Ethernet Networks in Review 42
Collision Domain 43
Broadcast Domain 44
CSMA/CD 45
Half- and Full-Duplex Ethernet 47
Ethernet at the Data Link Layer 49
Ethernet at the Physical Layer 55
Contents
Introduction
1
Chapter 1
Internetworking 3
Internetworking Basics
xii Contents
Ethernet Cabling 59
Straight-through Cable 60
Crossover Cable 60
Rolled Cable 62
Fiber Optic 64
Data Encapsulation 66
The Cisco three-layer HierarchicalModel 70
The Core Layer 70
The Distribution Layer 72
The Access Layer 72
Summary 73
Exam Essentials 73
Written Labs 74
Written Lab 2.1: Binary/Decimal/Hexadecimal Conversion 75
Written Lab 2.2: CSMA/CD Operations 79
Written Lab 2.3: Cabling 79
Written Lab 2.4: Encapsulation 80
Review Questions 81
Chapter 3 Introduction to TCP/IP 87
Introducing TCP/IP 88
A Brief History of TCP/IP 89
TCP/IP and the DoD Model 89
The Process/Application Layer Protocols 91
The Host-to-host Layer Protocols 101
The Internet Layer Protocols 110
IP Addressing 118
IP Terminology 118
The Hierarchical IP Addressing Scheme 119
Private IP Addresses (RFC 1918) 124
IPv4 Address Types 126
Layer 2 Broadcasts 126
Layer 3 Broadcasts 126
Unicast Address 127
Multicast Address 128
Summary 129
Exam Essentials 130
Written Labs 132
Written Lab 3.1: TCP/IP 132
Written Lab 3.2: Mapping Applications to the DoD Model 132
Review Questions 134
Chapter 4 Easy Subnetting 139
Subnetting Basics 140
How to Create Subnets 142
Subnet Masks 142
Contents xiii
Classless Inter-Domain Routing (CIDR) 144
IP Subnet-Zero 146
Subnetting Class C Addresses 146
Subnetting Class B Addresses 158
Subnetting Class A Addresses 167
Summary 170
Exam Essentials 170
Written Labs 171
Written Lab 4.1: Written Subnet Practice #1 171
Written Lab 4.2: Written Subnet Practice #2 172
Written Lab 4.3: Written Subnet Practice #3 173
Review Questions 174
Chapter 5 VLSMs, Summarization, and
Troubleshooting TCP/IP 181
Variable Length Subnet Masks (VLSMs) 182
VLSM Design 184
Implementing VLSM Networks 184
Summarization 193
Troubleshooting IP Addressing 196
Determining IP Address Problems 198
Summary 203
Exam Essentials 204
Written Lab 5 205
Review Questions 206
Chapter 6 Cisco’s Internetworking Operating System (IOS) 213
The IOS User Interface 215
Cisco IOS 216
Connecting to a Cisco IOS Device 216
Bringing Up a Switch 218
Command-line Interface (CLI) 219
Entering the CLI 219
Overview of Router Modes 219
CLI Prompts 220
Editing and Help Features 223
Administrative Configurations 228
Hostnames 228
Banners 229
Setting Passwords 231
Encrypting Your Passwords 237
Descriptions 239
Router and Switch Interfaces 241
Bringing Up an Interface 244
xiv Contents
Viewing, Saving, and Erasing Configurations 250
Deleting the Configuration and Reloading the Device 252
Verifying Your Configuration 253
Summary 264
Exam Essentials 265
Written Lab 6 268
Hands-on Labs 268
Hands-on Lab 6.1: Erasing an Existing Configuration 269
Hands-on Lab 6.2: Exploring User, Privileged,
and Configuration Modes 269
Hands-on Lab 6.3: Using the Help and Editing Features 270
Hands-on Lab 6.4: Saving a Configuration 271
Hands-on Lab 6.5: Setting Passwords 272
Hands-on Lab 6.6: Setting the Hostname, Descriptions,
IP Address, and Clock Rate 274
Review Questions 277
Chapter 7 Managing a Cisco Internetwork 283
The Internal Components of a Cisco Router and Switch 284
The Router and Switch Boot Sequence 285
Backing Up and Restoring the Cisco Configuration 286
Backing Up the Cisco Configuration 286
Restoring the Cisco Configuration 288
Erasing the Configuration 289
Configuring DHCP 290
DHCP Relay 291
Verifying DHCP on Cisco IOS 292
Network Time Protocol (NTP) 293
Using Cisco Discovery Protocol (CDP) 295
Getting CDP Timers and Holdtime Information 295
Gathering Neighbor Information 296
Documenting a Network Topology Using CDP 300
Using Telnet 303
Telnetting into Multiple Devices Simultaneously 305
Checking Telnet Connections 306
Checking Telnet Users 306
Closing Telnet Sessions 306
Resolving Hostnames 307
Building a Host Table 307
Using DNS to Resolve Names 309
Checking Network Connectivity and Troubleshooting 311
Using the ping Command 312
Using the traceroute Command 312
Contents xv
Debugging 314
Using the show processes Command 316
Summary 317
Exam Essentials 317
Written Lab 7 319
Written Lab 7.1: IOS Management 319
Written Lab 7.2: Router Memory 319
Hands-on Labs 320
Hands-on Lab 7.1: Backing Up the Router Configuration 320
Hands-on Lab 7.2: Using the Cisco Discovery
Protocol (CDP) 321
Hands-on Lab 7.3: Using Telnet 322
Hands-on Lab 7.4: Resolving Hostnames 323
Review Questions 325
Chapter 8 IP Routing 331
Routing Basics 333
The IP Routing Process 335
The Cisco Router Internal Process 341
Testing Your IP Routing Understanding 342
Configuring IP Routing 346
Configuring IP Routing in Our Network 356
Static Routing 357
Default Routing 362
Dynamic Routing 365
Routing Protocol Basics 365
Routing Information Protocol (RIP) 367
Configuring RIP Routing 367
Holding Down RIP Propagations 371
Summary 373
Exam Essentials 374
Written Lab 8 376
Hands-on Labs 376
Hands-on Lab 8.1: Creating Static Routes 377
Hands-on Lab 8.2: Configuring RIP Routing 378
Review Questions 380
Chapter 9 Open Shortest Path First (OSPF) 385
Open Shortest Path First (OSPF) Basics 386
OSPF Terminology 389
OSPF Operation 391
Configuring OSPF 393
Enabling OSPF 393
Configuring OSPF Areas 394
Configuring Our Network with OSPF 397
xvi Contents
OSPF and Loopback Interfaces 402
Configuring Loopback Interfaces 403
Verifying OSPF Configuration 405
The show ip ospf Command 406
The show ip ospf database Command 407
The show ip ospf interface Command 408
The show ip ospf neighbor Command 409
The show ip protocols Command 411
Summary 411
Exam Essentials 412
Written Lab 9 413
Hands-on Labs 413
Hands-on Lab 9.1: Enabling the OSPF Process 414
Hands-on Lab 9.2: Configuring OSPF Interfaces 415
Hands-on Lab 9.3: Verifying OSPF Operation 416
Review Questions 417
Chapter 10 Layer 2 Switching 425
Switching Services 426
Three Switch Functions at Layer 2 427
Port Security 431
Configuring Catalyst Switches 436
Catalyst Switch Configuration 436
Verifying Cisco Catalyst Switches 444
Summary 447
Exam Essentials 447
Written Lab 10 448
Hands-on Labs 448
Lab 10.1: Configuring Layer 2 Switches 449
Lab 10.2: Verifying Layer 2 Switches 450
Lab 10.3: Configuring Port Security 450
Review Questions 452
Chapter 11 VLANs and InterVLAN Routing 459
VLAN Basics 460
Broadcast Control 463
Security 464
Flexibility and Scalability 464
Identifying VLANs 465
Frame Tagging 467
VLAN Identification Methods 468
Routing between VLANs 469
Contents xvii
Configuring VLANs 472
Assigning Switch Ports to VLANs 475
Configuring Trunk Ports 476
Configuring Inter-VLAN Routing 480
Summary 487
Exam Essentials 488
Written Lab 11 489
Hands-on Labs 489
Hands-on Lab 11.1: Configuring and Verifying VLANs 490
Hands-on Lab 11.2: Configuring and Verifying
Trunk Links 491
Hands-on Lab 11.3: Configuring Router on a
Stick Routing 492
Hands-on Lab 11.4: Configuring IVR with a
Layer 3 Switch 492
Review Questions 494
Chapter 12 Security 501
Perimeter, Firewall, and Internal Routers 502
Introduction to Access Lists 503
Mitigating Security Issues with ACLs 506
Standard Access Lists 507
Wildcard Masking 508
Standard Access List Example 510
Controlling VTY (Telnet/SSH) Access 514
Extended Access Lists 515
Extended Access List Example 1 519
Extended Access List Example 2 521
Extended Access List Example 3 522
Named ACLs 523
Remarks 525
Monitoring Access Lists 526
Summary 528
Exam Essentials 529
Written Lab 12 530
Hands-on Labs 530
Hands-on Lab 12.1: Standard IP Access Lists 531
Hands-on Lab 12.2: Extended IP Access Lists 532
Review Questions 535
Chapter 13 Network Address Translation (NAT) 541
When Do We Use NAT? 542
Types of Network Address Translation 544
NAT Names 544
xviii Contents
How NAT Works 545
Static NAT Configuration 547
Dynamic NAT Configuration 548
PAT (Overloading) Configuration 548
Simple Verification of NAT 549
Testing and Troubleshooting NAT 550
Summary 555
Exam Essentials 556
Written Lab 13 557
Hands-on Labs 557
Lab 13.1: Preparing for NAT 558
Lab 13.2: Configuring Dynamic NAT 560
Lab 13.3: Configuring PAT 561
Review Questions 564
Chapter 14 Internet Protocol Version 6 (IPv6) 569
Why Do We Need IPv6? 571
The Benefits and Uses of IPv6 571
IPv6 Addressing and Expressions 573
Shortened Expression 574
Address Types 575
Special Addresses 576
How IPv6 Works in an Internetwork 577
Manual Address Assignment 578
Stateless Autoconfiguration (eui-64) 578
DHCPv6 (Stateful) 581
IPv6 Header 582
ICMPv6 583
IPv6 Routing Protocols 586
Static Routing with IPv6 586
OSPFv3 587
Configuring IPv6 on Our Internetwork 588
Configuring Routing on Our Internetwork 591
Verifying OSPFv3 594
Summary 599
Exam Essentials 599
Written Labs 601
Written Lab 14.1 601
Written Lab 14.2 601
Hands-on Labs 602
Hands-on Lab 14.1: Manual and
Stateful Autoconfiguration 602
Hands-on Lab 14.2: Static and Default Routing 604
Hands-on Lab 14.3: OSPFv3 605
Review Questions 607
Contents xix
Part II ICND2 613
Chapter 15 Enhanced Switched Technologies 615
Spanning Tree Protocol (STP) 616
Spanning-tree Terms 617
Spanning-tree Operations 621
Types of Spanning-tree Protocols 623
Common Spanning Tree 624
Per-VLAN Spanning Tree+ 625
Modifying and Verifying the Bridge ID 631
Spanning-tree Failure Consequences 636
PortFast and BPDU Guard 638
BPDU Guard 640
EtherChannel 642
Configuring and Verifying Port Channels 643
Summary 646
Exam Essentials 647
Written Lab 1 648
Hands-on Labs 648
Hands-on Lab 1.1: Verifying STP and Finding Your
RootBridge 649
Hands-on Lab 1.2: Configuring and Verifying Your
Root Bridge 651
Hands-on Lab 1.3: Configuring PortFast and
BPDU Guard 652
Hands-on Lab 1.4: Configuring and
Verifying EtherChannel 653
Review Questions 656
Chapter 16 Managing Cisco Devices 661
The Internal Components of a
Cisco Router 662
The Router Boot Sequence 663
Managing Configuration Register 664
Understanding the Configuration Register Bits 665
Checking the Current Configuration Register Value 666
Boot System Commands 667
Recovering Passwords 669
Backing Up and Restoring the Cisco IOS 671
Verifying Flash Memory 673
Backing Up the Cisco IOS 674
Restoring or Upgrading the Cisco Router IOS 675
Using the Cisco IOS File System (Cisco IFS) 677
xx Contents
Licensing 682
Right-To-Use Licenses (Evaluation Licenses) 684
Backing Up and Uninstalling the License 687
Summary 688
Exam Essentials 689
Written Labs 691
Written Lab 2.1: IOS Management 691
Written Lab 2.2: Router Memory 691
Hands-on Labs 692
Hands-on Lab 2.1: Backing Up Your Router IOS 692
Hands-on Lab 2.2: Upgrading or Restoring
Your Router IOS 692
Review Questions 694
Chapter 17 IP Services 699
Client Redundancy Issues 700
Introducing First Hop Redundancy Protocol (FHRP) 702
Hot Standby Router Protocol (HSRP) 704
Virtual MAC Address 705
HSRP Timers 706
Group Roles 708
Configuring and Verifying HSRP 709
Virtual Router Redundancy Protocol 715
Comparing VRRP and HSRP 715
VRRP Redundancy Characteristics 715
Gateway Load Balancing Protocol 716
GLBP Functions 717
GLBP Features 717
GLBP Per-host Traffic Balancing 718
Configuring GLBP 718
Syslog 721
Configuring and Verifying Syslog 723
SNMP 726
Management Information Base (MIB) 727
Configuring SNMP 728
NetFlow 730
NetFlow Overview and Flows 731
Configuring NetFlow 732
Summary 734
Exam Essentials 734
Written Lab 3 735
Review Questions 736
Contents xxi
Chapter 18 Troubleshooting IP, IPv6, and VLANs 741
Troubleshooting IP Network Connectivity 742
Troubleshooting IPv6 Network Connectivity 754
ICMPv6 755
Troubleshooting VLAN Connectivity 763
VLAN Troubleshooting 763
Trunk Troubleshooting 768
Summary 776
Exam Essentials 777
Written Lab 4 778
Review Questions 779
Chapter 19 Enhanced IGRP 783
EIGRP Features and Operations 784
Neighbor Discovery 785
Reliable Transport Protocol (RTP) 790
Diffusing Update Algorithm (DUAL) 791
Route Discovery and Maintenance 792
Configuring EIGRP 792
VLSM Support and Summarization 795
Controlling EIGRP Traffic 798
Split Horizon 809
Verifying and Troubleshooting EIGRP 811
Troubleshooting Example with EIGRP 818
EIGRPv6 828
Summary 833
Exam Essentials 834
Written Lab 5 835
Hands-on Labs 835
Hands-on Lab 5.1: Configuring and Verifying EIGRP 835
Hands-on Lab 5.2: Configuring and Verifying EIGRPv6 837
Review Questions 838
Chapter 20 Multi-Area OSPF 847
OSPF Scalability 848
Categories of Multi-area Components 850
Adjacency Requirements 850
OSPF Router Roles 851
Link-state Advertisements 852
OSPF Hello Protocol 854
Neighbor States 855
Basic Multi-area Configuration 857
xxii Contents
Verifying and Troubleshooting
Multi-area OSPF Networks 859
The show ip ospf Command 861
The show ip ospf interface Command 862
The show ip protocols Command 864
The show ip route Command 865
The show ip ospf database Command 866
Troubleshooting OSPF Scenario 868
OSPFv3 876
Verifying OSPFv3 879
Summary 881
Exam Essentials 881
Written Lab 6 883
Hands-on Labs 883
Hands-on Lab 6.1: Configuring and Verifying
OSPF Multi-Area 884
Hands-on Lab 6.2: Configuring and Verifying OSPFv3 887
Review Questions 890
Chapter 21 Wide Area Networks 897
Introduction to Wide Area Networks 898
Defining WAN Terms 899
WAN Connection Bandwidth 900
WAN Connection Types 900
WAN Support 902
Cable and DSL 905
Cable 906
Digital Subscriber Line (DSL) 907
Cabling the Serial Wide Area Network 910
Serial Transmission 910
Data Terminal Equipment and Data
Communication Equipment 911
High-Level Data-Link Control (HDLC) Protocol 912
Point-to-Point Protocol (PPP) 914
Link Control Protocol (LCP) Configuration Options 915
PPP Session Establishment 916
PPP Authentication Methods 917
Configuring PPP on Cisco Routers 917
Configuring PPP Authentication 917
Verifying and Troubleshooting Serial Links 918
Frame Relay 923
Introduction to Frame Relay Technology 924
Frame Relay Implementation and Monitoring 931
Contents xxiii
Virtual Private Networks 942
Benefits of VPNs 943
Introduction to Cisco IOS IPsec 944
IPsec Transforms 945
GRE Tunnels 946
Configuring GRE Tunnels 947
Verifying GRP Tunnels 949
Summary 951
Exam Essentials 951
Written Lab 7 953
Hands-on Labs 953
Hands-on Lab 7.1: Configuring PPP Encapsulation
and Authentication 953
Hands-on Lab 7.2: Configuring and Monitoring HDLC 955
Hands-on Lab 7.3: Configuring Frame Relay
and Subinterfaces 956
Hands-on Lab 7.4: Configuring a GRE Tunnel 958
Review Questions 961
Appendix A Answers to Written Labs 967
Chapter 1: Internetworking 968
Written Lab 1.1: OSI Questions 968
Written Lab 1.2: Defining the OSI Layers and Devices 969
Written Lab 1.3: Identifying Collision and
Broadcast Domains 970
Chapter 2: Ethernet Networking and Data Encapsulation 970
Written Lab 2.1: Binary/Decimal/Hexadecimal Conversion 970
Written Lab 2.2: CSMA/CD Operations 973
Written Lab 2.3: Cabling 973
Written Lab 2.4: Encapsulation 974
Chapter 3: Introduction to TCP/IP 974
Written Lab 3.1: TCP/IP 974
Written Lab 3.2: Mapping Applications to the DoD Model 974
Chapter 4: Easy Subnetting 975
Written Lab 4.1: Written Subnet Practice #1 975
Written Lab 4.2: Written Subnet Practice #2 976
Written Lab 4.3: Written Subnet Practice #3 977
Chapter 5: VLSMs, Summarization and
Troubleshooting TCP/IP 977
Chapter 6: Cisco’s Internetworking Operating System (IOS) 978
Written Lab 6 978
Chapter 7: Managing a Cisco Internetwork 978
Written Lab 7.1: IOS Management 978
Written Lab 7.2: Router Memory 979
xxiv Contents
Chapter 8: IP Routing 979
Chapter 9: Open Shortest Path First (OSPF) 980
Chapter 10: Layer 2 Switching 980
Chapter 11: VLANs and InterVLAN Routing 980
Chapter 12: Security 981
Chapter 13: Network Address Translation (NAT) 981
Chapter 14: Internet Protocol Version 6 (IPv6) 982
Written Lab 14.1 982
Written Lab 14.2 982
Chapter 15: Enhanced Switch Technologies 982
Chapter 16: Managing Cisco Devices 983
Written Lab 16.1: IOS Management 983
Written Lab 16.2: Router Memory 983
Chapter 17: IP Services 984
Chapter 18: Troubleshooting IP, IPv6, and VLANs 984
Chapter 19: Enhanced IGRP 984
Chapter 20: Multi-Area OSPF 985
Chapter 21: Wide Area Networks 985
Appendix B Answers to Review Questions 987
Chapter 1: Internetworking 988
Chapter 2: Ethernet Networking and Data Encapsulation 990
Chapter 3: Introduction to TCP/IP 991
Chapter 4: Easy Subnetting 993
Chapter 5: VLSMs, Summarization, and
Troubleshooting TCP/IP 995
Chapter 6: Cisco’s Internetworking Operating System (IOS) 997
Chapter 7: Managing a Cisco Internetwork 999
Chapter 8: IP Routing 1001
Chapter 9: Open Shortest Path First (OSPF) 1002
Chapter 10: Layer 2 Switching 1004
Chapter 11: VLANs and InterVLAN Routing 1007
Chapter 12: Security 1008
Chapter 13: Network Address Translation (NAT) 1011
Chapter 14: Internet Protocol Version 6 (IPv6) 1012
Chapter 15: Enhanced Switch Technologies 1014
Chapter 16: Managing Cisco Devices 1016
Chapter 17: IP Services 1018
Chapter 18: Troubleshooting IP, IPv6, andVLANs 1021
Chapter 19: Enhanced IGRP 1022
Chapter 20: Multi-Area OSPF 1024
Chapter 21: Wide Area Networks 1026
Contents xxv
Appendix C Disabling and Configuring Network Services 1029
Blocking SNMP Packets 1030
Disabling Echo 1030
Turning off BootP and Auto-Config 1031
Disabling the HTTP Interface 1032
Disabling IP Source Routing 1032
Disabling Proxy ARP 1032
Disabling Redirect Messages 1032
Disabling the Generation of ICMP Unreachable Messages 1033
Disabling Multicast Route Caching 1033
Disabling the Maintenance Operation Protocol (MOP) 1033
Turning Off the X.25 PAD Service 1034
Enabling the Nagle TCP Congestion Algorithm 1034
Logging Every Event 1034
Disabling Cisco Discovery Protocol 1035
Disabling the Default Forwarded UDP Protocols 1035
Cisco’s Auto Secure 1036
Appendix D About the Additional Study Tools 1041
Additional Study Tools 1042
Test Engine 1042
Electronic Flashcards 1042
Videos 1042
Network Simulator 1043
PDF of Glossary of Terms 1043
Adobe Reader 1043
Minimum System Requirements 1043
Using the Study Tools 1044
Troubleshooting 1044
Customer Care 1044
Index 1045
Introduction
Welcome to the exciting world of Cisco certication! If you’ve picked up this book because
you want to improve yourself and your life with a better, more satisfying, and secure job,
you’ve done the right thing. Whether you’re striving to enter the thriving, dynamic IT sector
or seeking to enhance your skill set and advance your position within it, being Cisco certied
can seriously stack the odds in your favor to help you attain your goals!
Cisco certications are powerful instruments of success that also markedly improve
your grasp of all things internetworking. As you progress through this book, you’ll gain a
complete understanding of networking that reaches far beyond Cisco devices. By the end of
this book, you’ll comprehensively know how disparate network topologies and technologies
work together to form the fully operational networks that are vital to today’s very way of
life in the developed world. The knowledge and expertise you’ll gain here is essential for and
relevant to every networking job and is why Cisco certications are in such high demand—
even at companies with few Cisco devices!
Although it’s now common knowledge that Cisco rules routing and switching, the fact
that it also rocks the voice, data center, and service provider worlds is also well recognized.
And Cisco certications reach way beyond the popular but less extensive certications like
those offered by CompTIA and Microsoft to equip you with indispensable insight into today’s
vastly complex networking realm. Essentially, by deciding to become Cisco certied, you’re
proudly announcing that you want to become an unrivaled networking expert—a goal that
this book will get you well on your way to achieving. Congratulations in advance on the
beginning of your brilliant future!
For up-to-the-minute updates covering additions or modifications to the
Cisco certification exams, as well as additional study tools, review questions,
and bonus materials, be sure to visit the Todd Lammle websites and forum at
www.lammle.com, www.lammlesim.com, and www.lammle.com/forum.
Cisco’s Network Certifications
It used to be that to secure the holy grail of Cisco certications—the CCIE—you passed only
one written test before being faced with a grueling, formidable hands-on lab. This intensely
daunting, all-or-nothing approach made it nearly impossible to succeed and predictably didn’t
work out too well for most people. Cisco responded to this issue by creating a series of new
certications, which not only made it easier to eventually win the highly coveted CCIE prize, it
gave employers a way to accurately rate and measure the skill levels of prospective and current
employees. This exciting paradigm shift in Cisco’s certication path truly opened doors that
few were allowed through before!
xxviii Introduction
Beginning in 1998, obtaining the Cisco Certied Network Associate (CCNA) certication
was the rst milestone in the Cisco certication climb, as well as the ofcial prerequisite to
each of the more advanced levels. But that changed in 2007, when Cisco announced the Cisco
Certied Entry Network Technician (CCENT) certication. And then in March 2013, Cisco
once again proclaimed updates to the CCENT and CCNA Routing and Switching (R/S) tests.
Now the Cisco certication process looks like Figure I.1.
FIGURE I.1 The Cisco certification path
Routing/Switching
CCIE
CCNP
CCNA
CCENT
Data Center
CCIE
CCNP
CCNA
No Pre-req
Voice
Cisco 2013 Certification Path Announcements
CCIE
CCNP
CCNA
CCENT
Security
CCIE
CCNP
CCNA
CCENT
Wireless
CCIE
CCNP
CCNA
CCENT
I have included only the most popular tracks in Figure I.1. In addition to the
ones in this image, there are also tracks for Design, Service Provider, Service
Provider Operations, and Video. Also note that the CCIE Voice certification
retirement will be announced shortly.
The Cisco R/S path is by far the most popular and could very well remain so, but soon
you’ll see the Data Center path become more and more of a focus as companies migrate to
data center technologies. The Voice track also actually does provide a good job opportunity.
Still, understanding the foundation of R/S before attempting any other certication track is
something I highly recommend.
Even so, and as the gure shows, you only need your CCENT certication to get under-
way for most of the tracks. Also, note that there are a few other certication tracks you
can go down that are not shown in the gure, although they’re not as popular as the ones
shown. You can nd information on all Cisco certication tracks at: www.cisco.com.
Cisco Certified Entry Network Technician (CCENT)
Don’t be fooled by the oh-so-misleading name of this rst certication because it absolutely
isn’t entry level! Okay—maybe entry level for Cisco’s certication path, but denitely not
for someone without experience trying to break into the highly lucrative yet challenging IT
Introduction xxix
job market! For the uninitiated, the CompTIA A+ and Network+ certications aren’t ofcial
prerequisites, but know that Cisco does expect you to have that type and level of experience
before embarking on your Cisco certication journey.
All of this gets us to 2013, when the climb to Cisco supremacy just got much harder
again. The innocuous-sounding siren’s call of the CCENT can lure you to some serious
trouble if you’re not prepared, because it’s actually much harder than the old CCNA ever
was. This will rapidly become apparent once you start studying, but be encouraged! The
fact that the certication process is getting harder really works better for you in the long
run, because that which is harder to obtain only becomes that much more valuable when
you nally do, right? Yes, indeed!
Another important factor to keep in mind is that the Interconnection Cisco Network
Devices Part 1 (ICND1) exam, which is the required exam for the CCENT certication,
costs $150 per attempt and it’s anything but easy to pass! The good news is that Part 1 of
this book (Chapters 1-14) will guide you step-by-step in building a strong foundation in
routing and switching technologies. You really need to build on a strong technical founda-
tion and stay away from exam cram type books, suspicious online material, and the like.
They can help somewhat, but understand that you’ll pass the Cisco certication exams only
if you have a strong foundation and that you’ll get that solid foundation only by reading as
much as you can, performing the written labs and review questions in this book, and prac-
ticing lots and lots of hands-on labs. Additional practice exam questions, videos, and labs
are offered on my website, and what seems like a million other sites offer additional material
that can help you study.
However, there is one way to skip the CCENT exam and still meet the prerequisite
before moving on to any other certication track, and that path is through the CCNA R/S
Composite exam. First, I’ll discuss the Interconnecting Cisco Network Devices Part 2
(ICND2) exam, and then I’ll tell you about the CCNA Composite exam, which will provide
you, when successful, with both the CCENT and the CCNA R/S certication.
Cisco Certified Network Associate Routing
andSwitching (CCNA R/S)
Once you have achieved your CCENT certication, you can take the ICND2 (200-101) exam
in order to achieve your CCNA R/S certication, which is the most popular certication
Cisco has by far because it’s the most sought-after certication of all employers.
As with the CCENT, the ICND2 exam is also $150 per attempt—although thinking you
can just skim a book and pass any of these exams would probably be a really expensive mis-
take! The CCENT/CCNA exams are extremely hard and cover a lot of material, so you have
to really know your stuff. Taking a Cisco class or spending months with hands-on experience
is denitely a requirement to succeed when faced with this monster!
And once you have your CCNA, you don’t have to stop there—you can choose to continue
and achieve an even higher certication, called the Cisco Certied Network Professional
(CCNP). There are various ones, as shown in Figure I.1. The CCNP R/S is still the most
popular, with Voice certications coming in at a close second. And I’ve got to tell you that
xxx Introduction
the Data Center certication will be catching up fast. Also good to know is that anyone with
a CCNP R/S has all the skills and knowledge needed to attempt the notoriously dreaded but
coveted CCIE R/S lab. But just becoming a CCNA R/S can land you that job you’ve dreamed
about and that’s what this book is all about: helping you to get and keep a great job!
Still, why take two exams to get your CCNA if you don’t have to? Cisco still has the
CCNA Composite (200-120) exam that, if passed, will land you with your CCENT and
your CCNA R/S via only one test priced accordingly at $300. Some people like the one-test
approach, and some people like the two-test approach. Part 2 of this book (Chapters 15-21)
covers the ICND2 exam topics.
Why Become a CCENT and CCNA R/S?
Cisco, like Microsoft and other vendors that provide certication, has created the certica-
tion process to give administrators a set of skills and to equip prospective employers with a
way to measure those skills or match certain criteria. And as you probably know, becoming
a CCNA R/S is certainly the initial, key step on a successful journey toward a new, highly
rewarding, and sustainable networking career.
The CCNA program was created to provide a solid introduction not only to the Cisco
Internetwork Operating System (IOS) and Cisco hardware but also to internetworking in gen-
eral, making it helpful to you in areas that are not exclusively Cisco’s. And regarding today’s
certication process, it’s not unrealistic that network managers—even those without Cisco
equipment—require Cisco certications for their job applicants.
Rest assured that if you make it through the CCNA and are still interested in Cisco and
internetworking, you’re headed down a path to certain success!
What Skills Do You Need to Become a CCNA R/S?
This ICND1 exam (100-101) tests a candidate for the knowledge and skills required to suc-
cessfully install, operate, and troubleshoot a small branch ofce network. The exam includes
questions on the operation of IP data networks, LAN switching technologies, IPv6, IP rout-
ing technologies, IP services network device security, and basic troubleshooting. The ICND2
exam (exam 200-101) tests a candidate for the knowledge and skills required to successfully
install, operate, and troubleshoot a small- to medium-size enterprise branch network. The
exam includes questions on LAN switching technologies, IP routing technologies, IP services
(FHRP, syslog, SNMP v2 and v3), troubleshooting, and WAN technologies.
How Do You Become a CCNA R/S
If you want to go straight for our CCNA R/S and take only one exam, all you have to do is
pass the CCNA Composite exam (200-120). Oh, but don’t you wish it were that easy? True,
it’s just one test, but it’s a whopper, and to pass it you must possess enough knowledge to
understand what the test writers are saying, and you need to know everything I mentioned
previously, in the sections on the ICND1 and ICND2 exams! Hey, it’s hard, but it can be done!
Introduction xxxi
What does the CCNA Composite exam (200-120) cover? Pretty much the same topics
covered in the ICND1 and ICND2 exams. Candidates can prepare for this exam by taking
the Todd Lammle authorized Cisco boot camps. 200-120 tests a candidate's knowledge
and skills required to install, operate, and troubleshoot a small- to medium-size enterprise
branch network.
While you can take the Composite exam to get your CCNA, it’s good to know that Cisco
offers the two-step process I discussed earlier in this Introduction. And this book covers both
those exams too! It may be easier than taking that one ginormous exam for you, but don’t
think the two-test method is easy. It takes work! However, it can be done; you just need to
stick with your studies.
The two-test method involves passing the following:
u Exam 100-101: Interconnecting Cisco Networking Devices Part 1 (ICND1)
u Exam 200-101: Interconnecting Cisco Networking Devices Part 2 (ICND2)
I can’t stress this point enough: It’s critical that you have some hands-on experience with
Cisco routers. If you can get a hold of some basic routers and switches, you’re set, but if you
can’t, I’ve worked hard to provide hundreds of conguration examples throughout this book
to help network administrators, or people who want to become network administrators,
learn the skills they need to pass the CCENT and CCNA R/S exams. In addition, a simulator
called LammleSim IOS version is available for free with the purchase of this book. This small
simulator will run through all the hands-on labs found in this book—nice huh?
For Cisco certification hands-on training with CCSI Todd Lammle, please
see: www.lammle.com. Each student will get hands-on experience by con-
figuring at least three routers and two switches—no sharing of equipment!
What Does This Book Cover?
This book covers everything you need to know to pass the ICND1 (100-101) and ICND2
(200-101) exams, as well as the CCNA Composite (200-120) exam. But regardless of which
path you choose, as I’ve said, taking plenty of time to study and practice with routers or a
router simulator is the real key to success.
You will learn the following information in this book:
Chapter 1: Internetworking Chapters 1-14 map to the ICND1 exam. In Chapter 1, you
will learn the basics of the Open Systems Interconnection (OSI) model the way Cisco wants
you to learn it. There are written labs and plenty of review questions to help you. Do not
even think of skipping the fundamental written labs in this chapter!
Chapter 2: Ethernet Networking and Data Encapsulation This chapter will provide
you with the Ethernet foundation you need in order to pass both the CCENT and CCNA
xxxii Introduction
exams. Data encapsulation is discussed in detail in this chapter as well. And as with the
other chapters, this chapter includes written labs and review questions to help you.
Chapter 3: Introduction to TCP/IP This chapter provides you with the background necessary
for success on the exam, as well as in the real world with a thorough presentation of TCP/IP.
This in-depth chapter covers the very beginnings of the Internet Protocol stack and goes all
the way to IP addressing and understanding the difference between a network address and a
broadcast address before nally ending with network troubleshooting.
Chapter 4: Easy Subnetting You’ll actually be able to subnet a network in your head after
reading this chapter if you really want to! And you’ll nd plenty of help in this chapter as
long as you don’t skip the written labs and review questions at the end.
Chapter 5: VLSMs, Summarization, and Troubleshooting TCP/IP Here, you’ll nd out all
about variable length subnet masks (VLSMs) and how to design a network using VLSMs.
This chapter will nish with summarization techniques and congurations. As with Chapter
4, plenty of help is there for you if you don’t skip the written lab and review questions.
Chapter 6: Cisco’s Internetworking Operating System (IOS) This chapter introduces you to
the Cisco Internetworking Operating System (IOS) and command-line interface (CLI). In this
chapter you’ll learn how to turn on a router and congure the basics of the IOS, including
setting passwords, banners, and more. Hands-on labs will help you gain a rm grasp of the
concepts taught in the chapter. Before you go through the hands-on labs, be sure to complete
the written lab and review questions.
Chapter 7: Managing a Cisco Internetwork This chapter provides you with the management
skills needed to run a Cisco IOS network. Backing up and restoring the IOS, as well as router
conguration, are covered, as are the troubleshooting tools necessary to keep a network up
and running. As always, before tackling the hands-on labs in this chapter, complete the writ-
ten labs and review questions.
Chapter 8: IP Routing This is a fun chapter because we will begin to build our network,
add IP addresses, and route data between routers. You will also learn about static, default,
and dynamic routing using RIP and RIPv2. Hands-on labs, a written lab, and the review
questions will help you fully nail down IP routing.
Chapter 9: Open Shortest Path First (OSPF) Chapter 9 dives into more complex dynamic
routing by covering OSPF routing. The written lab, hands-on labs, and review questions
will help you master this vital routing protocol.
Chapter 10: Layer 2 Switching This chapter sets you up with the solid background you
need on layer 2 switching, how switches perform address learning and make forwarding
and ltering decisions. In addition, switch port security with MAC addresses is covered in
detail. As always, go through the hands-on labs, written lab, and review questions to make
sure you’ve really got layer 2 switching down!
Chapter 11: VLANs and Inter-VLAN Routing Here I cover virtual VLANs and how
to use them in your internetwork. This chapter covers the nitty-gritty of VLANs and
the different concepts and protocols used with VLANs. I’ll also guide you through
Introduction xxxiii
troubleshooting techniques in this all-important chapter. The hands-on labs, written lab,
and review questions are there to reinforce the VLAN material.
Chapter 12: Security This chapter covers security and access lists, which are created on
routers to lter the network. IP standard, extended, and named access lists are covered in
detail. Written and hands-on labs, along with review questions, will help you study for the
security and access-list portion of the Cisco exams.
Chapter 13: Network Address Translation (NAT) New information, commands, trouble-
shooting, and detailed hands-on labs will help you nail the NAT CCENT objectives.
Chapter 14: Internet Protocol Version 6 (IPv6) This is a fun chapter chock-full of some
great information. IPv6 is not the big, bad scary creature that most people think it is, and
it’s a really important objective on the latest exam, so study this chapter carefully—don’t
just skim it. And make sure you hit those hands-on labs hard!
Chapter 15: Enhanced Switched Technologies Chapter 15 is the rst chapter of Part 2 of this
book, which maps to the ICND2 exam. This chapter will start off with STP protocols and
dive into the fundamentals, covering the modes, as well as the various avors of STP. VLANs,
trunks, and troubleshooting are covered as well. EtherChannel technologies, conguration,
and verication are also covered. There are hands-on labs, a written lab, and plenty of review
questions to help you. Do not even think of skipping the fundamental written and hands-on
labs in this chapter!
Chapter 16: Managing Cisco Devices This chapter describes the boot process of Cisco
routers, the conguration register, and how to manage Cisco IOS les. The chapter n-
ishes with a section on Cisco’s new licensing strategy for IOS. Hands-on and written labs,
along with review questions, will help you build a strong foundation for the objectives
covered in this chapter.
Chapter 17: IP Services This chapter mostly focuses on rst hop redundancy protocols
(FHRPs), such as HSRP and GLBP. Also covered are syslog, SNMP, and NetFlow. There
are review questions and a written lab.
Chapter 18: Troubleshooting IP, IPv6, and VLANs I want to say this is the most impor-
tant chapter in the book, but that’s hard to say. You can decide that yourself when you
take the exam! Be sure to go through all the troubleshooting steps for IP, IPv6, and
VLANs. The hands-on labs for this chapter will be included in the free bonus material
and dynamic labs that I’ll write and change as needed. Don’t skip the written lab and
review questions.
Chapter 19: Enhanced IGRP EIGRP was not covered in the ICND1 (CCENT) chapters,
so this is a full chapter on nothing but EIGRP and EIGRPv6. There are lots of examples,
including conguration, verication, and troubleshooting labs, with both IP and with IPv6.
Great hands-on labs are included, as well as a written lab and review questions.
Chapter 20: Multi-Area OSPF The ICND1 (CCENT) portion of this book had a large
chapter on OSPF, so before reading this chapter, be sure you have the CCENT objectives
down pat with a strong OSPF foundation. This chapter will take off where that ICND1
xxxiv Introduction
chapter left off and add multi-area networks along with advanced congurations and then
nish with OSPv3. Hands-on labs, a written lab, and challenging review questions await
you at the end of the chapter.
Chapter 21: Wide Area Networks This is the longest, and last, chapter in the book. It
covers multiple protocols in depth, especially HDLC, PPP, and Frame Relay, along with a
discussion on many other technologies. Good troubleshooting examples are provided in the
PPP and Frame Relay conguration sections, and these cannot be skipped! Hands-on labs
meant to focus squarely on the objectives are included at the end of the chapter, as well as
a written lab and challenging review questions.
Appendix A: Answers to Written Labs This appendix contains the answers to the book’s
written labs.
Appendix B: Answers to Chapter Review Questions This appendix provides the answers
to the end-of-chapter review questions.
Appendix C: Disabling and Configuring Network Services Appendix C takes a look at
the basic services you should disable on your routers to make your network less of a target
for denial of service (DoS) attacks and break-in attempts.
Appendix D: About the Additional Study Tools This describes the technical requirements
for the digital study tools that come with this book. (Those tools are described further in
the following section.)
Be sure to check the announcements section of my forum to find out how
to download bonus material I created specifically for this book.
What’s Available Online?
I have worked hard to provide some really great tools to help you with your certication
process. All of the following tools, most of them available at www.sybex.com/go/ccnarssg,
should be loaded on your workstation when you’re studying for the test. As a fantastic bonus,
I was able to add to the download link a preview section from my CCNA video series! Please
understand that these are not the full versions, but they’re still a great value for you included
free with this book.
Test Preparation Software The test preparation software prepares you to pass the ICND1
and ICND2 exams and the CCNA R/S Composite exam. You’ll nd all the review and
assessment questions from the book plus additional practice exam questions that appear
exclusively from the downloadable study tools.
Introduction xxxv
Electronic Flashcards The companion study tools include over 200 ashcards specically
written to hit you hard, so don’t get discouraged if you don’t ace your way through them at
rst! They’re there to ensure that you’re really ready for the exam. And no worries—armed
with the review questions, practice exams, and ashcards, you’ll be more than prepared
when exam day comes!
LammleSim IOS Version At
www.lammle.com or www.lammlesim.com, I have provided an
IOS simulator that can be used with all of the hands-on labs in this book. You can also
download this free simulator from www.sybex.com/go/ccnarssg.
Glossary A complete glossary of CCENT, ICND2, CCNA R/S and Cisco routing terms is
available at www.sybex.com/go/ccnarssg.
Todd Lammle Bonus Material and Labs Be sure to check the announcement section of my
forum at www.lammle.com/forum for directions on how to download all the latest bonus mate-
rial created specically to help you study for your ICND1, ICND2, and CCNA R/S exams.
Todd Lammle Videos I have created a full CCNA series of videos that can be purchased
in either DVD or downloadable format from www.lammle.com. As a bonus included with
this book, the rst module of six of the DVDs can be downloaded from the book’s web
page, www.sybex.com/go/ccnarssg, as a preview. Although this isn’t the full version, the
videos included with this book are over two hours of foundational CCNA information.
This is a $198 value, so don’t skip these videos because they cover key topics for the exams.
CBT Nuggets Videos
In addition, CBT Nuggets has created dozens of free MicroNugget videos that help rein-
force the chapter topics. Throughout the book, you will see QR codes and URLs that will
direct you to a dedicated website where you can view these videos created by CBT Nug-
gets author and expert trainer Jeremy Cioara. Just scan the QR code with your smart
phone or tablet, or type in the URL, to view the video.
How to Use This Book
If you want a solid foundation for the serious effort of preparing for the Interconnecting
Cisco Network Devices Part 1 and 2 exams, or the CCNA R/S Composite exam, then look
no further. I’ve spent hundreds of hours putting together this book with the sole intention
of helping you to pass the Cisco exams, as well as really learn how to correctly congure
Cisco routers and switches!
This book is loaded with valuable information, and you will get the most out of your
study time if you understand why the book is organized the way it is.
xxxvi Introduction
So to maximize your benet from this book, I recommend the following study method:
1. Take the assessment test that’s provided at the end of this introduction. (The answers
are at the end of the test.) It’s okay if you don’t know any of the answers; that’s why
you bought this book! Carefully read over the explanations for any questions you get
wrong and note the chapters in which the material relevant to them is covered. This
information should help you plan your study strategy.
2. Study each chapter carefully, making sure you fully understand the information and
the test objectives listed at the beginning of each one. Pay extra-close attention to any
chapter that includes material covered in questions you missed.
3. Complete the written labs at the end of each chapter. (Answers to these appear in
Appendix A.) Do not skip these written exercises because they directly relate to the
Cisco exams and what you must glean from the chapters in which they appear. Do
not just skim these labs! Make sure you completely understand the reason for each
correct answer.
4. Complete all hands-on labs in each chapter, referring to the text of the chapter so that
you understand the reason for each step you take. Try to get your hands on some real
equipment, but if you don’t have Cisco equipment available, try the LammleSim IOS
version, which you can use for the hands-on labs found only in this book. These labs
will equip you with everything you need for all your Cisco certification goals.
5. Answer all of the review questions related to each chapter. (The answers appear in
Appendix B.) Note the questions that confuse you, and study the topics they cover
again until the concepts are crystal clear. And again—do not just skim these questions!
Make sure you fully comprehend the reason for each correct answer. Remember that
these will not be the exact questions you will find on the exam, but they’re written to
help you understand the chapter material and ultimately pass the exam!
6. Try your hand at the practice questions that are exclusive to this book. The questions can
be found only at www.sybex.com/go/ccnarssg. And be sure to check out www.lammle.com
for the most up-to-date Cisco exam prep questions, videos, Todd Lammle boot camps,
and more.
7. Also on the download link is the first module from six videos from my complete
CCNA R/S video series.
Please understand that these are preview versions of the videos found
at www.lammle.com. The videos on the free download link are not the full
versions, but they’re still a great value loaded with information.
8. Test yourself using all the flashcards, which are also found on the download link.
These are brand-new and updated flashcards to help you prepare for the CCNA R/S
exam and a wonderful study tool!
Introduction xxxvii
To learn every bit of the material covered in this book, you’ll have to apply yourself
regularly, and with discipline. Try to set aside the same time period every day to study, and
select a comfortable and quiet place to do so. I’m condent that if you work hard, you’ll be
surprised at how quickly you learn this material!
If you follow these steps and really study—doing hands-on labs every single day in
addition to using the review questions, the practice exams, the Todd Lammle video sec-
tions, and the electronic ashcards, as well as all the written labs—it would actually be
hard to fail the Cisco exams. But understand that studying for the Cisco exams is a lot
like getting in shape—if you do not go to the gym every day, it’s not going to happen!
Where Do You Take the Exams?
You may take the ICND1, ICND2, or CCNA R/S Composite or any Cisco exam at any of
the Pearson VUE authorized testing centers. For information, check www.vue.com or call
877-404-EXAM (3926).
To register for a Cisco exam, follow these steps:
1. Determine the number of the exam you want to take. (The ICND1 exam number is
100-101, ICND2 is 100-201, and CCNA R/S Composite is 200-120.)
2. Register with the nearest Pearson VUE testing center. At this point, you will be asked
to pay in advance for the exam. At the time of this writing, the ICND1 and ICND2
exams are $150, and the CCNA R/S Composite exam is $300. The exams must be
taken within one year of payment. You can schedule exams up to six weeks in advance
or as late as the day you want to take it—but if you fail a Cisco exam, you must wait
five days before you will be allowed to retake it. If something comes up and you need
to cancel or reschedule your exam appointment, contact Pearson VUE at least 24 hours
in advance.
3. When you schedule the exam, you’ll get instructions regarding all appointment and
cancellation procedures, the ID requirements, and information about the testing-center
location.
Tips for Taking Your Cisco Exams
The Cisco exams contain about 50-60 questions and must be completed in about 90 minutes
or less. This information can change per exam. You must get a score of about 85 percent to
pass this exam, but again, each exam can be different.
Many questions on the exam have answer choices that at rst glance look identical—
especially the syntax questions! So remember to read through the choices carefully because
close just doesn’t cut it. If you get commands in the wrong order or forget one measly char-
acter, you’ll get the question wrong. So, to practice, do the hands-on exercises at the end of
this book’s chapters over and over again until they feel natural to you.
xxxviii Introduction
Also, never forget that the right answer is the Cisco answer. In many cases, more than one
appropriate answer is presented, but the correct answer is the one that Cisco recommends.
On the exam, you will always be told to pick one, two, or three options, never “choose all
that apply.” The Cisco exam may include the following test formats:
u Multiple-choice single answer
u Multiple-choice multiple answer
u Drag-and-drop
u Router simulations
Cisco proctored exams will not show the steps to follow in completing a router interface
conguration, but they do allow partial command responses. For example, show run, sho
running, or sh running-config would be acceptable.
Here are some general tips for exam success:
u Arrive early at the exam center so you can relax and review your study materials.
u Read the questions carefully. Don’t jump to conclusions. Make sure you’re clear
about exactly what each question asks. “Read twice, answer once,” is what I always
tell my students.
u When answering multiple-choice questions that you’re not sure about, use the process
of elimination to get rid of the obviously incorrect answers first. Doing this greatly
improves your odds if you need to make an educated guess.
u You can no longer move forward and backward through the Cisco exams, so double-
check your answer before clicking Next since you can’t change your mind.
After you complete an exam, you’ll get immediate, online notication of your pass
or fail status, a printed examination score report that indicates your pass or fail status,
and your exam results by section. (The test administrator will give you the printed score
report.) Test scores are automatically forwarded to Cisco within ve working days after
you take the test, so you don’t need to send your score to them. If you pass the exam,
you’ll receive conrmation from Cisco, typically within two to four weeks, sometimes a
bit longer.
ICND1 (100-101) Exam Objectives
Exam objectives are subject to change at any time without prior notice and at Cisco’s sole
discretion. Please visit Cisco’s certication website (www.cisco.com/web/learning) for the
latest information on the ICND1 exam.
Operation of IP Data Networks Chapter(s)
Recognize the purpose and functions of various network devices, such as
Routers, Switches, Bridges, and Hubs.
1, 2
Introduction xxxix
Operation of IP Data Networks Chapter(s)
Select the components required to meet a given network specification. 1, 2
Identify common applications and their impact on the network. 1, 3
Describe the purpose and basic operation of the protocols in the OSI and
TCP/IP models.
1, 3
Predict the data flow between two hosts across a network. 1, 2, 13
Identify the appropriate media, cables, ports, and connectors, to connect
Cisco network devices to other network devices and hosts in a LAN.
2
LAN Switching Technologies
Determine the technology and media access control method for Ethernet
networks.
2
Identify basic switching concepts and the operation of Cisco switches.
u Collision domains
u Broadcast domains
u Types of switching
u CAM table
2, 10
Configure and verify initial switch-configuration including remote access
management.
u Cisco IOS commands to perform basic switch setup
6, 10
Verify network status and switch-operation using basic utilities, such as
ping, Telnet, and SSH.
7, 10
Describe how VLANs create logically separate networks and the need for
routing between them.
u Explain network segmentation and basic traffic management concepts.
11
Configure and verify VLANs. 11
Configure and verify trunking on Cisco switches.
u DTP
u Auto negotiation
11
IP addressing (IPv4/IPv6)
Describe the operation and necessity of using private and public IP
addresses for IPv4 addressing.
3, 4
Identify the appropriate IPv6-addressing scheme to satisfy addressing
requirements in a LAN/WAN environment.
14
xl Introduction
Operation of IP Data Networks Chapter(s)
Identify the appropriate IPv4-addressing scheme using VLSM and summa-
rization to satisfy addressing requirements in a LAN/WAN environment.
5
Describe the technological requirements for running IPv6 in conjunction
with IPv4 such as dual stack.
14
Describe IPv6 addresses.
u Global unicast
u Multicast
u Link local
u Unique local
u eui-64
u Autoconfiguration
14
IP Routing Technologies
Describe basic routing concepts.
u CEF
u Packet forwarding
u Router lookup process
8
Configure and verify utilizing the CLI to set the basic router configuration.
u Cisco IOS commands to perform basic router setup
6, 7
Configure and verify the operation status of an Ethernet interface. 6
Verify router configuration and network connectivity.
u Cisco IOS commands to review basic router information and network
connectivity
6, 7
Configure and verify routing configuration for a static or default route
given specific routing requirements.
8
Differentiate methods of routing and routing protocols.
u Static vs dynamic
u Link state vs distance vector
u NExt-hop
u ip routing table
u Passive interfaces
8
(continued)
Introduction xli
Operation of IP Data Networks Chapter(s)
Configure and verify OSPF (single area)
u Benefit of single area
u Configure OSPFv2
u Configure OSPFv3
u Router ID
u Passive interface
9, 14
Configure and verify interVLAN routing (router on a stick).
u Subinterfaces
u Upstream routing
u Encapsulation
11
Configure SVI interfaces. 11
IP Services
Configure and verify DHCP (IOS Router).
u Configuring router interfaces to use DHCP
u DHCP options
u Excluded addresses
u Lease time
7
Describe the types, features, and applications of ACLs.
u Standard
u Sequence numbers
u Editing
u Extended
u Named
u Numbered
u Log option
12
Configure and verify ACLs in a network environment.
u Named
u Numbered
u Log option
12
xlii Introduction
Operation of IP Data Networks Chapter(s)
Identify the basic operation of NAT
u Purpose
u Pool
u Static
u 1 to 1
u Overloading
u Source addressing
u One -way NAT
13
Configure and verify NAT for given network requirements. 13
Configure and verify NTP as a client. 7
Network Device Security
Configure and verify network device security features such as:
u Device password security
u Enable secret vs enable
u Transport
u Disable Telnet
u SSH
u VT Ys
u Physical security
u Service password
u External authentication methods
6
Configure and verify switch port security features, such as:
u Sticky MAC
u MAC address limitation
u Static/dynamic
u Violation modes
u Err disable
u Shutdown
u Protect restrict
u Shutdown unused ports
u Err disable recovery
u Assign unused ports to an unused VLAN
u Setting native VLAN to other than VLAN 1
10
(continued)
Introduction xliii
Operation of IP Data Networks Chapter(s)
Configure and verify ACLs to filter network traffic. 12
Configure and verify ACLs to limit Telnet and SSH access to the router. 12
Troubleshooting
Troubleshoot and correct common problems associated with IP address-
ing and host configurations.
5
Troubleshoot and resolve VLAN problems.
u Identify that VLANs are configured
u Port membership correct
u IP address configured
11
Troubleshoot and resolve trunking problems on Cisco switches.
u Correct trunk states
u Correct encapsulation configured
u Correct VLANS allowed
11
Troubleshoot and resolve ACL issues.
u Statistics
u Permitted networks
u Direction
u Interface
12
Troubleshoot and resolve Layer 1 problems.
u Framing
u CRC
u Runts
u Giants
u Dropped packets
u Late collision
u Input/Output errors
6
xliv Introduction
ICND2 (200-101) Exam Objectives
Exam objectives are subject to change at any time without prior notice and at Cisco’s sole
discretion. Please visit Cisco’s certication website (www.cisco.com/web/learning) for the
latest information on the ICND2 exam.
LAN Switching Technologies Chapter(s)
Identify enhanced switching technologies.
u RSTP
u PVSTP
u EtherChannels
15
Configure and verify PVSTP operation.
u Describe root bridge election
u Spanning-tree mode
15
IP Routing Technologies
Describe the boot process of Cisco IOS routers.
u POST
u Router bootup process
16
Configure and verify operation status of a serial interface. 21
Manage Cisco IOS Files.
u Boot preferences
u Cisco IOS image(s)
u Licensing
u Show license
u Change license
16
Differentiate methods of routing and routing protocols.
u Administrative distance
u Split horizon
u Metric
u Next-hop
19
Introduction xlv
LAN Switching Technologies Chapter(s)
Configure and verify OSPF (single area).
u Neighbor adjacencies
u OSPF states
u Discuss Multi-area
u Configure OSPFv2
u Configure OSPFv3
u Router ID
u LSA types
20
Configure and verify EIGRP (single AS)
u Feasible Distance / Feasible Successors / Administrative distance
u Feasibility condition
u Metric composition
u Router ID
u Auto-summary
u Path selection
u Load balancing
u Equal
u Unequal
u Passive interface
19
IP Services
Recognize high availability (FHRP).
u VRRP
u HSRP
u GLBP
17
Configure and verify Syslog.
u Utilize Syslog Output
17
Describe SNMPv2 & SNMPv3 17
Troubleshooting
Identify and correct common network problems. 18
Utilize NetFlow data. 17
xlvi Introduction
LAN Switching Technologies Chapter(s)
Troubleshoot and resolve spanning-tree operation issues
u Root switch
u Priority
u Mode is correct
u Port states
15
Troubleshoot and resolve routing issues
u Routing is enabled
u Routing table is correct
u Correct path selection
18, 19, 20
Troubleshoot and resolve OSPF problems.
u Neighbor adjacencies
u Hello and dead timers
u OSPF area
u Interface MTU
u Network types
u Neighbor states
u OSPF topology database
20
Troubleshoot and resolve EIGRP problems.
u Neighbor adjacencies
u AS number
u Load balancing
u Split horizon
19
Troubleshoot and resolve interVLAN routing problems.
u Connectivity
u Encapsulation
u Subnet
u Native VLAN
u Port mode trunk status
15, 18
Troubleshoot and resolve WAN implementation issues.
u Serial interfaces
u PPP
u Frame relay
21
(continued)
Introduction xlvii
LAN Switching Technologies Chapter(s)
Monitor NetFlow statistics. 17
Troubleshoot EtherChannel problems. 15
WAN Technologies
Identify different WAN Technologies.
u Metro Ethernet
u VSAT
u Cellular 3G/4G
u MPLS
u T1/E1
u ISDN
u DSL
u Frame relay
u Cable
u VPN
21
Configure and verify a basic WAN serial connection. 21
Configure and verify a PPP connection between Cisco routers. 21
Configure and verify frame relay on Cisco routers. 21
Implement and troubleshoot PPPoE. 21
CCNA Composite (200-120)
ExamObjectives
Operation of IP Data Networks Chapter(s)
Recognize the purpose and functions of various network devices, such as
Routers, Switches, Bridges, and Hubs.
1, 2
Select the components required to meet a given network specification. 1, 2
Identify common applications and their impact on the network. 1, 3
xlviii Introduction
Operation of IP Data Networks Chapter(s)
Describe the purpose and basic operation of the protocols in the OSI and
TCP/IP models.
1, 3
Predict the data flow between two hosts across a network. 1, 2, 8, 9, 13
Identify the appropriate media, cables, ports, and connectors, to connect
Cisco network devices to other network devices and hosts in a LAN.
2
LAN Switching Technologies
Determine the technology and media access control method for Ethernet
networks.
2, 10
Identify basic switching concepts and the operation of Cisco switches. 2, 10
u Collision domains 1, 2
u Broadcast domains 1, 2
u Types of switching 1, 2, 10, 11
u CAM table 1, 2, 8, 10, 11
Configure and verify initial switch configuration including remote access
management.
6, 10
u Cisco IOS commands to perform basic switch setup
Verify network status and switch operation using basic utilities, such as
ping, Telnet, and SSH.
6, 7, 10
Describe how VLANs create logically separate networks and the need for
routing between them.
11, 15
u Explain network segmentation and basic traffic management concepts 1, 6, 8, 10
Configure and verify VLANs. 11, 15
Configure and verify trunking on Cisco switches. 11, 15
u DTP 11, 15
u Auto negotiation 11, 15
IP addressing (IPv4/IPv6)
Describe the operation and necessity of using private and public
IPaddresses for IPv4 addressing.
3, 4, 5
Identify the appropriate IPv6-addressing scheme to satisfy addressing
requirements in a LAN/WAN environment.
14
(continued)
Introduction xlix
Operation of IP Data Networks Chapter(s)
Identify the appropriate IPv4-addressing scheme using VLSM and summa-
rization to satisfy addressing requirements in a LAN/WAN environment.
5
Describe the technological requirements for running IPv6 in conjunction
with IPv4-such as dual stack.
14
Describe IPv6 addresses. 14
u Global unicast 14
u Multicast 14
u Link local 14
u Unique local 14
u eui 64 14
u Autoconfiguration 14
IP Routing Technologies
Describe basic routing concepts. 8
u CEF 8
u Packet forwarding 8
u Router lookup process 8
Configure and verify utilizing the CLI to set the basic router configuration. 6, 7
u Cisco IOS commands to perform basic router setup 6
Configure and verify operation status of an Ethernet interface. 6
Verify router configuration and network connectivity. 6, 7
u Cisco IOS commands to review basic router information and network
connectivity
6
Configure and verify routing configuration for a static or default route
given specific routing requirements.
8
Differentiate methods of routing and routing protocols. 8
u Static vs dynamic 8
u Link state vs distance vector 8
u Next-hop 8
l Introduction
Operation of IP Data Networks Chapter(s)
u ip routing table 8
u Passive interfaces 8
Configure and verify OSPF (single area). 9, 14
u Benefit of single area 9
u Configure OSPFv2 9
u Configure OSPFv3 14, 18, 20
u Router ID 14, 18, 19, 20
u Passive interface 8, 9, 14, 19, 20
Configure and verify interVLAN routing (router on a stick). 11, 15
u Subinterfaces 11, 15
u Upstream routing 11, 15
u Encapsulation 11, 15
Configure SVI interfaces. 11, 15
IP Services
Configure and verify DHCP (IOS Router). 6, 7, 17
u Configuring router interfaces to use DHCP 6, 8, 17
u DHCP options 6, 8, 17
u Excluded addresses 6, 8, 17
u Lease time 16, 8, 17
Describe the types, features, and applications of ACLs. 12
u Standard 12
u Sequence numbers 12
u Editing 12
u Extended 12
u Named 12
u Numbered 12
u Log option 12
(continued)
Introduction li
Operation of IP Data Networks Chapter(s)
Configure and verify ACLs in a network environment. 12
u Named 12
u Numbered 12
u Log option 12
Identify the basic operation of NAT. 13
u Purpose 13
u Pool 13
u Static 13
u 1 to 1 13
u Overloading 13
u Source addressing 13
u One-way NAT 13
Configure and verify NAT for given network requirements. 13
Configure and verify NTP as a client. 7
Network Device Security
Configure and verify network device security features such as: 6
u Device password security 6
u Enable secret vs enable 6
u Transport 6
u Disable Telnet 6
u SSH 6
u VT Ys 6
u Physical security 6
u Service password 6
u External authentication methods 6
lii Introduction
Operation of IP Data Networks Chapter(s)
Configure and verify switch port security features such as. 10
u Sticky MAC 10
u MAC address limitation 10
u Static/dynamic 10
u Violation modes 10
u Err disable 10
u Shutdown 10
u Protect restrict 10
u Shutdown unused ports 10
u Err disable recovery 10
u Assign unused ports to an unused VLAN 10
u Setting native VLAN to other than VLAN 1 10
Configure and verify ACLs to filter network traffic. 12
Configure and verify ACLs to limit Telnet and SSH access to the router. 12
Troubleshooting
Troubleshoot and correct common problems associated with IP address-
ing and host configurations.
4, 5, 7, 18
Troubleshoot and resolve VLAN problems. 11, 15
u Identify that VLANs are configured 11, 15
u Port membership correct 11, 15
u IP address configured 11, 15
Troubleshoot and resolve trunking problems on Cisco switches. 11, 15
u Correct trunk states 11, 15
u Correct encapsulation configured 11, 15
u Correct VLANS allowed 11, 15
Troubleshoot and resolve ACL issues. 12, 15
u Statistics 12, 15
u Permitted networks 12, 15
(continued)
Introduction liii
Operation of IP Data Networks Chapter(s)
u Direction 12, 15
u Interface 12, 15
Troubleshoot and resolve Layer 1 problems. 6, 18
u Framing 6, 18
u CRC 6, 18
u Runts 6, 18
u Giants 6, 18
u Dropped packets 6, 18
u Late collision 6, 18
u Input/Output errors 6, 18
LAN Switching Technologies
Identify enhanced switching technologies. 10, 11, 15
u RSTP 15
u PVSTP 15
u EtherChannels 15
Configure and verify PVSTP operation. 15
u Describe root bridge election 15
u Spanning-tree mode 15
IP Routing Technologies 8, 9
Describe the boot process of Cisco IOS routers. 7, 16
u POST 7, 16
u Router bootup process 16
u Configure and verify operation status of a serial interface 21
u Manage Cisco IOS Files 16
u Boot preferences 16
u Cisco IOS image(s) 16
liv Introduction
Operation of IP Data Networks Chapter(s)
u Licensing 16
u Show license 16
u Change license 16
Differentiate methods of routing and routing protocols. 8
u Administrative distance 8
u Split horizon 8
u Metric 8
u Next-hop 8
Configure and verify OSPF (single area). 9
u Neighbor adjacencies 9
u OSPF states 9
u Discuss Multi-area 9
u Configure OSPFv2 9
u Configure OSPFv3 14, 18, 20
u Router ID 9, 14, 18, 20
u LSA types 20
Configure and verify EIGRP (single AS). 19
u Feasible Distance/Feasible Successors/Administrative distance 19
u Feasibility condition 19
u Metric composition 19
u Router ID 19
u Auto-summary 19
u Path selection 19
u Load balancing 19
u Equal 19
u Unequal 19
u Passive interface 8, 19
(continued)
Introduction lv
Operation of IP Data Networks Chapter(s)
IP Services
Recognize high availability (FHRP). 17
u VRRP 17
u HSRP 17
u GLBP 17
Configure and verify Syslog. 17
u Utilize Syslog Output 17
Describe SNMPv2 & SNMPv3. 17
Troubleshooting
Identify and correct common network problems. 1, 2, 6, 7, 8
Utilize NetFlow data. 17
Troubleshoot and resolve spanning-tree operation issues 15
u Root switch 15
u Priority 15
u Mode is correct 15
u Port states 15
Troubleshoot and resolve routing issues. 8, 9, 18, 19, 20
u Routing is enabled 8, 9, 18, 19, 20
u Routing table is correct 8, 9, 18, 19, 20
u Correct path selection 8, 9, 18, 19, 20
Troubleshoot and resolve OSPF problems. 9, 18, 20
u Neighbor adjacencies 9, 18, 20
u Hello and dead timers 9, 18, 20
u OSPF area 9, 18, 20
u Interface MTU 9, 18, 20
u Network types 9, 18, 20
lvi Introduction
Operation of IP Data Networks Chapter(s)
u Neighbor states 9, 18, 20
u OSPF topology database 9, 18, 20
Troubleshoot and resolve EIGRP problems. 19
u Neighbor adjacencies 19
u AS number 19
u Load balancing 19
u Split horizon 19
Troubleshoot and resolve interVLAN routing problems. 11, 15
u Connectivity 11, 15
u Encapsulation 11, 15
u Subnet 11, 15
u Native VLAN 11, 15
u Port mode trunk status 11, 15
Troubleshoot and resolve WAN implementation issues. 21
u Serial interfaces 21
u PPP 21
u Frame relay 21
Monitor NetFlow statistics. 17
Troubleshoot EtherChannel problems. 15
WAN Technologies
Identify different WAN Technologies. 21
u Metro Ethernet 21
u VSAT 21
u Cellular 3G/4G 21
u MPLS 21
u T1/E1 21
(continued)
Introduction lvii
Operation of IP Data Networks Chapter(s)
u ISDN 21
u DSL 21
u Frame relay 21
u Cable 21
u VPN 21
Configure and verify a basic WAN serial connection. 21
Configure and verify a PPP connection between Cisco routers. 21
Configure and verify frame relay on Cisco routers. 21
Implement and troubleshoot PPPoE. 21
Assessment Test
1. What is the sys-id-ext field in a BPDU used for?
A. It is a 4-bit field inserted into an Ethernet frame to define trunking information
between switches.
B. It is a 12-bit field inserted into an Ethernet frame to define VLANs in an STP
instance.
C. It is a 4-bit field inserted into an non-Ethernet frame to define EtherChannel
options.
D. It is a 12-bit field inserted into an Ethernet frame to define STP root bridges.
2. You have four RSTP PVST+ links between switches and want to aggregate the band-
width. What solution will you use?
A. EtherChannel
B. PortFast
C. BPDU Channel
D. VLANs
E. EtherBundle
3. What configuration parameters must be configured the same between switches for
LACP to form a channel? (Choose three.)
A. Virtual MAC address
B. Port speeds
C. Duplex
D. PortFast enabled
E. Allowed VLAN information
4. You reload a router with a configuration register setting of 0x2101. What will the
router do when it reloads?
A. The router enters setup mode.
B. The router enters ROM monitor mode.
C. The router boots the mini-IOS in ROM.
D. The router expands the first IOS in flash memory into RAM.
Assessment Test lix
5. Which of the following commands provides the product ID and serial number of a router?
A. show license
B. show license feature
C. show version
D. show license udi
6. Which command allows you to view the technology options and licenses that are sup-
ported on your router along with several status variables?
A. show license
B. show license feature
C. show license udi
D. show version
7. Which of the following will you use to collect details about network traffic patterns on
your network, including protocols used.
A. SNMPv3
B. Syslogv2
C. NetFlow 9
D. logging host ip_address
8. You want to send a console message to a syslog server, but you only want to send status
messages of 3 and lower. Which of the following commands will you use?
A. logging trap emergencies
B. logging trap errors
C. logging trap debugging
D. logging trap notifications
E. logging trap critical
F. logging trap warnings
G. logging trap alerts
9. When is the AVR router used in GLBP?
A. When HSRP preempts GLBP
B. When a client needs the virtual MAC
C. When it’s set to passive mode and load balancing
D. When it’s load-balancing between VLANs with VVRP
E. When there is a redundancy failure
lx Assessment Test
10. You need to connect to a remote IPv6 server in your virtual server farm. You can con-
nect to the IPv4 servers, but not the critical IPv6 server you desperately need. Based on
the following output, what could your problem be?
C:\>ipconfig
Connection-specific DNS Suffix . : localdomain
IPv6 Address. . . . . . . . . . . : 2001:db8:3c4d:3:ac3b:2ef:1823:8938
Temporary IPv6 Address. . . . . . : 2001:db8:3c4d:3:2f33:44dd:211:1c3d
Link-local IPv6 Address . . . . . : fe80::ac3b:2ef:1823:8938%11
IPv4 Address. . . . . . . . . . . : 10.1.1.10
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 10.1.1.1
A. The global address is in the wrong subnet.
B. The IPv6 default gateway has not been configured or received from the router.
C. The link-local address has not been resolved so the host cannot communicate to
the router.
D. There are two IPv6 global addresses configured. One must be removed from the
configuration.
11. What command is used to view the IPv6-to-MAC-address resolution table on a
Ciscorouter?
A. show ip arp
B. show ipv6 arp
C. show ip neighbors
D. show ipv6 neighbors
E. show arp
12. An IPv6 ARP entry is listed as with a status of REACH. What can you conclude about
the IPv6-to-MAC-address mapping?
A. The interface has communicated with the neighbor address and the mapping is
current.
B. The interface has not communicated within the neighbor reachable time frame.
C. The ARP entry has timed out.
D. IPv6 can reach the neighbor address but the addresses has not yet been resolved.
Assessment Test lxi
13. Serial0/1 goes down. How will EIGRP send packets to the 10.1.1.0 network?
Corp#show ip eigrp topology
[output cut]
P 10.1.1.0/24, 2 successors, FD is 2681842
via 10.1.2.2 (2681842/2169856), Serial0/0
via 10.1.3.1 (2973467/2579243), Serial0/2
via 10.1.3.3 (2681842/2169856), Serial0/1
A. EIGRP will put the 10.1.1.0 network into active mode.
B. EIGRP will drop all packets destined for 10.1.1.0.
C. EIGRP will just keep sending packets out s0/0.
E. EIGRP will use s0/2 as the successor and keep routing to 10.1.1.0.
14. What command produced the following output?
via FE80::201:C9FF:FED0:3301 (29110112/33316), Serial0/0/0
via FE80::209:7CFF:FE51:B401 (4470112/42216), Serial0/0/1
via FE80::209:7CFF:FE51:B401 (2170112/2816), Serial0/0/2
A. show ip protocols
B. show ipv6 protocols
C. show ip eigrp neighbors
D. show ipv6 eigrp neighbors
E. show ip eigrp topology
F. show ipv6 eigrp topology
15. You need to troubleshoot an adjacency between two EIGRP configured routers? What
should you look for? (Choose four.)
A. Verify the AS numbers.
B. Verify that you have the proper interfaces enabled for EIGRP.
C. Make sure there are no mismatched K-values.
D. Check your passive interface settings.
E. Make sure your remote routers are not connected to the Internet.
F. If authentication is configured, make sure all routers use different passwords.
lxii Assessment Test
16. You have two OSPF directly configured routers that are not forming an adjacency.
What should you check? (Choose three.)
A. Process ID
B. Hello and dead timers
C. Link cost
D. Area
E. IP address/subnet mask
17. When do two adjacent routers-enter the 2WAY state?
A. After both routers have received Hello information
B. After they have exchanged topology databases
C. When they connect only to a DR or BDR
D. When they need to exchange RID information
18. Which type of LSAs are generated by ABRs and referred to summary link advertise-
ments (SLAs)?
A. Type 1
B. Type 2
C. Type 3
D. Type 4
E. Type 5
19. Which of the following is not provided by the AH portion of IPsec?
A. Integrity
B. Confidentiality
C. Authenticity
D. Anti-reply
20. Which statement about GRE is not true?
A. GRE is stateless and has no flow control.
B. GRE has security.
C. GRE has additional overhead for tunneled packets, at least 24 bytes.
D. GRE uses a protocol-type field in the GRE header so any layer 3 protocol can be
used through the tunnel.
Assessment Test lxiii
21. How should a router that is being used in a frame relay network be configured to keep
split horizon issues from preventing routing updates?
A. Configure a single subinterface to establish multiple PVC connections to multiple
remote router interfaces.
B. Combine multiple frame relay circuits as a point-to-point line to support multicast
and broadcast traffic.
C. Configure many subinterfaces in the same subnet.
D. Configure a separate subinterface for each PVC with a unique DLCI and subnet
assigned to the subinterface.
22. IPv6 unicast routing is running on the Corp router. Which of the following addresses
would show up with the show ipv6 int brief command?
Corp#sh int f0/0
FastEthernet0/0 is up, line protocol is up
Hardware is AmdFE, address is 000d.bd3b.0d80 (bia 000d.bd3b.0d80)
[output cut]
A. FF02::3c3d:0d:bdff:fe3b:0d80
B. FE80::3c3d:2d:bdff:fe3b:0d80
C. FE80::3c3d:0d:bdff:fe3b:0d80
D. FE80::3c3d:2d:ffbd:3bfe:0d80
23. A host sends a type of NDP message providing the MAC address that was requested.
Which type of NDP was sent?
A. NA
B. RS
C. RA
D. NS
24. Each field in an IPv6 address is how many bits long?
A. 4
B. 16
C. 32
D. 128
lxiv Assessment Test
25. To enable OSPFv3, which of the following would you use?
A. Router(config-if)#ipv6 ospf 10 area 0.0.0.0
B. Router(config-if)#ipv6 router rip 1
C. Router(config)#ipv6 router eigrp 10
D. Router(config-rtr)#no shutdown
E. Router(config-if)#ospf ipv6 10 area 0
26. What does the command routerA(config)#line cons 0 allow you to perform next?
A. Set the Telnet password.
B. Shut down the router.
C. Set your console password.
D. Disable console connections.
2 7. Which two statements describe the IP address 10.16.3.65/23? (Choose two.)
A. The subnet address is 10.16.3.0 255.255.254.0.
B. The lowest host address in the subnet is 10.16.2.1 255.255.254.0.
C. The last valid host address in the subnet is 10.16.2.254 255.255.254.0.
D. The broadcast address of the subnet is 10.16.3.255 255.255.254.0.
E. The network is not subnetted.
28. On which interface do you configure an IP address for a switch?
A. int fa0/0
B. int vty 0 15
C. int vlan 1
D. int s/0/0
29. Which of the following is the valid host range for the subnet on which the IP address
192.168.168.188 255.255.255.192 resides?
A. 192.168.168.129–190
B. 192.168.168.129–191
C. 192.168.168.128–190
D. 192.168.168.128–192
30. Which of the following is considered to be the inside host’s address after translation?
A. Inside local
B. Outside local
C. Inside global
D. Outside global
Assessment Test lxv
31. Your inside locals are not being translated to the inside global addresses. Which of the fol-
lowing commands will show you if your inside globals are allowed to use the NAT pool?
ip nat pool Corp 198.18.41.129 198.18.41.134 netmask 255.255.255.248
ip nat inside source list 100 int pool Corp overload
A. debug ip nat
B. show access-list
C. show ip nat translation
D. show ip nat statistics
32. How many collision domains are created when you segment a network with a
12-port switch?
A. 1
B. 2
C. 5
D. 12
33. Which of the following commands will allow you to set your Telnet password on a
Cisco router?
A. line telnet 0 4
B. line aux 0 4
C. line vty 0 4
D. line con 0
34. Which router command allows you to view the entire contents of all access lists?
A. show all access-lists
B. show access-lists
C. show ip interface
D. show interface
35. What does a VLAN do?
A. Acts as the fastest port to all servers
B. Provides multiple collision domains on one switch port
C. Breaks up broadcast domains in a layer 2 switch internetwork
D. Provides multiple broadcast domains within a single collision domain
lxvi Assessment Test
36. If you wanted to delete the configuration stored in NVRAM, choose the best answer
for the Cisco objectives.
A. erase startup
B. delete running
C. erase flash
D. erase running
3 7. Which protocol is used to send a destination network unknown message back to origi-
nating hosts?
A. TCP
B. ARP
C. ICMP
D. BootP
38. Which class of IP address provides 15 bits for subnetting?
A. A
B. B
C. C
D. D
39. There are three possible routes for a router to reach a destination network. The first
route is from OSPF with a metric of 782. The second route is from RIPv2 with a metric
of 4. The third is from EIGRP with a composite metric of 20514560. Which route will
be installed by the router in its routing table?
A. RIPv2
B. EIGRP
C. OSPF
D. All three
40. Which one of the following is true regarding VLANs?
A. Two VLANs are configured by default on all Cisco switches.
B. VLANs only work if you have a complete Cisco switched internetwork. No off-
brand switches are allowed.
C. You should not have more than 10 switches in the same VTP domain.
D. You need to have a trunk link configured between switches in order to send infor-
mation about more than one VLAN down the link.
Assessment Test lxvii
41. Which two of the following commands will place network 10.2.3.0/24 into area 0?
(Choose two.)
A. router eigrp 10
B. router ospf 10
C. router rip
D. network 10.0.0.0
E. network 10.2.3.0 255.255.255.0 area 0
F. network 10.2.3.0 0.0.0.255 area0
G. network 10.2.3.0 0.0.0.255 area 0
42. How many broadcast domains are created when you segment a network with a
12-port switch?
A. 1
B. 2
C. 5
D. 12
43. If routers in a single area are configured with the same priority value, what value does
a router use for the OSPF router ID in the absence of a loopback interface?
A. The lowest IP address of any physical interface
B. The highest IP address of any physical interface
C. The lowest IP address of any logical interface
D. The highest IP address of any logical interface
44. What protocols are used to configure trunking on a switch? (Choose two.)
A. VLAN Trunking Protocol
B. VLAN
C. 802.1q
D. ISL
45. What is a stub network?
A. A network with more than one exit point
B. A network with more than one exit and entry point
C. A network with only one entry and no exit point
D. A network that has only one entry and exit point
lxviii Assessment Test
46. Where is a hub specified in the OSI model?
A. Session layer
B. Physical layer
C. Data Link layer
D. Application layer
47. What are the two main types of access control lists (ACLs)? (Choose two.)
A. Standard
B. IEEE
C. Extended
D. Specialized
48. Which of the following is the best summarization of the following networks:
192.168.128.0 through 192.168.159.0?
A. 192.168.0.0/24
B. 192.168.128.0/16
C. 192.168.128.0/19
D. 192.168.128.0/20
49. What command is used to create a backup configuration?
A. copy running backup
B. copy running-config startup-config
C. config mem
D. wr net
50. 1000Base-T is which IEEE standard?
A. 802.3f
B. 802.3z
C. 802.3ab
D. 802.3ae
51. Which protocol does DHCP use at the Transport layer?
A. IP
B. TCP
C. UDP
D. ARP
Assessment Test lxix
52. If your router is facilitating a CSU/DSU, which of the following commands do you
need to use to provide the router with a 64000 bps serial link?
A. RouterA(config)#bandwidth 64
B. RouterA(config-if)#bandwidth 64000
C. RouterA(config)#clockrate 64000
D. RouterA(config-if)#clock rate 64
E. RouterA(config-if)#clock rate 64000
53. Which command is used to determine if an access list is enabled on a particular interface?
A. show access-lists
B. show interface
C. show ip interface
D. show interface access-lists
54. Which of the following statements is true with regard to ISL and 802.1q?
A. 802.1q encapsulates the frame with control information; ISL inserts an ISL field
along with tag control information.
B. 802.1q is Cisco proprietary.
C. ISL encapsulates the frame with control information; 802.1q inserts an 802.1q
field along with tag control information.
D. ISL is a standard.
55. The protocol data unit (PDU) encapsulation is completed in which order?
A. Bits, frames, packets, segments, data
B. Data, bits, segments, frames, packets
C. Data, segments, packets, frames, bits
D. Packets, frames, bits, segments, data
56. Based on the configuration shown below, what statement is true?
S1(config)#ip routing
S1(config)#int vlan 10
S1(config-if)#ip address 192.168.10.1 255.255.255.0
S1(config-if)#int vlan 20
S1(config-if)#ip address 192.168.20.1 255.255.255.0
A. This is a multilayer switch.
B. The two VLANs are in the same subnet.
C. Encapsulation must be configured.
D. VLAN 10 is the management VLAN.
lxx Answers to Assessment Test
Answers to Assessment Test
1. B. To allow for the PVST+ to operate, there’s a field inserted into the BPDU to accom-
modate the extended system ID so that PVST+ can have a root bridge configured on a
per-STP instance. The extended system ID (VLAN ID) is a 12-bit field, and we can even
see what this field is carrying via show spanning-tree command output. See Chapter 15
for more information.
2. A. Cisco’s EtherChannel can bundle up to eight ports between switches to provide resil-
iency and more bandwidth between switches. See Chapter 15 for more information.
3. B, C, E. All the ports on both sides of every link must be configured exactly the same
between switches or it will not work. Speed, duplex, and allowed VLANs must match.
See Chapter 15 for more information.
4. C. 2100 boots the router into ROM monitor mode, 2101 loads the mini-IOS from
ROM, and 2102 is the default and loads the IOS from flash. See Chapter 16 for more
information.
5. D. The show license udi command displays the unique device identifier (UDI) of
the router, which comprises the product ID (PID) and serial number of the router. See
Chapter 16 for more information.
6. B. The show license feature command allows you to view the technology package
licenses and feature licenses that are supported on your router along with several status
variables related to software activation and licensing, both licensed and unlicensed fea-
tures. See Chapter 16 for more information.
7. C. NetFlow allows the collection of flows of traffic on the network, including address
and protocols in use. A recent version of NetFlow is version 9, referred to as Flexible
NetFlow. See Chapter 17 for more information.
8. B. There are eight different trap levels. If you choose, for example level 3, level 0
through level 3 messages will be displayed. See Chapter 17 for more information.
9. B. The active virtual router (AVR) responds with the virtual MAC addresses to the
clients. See Chapter 17 for more information.
10. B. There is no IPv6 default gateway listed in the output, which will be the link-local
address of the router interface, sent to the host as a router advertisement. Until this
host receives the router address, the host will communicate with IPv6 only on the local
subnet. See Chapter 18 for more information.
11. D. The command show ipv6 neighbors provides the ARP cache for on a router. See
Chapter 18 for more information.
Answers to Assessment Test lxxi
12. A. If the state is STALE when the interface has not communicated within the neigh-
bor reachable time frame. The next time the neighbor communicates, the state will be
REACH. See Chapter 18 for more information.
13. C. There are two successor routes, so by default, EIGRP was load-balancing out s0/0 and
s0/1. When s0/1 goes down, EIGRP will just keep forwarding traffic out the second link
s0/0. s0/1 will be removed from the routing table. See Chapter 19 for more information.
14. F. There isn’t a lot to go on from with the output, but the only commands that provide
the FD and AD are show ip eigrp topology and show ipv6 eigrp topology. The
addresses in the output are link-local IPv6 addresses, so our answer is the latter. See
Chapter 19 for more information.
15. A, B, C, D. Cisco has documented steps, according to the objectives, that you must go
through when troubleshooting an adjacency. See Chapter 19 for more information.
16. B, D, E. In order for two OSPF routers to create an adjacency, the Hello and dead tim-
ers must match, and they must both be configured into the same area, as well as being
in the same subnet. See Chapter 20 for more information.
17. A. The process starts by sending out Hello packets. Every listening router will then
add the originating router to the neighbor database. The responding routers will reply
with all of their Hello information so that the originating router can add them to its
own neighbor table. At this point, we will have reached the 2WAY state—only certain
routers will advance beyond to this. See Chapter 20 for more information.
18. C. Referred to as summary link advertisements (SLAs), Type 3 LSAs are generated by
area border routers. These ABRs send Type 3 LSAs toward the area external to the one
where they were generated. See Chapter 20 for more information.
19. B. Authentication Header (AH) provides authentication of either all or part of the
IP packet through the addition of a header that is calculated based on the values in the
packet, but it doesn’t offer any encryption services. See Chapter 21 for more information.
20. B. Generic Routing Encapsulation (GRE) has no built-in security mechanisms. See
Chapter 21 for more information.
21. D. If you have a serial port configured with multiple DLCIs connected to multiple
remote sites, split horizon rules stop route updates received on an interface from being
sent out the same interface. By creating subinterfaces for each PVC, you can avoid the
split horizon issues when using Frame Relay. See Chapter 21 for more information.
22. B. This can be a hard question if you don’t remember to invert the 7th bit of the first
octet in the MAC address! Always look for the 7th bit when studying for the Cisco R/S,
and when using eui-64, invert it. The eui-64 autoconfiguration then inserts an FF:FE in
the middle of the 48-bit MAC address to create a unique IPv6 address. See Chapter 14
for more information.
lxxii Answers to Assessment Test
23. A. The NDP neighbor advertisement (NA) contains the MAC address. A neighbor
solicitation (NS) was initially sent asking for the MAC address. See Chapter 14 for
more information.
24. B. Each field in an IPv6 address is 16 bits long. An IPv6 address is a total of 128 bits.
See Chapter 14 for more information.
25. A. To enable OSPFv3, you enable the protocol at the interface level, as with RIPng.
The command string is area-id. It’s important to understand that area 0 and area
0.0.0.0 both describe area 0. See Chapter 14 for more information.
26. C. The command line console 0 places you at a prompt where you can then set your
console user-mode password. See Chapter 6 for more information.
2 7. B, D. The mask 255.255.254.0 (/23) used with a Class A address means that there are
15 subnet bits and 9 host bits. The block size in the third octet is 2 (256–254). So this
makes the subnets in the interesting octet 0, 2, 4, 6, etc., all the way to 254. The host
10.16.3.65 is in the 2.0 subnet. The next subnet is 4.0, so the broadcast address for the
2.0 subnet is 3.255. The valid host addresses are 2.1 through 3.254. See Chapter 4 for
more information.
28. C. The IP address is configured under a logical interface, called a management domain
or VLAN 1, by default. See Chapter 10 for more information.
29. A. 256 – 192 = 64, so 64 is our block size. Just count in increments of 64 to find our
subnet: 64 + 64 = 128. 128 + 64 = 192. The subnet is 128, the broadcast address is
191, and the valid host range is the numbers in between, or 129–190. See Chapter 4
for more information.
30. C. An inside global address is considered to be the IP address of the host on the private
network after translation. See Chapter 13 for more information.
31. B. Once you create your pool, the command ip nat inside source must be used to
say which inside locals are allowed to use the pool. In this question, we need to see if
access list 100 is configured correctly, if at all, so show access-list is the best answer.
See Chapter 13 for more information.
32. D. Layer 2 switching creates individual collision domains per port. See Chapter 1 for
more information.
33. C. The command line vty 0 4 places you in a prompt that will allow you to set or
change your Telnet password. See Chapter 6 for more information.
34. B. To see the contents of all access lists, use the show access-lists command. See
Chapter12 for more information.
35. C. VLANs break up broadcast domains at layer 2. See Chapter 11 for more information.
Answers to Assessment Test lxxiii
36. A. The command erase startup-config deletes the configuration stored in NVRAM.
See Chapter 6 for more information.
3 7. C. ICMP is the protocol at the Network layer that is used to send messages back to an
originating router. See Chapter 3 for more information.
38. A. Class A addressing provides 22 bits for host subnetting. Class B provides 16 bits,
but only 14 are available for subnetting. Class C provides only 6 bits for subnetting.
See Chapter 3 for more information.
39. B. Only the EIGRP route will be placed in the routing table because EIGRP has the low-
est administrative distance (AD), and that is always used before metrics. See Chapter 8
for more information.
40. D. Switches send information about only one VLAN down a link unless it is configured
as a trunk link. See Chapter 11 for more information.
41. B, G. To enable OSPF, you must first start OSPF using a process ID. The number is
irrelevant; just choose a number from 1 to 65,535 and you’re good to go. After you
start the OSPF process, you must configure interfaces on which to activate OSPF using
the network command with wildcards and specification of an area. Option F is wrong
because there must be a space after the parameter area and before you list the area
number. See Chapter 9 for more information.
42. A. By default, switches break up collision domains on a per-port basis but are one
large broadcast domain. See Chapter 1 for more information.
43. B. At the moment of OSPF process startup, the highest IP address on any active inter-
face will be the router ID (RID) of the router. If you have a loopback interface config-
ured (logical interface), then that will override the interface IP address and become the
RID of the router automatically. See Chapter 9 for more information.
44. C, D. VLAN Trunking Protocol (VTP) is not right because it has nothing to do with
trunking except that it sends VLAN information across a trunk link. 802.1q and ISL
encapsulations are used to configure trunking on a port. See Chapter 11 for more
information.
45. D. Stub networks have only one connection to an internetwork. Default routes should
be set on a stub network or network loops may occur; however, there are exceptions to
this rule. See Chapter 8 for more information.
46. B. Hubs regenerate electrical signals, which are specified at the Physical layer. See
Chapter 1 for more information.
47. A, C. Standard and extended access control lists (ACLs) are used to configure security
on a router. See Chapter 12 for more information.
lxxiv Answers to Assessment Test
48. C. If you start at 192.168.128.0 and go through 192.168.159.0, you can see that this is
a block of 32 in the third octet. Since the network address is always the first one in the
range, the summary address is 192.168.128.0. What mask provides a block of 32 in the
third octet? The answer is 255.255.224.0, or /19. See Chapter 5 for more information.
49. B. The command to back up the configuration on a router is copy running-config
startup-config. See Chapter 7 for more information.
50. C. IEEE 802.3ab is the standard for 1 Gbps on twisted-pair. See Chapter 2 for more
information.
51. C. User Datagram Protocol is a connection network service at the Transport layer, and
DHCP uses this connectionless service. See Chapter 3 for more information.
52. E. The clock rate command is two words, and the speed of the line is in bits per
second (bps). See Chapter 6 for more information.
53. C. The show ip interface command will show you if any interfaces have an outbound
or inbound access list set. See Chapter 12 for more information.
54. C. Unlike ISL, which encapsulates the frame with control information, 802.1q inserts an
802.1q field along with tag control information. See Chapter 11 for more information.
55. C. The PDU encapsulation method defines how data is encoded as it goes through
each layer of the TCP/IP model. Data is segmented at the Transport later, packets cre-
ated at the Network layer, frames at the Data Link layer, and finally, the Physical layer
encodes the 1s and 0s into a digital signal. See Chapter 2 for more information.
56. A. With a multilayer switch, enable IP routing and create one logical interface for each
VLAN using the interface vlan number command and you’re now doing inter-VLAN
routing on the backplane of the switch! See Chapter 11 for more information.
PART
I
ICND1
Chapter
1
Internetworking
THE FOLLOWING ICND1 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 Operation of IP Data Networks
■Recognize the purpose and functions of various network
devices such as Routers, Switches, Bridges and Hubs.
■Select the components required to meet a given network
specification.
■Identify common applications and their impact on the
network.
■Describe the purpose and basic operation of the protocols in
the OSI and TCP/IP models.
Welcome to the exciting world of internetworking. This rst
chapter will serve as an internetworking review by focusing
on how to connect networks together using Cisco routers and
switches, and I’ve written it with the assumption that you have some simple basic network-
ing knowledge. The emphasis of this review will be on the Cisco CCENT and/or CCNA
Routing and Switching (CCNA R/S) objectives you’ll need a solid grasp on in order to suc-
ceed in getting your certications.
Let’s start by dening exactly what an internetwork is: You create an internetwork when
you connect two or more networks via a router and congure a logical network addressing
scheme with a protocol such as IP or IPv6.
We’ll also dissect the Open Systems Interconnection (OSI) model, and I’ll describe
each part of it to you in detail because you really need complete, reliable knowledge of it.
Understanding the OSI model is key for the solid foundation you’ll need to build upon with
the more advanced Cisco networking knowledge gained as you become increasingly skilled.
The OSI model has seven hierarchical layers that were developed to enable different
networks to communicate reliably between disparate systems. Since this book is centering
upon all things CCNA, it’s crucial for you to understand the OSI model as Cisco sees it, so
that’s how I’ll be presenting the seven layers to you.
After you nish reading this chapter, you’ll encounter review questions and written labs.
These are given to you to really lock the information from this chapter into your memory.
So don’t skip them!
To find up-to-the-minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
Internetworking Basics
Before exploring internetworking models and the OSI model’s specications, you need to
grasp the big picture and the answer to this burning question: Why is it so important to learn
Cisco internetworking anyway?
Networks and networking have grown exponentially over the past 20 years, and under-
standably so. They’ve had to evolve at light speed just to keep up with huge increases in
basic, mission-critical user needs, (e.g. simple sharing data and printers), as well as greater
burdens like multimedia remote presentations and conferencing. Unless everyone who needs
Internetworking Basics 5
to share network resources is located in the same ofce space—an increasingly uncommon
situation—the challenge is to connect relevant networks so all users can share the wealth of
whatever services and resources are required.
Figure1.1 shows a basic local area network (LAN) that’s connected using a hub, which is
basically just an antiquated device that connects wires together. Keep in mind that a simple
network like this would be considered one collision domain and one broadcast domain. No
worries if you have no idea what I mean by that because coming up soon, I’m going to talk
about collision and broadcast domains enough to make you dream about them!
FIGURE1.1 A very basic network
Sally
Bob HEY SALLY?
Hub
Okay, things really can’t get much simpler than this. And yes, though you can still nd
this conguration in some home networks, even many of those as well as the smallest busi-
ness networks are more complicated today. As we move through this book, I’ll just keep
building upon this tiny network a bit at a time until we arrive at some really nice, robust
and current network designs—the types that will help you get your certication and a job!
But as I said, we’ll get there one step at a time, so let’s get back to the network shown
in Figure1.1 with this scenario: Bob wants to send Sally a le, and to complete that goal in
this kind of network, he’ll simply broadcast that he’s looking for her, which is basically just
shouting out over the network. Think of it like this, Bob walking out of his house and yell-
ing down a street called Chaos Court in order to contact Sally. This might work if Bob and
Sally were the only ones living there, but not so much if it’s crammed with homes and all
the others living there are always hollering up and down the street to their neighbors just
like Bob. Nope, Chaos Court would absolutely live up to its name, with all those residents
going off whenever they felt like it—and believe it or not, our networks actually still work
this way to a degree! So, given a choice, would you stay in Chaos, or would you pull up
stakes and move on over to a nice new modern community called Broadway Lanes, which
offers plenty of amenities and room for your home plus future additions all on nice, wide
streets that can easily handle all present and future trafc? Good choice… so did Sally, who
now lives a much quieter life, getting letters (packets) from Bob instead of a headache!
The scenario I just described brings me to the basic point of what this book and the
Cisco certication objectives are really all about. My goal of showing you how to create
efcient networks and segment them correctly in order to minimize all the chaotic yelling
and screaming going on in them is a universal theme throughout my CCENT and CCNA
series books. It’s just inevitable that you’ll have to break up a large network into a bunch
6 Chapter 1 u Internetworking
of smaller ones at some point to match a network’s equally inevitable growth, and as that
expansion occurs, user response time simultaneously dwindles to a frustrating crawl. But
if you master the vital technology and skills I have in store for you in this series, you’ll be
well equipped to rescue your network and its users by creating an efcient new network
neighborhood to give them key amenities like the bandwidth they need to meet their
evolving demands.
And this is no joke; most of us think of growth as good—and it can be—but as many
of us experience daily when commuting to work, school, etc., it can also mean your LAN’s
trafc congestion can reach critical mass and grind to a complete halt! Again, the solu-
tion to this problem begins with breaking up a massive network into a number of smaller
ones—something called network segmentation. This concept is a lot like planning a new
community or modernizing an existing one. More streets are added, complete with new
intersections and trafc signals, plus post ofces with ofcial maps documenting all those
street names and directions on how to get to each are built. You’ll need to effect new laws
to keep order to it all and provide a police station to protect this nice new neighborhood
as well. In a networking neighborhood environment, all of this is carried out using devices
like routers, switches, and bridges.
So let’s take a look at our new neighborhood now, because the word has gotten out; many
more hosts have moved into it, so it’s time to upgrade that new high-capacity infrastructure
that we promised to handle the increase in population. Figure1.2 shows a network that’s
been segmented with a switch, making each network segment that connects to the switch its
own separate collision domain. Doing this results in a lot less yelling!
FIGURE1.2 A switch can break up collision domains.
Sally
Bob
John
HEY JOHN!
Hub Switch
S1
This is a great start, but I really want you to make note of the fact that this network is
still one, single broadcast domain, meaning that we’ve really only decreased our scream-
ing and yelling, not eliminated it. For example, if there’s some sort of vital announcement
that everyone in our neighborhood needs to hear about, it will denitely still get loud!
You can see that the hub used in Figure1.2 just extended the one collision domain from
the switch port. The result is that John received the data from Bob but, happily, Sally did
not. This is good because Bob intended to talk with John directly, and if he had needed to
send a broadcast instead, everyone, including Sally, would have received it, possibly caus-
ing unnecessary congestion.
Internetworking Basics 7
Here’s a list of some of the things that commonly cause LAN trafc congestion:
u Too many hosts in a collision or broadcast domain
u Broadcast storms
u Too much multicast traffic
u Low bandwidth
u Adding hubs for connectivity to the network
u A bunch of ARP broadcasts
Take another look at Figure1.2 and make sure you see that I extended the main hub
from Figure1.1 to a switch in Figure1.2. I did that because hubs don’t segment a network;
they just connect network segments. Basically, it’s an inexpensive way to connect a couple
of PCs, and again, that’s great for home use and troubleshooting, but that’s about it!
As our planned community starts to grow, we’ll need to add more streets with trafc
control, and even some basic security. We’ll achieve this by adding routers because these con-
venient devices are used to connect networks and route packets of data from one network to
another. Cisco became the de facto standard for routers because of its unparalleled selection of
high-quality router products and fantastic service. So never forget that by default, routers are
basically employed to efciently break up a broadcast domain—the set of all devices on a net-
work segment, which are allowed to “hear” all broadcasts sent out on that specic segment.
Figure1.3 depicts a router in our growing network, creating an internetwork and break-
ing up broadcast domains.
FIGURE1.3 Routers create an internetwork.
I LOVE SHOUTING!
. . . HEY EVERYONE!
Sure is nice and quiet here.
S2 S2
R1
The network in Figure1.3 is actually a pretty cool little network. Each host is connected
to its own collision domain because of the switch, and the router has created two broadcast
domains. So now our Sally is happily living in peace in a completely different neighborhood,
no longer subjected to Bob’s incessant shouting! If Bob wants to talk with Sally, he has to
send a packet with a destination address using her IP address—he cannot broadcast for her!
But there’s more… routers provide connections to wide area network (WAN) services as
well via a serial interface for WAN connections—specically, a V.35 physical interface on a
Cisco router.
8 Chapter 1 u Internetworking
Let me make sure you understand why breaking up a broadcast domain is so important.
When a host or server sends a network broadcast, every device on the network must read
and process that broadcast—unless you have a router. When the router’s interface receives
this broadcast, it can respond by basically saying, “Thanks, but no thanks,” and discard
the broadcast without forwarding it on to other networks. Even though routers are known
for breaking up broadcast domains by default, it’s important to remember that they break
up collision domains as well.
There are two advantages to using routers in your network:
u They don’t forward broadcasts by default.
u They can filter the network based on layer 3, Network layer, information such as an
IP address.
Here are four ways a router functions in your network:
u Packet switching
u Packet filtering
u Internetwork communication
u Path selection
I’ll tell you all about the various layers later in this chapter, but for now, it’s helpful to
think of routers as layer 3 switches. Unlike plain-vanilla layer 2 switches, which forward
or lter frames, routers (layer 3 switches) use logical addressing and provide an important
capacity called packet switching. Routers can also provide packet ltering via access lists,
and when routers connect two or more networks together and use logical addressing (IP or
IPv6), you then have an internetwork. Finally, routers use a routing table, which is essen-
tially a map of the internetwork, to make best path selections for getting data to its proper
destination and properly forward packets to remote networks.
Conversely, we don’t use layer 2 switches to create internetworks because they don’t break
up broadcast domains by default. Instead, they’re employed to add functionality to a network
LAN. The main purpose of these switches is to make a LAN work better—to optimize its
performance—providing more bandwidth for the LAN’s users. Also, these switches don’t for-
ward packets to other networks like routers do. Instead, they only “switch” frames from one
port to another within the switched network. And don’t worry, even though you’re probably
thinking, “Wait—what are frames and packets?” I promise to completely ll you in later in
this chapter. For now, think of a packet as a package containing data.
Okay, so by default, switches break up collision domains, but what are these things?
Collision domain is an Ethernet term used to describe a network scenario in which one
device sends a packet out on a network segment and every other device on that same seg-
ment is forced to pay attention no matter what. This isn’t very efcient because if a differ-
ent device tries to transmit at the same time, a collision will occur, requiring both devices
to retransmit, one at a time—not good! This happens a lot in a hub environment, where
each host segment connects to a hub that represents only one collision domain and a single
broadcast domain. By contrast, each and every port on a switch represents its own collision
domain, allowing network trafc to ow much more smoothly.
Internetworking Basics 9
Switches create separate collision domains within a single broadcast
domain. Routers provide a separate broadcast domain for each interface.
Don’t let this ever confuse you!
The term bridging was introduced before routers and switches were implemented, so it’s
pretty common to hear people referring to switches as bridges. That’s because bridges and
switches basically do the same thing—break up collision domains on a LAN. Note to self
that you cannot buy a physical bridge these days, only LAN switches, which use bridging
technologies. This does mean that you’ll still hear Cisco and others refer to LAN switches
as multiport bridges now and then.
But does it mean that a switch is just a multiple-port bridge with more brainpower?
Well, pretty much, only there are still some key differences. Switches do provide a bridging
function, but they do that with greatly enhanced management ability and features. Plus,
most bridges had only 2 or 4 ports, which is severely limiting. Of course, it was possible to
get your hands on a bridge with up to 16 ports, but that’s nothing compared to the hun-
dreds of ports available on some switches!
You would use a bridge in a network to reduce collisions within broadcast
domains and to increase the number of collision domains in your network.
Doing this provides more bandwidth for users. And never forget that using
hubs in your Ethernet network can contribute to congestion. As always,
plan your network design carefully!
Figure1.4 shows how a network would look with all these internetwork devices in place.
Remember, a router doesn’t just break up broadcast domains for every LAN interface, it
breaks up collision domains too.
Looking at Figure1.4, did you notice that the router has the center stage position and
connects each physical network together? I’m stuck with using this layout because of the
ancient bridges and hubs involved. I really hope you don’t run across a network like this,
but it’s still really important to understand the strategic ideas that this gure represents!
See that bridge up at the top of our internetwork shown in Figure1.4? It’s there to connect
the hubs to a router. The bridge breaks up collision domains, but all the hosts connected to
both hubs are still crammed into the same broadcast domain. That bridge also created only
three collision domains, one for each port, which means that each device connected to a hub
is in the same collision domain as every other device connected to that same hub. This is
really lame and to be avoided if possible, but it’s still better than having one collision domain
for all hosts! So don’t do this at home; it’s a great museum piece and a wonderful example
of what not to do, but this inefcient design would be terrible for use in today’s networks! It
does show us how far we’ve come though, and again, the foundational concepts it illustrates
are really important for you to get.
10 Chapter 1 u Internetworking
FIGURE1.4 Internetworking devices
Hub
WAN Services ISP
Switch
Bridge
Router
And I want you to notice something else: The three interconnected hubs at the bottom
of the gure also connect to the router. This setup creates one collision domain and one
broadcast domain and makes that bridged network, with its two collision domains, look
majorly better by contrast!
Don’t misunderstand… bridges/switches are used to segment networks,
but they will not isolate broadcast or multicast packets.
The best network connected to the router is the LAN switched network on the left.
Why? Because each port on that switch breaks up collision domains. But it’s not all good—
all devices are still in the same broadcast domain. Do you remember why this can be really
bad? Because all devices must listen to all broadcasts transmitted, that’s why! And if your
broadcast domains are too large, the users have less bandwidth and are required to process
more broadcasts. Network response time eventually will slow to a level that could cause
riots and strikes, so it’s important to keep your broadcast domains small in the vast major-
ity of networks today.
Once there are only switches in our example network, things really change a lot! Figure1.5
demonstrates a network you’ll typically stumble upon today.
Internetworking Basics 11
Here I’ve placed the LAN switches at the center of this network world, with the routers
connecting the logical networks. If I went ahead and implemented this design, I’ve created
something called virtual LANs, or VLANs, which are used when you logically break up
broadcast domains in a layer 2, switched network. It’s really important to understand that
even in a switched network environment, you still need a router to provide communication
between VLANs. Don’t forget that!
FIGURE1.5 Switched networks creating an internetwork
Router
Router
Still, clearly the best network design is the one that’s perfectly congured to meet the
business requirements of the specic company or client it serves, and it’s usually one in
which LAN switches exist in harmony with routers strategically placed in the network. It’s
my hope that this book will help you understand the basics of routers and switches so you
can make solid, informed decisions on a case-by-case basis and be able to achieve that goal!
But I digress…
So let’s go back to Figure1.4 now for a minute and really scrutinize it because I want
to ask you this question: How many collision domains and broadcast domains are really
there in this internetwork? I hope you answered nine collision domains and three broad-
cast domains! The broadcast domains are denitely the easiest to spot because only routers
break up broadcast domains by default, and since there are three interface connections,
that gives you three broadcast domains. But do you see the nine collision domains? Just in
case that’s a no, I’ll explain. The all-hub network at the bottom is one collision domain; the
bridge network on top equals three collision domains. Add in the switch network of ve
collision domains—one for each switch port—and you get a total of nine!
While we’re at this, in Figure1.5, each port on the switch is a separate collision domain,
and each VLAN would be a separate broadcast domain. So how many collision domains do
you see here? I’m counting 12—remember that connections between the switches are con-
sidered a collision domain! Since the gure doesn’t show any VLAN information, we can
assume the default of one broadcast domain is in place.
12 Chapter 1 u Internetworking
Should I Replace My Existing 10/100 Mbps Switches?
Let’s say you’re a network administrator at a large company. The boss comes to you and
says that he got your requisition to buy a bunch of new switches but he’s really freaking
out about the price tag! Should you push it—do you really need to go this far?
If you can, absolutely! Make your case and go for it because the newest switches add
really huge capacity to a network that older 10/100 Mbps switches just can’t touch. And
yes, five-year-old switches are considered pretty Pleistocene these days. But in real-
ity, most of us just don’t have an unlimited budget to buy all new gigabit switches and
10/100 Mbps switches can still create a nice network—if you design and implement that
network correctly! Still, plan and budget accordingly because you’ll have to replace
those 10/100 switches eventually.
Another good question: Do you really need 1 Gbps or better switch ports for all your users,
servers, and other devices? Yes, you absolutely need new higher-end switches! This is
because servers and hosts are no longer the bottlenecks of our internetworks, our routers
and switches are—especially legacy ones. We now need gigabit on the desktop and on every
router interface; 10 Gbps would be better, and go even higher if you can afford it.
So, go ahead. Put that requisition for all new switches. You’ll be a hero before long!
Okay, so now that you’ve gotten a pretty thorough introduction to internetworking and
the various devices that populate an internetwork, it’s time to head into exploring the inter-
networking models.
Internetworking Models
First a little history: When networks rst came into being, computers could typically
communicate only with computers from the same manufacturer. For example, companies
ran either a complete DECnet solution or an IBM solution, never both together. In the
late 1970s, the Open Systems Interconnection (OSI) reference model was created by
the International Organization for Standardization (ISO) to break through this barrier.
The OSI model was meant to help vendors create interoperable network devices and
software in the form of protocols so that different vendor networks could work in peace-
able accord with each other. Like world peace, it’ll probably never happen completely, but
it’s still a great goal!
Anyway the OSI model is the primary architectural model for networks. It describes how
data and network information are communicated from an application on one computer
Internetworking Models 13
through the network media to an application on another computer. The OSI reference model
breaks this approach into layers.
Coming up, I’ll explain the layered approach to you plus how we can use it to help us
troubleshoot our internetworks.
Goodness! ISO, OSI, and soon you’ll hear about IOS! Just remember that the
ISO created the OSI and that Cisco created the Internetworking Operating
System (IOS), which is what this book is all-so-about.
The Layered Approach
Understand that a reference model is a conceptual blueprint of how communications should
take place. It addresses all the processes required for effective communication and divides
them into logical groupings called layers. When a communication system is designed in this
manner, it’s known as a hierarchical or layered architecture.
Think of it like this: You and some friends want to start a company. One of the rst
things you’ll do is sort out every task that must be done and decide who will do what. You
would move on to determine the order in which you would like everything to be done with
careful consideration of how all your specic operations relate to each other. You would
then organize everything into departments (e.g., sales, inventory, and shipping), with each
department dealing with its specic responsibilities and keeping its own staff busy enough
to focus on their own particular area of the enterprise.
In this scenario, departments are a metaphor for the layers in a communication system.
For things to run smoothly, the staff of each department has to trust in and rely heavily upon
those in the others to do their jobs well. During planning sessions, you would take notes,
recording the entire process to guide later discussions and clarify standards of operation,
thereby creating your business blueprint—your own reference model.
And once your business is launched, your department heads, each armed with the part of
the blueprint relevant to their own department, will develop practical ways to implement their
distinct tasks. These practical methods, or protocols, will then be compiled into a standard
operating procedures manual and followed closely because each procedure will have been
included for different reasons, delimiting their various degrees of importance and implemen-
tation. All of this will become vital if you form a partnership or acquire another company
because then it will be really important that the new company’s business model is compatible
with yours!
Models happen to be really important to software developers too. They often use a refer-
ence model to understand computer communication processes so they can determine which
functions should be accomplished on a given layer. This means that if someone is creating a
protocol for a certain layer, they only need to be concerned with their target layer’s function.
Software that maps to another layers’ protocols and is specically designed to be deployed
there will handle additional functions. The technical term for this idea is binding. The com-
munication processes that are related to each other are bound, or grouped together, at a
particular layer.
14 Chapter 1 u Internetworking
Advantages of Reference Models
The OSI model is hierarchical, and there are many advantages that can be applied to any
layered model, but as I said, the OSI model’s primary purpose is to allow different vendors’
networks to interoperate.
Here’s a list of some of the more important benets for using the OSI layered model:
u It divides the network communication process into smaller and simpler components,
facilitating component development, design, and troubleshooting.
u It allows multiple-vendor development through the standardization of network
components.
u It encourages industry standardization by clearly defining what functions occur at each
layer of the model.
u It allows various types of network hardware and software to communicate.
u It prevents changes in one layer from affecting other layers to expedite development.
The OSI Reference Model
One of best gifts the OSI specications gives us is paving the way for the data transfer
between disparate hosts running different operating systems, like Unix hosts, Windows
machines, Macs, smartphones, and so on.
And remember, the OSI is a logical model, not a physical one. It’s essentially a set of guide-
lines that developers can use to create and implement applications to run on a network. It also
provides a framework for creating and implementing networking standards, devices, and inter-
networking schemes.
The OSI has seven different layers, divided into two groups. The top three layers dene
how the applications within the end stations will communicate with each other as well as
with users. The bottom four layers dene how data is transmitted end to end.
Figure1.6 shows the three upper layers and their functions.
FIGURE1.6 The upper layers
Presentation
Application
Session
• Provides a user interface
• Presents data
• Handles processing such as encryption
• Keeps different applications’ data separate
When looking at Figure1.6, understand that users interact with the computer at
the Application layer and also that the upper layers are responsible for applications
The OSI Reference Model 15
communicating between hosts. None of the upper layers knows anything about network-
ing or network addresses because that’s the responsibility of the four bottom layers.
In Figure1.7, which shows the four lower layers and their functions, you can see that
it’s these four bottom layers that dene how data is transferred through physical media like
wire, cable, ber optics, switches, and routers. These bottom layers also determine how to
rebuild a data stream from a transmitting host to a destination host’s application.
FIGURE1.7 The lower layers
Transport
Network
• Provides reliable or unreliable delivery
• Performs error correction before retransmit
Data Link
• Combines packets into bytes and bytes into frames
• Provides access to media using MAC address
• Performs error detection not correction
Physical • Moves bits between devices
• Specifies voltage, wire speed, and pinout of cables
• Provides logical addressing, which routers use for path determination
The following network devices operate at all seven layers of the OSI model:
u Network management stations (NMSs)
u Web and application servers
u Gateways (not default gateways)
u Servers
u Network hosts
Basically, the ISO is pretty much the Emily Post of the network protocol world. Just as
Ms. Post wrote the book setting the standards—or protocols—for human social interac-
tion, the ISO developed the OSI reference model as the precedent and guide for an open
network protocol set. Dening the etiquette of communication models, it remains the most
popular means of comparison for protocol suites today.
The OSI reference model has the following seven layers:
u Application layer (layer 7)
u Presentation layer (layer 6)
u Session layer (layer 5)
u Transport layer (layer 4)
u Network layer (layer 3)
u Data Link layer (layer 2)
u Physical layer (layer 1)
16 Chapter 1 u Internetworking
Some people like to use a mnemonic to remember the seven layers, such as All People
Seem To Need Data Processing. Figure1.8 shows a summary of the functions dened at
each layer of the OSI model.
FIGURE1.8 OSI layer functions
Application • File, print, message, database, and application services
Presentation • Data encryption, compression, and translation services
Session • Dialog control
Transport • End-to-end connection
Network • Routing
Data Link • Framing
Physical • Physical topology
I’ve separated the 7-layer model into three different functions: the upper layers, the middle
layers and the bottom layers. The upper layers communicate with the user interface and appli-
cation, the middle layers do reliable communication and routing to a remote network, and the
bottom layers communicate to the local network.
With this in hand, you’re now ready to explore each layer’s function in detail!
The Application Layer
The Application layer of the OSI model marks the spot where users actually communi-
cate to the computer and comes into play only when it’s clear that access to the network
will be needed soon. Take the case of Internet Explorer (IE). You could actually uninstall
every trace of networking components like TCP/IP, the NIC card, and so on and still use
IE to view a local HTML document. But things would get ugly if you tried to do things
like view a remote HTML document that must be retrieved because IE and other browsers
act on these types of requests by attempting to access the Application layer. So basically,
the Application layer is working as the interface between the actual application program
and the next layer down by providing ways for the application to send information down
through the protocol stack. This isn’t actually part of the layered structure, because
browsers don’t live in the Application layer, but they interface with it as well as the rel-
evant protocols when asked to access remote resources.
Identifying and conrming the communication partner’s availability and verifying
the required resources to permit the specied type of communication to take place also
occurs at the Application layer. This is important because, like the lion’s share of browser
functions, computer applications sometimes need more than desktop resources. It’s more
typical than you would think for the communicating components of several network
The OSI Reference Model 17
applications to come together to carry out a requested function. Here are a few good
examples of these kinds of events:
u File transfers
u Email
u Enabling remote access
u Network management activities
u Client/server processes
u Information location
Many network applications provide services for communication over enterprise net-
works, but for present and future internetworking, the need is fast developing to reach
beyond the limits of current physical networking.
The Application layer works as the interface between actual application
programs. This means end-user programs like Microsoft Word don’t reside
at the Application layer, they interface with the Application layer protocols.
Later, in Chapter 3, “TCP/IP,” I’ll talk in detail about a few important programs
that actually reside at the Application layer, like Telnet, FTP and TFTP.
The Presentation Layer
The Presentation layer gets its name from its purpose: It presents data to the Application
layer and is responsible for data translation and code formatting. Think of it as the OSI
model’s translator, providing coding and conversion services. One very effective way of
ensuring a successful data transfer is to convert the data into a standard format before
transmission. Computers are congured to receive this generically formatted data and then
reformat it back into its native state to read it. An example of this type of translation service
occurs when translating old Extended Binary Coded Decimal Interchange Code (EBCDIC)
data to ASCII, the American Standard Code for Information Interchange (often pronounced
“askee”). So just remember that by providing translation services, the Presentation layer
ensures that data transferred from the Application layer of one system can be read by the
Application layer of another one.
With this in mind, it follows that the OSI would include protocols that dene how
standard data should be formatted, so key functions like data compression, decompres-
sion, encryption, and decryption are also associated with this layer. Some Presentation
layer standards are involved in multimedia operations as well.
The Session Layer
The Session layer is responsible for setting up, managing, and dismantling sessions between
Presentation layer entities and keeping user data separate. Dialog control between devices
also occurs at this layer.
18 Chapter 1 u Internetworking
Communication between hosts’ various applications at the Session layer, as from a
client to a server, is coordinated and organized via three different modes: simplex, half-
duplex, and full-duplex. Simplex is simple one-way communication, kind of like saying
something and not getting a reply. Half-duplex is actual two-way communication, but it
can take place in only one direction at a time, preventing the interruption of the trans-
mitting device. It’s like when pilots and ship captains communicate over their radios, or
even a walkie-talkie. But full-duplex is exactly like a real conversation where devices can
transmit and receive at the same time, much like two people arguing or interrupting each
other during a telephone conversation.
The Transport Layer
The Transport layer segments and reassembles data into a single data stream. Services
located at this layer take all the various data received from upper-layer applications, then
combine it into the same, concise data stream. These protocols provide end-to-end data
transport services and can establish a logical connection between the sending host and
destination host on an internetwork.
A pair of well-known protocols called TCP and UDP are integral to this layer, but no
worries if you’re not already familiar with them because I’ll bring you up to speed later, in
Chapter 3. For now, understand that although both work at the Transport layer, TCP known
as a reliable service but UDP is not. This distinction gives application developers more options
because they have a choice between the two protocols when they are designing products for
this layer.
The Transport layer is responsible for providing mechanisms for multiplexing upper-layer
applications, establishing sessions, and tearing down virtual circuits. It can also hide the
details of network-dependent information from the higher layers as well as provide transpar-
ent data transfer.
The term reliable networking can be used at the Transport layer. Reliable
networking requires that acknowledgments, sequencing, and flow control
will all be used.
The Transport layer can either be connectionless or connection-oriented, but because
Cisco really wants you to understand the connection-oriented function of the Transport
layer, I’m going to go into that in more detail here.
Connection-Oriented Communication
For reliable transport to occur, a device that wants to transmit must rst establish a connec-
tion-oriented communication session with a remote device—its peer system—known as a call
setup or a three-way handshake. Once this process is complete, the data transfer occurs, and
when it’s nished, a call termination takes place to tear down the virtual circuit.
Figure1.9 depicts a typical reliable session taking place between sending and receiving
systems. In it, you can see that both hosts’ application programs begin by notifying their
The OSI Reference Model 19
individual operating systems that a connection is about to be initiated. The two operating
systems communicate by sending messages over the network conrming that the transfer
is approved and that both sides are ready for it to take place. After all of this required syn-
chronization takes place, a connection is fully established and the data transfer begins. And
by the way, it’s really helpful to understand that this virtual circuit setup is often referred to
as overhead!
FIGURE1.9 Establishing a connection-oriented session
Sender SYN Receiver
SYN/ACK
ACK
Connection Established
Data transfer
(Send bytes of segments)
Okay, now while the information is being transferred between hosts, the two machines
periodically check in with each other, communicating through their protocol software to
ensure that all is going well and that the data is being received properly.
Here’s a summary of the steps in the connection-oriented session—that three-way
handshake—pictured in Figure1.9:
u The first “connection agreement” segment is a request for synchronization (SYN).
u The next segments acknowledge (ACK) the request and establish connection
parameters—the rules—between hosts. These segments request that the receiver’s
sequencing is synchronized here as well so that a bidirectional connection can
be formed.
u The final segment is also an acknowledgment, which notifies the destination host that
the connection agreement has been accepted and that the actual connection has been
established. Data transfer can now begin.
Sounds pretty simple, but things don’t always ow so smoothly. Sometimes during a trans-
fer, congestion can occur because a high-speed computer is generating data trafc a lot faster
than the network itself can process it! And a whole bunch of computers simultaneously send-
ing datagrams through a single gateway or destination can also jam things up pretty badly. In
the latter case, a gateway or destination can become congested even though no single source
caused the problem. Either way, the problem is basically akin to a freeway bottleneck—too
much trafc for too small a capacity. It’s not usually one car that’s the problem; it’s just that
there are way too many cars on that freeway at once!
20 Chapter 1 u Internetworking
But what actually happens when a machine receives a ood of datagrams too quickly for
it to process? It stores them in a memory section called a buffer. Sounds great; it’s just that
this buffering action can solve the problem only if the datagrams are part of a small burst. If
the datagram deluge continues, eventually exhausting the device’s memory, its ood capacity
will be exceeded and it will dump any and all additional datagrams it receives just like an
inundated overowing bucket!
Flow Control
Since oods and losing data can both be tragic, we have a fail-safe solution in place known as
ow control. Its job is to ensure data integrity at the Transport layer by allowing applications
to request reliable data transport between systems. Flow control prevents a sending host on
one side of the connection from overowing the buffers in the receiving host. Reliable data
transport employs a connection-oriented communications session between systems, and the
protocols involved ensure that the following will be achieved:
u The segments delivered are acknowledged back to the sender upon their reception.
u Any segments not acknowledged are retransmitted.
u Segments are sequenced back into their proper order upon arrival at their destination.
u A manageable data flow is maintained in order to avoid congestion, overloading, or
worse, data loss.
The purpose of flow control is to provide a way for the receiving device to
control the amount of data sent by the sender.
Because of the transport function, network ood control systems really work well.
Instead of dumping and losing data, the Transport layer can issue a “not ready” indicator
to the sender, or potential source of the ood. This mechanism works kind of like a stop-
light, signaling the sending device to stop transmitting segment trafc to its overwhelmed
peer. After the peer receiver processes the segments already in its memory reservoir—its
buffer—it sends out a “ready” transport indicator. When the machine waiting to transmit
the rest of its datagrams receives this “go” indicator, it resumes its transmission. The pro-
cess is pictured in Figure1.10.
In a reliable, connection-oriented data transfer, datagrams are delivered to the receiving
host hopefully in the same sequence they’re transmitted. A failure will occur if any data
segments are lost, duplicated, or damaged along the way—a problem solved by having the
receiving host acknowledge that it has received each and every data segment.
A service is considered connection-oriented if it has the following characteristics:
u A virtual circuit, or “three-way handshake” is set up.
u It uses sequencing.
u It uses acknowledgments.
u It uses flow control.
The OSI Reference Model 21
FIGURE1.10 Transmitting segments with flow control
Sender Receiver
Buffer full
Not ready –
STOP!
Segments
processed
GO!
The types of flow control are buffering, windowing, and congestion
avoidance.
Windowing
Ideally, data throughput happens quickly and efciently. And as you can imagine, it would
be painfully slow if the transmitting machine had to actually wait for an acknowledgment
after sending each and every segment! The quantity of data segments, measured in bytes,
that the transmitting machine is allowed to send without receiving an acknowledgment is
called a window.
Windows are used to control the amount of outstanding, unacknowledged
data segments.
The size of the window controls how much information is transferred from one end to the
other before an acknowledgement is required. While some protocols quantify information
depending on the number of packets, TCP/IP measures it by counting the number of bytes.
As you can see in Figure1.11, there are two window sizes—one set to 1 and one set to 3.
If you’ve congured a window size of 1, the sending machine will wait for an acknowl-
edgment for each data segment it transmits before transmitting another one but will allow
three to be transmitted before receiving an acknowledgement if the window size is set to 3.
In this simplied example, both the sending and receiving machines are workstations.
Remember that in reality, the transmission isn’t based on simple numbers but in the amount
of bytes that can be sent!
22 Chapter 1 u Internetworking
FIGURE1.11 Windowing
Sender
Window size of 1
Window size of 3
Receiver
Send 1 Receive 1
ACK 1
Send 1 Receive 2
ACK 2
Send 1
Send 2
Send 3
ACK 4
Send 4
If a receiving host fails to receive all the bytes that it should acknowl-
edge, the host can improve the communication session by decreasing
the window size.
Acknowledgments
Reliable data delivery ensures the integrity of a stream of data sent from one machine to the
other through a fully functional data link. It guarantees that the data won’t be duplicated or
lost. This is achieved through something called positive acknowledgment with retransmis-
sion—a technique that requires a receiving machine to communicate with the transmitting
source by sending an acknowledgment message back to the sender when it receives data. The
sender documents each segment measured in bytes, then sends and waits for this acknowledg-
ment before sending the next segment. Also important is that when it sends a segment, the
transmitting machine starts a timer and will retransmit if it expires before it gets an acknowl-
edgment back from the receiving end. Figure1.12 shows the process I just described.
In the gure, the sending machine transmits segments 1, 2, and 3. The receiving node
acknowledges that it has received them by requesting segment 4 (what it is expecting next).
When it receives the acknowledgment, the sender then transmits segments 4, 5, and 6. If
segment 5 doesn’t make it to the destination, the receiving node acknowledges that event
with a request for the segment to be re-sent. The sending machine will then resend the lost
segment and wait for an acknowledgment, which it must receive in order to move on to the
transmission of segment 7.
The Transport layer, working in tandem with the Session layer, also separates the data from
different applications, an activity known as session multiplexing, and it happens when a client
connects to a server with multiple browser sessions open. This is exactly what’s taking place
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The OSI Reference Model 23
when you go someplace online like Amazon and click multiple links, opening them simulta-
neously to get information when comparison shopping. The client data from each browser
session must be separate when the server application receives it, which is pretty slick techno-
logically speaking, and it’s the Transport layer to the rescue for that juggling act!
FIGURE1.12 Transport layer reliable delivery
Sender Receiver
Send 1
1
Send 3
Send 2
ACK 4
Send 4
Send 5
Send 6
ACK 5
Connection lost!
ACK 7
Send 5
23456
123456
The Network Layer
The Network layer, or layer 3, manages device addressing, tracks the location of devices
on the network, and determines the best way to move data. This means that it’s up to the
Network layer to transport trafc between devices that aren’t locally attached. Routers,
which are layer 3 devices, are specied at this layer and provide the routing services within
an internetwork.
Here’s how that works: rst, when a packet is received on a router interface, the desti-
nation IP address is checked. If the packet isn’t destined for that particular router, it will
look up the destination network address in the routing table. Once the router chooses an
exit interface, the packet will be sent to that interface to be framed and sent out on the
local network. If the router can’t nd an entry for the packet’s destination network in
the routing table, the router drops the packet.
Data and route update packets are the two types of packets used at the Network layer:
Data packets These are used to transport user data through the internetwork. Protocols
used to support data trafc are called routed protocols, and IP and IPv6 are key examples.
I’ll cover IP addressing in Chapter 3, “TCP/IP,” and Chapter 4, “Easy Subnetting,” and I’ll
cover IPv6 in Chapter 14, “Internet Protocol Version 6 (IPV6)”.
24 Chapter 1 u Internetworking
Route update packets These packets are used to update neighboring routers about the net-
works connected to all routers within the internetwork. Protocols that send route update pack-
ets are called routing protocols; the most critical ones for CCNA are RIP, RIPv2, EIGRP, and
OSPF. Route update packets are used to help build and maintain routing tables.
Figure1.13 shows an example of a routing table. The routing table each router keeps
and refers to includes the following information:
FIGURE1.13 Routing table used in a router
1.0 3.0
1.3
E0
2.1
S0
NET
2.2
S0
3.3
E0
1.1
1.2
3.1
3.2
1
2
3
INT
Routing table
E0
S0
S0
Metric
0
0
1
NET
1
2
3
INT
Routing table
S0
S0
E0
Metric
1
0
0
Network addresses Protocol-specic network addresses. A router must maintain a routing
table for individual routing protocols because each routed protocol keeps track of a net-
work with a different addressing scheme. For example, the routing tables for IP, IPv6, and
IPX are completely different, so the router keeps a table for each one. Think of it as a street
sign in each of the different languages spoken by the American, Spanish, and French people
living on a street; the street sign would read, Cat/Gato/Chat.
Interface The exit interface a packet will take when destined for a specic network.
Metric The distance to the remote network. Different routing protocols use different ways
of computing this distance. I’m going to cover routing protocols thoroughly in Chapter 8, “IP
Routing,” and Chapter 9, “Open Shortest Path First.” For now, know that some routing pro-
tocols like the Routing Information Protocol, or RIP, use hop count, which refers to the num-
ber of routers a packet passes through en route to a remote network. Others use bandwidth,
delay of the line, or even tick count (1⁄18 of a second) to determine the best path for data to get
to a given destination.
And as I mentioned earlier, routers break up broadcast domains, which means that by
default, broadcasts aren’t forwarded through a router. Do you remember why this is a good
thing? Routers also break up collision domains, but you can also do that using layer 2, Data
Link layer, switches. Because each interface in a router represents a separate network, it must
be assigned unique network identication numbers, and each host on the network connected
to that router must use the same network number. Figure1.14 shows how a router works in
an internetwork.
The OSI Reference Model 25
FIGURE1.14 A router in an internetwork. Each router LAN interface is a broadcast
domain. Routers break up broadcast domains by default and provide WAN services.
WAN services Internet
FastEthernet0/0
FastEthernet0/1
Serial0
Here are some router characteristics that you should never forget:
u Routers, by default, will not forward any broadcast or multicast packets.
u Routers use the logical address in a Network layer header to determine the next-hop
router to forward the packet to.
u Routers can use access lists, created by an administrator, to control security based on
the types of packets allowed to enter or exit an interface.
u Routers can provide layer 2 bridging functions if needed and can simultaneously route
through the same interface.
u Layer 3 devices—in this case, routers—provide connections between virtual
LANs (VLANs).
u Routers can provide quality of service (QoS) for specific types of network traffic.
The Data Link Layer
The Data Link layer provides for the physical transmission of data and handles error noti-
cation, network topology, and ow control. This means that the Data Link layer will ensure
that messages are delivered to the proper device on a LAN using hardware addresses and will
translate messages from the Network layer into bits for the Physical layer to transmit.
The Data Link layer formats the message, each called a data frame, and adds a customized
header containing the hardware destination and source address. This added information forms
a sort of capsule that surrounds the original message in much the same way that engines,
navigational devices, and other tools were attached to the lunar modules of the Apollo project.
These various pieces of equipment were useful only during certain stages of space ight and
were stripped off the module and discarded when their designated stage was completed. The
process of data traveling through networks is similar.
Figure1.15 shows the Data Link layer with the Ethernet and IEEE specications.
When you check it out, notice that the IEEE 802.2 standard is used in conjunction
with and adds functionality to the other IEEE standards. (You’ll read more about the
important IEEE 802 standards used with the Cisco objectives in Chapter 2 “Ethernet
Networking and Data Encapsulation.”)
It’s important for you to understand that routers, which work at the Network layer, don’t
care at all about where a particular host is located. They’re only concerned about where net-
works are located and the best way to reach them—including remote ones. Routers are totally
obsessive when it comes to networks, which in this case is a good thing! It’s the Data Link
layer that’s responsible for the actual unique identication of each device that resides on a
local network.
26 Chapter 1 u Internetworking
FIGURE1.15 Data Link layer
Media Access Control (MAC)
802.11 802.3 802.2
Logical Link Control (LLC)
For a host to send packets to individual hosts on a local network as well as transmit
packets between routers, the Data Link layer uses hardware addressing. Each time a packet
is sent between routers, it’s framed with control information at the Data Link layer, but
that information is stripped off at the receiving router and only the original packet is left
completely intact. This framing of the packet continues for each hop until the packet is
nally delivered to the correct receiving host. It’s really important to understand that the
packet itself is never altered along the route; it’s only encapsulated with the type of control
information required for it to be properly passed on to the different media types.
The IEEE Ethernet Data Link layer has two sublayers:
Media Access Control (MAC) Denes how packets are placed on the media. Contention
media access is “rst come/rst served” access where everyone shares the same band-
width—hence the name. Physical addressing is dened here as well as logical topologies.
What’s a logical topology? It’s the signal path through a physical topology. Line discipline,
error notication (but not correction), the ordered delivery of frames, and optional ow
control can also be used at this sublayer.
Logical Link Control (LLC) Responsible for identifying Network layer protocols and then
encapsulating them. An LLC header tells the Data Link layer what to do with a packet once
a frame is received. It works like this: a host receives a frame and looks in the LLC header
to nd out where the packet is destined—for instance, the IP protocol at the Network layer.
The LLC can also provide ow control and sequencing of control bits.
The switches and bridges I talked about near the beginning of the chapter both work at
the Data Link layer and lter the network using hardware (MAC) addresses. I’ll talk about
these next.
As data is encoded with control information at each layer of the OSI model,
the data is named with something called a Protocol Data Unit (PDU). At the
Transport layer the PDU is called a Segment, Network layer is Packet, Data
Link is Frame, and Physical layer is Bits. This method of naming the data at
each layer is covered thoroughly in Chapter 2.
The OSI Reference Model 27
Switches and Bridges at the Data Link Layer
Layer 2 switching is considered hardware-based bridging because it uses specialized hard-
ware called an application-specic integrated circuit (ASIC). ASICs can run up to high
gigabit speeds with very low latency rates.
Latency is the time measured from when a frame enters a port to when it
exits a port.
Bridges and switches read each frame as it passes through the network. The layer 2
device then puts the source hardware address in a lter table and keeps track of which port
the frame was received on. This information (logged in the bridge’s or switch’s lter table)
is what helps the machine determine the location of the specic sending device. Figure1.16
shows a switch in an internetwork and how John is sending packets to the Internet and
Sally doesn’t hear his frames because she is in a different collision domain. The destination
frame goes directly to the default gateway router, and Sally doesn’t see John’s trafc, much
to her relief.
FIGURE1.16 A switch in an internetwork
23
Mac Address—Table
F0/1: 00c0.1234.2211
F0/2: 00c0.1234.2212
F0/3: 00c0.1234.2213
F0/4: 00c0.1234.2214
14
The real estate business is all about location, location, location, and it’s the same way for
both layer 2 and layer 3 devices. Though both need to be able to negotiate the network, it’s
crucial to remember that they’re concerned with very different parts of it. Primarily, layer 3
machines (such as routers) need to locate specic networks, whereas layer 2 machines (switches
and bridges) need to eventually locate specic devices. So, networks are to routers as individual
devices are to switches and bridges. And routing tables that “map” the internetwork are for
routers, as lter tables that “map” individual devices are for switches and bridges.
After a lter table is built on the layer 2 device, it will forward frames only to the segment
where the destination hardware address is located. If the destination device is on the same seg-
ment as the frame, the layer 2 device will block the frame from going to any other segments.
If the destination is on a different segment, the frame can be transmitted only to that segment.
This is called transparent bridging.
28 Chapter 1 u Internetworking
When a switch interface receives a frame with a destination hardware address that isn’t
found in the device’s lter table, it will forward the frame to all connected segments. If the
unknown device that was sent the “mystery frame” replies to this forwarding action, the
switch updates its lter table regarding that device’s location. But in the event the destina-
tion address of the transmitting frame is a broadcast address, the switch will forward all
broadcasts to every connected segment by default.
All devices that the broadcast is forwarded to are considered to be in the same broadcast
domain. This can be a problem because layer 2 devices propagate layer 2 broadcast storms
that can seriously choke performance, and the only way to stop a broadcast storm from
propagating through an internetwork is with a layer 3 device—a router!
The biggest benet of using switches instead of hubs in your internetwork is that each
switch port is actually its own collision domain. Remember that a hub creates one large
collision domain, which is not a good thing! But even armed with a switch, you still don’t
get to just break up broadcast domains by default because neither switches nor bridges will
do that. They’ll simply forward all broadcasts instead.
Another benet of LAN switching over hub-centered implementations is that each device
on every segment plugged into a switch can transmit simultaneously. Well, at least they can
as long as there’s only one host on each port and there isn’t a hub plugged into a switch port!
As you might have guessed, this is because hubs allow only one device per network segment
to communicate at a time.
The Physical Layer
Finally arriving at the bottom, we nd that the Physical layer does two things: it sends bits
and receives bits. Bits come only in values of 1 or 0—a Morse code with numerical values.
The Physical layer communicates directly with the various types of actual communication
media. Different kinds of media represent these bit values in different ways. Some use audio
tones, while others employ state transitions—changes in voltage from high to low and low to
high. Specic protocols are needed for each type of media to describe the proper bit patterns
to be used, how data is encoded into media signals, and the various qualities of the physical
media’s attachment interface.
The Physical layer species the electrical, mechanical, procedural, and functional
requirements for activating, maintaining, and deactivating a physical link between end
systems. This layer is also where you identify the interface between the data terminal
equipment (DTE) and the data communication equipment (DCE). (Some old phone-
company employees still call DCE “data circuit-terminating equipment.”) The DCE is
usually located at the service provider, while the DTE is the attached device. The services
available to the DTE are most often accessed via a modem or channel service unit/data
service unit (CSU/DSU).
The Physical layer’s connectors and different physical topologies are dened by the OSI
as standards, allowing disparate systems to communicate. The Cisco exam objectives are
interested only in the IEEE Ethernet standards.
Summary 29
Hubs at the Physical Layer
A hub is really a multiple-port repeater. A repeater receives a digital signal, reamplies or
regenerates that signal, then forwards the signal out the other port without looking at any
data. A hub does the same thing across all active ports: any digital signal received from a
segment on a hub port is regenerated or reamplied and transmitted out all other ports on
the hub. This means all devices plugged into a hub are in the same collision domain as well
as in the same broadcast domain. Figure1.17 shows a hub in a network, and how when one
host transmits, all other hosts must stop and listen.
FIGURE1.17 A hub in a network
I love it when everyone has to listen to everything I say!
Hubs, like repeaters, don’t examine any of the trafc as it enters or before it’s transmit-
ted out to the other parts of the physical media. And every device connected to the hub, or
hubs, must listen if a device transmits. A physical star network, where the hub is a central
device and cables extend in all directions out from it, is the type of topology a hub creates.
Visually, the design really does resemble a star, whereas Ethernet networks run a logical
bus topology, meaning that the signal has to run through the network from end to end.
Hubs and repeaters can be used to enlarge the area covered by a single
LAN segment, but I really do not recommend going with this configura-
tion! LAN switches are affordable for almost every situation and will
make you much happier.
Summary
Whew! I know this seemed like the chapter that wouldn’t end, but it did—and you made it
through! You’re now armed with a ton of fundamental information; you’re ready to build
upon it and are well on your way to certication.
I started by discussing simple, basic networking and the differences between collision
and broadcast domains.
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I then discussed the OSI model—the seven-layer model used to help application developers
design applications that can run on any type of system or network. Each layer has its special
jobs and select responsibilities within the model to ensure that solid, effective communications
do, in fact, occur. I provided you with complete details of each layer and discussed how Cisco
views the specications of the OSI model.
In addition, each layer in the OSI model species different types of devices, and I described
the different devices used at each layer.
Remember that hubs are Physical layer devices and repeat the digital signal to all segments
except the one from which it was received. Switches segment the network using hardware
addresses and break up collision domains. Routers break up broadcast domains as well as
collision domains and use logical addressing to send packets through an internetwork.
Exam Essentials
Identify the possible causes of LAN traffic congestion. Too many hosts in a broadcast
domain, broadcast storms, multicasting, and low bandwidth are all possible causes of LAN
trafc congestion.
Describe the difference between a collision domain and a broadcast domain. Collision
domain is an Ethernet term used to describe a network collection of devices in which one
particular device sends a packet on a network segment, forcing every other device on that
same segment to pay attention to it. With a broadcast domain, a set of all devices on a net-
work hear all broadcasts sent on all segments.
Differentiate a MAC address and an IP address and describe how and when each address
type is used in a network. A MAC address is a hexadecimal number identifying the phys-
ical connection of a host. MAC addresses are said to operate on layer 2 of the OSI model.
IP addresses, which can be expressed in binary or decimal format, are logical identiers
that are said to be on layer 3 of the OSI model. Hosts on the same physical segment locate
one another with MAC addresses, while IP addresses are used when they reside on differ-
ent LAN segments or subnets.
Understand the difference between a hub, a bridge, a switch, and a router. A hub creates one
collision domain and one broadcast domain. A bridge breaks up collision domains but creates
one large broadcast domain. They use hardware addresses to lter the network. Switches are
really just multiple-port bridges with more intelligence; they break up collision domains but
creates one large broadcast domain by default. Bridges and switches use hardware addresses to
lter the network. Routers break up broadcast domains (and collision domains) and use logical
addressing to lter the network.
Identify the functions and advantages of routers. Routers perform packet switching, lter-
ing, and path selection, and they facilitate internetwork communication. One advantage of
routers is that they reduce broadcast trafc.
Exam Essentials 31
Differentiate connection-oriented and connectionless network services and describe
how each is handled during network communications. Connection-oriented services use
acknowledgments and ow control to create a reliable session. More overhead is used than
in a connectionless network service. Connectionless services are used to send data with no
acknowledgments or ow control. This is considered unreliable.
Define the OSI layers, understand the function of each, and describe how devices and net-
working protocols can be mapped to each layer. You must remember the seven layers of
the OSI model and what function each layer provides. The Application, Presentation, and
Session layers are upper layers and are responsible for communicating from a user inter-
face to an application. The Transport layer provides segmentation, sequencing, and virtual
circuits. The Network layer provides logical network addressing and routing through an
internetwork. The Data Link layer provides framing and placing of data on the network
medium. The Physical layer is responsible for taking 1s and 0s and encoding them into a
digital signal for transmission on the network segment.
32 Chapter 1 u Internetworking
Written Labs
In this section, you’ll complete the following labs to make sure you’ve got the information
and concepts contained within them fully dialed in:
Lab 1.1: OSI Questions
Lab 1.2: Dening the OSI Layers and Devices
Lab 1.3: Identifying Collision and Broadcast Domains
The answers to these labs can be found in Appendix A, “Answers to Written Labs.”
Written Lab 1.1: OSI Questions
Answer the following questions about the OSI model:
1. Which layer chooses and determines the availability of communicating partners along
with the resources necessary to make the connection; coordinates partnering applications;
and forms a consensus on procedures for controlling data integrity and error recovery?
2. Which layer is responsible for converting data packets from the Data Link layer into
electrical signals?
3. At which layer is routing implemented, enabling connections and path selection
between two end systems?
4. Which layer defines how data is formatted, presented, encoded, and converted for use
on the network?
5. Which layer is responsible for creating, managing, and terminating sessions between
applications?
6. Which layer ensures the trustworthy transmission of data across a physical link and is
primarily concerned with physical addressing, line discipline, network topology, error
notification, ordered delivery of frames, and flow control?
7. Which layer is used for reliable communication between end nodes over the network
and provides mechanisms for establishing, maintaining, and terminating virtual circuits;
transport-fault detection and recovery; and controlling the flow of information?
8. Which layer provides logical addressing that routers will use for path determination?
9. Which layer specifies voltage, wire speed, and cable pinouts and moves bits
between devices?
10. Which layer combines bits into bytes and bytes into frames, uses MAC addressing, and
provides error detection?
11. Which layer is responsible for keeping the data from different applications separate on
the network?
Written Labs 33
12. Which layer is represented by frames?
13. Which layer is represented by segments?
14. Which layer is represented by packets?
15. Which layer is represented by bits?
16. Put the following in order of encapsulation:
Packets
Frames
Bits
Segments
17. Which layer segments and reassembles data into a data stream?
18. Which layer provides the physical transmission of the data and handles error notifica-
tion, network topology, and flow control?
19. Which layer manages logical device addressing, tracks the location of devices on the
internetwork, and determines the best way to move data?
20. What is the bit length and expression form of a MAC address?
Written Lab 1.2: Defining the OSI Layers and Devices
Fill in the blanks with the appropriate layer of the OSI or hub, switch, or router device.
Description Device or OSI Layer
This device sends and receives information
about the Network layer.
This layer creates a virtual circuit before
transmitting between two end stations.
This device uses hardware addresses to filter a
network.
Ethernet is defined at these layers.
This layer supports flow control, sequencing,
and acknowledgments.
This device can measure the distance to a
remote network.
Logical addressing is used at this layer.
Hardware addresses are defined at this layer.
34 Chapter 1 u Internetworking
Description Device or OSI Layer
This device creates one big collision domain
and one large broadcast domain.
This device creates many smaller collision
domains, but the network is still one large
broadcast domain.
This device can never run full-duplex.
This device breaks up collision domains and
broadcast domains.
Written Lab 1.3: Identifying Collision and
Broadcast Domains
1. In the following exhibit, identify the number of collision domains and broadcast
domains in each specified device. Each device is represented by a letter:
A. Hub
B. Bridge
C. Switch
D. Router
A
B
Hub Switch
C
S1
Bridge
D
Router
Review Questions 35
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional ques-
tions, please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. Which of the following statements is/are true with regard to the device shown below?
(Choose all that apply.)
A. It includes one collision domain and one broadcast domain
B. It includes one collision domain and 10 broadcast domains
C. It includes 10 collision domains and one broadcast domain
D. It includes one collision domain and 10 broadcast domains
E. It includes 10 collision domains and 10 broadcast domains
2. With respect to the OSI model, which of the following are correct statements
about PDUs?
A. A segment contains IP addresses.
B. A packet contains IP addresses.
C. A segment contains MAC addresses.
D. A packet contains MAC addresses.
3. You are the Cisco administrator for your company. A new branch office is opening and
you are selecting the necessary hardware to support the network. There will be two
groups of computers, each organized by department. The Sales group computers will
be assigned IP addresses ranging from 192.168.1.2 to 192.168.1.50. The Accounting
group will be assigned IP addresses ranging from 10.0.0.2 to 10.0.0.50. What type of
device should you select to connect the two groups of computers so that data commu-
nication can occur?
A. Hub
B. Switch
C. Router
D. Bridge
36 Chapter 1 u Internetworking
4. The most effective way to mitigate congestion on a LAN would be to__________________?
A. Upgrade the network cards
B. Change the cabling to CAT 6
C. Replace the hubs with switches
D. Upgrade the CPUs in the routers
5. In the work area below draw a line from the OSI model layer to its PDU.
Transport Bits
Data Link Segment
Physical Packet
Network Frame
Layer Description
6. In the diagram below what procedure is shown?
Sender SYN Receiver
SYN/ACK
ACK
Connection Established
Data transfer
(Send bytes of segments)
A. flow control
B. windowing
C. TCP handshake
D. reliable delivery
Review Questions 37
7. You need to provide network connectivity to 150 client computers that will reside in
the same sub network, and each client computer must be allocated dedicated band-
width. Which device should you use to accomplish the task?
A. Hub
B. Switch
C. Router
D. Bridge
8. In the work area below, drag the OSI model layer on the left to its description on the right.
left list:
Layer
Transpor t
Physical
Data Link
Network
right list:
Description
Framing
End-to-end connection
Routing
Conversion to bits
9. What feature of TCP is illustrated below?
Sender Receiver
Send 1
1
Send 3
Send 2
ACK 2
Send 4
Send 5
Send 6
ACK 5
Connection lost!
ACK 7
Send 5
23456
123456
A. flow control
B. UDP handshake
C. TCP handshake
D. reliable delivery
38 Chapter 1 u Internetworking
10. Which of the following is an example of a routed protocol?
A. EIGRP
B. IP
C. OSPF
D. BGP
11. Which of the following is NOT a function carried out on the Application layer of the
OSI model?
A. email
B. data translation and code formatting
C. file transfers
D. client/server processes
12. Which of the following layers of the OSI model was later subdivided into two layers?
A. Presentation
B. Transport
C. Data Link
D. Physical
13. What feature of TCP is illustrated below?
Sender Receiver
Send 1
Send 3
Send 2
ACK 4
Send 4
A. flow control
B. windowing
C. TCP handshake
D. reliable delivery
Review Questions 39
14. An example of a device that operates on the physical layer is a ____________.
A. Hub
B. Switch
C. Router
D. Bridge
15. Which of the following is NOT a benefit of using a reference model?
A. divides the network communication process into smaller and simpler components
B. encourages industry standardization
C. enforces consistency across vendors
D. allows various types of network hardware and software to communicate
16. Which of the following statements is not true with regard to routers?
A. They forward broadcasts by default
B. They can filter the network based on Network layer information
C. They perform path selection
D. They perform packet switching
17. Switches break up _______________ domains and routers break up _____________
domains.
A. broadcast, broadcast
B. collision, collision
C. collision, broadcast
D. broadcast, collision
40 Chapter 1 u Internetworking
18. How many collision domains are present in the diagram below?
Hub
Switch
Bridge
Router
A. eight
B. nine
C. ten
D. eleven
19. Which of the following layers of the OSI model is not involved in defining how the appli-
cations within the end stations will communicate with each other as well as with users?
A. Transport
B. Application
C. Presentation
D. Session
20. Which if the following is the ONLY device that operates at all layers of the OSI model?
A. Network host
B. Switch
C. Router
D. Bridge
Chapter
2
Ethernet Networking
and Data
Encapsulation
THE FOLLOWING ICND1 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 Operation of IP Data Networks
■Recognize the purpose and functions of various network
devices such as Routers, Switches, Bridges and Hubs.
■Select the components required to meet a given network
specification.
■Predict the data flow between two hosts across a network.
■Identify the appropriate media, cables, ports, and connectors
to connect Cisco network devices to other network devices
and hosts in a LAN
1 LAN Switching Technologies
■Determine the technology and media access control method
for Ethernet networks
■Identify basic switching concepts and the operation of
Cisco switches.
■Collision Domains
■Broadcast Domains
Before we begin exploring a set of key foundational topics like
the TCP/IP DoD model, IP addressing, subnetting, and routing
in the upcoming chapters, I really want you to grasp the big
picture of LANs conceptually. The role Ethernet plays in today’s networks as well as what
Media Access Control (MAC) addresses are and how they are used are two more critical
networking basics you’ll want a solid understanding of as well.
We’ll cover these important subjects and more in this chapter, beginning with Ethernet
basics and the way MAC addresses are used on an Ethernet LAN, and then we’ll focus in
on the actual protocols used with Ethernet at the Data Link layer. To round out this discus-
sion, you’ll also learn about some very important Ethernet specications.
You know by now that there are a whole bunch of different devices specied at the vari-
ous layers of the OSI model and that it’s essential to be really familiar with the many types
of cables and connectors employed to hook them up to the network correctly. I’ll review the
types of cabling used with Cisco devices in this chapter, demonstrate how to connect to a
router or switch, plus show you how to connect a router or switch via a console connection.
I’ll also introduce you to a vital process of encoding data as it makes its way down the
OSI stack known as encapsulation.
I’m not nagging at all here—okay, maybe just a little, but promise that you’ll actually work
through the four written labs and 20 review questions I added to the end of this chapter just
for you. You’ll be so happy you did because they’re written strategically to make sure that all
the important material covered in this chapter gets locked in, vault-tight into your memory.
So don’t skip them!
To find up-to-the minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
Ethernet Networks in Review
Ethernet is a contention-based media access method that allows all hosts on a network
to share the same link’s bandwidth. Some reasons it’s so popular are that Ethernet is
really pretty simple to implement and it makes troubleshooting fairly straightforward as
well. Ethernet is so readily scalable, meaning that it eases the process of integrating new
Ethernet Networks in Review 43
technologies into an existing network infrastructure, like upgrading from Fast Ethernet
to Gigabit Ethernet.
Ethernet uses both Data Link and Physical layer specications, so you’ll be presented with
information relative to both layers, which you’ll need to effectively implement, troubleshoot,
and maintain an Ethernet network.
Collision Domain
In Chapter 1, “Internetworking,” you learned that the Ethernet term collision domain
refers to a network scenario wherein one device sends a frame out on a physical network
segment forcing every other device on the same segment to pay attention to it. This is bad
because if two devices on a single physical segment just happen to transmit simultaneously,
it will cause a collision and require these devices to retransmit. Think of a collision event as
a situation where each device’s digital signals totally interfere with one another on the wire.
Figure2.1 shows an old, legacy network that’s a single collision domain where only one
host can transmit at a time.
FIGURE2.1 Legacy collision domain design
One broadcast domain
One collision domain
Hub
Hub
The hosts connected to each hub are in the same collision domain, so if one of them
transmits, all the others must take the time to listen for and read the digital signal. It is easy
to see how collisions can be a serious drag on network performance, so I’ll show you how
to strategically avoid them soon!
Okay—take another look at the network pictured in Figure2.1. True, it has only one
collision domain, but worse, it’s also a single broadcast domain—what a mess! Let’s check
out an example, in Figure2.2, of a typical network design still used today and see if it’s
any better.
44 Chapter 2 u Ethernet Networking and Data Encapsulation
FIGURE2.2 A typical network you’d see today
Each connection on a switch creates a separate collision domain.
One broadcast domain by default
S1 S2
Because each port off a switch is a single collision domain, we gain more bandwidth for
users, which is a great start. But switches don’t break up broadcast domains by default, so
this is still only one broadcast domain, which is not so good. This can work in a really small
network, but to expand it at all, we would need to break up the network into smaller broad-
cast domains or our users won’t get enough bandwidth! And you’re probably wondering
about that device in the lower-right corner, right? Well, that’s a wireless access point, which is
sometimes referred as an AP (which stands for access point). It’s a wireless device that allows
hosts to connect wirelessly using the IEEE 802.11 specication and I added it to the gure to
demonstrate how these devices can be used to extend a collision domain. But still, understand
that APs don’t actually segment the network, they only extend them, meaning our LAN just
got a lot bigger, with an unknown amount of hosts that are all still part of one measly broad-
cast domain! This clearly demonstrates why understanding exactly what a broadcast domain
is, is so important, and now is a great time to talk about them in detail.
Broadcast Domain
Let me start by giving you the formal denition: broadcast domain refers to a group of
devices on a specic network segment that hear all the broadcasts sent out on that specic
network segment.
But even though a broadcast domain is usually a boundary delimited by physical media
like switches and routers, it can also refer to a logical division of a network segment, where
all hosts can communicate via a Data Link layer, hardware address broadcast.
Figure2.3 shows how a router would create a broadcast domain boundary.
Here you can see there are two router interfaces giving us two broadcast domains, and I
count 10 switch segments, meaning we’ve got 10 collision domains.
The design depicted in Figure2.3 is still in use today, and routers will be around
for a long time, but in the latest, modern switched networks, it’s important to create
small broadcast domains. We achieve this by building virtual LANs (VLANs) within
Ethernet Networks in Review 45
our switched networks, which I’ll demonstrate shortly. Without employing VLANs in
today’s switched environments, there wouldn’t be much bandwidth available to individual
users. Switches break up collision domains with each port, which is awesome, but they’re
still only one broadcast domain by default! It’s also one more reason why it’s extremely
important to design our networks very carefully.
FIGURE2.3 A router creates broadcast domain boundaries.
Two broadcast domains. How many collision domains do you see?
S2S1
R1
And key to carefully planning your network design is never to allow broadcast domains
to grow too large and get out of control. Both collision and broadcast domains can easily be
controlled with routers and VLANs, so there’s just no excuse to allow user bandwidth to slow
to a painful crawl when there are plenty of tools in your arsenal to prevent the suffering!
An important reason for this book’s existence is to ensure that you really get the founda-
tional basics of Cisco networks nailed down so you can affectively design, implement, cong-
ure, troubleshoot, and even dazzle colleagues and superiors with elegant designs that lavish
your users with all the bandwidth their hearts could possibly desire.
To make it to the top of that mountain, you need more than just the basic story, so let’s
move on to explore the collision detection mechanism used in half-duplex Ethernet.
CSMA/CD
Ethernet networking uses a protocol called Carrier Sense Multiple Access with Collision
Detection (CSMA/CD), which helps devices share the bandwidth evenly while preventing
two devices from transmitting simultaneously on the same network medium. CSMA/CD
was actually created to overcome the problem of the collisions that occur when packets
are transmitted from different nodes at the same time. And trust me—good collision man-
agement is crucial, because when a node transmits in a CSMA/CD network, all the other
nodes on the network receive and examine that transmission. Only switches and routers
can affectively prevent a transmission from propagating throughout the entire network!
So, how does the CSMA/CD protocol work? Let’s start by taking a look at Figure2.4.
46 Chapter 2 u Ethernet Networking and Data Encapsulation
FIGURE2.4 CSMA/CD
Collision
Jam Jam Jam Jam Jam Jam Jam Jam
ABCD
ABCD
ABCD
ABCD
When a host wants to transmit over the network, it rst checks for the presence of a
digital signal on the wire. If all is clear and no other host is transmitting, the host will then
proceed with its transmission.
But it doesn’t stop there. The transmitting host constantly monitors the wire to make
sure no other hosts begin transmitting. If the host detects another signal on the wire, it
sends out an extended jam signal that causes all nodes on the segment to stop sending
data—think busy signal.
The nodes respond to that jam signal by waiting a bit before attempting to transmit
again. Backoff algorithms determine when the colliding stations can retransmit. If colli-
sions keep occurring after 15 tries, the nodes attempting to transmit will then time out.
Half-duplex can be pretty messy!
When a collision occurs on an Ethernet LAN, the following happens:
1. A jam signal informs all devices that a collision occurred.
2. The collision invokes a random backoff algorithm.
3. Each device on the Ethernet segment stops transmitting for a short time until its back-
off timer expires.
4. All hosts have equal priority to transmit after the timers have expired.
Ethernet Networks in Review 47
The ugly effects of having a CSMA/CD network sustain heavy collisions are delay, low
throughput, and congestion.
Backoff on an Ethernet network is the retransmission delay that’s enforced
when a collision occurs. When that happens, a host will resume transmis-
sion only after the forced time delay has expired. Keep in mind that after
the backoff has elapsed, all stations have equal priority to transmit data.
At this point, let’s take a minute to talk about Ethernet in detail at both the Data Link
layer (layer 2) and the Physical layer (layer 1).
Half- and Full-Duplex Ethernet
Half-duplex Ethernet is dened in the original IEEE 802.3 Ethernet specication, which
differs a bit from how Cisco describes things. Cisco says Ethernet uses only one wire pair
with a digital signal running in both directions on the wire. Even though the IEEE speci-
cations discuss the half-duplex process somewhat differently, it’s not actually a full-blown
technical disagreement. Cisco is really just talking about a general sense of what’s happen-
ing with Ethernet.
Half-duplex also uses the CSMA/CD protocol I just discussed to help prevent collisions
and to permit retransmitting if one occurs. If a hub is attached to a switch, it must operate
in half-duplex mode because the end stations must be able to detect collisions. Figure2.5
shows a network with four hosts connected to a hub.
FIGURE2.5 Half-duplex example
Hub
Collision
The problem here is that we can only run half-duplex, and if two hosts communicate at
the same time there will be a collision. Also, half-duplex Ethernet is only about 30 to 40 per-
cent efcient because a large 100Base-T network will usually only give you 30 to 40 Mbps,
at most, due to overhead.
But full-duplex Ethernet uses two pairs of wires at the same time instead of a single
wire pair like half-duplex. And full-duplex uses a point-to-point connection between the
transmitter of the transmitting device and the receiver of the receiving device. This means
that full-duplex data transfers happen a lot faster when compared to half-duplex transfers.
48 Chapter 2 u Ethernet Networking and Data Encapsulation
Also, because the transmitted data is sent on a different set of wires than the received data,
collisions won’t happen. Figure2.6 shows four hosts connected to a switch, plus a hub, and
denitely try not to use hubs if you can help it!
FIGURE2.6 Full-duplex example
Switch
Half-duplex port
Hub
Collision
Theoretically all hosts connected to the switch in Figure2.6 can communicate at the
same time because they can run full-duplex. Just keep in mind that the switch port connect-
ing to the hub as well as the hosts connecting to that hub must run at half-duplex.
The reason you don’t need to worry about collisions is because now it’s like a freeway
with multiple lanes instead of the single-lane road provided by half-duplex. Full-duplex
Ethernet is supposed to offer 100-percent efciency in both directions—for example, you
can get 20 Mbps with a 10 Mbps Ethernet running full-duplex, or 200 Mbps for Fast
Ethernet. But this rate is known as an aggregate rate, which translates as “you’re supposed
to get” 100 percent efciency. No guarantees, in networking as in life!
You can use full-duplex Ethernet in at least the following six situations:
u With a connection from a switch to a host
u With a connection from a switch to a switch
u With a connection from a host to a host
u With a connection from a switch to a router
u With a connection from a router to a router
u With a connection from a router to a host
Full-duplex Ethernet requires a point-to-point connection when only two
nodes are present. You can run full-duplex with just about any device
except a hub.
Now this may be a little confusing because this begs the question that if it’s capable of
all that speed, why wouldn’t it actually deliver? Well, when a full-duplex Ethernet port is
powered on, it rst connects to the remote end and then negotiates with the other end of the
Fast Ethernet link. This is called an auto-detect mechanism. This mechanism rst decides on
the exchange capability, which means it checks to see if it can run at 10, 100, or even 1000
Mbps. It then checks to see if it can run full-duplex, and if it can’t, it will run half-duplex.
Ethernet Networks in Review 49
Remember that half-duplex Ethernet shares a collision domain and
provides a lower effective throughput than full-duplex Ethernet, which
typically has a private per-port collision domain plus a higher effective
throughput.
Last, remember these important points:
u There are no collisions in full-duplex mode.
u A dedicated switch port is required for each full-duplex node.
u The host network card and the switch port must be capable of operating in full-
duplex mode.
u The default behavior of 10Base-T and 100Base-T hosts is 10 Mbps half-duplex if the
autodetect mechanism fails, so it is always good practice to set the speed and duplex of
each port on a switch if you can.
Now let’s take a look at how Ethernet works at the Data Link layer.
Ethernet at the Data Link Layer
Ethernet at the Data Link layer is responsible for Ethernet addressing, commonly referred to
as MAC or hardware addressing. Ethernet is also responsible for framing packets received
from the Network layer and preparing them for transmission on the local network through
the Ethernet contention-based media access method.
Ethernet Addressing
Here’s where we get into how Ethernet addressing works. It uses the Media Access
Control (MAC) address burned into each and every Ethernet network interface card
(NIC). The MAC, or hardware, address is a 48-bit (6-byte) address written in a hexa-
decimal format.
Figure2.7 shows the 48-bit MAC addresses and how the bits are divided.
FIGURE2.7 Ethernet addressing using MAC addresses
24 bits
Organizationally Unique
Identifier (OUI)
(Assigned by IEEE)
Example: 0000.0c12.3456
G/L Vendor assigned
I/G
4647
24 bits
The organizationally unique identier (OUI) is assigned by the IEEE to an organization.
It’s composed of 24 bits, or 3 bytes, and it in turn assigns a globally administered address
also made up of 24 bits, or 3 bytes, that’s supposedly unique to each and every adapter an
50 Chapter 2 u Ethernet Networking and Data Encapsulation
organization manufactures. Surprisingly, there’s no guarantee when it comes to that unique
claim! Okay, now look closely at the gure. The high-order bit is the Individual/Group (I/G)
bit. When it has a value of 0, we can assume that the address is the MAC address of a device
and that it may well appear in the source portion of the MAC header. When it’s a 1, we can
assume that the address represents either a broadcast or multicast address in Ethernet.
The next bit is the global/local bit, sometimes called the G/L bit or U/L bit, where U
means universal. When set to 0, this bit represents a globally administered address, as
assigned by the IEEE, but when it’s a 1, it represents a locally governed and administered
address. The low-order 24 bits of an Ethernet address represent a locally administered or
manufacturer-assigned code. This portion commonly starts with 24 0s for the rst card
made and continues in order until there are 24 1s for the last (16,777,216th) card made.
You’ll nd that many manufacturers use these same six hex digits as the last six characters
of their serial number on the same card.
Let’s stop for a minute and go over some addressing schemes important in the
Ethernet world.
Binary to Decimal and Hexadecimal Conversion
Before we get into working with the TCP/IP protocol and IP addressing, which we’ll do in
Chapter 3, “TCP/IP,” it’s really important for you to truly grasp the differences between
binary, decimal, and hexadecimal numbers and how to convert one format into the other.
We’ll start with binary numbering, which is really pretty simple. The digits used are
limited to either a 1 or a 0, and each digit is called a bit, which is short for binary digit.
Typically, you group either 4 or 8 bits together, with these being referred to as a nibble
and a byte, respectively.
The interesting thing about binary numbering is how the value is represented in a decimal
format—the typical decimal format being the base-10 number scheme that we’ve all used
since kindergarten. The binary numbers are placed in a value spot, starting at the right and
moving left, with each spot having double the value of the previous spot.
Table2.1 shows the decimal values of each bit location in a nibble and a byte.
Remember, a nibble is 4 bits and a byte is 8 bits.
TABLE2.1 Binary values
Nibble Values Byte Values
8 4 2 1 128 64 32 16 8 4 2 1
What all this means is that if a one digit (1) is placed in a value spot, then the nibble or byte
takes on that decimal value and adds it to any other value spots that have a 1. If a zero (0) is
placed in a bit spot, you don’t count that value.
Let me clarify this a little. If we have a 1 placed in each spot of our nibble, we would then
add up 8 + 4 + 2 + 1 to give us a maximum value of 15. Another example for our nibble
values would be 1001, meaning that the 8 bit and the 1 bit are turned on, which equals a
Ethernet Networks in Review 51
decimal value of 9. If we have a nibble binary value of 0110, then our decimal value would be
6, because the 4 and 2 bits are turned on.
But the byte decimal values can add up to a number that’s signicantly higher than 15.
This is how: If we counted every bit as a one (1), then the byte binary value would look like
the following example because, remember, 8 bits equal a byte:
11111111
We would then count up every bit spot because each is turned on. It would look like this,
which demonstrates the maximum value of a byte:
128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255
There are plenty of other decimal values that a binary number can equal. Let’s work
through a few examples:
10010110
Which bits are on? The 128, 16, 4, and 2 bits are on, so we’ll just add them up: 128 + 16
+ 4 + 2 = 150.
01101100
Which bits are on? The 64, 32, 8, and 4 bits are on, so we just need to add them up:
64 + 32 + 8 + 4 = 108.
11101000
Which bits are on? The 128, 64, 32, and 8 bits are on, so just add the values up: 128 +
64 + 32 + 8 = 232.
I highly recommend that you memorize Table2.2 before braving the IP sections in
Chapter 3, “TCP/IP,” and Chapter 4, “Easy Subnetting”!
TABLE2.2 Binary to decimal memorization chart
Binary Value Decimal Value
10000000 128
11000000 192
11100000 224
1111000 0 240
11111000 248
11111100 252
11111110 254
11111111 255
52 Chapter 2 u Ethernet Networking and Data Encapsulation
Hexadecimal addressing is completely different than binary or decimal—it’s converted
by reading nibbles, not bytes. By using a nibble, we can convert these bits to hex pretty
simply. First, understand that the hexadecimal addressing scheme uses only the characters
0 through 9. Because the numbers 10, 11, 12, and so on can’t be used (because they are
two-digit numbers), the letters A, B, C, D, E, and F are used instead to represent 10, 11,
12, 13, 14, and 15, respectively.
Hex is short for hexadecimal, which is a numbering system that uses the
first six letters of the alphabet, A through F, to extend beyond the available
10 characters in the decimal system. These values are not case sensitive.
Table2.3 shows both the binary value and the decimal value for each hexadecimal digit.
TABLE2.3 Hex to binary to decimal chart
Hexadecimal Value Binary Value Decimal Value
00000 0
10001 1
20010 2
30011 3
40100 4
50101 5
60110 6
70111 7
81000 8
91001 9
A1010 10
B1011 11
C1100 12
D1101 13
Ethernet Networks in Review 53
Hexadecimal Value Binary Value Decimal Value
E1110 14
F1111 15
Did you notice that the rst 10 hexadecimal digits (0–9) are the same value as the decimal
values? If not, look again because this handy fact makes those values super easy to convert!
Okay, now suppose you have something like this: 0x6A. This is important because some-
times Cisco likes to put 0x in front of characters so you know that they are a hex value. It
doesn’t have any other special meaning. So what are the binary and decimal values? All you
have to remember is that each hex character is one nibble and that two hex characters joined
together make a byte. To gure out the binary value, put the hex characters into two nibbles
and then join them together into a byte. 6 = 0110; A, which is 10 in hex = 1010; so the com-
plete byte would be 01101010.
To convert from binary to hex, just take the byte and break it into nibbles. Let me
clarify this.
Say you have the binary number 01010101. First, break it into nibbles—0101 and
0101—with the value of each nibble being 5 since the 1 and 4 bits are on. This makes the
hex answer 0x55. And in decimal format, the binary number is 01010101, which converts
to 64 + 16 + 4 + 1 = 85.
Here’s another binary number:
11001100
Your answer would be 1100 = 12 and 1100 = 12, so therefore, it’s converted to CC in hex.
The decimal conversion answer would be 128 + 64 + 8 + 4 = 204.
One more example, then we need to get working on the Physical layer. Suppose you had
the following binary number:
10110101
The hex answer would be 0xB5, since 1011 converts to B and 0101 converts to 5 in hex
value. The decimal equivalent is 128 + 32 + 16 + 4 + 1 = 181.
Make sure you check out Written Lab 2.1 for more practice with binary/
decimal/hex conversion!
Ethernet Frames
The Data Link layer is responsible for combining bits into bytes and bytes into frames. Frames
are used at the Data Link layer to encapsulate packets handed down from the Network layer
for transmission on a type of media access.
54 Chapter 2 u Ethernet Networking and Data Encapsulation
The function of Ethernet stations is to pass data frames between each other using a
group of bits known as a MAC frame format. This provides error detection from a cyclic
redundancy check (CRC). But remember—this is error detection, not error correction. An
example of a typical Ethernet frame used today is shown in Figure2.8.
FIGURE2.8 Typical Ethernet frame format
Ethernet_II
Preamble
7 bytes
SFD
1 byte
Destination
6 bytes
Source
6 bytes
Type
2 bytes
Data and Pad
46 – 1500 bytes
Packet
FCS
4 bytes
Encapsulating a frame within a different type of frame is called tunneling.
Following are the details of the various elds in the typical Ethernet frame type:
Preamble An alternating 1,0 pattern provides a 5 MHz clock at the start of each packet,
which allows the receiving devices to lock the incoming bit stream.
Start Frame Delimiter (SFD)/Synch The preamble is seven octets and the SFD is one octet
(synch). The SFD is 10101011, where the last pair of 1s allows the receiver to come into the
alternating 1,0 pattern somewhere in the middle and still sync up to detect the beginning of
the data.
Destination Address (DA) This transmits a 48-bit value using the least signicant bit
(LSB) rst. The DA is used by receiving stations to determine whether an incoming packet
is addressed to a particular node. The destination address can be an individual address or a
broadcast or multicast MAC address. Remember that a broadcast is all 1s—all Fs in hex—
and is sent to all devices. A multicast is sent only to a similar subset of nodes on a network.
Source Address (SA) The SA is a 48-bit MAC address used to identify the transmitting
device, and it uses the least signicant bit rst. Broadcast and multicast address formats are
illegal within the SA eld.
Length or Type 802.3 uses a Length eld, but the Ethernet_II frame uses a Type eld to
identify the Network layer protocol. The old, original 802.3 cannot identify the upper-layer
protocol and must be used with a proprietary LAN—IPX, for example.
Data This is a packet sent down to the Data Link layer from the Network layer. The size
can vary from 46 to 1,500 bytes.
Ethernet Networks in Review 55
Frame Check Sequence (FCS) FCS is a eld at the end of the frame that’s used to store the
cyclic redundancy check (CRC) answer. The CRC is a mathematical algorithm that’s run
when each frame is built based on the data in the frame. When a receiving host receives the
frame and runs the CRC, the answer should be the same. If not, the frame is discarded,
assuming errors have occurred.
Let’s pause here for a minute and take a look at some frames caught on my trusty net-
work analyzer. You can see that the frame below has only three elds: Destination, Source,
and Type, which is shown as Protocol Type on this particular analyzer:
Destination: 00:60:f5:00:1f:27
Source: 00:60:f5:00:1f:2c
Protocol Type: 08-00 IP
This is an Ethernet_II frame. Notice that the Type eld is IP, or 08-00, mostly just
referred to as 0x800 in hexadecimal.
The next frame has the same elds, so it must be an Ethernet_II frame as well:
Destination: ff:ff:ff:ff:ff:ff Ethernet Broadcast
Source: 02:07:01:22:de:a4
Protocol Type: 08-00 IP
Did you notice that this frame was a broadcast? You can tell because the destination
hardware address is all 1s in binary, or all Fs in hexadecimal.
Let’s take a look at one more Ethernet_II frame. I’ll talk about this next example again
when we use IPv6 in Chapter 14, “IPv6 Routing,” but you can see that the Ethernet frame is
the same Ethernet_II frame used with the IPv4 routed protocol. The Type eld has 0x86dd
when the frame is carrying IPv6 data, and when we have IPv4 data, the frame uses 0x0800
in the protocol eld:
Destination: IPv6-Neighbor-Discovery_00:01:00:03 (33:33:00:01:00:03)
Source: Aopen_3e:7f:dd (00:01:80:3e:7f:dd)
Type: IPv6 (0x86dd)
This is the beauty of the Ethernet_II frame. Because of the Type eld, we can run any
Network layer routed protocol and the frame will carry the data because it can identify
the Network layer protocol!
Ethernet at the Physical Layer
Ethernet was rst implemented by a group called DIX, which stands for Digital, Intel, and
Xerox. They created and implemented the rst Ethernet LAN specication, which the IEEE
used to create the IEEE 802.3 committee. This was a 10 Mbps network that ran on coax
and then eventually twisted-pair and ber physical media.
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56 Chapter 2 u Ethernet Networking and Data Encapsulation
The IEEE extended the 802.3 committee to three new committees known as 802.3u
(Fast Ethernet), 802.3ab (Gigabit Ethernet on category 5) and then nally one more,
802.3ae (10 Gbps over ber and coax). There are more standards evolving almost daily,
such as the new 100 Gbps Ethernet (802.3ba)!
When designing your LAN, it’s really important to understand the different types of
Ethernet media available to you. Sure, it would be great to run Gigabit Ethernet to each
desktop and 10 Gbps between switches, but you would need to gure out how to justify the
cost of that network today! However, if you mix and match the different types of Ethernet
media methods currently available, you can come up with a cost-effective network solution
that works really great.
The EIA/TIA (Electronic Industries Alliance and the newer Telecommunications
Industry Association) is the standards body that creates the Physical layer specications
for Ethernet. The EIA/TIA species that Ethernet use a registered jack (RJ) connector on
unshielded twisted-pair (UTP) cabling (RJ45). But the industry is moving toward simply
calling this an 8-pin modular connector.
Every Ethernet cable type that’s specied by the EIA/TIA has inherent attenuation, which is
dened as the loss of signal strength as it travels the length of a cable and is measured in deci-
bels (dB). The cabling used in corporate and home markets is measured in categories. A higher-
quality cable will have a higher-rated category and lower attenuation. For example, category 5
is better than category 3 because category 5 cables have more wire twists per foot and therefore
less crosstalk. Crosstalk is the unwanted signal interference from adjacent pairs in the cable.
Here is a list of some of the most common IEEE Ethernet standards, starting with
10 Mbps Ethernet:
10Base-T (IEEE 802.3) 10 Mbps using category 3 unshielded twisted pair (UTP) wiring
for runs up to 100 meters. Unlike with the 10Base-2 and 10Base-5 networks, each device
must connect into a hub or switch, and you can have only one host per segment or wire.
It uses an RJ45 connector (8-pin modular connector) with a physical star topology and a
logical bus.
100Base-TX (IEEE 802.3u) 100Base-TX, most commonly known as Fast Ethernet, uses
EIA/TIA category 5, 5E, or 6 UTP two-pair wiring. One user per segment; up to 100 meters
long. It uses an RJ45 connector with a physical star topology and a logical bus.
100Base-FX (IEEE 802.3u) Uses ber cabling 62.5/125-micron multimode ber. Point-
to-point topology; up to 412 meters long. It uses ST and SC connectors, which are media-
interface connectors.
1000Base-CX (IEEE 802.3z) Copper twisted-pair, called twinax, is a balanced coaxial
pair that can run only up to 25 meters and uses a special 9-pin connector known as the High
Speed Serial Data Connector (HSSDC). This is used in Cisco’s new Data Center technologies.
1000Base-T (IEEE 802.3ab) Category 5, four-pair UTP wiring up to 100 meters long and
up to 1 Gbps.
Ethernet Networks in Review 57
1000Base-SX (IEEE 802.3z) The implementation of 1 Gigabit Ethernet running over
multimode ber-optic cable instead of copper twisted-pair cable, using short wavelength
laser. Multimode ber (MMF) using 62.5- and 50-micron core; uses an 850 nanometer
(nm) laser and can go up to 220 meters with 62.5-micron, 550 meters with 50-micron.
1000Base-LX (IEEE 802.3z) Single-mode ber that uses a 9-micron core and 1300 nm
laser and can go from 3 kilometers up to 10 kilometers.
1000Base-ZX (Cisco standard) 1000BaseZX, or 1000Base-ZX, is a Cisco specied stan-
dard for Gigabit Ethernet communication. 1000BaseZX operates on ordinary single-mode
ber-optic links with spans up to 43.5 miles (70 km).
10GBase-T (802.3.an) 10GBase-T is a standard proposed by the IEEE 802.3an committee
to provide 10 Gbps connections over conventional UTP cables, (category 5e, 6, or 7 cables).
10GBase-T allows the conventional RJ45 used for Ethernet LANs and can support signal
transmission at the full 100-meter distance specied for LAN wiring.
If you want to implement a network medium that is not susceptible to elec-
tromagnetic interference (EMI), fiber-optic cable provides a more secure,
long-distance cable that is not susceptible to EMI at high speeds.
Armed with the basics covered so far in this chapter, you’re equipped to go to the next
level and put Ethernet to work using various Ethernet cabling.
Interference or Host Distance Issue?
Quite a few years ago, I was consulting at a very large aerospace company in the Los
Angeles area. In the very busy warehouse, they had hundreds of hosts provide many
different services to the various departments working in that area.
However, a small group of hosts have been experiencing intermittent outages that no one
could explain since most hosts in the same area have no problems whatsoever. So I decided
to take a crack at this problem and see what I could find.
First, I traced the backbone connection from the main switch to multiple switches in
the warehouse area. Assuming that the hosts with the issues were connected to the
same switch, I traced each cable, and much to my surprise they were connected to
various switches! Now my interest really peaked because the simplest issue had been
eliminated right off the bat. It wasn’t a simple switch problem!
58 Chapter 2 u Ethernet Networking and Data Encapsulation
I continued to trace each cable one-by-one, and this is what I found:
Working
PC
Working
PC
Warehouse area
Backbone area
Intermittent
PC
As I drew this network out, I noticed that they had many repeaters in place, which isn’t a
cause for immediate suspicion since bandwidth was not their biggest requirement here.
So I looked deeper still. At this point, I decided to measure the distance of one of the
intermittent hosts connecting to their hub/repeater.
This is what I measured. Can you see the problem?
Working
PC
Working
PC
Warehouse area
Backbone area
75 meters
Intermittent
PC
115 meters 90 meters
Ethernet Cabling 59
Having a hub or repeater in your network isn’t a problem, unless you need better band-
width (which they didn’t in this case), but the distance was! It’s not always easy to tell
how far away a host is from their connection in an extremely large area, so these hosts
ended up having a connection past the 100-meter Ethernet specification which created
a problem for the hosts not cabled correctly. Understand that this didn’t stop the hosts
from completely working, but the workers felt the hosts stopped working when they
were at their most stressful point of the day. Sure, that makes sense, because whenever
my host stops working, that becomes my most stressful part of the day!
Ethernet Cabling
A discussion about Ethernet cabling is an important one, especially if you are planning on
taking the Cisco exams. You need to really understand the following three types of cables:
u Straight-through cable
u Crossover cable
u Rolled cable
We will look at each in the following sections, but rst, let’s take a look at the most
common Ethernet cable used today, the category 5 Enhanced Unshielded Twisted Pair
(UTP), shown in Figure2.9.
FIGURE2.9 Category 5 Enhanced UTP cable
The category 5 Enhanced UTP cable can handle speeds up to a gigabit with a distance
of up to 100 meters. Typically we’d use this cable for 100 Mbps and category 6 for a gigabit,
but the category 5 Enhanced is rated for gigabit speeds and category 6 is rated for 10 Gbps!
60 Chapter 2 u Ethernet Networking and Data Encapsulation
Straight-through Cable
The straight-through cable is used to connect the following devices:
u Host to switch or hub
u Router to switch or hub
Four wires are used in straight-through cable to connect Ethernet devices. It’s relatively
simple to create this type, and Figure2.10 shows the four wires used in a straight-through
Ethernet cable.
FIGURE2.10 Straight-through Ethernet cable
Transmit on pins 1 & 2
Receive on pins 3 & 6
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
Receive on pins 1 & 2
Transmit on pins 3 & 6
Notice that only pins 1, 2, 3, and 6 are used. Just connect 1 to 1, 2 to 2, 3 to 3, and 6
to 6 and you’ll be up and networking in no time. However, remember that this would be a
10/100 Mbps Ethernet-only cable and wouldn’t work with gigabit, voice, or other LAN or
WAN technology.
Crossover Cable
The crossover cable can be used to connect the following devices:
u Switch to switch
u Hub to hub
u Host to host
u Hub to switch
u Router direct to host
u Router to router
The same four wires used in the straight-through cable are used in this cable—we just
connect different pins together. Figure2.11 shows how the four wires are used in a cross-
over Ethernet cable.
Ethernet Cabling 61
FIGURE2.11 Crossover Ethernet cable
Receive on pins 1 & 2
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
Transmit on pins 3 & 6
Notice that instead of connecting 1 to 1, 2 to 2, and so on, here we connect pins 1 to 3
and 2 to 6 on each side of the cable. Figure2.12 shows some typical uses of straight-through
and crossover cables.
FIGURE2.12 Typical uses for straight-through and cross-over Ethernet cables
Fa0/1
Fa0/1
Fa0/0 Fa0/0
Fa0/0
Crossover cable
Fa0/1
Fa0/0
Fa0/1
Straight-through cable
S1
S1
R1
R1
R2 R1
S1 S2
The crossover examples in Figure2.12 are switch port to switch port, router Ethernet port
to router Ethernet port, and PC Ethernet to router Ethernet port. For the straight-through
examples I used PC Ethernet to switch port and router Ethernet port to switch port.
It’s very possible to connect a straight-through cable between two switches,
and it will start working because of autodetect mechanisms called auto-
mdix. But be advised that the CCNA objectives do not typically consider
autodetect mechanisms valid between devices!
UTP Gigabit Wiring (1000Base-T)
In the previous examples of 10Base-T and 100Base-T UTP wiring, only two wire pairs were
used, but that is not good enough for Gigabit UTP transmission.
62 Chapter 2 u Ethernet Networking and Data Encapsulation
1000Base-T UTP wiring (Figure2.13) requires four wire pairs and uses more advanced
electronics so that each and every pair in the cable can transmit simultaneously. Even so,
gigabit wiring is almost identical to my earlier 10/100 example, except that we’ll use the
other two pairs in the cable.
FIGURE2.13 UTP Gigabit crossover Ethernet cable
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
For a straight-through cable it’s still 1 to 1, 2 to 2, and so on up to pin 8. And in creat-
ing the gigabit crossover cable, you’d still cross 1 to 3 and 2 to 6, but you would add 4 to 7
and 5 to 8—pretty straightforward!
Rolled Cable
Although rolled cable isn’t used to connect any Ethernet connections together, you can use
a rolled Ethernet cable to connect a host EIA-TIA 232 interface to a router console serial
communication (COM) port.
If you have a Cisco router or switch, you would use this cable to connect your PC, Mac,
or a device like an iPad to the Cisco hardware. Eight wires are used in this cable to connect
serial devices, although not all eight are used to send information, just as in Ethernet net-
working. Figure2.14 shows the eight wires used in a rolled cable.
FIGURE2.14 Rolled Ethernet cable
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
Console port
RS-232 port
These are probably the easiest cables to make because you just cut the end off on one side
of a straight-through cable, turn it over, and put it back on—with a new connector, of course!
Ethernet Cabling 63
Okay, once you have the correct cable connected from your PC to the Cisco router or
switch console port, you can start your emulation program such as putty or SecureCRT
to create a console connection and congure the device. Set the conguration as shown in
Figure2.15.
FIGURE2.15 Configuring your console emulation program
Notice that Bit Rate is set to 9600, Data Bits to 8, Parity to None, and Flow Control is
set to None. At this point, you can click Connect and press the Enter key and you should be
connected to your Cisco device console port.
Figure2.16 shows a nice new 2960 switch with two console ports.
FIGURE2.16 A Cisco 2960 console connections
Console
Notice there are two console connections on this new switch—a typical original RJ45
connection, and the newer mini type-B USB console. Remember that the new USB port super-
sedes the RJ45 port if you just happen to plug into both at the same time, and the USB port
can have speeds up to 115,200 Kbps, which is awesome if you have to use Xmodem to update
an IOS. I’ve even seen some cables that work on iPhones and iPads and allow them to connect
to these mini USB ports!
64 Chapter 2 u Ethernet Networking and Data Encapsulation
Now that you’ve seen the various RJ45 unshielded twisted-pair (UTP) cables, what type
of cable is used between the switches in Figure2.17?
FIGURE2.17 RJ45 UTP cable question #1
Switch
A
Switch
B
?
In order for host A to ping host B, you need a crossover cable to connect the two switches
together. But what types of cables are used in the network shown in Figure2.18?
FIGURE2.18 RJ45 UTP cable question #2
Router
Console
In Figure2.18, there’s a whole menu of cables in use. For the connection between the
switches, we’d obviously use a crossover cable like we saw in Figure2.13. The trouble is
that you must understand that we have a console connection that uses a rolled cable. Plus,
the connection from the router to the switch is a straight-through cable, as is true for the
hosts to the switches. Keep in mind that if we had a serial connection, which we don’t, we
would use a V.35 to connect us to a WAN.
Fiber Optic
Fiber-optic cabling has been around for a long time and has some solid standards. The cable
allows for very fast transmission of data, is made of glass (or even plastic!), is very thin, and
works as a waveguide to transmit light between two ends of the ber. Fiber optics has been
used to go very long distances, as in intercontinental connections, but it is becoming more
and more popular in Ethernet LAN networks due to the fast speeds available and because,
unlike UTP, it’s immune to interference like cross-talk.
Ethernet Cabling 65
Some main components of this cable are the core and the cladding. The core will
hold the light and the cladding connes the light in the core. The tighter the cladding,
the smaller the core, and when the core is small, less light will be sent, but it can go
faster and farther!
In Figure2.19 you can see that there is a 9-micron core, which is very small and can be
measured against a human hair, which is 50 microns.
FIGURE2.19 Typical fiber cable
Core
Cladding
Buffer
250
125
9
Dimensions are in um (10–6 meters). Not to scale.
The cladding is 125 microns, which is actually a ber standard that allows manufacturers
to make connectors for all ber cables. The last piece of this cable is the buffer, which is there
to protect the delicate glass.
There are two major types of ber optics: single-mode and multimode. Figure2.20
shows the differences between multimode and single-mode bers.
FIGURE2.20 Multimode and single-mode fibers
Cladding
Multi-mode fiber
Core
Cladding
Single-mode fiber
Core
66 Chapter 2 u Ethernet Networking and Data Encapsulation
Single-mode is more expensive, has a tighter cladding, and can go much farther distances
than multimode. The difference comes in the tightness of the cladding, which makes a smaller
core, meaning that only one mode of light will propagate down the ber. Multimode is looser
and has a larger core so it allows multiple light particles to travel down the glass. These parti-
cles have to be put back together at the receiving end, so distance is less than that with single-
mode ber, which allows only very few light particles to travel down the ber.
There are about 70 different connectors for ber, and Cisco uses a few different types.
Looking back at Figure2.16, the two bottom ports are referred to as Small Form-Factor
Pluggable, or SFPs.
Data Encapsulation
When a host transmits data across a network to another device, the data goes through a
process called encapsulation and is wrapped with protocol information at each layer of the
OSI model. Each layer communicates only with its peer layer on the receiving device.
To communicate and exchange information, each layer uses protocol data units (PDUs).
These hold the control information attached to the data at each layer of the model. They are
usually attached to the header in front of the data eld but can also be at the trailer, or end,
of it.
Each PDU attaches to the data by encapsulating it at each layer of the OSI model, and
each has a specic name depending on the information provided in each header. This PDU
information is read-only by the peer layer on the receiving device. After its read, it’s stripped
off and the data is then handed to the next layer up.
Figure2.21 shows the PDUs and how they attach control information to each layer.
This gure demonstrates how the upper-layer user data is converted for transmission on
the network. The data stream is then handed down to the Transport layer, which sets up a
virtual circuit to the receiving device by sending over a synch packet. Next, the data stream
is broken up into smaller pieces, and a Transport layer header is created and attached to the
header of the data eld; now the piece of data is called a segment (a PDU). Each segment
can be sequenced so the data stream can be put back together on the receiving side exactly
as it was transmitted.
Each segment is then handed to the Network layer for network addressing and routing
through the internetwork. Logical addressing (for example, IP and IPv6) is used to get each
segment to the correct network. The Network layer protocol adds a control header to the
segment handed down from the Transport layer, and what we have now is called a packet
or datagram. Remember that the Transport and Network layers work together to rebuild
a data stream on a receiving host, but it’s not part of their work to place their PDUs on a
local network segment—which is the only way to get the information to a router or host.
It’s the Data Link layer that’s responsible for taking packets from the Network layer and
placing them on the network medium (cable or wireless). The Data Link layer encapsulates
each packet in a frame, and the frame’s header carries the hardware addresses of the source
and destination hosts. If the destination device is on a remote network, then the frame is
Data Encapsulation 67
sent to a router to be routed through an internetwork. Once it gets to the destination net-
work, a new frame is used to get the packet to the destination host.
FIGURE2.21 Data encapsulation
Upper-layer data
Upper-layer data
TCP header
DataIP header
Data
0101110101001000010
Transport
Session
Presentation
Application
Network
Data Link
Physical
segment
packet
frame
bits
MAC LLC FCS
To put this frame on the network, it must rst be put into a digital signal. Since a frame
is really a logical group of 1s and 0s, the physical layer is responsible for encoding these dig-
its into a digital signal, which is read by devices on the same local network. The receiving
devices will synchronize on the digital signal and extract (decode) the 1s and 0s from the digi-
tal signal. At this point, the devices reconstruct the frames, run a CRC, and then check their
answer against the answer in the frame’s FCS eld. If it matches, the packet is pulled from the
frame and what’s left of the frame is discarded. This process is called de-encapsulation. The
packet is handed to the Network layer, where the address is checked. If the address matches,
the segment is pulled from the packet and what’s left of the packet is discarded. The segment
is processed at the Transport layer, which rebuilds the data stream and acknowledges to the
transmitting station that it received each piece. It then happily hands the data stream to the
upper-layer application.
At a transmitting device, the data encapsulation method works like this:
1. User information is converted to data for transmission on the network.
2. Data is converted to segments, and a reliable connection is set up between the trans-
mitting and receiving hosts.
3. Segments are converted to packets or datagrams, and a logical address is placed in the
header so each packet can be routed through an internetwork.
4. Packets or datagrams are converted to frames for transmission on the local network.
Hardware (Ethernet) addresses are used to uniquely identify hosts on a local net-
work segment.
5. Frames are converted to bits, and a digital encoding and clocking scheme is used.
68 Chapter 2 u Ethernet Networking and Data Encapsulation
To explain this in more detail using the layer addressing, I’ll use Figure2.22.
FIGURE2.22 PDU and layer addressing
Segment Source
port
Destination
port ... Data
Packet Source IP Destination
IP Protocol ... Segment
Frame Destination
MAC
Source
MAC Ether-Field FCS
Bits 1011011100011110000
Packet
Remember that a data stream is handed down from the upper layer to the Transport
layer. As technicians, we really don’t care who the data stream comes from because that’s
really a programmer’s problem. Our job is to rebuild the data stream reliably and hand it
to the upper layers on the receiving device.
Before we go further in our discussion of Figure2.22, let’s discuss port numbers and
make sure you understand them. The Transport layer uses port numbers to dene both the
virtual circuit and the upper-layer processes, as you can see from Figure2.23.
FIGURE2.23 Port numbers at the Transport layer
Source
port
Destination
port ...
SP DP
1028 23 ...
Used in part to define
the virtual circuit
Defines upper layer
process or application
Data Encapsulation 69
When using a connection-oriented protocol like TCP, the Transport layer takes the data
stream, makes segments out of it, and establishes a reliable session by creating a virtual cir-
cuit. It then sequences (numbers) each segment and uses acknowledgments and ow control. If
you’re using TCP, the virtual circuit is dened by the source and destination port number plus
the source and destination IP address and called a socket. Understand that the host just makes
this up, starting at port number 1024 because 0 through 1023 are reserved for well-known
port numbers. The destination port number denes the upper-layer process or application that
the data stream is handed to when the data stream is reliably rebuilt on the receiving host.
Now that you understand port numbers and how they are used at the Transport layer,
let’s go back to Figure2.22. Once the Transport layer header information is added to the
piece of data, it becomes a segment that’s handed down to the Network layer along with
the destination IP address. As you know, the destination IP address was handed down from
the upper layers to the Transport layer with the data stream and was identied via name
resolution at the upper layers—probably with DNS.
The Network layer adds a header and adds the logical addressing such as IP addresses
to the front of each segment. Once the header is added to the segment, the PDU is called a
packet. The packet has a protocol eld that describes where the segment came from (either
UDP or TCP) so it can hand the segment to the correct protocol at the Transport layer
when it reaches the receiving host.
The Network layer is responsible for nding the destination hardware address that dic-
tates where the packet should be sent on the local network. It does this by using the Address
Resolution Protocol (ARP)—something I’ll talk about more in Chapter 3. IP at the Network
layer looks at the destination IP address and compares that address to its own source IP
address and subnet mask. If it turns out to be a local network request, the hardware address
of the local host is requested via an ARP request. If the packet is destined for a remote host,
IP will look for the IP address of the default gateway (router) instead.
The packet, along with the destination hardware address of either the local host or
default gateway, is then handed down to the Data Link layer. The Data Link layer will add
a header to the front of the packet and the piece of data then becomes a frame. It’s called a
frame because both a header and a trailer are added to the packet, which makes it look like
it’s within bookends—a frame—as shown in Figure2.22. The frame uses an Ether-Type
eld to describe which protocol the packet came from at the Network layer. Now a cyclic
redundancy check is run on the frame, and the answer to the CRC is placed in the Frame
Check Sequence eld found in the trailer of the frame.
The frame is now ready to be handed down, one bit at a time, to the Physical layer,
which will use bit-timing rules to encode the data in a digital signal. Every device on the
network segment will receive the digital signal and synchronize with the clock and extract
the 1s and 0s from the digital signal to build a frame. After the frame is rebuilt, a CRC is
run to make sure the frame is in proper order. If everything turns out to be all good, the
hosts will check the destination MAC and IP addresses to see if the frame is for them.
If all this is making your eyes cross and your brain freeze, don’t freak. I’ll be going over
exactly how data is encapsulated and routed through an internetwork later, in Chapter 8,
“IP Routing.”
70 Chapter 2 u Ethernet Networking and Data Encapsulation
The Cisco three-Layer
HierarchicalModel
Most of us were exposed to hierarchy early in life. Anyone with older siblings learned what
it was like to be at the bottom of the hierarchy. Regardless of where you rst discovered the
concept of hierarchy, most of us experience it in many aspects of our lives. It’s hierarchy that
helps us understand where things belong, how things t together, and what functions go
where. It brings order to otherwise complex models. If you want a pay raise, for instance,
hierarchy dictates that you ask your boss, not your subordinate, because that’s the person
whose role it is to grant or deny your request. So basically, understanding hierarchy helps us
discern where we should go to get what we need.
Hierarchy has many of the same benets in network design that it does in other areas
of life. When used properly, it makes networks more predictable and helps us dene which
areas should perform certain functions. Likewise, you can use tools such as access lists at
certain levels in hierarchical networks and avoid them at others.
Let’s face it: Large networks can be extremely complicated, with multiple protocols,
detailed congurations, and diverse technologies. Hierarchy helps us summarize a complex
collection of details into an understandable model, bringing order from the chaos. Then, as
specic congurations are needed, the model dictates the appropriate manner in which to
apply them.
The Cisco hierarchical model can help you design, implement, and maintain a scalable,
reliable, cost-effective hierarchical internetwork. Cisco denes three layers of hierarchy, as
shown in Figure2.24, each with specic functions.
Each layer has specic responsibilities. Keep in mind that the three layers are logical and
are not necessarily physical devices. Consider the OSI model, another logical hierarchy. Its
seven layers describe functions but not necessarily protocols, right? Sometimes a protocol
maps to more than one layer of the OSI model, and sometimes multiple protocols commu-
nicate within a single layer. In the same way, when we build physical implementations of
hierarchical networks, we may have many devices in a single layer, or there may be a single
device performing functions at two layers. Just remember that the denition of the layers is
logical, not physical!
So let’s take a closer look at each of the layers now.
The Core Layer
The core layer is literally the core of the network. At the top of the hierarchy, the core layer
is responsible for transporting large amounts of trafc both reliably and quickly. The only
purpose of the network’s core layer is to switch trafc as fast as possible. The trafc trans-
ported across the core is common to a majority of users. But remember that user data is
processed at the distribution layer, which forwards the requests to the core if needed.
The Cisco three-Layer HierarchicalModel 71
FIGURE2.24 The Cisco hierarchical model
Core
Backbone
Distribution
Access
Web server Email server PC1 PC2
If there’s a failure in the core, every single user can be affected! This is why fault toler-
ance at this layer is so important. The core is likely to see large volumes of trafc, so speed
and latency are driving concerns here. Given the function of the core, we can now consider
some design specics. Let’s start with some things we don’t want to do:
u Never do anything to slow down traffic. This includes using access lists, routing
between virtual local area networks and implementing packet filtering.
u Don’t support workgroup access here.
u Avoid expanding the core (e.g., adding routers when the internetwork grows). If perfor-
mance becomes an issue in the core, give preference to upgrades over expansion.
Here’s a list of things that we want to achieve as we design the core:
u Design the core for high reliability. Consider data-link technologies that facilitate
both speed and redundancy, like Gigabit Ethernet with redundant links or even
10 Gigabit Ethernet.
u Design with speed in mind. The core should have very little latency.
u Select routing protocols with lower convergence times. Fast and redundant data-link
connectivity is no help if your routing tables are shot!
72 Chapter 2 u Ethernet Networking and Data Encapsulation
The Distribution Layer
The distribution layer is sometimes referred to as the workgroup layer and is the communica-
tion point between the access layer and the core. The primary functions of the distribution
layer are to provide routing, ltering, and WAN access and to determine how packets can
access the core, if needed. The distribution layer must determine the fastest way that network
service requests are handled—for example, how a le request is forwarded to a server. After
the distribution layer determines the best path, it forwards the request to the core layer if nec-
essary. The core layer then quickly transports the request to the correct service.
The distribution layer is where we want to implement policies for the network because
we are allowed a lot of exibility in dening network operation here. There are several
things that should generally be handled at the distribution layer:
u Routing
u Implementing tools (such as access lists), packet filtering, and queuing
u Implementing security and network policies, including address translation and firewalls
u Redistributing between routing protocols, including static routing
u Routing between VLANs and other workgroup support functions
u Defining broadcast and multicast domains
Key things to avoid at the distribution layer are those that are limited to functions that
exclusively belong to one of the other layers!
The Access Layer
The access layer controls user and workgroup access to internetwork resources. The access
layer is sometimes referred to as the desktop layer. The network resources most users need
will be available locally because the distribution layer handles any trafc for remote services.
The following are some of the functions to be included at the access layer:
u Continued (from distribution layer) use of access control and policies
u Creation of separate collision domains (segmentation)
u Workgroup connectivity into the distribution layer
Technologies like Gigabit or Fast Ethernet switching are frequently seen in the access layer.
I can’t stress this enough—just because there are three separate levels does not imply
three separate devices! There could be fewer or there could be more. After all, this is a
layered approach.
Visit ccna
.gg/ch2/b
for a
companion
MicroNugget
from CBT
Nuggets.
Exam Essentials 73
Summary
In this chapter, you learned the fundamentals of Ethernet networking, how hosts communicate
on a network. You discovered how CSMA/CD works in an Ethernet half-duplex network.
I also talked about the differences between half- and full-duplex modes, and we discussed
the collision detection mechanism called CSMA/CD.
I described the common Ethernet cable types used in today’s networks in this chapter as
well, and by the way, you’d be wise to study that section really well!
Important enough to not gloss over, this chapter provided an introduction to encapsulation.
Encapsulation is the process of encoding data as it goes down the OSI stack.
Last, I covered the Cisco three-layer hierarchical model. I described in detail the three
layers and how each is used to help design and implement a Cisco internetwork.
Exam Essentials
Describe the operation of Carrier Sense Multiple Access with Collision Detection
(CSMA/CD). CSMA/CD is a protocol that helps devices share the bandwidth evenly
without having two devices transmit at the same time on the network medium. Although
it does not eliminate collisions, it helps to greatly reduce them, which reduces retransmis-
sions, resulting in a more efcient transmission of data for all devices.
Differentiate half-duplex and full-duplex communication and define the requirements to
utilize each method. Full-duplex Ethernet uses two pairs of wires at the same time instead
of one wire pair like half-duplex. Full-duplex allows for sending and receiving at the same
time, using different wires to eliminate collisions, while half-duplex can send or receive but
not at the same time and still can suffer collisions. To use full-duplex, the devices at both
ends of the cable must be capable of and congured to perform full-duplex.
Describe the sections of a MAC address and the information contained in each section.
The MAC, or hardware, address is a 48-bit (6-byte) address written in a hexadecimal for-
mat. The rst 24 bits, or 3 bytes, are called the organizationally unique identier (OUI),
which is assigned by the IEEE to the manufacturer of the NIC. The balance of the number
uniquely identies the NIC.
Identify the binary and hexadecimal equivalent of a decimal number. Any number expressed
in one format can also be expressed in the other two. The ability to perform this conversion is
critical to understanding IP addressing and subnetting. Be sure to go through the written labs
covering binary to decimal to hexadecimal conversion.
Identify the fields in the Data Link portion of an Ethernet frame. The elds in the Data Link
portion of a frame include the preamble, Start Frame Delimiter, destination MAC address,
source MAC address, Length or Type, Data, and Frame Check Sequence.
74 Chapter 2 u Ethernet Networking and Data Encapsulation
Identify the IEEE physical standards for Ethernet cabling. These standards describe the
capabilities and physical characteristics of various cable types and include but are not limited
to 10Base-2, 10Base-5, and 10Base-T.
Differentiate types of Ethernet cabling and identify their proper application. The three
types of cables that can be created from an Ethernet cable are straight-through (to connect
a PC’s or router’s Ethernet interface to a hub or switch), crossover (to connect hub to hub,
hub to switch, switch to switch, or PC to PC), and rolled (for a console connection from a
PC to a router or switch).
Describe the data encapsulation process and the role it plays in packet creation. Data
encapsulation is a process whereby information is added to the frame from each layer of
the OSI model. This is also called packet creation. Each layer communicates only with its
peer layer on the receiving device.
Understand how to connect a console cable from a PC to a router and switch. Take a
rolled cable and connect it from the COM port of the host to the console port of a router.
Start your emulations program such as putty or SecureCRT and set the bits per second to
9600 and ow control to None.
Identify the layers in the Cisco three-layer model and describe the ideal function of each layer.
The three layers in the Cisco hierarchical model are the core (responsible for transporting large
amounts of trafc both reliably and quickly), distribution (provides routing, ltering, and
WAN access), and access (workgroup connectivity into the distribution layer).
Written Labs
In this section, you’ll complete the following labs to make sure you’ve got the information
and concepts contained within them fully dialed in:
Lab 2.1: Binary/Decimal/Hexadecimal Conversion
Lab 2.2: CSMA/CD Operations
Lab 2.3: Cabling
Lab 2.4: Encapsulation
The answers to these labs can be found in Appendix A, “Answers to Written Labs.”
Written Labs 75
Written Lab 2.1: Binary/Decimal/Hexadecimal Conversion
1. Convert from decimal IP address to binary format.
Complete the following table to express 192.168.10.15 in binary format.
128 64 32 16 8421Binary
Complete the following table to express 172.16.20.55 in binary format.
128 64 32 16 8421Binary
76 Chapter 2 u Ethernet Networking and Data Encapsulation
Complete the following table to express 10.11.12.99 in binary format.
128 64 32 16 8421Binary
2. Convert the following from binary format to decimal IP address.
Complete the following table to express 11001100.00110011.10101010.01010101 in
decimal IP address format.
128 64 32 16 8421Decimal
Written Labs 77
Complete the following table to express 11000110.11010011.00111001.11010001 in
decimal IP address format.
128 64 32 16 8421Decimal
Complete the following table to express 10000100.11010010.10111000.10100110 in
decimal IP address format.
128 64 32 16 8421Decimal
78 Chapter 2 u Ethernet Networking and Data Encapsulation
3. Convert the following from binary format to hexadecimal.
Complete the following table to express 11011000.00011011.00111101.01110110 in
hexadecimal.
128 64 32 16 8421Hexadecimal
Complete the following table to express 11001010.11110101.10000011.11101011 in
hexadecimal.
128 64 32 16 8421Hexadecimal
Written Labs 79
Complete the following table to express 10000100.11010010.01000011.10110011 in
hexadecimal.
128 64 32 16 8421Hexadecimal
Written Lab 2.2: CSMA/CD Operations
Carrier Sense Multiple Access with Collision Detection (CSMA/CD) helps to minimize col-
lisions in the network, thereby increasing data transmission efciency. Place the following
steps of its operation in the order in which they occur.
u All hosts have equal priority to transmit after the timers have expired.
u Each device on the Ethernet segment stops transmitting for a short time until the tim-
ers expire.
u The collision invokes a random backoff algorithm.
u A jam signal informs all devices that a collision occurred.
Written Lab 2.3: Cabling
For each of the following situations, determine whether a straight-through, crossover, or
rolled cable would be used.
1. Host to host
2. Host to switch or hub
3. Router direct to host
4. Switch to switch
5. Router to switch or hub
80 Chapter 2 u Ethernet Networking and Data Encapsulation
6. Hub to hub
7. Hub to switch
8. Host to a router console serial communication (COM) port
Written Lab 2.4: Encapsulation
Place the following steps of the encapsulation process in the proper order.
u Packets or datagrams are converted to frames for transmission on the local network.
Hardware (Ethernet) addresses are used to uniquely identify hosts on a local network
segment.
u Segments are converted to packets or datagrams, and a logical address is placed in the
header so each packet can be routed through an internetwork.
u User information is converted to data for transmission on the network.
u Frames are converted to bits, and a digital encoding and clocking scheme is used.
u Data is converted to segments, and a reliable connection is set up between the trans-
mitting and receiving hosts.
Review Questions 81
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional ques-
tions, please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. In the accompanying graphic, what is the name for the section of the MAC address
marked as unknown?
24 bits
?????????????
Example: 0000.0c12.3456
G/L Vendor assigned
I/G
4647
24 bits
A. IOS
B. OSI
C. ISO
D. OUI
2. _____________on an Ethernet network is the retransmission delay that’s enforced when
a collision occurs.
A. Backoff
B. Carrier sense
C. Forward delay
D. Jamming
82 Chapter 2 u Ethernet Networking and Data Encapsulation
3. On which type of device could the situation shown in the diagram occur?
Collision
?
A. Hub
B. Switch
C. Router
D. Bridge
4. In the Ethernet II frame shown here, what is the function of the section labeled “FCS”?
Ethernet_II
Preamble
7 bytes
SFD
1 byte
Destination
6 bytes
Source
6 bytes
Type
2 bytes
Data and Pad
46 – 1500 bytes
FCS
4 bytes
A. Allows the receiving devices to lock the incoming bit stream.
B. Error detection
C. Identifies the upper-layer protocol
D. Identifies the transmitting device
5. The contention method used by Ethernet is called ____________.
A. Token passing
B. CSMA/CD
C. Polling
D. CSMA/CA
6. In which if the following situations can you not use full-duplex?
A. With a connection from a switch to a switch
B. With a connection from a router to a router
C. With a connection from a host to a host
D. With a connection from a host to a hub
Review Questions 83
7. Between which systems could you use a cable that uses the pinout pattern shown below?
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
A. With a connection from a switch to a switch
B. With a connection from a router to a router
C. With a connection from a host to a host
D. With a connection from a host to a switch
8. When the I/G bit in a MAC address is set to 1 the transmission is ____________.
(Choose all that apply.)
A. Unicast
B. Broadcast
C. Multicast
D. Anycast
9. What type of cable uses the pinout shown here?
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
A. Fiber optic
B. Crossover Gigabit Ethernet cable
C. Straight-through FastEthernet
D. Coaxial
10. When configuring a terminal emulation program which of the following is an
incorrect setting?
A. Bit rate: 9600
B. Parity: None
C. Flow control: None
D. Data bits: 1
84 Chapter 2 u Ethernet Networking and Data Encapsulation
11. Which part of a MAC address indicates whether the address is a locally or globally
administered address?
A. FCS
B. I/G bit
C. OUI
D. U/L bit
12. What cable type uses the pinout arrangement shown below?
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
A. Fiber optic
B. Rolled
C. Straight through
D. Crossover
13. Which of the following is not one of the actions taken in the operation of CSMA/CD
when a collision occurs?
A. A jam signal informs all devices that a collision occurred.
B. The collision invokes a random backoff algorithm on the systems involved in
the collision.
C. Each device on the Ethernet segment stops transmitting for a short time until their
backoff timers expire.
D. All hosts have equal priority to transmit after the timers have expired.
14. Which of the following statements is false with regard to Ethernet?
A. There are very few collisions in full-duplex mode.
B. A dedicated switch port is required for each full-duplex node.
C. The host network card and the switch port must be capable of operating in full-
duplex mode to use full-duplex.
D. The default behavior of 10Base-T and 100Base-T hosts is 10 Mbps half-duplex if
the autodetect mechanism fails.
Review Questions 85
15. In the diagram below, identify the cable types required for connections A and B.
Switch
A
Switch
BB
A. A crossover, B crossover
B. A crossover, B straight through
C. A straight through, B straight through
D. A straight through, B crossover
16. In the work area below match the cable type to the standard with which it goes.
IEEE 802.3u
IEEE 802.3
IEEE 802.3ab
IEEE 802.3z
1000Base-T
1000Base-SX
10Base-T
100Base-TX
17. The cable used to connect to the console port on a router or switch is called
a __________ cable.
A. Crossover
B. Rollover
C. Straight-through
D. Full-duplex
18. Which of the following items comprise a socket?
A. IP address and MAC address
B. IP address and port number
C. Port number and MAC address
D. MAC address and DLCI
86 Chapter 2 u Ethernet Networking and Data Encapsulation
19. Which of the following hexadecimal numbers converts to 28 in decimal?
A. 1c
B. 12
C. 15
D. ab
20. What cable type is shown in the below graphic?
Core
Cladding
Buffer
A. Fiber optic
B. Rollover
C. Coaxial
D. Full-duplex
Chapter
3
Introduction to TCP/IP
THE FOLLOWING ICND1 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 Operation of IP Data Networks
■Identify common applications and their impact on the network
■Describe the purpose and basic operation of the protocols in
the OSI and TCP/IP models.
1 IP addressing (IPv4 / IPv6)
■Describe the operation and necessity of using private and
public IP addresses for IPv4 addressing
The Transmission Control Protocol/Internet Protocol
(TCP/IP) suite was designed and implemented by the
Department of Defense (DoD) to ensure and preserve data
integrity as well as maintain communications in the event of catastrophic war. So it
follows that if designed and implemented correctly, a TCP/IP network can be a secure,
dependable and resilient one. In this chapter, I’ll cover the protocols of TCP/IP, and
throughout this book, you’ll learn how to create a solid TCP/IP network with Cisco
routers and switches.
We’ll begin by exploring the DoD’s version of TCP/IP, then compare that version and its
protocols with the OSI reference model that we discussed earlier.
Once you understand the protocols and processes used at the various levels of the DoD
model, we’ll take the next logical step by delving into the world of IP addressing and the
different classes of IP addresses used in networks today.
Subnetting is so vital, it will be covered in its own Chapter 4,
“Easy Subnetting.”
Because having a good grasp of the various IPv4 address types is critical to understanding
IP addressing, subnetting and variable length subnet masks (VLSMs), we’ll explore these key
topics in detail, ending this chapter by discussing the various types of IPv4 addresses that
you’ll need to have down for the exam.
I’m not going to cover Internet Protocol version 6 in this chapter because we’ll get into
that later, in Chapter 14, “Internet Protocol Version 6 (IPv6).” And just so you know, you’ll
simply see Internet Protocol version 4 written as just IP, rarely as IPv4.
To find up-to-the minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
Introducing TCP/IP
TCP/IP is at the very core of all things networking, so I really want to ensure that you have
a comprehensive and functional command of it. I’ll start by giving you the whole TCP/IP
backstory, including its inception, and then move on to describe the important technical
TCP/IP and the DoD Model 89
goals as dened by its original architects. And of course I’ll include how TCP/IP compares
to the theoretical OSI model.
A Brief History of TCP/IP
TCP rst came on the scene way back in 1973, and in 1978, it was divided into two dis-
tinct protocols: TCP and IP. Later, in 1983, TCP/IP replaced the Network Control Protocol
(NCP) and was authorized as the ofcial means of data transport for anything connecting to
ARPAnet, the Internet’s ancestor. The DoD’s Advanced Research Projects Agency (ARPA)
created this ancient network way back in 1957 in a cold war reaction to the Soviet’s launch-
ing of Sputnik. Also in 1983, ARPA was redubbed DARPA and divided into ARPAnet and
MILNET until both were nally dissolved in 1990.
It may be counterintuitive, but most of the development work on TCP/IP happened at
UC Berkeley in Northern California, where a group of scientists were simultaneously work-
ing on the Berkeley version of UNIX, which soon became known as the Berkeley Software
Distribution (BSD) series of UNIX versions. Of course, because TCP/IP worked so well, it
was packaged into subsequent releases of BSD Unix and offered to other universities and
institutions if they bought the distribution tape. So basically, BSD Unix bundled with TCP/
IP began as shareware in the world of academia. As a result, it became the foundation for
the tremendous success and unprecedented growth of today’s Internet as well as smaller,
private and corporate intranets.
As usual, what started as a small group of TCP/IP acionados evolved, and as it did,
the US government created a program to test any new published standards and make
sure they passed certain criteria. This was to protect TCP/IP’s integrity and to ensure
that no developer changed anything too dramatically or added any proprietary features.
It’s this very quality—this open-systems approach to the TCP/IP family of protocols—
that sealed its popularity because this quality guarantees a solid connection between
myriad hardware and software platforms with no strings attached.
TCP/IP and the DoD Model
The DoD model is basically a condensed version of the OSI model that comprises four instead
of seven layers:
u Process/Application layer
u Host-to-Host layer/or Transport
u Internet layer
u Network Access layer/or Link
Figure3.1 offers a comparison of the DoD model and the OSI reference model. As you can
see, the two are similar in concept, but each has a different number of layers with different
names. Cisco may at times use different names for the same layer, such as both “Network
Access” and “Link” used to describe the bottom layer.
90 Chapter 3 u Introduction to TCP/IP
FIGURE3.1 The DoD and OSI models
Application
OSI Model
Session
Presentation
Transport
Network
Data Link
Physical
Process/
Application
DoD Model
Host-to-Host
Internet
Network
Access
When the different protocols in the IP stack are discussed, the layers of the
OSI and DoD models are interchangeable. In other words, be prepared for
the exam objectives to call the Host-to-Host layer the Transport layer!
A vast array of protocols join forces at the DoD model’s Process/Application layer.
These processes integrate the various activities and duties spanning the focus of the OSI’s
corresponding top three layers (Application, Presentation, and Session). We’ll focus on
a few of the most important applications found in the CCNA objectives. In short, the
Process/Application layer denes protocols for node-to-node application communication
and controls user-interface specications.
The Host-to-Host layer parallels the functions of the OSI’s Transport layer, dening
protocols for setting up the level of transmission service for applications. It tackles issues
like creating reliable end-to-end communication and ensuring the error-free delivery of
data. It handles packet sequencing and maintains data integrity.
The Internet layer corresponds to the OSI’s Network layer, designating the protocols
relating to the logical transmission of packets over the entire network. It takes care of the
addressing of hosts by giving them an IP (Internet Protocol) address and handles the routing
of packets among multiple networks.
At the bottom of the DoD model, the Network Access layer implements the data exchange
between the host and the network. The equivalent of the Data Link and Physical layers of the
OSI model, the Network Access layer oversees hardware addressing and denes protocols for
the physical transmission of data. The reason TCP/IP became so popular is because there were
no set physical layer specications, so it could run on any existing or future physical network!
The DoD and OSI models are alike in design and concept and have similar functions in
similar layers. Figure3.2 shows the TCP/IP protocol suite and how its protocols relate to
the DoD model layers.
In the following sections, we will look at the different protocols in more detail, beginning
with those found at the Process/Application layer.
TCP/IP and the DoD Model 91
FIGURE3.2 The TCP/IP protocol suite
DoD Model
Telnet
TFTP
FTP
SMTP
LPD
NFS
SNMP
X Window
Process/
Application
Host-to-Host
ICMP ARP RARP
IP
Internet
Network
Access
TCP UDP
Ethernet Fast
Ethernet
Gigabit
Ethernet
10 Gig
Ethernet
The Process/Application Layer Protocols
Coming up, I’ll describe the different applications and services typically used in IP networks,
and although there are many more protocols dened here, we’ll focus in on the protocols most
relevant to the CCNA objectives. Here’s a list of the protocols and applications we’ll cover in
this section:
u Telnet
u SSH
u FTP
u TFTP
u SNMP
u HTTP
u HTTPS
u NTP
u DNS
u DHCP/BootP
Telnet
Telnet was one of the rst Internet standards, developed in 1969, and is the chameleon of
protocols—its specialty is terminal emulation. It allows a user on a remote client machine,
called the Telnet client, to access the resources of another machine, the Telnet server, in order
to access a command-line interface. Telnet achieves this by pulling a fast one on the Telnet
server and making the client machine appear as though it were a terminal directly attached to
the local network. This projection is actually a software image—a virtual terminal that can
interact with the chosen remote host. A drawback is that there are no encryption techniques
92 Chapter 3 u Introduction to TCP/IP
available within the Telnet protocol, so everything must be sent in clear text, including pass-
words! Figure3.3 shows an example of a Telnet client trying to connect to a Telnet server.
FIGURE3.3 Telnet
>telnet 1.1.1.2
Can I have access to
your command line?
Okay! Configure me!
I’ll send everything in
clear text, including
passwords...
Good, because I can’t
do encryption!
These emulated terminals are of the text-mode type and can execute dened procedures
such as displaying menus that give users the opportunity to choose options and access the
applications on the duped server. Users begin a Telnet session by running the Telnet client
software and then logging into the Telnet server. Telnet uses an 8-bit, byte-oriented data
connection over TCP, which makes it very thorough. It’s still in use today because it is so
simple and easy to use, with very low overhead, but again, with everything sent in clear
text, it’s not recommended in production.
Secure Shell (SSH)
Secure Shell (SSH) protocol sets up a secure session that’s similar to Telnet over a standard
TCP/IP connection and is employed for doing things like logging into systems, running
programs on remote systems, and moving les from one system to another. And it does all
of this while maintaining an encrypted connection. Figure3.4 shows a SSH client trying to
connect to a SSH server. The client must send the data encrypted!
You can think of it as the new-generation protocol that’s now used in place of the anti-
quated and very unused rsh and rlogin—even Telnet.
File Transfer Protocol (FTP)
File Transfer Protocol (FTP) actually lets us transfer les, and it can accomplish this between
any two machines using it. But FTP isn’t just a protocol; it’s also a program. Operating as a
protocol, FTP is used by applications. As a program, it’s employed by users to perform le
tasks by hand. FTP also allows for access to both directories and les and can accomplish
certain types of directory operations, such as relocating into different ones (Figure3.5).
But accessing a host through FTP is only the rst step. Users must then be subjected to
an authentication login that’s usually secured with passwords and usernames implemented
by system administrators to restrict access. You can get around this somewhat by adopting
the username anonymous, but you’ll be limited in what you’ll be able to access.
TCP/IP and the DoD Model 93
FIGURE3.4 Secure Shell
>ssh admin@1.1.1.2
Can I have access to
your command line?
I accept only
encrypted data!
Here is my encrypted
username, password and
key: a@#$alskdjf2H!Vm34
Here is my response:
eSgkh2g42#$!@!#!$kjka12s
FIGURE3.5 FTP
>ftp@1.1.1.2
I want to get a file!
Which one?
I don’t know, what do
you have available?
No problem! Here is my
directory of available files!
Even when employed by users manually as a program, FTP’s functions are limited to
listing and manipulating directories, typing le contents, and copying les between hosts.
It can’t execute remote les as programs.
Trivial File Transfer Protocol (TFTP)
Trivial File Transfer Protocol (TFTP) is the stripped-down, stock version of FTP, but it’s the
protocol of choice if you know exactly what you want and where to nd it because it’s fast
and so easy to use!
But TFTP doesn’t offer the abundance of functions that FTP does because it has no
directory-browsing abilities, meaning that it can only send and receive les (Figure3.6). Still,
it’s heavily used for managing le systems on Cisco devices, as I’ll show you in Chapter 7,
“Managing and Troubleshooting a Cisco Internetwork.”
94 Chapter 3 u Introduction to TCP/IP
FIGURE3.6 TFTP
tftp 10.1.1.2
I want to get a file!
I’m not smart enough to
know what that means!
What files do you
have?
I’m TFTP! I don’t
know!
Fine! Give me a specific file
from Flash memory. Here is
the name of the file...
I can do that!
This compact little protocol also skimps in the data department, sending much smaller
blocks of data than FTP. Also, there’s no authentication as with FTP, so it’s even more
insecure, and few sites support it because of the inherent security risks.
When Should You Use FTP?
Let’s say everyone at your San Francisco office needs a 50 GB file emailed to them right
away. What do you do? Many email servers would reject that email due to size limits (a lot
of ISPs don’t allow files larger than 5 MB or 10 MB to be emailed) and, even if there are no
size limits on the server, it would still take a while to send this huge file. FTP to the rescue!
If you need to give someone a large file or you need to get a large file from someone, FTP
is a nice choice. To use FTP, you would need to set up an FTP server on the Internet so
that the files can be shared.
Besides resolving size issues, FTP is faster than email. In addition, because it uses TCP
and is connection-oriented, if the session dies, FTP can sometimes start up where it left
off. Try that with your email client!
Simple Network Management Protocol (SNMP)
Simple Network Management Protocol (SNMP) collects and manipulates valuable net-
work information, as you can see in Figure3.7. It gathers data by polling the devices on
the network from a network management station (NMS) at xed or random intervals,
TCP/IP and the DoD Model 95
requiring them to disclose certain information, or even asking for certain information
from the device. In addition, network devices can inform the NMS station about prob-
lems as they occur so the network administrator is alerted.
FIGURE3.7 SNMP
MY FAN DIED! I’M BURNING
UP!! UGH!!
OKAY! I’ll sound
the alarm!
NMS Station
When all is well, SNMP receives something called a baseline—a report delimiting the
operational traits of a healthy network. This protocol can also stand as a watchdog over
the network, quickly notifying managers of any sudden turn of events. These network
watchdogs are called agents, and when aberrations occur, agents send an alert called a
trap to the management station.
SNMP Versions 1, 2, and 3
SNMP versions 1 and 2 are pretty much obsolete. This doesn’t mean you won’t see
them in a network now and then, but you’ll only come across v1 rarely, if ever. SNMPv2
provided improvements, especially in performance. But one of the best additions was
called GETBULK, which allowed a host to retrieve a large amount of data at once. Even
so, v2 never really caught on in the networking world and SNMPv3 is now the standard.
Unlike v1, which used only UDP, v3 uses both TCP and UDP and added even more secu-
rity, message integrity, authentication, and encryption.
Hypertext Transfer Protocol (HTTP)
All those snappy websites comprising a mélange of graphics, text, links, ads and so on rely
on the Hypertext Transfer Protocol (HTTP) to make it all possible (Figure3.8). It’s used
to manage communications between web browsers and web servers and opens the right
resource when you click a link, wherever that resource may actually reside.
In order for a browser to display a web page, it must nd the exact server that has the
right web page, plus the exact details that identify the information requested. This informa-
tion must be then be sent back to the browser. Nowadays, it’s highly doubtful that a web
server would have only one page to display!
96 Chapter 3 u Introduction to TCP/IP
FIGURE3.8 HTTP
Okay! Here is the web page for
www.lammle.com
I want to get some awesome
Cisco training! I want URL
www.lammle.com!
http://www.Lammle.com
Your browser can understand what you need when you enter a Uniform Resource
Locator (URL), which we usually refer to as a web address, e.g. http://www.lammle.com/
forum and http://www.lammle.com/blog.
So basically, each URL denes the protocol used to transfer data, the name of the server,
and the particular web page on that server.
Hypertext Transfer Protocol Secure (HTTPS)
Hypertext Transfer Protocol Secure (HTTPS) is also known as Secure Hypertext Transfer
Protocol. It uses Secure Sockets Layer (SSL). Sometimes you’ll see it referred to as SHTTP or
S-HTTP, which were slightly different protocols, but since Microsoft supported HTTPS, it
became the de facto standard for securing web communication. But no matter—as indicated,
it’s a secure version of HTTP that arms you with a whole bunch of security tools for keeping
transactions between a web browser and a server secure.
It’s what your browser needs to ll out forms, sign in, authenticate, and encrypt an
HTTP message when you do things online like make a reservation, access your bank, or
buy something.
Network Time Protocol (NTP)
Kudos to Professor David Mills of the University of Delaware for coming up with this handy
protocol that’s used to synchronize the clocks on our computers to one standard time source
(typically, an atomic clock). Network Time Protocol (NTP) works by synchronizing devices
to ensure that all computers on a given network agree on the time (Figure3.9).
This may sound pretty simple, but it’s very important because so many of the transac-
tions done today are time and date stamped. Think about databases—a server can get
messed up pretty badly and even crash if it’s out of sync with the machines connected to it
by even mere seconds! You can’t have a transaction entered by a machine at, say, 1:50 a.m.
when the server records that transaction as having occurred at 1:45 a.m. So basically, NTP
works to prevent a “back to the future sans DeLorean” scenario from bringing down the
network—very important indeed!
I’ll tell you a lot more about NTP in Chapter 7, including how to congure this protocol
in a Cisco environment.
TCP/IP and the DoD Model 97
FIGURE3.9 NTP
I’m connected to an atomic
clock on the Internet – here is
the exact time and date!
I have some errors to report but
I need to record them with the
correct time and date!
What is the exact time?
NTP Server
Domain Name Service (DNS)
Domain Name Service (DNS) resolves hostnames—specically, Internet names, such as www
.lammle.com. But you don’t have to actually use DNS. You just type in the IP address of any
device you want to communicate with and nd the IP address of a URL by using the Ping
program. For example, >ping www.cisco.com will return the IP address resolved by DNS.
An IP address identies hosts on a network and the Internet as well, but DNS was designed
to make our lives easier. Think about this: What would happen if you wanted to move your
web page to a different service provider? The IP address would change and no one would
know what the new one was. DNS allows you to use a domain name to specify an IP address.
You can change the IP address as often as you want and no one will know the difference.
To resolve a DNS address from a host, you’d typically type in the URL from your favor-
ite browser, which would hand the data to the Application layer interface to be transmitted
on the network. The application would look up the DNS address and send a UDP request
to your DNS server to resolve the name (Figure3.10).
FIGURE3.10 DNS
What is the address for
Lammle.com?
I don’t know. Let me check
with another server!
I know the answer!
DNS Root Server
Lammle.com is
198.1.78.115
Get me to 198.1.78.115 ASAP!
DNS Server Lammle.com is:
198.1.78.115
1 2
54 3
98 Chapter 3 u Introduction to TCP/IP
If your rst DNS server doesn’t know the answer to the query, then the DNS server
forwards a TCP request to its root DNS server. Once the query is resolved, the answer
is transmitted back to the originating host, which means the host can now request the
information from the correct web server.
DNS is used to resolve a fully qualied domain name (FQDN)—for example, www.lammle
.com or todd.lammle.com. An FQDN is a hierarchy that can logically locate a system based on
its domain identier.
If you want to resolve the name todd, you either must type in the FQDN of todd.lammle
.com or have a device such as a PC or router add the sufx for you. For example, on a Cisco
router, you can use the command ip domain-name lammle.com to append each request with
the lammle.com domain. If you don’t do that, you’ll have to type in the FQDN to get DNS
to resolve the name.
An important thing to remember about DNS is that if you can ping a device
with an IP address but cannot use its FQDN, then you might have some
type of DNS configuration failure.
Dynamic Host Configuration Protocol (DHCP)/Bootstrap
Protocol (BootP)
Dynamic Host Conguration Protocol (DHCP) assigns IP addresses to hosts. It allows for
easier administration and works well in small to very large network environments. Many
types of hardware can be used as a DHCP server, including a Cisco router.
DHCP differs from BootP in that BootP assigns an IP address to a host but the host’s
hardware address must be entered manually in a BootP table. You can think of DHCP as
a dynamic BootP. But remember that BootP is also used to send an operating system that
a host can boot from. DHCP can’t do that.
But there’s still a lot of information a DHCP server can provide to a host when the host
is requesting an IP address from the DHCP server. Here’s a list of the most common types
of information a DHCP server can provide:
u IP address
u Subnet mask
u Domain name
u Default gateway (routers)
u DNS server address
u WINS server address
A client that sends out a DHCP Discover message in order to receive an IP address sends
out a broadcast at both layer 2 and layer 3.
u The layer 2 broadcast is all Fs in hex, which looks like this: ff:ff:ff:ff:ff:ff.
u The layer 3 broadcast is 255.255.255.255, which means all networks and all hosts.
TCP/IP and the DoD Model 99
DHCP is connectionless, which means it uses User Datagram Protocol (UDP) at the
Transport layer, also known as the Host-to-Host layer, which we’ll talk about later.
Seeing is believing, so here’s an example of output from my analyzer showing the layer 2
and layer 3 broadcasts:
Ethernet II, Src: 0.0.0.0 (00:0b:db:99:d3:5e),Dst: Broadcast(ff:ff:ff:ff:ff:ff)
Internet Protocol, Src: 0.0.0.0 (0.0.0.0),Dst: 255.255.255.255(255.255.255.255)
The Data Link and Network layers are both sending out “all hands” broadcasts saying,
“Help—I don’t know my IP address!”
DHCP will be discussed in more detail, including configuration on a Cisco
router and switch, in Chapter 7 and Chapter 8, “IP Routing.”
Figure3.11 shows the process of a client-server relationship using a DHCP connection.
FIGURE3.11 DHCP client four-step process
Help! I don’t have an address!
Client broadcast
DHCPDiscover
Server unicast
DHCPOffer
How about this address?
Can’t I just use the
address I had before?
Awesome! I’m somebody now!
Client broadcast
DHCPRequest
Server unicast
DHCPACK
Fine! Here it is.
100 Chapter 3 u Introduction to TCP/IP
This is the four-step process a client takes to receive an IP address from a DHCP server:
1. The DHCP client broadcasts a DHCP Discover message looking for a DHCP server
(Port 67).
2. The DHCP server that received the DHCP Discover message sends a layer 2 unicast
DHCP Offer message back to the host.
3. The client then broadcasts to the server a DHCP Request message asking for the
offered IP address and possibly other information.
4. The server finalizes the exchange with a unicast DHCP Acknowledgment message.
DHCP Conflicts
A DHCP address conict occurs when two hosts use the same IP address. This sounds bad,
and it is! We’ll never even have to discuss this problem once we get to the chapter on IPv6!
During IP address assignment, a DHCP server checks for conicts using the Ping program
to test the availability of the address before it’s assigned from the pool. If no host replies, then
the DHCP server assumes that the IP address is not already allocated. This helps the server
know that it’s providing a good address, but what about the host? To provide extra protection
against that terrible IP conict issue, the host can broadcast for its own address!
A host uses something called a gratuitous ARP to help avoid a possible duplicate address.
The DHCP client sends an ARP broadcast out on the local LAN or VLAN using its newly
assigned address to solve conicts before they occur.
So, if an IP address conict is detected, the address is removed from the DHCP pool
(scope), and it’s really important to remember that the address will not be assigned to a
host until the administrator resolves the conict by hand!
Please see Chapter 8 to check out a DHCP configuration on a Cisco router
and also to find out what happens when a DHCP client is on one side of a
router but the DHCP server is on the other side on a different network!
Automatic Private IP Addressing (APIPA)
Okay, so what happens if you have a few hosts connected together with a switch or hub
and you don’t have a DHCP server? You can add IP information by hand, known as static
IP addressing, but later Windows operating systems provide a feature called Automatic
Private IP Addressing (APIPA). With APIPA, clients can automatically self-congure an
IP address and subnet mask—basic IP information that hosts use to communicate—when
a DHCP server isn’t available. The IP address range for APIPA is 169.254.0.1 through
169.254.255.254. The client also congures itself with a default Class B subnet mask of
255.255.0.0.
But when you’re in your corporate network working and you have a DHCP server run-
ning, and your host shows that it’s using this IP address range, it means that either your
DHCP client on the host is not working or the server is down or can’t be reached due to
TCP/IP and the DoD Model 101
some network issue. Believe me—I don’t know anyone who’s seen a host in this address
range and has been happy about it!
Now, let’s take a look at the Transport layer, or what the DoD calls the Host-to-Host
layer.
The Host-to-host Layer Protocols
The main purpose of the Host-to-Host layer is to shield the upper-layer applications from
the complexities of the network. This layer says to the upper layer, “Just give me your data
stream, with any instructions, and I’ll begin the process of getting your information ready
to send.”
Coming up, I’ll introduce you to the two protocols at this layer:
u Transmission Control Protocol (TCP)
u User Datagram Protocol (UDP)
In addition, we’ll look at some of the key host-to-host protocol concepts, as well as the
port numbers.
Remember, this is still considered layer 4, and Cisco really likes the way
layer 4 can use acknowledgments, sequencing, and flow control.
Transmission Control Protocol (TCP)
Transmission Control Protocol (TCP) takes large blocks of information from an applica-
tion and breaks them into segments. It numbers and sequences each segment so that the
destination’s TCP stack can put the segments back into the order the application intended.
After these segments are sent on the transmitting host, TCP waits for an acknowledgment
of the receiving end’s TCP virtual circuit session, retransmitting any segments that aren’t
acknowledged.
Before a transmitting host starts to send segments down the model, the sender’s TCP
stack contacts the destination’s TCP stack to establish a connection. This creates a virtual
circuit, and this type of communication is known as connection-oriented. During this ini-
tial handshake, the two TCP layers also agree on the amount of information that’s going to
be sent before the recipient’s TCP sends back an acknowledgment. With everything agreed
upon in advance, the path is paved for reliable communication to take place.
TCP is a full-duplex, connection-oriented, reliable, and accurate protocol, but estab-
lishing all these terms and conditions, in addition to error checking, is no small task. TCP
is very complicated, and so not surprisingly, it’s costly in terms of network overhead. And
since today’s networks are much more reliable than those of yore, this added reliability is
often unnecessary. Most programmers use TCP because it removes a lot of programming
work, but for real-time video and VoIP, User Datagram Protocol (UDP) is often better
because using it results in less overhead.
102 Chapter 3 u Introduction to TCP/IP
TCP Segment Format
Since the upper layers just send a data stream to the protocols in the Transport layers, I’ll use
Figure3.12 to demonstrate how TCP segments a data stream and prepares it for the Internet
layer. When the Internet layer receives the data stream, it routes the segments as packets
through an internetwork. The segments are handed to the receiving host’s Host-to-Host layer
protocol, which rebuilds the data stream for the upper-layer applications or protocols.
FIGURE3.12 TCP segment format
4-bit
header
length
Reserved Flags 16-bit window size
Host-to-Host
Host-to-Host
16-bit destination port16-bit source port
16-bit urgent pointer16-bit TCP checksum
32-bit sequence number
32-Bit Acknowledgment Number
Options
Data
Figure3.12 shows the TCP segment format and shows the different elds within the
TCP header. This isn’t important to memorize for the Cisco exam objectives, but you need
to understand it well because it’s really good foundational information.
The TCP header is 20 bytes long, or up to 24 bytes with options. You need to understand
what each eld in the TCP segment is in order to build a strong educational foundation:
Source port This is the port number of the application on the host sending the data,
which I’ll talk about more thoroughly a little later in this chapter.
Destination port This is the port number of the application requested on the destina-
tion host.
Sequence number A number used by TCP that puts the data back in the correct order or
retransmits missing or damaged data during a process called sequencing.
Acknowledgment number The value is the TCP octet that is expected next.
Header length The number of 32-bit words in the TCP header, which indicates where the
data begins. The TCP header (even one including options) is an integral number of 32 bits
in length.
Reserved Always set to zero.
Code bits/flags Controls functions used to set up and terminate a session.
Window The window size the sender is willing to accept, in octets.
Checksum The cyclic redundancy check (CRC), used because TCP doesn’t trust the lower
layers and checks everything. The CRC checks the header and data elds.
TCP/IP and the DoD Model 103
Urgent A valid eld only if the Urgent pointer in the code bits is set. If so, this value
indicates the offset from the current sequence number, in octets, where the segment of
non-urgent data begins.
Options May be 0, meaning that no options have to be present, or a multiple of 32 bits.
However, if any options are used that do not cause the option eld to total a multiple of
32 bits, padding of 0s must be used to make sure the data begins on a 32-bit boundary.
These boundaries are known as words.
Data Handed down to the TCP protocol at the Transport layer, which includes the
upper-layer headers.
Let’s take a look at a TCP segment copied from a network analyzer:
TCP - Transport Control Protocol
Source Port: 5973
Destination Port: 23
Sequence Number: 1456389907
Ack Number: 1242056456
Offset: 5
Reserved: %000000
Code: %011000
Ack is valid
Push Request
Window: 61320
Checksum: 0x61a6
Urgent Pointer: 0
No TCP Options
TCP Data Area:
vL.5.+.5.+.5.+.5 76 4c 19 35 11 2b 19 35 11 2b 19 35 11
2b 19 35 +. 11 2b 19
Frame Check Sequence: 0x0d00000f
Did you notice that everything I talked about earlier is in the segment? As you can see
from the number of elds in the header, TCP creates a lot of overhead. Again, this is why
application developers may opt for efciency over reliability to save overhead and go with
UDP instead. It’s also dened at the Transport layer as an alternative to TCP.
User Datagram Protocol (UDP)
User Datagram Protocol (UDP) is basically the scaled-down economy model of TCP,
which is why UDP is sometimes referred to as a thin protocol. Like a thin person on a
park bench, a thin protocol doesn’t take up a lot of room—or in this case, require much
bandwidth on a network.
104 Chapter 3 u Introduction to TCP/IP
UDP doesn’t offer all the bells and whistles of TCP either, but it does do a fabulous job
of transporting information that doesn’t require reliable delivery, using far less network
resources. (UDP is covered thoroughly in Request for Comments 768.)
So clearly, there are times that it’s wise for developers to opt for UDP rather than TCP,
one of them being when reliability is already taken care of at the Process/Application layer.
Network File System (NFS) handles its own reliability issues, making the use of TCP both
impractical and redundant. But ultimately, it’s up to the application developer to opt for
using UDP or TCP, not the user who wants to transfer data faster!
UDP does not sequence the segments and does not care about the order in which the seg-
ments arrive at the destination. UDP just sends the segments off and forgets about them.
It doesn’t follow through, check up on them, or even allow for an acknowledgment of safe
arrival—complete abandonment. Because of this, it’s referred to as an unreliable protocol. This
does not mean that UDP is ineffective, only that it doesn’t deal with reliability issues at all.
Furthermore, UDP doesn’t create a virtual circuit, nor does it contact the destination
before delivering information to it. Because of this, it’s also considered a connectionless
protocol. Since UDP assumes that the application will use its own reliability method, it
doesn’t use any itself. This presents an application developer with a choice when running
the Internet Protocol stack: TCP for reliability or UDP for faster transfers.
It’s important to know how this process works because if the segments arrive out of
order, which is commonplace in IP networks, they’ll simply be passed up to the next layer
in whatever order they were received. This can result in some seriously garbled data! On the
other hand, TCP sequences the segments so they get put back together in exactly the right
order, which is something UDP just can’t do.
UDP Segment Format
Figure3.13 clearly illustrates UDP’s markedly lean overhead as compared to TCP’s hungry
requirements. Look at the gure carefully—can you see that UDP doesn’t use windowing
or provide for acknowledgments in the UDP header?
FIGURE3.13 UDP segment
16-bit destination port16-bit source port
Bit 31Bit 15Bit 0 Bit 16
16-bit checksum16-bit length
Data
8 bytes
It’s important for you to understand what each eld in the UDP segment is:
Source port Port number of the application on the host sending the data
Destination port Port number of the application requested on the destination host
Length Length of UDP header and UDP data
Checksum Checksum of both the UDP header and UDP data elds
Data Upper-layer data
TCP/IP and the DoD Model 105
UDP, like TCP, doesn’t trust the lower layers and runs its own CRC. Remember that
the Frame Check Sequence (FCS) is the eld that houses the CRC, which is why you can
see the FCS information.
The following shows a UDP segment caught on a network analyzer:
UDP - User Datagram Protocol
Source Port: 1085
Destination Port: 5136
Length: 41
Checksum: 0x7a3c
UDP Data Area:
..Z......00 01 5a 96 00 01 00 00 00 00 00 11 0000 00
...C..2._C._C 2e 03 00 43 02 1e 32 0a 00 0a 00 80 43 00 80
Frame Check Sequence: 0x00000000
Notice that low overhead! Try to nd the sequence number, ack number, and window
size in the UDP segment. You can’t because they just aren’t there!
Key Concepts of Host-to-Host Protocols
Since you’ve now seen both a connection-oriented (TCP) and connectionless (UDP) protocol
in action, it’s a good time to summarize the two here. Table3.1 highlights some of the key
concepts about these two protocols for you to memorize.
TABLE3.1 Key features of TCP and UDP
TCP UDP
Sequenced Unsequenced
Reliable Unreliable
Connection-oriented Connectionless
Virtual circuit Low overhead
Acknowledgments No acknowledgment
Windowing flow control No windowing or flow control of any type
And if all this isn’t quite clear yet, a telephone analogy will really help you understand
how TCP works. Most of us know that before you speak to someone on a phone, you must
rst establish a connection with that other person no matter where they are. This is akin to
establishing a virtual circuit with the TCP protocol. If you were giving someone important
106 Chapter 3 u Introduction to TCP/IP
information during your conversation, you might say things like, “You know? or “Did you
get that?” Saying things like this is a lot like a TCP acknowledgment—it’s designed to get
you verication. From time to time, especially on mobile phones, people ask, “Are you still
there?” People end their conversations with a “Goodbye” of some kind, putting closure on
the phone call, which you can think of as tearing down the virtual circuit that was created
for your communication session. TCP performs these types of functions.
Conversely, using UDP is more like sending a postcard. To do that, you don’t need to con-
tact the other party rst, you simply write your message, address the postcard, and send it
off. This is analogous to UDP’s connectionless orientation. Since the message on the postcard
is probably not a matter of life or death, you don’t need an acknowledgment of its receipt.
Similarly, UDP does not involve acknowledgments.
Let’s take a look at another gure, one that includes TCP, UDP, and the applications
associated to each protocol: Figure3.14 (discussed in the next section).
FIGURE3.14 Port numbers for TCP and UDP
FTP Telnet POP3 DNS TFTP BootPS
TCP UDP
Application
layer
Port numbers
Transport
layer
21 23 110 53 69 67
Port Numbers
TCP and UDP must use port numbers to communicate with the upper layers because
these are what keep track of different conversations crossing the network simultaneously.
Originating-source port numbers are dynamically assigned by the source host and will
equal some number starting at 1024. Port number 1023 and below are dened in RFC
3232 (or just see www.iana.org), which discusses what we call well-known port numbers.
Virtual circuits that don’t use an application with a well-known port number are assigned
port numbers randomly from a specic range instead. These port numbers identify the source
and destination application or process in the TCP segment.
The Requests for Comments (RFCs) form a series of notes about the
Internet (originally the ARPAnet) began in 1969. These notes discuss
many aspects of computer communication, focusing on networking pro-
tocols, procedures, programs, and concepts; but they also include meet-
ing notes, opinions, and sometimes even humor. You can find the RFCs
by visiting www.iana.org.
TCP/IP and the DoD Model 107
Figure3.14 illustrates how both TCP and UDP use port numbers. I’ll cover the different
port numbers that can be used next:
u Numbers below 1024 are considered well-known port numbers and are defined in
RFC 3232.
u Numbers 1024 and above are used by the upper layers to set up sessions with other
hosts and by TCP and UDP to use as source and destination addresses in the segment.
TCP Session: Source Port
Let’s take a minute to check out analyzer output showing a TCP session I captured with my
analyzer software session now:
TCP - Transport Control Protocol
Source Port: 5973
Destination Port: 23
Sequence Number: 1456389907
Ack Number: 1242056456
Offset: 5
Reserved: %000000
Code: %011000
Ack is valid
Push Request
Window: 61320
Checksum: 0x61a6
Urgent Pointer: 0
No TCP Options
TCP Data Area:
vL.5.+.5.+.5.+.5 76 4c 19 35 11 2b 19 35 11 2b 19 35 11
2b 19 35 +. 11 2b 19
Frame Check Sequence: 0x0d00000f
Notice that the source host makes up the source port, which in this case is 5973. The
destination port is 23, which is used to tell the receiving host the purpose of the intended
connection (Telnet).
By looking at this session, you can see that the source host makes up the source port
by using numbers from 1024 to 65535. But why does the source make up a port number?
To differentiate between sessions with different hosts because how would a server know
where information is coming from if it didn’t have a different number from a sending
host? TCP and the upper layers don’t use hardware and logical addresses to understand
the sending host’s address as the Data Link and Network layer protocols do. Instead,
they use port numbers.
108 Chapter 3 u Introduction to TCP/IP
TCP Session: Destination Port
You’ll sometimes look at an analyzer and see that only the source port is above 1024 and
the destination port is a well-known port, as shown in the following trace:
TCP - Transport Control Protocol
Source Port: 1144
Destination Port: 80 World Wide Web HTTP
Sequence Number: 9356570
Ack Number: 0
Offset: 7
Reserved: %000000
Code: %000010
Synch Sequence
Window: 8192
Checksum: 0x57E7
Urgent Pointer: 0
TCP Options:
Option Type: 2 Maximum Segment Size
Length: 4
MSS: 536
Option Type: 1 No Operation
Option Type: 1 No Operation
Option Type: 4
Length: 2
Opt Value:
No More HTTP Data
Frame Check Sequence: 0x43697363
And sure enough, the source port is over 1024, but the destination port is 80, indicating
an HTTP service. The server, or receiving host, will change the destination port if it needs to.
In the preceding trace, a “SYN” packet is sent to the destination device. This Synch (as
shown in the output) sequence is what’s used to inform the remote destination device that it
wants to create a session.
TCP Session: Syn Packet Acknowledgment
The next trace shows an acknowledgment to the syn packet:
TCP - Transport Control Protocol
Source Port: 80 World Wide Web HTTP
Destination Port: 1144
Sequence Number: 2873580788
Ack Number: 9356571
Offset: 6
TCP/IP and the DoD Model 109
Reserved: %000000
Code: %010010
Ack is valid
Synch Sequence
Window: 8576
Checksum: 0x5F85
Urgent Pointer: 0
TCP Options:
Option Type: 2 Maximum Segment Size
Length: 4
MSS: 1460
No More HTTP Data
Frame Check Sequence: 0x6E203132
Notice the Ack is valid, which means that the source port was accepted and the device
agreed to create a virtual circuit with the originating host.
And here again, you can see that the response from the server shows that the source is
80 and the destination is the 1144 sent from the originating host—all’s well!
Table3.2 gives you a list of the typical applications used in the TCP/IP suite by showing
their well-known port numbers, and the Transport layer protocols used by each application
or process. It’s really key to memorize this table.
TABLE3.2 Key protocols that use TCP and UDP
TCP UDP
Telnet 23 SNMP 161
SMTP 25 TFTP 69
HTTP 80 DNS 53
FTP 20, 21 BooTPS/DHCP 67
DNS 53
HTTPS 443
SSH 22
POP3 110
NTP 123
IMAP4 143
110 Chapter 3 u Introduction to TCP/IP
Notice that DNS uses both TCP and UDP. Whether it opts for one or the other depends
on what it’s trying to do. Even though it’s not the only application that can use both proto-
cols, it’s certainly one that you should make sure to remember in your studies.
What makes TCP reliable is sequencing, acknowledgments, and flow con-
trol (windowing). UDP does not have reliability.
Okay—I want to discuss one more item before we move down to the Internet layer—
session multiplexing. Session multiplexing is used by both TCP and UDP and basically
allows a single computer, with a single IP address, to have multiple sessions occurring
simultaneously. Say you go to www.lammle.com and are browsing and then you click a
link to another page. Doing this opens another session to your host. Now you go to www
.lammle.com/forum from another window and that site opens a window as well. Now
you have three sessions open using one IP address because the Session layer is sorting the
separate request based on the Transport layer port number. This is the job of the Session
layer: to keep application layer data separate!
The Internet Layer Protocols
In the DoD model, there are two main reasons for the Internet layer’s existence: routing
and providing a single network interface to the upper layers.
None of the other upper- or lower-layer protocols have any functions relating to routing—
that complex and important task belongs entirely to the Internet layer. The Internet layer’s
second duty is to provide a single network interface to the upper-layer protocols. Without
this layer, application programmers would need to write “hooks” into every one of their
applications for each different Network Access protocol. This would not only be a pain in the
neck, but it would lead to different versions of each application—one for Ethernet, another
one for wireless, and so on. To prevent this, IP provides one single network interface for the
upper-layer protocols. With that mission accomplished, it’s then the job of IP and the various
Network Access protocols to get along and work together.
All network roads don’t lead to Rome—they lead to IP. And all the other protocols at this
layer, as well as all those at the upper layers, use it. Never forget that. All paths through the
DoD model go through IP. Here’s a list of the important protocols at the Internet layer that
I’ll cover individually in detail coming up:
u Internet Protocol (IP)
u Internet Control Message Protocol (ICMP)
u Address Resolution Protocol (ARP)
Internet Protocol (IP)
Internet Protocol (IP) essentially is the Internet layer. The other protocols found here merely
exist to support it. IP holds the big picture and could be said to “see all,” because it’s aware
of all the interconnected networks. It can do this because all the machines on the network
TCP/IP and the DoD Model 111
have a software, or logical, address called an IP address, which we’ll explore more thor-
oughly later in this chapter.
For now, understand that IP looks at each packet’s address. Then, using a routing table, it
decides where a packet is to be sent next, choosing the best path to send it upon. The protocols
of the Network Access layer at the bottom of the DoD model don’t possess IP’s enlightened
scope of the entire network; they deal only with physical links (local networks).
Identifying devices on networks requires answering these two questions: Which network
is it on? And what is its ID on that network? The rst answer is the software address, or
logical address. You can think of this as the part of the address that species the correct
street. The second answer is the hardware address, which goes a step further to specify the
correct mailbox. All hosts on a network have a logical ID called an IP address. This is the
software, or logical, address and contains valuable encoded information, greatly simplify-
ing the complex task of routing. (IP is discussed in RFC 791.)
IP receives segments from the Host-to-Host layer and fragments them into datagrams
(packets) if necessary. IP then reassembles datagrams back into segments on the receiving
side. Each datagram is assigned the IP address of the sender and that of the recipient. Each
router or switch (layer 3 device) that receives a datagram makes routing decisions based on
the packet’s destination IP address.
Figure3.15 shows an IP header. This will give you a picture of what the IP protocol has
to go through every time user data that is destined for a remote network is sent from the
upper layers.
FIGURE3.15 IP header
Total length (16)
Version
(4)
Header
length (4)
Priority and
Type of Service (8)
Header checksum (16)Time to live (8) Protocol (8)
Identification (16) Flags (3) Fragmented offset (13)
Source IP address (32)
Destination IP address (32)
Options (0 or 32 if any)
Data (varies if any)
Bit 0Bit 15 Bit 16 Bit 31
20 bytes
The following elds make up the IP header:
Version IP version number.
Header length Header length (HLEN) in 32-bit words.
Priority and Type of Service Type of Service tells how the datagram should be handled.
The rst 3 bits are the priority bits, now called the differentiated services bits.
Total length Length of the packet, including header and data.
Identification Unique IP-packet value used to differentiate fragmented packets from dif-
ferent datagrams.
112 Chapter 3 u Introduction to TCP/IP
Flags Species whether fragmentation should occur.
Fragment offset Provides fragmentation and reassembly if the packet is too large to put in
a frame. It also allows different maximum transmission units (MTUs) on the Internet.
Time To Live The time to live (TTL) is set into a packet when it is originally generated.
If it doesn’t get to where it’s supposed to go before the TTL expires, boom—it’s gone. This
stops IP packets from continuously circling the network looking for a home.
Protocol Port of upper-layer protocol; for example, TCP is port 6 or UDP is port 17. Also
supports Network layer protocols, like ARP and ICMP, and can referred to as the Type
eld in some analyzers. We’ll talk about this eld more in a minute.
Header checksum Cyclic redundancy check (CRC) on header only.
Source IP address 32-bit IP address of sending station.
Destination IP address 32-bit IP address of the station this packet is destined for.
Options Used for network testing, debugging, security, and more.
Data After the IP option eld, will be the upper-layer data.
Here’s a snapshot of an IP packet caught on a network analyzer. Notice that all the
header information discussed previously appears here:
IP Header - Internet Protocol Datagram
Version: 4
Header Length: 5
Precedence: 0
Type of Service: %000
Unused: %00
Total Length: 187
Identifier: 22486
Fragmentation Flags: %010 Do Not Fragment
Fragment Offset: 0
Time To Live: 60
IP Type: 0x06 TCP
Header Checksum: 0xd031
Source IP Address: 10.7.1.30
Dest. IP Address: 10.7.1.10
No Internet Datagram Options
The Type eld is typically a Protocol eld, but this analyzer sees it as an IP Type eld.
This is important. If the header didn’t carry the protocol information for the next layer,
IP wouldn’t know what to do with the data carried in the packet. The preceding example
clearly tells IP to hand the segment to TCP.
Figure3.16 demonstrates how the Network layer sees the protocols at the Transport
layer when it needs to hand a packet up to the upper-layer protocols.
TCP/IP and the DoD Model 113
FIGURE3.16 The Protocol field in an IP header
TCP UDP
IP
Transport
layer
Port numbers
Network
layer
617
In this example, the Protocol eld tells IP to send the data to either TCP port 6 or
UDP port 17. But it will be UDP or TCP only if the data is part of a data stream headed
for an upper-layer service or application. It could just as easily be destined for Internet
Control Message Protocol (ICMP), Address Resolution Protocol (ARP), or some other
type of Network layer protocol.
Table3.3 is a list of some other popular protocols that can be specied in the Protocol eld.
TABLE3.3 Possible protocols found in the Protocol field of an IP header
Protocol Protocol Number
ICMP 1
IP in IP (tunneling) 4
TCP 6
UDP 17
EIGRP 88
OSPF 89
IPv6 41
GRE 47
Layer 2 tunnel (L2TP) 115
You can find a complete list of Protocol field numbers at www.iana.org/
assignments/protocol-numbers.
114 Chapter 3 u Introduction to TCP/IP
Internet Control Message Protocol (ICMP)
Internet Control Message Protocol (ICMP) works at the Network layer and is used by
IP for many different services. ICMP is basically a management protocol and messaging
service provider for IP. Its messages are carried as IP datagrams. RFC 1256 is an annex to
ICMP, which gives hosts extended capability in discovering routes to gateways.
ICMP packets have the following characteristics:
u They can provide hosts with information about network problems.
u They are encapsulated within IP datagrams.
The following are some common events and messages that ICMP relates to:
Destination unreachable If a router can’t send an IP datagram any further, it uses ICMP
to send a message back to the sender, advising it of the situation. For example, take a look
at Figure3.17, which shows that interface E0 of the Lab_B router is down.
FIGURE3.17 ICMP error message is sent to the sending host from the
remote router.
Lab_A
Host A
ICMP packet
e0
Lab_B
Host B
e0
When Host A sends a packet destined for Host B, the Lab_B router will send an ICMP des-
tination unreachable message back to the sending device, which is Host A in this example.
Buffer full/source quench If a router’s memory buffer for receiving incoming datagrams is
full, it will use ICMP to send out this message alert until the congestion abates.
Hops/time exceeded Each IP datagram is allotted a certain number of routers, called
hops, to pass through. If it reaches its limit of hops before arriving at its destination, the
last router to receive that datagram deletes it. The executioner router then uses ICMP to
send an obituary message, informing the sending machine of the demise of its datagram.
Ping Packet Internet Groper (Ping) uses ICMP echo request and reply messages to check
the physical and logical connectivity of machines on an internetwork.
Traceroute Using ICMP time-outs, Traceroute is used to discover the path a packet takes
as it traverses an internetwork.
TCP/IP and the DoD Model 115
Both Ping and Traceroute are usually just called Trace. Microsoft Win-
dows uses tracert to allow you to verify address configurations in your
internetwork.
The following data is from a network analyzer catching an ICMP echo request:
Flags: 0x00
Status: 0x00
Packet Length: 78
Timestamp: 14:04:25.967000 12/20/03
Ethernet Header
Destination: 00:a0:24:6e:0f:a8
Source: 00:80:c7:a8:f0:3d
Ether-Type: 08-00 IP
IP Header - Internet Protocol Datagram
Version: 4
Header Length: 5
Precedence: 0
Type of Service: %000
Unused: %00
Total Length: 60
Identifier: 56325
Fragmentation Flags: %000
Fragment Offset: 0
Time To Live: 32
IP Type: 0x01 ICMP
Header Checksum: 0x2df0
Source IP Address: 100.100.100.2
Dest. IP Address: 100.100.100.1
No Internet Datagram Options
ICMP - Internet Control Messages Protocol
ICMP Type: 8 Echo Request
Code: 0
Checksum: 0x395c
Identifier: 0x0300
Sequence Number: 4352
ICMP Data Area:
abcdefghijklmnop 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70
qrstuvwabcdefghi 71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69
Frame Check Sequence: 0x00000000
116 Chapter 3 u Introduction to TCP/IP
Notice anything unusual? Did you catch the fact that even though ICMP works at the
Internet (Network) layer, it still uses IP to do the Ping request? The Type eld in the IP
header is 0x01, which species that the data we’re carrying is owned by the ICMP protocol.
Remember, just as all roads lead to Rome, all segments or data must go through IP!
The Ping program uses the alphabet in the data portion of the packet as a
payload, typically around 100 bytes by default, unless, of course, you are
pinging from a Windows device, which thinks the alphabet stops at the let-
ter W (and doesn’t include X, Y, or Z) and then starts at A again. Go figure!
If you remember reading about the Data Link layer and the different frame types in
Chapter 2, “Ethernet Technologies and Data Encapsulation,” you should be able to look at
the preceding trace and tell what type of Ethernet frame this is. The only elds are destina-
tion hardware address, source hardware address, and Ether-Type. The only frame that uses
an Ether-Type eld exclusively is an Ethernet_II frame.
We’ll move on soon, but before we get into the ARP protocol, let’s take another look
at ICMP in action. Figure3.18 shows an internetwork—it has a router, so it’s an internet-
work, right?
FIGURE3.18 ICMP in action
Server 1
10.1.2.2/24 10.1.4.2/24
I’m trying to
telnet to
10.1.1.5—
can you
forward this
request?
10.1.5.2/24
10.1.5.3/24
10.1.5.4/24
10.1.5.5/24
10.1.1.0 network?
No, never heard of it!
Discard! Create ICMP packet!
Server1 (10.1.2.2) telnets to 10.1.1.5 from a DOS prompt. What do you think Server1
will receive as a response? Server1 will send the Telnet data to the default gateway, which is
the router, and the router will drop the packet because there isn’t a network 10.1.1.0 in the
routing table. Because of this, Server1 will receive an ICMP destination unreachable back
from the router.
TCP/IP and the DoD Model 117
Address Resolution Protocol (ARP)
Address Resolution Protocol (ARP)nds the hardware address of a host from a known IP
address. Here’s how it works: When IP has a datagram to send, it must inform a Network
Access protocol, such as Ethernet or wireless, of the destination’s hardware address on the
local network. Remember that it has already been informed by upper-layer protocols of
the destination’s IP address. If IP doesn’t nd the destination host’s hardware address in
the ARP cache, it uses ARP to nd this information.
As IP’s detective, ARP interrogates the local network by sending out a broadcast ask-
ing the machine with the specied IP address to reply with its hardware address. So basi-
cally, ARP translates the software (IP) address into a hardware address—for example, the
destination machine’s Ethernet adapter address—and from it, deduces its whereabouts
on the LAN by broadcasting for this address. Figure3.19 shows how an ARP broadcast
looks to a local network.
FIGURE3.19 Local ARP broadcast
I need the Ethernet
address of 10.1.1.2.
10.1.1.1
IP: 10.1.1.2 = ???
IP: 10.1.1.2
Ethernet: 45:AC:24:E3:60:A5
10.1.1.2
I heard that broadcast.
The message is for me.
Here is my Ethernet address.
ARP resolves IP addresses to Ethernet (MAC) addresses.
The following trace shows an ARP broadcast—notice that the destination hardware
address is unknown and is all Fs in hex (all 1s in binary)—and is a hardware address
broadcast:
Flags: 0x00
Status: 0x00
Packet Length: 64
Timestamp: 09:17:29.574000 12/06/03
118 Chapter 3 u Introduction to TCP/IP
Ethernet Header
Destination: FF:FF:FF:FF:FF:FF Ethernet Broadcast
Source: 00:A0:24:48:60:A5
Protocol Type: 0x0806 IP ARP
ARP - Address Resolution Protocol
Hardware: 1 Ethernet (10Mb)
Protocol: 0x0800 IP
Hardware Address Length: 6
Protocol Address Length: 4
Operation: 1 ARP Request
Sender Hardware Address: 00:A0:24:48:60:A5
Sender Internet Address: 172.16.10.3
Target Hardware Address: 00:00:00:00:00:00 (ignored)
Target Internet Address: 172.16.10.10
Extra bytes (Padding):
................ 0A 0A 0A 0A 0A 0A 0A 0A 0A 0A 0A 0A 0A
0A 0A 0A 0A 0A
Frame Check Sequence: 0x00000000
IP Addressing
One of the most important topics in any discussion of TCP/IP is IP addressing. An IP address
is a numeric identier assigned to each machine on an IP network. It designates the specic
location of a device on the network.
An IP address is a software address, not a hardware address—the latter is hard-coded on
a network interface card (NIC) and used for nding hosts on a local network. IP addressing
was designed to allow hosts on one network to communicate with a host on a different net-
work regardless of the type of LANs the hosts are participating in.
Before we get into the more complicated aspects of IP addressing, you need to understand
some of the basics. First I’m going to explain some of the fundamentals of IP addressing and
its terminology. Then you’ll learn about the hierarchical IP addressing scheme and private
IP addresses.
IP Terminology
Throughout this chapter you’re being introduced to several important terms that are vital
to understanding the Internet Protocol. Here are a few to get you started:
Bit A bit is one digit, either a 1 or a 0.
Byte A byte is 7 or 8 bits, depending on whether parity is used. For the rest of this chapter,
always assume a byte is 8 bits.
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IP Addressing 119
Octet An octet, made up of 8 bits, is just an ordinary 8-bit binary number. In this chapter,
the terms byte and octet are completely interchangeable.
Network address This is the designation used in routing to send packets to a remote
network—for example, 10.0.0.0, 172.16.0.0, and 192.168.10.0.
Broadcast address The address used by applications and hosts to send information to all
nodes on a network is called the broadcast address. Examples of layer 3 broadcasts include
255.255.255.255, which is any network, all nodes; 172.16.255.255, which is all subnets and
hosts on network 172.16.0.0; and 10.255.255.255, which broadcasts to all subnets and hosts
on network 10.0.0.0.
The Hierarchical IP Addressing Scheme
An IP address consists of 32 bits of information. These bits are divided into four sections,
referred to as octets or bytes, with each containing 1 byte (8 bits). You can depict an IP
address using one of three methods:
u Dotted-decimal, as in 172.16.30.56
u Binary, as in 10101100.00010000.00011110.00111000
u Hexadecimal, as in AC.10.1E.38
All these examples represent the same IP address. Pertaining to IP addressing, hexadeci-
mal isn’t used as often as dotted-decimal or binary, but you still might nd an IP address
stored in hexadecimal in some programs.
The 32-bit IP address is a structured or hierarchical address, as opposed to a at or
nonhierarchical address. Although either type of addressing scheme could have been used,
hierarchical addressing was chosen for a good reason. The advantage of this scheme is that
it can handle a large number of addresses, namely 4.3 billion (a 32-bit address space with
two possible values for each position—either 0 or 1—gives you 232, or 4,294,967,296).
The disadvantage of the at addressing scheme, and the reason it’s not used for IP address-
ing, relates to routing. If every address were unique, all routers on the Internet would need
to store the address of each and every machine on the Internet. This would make efcient
routing impossible, even if only a fraction of the possible addresses were used!
The solution to this problem is to use a two- or three-level hierarchical addressing
scheme that is structured by network and host or by network, subnet, and host.
This two- or three-level scheme can also be compared to a telephone number. The rst
section, the area code, designates a very large area. The second section, the prex, narrows
the scope to a local calling area. The nal segment, the customer number, zooms in on the
specic connection. IP addresses use the same type of layered structure. Rather than all
32 bits being treated as a unique identier, as in at addressing, a part of the address is des-
ignated as the network address and the other part is designated as either the subnet
and host or just the node address.
Next, we’ll cover IP network addressing and the different classes of address we can use
to address our networks.
120 Chapter 3 u Introduction to TCP/IP
Network Addressing
The network address (which can also be called the network number) uniquely identies
each network. Every machine on the same network shares that network address as part of
its IP address. For example, in the IP address 172.16.30.56, 172.16 is the network address.
The node address is assigned to, and uniquely identies, each machine on a network. This
part of the address must be unique because it identies a particular machine—an individual—
as opposed to a network, which is a group. This number can also be referred to as a host
address. In the sample IP address 172.16.30.56, the 30.56 species the node address.
The designers of the Internet decided to create classes of networks based on network size.
For the small number of networks possessing a very large number of nodes, they created the
rank Class A network. At the other extreme is the Class C network, which is reserved for
the numerous networks with a small number of nodes. The class distinction for networks
between very large and very small is predictably called the Class B network.
Subdividing an IP address into a network and node address is determined by the class
designation of one’s network. Figure3.20 summarizes the three classes of networks used
to address hosts—a subject I’ll explain in much greater detail throughout this chapter.
FIGURE3.20 Summary of the three classes of networks
Network
8 bits
Network
Network
Host
8 bits
Network
Network
Host
8 bits
Host
Network
Host
8 bits
Host
Host
Multicast
Research
Class A:
Class B:
Class C:
Class D:
Class E:
To ensure efcient routing, Internet designers dened a mandate for the leading-bits
section of the address for each different network class. For example, since a router knows
that a Class A network address always starts with a 0, the router might be able to speed
a packet on its way after reading only the rst bit of its address. This is where the address
schemes dene the difference between a Class A, a Class B, and a Class C address. Coming
up, I’ll discuss the differences between these three classes, followed by a discussion of the
Class D and Class E addresses. Classes A, B, and C are the only ranges that are used to
address hosts in our networks.
Network Address Range: Class A
The designers of the IP address scheme decided that the rst bit of the rst byte in a Class A
network address must always be off, or 0. This means a Class A address must be between 0
and 127 in the rst byte, inclusive.
IP Addressing 121
Consider the following network address:
0xxxxxxx
If we turn the other 7 bits all off and then turn them all on, we’ll nd the Class A range
of network addresses:
00000000 = 0
01111111 = 127
So, a Class A network is dened in the rst octet between 0 and 127, and it can’t
be less or more. Understand that 0 and 127 are not valid in a Class A network because
they’re reserved addresses, which I’ll explain soon.
Network Address Range: Class B
In a Class B network, the RFCs state that the rst bit of the rst byte must always be turned
on but the second bit must always be turned off. If you turn the other 6 bits all off and then
all on, you will nd the range for a Class B network:
10000000 = 128
10111111 = 191
As you can see, a Class B network is dened when the rst byte is congured from
128 to 191.
Network Address Range: Class C
For Class C networks, the RFCs dene the rst 2 bits of the rst octet as always turned on,
but the third bit can never be on. Following the same process as the previous classes, convert
from binary to decimal to nd the range. Here’s the range for a Class C network:
11000000 = 192
11011111 = 223
So, if you see an IP address that starts at 192 and goes to 223, you’ll know it is a Class C
IP address.
Network Address Ranges: Classes D and E
The addresses between 224 to 255 are reserved for Class D and E networks. Class D (224–
239) is used for multicast addresses and Class E (240–255) for scientic purposes, but I’m not
going into these types of addresses because they are beyond the scope of knowledge you need
to gain from this book.
Network Addresses: Special Purpose
Some IP addresses are reserved for special purposes, so network administrators can’t ever
assign these addresses to nodes. Table3.4 lists the members of this exclusive little club and
the reasons why they’re included in it.
122 Chapter 3 u Introduction to TCP/IP
TABLE3.4 Reserved IP addresses
Address Function
Network address of all 0s Interpreted to mean “this network or segment.”
Network address of all 1s Interpreted to mean “all networks.”
Network 127.0.0.1 Reserved for loopback tests. Designates the local
node and allows that node to send a test packet to
itself without generating network traffic.
Node address of all 0s Interpreted to mean “network address” or any
host on a specified network.
Node address of all 1s Interpreted to mean “all nodes” on the specified
network; for example, 128.2.255.255 means “all
nodes” on network 128.2 (Class B address).
Entire IP address set to all 0s Used by Cisco routers to designate the default
route. Could also mean “any network.”
Entire IP address set to all 1s (same as
255.255.255.255)
Broadcast to all nodes on the current network;
sometimes called an “all 1s broadcast” or local
broadcast.
Class A Addresses
In a Class A network address, the rst byte is assigned to the network address and the three
remaining bytes are used for the node addresses. The Class A format is as follows:
network.node.node.node
For example, in the IP address 49.22.102.70, the 49 is the network address and 22.102.70
is the node address. Every machine on this particular network would have the distinctive net-
work address of 49.
Class A network addresses are 1 byte long, with the rst bit of that byte reserved and the
7 remaining bits available for manipulation (addressing). As a result, the maximum number
of Class A networks that can be created is 128. Why? Because each of the 7 bit positions
can be either a 0 or a 1, thus 27, or 128.
To complicate matters further, the network address of all 0s (0000 0000) is reserved to
designate the default route (see Table3.4 in the previous section). Additionally, the address
127, which is reserved for diagnostics, can’t be used either, which means that you can really
only use the numbers 1 to 126 to designate Class A network addresses. This means the
actual number of usable Class A network addresses is 128 minus 2, or 126.
IP Addressing 123
The IP address 127.0.0.1 is used to test the IP stack on an individual node
and cannot be used as a valid host address. However, the loopback address
creates a shortcut method for TCP/IP applications and services that run on
the same device to communicate with each other.
Each Class A address has 3 bytes (24-bit positions) for the node address of a machine.
This means there are 224—or 16,777,216—unique combinations and, therefore, precisely
that many possible unique node addresses for each Class A network. Because node addresses
with the two patterns of all 0s and all 1s are reserved, the actual maximum usable number
of nodes for a Class A network is 224 minus 2, which equals 16,777,214. Either way, that’s a
huge number of hosts on a single network segment!
Class A Valid Host IDs
Here’s an example of how to gure out the valid host IDs in a Class A network address:
u All host bits off is the network address: 10.0.0.0.
u All host bits on is the broadcast address: 10.255.255.255.
The valid hosts are the numbers in between the network address and the broadcast
address: 10.0.0.1 through 10.255.255.254. Notice that 0s and 255s can be valid host IDs.
All you need to remember when trying to nd valid host addresses is that the host bits can’t
all be turned off or on at the same time.
Class B Addresses
In a Class B network address, the rst 2 bytes are assigned to the network address and the
remaining 2 bytes are used for node addresses. The format is as follows:
network.network.node.node
For example, in the IP address 172.16.30.56, the network address is 172.16 and the
node address is 30.56.
With a network address being 2 bytes (8 bits each), you get 216 unique combinations.
But the Internet designers decided that all Class B network addresses should start with the
binary digit 1, then 0. This leaves 14 bit positions to manipulate, therefore 16,384, or 214
unique Class B network addresses.
A Class B address uses 2 bytes for node addresses. This is 216 minus the two reserved
patterns of all 0s and all 1s for a total of 65,534 possible node addresses for each Class B
network.
Class B Valid Host IDs
Here’s an example of how to nd the valid hosts in a Class B network:
u All host bits turned off is the network address: 172.16.0.0.
u All host bits turned on is the broadcast address: 172.16.255.255.
124 Chapter 3 u Introduction to TCP/IP
The valid hosts would be the numbers in between the network address and the broadcast
address: 172.16.0.1 through 172.16.255.254.
Class C Addresses
The rst 3 bytes of a Class C network address are dedicated to the network portion of the
address, with only 1 measly byte remaining for the node address. Here’s the format:
network.network.network.node
Using the example IP address 192.168.100.102, the network address is 192.168.100 and
the node address is 102.
In a Class C network address, the rst three bit positions are always the binary 110. The
calculation is as follows: 3 bytes, or 24 bits, minus 3 reserved positions leaves 21 positions.
Hence, there are 221, or 2,097,152, possible Class C networks.
Each unique Class C network has 1 byte to use for node addresses. This leads to 28, or
256, minus the two reserved patterns of all 0s and all 1s, for a total of 254 node addresses
for each Class C network.
Class C Valid Host IDs
Here’s an example of how to nd a valid host ID in a Class C network:
u All host bits turned off is the network ID: 192.168.100.0.
u All host bits turned on is the broadcast address: 192.168.100.255.
The valid hosts would be the numbers in between the network address and the broadcast
address: 192.168.100.1 through 192.168.100.254.
Private IP Addresses (RFC 1918)
The people who created the IP addressing scheme also created private IP addresses. These
addresses can be used on a private network, but they’re not routable through the Internet.
This is designed for the purpose of creating a measure of well-needed security, but it also
conveniently saves valuable IP address space.
If every host on every network was required to have real routable IP addresses, we would
have run out of IP addresses to hand out years ago. But by using private IP addresses, ISPs,
corporations, and home users only need a relatively tiny group of bona de IP addresses to
connect their networks to the Internet. This is economical because they can use private IP
addresses on their inside networks and get along just ne.
To accomplish this task, the ISP and the corporation—the end user, no matter who
they are—need to use something called Network Address Translation (NAT), which basi-
cally takes a private IP address and converts it for use on the Internet. (NAT is covered
in Chapter 13, “Network Address Translation.”) Many people can use the same real IP
IP Addressing 125
address to transmit out onto the Internet. Doing things this way saves megatons of address
space—good for us all!
The reserved private addresses are listed in Table3.5.
TABLE3.5 Reserved IP address space
Address Class Reserved Address Space
Class A 10.0.0.0 through 10.255.255.255
Class B 172.16.0.0 through 172.31.255.255
Class C 192.168.0.0 through 192.168.255.255
You must know your private address space to become Cisco certified!
So, What Private IP Address Should I Use?
That’s a really great question: Should you use Class A, Class B, or even Class C private
addressing when setting up your network? Let’s take Acme Corporation in SF as an
example. This company is moving into a new building and needs a whole new network. It
has 14 departments, with about 70 users in each. You could probably squeeze one or two
Class C addresses to use, or maybe you could use a Class B, or even a Class A just for fun.
The rule of thumb in the consulting world is, when you’re setting up a corporate network—
regardless of how small it is—you should use a Class A network address because it gives
you the most flexibility and growth options. For example, if you used the 10.0.0.0 network
address with a /24 mask, then you’d have 65,536 networks, each with 254 hosts. Lots of
room for growth with that network!
But if you’re setting up a home network, you’d opt for a Class C address because it is the
easiest for people to understand and configure. Using the default Class C mask gives you
one network with 254 hosts—plenty for a home network.
With the Acme Corporation, a nice 10.1.x.0 with a /24 mask (the x is the subnet for each
department) makes this easy to design, install, and troubleshoot.
126 Chapter 3 u Introduction to TCP/IP
IPv4 Address Types
Most people use the term broadcast as a generic term, and most of the time, we understand
what they mean—but not always! For example, you might say, “The host broadcasted through
a router to a DHCP server,” but, well, it’s pretty unlikely that this would ever really happen.
What you probably mean—using the correct technical jargon—is, “The DHCP client broad-
casted for an IP address and a router then forwarded this as a unicast packet to the DHCP
server.” Oh, and remember that with IPv4, broadcasts are pretty important, but with IPv6,
there aren’t any broadcasts sent at all—now there’s something to look forward to reading
about in Chapter 14!
Okay, I’ve referred to IP addresses throughout the preceding chapters and now all
throughout this chapter, and even showed you some examples. But I really haven’t gone
into the different terms and uses associated with them yet, and it’s about time I did. So
here are the address types that I’d like to dene for you:
Loopback (localhost) Used to test the IP stack on the local computer. Can be any address
from 127.0.0.1 through 127.255.255.254.
Layer 2 broadcasts These are sent to all nodes on a LAN.
Broadcasts (layer 3) These are sent to all nodes on the network.
Unicast This is an address for a single interface, and these are used to send packets to a
single destination host.
Multicast These are packets sent from a single source and transmitted to many devices on
different networks. Referred to as “one-to-many.”
Layer 2 Broadcasts
First, understand that layer 2 broadcasts are also known as hardware broadcasts—they
only go out on a LAN, but they don’t go past the LAN boundary (router).
The typical hardware address is 6 bytes (48 bits) and looks something like
45:AC:24:E3:60:A5. The broadcast would be all 1s in binary, which would be
all Fs in hexadecimal, as in ff:ff:ff:ff:ff:ff and shown in Figure3.21.
Every network interface card (NIC) will receive and read the frame, including the router,
since this was a layer 2 broadcast, but the router would never, ever forward this!
Layer 3 Broadcasts
Then there are the plain old broadcast addresses at layer 3. Broadcast messages are meant
to reach all hosts on a broadcast domain. These are the network broadcasts that have all
host bits on.
Here’s an example that you’re already familiar with: The network address of 172.16.0.0
255.255.0.0 would have a broadcast address of 172.16.255.255—all host bits on. Broadcasts
IPv4 Address Types 127
can also be “any network and all hosts,” as indicated by 255.255.255.255, and shown in
Figure3.22.
FIGURE3.21 Local layer 2 broadcasts
I need to send this
to everyone on my
local network!
Ethernet: FF:FF:FF:FF:FF:FF
I heard that local
LAN broadcast.
I need to read this!
FIGURE3.22 Layer 3 broadcasts
I need to send this to
everyone on my local network,
as well as to every
network I can get to!
Ethernet: FF:FF:FF:FF:FF:FF
I heard that all-
networks broadcast.
I need to read this!
IP: 255.255.255.255
In Figure3.22, all hosts on the LAN will get this broadcast on their NIC, including the
router, but by default the router would never forward this packet.
Unicast Address
A unicast is dened as a single IP address that’s assigned to a network interface card and is
the destination IP address in a packet—in other words, it’s used for directing packets to a
specic host.
128 Chapter 3 u Introduction to TCP/IP
In Figure3.23 both the MAC address and the destination IP address are for a single NIC
on the network. All hosts on the broadcast domain would receive this frame and accept it.
Only the destination NIC of 10.1.1.2 would accept the packet; the other NICs would discard
the packet.
FIGURE3.23 Unicast address
I need to send a packet to
a host on a remote network!
This needs to go to
my router!
Ethernet: 00.00.0c.12.34.56
I see the frame, but
this is not my MAC!
Discard!
IP: 10.1.1.2
I see that frame. It’s
for me! Now I need
to read the packet!
Multicast Address
Multicast is a different beast entirely. At rst glance, it appears to be a hybrid of unicast
and broadcast communication, but that isn’t quite the case. Multicast does allow point-
to-multipoint communication, which is similar to broadcasts, but it happens in a different
manner. The crux of multicast is that it enables multiple recipients to receive messages
without ooding the messages to all hosts on a broadcast domain. However, this is not the
default behavior—it’s what we can do with multicasting if it’s congured correctly!
Multicast works by sending messages or data to IP multicast group addresses. Unlike with
broadcasts, which aren’t forwarded, routers then forward copies of the packet out to every
interface that has hosts subscribed to that group address. This is where multicast differs from
broadcast messages—with multicast communication, copies of packets, in theory, are sent
only to subscribed hosts. For example, when I say in theory, I mean that the hosts will receive
a multicast packet destined for 224.0.0.10. This is an EIGRP packet, and only a router run-
ning the EIGRP protocol will read these. All hosts on the broadcast LAN, and Ethernet is a
broadcast multi-access LAN technology, will pick up the frame, read the destination address,
then immediately discard the frame unless they’re in the multicast group. This saves PC
processing, not LAN bandwidth. Be warned though—multicasting can cause some serious
LAN congestion if it’s not implemented carefully! Figure3.24 shows a Cisco router sending
an EIGRP multicast packet on the local LAN and only the other Cisco router will accept and
read this packet.
Summary 129
FIGURE3.24 EIGRP multicast example
We see that
multicast frame, but
this is not for us!
I run EIGRP! This is
for me!
I have EIGRP data!
224.0.0.10
There are several different groups that users or applications can subscribe to. The
range of multicast addresses starts with 224.0.0.0 and goes through 239.255.255.255.
As you can see, this range of addresses falls within IP Class D address space based on
classful IP assignment.
Summary
If you made it this far and understood everything the rst time through, you should be
extremely proud of yourself! We really covered a lot of ground in this chapter, but under-
stand that the information in it is critical to being able to navigate well through the rest of
this book.
If you didn’t get a complete understanding the rst time around, don’t stress. It really
wouldn’t hurt you to read this chapter more than once. There is still a lot of ground to cover,
so make sure you’ve got this material all nailed down. That way, you’ll be ready for more,
and just so you know, there’s a lot more! What we’re doing up to this point is building a
solid foundation to build upon as you advance.
With that in mind, after you learned about the DoD model, the layers, and associ-
ated protocols, you learned about the oh-so-important topic of IP addressing. I discussed
in detail the difference between each address class, how to nd a network address and
broadcast address, and what denotes a valid host address range. I can’t stress enough how
important it is for you to have this critical information unshakably understood before
moving on to Chapter 4!
Since you’ve already come this far, there’s no reason to stop now and waste all those
brainwaves and new neural connections. So don’t stop—go through the written lab and
review questions at the end of this chapter and make sure you understand each answer’s
explanation. The best is yet to come!
Visit ccna
.gg/ch3/b
for a
companion
MicroNugget
from CBT
Nuggets.
130 Chapter 3 u Introduction to TCP/IP
Exam Essentials
Differentiate the DoD and the OSI network models. The DoD model is a condensed ver-
sion of the OSI model, composed of four layers instead of seven, but is nonetheless like the
OSI model in that it can be used to describe packet creation and devices and protocols can
be mapped to its layers.
Identify Process/Application layer protocols. Telnet is a terminal emulation program that
allows you to log into a remote host and run programs. File Transfer Protocol (FTP) is a con-
nection-oriented service that allows you to transfer les. Trivial FTP (TFTP) is a connection-
less le transfer program. Simple Mail Transfer Protocol (SMTP) is a sendmail program.
Identify Host-to-Host layer protocols. Transmission Control Protocol (TCP) is a connection-
oriented protocol that provides reliable network service by using acknowledgments and ow
control. User Datagram Protocol (UDP) is a connectionless protocol that provides low over-
head and is considered unreliable.
Identify Internet layer protocols. Internet Protocol (IP) is a connectionless protocol
that provides network address and routing through an internetwork. Address Resolution
Protocol (ARP) nds a hardware address from a known IP address. Reverse ARP (RARP)
nds an IP address from a known hardware address. Internet Control Message Protocol
(ICMP) provides diagnostics and destination unreachable messages.
Describe the functions of DNS and DHCP in the network. Dynamic Host Conguration
Protocol (DHCP) provides network conguration information (including IP addresses) to
hosts, eliminating the need to perform the congurations manually. Domain Name Service
(DNS) resolves hostnames—both Internet names such as www.lammle.com and device names
such as Workstation 2—to IP addresses, eliminating the need to know the IP address of a
device for connection purposes.
Identify what is contained in the TCP header of a connection-oriented transmission. The
elds in the TCP header include the source port, destination port, sequence number, acknowl-
edgment number, header length, a eld reserved for future use, code bits, window size, check-
sum, urgent pointer, options eld, and nally, the data eld.
Identify what is contained in the UDP header of a connectionless transmission. The elds
in the UDP header include only the source port, destination port, length, checksum, and
data. The smaller number of elds as compared to the TCP header comes at the expense
of providing none of the more advanced functions of the TCP frame.
Identify what is contained in the IP header. The elds of an IP header include version,
header length, priority or type of service, total length, identication, ags, fragment offset,
time to live, protocol, header checksum, source IP address, destination IP address, options,
and nally, data.
Compare and contrast UDP and TCP characteristics and features. TCP is connection-
oriented, acknowledged, and sequenced and has ow and error control, while UDP is con-
nectionless, unacknowledged, and not sequenced and provides no error or ow control.
Exam Essentials 131
Understand the role of port numbers. Port numbers are used to identify the protocol or
service that is to be used in the transmission.
Identify the role of ICMP. Internet Control Message Protocol (ICMP) works at the Network
layer and is used by IP for many different services. ICMP is a management protocol and mes-
saging service provider for IP.
Define the Class A IP address range. The IP range for a Class A network is 1–126. This
provides 8 bits of network addressing and 24 bits of host addressing by default.
Define the Class B IP address range. The IP range for a Class B network is 128–191.
Class B addressing provides 16 bits of network addressing and 16 bits of host addressing
by default.
Define the Class C IP address range. The IP range for a Class C network is 192 through
223. Class C addressing provides 24 bits of network addressing and 8 bits of host addressing
by default.
Identify the private IP ranges. The Class A private address range is 10.0.0.0 through
10.255.255.255. The Class B private address range is 172.16.0.0 through 172.31.255.255.
The Class C private address range is 192.168.0.0 through 192.168.255.255.
Understand the difference between a broadcast, unicast, and multicast address. A broad-
cast is to all devices in a subnet, a unicast is to one device, and a multicast is to some but
not all devices.
132 Chapter 3 u Introduction to TCP/IP
Written Labs
In this section, you’ll complete the following labs to make sure you’ve got the information
and concepts contained within them fully dialed in:
Lab 3.1: TCP/IP
Lab 3.2: Mapping Applications to the DoD Model
The answers to these labs can be found in Appendix A, “Answers to Written Labs.”
Written Lab 3.1: TCP/IP
Answer the following questions about TCP/IP:
1. What is the Class C address range in decimal and in binary?
2. What layer of the DoD model is equivalent to the Transport layer of the OSI model?
3. What is the valid range of a Class A network address?
4. What is the 127.0.0.1 address used for?
5. How do you find the network address from a listed IP address?
6. How do you find the broadcast address from a listed IP address?
7. What is the Class A private IP address space?
8. What is the Class B private IP address space?
9. What is the Class C private IP address space?
10. What are all the available characters that you can use in hexadecimal addressing?
Written Lab 3.2: Mapping Applications to the DoD Model
The four layers of the DoD model are Process/Application, Host-to-Host, Internet, and
Network Access. Identify the layer of the DoD model on which each of these protocols
operates.
1. Internet Protocol (IP)
2. Telnet
3. FTP
4. SNMP
5. DNS
6. Address Resolution Protocol (ARP)
7. DHCP/BootP
8. Transmission Control Protocol (TCP)
Written Labs 133
9. X Window
10. User Datagram Protocol (UDP)
11. NFS
12. Internet Control Message Protocol (ICMP)
13. Reverse Address Resolution Protocol (RARP)
14. Proxy ARP
15. TFTP
16. SMTP
17. LPD
134 Chapter 3 u Introduction to TCP/IP
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional ques-
tions, please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. What must happen if a DHCP IP conflict occurs?
A. Proxy ARP will fix the issue.
B. The client uses a gratuitous ARP to fix the issue.
C. The administrator must fix the conflict by hand at the DHCP server.
D. The DHCP server will reassign new IP addresses to both computers.
2. Which of the following Application layer protocols sets up a secure session that’s simi-
lar to Telnet?
A. FTP
B. SSH
C. DNS
D. DHCP
3. Which of the following mechanisms is used by the client to avoid a duplicate IP address
during the DHCP process?
A. ping
B. traceroute
C. gratuitous arp
D. pathping
4. What protocol is used to find the hardware address of a local device?
A. RARP
B. ARP
C. IP
D. ICMP
E. BootP
Review Questions 135
5. Which of the following are layers in the TCP/IP model? (Choose three.)
A. Application
B. Session
C. Transport
D. Internet
E. Data Link
F. Physical
6. Which class of IP address provides a maximum of only 254 host addresses per
network ID?
A. Class A
B. Class B
C. Class C
D. Class D
E. Class E
7. Which of the following describe the DHCP Discover message? (Choose two.)
A. It uses FF:FF:FF:FF:FF:FF as a layer 2 broadcast.
B. It uses UDP as the Transport layer protocol.
C. It uses TCP as the Transport layer protocol.
D. It does not use a layer 2 destination address.
8. Which layer 4 protocol is used for a Telnet connection?
A. IP
B. TCP
C. TCP/IP
D. UDP
E. ICMP
9. Private IP addressing was specified in RFC __________.
10. Which of the following services use TCP? (Choose three.)
A. DHCP
B. SMTP
C. SNMP
D. FTP
E. HTTP
F. TFTP
136 Chapter 3 u Introduction to TCP/IP
11. Which Class of IP addresses uses the pattern shown below?
Network Network Network Host
A. Class A
B. Class B
C. Class C
D. Class D
12. Which of the following is an example of a multicast address?
A. 10.6.9.1
B. 192.168.10.6
C. 224.0.0.10
D. 172.16.9.5
13. The following illustration shows a data structure header. What protocol is this
header from?
4-Bit
Header
Length
Reserved Flags 16-Bit Window Size
Host-to-Host
Host-to-Host
16-Bit Destination Port16-Bit Source Port
16-bit Urgent Pointer16-bit TCP Checksum
32-Bit Sequence Number
32-Bit Acknowledgement Number
Options
Data
A. IP
B. ICMP
C. TCP
D. UDP
E. ARP
F. RARP
14. If you use either Telnet or FTP, what layer are you using to generate the data?
A. Application
B. Presentation
C. Session
D. Transport
Review Questions 137
15. The DoD model (also called the TCP/IP stack) has four layers. Which layer of the DoD
model is equivalent to the Network layer of the OSI model?
A. Application
B. Host-to-Host
C. Internet
D. Network Access
16. Which two of the following are private IP addresses?
A. 12.0.0.1
B. 168.172.19.39
C. 172.20.14.36
D. 172.33.194.30
E. 192.168.24.43
17. What layer in the TCP/IP stack is equivalent to the Transport layer of the OSI model?
A. Application
B. Host-to-Host
C. Internet
D. Network Access
18. Which statements are true regarding ICMP packets? (Choose two).
A. ICMP guarantees datagram delivery.
B. ICMP can provide hosts with information about network problems.
C. ICMP is encapsulated within IP datagrams.
D. ICMP is encapsulated within UDP datagrams.
19. What is the address range of a Class B network address in binary?
A. 01xxxxxx
B. 0xxxxxxx
C. 10xxxxxx
D. 110xxxxx
138 Chapter 3 u Introduction to TCP/IP
20. Drag the steps in the DHCP process and place them in the correct order on the right.
left list:
DHCPOffer
DHCPDiscover
DHCPAck
DHCPRequest
right list:
Drop Target A
Drop Target B
Drop Target C
Drop Target D
Chapter
4
Easy Subnetting
THE FOLLOWING ICND1 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 IP addressing (IPv4 / IPv6)
■Describe the operation and necessity of using private and
public IP addresses for IPv4 addressing
We’ll pick up right where we left off in the last chapter and con-
tinue to explore the world of IP addressing. I’ll open this chapter
by telling you how to subnet an IP network—an indispensably
crucial skill that’s central to mastering networking in general! Forewarned is forearmed, so
prepare yourself because being able to subnet quickly and accurately is pretty challenging and
you’ll need time to practice what you’ve learned to really nail it. So be patient and don’t give
up on this key aspect of networking until your skills are seriously sharp. I’m not kidding—
this chapter is so important you should really just graft it into your brain!
So be ready because we’re going to hit the ground running and thoroughly cover IP
subnetting from the very start. And though I know this will sound weird to you, you’ll be
much better off if you just try to forget everything you’ve learned about subnetting before
reading this chapter—especially if you’ve been to an ofcial Cisco or Microsoft class! I
think these forms of special torture often do more harm than good and sometimes even
scare people away from networking completely. Those that survive and persevere usually
at least question the sanity of continuing to study in this eld. If this is you, relax, breathe,
and know that you’ll nd that the way I tackle the issue of subnetting is relatively painless
because I’m going to show you a whole new, much easier method to conquer this monster!
After working through this chapter, and I can’t say this enough, after working through
the extra study material at the end as well, you’ll be able to tame the IP addressing/subnetting
beast—just don’t give up! I promise that you’ll be really glad you didn’t. It’s one of those things
that once you get it down, you’ll wonder why you used to think it was so hard!
To find up-to-the minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
Subnetting Basics
In Chapter 3, “TCP/IP,” you learned how to dene and nd the valid host ranges used in a
Class A, Class B, and Class C network address by turning the host bits all off and then all
on. This is very good, but here’s the catch: you were dening only one network, as shown
in Figure4.1.
Subnetting Basics 141
FIGURE4.1 One network
.130 .130 .131 .132.2 .3 .4 .5
192.168.10.0/24
. . . . . . . . .
One large broadcast domain!
By now you know that having one large network is not a good thing because the rst
three chapters you just read were veritably peppered with me incessantly telling you that!
But how would you x the out of control problem that Figure4.1 illustrates? Wouldn’t it
be nice to be able to break up that one, huge network address and create four manageable
networks from it? You betcha it would, but to make that happen, you would need to apply
the infamous trick of subnetting because it’s the best way to break up a giant network into
a bunch of smaller ones. Take a look at Figure4.2 and see how this might look.
FIGURE4.2 Multiple networks connected together
192.168.10.64 192.168.10.96
192.168.10.32
192.168.10.0
What are those 192.168.10.x addresses shown in the gure? Well that is what this chapter
will explain! How to make one network into many networks!
Let’s take off from where we left in Chapter 3 and start working in the host section
(host bits) of a network address, where we can borrow bits to create subnets.
142 Chapter 4 u Easy Subnetting
How to Create Subnets
Creating subnetworks is essentially the act of taking bits from the host portion of the address
and reserving them to dene the subnet address instead. Clearly this will result in fewer bits
being available for dening your hosts, which is something you’ll always want to keep in mind.
Later in this chapter, I’ll guide you through the entire process of creating subnets starting
with Class C addresses. As always in networking, before you actually implement anything,
including subnetting, you must rst determine your current requirements and make sure to
plan for future conditions as well.
In this first section, we’ll be discussing classful routing, which refers to
the fact that all hosts (nodes) in the network are using the exact same sub-
net mask. Later, when we move on to cover variable length subnet masks
(VLSMs), I’ll tell you all about classless routing, which is an environment
wherein each network segment can use a different subnet mask.
Okay—to create a subnet, we’ll start by fullling these three steps:
1. Determine the number of required network IDs:
u One for each LAN subnet
u One for each wide area network connection
2. Determine the number of required host IDs per subnet:
u One for each TCP/IP host
u One for each router interface
3. Based on the above requirements, create the following:
u A unique subnet mask for your entire network
u A unique subnet ID for each physical segment
u A range of host IDs for each subnet
Subnet Masks
For the subnet address scheme to work, every machine on the network must know which
part of the host address will be used as the subnet address. This condition is met by assign-
ing a subnet mask to each machine. A subnet mask is a 32-bit value that allows the device
that’s receiving IP packets to distinguish the network ID portion of the IP address from the
host ID portion of the IP address. This 32-bit subnet mask is composed of 1s and 0s, where
the 1s represent the positions that refer to the network subnet addresses.
Not all networks need subnets, and if not, it really means that they’re using the default
subnet mask, which is basically the same as saying that a network doesn’t have a subnet
address. Table4.1 shows the default subnet masks for Classes A, B, and C.
Subnetting Basics 143
TABLE4.1 Default subnet mask
Class Format Default Subnet Mask
Anetwork.node.node.node 255.0.0.0
Bnetwork.network.node.node 255.255.0.0
Cnetwork.network.network.node 255.255.255.0
Although you can use any mask in any way on an interface, typically it’s not usually
good to mess with the default masks. In other words, you don’t want to make a Class B
subnet mask read 255.0.0.0, and some hosts won’t even let you type it in. But these days,
most devices will. For a Class A network, you wouldn’t change the rst byte in a subnet
mask because it should read 255.0.0.0 at a minimum. Similarly, you wouldn’t assign
255.255.255.255 because this is all 1s, which is a broadcast address. A Class B address
starts with 255.255.0.0, and a Class C starts with 255.255.255.0, and for the CCNA
especially, there is no reason to change the defaults!
Understanding the Powers of 2
Powers of 2 are important to understand and memorize for use with IP subnetting. Review-
ing powers of 2, remember that when you see a number noted with an exponent, it means
you should multiply the number by itself as many times as the upper number specifies. For
example, 23 is 2 x 2 x 2, which equals 8. Here’s a list of powers of 2 to commit to memory:
21 = 2
22 = 4
23 = 8
24 = 16
25 = 32
26 = 64
27 = 128
28 = 256
29 = 512
210 = 1,024
211 = 2,048
212 = 4,096
213 = 8,192
214 = 16,384
Memorizing these powers of 2 is a good idea, but it’s not absolutely necessary. Just
remember that since you’re working with powers of 2, each successive power of 2 is
double the previous one.
144 Chapter 4 u Easy Subnetting
It works like this—all you have to do to remember the value of 29 is to first know that 28 = 256.
Why? Because when you double 2 to the eighth power (256), you get 29 (or 512). To deter-
mine the value of 210, simply start at 28 = 256, and then double it twice.
You can go the other way as well. If you needed to know what 26 is, for example, you just
cut 256 in half two times: once to reach 27 and then one more time to reach 26.
Classless Inter-Domain Routing (CIDR)
Another term you need to familiarize yourself with is Classless Inter-Domain Routing
(CIDR). It’s basically the method that Internet service providers (ISPs) use to allocate a
number of addresses to a company, a home—their customers. They provide addresses in a
certain block size, something I’ll talk about in greater detail soon.
When you receive a block of addresses from an ISP, what you get will look something
like this: 192.168.10.32/28. This is telling you what your subnet mask is. The slash nota-
tion (/) means how many bits are turned on (1s). Obviously, the maximum could only be
/32 because a byte is 8 bits and there are 4 bytes in an IP address: (4 n 8 = 32). But keep in
mind that regardless of the class of address, the largest subnet mask available relevant to
the Cisco exam objectives can only be a /30 because you’ve got to keep at least 2 bits for
host bits.
Take, for example, a Class A default subnet mask, which is 255.0.0.0. This tells us that
the rst byte of the subnet mask is all ones (1s), or 11111111. When referring to a slash
notation, you need to count all the 1-bits to gure out your mask. The 255.0.0.0 is consid-
ered a /8 because it has 8 bits that are 1s—that is, 8 bits that are turned on.
A Class B default mask would be 255.255.0.0, which is a /16 because 16 bits are ones
(1s): 11111111.11111111.00000000.00000000.
Table4.2 has a listing of every available subnet mask and its equivalent CIDR slash
notation.
TABLE4.2 CIDR values
Subnet Mask CIDR Value
255.0.0.0 /8
255.128.0.0 /9
255.192.0.0 /10
255.224.0.0 /11
Subnetting Basics 145
Subnet Mask CIDR Value
255.240.0.0 /12
255.248.0.0 /13
255.252.0.0 /14
255.254.0.0 /15
255.255.0.0 /16
255.255.128.0 /17
255.255.192.0 /18
255.255.224.0 /19
255.255.240.0 /20
255.255.248.0 /21
255.255.252.0 /22
255.255.254.0 /23
255.255.255.0 /24
255.255.255.128 /25
255.255.255.192 /26
255.255.255.224 /27
255.255.255.240 /28
255.255.255.248 /29
255.255.255.252 /30
The /8 through /15 can only be used with Class A network addresses. /16 through /23 can
be used by Class A and B network addresses. /24 through /30 can be used by Class A, B, and
C network addresses. This is a big reason why most companies use Class A network addresses.
Since they can use all subnet masks, they get the maximum exibility in network design.
146 Chapter 4 u Easy Subnetting
No, you cannot configure a Cisco router using this slash format. But
wouldn’t that be nice? Nevertheless, it’s really important for you to know
subnet masks in the slash notation (CIDR).
IP Subnet-Zero
Even though IP subnet-zero is not a new command, Cisco courseware and Cisco exam
objectives didn’t used to cover it. Know that Cisco certainly covers it now! This command
allows you to use the rst and last subnet in your network design. For instance, the Class
C mask of 255.255.255.192 provides subnets 64 and 128, another facet of subnetting that
we’ll discuss more thoroughly later in this chapter. But with the ip subnet-zero command,
you now get to use subnets 0, 64, 128, and 192. It may not seem like a lot, but this provides
two more subnets for every subnet mask we use.
Even though we don’t discuss the command-line interface (CLI) until Chapter 6, “Cisco’s
Internetworking Operating System (IOS),” it’s important for you to be at least a little familiar
with this command at this point:
Router#sh running-config
Building configuration...
Current configuration : 827 bytes
!
hostname Pod1R1
!
ip subnet-zero
!
This router output shows that the command ip subnet-zero is enabled on the router.
Cisco has turned this command on by default starting with Cisco IOS version 12.x and
now we’re running 15.x code.
When taking your Cisco exams, make sure you read very carefully to see if Cisco is ask-
ing you not to use ip subnet-zero. There are actually instances where this may happen.
Subnetting Class C Addresses
There are many different ways to subnet a network. The right way is the way that works best
for you. In a Class C address, only 8 bits are available for dening the hosts. Remember that
subnet bits start at the left and move to the right, without skipping bits. This means that the
only Class C subnet masks can be the following:
Binary Decimal CIDR
---------------------------------------------------------
00000000 = 255.255.255.0 /24
Subnetting Basics 147
10000000 = 255.255.255.128 /25
11000000 = 255.255.255.192 /26
11100000 = 255.255.255.224 /27
11110000 = 255.255.255.240 /28
11111000 = 255.255.255.248 /29
11111100 = 255.255.255.252 /30
We can’t use a /31 or /32 because, as I’ve said, we must have at least 2 host bits for
assigning IP addresses to hosts. But this is only mostly true. Certainly we can never use
a /32 because that would mean zero host bits available, yet Cisco has various forms of
the IOS, as well as the new Cisco Nexus switches operating system, that support the /31
mask. The /31 is above the scope of the CCENT and CCNA objectives, so we won’t be
covering it in this book.
Coming up, I’m going to teach you that signicantly less painful method of subnetting I
promised you at the beginning of this chapter, which makes it ever so much easier to subnet
larger numbers in a ash. Excited—good! Because I’m not kidding when I tell you that you
absolutely need to be able to subnet quickly and accurately to succeed in the networking
real world and on the exam too!
Subnetting a Class C Address—The Fast Way!
When you’ve chosen a possible subnet mask for your network and need to determine the
number of subnets, valid hosts, and the broadcast addresses of a subnet that mask will
provide, all you need to do is answer ve simple questions:
u How many subnets does the chosen subnet mask produce?
u How many valid hosts per subnet are available?
u What are the valid subnets?
u What’s the broadcast address of each subnet?
u What are the valid hosts in each subnet?
This is where you’ll be really glad you followed my advice and took the time to memorize
your powers of 2. If you didn’t, now would be a good time… Just refer back to the sidebar,
“Understanding the Powers of 2,” earlier if you need to brush up. Here’s how you arrive at
the answers to those ve big questions:
u How many subnets? 2x = number of subnets. x is the number of masked bits, or the 1s.
For example, in 11000000, the number of 1s gives us 22 subnets. So in this example,
there are 4 subnets.
u How many hosts per subnet? 2y – 2 = number of hosts per subnet. y is the number of
unmasked bits, or the 0s. For example, in 11000000, the number of 0s gives us 26 – 2
hosts, or 62 hosts per subnet. You need to subtract 2 for the subnet address and the
broadcast address, which are not valid hosts.
u What are the valid subnets? 256 – subnet mask = block size, or increment number. An
example would be the 255.255.255.192 mask, where the interesting octet is the fourth
148 Chapter 4 u Easy Subnetting
octet (interesting because that is where our subnet numbers are). Just use this math:
256 – 192 = 64. The block size of a 192 mask is always 64. Start counting at zero in
blocks of 64 until you reach the subnet mask value and these are your subnets in the
fourth octet: 0, 64, 128, 192. Easy, huh?
u What’s the broadcast address for each subnet? Now here’s the really easy part. Since we
counted our subnets in the last section as 0, 64, 128, and 192, the broadcast address is
always the number right before the next subnet. For example, the 0 subnet has a broad-
cast address of 63 because the next subnet is 64. The 64 subnet has a broadcast address
of 127 because the next subnet is 128, and so on. Remember, the broadcast address of
the last subnet is always 255.
u What are the valid hosts? Valid hosts are the numbers between the subnets, omitting
the all-0s and all-1s. For example, if 64 is the subnet number and 127 is the broadcast
address, then 65–126 is the valid host range. Your valid range is always the group of
numbers between the subnet address and the broadcast address.
If you’re still confused, don’t worry because it really isn’t as hard as it seems to be at rst—
just hang in there! To help lift any mental fog, try a few of the practice examples next.
Subnetting Practice Examples: Class C Addresses
Here’s your opportunity to practice subnetting Class C addresses using the method I just
described. This is so cool. We’re going to start with the rst Class C subnet mask and work
through every subnet that we can, using a Class C address. When we’re done, I’ll show you
how easy this is with Class A and B networks too!
Practice Example #1C: 255.255.255.128 (/25)
Since 128 is 10000000 in binary, there is only 1 bit for subnetting and 7 bits for hosts. We’re
going to subnet the Class C network address 192.168.10.0.
192.168.10.0 = Network address
255.255.255.128 = Subnet mask
Now, let’s answer our big ve:
u How many subnets? Since 128 is 1 bit on (10000000), the answer would be 21 = 2.
u How many hosts per subnet? We have 7 host bits off (10000000), so the equation would
be 27 – 2 = 126 hosts. Once you figure out the block size of a mask, the amount of hosts
is always the block size minus 2. No need to do extra math if you don’t need to!
u What are the valid subnets? 256 – 128 = 128. Remember, we’ll start at zero and count in
our block size, so our subnets are 0, 128. By just counting your subnets when counting
in your block size, you really don’t need to do steps 1 and 2. We can see we have two sub-
nets, and in the step before this one, just remember that the amount of hosts is always the
block size minus 2, and in this example, that gives us 2 subnets, each with 126 hosts.
u What’s the broadcast address for each subnet? The number right before the value of
the next subnet is all host bits turned on and equals the broadcast address. For the zero
subnet, the next subnet is 128, so the broadcast of the 0 subnet is 127.
Subnetting Basics 149
u What are the valid hosts? These are the numbers between the subnet and broadcast
address. The easiest way to find the hosts is to write out the subnet address and the
broadcast address, which makes valid hosts completely obvious. The following table
shows the 0 and 128 subnets, the valid host ranges of each, and the broadcast address
of both subnets:
Subnet 0128
First host 1129
Last host 126 254
Broadcast 127 255
Okay, looking at a Class C /25, it’s pretty clear that there are two subnets. But so what—
why is this signicant? Well actually, it’s not because that’s not the right question. What you
really want to know is what you would do with this information!
I know this isn’t exactly everyone’s favorite pastime, but what we’re about to do is really
important, so bear with me; we’re going to talk about subnetting—period. The key to under-
standing subnetting is to understand the very reason you need to do it, and I’m going to dem-
onstrate this by going through the process of building a physical network.
Okay—because we added that router shown in Figure4.3, in order for the hosts on our
internetwork to communicate, they must now have a logical network addressing scheme.
We could use IPv6, but IPv4 is still the most popular for now. It’s also what we’re studying
at the moment, so that’s what we’re going with.
FIGURE4.3 Implementing a Class C /25 logical network
192.168.10.0 192.168.10.128.129.1
Router#show ip route
[output cut]
C 192.168.10.0 is directly connected to Ethernet 0
C 192.168.10.128 is directly connected to Ethernet 1
.4.3.2 .132.131.130
Looking at Figure4.3, you can see that there are two physical networks, so we’re going to
implement a logical addressing scheme that allows for two logical networks. As always, it’s
a really good idea to look ahead and consider likely short- and long-term growth scenarios,
but for this example in this book, a /25 gets it done.
150 Chapter 4 u Easy Subnetting
Figure4.3 shows us that both subnets have been assigned to a router interface, which cre-
ates our broadcast domains and assigns our subnets. Use the command show ip route to see
the routing table on a router. Notice that instead of one large broadcast domain, there are now
two smaller broadcast domains, providing for up to 126 hosts in each. The C in the router out-
put translates to “directly connected network,” and we can see we have two of those with two
broadcast domains and that we created and implemented them. So congratulations—you did
it! You have successfully subnetted a network and applied it to a network design. Nice! Let’s
do it again.
Practice Example #2C: 255.255.255.192 (/26)
This time, we’re going to subnet the network address 192.168.10.0 using the subnet mask
255.255.255.192.
192.168.10.0 = Network address
255.255.255.192 = Subnet mask
Now, let’s answer the big ve:
u How many subnets? Since 192 is 2 bits on (11000000), the answer would be
22 = 4 subnets.
u How many hosts per subnet? We have 6 host bits off (11000000), giving us 26 – 2 = 62
hosts. The amount of hosts is always the block-size minus 2.
u What are the valid subnets? 256 – 192 = 64. Remember, to start at zero and count in
our block size. This means our subnets are 0, 64, 128, and 192. We can see we have a
block size of 64, so we have 4 subnets, each with 62 hosts.
u What’s the broadcast address for each subnet? The number right before the value of
the next subnet is all host bits turned on and equals the broadcast address. For the zero
subnet, the next subnet is 64, so the broadcast address for the zero subnet is 63.
u What are the valid hosts? These are the numbers between the subnet and broadcast
address. As I said, the easiest way to find the hosts is to write out the subnet address
and the broadcast address, which clearly delimits our valid hosts. The following table
shows the 0, 64, 128, and 192 subnets, the valid host ranges of each, and the broadcast
address of each subnet:
The subnets (do this first) 0 64 128 192
Our first host (perform host
addressing last)
165 129 193
Our last host 62 126 190 254
The broadcast address
(do this second)
63 127 191 255
Subnetting Basics 151
Okay, again, before getting into the next example, you can see that we can now subnet
a /26 as long as we can count in increments of 64. And what are you going to do with
this fascinating information? Implement it! We’ll use Figure4.4 to practice a /26 network
implementation.
FIGURE4.4 Implementing a class C /26 (with three networks)
192.168.10.64 192.168.10.128.129
.1
.65
Router#show ip route
[output cut]
C 192.168.10.0 is directly connected to Ethernet 0
C 192.168.10.64 is directly connected to Ethernet 1
C 192.168.10.128 is directly connected to Ethernet 2
.68.67.66 .132.131.130
192.168.10.0
.3.2 .5.4
The /26 mask provides four subnetworks, and we need a subnet for each router interface.
With this mask, in this example, we actually have room with a spare subnet to add to another
router interface in the future. Always plan for growth if possible!
Practice Example #3C: 255.255.255.224 (/27)
This time, we’ll subnet the network address 192.168.10.0 and subnet mask
255.255.255.224.
192.168.10.0 = Network address
255.255.255.224 = Subnet mask
u How many subnets? 224 is 11100000, so our equation would be 23 = 8.
u How many hosts? 25 – 2 = 30.
u What are the valid subnets? 256 – 224 = 32. We just start at zero and count to the sub-
net mask value in blocks (increments) of 32: 0, 32, 64, 96, 128, 160, 192, and 224.
u What’s the broadcast address for each subnet (always the number right before the
next subnet)?
u What are the valid hosts (the numbers between the subnet number and the broadcast
address)?
152 Chapter 4 u Easy Subnetting
To answer the last two questions, rst just write out the subnets, then write out the broad-
cast addresses—the number right before the next subnet. Last, ll in the host addresses. The
following table gives you all the subnets for the 255.255.255.224 Class C subnet mask:
The subnet address 0 32 64 96 128 160 192 224
The first valid host 1 33 65 97 129 161 193 225
The last valid host 30 62 94 126 158 190 222 254
The broadcast address 31 63 95 127 159 191 223 255
In practice example #3C, we’re using a 255.255.255.224 (/27) network, which provides
eight subnets as shown above. We can take these subnets and implement them as shown in
Figure4.5 using any of the subnets available.
FIGURE4.5 Implementing a Class C /27 logical network
192.168.10.128
.129
Router#show ip route
[output cut]
C 192.168.10.0 is directly connected to Ethernet 0
C 192.168.10.32 is directly connected to Ethernet 1
C 192.168.10.64 is directly connected to Ethernet 2
C 192.168.10.96 is directly connected to Serial 0
.161
.98
.97
192.168.10.0
192.168.10.160
192.168.10.96
192.168.10.32 .33 .65
.1 192.168.10.64
Noticed I used six of the eight subnets available for my network design. The lightning
bolt symbol in the gure represents a wide area network (WAN) such as a T1 or other serial
connection through an ISP or telco. In other words, something you don’t own, but it’s still
a subnet just like any LAN connection on a router. As usual, I used the rst valid host in
each subnet as the router’s interface address. This is just a rule of thumb; you can use any
address in the valid host range as long as you remember what address you congured so
you can set the default gateways on your hosts to the router address.
Subnetting Basics 153
Practice Example #4C: 255.255.255.240 (/28)
Let’s practice another one:
192.168.10.0 = Network address
255.255.255.240 = Subnet mask
u Subnets? 240 is 11110000 in binary. 24 = 16.
u Hosts? 4 host bits, or 24 – 2 = 14.
u Valid subnets? 256 – 240 = 16. Start at 0: 0 + 16 = 16. 16 + 16 = 32. 32 + 16 = 48. 48
+ 16 = 64. 64 + 16 = 80. 80 + 16 = 96. 96 + 16 = 112. 112 + 16 = 128. 128 + 16 = 144.
144 + 16 = 160. 160 + 16 = 176. 176 + 16 = 192. 192 + 16 = 208. 208 + 16 = 224. 224
+ 16 = 240.
u Broadcast address for each subnet?
u Valid hosts?
To answer the last two questions, check out the following table. It gives you the subnets,
valid hosts, and broadcast addresses for each subnet. First, nd the address of each subnet using
the block size (increment). Second, nd the broadcast address of each subnet increment, which
is always the number right before the next valid subnet, and then just ll in the host addresses.
The following table shows the available subnets, hosts, and broadcast addresses provided from
a Class C 255.255.255.240 mask.
Subnet
First host
Last host
0
1
14
15
16
17
30
31
32
33
46
47
48
49
62
63
64
65
78
79
80
81
94
95
96
97
110
111
112
113
126
127
128
129
142
143
144
145
158
159
160
161
174
175
176
177
190
191
192
193
206
207
208
209
222
223
224
225
238
239
240
241
254
255Broadcast
Cisco has figured out that most people cannot count in 16s and therefore
have a hard time finding valid subnets, hosts, and broadcast addresses
with the Class C 255.255.255.240 mask. You’d be wise to study this mask.
Practice Example #5C: 255.255.255.248 (/29)
Let’s keep practicing:
192.168.10.0 = Network address
255.255.255.248 = Subnet mask
u Subnets? 248 in binary = 11111000. 25 = 32.
u Hosts? 23 – 2 = 6.
u Valid subnets? 256 – 248 = 0, 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 88, 96, 104, 112, 120,
128, 136, 144, 152, 160, 168, 176, 184, 192, 200, 208, 216, 224, 232, 240, and 248.
154 Chapter 4 u Easy Subnetting
u Broadcast address for each subnet?
u Valid hosts?
Take a look at the following table. It shows some of the subnets (rst four and last four
only), valid hosts, and broadcast addresses for the Class C 255.255.255.248 mask:
Subnet 0816 24 …224 232 240 248
First host 1917 25 …225 233 241 249
Last host 614 22 30 …230 238 246 254
Broadcast 715 23 31 …231 239 247 255
If you try to configure a router interface with the address 192.168.10.6
255.255.255.248 and receive this error:
Bad mask /29 for address 192.168.10.6
It means that ip subnet-zero is not enabled. You must be able to subnet
to see that the address used in this example is in the zero subnet!
Practice Example #6C: 255.255.255.252 (/30)
Okay—just one more:
192.168.10.0 = Network address
255.255.255.252 = Subnet mask
u Subnets? 64.
u Hosts? 2.
u Valid subnets? 0, 4, 8, 12, etc., all the way to 252.
u Broadcast address for each subnet? (always the number right before the next subnet)
u Valid hosts? (the numbers between the subnet number and the broadcast address)
The following table shows you the subnet, valid host, and broadcast address of the rst
four and last four subnets in the 255.255.255.252 Class C subnet:
Subnet 04812 …240 244 248 252
First host 15913 …241 245 249 253
Last host 2610 14 …242 246 250 254
Broadcast 3711 15 …243 247 251 255
Subnetting Basics 155
Should We Really Use This Mask That Provides Only Two Hosts?
You are the network administrator for Acme Corporation in San Francisco, with dozens
of WAN links connecting to your corporate office. Right now your network is a classful
network, which means that the same subnet mask is on each host and router interface.
You’ve read about classless routing, where you can have different sized masks, but don’t
know what to use on your point-to-point WAN links. Is the 255.255.255.252 (/30) a helpful
mask in this situation?
Yes, this is a very helpful mask in wide area networks!
If you were to use the 255.255.255.0 mask in this situation, then each network would have
254 hosts. But you use only 2 addresses with a WAN link, which is a waste of 252 hosts per
subnet! If you use the 255.255.255.252 mask, then each subnet has only 2 hosts, and you
don’t want to waste precious addresses. This is a really important subject, one that we’ll
address in a lot more detail in the section on VLSM network design in the next chapter!
Subnetting in Your Head: Class C Addresses
It really is possible to subnet in your head? Yes, and it’s not all that hard either—take the
following example:
192.168.10.50 = Node address
255.255.255.224 = Subnet mask
First, determine the subnet and broadcast address of the network in which the above
IP address resides. You can do this by answering question 3 of the big 5 questions: 256 –
224 = 32. 0, 32, 64, and so on. The address of 33 falls between the two subnets of 32 and
64 and must be part of the 192.168.10.32 subnet. The next subnet is 64, so the broadcast
address of the 32 subnet is 63. Don’t forget that the broadcast address of a subnet is always
the number right before the next subnet. The valid host range equals the numbers between
the subnet and broadcast address, or 33–62. This is too easy!
Okay, let’s try another one. We’ll subnet another Class C address:
192.168.10.50 = Node address
255.255.255.240 = Subnet mask
What is the subnet and broadcast address of the network of which the above IP address
is a member? 256 – 240 = 16. Now just count by our increments of 16 until we pass the
host address: 0, 16, 32, 48, 64. Bingo—the host address is between the 48 and 64 subnets.
The subnet is 192.168.10.48, and the broadcast address is 63 because the next subnet is
64. The valid host range equals the numbers between the subnet number and the broadcast
address, or 49–62.
156 Chapter 4 u Easy Subnetting
Let’s do a couple more to make sure you have this down.
You have a node address of 192.168.10.174 with a mask of 255.255.255.240. What is
the valid host range?
The mask is 240, so we’d do a 256 – 240 = 16. This is our block size. Just keep adding
16 until we pass the host address of 174, starting at zero, of course: 0, 16, 32, 48, 64, 80,
96, 112, 128, 144, 160, 176. The host address of 174 is between 160 and 176, so the subnet
is 160. The broadcast address is 175; the valid host range is 161–174. That was a tough one!
One more—just for fun. This one is the easiest of all Class C subnetting:
192.168.10.17 = Node address
255.255.255.252 = Subnet mask
What is the subnet and broadcast address of the subnet in which the above IP address
resides? 256 – 252 = 0 (always start at zero unless told otherwise). 0, 4, 8, 12, 16, 20,
etc. You’ve got it! The host address is between the 16 and 20 subnets. The subnet is
192.168.10.16, and the broadcast address is 19. The valid host range is 17–18.
Now that you’re all over Class C subnetting, let’s move on to Class B subnetting. But
before we do, let’s go through a quick review.
What Do We Know?
Okay—here’s where you can really apply what you’ve learned so far and begin committing
it all to memory. This is a very cool section that I’ve been using in my classes for years. It
will really help you nail down subnetting for good!
When you see a subnet mask or slash notation (CIDR), you should know the following:
/25 What do we know about a /25?
u 128 mask
u 1 bit on and 7 bits off (10000000)
u Block size of 128
u Subnets 0 and 128
u 2 subnets, each with 126 hosts
/26 What do we know about a /26?
u 192 mask
u 2 bits on and 6 bits off (11000000)
u Block size of 64
u Subnet 0, 64, 128, 192
u 4 subnets, each with 62 hosts
/27 What do we know about a /27?
u 224 mask
u 3 bits on and 5 bits off (11100000)
Subnetting Basics 157
u Block size of 32
u Subnets 0, 32, 64, 96, 128, 160, 192, 224
u 8 subnets, each with 30 hosts
/28 What do we know about a /28?
u 240 mask
u 4 bits on and 4 bits off
u Block size of 16
u Subnets 0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240
u 16 subnets, each with 14 hosts
/29 What do we know about a /29?
u 248 mask
u 5 bits on and 3 bits off
u Block size of 8
u Subnets 0, 8, 16, 24, 32, 40, 48, etc.
u 32 subnets, each with 6 hosts
/30 What do we know about a /30?
u 252 mask
u 6 bits on and 2 bits off
u Block size of 4
u Subnets 0, 4, 8, 12, 16, 20, 24, etc.
u 64 subnets, each with 2 hosts
The below table 4.3 puts all of the above information into one compact little table. You
should practice writing this table out on scratch paper, and if you can do it, write it down
before you start your exam!
TABLE4.3 What do you know?
CIDR Notation Mask Bits Block Size Subnets Hosts
/25 128 1 bit on and
7 bits off
128 0 and 128 2 subnets, each
with 126 hosts
/26 192 2 bits on and
6 bits off
64 0, 64, 128, 192 4 subnets, each
with 62 hosts
/27 224 3 bits on and
5 bits off
32 0, 32, 64, 96, 128,
160, 192, 224
8 subnets, each
with 30 hosts
158 Chapter 4 u Easy Subnetting
CIDR Notation Mask Bits Block Size Subnets Hosts
/28 240 4 bits on and
4 bits off
16 0, 16, 32, 48, 64,
80, 96, 112, 128,
144, 160, 176, 192,
208, 224, 240
16 subnets, each
with 14 hosts
/29 248 5 bits on and
3 bits off
8 0, 8, 16, 24, 32, 40,
48, etc.
32 subnets, each
with 6 hosts
/30 252 6 bits on and
2 bits off
4 0, 4, 8, 12, 16, 20,
24, etc.
64 subnets, each
with 2 hosts
Regardless of whether you have a Class A, Class B, or Class C address, the /30 mask will
provide you with only two hosts, ever. As suggested by Cisco, this mask is suited almost
exclusively for use on point-to-point links.
If you can memorize this “What Do We Know?” section, you’ll be much better off in
your day-to-day job and in your studies. Try saying it out loud, which helps you memorize
things—yes, your signicant other and/or coworkers will think you’ve lost it, but they
probably already do if you’re in the networking eld anyway. And if you’re not yet in the
networking eld but are studying all this to break into it, get used to it!
It’s also helpful to write these on some type of ashcards and have people test your skill.
You’d be amazed at how fast you can get subnetting down if you memorize block sizes as
well as this “What Do We Know?” section.
Subnetting Class B Addresses
Before we dive into this, let’s look at all the possible Class B subnet masks rst. Notice that
we have a lot more possible subnet masks than we do with a Class C network address:
255.255.0.0 (/16)
255.255.128.0 (/17) 255.255.255.0 (/24)
255.255.192.0 (/18) 255.255.255.128 (/25)
255.255.224.0 (/19) 255.255.255.192 (/26)
255.255.240.0 (/20) 255.255.255.224 (/27)
255.255.248.0 (/21) 255.255.255.240 (/28)
255.255.252.0 (/22) 255.255.255.248 (/29)
255.255.254.0 (/23) 255.255.255.252 (/30)
We know the Class B network address has 16 bits available for host addressing. This
means we can use up to 14 bits for subnetting because we need to leave at least 2 bits for
host addressing. Using a /16 means you are not subnetting with Class B, but it is a mask
you can use!
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TABLE4.3 What do you know?
(continued)
Subnetting Basics 159
By the way, do you notice anything interesting about that list of subnet
values—a pattern, maybe? Ah ha! That’s exactly why I had you memorize
the binary-to-decimal numbers earlier in Chapter 2. Since subnet mask bits
start on the left and move to the right and bits can’t be skipped, the num-
bers are always the same regardless of the class of address. If you haven’t
already, memorize this pattern!
The process of subnetting a Class B network is pretty much the same as it is for a Class C,
except that you have more host bits and you start in the third octet.
Use the same subnet numbers for the third octet with Class B that you used for the
fourth octet with Class C, but add a zero to the network portion and a 255 to the broad-
cast section in the fourth octet. The following table shows you an example host range of
two subnets used in a Class B 240 (/20) subnet mask:
Subnet address 16.0 32.0
Broadcast address 31.255 47. 255
Just add the valid hosts between the numbers and you’re set!
The preceding example is true only until you get up to /24. After that, it’s
numerically exactly like Class C.
Subnetting Practice Examples: Class B Addresses
The following sections will give you an opportunity to practice subnetting Class B addresses.
Again, I have to mention that this is the same as subnetting with Class C, except we start in
the third octet—with the exact same numbers!
Practice Example #1B: 255.255.128.0 (/17)
172.16.0.0 = Network address
255.255.128.0 = Subnet mask
u Subnets? 21 = 2 (same amount as Class C).
u Hosts? 215 – 2 = 32,766 (7 bits in the third octet, and 8 in the fourth).
u Valid subnets? 256 – 128 = 128. 0, 128. Remember that subnetting is performed in the
third octet, so the subnet numbers are really 0.0 and 128.0, as shown in the next table.
These are the exact numbers we used with Class C; we use them in the third octet and
add a 0 in the fourth octet for the network address.
u Broadcast address for each subnet?
u Valid hosts?
160 Chapter 4 u Easy Subnetting
The following table shows the two subnets available, the valid host range, and the
broadcast address of each:
Subnet 0.0 128.0
First host 0.1 128.1
Last host 127. 254 255.254
Broadcast 127. 255 255.255
Okay, notice that we just added the fourth octet’s lowest and highest values and came
up with the answers. And again, it’s done exactly the same way as for a Class C subnet. We
just used the same numbers in the third octet and added 0 and 255 in the fourth octet—
pretty simple, huh? I really can’t say this enough: it’s just not that hard. The numbers never
change; we just use them in different octets!
Question: Using the above subnet mask, do you think 172.16.10.0 is a valid host
address? What about 172.16.10.255? Can 0 and 255 in the fourth octet ever be a valid
host address? The answer is absolutely, yes, those are valid hosts! Any number between
the subnet number and the broadcast address is always a valid host.
Practice Example #2B: 255.255.192.0 (/18)
172.16.0.0 = Network address
255.255.192.0 = Subnet mask
u Subnets? 22 = 4.
u Hosts? 214 – 2 = 16,382 (6 bits in the third octet, and 8 in the fourth).
u Valid subnets? 256 – 192 = 64. 0, 64, 128, 192. Remember that the subnetting is per-
formed in the third octet, so the subnet numbers are really 0.0, 64.0, 128.0, and 192.0,
as shown in the next table.
u Broadcast address for each subnet?
u Valid hosts?
The following table shows the four subnets available, the valid host range, and the
broadcast address of each:
Subnet 0.0 64.0 128.0 192.0
First host 0.1 64.1 128.1 192.1
Last host 63.254 127. 254 191.254 255.254
Broadcast 63.255 127.255 191.255 255.255
Subnetting Basics 161
Again, it’s pretty much the same as it is for a Class C subnet—we just added 0 and 255
in the fourth octet for each subnet in the third octet.
Practice Example #3B: 255.255.240.0 (/20)
172.16.0.0 = Network address
255.255.240.0 = Subnet mask
u Subnets? 24 = 16.
u Hosts? 212 – 2 = 4094.
u Valid subnets? 256 – 240 = 0, 16, 32, 48, etc., up to 240. Notice that these are the
same numbers as a Class C 240 mask—we just put them in the third octet and add a
0 and 255 in the fourth octet.
u Broadcast address for each subnet?
u Valid hosts?
The following table shows the rst four subnets, valid hosts, and broadcast addresses in
a Class B 255.255.240.0 mask:
Subnet 0.0 16.0 32.0 48.0
First host 0.1 16.1 32.1 48.1
Last host 15.254 31.254 47.2 54 63.254
Broadcast 15.255 31.255 47. 255 63.255
Practice Example #4B: 255.255.248.0 (/21)
172.16.0.0 = Network address
255.255.248.0 = Subnet mask
u Subnets? 25 = 32.
u Hosts? 211 – 2 = 2046.
u Valid subnets? 256 – 248 = 0, 8, 16, 24, 32, etc., up to 248.
u Broadcast address for each subnet?
u Valid hosts?
162 Chapter 4 u Easy Subnetting
The following table shows the rst ve subnets, valid hosts, and broadcast addresses in a
Class B 255.255.248.0 mask:
Subnet 0.0 8.0 16.0 24.0 32.0
First host 0.1 8.1 16.1 24.1 32.1
Last host 7.254 15.254 23.254 31.254 39.254
Broadcast 7. 255 15.255 23.255 31.255 39.255
Practice Example #5B: 255.255.252.0 (/22)
172.16.0.0 = Network address
255.255.252.0 = Subnet mask
u Subnets? 26 = 64.
u Hosts? 210 – 2 = 1022.
u Valid subnets? 256 – 252 = 0, 4, 8, 12, 16, etc., up to 252.
u Broadcast address for each subnet?
u Valid hosts?
The following table shows the rst ve subnets, valid hosts, and broadcast addresses in a
Class B 255.255.252.0 mask:
Subnet 0.0 4.0 8.0 12.0 16.0
First host 0.1 4.1 8.1 12.1 16.1
Last host 3.254 7. 254 11.254 15.254 19.254
Broadcast 3.255 7. 255 11.255 15.255 19.255
Practice Example #6B: 255.255.254.0 (/23)
172.16.0.0 = Network address
255.255.254.0 = Subnet mask
u Subnets? 27 = 128.
u Hosts? 29 – 2 = 510.
u Valid subnets? 256 – 254 = 0, 2, 4, 6, 8, etc., up to 254.
Subnetting Basics 163
u Broadcast address for each subnet?
u Valid hosts?
The following table shows the rst ve subnets, valid hosts, and broadcast addresses in a
Class B 255.255.254.0 mask:
Subnet 0.0 2.0 4.0 6.0 8.0
First host 0.1 2.1 4.1 6.1 8.1
Last host 1.254 3.254 5.254 7. 254 9.254
Broadcast 1.255 3.255 5.255 7. 255 9.255
Practice Example #7B: 255.255.255.0 (/24)
Contrary to popular belief, 255.255.255.0 used with a Class B network address is not called
a Class B network with a Class C subnet mask. It’s amazing how many people see this mask
used in a Class B network and think it’s a Class C subnet mask. This is a Class B subnet
mask with 8 bits of subnetting—it’s logically different from a Class C mask. Subnetting this
address is fairly simple:
172.16.0.0 = Network address
255.255.255.0 = Subnet mask
u Subnets? 28 = 256.
u Hosts? 28 – 2 = 254.
u Valid subnets? 256 – 255 = 1. 0, 1, 2, 3, etc., all the way to 255.
u Broadcast address for each subnet?
u Valid hosts?
The following table shows the rst four and last two subnets, the valid hosts, and the
broadcast addresses in a Class B 255.255.255.0 mask:
Subnet 0.0 1.0 2.0 3.0 ... 254.0 255.0
First host 0.1 1.1 2.1 3.1 ... 254.1 255.1
Last host 0.254 1.254 2.254 3.254 ... 254.254 255.254
Broadcast 0.255 1.255 2.255 3.255 ... 254.255 255.255
164 Chapter 4 u Easy Subnetting
Practice Example #8B: 255.255.255.128 (/25)
This is actually one of the hardest subnet masks you can play with. And worse, it actually is
a really good subnet to use in production because it creates over 500 subnets with 126 hosts
for each subnet—a nice mixture. So, don’t skip over it!
172.16.0.0 = Network address
255.255.255.128 = Subnet mask
u Subnets? 29 = 512.
u Hosts? 27 – 2 = 126.
u Valid subnets? Okay, now for the tricky part. 256 – 255 = 1. 0, 1, 2, 3, etc., for the
third octet. But you can’t forget the one subnet bit used in the fourth octet. Remember
when I showed you how to figure one subnet bit with a Class C mask? You figure this
the same way. You actually get two subnets for each third octet value, hence the 512
subnets. For example, if the third octet is showing subnet 3, the two subnets would
actually be 3.0 and 3.128.
u Broadcast address for each subnet? The numbers right before the next subnet.
u Valid hosts? The numbers between the subnet numbers and the broadcast address.
The following graphic shows how you can create subnets, valid hosts, and broadcast
addresses using the Class B 255.255.255.128 subnet mask. The rst eight subnets are
shown, followed by the last two subnets:
Subnet
First host
Last host
0.0
0.1
0.126
0.127
0.128
0.129
0.254
0.255
1.0
1.1
1.126
1.127
1.128
1.129
1.254
1.255
2.0
2.1
2.126
2.127
2.128
2.129
2.254
2.255
3.0
3.1
3.126
3.127
3.128
3.129
3.254
3.255
...
...
...
...
255.0
255.1
255.126
255.127
255.128
255.129
255.254
255.255Broadcast
Practice Example #9B: 255.255.255.192 (/26)
Now, this is where Class B subnetting gets easy. Since the third octet has a 255 in the
mask section, whatever number is listed in the third octet is a subnet number. And now
that we have a subnet number in the fourth octet, we can subnet this octet just as we did
with Class C subnetting. Let’s try it out:
172.16.0.0 = Network address
255.255.255.192 = Subnet mask
u Subnets? 210 = 1024.
u Hosts? 26 – 2 = 62.
u Valid subnets? 256 – 192 = 64. The subnets are shown in the following table. Do these
numbers look familiar?
Subnetting Basics 165
u Broadcast address for each subnet?
u Valid hosts?
The following table shows the rst eight subnet ranges, valid hosts, and broadcast
addresses:
Subnet 0.0 0.64 0.128 0.192 1.0 1.64 1.128 1.192
First host 0.1 0.65 0.129 0.193 1.1 1.65 1.129 1.193
Last host 0.62 0.126 0.190 0.254 1.62 1.126 1.190 1.254
Broadcast 0.63 0.127 0.191 0.255 1.63 1.127 1.191 1.255
Notice that for each subnet value in the third octet, you get subnets 0, 64, 128, and 192
in the fourth octet.
Practice Example #10B: 255.255.255.224 (/27)
This one is done the same way as the preceding subnet mask, except that we just have more
subnets and fewer hosts per subnet available.
172.16.0.0 = Network address
255.255.255.224 = Subnet mask
u Subnets? 211 = 2048.
u Hosts? 25 – 2 = 30.
u Valid subnets? 256 – 224 = 32. 0, 32, 64, 96, 128, 160, 192, 224.
u Broadcast address for each subnet?
u Valid hosts?
The following table shows the rst eight subnets:
Subnet 0.0 0.32 0.64 0.96 0.128 0.160 0.192 0.224
First host 0.1 0.33 0.65 0.97 0.129 0.161 0.193 0.225
Last host 0.30 0.62 0.94 0.126 0.158 0.190 0.222 0.254
Broadcast 0.31 0.63 0.95 0.127 0.159 0.191 0.223 0.255
166 Chapter 4 u Easy Subnetting
This next table shows the last eight subnets:
Subnet 255.0 255.32 255.64 255.96 255.128 255.160 255.192 255.224
First host 255.1 255.33 255.65 255.97 255.129 255.161 255.193 255.225
Last host 255.30 255.62 255.94 255.126 255.158 255.190 255.222 255.254
Broadcast 255.31 255.63 255.95 255.127 255.159 255.191 255.223 255.255
Subnetting in Your Head: Class B Addresses
Are you nuts? Subnet Class B addresses in our heads? It’s actually easier than writing it out—
I’m not kidding! Let me show you how:
Question: What is the subnet and broadcast address of the subnet in which
172.16.10.33 /27 resides?
Answer: The interesting octet is the fourth one. 256 – 224 = 32. 32 + 32 = 64. You’ve
got it: 33 is between 32 and 64. But remember that the third octet is considered part
of the subnet, so the answer would be the 10.32 subnet. The broadcast is 10.63, since
10.64 is the next subnet. That was a pretty easy one.
Question: What subnet and broadcast address is the IP address 172.16.66.10
255.255.192.0 (/18) a member of?
Answer: The interesting octet here is the third octet instead of the fourth one.
256 – 192 = 64. 0, 64, 128. The subnet is 172.16.64.0. The broadcast must be
172.16.127.255 since 128.0 is the next subnet.
Question: What subnet and broadcast address is the IP address 172.16.50.10
255.255.224.0 (/19) a member of?
Answer: 256 – 224 = 0, 32, 64 (remember, we always start counting at 0). The subnet
is 172.16.32.0, and the broadcast must be 172.16.63.255 since 64.0 is the next subnet.
Question: What subnet and broadcast address is the IP address 172.16.46.255
255.255.240.0 (/20) a member of?
Answer: 256 – 240 = 16. The third octet is important here: 0, 16, 32, 48. This subnet
address must be in the 172.16.32.0 subnet, and the broadcast must be 172.16.47.255
since 48.0 is the next subnet. So, yes, 172.16.46.255 is a valid host.
Question: What subnet and broadcast address is the IP address 172.16.45.14
255.255.255.252 (/30) a member of?
Answer: Where is our interesting octet? 256 – 252 = 0, 4, 8, 12, 16—the fourth. The
subnet is 172.16.45.12, with a broadcast of 172.16.45.15 because the next subnet is
172.16.45.16.
Question: What is the subnet and broadcast address of the host 172.16.88.255/20?
Subnetting Basics 167
Answer: What is a /20 written out in dotted decimal? If you can’t answer this, you
can’t answer this question, can you? A /20 is 255.255.240.0, gives us a block size of
16 in the third octet, and since no subnet bits are on in the fourth octet, the answer is
always 0 and 255 in the fourth octet: 0, 16, 32, 48, 64, 80, 96. Because 88 is between
80 and 96, the subnet is 80.0 and the broadcast address is 95.255.
Question: A router receives a packet on an interface with a destination address of
172.16.46.191/26. What will the router do with this packet?
Answer: Discard it. Do you know why? 172.16.46.191/26 is a 255.255.255.192 mask,
which gives us a block size of 64. Our subnets are then 0, 64, 128 and 192. 191 is the
broadcast address of the 128 subnet, and by default, a router will discard any broad-
cast packets.
Subnetting Class A Addresses
You don’t go about Class A subnetting any differently than Classes B and C, but there are
24 bits to play with instead of the 16 in a Class B address and the 8 in a Class C address.
Let’s start by listing all the Class A masks:
255.0.0.0 (/8)
255.128.0.0 (/9) 255.255.240.0 (/20)
255.192.0.0 (/10) 255.255.248.0 (/21)
255.224.0.0 (/11) 255.255.252.0 (/22)
255.240.0.0 (/12) 255.255.254.0 (/23)
255.248.0.0 (/13) 255.255.255.0 (/24)
255.252.0.0 (/14) 255.255.255.128 (/25)
255.254.0.0 (/15) 255.255.255.192 (/26)
255.255.0.0 (/16) 255.255.255.224 (/27)
255.255.128.0 (/17) 255.255.255.240 (/28)
255.255.192.0 (/18) 255.255.255.248 (/29)
255.255.224.0 (/19) 255.255.255.252 (/30)
That’s it. You must leave at least 2 bits for dening hosts. I hope you can see the pattern
by now. Remember, we’re going to do this the same way as a Class B or C subnet. It’s just
that, again, we simply have more host bits and we just use the same subnet numbers we used
with Class B and C, but we start using these numbers in the second octet. However, the rea-
son class A addresses are so popular to implement is because they give the most exibility.
You can subnet in the 2nd, 3rd or 4th octet. I’ll show you this in the next examples.
Subnetting Practice Examples: Class A Addresses
When you look at an IP address and a subnet mask, you must be able to distinguish the
bits used for subnets from the bits used for determining hosts. This is imperative. If you’re
still struggling with this concept, please reread the section, “IP Addressing” in Chapter 3.
It shows you how to determine the difference between the subnet and host bits and should
help clear things up.
168 Chapter 4 u Easy Subnetting
Practice Example #1A: 255.255.0.0 (/16)
Class A addresses use a default mask of 255.0.0.0, which leaves 22 bits for subnetting
because you must leave 2 bits for host addressing. The 255.255.0.0 mask with a Class A
address is using 8 subnet bits:
u Subnets? 28 = 256.
u Hosts? 216 – 2 = 65,534.
u Valid subnets? What is the interesting octet? 256 – 255 = 1. 0, 1, 2, 3, etc. (all in the
second octet). The subnets would be 10.0.0.0, 10.1.0.0, 10.2.0.0, 10.3.0.0, etc., up to
10.255.0.0.
u Broadcast address for each subnet?
u Valid hosts?
The following table shows the rst two and the last two subnets, the valid host range
and the broadcast addresses for the private Class A 10.0.0.0 network:
Subnet 10.0.0.0 10.1.0.0 … 10.254.0.0 10.255.0.0
First host 10.0.0.1 10.1.0.1 …10.254.0.1 10.255.0.1
Last host 10.0.255.254 10.1.255.254 …10.254.255.254 10.255.255.254
Broadcast 10.0.255.255 10.1.255.255 …10.254.255.255 10.255.255.255
Practice Example #2A: 255.255.240.0 (/20)
255.255.240.0 gives us 12 bits of subnetting and leaves us 12 bits for host addressing.
u Subnets? 212 = 4096.
u Hosts? 212 – 2 = 4094.
u Valid subnets? What is your interesting octet? 256 – 240 = 16. The subnets in the sec-
ond octet are a block size of 1 and the subnets in the third octet are 0, 16, 32, etc.
u Broadcast address for each subnet?
u Valid hosts?
The following table shows some examples of the host ranges—the rst three subnets and
the last subnet:
Subnet 10.0.0.0 10.0.16.0 10.0.32.0 … 10.255.240.0
First host 10.0.0.1 10.0.16.1 10.0.32.1 …10.255.240.1
Last host 10.0.15.254 10.0.31.254 10.0.47.254 …10.255.255.254
Subnetting Basics 169
Subnet 10.0.0.0 10.0.16.0 10.0.32.0 … 10.255.240.0
Broadcast 10.0.15.255 10.0.31.255 10.0.47.255 …10.255.255.255
Practice Example #3A: 255.255.255.192 (/26)
Let’s do one more example using the second, third, and fourth octets for subnetting:
u Subnets? 218 = 262,144.
u Hosts? 26 – 2 = 62.
u Valid subnets? In the second and third octet, the block size is 1, and in the fourth
octet, the block size is 64.
u Broadcast address for each subnet?
u Valid hosts?
The following table shows the rst four subnets and their valid hosts and broadcast
addresses in the Class A 255.255.255.192 mask:
Subnet 10.0.0.0 10.0.0.64 10.0.0.128 10.0.0.192
First host 10.0.0.1 10.0.0.65 10.0.0.129 10.0.0.193
Last host 10.0.0.62 10.0.0.126 10.0.0.190 10.0.0.254
Broadcast 10.0.0.63 10.0.0.127 10.0.0.191 10.0.0.255
This table shows the last four subnets and their valid hosts and broadcast addresses:
Subnet 10.255.255.0 10.255.255.64 10.255.255.128 10.255.255.192
First host 10.255.255.1 10.255.255.65 10.255.255.129 10.255.255.193
Last host 10.255.255.62 10.255.255.126 10.255.255.190 10.255.255.254
Broadcast 10.255.255.63 10.255.255.127 10.255.255.191 10.255.255.255
Subnetting in Your Head: Class A Addresses
Again, I know this sounds hard, but as with Class C and Class B, the numbers are the same;
we just start in the second octet. What makes this easy? You only need to worry about the
octet that has the largest block size, which is typically called the interesting octet, and one
that is something other than 0 or 255, such as, for example, 255.255.240.0 (/20) with a Class
170 Chapter 4 u Easy Subnetting
A network. The second octet has a block size of 1, so any number listed in that octet is a sub-
net. The third octet is a 240 mask, which means we have a block size of 16 in the third octet.
If your host ID is 10.20.80.30, what is your subnet, broadcast address, and valid host range?
The subnet in the second octet is 20 with a block size of 1, but the third octet is in block
sizes of 16, so we’ll just count them out: 0, 16, 32, 48, 64, 80, 96… voilà! By the way, you
can count by 16s by now, right? Good! This makes our subnet 10.20.80.0, with a broadcast
of 10.20.95.255 because the next subnet is 10.20.96.0. The valid host range is 10.20.80.1
through 10.20.95.254. And yes, no lie! You really can do this in your head if you just get
your block sizes nailed!
Okay, let’s practice on one more, just for fun!
Host IP: 10.1.3.65/23
First, you can’t answer this question if you don’t know what a /23 is. It’s 255.255.254.0.
The interesting octet here is the third one: 256 – 254 = 2. Our subnets in the third octet are
0, 2, 4, 6, etc. The host in this question is in subnet 2.0, and the next subnet is 4.0, so that
makes the broadcast address 3.255. And any address between 10.1.2.1 and 10.1.3.254 is
considered a valid host.
Summary
Did you read Chapters 3 and 4 and understand everything on the rst pass? If so, that is
fantastic—congratulations! However, you probably really did get lost a couple of times. No
worries because as I told you, that’s what usually happens. Don’t waste time feeling bad if
you have to read each chapter more than once, or even 10 times, before you’re truly good to
go. If you do have to read the chapters more than once, you’ll be seriously better off in the
long run even if you were pretty comfortable the rst time through!
This chapter provided you with an important understanding of IP subnetting—the painless
way! And when you’ve got the key material presented in this chapter really nailed down, you
should be able to subnet IP addresses in your head.
This chapter is extremely essential to your Cisco certication process, so if you just
skimmed it, please go back, read it thoroughly, and don’t forget to do all the written labs too!
Exam Essentials
Identify the advantages of subnetting. Benets of subnetting a physical network include
reduced network trafc, optimized network performance, simplied management, and
facilitated spanning of large geographical distances.
Describe the effect of the ip subnet-zero command. This command allows you to use
the rst and last subnet in your network design.
Visit ccna
.gg/ch4/b
for a
companion
MicroNugget
from CBT
Nuggets.
Written Labs 171
Identify the steps to subnet a classful network. Understand how IP addressing and subnet-
ting work. First, determine your block size by using the 256-subnet mask math. Then count
your subnets and determine the broadcast address of each subnet—it is always the number
right before the next subnet. Your valid hosts are the numbers between the subnet address
and the broadcast address.
Determine possible block sizes. This is an important part of understanding IP address-
ing and subnetting. The valid block sizes are always 2, 4, 8, 16, 32, 64, 128, etc. You can
determine your block size by using the 256-subnet mask math.
Describe the role of a subnet mask in IP addressing. A subnet mask is a 32-bit value that
allows the recipient of IP packets to distinguish the network ID portion of the IP address
from the host ID portion of the IP address.
Understand and apply the 2x – 2 formula. Use this formula to determine the proper sub-
net mask for a particular size network given the application of that subnet mask to a par-
ticular classful network.
Explain the impact of Classless Inter-Domain Routing (CIDR). CIDR allows the creation
of networks of a size other than those allowed with the classful subnetting by allowing
more than the three classful subnet masks.
Written Labs
In this section, you’ll complete the following labs to make sure you’ve got the information
and concepts contained within them fully dialed in:
Lab 4.1: Written Subnet Practice #1
Lab 4.2: Written Subnet Practice #2
Lab 4.3: Written Subnet Practice #3
The answers to these labs can be found in Appendix A, “Answers to Written Labs.”
Written Lab 4.1: Written Subnet Practice #1
Write the subnet, broadcast address, and a valid host range for question 1 through question
6. Then answer the remaining questions.
1. 192.168.100.25/30
2. 192.168.100.37/28
3. 192.168.100.66/27
4. 192.168.100.17/29
5. 192.168.100.99/26
172 Chapter 4 u Easy Subnetting
6. 192.168.100.99/25
7. You have a Class B network and need 29 subnets. What is your mask?
8. What is the broadcast address of 192.168.192.10/29?
9. How many hosts are available with a Class C /29 mask?
10. What is the subnet for host ID 10.16.3.65/23?
Written Lab 4.2: Written Subnet Practice #2
Given a Class B network and the net bits identied (CIDR), complete the following table to
identify the subnet mask and the number of host addresses possible for each mask.
Classful Address Subnet Mask Number of Hosts per Subnet (2x – 2)
/16
/17
/18
/19
/20
/21
/22
/23
/24
/25
/26
/27
/28
/29
/30
Written Labs 173
Written Lab 4.3: Written Subnet Practice #3
Complete the following based on the decimal IP address.
Decimal IP Address Address Class
Number of Subnet
and Host Bits
Number of
Subnets (2x )
Number of
Hosts (2x – 2)
10.25.66.154/23
172.31.254.12/24
192.168.20.123/28
63.24.89.21/18
128.1.1.254/20
208.100.54.209/30
174 Chapter 4 u Easy Subnetting
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional questions,
please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. What is the maximum number of IP addresses that can be assigned to hosts on a local
subnet that uses the 255.255.255.224 subnet mask?
A. 14
B. 15
C. 16
D. 30
E. 31
F. 62
2. You have a network that needs 29 subnets while maximizing the number of host addresses
available on each subnet. How many bits must you borrow from the host field to provide
the correct subnet mask?
A. 2
B. 3
C. 4
D. 5
E. 6
F. 7
3. What is the subnetwork address for a host with the IP address 200.10.5.68/28?
A. 200.10.5.56
B. 200.10.5.32
C. 200.10.5.64
D. 200.10.5.0
Review Questions 175
4. The network address of 172.16.0.0/19 provides how many subnets and hosts?
A. 7 subnets, 30 hosts each
B. 7 subnets, 2,046 hosts each
C. 7 subnets, 8,190 hosts each
D. 8 subnets, 30 hosts each
E. 8 subnets, 2,046 hosts each
F. 8 subnets, 8,190 hosts each
5. Which two statements describe the IP address 10.16.3.65/23? (Choose two.)
A. The subnet address is 10.16.3.0 255.255.254.0.
B. The lowest host address in the subnet is 10.16.2.1 255.255.254.0.
C. The last valid host address in the subnet is 10.16.2.254 255.255.254.0.
D. The broadcast address of the subnet is 10.16.3.255 255.255.254.0.
E. The network is not subnetted.
6. If a host on a network has the address 172.16.45.14/30, what is the subnetwork this
host belongs to?
A. 172.16.45.0
B. 172.16.45.4
C. 172.16.45.8
D. 172.16.45.12
E. 172.16.45.16
7. Which mask should you use on point-to-point WAN links in order to reduce the waste
of IP addresses?
A. /27
B. /28
C. /29
D. /30
E. /31
8. What is the subnetwork number of a host with an IP address of 172.16.66.0/21?
A. 172.16.36.0
B. 172.16.48.0
C. 172.16.64.0
D. 172.16.0.0
176 Chapter 4 u Easy Subnetting
9. You have an interface on a router with the IP address of 192.168.192.10/29. Including
the router interface, how many hosts can have IP addresses on the LAN attached to the
router interface?
A. 6
B. 8
C. 30
D. 62
E. 126
10. You need to configure a server that is on the subnet 192.168.19.24/29. The router has
the first available host address. Which of the following should you assign to the server?
A. 192.168.19.0 255.255.255.0
B. 192.168.19.33 255.255.255.240
C. 192.168.19.26 255.255.255.248
D. 192.168.19.31 255.255.255.248
E. 192.168.19.34 255.255.255.240
11. You have an interface on a router with the IP address of 192.168.192.10/29. What is
the broadcast address the hosts will use on this LAN?
A. 192.168.192.15
B. 192.168.192.31
C. 192.168.192.63
D. 192.168.192.127
E. 192.168.192.255
12. You need to subnet a network that has 5 subnets, each with at least 16 hosts. Which
classful subnet mask would you use?
A. 255.255.255.192
B. 255.255.255.224
C. 255.255.255.240
D. 255.255.255.248
Review Questions 177
13. You configure a router interface with the IP address 192.168.10.62 255.255.255.192
and receive the following error:
Bad mask /26 for address 192.168.10.62
Why did you receive this error?
A. You typed this mask on a WAN link and that is not allowed.
B. This is not a valid host and subnet mask combination.
C. ip subnet-zero is not enabled on the router.
D. The router does not support IP.
14. If an Ethernet port on a router were assigned an IP address of 172.16.112.1/25, what
would be the valid subnet address of this interface?
A. 172.16.112.0
B. 172.16.0.0
C. 172.16.96.0
D. 172.16.255.0
E. 172.16.128.0
15. Using the following illustration, what would be the IP address of E0 if you were using
the eighth subnet? The network ID is 192.168.10.0/28 and you need to use the last
available IP address in the range. The zero subnet should not be considered valid for
this question.
S0
192.168.10.0/28
E0
A. 192.168.10.142
B. 192.168.10.66
C. 192.168.100.254
D. 192.168.10.143
E. 192.168.10.126
178 Chapter 4 u Easy Subnetting
16. Using the illustration from the previous question, what would be the IP address of S0
if you were using the first subnet? The network ID is 192.168.10.0/28 and you need to
use the last available IP address in the range. Again, the zero subnet should not be con-
sidered valid for this question.
A. 192.168.10.24
B. 192.168.10.62
C. 192.168.10.30
D. 192.168.10.127
17. Which configuration command must be in effect to allow the use of 8 subnets if the
Class C subnet mask is 255.255.255.224?
A. Router(config)#ip classless
B. Router(config)#ip version 6
C. Router(config)#no ip classful
D. Router(config)#ip unnumbered
E. Router(config)#ip subnet-zero
F. Router(config)#ip all-nets
18. You have a network with a subnet of 172.16.17.0/22. Which is the valid host address?
A. 172.16.17.1 255.255.255.252
B. 172.16.0.1 255.255.240.0
C. 172.16.20.1 255.255.254.0
D. 172.16.16.1 255.255.255.240
E. 172.16.18.255 255.255.252.0
F. 172.16.0.1 255.255.255.0
19. Your router has the following IP address on Ethernet0: 172.16.2.1/23. Which of the fol-
lowing can be valid host IDs on the LAN interface attached to the router? (Choose two.)
A. 172.16.0.5
B. 172.16.1.100
C. 172.16.1.198
D. 172.16.2.255
E. 172.16.3.0
F. 172.16.3.255
Review Questions 179
20. To test the IP stack on your local host, which IP address would you ping?
A. 172.0.0.1
B. 1.0.0.127
C. 127.0.0.1
D. 127.255.255.255
E. 255.255.255.255
Chapter
5
VLSMs,
Summarization,
and Troubleshooting
TCP/IP
THE FOLLOWING ICND1 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 IP addressing (IPv4 / IPv6)
■Identify the appropriate IPv4 addressing scheme using VLSM
and summarization to satisfy addressing requirements in a
LAN/WAN environment.
1 Troubleshooting
■Troubleshoot and correct common problems associated with
IP addressing and host configurations.
Now that IP addressing and subnetting have been thoroughly
covered in the last two chapters, you’re now fully prepared and
ready to learn all about variable length subnet masks (VLSMs).
I’ll also show you how to design and implement a network using VLSM in this chapter. After
ensuring you’ve mastered VLSM design and implementation, I’ll demonstrate how to summa-
rize classful boundaries.
We’ll wrap up the chapter by going over IP address troubleshooting, focusing on the steps
Cisco recommends to follow when troubleshooting an IP network.
So get psyched because this chapter will give you powerful tools to hone your knowledge
of IP addressing and networking and seriously rene the important skills you’ve gained so
far. So stay with me—I guarantee that your hard work will pay off! Ready? Let’s go!
To find up-to-the minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
Variable Length Subnet Masks (VLSMs)
Teaching you a simple way to create many networks from a large single network using subnet
masks of different lengths in various kinds of network designs is what my primary focus will
be in this chapter. Doing this is called VLSM networking, and it brings up another important
subject I mentioned in Chapter 4, “Easy Subnetting,” classful and classless networking.
Older routing protocols like Routing Information Protocol version 1 (RIPv1) do not have
a eld for subnet information, so the subnet information gets dropped. This means that if a
router running RIP has a subnet mask of a certain value, it assumes that all interfaces within
the classful address space have the same subnet mask. This is called classful routing, and RIP
is considered a classful routing protocol. We’ll cover RIP and the difference between classful
and classless networks later on in Chapter 8, “IP Routing,” but for now, just remember that
if you try to mix and match subnet mask lengths in a network that’s running an old routing
protocol, such as RIP, it just won’t work!
However, classless routing protocols do support the advertisement of subnet information,
which means you can use VLSM with routing protocols such as RIPv2, Enhanced Interior
Gateway Protocol (EIGRP), and Open Shortest Path First (OSPF). The benet of this type of
network is that it saves a bunch of IP address space.
Variable Length Subnet Masks (VLSMs) 183
As the name suggests, VLSMs can use subnet masks with different lengths for different
router interfaces. Check out Figure5.1 to see an example of why classful network designs
are inefcient.
FIGURE5.1 Typical classful network
192.168.10.32/28
192.168.10.0/28
(6 hosts) (10 hosts)
.33
.1
.49
.34 .35
192.168.10.64/28
.66 .67
(25 hosts) (12 hosts)
.2 .3 .18 .19
.50
.17
.65
2 hosts
192.168.10.48/28
192.168.10.16/28
Looking at Figure5.1, you can see that there are two routers, each with two LANs and
connected together with a WAN serial link. In a typical classful network design that’s running
RIP, you could subnet a network like this:
192.168.10.0 = Network
255.255.255.240 (/28) = Mask
Our subnets would be—you know this part, right?— 0, 16, 32, 48, 64, 80, etc., which
allows us to assign 16 subnets to our internetwork. But how many hosts would be avail-
able on each network? Well, as you know by now, each subnet provides only 14 hosts, so
each LAN has only 14 valid hosts available (don’t forget that the router interface needs an
address too and is included in the amount of needed valid hosts). This means that one LAN
doesn’t even have enough addresses needed for all the hosts, and this network as it is shown
would not work as addressed in the gure! Since the point-to-point WAN link also has
14 valid hosts, it would be great to be able to nick a few valid hosts from that WAN link
to give to our LANs!
All hosts and router interfaces have the same subnet mask—again, known as classful
routing—and if we want this network to be efcient, we would denitely need to add dif-
ferent masks to each router interface.
But that’s not our only problem—the link between the two routers will never use more
than two valid hosts! This wastes valuable IP address space, and it’s the big reason you
need to learn about VLSM network design.
184 Chapter 5 u VLSMs, Summarization, and Troubleshooting TCP/IP
VLSM Design
Let’s take Figure5.1 and use a classless design instead, which will become the new network
shown in Figure5.2. In the previous example, we wasted address space—one LAN didn’t
have enough addresses because every router interface and host used the same subnet mask.
Not so good. A better solution would be to provide for only the needed number of hosts on
each router interface, and we’re going to use VLSMs to achieve that goal.
FIGURE5.2 Classless network design
192.168.10.64/29
192.168.10.0/27
(6 hosts) (10 hosts)
.65
.1
.73
.66 .67
192.168.10.48/28
.50 .51
(25 hosts) (12 hosts)
.2 .3 .34 .35
.74
.33
.49
2 hosts
192.168.10.72/30
192.168.10.32/28
Now remember that we can use different size masks on each router interface. If we use
a /30 on our WAN links and a /27, /28, and /29 on our LANs, we’ll get 2 hosts per WAN
interface and 30, 14, and 6 hosts per LAN interface—nice! This makes a huge difference—
not only can we get just the right amount of hosts on each LAN, we still have room to add
more WANs and LANs using this same network!
To implement a VLSM design on your network, you need to have a routing
protocol that sends subnet mask information with the route updates. The
protocols that do that are RIPv2, EIGRP, and OSPF. Remember, RIPv1 will not
work in classless networks, so it’s considered a classful routing protocol.
Implementing VLSM Networks
To create VLSMs quickly and efciently, you need to understand how block sizes and charts
work together to create the VLSM masks. Table5.1 shows you the block sizes used when
Variable Length Subnet Masks (VLSMs) 185
creating VLSMs with Class C networks. For example, if you need 25 hosts, then you’ll need
a block size of 32. If you need 11 hosts, you’ll use a block size of 16. Need 40 hosts? Then
you’ll need a block of 64. You cannot just make up block sizes—they’ve got to be the block
sizes shown in Table5.1. So memorize the block sizes in this table—it’s easy. They’re the
same numbers we used with subnetting!
TABLE5.1 Block sizes
Prefix Mask Hosts Block Size
/25 128 126 128
/26 192 62 64
/27 224 30 32
/28 240 14 16
/29 248 6 8
/30 252 2 4
The next step is to create a VLSM table. Figure5.3 shows you the table used in creat-
ing a VLSM network. The reason we use this table is so we don’t accidentally overlap
networks.
You’ll nd the sheet shown in Figure5.3 very valuable because it lists every block size
you can use for a network address. Notice that the block sizes start at 4 and advance all
the way up to a block size of 128. If you have two networks with block sizes of 128, you
can have only 2 networks. With a block size of 64, you can have only 4, and so on, all the
way to 64 networks using a block size of 4. Of course, this is assuming you’re using the ip
subnet-zero command in your network design.
So now all you need to do is ll in the chart in the lower-left corner, then add the sub-
nets to the worksheet and you’re good to go!
Based on what you’ve learned so far about block sizes and the VLSM table, let’s create a
VLSM network using a Class C network address 192.168.10.0 for the network in Figure5.4,
then ll out the VLSM table, as shown in Figure5.5.
In Figure5.4, we have four WAN links and four LANs connected together, so we need
to create a VLSM network that will save address space. Looks like we have two block sizes
of 32, a block size of 16, and a block size of 8, and our WANs each have a block size of 4.
Take a look and see how I lled out our VLSM chart in Figure5.5.
There are two important things to note here, the rst is that we still have plenty of room
for growth with this VLSM network design. The second point is that we could never achieve
this goal with one subnet mask using classful routing.
186 Chapter 5 u VLSMs, Summarization, and Troubleshooting TCP/IP
FIGURE5.3 The VLSM table
Subnet
/25
/26
/27
/28
/29
/30
Mask
128
192
224
240
248
252
Subnets
2
4
8
16
32
64
Hosts
126
62
30
14
6
2
Block
128
64
32
16
8
4
Network
A
B
C
D
E
F
G
H
I
J
K
L
Hosts Block Subnet Mask
0
4
8
12
16
20
24
28
32
36
40
44
48
52
56
60
64
68
72
76
80
84
88
92
96
100
104
108
112
116
120
124
128
132
136
140
144
148
152
156
160
164
168
172
176
180
184
188
192
196
200
204
208
212
216
220
224
228
232
236
240
244
248
252
256
Variable Length Subnet Masks (VLSMs) 187
FIGURE5.4 VLSM network example 1
192.168.10.32/27 192.168.10.64/27
192.168.10.100/30
192.168.10.108/30
192.168.10.96/30 192.168.10.104/30
192.168.10.8/29 192.168.10.16/28
30 hosts
Network B
20 hosts
Network C
6 hosts
Network D
14 hosts
Network A
Fa0/0
Fa0/0 Fa0/0
2 hosts
Network F
2 hosts
Network H
2 hosts
Network E
2 hosts
Network G
Lab D
Fa0/0
Lab E
Lab A Lab B
Let’s do another one. Figure5.6 shows a network with 11 networks, two block sizes of
64, one of 32, ve of 16, and three of 4.
First, create your VLSM table and use your block size chart to ll in the table with the
subnets you need. Figure5.7 shows a possible solution.
Notice that I lled in this entire chart and only have room for one more block size of 4.
You can only gain that amount of address space savings with a VLSM network!
Keep in mind that it doesn’t matter where you start your block sizes as long as you
always begin counting from zero. For example, if you had a block size of 16, you must
start at 0 and incrementally progress from there—0, 16, 32, 48, and so on. You can’t
start with a block size of 16 or some value like 40, and you can’t progress using any-
thing but increments of 16.
Here’s another example. If you had block sizes of 32, start at zero like this: 0, 32, 64, 96, etc.
Again, you don’t get to start wherever you want; you must always start counting from zero. In
the example in Figure5.7, I started at 64 and 128, with my two block sizes of 64. I didn’t have
much choice because my options are 0, 64, 128, and 192. However, I added the block size of
32, 16, 8, and 4 elsewhere, but they were always in the correct increments required of the spe-
cic block size. Remember that if you always start with the largest blocks rst, then make your
way to the smaller blocks sizes, you will automatically fall on an increment boundary. It also
guarantees that you are using your address space in the most effective way.
Okay—you have three locations you need to address, and the IP network you have
received is 192.168.55.0 to use as the addressing for the entire network. You’ll use
ip subnet-zero and RIPv2 as the routing protocol because RIPv2 supports VLSM net-
works but RIPv1 does not. Figure5.8 shows the network diagram and the IP address of
the RouterA S0/0 interface.
188 Chapter 5 u VLSMs, Summarization, and Troubleshooting TCP/IP
FIGURE5.5 VLSM table example 1
0
D — 192.168.10.8/29
A — 192.168.10.16/28
B — 192.168.10.32/27
C — 192.168.10.64/27
E — 192.168.10.96/30
F — 192.168.10.100/30
G — 192.168.10.104/30
H — 192.168.10.108/30
4
8
12
16
20
24
28
32
36
40
44
48
52
56
60
64
68
72
76
80
84
88
92
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112
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128
132
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256
Subnet
/25
/26
/27
/28
/29
/30
Mask
128
192
224
240
248
252
Subnets
2
4
8
16
32
64
Hosts
126
62
30
14
6
2
Block
128
64
32
16
8
4
Network
A
B
C
D
E
F
G
H
12
20
25
4
2
2
2
2
16
32
32
8
4
4
4
4
/28
/27
/27
/29
/30
/30
/30
/30
240
224
224
248
252
252
252
252
Hosts Block Subnet Mask
Variable Length Subnet Masks (VLSMs) 189
FIGURE5.6 VLSM network example 2
Net=C
12 hosts
Core
SF NY
Bldg1
Net=B
10 hosts
Fa0/1 Fa0/2
Fa0/3Fa0/0
Fa0/0
Net=G
12 hosts
Fa0/0 Fa0/1 Fa0/1
Fa0/0
Fa0/1
Net=A
30 hosts
A: /27
B: /28
C: /28
D: /30
E: /30
F: /30
G: /28
H: /26
I: /28
J: /26
K: /28
Net=E
2 hosts
Net=F
2 hosts
Net=D
2 hosts
Net=H
60 hosts
Net=I
14 hosts
Net=J
60 hosts
Net=K
8 hosts
From the list of IP addresses on the right of the gure, which IP address do you think
will be placed in each router’s FastEthernet 0/0 interface and serial 0/1 of RouterB?
To answer this, look for clues in Figure5.8. The rst is that interface S0/0 on RouterA
has IP address 192.168.55.2/30 assigned, which makes for an easy answer because A /30 is
255.255.255.252, which gives you a block size of 4. Your subnets are 0, 4, 8, etc. Since the
known host has an IP address of 2, the only other valid host in the zero subnet is 1, so the
third answer down is the right one for the S0/1 interface of RouterB.
The next clues are the listed number of hosts for each of the LANs. RouterA needs 7
hosts—a block size of 16 (/28). RouterB needs 90 hosts—a block size of 128 (/25). And
RouterC needs 23 hosts—a block size of 32 (/27).
Figure5.9 illustrates this solution.
This is actually pretty simple because once you’ve gured out the block size needed for
each LAN, all you need to get to the right solution is to identify proper clues and, of course,
know your block sizes well!
One last example of VLSM design before we move on to summarization. Figure5.10
shows three routers, all running RIPv2. Which Class C addressing scheme would you use
to maintain the needs of this network while saving as much address space as possible?
This is actually a pretty clean network design that’s just waiting for you to ll out the
chart. There are block sizes of 64, 32, and 16 and two block sizes of 4. Coming up with the
right solution should be a slam dunk! Take a look at my answer in Figure5.11.
My solution began at subnet 0, and I used the block size of 64. Clearly, I didn’t have
to go with a block size of 64 because I could’ve chosen a block size of 4 instead. But I
didn’t because I usually like to start with the largest block size and move to the small-
est. With that done, I added the block sizes of 32 and 16 as well as the two block sizes
of 4. This solution is optimal because it still leaves lots of room to add subnets to this
network!
190 Chapter 5 u VLSMs, Summarization, and Troubleshooting TCP/IP
FIGURE5.7 VLSM table example 2
0
B — 192.168.10.0/28
C — 192.168.10.16/28
A — 192.168.10.32/27
H — 192.168.10.64/26
J — 192.168.10.128/26
D — 192.168.10.244/30
I — 192.168.10.192/28
G — 192.168.10.208/28
K — 192.168.10.224/28
E — 192.168.10.248/30
F — 192.168.10.252/30
4
8
12
16
20
24
28
32
36
40
44
48
52
56
60
64
68
72
76
80
84
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108
112
116
120
124
128
132
136
140
144
148
152
156
160
164
168
172
176
180
184
188
192
196
200
204
208
212
216
220
224
228
232
236
240
244
248
252
256
Subnet
/25
/26
/27
/28
/29
/30
Mask
128
192
224
240
248
252
Subnets
2
4
8
16
32
64
Hosts
126
62
30
14
6
2
Block
128
64
32
16
8
4
Network
A
B
C
D
E
F
G
H
I
J
K
Hosts Block Subnet Mask
Variable Length Subnet Masks (VLSMs) 191
FIGURE5.8 VLSM design example 1
192.168.55.2/30
192.168.55.57/27
192.168.55.29/28
192.168.55.1/30
192.168.55.132/25
192.168.55.3/30
192.168.55.127/26
S0/0
Fa0/0
RouterA
S0/0:
7 hosts
Fa0/0
RouterB
90 hosts
Fa0/0
RouterC
23 hosts
FIGURE5.9 Solution to VLSM design example 1
192.168.55.2/30
192.168.55.57/27
192.168.55.29/28
192.168.55.1/30
192.168.55.132/25
192.168.55.3/30
192.168.55.127/26
S0/0
Fa0/0
RouterA
S0/0:
7 hosts
Fa0/0
RouterB
90 hosts
Fa0/0
RouterC
23 hosts
192.168.55.29/28 192.168.55.132/25
192.168.55.1/30
192.168.55.57/27
FIGURE5.10 VLSM design example 2
60 hosts
Net 1
30 hosts
Net 2
12 hosts
Net 3
4: Serial 1 5: Serial 2
192 Chapter 5 u VLSMs, Summarization, and Troubleshooting TCP/IP
FIGURE5.11 Solution to VLSM design example 2
1: 192.168.10.0/26
2: 192.168.10.64/27
3: 192.168.10.96/28
–chart cut in interest of brevity–
4: 192.168.10.112/30
5: 192.168.10.116/30
0
4
8
12
16
20
24
28
32
36
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48
52
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64
68
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96
100
104
108
112
116
120
124
128
Why Bother with VLSM Design?
You have just been hired by a new company and need to add on to their existing network.
There are no restrictions to prevent you from starting over with a completely new IP address
scheme. Should you use a VLSM classless network or opt for a classful network?
Let’s say you happen to have plenty of address space because you’re using the Class A
10.0.0.0 private network address, so you really can’t imagine that you’d ever run out of
IP addresses. So why would you want to bother with the VLSM design process in this
environment?
Good question! Here’s your answer…
By creating contiguous blocks of addresses to specific areas of your network, you can
then easily summarize the network and keep route updates with a routing protocol to a
minimum. Why would anyone want to advertise hundreds of networks between buildings
when you can just send one summary route between buildings and achieve the same
result? This approach will optimize the network’s performance dramatically!
Summarization 193
To make sure this is clear, let me take a second to explain summary routes. Summariza-
tion, also called supernetting, provides route updates in the most efficient way possible
by advertising many routes in one advertisement instead of individually. This saves a
ton of bandwidth and minimizes router processing. As always, you need to use blocks of
addresses to configure your summary routes and watch your network’s performance hum
along efficiently! And remember, block sizes are used in all sorts of networks anyway.
Still, it’s important to understand that summarization works only if you design your net-
work properly. If you carelessly hand out IP subnets to any location on the network, you’ll
quickly notice that you no longer have any summary boundaries. And you won’t get very
far creating summary routes without those, so watch your step!
Summarization
Summarization, also called route aggregation, allows routing protocols to advertise many
networks as one address. The purpose of this is to reduce the size of routing tables on routers
to save memory, which also shortens the amount of time IP requires to parse the routing table
when determining the best path to a remote network.
Figure5.12 shows how a summary address would be used in an internetwork.
FIGURE5.12 Summary address used in an internetwork
10.0.0.0/8
10.0.0.0/16
10.1.0.0/16
10.2.0.0/16
10.255.0.0/16
Summarization is pretty straightforward because all you really need to have down is a
solid understanding of the block sizes we’ve been using for subnetting and VLSM design.
For example, if you wanted to summarize the following networks into one network advertise-
ment, you just have to nd the block size rst, which will make it easy to nd your answer:
192.168.16.0 through network 192.168.31.0
Okay—so what’s the block size? Well, there are exactly 16 Class C networks, which t
neatly into a block size of 16.
Now that we’ve determined the block size, we just need to nd the network address and
mask used to summarize these networks into one advertisement. The network address used
to advertise the summary address is always the rst network address in the block—in this
example, 192.168.16.0. To gure out a summary mask, we just need to gure out which
mask will get us a block size of 16. If you came up with 240, you got it right! 240 would be
placed in the third octet, which is exactly the octet where we’re summarizing, so the mask
would be 255.255.240.0.
194 Chapter 5 u VLSMs, Summarization, and Troubleshooting TCP/IP
Here’s another example:
Networks 172.16.32.0 through 172.16.50.0
This isn’t as clean as the previous example because there are two possible answers. Here’s
why: Since you’re starting at network 32, your options for block sizes are 4, 8, 16, 32, 64,
etc., and block sizes of 16 and 32 could work as this summary address. Let’s explore your
two options:
u If you went with a block size of 16, then the network address would be 172.16.32.0
with a mask of 255.255.240.0 (240 provides a block of 16). The problem is that this
only summarizes from 32 to 47, which means that networks 48 through 50 would be
advertised as single networks. Even so, this could still be a good solution depending
on your network design.
u If you decided to go with a block size of 32 instead, then your summary address would
still be 172.16.32.0, but the mask would be 255.255.224.0 (224 provides a block of 32).
The possible problem with this answer is that it will summarize networks 32 through 63
and we only have networks 32 to 50. No worries if you’re planning on adding networks
51 to 63 later into the same network, but you could have serious problems in your inter-
network if somehow networks 51 to 63 were to show up and be advertised from some-
where else in your network! So even though this option does allow for growth, it’s a lot
safer to go with option #1.
Let’s take a look at another example: Your summary address is 192.168.144.0/20, so
what’s the range of host addresses that would be forwarded according to this summary?
The /20 provides a summary address of 192.168.144.0 and mask of 255.255.240.0.
The third octet has a block size of 16, and starting at summary address 144, the next
block of 16 is 160, so your network summary range is 144 to 159 in the third octet. This
is why it comes in handy to be able to count in 16s!
A router with this summary address in the routing table will forward any packet having
destination IP addresses of 192.168.144.1 through 192.168.159.254.
Only two more summarization examples, then we’ll move on to troubleshooting.
In summarization example 4, Figure5.13, the Ethernet networks connected to router R1
are being summarized to R2 as 192.168.144.0/20. Which range of IP addresses will R2 for-
ward to R1 according to this summary?
FIGURE5.13 Summarization example 4. The Ethernet networks connected to
router R1 are being summarized to R2 as 192.168.144.0/20. Which IP addresses will R2
forward to R1 according to this summary?
192.168.144.0/20
R1 R2
Summarization 195
No worries—solving this is easier than it looks initially. The question actually has the
summary address listed in it: 192.168.144.0/20. You already know that /20 is 255.255.240.0,
which means you’ve got a block size of 16 in the third octet. Starting at 144, which is also
right there in the question, makes the next block size of 16 equal 160. You can’t go above
159 in the third octet, so the IP addresses that will be forwarded are 192.168.144.1 through
192.168.159.254.
Okay, last one. In Figure5.14, there are ve networks connected to router R1. What’s
the best summary address to R2?
FIGURE5.14 Summarization example 5
What is the best summary to R2?
R2
172.1.4.0/25
172.1.4.128/25
172.1.5.0/24
172.1.6.0/24
172.1.7.0/24
R1
I’ll be honest with you—this is a much harder question than the one in Figure5.13, so
you’re going to have to look carefully to see the answer. A good approach here would be to
write down all the networks and see if you can nd anything in common with all of them:
u 172.1.4.128/25
u 172 .1.7.0/ 24
u 172.1.6.0/24
u 172.1.5.0/24
u 172.1.4.0/25
Do you see an octet that looks interesting to you? I do. It’s the third octet. 4, 5, 6, 7, and
yes, it’s a block size of 4. So you can summarize 172.1.4.0 using a mask of 255.255.252.0,
meaning you would use a block size of 4 in the third octet. The IP addresses forwarded
with this summary would be 172.1.4.1 through 172.1.7.254.
To summarize the summarization section, if you’ve nailed down your block sizes, then
nding and applying summary addresses and masks is a relatively straightforward task. But
you’re going to get bogged down pretty quickly if you don’t know what a /20 is or if you
can’t count by 16s! Visit ccna
.gg/ch5/a
for a
companion
MicroNugget
from CBT
Nuggets.
196 Chapter 5 u VLSMs, Summarization, and Troubleshooting TCP/IP
Troubleshooting IP Addressing
Because running into trouble now and then in networking is a given, being able to trouble-
shoot IP addressing is clearly a vital skill. I’m not being negative here—just realistic. The
positive side to this is that if you’re the one equipped with the tools to diagnose and clear up
the inevitable trouble, you get to be the hero when you save the day! Even better? You can
usually x an IP network regardless of whether you’re on site or at home!
So this is where I’m going to show you the “Cisco way” of troubleshooting IP addressing.
Let’s use Figure5.15 as an example of your basic IP trouble—poor Sally can’t log in to the
Windows server. Do you deal with this by calling the Microsoft team to tell them their server
is a pile of junk and causing all your problems? Though tempting, a better approach is to rst
double-check and verify your network instead.
FIGURE5.15 Basic IP troubleshooting
E0
172.16.10.1
Sally
172.16.10.2 Server
172.16.20.2
Okay, let’s get started by going through the troubleshooting steps that Cisco recommends.
They’re pretty simple, but important nonetheless. Pretend you’re at a customer host and they’re
complaining that they can’t communicate to a server that just happens to be on a remote net-
work. Here are the four troubleshooting steps Cisco recommends:
1. Open a Command window and ping 127.0.0.1. This is the diagnostic, or loopback,
address, and if you get a successful ping, your IP stack is considered initialized. If it
fails, then you have an IP stack failure and need to reinstall TCP/IP on the host.
C:\>ping 127.0.0.1
Pinging 127.0.0.1 with 32 bytes of data:
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Ping statistics for 127.0.0.1:
Troubleshooting IP Addressing 197
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
2. From the Command window, ping the IP address of the local host (we’ll assume cor-
rect configuration here, but always check the IP configuration too!). If that’s successful,
your network interface card (NIC) is functioning. If it fails, there is a problem with the
NIC. Success here doesn’t just mean that a cable is plugged into the NIC, only that the
IP protocol stack on the host can communicate to the NIC via the LAN driver.
C:\>ping 172.16.10.2
Pinging 172.16.10.2 with 32 bytes of data:
Reply from 172.16.10.2: bytes=32 time<1ms TTL=128
Reply from 172.16.10.2: bytes=32 time<1ms TTL=128
Reply from 172.16.10.2: bytes=32 time<1ms TTL=128
Reply from 172.16.10.2: bytes=32 time<1ms TTL=128
Ping statistics for 172.16.10.2:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
3. From the CMD window, ping the default gateway (router). If the ping works, it means
that the NIC is plugged into the network and can communicate on the local network.
If it fails, you have a local physical network problem that could be anywhere from the
NIC to the router.
C:\>ping 172.16.10.1
Pinging 172.16.10.1 with 32 bytes of data:
Reply from 172.16.10.1: bytes=32 time<1ms TTL=128
Reply from 172.16.10.1: bytes=32 time<1ms TTL=128
Reply from 172.16.10.1: bytes=32 time<1ms TTL=128
Reply from 172.16.10.1: bytes=32 time<1ms TTL=128
Ping statistics for 172.16.10.1:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
4. If steps 1 through 3 were successful, try to ping the remote server. If that works, then
you know that you have IP communication between the local host and the remote
server. You also know that the remote physical network is working.
C:\>ping 172.16.20.2
Pinging 172.16.20.2 with 32 bytes of data:
198 Chapter 5 u VLSMs, Summarization, and Troubleshooting TCP/IP
Reply from 172.16.20.2: bytes=32 time<1ms TTL=128
Reply from 172.16.20.2: bytes=32 time<1ms TTL=128
Reply from 172.16.20.2: bytes=32 time<1ms TTL=128
Reply from 172.16.20.2: bytes=32 time<1ms TTL=128
Ping statistics for 172.16.20.2:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
If the user still can’t communicate with the server after steps 1 through 4 have been
completed successfully, you probably have some type of name resolution problem and need
to check your Domain Name System (DNS) settings. But if the ping to the remote server
fails, then you know you have some type of remote physical network problem and need to
go to the server and work through steps 1 through 3 until you nd the snag.
Before we move on to determining IP address problems and how to x them, I just want
to mention some basic commands that you can use to help troubleshoot your network from
both a PC and a Cisco router. Keep in mind that though these commands may do the same
thing, they’re implemented differently.
ping Uses ICMP echo request and replies to test if a node IP stack is initialized and alive
on the network.
traceroute Displays the list of routers on a path to a network destination by using TTL
time-outs and ICMP error messages. This command will not work from a command prompt.
tracert Same function as traceroute, but it’s a Microsoft Windows command and will
not work on a Cisco router.
arp -a Displays IP-to-MAC-address mappings on a Windows PC.
show ip arp Same function as arp -a, but displays the ARP table on a Cisco router. Like
the commands traceroute and tracert, arp -a and show ip arp are not interchangeable
through DOS and Cisco.
ipconfig /all Used only from a Windows command prompt; shows you the PC network
conguration.
Once you’ve gone through all these steps and, if necessary, used the appropriate com-
mands, what do you do when you nd a problem? How do you go about xing an IP address
conguration error? Time to cover the next step—determining and xing the issue at hand!
Determining IP Address Problems
It’s common for a host, router, or other network device to be congured with the wrong
IP address, subnet mask, or default gateway. Because this happens way too often, you
must know how to nd and x IP address conguration errors.
A good way to start is to draw out the network and IP addressing scheme. If that’s
already been done, consider yourself lucky because though sensible, it’s rarely done. Even
Troubleshooting IP Addressing 199
if it is, it’s usually outdated or inaccurate anyway. So either way, it’s a good idea to bite the
bullet and start from scratch.
I’ll show you how a great way to draw out your network using the Cisco
Discovery Protocol (CDP) soon, in Chapter 7, “Managing and Trouble-
shooting a Cisco Internetwork.”
Once you have your network accurately drawn out, including the IP addressing scheme,
you need to verify each host’s IP address, mask, and default gateway address to establish
the problem. Of course, this is assuming that you don’t have a physical layer problem, or if
you did, that you’ve already xed it.
Let’s check out the example illustrated in Figure5.16. A user in the sales department
calls and tells you that she can’t get to ServerA in the marketing department. You ask her
if she can get to ServerB in the marketing department, but she doesn’t know because she
doesn’t have rights to log on to that server. What do you do?
First, guide your user through the four troubleshooting steps you learned in the pre-
ceding section. Okay—let’s say steps 1 through 3 work but step 4 fails. By looking at the
gure, can you determine the problem? Look for clues in the network drawing. First, the
WAN link between the Lab_A router and the Lab_B router shows the mask as a /27. You
should already know that this mask is 255.255.255.224 and determine that all networks
are using this mask. The network address is 192.168.1.0. What are our valid subnets and
hosts? 256 – 224 = 32, so this makes our subnets 0, 32, 64, 96, 128, etc. So, by looking at
the gure, you can see that subnet 32 is being used by the sales department. The WAN link
is using subnet 96, and the marketing department is using subnet 64.
FIGURE5.16 IP address problem 1
Lab A Lab B
192.168.1.33
Default gateway:
192.168.1.62
Fa0/0 192.168.1.95
192.168.1.100/27
DCE
S0/0
Fa0/0 192.168.1.62
S0/0
192.168.1.97/27
ServerA
192.168.1.66
Default gateway:
192.168.1.95
ServerB
192.168.1.65
Default gateway:
192.168.1.95
Sales Marketing
200 Chapter 5 u VLSMs, Summarization, and Troubleshooting TCP/IP
Now you’ve got to establish what the valid host ranges are for each subnet. From what
you learned at the beginning of this chapter, you should now be able to easily determine the
subnet address, broadcast addresses, and valid host ranges. The valid hosts for the Sales LAN
are 33 through 62, and the broadcast address is 63 because the next subnet is 64, right? For
the Marketing LAN, the valid hosts are 65 through 94 (broadcast 95), and for the WAN link,
97 through 126 (broadcast 127). By closely examining the gure, you can determine that the
default gateway on the Lab_B router is incorrect. That address is the broadcast address for
subnet 64, so there’s no way it could be a valid host!
If you tried to configure that address on the Lab_B router interface, you’d
receive a “bad mask error.” Cisco routers don’t let you type in subnet and
broadcast addresses as valid hosts!
Did you get all that? Let’s try another one to make sure. Figure5.17 shows a network
problem. A user in the Sales LAN can’t get to ServerB. You have the user run through the
four basic troubleshooting steps and nd that the host can communicate to the local net-
work but not to the remote network. Find and dene the IP addressing problem.
If you went through the same steps used to solve the last problem, you can see that rst,
the WAN link again provides the subnet mask to use— /29, or 255.255.255.248. Assuming
classful addressing, you need to determine what the valid subnets, broadcast addresses, and
valid host ranges are to solve this problem.
FIGURE5.17 IP address problem 2
Lab A Lab B
192.168.1.25
Default gateway:
192.168.1.30
Fa0/0 192.168.1.81
192.168.1.46/29
DCE
S0/0
Fa0/0 192.168.1.30
S0/0
192.168.1.41/29
ServerA
192.168.1.86
Default gateway:
192.168.1.81
ServerB
192.168.1.87
Default gateway:
192.168.1.81
Sales Marketing
Troubleshooting IP Addressing 201
The 248 mask is a block size of 8 (256 – 248 = 8, as discussed in Chapter 4), so the sub-
nets both start and increment in multiples of 8. By looking at the gure, you see that the Sales
LAN is in the 24 subnet, the WAN is in the 40 subnet, and the Marketing LAN is in the
80 subnet. Can you see the problem yet? The valid host range for the Sales LAN is 25–30,
and the conguration appears correct. The valid host range for the WAN link is 41–46, and
this also appears correct. The valid host range for the 80 subnet is 81–86, with a broadcast
address of 87 because the next subnet is 88. ServerB has been congured with the broadcast
address of the subnet.
Okay, now that you can gure out miscongured IP addresses on hosts, what do you
do if a host doesn’t have an IP address and you need to assign one? What you need to do
is scrutinize the other hosts on the LAN and gure out the network, mask, and default
gateway. Let’s take a look at a couple of examples of how to nd and apply valid IP
addresses to hosts.
You need to assign a server and router IP addresses on a LAN. The subnet assigned on
that segment is 192.168.20.24/29. The router needs to be assigned the rst usable address
and the server needs the last valid host ID. What is the IP address, mask, and default gate-
way assigned to the server?
To answer this, you must know that a /29 is a 255.255.255.248 mask, which provides
a block size of 8. The subnet is known as 24, the next subnet in a block of 8 is 32, so the
broadcast address of the 24 subnet is 31 and the valid host range is 25–30.
Server IP address: 192.168.20.30
Server mask: 255.255.255.248
Default gateway: 192.168.20.25 (router’s IP address)
Take a look at Figure5.18 and solve this problem.
FIGURE5.18 Find the valid host #1
Router A
E0: 192.168.10.33/27
Look at the router’s IP address on Ethernet0. What IP address, subnet mask, and valid
host range could be assigned to the host?
The IP address of the router’s Ethernet0 is 192.168.10.33/27. As you already know, a
/27 is a 224 mask with a block size of 32. The router’s interface is in the 32 subnet. The
next subnet is 64, so that makes the broadcast address of the 32 subnet 63 and the valid
host range 33–62.
202 Chapter 5 u VLSMs, Summarization, and Troubleshooting TCP/IP
Host IP address: 192.168.10.34–62 (any address in the range except for 33, which is
assigned to the router)
Mask: 255.255.255.224
Default gateway: 192.168.10.33
Figure5.19 shows two routers with Ethernet congurations already assigned. What are
the host addresses and subnet masks of HostA and HostB?
FIGURE5.19 Find the valid host #2
E0: 192.168.10.65/26
HostA HostB
Router A
E0: 192.168.10.33/28
Router B
RouterA has an IP address of 192.168.10.65/26 and RouterB has an IP address
of 192.168.10.33/28. What are the host congurations? RouterA Ethernet0 is in the
192.168.10.64 subnet and RouterB Ethernet0 is in the 192.168.10.32 network.
Host A IP address: 192.168.10.66–126
Host A mask: 255.255.255.192
Host A default gateway: 192.168.10.65
Host B IP address: 192.168.10.34–46
Host B mask: 255.255.255.240
Host B default gateway: 192.168.10.33
Just a couple more examples before you can put this chapter behind you—hang in there!
Figure5.20 shows two routers. You need to congure the S0/0 interface on RouterA.
The network assigned to the serial link is 172.16.17.0/22. What IP address can be assigned?
FIGURE5.20 Find the valid host address #3
172.16.17.0/22
RouterA
S0/0 S0/0
RouterB
Summary 203
First, know that a /22 CIDR is 255.255.252.0, which makes a block size of 4 in the
third octet. Since 17 is listed, the available range is 16.1 through 19.254, so in this example,
the IP address S0/0 could be 172.16.18.255 since that’s within the range.
Okay, last one! You need to nd a classful network add that has one Class C network
ID and you need to provide one usable subnet per city while allowing enough usable host
addresses for each city specied in Figure5.21. What is your mask?
FIGURE5.21 Find the valid subnet mask.
N.Y.
7 Users
Corporate
7 Users
S.F.
13 Users
L.A.
15 Users
Wy.
16 Users
Actually, this is probably the easiest thing you’ve done all day! I count 5 subnets
needed, and the Wyoming ofce needs 16 users—always look for the network that needs
the most hosts! What block size is needed for the Wyoming ofce? Your answer is 32.
You can’t use a block size of 16 because you always have to subtract 2. What mask pro-
vides you with a block size of 32? 224 is your answer because this provides 8 subnets,
each with 30 hosts.
You’re done—the diva has sung and the chicken has safely crossed the road…whew!
Time to take a break, but skip the shot and the beer if that’s what you had in mind
because you need to have your head straight to go through the written lab and review
questions next!
Summary
Again, if you got to this point without getting lost along the way a few times, you’re awesome,
but if you did get lost, don’t stress because most people do! Just be patient with yourself and go
back over the material that tripped you up until it’s all crystal clear. You’ll get there!
This chapter provided you with keys to understanding the oh-so-very-important topic of
variable length subnet masks. You should also know how to design and implement simple
VLSM networks and be clear on summarization as well.
And make sure you understand and memorize Cisco’s troubleshooting methods. You
must remember the four steps that Cisco recommends to take when trying to narrow down
exactly where a network and/or IP addressing problem is and then know how to proceed
systematically to x it. In addition, you should be able to nd valid IP addresses and subnet
masks by looking at a network diagram.
Visit ccna
.gg/ch5/b
for a
companion
MicroNugget
from CBT
Nuggets.
204 Chapter 5 u VLSMs, Summarization, and Troubleshooting TCP/IP
Exam Essentials
Describe the benefits of variable length subnet masks (VLSMs). VLSMs enable the creation
of subnets of specic sizes and allow the division of a classless network into smaller networks
that do not need to be equal in size. This makes use of the address space more efcient because
many times IP addresses are wasted with classful subnetting.
Understand the relationship between the subnet mask value and the resulting block size and
the allowable IP addresses in each resulting subnet. The relationship between the classful
network being subdivided and the subnet mask used determines the number of possible hosts
or the block size. It also determines where each subnet begins and ends and which IP addresses
cannot be assigned to a host within each subnet.
Describe the process of summarization or route aggregation and its relationship to
subnetting. Summarization is the combining of subnets derived from a classful network
for the purpose of advertising a single route to neighboring routers instead of multiple
routes, reducing the size of routing tables and speeding the route process.
Calculate the summary mask that will advertise a single network representing all subnets.
The network address used to advertise the summary address is always the rst network
address in the block of subnets. The mask is the subnet mask value that yields the same
block size.
Remember the four diagnostic steps. The four simple steps that Cisco recommends for
troubleshooting are ping the loopback address, ping the NIC, ping the default gateway, and
ping the remote device.
Identify and mitigate an IP addressing problem. Once you go through the four trouble-
shooting steps that Cisco recommends, you must be able to determine the IP addressing
problem by drawing out the network and nding the valid and invalid hosts addressed in
your network.
Understand the troubleshooting tools that you can use from your host and a Cisco router.
The ping 127.0.0.1 command tests your local IP stack, and tracert is a Windows com-
mand to track the path a packet takes through an internetwork to a destination. Cisco routers
use the command traceroute, or just trace for short. Don’t confuse the Windows and Cisco
commands. Although they produce the same output, they don’t work from the same prompts.
The command ipconfig /all will display your PC network conguration from a DOS
prompt, and arp -a (again from a DOS prompt) will display IP-to-MAC-address mapping
on a Windows PC.
Written Lab 5 205
Written Lab 5
The answers to this lab can be found in Appendix A, “Answers to Written Labs.”
For each of the following sets of networks, determine the summary address and the
mask to be used that will summarize the subnets.
1. 192.168.1.0/24 through 192.168.12.0/24
2. 172.144.0.0 through 172.159.0.0
3. 192.168.32.0 through 192.168.63.0
4. 192.168.96.0 through 192.168.111.0
5. 66.66.0.0 through 66.66.15.0
6. 192.168.1.0 through 192.168.120.0
7. 172.16.1.0 through 172.16.7.0
8. 192.168.128.0 through 192.168.190.0
9. 53.60.96.0 through 53.60.127.0
10. 172.16.10.0 through 172.16.63.0
206 Chapter 5 u VLSMs, Summarization, and Troubleshooting TCP/IP
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional questions,
please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. On a VLSM network, which mask should you use on point-to-point WAN links in
order to reduce the waste of IP addresses?
A. /27
B. /28
C. /29
D. /30
E. /31
2. In the network shown in the diagram, how many computers could be in subnet B?
192.168.10.64/29
192.168.10.0/27
Network A Network B
.65
.1
.73
.66 .67
192.168.10.48/28
.50 .51
Network D Network E
.2 .3 .34 .35
.74
.33
.49
Network C
192.168.10.72/30
192.168.10.32/28
A. 6
B. 12
C. 14
D. 30
Review Questions 207
3. In the diagram below, in order to have as efficient IP addressing as possible, which
network should use a /29 mask?
30 hosts
Network B
20 hosts
Network C
6 hosts
Network D
14 hosts
Network A
Fa0/0
Fa0/0 Fa0/0
2 hosts
Network F
2 hosts
Network H
2 hosts
Network E
2 hosts
Network G
Lab D
Fa0/0
Lab E
Lab A Lab B
A. A
B. B
C. C
D. D
4. To use VLSM, what capability must the routing protocols in use possess?
A. Support for multicast
B. Multiprotocol support
C. Transmission of subnet mask information
D. Support for unequal load balancing
5. What summary address would cover all the networks shown and advertise a single,
efficient route to Router B that won’t advertise more networks than needed?
172.16.1.0/24
172.16.10.0/24
172.16.5.0/24
172.16.15.0/24
172.16.7.0/24
172.16.11.0/24
172.16.6.0/24
Router B
?
A. 172.16.0.0/24
B. 172.16.1.0/24
C. 172.16.0.0/24
D. 172.16.0.0/20
E. 172.16.16.0/28
F. 172.16.0.0/27
208 Chapter 5 u VLSMs, Summarization, and Troubleshooting TCP/IP
6. In the diagram below what is the most likely reason the station cannot ping outside of
its network?
RouterA
E0: 192.168.10.33/27
IP 192.168.10.28/27
Default gateway 192.168.10.33/27
A. The IP address is incorrect on E0 of the router.
B. The default gateway address is incorrect on the station.
C. The IP address on the station is incorrect.
D. The router is malfunctioning.
7. If Host A is configured with an incorrect default gateway and all other computers and the
router are known to be configured correctly, which of the following statements is TRUE?
A. Host A cannot communicate with the router.
B. Host A can communicate with other hosts in the same subnet.
C. Host A can communicate with hosts in other subnets.
D. Host A can communicate with no other systems.
8. Which of the following troubleshooting steps, if completed successfully, also confirms
the other steps will succeed as well?
A. ping a remote computer
B. ping the loopback address
C. ping the NIC
D. ping the default gateway
9. When a ping to the local host IP address fails, what can you assume?
A. The IP address of the local host is incorrect.
B. The IP address of the remote host is incorrect.
C. The NIC is not functional.
D. The IP stack has failed to initialize.
Review Questions 209
10. When a ping to the local host IP address succeeds but a ping to the default gateway
IP address fails, what can you rule out? (Choose all that apply.)
A. The IP address of the local host is incorrect.
B. The IP address of the gateway is incorrect.
C. The NIC is not functional.
D. The IP stack has failed to initialize.
11. Which of the networks in the diagram could use a /29 mask?
Corporate
7 Users
L.A.
15 Users
S.F.
13 Users
N.Y.
7 Users
Wy.
16 Users
A. Corporate
B. LA
C. SF
D. NY
E. none
12. What network service is the most likely problem if you can ping a computer by IP
address but not by name?
A. DNS
B. DHCP
C. ARP
D. ICMP
13. When you issue the ping command, what protocol are you using?
A. DNS
B. DHCP
C. ARP
D. ICMP
210 Chapter 5 u VLSMs, Summarization, and Troubleshooting TCP/IP
14. Which of the following commands displays the networks traversed on a path to a
network destination?
A. ping
B. traceroute
C. pingroute
D. pathroute
15. What command generated the output shown below?
Reply from 172.16.10.2: bytes=32 time<1ms TTL=128
Reply from 172.16.10.2: bytes=32 time<1ms TTL=128
Reply from 172.16.10.2: bytes=32 time<1ms TTL=128
Reply from 172.16.10.2: bytes=32 time<1ms TTL=128
A. traceroute
B. show ip route
C. ping
D. pathping
16. In the work area, match the command to its function on the right.
traceroute
arp -a
show ip arp
ipconfig /all
Displays the ARP table on a Cisco router
Displays the list of routers on a path to a network destination
Shows you the PC network configuration
Displays IP-to-MAC-address mappings on a Windows PC
17. Which of the following network addresses correctly summarizes the three networks
shown below efficiently?
10.0.0.0/16
10.1.0.0/16
10.2.0.0/16
A. 10.0.0.0/15
B. 10.1.0.0/8
C. 10.0.0.0/14
D. 10.0.0.8/16
Review Questions 211
18. What command displays the ARP table on a Cisco router?
A. show ip arp
B. traceroute
C. arp -a
D. tracert
19. What switch must be added to the ipconfig command on a PC to verify DNS
configuration?
A. /dns
B. -dns
C. /all
D. showall
20. Which of the following is the best summarization of the following networks:
192.168.128.0 through 192.168.159.0
A. 192.168.0.0/24
B. 192.168.128.0/16
C. 192.168.128.0/19
D. 192.168.128.0/20
Chapter
6
Cisco’s
Internetworking
Operating
System (IOS)
THE FOLLOWING ICND1 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 LAN Switching Technologies
■Configure and verify initial switch configuration including
remote access management.
■Cisco IOS commands to perform basic switch setup
1 IP Routing Technologies
■Configure and verify utilizing the CLI to set basic Router con-
figuration
■Cisco IOS commands to perform basic router setup
■Configure and verify operation status of an ethernet interface
■Verify router configuration and network connectivity
■Cisco IOS commands to review basic router information
and network connectivity
1 Network Device Security
■Configure and verify network device security features such as
■Device password security
■Enable secret vs enable
■Transport
■Disable telnet
■SSH
■VTYs
■Physical security
■Service password
■Describe external authentication methods
1 Troubleshooting
■Troubleshoot and resolve Layer 1 problems
■Framing
■CRC
■Runts
■Giants
■Dropped packets
■Late collision
■Input / Output errors
It’s time to introduce you to the Cisco Internetwork
Operating System (IOS). The IOS is what runs Cisco
routers as well as Cisco’s switches, and it’s also what
we use to congure these devices.
So that’s what you’re going to learn about in this chapter. I’m going to show you how to
congure a Cisco IOS device using the Cisco IOS command-line interface (CLI). Once pro-
cient with this interface, you’ll be able to congure hostnames, banners, passwords, and
more as well as troubleshoot skillfully using the Cisco IOS.
We’ll also begin the journey to mastering the basics of router and switch congurations
plus command verications in this chapter.
I’ll start with a basic IOS switch to begin building the network we’ll use throughout
this book for conguration examples. Don’t forget—I’ll be using both switches and routers
throughout this chapter, and we congure these devices pretty much the same way. Things
diverge when we get to the interfaces where the differences between the two become key, so
pay attention closely when we get to that point!
Just as it was with preceding chapters, the fundamentals presented in this chapter are
important building blocks to have solidly in place before moving on to the more advanced
material coming up in the next ones.
To find up-to-the minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
The IOS User Interface
The Cisco Internetwork Operating System (IOS) is the kernel of Cisco routers as well as
all current Catalyst switches. In case you didn’t know, a kernel is the elemental, indispens-
able part of an operating system that allocates resources and manages tasks like low-level
hardware interfaces and security.
Coming up, I’ll show you the Cisco IOS and how to congure a Cisco switch using the
command-line interface (CLI). The congurations you’ll see in this chapter are exactly
the same as they are on a Cisco router.
216 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
Cisco IOS
The Cisco IOS is a proprietary kernel that provides routing, switching, internetworking,
and telecommunications features. The rst IOS was written by William Yeager in 1986 and
enabled networked applications. It runs on most Cisco routers as well as a growing number
of Cisco Catalyst switches, like the Catalyst 2960 and 3560 series switches used in this
book. And it’s an essential for the Cisco exam objectives!
Here’s a short list of some important things that the Cisco router IOS software is
responsible for:
u Carrying network protocols and functions
u Connecting high-speed traffic between devices
u Adding security to control access and stop unauthorized network use
u Providing scalability for ease of network growth and redundancy
u Supplying network reliability for connecting to network resources
You can access the Cisco IOS through the console port of a router or switch, from a
modem into the auxiliary (or aux) port on a router, or even through Telnet and Secure
Shell (SSH). Access to the IOS command line is called an EXEC session.
Connecting to a Cisco IOS Device
We connect to a Cisco device to congure it, verify its conguration, and check statis-
tics, and although there are different approaches to this, the rst place you would usu-
ally connect to is the console port. The console port is usually an RJ-45, 8-pin modular
connection located at the back of the device, and there may or may not be a password
set on it by default.
Look back into Chapter 2, “Ethernet Networking and Data Encapsulation,”
to review how to configure a PC and enable it to connect to a router
console port.
You can also connect to a Cisco router through an auxiliary port, which is really the
same thing as a console port, so it follows that you can use it as one. The main difference
with an auxiliary port is that it also allows you to congure modem commands so that a
modem can be connected to the router. This is a cool feature because it lets you dial up a
remote router and attach to the auxiliary port if the router is down and you need to cong-
ure it remotely, out-of-band. One of the differences between Cisco routers and switches is
that switches do not have an auxiliary port.
The third way to connect to a Cisco device is in-band, through the program Telnet or
Secure Shell (SSH). In-band means conguring the device via the network, the opposite of
out-of-band. We covered Telnet and SSH back in Chapter 3, and in this chapter, I’ll show
you how to congure access to both of these protocols on a Cisco device.
The IOS User Interface 217
Figure6.1 shows an illustration of a Cisco 2960 switch. Really focus in on all the differ-
ent kinds of interfaces and connections! On the right side is the 10/100/1000 uplink. You
can use either the UTP port or the ber port, but not both at the same time.
FIGURE6.1 A Cisco 2960 switch
The 3560 switch I’ll be using in this book looks a lot like the 2960, but it can perform
layer 3 switching, unlike the 2960, which is limited to only layer 2 functions.
I also want to take a moment and tell you about the 2800 series router because that’s
the router series I’ll be using in this book. This router is known as an Integrated Services
Router (ISR) and Cisco has updated this to the 2900 series but I still have plenty of 2800
series routers in my production networks. Figure6.2 shows a new 1900 series router. The
new ISR series of routers are nice; they get their name because many services, like security,
are built into it. It’s a modular device, much faster and a lot sleeker than the older 2600
series routers, and it’s elegantly designed to support a broad new range of interface options.
The new ISR series router can offer multiple serial interfaces, which can be used for con-
necting a T1 using a serial V.35 WAN connection. And multiple Fast Ethernet or Gigabit
Ethernet ports can be used on the router, depending on the model. This router also has one
console via an RJ-45 connector and another through the USB port. There is also an auxil-
iary connection to allow a console connection via a remote modem.
FIGURE6.2 A new Cisco 1900 router
You need to keep in mind that for the most part, you get some serious bang for your
buck with the 2800/2900—unless you start adding a bunch of interfaces to it. You’ve got
to pony up for each one of those little beauties, so this can really start to add up and fast!
A couple of other series of routers that will set you back a lot less than the 2800 series
are the 1800/1900s, so look into these routers if you want a less-expensive alternative to
the 2800/2900 but still want to run the same IOS.
So even though I’m going to be using mostly 2800 series routers and 2960/3560 switches
throughout this book to demonstrate examples of IOS congurations, I want to point out
that the particular router model you use to practice for the Cisco exam isn’t really important.
The switch types are, though—you denitely need 2950 or 2960 switches, as well as a 3560
switch if you want to measure up to the exam objectives!
218 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
You can find more information about all Cisco routers at www.cisco.com/
en/US/products/hw/routers/index.html.
Bringing Up a Switch
When you rst bring up a Cisco IOS device, it will run a power-on self-test—a POST. Upon
passing that, the machine will look for and then load the Cisco IOS from ash memory if an
IOS le is present, then expand it into RAM. As you probably know, ash memory is elec-
tronically erasable programmable read-only memory—an EEPROM. The next step is for the
IOS to locate and load a valid conguration known as the startup-cong that will be stored
in nonvolatile RAM (NVRAM).
Once the IOS is loaded and up and running, the startup-cong will be copied from
NVRAM into RAM and from then on referred to as the running-cong.
But if a valid startup-cong isn’t found in NVRAM, your switch will enter setup mode,
giving you a step-by-step dialog to help congure some basic parameters on it.
You can also enter setup mode at any time from the command line by typing the command
setup from privileged mode, which I’ll get to in a minute. Setup mode only covers some basic
commands and generally isn’t really all that helpful. Here’s an example:
Would you like to enter the initial configuration dialog? [yes/no]: y
At any point you may enter a question mark ‘?’ for help.
Use ctrl-c to abort configuration dialog at any prompt.
Default settings are in square brackets ‘[]’.
Basic management setup configures only enough connectivity
for management of the system, extended setup will ask you
to configure each interface on the system
Would you like to enter basic management setup? [yes/no]: y
Configuring global parameters:
Enter host name [Switch]: Ctrl+C
Configuration aborted, no changes made.
You can exit setup mode at any time by pressing Ctrl+C.
Command-line Interface (CLI) 219
I highly recommend going through setup mode once, then never again because you
should always use the CLI instead!
Command-line Interface (CLI)
I sometimes refer to the CLI as “cash line interface” because the ability to create advanced
congurations on Cisco routers and switches using the CLI will earn you some decent cash!
Entering the CLI
After the interface status messages appear and you press Enter, the Switch> prompt will
pop up. This is called user exec mode, or user mode for short, and although it’s mostly used
to view statistics, it is also a stepping stone along the way to logging in to privileged exec
mode, called privileged mode for short.
You can view and change the conguration of a Cisco router only while in privileged
mode, and you enter it via the enable command like this:
Switch>enable
Switch#
The Switch# prompt signals you’re in privileged mode where you can both view and
change the switch conguration. You can go back from privileged mode into user mode by
using the disable command:
Switch#disable
Switch>
You can type logout from either mode to exit the console:
Switch>logout
Switch con0 is now available
Press RETURN to get started.
Next, I’ll show how to perform some basic administrative congurations.
Overview of Router Modes
To congure from a CLI, you can make global changes to the router by typing configure
terminal or just config t. This will get you into global conguration mode where you can
make changes to the running-cong. Commands run from global conguration mode are
predictably referred to as global commands, and they are typically set only once and affect
the entire router.
220 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
Type config from the privileged-mode prompt and then press Enter to opt for the default
of terminal like this:
Switch#config
Configuring from terminal, memory, or network [terminal]? [press enter]
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)#
At this point, you make changes that affect the router as a whole (globally), hence the
term global conguration mode. For instance, to change the running-cong—the current
conguration running in dynamic RAM (DRAM)—use the configure terminal command,
as I just demonstrated.
CLI Prompts
Let’s explore the different prompts you’ll encounter when conguring a switch or router now,
because knowing them well will really help you orient yourself and recognize exactly where
you are at any given time while in conguration mode. I’m going to demonstrate some of the
prompts used on a Cisco switch and cover the various terms used along the way. Make sure
you’re very familiar with them, and always check your prompts before making any changes to
a router’s conguration!
We’re not going to venture into every last obscure command prompt you could potentially
come across in the conguration mode world because that would get us deep into territory
that’s beyond the scope of this book. Instead, I’m going to focus on the prompts you absolutely
must know to pass the exam plus the very handy and seriously vital ones you’ll need and use
the most in real-life networking—the cream of the crop.
Don’t freak! It’s not important that you understand exactly what each of
these command prompts accomplishes just yet because I’m going to com-
pletely fill you in on all of them really soon. For now, relax and focus on just
becoming familiar with the different prompts available and all will be well!
Interfaces
To make changes to an interface, you use the interface command from global congura-
tion mode:
Switch(config)#interface ?
Async Async interface
BVI Bridge-Group Virtual Interface
CTunnel CTunnel interface
Dialer Dialer interface
Command-line Interface (CLI) 221
FastEthernet FastEthernet IEEE 802.3
Filter Filter interface
Filtergroup Filter Group interface
GigabitEthernet GigabitEthernet IEEE 802.3z
Group-Async Async Group interface
Lex Lex interface
Loopback Loopback interface
Null Null interface
Port-channel Ethernet Channel of interfaces
Portgroup Portgroup interface
Pos-channel POS Channel of interfaces
Tunnel Tunnel interface
Vif PGM Multicast Host interface
Virtual-Template Virtual Template interface
Virtual-TokenRing Virtual TokenRing
Vlan Catalyst Vlans
fcpa Fiber Channel
range interface range command
Switch(config)#interface fastEthernet 0/1
Switch(config-if)#)
Did you notice that the prompt changed to Switch(config-if)#? This tells you that you’re
in interface conguration mode. And wouldn’t it be nice if the prompt also gave you an indica-
tion of what interface you were conguring? Well, at least for now we’ll have to live without
the prompt information, because it doesn’t. But it should already be clear to you that you really
need to pay attention when conguring an IOS device!
Line Commands
To congure user-mode passwords, use the line command. The prompt then becomes
Switch(config-line)#:
Switch(config)#line ?
<0-16> First Line number
console Primary terminal line
vty Virtual terminal
Switch(config)#line console 0
Switch(config-line)#
The line console 0 command is a global command, and sometimes you’ll also hear
people refer to global commands as major commands. In this example, any command
typed from the (config-line) prompt is known as a subcommand.
222 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
Access List Configurations
To congure a standard named access list, you’ll need to get to the prompt
Switch(config-std-nacl)#:
Switch#config t
Switch(config)#ip access-list standard Todd
Switch(config-std-nacl)#
What you see here is a typical basic standard ACL prompt. There are various ways to con-
gure access lists, and the prompts are only slightly different from this particular example.
Routing Protocol Configurations
I need to point out that we don’t use routing or router protocols on 2960 switches, but we
can and will use them on my 3560 switches. Here is an example of conguring routing on
a layer 3 switch:
Switch(config)#router rip
IP routing not enabled
Switch(config)#ip routing
Switch(config)#router rip
Switch(config-router)#
Did you notice that the prompt changed to Switch(config-router)#? To make sure you
achieve the objectives specic to the Cisco exam and this book, I’ll congure RIPv2, OSPF,
RIPng, and OSPFv3. And don’t worry—I’ll explain all of these in detail soon, in Chapter 8,
“IP Routing,” and Chapter 14, “Internet Protocol Version 6 (IPv6)”!
Defining Router Terms
Table6.1 denes some of the terms I’ve used so far.
TABLE6.1 Router terms
Mode Definition
User exec mode Limited to basic monitoring commands
Privileged exec mode Provides access to all other router commands
Global configuration mode Commands that affect the entire system
Specific configuration modes Commands that affect interfaces/processes only
Setup mode Interactive configuration dialog
Command-line Interface (CLI) 223
Editing and Help Features
The Cisco advanced editing features can also help you congure your router. If you type in
a question mark (?) at any prompt, you’ll be given a list of all the commands available from
that prompt:
Switch#?
Exec commands:
access-enable Create a temporary Access-List entry
access-template Create a temporary Access-List entry
archive manage archive files
cd Change current directory
clear Reset functions
clock Manage the system clock
cns CNS agents
configure Enter configuration mode
connect Open a terminal connection
copy Copy from one file to another
debug Debugging functions (see also 'undebug')
delete Delete a file
diagnostic Diagnostic commands
dir List files on a filesystem
disable Turn off privileged commands
disconnect Disconnect an existing network connection
dot1x IEEE 802.1X Exec Commands
enable Turn on privileged commands
eou EAPoUDP
erase Erase a filesystem
exit Exit from the EXEC
--More-- ?
Press RETURN for another line, SPACE for another page, anything else to quit
And if this is not enough information for you, you can press the spacebar to get another
whole page of information, or you can press Enter to go one command at a time. You can
also press Q, or any other key for that matter, to quit and return to the prompt. Notice that
I typed a question mark (?) at the more prompt and it told me what my options were from
that prompt.
Here’s a shortcut: To nd commands that start with a certain letter, use the letter and
the question mark with no space between them, like this:
Switch#c?
cd clear clock cns configure
224 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
connect copy
Switch#c
Okay, see that? By typing c?, I got a response listing all the commands that start with c.
Also notice that the Switch#c prompt reappears after the list of commands is displayed. This
can be really helpful when you happen to be working with long commands but you’re short
on patience and still need the next possible one. It would get old fast if you actually had to
retype the entire command every time you used a question mark!
So with that, let’s nd the next command in a string by typing the rst command and
then a question mark:
Switch#clock ?
set Set the time and date
Switch#clock set ?
hh:mm:ss Current Time
Switch#clock set 2:34 ?
% Unrecognized command
Switch#clock set 2:34:01 ?
<1-31> Day of the month
MONTH Month of the year
Switch#clock set 2:34:01 21 july ?
<1993-2035> Year
Switch#clock set 2:34:01 21 august 2013
Switch#
00:19:45: %SYS-6-CLOCKUPDATE: System clock has been updated from 00:19:45 UTC Mon
Mar 1 1993 to 02:34:01 UTC Wed Aug 21 2013, configured from console by console.
I entered the clock ? command and got a list of the next possible parameters plus what
they do. Make note of the fact that you can just keep typing a command, a space, and then
a question mark until <cr> (carriage return) is your only option left.
And if you’re typing commands and receive
Switch#clock set 11:15:11
% Incomplete command.
no worries—that’s only telling you that the command string simply isn’t complete quite yet.
All you need to do is to press the up arrow key to redisplay the last command entered and
then continue with the command by using your question mark.
Command-line Interface (CLI) 225
But if you get the error
Switch(config)#access-list 100 permit host 1.1.1.1 host 2.2.2.2
^
% Invalid input detected at '^' marker.
all is not well because it means you actually have entered a command incorrectly. See that
little caret—the ^? It’s a very helpful tool that marks the exact point where you blew it and
made a mess.
Here’s another example of when you’ll see that caret:
Switch#sh fastethernet 0/0
^
% Invalid input detected at ‘^’ marker.
This command looks right, but be careful! The problem is that the full command is show
interface fastethernet 0/0.
Now if you receive the error
Switch#sh cl
% Ambiguous command: "sh cl"
you’re being told that there are multiple commands that begin with the string you entered
and it’s not unique. Use the question mark to nd the exact command you need:
Switch#sh cl?
class-map clock cluster
Case in point: There are three commands that start with show cl.
Table6.2 lists the enhanced editing commands available on a Cisco router.
TABLE6.2 Enhanced editing commands
Command Meaning
Ctrl+A Moves your cursor to the beginning of the line
Ctrl+E Moves your cursor to the end of the line
Esc+B Moves back one word
Ctrl+B Moves back one character
Ctrl+F Moves forward one character
Esc+F Moves forward one word
226 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
Command Meaning
Ctrl+D Deletes a single character
Backspace Deletes a single character
Ctrl+R Redisplays a line
Ctrl+U Erases a line
Ctrl+W Erases a word
Ctrl+Z Ends configuration mode and returns to EXEC
Tab Finishes typing a command for you
Another really cool editing feature you need to know about is the automatic scrolling
of long lines. In the following example, the command I typed reached the right margin and
automatically moved 11 spaces to the left. How do I know this? Because the dollar sign [$]
is telling me that the line has been scrolled to the left:
Switch#config t
Switch(config)#$ 100 permit ip host 192.168.10.1 192.168.10.0 0.0.0.255
You can review the router-command history with the commands shown in Table6.3.
TABLE6.3 Router-command history
Command Meaning
Ctrl+P or up arrow Shows last command entered
Ctrl+N or down arrow Shows previous commands entered
show history Shows last 20 commands entered by default
show terminal Shows terminal configurations and history buffer size
terminal history size Changes buffer size (max 256)
TABLE 6.2 Enhanced editing commands
(continued)
Command-line Interface (CLI) 227
The following example demonstrates the show history command as well as how to change
the history’s size. It also shows how to verify the history with the show terminal command.
First, use the show history command, which will allow you to see the last 20 commands
that were entered on the router (even though my particular router reveals only 10 commands
because that’s all I’ve entered since rebooting it). Check it out:
Switch#sh history
sh fastethernet 0/0
sh ru
sh cl
config t
sh history
sh flash
sh running-config
sh startup-config
sh ver
sh history
Okay—now, we’ll use the show terminal command to verify the terminal history size:
Switch#sh terminal
Line 0, Location: "", Type: ""
Length: 24 lines, Width: 80 columns
Baud rate (TX/RX) is 9600/9600, no parity, 2 stopbits, 8 databits
Status: PSI Enabled, Ready, Active, Ctrl-c Enabled, Automore On
0x40000
Capabilities: none
Modem state: Ready
[output cut]
Modem type is unknown.
Session limit is not set.
Time since activation: 00:17:22
Editing is enabled.
History is enabled, history size is 10.
DNS resolution in show commands is enabled
Full user help is disabled
Allowed input transports are none.
Allowed output transports are telnet.
Preferred transport is telnet.
No output characters are padded
No special data dispatching characters
228 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
When Should I Use the Cisco Editing Features?
You’ll find yourself using a couple of editing features quite often and some not so much,
if at all. Understand that Cisco didn’t make these up; these are just old Unix commands!
Even so, Ctrl+A is still a really helpful way to negate a command.
For example, if you were to put in a long command and then decide you didn’t want to
use that command in your configuration after all, or if it didn’t work, then you could just
press your up arrow key to show the last command entered, press Ctrl+A, type no and
then a space, press Enter—and poof! The command is negated. This doesn’t work on
every command, but it works on a lot of them and saves some serious time!
Administrative Configurations
Even though the following sections aren’t critical to making a router or switch work on a
network, they’re still really important. I’m going to guide you through conguring specic
commands that are particularly helpful when administering your network.
You can congure the following administrative functions on a router and switch:
u Hostnames
u Banners
u Passwords
u Interface descriptions
Remember, none of these will make your routers or switches work better or faster, but
trust me, your life will be a whole lot better if you just take the time to set these congura-
tions on each of your network devices. This is because doing so makes troubleshooting and
maintaining your network a great deal easier—seriously! In this next section, I’ll be dem-
onstrating commands on a Cisco switch, but understand that these commands are used in
the exact same way on a Cisco router.
Hostnames
We use the hostname command to set the identity of the router. This is only locally signi-
cant, meaning it doesn’t affect how the router performs name lookups or how the device
actually works on the internetwork. But the hostname is still important because it’s often
used for authentication in many wide area networks (WANs). Here’s an example:
Switch#config t
Switch(config)#hostname Todd
Administrative Configurations 229
Todd(config)#hostname Chicago
Chicago(config)#hostname Todd
Todd(config)#
I know it’s pretty tempting to congure the hostname after your own name, but it’s usually
a much better idea to name the router something that relates to its physical location. A name
that maps to where the device lives will make nding it a whole lot easier, which among other
things, conrms that you’re actually conguring the correct device. Even though it seems like
I’m completely ditching my own advice by naming mine Todd, I’m not, because this particu-
lar device really does live in “Todd’s” ofce. Its name perfectly maps to where it is, so it won’t
be confused with those in the other networks I work with!
Banners
A very good reason for having a banner is to give any and all who dare attempt to telnet or
sneak into your internetwork a little security notice. And they’re very cool because you can
create and customize them so that they’ll greet anyone who shows up on the router with
exactly the information you want them to have!
Here are the three types of banners you need to be sure you’re familiar with:
u Exec process creation banner
u Login banner
u Message of the day banner
And you can see them all illustrated in the following code:
Todd(config)#banner ?
LINE c banner-text c, where 'c' is a delimiting character
exec Set EXEC process creation banner
incoming Set incoming terminal line banner
login Set login banner
motd Set Message of the Day banner
prompt-timeout Set Message for login authentication timeout
slip-ppp Set Message for SLIP/PPP
Message of the day (MOTD) banners are the most widely used banners because they
give a message to anyone connecting to the router via Telnet or an auxiliary port or even
through a console port as seen here:
Todd(config)#banner motd ?
LINE c banner-text c, where ‘c’ is a delimiting character
Todd(config)#banner motd #
Enter TEXT message. End with the character ‘#’.
$ Acme.com network, then you must disconnect immediately.
230 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
#
Todd(config)#^Z (Press the control key + z keys to return to privileged mode)
Todd#exit
Router con0 is now available
Press RETURN to get started.
If you are not authorized to be in Acme.com network, then you
must disconnect immediately.
Todd#
This MOTD banner essentially tells anyone connecting to the router to get lost if they’re
not on the guest list. The part to focus upon here is the delimiting character, which is what
informs the router the message is done. Clearly, you can use any character you want for it
except for the delimiting character in the message itself. Once the message is complete, press
Enter, then the delimiting character, and then press Enter again. Everything will still work if
you don’t follow this routine unless you have more than one banner. If that’s the case, make
sure you do follow it or your banners will all be combined into one message and put on a
single line!
You can set a banner on one line like this:
Todd(config)#banner motd x Unauthorized access prohibited! x
Let’s take a minute to go into more detail about the other two types of banners I mentioned:
Exec banner You can congure a line-activation (exec) banner to be displayed when EXEC
processes such as a line activation or an incoming connection to a VTY line have been created.
Simply initiating a user exec session through a console port will activate the exec banner.
Login banner You can congure a login banner for display on all connected terminals. It
will show up after the MOTD banner but before the login prompts. This login banner can’t
be disabled on a per-line basis, so to globally disable it you’ve got to delete it with the no
banner login command.
Here’s what a login banner output looks like:
!
banner login ^C
-----------------------------------------------------------------
Cisco Router and Security Device Manager (SDM) is installed on this device.
This feature requires the one-time use of the username “cisco”
with the password “cisco”. The default username and password
have a privilege level of 15.
Please change these publicly known initial credentials using
Administrative Configurations 231
SDM or the IOS CLI.
Here are the Cisco IOS commands.
username <myuser> privilege 15 secret 0 <mypassword>
no username cisco
Replace <myuser> and <mypassword> with the username and
password you want to use.
For more information about SDM please follow the instructions
in the QUICK START GUIDE for your router or go to http://www.cisco.com/go/sdm
-----------------------------------------------------------------
^C
!
The above login banner should look pretty familiar to anyone who’s ever logged into an
ISR router because it’s the banner Cisco has in the default conguration for its ISR routers.
Remember that the login banner is displayed before the login prompts and
after the MOTD banner.
Setting Passwords
There are ve passwords you’ll need to secure your Cisco routers: console, auxiliary, telnet
(VTY), enable password, and enable secret. The enable secret and enable password are the
ones used to set the password for securing privileged mode. Once the enable commands
are set, users will be prompted for a password. The other three are used to congure a
password when user mode is accessed through the console port, through the auxiliary
port, or via Telnet.
Let’s take a look at each of these now.
Enable Passwords
You set the enable passwords from global conguration mode like this:
Todd(config)#enable ?
last-resort Define enable action if no TACACS servers
respond
password Assign the privileged level password
secret Assign the privileged level secret
use-tacacs Use TACACS to check enable passwords
232 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
The following list describes the enable password parameters:
last-resort This allows you to still enter the router if you set up authentication through
a TACACS server and it’s not available. It won’t be used if the TACACS server is working.
password This sets the enable password on older, pre-10.3 systems, and isn’t ever used if
an enable secret is set.
secret The newer, encrypted password that overrides the enable password if it has been set.
use-tacacs This tells the router to authenticate through a TACACS server. It comes in
really handy when you have lots of routers because changing the password on a multitude
of them can be insanely tedious. It’s much easier to simply go through the TACACS server
and change the password only once!
Here’s an example that shows how to set the enable passwords:
Todd(config)#enable secret todd
Todd(config)#enable password todd
The enable password you have chosen is the same as your
enable secret. This is not recommended. Re-enter the
enable password.
If you try to set the enable secret and enable passwords the same, the router will give you
a polite warning to change the second password. Make a note to yourself that if there aren’t
any old legacy routers involved, you don’t even bother to use the enable password!
User-mode passwords are assigned via the line command like this:
Todd(config)#line ?
<0-16> First Line number
console Primary terminal line
vty Virtual terminal
And these two lines are especially important for the exam objectives:
console Sets a console user-mode password.
vty Sets a Telnet password on the router. If this password isn’t set, then by default, Telnet
can’t be used.
To congure user-mode passwords, choose the line you want and congure it using the
login command to make the switch prompt for authentication. Let’s focus in on the con-
guration of individual lines now.
Console Password
We set the console password with the line console 0 command, but look at what hap-
pened when I tried to type line console ? from the (config-line)# prompt—I received
an error! Here’s the example:
Todd(config-line)#line console ?
% Unrecognized command
Administrative Configurations 233
Todd(config-line)#exit
Todd(config)#line console ?
<0-0> First Line number
Todd(config-line)#password console
Todd(config-line)#login
You can still type line console 0 and that will be accepted, but the help screens just
don’t work from that prompt. Type exit to go back one level, and you’ll nd that your help
screens now work. This is a “feature.” Really.
Because there’s only one console port, I can only choose line console 0. You can set all your
line passwords to the same password, but doing this isn’t exactly a brilliant security move!
And it’s also important to remember to apply the login command or the console port
won’t prompt for authentication. The way Cisco has this process set up means you can’t set
the login command before a password is set on a line because if you set it but don’t then
set a password, that line won’t be usable. You’ll actually get prompted for a password that
doesn’t exist, so Cisco’s method isn’t just a hassle; it makes sense and is a feature after all!
Definitely remember that although Cisco has this “password feature” on its
routers starting with IOS 12.2 and above, it’s not in included in older IOSs.
Okay, there are a few other important commands you need to know regarding the con-
sole port.
For one, the exec-timeout 0 0 command sets the time-out for the console EXEC session
to zero, ensuring that it never times out. The default time-out is 10 minutes.
If you’re feeling mischievous, try this on people at work: Set the
exec-timeout command to 0 1. This will make the console time out in
1 second, and to fix it, you have to continually press the down arrow key
while changing the time-out time with your free hand!
Logging synchronous is such a cool command that it should be a default, but it’s not.
It’s great because it’s the antidote for those annoying console messages that disrupt the
input you’re trying to type. The messages will still pop up, but at least you get returned to
your router prompt without your input being interrupted! This makes your input messages
oh-so-much easier to read!
Here’s an example of how to congure both commands:
Todd(config-line)#line con 0
Todd(config-line)#exec-timeout ?
<0-35791> Timeout in minutes
Todd(config-line)#exec-timeout 0 ?
<0-2147483> Timeout in seconds
<cr>
234 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
Todd(config-line)#exec-timeout 0 0
Todd(config-line)#logging synchronous
You can set the console to go from never timing out (0 0) to timing out
in 35,791 minutes and 2,147,483 seconds. Remember that the default is
10 minutes.
Telnet Password
To set the user-mode password for Telnet access into the router or switch, use the line vty
command. IOS switches typically have 16 lines, but routers running the Enterprise edition
have considerably more. The best way to nd out how many lines you have is to use that
handy question mark like this:
Todd(config-line)#line vty 0 ?
% Unrecognized command
Todd(config-line)#exit
Todd(config)#line vty 0 ?
<1-15> Last Line number
<cr>
Todd(config)#line vty 0 15
Todd(config-line)#password telnet
Todd(config-line)#login
This output clearly shows that you cannot get help from your (config-line)# prompt.
You must go back to global cong mode in order to use the question mark (?).
So what will happen if you try to telnet into a device that doesn’t have a VTY password
set? You’ll receive an error saying the connection has been refused because the password isn’t
set. So, if you telnet into a switch and receive a message like this one that I got from Switch B
Todd#telnet SwitchB
Trying SwitchB (10.0.0.1)…Open
Password required, but none set
[Connection to SwitchB closed by foreign host]
Todd#
it means the switch doesn’t have the VTY password set. But you can still get around this
and tell the switch to allow Telnet connections without a password by using the no login
command:
SwitchB(config-line)#line vty 0 15
SwitchB(config-line)#no login
Administrative Configurations 235
I definitely do not recommend using the no login command to allow Telnet
connections without a password, unless you’re in a testing or classroom
environment. In a production network, always set your VTY password!
After your IOS devices are congured with an IP address, you can use the Telnet pro-
gram to congure and check your routers instead of having to use a console cable. You can
use the Telnet program by typing telnet from any command prompt (DOS or Cisco). I’ll
cover all things Telnet more thoroughly in Chapter 7, “Managing a Cisco Internetwork.”
Auxiliary Password
To congure the auxiliary password on a router, go into global conguration mode and type
line aux ?. And by the way, you won’t nd these ports on a switch. This output shows that
you only get a choice of 0–0, which is because there’s only one port:
Todd#config t
Todd(config)#line aux ?
<0-0> First Line number
Todd(config)#line aux 0
Todd(config-line)#login
% Login disabled on line 1, until ‘password’ is set
Todd(config-line)#password aux
Todd(config-line)#login
Setting Up Secure Shell (SSH)
I strongly recommend using Secure Shell (SSH) instead of Telnet because it creates a more
secure session. The Telnet application uses an unencrypted data stream, but SSH uses encryp-
tion keys to send data so your username and password aren’t sent in the clear, vulnerable to
anyone lurking around!
Here are the steps for setting up SSH:
1. Set your hostname:
Router(config)#hostname Todd
2. Set the domain name—both the hostname and domain name are required for the
encryption keys to be generated:
Todd(config)#ip domain-name Lammle.com
3. Set the username to allow SSH client access:
Todd(config)#username Todd password Lammle
236 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
4. Generate the encryption keys for securing the session:
Todd(config)#crypto key generate rsa
The name for the keys will be: Todd.Lammle.com
Choose the size of the key modulus in the range of 360 to
4096 for your General Purpose Keys. Choosing a key modulus
Greater than 512 may take a few minutes.
How many bits in the modulus [512]: 1024
% Generating 1024 bit RSA keys, keys will be non-exportable...
[OK] (elapsed time was 6 seconds)
Todd(config)#
1d14h: %SSH-5-ENABLED: SSH 1.99 has been enabled*June 24
19:25:30.035: %SSH-5-ENABLED: SSH 1.99 has been enabled
5. Enable SSH version 2 on the router—not mandatory, but strongly suggested:
Todd(config)#ip ssh version 2
6. Connect to the VTY lines of the switch:
Todd(config)#line vty 0 15
7. Configure your access protocols:
Todd(config-line)#transport input ?
all All protocols
none No protocols
ssh TCP/IP SSH protocol
telnet TCP/IP Telnet protocol
Beware of this next line, and make sure you never use it in production because it’s a
horrendous security risk:
Todd(config-line)#transport input all
I recommend using the next line to secure your VTY lines with SSH:
Todd(config-line)#transport input ssh ?
telnet TCP/IP Telnet protocol
<cr>
Administrative Configurations 237
I actually do use Telnet once in a while when a situation arises that specically calls
for it. It just doesn’t happen very often. But if you want to go with Telnet, here’s how
you do that:
Todd(config-line)#transport input ssh telnet
Know that if you don’t use the keyword telnet at the end of the command string,
then only SSH will work on the device. You can go with either, just so long as you
understand that SSH is way more secure than Telnet.
Encrypting Your Passwords
Because only the enable secret password is encrypted by default, you’ll need to manually
congure the user-mode and enable passwords for encryption.
Notice that you can see all the passwords except the enable secret when performing a
show running-config on a switch:
Todd#sh running-config
Building configuration...
Current configuration : 1020 bytes
!
! Last configuration change at 00:03:11 UTC Mon Mar 1 1993
!
version 15.0
no service pad
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname Todd
!
enable secret 4 ykw.3/tgsOuy9.6qmgG/EeYOYgBvfX4v.S8UNA9Rddg
enable password todd
!
[output cut]
!
line con 0
password console
login
line vty 0 4
238 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
password telnet
login
line vty 5 15
password telnet
login
!
end
To manually encrypt your passwords, use the service password-encryption command.
Here’s how:
Todd#config t
Todd(config)#service password-encryption
Todd(config)#exit
Todd#show run
Building configuration...
!
!
enable secret 4 ykw.3/tgsOuy9.6qmgG/EeYOYgBvfX4v.S8UNA9Rddg
enable password 7 1506040800
!
[output cut]
!
!
line con 0
password 7 050809013243420C
login
line vty 0 4
password 7 06120A2D424B1D
login
line vty 5 15
password 7 06120A2D424B1D
login
!
end
Todd#config t
Todd(config)#no service password-encryption
Todd(config)#^Z
Todd#
Administrative Configurations 239
Nicely done—the passwords will now be encrypted. All you need to do is encrypt the
passwords, perform a show run, then turn off the command if you want. This output clearly
shows us that the enable password and the line passwords are all encrypted.
Before we move on to nd out how to set descriptions on your interfaces, I want to stress
some points about password encryption. As I said, if you set your passwords and then turn
on the service password-encryption command, you have to perform a show running-
config before you turn off the encryption service or your passwords won’t be encrypted.
You don’t have to turn off the encryption service at all—you’d only do that if your switch
is running low on processes. And if you turn on the service before you set your passwords,
then you don’t even have to view them to have them encrypted.
Descriptions
Setting descriptions on an interface is another administratively helpful thing, and like the
hostname, it’s also only locally signicant. One case where the description command comes
in really handy is when you want to keep track of circuit numbers on a switch or a router’s
serial WAN port.
Here’s an example on my switch:
Todd#config t
Todd(config)#int fa0/1
Todd(config-if)#description Sales VLAN Trunk Link
Todd(config-if)#^Z
Todd#
And on a router serial WAN:
Router#config t
Router(config)#int s0/0/0
Router(config-if)#description WAN to Miami
Router(config-if)#^Z
You can view an interface’s description with either the show running-config command
or the show interface—even with the show interface description command:
Todd#sh run
Building configuration...
Current configuration : 855 bytes
!
interface FastEthernet0/1
description Sales VLAN Trunk Link
!
[output cut]
Visit ccna
.gg/ch6/a
for a
companion
MicroNugget
from CBT
Nuggets.
240 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
Todd#sh int f0/1
FastEthernet0/1 is up, line protocol is up (connected)
Hardware is Fast Ethernet, address is ecc8.8202.8282 (bia ecc8.8202.8282)
Description: Sales VLAN Trunk Link
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
[output cut]
Todd#sh int description
Interface Status Protocol Description
Vl1 up up
Fa0/1 up up Sales VLAN Trunk Link
Fa0/2 up up
description: A Helpful Command
Bob, a senior network admin at Acme Corporation in San Francisco, has over 50 WAN
links to branches throughout the United States and Canada. Whenever an interface goes
down, Bob wastes lots of time trying to figure out the circuit number and the phone num-
ber of the provider of his ailing WAN link.
This kind of scenario shows just how helpful the interface description command can be.
It would save Bob a lot of work because he could use it on his most important switch LAN
links to find out exactly where every interface is connected. Bob’s life would also be made
a lot easier by adding circuit numbers to each and every WAN interface on his routers,
along with the phone number of the responsible provider.
So if Bob had just taken time in advance to preventively add this information to his inter-
faces, he would have saved himself an ocean of stress and a ton of precious time when
his WAN links inevitably go down!
Doing the do Command
In every previous example so far, we’ve had to run all show from privileged mode. But
I’ve got great news—beginning with IOS version 12.3, Cisco has nally added a com-
mand to the IOS that allows you to view the conguration and statistics from within
conguration mode!
Router and Switch Interfaces 241
In fact, with any IOS, you’d get the following error if you tried to view the conguration
from global cong:
Todd(config)#sh run
^
% Invalid input detected at ‘^’ marker.
Compare that to the output I get from entering that same command on my router that’s
running the 15.0 IOS using the “do” syntax:
Todd(config)#do show run
Building configuration...
Current configuration : 759 bytes
!
version 15.0
no service pad
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname Todd
!
boot-start-marker
boot-end-marker
!
[output cut]
So now you can pretty much run any command from any conguration prompt—nice,
huh? Looking back through all those examples for encrypting our passwords, you can see
that the do command would denitely have gotten the party started sooner, making this
innovation one to celebrate for sure!
Router and Switch Interfaces
Interface conguration is arguably the most important router conguration because
without interfaces, a router is a pretty useless object. Furthermore, interface congura-
tions must be totally precise to enable communication with other devices. Network layer
addresses, media type, bandwidth, and other administrator commands are all used to
congure an interface.
242 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
On a layer 2 switch, interface congurations typically involve a lot less work than router
interface conguration. Check out the output from the powerful verication command
show ip interface brief, which reveals all the interfaces on my 3560 switch:
Todd#sh ip interface brief
Interface IP-Address OK? Method Status Protocol
Vlan1 192.168.255.8 YES DHCP up up
FastEthernet0/1 unassigned YES unset up up
FastEthernet0/2 unassigned YES unset up up
FastEthernet0/3 unassigned YES unset down down
FastEthernet0/4 unassigned YES unset down down
FastEthernet0/5 unassigned YES unset up up
FastEthernet0/6 unassigned YES unset up up
FastEthernet0/7 unassigned YES unset down down
FastEthernet0/8 unassigned YES unset down down
GigabitEthernet0/1 unassigned YES unset down down
The above output shows the default routed port found on all Cisco switches (VLAN 1),
plus nine switch FastEthernet interface ports, with one port being a Gigabit Ethernet port
used for uplinks to other switches.
Different routers use different methods to choose the interfaces used on them. For
instance, the following command shows one of my 2800 ISR Cisco routers with two
FastEthernet interfaces along with two serial WAN interfaces:
Router>sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.255.11 YES DHCP up up
FastEthernet0/1 unassigned YES unset administratively down down
Serial0/0/0 unassigned YES unset administratively down down
Serial0/1/0 unassigned YES unset administratively down down
Router>
Previously, we always used the interface type number sequence to congure an inter-
face, but the newer routers come with an actual physical slot and include a port number on
the module plugged into it. So on a modular router, the conguration would be interface
typ e slot/port, as demonstrated here:
Todd#config t
Todd(config)#interface GigabitEthernet 0/1
Todd(config-if)#
You can see that we are now at the Gigabit Ethernet slot 0, port 1 prompt, and from here
we can make conguration changes to the interface. Make note of the fact that you can’t just
type int gigabitethernet 0. No shortcuts on the slot/port—you’ve got to type the slot/
port variables in the command: type slot/port or, for example, int gigabitethernet 0/1
(or just int g0/1).
Router and Switch Interfaces 243
Once in interface conguration mode, we can congure various options. Keep in mind
that speed and duplex are the two factors to be concerned with for the LAN:
Todd#config t
Todd(config)#interface GigabitEthernet 0/1
Todd(config-if)#speed 1000
Todd(config-if)#duplex full
So what’s happened here? Well basically, this has shut off the auto-detect mechanism on
the port, forcing it to only run gigabit speeds at full duplex. For the ISR series router, it’s basi-
cally the same, but you get even more options! The LAN interfaces are the same, but the rest
of the modules are different—they use three numbers instead of two. The three numbers used
here can represent slot/subslot/port, but this depends on the card used in the ISR router.
For the objectives you just need to remember this: The rst 0 is the router itself. You then
choose the slot and then the port. Here’s an example of a serial interface on my 2811:
Todd(config)#interface serial ?
<0-2> Serial interface number
Todd(config)#interface serial 0/0/?
<0-1> Serial interface number
Todd(config)#interface serial 0/0/0
Todd(config-if)#
This might look a little dicey to you, but I promise it’s really not that hard! It helps to
remember that you should always view the output of the show ip interface brief command
or a show running-config output rst so you know the exact interfaces you have to deal with.
Here’s one of my 2811’s output that has even more serial interfaces installed:
Todd(config-if)#do show run
Building configuration...
[output cut]
!
interface FastEthernet0/0
no ip address
shutdown
duplex auto
speed auto
!
interface FastEthernet0/1
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/0/0
244 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
no ip address
shutdown
no fair-queue
!
interface Serial0/0/1
no ip address
shutdown
!
interface Serial0/1/0
no ip address
shutdown
!
interface Serial0/2/0
no ip address
shutdown
clock rate 2000000
!
[output cut]
For the sake of brevity, I didn’t include my complete running-cong, but I’ve displayed all
you really need. You can see the two built-in FastEthernet interfaces, the two serial interfaces
in slot 0 (0/0/0 and 0/0/1), the serial interface in slot 1 (0/1/0), and the serial interface in slot
2 (0/2/0). And once you see the interfaces like this, it makes it a lot easier to understand how
the modules are inserted into the router.
Just understand that if you type interface e0 on an old 2500 series router, interface
fastethernet 0/0 on a modular router (such as the 2800 series router), or interface
serial 0/1/0 on a ISR router, all you’re actually doing is choosing an interface to cong-
ure. Essentially, they’re all congured the same way after that.
Let’s delve deeper into our router interface discussion by exploring how to bring up the
interface and set an IP address on it next.
Bringing Up an Interface
You can disable an interface with the interface command shutdown and enable it with the
no shutdown command. Just to remind you, all switch ports are enabled by default and all
router ports are disabled by default, so we’re going to talk more about router ports than
switch ports in the next few sections.
If an interface is shut down, it’ll display as administratively down when you use the show
interfaces command (sh int for short):
Router#sh int f0/0
FastEthernet0/1 is administratively down, line protocol is down
[output cut]
Router and Switch Interfaces 245
Another way to check an interface’s status is via the show running-config command.
You can bring up the router interface with the no shutdown command (no shut for short):
Router(config)#int f0/0
Router(config-if)#no shutdown
*August 21 13:45:08.455: %LINK-3-UPDOWN: Interface FastEthernet0/0,
changed state to up
Router(config-if)#do show int f0/0
FastEthernet0/0 is up, line protocol is up
[output cut]
Configuring an IP Address on an Interface
Even though you don’t have to use IP on your routers, it’s usually what everyone uses. To
congure IP addresses on an interface, use the ip address command from interface con-
guration mode and remember that you do not set an IP address on a layer 2 switch port!
Todd(config)#int f0/1
Todd(config-if)#ip address 172.16.10.2 255.255.255.0
Also, don’t forget to enable the interface with the no shutdown command. Remember to
look at the command show interface int output to see if the interface is administratively
shut down or not. Show ip int brief and show running-config will also give you this
information.
The ip address address mask command starts the IP processing on the
router interface. Again, you do not configure an IP address on a layer 2
switch interface!
Okay—now if you want to add a second subnet address to an interface, you have
to use the secondary parameter. If you type another IP address and press Enter, it will
replace the existing primary IP address and mask. This is denitely one of the Cisco
IOS’s coolest features!
So let’s try it. To add a secondary IP address, just use the secondary parameter:
Todd(config-if)#ip address 172.16.20.2 255.255.255.0 ?
secondary Make this IP address a secondary address
<cr>
Todd(config-if)#ip address 172.16.20.2 255.255.255.0 secondary
Todd(config-if)#^Z
Todd(config-if)#do sh run
Building configuration...
[output cut]
246 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
interface FastEthernet0/1
ip address 172.16.20.2 255.255.255.0 secondary
ip address 172.16.10.2 255.255.255.0
duplex auto
speed auto
!
But I’ve got to stop here to tell you that I really wouldn’t recommend having multiple
IP addresses on an interface because it’s really inefcient. I showed you how anyway just in
case you someday nd yourself dealing with an MIS manager who’s in love with really bad
network design and makes you administer it! And who knows? Maybe someone will ask
you about it someday and you’ll get to seem really smart because you know this.
Using the Pipe
No, not that pipe. I mean the output modier. Although, I’ve got to say that some of the
router congurations I’ve seen in my career make me wonder! Anyway, this pipe ( | ) allows
us to wade through all the congurations or other long outputs and get straight to our
goods fast. Here’s an example:
Router#sh run | ?
append Append redirected output to URL (URLs supporting append
operation only)
begin Begin with the line that matches
exclude Exclude lines that match
include Include lines that match
redirect Redirect output to URL
section Filter a section of output
tee Copy output to URL
Router#sh run | begin interface
interface FastEthernet0/0
description Sales VLAN
ip address 10.10.10.1 255.255.255.248
duplex auto
speed auto
!
interface FastEthernet0/1
ip address 172.16.20.2 255.255.255.0 secondary
ip address 172.16.10.2 255.255.255.0
duplex auto
speed auto
!
Router and Switch Interfaces 247
interface Serial0/0/0
description Wan to SF circuit number 6fdda 12345678
no ip address
!
So basically, the pipe symbol—the output modier—is what you need to help you get
where you want to go light years faster than mucking around in a router’s entire congura-
tion. I use it a lot when scrutinizing a large routing table to nd out whether a certain route
is in the routing table. Here’s an example:
Todd#sh ip route | include 192.168.3.32
R 192.168.3.32 [120/2] via 10.10.10.8, 00:00:25, FastEthernet0/0
Todd#
First, you need to know that this routing table had over 100 entries, so without my
trusty pipe, I’d probably still be looking through that output! It’s a powerfully efcient tool
that saves you major time and effort by quickly nding a line in a conguration—or as the
preceding example shows, a single route within a huge routing table.
Give yourself a little time to play around with the pipe command to get the hang of it and
you’ll be naturally high on your newfound ability to quickly parse through router output!
Serial Interface Commands
But wait! Before you just jump in and congure a serial interface, you need some key infor-
mation, like knowing the interface will usually be attached to a CSU/DSU type of device
that provides clocking for the line to the router. Check out Figure6.3 for an example.
FIGURE6.3 A typical WAN connection. Clocking is typically provided by a DCE
network to routers. In nonproduction environments, a DCE network is not always present.
DTE
DTE
DCE
CSU/DSU CSU/DSU
Here you can see that the serial interface is used to connect to a DCE network via a
CSU/DSU that provides the clocking to the router interface. But if you have a back-to-back
248 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
conguration, such as one that’s used in a lab environment like the one in Figure6.4, one
end—the data communication equipment (DCE) end of the cable—must provide clocking!
FIGURE6.4 Providing clocking on a nonproduction network
Set clock rate if needed
Todd# config t
Todd(config)# interface serial 0
Todd(config-if)#clock rate 1000000
DCE side determined by the cable.
Add clocking to DCE side only.
>show controllers int will show the cable connection type
DCE
DTE
By default, Cisco router serial interfaces are all data terminal equipment (DTE) devices,
which means that you must congure an interface to provide clocking if you need it to act like
a DCE device. Again, you would not provide clocking on a production T1 connection because
you would have a CSU/DSU connected to your serial interface, as shown in Figure6.3.
You congure a DCE serial interface with the clock rate command:
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#int s0/0/0
Router(config-if)#clock rate ?
Speed (bits per second)
1200
2400
4800
9600
14400
19200
28800
32000
38400
48000
56000
57600
64000
72000
115200
Router and Switch Interfaces 249
125000
128000
148000
192000
250000
256000
384000
500000
512000
768000
800000
1000000
2000000
4000000
5300000
8000000
<300-8000000> Choose clockrate from list above
Router(config-if)#clock rate 1000000
The clock rate command is set in bits per second. Besides looking at the cable end to
check for a label of DCE or DTE, you can see if a router’s serial interface has a DCE cable
connected with the show controllers int command:
Router#sh controllers s0/0/0
Interface Serial0/0/0
Hardware is GT96K
DTE V.35idb at 0x4342FCB0, driver data structure at 0x434373D4
Here is an example of an output depicting a DCE connection:
Router#sh controllers s0/2/0
Interface Serial0/2/0
Hardware is GT96K
DCE V.35, clock rate 1000000
The next command you need to get acquainted with is the bandwidth command. Every
Cisco router ships with a default serial link bandwidth of T1 (1.544 Mbps). But this has
nothing to do with how data is transferred over a link. The bandwidth of a serial link is
used by routing protocols like EIGRP and OSPF to calculate the best cost path to a remote
network. So if you’re using RIP routing, the bandwidth setting of a serial link is irrelevant
since RIP uses only hop count to determine this.
250 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
You may be rereading this part and thinking, “Huh? What? Routing proto-
cols? Metrics?” But don’t freak! I’m going over all of that soon in Chapter 8,
“IP Routing.”
Here’s an example of using the bandwidth command:
Router#config t
Router(config)#int s0/0/0
Router(config-if)#bandwidth ?
<1-10000000> Bandwidth in kilobits
inherit Specify that bandwidth is inherited
receive Specify receive-side bandwidth
Router(config-if)#bandwidth 1000
Did you notice that, unlike the clock rate command, the bandwidth command is con-
gured in kilobits per second?
After going through all these configuration examples regarding the clock
rate command, understand that the new ISR routers automatically detect
DCE connections and set clock rate to 2000000. But know that you still
need to understand the clock rate command for the Cisco objectives,
even though the new routers set it for you automatically!
Viewing, Saving, and Erasing
Configurations
If you run through setup mode, you’ll be asked if you want to use the conguration you just
created. If you say yes, the conguration running in DRAM that’s known as the running-
cong will be copied into NVRAM, and the le will be named startup-config. Hopefully,
you’ll be smart and always use the CLI, not setup mode!
You can manually save the le from DRAM, which is usually just called RAM, to
NVRAM by using the copy running-config startup-config command. You can use
the shortcut copy run start as well:
Todd#copy running-config startup-config
Destination filename [startup-config]? [press enter]
Building configuration...
[OK]
Viewing, Saving, and Erasing Configurations 251
Todd#
Building configuration...
When you see a question with an answer in [], it means that if you just press Enter, you’re
choosing the default answer.
Also, when the command asks for the destination lename, the default answer is startup-
cong. The reason it asks is because you can copy the conguration to pretty much anywhere
you want. Take a look at the output from my switch:
Todd#copy running-config ?
flash: Copy to flash: file system
ftp: Copy to ftp: file system
http: Copy to http: file system
https: Copy to https: file system
null: Copy to null: file system
nvram: Copy to nvram: file system
rcp: Copy to rcp: file system
running-config Update (merge with) current system configuration
scp: Copy to scp: file system
startup-config Copy to startup configuration
syslog: Copy to syslog: file system
system: Copy to system: file system
tftp: Copy to tftp: file system
tmpsys: Copy to tmpsys: file system
vb: Copy to vb: file system
To reassure you, we’ll get deeper into how and where to copy les in Chapter 7.
For now, you can view the les by typing show running-config or show startup-
config from privileged mode. The sh run command, which is a shortcut for show
running-config, tells us that we’re viewing the current conguration:
Todd#sh run
Building configuration...
Current configuration : 855 bytes
!
! Last configuration change at 23:20:06 UTC Mon Mar 1 1993
!
version 15.0
[output cut]
252 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
The sh start command—one of the shortcuts for the show startup-config command—
shows us the conguration that will be used the next time the router is reloaded. It also tells
us how much NVRAM is being used to store the startup-cong le. Here’s an example:
Todd#sh start
Using 855 out of 524288 bytes
!
! Last configuration change at 23:20:06 UTC Mon Mar 1 1993
!
version 15.0
[output cut]
But beware—if you try and view the conguration and see
Todd#sh start
startup-config is not present
you have not saved your running-cong to NVRAM, or you’ve deleted the backup congu-
ration! Let me talk about just how you would do that now.
Deleting the Configuration and Reloading the Device
You can delete the startup-cong le by using the erase startup-config command:
Todd#erase start
% Incomplete command.
First, notice that you can no longer use the shortcut commands for erasing the backup
conguration. This started in IOS 12.4 with the ISR routers.
Todd#erase startup-config
Erasing the nvram filesystem will remove all configuration files! Continue?
[confirm]
[OK]
Erase of nvram: complete
Todd#
*Mar 5 01:59:45.206: %SYS-7-NV_BLOCK_INIT: Initialized the geometry of nvram
Todd#reload
Proceed with reload? [confirm]
Now if you reload or power the router down after using the erase startup-config com-
mand, you’ll be offered setup mode because there’s no conguration saved in NVRAM. You
can press Ctrl+C to exit setup mode at any time, but the reload command can only be used
from privileged mode.
Viewing, Saving, and Erasing Configurations 253
At this point, you shouldn’t use setup mode to congure your router. So just say no to
setup mode, because it’s there to help people who don’t know how to use the cash line
interface (CLI), and this no longer applies to you. Be strong—you can do it!
Verifying Your Configuration
Obviously, show running-config would be the best way to verify your conguration and
show startup-config would be the best way to verify the conguration that’ll be used the
next time the router is reloaded—right?
Well, once you take a look at the running-cong, if all appears well, you can verify
your conguration with utilities like Ping and Telnet. Ping is a program that uses ICMP
echo requests and replies, which we covered in Chapter 3, “TCP/IP.” For review, Ping
sends a packet to a remote host, and if that host responds, you know that it’s alive. But
you don’t know if it’s alive and also well; just because you can ping a Microsoft server
does not mean you can log in! Even so, Ping is an awesome starting point for trouble-
shooting an internetwork.
Did you know that you can ping with different protocols? You can, and you can test this
by typing ping ? at either the router user-mode or privileged-mode prompt:
Todd#ping ?
WORD Ping destination address or hostname
clns CLNS echo
ip IP echo
ipv6 IPv6 echo
tag Tag encapsulated IP echo
<cr>
If you want to nd a neighbor’s Network layer address, either you go straight to the
router or switch itself or you can type show cdp entry * protocol to get the Network
layer addresses you need for pinging.
You can also use an extended ping to change the default variables, as shown here:
Todd#ping
Protocol [ip]:
Target IP address: 10.1.1.1
Repeat count [5]:
% A decimal number between 1 and 2147483647.
Repeat count [5]: 5000
Datagram size [100]:
% A decimal number between 36 and 18024.
Datagram size [100]: 1500
Timeout in seconds [2]:
254 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
Extended commands [n]: y
Source address or interface: FastEthernet 0/1
Source address or interface: Vlan 1
Type of service [0]:
Set DF bit in IP header? [no]:
Validate reply data? [no]:
Data pattern [0xABCD]:
Loose, Strict, Record, Timestamp, Verbose[none]:
Sweep range of sizes [n]:
Type escape sequence to abort.
Sending 5000, 1500-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:
Packet sent with a source address of 10.10.10.1
Notice that by using the question mark, I was able to determine that extended ping
allows you to set the repeat count higher than the default of 5 and the datagram size larger.
This raises the MTU and allows for a more accurate testing of throughput. The source
interface is one last important piece of information I’ll pull out of the output. You can
choose which interface the ping is sourced from, which is really helpful in certain diagnos-
tic situations. Using my switch to display the extended ping capabilities, I had to use my
only routed port, which is named VLAN 1, by default.
Cisco Discovery Protocol (CDP) is covered in Chapter 7.
Traceroute uses ICMP with IP time to live (TTL) time-outs to track the path a given
packet takes through an internetwork. This is in contrast to Ping, which just nds the host
and responds. Traceroute can also be used with multiple protocols. Check out this output:
Todd#traceroute ?
WORD Trace route to destination address or hostname
aaa Define trace options for AAA events/actions/errors
appletalk AppleTalk Trace
clns ISO CLNS Trace
ip IP Trace
ipv6 IPv6 Trace
ipx IPX Trace
mac Trace Layer2 path between 2 endpoints
oldvines Vines Trace (Cisco)
vines Vines Trace (Banyan)
<cr>
Viewing, Saving, and Erasing Configurations 255
Telnet, FTP, and HTTP are really the best tools because they use IP at the Network layer
and TCP at the Transport layer to create a session with a remote host. If you can telnet, ftp,
or http into a device, you know that your IP connectivity just has to be solid!
Todd#telnet ?
WORD IP address or hostname of a remote system
<cr>
From the switch or router prompt, you just type a hostname or IP address and it will
assume you want to telnet—you don’t need to type the actual command, telnet.
Coming up, I’ll show you how to verify the interface statistics.
Verifying with the show interface Command
Another way to verify your conguration is by typing show interface commands, the rst
of which is the show interface ? command. Doing this will reveal all the available inter-
faces to verify and congure.
The show interfaces command, with an s, displays the configurable
parameters and statistics of all interfaces on a router.
This command comes in really handy when you’re verifying and troubleshooting router
and network issues.
The following output is from my freshly erased and rebooted 2811 router:
Router#sh int ?
Async Async interface
BVI Bridge-Group Virtual Interface
CDMA-Ix CDMA Ix interface
CTunnel CTunnel interface
Dialer Dialer interface
FastEthernet FastEthernet IEEE 802.3
Loopback Loopback interface
MFR Multilink Frame Relay bundle interface
Multilink Multilink-group interface
Null Null interface
Port-channel Ethernet Channel of interfaces
Serial Serial
Tunnel Tunnel interface
Vif PGM Multicast Host interface
Virtual-PPP Virtual PPP interface
Virtual-Template Virtual Template interface
Virtual-TokenRing Virtual TokenRing
256 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
accounting Show interface accounting
counters Show interface counters
crb Show interface routing/bridging info
dampening Show interface dampening info
description Show interface description
etherchannel Show interface etherchannel information
irb Show interface routing/bridging info
mac-accounting Show interface MAC accounting info
mpls-exp Show interface MPLS experimental accounting info
precedence Show interface precedence accounting info
pruning Show interface trunk VTP pruning information
rate-limit Show interface rate-limit info
stats Show interface packets & octets, in & out, by switching path
status Show interface line status
summary Show interface summary
switching Show interface switching
switchport Show interface switchport information
trunk Show interface trunk information
| Output modifiers
<cr>
The only “real” physical interfaces are FastEthernet, Serial, and Async—the rest are all
logical interfaces or commands you can use to verify with.
The next command is show interface fastethernet 0/0. It reveals the hardware address,
logical address, and encapsulation method as well as statistics on collisions, as seen here:
Router#sh int f0/0
FastEthernet0/0 is up, line protocol is up
Hardware is MV96340 Ethernet, address is 001a.2f55.c9e8 (bia 001a.2f55.c9e8)
Internet address is 192.168.1.33/27
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Auto-duplex, Auto Speed, 100BaseTX/FX
ARP type: ARPA, ARP Timeout 04:00:00
Last input never, output 00:02:07, output hang never
Last clearing of “show interface” counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
Viewing, Saving, and Erasing Configurations 257
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes
Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog
0 input packets with dribble condition detected
16 packets output, 960 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier
0 output buffer failures, 0 output buffers swapped out
Router#
You probably guessed that we’re going to go over the important statistics from this output,
but rst, just for fun, I’ve got to ask you, which subnet is FastEthernet 0/0 a member of and
what’s the broadcast address and valid host range?
I’m serious—you really have to be able to nail these things NASCAR-fast! Just in case you
didn’t, the address is 192.168.1.33/27. And I’ve gotta be honest—if you don’t know what a
/27 is at this point, you’ll need a miracle to pass the exam! That or you need to actually read
this book. (As a quick reminder, a /27 is 255.255.255.224.) The fourth octet is a block size of
32. The subnets are 0, 32, 64, etc.; the FastEthernet interface is in the 32 subnet; the broad-
cast address is 63; and the valid hosts are 33–62. All good now?
If you struggled with any of this, please save yourself from certain doom
and get yourself back into Chapter 4, “Easy Subnetting,” now! Read and
reread it until you’ve got it dialed in!
Okay—back to the output. The preceding interface is working and looks to be in good
shape. The show interfaces command will show you if you’re receiving errors on the inter-
face, and it will also show you the maximum transmission unit (MTU). MTU is the maximum
packet size allowed to transmit on that interface, bandwidth (BW) is for use with routing pro-
tocols, and 255/255 means that reliability is perfect! The load is 1/255, meaning no load.
Continuing through the output, can you gure out the bandwidth of the interface? Well,
other than the easy giveaway of the interface being called a “FastEthernet” interface, we
can see that the bandwidth is 100000 Kbit, which is 100,000,000. Kbit means to add three
zeros, which is 100 Mbits per second, or FastEthernet. Gigabit would be 1000000 Kbits
per second.
Be sure you don’t miss the output errors and collisions, which show 0 in my output. If
these numbers are increasing, then you have some sort of Physical or Data Link layer issue.
Check your duplex! If you have one side as half-duplex and one at full-duplex, your interface
will work, albeit really slow and those numbers will be increasing fast!
258 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
The most important statistic of the show interface command is the output of the line and
Data Link protocol status. If the output reveals that FastEthernet 0/0 is up and the line proto-
col is up, then the interface is up and running:
Router#sh int fa0/0
FastEthernet0/0 is up, line protocol is up
The rst parameter refers to the Physical layer, and it’s up when it receives carrier detect.
The second parameter refers to the Data Link layer, and it looks for keepalives from the
connecting end. Keepalives are important because they’re used between devices to make
sure connectivity hasn’t been dropped.
Here’s an example of where your problem will often be found—on serial interfaces:
Router#sh int s0/0/0
Serial0/0 is up, line protocol is down
If you see that the line is up but the protocol is down, as displayed here, you’re experienc-
ing a clocking (keepalive) or framing problem—possibly an encapsulation mismatch. Check
the keepalives on both ends to make sure they match. Make sure that the clock rate is set, if
needed, and that the encapsulation type is equal on both ends. The preceding output tells us
that there’s a Data Link layer problem.
If you discover that both the line interface and the protocol are down, it’s a cable or
interface problem. The following output would indicate a Physical layer problem:
Router#sh int s0/0/0
Serial0/0 is down, line protocol is down
As you’ll see next, if one end is administratively shut down, the remote end would
present as down and down:
Router#sh int s0/0/0
Serial0/0 is administratively down, line protocol is down
To enable the interface, use the command no shutdown from interface conguration mode.
The next show interface serial 0/0/0 command demonstrates the serial line and
the maximum transmission unit (MTU)—1,500 bytes by default. It also shows the default
bandwidth (BW) on all Cisco serial links, which is 1.544 Kbps. This is used to determine
the bandwidth of the line for routing protocols like EIGRP and OSPF. Another important
conguration to notice is the keepalive, which is 10 seconds by default. Each router sends
a keepalive message to its neighbor every 10 seconds, and if both routers aren’t congured
for the same keepalive time, it won’t work! Check out this output:
Router#sh int s0/0/0
Serial0/0 is up, line protocol is up
Hardware is HD64570
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Viewing, Saving, and Erasing Configurations 259
Encapsulation HDLC, loopback not set, keepalive set
(10 sec)
Last input never, output never, output hang never
Last clearing of “show interface” counters never
Queueing strategy: fifo
Output queue 0/40, 0 drops; input queue 0/75, 0 drops
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored,
0 abort
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 16 interface resets
0 output buffer failures, 0 output buffers swapped out
0 carrier transitions
DCD=down DSR=down DTR=down RTS=down CTS=down
You can clear the counters on the interface by typing the command clear counters:
Router#clear counters ?
Async Async interface
BVI Bridge-Group Virtual Interface
CTunnel CTunnel interface
Dialer Dialer interface
FastEthernet FastEthernet IEEE 802.3
Group-Async Async Group interface
Line Terminal line
Loopback Loopback interface
MFR Multilink Frame Relay bundle interface
Multilink Multilink-group interface
Null Null interface
Serial Serial
Tunnel Tunnel interface
Vif PGM Multicast Host interface
Virtual-Template Virtual Template interface
Virtual-TokenRing Virtual TokenRing
<cr>
Router#clear counters s0/0/0
Clear “show interface” counters on this interface
[confirm][enter]
260 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
Router#
00:17:35: %CLEAR-5-COUNTERS: Clear counter on interface
Serial0/0/0 by console
Router#
Troubleshooting with the show interfaces command
Let’s take a look at the output of the show interfaces command one more time before
I move on. There are some important statistics in this output that are important for the
Cisco objectives.
275496 packets input, 35226811 bytes, 0 no buffer
Received 69748 broadcasts (58822 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 58822 multicast, 0 pause input
0 input packets with dribble condition detected
2392529 packets output, 337933522 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 PAUSE output
0 output buffer failures, 0 output buffers swapped out
Finding where to start when troubleshooting an interface can be the difcult part, but
certainly we’ll look for the number of input errors and CRCs right away. Typically we’d
see those statistics increase with a duplex error, but it could be another Physical layer issue
such as the cable might be receiving excessive interference or the network interface cards
might have a failure. Typically you can tell if it is interference when the CRC and input
errors output grow but the collision counters do not.
Let’s take a look at some of the output:
No buffer This isn’t a number you want to see incrementing. This means you don’t have
any buffer room left for incoming packets. Any packets received once the buffers are full are
discarded. You can see how many packets are dropped with the ignored output.
Ignored If the packet buffers are full, packets will be dropped. You see this increment along
with the no buffer output. Typically if the no buffer and ignored outputs are incrementing,
you have some sort of broadcast storm on your LAN. This can be caused by a bad NIC or
even a bad network design.
Runts Frames that did not meet the minimum frame size requirement of 64 bytes. Typically
caused by collisions.
Giants Frames received that are larger than 1518 bytes
Viewing, Saving, and Erasing Configurations 261
Input Errors This is the total of many counters: runts, giants, no buffer, CRC, frame,
overrun, and ignored counts.
CRC At the end of each frame is a Frame Check Sequence (FCS) eld that holds the answer
to a cyclic redundancy check (CRC). If the receiving host’s answer to the CRC does not match
the sending host’s answer, then a CRC error will occur.
Frame This output increments when frames received are of an illegal format, or not
complete, which is typically incremented when a collision occurs.
Packets Output Total number of packets (frames) forwarded out to the interface.
Output Errors Total number of packets (frames) that the switch port tried to transmit but
for which some problem occurred.
Collisions When transmitting a frame in half-duplex, the NIC listens on the receiving
pair of the cable for another signal. If a signal is transmitted from another host, a collision
has occurred. This output should not increment if you are running full-duplex.
Late Collisions If all Ethernet specications are followed during the cable install, all
collisions should occur by the 64th byte of the frame. If a collision occurs after 64 bytes,
the late collisions counter increments. This counter will increment on a duplex mis-
matched interface, or if cable length exceeds specications.
Verifying with the show ip interface Command
The show ip interface command will provide you with information regarding the layer 3
congurations of a router’s interfaces:
Router#sh ip interface
FastEthernet0/0 is up, line protocol is up
Internet address is 1.1.1.1/24
Broadcast address is 255.255.255.255
Address determined by setup command
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is enabled
Security level is default
Split horizon is enabled
[output cut]
The status of the interface, the IP address and mask, information on whether an access
list is set on the interface, and basic IP information are all included in this output.
262 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
Using the show ip interface brief Command
The show ip interface brief command is probably one of the best commands that you
can ever use on a Cisco router. This command provides a quick overview of the router’s
interfaces, including the logical address and status:
Router#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 unassigned YES unset up up
FastEthernet0/1 unassigned YES unset up up
Serial0/0/0 unassigned YES unset up down
Serial0/0/1 unassigned YES unset administratively down down
Serial0/1/0 unassigned YES unset administratively down down
Serial0/2/0 unassigned YES unset administratively down down
Remember, administratively down means that you need to type no shutdown in order to
enable the interface. Notice that Serial0/0/0 is up/down, which means that the Physical layer
is good and carrier detect is sensed but no keepalives are being received from the remote
end. In a nonproduction network, like the one I am working with, this tells us the clock rate
hasn’t been set.
Verifying with the show protocols Command
The show protocols command is also a really helpful command that you’d use in order to
quickly see the status of layers 1 and 2 of each interface as well as the IP addresses used.
Here’s a look at one of my production routers:
Router#sh protocols
Global values:
Internet Protocol routing is enabled
Ethernet0/0 is administratively down, line protocol is down
Serial0/0 is up, line protocol is up
Internet address is 100.30.31.5/24
Serial0/1 is administratively down, line protocol is down
Serial0/2 is up, line protocol is up
Internet address is 100.50.31.2/24
Loopback0 is up, line protocol is up
Internet address is 100.20.31.1/24
The show ip interface brief and show protocols command provides the layer 1 and layer 2
statistics of an interface as well as the IP addresses. The next command, show controllers, only
provides layer 1 information. Let’s take a look.
Viewing, Saving, and Erasing Configurations 263
Using the show controllers Command
The show controllers command displays information about the physical interface itself.
It’ll also give you the type of serial cable plugged into a serial port. Usually, this will only
be a DTE cable that plugs into a type of data service unit (DSU).
Router#sh controllers serial 0/0
HD unit 0, idb = 0x1229E4, driver structure at 0x127E70
buffer size 1524 HD unit 0, V.35 DTE cable
Router#sh controllers serial 0/1
HD unit 1, idb = 0x12C174, driver structure at 0x131600
buffer size 1524 HD unit 1, V.35 DCE cable
Notice that serial 0/0 has a DTE cable, whereas the serial 0/1 connection has a DCE
cable. Serial 0/1 would have to provide clocking with the clock rate command. Serial 0/0
would get its clocking from the DSU.
Let’s look at this command again. In Figure6.5, see the DTE/DCE cable between the
two routers? Know that you will not see this in production networks!
FIGURE6.5 Where do you configure clocking? Use the show controllers
command on each router’s serial interface to find out.
R1
DTE DCE
R2
Router R1 has a DTE connection, which is typically the default for all Cisco routers.
Routers R1 and R2 can’t communicate. Check out the output of the show controllers
s0/0 command here:
R1#sh controllers serial 0/0
HD unit 0, idb = 0x1229E4, driver structure at 0x127E70
buffer size 1524 HD unit 0, V.35 DCE cable
The show controllers s0/0 command reveals that the interface is a V.35 DCE cable.
This means that R1 needs to provide clocking of the line to router R2. Basically, the inter-
face has the wrong label on the cable on the R1 router’s serial interface. But if you add
clocking on the R1 router’s serial interface, the network should come right up.
Let’s check out another issue in Figure6.6 that you can solve by using the show
controllers command. Again, routers R1 and R2 can’t communicate.
264 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
FIGURE6.6 By looking at R1, the show controllers command reveals that R1 and
R2 can’t communicate.
R1
S0/0 S0/0
R2
Here’s the output of R1’s show controllers s0/0 command and show ip
interface s0/0:
R1#sh controllers s0/0
HD unit 0, idb = 0x1229E4, driver structure at 0x127E70
buffer size 1524 HD unit 0,
DTE V.35 clocks stopped
cpb = 0xE2, eda = 0x4140, cda = 0x4000
R1#sh ip interface s0/0
Serial0/0 is up, line protocol is down
Internet address is 192.168.10.2/24
Broadcast address is 255.255.255.255
If you use the show controllers command and the show ip interface command, you’ll
see that router R1 isn’t receiving the clocking of the line. This network is a nonproduction
network, so no CSU/DSU is connected to provide clocking for it. This means the DCE end of
the cable will be providing the clock rate—in this case, the R2 router. The show ip interface
indicates that the interface is up but the protocol is down, which means that no keepalives are
being received from the far end. In this example, the likely culprit is the result of bad cable, or
simply the lack of clocking.
Summary
This was a fun chapter! I showed you a lot about the Cisco IOS, and I really hope you gained
a lot of insight into the Cisco router world. I started off by explaining the Cisco Internetwork
Operating System (IOS) and how you can use the IOS to run and congure Cisco routers. You
learned how to bring a router up and what setup mode does. Oh, and by the way, since you
can now basically congure Cisco routers, you should never use setup mode, right?
After I discussed how to connect to a router with a console and LAN connection, I
covered the Cisco help features and how to use the CLI to nd commands and command
parameters. In addition, I discussed some basic show commands to help you verify your
congurations.
Administrative functions on a router help you administer your network and verify
that you are conguring the correct device. Setting router passwords is one of the most
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Exam Essentials 265
important congurations you can perform on your routers. I showed you the ve passwords
you must set, plus I introduced you to the hostname, interface description, and banners as
tools to help you administer your router.
Well, that concludes your introduction to the Cisco IOS. And, as usual, it’s super-important
for you to have the basics that we went over in this chapter down rock-solid before you move
on to the following chapters!
Exam Essentials
Describe the responsibilities of the IOS. The Cisco router IOS software is responsible
for network protocols and providing supporting functions, connecting high-speed trafc
between devices, adding security to control access and preventing unauthorized network
use, providing scalability for ease of network growth and redundancy, and supplying net-
work reliability for connecting to network resources.
List the options available to connect to a Cisco device for management purposes. The
three options available are the console port, auxiliary port, and in-band communication,
such as Telnet, SSH and HTTP. Don’t forget, a Telnet connection is not possible until an IP
address has been congured and a Telnet password has been congured.
Understand the boot sequence of a router. When you rst bring up a Cisco router, it will
run a power-on self-test (POST), and if that passes, it will look for and load the Cisco IOS
from ash memory, if a le is present. The IOS then proceeds to load and looks for a valid
conguration in NVRAM called the startup-cong. If no le is present in NVRAM, the
router will go into setup mode.
Describe the use of setup mode. Setup mode is automatically started if a router boots and
no startup-cong is in NVRAM. You can also bring up setup mode by typing setup from
privileged mode. Setup provides a minimum amount of conguration in an easy format for
someone who does not understand how to congure a Cisco router from the command line.
Differentiate user, privileged, and global configuration modes, both visually and from a com-
mand capabilities perspective. User mode, indicated by the routername> prompt, provides
a command-line interface with very few available commands by default. User mode does not
allow the conguration to be viewed or changed. Privileged mode, indicated by the routername#
prompt, allows a user to both view and change the conguration of a router. You can enter privi-
leged mode by typing the command enable and entering the enable password or enable secret
password, if set. Global conguration mode, indicated by the routername(config)# prompt,
allows conguration changes to be made that apply to the entire router (as opposed to a congu-
ration change that might affect only one interface, for example).
Recognize additional prompts available in other modes and describe their use. Additional
modes are reached via the global conguration prompt, routername(config)#, and
their prompts include interface, router(config-if)#, for making interface settings; line
266 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
conguration mode, router(config-line)#, used to set passwords and make other settings
to various connection methods; and routing protocol modes for various routing protocols,
router(config-router)#, used to enable and congure routing protocols.
Access and utilize editing and help features. Make use of typing a question mark at the end
of commands for help in using the commands. Additionally, understand how to lter com-
mand help with the same question mark and letters. Use the command history to retrieve
commands previously utilized without retyping. Understand the meaning of the caret when
an incorrect command is rejected. Finally, identify useful hot key combinations.
Identify the information provided by the show version command. The show version
command will provide basic conguration for the system hardware as well as the software
version, the names and sources of conguration les, the conguration register setting,
and the boot images.
Set the hostname of a router. The command sequence to set the hostname of a router is
as follows:
enable
config t
hostname Todd
Differentiate the enable password and enable secret password. Both of these passwords
are used to gain access into privileged mode. However, the enable secret password is newer
and is always encrypted by default. Also, if you set the enable password and then set the
enable secret, only the enable secret will be used.
Describe the configuration and use of banners. Banners provide information to users
accessing the device and can be displayed at various login prompts. They are congured
with the banner command and a keyword describing the specic type of banner.
Set the enable secret on a router. To set the enable secret, you use the global cong com-
mand enable secret. Do not use enable secret password password or you will set your
password to password password. Here is an example:
enable
config t
enable secret todd
Set the console password on a router. To set the console password, use the following
sequence:
enable
config t
line console 0
password todd
login
Exam Essentials 267
Set the Telnet password on a router. To set the Telnet password, the sequence is as follows:
enable
config t
line vty 0 4
password todd
login
Describe the advantages of using Secure Shell and list its requirements. Secure Shell (SSH)
uses encrypted keys to send data so that usernames and passwords are not sent in the clear.
It requires that a hostname and domain name be congured and that encryption keys be
generated.
Describe the process of preparing an interface for use. To use an interface, you must
congure it with an IP address and subnet mask in the same subnet of the hosts that will
be connecting to the switch that is connected to that interface. It also must be enabled
with the no shutdown command. A serial interface that is connected back to back with
another router serial interface must also be congured with a clock rate on the DCE end
of the serial cable.
Understand how to troubleshoot a serial link problem. If you type show interface serial
0/0 and see down, line protocol is down, this will be considered a Physical layer problem. If
you see it as up, line protocol is down, then you have a Data Link layer problem.
Understand how to verify your router with the show interfaces command. If you type
show interfaces, you can view the statistics for the interfaces on the router, verify whether
the interfaces are shut down, and see the IP address of each interface.
Describe how to view, edit, delete, and save a configuration. The
show running-config
command is used to view the current conguration being used by the router. The show
startup-config command displays the last conguration that was saved and is the one that
will be used at next startup. The copy running-config startup-config command is used
to save changes made to the running conguration in NVRAM. The erase startup-config
command deletes the saved conguration and will result in the invocation of the setup menu
when the router is rebooted because there will be no conguration present.
268 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
Written Lab 6
The answers to this lab can be found in Appendix A, “Answers to Written Labs.”
Write out the command or commands for the following questions:
1. What command is used to set a serial interface to provide clocking to another router at
1000 Kb?
2. If you telnet into a switch and get the response connection refused, password not
set, what commands would you execute on the destination device to stop receiving this
message and not be prompted for a password?
3. If you type show int fastethernet 0/1 and notice the port is administratively down,
what commands would you execute to enable the interface?
4. If you wanted to delete the configuration stored in NVRAM, what command(s) would
you type?
5. If you wanted to set the user-mode password to todd for the console port, what
command(s) would you type?
6. If you wanted to set the enable secret password to cisco, what command(s) would you
type?
7. If you wanted to determine if serial interface 0/2 on your router should provide clock-
ing, what command would you use?
8. What command would you use to see the terminal history size?
9. You want to reinitialize the switch and totally replace the running-config with the cur-
rent startup-config. What command will you use?
10. How would you set the name of a switch to Sales?
Hands-on Labs
In this section, you will perform commands on a Cisco switch (or you can use a router) that
will help you understand what you learned in this chapter.
You’ll need at least one Cisco device—two would be better, three would be outstanding.
The hands-on labs in this section are included for use with real Cisco routers, but all of these
labs work with the LammleSim IOS Version or Cisco Packet Tracer router simulator. Last, for
the Cisco exam it doesn’t matter what model of switch or router you use with these labs, as
long as you’re running IOS 12.2 or newer. Yes, I know the objectives are 15 code, but that is
not important for any of these labs.
Hands-on Labs 269
It is assumed that the device you’re going to use has no current conguration present. If
necessary, erase any existing conguration with Hands-on Lab 6.1; otherwise, proceed to
Hands-on Lab 6.2:
Lab 6.1: Erasing an Existing Conguration
Lab 6.2: Exploring User, Privileged, and Conguration Modes
Lab 6.3: Using the Help and Editing Features
Lab 6.4: Saving a Conguration
Lab 6.5: Setting Passwords
Lab 6.6: Setting the Hostname, Descriptions, IP Address, and Clock Rate
Hands-on Lab 6.1: Erasing an Existing Configuration
The following lab may require the knowledge of a username and password to enter privi-
leged mode. If the router has a conguration with an unknown username and password for
privileged mode, this procedure will not be possible. It is possible to erase a conguration
without a privileged mode password, but the exact steps depend on the model and will not
be covered until Chapter 7.
1. Start the switch up and when prompted, press Enter.
2. At the Switch> prompt, type enable.
3. If prompted, enter the username and press Enter. Then enter the correct password and
press Enter.
4. At the privileged mode prompt, type erase startup-config.
5. At the privileged mode prompt, type reload, and when prompted to save the configu-
ration, type n for no.
Hands-on Lab 6.2: Exploring User, Privileged, and
Configuration Modes
In the following lab, you’ll explore user, privileged, and conguration modes:
1. Plug the switch in, or turn the router on. If you just erased the configuration as in
Hands-on Lab 6.1, when prompted to continue with the configuration dialog, enter n
for no and press Enter. When prompted, press Enter to connect to your router. This
will put you into user mode.
2. At the Switch> prompt, type a question mark (?).
3. Notice the –more– at the bottom of the screen.
4. Press the Enter key to view the commands line by line. Press the spacebar to view the
commands a full screen at a time. You can type q at any time to quit.
270 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
5. Type enable or en and press Enter. This will put you into privileged mode where you
can change and view the router configuration.
6. At the Switch# prompt, type a question mark (?). Notice how many options are avail-
able to you in privileged mode.
7. Type q to quit.
8. Type config and press Enter.
9. When prompted for a method, press Enter to configure your router using your terminal
(which is the default).
10. At the Switch(config)# prompt, type a question mark (?), then q to quit, or press the
spacebar to view the commands.
11. Type interface f0/1 or int f0/1 (or even int gig0/1) and press Enter. This will
allow you to configure interface FastEthernet 0/1 or Gigabit 0/1.
12. At the Switch(config-if)# prompt, type a question mark (?).
13. If using a router, type int s0/0, interface s0/0 or even interface s0/0/0 and press
Enter. This will allow you to configure interface serial 0/0. Notice that you can go
from interface to interface easily.
14. Ty pe encapsulation ?.
15. Ty pe exit. Notice how this brings you back one level.
16. Press Ctrl+Z. Notice how this brings you out of configuration mode and places you
back into privileged mode.
17. Ty pe disable. This will put you into user mode.
18. Type exit, which will log you out of the router or switch.
Hands-on Lab 6.3: Using the Help and Editing Features
This lab will provide hands-on experience with Cisco’s help and editing features.
1. Log into your device and go to privileged mode by typing en or enable.
2. Type a question mark (?).
3. Type cl? and then press Enter. Notice that you can see all the commands that start
with cl.
4. Type clock ? and press Enter.
Notice the difference between steps 3 and 4. Step 3 has you type letters
with no space and a question mark, which will give you all the commands
that start with cl. Step 4 has you type a command, space, and question
mark. By doing this, you will see the next available parameter.
Hands-on Labs 271
5. Set the clock by typing clock ? and, following the help screens, setting the time and
date. The following steps walk you through setting the date and time:
6. Type clock ?.
7. Type clock set ?.
8. Type clock set 10:30:30 ?.
9. Type clock set 10:30:30 14 May ?.
10. Ty pe clock set 10:30:30 14 May 2011.
11. Press Enter.
12. Type show clock to see the time and date.
13. From privileged mode, type show access-list 10. Don’t press Enter.
14. Press Ctrl+A. This takes you to the beginning of the line.
15. Press Ctrl+E. This should take you back to the end of the line.
16. The Ctrl+A takes your cursor back to the beginning of the line, and then the Ctrl+F
moves your cursor forward one character.
17. Press Ctrl+B, which will move you back one character.
18. Press Enter, then press Ctrl+P. This will repeat the last command.
19. Press the up arrow key on your keyboard. This will also repeat the last command.
20. Type sh history. This shows you the last 10 commands entered.
21. Type terminal history size ?. This changes the history entry size. The ? is the num-
ber of allowed lines.
22. Type show terminal to gather terminal statistics and history size.
23. Type terminal no editing. This turns off advanced editing. Repeat steps 14 through
18 to see that the shortcut editing keys have no effect until you type terminal editing.
24. Type terminal editing and press Enter to re-enable advanced editing.
25. Type sh run, then press your Tab key. This will finish typing the command for you.
26. Type sh start, then press your Tab key. This will finish typing the command for you.
Hands-on Lab 6.4: Saving a Configuration
In this lab, you will get hands-on experience saving a conguration:
1. Log into your device and go into privileged mode by typing en or enable, then
press Enter.
2. To see the configuration stored in NVRAM, type sh start and press Tab and Enter,
or type show startup-config and press Enter. However, if no configuration has been
saved, you will get an error message.
272 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
3. To save a configuration to NVRAM, which is known as startup-config, you can do
one of the following:
u Type copy run start and press Enter.
u Type copy running, press Tab, type start, press Tab, and press Enter.
u Type copy running-config startup-config and press Enter.
4. Type sh start, press Tab, then press Enter.
5. Type sh run, press Tab, then press Enter.
6. Type erase startup-config, press Tab, then press Enter.
7. Type sh start, press Tab, then press Enter. The router will either tell you that
NVRAM is not present or display some other type of message, depending on the IOS
and hardware.
8. Type reload, then press Enter. Acknowledge the reload by pressing Enter. Wait for the
device to reload.
9. Say no to entering setup mode, or just press Ctrl+C.
Hands-on Lab 6.5: Setting Passwords
This hands-on lab will have you set your passwords.
1. Log into the router and go into privileged mode by typing en or enable.
2. Type config t and press Enter.
3. Type enable ?.
4. Set your enable secret password by typing enable secret password (the third word
should be your own personalized password) and pressing Enter. Do not add the param-
eter password after the parameter secret (this would make your password the word
password). An example would be enable secret todd.
5. Now let’s see what happens when you log all the way out of the router and then log
in. Log out by pressing Ctrl+Z, and then type exit and press Enter. Go to privileged
mode. Before you are allowed to enter privileged mode, you will be asked for a pass-
word. If you successfully enter the secret password, you can proceed.
6. Remove the secret password. Go to privileged mode, type config t, and press Enter.
Type no enable secret and press Enter. Log out and then log back in again; now you
should not be asked for a password.
7. One more password used to enter privileged mode is called the enable password. It is
an older, less secure password and is not used if an enable secret password is set. Here
is an example of how to set it:
config t
enable password todd1
Hands-on Labs 273
8. Notice that the enable secret and enable passwords are different. They should never be
set the same. Actually, you should never use the enable password, only enable secret.
9. Type config t to be at the right level to set your console and auxiliary passwords, then
type line ?.
10. Notice that the parameters for the line commands are auxiliary, vty, and console.
You will set all three if you’re on a router; if you’re on a switch, only the console and
VTY lines are available.
11. To set the Telnet or VTY password, type line vty 0 4 and then press Enter. The 0 4
is the range of the five available virtual lines used to connect with Telnet. If you have
an enterprise IOS, the number of lines may vary. Use the question mark to determine
the last line number available on your router.
12. The next command is used to set the authentication on or off. Type login and press
Enter to prompt for a user-mode password when telnetting into the device. You will
not be able to telnet into a Cisco device if the password is not set.
You can use the no login command to disable the user-mode password
prompt when using Telnet. Do not do this in production!
13. One more command you need to set for your VTY password is password. Type password
password to set the password. (password is your password.)
14. Here is an example of how to set the VTY password:
config t
line vty 0 4
password todd
login
15. Set your auxiliary password by first typing line auxiliary 0 or line aux 0 (if you
are using a router).
16. Type login.
17. Ty pe password password.
18. Set your console password by first typing line console 0 or line con 0.
19. Ty pe login.
20. Type password password. Here is an example of the last two command sequences:
config t
line con 0
password todd1
login
line aux 0
274 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
password todd
login
21. You can add the Exec-timeout 0 0 command to the console 0 line. This will stop the
console from timing out and logging you out. The command sequence will now look
like this:
config t
line con 0
password todd2
login
exec-timeout 0 0
22. Set the console prompt to not overwrite the command you’re typing with console
messages by using the command logging synchronous.
config t
line con 0
logging synchronous
Hands-on Lab 6.6: Setting the Hostname, Descriptions,
IP Address, and Clock Rate
This lab will have you set your administrative functions on each device.
1. Log into the switch or router and go into privileged mode by typing en or enable. If
required, enter a username and password.
2. Set your hostname by using the hostname command. Notice that it is one word. Here
is an example of setting your hostname on your router, but the switch uses the exact
same command:
Router#config t
Router(config)#hostname RouterA
RouterA(config)#
Notice that the hostname of the router changed in the prompt as soon as you
pressed Enter.
3. Set a banner that the network administrators will see by using the banner command,
as shown in the following steps.
4. Type config t, then banner ?.
5. Notice that you can set at least four different banners. For this lab we are only inter-
ested in the login and message of the day (MOTD) banners.
Hands-on Labs 275
6. Set your MOTD banner, which will be displayed when a console, auxiliary, or Telnet
connection is made to the router, by typing this:
config t
banner motd #
This is an motd banner
#
7. The preceding example used a # sign as a delimiting character. This tells the router when
the message is done. You cannot use the delimiting character in the message itself.
8. You can remove the MOTD banner by typing the following command:
config t
no banner motd
9. Set the login banner by typing this:
config t
banner login #
This is a login banner
#
10. The login banner will display immediately after the MOTD but before the user-mode
password prompt. Remember that you set your user-mode passwords by setting the
console, auxiliary, and VTY line passwords.
11. You can remove the login banner by typing this:
config t
no banner login
12. You can add an IP address to an interface with the ip address command if you are using
a router. You need to get into interface configuration mode first; here is an example of
how you do that:
config t
int f0/1
ip address 1.1.1.1 255.255.0.0
no shutdown
Notice that the IP address (1.1.1.1) and subnet mask (255.255.0.0) are congured on
one line. The no shutdown (or no shut for short) command is used to enable the inter-
face. All interfaces are shut down by default on a router. If you are on a layer 2 switch,
you can set an IP address only on the VLAN 1 interface.
276 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
13. You can add identification to an interface by using the description command. This is
useful for adding information about the connection. Here is an example:
config t
int f0/1
ip address 2.2.2.1 255.255.0.0
no shut
description LAN link to Finance
14. You can add the bandwidth of a serial link as well as the clock rate when simulating a
DCE WAN link on a router. Here is an example:
config t
int s0/0
bandwidth 1000
clock rate 1000000
Review Questions 277
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional ques-
tions, please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. You type show interfaces fa0/1 and get this output:
275496 packets input, 35226811 bytes, 0 no buffer
Received 69748 broadcasts (58822 multicasts)
0 runts, 0 giants, 0 throttles
111395 input errors, 511987 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 58822 multicast, 0 pause input
0 input packets with dribble condition detected
2392529 packets output, 337933522 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 PAUSE output
0 output buffer failures, 0 output buffers swapped out
What could the problem possibly be with this interface?
A. Speed mismatch on directly connected interfaces
B. Collisions causing CRC errors
C. Frames received are too large
D. Interference on the Ethernet cable
2. The output of the show running-config command comes from ___________.
A. NVRAM
B. Flash
C. RAM
D. Firmware
278 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
3. Which two of the following commands are required when configuring SSH on your
router? (Choose two.)
A. enable secret password
B. exec-timeout 0 0
C. ip domain-name name
D. username name password password
E. ip ssh version 2
4. Which command will show you whether a DTE or a DCE cable is plugged into serial
0/0 on your router’s WAN port?
A. sh int s0/0
B. sh int serial 0/0
C. show controllers s 0/0
D. show serial 0/0 controllers
5. In the work area, drag the router term to its definition on the right.
Mode Definition
User EXEC mode Commands that affect the entire system
Privileged EXEC mode Commands that affect interfaces/processes only
Global configuration mode Interactive configuration dialog
Specific configuration modes Provides access to all other router commands
Setup mode Limited to basic monitoring commands
6. Using the given output, what type of interface is f0/0?
[output cut]
Hardware is MV96340 Ethernet, address is 001a.2f55.c9e8 (bia 001a.2f55.c9e8)
Internet address is 192.168.1.33/27
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
A. 10 MB
B. 100 MB
C. 1000 MB
D. 1000 MB
Review Questions 279
7. Which of the following commands will configure all the default VTY ports on a
switch?
A. Switch#line vty 0 4
B. Switch(config)#line vty 0 4
C. Switch(config-if)#line console 0
D. Switch(config)#line vty all
8. Which of the following commands sets the privileged mode password to Cisco and
encrypts the password?
A. enable secret password Cisco
B. enable secret cisco
C. enable secret Cisco
D. enable password Cisco
9. If you wanted administrators to see a message when logging into the switch, which
command would you use?
A. message banner motd
B. banner message motd
C. banner motd
D. message motd
10. Which of the following prompts indicates that the switch is currently in privileged
mode?
A. Switch(config)#
B. Switch>
C. Switch#
D. Switch(config-if)
11. What command do you type to save the configuration stored in RAM to NVRAM?
A. Switch(config)#copy current to starting
B. Switch#copy starting to running
C. Switch(config)#copy running-config startup-config
D. Switch#copy run start
280 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
12. You try to telnet into SF from router Corp and receive this message:
Corp#telnet SF
Trying SF (10.0.0.1)…Open
Password required, but none set
[Connection to SF closed by foreign host]
Corp#
Which of the following sequences will address this problem correctly?
A. Corp(config)#line console 0
Corp (config-line)#password password
Corp (config-line)#login
B. SF (config)#line console 0
SF(config-line)#enable secret password
SF(config-line)#login
C. Corp(config)#line vty 0 4
Corp (config-line)#password password
Corp (config-line)#login
D. SF (config)#line vty 0 4
SF(config-line)#password password
SF(config-line)#login
13. Which command will delete the contents of NVRAM on a switch?
A. delete NVRAM
B. delete startup-config
C. erase flash
D. erase startup-config
E. erase start
14. What is the problem with an interface if you type show interface g0/1 and receive
the following message?
Gigabit 0/1 is administratively down, line protocol is down
A. The keepalives are different times.
B. The administrator has the interface shut down.
C. The administrator is pinging from the interface.
D. No cable is attached.
Review Questions 281
15. Which of the following commands displays the configurable parameters and statistics
of all interfaces on a switch?
A. show running-config
B. show startup-config
C. show interfaces
D. show versions
16. If you delete the contents of NVRAM and reboot the switch, what mode will you be in?
A. Privileged mode
B. Global mode
C. Setup mode
D. NVRAM loaded mode
17. You type the following command into the switch and receive the following output:
Switch#show fastethernet 0/1
^
% Invalid input detected at ‘^’ marker.
Why was this error message displayed?
A. You need to be in privileged mode.
B. You cannot have a space between fastethernet and 0/1.
C. The switch does not have a Fastethernet 0/1 interface.
D. Part of the command is missing.
18. You type Switch#sh r and receive a % ambiguous command error. Why did you receive
this message?
A. The command requires additional options or parameters.
B. There is more than one show command that starts with the letter r.
C. There is no show command that starts with r.
D. The command is being executed from the wrong mode.
19. Which of the following commands will display the current IP addressing and the
layer 1 and 2 status of an interface? (Choose two.)
A. show version
B. show interfaces
C. show controllers
D. show ip interface
E. show running-config
282 Chapter 6 u Cisco’s Internetworking Operating System (IOS)
20. At which layer of the OSI model would you assume the problem is if you type show
interface serial 1 and receive the following message?
Serial1 is down, line protocol is down
A. Physical layer
B. Data Link layer
C. Network layer
D. None; it is a router problem.
Chapter
7
Managing a Cisco
Internetwork
THE FOLLOWING ICND1 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 LAN Switching Technologies
■Verify network status and switch operation using basic
utilities such as ping, telnet, and ssh
1 IP Routing Technologies
■Configure and verify utilizing the CLI to set basic Router
configuration
■Cisco IOS commands to perform basic router setup
■Verify router configuration and network connectivity
■Cisco IOS commands to review basic router information
and network connectivity
1 IP Services
■Configure and verify DHCP (IOS Router)
■configuring router interfaces to use DHCP
■DHCP options
■excluded addresses
■lease time
■Configure and verify NTP as a client
Here in Chapter 7, I’m going to show you how to manage Cisco
routers and switches on an internetwork. You’ll nd out how to
manage Cisco devices by using the copy command with a TFTP
host and how to congure DHCP and NTP, plus you’ll get a survey of the Cisco Discovery
Protocol (CDP). I’ll also show you how to resolve hostnames.
I’ll wrap up the chapter by guiding you through some important Cisco IOS troubleshooting
techniques to ensure that you’re well equipped with these key skills.
To find up-to-the minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
The Internal Components of a
Cisco Router and Switch
Unless you happen to be really savvy about the inner and outer workings of all your car’s
systems and its machinery and how all of that technology works together, you’ll take it to
someone who does know how to keep it maintained, gure out what’s wrong when it stops
running, and get it up and running again. It’s the same deal with Cisco networking devices—
you need to know all about their major components, pieces, and parts as well as what they all
do and why and how they all work together to make a network work. The more solid your
knowledge, the more expert you are about these things and the better equipped you’ll be to
congure and troubleshoot a Cisco internetwork. Toward that goal, study Table7.1 for an
introductory description of a Cisco router’s major components.
TABLE7.1 Cisco router components
Component Description
Bootstrap Stored in the microcode of the ROM, the bootstrap is
used to bring a router up during initialization. It boots the
router up and then loads the IOS.
The Internal Components of a Cisco Router and Switch 285
Component Description
POST (power-on self-test) Also stored in the microcode of the ROM, the POST is
used to check the basic functionality of the router hard-
ware and determines which interfaces are present.
ROM monitor Again, stored in the microcode of the ROM, the ROM
monitor is used for manufacturing, testing, and trouble-
shooting, as well as running a mini-IOS when the IOS in
flash fails to load.
Mini-IOS Called the RXBOOT or bootloader by Cisco, the mini-IOS is
a small IOS in ROM that can be used to bring up an inter-
face and load a Cisco IOS into flash memory. The mini-IOS
can also perform a few other maintenance operations.
RAM (random access memory) Used to hold packet buffers, ARP cache, routing tables, and
also the software and data structures that allow the router
to function. Running-config is stored in RAM, and most
routers expand the IOS from flash into RAM upon boot.
ROM (read-only memory) Used to start and maintain the router. Holds the POST and
the bootstrap program as well as the mini-IOS.
Flash memory Stores the Cisco IOS by default. Flash memory is not
erased when the router is reloaded. It is EEPROM (elec-
tronically erasable programmable read-only memory)
created by Intel.
NVRAM (nonvolatile RAM) Used to hold the router and switch configuration.
NVRAM is not erased when the router or switch is
reloaded. Does not store an IOS. The configuration
register is stored in NVRAM.
Configuration register Used to control how the router boots up. This value can
be found as the last line of the show version command
output and by default is set to 0x2102, which tells the
router to load the IOS from flash memory as well as to
load the configuration from NVRAM.
The Router and Switch Boot Sequence
When a Cisco device boots up, it performs a series of steps, called the boot sequence, to test the
hardware and load the necessary software. The boot sequence comprises the following steps:
1. The IOS device performs a POST, which tests the hardware to verify that all components
of the device are present and operational. The post takes stock of the different interfaces
on the switch or router, and it’s stored in and runs from read-only memory (ROM).
286 Chapter 7 u Managing a Cisco Internetwork
2. The bootstrap in ROM then locates and loads the Cisco IOS software by executing
programs responsible for finding where each IOS program is located. Once they are
found, it then loads the proper files. By default, the IOS software is loaded from flash
memory in all Cisco devices.
The default order of an IOS loading from a Cisco device begins with flash,
then TFTP server, and finally, ROM.
3. The IOS software then looks for a valid configuration file stored in NVRAM. This
file is called startup-config and will be present only if an administrator has copied the
running-config file into NVRAM.
4. If a startup-config file is found in NVRAM, the router or switch will copy it, place it in
RAM, and name the file the running-config. The device will use this file to run, and the
router/switch should now be operational. If no startup-config file is found in NVRAM,
the router/switch reacts by broadcasting out any interface that detects carrier detect (CD)
to locate a TFTP host in its search for a configuration. When that fails, which it typically
does, the device will begin the setup mode configuration process. Most people don’t even
realize the device has attempted this process!
Backing Up and Restoring the
Cisco Configuration
Any changes that you make to the conguration are stored in the running-cong le. And
if you don’t enter a copy run start command after you make a change to running-cong,
that change will totally disappear if the device reboots or gets powered down. As always,
backups are good, so you’ll want to make another backup of the conguration information
just in case the router or switch completely dies on you. Even if your machine is healthy and
happy, it’s good to have a backup for reference and documentation reasons!
Next, I’ll cover how to copy the conguration of a router to a TFTP server as well as
how to restore that conguration.
Backing Up the Cisco Configuration
To copy the conguration from an IOS device to a TFTP server, you can use either the
copy running-config tftp or the copy startup-config tftp command. Either one
will back up the router conguration that’s currently running in DRAM or one that’s
stored in NVRAM.
Backing Up and Restoring the Cisco Configuration 287
Verifying the Current Configuration
To verify the conguration in DRAM, use the show running-config command (sh run for
short) like this:
Router#show running-config
Building configuration...
Current configuration : 855 bytes
!
version 15.0
The current conguration information indicates that the router is running version 15.0
of the IOS.
Verifying the Stored Configuration
Next, you should check the conguration stored in NVRAM. To see this, use the show
startup-config command (sh start for short) like this:
Router#sh start
Using 855 out of 524288 bytes
!
! Last configuration change at 04:49:14 UTC Fri Mar 5 1993
!
version 15.0
The rst line shows you how much room your backup conguration is taking up. Here,
we can see that NVRAM is about 524 KB and that only 855 bytes of it are being used. But
memory is easier to reveal via the show version command when you’re using an ISR router.
If you’re not sure that the les are the same and the running-cong le is what you want to
go with, then use the copy running-config startup-config command. This will help you
ensure that both les are in fact the same. I’ll guide you through this in the next section.
Copying the Current Configuration to NVRAM
By copying running-cong to NVRAM as a backup, as shown in the following output,
you ensure that your running-cong will always be reloaded if the router gets rebooted.
Starting in the 12.0 IOS, you’ll be prompted for the lename you want to use:
Router#copy running-config startup-config
Destination filename [startup-config]?[enter]
Building configuration...
[OK]
288 Chapter 7 u Managing a Cisco Internetwork
The reason the lename prompt appears is that there are now so many options you can
use when using the copy command—check it out:
Router#copy running-config ?
flash: Copy to flash: file system
ftp: Copy to ftp: file system
http: Copy to http: file system
https: Copy to https: file system
null: Copy to null: file system
nvram: Copy to nvram: file system
rcp: Copy to rcp: file system
running-config Update (merge with) current system configuration
scp: Copy to scp: file system
startup-config Copy to startup configuration
syslog: Copy to syslog: file system
system: Copy to system: file system
tftp: Copy to tftp: file system
tmpsys: Copy to tmpsys: file system
We’ll go over the copy command in more detail in the Sybex ICND2 Study Guide.
Copying the Configuration to a TFTP Server
Once the le is copied to NVRAM, you can make a second backup to a TFTP server by
using the copy running-config tftp command, or copy run tftp for short. I’m going to
set the hostname to Todd before I run this command:
Todd#copy running-config tftp
Address or name of remote host []? 10.10.10.254
Destination filename [todd-confg]?
!!
776 bytes copied in 0.800 secs (970 bytes/sec)
If you have a hostname already congured, the command will automatically use the
hostname plus the extension -confg as the name of the le.
Restoring the Cisco Configuration
What do you do if you’ve changed your running-cong le and want to restore the congu-
ration to the version in the startup-cong le? The easiest way to get this done is to use the
copy startup-config running-config command, or copy start run for short, but this
will work only if you copied running-cong into NVRAM before you made any changes!
Of course, a reload of the device will work too!
If you did copy the conguration to a TFTP server as a second backup, you can restore the
conguration using the copy tftp running-config command (copy tftp run for short), or
Backing Up and Restoring the Cisco Configuration 289
the copy tftp startup-config command (copy tftp start for short), as shown in the out-
put below. Just so you know, the old command we used to use for this is config net:
Todd#copy tftp running-config
Address or name of remote host []?10.10.10.254
Source filename []?todd-confg
Destination filename[running-config]?[enter]
Accessing tftp://10.10.10.254/todd-confg...
Loading todd-confg from 10.10.10.254 (via FastEthernet0/0):
!!
[OK - 776 bytes]
776 bytes copied in 9.212 secs (84 bytes/sec)
Todd#
*Mar 7 17:53:34.071: %SYS-5-CONFIG_I: Configured from
tftp://10.10.10.254/todd-confg by console
Okay, here we can see that the conguration le is an ASCII text le, meaning that
before you copy the conguration stored on a TFTP server back to a router, you can make
changes to the le with any text editor.
Remember that when you copy or merge a configuration from a TFTP
server to a freshly erased and rebooted router’s RAM, the interfaces are
shut down by default and you must manually enable each interface with
the no shutdown command.
Erasing the Configuration
To delete the startup-cong le on a Cisco router or switch, use the command erase
startup-config, like this:
Todd#erase startup-config
Erasing the nvram filesystem will remove all configuration files!
Continue? [confirm][enter]
[OK]
Erase of nvram: complete
*Mar 7 17:56:20.407: %SYS-7-NV_BLOCK_INIT: Initialized the geometry of nvram
Todd#reload
System configuration has been modified. Save? [yes/no]:n
Proceed with reload? [confirm][enter]
*Mar 7 17:56:31.059: %SYS-5-RELOAD: Reload requested by console.
Reload Reason: Reload Command.
290 Chapter 7 u Managing a Cisco Internetwork
This command deletes the contents of NVRAM on the switch and router. If you type
reload while in privileged mode and say no to saving changes, the switch or router will
reload and come up into setup mode.
Configuring DHCP
We went over DHCP in Chapter 3, “Introduction to TCP/IP”, where I described how it
works and what happens when there’s a conict. At this point, you’re ready to learn how
to congure DHCP on Cisco’s IOS as well as how to congure a DHCP forwarder for
when your hosts don’t live on the same LAN as the DHCP server. Do you remember the
four-way handshake hosts used to get an address from a server? If not, now would be a
really great time to head back to Chapter 3 and thoroughly review that before moving
on with this!
To congure a DHCP server for your hosts, you need the following information at
minimum:
Network and mask for each LAN Network ID, also called a scope. All addresses in a
subnet can be leased to hosts by default.
Reserved/excluded addresses Reserved addresses for printers, servers, routers, etc.
These addresses will not be handed out to hosts. I usually reserve the rst address of
each subnet for the router, but you don’t have to do this.
Default router This is the router’s address for each LAN.
DNS address A list of DNS server addresses provided to hosts so they can resolve names.
Here are your conguration steps:
1. Exclude the addresses you want to reserve. The reason you do this step first is
because as soon as you set a network ID, the DHCP service will start responding
to client requests.
2. Create your pool for each LAN using a unique name.
3. Choose the network ID and subnet mask for the DHCP pool that the server will use to
provide addresses to hosts.
4. Add the address used for the default gateway of the subnet.
5. Provide the DNS server address(es).
6. If you don’t want to use the default lease time of 24 hours, you need to set the lease
time in days, hours, and minutes.
I’ll congure the switch in Figure7.1 to be the DHCP server for the Sales Wireless LAN.
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Configuring DHCP 291
FIGURE7.1 DHCP Configuration example on a switch
Int f0/0 192.168.10.1
S1
Sales wireless area
DHCP server
Understand that this conguration could just have easily been placed on the router in
Figure7.1. Here’s how we’ll congure DHCP using the 192.168.10.0/24 network ID:
Switch(config)#ip dhcp excluded-address 192.168.10.1 192.168.10.10
Switch(config)#ip dhcp pool Sales_Wireless
Switch(dhcp-config)#network 192.168.10.0 255.255.255.0
Switch(dhcp-config)#default-router 192.168.10.1
Switch(dhcp-config)#dns-server 4.4.4.4
Switch(dhcp-config)#lease 3 12 15
First, you can see that I reserved 10 addresses in the range for the router, servers, and
printers, etc. I then created the pool named Sales_Wireless, added the default gateway and
DNS server, and set the lease to 3 days, 12 hours, and 15 minutes (which isn’t really sig-
nicant because I just set it that way for demonstration purposes). Pretty straightforward,
right? The switch will now respond to DHCP client requests. But what happens if we need
to provide an IP address from a DHCP server to a host that’s not in our broadcast domain,
or if we want to receive a DHCP address for a client from a remote server?
DHCP Relay
If you need to provide addresses from a DHCP server to hosts that aren’t on the same
LAN as the DHCP server, you can congure your router interface to relay or forward
the DHCP client requests, as shown in Figure7.2. If we don’t provide this service, our
router would receive the DHCP client broadcast, promptly discard it, and the remote
host would never receive an address—unless we added a DHCP server on every broad-
cast domain! Let’s take a look at how we would typically congure DHCP service in
today’s networks.
So we know that because the hosts off the router don’t have access to a DHCP server,
the router will simply drop their client request broadcast messages by default. To solve
292 Chapter 7 u Managing a Cisco Internetwork
this problem, we can congure the F0/0 interface of the router to accept the DHCP client
requests and forward them to the DHCP server like this:
Router#config t
Router(config)#interface fa0/0
Router(config-if)#ip helper-address 10.10.10.254
FIGURE7.2 Configuring a DHCP relay
Int Fa0/0 192.168.10.1
I hear the port 67 request on int Fa0/0!
I’ll forward this broadcast as a unicast to
10.10.10.254 ASAP!
Sales
Admin
DHCP client broadcast request
DHCP server
10.10.10.254
Now I know that was a pretty simple example, and there are denitely other ways to
congure the relay, but rest assured that I’ve covered the objectives for you. Also, I want
you to know that ip helper-address forwards more than just DHCP client requests, so be
sure to research this command before you implement it! Now that I’ve demonstrated how
to create the DHCP service, let’s take a minute to verify DHCP before moving on to NTP.
Verifying DHCP on Cisco IOS
There are some really useful verication commands to use on a Cisco IOS device for moni-
toring and verifying a DHCP service. You’ll get to see the output for these commands when
I build the network in Chapter 8, “IP Routing,” and add DHCP to the two remote LANs. I
just want you to begin getting familiar with them, so here’s a list of four very important ones
and what they do:
show ip dhcp binding Lists state information about each IP address currently leased to
a client.
show ip dhcp pool [poolname] Lists the congured range of IP addresses, plus statis-
tics for the number of currently leased addresses and the high watermark for leases from
each pool.
Network Time Protocol (NTP) 293
show ip dhcp server statistics Lists DHCP server statistics—a lot of them!
show ip dhcp conflict If someone statically congures an IP address on a LAN and the
DHCP server hands out that same address, you’ll end up with a duplicate address. This isn’t
good, which is why this command is so helpful!
Again, no worries because we’ll cover these vital commands thoroughly in the next chapter.
Network Time Protocol (NTP)
Network Time Protocol provides pretty much what it describes: time to all your network
devices. To be more precise, NTP synchronizes clocks of computer systems over packet-
switched, variable-latency data networks.
Typically you’ll have an NTP server that connects through the Internet to an atomic clock.
This time can then be synchronized through the network to keep all routers, switches, servers,
etc. receiving the same time information.
Correct network time within the network is important:
u Correct time allows the tracking of events in the network in the correct order.
u Clock synchronization is critical for the correct interpretation of events within the
syslog data.
u Clock synchronization is critical for digital certificates.
Making sure all your devices have the correct time is especially helpful for your routers and
switches for looking at logs regarding security issues or other maintenance issues. Routers and
switches issue log messages when different events take place—for example, when an interface
goes down and then back up. As you already know, all messages generated by the IOS go only
to the console port by default. However, as shown in Figure7.3, those console messages can be
directed to a syslog server.
FIGURE7.3 Sending console messages to a syslog server
Syslog server
172.16.10.1 Console messages
SF
A syslog server saves copies of console messages and can time-stamp them so you can
view them at a later time. This is actually rather easy to do. Here would be your congura-
tion on the SF router:
SF(config)#logging host 172.16.10.1
SF(config)#service timestamps log datetime msec
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294 Chapter 7 u Managing a Cisco Internetwork
Now all the console messages will be stored in one location that you can view at your
convenience. However, even though I had the messages time-stamped in Figure7.3 with the
command service timestamps log datetime msec, this doesn’t mean that we’ll know the
exact time if using default clock sources. To make sure all devices are synchronized with
the same time information, we’ll congure our devices to receive the accurate time informa-
tion from a centralized server, as shown in Figure7.4:
SF(config)#ntp server 172.16.10.1 version 4
FIGURE7.4 Synchronizing time information
NTP Client
Here is the exact time and date!
172.16.10.1
10.1.1.2
NTP server
Corp
SF
Just use that one simple command on all your devices and each network device on
your network will then have the same exact time and date information. You can then rest
assured that your time stamps are accurate. You can also make your router or switch be an
NTP server with the ntp master command.
To verify our VTP client is receiving clocking information, we use the following
commands:
SF#sh ntp ?
associations NTP associations
status NTP status status VTP domain status
SF#sh ntp status
Clock is unsynchronized, stratum 16, no reference clock
nominal freq is 119.2092 Hz, actual freq is 119.2092 Hz, precision is 2**18
reference time is 00000000.00000000 (00:00:00.000 UTC Mon Jan 1 1900)
clock offset is 0.0000 msec, root delay is 0.00 msec
S1#sh ntp associations
address ref clock st when poll reach delay offset disp
~172.16.10.1 0.0.0.0 16 - 64 0 0.0 0.00 16000.
* master (synced), # master (unsynced), + selected, - candidate, ~ configured
Using Cisco Discovery Protocol (CDP) 295
You can see in the example that the NTP client in SF is not synchronized with the server
by using the show ntp status command. The stratum value is a number from 1 to 15, and a
lower stratum value indicates a higher NTP priority; 16 means there is no clocking received.
There are many other congurations of an NTP client that are available, such as
authentication of NTP so a router or switch isn’t fooled into changing the time of an
attack, for example.
Using Cisco Discovery Protocol (CDP)
Cisco Discovery Protocol (CDP) is a proprietary protocol designed by Cisco to help adminis-
trators collect information about locally attached devices. Armed with CDP, you can gather
hardware and protocol information about neighbor devices, which is crucial information to
have when troubleshooting and documenting the network.
Let’s start by exploring the CDP timer and CDP commands we’ll need to verify our
network.
Getting CDP Timers and Holdtime Information
The show cdp command (sh cdp for short) gives you information about two CDP global
parameters that can be congured on Cisco devices:
u CDP timer delimits how often CDP packets are transmitted out all active interfaces.
u CDP holdtime delimits the amount of time that the device will hold packets received
from neighbor devices.
Both Cisco routers and switches use the same parameters. Check out Figure7.5 to see
how CDP works within a switched network that I set up for my switching labs in this book.
FIGURE7.5 Cisco Discovery Protocol
F0/1 F0/2
F0/16 F0/18
F0/15 F0/17
F0/5F0/5
F0/6F0/6
F0/2 F0/1
10.100.128.9
SW–2
10.100.128.10
SW–1
10.100.128.8
SW–3
The output on my 3560 SW-3 looks like this:
SW-3#sh cdp
Global CDP information:
296 Chapter 7 u Managing a Cisco Internetwork
Sending CDP packets every 60 seconds
Sending a holdtime value of 180 seconds
Sending CDPv2 advertisements is enabled
This output tells us that the default transmits every 60 seconds and will hold packets from
a neighbor in the CDP table for 180 seconds. I can use the global commands cdp holdtime
and cdp timer to congure the CDP holdtime and timer on a router if necessary like this:
SW-3(config)#cdp ?
advertise-v2 CDP sends version-2 advertisements
holdtime Specify the holdtime (in sec) to be sent in packets
run Enable CDP
timer Specify the rate at which CDP packets are sent (in sec)
tlv Enable exchange of specific tlv information
SW-3(config)#cdp holdtime ?
<10-255> Length of time (in sec) that receiver must keep this packet
SW-3(config)#cdp timer ?
<5-254> Rate at which CDP packets are sent (in sec)
You can turn off CDP completely with the no cdp run command from global conguration
mode of a router and enable it with the cdp run command:
SW-3(config)#no cdp run
SW-3(config)#cdp run
To turn CDP off or on for an interface, use the no cdp enable and cdp enable commands.
Gathering Neighbor Information
The show cdp neighbor command (sh cdp nei for short) delivers information about directly
connected devices. It’s important to remember that CDP packets aren’t passed through a Cisco
switch and that you only see what’s directly attached. So this means that if your router is con-
nected to a switch, you won’t see any of the Cisco devices connected to that switch!
The following output shows the show cdp neighbor command I used on my SW-3:
SW-3#sh cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
D - Remote, C - CVTA, M - Two-port Mac Relay Device ID
Local Intrfce Holdtme Capability Platform Port ID
SW-1 Fas 0/1 170 S I WS-C3560- Fas 0/15
Using Cisco Discovery Protocol (CDP) 297
SW-1 Fas 0/2 170 S I WS-C3560- Fas 0/16
SW-2 Fas 0/5 162 S I WS-C3560- Fas 0/5
SW-2 Fas 0/6 162 S I WS-C3560- Fas 0/6
Okay—we can see that I’m directly connected with a console cable to the SW-3 switch and
also that SW-3 is directly connected to two other switches. However, do we really need the
gure to draw out our network? We don’t! CDP allows me to see who my directly connected
neighbors are and gather information about them. From the SW-3 switch, we can see that
there are two connections to SW-1 and two connections to SW-2. SW-3 connects to SW-1 with
ports Fas 0/1 and Fas 0/2, and we have connections to SW-2 with local interfaces Fas 0/5 and
Fas 0/6. Both the SW-1 and SW-2 switches are 3650 switches, and SW-1 is using ports Fas
0/15 and Fas 0/16 to connect to SW-3. SW-2 is using ports Fas 0/5 and Fas 0/6.
To sum this up, the device ID shows the congured hostname of the connected device,
that the local interface is our interface, and the port ID is the remote devices’ directly con-
nected interface. Remember that all you get to view are directly connected devices!
Table7.2 summarizes the information displayed by the show cdp neighbor command
for each device.
TABLE7.2 Output of the show cdp neighbors command
Field Description
Device ID The hostname of the device directly connected.
Local Interface The port or interface on which you are receiving the CDP packet.
Holdtime The remaining amount of time the router will hold the information
before discarding it if no more CDP packets are received.
Capability The capability of the neighbor—the router, switch, or repeater. The
capability codes are listed at the top of the command output.
Platform The type of Cisco device directly connected. In the previous out-
put, the SW-3 shows it’s directly connected to two 3560 switches.
Port ID The neighbor device’s port or interface on which the CDP packets
are multicast.
It’s imperative that you can look at the output of a show cdp neighbors
command and decipher the information gained about the neighbor device’s
capability, whether it’s a router or switch, the model number (platform), your
port connecting to that device (local interface), and the port of the neighbor
connecting to you (port ID).
298 Chapter 7 u Managing a Cisco Internetwork
Another command that will deliver the goods on neighbor information is the show
cdp neighbors detail command (show cdp nei de for short). This command can be
run on both routers and switches, and it displays detailed information about each device
connected to the device you’re running the command on. Check out the router output in
Listing7.1.
Listing7.1: Showing CDP Neighbors
SW-3#sh cdp neighbors detail
-------------------------
Device ID: SW-1
Entry address(es):
IP address: 10.100.128.10
Platform: cisco WS-C3560-24TS, Capabilities: Switch IGMP
Interface: FastEthernet0/1, Port ID (outgoing port): FastEthernet0/15
Holdtime : 137 sec
Version :
Cisco IOS Software, C3560 Software (C3560-IPSERVICESK9-M), Version 12.2(55)SE7,
RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2013 by Cisco Systems, Inc.
Compiled Mon 28-Jan-13 10:10 by prod_rel_team
advertisement version: 2
Protocol Hello: OUI=0x00000C, Protocol ID=0x0112; payload len=27, value=00000000
FFFFFFFF010221FF000000000000001C575EC880FF0000
VTP Management Domain: 'NULL'
Native VLAN: 1
Duplex: full
Power Available TLV:
Power request id: 0, Power management id: 1, Power available: 0, Power
management level: -1
Management address(es):
IP address: 10.100.128.10
-------------------------
Using Cisco Discovery Protocol (CDP) 299
[ouput cut]
-------------------------
Device ID: SW-2
Entry address(es):
IP address: 10.100.128.9
Platform: cisco WS-C3560-8PC, Capabilities: Switch IGMP
Interface: FastEthernet0/5, Port ID (outgoing port): FastEthernet0/5
Holdtime : 129 sec
Version :
Cisco IOS Software, C3560 Software (C3560-IPBASE-M), Version 12.2(35)SE5, RELEASE
SOFTWARE (fc1)
Copyright (c) 1986-2007 by Cisco Systems, Inc.
Compiled Thu 19-Jul-07 18:15 by nachen
advertisement version: 2
Protocol Hello: OUI=0x00000C, Protocol ID=0x0112; payload len=27, value=00000000
FFFFFFFF010221FF000000000000B41489D91880FF0000
VTP Management Domain: 'NULL'
Native VLAN: 1
Duplex: full
Power Available TLV:
Power request id: 0, Power management id: 1, Power available: 0, Power
management level: -1
Management address(es):
IP address: 10.100.128.9
[output cut]
So what’s revealed here? First, we’ve been given the hostname and IP address of all
directly connected devices. And in addition to the same information displayed by the show
cdp neighbor command (see Table7.2), the show cdp neighbor detail command tells us
about the IOS version and IP address of the neighbor device—that’s quite a bit!
The show cdp entry * command displays the same information as the show cdp
neighbors detail command. There isn’t any difference between these commands.
300 Chapter 7 u Managing a Cisco Internetwork
CDP Can Save Lives!
Karen has just been hired as a senior network consultant at a large hospital in Dallas,
Texas, so she’s expected to be able to take care of any problem that rears its ugly head.
As if that weren’t enough pressure, she also has to worry about the horrid possibility that
people won’t receive correct health care solutions—even the correct medications—if the
network goes down. Talk about a potential life-or-death situation!
But Karen is confident and begins her job optimistically. Of course, it’s not long before the
network reveals that it has a few problems. Unfazed, she asks one of the junior administra-
tors for a network map so she can troubleshoot the network. This person tells her that the
old senior administrator, who she replaced, had them with him and now no one can find
them. The sky begins to darken!
Doctors are calling every couple of minutes because they can’t get the necessary infor-
mation they need to take care of their patients. What should she do?
It’s CDP to the rescue! And it’s a gift that this hospital happens to be running Cisco routers
and switches exclusively, because CDP is enabled by default on all Cisco devices. Karen
is also in luck because the disgruntled former administrator didn’t turn off CDP on any
devices before he left!
So all Karen has to do now is to use the show cdp neighbor detail command to find
all the information she needs about each device to help draw out the hospital network,
bringing it back up to speed so the personnel who rely upon it can get on to the important
business of saving lives!
The only snag for you nailing this in your own network is if you don’t know the passwords
of all those devices. Your only hope then is to somehow find out the access passwords or
to perform password recovery on them.
So, use CDP—you never know when you may end up saving someone’s life.
By the way, this is a true story!
Documenting a Network Topology Using CDP
With that moving real-life scenario in mind, I’m now going to show you how to document a
sample network by using CDP. You’ll learn to determine the appropriate router types, inter-
face types, and IP addresses of various interfaces using only CDP commands and the show
running-config command. And you can only console into the Lab_A router to document
Using Cisco Discovery Protocol (CDP) 301
the network. You’ll have to assign any remote routers the next IP address in each range.
We’ll use a different gure for this example—Figure7.6— to help us to complete the neces-
sary documentation.
FIGURE7.6 Documenting a network topology using CDP
.1
Int
Router
IP address
Int
Router
IP address
Int
Router
IP address
Int
Router
IP address
Fa0/0
Fa0/1
S0/1
S0/0
.1
.1 .1
In this output, you can see that you have a router with four interfaces: two Fast Ethernet
and two serial. First, determine the IP addresses of each interface by using the show running-
config command like this:
Lab_A#sh running-config
Building configuration...
Current configuration : 960 bytes
!
version 12.2
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname Lab_A
!
ip subnet-zero
!
!
interface FastEthernet0/0
ip address 192.168.21.1 255.255.255.0
duplex auto
!
302 Chapter 7 u Managing a Cisco Internetwork
interface FastEthernet0/1
ip address 192.168.18.1 255.255.255.0
duplex auto
!
interface Serial0/0
ip address 192.168.23.1 255.255.255.0
!
interface Serial0/1
ip address 192.168.28.1 255.255.255.0
!
ip classless
!
line con 0
line aux 0
line vty 0 4
!
end
With this step completed, you can now write down the IP addresses of the Lab_A router’s
four interfaces. Next, you must determine the type of device on the other end of each of these
interfaces. It’s easy—just use the show cdp neighbors command:
Lab_A#sh cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater
Device ID Local Intrfce Holdtme Capability Platform Port ID
Lab_B Fas 0/0 178 R 2501 E0
Lab_C Fas 0/1 137 R 2621 Fa0/0
Lab_D Ser 0/0 178 R 2514 S1
Lab_E Ser 0/1 137 R 2620 S0/1
Wow—looks like we’re connected to some old routers! But it’s not our job to judge. Our
mission is to draw out our network, so it’s good that we’ve got some nice information to meet
the challenge with now. By using both the show running-config and show cdp neighbors
commands, we know about all the IP addresses of the Lab_A router, the types of routers con-
nected to each of the Lab_A router’s links, and all the interfaces of the remote routers.
Now that we’re equipped with all the information gathered via show running-config
and show cdp neighbors, we can accurately create the topology in Figure7.7.
If we needed to, we could’ve also used the show cdp neighbors detail command to
view the neighbor’s IP addresses. But since we know the IP addresses of each link on the
Lab_A router, we already know what the next available IP address is going to be.
Using Telnet 303
FIGURE7.7 Network topology documented
.1
Fa0/0
Fa0/1
S0/1
S0/0
.1
.1 .1
192.168.21.2/24 192.168.23.2/24
E0 S1
192.168.18.2/24 192.168.28.2/24
Fa0/0 S0/1
Lab A
2501 2514
2621 2620
Link Layer Discovery Protocol (LLDP)
Before moving on from CDP, I want to tell you about a nonproprietary discovery protocol
that provides pretty much the same information as CDP but works in multi-vendor networks.
The IEEE created a new standardized discovery protocol called 802.1AB for Station
and Media Access Control Connectivity Discovery. We’ll just call it Link Layer Discovery
Protocol (LLDP).
LLDP denes basic discovery capabilities, but it was also enhanced to specically
address the voice application, and this version is called LLDP-MED (Media Endpoint
Discovery). It’s good to remember that LLDP and LLDP-MED are not compatible.
You can nd out more about these protocols at these two locations:
www.cisco.com/en/US/docs/ios/cether/configuration/guide/ce_lldp-med.html
www.cisco.com/en/US/technologies/tk652/tk701/technologies_white_
paper0900aecd804cd46d.html
But it’s probably easier to go to www.cisco.com and search on LLDP than type in the
URLs above.
Using Telnet
As part of the TCP/IP protocol suite, Telnet is a virtual terminal protocol that allows you
to make connections to remote devices, gather information, and run programs.
After your routers and switches are congured, you can use the Telnet program to
recongure and/or check up on them without using a console cable. You run the Telnet pro-
gram by typing telnet from any command prompt (Windows or Cisco), but you need to
have VTY passwords set on the IOS devices for this to work.
304 Chapter 7 u Managing a Cisco Internetwork
Remember, you can’t use CDP to gather information about routers and switches that
aren’t directly connected to your device. But you can use the Telnet application to connect to
your neighbor devices and then run CDP on those remote devices to get information on them.
You can issue the telnet command from any router or switch prompt. Below, I’m trying
to telnet from switch 1 to switch 3:
SW-1#telnet 10.100.128.8
Trying 10.100.128.8 ... Open
Password required, but none set
[Connection to 10.100.128.8 closed by foreign host]
Oops—clearly, I didn’t set my passwords—how embarrassing! Remember that the VTY
ports are default congured as login, meaning that we have to either set the VTY passwords
or use the no login command. If you need to review the process of setting passwords, take a
quick look back in Chapter 6, “Cisco’s Internetworking Operating System (IOS).”
If you can’t telnet into a device, it could be that the password on the remote
device hasn’t been set. It’s also quite possible that an access control list is
filtering the Telnet session.
On a Cisco device, you don’t need to use the telnet command; you can just type in an
IP address from a command prompt and the router will assume that you want to telnet to
the device. Here’s how that looks using just the IP address:
SW-1#10.100.128.8
Trying 10.100.128.8... Open
Password required, but none set
[Connection to 10.100.128.8 closed by foreign host]
SW-1#
Now would be a great time to set those VTY passwords on the SW-3 that I want to telnet
into. Here’s what I did on the switch named SW-3:
SW-3(config)#line vty 0 15
SW-3(config-line)#login
SW-3(config-line)#password telnet
SW-3(config-line)#login
SW-3(config-line)#^Z
Using Telnet 305
Now let’s try this again. This time, I’m connecting to SW-3 from the SW-1 console:
SW-1#10.100.128.8
Trying 10.100.128.8 ... Open
User Access Verification
Password:
SW-3>
Remember that the VTY password is the user-mode password, not the enable-mode
password. Watch what happens when I try to go into privileged mode after telnetting into
the switch:
SW-3>en
% No password set
SW-3>
It’s totally slamming the door in my face, which happens to be a really nice security fea-
ture! After all, you don’t want just anyone telnetting into your device and typing the enable
command to get into privileged mode now, do you? You’ve got to set your enable-mode pass-
word or enable secret password to use Telnet to congure remote devices.
When you telnet into a remote device, you won’t see console messages by
default. For example, you will not see debugging output. To allow console
messages to be sent to your Telnet session, use the terminal monitor
command.
Using the next group of examples, I’ll show you how to telnet into multiple devices
simultaneously as well as how to use hostnames instead of IP addresses.
Telnetting into Multiple Devices Simultaneously
If you telnet to a router or switch, you can end the connection by typing exit at any time.
But what if you want to keep your connection to a remote device going while still coming
back to your original router console? To do that, you can press the Ctrl+Shift+6 key combi-
nation, release it, and then press X.
Here’s an example of connecting to multiple devices from my SW-1 console:
SW-1#10.100.128.8
Trying 10.100.128.8... Open
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User Access Verification
Password:
SW-3>Ctrl+Shift+6
SW-1#
Here you can see that I telnetted to SW-1 and then typed the password to enter user mode.
Next, I pressed Ctrl+Shift+6, then X, but you won’t see any of that because it doesn’t show on
the screen output. Notice that my command prompt now has me back at the SW-1 switch.
Now let’s run through some verication commands.
Checking Telnet Connections
If you want to view the connections from your router or switch to a remote device, just
use the show sessions command. In this case, I’ve telnetted into both the SW-3 and SW-2
switches from SW1:
SW-1#sh sessions
Conn Host Address Byte Idle Conn Name
1 10.100.128.9 10.100.128.9 0 10.100.128.9
* 2 10.100.128.8 10.100.128.8 0 10.100.128.8
SW-1#
See that asterisk (*) next to connection 2? It means that session 2 was the last session
I connected to. You can return to your last session by pressing Enter twice. You can also
return to any session by typing the number of the connection and then Enter.
Checking Telnet Users
You can reveal all active consoles and VTY ports in use on your router with the show users
command:
SW-1#sh users
Line User Host(s) Idle Location
* 0 con 0 10.100.128.9 00:00:01
10.100.128.8 00:01:06
In the command’s output, con represents the local console, and we can see that the con-
sole session is connected to two remote IP addresses—in other words, two devices.
Closing Telnet Sessions
You can end Telnet sessions a few different ways. Typing exit or disconnect are probably
the two quickest and easiest.
Resolving Hostnames 307
To end a session from a remote device, use the exit command:
SW-3>exit
[Connection to 10.100.128.8 closed by foreign host]
SW-1#
To end a session from a local device, use the disconnect command:
SW-1#sh session
Conn Host Address Byte Idle Conn Name
*2 10.100.128.9 10.100.128.9 0 10.100.128.9
SW-1#disconnect ?
<2-2> The number of an active network connection
qdm Disconnect QDM web-based clients
ssh Disconnect an active SSH connection
SW-1#disconnect 2
Closing connection to 10.100.128.9 [confirm][enter]
In this example, I used session number 2 because that was the connection I wanted to
conclude. As demonstrated, you can use the show sessions command to see the connec-
tion number.
Resolving Hostnames
If you want to use a hostname instead of an IP address to connect to a remote device, the
device that you’re using to make the connection must be able to translate the hostname to
an IP address.
There are two ways to resolve hostnames to IP addresses. The rst is by building a host
table on each router, and the second is to build a Domain Name System (DNS) server. The
latter method is similar to creating a dynamic host table assuming that you’re dealing with
dynamic DNS.
Building a Host Table
An important factor to remember is that although a host table provides name resolution, it
does that only on the specic router that it was built upon. The command you use to build
a host table on a router looks this:
ip host host_name [tcp_port_number] ip_address
The default is TCP port number 23, but you can create a session using Telnet with a
different TCP port number if you want. You can also assign up to eight IP addresses to
a hostname.
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Here’s how I congured a host table on the SW-1 switch with two entries to resolve the
names for the SW-2 and SW-3:
SW-1#config t
SW-1(config)#ip host SW-2 ?
<0-65535> Default telnet port number
A.B.C.D Host IP address
additional Append addresses
SW-1(config)#ip host SW-2 10.100.128.9
SW-1(config)#ip host SW-3 10.100.128.8
Notice that I can just keep adding IP addresses to reference a unique host, one after
another. To view our newly built host table, I’ll just use the show hosts command:
SW-1(config)#do sho hosts
Default domain is not set
Name/address lookup uses domain service
Name servers are 255.255.255.255
Codes: u - unknown, e - expired, * - OK, ? - revalidate
t - temporary, p - permanent
Host Port Flags Age Type Address(es)
SW-3 None (perm, OK) 0 IP 10.100.128.8
SW-2 None (perm, OK) 0 IP 10.100.128.9
In this output, you can see the two hostnames plus their associated IP addresses. The
perm in the Flags column means that the entry has been manually congured. If it read
temp, it would be an entry that was resolved by DNS.
The show hosts command provides information on temporary DNS entries
and permanent name-to-address mappings created using the ip host
command.
To verify that the host table resolves names, try typing the hostnames at a router
prompt. Remember that if you don’t specify the command, the router will assume you
want to telnet.
In the following example, I’ll use the hostnames to telnet into the remote devices and
press Ctrl+Shift+6 and then X to return to the main console of the SW-1 router:
SW-1#sw-3
Trying SW-3 (10.100.128.8)... Open
Resolving Hostnames 309
User Access Verification
Password:
SW-3> Ctrl+Shift+6
SW-1#
It worked—I successfully used entries in the host table to create a session to the SW-3
device by using the name to telnet into it. And just so you know, names in the host table
are not case-sensitive.
Notice that the entries in the following show sessions output now display the hostnames
and IP addresses instead of just the IP addresses:
SW-1#sh sessions
Conn Host Address Byte Idle Conn Name
1 SW-3 10.100.128.8 0 1 SW-3
* 2 SW-2 10.100.128.9 0 1 SW-2
SW-1#
If you want to remove a hostname from the table, all you need to do is use the no ip host
command like this:
SW-1(config)#no ip host SW-3
The drawback to going with this host table method is that you must create a host table
on each router in order to be able to resolve names. So clearly, if you have a whole bunch of
routers and want to resolve names, using DNS is a much better option!
Using DNS to Resolve Names
If you have a lot of devices, you don’t want to create a host table in each one of them unless
you’ve also got a lot of time to waste. Since most of us don’t, I highly recommend using a
DNS server to resolve hostnames instead!
Anytime a Cisco device receives a command it doesn’t understand, it will try to resolve it
through DNS by default. Watch what happens when I type the special command todd at a
Cisco router prompt:
SW-1#todd
Translating "todd"...domain server (255.255.255.255)
% Unknown command or computer name, or unable to find
computer address
SW-1#
Because it doesn’t know my name or the command I’m trying to type, it tries to resolve
this through DNS. This is really annoying for two reasons: rst, because it doesn’t know
my name <grin>, and second, because I need to hang out and wait for the name lookup to
310 Chapter 7 u Managing a Cisco Internetwork
time out. You can get around this and prevent a time-consuming DNS lookup by using the
no ip domain-lookup command on your router from global conguration mode.
So if you have a DNS server on your network, you’ll need to add a few commands to
make DNS name resolution work well for you:
u The first command is ip domain-lookup, which is turned on by default. It needs to be
entered only if you previously turned it off with the no ip domain-lookup command. The
command can be used without the hyphen as well with the syntax ip domain lookup.
u The second command is ip name-server. This sets the IP address of the DNS server.
You can enter the IP addresses of up to six servers.
u The last command is ip domain-name. Although this command is optional, you really
set it because it appends the domain name to the hostname you type in. Since DNS
uses a fully qualified domain name (FQDN) system, you must have a second-level DNS
name, in the form domain.com.
Here’s an example of using these three commands:
SW-1#config t
SW-1(config)#ip domain-lookup
SW-1(config)#ip name-server ?
A.B.C.D Domain server IP address (maximum of 6)
SW-1(config)#ip name-server 4.4.4.4
SW-1(config)#ip domain-name lammle.com
SW-1(config)#^Z
After the DNS congurations have been set, you can test the DNS server by using a host-
name to ping or telnet into a device like this:
SW-1#ping SW-3
Translating "SW-3"...domain server (4.4.4.4) [OK]
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.100.128.8, timeout is
2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max
= 28/31/32 ms
Notice that the router uses the DNS server to resolve the name.
After a name is resolved using DNS, use the show hosts command to verify that the
device cached this information in the host table. If I hadn’t used the ip domain-name
lammle.com command, I would have needed to type in ping sw-3.lammle.com, which is
kind of a hassle.
Checking Network Connectivity and Troubleshooting 311
Should You Use a Host Table or a DNS Server?
Karen has finally finished mapping her network via CDP and the hospital’s staff is now
much happier. But Karen is still having a difficult time administering the network because
she has to look at the network drawing to find an IP address every time she needs to tel-
net to a remote router.
Karen was thinking about putting host tables on each router, but with literally hundreds
of routers, this is a daunting task and not the best solution. What should she do?
Most networks have a DNS server now anyway, so adding a hundred or so hostnames
into it would be much easier—certainly better than adding these hostnames to each
and every router! She can just add the three commands on each router and voilà—she’s
resolving names!
Using a DNS server makes it easy to update any old entries too. Remember, for even one
little change, her alternative would be to go to each and every router to manually update
its table if she’s using static host tables.
Keep in mind that this has nothing to do with name resolution on the network and nothing
to do with what a host on the network is trying to accomplish. You only use this method
when you’re trying to resolve names from the router console.
Checking Network Connectivity
and Troubleshooting
You can use the ping and traceroute commands to test connectivity to remote devices,
and both of them can be used with many protocols, not just IP. But don’t forget that the
show ip route command is a great troubleshooting command for verifying your routing
table and the show interfaces command will reveal the status of each interface to you.
I’m not going to get into the show interfaces commands here because we’ve already been
over that in Chapter 6. But I am going to go over both the debug command and the show
processes command that come in very handy when you need to troubleshoot a router.
312 Chapter 7 u Managing a Cisco Internetwork
Using the ping Command
So far, you’ve seen lots of examples of pinging devices to test IP connectivity and name
resolution using the DNS server. To see all the different protocols that you can use with
the Ping program, type ping ?:
SW-1#ping ?
WORD Ping destination address or hostname
clns CLNS echo
ip IP echo
ipv6 IPv6 echo
tag Tag encapsulated IP echo
<cr>
The ping output displays the minimum, average, and maximum times it takes for a ping
packet to nd a specied system and return. Here’s an example:
SW-1#ping SW-3
Translating "SW-3"...domain server (4.4.4.4) [OK]
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.100.128.8, timeout is
2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max
= 28/31/32 ms
This output tells us that the DNS server was used to resolve the name, and the device
was pinged in a minimum of 28 ms (milliseconds), an average of 31 ms, and up to 32 ms.
This network has some latency!
The ping command can be used in user and privileged mode but not con-
figuration mode!
Using the traceroute Command
Traceroute—the traceroute command, or trace for short—shows the path a packet takes
to get to a remote device. It uses time to live (TTL), time-outs, and ICMP error messages to
outline the path a packet takes through an internetwork to arrive at a remote host.
The trace command, which you can deploy from either user mode or privileged mode,
allows you to gure out which router in the path to an unreachable network host should be
examined more closely as the probable cause of your network’s failure.
Checking Network Connectivity and Troubleshooting 313
To see the protocols that you can use with the traceroute command, type traceroute ?:
SW-1#traceroute ?
WORD Trace route to destination address or hostname
appletalk AppleTalk Trace
clns ISO CLNS Trace
ip IP Trace
ipv6 IPv6 Trace
ipx IPX Trace
mac Trace Layer2 path between 2 endpoints
oldvines Vines Trace (Cisco)
vines Vines Trace (Banyan)
<cr>
The traceroute command shows the hop or hops that a packet traverses on its way to a
remote device.
Do not get confused! You can’t use the tracert command; that’s a Win-
dows command. For a router, use the traceroute command!
Here’s an example of using tracert on a Windows prompt—notice that the command is
tracert, not traceroute:
C:\>tracert www.whitehouse.gov
Tracing route to a1289.g.akamai.net [69.8.201.107]
over a maximum of 30 hops:
1 * * * Request timed out.
2 53 ms 61 ms 53 ms hlrn-dsl-gw15-207.hlrn.qwest.net [207.225.112.207]
3 53 ms 55 ms 54 ms hlrn-agw1.inet.qwest.net [71.217.188.113]
4 54 ms 53 ms 54 ms hlr-core-01.inet.qwest.net [205.171.253.97]
5 54 ms 53 ms 54 ms apa-cntr-01.inet.qwest.net [205.171.253.26]
6 54 ms 53 ms 53 ms 63.150.160.34
7 54 ms 54 ms 53 ms www.whitehouse.gov [69.8.201.107]
Trace complete.
Okay, let’s move on now and talk about how to troubleshoot your network using the
debug command.
314 Chapter 7 u Managing a Cisco Internetwork
Debugging
Debug is a useful troubleshooting command that’s available from the privileged exec mode
of Cisco IOS. It’s used to display information about various router operations and the related
trafc generated or received by the router, plus any error messages.
Even though it’s a helpful, informative tool, there are a few important facts that you need
to know about it. Debug is regarded as a very high-overhead task because it can consume
a huge amount of resources and the router is forced to process-switch the packets being
debugged. So you don’t just use debug as a monitoring tool—it’s meant to be used for a short
period of time and only as a troubleshooting tool. It’s highly useful for discovering some truly
signicant facts about both working and faulty software and/or hardware components, but
remember to limit its use as the benecial troubleshooting tool it’s designed to be.
Because debugging output takes priority over other network trafc, and because the
debug all command generates more output than any other debug command, it can severely
diminish the router’s performance—even render it unusable! Because of this, it’s nearly
always best to use more specic debug commands.
As you can see from the following output, you can’t enable debugging from user mode,
only privileged mode:
SW-1>debug ?
% Unrecognized command
SW-1>en
SW-1#debug ?
aaa AAA Authentication, Authorization and Accounting
access-expression Boolean access expression
adjacency adjacency
aim Attachment Information Manager
all Enable all debugging
archive debug archive commands
arp IP ARP and HP Probe transactions
authentication Auth Manager debugging
auto Debug Automation
beep BEEP debugging
bgp BGP information
bing Bing(d) debugging
call-admission Call admission control
cca CCA activity
cdp CDP information
cef CEF address family independent operations
cfgdiff debug cfgdiff commands
cisp CISP debugging
clns CLNS information
Checking Network Connectivity and Troubleshooting 315
cluster Cluster information
cmdhd Command Handler
cns CNS agents
condition Condition
configuration Debug Configuration behavior
[output cut]
If you’ve got the freedom to pretty much take out a router or switch and you really want
to have some fun with debugging, use the debug all command:
Sw-1#debug all
This may severely impact network performance. Continue? (yes/[no]):yes
All possible debugging has been turned on
At this point my switch overloaded and crashed and I had to reboot it. Try this on your
switch at work and see if you get the same results. Just kidding!
To disable debugging on a router, just use the command no in front of the debug command:
SW-1#no debug all
I typically just use the undebug all command since it is so easy when using the shortcut:
SW-1#un all
Remember that instead of using the debug all command, it’s usually a much better idea
to use specic commands—and only for short periods of time. Here’s an example:
S1#debug ip icmp
ICMP packet debugging is on
S1#ping 192.168.10.17
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.10.17, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms
S1#
1w4d: ICMP: echo reply sent, src 192.168.10.17, dst 192.168.10.17
1w4d: ICMP: echo reply rcvd, src 192.168.10.17, dst 192.168.10.17
1w4d: ICMP: echo reply sent, src 192.168.10.17, dst 192.168.10.17
1w4d: ICMP: echo reply rcvd, src 192.168.10.17, dst 192.168.10.17
1w4d: ICMP: echo reply sent, src 192.168.10.17, dst 192.168.10.17
1w4d: ICMP: echo reply rcvd, src 192.168.10.17, dst 192.168.10.17
316 Chapter 7 u Managing a Cisco Internetwork
1w4d: ICMP: echo reply sent, src 192.168.10.17, dst 192.168.10.17
1w4d: ICMP: echo reply rcvd, src 192.168.10.17, dst 192.168.10.17
1w4d: ICMP: echo reply sent, src 192.168.10.17, dst 192.168.10.17
1w4d: ICMP: echo reply rcvd, src 192.168.10.17, dst 192.168.10.17
SW-1#un all
I’m sure you can see that the debug command is one powerful command. And because
of this, I’m also sure you realize that before you use any of the debugging commands, you
should make sure you check the CPU utilization capacity of your router. This is important
because in most cases, you don’t want to negatively impact the device’s ability to process
the packets on your internetwork. You can determine a specic router’s CPU utilization
information by using the show processes command.
Remember, when you telnet into a remote device, you will not see console
messages by default! For example, you will not see debugging output. To
allow console messages to be sent to your Telnet session, use the terminal
monitor command.
Using the show processes Command
As I’ve said, you’ve really got to be careful when using the debug command on your devices.
If your router’s CPU utilization is consistently at 50 percent or more, it’s probably not a good
idea to type in the debug all command unless you want to see what a router looks like when
it crashes!
So what other approaches can you use? Well, the show processes (or show processes
cpu) is a good tool for determining a given router’s CPU utilization. Plus, it’ll give you a
list of active processes along with their corresponding process ID, priority, scheduler test
(status), CPU time used, number of times invoked, and so on. Lots of great stuff! Plus, this
command is super handy when you want to evaluate your router’s performance and CPU
utilization and are otherwise tempted to reach for the debug command!
Okay—what do you see in the following output? The rst line shows the CPU utilization
output for the last 5 seconds, 1 minute, and 5 minutes. The output provides 5%/0% in front
of the CPU utilization for the last 5 seconds: The rst number equals the total utilization,
and the second one indicates the utilization due to interrupt routines. Take a look:
SW-1#sh processes
CPU utilization for five seconds: 5%/0%; one minute: 7%; five minutes: 8%
PID QTy PC Runtime(ms) Invoked uSecs Stacks TTY Process
1 Cwe 29EBC58 0 22 0 5236/6000 0 Chunk Manager
2 Csp 1B9CF10 241 206881 1 2516/3000 0 Load Meter
3 Hwe 1F108D0 0 1 0 8768/9000 0 Connection Mgr
4 Lst 29FA5C4 9437909 454026 20787 5540/6000 0 Check heaps
Exam Essentials 317
5 Cwe 2A02468 0 2 0 5476/6000 0 Pool Manager
6 Mst 1E98F04 0 2 0 5488/6000 0 Timers
7 Hwe 13EB1B4 3686 101399 36 5740/6000 0 Net Input
8 Mwe 13BCD84 0 1 0 23668/24000 0 Crash writer
9 Mwe 1C591B4 4346 53691 80 4896/6000 0 ARP Input
10 Lwe 1DA1504 0 1 0 5760/6000 0 CEF MIB API
11 Lwe 1E76ACC 0 1 0 5764/6000 0 AAA_SERVER_DEADT
12 Mwe 1E6F980 0 2 0 5476/6000 0 AAA high-capacit
13 Mwe 1F56F24 0 1 0 11732/12000 0 Policy Manager [output
cut]
So basically, the output from the show processes command reveals that our router is
happily able to process debugging commands without being overloaded—nice!
Summary
In this chapter, you learned how Cisco routers are congured and how to manage those
congurations.
We covered the internal components of a router, including ROM, RAM, NVRAM,
and ash.
Next, you found out how to back up and restore the conguration of a Cisco router
and switch.
You also learned how to use CDP and Telnet to gather information about remote devices.
Finally, you discovered how to resolve hostnames and use the ping and trace commands to
test network connectivity as well as how to use the debug and show processes commands—
well done!
Exam Essentials
Define the Cisco Router components. Describe the functions of the bootstrap, POST, ROM
monitor, mini-IOS, RAM, ROM, ash memory, NVRAM, and the conguration register.
Identify the steps in the router boot sequence. The steps in the boot sequence are POST,
loading the IOS, and copying the startup conguration from NVRAM to RAM.
Save the configuration of a router or switch. There are a couple of ways to do this, but the
most common, as well as the most tested, method is copy running-config startup-config.
Erase the configuration of a router or switch. Type the privileged-mode command erase
startup-config and reload the router.
Describe the value of CDP. Cisco Discovery Protocol can be used to help you document as
well as troubleshoot your network.
318 Chapter 7 u Managing a Cisco Internetwork
List the information provided by the output of the show cdp neighbors command. The
show cdp neighbors command provides the following information: device ID, local inter-
face, holdtime, capability, platform, and port ID (remote interface).
Understand how to establish a Telnet session with multiple routers simultaneously. If
you telnet to a router or switch, you can end the connection by typing exit at any time.
However, if you want to keep your connection to a remote device but still come back to
your original router console, you can press the Ctrl+Shift+6 key combination, release it,
and then press X.
Identify current Telnet sessions. The command show sessions will provide you with infor-
mation about all the currently active sessions your router has with other routers.
Build a static host table on a router. By using the global conguration command ip host
host_name ip_address, you can build a static host table on your router. You can apply
multiple IP addresses against the same host entry.
Verify the host table on a router. You can verify the host table with the show hosts
command.
Describe the function of the ping command. Packet Internet Groper (ping) uses ICMP
echo requests and ICMP echo replies to verify an active IP address on a network.
Ping a valid host ID from the correct prompt. You can ping an IP address from a router’s
user mode or privileged mode but not from conguration mode, unless you use the do
command. You must ping a valid address, such as 1.1.1.1.
Written Lab 7 319
Written Lab 7
In this section, you’ll complete the following labs to make sure you’ve got the information
and concepts contained within them fully dialed in:
Lab 7.1: IOS Management
Lab 7.2: Router Memory
The answers to these labs can be found in Appendix A, “Answers to Written Labs.”
Written Lab 7.1: IOS Management
Write the answers to the following questions:
1. What is the command to copy the startup-config file to DRAM?
2. What command can you use to see the neighbor router’s IP address from your router
prompt?
3. What command can you use to see the hostname, local interface, platform, and remote
port of a neighbor router?
4. What keystrokes can you use to telnet into multiple devices simultaneously?
5. What command will show you your active Telnet connections to neighbor and
remote devices?
6. What command can you use to merge a backup configuration with the configuration
in RAM?
7. What protocol can be used on a network to synchronize clock and date information?
8. What command is used by a router to forward a DHCP client request to a remote
DHCP server?
9. What command enables your switch or router to receive clock and date information
and synchronize with the NTP server?
10. Which NTP verification command will show the reference master for the client?
Written Lab 7.2: Router Memory
Identify the location in a router where each of the following les is stored by default.
1. Cisco IOS
2. Bootstrap
3. Startup configuration
4. POST routine
5. Running configuration
320 Chapter 7 u Managing a Cisco Internetwork
6. ARP cache
7. Mini IOS
8. ROM Monitor
9. Routing tables
10. Packet buffers
Hands-on Labs
To complete the labs in this section, you need at least one router or switch (three would be
best) and at least one PC running as a TFTP server. TFTP server software must be installed
and running on the PC. For this lab, it is also assumed that your PC and the Cisco devices are
connected together with a switch and that all interfaces (PC NIC and router interfaces) are
in the same subnet. You can alternately connect the PC directly to the router or connect the
routers directly to one another (use a crossover cable in that case). Remember that the labs
listed here were created for use with real routers but can easily be used with LammleSim IOS
Version or Cisco’s Packet Tracer program. Last, although it doesn’t matter if you are using a
switch or router in these labs, I’m just going to use my routers, but feel free to use your switch
to go through these labs!
Here is a list of the labs in this chapter:
Lab 7.1: Backing Up the Router Conguration
Lab 7.2: Using the Cisco Discovery Protocol (CDP)
Lab 7.3: Using Telnet
Lab 7.4: Resolving Hostnames
Hands-on Lab 7.1: Backing Up the Router Configuration
In this lab, you’ll back up the router conguration:
1. Log into your router and go into privileged mode by typing en or enable.
2. Ping the TFTP server to make sure you have IP connectivity.
3. From RouterB, type copy run tftp.
4. When prompted, type the IP address of the TFTP server (for example, 172.16.30.2)
and press Enter.
5. By default, the router will prompt you for a filename. The hostname of the router is fol-
lowed by the suffix -confg (yes, I spelled that correctly). You can use any name you want.
Name of configuration file to write [RouterB-confg]?
Hands-on Labs 321
Press Enter to accept the default name.
Write file RouterB-confg on host 172.16.30.2? [confirm]
Press Enter to conrm.
Hands-on Lab 7.2: Using the Cisco Discovery
Protocol (CDP)
CDP is an important objective for the Cisco exams. Please go through this lab and use CDP
as much as possible during your studies.
1. Log into your router and go into privileged mode by typing en or enable.
2. From the router, type sh cdp and press Enter. You should see that CDP packets are
being sent out to all active interfaces every 60 seconds and the holdtime is 180 seconds
(these are the defaults).
3. To change the CDP update frequency to 90 seconds, type cdp timer 90 in global
configuration mode.
Router#config t
Enter configuration commands, one per line. End with
CNTL/Z.
Router(config)#cdp timer ?
<5-900> Rate at which CDP packets are sent (in sec)
Router(config)#cdp timer 90
4. Verify that your CDP timer frequency has changed by using the command show cdp in
privileged mode.
Router#sh cdp
Global CDP information:
Sending CDP packets every 90 seconds
Sending a holdtime value of 180 seconds
5. Now use CDP to gather information about neighbor routers. You can get the list of
available commands by typing sh cdp ?.
Router#sh cdp ?
entry Information for specific neighbor entry
interface CDP interface status and configuration
neighbors CDP neighbor entries
322 Chapter 7 u Managing a Cisco Internetwork
traffic CDP statistics
<cr>
6. Type sh cdp int to see the interface information plus the default encapsulation used
by the interface. It also shows the CDP timer information.
7. Type sh cdp entry * to see complete CDP information received from all devices.
8. Type show cdp neighbors to gather information about all connected neighbors. (You
should know the specific information output by this command.)
9. Type show cdp neighbors detail. Notice that it produces the same output as show
cdp entry *.
Hands-on Lab 7.3: Using Telnet
Secure Shell was covered in Chapter 6, which is what you should use for remote access
into a Cisco device. However, the Cisco objectives cover telnet conguration, so let’s do
a lab on telnet!
1. Log into your router and go into privileged mode by typing en or enable.
2. From RouterA, telnet into your remote router (RouterB) by typing telnet ip_address
from the command prompt. Type exit to disconnect.
3. Now type in RouterB’s IP address from RouterA’s command prompt. Notice that the
router automatically tries to telnet to the IP address you specified. You can use the telnet
command or just type in the IP address.
4. From RouterB, press Ctrl+Shift+6 and then X to return to RouterA’s command prompt.
Now telnet into your third router, RouterC. Press Ctrl+Shift+6 and then X to return to
RouterA.
5. From RouterA, type show sessions. Notice your two sessions. You can press the
number displayed to the left of the session and press Enter twice to return to that ses-
sion. The asterisk shows the default session. You can press Enter twice to return to
that session.
6. Go to the session for your RouterB. Type show users. This shows the console connec-
tion and the remote connection. You can use the disconnect command to clear the
session or just type exit from the prompt to close your session with RouterB.
7. Go to RouterC’s console port by typing show sessions on the first router and using
the connection number to return to RouterC. Type show user and notice the connec-
tion to your first router, RouterA.
8. Type clear line line_number to disconnect the Telnet session.
Hands-on Labs 323
Hands-on Lab 7.4: Resolving Hostnames
It’s best to use a DNS server for name resolution, but you can also create a local hosts table
to resolve names. Let’s take a look.
1. Log into your router and go into privileged mode by typing en or enable.
2. From RouterA, type todd and press Enter at the command prompt. Notice the error
you receive and the delay. The router is trying to resolve the hostname to an IP address
by looking for a DNS server. You can turn this feature off by using the no ip domain-
lookup command from global configuration mode.
3. To build a host table, you use the ip host command. From RouterA, add a host table
entry for RouterB and RouterC by entering the following commands:
ip host routerb ip_address
ip host routerc ip_address
Here is an example:
ip host routerb 172.16.20.2
ip host routerc 172.16.40.2
4. Test your host table by typing ping routerb from the privileged mode prompt (not the
config prompt).
RouterA#ping routerb
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.20.2, timeout
is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip
min/avg/max = 4/4/4 ms
5. Test your host table by typing ping routerc.
RouterA#ping routerc
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.40.2, timeout
is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip
min/avg/max = 4/6/8 ms
324 Chapter 7 u Managing a Cisco Internetwork
6. Telnet to RouterB and keep your session to RouterB open to RouterA by pressing
Ctrl+Shift+6, then X.
7. Telnet to RouterC by typing routerc at the command prompt.
8. Return to RouterA and keep the session to RouterC open by pressing Ctrl+Shift+6,
then X.
9. View the host table by typing show hosts and pressing Enter.
Default domain is not set
Name/address lookup uses domain service
Name servers are 255.255.255.255
Host Flags Age Type Address(es)
routerb (perm, OK) 0 IP 172.16.20.2
routerc (perm, OK) 0 IP 172.16.40.2
Review Questions 325
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional questions,
please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. Which of the following is a standards-based protocol that works much like CDP?
A. DHCP
B. LLDP
C. DDNS
D. SSTP
2. Which command can be used to determine a router’s capacity to generate debug output?
A. show version
B. show controllers
C. show processes cpu
D. show memory
3. You are troubleshooting a connectivity problem in your corporate network and want to
isolate the problem. You suspect that a router on the route to an unreachable network
is at fault. What IOS user exec command should you issue?
A. Router>ping
B. Router>trace
C. Router>show ip route
D. Router>show interface
E. Router>show cdp neighbors
4. You copy a configuration from a network host to a router’s RAM. The configuration
looks correct, yet it is not working at all. What could the problem be?
A. You copied the wrong configuration into RAM.
B. You copied the configuration into flash memory instead.
C. The copy did not override the shutdown command in running-config.
D. The IOS became corrupted after the copy command was initiated.
326 Chapter 7 u Managing a Cisco Internetwork
5. In the following command, what does the IP address 10.10.10.254 refer to?
Router#config t
Router(config)#interface fa0/0
Router(config-if)#ip helper-address 10.10.10.254
A. IP address of the ingress interface on the router
B. IP address of the egress interface on the router
C. IP address of the next hop on the path to the DHCP server
D. IP address of the DHCP server
6. The corporate office sends you a new router to connect, but upon connecting the con-
sole cable, you see that there is already a configuration on the router. What should be
done before a new configuration is entered in the router?
A. RAM should be erased and the router restarted.
B. Flash should be erased and the router restarted.
C. NVRAM should be erased and the router restarted.
D. The new configuration should be entered and saved.
7. What command can you use to determine the IP address of a directly connected neighbor?
A. show cdp
B. show cdp neighbors
C. show cdp neighbors detail
D. show neighbor detail
8. According to the output, what interface does SW-2 use to connect to SW-3?
SW-3#sh cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route BridgeS -
Switch, H - Host, I - IGMP, r - Repeater, P - Phone, D - Remote, C - CVTA, M
- Two-port Mac Relay Device ID
Local Intrfce Holdtme Capability Platform Port ID
SW-1 Fas 0/1 170 S I WS-C3560- Fas 0/15
SW-1 Fas 0/2 170 S I WS-C3560- Fas 0/16
SW-2 Fas 0/5 162 S I WS-C3560- Fas 0/2
A. Fas 0/1
B. Fas 0/16
C. Fas 0/2
D. Fas 0/5
Review Questions 327
9. What command can you use to determine the IP address of a directly connected neighbor?
A. show cdp
B. show cdp neighbors
C. show cdp neighbors detail
D. show neighbor detail
10. You save the configuration on a router with the copy running-config startup-config
command and reboot the router. The router, however, comes up with a blank configura-
tion. What can the problem be?
A. You didn’t boot the router with the correct command.
B. NVRAM is corrupted.
C. The configuration register setting is incorrect.
D. The newly upgraded IOS is not compatible with the hardware of the router.
E. The configuration you saved is not compatible with the hardware.
11. If you want to have more than one Telnet session open at the same time, what key-
stroke combination would you use?
A. Tab+spacebar
B. Ctrl+X, then 6
C. Ctrl+Shift+X, then 6
D. Ctrl+Shift+6, then X
12. You are unsuccessful in telnetting into a remote device from your switch, but you could
telnet to the router earlier. However, you can still ping the remote device. What could the
problem be? (Choose two.)
A. IP addresses are incorrect.
B. Access control list is filtering Telnet.
C. There is a defective serial cable.
D. The VTY password is missing.
13. What information is displayed by the show hosts command? (Choose two.)
A. Temporary DNS entries
B. The names of the routers created using the hostname command
C. The IP addresses of workstations allowed to access the router
D. Permanent name-to-address mappings created using the ip host command
E. The length of time a host has been connected to the router via Telnet
328 Chapter 7 u Managing a Cisco Internetwork
14. Which three commands can be used to check LAN connectivity problems on a switch?
(Choose three.)
A. show interfaces
B. show ip route
C. tracert
D. ping
E. dns lookups
15. You telnet to a router and make your necessary changes; now you want to end the
Telnet session. What command do you type in?
A. close
B. disable
C. disconnect
D. exit
16. You telnet into a remote device and type debug ip icmp, but no output from the debug
command is seen. What could the problem be?
A. You must type the show ip icmp command first.
B. IP addressing on the network is incorrect.
C. You must use the terminal monitor command.
D. Debug output is sent only to the console.
17. You need to view console messages on a device to which you have connected through
telnet. The command you need to execute to see these is ___________.
18. You need to gather the IP address of a remote switch that is located in Hawaii. What
can you do to find the address?
A. Fly to Hawaii, console into the switch, then relax and have a drink with an
umbrella in it.
B. Issue the show ip route command on the router connected to the switch.
C. Issue the show cdp neighbor command on the router connected to the switch.
D. Issue the show ip arp command on the router connected to the switch.
E. Issue the show cdp neighbors detail command on the router connected to
the switch.
Review Questions 329
19. You need to configure all your routers and switches so they synchronize their clocks
from one time source. What command will you type for each device?
A. clock synchronization ip_address
B. ntp master ip_address
C. sync ntp ip_address
D. ntp server ip_address version number
20. What two commands can you use to verify your NTP client?
A. show ntp server
B. show ntp status
C. show vtp status
D. show ntp associations
E. show clock source
Chapter
8
IP Routing
THE FOLLOWING ICND1 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 IP Routing Technologies
■Describe basic routing concepts
■CEF
■Packet forwarding
■Router lookup process
■Configure and verify routing configuration for a static or
default route given specific routing requirements
■Differentiate methods of routing and routing protocols
■Static vs. Dynamic
■Link State vs. Distance Vector
■next hop
■ip routing table
■Passive interfaces
It’s time now to turn our focus toward the core topic of the
ubiquitous IP routing process. It’s integral to networking
because it pertains to all routers and congurations that use
it, which is easily the lion’s share. IP routing is basically the process of moving packets
from one network to another network using routers. And by routers I mean Cisco routers,
of course! However, the terms router and layer 3 device are interchangeable, and through-
out this chapter when I use the term router, I am referring to any layer 3 device.
Before jumping into this chapter, I want to make sure you understand the difference
between a routing protocol and a routed protocol. Routers use routing protocols to dynam-
ically nd all networks within the greater internetwork and to ensure that all routers have
the same routing table. Routing protocols are also employed to determine the best path a
packet should take through an internetwork to get to its destination most efciently. RIP,
RIPv2, EIGRP, and OSPF are great examples of the most common routing protocols.
Once all routers know about all networks, a routed protocol can be used to send user data
(packets) through the established enterprise. Routed protocols are assigned to an interface
and determine the method of packet delivery. Examples of routed protocols are IP and IPv6.
I’m going to discuss the RIP routing protocol in this chapter even though I know that it
isn’t included in any objective. We’ll cover it anyway because it will help you gain a solid
understanding of the routing process before we get into a study on the much more complex
OSPF protocol in the next chapter. We’ll also discuss RIP because you just never know when
Cisco will toss in exam questions that include topics they haven’t bothered to list as a bona
de objective either! So even though I want you to be prepared just in case, I’ll promise to
keep the RIP section reasonably short.
I’m pretty condent I don’t have to underscore how crucial it is for you have this chapter’s
material down to a near instinctive level. IP routing is innately what Cisco routers do, and they
do it very well, so having a rm grasp of the fundamentals and basics of this topic are vital if
you want to excel during the exam and in a real-world networking environment as well!
In this chapter, I’m going to show you how to congure and verify IP routing with Cisco
routers and guide you through these ve key subjects:
u Routing basics
u The IP routing process
u Static routing
u Default routing
u Dynamic routing
Routing Basics 333
We’ll get into more the advanced aspects of dynamic routing next in Chapter 9, “Open
Shortest Path First (OSPF).” But rst, I want to nail down the basics of how packets actually
move through an internetwork, so let’s get started!
To find up-to-the-minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
Routing Basics
Once you create an internetwork by connecting your WANs and LANs to a router, you’ll
need to congure logical network addresses, like IP addresses, to all hosts on that internet-
work for them to communicate successfully throughout it.
The term routing refers to taking a packet from one device and sending it through the
network to another device on a different network. Routers don’t really care about hosts—
they only care about networks and the best path to each one of them. The logical network
address of the destination host is key to get packets through a routed network. It’s the hard-
ware address of the host that’s used to deliver the packet from a router and ensure it arrives
at the correct destination host.
Routing is irrelevant if your network has no routers because their job is to route trafc
to all the networks in your internetwork, but this is rarely the case! So here’s an important
list of the minimum factors a router must know to be able to affectively route packets:
u Destination address
u Neighbor routers from which it can learn about remote networks
u Possible routes to all remote networks
u The best route to each remote network
u How to maintain and verify routing information
The router learns about remote networks from neighboring routers or from an adminis-
trator. The router then builds a routing table, which is basically a map of the internetwork,
and it describes how to nd remote networks. If a network is directly connected, then the
router already knows how to get to it.
But if a network isn’t directly connected to the router, the router must use one of two
ways to learn how to get to the remote network. The static routing method requires some-
one to hand-type all network locations into the routing table, which can be a pretty daunt-
ing task when used on all but the smallest of networks!
Conversely, when dynamic routing is used, a protocol on one router communicates with the
same protocol running on neighboring routers. The routers then update each other about all
the networks they know about and place this information into the routing table. If a change
occurs in the network, the dynamic routing protocols automatically inform all routers about
the event. If static routing is used, the administrator is responsible for updating all changes by
334 Chapter 8 u IP Routing
hand onto all routers. Most people usually use a combination of dynamic and static routing to
administer a large network.
Before we jump into the IP routing process, let’s take a look at a very simple example that
demonstrates how a router uses the routing table to route packets out of an interface. We’ll
be going into a more detailed study of the process soon, but I want to show you something
called the “longest match rule” rst. With it, IP will scan a routing table to nd the longest
match as compared to the destination address of a packet. Let’s take a look at Figure8.1 to
get a picture of this process.
FIGURE8.1 A simple routing example
Fa0/2
10.10.0.1/16
Fa0/3
10.0.0.1/8
Fa0/0 Fa0/1
10.10.10.1/24
Lab A
Figure8.1 shows a simple network. Lab_A has four interfaces. Can you see which
interface will be used to forward an IP datagram to a host with a destination IP address
of 10.10.10.30?
By using the command show ip route on a router, we can see the routing table (map of
the internetwork) that Lab_A has used to make its forwarding decisions:
Lab_A#sh ip route
Codes: L - local, C - connected, S - static,
[output cut]
  10.0.0.0/8 is variably subnetted, 6 subnets, 4 masks
C 10.0.0.0/8 is directly connected, FastEthernet0/3
L 10.0.0.1/32 is directly connected, FastEthernet0/3
C 10.10.0.0/16 is directly connected, FastEthernet0/2
L 10.10.0.1/32 is directly connected, FastEthernet0/2
C 10.10.10.0/24 is directly connected, FastEthernet0/1
L 10.10.10.1/32 is directly connected, FastEthernet0/1
S* 0.0.0.0/0 is directly connected, FastEthernet0/0
The IP Routing Process 335
The C in the routing table output means that the networks listed are “directly con-
nected,” and until we add a routing protocol like RIPv2, OSPF, etc. to the routers in our
internetwork, or enter static routes, only directly connected networks will show up in
our routing table. But wait—what about that L in the routing table—that’s new, isn’t it?
Yes it is, because in the new Cisco IOS 15 code, Cisco denes a different route, called
a local route. Each has a /32 prex dening a route just for the one address. So in this
example, the router has relied upon these routes that list their own local IP addresses to
more efciently forward packets to the router itself.
So let’s get back to the original question: By looking at the gure and the output of the
routing table, can you determine what IP will do with a received packet that has a destina-
tion IP address of 10.10.10.30? The answer is that the router will packet-switch the packet
to interface FastEthernet 0/1, which will frame the packet and then send it out on the net-
work segment. Based upon the longest match rule, IP would look for 10.10.10.30, and if
that isn’t found in the table, then IP would search for 10.10.10.0, then 10.10.0.0, and so on
until a route is discovered.
Here’s another example: Based on the output of the next routing table, which interface
will a packet with a destination address of 10.10.10.14 be forwarded from?
Lab_A#sh ip route
[output cut]
Gateway of last resort is not set
C 10.10.10.16/28 is directly connected, FastEthernet0/0
L 10.10.10.17/32 is directly connected, FastEthernet0/0
C 10.10.10.8/29 is directly connected, FastEthernet0/1
L 10.10.10.9/32 is directly connected, FastEthernet0/1
C 10.10.10.4/30 is directly connected, FastEthernet0/2
L 10.10.10.5/32 is directly connected, FastEthernet0/2
C 10.10.10.0/30 is directly connected, Serial 0/0
L 10.10.10.1/32 is directly connected, Serial0/0
To gure this out, look closely at the output until you see that the network is subnetted
and each interface has a different mask. And I have to tell you—you just can’t answer this
question if you can’t subnet! 10.10.10.14 would be a host in the 10.10.10.8/29 subnet that’s
connected to the FastEthernet0/1 interface. Don’t freak if you’re struggling and don’t get
this! Instead, just go back and reread Chapter 4, “Easy Subnetting,” until it becomes clear
to you.
The IP Routing Process
The IP routing process is fairly simple and doesn’t change, regardless of the size of your
network. For a good example of this fact, I’ll use Figure8.2 to describe step-by-step what
happens when Host_A wants to communicate with Host_B on a different network.
336 Chapter 8 u IP Routing
FIGURE8.2 IP routing example using two hosts and one router
E0 E1
Host A
172.16.10.1 172.16.20.1
172.16.10.2
Host B
172.16.20.2
Lab A
In Figure8.2 a user on Host_A pinged Host_B’s IP address. Routing doesn’t get any
simpler than this, but it still involves a lot of steps, so let’s work through them now:
1. Internet Control Message Protocol (ICMP) creates an echo request payload, which is
simply the alphabet in the data field.
2. ICMP hands that payload to Internet Protocol (IP), which then creates a packet. At a
minimum, this packet contains an IP source address, an IP destination address, and a
Protocol field with 01h. Don’t forget that Cisco likes to use 0x in front of hex charac-
ters, so this could also look like 0x01. This tells the receiving host to whom it should
hand the payload when the destination is reached—in this example, ICMP.
3. Once the packet is created, IP determines whether the destination IP address is on the
local network or a remote one.
4. Since IP has determined that this is a remote request, the packet must be sent to the
default gateway so it can be routed to the remote network. The Registry in Windows is
parsed to find the configured default gateway.
5. The default gateway of Host_A is configured to 172.16.10.1. For this packet to be sent
to the default gateway, the hardware address of the router’s interface Ethernet 0, which
is configured with the IP address of 172.16.10.1, must be known. Why? So the packet
can be handed down to the Data Link layer, framed, and sent to the router’s interface
that’s connected to the 172.16.10.0 network. Because hosts communicate only via
hardware addresses on the local LAN, it’s important to recognize that for Host_A to
communicate to Host_B, it has to send packets to the Media Access Control (MAC)
address of the default gateway on the local network.
MAC addresses are always local on the LAN and never go through and
past a router.
6. Next, the Address Resolution Protocol (ARP) cache of the host is checked to see if the
IP address of the default gateway has already been resolved to a hardware address.
If it has, the packet is then free to be handed to the Data Link layer for framing.
Remember that the hardware destination address is also handed down with that
packet. To view the ARP cache on your host, use the following command:
C:\>arp -a
Interface: 172.16.10.2 --- 0x3
The IP Routing Process 337
Internet Address Physical Address Type
172.16.10.1 00-15-05-06-31-b0 dynamic
If the hardware address isn’t already in the ARP cache of the host, an ARP broadcast
will be sent out onto the local network to search for the 172.16.10.1 hardware address.
The router then responds to the request and provides the hardware address of Ethernet
0, and the host caches this address.
7. Once the packet and destination hardware address are handed to the Data Link layer,
the LAN driver is used to provide media access via the type of LAN being used, which
is Ethernet in this case. A frame is then generated, encapsulating the packet with control
information. Within that frame are the hardware destination and source addresses plus,
in this case, an Ether-Type field, which identifies the specific Network layer protocol
that handed the packet to the Data Link layer. In this instance, it’s IP. At the end of the
frame is something called a Frame Check Sequence (FCS) field that houses the result
of the cyclic redundancy check (CRC). The frame would look something like what I’ve
detailed in Figure8.3. It contains Host_A’s hardware (MAC) address and the destina-
tion hardware address of the default gateway. It does not include the remote host’s MAC
address—remember that!
FIGURE8.3 Frame used from Host_A to the Lab_A router when Host_B is pinged
Destination MAC
(router’s E0 MAC address)
Source MAC
(Host A MAC address)
Ether-Type
field Packet FCS
CRC
8. Once the frame is completed, it’s handed down to the Physical layer to be put on the
physical medium (in this example, twisted-pair wire) one bit at a time.
9. Every device in the collision domain receives these bits and builds the frame. They
each run a CRC and check the answer in the FCS field. If the answers don’t match,
the frame is discarded.
u If the CRC matches, then the hardware destination address is checked to see if it
matches (which, in this example, is the router’s interface Ethernet 0).
u If it’s a match, then the Ether-Type field is checked to find the protocol used at the
Network layer.
10. The packet is pulled from the frame, and what is left of the frame is discarded. The
packet is handed to the protocol listed in the Ether-Type field—it’s given to IP.
11. IP receives the packet and checks the IP destination address. Since the packet’s desti-
nation address doesn’t match any of the addresses configured on the receiving router
itself, the router will look up the destination IP network address in its routing table.
12. The routing table must have an entry for the network 172.16.20.0 or the packet will
be discarded immediately and an ICMP message will be sent back to the originating
device with a destination network unreachable message.
338 Chapter 8 u IP Routing
13. If the router does find an entry for the destination network in its table, the packet
is switched to the exit interface—in this example, interface Ethernet 1. The output
below displays the Lab_A router’s routing table. The C means “directly connected.”
No routing protocols are needed in this network since all networks (all two of them)
are directly connected.
Lab_A>sh ip route
C 172.16.10.0 is directly connected, Ethernet0
L 172.16.10.1/32 is directly connected, Ethernet0
C 172.16.20.0 is directly connected, Ethernet1
L 172.16.20.1/32 is directly connected, Ethernet1
14. The router packet-switches the packet to the Ethernet 1 buffer.
15. The Ethernet 1 buffer needs to know the hardware address of the destination host and
first checks the ARP cache.
u If the hardware address of Host_B has already been resolved and is in the router’s
ARP cache, then the packet and the hardware address will be handed down to the
Data Link layer to be framed. Let’s take a look at the ARP cache on the Lab_A
router by using the show ip arp command:
Lab_A#sh ip arp
Protocol Address Age(min) Hardware Addr Type Interface
Internet 172.16.20.1 - 00d0.58ad.05f4 ARPA Ethernet1
Internet 172.16.20.2 3 0030.9492.a5dd ARPA Ethernet1
Internet 172.16.10.1 - 00d0.58ad.06aa ARPA Ethernet0
Internet 172.16.10.2 12 0030.9492.a4ac ARPA Ethernet0
The dash (-) signies that this is the physical interface on the router. This out-
put shows us that the router knows the 172.16.10.2 (Host_A) and 172.16.20.2
(Host_B) hardware addresses. Cisco routers will keep an entry in the ARP table
for 4 hours.
u Now if the hardware address hasn’t already been resolved, the router will send
an ARP request out E1 looking for the 172.16.20.2 hardware address. Host_B
responds with its hardware address, and the packet and destination hardware
addresses are then both sent to the Data Link layer for framing.
16. The Data Link layer creates a frame with the destination and source hardware addresses,
Ether-Type field, and FCS field at the end. The frame is then handed to the Physical layer
to be sent out on the physical medium one bit at a time.
17. Host_B receives the frame and immediately runs a CRC. If the result matches the
information in the FCS field, the hardware destination address will be then checked
The IP Routing Process 339
next. If the host finds a match, the Ether-Type field is then checked to determine the
protocol that the packet should be handed to at the Network layer—IP in this example.
18. At the Network layer, IP receives the packet and runs a CRC on the IP header. If that
passes, IP then checks the destination address. Since a match has finally been made, the
Protocol field is checked to find out to whom the payload should be given.
19. The payload is handed to ICMP, which understands that this is an echo request. ICMP
responds to this by immediately discarding the packet and generating a new payload as
an echo reply.
20. A packet is then created including the source and destination addresses, Protocol field,
and payload. The destination device is now Host_A.
21. IP then checks to see whether the destination IP address is a device on the local LAN
or on a remote network. Since the destination device is on a remote network, the
packet needs to be sent to the default gateway.
22. The default gateway IP address is found in the Registry of the Windows device, and the
ARP cache is checked to see if the hardware address has already been resolved from an
IP address.
23. Once the hardware address of the default gateway is found, the packet and destination
hardware addresses are handed down to the Data Link layer for framing.
24. The Data Link layer frames the packet of information and includes the following in
the header:
u The destination and source hardware addresses
u The Ether-Type field with 0x0800 (IP) in it
u The FCS field with the CRC result in tow
25. The frame is now handed down to the Physical layer to be sent out over the network
medium one bit at a time.
26. The router’s Ethernet 1 interface receives the bits and builds a frame. The CRC is run,
and the FCS field is checked to make sure the answers match.
2 7. Once the CRC is found to be okay, the hardware destination address is checked. Since
the router’s interface is a match, the packet is pulled from the frame and the Ether-Type
field is checked to determine which protocol the packet should be delivered to at the
Network layer.
28. The protocol is determined to be IP, so it gets the packet. IP runs a CRC check on the
IP header first and then checks the destination IP address.
IP does not run a complete CRC as the Data Link layer does—it only checks
the header for errors.
340 Chapter 8 u IP Routing
Since the IP destination address doesn’t match any of the router’s interfaces, the routing
table is checked to see whether it has a route to 172.16.10.0. If it doesn’t have a route
over to the destination network, the packet will be discarded immediately. I want to
take a minute to point out that this is exactly where the source of confusion begins for
a lot of administrators because when a ping fails, most people think the packet never
reached the destination host. But as we see here, that’s not always the case. All it takes
for this to happen is for even just one of the remote routers to lack a route back to the
originating host’s network and—poof!—the packet is dropped on the return trip, not
on its way to the host!
Just a quick note to mention that when (and if) the packet is lost on the
way back to the originating host, you will typically see a request timed-out
message because it is an unknown error. If the error occurs because of a
known issue, such as if a route is not in the routing table on the way to the
destination device, you will see a destination unreachable message. This
should help you determine if the problem occurred on the way to the desti-
nation or on the way back.
29. In this case, the router happens to know how to get to network 172.16.10.0—the exit
interface is Ethernet 0—so the packet is switched to interface Ethernet 0.
30. The router then checks the ARP cache to determine whether the hardware address for
172.16.10.2 has already been resolved.
31. Since the hardware address to 172.16.10.2 is already cached from the originating trip
to Host_B, the hardware address and packet are then handed to the Data Link layer.
32. The Data Link layer builds a frame with the destination hardware address and source
hardware address and then puts IP in the Ether-Type field. A CRC is run on the frame
and the result is placed in the FCS field.
33. The frame is then handed to the Physical layer to be sent out onto the local network
one bit at a time.
34. The destination host receives the frame, runs a CRC, checks the destination hardware
address, then looks into the Ether-Type field to find out to whom to hand the packet.
35. IP is the designated receiver, and after the packet is handed to IP at the Network layer,
it checks the Protocol field for further direction. IP finds instructions to give the pay-
load to ICMP, and ICMP determines the packet to be an ICMP echo reply.
36. ICMP acknowledges that it has received the reply by sending an exclamation point (!)
to the user interface. ICMP then attempts to send four more echo requests to the desti-
nation host.
You’ve just experienced Todd’s 36 easy steps to understanding IP routing. The key point
here is that if you had a much larger network, the process would be the same. It’s just that
the larger the internetwork, the more hops the packet goes through before it nds the desti-
nation host.
The IP Routing Process 341
It’s super-important to remember that when Host_A sends a packet to Host_B, the des-
tination hardware address used is the default gateway’s Ethernet interface. Why? Because
frames can’t be placed on remote networks—only local networks. So packets destined for
remote networks must go through the default gateway.
Let’s take a look at Host_A’s ARP cache now:
C:\ >arp -a
Interface: 172.16.10.2 --- 0x3
Internet Address Physical Address Type
172.16.10.1 00-15-05-06-31-b0 dynamic
172.16.20.1 00-15-05-06-31-b0 dynamic
Did you notice that the hardware (MAC) address that Host_A uses to get to Host_B is
the Lab_A E0 interface? Hardware addresses are always local, and they never pass through
a router’s interface. Understanding this process is as important as air to you, so carve this
into your memory!
The Cisco Router Internal Process
One more thing before we get to testing your understanding of my 36 steps of IP routing. I
think it’s important to explain how a router forwards packets internally. For IP to look up a
destination address in a routing table on a router, processing in the router must take place,
and if there are tens of thousands of routes in that table, the amount of CPU time would be
enormous. It results in a potentially overwhelming amount of overhead—think about a router
at your ISP that has to calculate millions of packets per second and even subnet to nd the
correct exit interface! Even with the little network I’m using in this book, lots of processing
would need to be done if there were actual hosts connected and sending data.
Cisco uses three types of packet-forwarding techniques.
Process Switching This is actually how many people see routers to this day, because it’s
true that routers actually did perform this type of bare-bones packet switching back in
1990 when Cisco released their very rst router. But those days when trafc demands were
unimaginably light are long gone—not in today’s networks! This process is now extremely
complex and involves looking up every destination in the routing table and nding the exit
interface for every packet. This is pretty much how I just explained the process in my 36
steps. But even though what I wrote was absolutely true in concept, the internal process
requires much more than packet-switching technology today because of the millions of
packets per second that must now be processed. So Cisco came up with some other tech-
nologies to help with the “big process problem.”
Fast Switching This solution was created to make the slow performance of process switch-
ing faster and more efcient. Fast-switching uses a cache to store the most recently used
destinations so that lookups are not required for every packet. By caching the exit inter-
face of the destination device, as well as the layer 2 header, performance was dramatically
improved, but as our networks evolved with the need for even more speed, Cisco created
yet another technology!
342 Chapter 8 u IP Routing
Cisco Express Forwarding (CEF) This is Cisco’s newer creation, and it’s the default packet-
forwarding method used on all the latest Cisco routers. CEF makes many different cache
tables to help improve performance and is change triggered, not packet triggered. Translated,
this means that when the network topology changes, the cache changes along with it.
Testing Your IP Routing Understanding
Since understanding IP routing is super-important, it’s time for that little test I talked about
earlier on how well you’ve got the IP routing process down so far. I’m going to do that by
having you look at a couple of gures and answer some very basic IP routing questions
based upon them.
Figure8.4 shows a LAN connected to RouterA that’s connected via a WAN link to
RouterB. RouterB has a LAN connected with an HTTP server attached.
FIGURE8.4 IP routing example 1
HostA
RouterA
MAC
RouterB
S0/0
Fa0/0
HTTP Server
IP
TCP Dest Port 80
S0/0 Fa0/0
The critical information you want to obtain by looking at this gure is exactly how IP
routing will occur in this example. Let’s determine the characteristics of a frame as it leaves
HostA. Okay—we’ll cheat a bit. I’ll give you the answer, but then you should go back over
the gure and see if you can answer example 2 without looking at my three-step answer!
1. The destination address of a frame from HostA would be the MAC address of
RouterA’s Fa0/0 interface.
2. The destination address of a packet would be the IP address of the HTTP server’s
network interface card (NIC).
3. The destination port number in the segment header would be 80.
That was a pretty simple, straightforward scenario. One thing to remember is that when
multiple hosts are communicating to a server using HTTP, they must all use a different
source port number. The source and destination IP addresses and port numbers are how
the server keeps the data separated at the Transport layer.
The IP Routing Process 343
Okay—let’s complicate matters by adding another device into the network and then see if
you can nd the answers. Figure8.5 shows a network with only one router but two switches.
FIGURE8.5 IP routing example 2
HostA
RouterA
Fa0/1
Fa0/0
HTTPS Server
The key thing to understand about the IP routing process in this scenario is what hap-
pens when HostA sends data to the HTTPS server? Here’s your answer:
1. The destination address of a frame from HostA would be the MAC address of
RouterA’s Fa0/0 interface.
2. The destination address of a packet is the IP address of the HTTPS server’s network
interface card (NIC).
3. The destination port number in the segment header will have a value of 443.
Did you notice that the switches weren’t used as either a default gateway or any other des-
tination? That’s because switches have nothing to do with routing. I wonder how many of
you chose the switch as the default gateway (destination) MAC address for HostA? If you did,
don’t feel bad—just take another look to see where you went wrong and why. It’s very impor-
tant to remember that the destination MAC address will always be the router’s interface—if
your packets are destined for outside the LAN, as they were in these last two examples!
Before moving on into some of the more advanced aspects of IP routing, let’s look at
another issue. Take a look at the output of this router’s routing table:
Corp#sh ip route
[output cut]
R 192.168.215.0 [120/2] via 192.168.20.2, 00:00:23, Serial0/0
R 192.168.115.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0
R 192.168.30.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0
C 192.168.20.0 is directly connected, Serial0/0
L 192.168.20.1/32 is directly connected, Serial0/0
C 192.168.214.0 is directly connected, FastEthernet0/0
L 192.168.214.1/32 is directly connected, FastEthernet0/0
344 Chapter 8 u IP Routing
What do we see here? If I were to tell you that the corporate router received an IP packet
with a source IP address of 192.168.214.20 and a destination address of 192.168.22.3,
what do you think the Corp router will do with this packet?
If you said, “The packet came in on the FastEthernet 0/0 interface, but because the
routing table doesn’t show a route to network 192.168.22.0 (or a default route), the router
will discard the packet and send an ICMP destination unreachable message back out to
interface FastEthernet 0/0,” you’re a genius! The reason that’s the correct answer is because
that’s the source LAN where the packet originated from.
Now, let’s check out the next gure and talk about the frames and packets in detail.
We’re not really going over anything new here; I’m just making sure you totally, completely,
thoroughly, fully understand basic IP routing! It is the crux of this book, and the topic the
exam objectives are geared toward. It’s all about IP routing, which means you need to be
all over this stuff! We’ll use Figure8.6 for the next few scenarios.
FIGURE8.6 Basic IP routing using MAC and IP addresses
Fa0/0
S0/0
12 3 4
S0/0 Fa0/0
Sales
Server
Lab A Lab B
Referring to Figure8.6, here’s a list of all the answers to questions you need inscribed in
your brain:
1. In order to begin communicating with the Sales server, Host 4 sends out an ARP
request. How will the devices exhibited in the topology respond to this request?
2. Host 4 has received an ARP reply. Host 4 will now build a packet, then place this
packet in the frame. What information will be placed in the header of the packet that
leaves Host 4 if Host 4 is going to communicate to the Sales server?
3. The Lab_A router has received the packet and will send it out Fa0/0 onto the LAN
toward the server. What will the frame have in the header as the source and destination
addresses?
4. Host 4 is displaying two web documents from the Sales server in two browser windows
at the same time. How did the data find its way to the correct browser windows?
The following should probably be written in a teensy font and put upside down in another
part of the book so it would be really hard for you to cheat and peek, but since I’m not that
The IP Routing Process 345
mean and you really need to have this down, here are your answers in the same order that the
scenarios were just presented:
1. In order to begin communicating with the server, Host 4 sends out an ARP request.
How will the devices exhibited in the topology respond to this request? Since MAC
addresses must stay on the local network, the Lab_B router will respond with the
MAC address of the Fa0/0 interface and Host 4 will send all frames to the MAC
address of the Lab_B Fa0/0 interface when sending packets to the Sales server.
2. Host 4 has received an ARP reply. Host 4 will now build a packet, then place this
packet in the frame. What information will be placed in the header of the packet that
leaves Host 4 if Host 4 is going to communicate to the Sales server? Since we’re now
talking about packets, not frames, the source address will be the IP address of Host 4
and the destination address will be the IP address of the Sales server.
3. Finally, the Lab_A router has received the packet and will send it out Fa0/0 onto the
LAN toward the server. What will the frame have in the header as the source and desti-
nation addresses? The source MAC address will be the Lab_A router’s Fa0/0 interface,
and the destination MAC address will be the Sales server’s MAC address because all
MAC addresses must be local on the LAN.
4. Host 4 is displaying two web documents from the Sales server in two different
browser windows at the same time. How did the data find its way to the correct
browser windows? TCP port numbers are used to direct the data to the correct
application window.
Okay—great! But we’re not quite done yet. I’ve got a few more questions for you before
you actually get to congure routing in a real network. Ready? Figure8.7 shows a basic
network, and Host 4 needs to get email. Which address will be placed in the destination
address eld of the frame when it leaves Host 4?
FIGURE8.7 Testing basic routing knowledge
Fa0/0
S0/0
12 3 4
S0/0 Fa0/0
Email
server
Lab A Lab B
IP is end to end
ARP request/reply
Frames are local only
ARP request/reply
Frames are local only
346 Chapter 8 u IP Routing
The answer is that Host 4 will use the destination MAC address of the Fa0/0 interface
on the Lab_B router—you knew that, right? Look at Figure8.7 again: What if Host 4
needs to communicate with Host 1—not the server, but with Host 1. Which OSI layer 3
source address will be found in the packet header when it reaches Host 1?
Hopefully you’ve got this: At layer 3, the source IP address will be Host 4 and the des-
tination address in the packet will be the IP address of Host 1. Of course, the destination
MAC address from Host 4 will always be the Fa0/0 address of the Lab_B router, right?
And since we have more than one router, we’ll need a routing protocol that communicates
between both of them so that trafc can be forwarded in the right direction to reach the
network that Host 1 is connected to.
Okay—one more scenario and you’re on your way to being an IP routing machine! Again,
using Figure8.7, Host 4 is transferring a le to the email server connected to the Lab_A router.
What would be the layer 2 destination address leaving Host 4? Yes, I’ve asked this question
more than once. But not this one: What will be the source MAC address when the frame is
received at the email server?
Hopefully, you answered that the layer 2 destination address leaving Host 4 is the MAC
address of the Fa0/0 interface on the Lab_B router and that the source layer 2 address that
the email server will receive is the Fa0/0 interface of the Lab_A router.
If you did, you’re ready to discover how IP routing is handled in a larger network
environment!
Configuring IP Routing
It’s time to get serious and congure a real network. Figure8.8 shows three routers: Corp,
SF, and LA. Remember that, by default, these routers only know about networks that are
directly connected to them. I’ll continue to use this gure and network throughout the rest
of the chapters in this book. As I progress through this book, I’ll add more routers and
switches as needed.
As you might guess, I’ve got quite a nice collection of routers for us to play with. But you
don’t need a closet full of devices to perform most, if not all, of the commands we’ll use in
this book. You can get by nicely with pretty much any router or even with a good router
simulator.
Getting back to business, the Corp router has two serial interfaces, which will provide
a WAN connection to the SF and LA router and two Fast Ethernet interfaces as well. The
two remote routers have two serial interfaces and two Fast Ethernet interfaces.
The rst step for this project is to correctly congure each router with an IP address on
each interface. The following list shows the IP address scheme I’m going to use to congure
the network. After we go over how the network is congured, I’ll cover how to congure
IP routing. Pay attention to the subnet masks—they’re important! The LANs all use a /24
mask, but the WANs are using a /30.
Configuring IP Routing 347
FIGURE8.8 Configuring IP routing
SF
Corp
10.10.10.1/24Fa0/0
Fa0/0192.168.10.1/24
172.16.10.2/30 S0/0/0
LA
Fa0/0 192.168.20.1/24
172.16.10.6/30
S0/0/1
172.16.10.1/30 S0/0 172.16.10.5/30S0/1
Corp
u Serial 0/0: 172.16.10.1/30
u Serial 0/1: 172.16.10.5/30
u Fa0/0: 10.10.10.1/24
SF
u S0/0/0: 172.16.10.2/30
u Fa0/0: 192.168.10.1/24
LA
u S0/0/0: 172.16.10.6/30
u Fa0/0: 192.168.20.1/24
The router conguration is really a pretty straightforward process since you just need
to add IP addresses to your interfaces and then perform a no shutdown on those same
interfaces. It gets a tad more complex later on, but for right now, let’s congure the IP
addresses in the network.
Corp Configuration
We need to congure three interfaces to congure the Corp router. And conguring the
hostnames of each router will make identication much easier. While we’re at it, let’s set
the interface descriptions, banner, and router passwords too because it’s a really good idea
to make a habit of conguring these commands on every router!
348 Chapter 8 u IP Routing
To get started, I performed an erase startup-config on the router and reloaded, so
we’ll start in setup mode. I chose no when prompted to enter setup mode, which will get us
straight to the username prompt of the console. I’m going to congure all my routers this
same way.
Here’s how what I just did looks:
--- System Configuration Dialog ---
Would you like to enter the initial configuration dialog? [yes/no]: n
Press RETURN to get started!
Router>en
Router#config t
Router(config)#hostname Corp
Corp(config)#enable secret GlobalNet
Corp(config)#no ip domain-lookup
Corp(config)#int f0/0
Corp(config-if)#desc Connection to LAN BackBone
Corp(config-if)#ip address 10.10.10.1 255.255.255.0
Corp(config-if)#no shut
Corp(config-if)#int s0/0
Corp(config-if)#desc WAN connection to SF
Corp(config-if)#ip address 172.16.10.1 255.255.255.252
Corp(config-if)#no shut
Corp(config-if)#int s0/1
Corp(config-if)#desc WAN connection to LA
Corp(config-if)#ip address 172.16.10.5 255.255.255.252
Corp(config-if)#no shut
Corp(config-if)#line con 0
Corp(config-line)#password console
Corp(config-line)#login
Corp(config-line)#loggin sync
Corp(config-line)#exit
Corp(config)#line vty 0 ?
<1-181> Last Line number
<cr>
Corp(config)#line vty 0 181
Corp(config-line)#password telnet
Corp(config-line)#login
Corp(config-line)#exit
Configuring IP Routing 349
Corp(config)#banner motd # This is my Corp Router #
Corp(config)#^Z
Corp#copy run start
Destination filename [startup-config]?
Building configuration...
[OK]
Corp# [OK]
Let’s talk about the conguration of the Corp router. First, I set the hostname and
enable secret, but what is that no ip domain-lookup command? That command stops the
router from trying to resolve hostnames, which is an annoying feature unless you’ve con-
gured a host table or DNS. Next, I congured the three interfaces with descriptions and
IP addresses and enabled them with the no shutdown command. The console and VTY
passwords came next, but what is that logging sync command under the console line?
The logging synchronous command stops console messages from writing over what you
are typing in, meaning it’s a sanity-saving command that you’ll come to love! Last, I set
my banner and then saved my congs.
If you’re having a hard time understanding this configuration process, refer
back to Chapter 6, “Cisco’s Internetworking Operating System (IOS).”
To view the IP routing tables created on a Cisco router, use the command show ip route.
Here’s the command’s output:
Corp#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 1 subnets
C 10.10.10.0 is directly connected, FastEthernet0/0
L 10.10.10.1/32 is directly connected, FastEthernet0/0
Corp#
It’s important to remember that only congured, directly connected networks are going
to show up in the routing table. So why is it that only the FastEthernet 0/0 interface shows
350 Chapter 8 u IP Routing
up in the table? No worries—that’s just because you won’t see the serial interfaces come up
until the other side of the links are operational. As soon as we congure our SF and NY
routers, those interfaces should pop right up!
But did you notice the C on the left side of the output of the routing table? When you see
that there, it means that the network is directly connected. The codes for each type of con-
nection are listed at the top of the show ip route command, along with their descriptions.
For brevity, the codes at the top of the output will be cut in the rest of
this chapter.
SF Configuration
Now we’re ready to congure the next router—SF. To make that happen correctly, keep in
mind that we have two interfaces to deal with: serial 0/0/0 and FastEthernet 0/0. So let’s make
sure we don’t forget to add the hostname, passwords, interface descriptions, and banners to
the router conguration. As I did with the Corp router, I erased the conguration and reloaded
since this router had already been congured before.
Here’s the conguration I used:
R1#erase start
% Incomplete command.
R1#erase startup-config
Erasing the nvram filesystem will remove all configuration files!
Continue? [confirm][enter]
[OK]
Erase of nvram: complete
R1#reload
Proceed with reload? [confirm][enter]
[output cut]
%Error opening tftp://255.255.255.255/network-confg (Timed out)
%Error opening tftp://255.255.255.255/cisconet.cfg (Timed out)
--- System Configuration Dialog ---
Would you like to enter the initial configuration dialog? [yes/no]: n
Before we move on, let’s talk about this output for a second. First, notice that beginning
with IOS 12.4, ISR routers will no longer take the command erase start. The router has
only one command after erase that starts with s, as shown here:
Router#erase s?
startup-config
Configuring IP Routing 351
I know, you’d think that the IOS would continue to accept the command, but nope—
sorry! The second thing I want to point out is that the output tells us the router is looking
for a TFTP host to see if it can download a conguration. When that fails, it goes straight
into setup mode. This gives you a great picture of the Cisco router default boot sequence we
talked about in Chapter 7, “Managing and Troubleshooting a Cisco Internetwork.”
Okay, let’s get back to conguring our router:
Press RETURN to get started!
Router#config t
Router(config)#hostname SF
SF(config)#enable secret GlobalNet
SF(config)#no ip domain-lookup
SF(config)#int s0/0/0
SF(config-if)#desc WAN Connection to Corp
SF(config-if)#ip address 172.16.10.2 255.255.255.252
SF(config-if)#no shut
SF(config-if)#clock rate 1000000
SF(config-if)#int f0/0
SF(config-if)#desc SF LAN
SF(config-if)#ip address 192.168.10.1 255.255.255.0
SF(config-if)#no shut
SF(config-if)#line con 0
SF(config-line)#password console
SF(config-line)#login
SF(config-line)#logging sync
SF(config-line)#exit
SF(config)#line vty 0 ?
<1-1180> Last Line number
<cr>
SF(config)#line vty 0 1180
SF(config-line)#password telnet
SF(config-line)#login
SF(config-line)#banner motd #This is the SF Branch router#
SF(config)#exit
SF#copy run start
Destination filename [startup-config]?
Building configuration...
[OK]
Let’s take a look at our conguration of the interfaces with the following two commands:
SF#sh run | begin int
interface FastEthernet0/0
352 Chapter 8 u IP Routing
description SF LAN
ip address 192.168.10.1 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet0/1
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/0/0
description WAN Connection to Corp
ip address 172.16.10.2 255.255.255.252
clock rate 1000000
!
SF#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.10.1 YES manual up up
FastEthernet0/1 unassigned YES unset administratively down down
Serial0/0/0 172.16.10.2 YES manual up up
Serial0/0/1 unassigned YES unset administratively down down
SF#
Now that both ends of the serial link are congured, the link comes up. Remember,
the up/up status for the interfaces are Physical/Data Link layer status indicators that don’t
reect the layer 3 status! I ask students in my classes, “If the link shows up/up, can you
ping the directly connected network?” And they say, “Yes!” The correct answer is, “I don’t
know,” because we can’t see the layer 3 status with this command. We only see layers 1 and
2 and verify that the IP addresses don’t have a typo. This is really important to understand!
The show ip route command for the SF router reveals the following:
SF#sh ip route
C 192.168.10.0/24 is directly connected, FastEthernet0/0
L 192.168.10.1/32 is directly connected, FastEthernet0/0
172.16.0.0/30 is subnetted, 1 subnets
C 172.16.10.0 is directly connected, Serial0/0/0
L 172.16.10.2/32 is directly connected, Serial0/0/0
Configuring IP Routing 353
Notice that router SF knows how to get to networks 172.16.10.0/30 and 192.168.10.0/24;
we can now ping to the Corp router from SF:
SF#ping 172.16.10.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.10.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms
Okay—now let’s head back to the Corp router and check out the routing table:
Corp>sh ip route
172.16.0.0/30 is subnetted, 1 subnets
C 172.16.10.0 is directly connected, Serial0/0
L 172.16.10.1/32 is directly connected, Serial0/0
10.0.0.0/24 is subnetted, 1 subnets
C 10.10.10.0 is directly connected, FastEthernet0/0
L 10.10.10.1/32 is directly connected, FastEthernet0/0
On the SF router’s serial interface 0/0/0 is a DCE connection, which means a clock rate
needs to be set on the interface. Remember that you don’t need to use the clock rate com-
mand in production. While true, it’s still imperative that you know how/when you can use it
and that you understand it really well when studying for your CCNA exam!
We can see our clocking with the show controllers command:
SF#sh controllers s0/0/0
Interface Serial0/0/0
Hardware is GT96K
DCE V.35, clock rate 1000000
Corp>sh controllers s0/0
Interface Serial0/0
Hardware is PowerQUICC MPC860
DTE V.35 TX and RX clocks detected.
Since the SF router has a DCE cable connection, I needed to add clock rate to this inter-
face because DTE receives clock. Keep in mind that the new ISR routers will autodetect this
and set the clock rate to 2000000. And you still need to make sure you’re able to nd an
interface that is DCE and set clocking to meet the objectives.
Since the serial links are showing up, we can now see both networks in the Corp routing
table. And once we congure LA, we’ll see one more network in the routing table of the Corp
router. The Corp router can’t see the 192.168.10.0 network because we don’t have any rout-
ing congured yet—routers see only directly connected networks by default.
354 Chapter 8 u IP Routing
LA Configuration
To congure LA, we’re going to do pretty much the same thing we did with the other two
routers. There are two interfaces to deal with, serial 0/0/1 and FastEthernet 0/0, and again,
we’ll be sure to add the hostname, passwords, interface descriptions, and a banner to the
router conguration:
Router(config)#hostname LA
LA(config)#enable secret GlobalNet
LA(config)#no ip domain-lookup
LA(config)#int s0/0/1
LA(config-if)#ip address 172.16.10.6 255.255.255.252
LA(config-if)#no shut
LA(config-if)#clock rate 1000000
LA(config-if)#description WAN To Corporate
LA(config-if)#int f0/0
LA(config-if)#ip address 192.168.20.1 255.255.255.0
LA(config-if)#no shut
LA(config-if)#description LA LAN
LA(config-if)#line con 0
LA(config-line)#password console
LA(config-line)#login
LA(config-line)#loggin sync
LA(config-line)#exit
LA(config)#line vty 0 ?
<1-1180> Last Line number
<cr>
LA(config)#line vty 0 1180
LA(config-line)#password telnet
LA(config-line)#login
LA(config-line)#exit
LA(config)#banner motd #This is my LA Router#
LA(config)#exit
LA#copy run start
Destination filename [startup-config]?
Building configuration...
[OK]
Nice—everything was pretty straightforward. The output below, which I gained via the
show ip route command, displays the directly connected networks of 192.168.20.0 and
172.16.10.0:
LA#sh ip route
172.16.0.0/30 is subnetted, 1 subnets
Configuring IP Routing 355
C 172.16.10.4 is directly connected, Serial0/0/1
L 172.16.10.6/32 is directly connected, Serial0/0/1
C 192.168.20.0/24 is directly connected, FastEthernet0/0
L 192.168.20.1/32 is directly connected, FastEthernet0/0
Okay, so now that we’ve congured all three routers with IP addresses and administra-
tive functions, we can move on to deal with routing. But I want to do one more thing on
the SF and LA routers—since this is a very small network, let’s build a DHCP server on the
Corp router for each LAN.
Configuring DHCP on Our Corp Router
While it’s true that I could approach this task by going to each remote router and creating
a pool, why bother with all that when I can easily create two pools on the Corp router and
have the remote routers forward requests to the Corp router? Of course, you remember how
to do this from Chapter 7!
Let’s give it a shot:
Corp#config t
Corp(config)#ip dhcp excluded-address 192.168.10.1
Corp(config)#ip dhcp excluded-address 192.168.20.1
Corp(config)#ip dhcp pool SF_LAN
Corp(dhcp-config)#network 192.168.10.0 255.255.255.0
Corp(dhcp-config)#default-router 192.168.10.1
Corp(dhcp-config)#dns-server 4.4.4.4
Corp(dhcp-config)#exit
Corp(config)#ip dhcp pool LA_LAN
Corp(dhcp-config)#network 192.168.20.0 255.255.255.0
Corp(dhcp-config)#default-router 192.168.20.1
Corp(dhcp-config)#dns-server 4.4.4.4
Corp(dhcp-config)#exit
Corp(config)#exit
Corp#copy run start
Destination filename [startup-config]?
Building configuration...
Creating DHCP pools on a router is actually a simple process, and you would go about
the conguration the same way on any router you wish to add a DHCP pool to. To designate
a router as a DHCP server, you just create the pool name, add the network/subnet and the
default gateway, and then exclude any addresses that you don’t want handed out. You de-
nitely want to make sure you’ve excluded the default gateway address, and you’d usually add
a DNS server as well. I always add any exclusions rst, and remember that you can conve-
niently exclude a range of addresses on a single line. Soon, I’ll demonstrate those verication
commands I promised I’d show you back in Chapter 7, but rst, we need to gure out why
the Corp router still can’t get to the remote networks by default!
356 Chapter 8 u IP Routing
Now I’m pretty sure I congured DHCP correctly, but I just have this nagging feeling
I forgot something important. What could that be? Well, the hosts are remote across a
router, so what would I need to do that would allow them to get an address from a DHCP
server? If you concluded that I’ve got to congure the SF and LA F0/0 interfaces to forward
the DHCP client requests to the server, you got it!
Here’s how we’d go about doing that:
LA#config t
LA(config)#int f0/0
LA(config-if)#ip helper-address 172.16.10.5
SF#config t
SF(config)#int f0/0
SF(config-if)#ip helper-address 172.16.10.1
I’m pretty sure I did this correctly, but we won’t know until I have some type of routing
congured and working. So let’s get to that next!
Configuring IP Routing in Our Network
So is our network really good to go? After all, I’ve congured it with IP addressing, admin-
istrative functions, and even clocking that will automatically occur with the ISR routers.
But how will our routers send packets to remote networks when they get their destination
information by looking into their tables that only include directions about directly connected
networks? And you know routers promptly discard packets they receive with addresses for
networks that aren’t listed in their routing table!
So we’re not exactly ready to rock after all. But we will be soon because there are several
ways to congure the routing tables to include all the networks in our little internetwork
so that packets will be properly forwarded. As usual, one size ts all rarely ts at all, and
what’s best for one network isn’t necessarily what’s best for another. That’s why understand-
ing the different types of routing will be really helpful when choosing the best solution for
your specic environment and business requirements.
These are the three routing methods I’m going to cover with you:
u Static routing
u Default routing
u Dynamic routing
We’re going to start with the rst way and implement static routing on our network,
because if you can implement static routing and make it work, you’ve demonstrated that
you denitely have a solid understanding of the internetwork. So let’s get started.
Configuring IP Routing in Our Network 357
Static Routing
Static routing is the process that ensues when you manually add routes in each router’s rout-
ing table. Predictably, there are pros and cons to static routing, but that’s true for all routing
approaches.
Here are the pros:
u There is no overhead on the router CPU, which means you could probably make do
with a cheaper router than you would need for dynamic routing.
u There is no bandwidth usage between routers, saving you money on WAN links as well
as minimizing overhead on the router since you’re not using a routing protocol.
u It adds security because you, the administrator, can be very exclusive and choose to
allow routing access to certain networks only.
And here are the cons:
u Whoever the administrator is must have a vault-tight knowledge of the internetwork
and how each router is connected in order to configure routes correctly. If you don’t
have a good, accurate map of your internetwork, things will get very messy quickly!
u If you add a network to the internetwork, you have to tediously add a route to it on all
routers by hand, which only gets increasingly insane as the network grows.
u Due to the last point, it’s just not feasible to use it in most large networks because
maintaining it would be a full-time job in itself.
But that list of cons doesn’t mean you get to skip learning all about it mainly because of
that rst disadvantage I listed—the fact that you must have such a solid understanding of a
network to congure it properly and that your administrative knowledge has to practically
verge on the supernatural! So let’s dive in and develop those skills. Starting at the beginning,
here’s the command syntax you use to add a static route to a routing table from global cong:
ip route [destination_network] [mask] [next-hop_address or
exitinterface] [administrative_distance] [permanent]
This list describes each command in the string:
ip route The command used to create the static route.
destination_network The network you’re placing in the routing table.
mask The subnet mask being used on the network.
n e xt- h o p_ a d d re s s This is the IP address of the next-hop router that will receive packets
and forward them to the remote network, which must signify a router interface that’s on
a directly connected network. You must be able to successfully ping the router interface
before you can add the route. Important note to self is that if you type in the wrong next-
hop address or the interface to the correct router is down, the static route will show up in
the router’s conguration but not in the routing table.
358 Chapter 8 u IP Routing
exitinterface Used in place of the next-hop address if you want, and shows up as a
directly connected route.
administrative_ distance By default, static routes have an administrative distance of 1
or 0 if you use an exit interface instead of a next-hop address. You can change the default
value by adding an administrative weight at the end of the command. I’ll talk a lot more
about this later in the chapter when we get to the section on dynamic routing.
permanent If the interface is shut down or the router can’t communicate to the next-hop
router, the route will automatically be discarded from the routing table by default. Choosing
the permanent option keeps the entry in the routing table no matter what happens.
Before I guide you through conguring static routes, let’s take a look at a sample static
route to see what we can nd out about it:
Router(config)#ip route 172.16.3.0 255.255.255.0 192.168.2.4
u The ip route command tells us simply that it’s a static route.
u 172.16.3.0 is the remote network we want to send packets to.
u 255.255.255.0 is the mask of the remote network.
u 192.168.2.4 is the next hop, or router, that packets will be sent to.
But what if the static route looked like this instead?
Router(config)#ip route 172.16.3.0 255.255.255.0 192.168.2.4 150
That 150 at the end changes the default administrative distance (AD) of 1 to 150. As
said, I’ll talk much more about AD when we get into dynamic routing, but for now, just
remember that the AD is the trustworthiness of a route, where 0 is best and 255 is worst.
One more example, then we’ll start conguring:
Router(config)#ip route 172.16.3.0 255.255.255.0 s0/0/0
Instead of using a next-hop address, we can use an exit interface that will make the route
show up as a directly connected network. Functionally, the next hop and exit interface work
exactly the same.
To help you understand how static routes work, I’ll demonstrate the conguration on
the internetwork shown previously in Figure8.8. Here it is again in Figure8.9 to save
you the trouble of having to go back and forth to view the same gure.
Corp
Each routing table automatically includes directly connected networks. To be able to route
to all indirectly connected networks within the internetwork, the routing table must include
information that describes where these other networks are located and how to get to them.
The Corp router is connected to three networks. For the Corp router to be able to route
to all networks, the following networks have to be congured into its routing table:
u 192.168.10.0
u 192.168.20.0
Configuring IP Routing in Our Network 359
FIGURE8.9 Our internetwork
SF
Corp
10.10.10.1/24Fa0/0
Fa0/0192.168.10.1/24
172.16.10.2/30 S0/0/0
LA
Fa0/0 192.168.20.1/24
172.16.10.6/30
S0/0/1
172.16.10.1/30 S0/0 172.16.10.5/30S0/1
The following router output shows the static routes on the Corp router and the routing
table after the conguration. For the Corp router to nd the remote networks, I had to place
an entry into the routing table describing the remote network, the remote mask, and where
to send the packets. I am going to add a 150 at the end of each line to raise the administra-
tive distance. You’ll see why soon when we get to dynamic routing. Here’s the output:
Corp#config t
Corp(config)#ip route 192.168.10.0 255.255.255.0 172.16.10.2 150
Corp(config)#ip route 192.168.20.0 255.255.255.0 s0/1 150
Corp(config)#do show run | begin ip route
ip route 192.168.10.0 255.255.255.0 172.16.10.2 150
ip route 192.168.20.0 255.255.255.0 Serial0/1 150
I needed to use different paths for networks 192.168.10.0 and 192.168.20.0, so I used a
next-hop address for the SF router and an exit interface for the LA router. After the router
has been congured, you can just type show ip route to see the static routes:
Corp(config)#do show ip route
S 192.168.10.0/24 [150/0] via 172.16.10.2
172.16.0.0/30 is subnetted, 2 subnets
C 172.16.10.4 is directly connected, Serial0/1
L 172.16.10.5/32 is directly connected, Serial0/1
C 172.16.10.0 is directly connected, Serial0/0
L 172.16.10.1/32 is directly connected, Serial0/0
S 192.168.20.0/24 is directly connected, Serial0/1
360 Chapter 8 u IP Routing
10.0.0.0/24 is subnetted, 1 subnets
C 10.10.10.0 is directly connected, FastEthernet0/0
L 10.10.10.1/32 is directly connected, FastEthernet0/0
The Corp router is congured to route and know all routes to all networks. But can you
see a difference in the routing table for the routes to SF and LA? That’s right! The next-hop
conguration showed up as via, and the route congured with an exit interface congura-
tion shows up as static but also as directly connected! This demonstrates how they are
functionally the same but will display differently in the routing table.
Understand that if the routes don’t appear in the routing table, it’s because the router
can’t communicate with the next-hop address you’ve congured. But you can still use the
permanent parameter to keep the route in the routing table even if the next-hop device can’t
be contacted.
The S in the rst routing table entry means that the route is a static entry. The [150/0]
stands for the administrative distance and metric to the remote network, respectively.
Okay—we’re good. The Corp router now has all the information it needs to communi-
cate with the other remote networks. Still, keep in mind that if the SF and LA routers aren’t
congured with all the same information, the packets will be discarded. We can x this by
conguring static routes.
Don’t stress about the 150 at the end of the static route configuration at all,
because I promise to get to it really soon in this chapter, not a later one!
You really don’t need to worry about it at this point.
SF
The SF router is directly connected to networks 172.16.10.0/30 and 192.168.10.0/24, which
means I’ve got to congure the following static routes on the SF router:
u 10.10.10.0/24
u 192.168.20.0/24
u 172.16.10.4/30
The conguration for the SF router is revealed in the output below. Remember that we’ll
never create a static route to any network we’re directly connected to as well as the fact
that we must use the next hop of 172.16.10.1 since that’s our only router connection. Let’s
check out the commands:
SF(config)#ip route 10.10.10.0 255.255.255.0 172.16.10.1 150
SF(config)#ip route 172.16.10.4 255.255.255.252 172.16.10.1 150
SF(config)#ip route 192.168.20.0 255.255.255.0 172.16.10.1 150
SF(config)#do show run | begin ip route
ip route 10.10.10.0 255.255.255.0 172.16.10.1 150
ip route 172.16.10.4 255.255.255.252 172.16.10.1 150
ip route 192.168.20.0 255.255.255.0 172.16.10.1 150
Configuring IP Routing in Our Network 361
By looking at the routing table, you can see that the SF router now understands how to
nd each network:
SF(config)#do show ip route
C 192.168.10.0/24 is directly connected, FastEthernet0/0
L 192.168.10.1/32 is directly connected, FastEthernet0/0
172.16.0.0/30 is subnetted, 3 subnets
S 172.16.10.4 [150/0] via 172.16.10.1
C 172.16.10.0 is directly connected, Serial0/0/0
L 172.16.10.2/32 is directly connected, Serial0/0
S 192.168.20.0/24 [150/0] via 172.16.10.1
10.0.0.0/24 is subnetted, 1 subnets
S 10.10.10.0 [150/0] via 172.16.10.1
And we now can rest assured that the SF router has a complete routing table as well. As
soon as the LA router has all the networks in its routing table, SF will be able to communi-
cate with all remote networks!
LA
The LA router is directly connected to 192.168.20.0/24 and 172.16.10.4/30, so these are
the routes that must be added:
u 10.10.10.0/24
u 172.16.10.0/30
u 192.168.10.0/24
And here’s the LA router’s conguration:
LA#config t
LA(config)#ip route 10.10.10.0 255.255.255.0 172.16.10.5 150
LA(config)#ip route 172.16.10.0 255.255.255.252 172.16.10.5 150
LA(config)#ip route 192.168.10.0 255.255.255.0 172.16.10.5 150
LA(config)#do show run | begin ip route
ip route 10.10.10.0 255.255.255.0 172.16.10.5 150
ip route 172.16.10.0 255.255.255.252 172.16.10.5 150
ip route 192.168.10.0 255.255.255.0 172.16.10.5 150
This output displays the routing table on the LA router:
LA(config)#do sho ip route
S 192.168.10.0/24 [150/0] via 172.16.10.5
172.16.0.0/30 is subnetted, 3 subnets
C 172.16.10.4 is directly connected, Serial0/0/1
L 172.16.10.6/32 is directly connected, Serial0/0/1
362 Chapter 8 u IP Routing
S 172.16.10.0 [150/0] via 172.16.10.5
C 192.168.20.0/24 is directly connected, FastEthernet0/0
L 192.168.20.1/32 is directly connected, FastEthernet0/0
10.0.0.0/24 is subnetted, 1 subnets
S 10.10.10.0 [150/0] via 172.16.10.5
LA now shows all ve networks in the internetwork, so it too can now communicate
with all routers and networks. But before we test our little network, as well as our DHCP
server, let’s cover one more topic.
Default Routing
The SF and LA routers that I’ve connected to the Corp router are considered stub routers.
A stub indicates that the networks in this design have only one way out to reach all other
networks, which means that instead of creating multiple static routes, we can just use a
single default route. This default route is used by IP to forward any packet with a destina-
tion not found in the routing table, which is why it is also called a gateway of last resort.
Here’s the conguration I could have done on the LA router instead of typing in the static
routes due to its stub status:
LA#config t
LA(config)#no ip route 10.10.10.0 255.255.255.0 172.16.10.5 150
LA(config)#no ip route 172.16.10.0 255.255.255.252 172.16.10.5 150
LA(config)#no ip route 192.168.10.0 255.255.255.0 172.16.10.5 150
LA(config)#ip route 0.0.0.0 0.0.0.0 172.16.10.5
LA(config)#do sho ip route
[output cut]
Gateway of last resort is 172.16.10.5 to network 0.0.0.0
172.16.0.0/30 is subnetted, 1 subnets
C 172.16.10.4 is directly connected, Serial0/0/1
L 172.16.10.6/32 is directly connected, Serial0/0/1
C 192.168.20.0/24 is directly connected, FastEthernet0/0
L 192.168.20.0/32 is directly connected, FastEthernet0/0
S* 0.0.0.0/0 [1/0] via 172.16.10.5
Okay—I’ve removed all the initial static routes I had congured and adding a default route
is a lot easier than typing a bunch of static routes! Can you see the default route listed last in
the routing table? The S* shows that as a candidate for the default route. And I really want
you to notice that the gateway of last resort is now set too. Everything the router receives with
a destination not found in the routing table will be forwarded to 172.16.10.5. You need to be
careful where you place default routes because you can easily create a network loop!
So we’re there—we’ve congured all our routing tables! All the routers have the correct
routing table, so all routers and hosts should be able to communicate without a hitch—for
now. But if you add even one more network or another router to the internetwork, you’ll
Configuring IP Routing in Our Network 363
have to update each and every router’s routing tables by hand—ugh! Not really a problem
at all if you’ve got a small network like we do, but it would be a time-consuming monster if
you’re dealing with a large internetwork!
Verifying Your Configuration
We’re still not done yet—once all the routers’ routing tables are congured, they must be
veried. The best way to do this, besides using the show ip route command, is via Ping.
I’ll start by pinging from the Corp router to the SF router.
Here’s the output I got:
Corp#ping 192.168.10.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.10.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/4 ms
Corp#
Here you can see that I pinged from the Corp router to the remote interface of the SF
router. Now let’s ping the remote network on the LA router, and after that, we’ll test our
DHCP server and see if that is working too!
Corp#ping 192.168.20.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.20.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
Corp#
And why not test my conguration of the DHCP server on the Corp router while we’re
at it? I’m going to go to each host on the SF and LA routers and make them DHCP clients.
By the way, I’m using an old router to represent “hosts,” which just happens to work great
for studying purposes. Here’s how I did that:
SF_PC(config)#int e0
SF_PC(config-if)#ip address dhcp
SF_PC(config-if)#no shut
Interface Ethernet0 assigned DHCP address 192.168.10.8, mask 255.255.255.0
LA_PC(config)#int e0
LA_PC(config-if)#ip addr dhcp
LA_PC(config-if)#no shut
Interface Ethernet0 assigned DHCP address 192.168.20.4, mask 255.255.255.0
364 Chapter 8 u IP Routing
Nice! Don’t you love it when things just work the rst time? Sadly, this just isn’t exactly
a realistic expectation in the networking world, so we must be able to troubleshoot and
verify our networks. Let’s verify our DHCP server with a few of the commands you learned
back in Chapter 7:
Corp#sh ip dhcp binding
Bindings from all pools not associated with VRF:
IP address Client-ID/ Lease expiration Type
Hardware address/
User name
192.168.10.8 0063.6973.636f.2d30. Sept 16 2013 10:34 AM Automatic
3035.302e.3062.6330.
2e30.3063.632d.4574.
30
192.168.20.4 0063.6973.636f.2d30. Sept 16 2013 10:46 AM Automatic
3030.322e.3137.3632.
2e64.3032.372d.4574.
30
We can see from above that our little DHCP server is working! Let’s try another couple
of commands:
Corp#sh ip dhcp pool SF_LAN
Pool SF_LAN :
Utilization mark (high/low) : 100 / 0
Subnet size (first/next) : 0 / 0
Total addresses : 254
Leased addresses : 3
Pending event : none
1 subnet is currently in the pool :
Current index IP address range Leased addresses
192.168.10.9 192.168.10.1 - 192.168.10.254 3
Corp#sh ip dhcp conflict
IP address Detection method Detection time VRF
The last command would tell us if we had two hosts with the same IP address, so it’s
good news because there are no conicts reported! Two detection methods are used to con-
rm this:
u Ping from the DHCP server to make sure no other host responds before handing out
an address.
u A gratuitous ARP from a host that receives a DHCP address from the server.
Dynamic Routing 365
The DHCP client will send an ARP request with its new IP address looking to see if any-
one responds, and if so, it will report the conict to the server.
Okay, since we can communicate from end to end and to each host without a problem
while receiving DHCP addresses from our server, I’d say our static and default route con-
gurations have been a success—cheers!
Dynamic Routing
Dynamic routing is when protocols are used to nd networks and update routing tables
on routers. This is whole lot easier than using static or default routing, but it will cost you
in terms of router CPU processing and bandwidth on network links. A routing protocol
denes the set of rules used by a router when it communicates routing information between
neighboring routers.
The routing protocol I’m going to talk about in this chapter is Routing Information
Protocol (RIP) versions 1 and 2.
Two types of routing protocols are used in internetworks: interior gateway protocols
(IGPs) and exterior gateway protocols (EGPs). IGPs are used to exchange routing informa-
tion with routers in the same autonomous system (AS). An AS is either a single network or
a collection of networks under a common administrative domain, which basically means
that all routers sharing the same routing-table information are in the same AS. EGPs are
used to communicate between ASs. An example of an EGP is Border Gateway Protocol
(BGP), which we’re not going to bother with because it’s beyond the scope of this book.
Since routing protocols are so essential to dynamic routing, I’m going to give you the
basic information you need to know about them next. Later on in this chapter, we’ll focus
on conguration.
Routing Protocol Basics
There are some important things you should know about routing protocols before we get
deeper into them. Being familiar with administrative distances, the three different kinds of
routing protocols, and routing loops are three of the most important.
Administrative Distances
The administrative distance (AD) is used to rate the trustworthiness of routing information
received on a router from a neighbor router. An administrative distance is an integer from
0 to 255, where 0 is the most trusted and 255 means no trafc will be passed via this route.
If a router receives two updates listing the same remote network, the rst thing the
router checks is the AD. If one of the advertised routes has a lower AD than the other,
then the route with the lowest AD will be chosen and placed in the routing table.
If both advertised routes to the same network have the same AD, then routing protocol
metrics like hop count and/or the bandwidth of the lines will be used to nd the best path
to the remote network. The advertised route with the lowest metric will be placed in the
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366 Chapter 8 u IP Routing
routing table, but if both advertised routes have the same AD as well as the same metrics,
then the routing protocol will load-balance to the remote network, meaning the protocol
will send data down each link.
Table8.1 shows the default administrative distances that a Cisco router uses to decide
which route to take to a remote network.
TABLE8.1 Default administrative distances
Route Source Default AD
Connected interface 0
Static route 1
EIGRP 90
OSPF 110
RIP 120
External EIGRP 170
Unknown 255 (This route will never be used.)
If a network is directly connected, the router will always use the interface connected
to the network. If you congure a static route, the router will then believe that route over
any other ones it learns about. You can change the administrative distance of static routes,
but by default, they have an AD of 1. In our previous static route conguration, the AD of
each route is set at 150. This AD allows us to congure routing protocols without having to
remove the static routes because it’s nice to have them there for backup in case the routing
protocol experiences some kind of failure.
If you have a static route, an RIP-advertised route, and an EIGRP-advertised route list-
ing the same network, which route will the router go with? That’s right—by default, the
router will always use the static route unless you change its AD—which we did!
Routing Protocols
There are three classes of routing protocols:
Distance vector The distance-vector protocols in use today nd the best path to a remote
network by judging distance. In RIP routing, each instance where a packet goes through
a router is called a hop, and the route with the least number of hops to the network will
be chosen as the best one. The vector indicates the direction to the remote network. RIP
is a distance-vector routing protocol and periodically sends out the entire routing table to
directly connected neighbors.
Routing Information Protocol (RIP) 367
Link state In link-state protocols, also called shortest-path-rst protocols, the routers
each create three separate tables. One of these tables keeps track of directly attached neigh-
bors, one determines the topology of the entire internetwork, and one is used as the routing
table. Link-state routers know more about the internetwork than any distance-vector rout-
ing protocol ever could. OSPF is an IP routing protocol that’s completely link-state. Link-
state protocols send updates containing the state of their own links to all other directly
connected routers on the network. This is then propagated to their neighbors.
Hybrid Hybrid protocols use aspects of both distance-vector and link-state protocols,
and EIGRP is a great example—even though Cisco typically just calls EIGRP an advanced
distance-vector routing protocol!
There’s no set of rules to follow that dictate exactly how to broadly congure routing
protocols for every situation. It’s a task that really must be undertaken on a case-by-case
basis, with an eye on specic requirements of each one. If you understand how the different
routing protocols work, you can make good, solid decisions that will solidly meet the indi-
vidual needs of any business!
Routing Information Protocol (RIP)
Routing Information Protocol (RIP) is a true distance-vector routing protocol. RIP sends the
complete routing table out of all active interfaces every 30 seconds. It relies on hop count to
determine the best way to a remote network, but it has a maximum allowable hop count of
15 by default, so a destination of 16 would be considered unreachable. RIP works okay in
very small networks, but it’s super inefcient on large networks with slow WAN links or on
networks with a large number of routers installed and completely useless on networks that
have links with variable bandwidths!
RIP version 1 uses only classful routing, which means that all devices in the network must
use the same subnet mask. This is because RIP version 1 doesn’t send updates with subnet
mask information in tow. RIP version 2 provides something called prex routing and does
send subnet mask information with its route updates. This is called classless routing. You’ll
rarely see RIPv1 used in today’s networks, and it’s not considered in any CCENT and CCNA
objective. Even RIPv2 doesn’t get much attention in the objectives. So why am I even telling
you about them? Because it helps me explain routing protocols a little better before we get
into the much more advanced, and very much focused upon, OSPF protocol.
So, with that let’s congure our current network with RIPv2, before we move onto
OSPF in the next chapter.
Configuring RIP Routing
To congure RIP routing, just turn on the protocol with the router rip command and tell the
RIP routing protocol the networks to advertise. Remember that with static routing, we always
congured remote networks and never typed a route to our directly connected networks? Well,
368 Chapter 8 u IP Routing
dynamic routing is carried out the complete opposite way. You would never type a remote net-
work under your routing protocol—only enter your directly connected networks! Let’s cong-
ure our three-router internetwork, revisited in Figure8.9, with RIP routing.
Corp
RIP has an administrative distance of 120. Static routes have an administrative distance of
1 by default, and since we currently have static routes congured, the routing tables won’t
be populated with RIP information. We’re still good though because I added the 150 to the
end of each static route!
You can add the RIP routing protocol by using the router rip command and the network
command. The network command tells the routing protocol which classful network to adver-
tise. By doing this, you’re activating the RIP routing process on the interfaces whose address-
ing falls within the specied classful networks congured with the network command under
the RIP routing process.
Look at the Corp router conguration to see how easy this is. Oh wait—rst, I want to
verify my directly connected networks so I know what to congure RIP with:
Corp#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.10.10.1 YES manual up up
Serial0/0 172.16.10.1 YES manual up up
FastEthernet0/1 unassigned YES unset administratively down down
Serial0/1 172.16.10.5 YES manual up up
Corp#config t
Corp(config)#router rip
Corp(config-router)#network 10.0.0.0
Corp(config-router)#network 172.16.0.0
Corp(config-router)#version 2
Corp(config-router)#no auto-summary
That’s it—really! Typically just two or three commands and you’re done, which sure
makes your job a lot easier than dealing with static routes, doesn’t it? Be sure to keep in
mind the extra router CPU process and bandwidth that you’re consuming.
Anyway, so what exactly did I do here? I enabled the RIP routing protocol, added my
directly connected networks, made sure I was only running RIPv2, which is a classless
routing protocol, and then I disabled auto-summary. We typically don’t want our routing
protocols summarizing for us because it’s better to do that manually and both RIP and
EIGRP auto-summarize by default. So a general rule of thumb is to disable auto-summary,
which allows them to advertise subnets.
Notice I didn’t type in subnets, only the classful network address, which is betrayed by
the fact that all subnet bits and host bits are off! That’s because with dynamic routing, it’s
not my job and it’s up to the routing protocol to nd the subnets and populate the routing
tables. And since we have no router buddies running RIP, we won’t see any RIP routes in
the routing table yet.
Routing Information Protocol (RIP) 369
Remember that RIP uses the classful address when configuring the net-
work address. To clarify this, refer to the example in our network with an
address of 172.16.0.0/24 using subnets 172.16.10.0 and 172.16.20.0. You
would only type in the classful network address of 172.16.0.0 and let RIP
find the subnets and place them in the routing table. This doesn’t mean
you are running a classful routing protocol; this is just the way that both
RIP and EIGRP are configured.
SF
Okay, let’s congure our SF router now, which is connected to two networks. We need to
congure both directly connected classful networks, not subnets:
SF#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.10.1 YES manual up up
FastEthernet0/1 unassigned YES unset administratively down down
Serial0/0/0 172.16.10.2 YES manual up up
Serial0/0/1 unassigned YES unset administratively down down
SF#config
SF(config)#router rip
SF(config-router)#network 192.168.10.0
SF(config-router)#network 172.16.0.0
SF(config-router)#version 2
SF(config-router)#no auto-summary
SF(config-router)#do show ip route
C 192.168.10.0/24 is directly connected, FastEthernet0/0
L 192.168.10.1/32 is directly connected, FastEthernet0/0
172.16.0.0/30 is subnetted, 3 subnets
R 172.16.10.4 [120/1] via 172.16.10.1, 00:00:08, Serial0/0/0
C 172.16.10.0 is directly connected, Serial0/0/0
L 172.16.10.2/32 is directly connected, Serial0/0
S 192.168.20.0/24 [150/0] via 172.16.10.1
10.0.0.0/24 is subnetted, 1 subnets
R 10.10.10.0 [120/1] via 172.16.10.1, 00:00:08, Serial0/0/0
That was pretty straightforward. Let’s talk about this routing table. Since we have one
RIP buddy out there whom we are exchanging routing tables with, we can see the RIP net-
works coming from the Corp router. All the other routes still show up as static and local.
RIP also found both connections through the Corp router to networks 10.10.10.0 and
172.16.10.4. But we’re not done yet!
370 Chapter 8 u IP Routing
LA
Let’s congure our LA router with RIP, only I’m going to remove the default route rst,
even though I don’t have to. You’ll see why soon:
LA#config t
LA(config)#no ip route 0.0.0.0 0.0.0.0
LA(config)#router rip
LA(config-router)#network 192.168.20.0
LA(config-router)#network 172.16.0.0
LA(config-router)#no auto
LA(config-router)#vers 2
LA(config-router)#do show ip route
R 192.168.10.0/24 [120/2] via 172.16.10.5, 00:00:10, Serial0/0/1
172.16.0.0/30 is subnetted, 3 subnets
C 172.16.10.4 is directly connected, Serial0/0/1
L 172.16.10.6/32 is directly connected, Serial0/0/1
R 172.16.10.0 [120/1] via 172.16.10.5, 00:00:10, Serial0/0/1
C 192.168.20.0/24 is directly connected, FastEthernet0/0
L 192.168.20.1/32 is directly connected, FastEthernet0/0
10.0.0.0/24 is subnetted, 1 subnets
R 10.10.10.0 [120/1] via 172.16.10.5, 00:00:10, Serial0/0/1
The routing table is sprouting new Rs as we add RIP buddies! We can still see that all
routes are in the routing table.
This output shows us basically the same routing table and the same entries that it had
when we were using static routes—except for those Rs. An R indicates that the networks
were added dynamically using the RIP routing protocol. The [120/1] is the administrative
distance of the route (120) along with the metric, which for RIP is the number of hops to
that remote network (1). From the Corp router, all networks are one hop away.
So, while yes, it’s true that RIP has worked in our little internetwork, it’s just not a great
solution for most enterprises. Its maximum hop count of only 15 is a highly limiting factor.
And it performs full routing-table updates every 30 seconds, which would bring a larger
internetwork to a painful crawl in no time!
There’s still one more thing I want to show you about RIP routing tables and the param-
eters used to advertise remote networks. Using a different router on a different network as an
example for a second, look into the output below. Can you spot where the following routing
table shows [120/15] in the 10.1.3.0 network metric? This means that the administrative dis-
tance is 120, the default for RIP, but the hop count is 15. Remember that each time a router
sends out an update to a neighbor router, the hop count goes up by one incrementally for each
route! Here’s that output now:
Router#sh ip route
10.0.0.0/24 is subnetted, 12 subnets
Routing Information Protocol (RIP) 371
C 10.1.11.0 is directly connected, FastEthernet0/1
L 10.1.11.1/32 is directly connected, FastEthernet0/1
C 10.1.10.0 is directly connected, FastEthernet0/0
L 10.1.10.1/32 is directly connected, FastEthernet/0/0
R 10.1.9.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1
R 10.1.8.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1
R 10.1.12.0 [120/1] via 10.1.11.2, 00:00:00, FastEthernet0/1
R 10.1.3.0 [120/15] via 10.1.5.1, 00:00:15, Serial0/0/1
R 10.1.2.0 [120/1] via 10.1.5.1, 00:00:15, Serial0/0/1
R 10.1.1.0 [120/1] via 10.1.5.1, 00:00:15, Serial0/0/1
R 10.1.7.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1
R 10.1.6.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1
C 10.1.5.0 is directly connected, Serial0/0/1
L 10.1.5.1/32 is directly connected, Serial0/0/1
R 10.1.4.0 [120/1] via 10.1.5.1, 00:00:15, Serial0/0/1
So this [120/15] is really bad. We’re basically doomed because the next router that
receives the table from this router will just discard the route to network 10.1.3.0 since the
hop count would rise to 16, which is invalid!
If a router receives a routing update that contains a higher-cost path to a
network that’s already in its routing table, the update will be ignored.
Holding Down RIP Propagations
You probably don’t want your RIP network advertised everywhere on your LAN and
WAN. There’s enough stress in networking already and not a whole lot to be gained by
advertising your RIP network to the Internet!
There are a few different ways to stop unwanted RIP updates from propagating across
your LANs and WANs, and the easiest one is through the passive-interface command.
This command prevents RIP update broadcasts from being sent out of a specied interface
but still allows that same interface to receive RIP updates.
Here’s an example of how to congure a passive-interface on the Corp routers Fa0/1
interface, which we will pretend is connected to a LAN that we don’t want RIP on:
Corp#config t
Corp(config)#router rip
Corp(config-router)#passive-interface FastEthernet 0/1
This command will stop RIP updates from being propagated out of FastEthernet
interface 0/0, but this can still receive RIP updates.
372 Chapter 8 u IP Routing
Should We Really Use RIP in an Internetwork?
You have been hired as a consultant to install a couple of Cisco routers into a growing net-
work. They have a couple of old Unix routers that they want to keep in the network. These
routers do not support any routing protocol except RIP. I guess this means you just have to
run RIP on the entire network. If you were balding before, your head now shines like chrome.
No need for hairs abandoning ship though—you can run RIP on a router connecting that
old network, but you certainly don’t need to run RIP throughout the whole internetwork!
You can do what is called redistribution, which is basically translating from one type of
routing protocol to another. This means that you can support those old routers using RIP
but use something much better like Enhanced IGRP on the rest of your network.
This will prevent RIP routes from being sent all over the internetwork gobbling up all that
precious bandwidth!
Advertising a Default Route Using RIP
Okay, now I’m going to guide you through how to advertise a way out of your autonomous
system to other routers, and you’ll see this is completed the same way with OSPF. Imagine
that our Corp router’s Fa0/0 interface is connected to some type of Metro-Ethernet as a
connection to the Internet. This is a pretty common conguration today that uses a LAN
interface to connect to the ISP instead of a serial interface.
If we do add an Internet connection to Corp, all routers in our AS (SF and LA) must
know where to send packets destined for networks on the Internet or they’ll just drop the
packets when they get a remote request. One solution to this little hitch would be to place a
default route on every router and funnel the information to Corp, which in turn would have
a default route to the ISP. Most people do this type of conguration in small- to medium-
size networks because it actually works pretty well!
But since I’m running RIPv2 on all routers, I’ll just add a default route on the Corp
router to our ISP, as I would normally. I’ll then add another command to advertise my net-
work to the other routers in the AS as the default route to show them where to send packets
destined for the Internet.
Here’s my new Corp conguration:
Corp(config)#ip route 0.0.0.0 0.0.0.0 fa0/0
Corp(config)#router rip
Corp(config-router)#default-information originate
Summary 373
Now, let’s take a look at the last entry found in the Corp routing table:
S* 0.0.0.0/0 is directly connected, FastEthernet0/0
Let’s see if the LA router can see this same entry:
LA#sh ip route
Gateway of last resort is 172.16.10.5 to network 0.0.0.0
R 192.168.10.0/24 [120/2] via 172.16.10.5, 00:00:04, Serial0/0/1
172.16.0.0/30 is subnetted, 2 subnets
C 172.16.10.4 is directly connected, Serial0/0/1
L 172.16.10.5/32 is directly connected, Serial0/0/1
R 172.16.10.0 [120/1] via 172.16.10.5, 00:00:04, Serial0/0/1
C 192.168.20.0/24 is directly connected, FastEthernet0/0
L 192.168.20.1/32 is directly connected, FastEthernet0/0
10.0.0.0/24 is subnetted, 1 subnets
R 10.10.10.0 [120/1] via 172.16.10.5, 00:00:04, Serial0/0/1
R 192.168.218.0/24 [120/3] via 172.16.10.5, 00:00:04, Serial0/0/1
R 192.168.118.0/24 [120/2] via 172.16.10.5, 00:00:05, Serial0/0/1
R* 0.0.0.0/0 [120/1] via 172.16.10.5, 00:00:05, Serial0/0/1R2#
Can you see that last entry? It screams that it’s an RIP injected route, but it’s also a
default route so our default-information originate command is working! Last, notice
that the gateway of last resort is now set as well.
If all of what you’ve learned is clear and understood, congratulations—you’re ready to
move on to the next chapter right after you go through the written and hands-on labs, and
while you’re at it, don’t forget the review questions!
Summary
This chapter covered IP routing in detail. Again, it’s extremely important to fully under-
stand the basics we covered in this chapter because everything that’s done on a Cisco router
will typically have some kind of IP routing congured and running.
You learned how IP routing uses frames to transport packets between routers and to
the destination host. From there, we congured static routing on our routers and dis-
cussed the administrative distance used by IP to determine the best route to a destination
network. You found out that if you have a stub network, you can congure default rout-
ing, which sets the gateway of last resort on a router.
We then discussed dynamic routing, specically RIP and how it works on an internet-
work, which is not very well!
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MicroNugget
from CBT
Nuggets.
374 Chapter 8 u IP Routing
Exam Essentials
Describe the basic IP routing process. You need to remember that the frame changes at each
hop but that the packet is never changed or manipulated in any way until it reaches the desti-
nation device (the TTL eld in the IP header is decremented for each hop, but that’s it!).
List the information required by a router to successfully route packets. To be able to route
packets, a router must know, at a minimum, the destination address, the location of neigh-
boring routers through which it can reach remote networks, possible routes to all remote
networks, the best route to each remote network, and how to maintain and verify routing
information.
Describe how MAC addresses are used during the routing process. A MAC (hardware)
address will only be used on a local LAN. It will never pass a router’s interface. A frame
uses MAC (hardware) addresses to send a packet on a LAN. The frame will take the packet
to either a host on the LAN or a router’s interface (if the packet is destined for a remote net-
work). As packets move from one router to another, the MAC addresses used will change,
but normally the original source and destination IP addresses within the packet will not.
View and interpret the routing table of a router. Use the show ip route command to
view the routing table. Each route will be listed along with the source of the routing infor-
mation. A C to the left of the route will indicate directly connected routes, and other letters
next to the route can also indicate a particular routing protocol that provided the informa-
tion, such as, for example, R for RIP.
Differentiate the three types of routing. The three types of routing are static (in which
routes are manually congured at the CLI), dynamic (in which the routers share routing
information via a routing protocol), and default routing (in which a special route is cong-
ured for all trafc without a more specic destination network found in the table).
Compare and contrast static and dynamic routing. Static routing creates no routing
update trafc and creates less overhead on the router and network links, but it must be con-
gured manually and does not have the ability to react to link outages. Dynamic routing
creates routing update trafc and uses more overhead on the router and network links.
Configure static routes at the CLI. The command syntax to add a route is
ip route [destination_network] [mask] [next-hop_address or exitinterface]
[administrative_distance] [permanent].
Create a default route. To add a default route, use the command syntax ip route 0.0.0.0
0.0.0.0 ip-address or exit interface type and number.
Understand administrative distance and its role in the selection of the best route.
Administrative distance (AD) is used to rate the trustworthiness of routing information
received on a router from a neighbor router. Administrative distance is an integer from
0 to 255, where 0 is the most trusted and 255 means no trafc will be passed via this
route. All routing protocols are assigned a default AD, but it can be changed at the CLI.
Exam Essentials 375
Differentiate distance-vector, link-state and hybrid routing protocols. Distance-vector rout-
ing protocols make routing decisions based on hop count (think RIP), while link-state routing
protocols are able to consider multiple factors such as bandwidth available and building a
topology table. Hybrid routing protocols exhibit characteristics of both types.
Configure RIP routing. To congure RIP routing, rst you must be in global conguration
mode and then you type the command router rip. Then you add all directly connected net-
works, making sure to use the classful address and the version 2 command and to disable
auto-summarization.
376 Chapter 8 u IP Routing
Written Lab 8
The answers to this lab can be found in Appendix A, “Answers to Written Labs.”
Write the answers to the following questions:
1. At the appropriate command prompt, create a static route to network 172.16.10.0/24
with a next-hop gateway of 172.16.20.1 and an administrative distance of 150.
2. When a PC sends a packet to another PC in a remote network, what destination
addresses will be in the frame that it sends to its default gateway?
3. At the appropriate command prompt, create a default route to 172.16.40.1.
4. On which type of network is a default route most beneficial?
5. At the appropriate command prompt, display the routing table on your router.
6. When creating a static or default route, you don’t have to use the next-hop IP address;
you can use the ___________________.
7. True/False: To reach a remote host, you must know the MAC address of the remote host.
8. True/False: To reach a remote host, you must know the IP address of the remote host.
9. At the appropriate command prompt(s), prevent a router from propagating RIP infor-
mation out serial 1.
10. True/False: RIPv2 is considered classless.
Hands-on Labs
In the following hands-on labs, you will congure a network with three routers. These
exercises assume all the same setup requirements as the labs found in earlier chapters.
This chapter includes the following labs:
Lab 8.1: Creating Static Routes
Lab 8.2: Conguring RIP Routing
The internetwork shown in the following graphic will be used to congure all routers.
Fa0/0 S0/1 Fa0/0
S0/0
DCE S0/0
S0/0
DCE
Lab A Lab B Lab C
Hands-on Labs 377
Table8.2 shows our IP addresses for each router (each interface uses a /24 mask).
TABLE8.2 Our IP addresses
Router Interface IP Address
Lab_A Fa0/0 172.16 .10.1
Lab_A S0/0 172.16.20.1
Lab_B S0/0 172.16.20.2
Lab_B S0/1 172.16. 30.1
Lab_C S0/0 172.16.30.2
Lab_C Fa0/0 172.16.40.1
These labs were written without using the LAN interface on the Lab_B router. You can
choose to add that LAN into the labs if necessary. Also, if you have enough LAN interfaces,
then you don’t need to add the serial interfaces into this lab. Using all LAN interfaces is ne.
Hands-on Lab 8.1: Creating Static Routes
In this lab, you will create a static route in all three routers so that the routers see all net-
works. Verify with the Ping program when complete.
1. The Lab_A router is connected to two networks, 172.16.10.0 and 172.16.20.0. You
need to add routes to networks 172.16.30.0 and 172.16.40.0. Use the following com-
mands to add the static routes:
Lab_A#config t
Lab_A(config)#ip route 172.16.30.0 255.255.255.0
172.16.20.2
Lab_A(config)#ip route 172.16.40.0 255.255.255.0
172.16.20.2
2. Save the current configuration for the Lab_A router by going to privileged mode, typing
copy run start, and pressing Enter.
378 Chapter 8 u IP Routing
3. On the Lab_B router, you have direct connections to networks 172.16.20.0 and
172.16.30.0. You need to add routes to networks 172.16.10.0 and 172.16.40.0. Use
the following commands to add the static routes:
Lab_B#config t
Lab_B(config)#ip route 172.16.10.0 255.255.255.0
172.16.20.1
Lab_B(config)#ip route 172.16.40.0 255.255.255.0
172.16.30.2
4. Save the current configuration for router Lab_B by going to the enabled mode, typing
copy run start, and pressing Enter.
5. On router Lab_C, create a static route to networks 172.16.10.0 and 172.16.20.0,
which are not directly connected. Create static routes so that router Lab_C can see all
networks, using the commands shown here:
Lab_C#config t
Lab_C(config)#ip route 172.16.10.0 255.255.255.0
172.16.30.1
Lab_C(config)#ip route 172.16.20.0 255.255.255.0
172.16.30.1
6. Save the current configuration for router Lab_C by going to the enable mode, typing
copy run start, and pressing Enter.
7. Check your routing tables to make sure all four networks show up by executing the
show ip route command.
8. Now ping from each router to your hosts and from each router to each router. If it is
set up correctly, it will work.
Hands-on Lab 8.2: Configuring RIP Routing
In this lab, we will use the dynamic routing protocol RIP instead of static routing.
1. Remove any static routes or default routes configured on your routers by using the no
ip route command. For example, here is how you would remove the static routes on
the Lab_A router:
Lab_A#config t
Lab_A(config)#no ip route 172.16.30.0 255.255.255.0
172.16.20.2
Lab_A(config)#no ip route 172.16.40.0 255.255.255.0
172.16.20.2
Do the same thing for routers Lab_B and Lab_C. Verify that only your directly con-
nected networks are in the routing tables.
Hands-on Labs 379
2. After your static and default routes are clear, go into configuration mode on router
Lab_A by typing config t.
3. Tell your router to use RIP routing by typing router rip and pressing Enter, as
shown here:
config t
router rip
4. Add the network number for the networks you want to advertise. Since router Lab_A
has two interfaces that are in two different networks, you must enter a network state-
ment using the network ID of the network in which each interface resides. Alternately,
you could use a summarization of these networks and use a single statement, mini-
mizing the size of the routing table. Since the two networks are 172.16.10.0/24 and
172.16.20.0/24, the network summarization 172.16.0.0 would include both subnets.
Do this by typing network 172.16.0.0 and pressing Enter.
5. Press Ctrl+Z to get out of configuration mode.
6. The interfaces on Lab_B and Lab_C are in the 172.16.20.0/24 and 172.16.30.0/24
networks; therefore, the same summarized network statement will work there as well.
Type the same commands, as shown here:
Config t
Router rip
network 172.16.0.0
7. Verify that RIP is running at each router by typing the following commands at
each router:
show ip protocols
(Should indicate to you that RIP is present on the router.)
show ip route
(Should have routes present with an R to the left of them.)
show running-config or show run
(Should indicate that RIP is present and the networks are being advertised.)
8. Save your configurations by typing copy run start or copy running-config startup-
config and pressing Enter at each router.
9. Verify the network by pinging all remote networks and hosts.
380 Chapter 8 u IP Routing
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional ques-
tions, please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. What command was used to generate the following output?
Codes: L - local, C - connected, S - static,
[output cut]
10.0.0.0/8 is variably subnetted, 6 subnets, 4 masks
C 10.0.0.0/8 is directly connected, FastEthernet0/3
L 10.0.0.1/32 is directly connected, FastEthernet0/3
C 10.10.0.0/16 is directly connected, FastEthernet0/2
L 10.10.0.1/32 is directly connected, FastEthernet0/2
C 10.10.10.0/24 is directly connected, FastEthernet0/1
L 10.10.10.1/32 is directly connected, FastEthernet0/1
S* 0.0.0.0/0 is directly connected, FastEthernet0/0
2. You are viewing the routing table and you see an entry 10.1.1.1/32. What legend code
would you expect to see next to this route?
A. C
B. L
C. S
D. D
3. Which of the following statements are true regarding the command ip route
172.16.4.0 255.255.255.0 192.168.4.2? (Choose two.)
A. The command is used to establish a static route.
B. The default administrative distance is used.
C. The command is used to configure the default route.
D. The subnet mask for the source address is 255.255.255.0.
E. The command is used to establish a stub network.
Review Questions 381
4. What destination addresses will be used by HostA to send data to the HTTPS server as
shown in the following network? (Choose two.)
HostA
RouterA
Fa0/1
Fa0/0
HTTPS Server
A. The IP address of the switch
B. The MAC address of the remote switch
C. The IP address of the HTTPS server
D. The MAC address of the HTTPS server
E. The IP address of RouterA’s Fa0/0 interface
F. The MAC address of RouterA’s Fa0/0 interface
5. Using the output shown, what protocol was used to learn the MAC address for
172.16.10.1?
Interface: 172.16.10.2 --- 0x3
Internet Address Physical Address Type
172.16.10.1 00-15-05-06-31-b0 dynamic
A. ICMP
B. ARP
C. TCP
D. UDP
6. Which of the following is called an advanced distance-vector routing protocol?
A. OSPF
B. EIGRP
C. BGP
D. RIP
382 Chapter 8 u IP Routing
7. When a packet is routed across a network, the ______________ in the packet changes at
every hop while the ____ does not.
A. MAC address, IP address
B. IP address, MAC address
C. Port number, IP address
D. IP address, port number
8. Which statement is true regarding classless routing protocols? (Choose two.)
A. The use of discontiguous networks is not allowed.
B. The use of variable length subnet masks is permitted.
C. RIPv1 is a classless routing protocol.
D. IGRP supports classless routing within the same autonomous system.
E. RIPv2 supports classless routing.
9. Which two of the following are true regarding the distance-vector and link-state rout-
ing protocols? (Choose two.)
A. Link state sends its complete routing table out of all active interfaces at periodic
time intervals.
B. Distance vector sends its complete routing table out of all active interfaces at peri-
odic time intervals.
C. Link state sends updates containing the state of its own links to all routers in the
internetwork.
D. Distance vector sends updates containing the state of its own links to all routers in
the internetwork.
10. When a router looks up the destination in the routing table for every single packet it is
called _____________ .
A. dynamic switching
B. fast switching
C. process switching
D. Cisco Express Forwarding
11. What type(s) of route is the following? Choose all that apply.
S* 0.0.0.0/0 [1/0] via 172.16.10.5
A. Default
B. Subnetted
C. Static
D. Local
Review Questions 383
12. A network administrator views the output from the show ip route command. A net-
work that is advertised by both RIP and EIGRP appears in the routing table flagged as
an EIGRP route. Why is the RIP route to this network not used in the routing table?
A. EIGRP has a faster update timer.
B. EIGRP has a lower administrative distance.
C. RIP has a higher metric value for that route.
D. The EIGRP route has fewer hops.
E. The RIP path has a routing loop.
13. Which of the following is NOT an advantage of static routing?
A. Less overhead on the router CPU
B. No bandwidth usage between routers
C. Adds security
D. Recovers automatically from lost routes
14. What metric does RIPv2 use to find the best path to a remote network?
A. Hop count
B. MTU
C. Cumulative interface delay
D. Load
E. Path bandwidth value
15. The Corporate router receives an IP packet with a source IP address of 192.168.214.20
and a destination address of 192.168.22.3. Looking at the output from the Corp
router, what will the router do with this packet?
Corp#sh ip route
[output cut]
R 192.168.215.0 [120/2] via 192.168.20.2, 00:00:23, Serial0/0
R 192.168.115.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0
R 192.168.30.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0
C 192.168.20.0 is directly connected, Serial0/0
C 192.168.214.0 is directly connected, FastEthernet0/0
A. The packet will be discarded.
B. The packet will be routed out of the S0/0 interface.
C. The router will broadcast looking for the destination.
D. The packet will be routed out of the Fa0/0 interface.
384 Chapter 8 u IP Routing
16. If your routing table has a static, an RIP, and an EIGRP route to the same network,
which route will be used to route packets by default?
A. Any available route
B. RIP route
C. Static route
D. EIGRP route
E. They will all load-balance.
17. Which of the following is an EGP?
A. RIPv2
B. EIGRP
C. BGP
D. RIP
18. Which of the following is an advantage of static routing?
A. Less overhead on the router CPU
B. No bandwidth usage between routers
C. Adds security
D. Recovers automatically from lost routes
19. What command produced the following output?
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.10.1 YES manual up up
FastEthernet0/1 unassigned YES unset administratively down down
Serial0/0/0 172.16.10.2 YES manual up up
Serial0/0/1 unassigned YES unset administratively down down
A. show ip route
B. show interfaces
C. show ip interface brief
D. show ip arp
20. In the following command what does the 150 at the end of the command mean?
Router(config)#ip route 172.16.3.0 255.255.255.0 192.168.2.4 150
A. Metric
B. Administrative distance
C. Hop count
D. Cost
Chapter
9
Open Shortest Path
First (OSPF)
THE FOLLOWING ICND1 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 IP Routing Technologies
■Configure and verify OSPF (single area)
■Benefit of single area
■Configure OSPF v2
■Router ID
■Passive interface
Open Shortest Path First (OSPF) is by far the most popular
and important routing protocol in use today—so important,
I’m devoting this entire chapter to it! Sticking with the same
approach we’ve adhered to throughout this book, we’ll begin with the basics by completely
familiarizing you with key OSPF terminology. Once we’ve covered that thoroughly, I’ll guide
you through OSPF’s internal operation and then move on to tell you all about OSPF’s many
advantages over RIP.
This chapter is going to be more than chock full of vitally important information and it’s
also going to be really exciting because together, we’ll explore some seriously critical factors
and issues innate to implementing OSPF! I’ll walk you through exactly how to implement
single-area OSPF in a variety of networking environments and then demonstrate some great
techniques you’ll need to verify that everything is congured correctly and running smoothly.
To find up-to-the-minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
Open Shortest Path First (OSPF) Basics
Open Shortest Path First is an open standard routing protocol that’s been implemented by
a wide variety of network vendors, including Cisco. And it’s that open standard character-
istic that’s the key to OSPF’s exibility and popularity.
Most people opt for OSPF, which works by using the Dijkstra algorithm to initially
construct a shortest path tree and follows that by populating the routing table with the
resulting best paths. EIGRP’s convergence time may be blindingly fast, but OSPF isn’t that
far behind, and its quick convergence is another reason it’s a favorite. Another two great
advantages OSPF offers are that it supports multiple, equal-cost routes to the same destina-
tion, and like EIGRP, it also supports both IP and IPv6 routed protocols.
Here’s a list that summarizes some of OSPF’s best features:
u Allows for the creation of areas and autonomous systems
u Minimizes routing update traffic
Open Shortest Path First (OSPF) Basics 387
u Is highly flexible, versatile, and scalable
u Supports VLSM/CIDR
u Offers an unlimited hop count
u Is open standard and supports multi-vendor deployment
Because OSPF is the rst link-state routing protocol that most people run into, it’s a
good idea to size it up against more traditional distance-vector protocols like RIPv2 and
RIPv1. Table9.1 presents a nice comparison of all three of these common protocols.
TABLE9.1 OSPF and RIP comparison
Characteristic OSPF RIPv2 RIPv1
Type of protocol Link state Distance vector Distance vector
Classless support Yes Yes No
VLSM support Yes Yes No
Auto-summarization No Yes Yes
Manual summarization Yes Yes No
Noncontiguous support Yes Yes No
Route propagation Multicast on change Periodic multicast Periodic broadcast
Path metric Bandwidth Hops Hops
Hop count limit None 15 15
Convergence Fast Slow Slow
Peer authentication Yes Yes No
Hierarchical network
requirement
Yes (using areas) No (flat only) No (flat only)
Updates Event triggered Periodic Periodic
Route computation Dijkstra Bellman-Ford Bellman-Ford
388 Chapter 9 u Open Shortest Path First (OSPF)
I want you know that OSPF has many features beyond the few I’ve listed in Table9.1 and
all of them combine to produce a fast, scalable, robust protocol that’s also exible enough to
be actively deployed in a vast array of production networks!
One of OSPF’s most useful traits is that its design is intended to be hierarchical in use,
meaning that it allows us to subdivide the larger internetwork into smaller internetworks
called areas. It’s a really powerful feature that I recommend using, and I promise to show
you how to do that later in the chapter.
Here are three of the biggest reasons to implement OSPF in a way that makes full use of
its intentional, hierarchical design:
u To decrease routing overhead
u To speed up convergence
u To confine network instability to single areas of the network
Because free lunches are invariably hard to come by, all this wonderful functionality
predictably comes at a price and doesn’t exactly make conguring OSPF any easier. But
no worries—we’ll crush it!
Let’s start by checking out Figure9.1, which shows a very typical, yet simple OSPF design.
I really want to point out the fact that some routers connect to the backbone—called area 0—
the backbone area. OSPF absolutely must have an area 0, and all other areas should connect
to it except for those connected via virtual links, which are beyond the scope of this book. A
router that connects other areas to the backbone area within an AS is called an area border
router (ABR), and even these must have at least one of their interfaces connected to area 0.
FIGURE9.1 OSPF design example. An OSPF hierarchical design minimizes
routing table entries and keeps the impact of any topology changes contained within a
specific area.
External
routing
domain
Area 1
Autonomous System
F
Area 2
G
Area 3
H
CD
Backbone Area
ASBR
E
OSPF runs great inside an autonomous system, but it can also connect multiple autonomous
systems together. The router that connects these ASs is called an autonomous system bound-
ary router (ASBR). Ideally, your aim is to create other areas of networks to help keep route
Open Shortest Path First (OSPF) Basics 389
updates to a minimum, especially in larger networks. Doing this also keeps problems from
propagating throughout the network, affectively isolating them to a single area.
But let’s pause here to cover some key OSPF terms that are really essential for you to nail
down before we move on any further.
OSPF Terminology
Imagine being given a map and compass with no prior concept of east, west, north or south—
not even what rivers, mountains, lakes, or deserts are. I’m guessing that without any ability to
orient yourself in a basic way, your cool, new tools wouldn’t help you get anywhere but com-
pletely lost, right? This is exactly why we’re going to begin exploring OSPF by getting you sol-
idly acquainted with a fairly long list of terms before setting out from base camp into the great
unknown! Here are those vital terms to commit to memory now:
Link A link is a network or router interface assigned to any given network. When an inter-
face is added to the OSPF process, it’s considered to be a link. This link, or interface, will
have up or down state information associated with it as well as one or more IP addresses.
Router ID The router ID (RID) is an IP address used to identify the router. Cisco chooses
the router ID by using the highest IP address of all congured loopback interfaces. If no
loopback interfaces are congured with addresses, OSPF will choose the highest IP address
out of all active physical interfaces. To OSPF, this is basically the “name” of each router.
Neighbor Neighbors are two or more routers that have an interface on a common net-
work, such as two routers connected on a point-to-point serial link. OSPF neighbors must
have a number of common conguration options to be able to successfully establish a
neighbor relationship, and all of these options must be congured exactly the same way:
u Area ID
u Stub area flag
u Authentication password (if using one)
u Hello and Dead intervals
Adjacency An adjacency is a relationship between two OSPF routers that permits the
direct exchange of route updates. Unlike EIGRP, which directly shares routes with all of its
neighbors, OSPF is really picky about sharing routing information and will directly share
routes only with neighbors that have also established adjacencies. And not all neighbors will
become adjacent—this depends upon both the type of network and the conguration of the
routers. In multi-access networks, routers form adjacencies with designated and backup des-
ignated routers. In point-to-point and point-to-multipoint networks, routers form adjacen-
cies with the router on the opposite side of the connection.
Designated router A designated router (DR) is elected whenever OSPF routers are con-
nected to the same broadcast network to minimize the number of adjacencies formed and to
publicize received routing information to and from the remaining routers on the broadcast
network or link. Elections are won based upon a router’s priority level, with the one having
390 Chapter 9 u Open Shortest Path First (OSPF)
the highest priority becoming the winner. If there’s a tie, the router ID will be used to break
it. All routers on the shared network will establish adjacencies with the DR and the BDR,
which ensures that all routers’ topology tables are synchronized.
Backup designated router A backup designated router (BDR) is a hot standby for the
DR on broadcast, or multi-access, links. The BDR receives all routing updates from OSPF
adjacent routers but does not disperse LSA updates.
Hello protocol The OSPF Hello protocol provides dynamic neighbor discovery and main-
tains neighbor relationships. Hello packets and Link State Advertisements (LSAs) build and
maintain the topological database. Hello packets are addressed to multicast address 224.0.0.5.
Neighborship database The neighborship database is a list of all OSPF routers for which
Hello packets have been seen. A variety of details, including the router ID and state, are
maintained on each router in the neighborship database.
Topological database The topological database contains information from all of the Link
State Advertisement packets that have been received for an area. The router uses the infor-
mation from the topology database as input into the Dijkstra algorithm that computes the
shortest path to every network.
LSA packets are used to update and maintain the topological database.
Link State Advertisement A Link State Advertisement (LSA) is an OSPF data packet con-
taining link-state and routing information that’s shared among OSPF routers. There are dif-
ferent types of LSA packets, and I’ll cover these in the ICND2 book. An OSPF router will
exchange LSA packets only with routers to which it has established adjacencies.
OSPF areas An OSPF area is a grouping of contiguous networks and routers. All routers in
the same area share a common area ID. Because a router can be a member of more than one
area at a time, the area ID is associated with specic interfaces on the router. This would allow
some interfaces to belong to area 1 while the remaining interfaces can belong to area0. All of
the routers within the same area have the same topology table. When conguring OSPF with
multiple areas, you’ve got to remember that there must be an area 0 and that this is typically
considered the backbone area. Areas also play a role in establishing a hierarchical network
organization—something that really enhances the scalability of OSPF!
Broadcast (multi-access) Broadcast (multi-access) networks such as Ethernet allow multi-
ple devices to connect to or access the same network, enabling a broadcast ability in which
a single packet is delivered to all nodes on the network. In OSPF, a DR and BDR must be
elected for each broadcast multi-access network.
Nonbroadcast multi-access Nonbroadcast multi-access (NBMA) networks are networks
such as Frame Relay, X.25, and Asynchronous Transfer Mode (ATM). These types of
networks allow for multi-access without broadcast ability like Ethernet. NBMA networks
require special OSPF conguration to function properly.
Open Shortest Path First (OSPF) Basics 391
Point-to-point Point-to-point refers to a type of network topology made up of a direct
connection between two routers that provides a single communication path. The point-to-
point connection can be physical—for example, a serial cable that directly connects two
routers—or logical, where two routers thousands of miles apart are connected by a circuit
in a Frame Relay network. Either way, point-to-point congurations eliminate the need for
DRs or BDRs.
Point-to-multipoint Point-to-multipoint refers to a type of network topology made up
of a series of connections between a single interface on one router and multiple destination
routers. All interfaces on all routers share the point-to-multipoint connection and belong
to the same network. Point-to-multipoint networks can be further classied according to
whether they support broadcasts or not. This is important because it denes the kind of
OSPF congurations you can deploy.
All of these terms play a critical role when you’re trying to understand how OSPF actually
works, so again, make sure you’re familiar with each of them. Having these terms down will
enable you to condently place them in their proper context as we progress on our journey
through the rest of this chapter!
OSPF Operation
Fully equipped with your newly acquired knowledge of the terms and technologies we
just covered, it’s now time to delve into how OSPF discovers, propagates, and ultimately
chooses routes. Once you know how OSPF achieves these tasks, you’ll understand how
OSPF operates internally really well.
OSPF operation is basically divided into these three categories:
u Neighbor and adjacency initialization
u LSA flooding
u SPF tree calculation
The beginning neighbor/adjacency formation stage is a very big part of OSPF operation.
When OSPF is initialized on a router, the router allocates memory for it, as well as for the
maintenance of both neighbor and topology tables. Once the router determines which inter-
faces have been congured for OSPF, it will then check to see if they’re active and begin
sending Hello packets.
FIGURE9.2 The Hello protocol
Hello?
224.0.0.5
The Hello protocol is used to discover neighbors, establish adjacencies, and maintain
relationships with other OSPF routers. Hello packets are periodically sent out of each
enabled OSPF interface and in environments that support multicast.
392 Chapter 9 u Open Shortest Path First (OSPF)
The address used for this is 224.0.0.5, and the frequency with which Hello packets are sent
out depends upon the network type and topology. Broadcast and point-to-point networks send
Hellos every 10 seconds, whereas non-broadcast and point-to-multipoint networks send them
every 30 seconds.
LSA Flooding
LSA ooding is the method OSPF uses to share routing information. Via LSU packets, LSA
information containing link-state data is shared with all OSPF routers within an area. The
network topology is created from the LSA updates, and ooding is used so that all OSPF
routers have the same topology map to make SPF calculations with.
Efcient ooding is achieved through the use of a reserved multicast address: 224.0.0.5
(AllSPFRouters). LSA updates, which indicate that something in the topology has changed,
are handled a bit differently. The network type determines the multicast address used for
sending updates. Table9.2 contains the multicast addresses associated with LSA ooding.
Point-to-multipoint networks use the adjacent router’s unicast IP address.
TABLE9.2 LSA update multicast addresses
Network Type Multicast Address Description
Point-to-point 224.0.0.5 AllSPFRouters
Broadcast 224.0.0.6 AllDRouters
Point-to-multipoint NA NA
Once the LSA updates have been ooded throughout the network, each recipient must
acknowledge that the ooded update has been received. It’s also important for recipients to
validate the LSA update.
SPF Tree Calculation
Within an area, each router calculates the best/shortest path to every network in that same
area. This calculation is based upon the information collected in the topology database and
an algorithm called shortest path rst (SPF). Picture each router in an area constructing
a tree—much like a family tree—where the router is the root and all other networks are
arranged along the branches and leaves. This is the shortest path tree used by the router to
insert OSPF routes into the routing table.
It’s important to understand that this tree contains only networks that exist in the same
area as the router itself does. If a router has interfaces in multiple areas, then separate trees
will be constructed for each area. One of the key criteria considered during the route selec-
tion process of the SPF algorithm is the metric or cost of each potential path to a network.
But this SPF calculation doesn’t apply to routes from other areas.
Configuring OSPF 393
OSPF Metrics
OSPF uses a metric referred to as cost. A cost is associated with every outgoing interface
included in an SPF tree. The cost of the entire path is the sum of the costs of the outgoing
interfaces along the path. Because cost is an arbitrary value as dened in RFC 2338, Cisco
had to implement its own method of calculating the cost for each OSPF-enabled interface.
Cisco uses a simple equation of 108/bandwidth, where bandwidth is the congured band-
width for the interface. Using this rule, a 100 Mbps Fast Ethernet interface would have a
default OSPF cost of 1 and a 1,000 Mbps Ethernet interface would have a cost of 1.
Important to note is that this value can be overridden with the ip ospf cost command.
The cost is manipulated by changing the value to a number within the range of 1 to 65,535.
Because the cost is assigned to each link, the value must be changed on the specic interface
you want to change the cost on.
Cisco bases link cost on bandwidth. Other vendors may use other metrics
to calculate a given link’s cost. When connecting links between routers
from different vendors, you’ll probably have to adjust the cost to match
another vendor’s router because both routers must assign the same cost
to the link for OSPF to work properly.
Configuring OSPF
Conguring basic OSPF isn’t as simple as conguring RIP and EIGRP, and it can get really
complex once the many options that are allowed within OSPF are factored in. But that’s
okay because you really only need to focus on basic, single-area OSPF conguration at this
point. Coming up, I’ll show you how to congure single-area OSPF.
The two factors that are foundational to OSPF conguration are enabling OSPF and
conguring OSPF areas.
Enabling OSPF
The easiest and also least scalable way to congure OSPF is to just use a single area. Doing
this requires a minimum of two commands.
The rst command used to activate the OSPF routing process is as follows:
Router(config)#router ospf ?
<1-65535> Process ID
A value in the range from 1 to 65,535 identies the OSPF process ID. It’s a unique number
on this router that groups a series of OSPF conguration commands under a specic running
process. Different OSPF routers don’t have to use the same process ID to communicate. It’s a
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394 Chapter 9 u Open Shortest Path First (OSPF)
purely local value that doesn’t mean a lot, but you still need to remember that it cannot start
at 0; it has to start at a minimum of 1.
You can have more than one OSPF process running simultaneously on the same router
if you want, but this isn’t the same as running multi-area OSPF. The second process will
maintain an entirely separate copy of its topology table and manage its communications
independently of the rst one and you use it when you want OSPF to connect multiple ASs
together. Also, because the Cisco exam objectives only cover single-area OSPF with each
router running a single OSPF process, that’s what we’ll focus on in this book.
The OSPF process ID is needed to identify a unique instance of an OSPF
database and is locally significant.
Configuring OSPF Areas
After identifying the OSPF process, you need to identify the interfaces that you want to
activate OSPF communications on as well as the area in which each resides. This will
also congure the networks you’re going to advertise to others. OSPF uses wildcards in
the conguration, which are also used in the access list congurations that we’ll cover
in Chapter 12, “Security.”
Here’s an example of a basic OSPF conguration for you, showing our second mini-
mum command needed, the network command:
Router#config t
Router(config)#router ospf 1
Router(config-router)#network 10.0.0.0 0.255.255.255 area ?
<0-4294967295> OSPF area ID as a decimal value
A.B.C.D OSPF area ID in IP address format
Router(config-router)#network 10.0.0.0 0.255.255.255 area 0
The areas can be any number from 0 to 4.2 billion. Don’t get these numbers
confused with the process ID, which ranges from 1 to 65,535.
Remember, the OSPF process ID number is irrelevant. It can be the same on every router
on the network, or it can be different—doesn’t matter. It’s locally signicant and just enables
the OSPF routing on the router.
The arguments of the network command are the network number (10.0.0.0) and the wild-
card mask (0.255.255.255). The combination of these two numbers identies the interfaces
that OSPF will operate on and will also be included in its OSPF LSA advertisements. Based
on my sample conguration, OSPF will use this command to nd any interface on the router
congured in the 10.0.0.0 network and will place any interface it nds into area 0. Notice
Configuring OSPF 395
that you can create about 4.2 billion areas! In reality, a router wouldn’t let you create that
many, but you can certainly name them using the numbers up to 4.2 billion. You can also
label an area using an IP address format.
Let me stop here a minute to give you a quick explanation of wildcards: A 0 octet in the
wildcard mask indicates that the corresponding octet in the network must match exactly.
On the other hand, a 255 indicates that you don’t care what the corresponding octet is in
the network number. A network and wildcard mask combination of 1.1.1.1 0.0.0.0 would
match an interface congured exactly with 1.1.1.1 only, and nothing else. This is really use-
ful if you want to activate OSPF on a specic interface in a very clear and simple way. If
you insist on matching a range of networks, the network and wildcard mask combination
of 1.1.0.0 0.0.255.255 would match any interface in the range of 1.1.0.0 to 1.1.255.255.
Because of this, it’s simpler and safer to stick to using wildcard masks of 0.0.0.0 and iden-
tify each OSPF interface individually. Once congured, they’ll function exactly the same—
one way is really isn’t better than the other.
The nal argument is the area number. It indicates the area to which the interfaces
identied in the network and wildcard mask portion belong. Remember that OSPF routers
will become neighbors only if their interfaces share a network that’s congured to belong
to the same area number. The format of the area number is either a decimal value from the
range 1 to 4,294,967,295 or a value represented in standard dotted-decimal notation. For
example, area 0.0.0.0 is a legitimate area and is identical to area 0.
Wildcard Example
Before getting down to conguring our network, let’s take a quick peek at a more complex
OSPF network conguration to nd out what our OSPF network statements would be if we
were using subnets and wildcards.
In this scenario, you have a router with these four subnets connected to four different
interfaces:
u 192.168.10.64/28
u 192.168.10.80/28
u 192.168.10.96/28
u 192.168.10.8/30
All interfaces need to be in area 0, so it seems to me the easiest conguration would look
like this:
Test#config t
Test(config)#router ospf 1
Test(config-router)#network 192.168.10.0 0.0.0.255 area 0
Okay—I’ll admit that preceding example is actually pretty simple, but easy isn’t always
best—especially when dealing with OSPF! So even though this is an easy-button way to con-
gure OSPF, it doesn’t make good use of its capabilities and what fun is that? Worse yet, the
objectives aren’t very likely to present something this simple for you! So let’s create a separate
396 Chapter 9 u Open Shortest Path First (OSPF)
network statement for each interface using the subnet numbers and wildcards. Doing that
would look something like this:
Test#config t
Test(config)#router ospf 1
Test(config-router)#network 192.168.10.64 0.0.0.15 area 0
Test(config-router)#network 192.168.10.80 0.0.0.15 area 0
Test(config-router)#network 192.168.10.96 0.0.0.15 area 0
Test(config-router)#network 192.168.10.8 0.0.0.3 area 0
Wow, now that’s a different looking cong! Truthfully, OSPF would work exactly the
same way as it would with the easy conguration I showed you rst—but unlike the easy
conguration, this one covers the objectives!
And although this looks a bit complicated, trust me, it really isn’t. All you need for clarity is
to fully understand your block sizes! Just remember that when conguring wildcards, they’re
always one less than the block size. A /28 is a block size of 16, so we would add our network
statement using the subnet number and then add a wildcard of 15 in the interesting octet. For
the /30, which is a block size of 4, we would go with a wildcard of 3. Once you practice this a
few times, it gets really easy. And do practice because we’ll deal with them again when we get
to access lists later on!
Let’s use Figure9.3 as an example and congure that network with OSPF using wild-
cards to make sure you have a solid grip on this. The gure shows a three-router network
with the IP addresses of each interface.
FIGURE9.3 Sample OSPF wildcard configuration
10.255.255.81/30 10.255.255.82/30
Fa0/0 Fa0/0 Fa0/0
192.168.10.65/29 192.168.10.49/29 192.168.10.17/29
10.255.255.9/30 10.255.255.10/30
G0/0 G0/1 G0/0 G0/0
Lab A Lab B Lab C
The very rst thing you need to be able to do is to look at each interface and deter-
mine the subnet that the addresses are in. Hold on, I know what you’re thinking: “Why
don’t I just use the exact IP addresses of the interface with the 0.0.0.0 wildcard?” Well,
you can, but we’re paying attention to Cisco exam objectives here, not just what’s easi-
est, remember?
Configuring OSPF 397
The IP addresses for each interface are shown in the gure. The Lab_A router has two
directly connected subnets: 192.168.10.64/29 and 10.255.255.80/30. Here’s the OSPF con-
guration using wildcards:
Lab_A#config t
Lab_A(config)#router ospf 1
Lab_A(config-router)#network 192.168.10.64 0.0.0.7 area 0
Lab_A(config-router)#network 10.255.255.80 0.0.0.3 area 0
The Lab_A router is using a /29, or 255.255.255.248, mask on the Fa0/0 interface. This is
a block size of 8, which is a wildcard of 7. The G0/0 interface is a mask of 255.255.255.252—
block size of 4, with a wildcard of 3. Notice that I typed in the network number, not the
interface number. You can’t congure OSPF this way if you can’t look at the IP address and
slash notation and then gure out the subnet, mask, and wildcard, can you? So don’t take your
exam until you can do this.
Here are other two congurations to help you practice:
Lab_B#config t
Lab_B(config)#router ospf 1
Lab_B(config-router)#network 192.168.10.48 0.0.0.7 area 0
Lab_B(config-router)#network 10.255.255.80 0.0.0.3 area 0
Lab_B(config-router)#network 10.255.255.8 0.0.0.3 area 0
Lab_C#config t
Lab_C(config)#router ospf 1
Lab_C(config-router)#network 192.168.10.16 0.0.0.7 area 0
Lab_C(config-router)#network 10.255.255.8 0.0.0.3 area 0
As I mentioned with the Lab_A conguration, you’ve got to be able to determine the sub-
net, mask, and wildcard just by looking at the IP address and mask of an interface. If you
can’t do that, you won’t be able to congure OSPF using wildcards as I just demonstrated.
So go over this until you’re really comfortable with it!
Configuring Our Network with OSPF
Okay—now we get to have some fun! Let’s congure our internetwork with OSPF using
just area 0. OSPF has an administrative distance of 110, but let’s remove RIP while we’re
at it because I don’t want you to get in the habit of having RIP running on your network.
There’s a bunch of different ways to congure OSPF, and as I said, the simplest
and easiest is to use the wildcard mask 0.0.0.0. But I want to demonstrate that we can
398 Chapter 9 u Open Shortest Path First (OSPF)
congure each router differently with OSPF and still come up with the exact same result.
This is one reason why OSPF is more fun and challenging than other routing protocols—
it gives us all a lot more ways to screw things up, which automatically provides a trouble-
shooting opportunity! We’ll use our network as shown in Figure9.4 to congure OSPF
and by the way, notice I added a new router!
FIGURE9.4 Our new network layout
SF
Corp
10.10.10.1/24Fa0/0
Boulder
10.10.10.2/24Fa0/0
Fa0/0192.168.10.1/24
172.16.10.2/30 S0/0/0
LA
Fa0/0 192.168.20.1/24
172.16.10.6/30
S0/0/1
S0/0 S0/1172.16.10.1/30 172.16.10.5/30
Corp
Here’s the Corp router’s conguration:
Corp#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.10.10.1 YES manual up up
Serial0/0 172.16.10.1 YES manual up up
FastEthernet0/1 unassigned YES unset administratively down down
Serial0/1 172.16.10.5 YES manual up up
Corp#config t
Corp(config)#no router rip
Corp(config)#router ospf 132
Corp(config-router)#network 10.10.10.1 0.0.0.0 area 0
Corp(config-router)#network 172.16.10.1 0.0.0.0 area 0
Corp(config-router)#network 172.16.10.5 0.0.0.0 area 0
Configuring OSPF 399
Alright—it looks like we have a few things to talk about here. First, I removed RIP and
then added OSPF. Why did I use OSPF 132? It really doesn’t matter—the number is irrele-
vant. I guess it just felt good to use 132. But notice that I started with the show ip int brief
command, just like when I was conguring RIP. I did this because it’s always important to
verify exactly what you are directly connected to. Doing this really helps prevent typos!
The network commands are pretty straightforward. I typed in the IP address of each inter-
face and used the wildcard mask of 0.0.0.0, which means that the IP address must precisely
match each octet. This is actually one of those times where easier is better, so just do this:
Corp(config)#router ospf 132
Corp(config-router)#network 172.16.10.0 0.0.0.255 area 0
Nice—there’s only one line instead of two for the 172.16.10.0 network! I really want
you to understand that OSPF will work the same here no matter which way you congure
the network statement. Now, let’s move on to SF. To simplify things, we’re going to use our
same sample conguration.
SF
The SF router has two directly connected networks. I’ll use the IP addresses on each inter-
face to congure this router.
SF#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.10.1 YES manual up up
FastEthernet0/1 unassigned YES unset administratively down down
Serial0/0/0 172.16.10.2 YES manual up up
Serial0/0/1 unassigned YES unset administratively down down
SF#config t
SF(config)#no router rip
SF(config)#router ospf 300
SF(config-router)#network 192.168.10.1 0.0.0.0 area 0
SF(config-router)#network 172.16.10.2 0.0.0.0 area 0
*Apr 30 00:25:43.810: %OSPF-5-ADJCHG: Process 300, Nbr 172.16.10.5 on Serial0/0/0
from LOADING to FULL, Loading Done
Here, all I did was to rst disable RIP, turn on OSPF routing process 300, and then I
added my two directly connected networks. Now let’s move on to LA!
LA
We’re going to give some attention to the LA router that’s directly connected to two networks:
LA#sh ip int brief
Interface IP-Address OK? Method Status Protocol
400 Chapter 9 u Open Shortest Path First (OSPF)
FastEthernet0/0 192.168.20.1 YES manual up up
FastEthernet0/1 unassigned YES unset administratively down down
Serial0/0/0 unassigned YES unset administratively down down
Serial0/0/1 172.16.10.6 YES manual up up
LA#config t
LA(config)#router ospf 100
LA(config-router)#network 192.168.20.0 0.0.0.255 area 0
LA(config-router)#network 172.16.0.0 0.0.255.255 area 0
*Apr 30 00:56:37.090: %OSPF-5-ADJCHG: Process 100, Nbr 172.16.10.5 on Serial0/0/1
from LOADING to FULL, Loading Done
Remember that when you’re conguring dynamic routing, using the show ip int brief
command rst will make it all so much easier!
And don’t forget, I can use any process ID I want, as long as it’s a value from 1 to 65,535,
because it doesn’t matter if all routers use the same process ID. Also, notice that I used differ-
ent wildcards in this example. Doing this works really well too.
Okay, I want you to think about something for a second before we move onto more
advanced OSPF topics: What if the Fa0/1 interface of the LA router was connected to a
link that we didn’t want, or need to have on in order to have OSPF working, as shown in
Figure9.5?
FIGURE9.5 Adding a non-OSPF network to LA router
LA
OSPF network
Fa0/0
Fa0/1
Non-OSPF network
You’ve seen this before because I demonstrated this already back in Chapter 8, in the
RIP section. We can use the same command that we did under that routing process here as
well! Take a look:
LA(config)#router ospf 100
LA(config-router)#passive-interface fastEthernet 0/1
Even though this is pretty simple, you’ve really got to be careful before you congure
this command on your router! I added this command as an example on interface Fa0/1,
which happens to be an interface we’re not using in this network because I want OSPF to
work on my other router’s interfaces.
Configuring OSPF 401
Now it’s time to congure our Corp router to advertise a default route to the SF and LA
routers because doing so will make our lives a lot easier. Instead of having to congure all
our routers with a default route, we’ll only congure one router and then advertise that this
router is the one that holds the default route—elegant!
In Figure9.4, keep in mind that, for now, the corporate router is connected to the Internet
off of Fa0/0. We’ll create a default route toward this imaginary Internet and then tell the
other routers that this is the route they’ll use to get to the Internet. Here is the conguration:
Corp#config t
Corp(config)#ip route 0.0.0.0 0.0.0.0 Fa0/0
Corp(config)#router ospf 1
Corp(config-router)#default-information originate
Now, let’s check and see if our other routers have received this default route from the
Corp router:
SF#show ip route
[output cut]
E1 - OSPF external type 1, E2 - OSPF external type 2
[output cut]
O*E2 0.0.0.0/0 [110/1] via 172.16.10.1, 00:01:54, Serial0/0/0
SF#
Sure enough—the last line in the SF router shows that it received the advertisement from
the Corp router regarding the fact that the corporate router is the one holding the default
route out of the AS.
But hold on a second! I need to congure our new router into my lab to create the example
network we’ll use from here on. Here’s the conguration of the new router that I connected to
the same network that the Corp router is connected to via the Fa0/0 interface:
Router#config t
Router(config)#hostname Boulder
Boulder(config)#int f0/0
Boulder(config-if)#ip address 10.10.10.2 255.255.255.0
Boulder(config-if)#no shut
*Apr 6 18:01:38.007: %LINEPROTO-5-UPDOWN: Line protocol on Interface
FastEthernet0/0, changed state to up
Boulder(config-if)#router ospf 2
Boulder(config-router)#network 10.0.0.0 0.255.255.255 area 0
*Apr 6 18:03:27.267: %OSPF-5-ADJCHG: Process 2, Nbr 223.255.255.254 on
FastEthernet0/0 from LOADING to FULL, Loading Done
402 Chapter 9 u Open Shortest Path First (OSPF)
This is all good, but I need to make sure that you don’t follow my example to a tee
because here, I just quickly brought a router up without setting my passwords rst. I can
get away with this only because I am in a nonproduction network, so don’t do this in the
real world where security is key!
Anyway, now that I have my new router nicely connected with a basic conguration,
we’re going to move on to cover loopback interfaces, how to set the router ID (RID) used
with OSPF, and nally, how to verify OSPF.
OSPF and Loopback Interfaces
It’s really vital to congure loopback interfaces when using OSPF. In fact, Cisco suggests
using them whenever you congure OSPF on a router for stability purposes.
Loopback interfaces are logical interfaces, which means they’re virtual, software-only
interfaces, not actual, physical router interfaces. A big reason we use loopback interfaces
with OSPF congurations is because they ensure that an interface is always active and
available for OSPF processes.
Loopback interfaces also come in very handy for diagnostic purposes as well as for OSPF
conguration. Understand that if you don’t congure a loopback interface on a router, the
highest active IP address on a router will become that router’s RID during bootup! Figure9.6
illustrates how routers know each other by their router ID.
FIGURE9.6 OSPF router ID (RID)
I need your RID so I can
place you properly in my
neighbor table!
The RID is not only used to advertise routes, it’s also used to elect the designated router
(DR) and the backup designated router (BDR). These designated routers create adjacencies
when a new router comes up and exchanges LSAs to build topological databases.
By default, OSPF uses the highest IP address on any active interface at the
moment OSPF starts up to determine the RID of the router. But this behav-
ior can be overridden via a logical interface. Remember—the highest IP
address of any logical interface will always become a router’s RID!
OSPF and Loopback Interfaces 403
Now it’s time to show you how to congure these logical loopback interfaces and how to
verify them, as well as verify RIDs.
Configuring Loopback Interfaces
Conguring loopback interfaces rocks mostly because it’s the easiest part of OSPF congu-
ration, and we all need a break about now—right? So hang on—we’re in the home stretch!
First, let’s see what the RID is on the Corp router with the show ip ospf command:
Corp#sh ip ospf
Routing Process "ospf 1" with ID 172.16.10.5
[output cut]
Okay—we can see that the RID is 172.16.10.5—the Serial0/0 interface of the router. So
let’s congure a loopback interface using a completely different IP addressing scheme:
Corp(config)#int loopback 0
*Mar 22 01:23:14.206: %LINEPROTO-5-UPDOWN: Line protocol on Interface
Loopback0, changed state to up
Corp(config-if)#ip address 172.31.1.1 255.255.255.255
The IP scheme really doesn’t matter here, but each one being in a separate subnet does!
By using the /32 mask, we can use any IP address we want as long as the addresses are
never the same on any two routers.
Let’s congure the other routers now:
SF#config t
SF(config)#int loopback 0
*Mar 22 01:25:11.206: %LINEPROTO-5-UPDOWN: Line protocol on Interface
Loopback0, changed state to up
SF(config-if)#ip address 172.31.1.2 255.255.255.255
Here’s the conguration of the loopback interface on LA:
LA#config t
LA(config)#int loopback 0
*Mar 22 02:21:59.686: %LINEPROTO-5-UPDOWN: Line protocol on Interface
Loopback0, changed state to up
LA(config-if)#ip address 172.31.1.3 255.255.255.255
I’m pretty sure you’re wondering what the IP address mask of 255.255.255.255 (/32)
means and why we don’t just use 255.255.255.0 instead. While it’s true that either mask
works, the /32 mask is called a host mask and works ne for loopback interfaces. It also
404 Chapter 9 u Open Shortest Path First (OSPF)
allows us to save subnets. Notice how I was able to use 172.31.1.1, .2, .3, and .4? If I didn’t
use the /32, I’d have to use a separate subnet for each and every router—not good!
One important question to answer before we move on is did we actually change the
RIDs of our router by setting the loopback interfaces? Let’s nd out by taking a look at
the Corp’s RID:
Corp#sh ip ospf
Routing Process "ospf 1" with ID 172.16.10.5
Okay—what happened here? You would think that because we set logical interfaces, the
IP addresses under them would automatically become the RID of the router, right? Well, sort
of, but only if you do one of two things: either reboot the router or delete OSPF and re-create
the database on your router. Neither is all that great an option, so try to remember to create
your logical interfaces before you start OSPF routing. That way, the loopback interface would
always become your RID straight away!
With all this in mind, I’m going with rebooting the Corp router because it’s the easier of
the two options I have right now.
Now let’s look and see what our RID is:
Corp#sh ip ospf
Routing Process "ospf 1" with ID 172.31.1.1
Okay, that did the trick! The Corp router now has a new RID, so I guess I’ll just go
ahead and reboot all my routers to get their RIDs reset to our logical addresses. But should
I really do that?
Maybe not because there is one other way. What do you think about adding a new RID
for the router right under the router ospf process-id command instead? Sounds good, so
I’d say let’s give that a shot! Here’s an example of doing that on the Corp router:
Corp#config t
Corp(config)#router ospf 1
Corp(config-router)#router-id 223.255.255.254
Reload or use "clear ip ospf process" command, for this to take effect
Corp(config-router)#do clear ip ospf process
Reset ALL OSPF processes? [no]: yes
*Jan 16 14:20:36.906: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.20.1
on Serial0/1 from FULL to DOWN, Neighbor Down: Interface down
or detached
*Jan 16 14:20:36.906: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.10.1
on Serial0/0 from FULL to DOWN, Neighbor Down: Interface down
or detached
*Jan 16 14:20:36.982: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.20.1
Verifying OSPF Configuration 405
on Serial0/1 from LOADING to FULL, Loading Done
*Jan 16 14:20:36.982: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.10.1
on Serial0/0 from LOADING to FULL, Loading Done
Corp(config-router)#do sh ip ospf
Routing Process "ospf 1" with ID 223.255.255.254
Now look at that—it worked! We changed the RID without reloading the router! But
wait—remember, we set a logical loopback interface earlier. Does that mean the loopback
interface will win over the router-id command? Well, we can see our answer… A loop-
back interface will not override the router-id command, and we don’t have to reboot the
router to make it take effect as the RID!
So this process follows this hierarchy:
1. Highest active interface by default.
2. Highest logical interface overrides a physical interface.
3. The router-id overrides the interface and loopback interface.
The only thing left now is to decide whether you want to advertise the loopback inter-
faces under OSPF. There are pros and cons to using an address that won’t be advertised
versus using an address that will be. Using an unadvertised address saves on real IP address
space, but the address won’t appear in the OSPF table, which means you can’t ping it.
So basically, what you’re faced with here is a choice that equals a trade-off between the
ease of debugging the network and conservation of address space—what to do? A really
tight strategy is to use a private IP address scheme as I did. Do this and all will be well!
Now that we’ve congured all the routers with OSPF, what’s next? Miller time? Nope—
not yet. It’s that verication thing again. We still have to make sure that OSPF is really work-
ing, and that’s exactly what we’re going to do next.
Verifying OSPF Configuration
There are several ways to verify proper OSPF conguration and operation, so next, I’m going
to demonstrate the various OSPF show commands you need to know in order to achieve this.
We’re going to start by taking a quick look at the routing table of the Corp router.
First, let’s issue a show ip route command on the Corp router:
O 192.168.10.0/24 [110/65] via 172.16.10.2, 1d17h, Serial0/0
172.131.0.0/32 is subnetted, 1 subnets
172.131.0.0/32 is subnetted, 1 subnets
C 172.131.1.1 is directly connected, Loopback0
172.16.0.0/30 is subnetted, 4 subnets
C 172.16.10.4 is directly connected, Serial0/1
L 172.16.10.5/32 is directly connected, Serial0/1
406 Chapter 9 u Open Shortest Path First (OSPF)
C 172.16.10.0 is directly connected, Serial0/0
L 172.16.10.1/32 is directly connected, Serial0/0
O 192.168.20.0/24 [110/65] via 172.16.10.6, 1d17h, Serial0/1
10.0.0.0/24 is subnetted, 2 subnets
C 10.10.10.0 is directly connected, FastEthernet0/0
L 10.10.10.1/32 is directly connected, FastEthernet0/0
The Corp router shows only two dynamic routes for the internetwork, with the O repre-
senting OSPF internal routes. The Cs are clearly our directly connected networks, and our
two remote networks are showing up too—nice! Notice the 110/65, which is our adminis-
trative distance/metric.
Now that’s a really sweet-looking OSPF routing table! It’s important to make it easier to
troubleshoot and x an OSPF network, which is why I always use the show ip int brief
command when conguring my routing protocols. It’s very easy to make little mistakes
with OSPF, so keep your eyes on the details!
It’s time to show you all the OSPF verication commands that you need in your toolbox
for now.
The show ip ospf Command
The show ip ospf command is what you’ll need to display OSPF information for one or all
OSPF processes running on the router. Information contained therein includes the router
ID, area information, SPF statistics, and LSA timer information. Let’s check out the output
from the Corp router:
Corp#sh ip ospf
Routing Process "ospf 1" with ID 223.255.255.254
Start time: 00:08:41.724, Time elapsed: 2d16h
Supports only single TOS(TOS0) routes
Supports opaque LSA
Supports Link-local Signaling (LLS)
Supports area transit capability
Router is not originating router-LSAs with maximum metric
Initial SPF schedule delay 5000 msecs
Minimum hold time between two consecutive SPFs 10000 msecs
Maximum wait time between two consecutive SPFs 10000 msecs
Incremental-SPF disabled
Minimum LSA interval 5 secs
Minimum LSA arrival 1000 msecs
LSA group pacing timer 240 secs
Interface flood pacing timer 33 msecs
Retransmission pacing timer 66 msecs
Number of external LSA 0. Checksum Sum 0x000000
Verifying OSPF Configuration 407
Number of opaque AS LSA 0. Checksum Sum 0x000000
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Number of areas transit capable is 0
External flood list length 0
IETF NSF helper support enabled
Cisco NSF helper support enabled
Area BACKBONE(0)
Number of interfaces in this area is 3
Area has no authentication
SPF algorithm last executed 00:11:08.760 ago
SPF algorithm executed 5 times
Area ranges are
Number of LSA 6. Checksum Sum 0x03B054
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
Notice the router ID (RID) of 223.255.255.254, which is the highest IP address cong-
ured on the router. Hopefully, you also noticed that I set the RID of the corporate router to
the highest available IP address available with IPv4.
The show ip ospf database Command
Using the show ip ospf database command will give you information about the number
of routers in the internetwork (AS) plus the neighboring router’s ID—the topology database
I mentioned earlier. Unlike the show ip eigrp topology command, this command reveals
the OSPF routers, but not each and every link in the AS like EIGRP does.
The output is broken down by area. Here’s a sample output, again from Corp:
Corp#sh ip ospf database
OSPF Router with ID (223.255.255.254) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
10.10.10.2 10.10.10.2 966 0x80000001 0x007162 1
408 Chapter 9 u Open Shortest Path First (OSPF)
172.31.1.4 172.31.1.4 885 0x80000002 0x00D27E 1
192.168.10.1 192.168.10.1 886 0x8000007A 0x00BC95 3
192.168.20.1 192.168.20.1 1133 0x8000007A 0x00E348 3
223.255.255.254 223.255.255.254 925 0x8000004D 0x000B90 5
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.10.10.1 223.255.255.254 884 0x80000002 0x008CFE
You can see all the routers and the RID of each router—the highest IP address on each
of them. For example, the Link ID and ADV Router of my new Boulder router shows up
twice: once with the directly connected IP address (10.10.10.2) and as the RID that I set
under the OSPF process (172.31.1.4).
The router output shows the link ID—remember that an interface is also a link—and the
RID of the router on that link under the ADV router, or advertising router.
The show ip ospf interface Command
The show ip ospf interface command reveals all interface-related OSPF information.
Data is displayed about OSPF information for all OSPF-enabled interfaces or for specied
interfaces. I’ll highlight some of the more important factors for you. Check it out:
Corp#sh ip ospf int f0/0
FastEthernet0/0 is up, line protocol is up
Internet Address 10.10.10.1/24, Area 0
Process ID 1, Router ID 223.255.255.254, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 223.255.255.254, Interface address 10.10.10.1
Backup Designated router (ID) 172.31.1.4, Interface address 10.10.10.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:08
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 3/3, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Verifying OSPF Configuration 409
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 172.31.1. Suppress hello for 0 neighbor(s)
Okay—so this command has given us the following information:
u Interface IP address
u Area assignment
u Process ID
u Router ID
u Network type
u Cost
u Priority
u DR/BDR election information (if applicable)
u Hello and Dead timer intervals
u Adjacent neighbor information
The reason I used the show ip ospf interface f0/0 command is because I knew that
there would be a designated router elected on the FastEthernet broadcast multi-access net-
work between our Corp and Boulder routers. The information that I highlighted is all very
important, so make sure you’ve noted it! A good question to ask you here is what are the
Hello and Dead timers set to by default?
Type in the show ip ospf interface command and receive this response:
Corp#sh ip ospf int f0/0
%OSPF: OSPF not enabled on FastEthernet0/0
This error occurs when OSPF is enabled on the router, but not the interface. When this
happens, you need to check your network statements because it means that the interface
you’re trying to verify is not in your OSPF process!
The show ip ospf neighbor Command
The show ip ospf neighbor command is super-useful because it summarizes the pertinent
OSPF information regarding neighbors and the adjacency state. If a DR or BDR exists, that
information will also be displayed. Here’s a sample:
Corp#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
172.31.1.4 1 FULL/BDR 00:00:34 10.10.10.2 FastEthernet0/0
192.168.20.1 0 FULL/ - 00:00:31 172.16.10.6 Serial0/1
192.168.10.1 0 FULL/ - 00:00:32 172.16.10.2 Serial0/0
410 Chapter 9 u Open Shortest Path First (OSPF)
An Admin Connects Two Disparate Routers Together with OSPF and the
Link Between them Never Comes Up
Quite a few years ago, an admin called me in a panic because he couldn’t get OSPF work-
ing between two routers, one of which was an older router that they needed to use while
they were waiting for their new router to be shipped to them.
OSPF can be used in a multi-vendor network so he was confused as to why this wasn’t
working. He turned on RIP and it worked so he was super confused with why OSPF was
not creating adjacencies. I had him use the show ip ospf interface command to look at
the link between the two routers and sure enough, the hello and dead timers didn’t match.
I had him configure the mismatched parameters so they would match, but it still wouldn’t
create an adjacency. Looking more closely at the show ip ospf interface command, I
noticed the cost did not match! Cisco calculated the bandwidth differently than the other
vendor. Once I had him configure both as the same value, the link came up! Always remem-
ber, just because OSPF can be used in a multi-vendor network, does not mean it will work
out of the box!
This is a critical command to understand because it’s extremely useful in production
networks. Let’s take a look at the Boulder router output:
Boulder>sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
223.255.255.254 1 FULL/DR 00:00:31 10.10.10.1 FastEthernet0/0
Okay—here we can see that since there’s an Ethernet link (broadcast multi-access) on
the link between the Boulder and the Corp router, there’s going to be an election to deter-
mine who will be the designated router (DR) and who will be the backup designated router
(BDR). We can see that the Corp became the designated router, and it won because it had
the highest IP address on the network—the highest RID.
Now the reason that the Corp connections to SF and LA don’t have a DR or BDR listed
in the output is that by default, elections don’t happen on point-to-point links and they
show FULL/ - . But we can still determine that the Corp router is fully adjacent to all three
routers from its output.
Summary 411
The show ip protocols Command
The show ip protocols command is also highly useful, whether you’re running OSPF,
EIGRP, RIP, BGP, IS-IS, or any other routing protocol that can be congured on your router.
It provides an excellent overview of the actual operation of all currently running protocols!
Check out the output from the Corp router:
Corp#sh ip protocols
Routing Protocol is "ospf 1"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Router ID 223.255.255.254
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Maximum path: 4
Routing for Networks:
10.10.10.1 0.0.0.0 area 0
172.16.10.1 0.0.0.0 area 0
172.16.10.5 0.0.0.0 area 0
Reference bandwidth unit is 100 mbps
Routing Information Sources:
Gateway Distance Last Update
192.168.10.1 110 00:21:53
192.168.20.1 110 00:21:53
Distance: (default is 110) Distance: (default is 110)
From looking at this output, you can determine the OSPF process ID, OSPF router ID,
type of OSPF area, networks and areas congured for OSPF, and the OSPF router IDs of
neighbors—that’s a lot. It’s super-efcient!
Summary
This chapter gave you a great deal of information about OSPF. It’s really difcult to include
everything about OSPF because so much of it falls outside the scope of this chapter and book,
but I’ve given you a few tips here and there, so you’re good to go—as long as you make sure
you’ve got what I presented to you dialed in, that is!
I talked about a lot of OSPF topics, including terminology, operations, and conguration
as well as verication and monitoring.
Visit ccna
.gg/ch9/b
for a
companion
MicroNugget
from CBT
Nuggets.
412 Chapter 9 u Open Shortest Path First (OSPF)
Each of these topics encompasses quite a bit of information—the terminology section
just scratched the surface of OSPF. But you’ve got the goods you really need for your
studies. Finally, I gave you a tight survey of commands highly useful for observing the
operation of OSPF so you can verify that things are moving along as they should. So eat
it all up, and you’re set!
Exam Essentials
Compare OSPF and RIPv1. OSPF is a link-state protocol that supports VLSM and classless
routing; RIPv1 is a distance-vector protocol that does not support VLSM and supports only
classful routing.
Know how OSPF routers become neighbors and/or adjacent. OSPF routers become
neighbors when each router sees the other’s Hello packets.
Be able to configure single-area OSPF. A minimal single-area conguration involves only
two commands: router ospf process-id and network x.x.x.x y.y.y.y area Z .
Be able to verify the operation of OSPF. There are many show commands that provide use-
ful details on OSPF, and it is useful to be completely familiar with the output of each: show
ip ospf, show ip ospf database, show ip ospf interface, show ip ospf neighbor, and
show ip protocols.
Hands-on Labs 413
Written Lab 9
The answers to this lab can be found in Appendix A, “Answers to Written Labs.”
1. Write the command that will enable the OSPF process 101 on a router.
2. Write the command that will display details of all OSPF routing processes enabled on
a router.
3. Write the command that will display interface-specific OSPF information.
4. Write the command that will display all OSPF neighbors.
5. Write the command that will display all different OSPF route types that are currently
known by the router.
Hands-on Labs
In this section, you will use the following network and add OSPF routing.
S0/0
DCE
Fa0/0 Fa0/0
S0/0
DCES0/0
Lab A Lab B Lab C
S0/1
The rst lab (Lab 9.1) requires you to congure three routers for OSPF and then view
the conguration. Note that the labs in this chapter were written to be used with real
equipment—but they can be used with any router simulator. You can replace the WAN
links with Ethernet links if you want to.
The labs in this chapter are as follows:
Lab 9.1: Enabling the OSPF Process
Lab 9.2: Conguring OSPF Interfaces
Lab 9.3: Verifying OSPF Operation
Table9.5 shows our IP addresses for each router (each interface uses a /24 mask).
414 Chapter 9 u Open Shortest Path First (OSPF)
TABLE9.5 Our IP addresses
Router Interface IP address
Lab_A Fa0/0 172.16 .10.1
Lab_A S0/0 172.16.20.1
Lab_B S0/2 172.16.20.2
Lab_B S0/1 172.16. 30.1
Lab_C S0/0 172.16.30.2
Lab_C Fa0/0 172.16.40.1
Hands-on Lab 9.1: Enabling the OSPF Process
This is the rst mandatory step in OSPF conguration.
1. Enable OSPF process 100 on Lab_A:
Lab_A#conf t
Enter configuration commands, one per line.
End with CNTL/Z.
Lab_A (config)#router ospf 100
Lab_A (config-router)#^Z
2. Enable OSPF process 101 on Lab_B:
Lab_B#conf t
Enter configuration commands, one per line.
End with CNTL/Z.
Lab_B (config)#router ospf 101
Lab_B (config-router)#^Z
3. Enable OSPF process 102 on Lab_C:
Lab_C#conf t
Enter configuration commands, one per line.
End with CNTL/Z.
Lab_C (config)#router ospf 102
Lab_C (config-router)#^Z
Hands-on Labs 415
Hands-on Lab 9.2: Configuring OSPF Interfaces
The second mandatory step in OSPF is adding your network statements.
1. Configure the LAN and the network between Lab_A and Lab_B. Assign it to area 0.
Lab_A#conf t
Enter configuration commands, one per line.
End with CNTL/Z.
Lab_A (config)#router ospf 100
Lab_A (config-router)#network 172.16.10.1 0.0.0.0 area 0
Lab_A (config-router)#network 172.16.20.1 0.0.0.0 area 0
Lab_A (config-router)#^Z
Lab_A #
2. Configure the networks on the Lab_B router. Assign them to area 0.
Lab_B#conf t
Enter configuration commands, one per line.
End with CNTL/Z.
Lab_B(config)#router ospf 101
Lab_B(config-router)#network 172.16.20.2 0.0.0.0 area 0
Lab_B(config-router)#network 172.16.30.1 0.0.0.0 area 0
Lab_B(config-router)#^Z
Lab_B #
3. Configure the networks on the Lab_C router. Assign them to area 0.
Lab_C#conf t
Enter configuration commands, one per line.
End with CNTL/Z.
Lab_C(config)#router ospf 102
Lab_C(config-router)#network 172.16.30.2 0.0.0.0 area 0
Lab_C(config-router)#network 172.16.40.1 0.0.0.0 area 0
Lab_C(config-router)#^Z
Lab_C#
416 Chapter 9 u Open Shortest Path First (OSPF)
Hands-on Lab 9.3: Verifying OSPF Operation
You need to be able to verify what you congure.
1. Execute a show ip ospf neighbors command from the Lab_A router and view
the results.
Lab_A#sho ip ospf neig
2. Execute a show ip route command to verify that all other routers are learning
all routes.
Lab_A#sho ip route
3. Execute a show ip protocols command to verify OSPF information.
Lab_A#sho ip protocols
4. Execute a show ip OSPF command to verify your RID.
Lab_A#sho ip ospf
5. Execute a show ip ospf interface f0/0 command to verify your timers.
Lab_A#sho ip ospf int f0/0
Review Questions 417
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional ques-
tions, please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. There are three possible routes for a router to reach a destination network. The first
route is from OSPF with a metric of 782. The second route is from RIPv2 with a metric
of 4. The third is from EIGRP with a composite metric of 20514560. Which route will
be installed by the router in its routing table?
A. RIPv2
B. EIGRP
C. OSPF
D. All three
2. In the accompanying diagram, which of the routers must be ABRs? (Choose all
that apply.)
Area 1
F
Area 2
G
Area 3
H
CD
Backbone Area
E
A. C
B. D
C. E
D. F
E. G
F. H
418 Chapter 9 u Open Shortest Path First (OSPF)
3. Which of the following describe the process identifier that is used to run OSPF on a
router? (Choose two.)
A. It is locally significant.
B. It is globally significant.
C. It is needed to identify a unique instance of an OSPF database.
D. It is an optional parameter required only if multiple OSPF processes are running
on the router.
E. All routes in the same OSPF area must have the same process ID if they are to
exchange routing information.
4. All of the following must match for two OSPF routers to become neighbors except which?
A. Area ID
B. Router ID
C. Stub area flag
D. Authentication password if using one
5. In the diagram, by default what will be the router ID of Lab_B?
10.255.255.81/30 10.255.255.82/30
Fa0/0 Fa0/0 Fa0/0
192.168.10.65/29 192.168.10.49/29 192.168.10.17/29
10.255.255.9/30 10.255.255.10/30
G0/0 G0/1 G0/1 G0/0
Lab A Lab B Lab C
A. 10.255.255.82
B. 10.255.255.9
C. 192.168.10.49
D. 10.255.255.81
Review Questions 419
6. You get a call from a network administrator who tells you that he typed the following
into his router:
Router(config)#router ospf 1
Router(config-router)#network 10.0.0.0 255.0.0.0 area 0
He tells you he still can’t see any routes in the routing table. What configuration error
did the administrator make?
A. The wildcard mask is incorrect.
B. The OSPF area is wrong.
C. The OSPF process ID is incorrect.
D. The AS configuration is wrong.
7. Which of the following statements is true with regard to the output shown?
Corp#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
172.31.1.4 1 FULL/BDR 00:00:34 10.10.10.2 FastEthernet0/0
192.168.20.1 0 FULL/ - 00:00:31 172.16.10.6 Serial0/1
192.168.10.1 0 FULL/ - 00:00:32 172.16.10.2 Serial0/0
A. There is no DR on the link to 192.168.20.1.
B. The Corp router is the BDR on the link to 172.31.1.4.
C. The Corp router is the DR on the link to 192.168.20.1.
D. The link to 192.168.10.1 is Active.
8. What is the administrative distance of OSPF?
A. 90
B. 100
C. 120
D. 110
9. In OSPF, Hellos are sent to what IP address?
A. 224.0.0.5
B. 224.0.0.9
C. 224.0.0.10
D. 224.0.0.1
420 Chapter 9 u Open Shortest Path First (OSPF)
10. What command generated the following output?
172.31.1.4 1 FULL/BDR 00:00:34 10.10.10.2 FastEthernet0/0
192.168.20.1 0 FULL/ - 00:00:31 172.16.10.6 Serial0/1
192.168.10.1 0 FULL/ - 00:00:32 172.16.10.2 Serial0/0
A. show ip ospf neighbor
B. show ip ospf database
C. show ip route
D. show ip ospf interface
11. Updates addressed to 224.0.0.6 are destined for which type of OSPF router?
A. DR
B. ASBR
C. ABR
D. All OSPF routers
12. For some reason, you cannot establish an adjacency relationship on a common Ethernet
link between two routers. Looking at this output, what is the cause of the problem?
RouterA#
Ethernet0/0 is up, line protocol is up
Internet Address 172.16.1.2/16, Area 0
Process ID 2, Router ID 172.126.1.2, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 172.16.1.2, interface address 172.16.1.1
No backup designated router on this network
Timer intervals configured, Hello 5, Dead 20, Wait 20, Retransmit 5
RouterB#
Ethernet0/0 is up, line protocol is up
Internet Address 172.16.1.1/16, Area 0
Process ID 2, Router ID 172.126.1.1, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 172.16.1.1, interface address 172.16.1.2
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Review Questions 421
A. The OSPF area is not configured properly.
B. The priority on RouterA should be set higher.
C. The cost on RouterA should be set higher.
D. The Hello and Dead timers are not configured properly.
E. A backup designated router needs to be added to the network.
F. The OSPF process ID numbers must match.
13. In the work area match each OSPF term (by line) to its definition.
Designated router contains only the best routes
Topological database elected on broadcast networks
Hello protocol contains all routes learned
Routing table provides dynamic neighbor discovery
14. Type the command that will disable OSPF on the Fa0/1 interface under the routing
process. Write only the command and not the prompt.
15. Which two of the following commands will place network 10.2.3.0/24 into area 0?
(Choose two.)
A. router eigrp 10
B. router ospf 10
C. router rip
D. network 10.0.0.0
E. network 10.2.3.0 255.255.255.0 area 0
F. network 10.2.3.0 0.0.0.255 area0
G. network 10.2.3.0 0.0.0.255 area 0
422 Chapter 9 u Open Shortest Path First (OSPF)
16. Given the following output, which statement or statements can be determined to be
true? (Choose all that apply.)
RouterA2# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.23.2 1 FULL/BDR 00:00:29 10.24.4.2 FastEthernet1/0
192.168.45.2 2 FULL/BDR 00:00:24 10.1.0.5 FastEthernet0/0
192.168.85.1 1 FULL/- 00:00:33 10.6.4.10 Serial0/1
192.168.90.3 1 FULL/DR 00:00:32 10.5.5.2 FastEthernet0/1
192.168.67.3 1 FULL/DR 00:00:20 10.4.9.20 FastEthernet0/2
192.168.90.1 1 FULL/BDR 00:00:23 10.5.5.4 FastEthernet0/1
<<output omitted>>
A. The DR for the network connected to Fa0/0 has an interface priority higher than 2.
B. This router (A2) is the BDR for subnet 10.1.0.0.
C. The DR for the network connected to Fa0/1 has a router ID of 10.5.5.2.
D. The DR for the serial subnet is 192.168.85.1.
17. What are three reasons for creating OSPF in a hierarchical design? (Choose three.)
A. To decrease routing overhead
B. To speed up convergence
C. To confine network instability to single areas of the network
D. To make configuring OSPF easier
Review Questions 423
18. Type the command that produced the following output. Write only the command and
not the prompt.
FastEthernet0/0 is up, line protocol is up
Internet Address 10.10.10.1/24, Area 0
Process ID 1, Router ID 223.255.255.254, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 223.255.255.254, Interface address 10.10.10.1
Backup Designated router (ID) 172.31.1.4, Interface address 10.10.10.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:08
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 3/3, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 172.31.1. Suppress hello for 0 neighbor(s)
19. A(n) ____________is an OSPF data packet containing link-state and routing informa-
tion that are shared among OSPF routers.
A. LSA
B. TSA
C. Hello
D. SPF
20. If routers in a single area are configured with the same priority value, what value does
a router use for the OSPF router ID in the absence of a loopback interface?
A. The lowest IP address of any physical interface
B. The highest IP address of any physical interface
C. The lowest IP address of any logical interface
D. The highest IP address of any logical interface
Chapter
10
Layer 2 Switching
THE FOLLOWING ICND1 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 LAN Switching Technologies
■Identify basic switching concepts and the operation of
Cisco switches.
■Collision Domains
■Broadcast Domains
■Types of switching
■CAM Table
■Configure and verify initial switch configuration including
remote access management.
■Cisco IOS commands to perform basic switch setup
■Verify network status and switch operation using basic
utilities such as Ping, Telnet and SSH.
1 Network Device Security
■Configure and verify Switch Port Security features such as:
■Sticky MAC
■MAC address limitation
■Static/dynamic
■Violation modes
■Err-disable
■Shutdown
■Protect restrict
■Shutdown unused ports
■Err-disable recovery
When people at Cisco discuss switching in regards to the Cisco
exam objectives, they’re talking about layer 2 switching unless
they say otherwise. Layer 2 switching is the process of using
the hardware address of devices on a LAN to segment a network. Since you’ve got the basic
idea of how that works nailed down by now, we’re going to dive deeper into the particulars
of layer 2 switching to ensure that your concept of how it works is solid and complete.
You already know that we rely on switching to break up large collision domains into
smaller ones and that a collision domain is a network segment with two or more devices
sharing the same bandwidth. A hub network is a typical example of this type of technology.
But since each port on a switch is actually its own collision domain, we were able to create
a much better Ethernet LAN network by simply replacing our hubs with switches!
Switches truly have changed the way networks are designed and implemented. If a pure
switched design is properly implemented, it absolutely will result in a clean, cost-effective,
and resilient internetwork. In this chapter, we’ll survey and compare how networks were
designed before and after switching technologies were introduced.
I’ll be using three switches to begin our conguration of a switched network, and we’ll actu-
ally continue with their congurations in Chapter 11, “VLANs and Inter-VLAN Routing.”
To find up-to-the-minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
Switching Services
Unlike old bridges, which used software to create and manage a Content Addressable
Memory (CAM) lter table, our new, fast switches use application-specic integrated cir-
cuits (ASICs) to build and maintain their MAC lter tables. But it’s still okay to think of
a layer 2 switch as a multiport bridge because their basic reason for being is the same: to
break up collision domains.
Layer 2 switches and bridges are faster than routers because they don’t take up time looking
at the Network layer header information. Instead, they look at the frame’s hardware addresses
before deciding to either forward, ood, or drop the frame.
Unlike hubs, switches create private, dedicated collision domains and provide indepen-
dent bandwidth exclusive on each port.
Switching Services 427
Here’s a list of four important advantages we gain when using Layer 2 switching:
u Hardware-based bridging (ASICs)
u Wire speed
u Low latency
u Low cost
A big reason layer 2 switching is so efcient is that no modication to the data packet
takes place. The device only reads the frame encapsulating the packet, which makes the
switching process considerably faster and less error-prone than routing processes are.
And if you use layer 2 switching for both workgroup connectivity and network segmen-
tation (breaking up collision domains), you can create more network segments than you
can with traditional routed networks. Plus, layer 2 switching increases bandwidth for each
user because, again, each connection, or interface into the switch, is its own, self-contained
collision domain.
Three Switch Functions at Layer 2
There are three distinct functions of layer 2 switching that are vital for you to remember:
address learning, forward/lter decisions, and loop avoidance.
Address learning Layer 2 switches remember the source hardware address of each frame
received on an interface and enter this information into a MAC database called a forward/
lter table.
Forward/filter decisions When a frame is received on an interface, the switch looks at the
destination hardware address, then chooses the appropriate exit interface for it in the MAC
database. This way, the frame is only forwarded out of the correct destination port.
Loop avoidance If multiple connections between switches are created for redundancy pur-
poses, network loops can occur. Spanning Tree Protocol (STP) is used to prevent network
loops while still permitting redundancy.
Next, I’m going to talk about address learning and forward/ltering decisions. Loop
avoidance is beyond the scope of the objectives being covered in this chapter.
Address Learning
When a switch is rst powered on, the MAC forward/lter table (CAM) is empty, as shown
in Figure10.1.
When a device transmits and an interface receives a frame, the switch places the frame’s
source address in the MAC forward/lter table, allowing it to refer to the precise interface
the sending device is located on. The switch then has no choice but to ood the network
with this frame out of every port except the source port because it has no idea where the
destination device is actually located.
If a device answers this ooded frame and sends a frame back, then the switch will take
the source address from that frame and place that MAC address in its database as well,
428 Chapter 10 u Layer 2 Switching
associating this address with the interface that received the frame. Because the switch now
has both of the relevant MAC addresses in its ltering table, the two devices can now make
a point-to-point connection. The switch doesn’t need to ood the frame as it did the rst
time because now the frames can and will only be forwarded between these two devices.
This is exactly why layer 2 switches are so superior to hubs. In a hub network, all frames
are forwarded out all ports every time—no matter what. Figure10.2 shows the processes
involved in building a MAC database.
FIGURE10.1 Empty forward/filter table on a switch
MAC forward/filter table
Fa0/0:
Fa0/1:
Fa0/2:
Fa0/3:
Fa0/0
Fa0/1 Fa0/2
Fa0/3
FIGURE10.2 How switches learn hosts’ locations
Fa0/0
Step 1 Fa0/1 Fa0/2
Fa0/3
CAM/MAC forward/filter table
Fa0/0:
Fa0/1:
0000.8c01.000A
0000.8c01.000B
Step 2
Step 4
Fa0/2:
Fa0/3:
34 3 3
ABCD
In this gure, you can see four hosts attached to a switch. When the switch is powered
on, it has nothing in its MAC address forward/lter table, just as in Figure10.1. But when
the hosts start communicating, the switch places the source hardware address of each frame
into the table along with the port that the frame’s source address corresponds to.
Let me give you an example of how a forward/lter table is populated using Figure10.2:
1. Host A sends a frame to Host B. Host A’s MAC address is 0000.8c01.000A; Host B’s
MAC address is 0000.8c01.000B.
2. The switch receives the frame on the Fa0/0 interface and places the source address in
the MAC address table.
Switching Services 429
3. Since the destination address isn’t in the MAC database, the frame is forwarded out all
interfaces except the source port.
4. Host B receives the frame and responds to Host A. The switch receives this frame on
interface Fa0/1 and places the source hardware address in the MAC database.
5. Host A and Host B can now make a point-to-point connection and only these specific
devices will receive the frames. Hosts C and D won’t see the frames, nor will their MAC
addresses be found in the database because they haven’t sent a frame to the switch yet.
If Host A and Host B don’t communicate to the switch again within a certain time
period, the switch will ush their entries from the database to keep it as current as possible.
Forward/Filter Decisions
When a frame arrives at a switch interface, the destination hardware address is compared to
the forward/lter MAC database. If the destination hardware address is known and listed in
the database, the frame is only sent out of the appropriate exit interface. The switch won’t
transmit the frame out any interface except for the destination interface, which preserves
bandwidth on the other network segments. This process is called frame ltering.
But if the destination hardware address isn’t listed in the MAC database, then the frame
will be ooded out all active interfaces except the interface it was received on. If a device
answers the ooded frame, the MAC database is then updated with the device’s location—
its correct interface.
If a host or server sends a broadcast on the LAN, by default, the switch will ood the
frame out all active ports except the source port. Remember, the switch creates smaller
collision domains, but it’s always still one large broadcast domain by default.
In Figure10.3, Host A sends a data frame to Host D. What do you think the switch will
do when it receives the frame from Host A?
FIGURE10.3 Forward/filter table
Fa0/3
Fa0/4 Fa0/5
Fa0/6
Switch# show mac address-table
VLAN
1
Mac Address
0005.dccb.d74b
Ports
Fa0/4
1 000a.f467.9e80 Fa0/5
1 000a.f467.9e8b Fa0/6
ABCD
Let’s examine Figure10.4 to nd the answer.
Since Host A’s MAC address is not in the forward/lter table, the switch will add the
source address and port to the MAC address table, then forward the frame to Host D. It’s
430 Chapter 10 u Layer 2 Switching
really important to remember that the source MAC is always checked rst to make sure it’s
in the CAM table. After that, if Host D’s MAC address wasn’t found in the forward/lter
table, the switch would’ve ooded the frame out all ports except for port Fa0/3 because
that’s the specic port the frame was received on.
FIGURE10.4 Forward/filter table answer
Fa0/3
ABCD
Fa0/4 Fa0/5
Fa0/6
Switch# show mac address-table
VLAN
1
Mac Address
00ca.345a.c7b9
Ports
Fa0/3
1 0005.dccb.d74b Fa0/4
1 000a.f467.9e80 Fa0/5
1 000a.f467.9e8b Fa0/6
Now let’s take a look at the output that results from using a show mac address-table
command:
Switch#sh mac address-table
Vlan Mac Address Type Ports
---- ----------- -------- -----
1 0005.dccb.d74b DYNAMIC Fa0/1
1 000a.f467.9e80 DYNAMIC Fa0/3
1 000a.f467.9e8b DYNAMIC Fa0/4
1 000a.f467.9e8c DYNAMIC Fa0/3
1 0010.7b7f.c2b0 DYNAMIC Fa0/3
1 0030.80dc.460b DYNAMIC Fa0/3
1 0030.9492.a5dd DYNAMIC Fa0/1
1 00d0.58ad.05f4 DYNAMIC Fa0/1
But let’s say the preceding switch received a frame with the following MAC addresses:
Source MAC: 0005.dccb.d74b
Destination MAC: 000a.f467.9e8c
How will the switch handle this frame? The right answer is that the destination MAC
address will be found in the MAC address table and the frame will only be forwarded out
Fa0/3. Never forget that if the destination MAC address isn’t found in the forward/lter
table, the frame will be forwarded out all of the switch’s ports except for the one on which
it was originally received in an attempt to locate the destination device. Now that you can
Switching Services 431
see the MAC address table and how switches add host addresses to the forward lter table,
how do think we can secure it from unauthorized users?
Port Security
It’s usually not a good thing to have your switches available for anyone to just plug into and
play around with. I mean, we worry about wireless security, so why wouldn’t we demand
switch security just as much, if not more?
But just how do we actually prevent someone from simply plugging a host into one of
our switch ports—or worse, adding a hub, switch, or access point into the Ethernet jack
in their ofce? By default, MAC addresses will just dynamically appear in your MAC
forward/lter database and you can stop them in their tracks by using port security!
Figure10.5 shows two hosts connected to the single switch port Fa0/3 via either a hub
or access point (AP).
FIGURE10.5 “Port security” on a switch port restricts port access by
MAC address.
Fa0/3
NO! Okay!
AB
Port Fa0/3 is congured to observe and allow only certain MAC addresses to associate
with the specic port, so in this example, Host A is denied access, but Host B is allowed to
associate with the port.
By using port security, you can limit the number of MAC addresses that can be assigned
dynamically to a port, set static MAC addresses, and—here’s my favorite part—set penalties
for users who abuse your policy! Personally, I like to have the port shut down when the secu-
rity policy is violated. Making abusers bring me a memo from their boss explaining why they
violated the security policy brings with it a certain poetic justice, which is nice. And I’ll also
require something like that before I’ll enable their port again. Things like this really seem to
help people remember to behave!
This is all good, but you still need to balance your particular security needs with the
time that implementing and managing them will realistically require. If you have tons of
time on your hands, then go ahead and seriously lock your network down vault-tight!
If you’re busy like the rest of us, I’m here to reassure you that there are ways to secure
things nicely without being totally overwhelmed with a massive amount of administrative
432 Chapter 10 u Layer 2 Switching
overhead. First, and painlessly, always remember to shut down unused ports or assign them
to an unused VLAN. All ports are enabled by default, so you need to make sure there’s no
access to unused switch ports!
Here are your options for conguring port security:
Switch#config t
Switch(config)#int f0/1
Switch(config-if)#switchport mode access
Switch(config-if)#switchport port-security
Switch(config-if)#switchport port-security ?
aging Port-security aging commands
mac-address Secure mac address
maximum Max secure addresses
violation Security violation mode
<cr>
Most Cisco switches ship with their ports in desirable mode, which means that those
ports will desire to trunk when sensing that another switch has just been connected. So
rst, we need to change the port out from desirable mode and make it an access port
instead. If we don’t do that, we won’t be able to congure port security on it at all! Once
that’s out of the way, we can move on using our port-security commands, never forget-
ting that we must enable port security on the interface. Notice that I did this after I made
the port an access port!
The preceding output clearly illustrates that the switchport port-security command
can be used with four options. You can use the switchport port-security mac-address
mac-address command to assign individual MAC addresses to each switch port, but be
warned because if you go with that option, you had better have boatloads of time on
your hands!
If you want to set up a switch port to allow only one host per port and make sure the
port will shut down if this rule is violated, use the following commands like this:
Switch(config-if)#switchport port-security maximum 1
Switch(config-if)#switchport port-security violation shutdown
These commands really are probably the most popular because they prevent random
users from connecting to a specic switch or access point that’s in their ofce. The
maximum setting is 1, which is the port security default that’s immediately set on a port
when it’s enabled. Sounds okay, but the drawback to this is that it only allows a single
MAC address to be used on the port, so if anyone, including you, tries to add another host
on that segment, the switch port will immediately shut down. And when that happens,
you have to manually go into the switch and re-enable the port by cycling it with a
shutdown and then a no shutdown command.
Switching Services 433
Probably one of my favorite commands is the sticky command and not just because it’s
got a cool name. It also makes very cool things happen! You can nd this command under
the mac-address command:
Switch(config-if)#switchport port-security mac-address sticky
Switch(config-if)#switchport port-security maximum 2
Switch(config-if)#switchport port-security violation shutdown
Basically, with the sticky command you can provide static MAC address security with-
out having to type in absolutely everyone’s MAC address on the network. I like things that
save me time like that!
In the preceding example, the rst two MAC addresses coming into the port “stick” to it
as static addresses and will be placed in the running-cong, but when a third address tried
to connect, the port would shut down immediately.
Let me show you one more example. Figure10.6 displays a host in a company lobby
that needs to be secured against the Ethernet cable used by anyone other than a single
authorized individual.
FIGURE10.6 Protecting a PC in a lobby
Fa0/1
What can you do to ensure that only the MAC address of the lobby PC is allowed by
switch port Fa0/1?
The solution is pretty straightforward because in this case, the defaults for port security
will work well. All I have left to do is add a static MAC entry:
Switch(config-if)#switchport port-security
Switch(config-if)#switchport port-security violation restrict
Switch(config-if)#switchport port-security mac-address aa.bb.cc.dd.ee.ff
To protect the lobby PC, we would set the maximum allowed MAC addresses to 1 and
the violation to restrict so the port didn’t get shut down every time someone tried to use
the Ethernet cable (which would be constantly). By using violation restrict, the unau-
thorized frames would just be dropped. But did you notice that I enabled port-security
and then set a static MAC address? Remember that as soon as you enable port-security
on a port, it defaults to violation shutdown and a maximum of 1. So all I needed to do
was change the violation mode and add the static MAC address and our business require-
ment is solidly met!
434 Chapter 10 u Layer 2 Switching
I’ll be going over port security again in the configuration examples later in
this chapter.
Lobby PC Always Being Disconnected Becomes a Security Risk
At a large Fortune 50 company in San Jose, CA, there was a PC in the lobby that held the
company directory. With no security guard present in the lobby, the Ethernet cable con-
necting the PC was free game to all vendors, contractors, and visitors waiting in the lobby.
Port security to the rescue! By enabling port security on the port with the switchport
port-security command, the switch port connecting to the PC was automatically secured
with the defaults of allowing only one MAC address to associate to the port and violation
shutdown. However, the port was always going into err-shutdown mode whenever anyone
tried to use the Ethernet port. By changing the violation mode to restrict and setting a static
MAC address for the port with the switchport port-security mac-address mac-address
command, only the Lobby PC was able to connect and communicate on the network! Prob-
lem solved!
Loop Avoidance
Redundant links between switches are important to have in place because they help prevent
nasty network failures in the event that one link stops working.
But while it’s true that redundant links can be extremely helpful, they can also cause
more problems than they solve! This is because frames can be ooded down all redundant
links simultaneously, creating network loops as well as other evils. Here’s a list of some of
the ugliest problems that can occur:
u If no loop avoidance schemes are put in place, the switches will flood broadcasts endlessly
throughout the internetwork. This is sometimes referred to as a broadcast storm. Most of
the time, they’re referred to in very unprintable ways! Figure10.7 illustrates how a broad-
cast can be propagated throughout the network. Observe how a frame is continually being
flooded through the internetwork’s physical network media.
u A device can receive multiple copies of the same frame because that frame can arrive
from different segments at the same time. Figure10.8 demonstrates how a whole bunch
of frames can arrive from multiple segments simultaneously. The server in the figure
sends a unicast frame to Router C. Because it’s a unicast frame, Switch A forwards the
frame and Switch B provides the same service—it forwards the unicast. This is bad
because it means that Router C receives that unicast frame twice, causing additional
overhead on the network.
Switching Services 435
u You may have thought of this one: The MAC address filter table could be totally con-
fused about the source device’s location because the switch can receive the frame from
more than one link. Worse, the bewildered switch could get so caught up in constantly
updating the MAC filter table with source hardware address locations that it will fail
to forward a frame! This is called thrashing the MAC table.
u One of the most vile events is when multiple loops propagate throughout a network.
Loops can occur within other loops, and if a broadcast storm were to occur simultane-
ously, the network wouldn’t be able to perform frame switching—period!
FIGURE10.7 Broadcast storm
Segment 1
Segment 2
Switch BSwitch A Broadcast
FIGURE10.8 Multiple frame copies
Segment 1
Segment 2
Switch ASwitch B
Unicast
Unicast
Unicast
Router C
436 Chapter 10 u Layer 2 Switching
All of these problems spell disaster or close and are all evil situations that must be avoided
or xed somehow. That’s where the Spanning Tree Protocol comes into play. It was actually
developed to solve each and every one of the problems I just told you about!
Now that I explained the issues that can occur when you have redundant links, or when
you have links that are improperly implemented, I’m sure you understand how vital it is to
prevent them. However, the best solutions are beyond the scope of this chapter and among
the territory covered in the more advanced Cisco exam objectives. For now, let’s focus on
conguring some switching!
Configuring Catalyst Switches
Cisco Catalyst switches come in many avors; some run 10 Mbps, while others can speed all
the way up to 10 Gbps switched ports with a combination of twisted-pair and ber. These
newer switches, like the 2960s and 3560s, also have more intelligence, so they can give you
data fast—mixed media services, too!
With that in mind, it’s time to show you how to start up and congure a Cisco Catalyst
switch using the command-line interface (CLI). After you get the basic commands down in
this chapter, I’ll show you how to congure virtual LANs (VLANs) plus Inter-Switch Link
(ISL), and 802.1q trunking in the next one.
Here’s a list of the basic tasks we’ll be covering next:
u Administrative functions
u Configuring the IP address and subnet mask
u Setting the IP default gateway
u Setting port security
u Testing and verifying the network
You can learn all about the Cisco family of Catalyst switches at
www.cisco.com/en/US/products/hw/switches/index.html.
Catalyst Switch Configuration
But before we actually get into conguring one of the Catalyst switches, I’ve got to ll you
in regarding the boot process of these switches, just as I did with the routers in Chapter 7,
“Managing a Cisco Internetwork.” Figure10.9 shows a typical Cisco Catalyst switch and
I need to tell you about the different interfaces and features of this device.
The rst thing I want to point out is that the console port for the Catalyst switches are
typically located on the back of the switch. Yet, on a smaller switch like the 3560 shown in
the gure, the console is right in the front to make it easier to use. (The eight-port 2960 looks
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Configuring Catalyst Switches 437
exactly the same.) If the POST completes successfully, the system LED turns green, but if the
POST fails, it will turn amber. And seeing that amber glow is an ominous thing—typically
fatal. So you may just want to keep a spare switch around—especially in case it’s a produc-
tion switch that’s croaked! The bottom button is used to show you which lights are providing
Power over Ethernet (PoE). You can see this by pressing the Mode button. The PoE is a very
nice feature of these switches. It allows me to power my access point and phone by just con-
necting them into the switch with an Ethernet cable—sweet.
FIGURE10.9 A Cisco Catalyst switch
Just as we did with the routers we congured in Chapter 8, “IP Routing,” and Chapter 9,
“Open Shortest Path First (OSPF),” we’ll use a diagram and switch setup to congure in
this chapter as well as in Chapter 11. Figure10.10 shows the switched network we’ll be
working on.
FIGURE10.10 Our switched network
F0/1
F0/2
F0/16 F0/18
F0/15 F0/17
F0/5F0/5
F0/6F0/6
F0/2
F0/1
192.168.10.18/28
192.168.10.17/28
192.168.10.19/28
F0/0
F0/8
F0/4F0/3 F0/4F0/3
S1
S3 S2
I’m going to use three 3560 switches, which I also used for demonstration in Chapter 6,
“Cisco’s Internetworking Operating System (IOS),” and Chapter 7. You can use any layer 2
switches for this chapter to follow the conguration, but when we get to Chapter 11, you’ll
need at least one router as well as a layer 3 switch, like my 3560.
Now if we connect our switches to each other, as shown in Figure10.10, remember that
rst we’ll need a crossover cable between the switches. My 3560 switches autodetect the
438 Chapter 10 u Layer 2 Switching
connection type, so I was able to use straight-through cables. But not all switches autode-
tect the cable type. Different switches have different needs and abilities, so just keep this in
mind when connecting your various switches together. Make a note that in the Cisco exam
objectives, switches never autodetect!
When you rst connect the switch ports to each other, the link lights are amber and then
turn green indicating normal operation. What you’re actually watching is spanning-tree
converging, and this process takes around 50 seconds with no extensions enabled. But if
you connect into a switch port and the switch port LED is alternating green and amber, it
means the port is experiencing errors. If this happens, check the host NIC or the cabling,
possibly even the duplex settings on the port to make sure they match the host setting.
Do We Need to Put an IP Address on a Switch?
Absolutely not! Switches have all ports enabled and ready to rock. Take the switch out of the
box, plug it in, and the switch starts learning MAC addresses in the CAM. So why would I
need an IP address since switches are providing layer 2 services? Because you still need it for
in-band management purposes! Telnet, SSH, SNMP, etc. all need an IP address in order to
communicate with the switch through the network (in-band). Remember, since all ports are
enabled by default, you need to shut down unused ports or assign them to an unused VLAN.
So where do we put this management IP address the switch needs for management pur-
poses? On what is predictably called the management VLAN interface—a routed interface on
every Cisco switch and called interface VLAN 1. This management interface can be changed,
and Cisco recommends that you do change this to a different management interface for secu-
rity purposes. No worries—I’ll demonstrate how to do this in Chapter 11.
Let’s congure our switches now so you can watch how I congure the management
interfaces on each switch.
S1
We’re going to begin our conguration by connecting into each switch and setting the admin-
istrative functions. We’ll also assign an IP address to each switch, but as I said, doing that isn’t
really necessary to make our network function. The only reason we’re going to do that is so
we can manage/administer it remotely, via Telnet for example. Let’s use a simple IP scheme like
192.168.10.16/28. This mask should be familiar to you! Check out the following output:
Switch>en
Switch#config t
Switch(config)#hostname S1
S1(config)#enable secret todd
S1(config)#int f0/15
S1(config-if)#description 1st connection to S3
S1(config-if)#int f0/16
S1(config-if)#description 2nd connection to S3
S1(config-if)#int f0/17
S1(config-if)#description 1st connection to S2
Configuring Catalyst Switches 439
S1(config-if)#int f0/18
S1(config-if)#description 2nd connection to S2
S1(config-if)#int f0/8
S1(config-if)#desc Connection to IVR
S1(config-if)#line con 0
S1(config-line)#password console
S1(config-line)#login
S1(config-line)#line vty 0 15
S1(config-line)#password telnet
S1(config-line)#login
S1(config-line)#int vlan 1
S1(config-if)#ip address 192.168.10.17 255.255.255.240
S1(config-if)#no shut
S1(config-if)#exit
S1(config)#banner motd #this is my S1 switch#
S1(config)#exit
S1#copy run start
Destination filename [startup-config]? [enter]
Building configuration...
[OK]
S1#
The rst thing to notice about this is that there’s no IP address congured on the switch’s
physical interfaces. Since all ports on a switch are enabled by default, there’s not really a whole
lot to congure! The IP address is congured under a logical interface, called a management
domain or VLAN. You can use the default VLAN 1 to manage a switched network just as
we’re doing here, or you can opt to use a different VLAN for management.
The rest of the conguration is basically the same as the process you go through for router
conguration. So remember… no IP addresses on physical switch interfaces, no routing pro-
tocols, and so on. We’re performing layer 2 switching at this point, not routing! Also, make a
note to self that there is no AUX port on Cisco switches.
S2
Here is the S2 conguration:
Switch#config t
Switch(config)#hostname S2
S2(config)#enable secret todd
S2(config)#int f0/1
S2(config-if)#desc 1st connection to S1
S2(config-if)#int f0/2
S2(config-if)#desc 2nd connection to s2
440 Chapter 10 u Layer 2 Switching
S2(config-if)#int f0/5
S2(config-if)#desc 1st connection to S3
S2(config-if)#int f0/6
S2(config-if)#desc 2nd connection to s3
S2(config-if)#line con 0
S2(config-line)#password console
S2(config-line)#login
S2(config-line)#line vty 0 15
S2(config-line)#password telnet
S2(config-line)#login
S2(config-line)#int vlan 1
S2(config-if)#ip address 192.168.10.18 255.255.255.240
S2(config)#exit
S2#copy run start
Destination filename [startup-config]?[enter]
Building configuration...
[OK]
S2#
We should now be able to ping from S2 to S1. Let’s try it:
S2#ping 192.168.10.17
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.10.17, timeout is 2 seconds:
.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 1/1/1 ms
S2#
Okay—now why did I get only four pings to work instead of ve? The rst period [.] is a
timeout, but the exclamation point [!] is a success.
It’s a good question, and here’s your answer: the rst ping didn’t work because of the time
that ARP takes to resolve the IP address to its corresponding hardware MAC address.
S3
Check out the S3 switch conguration:
Switch>en
Switch#config t
SW-3(config)#hostname S3
S3(config)#enable secret todd
S3(config)#int f0/1
S3(config-if)#desc 1st connection to S1
Configuring Catalyst Switches 441
S3(config-if)#int f0/2
S3(config-if)#desc 2nd connection to S1
S3(config-if)#int f0/5
S3(config-if)#desc 1st connection to S2
S3(config-if)#int f0/6
S3(config-if)#desc 2nd connection to S2
S3(config-if)#line con 0
S3(config-line)#password console
S3(config-line)#login
S3(config-line)#line vty 0 15
S3(config-line)#password telnet
S3(config-line)#login
S3(config-line)#int vlan 1
S3(config-if)#ip address 192.168.10.19 255.255.255.240
S3(config-if)#no shut
S3(config-if)#banner motd #This is the S3 switch#
S3(config)#exit
S3#copy run start
Destination filename [startup-config]?[enter]
Building configuration...
[OK]
S3#
Now let’s ping to S1 and S2 from the S3 switch and see what happens:
S3#ping 192.168.10.17
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.10.17, timeout is 2 seconds:
.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 1/3/9 ms
S3#ping 192.168.10.18
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.10.18, timeout is 2 seconds:
.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 1/3/9 ms
S3#sh ip arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 192.168.10.17 0 001c.575e.c8c0 ARPA Vlan1
Internet 192.168.10.18 0 b414.89d9.18c0 ARPA Vlan1
Internet 192.168.10.19 - ecc8.8202.82c0 ARPA Vlan1
S3#
442 Chapter 10 u Layer 2 Switching
In the output of the show ip arp command, the dash (-) in the minutes column means
that it is the physical interface of the device.
Now, before we move on to verifying the switch congurations, there’s one more com-
mand you need to know about, even though we don’t really need it in our current network
because we don’t have a router involved. It’s the ip default-gateway command. If you
want to manage your switches from outside your LAN, you must set a default gateway on
the switches just as you would with a host, and you do this from global cong. Here’s an
example where we introduce our router with an IP address using the last IP address in our
subnet range:
S3#config t
S3(config)#ip default-gateway 192.168.10.30
Now that we have all three switches basically congured, let’s have some fun with them!
Port Security
A secured switch port can associate anywhere from 1 to 8,192 MAC addresses, but the 3560s
I am using can support only 6,144, which seems like way more than enough to me. You can
choose to allow the switch to learn these values dynamically, or you can set static addresses
for each port using the switchport port-security mac-address mac-address command.
So let’s set port security on our S3 switch now. Ports Fa0/3 and Fa0/4 will have only one
device connected in our lab. By using port security, we’re assured that no other device can
connect once our hosts in ports Fa0/3 and in Fa0/4 are connected. Here’s how to easily do
that with just a couple commands:
S3#config t
S3(config)#int range f0/3-4
S3(config-if-range)#switchport mode access
S3(config-if-range)#switchport port-security
S3(config-if-range)#do show port-security int f0/3
Port Security : Enabled
Port Status : Secure-down
Violation Mode : Shutdown
Aging Time : 0 mins
Aging Type : Absolute
SecureStatic Address Aging : Disabled
Maximum MAC Addresses : 1
Total MAC Addresses : 0
Configured MAC Addresses : 0
Sticky MAC Addresses : 0
Last Source Address:Vlan : 0000.0000.0000:0
Security Violation Count : 0
Configuring Catalyst Switches 443
The rst command sets the mode of the ports to “access” ports. These ports must be access
or trunk ports to enable port security. By using the command switchport port-security on
the interface, I’ve enabled port security with a maximum MAC address of 1 and violation of
shutdown. These are the defaults, and you can see them in the highlighted output of the show
port-security int f0/3 command in the preceding code.
Port security is enabled, as displayed on the rst line, but the second line shows
Secure-down because I haven’t connected my hosts into the ports yet. Once I do, the
status will show Secure-up and would become Secure-shutdown if a violation occurs.
I’ve just got to point out this all-so-important fact one more time: It’s very important to
remember that you can set parameters for port security but it won’t work until you enable
port security at the interface level. Notice the output for port F0/6:
S3#config t
S3(config)#int range f0/6
S3(config-if-range)#switchport mode access
S3(config-if-range)#switchport port-security violation restrict
S3(config-if-range)#do show port-security int f0/6
Port Security : Disabled
Port Status : Secure-up
Violation Mode : restrict
[output cut]
Port Fa0/6 has been congured with a violation of shutdown, but the rst line shows that
port security has not been enabled on the port yet. Remember, you must use this command at
interface level to enable port security on a port:
S3(config-if-range)#switchport port-security
There are two other modes you can use instead of just shutting down the port. The
restrict and protect modes mean that another host can connect up to the maximum MAC
addresses allowed, but after the maximum has been met, all frames will just be dropped
and the port won’t be shut down. Additionally, both the restrict mode and shutdown vio-
lation modes alert you via SNMP that a violation has occurred on a port. You can then
call the abuser and tell them they’re so busted—you can see them, you know what they
did, and they’re in serious trouble!
If you’ve congured ports with the violation shutdown command, then the ports will
look like this when a violation occurs:
S3#sh port-security int f0/3
Port Security : Enabled
Port Status : Secure-shutdown
Violation Mode : Shutdown
Aging Time : 0 mins
Aging Type : Absolute
SecureStatic Address Aging : Disabled
444 Chapter 10 u Layer 2 Switching
Maximum MAC Addresses : 1
Total MAC Addresses : 2
Configured MAC Addresses : 0
Sticky MAC Addresses : 0
Last Source Address:Vlan : 0013:0ca69:00bb3:00ba8:1
Security Violation Count : 1
Here you can see that the port is in Secure-shutdown mode and the light for the port
would be amber. To enable the port again, you’d need to do the following:
S3(config-if)#shutdown
S3(config-if)#no shutdown
Let’s verify our switch congurations before we move onto VLANs in the next chapter.
Beware that even though some switches will show err-disabled instead of Secure-shutdown
as my switch shows, there is no difference between the two.
Verifying Cisco Catalyst Switches
The rst thing I like to do with any router or switch is to run through the congurations
with a show running-config command. Why? Because doing this gives me a really great
overview of each device. But it is time consuming, and showing you all the congs would
take up way too many pages in this book. Besides, we can instead run other commands
that will still stock us up with really good information.
For example, to verify the IP address set on a switch, we can use the show interface
command. Here’s the output:
S3#sh int vlan 1
Vlan1 is up, line protocol is up
Hardware is EtherSVI, address is ecc8.8202.82c0 (bia ecc8.8202.82c0)
Internet address is 192.168.10.19/28
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
[output cut]
The above output shows the interface is in up/up status. Remember to always check this
interface, either with this command or the show ip interface brief command. Lots of
people tend to forget that this interface is shutdown by default.
Never forget that IP addresses aren’t needed on a switch for it to operate.
The only reason we would set an IP address, mask, and default gateway is
for management purposes.
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Configuring Catalyst Switches 445
show mac address-table
I’m sure you remember being shown this command earlier in the chapter. Using it displays
the forward lter table, also called a content addressable memory (CAM) table. Here’s the
output from the S1 switch:
S3#sh mac address-table
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
All 0100.0ccc.cccc STATIC CPU
[output cut]
1 000e.83b2.e34b DYNAMIC Fa0/1
1 0011.1191.556f DYNAMIC Fa0/1
1 0011.3206.25cb DYNAMIC Fa0/1
1 001a.2f55.c9e8 DYNAMIC Fa0/1
1 001a.4d55.2f7e DYNAMIC Fa0/1
1 001c.575e.c891 DYNAMIC Fa0/1
1 b414.89d9.1886 DYNAMIC Fa0/5
1 b414.89d9.1887 DYNAMIC Fa0/6
The switches use things called base MAC addresses, which are assigned to the CPU.
The rst one listed is the base mac address of the switch. From the preceding output, you
can see that we have six MAC addresses dynamically assigned to Fa0/1, meaning that port
Fa0/1 is connected to another switch. Ports Fa0/5 and Fa0/6 only have one MAC address
assigned, and all ports are assigned to VLAN 1.
Let’s take a look at the S2 switch CAM and see what we can nd out.
S2#sh mac address-table
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
All 0100.0ccc.cccc STATIC CPU
[output cut
1 000e.83b2.e34b DYNAMIC Fa0/5
1 0011.1191.556f DYNAMIC Fa0/5
1 0011.3206.25cb DYNAMIC Fa0/5
1 001a.4d55.2f7e DYNAMIC Fa0/5
446 Chapter 10 u Layer 2 Switching
1 581f.aaff.86b8 DYNAMIC Fa0/5
1 ecc8.8202.8286 DYNAMIC Fa0/5
1 ecc8.8202.82c0 DYNAMIC Fa0/5
Total Mac Addresses for this criterion: 27
S2#
This output tells us that we have seven MAC addresses assigned to Fa0/5, which is our
connection to S3. But where’s port 6? Since port 6 is a redundant link to S3, STP placed
Fa0/6 into blocking mode.
Assigning Static MAC Addresses
You can set a static MAC address in the MAC address table, but like setting static MAC
port security without the sticky command, it’s a ton of work. Just in case you want to do
it, here’s how it’s done:
S3(config)#mac address-table ?
aging-time Set MAC address table entry maximum age
learning Enable MAC table learning feature
move Move keyword
notification Enable/Disable MAC Notification on the switch
static static keyword
S3(config)#mac address-table static aaaa.bbbb.ccc vlan 1 int fa0/7
S3(config)#do show mac address-table
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
All 0100.0ccc.cccc STATIC CPU
[output cut]
1 000e.83b2.e34b DYNAMIC Fa0/1
1 0011.1191.556f DYNAMIC Fa0/1
1 0011.3206.25cb DYNAMIC Fa0/1
1 001a.4d55.2f7e DYNAMIC Fa0/1
1 001b.d40a.0538 DYNAMIC Fa0/1
1 001c.575e.c891 DYNAMIC Fa0/1
1 aaaa.bbbb.0ccc STATIC Fa0/7
[output cut]
Total Mac Addresses for this criterion: 59
Exam Essentials 447
As shown on the left side of the output, you can see that a static MAC address has now
been assigned permanently to interface Fa0/7 and that it’s also been assigned to VLAN 1 only.
Now admit it—this chapter had a lot of great information, and you really did learn a lot
and, well, maybe even had a little fun along the way too! You’ve now congured and veried
all switches and set port security. That means you’re now ready to learn all about virtual
LANs! I’m going to save all our switch congurations so we’ll be able to start right from
here in Chapter 11.
Summary
In this chapter, I talked about the differences between switches and bridges and how they both
work at layer 2. They create MAC address forward/lter tables in order to make decisions on
whether to forward or ood a frame.
I also covered some problems that can occur if you have multiple links between bridges
(switches).
Finally, I covered detailed conguration of Cisco’s Catalyst switches, including verifying
the conguration.
Exam Essentials
Remember the three switch functions. Address learning, forward/lter decisions, and
loop avoidance are the functions of a switch.
Remember the command show mac address-table. The command show mac address-
table will show you the forward/lter table used on the LAN switch.
Understand the reason for port security. Port security restricts access to a switch based on
MAC addresses.
Know the command to enable port security. To enable port security on a port, you must
rst make sure the port is an access port and then use the switchport port-security
command at interface level. You can set the port security parameters before or after
enabling port security.
Know the commands to verify port security. To verify port security, use the show
port-security, show port-security interface interface, and show running-config
commands.
448 Chapter 10 u Layer 2 Switching
Written Lab 10
The answers to this lab can be found in Appendix A, “Answers to Written Labs.”
Write the answers to the following questions:
1. What command will show you the forward/filter table?
2. If a destination MAC address is not in the forward/filter table, what will the switch do
with the frame?
3. What are the three switch functions at layer 2?
4. If a frame is received on a switch port and the source MAC address is not in the for-
ward/filter table, what will the switch do?
5. What are the default modes for a switch port configured with port security?
Hands-on Labs
In this section, you will use the following switched network to congure your switching
labs. You can use any Cisco switches to do this lab, as well as LammleSim IOS version.
They do not need to be multilayer switches, just layer 2 switches.
F0/1
F0/2
F0/16 F0/18
F0/15 F0/17
F0/5F0/5
F0/6F0/6
F0/2
F0/1
192.168.10.18/28
192.168.10.17/28
192.168.10.19/28
F0/0
F0/8
F0/4F0/3 F0/4F0/3
S1
S3 S2
The rst lab (Lab 10.1) requires you to congure three switches, and then you will verify
them in Lab 10.2.
The labs in this chapter are as follows:
Hands-on Lab 10.1: Conguring Layer 2 Switches
Hands-on Lab 10.2 Verifying Layer 2 Switches
Hands-on Lab 10.3: Conguring Port Security
Hands-on Labs 449
Lab 10.1: Configuring Layer 2 Switches
In this lab, you will congure the three switches in the graphic:
1. Connect to the S1 switch and configure the following, not in any particular order:
u Hostname
u Banner
u Interface description
u Passwords
u IP address, subnet mask, default gateway
Switch>en
Switch#config t
Switch(config)#hostname S1
S1(config)#enable secret todd
S1(config)#int f0/15
S1(config-if)#description 1st connection to S3
S1(config-if)#int f0/16
S1(config-if)#description 2nd connection to S3
S1(config-if)#int f0/17
S1(config-if)#description 1st connection to S2
S1(config-if)#int f0/18
S1(config-if)#description 2nd connection to S2
S1(config-if)#int f0/8
S1(config-if)#desc Connection to IVR
S1(config-if)#line con 0
S1(config-line)#password console
S1(config-line)#login
S1(config-line)#line vty 0 15
S1(config-line)#password telnet
S1(config-line)#login
S1(config-line)#int vlan 1
S1(config-if)#ip address 192.168.10.17 255.255.255.240
S1(config-if)#no shut
S1(config-if)#exit
S1(config)#banner motd #this is my S1 switch#
S1(config)#exit
S1#copy run start
Destination filename [startup-config]? [enter]
Building configuration...
450 Chapter 10 u Layer 2 Switching
2. Connect to the S2 switch and configure all the settings you used in step 1. Do not for-
get to use a different IP address on the switch.
3. Connect to the S3 switch and configure all the settings you used in step 1 and 2. Do
not forget to use a different IP address on the switch.
Lab 10.2: Verifying Layer 2 Switches
Once you congure a device, you must be able to verify it.
1. Connect to each switch and verify the management interface.
S1#sh interface vlan 1
2. Connect to each switch and verify the CAM.
S1#sh mac address-table
3. Verify your configurations with the following commands:
S1#sh running-config
S1#sh ip int brief
Lab 10.3: Configuring Port Security
Port security is a big Cisco objective. Do not skip this lab!
1. Connect to your S3 switch.
2. Configure port Fa0/3 with port security.
S3#config t
S(config)#int fa0/3
S3(config-if#Switchport mode access
S3(config-if#switchport port-security
3. Check your default setting for port security.
S3#show port-security int f0/3
4. Change the settings to have a maximum of two MAC addresses that can associate to
interface Fa0/3.
S3#config t
S(config)#int fa0/3
S3(config-if#switchport port-security maximum 2
Hands-on Labs 451
5. Change the violation mode to restrict.
S3#config t
S(config)#int fa0/3
S3(config-if#switchport port-security violation restrict
6. Verify your configuration with the following commands:
S3#show port-security
S3#show port-security int fa0/3
S3#show running-config
452 Chapter 10 u Layer 2 Switching
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional questions,
please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. Which of the following statements is not true with regard to layer 2 switching?
A. Layer 2 switches and bridges are faster than routers because they don’t take up
time looking at the Data Link layer header information.
B. Layer 2 switches and bridges look at the frame’s hardware addresses before deciding
to either forward, flood, or drop the frame.
C. Switches create private, dedicated collision domains and provide independent
bandwidth on each port.
D. Switches use application-specific integrated circuits (ASICs) to build and maintain
their MAC filter tables.
2. Type the command that generated the last entry in the MAC address table shown.
Type the command only, without the prompt.
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
All 0100.0ccc.cccc STATIC CPU
[output cut]
1 000e.83b2.e34b DYNAMIC Fa0/1
1 0011.1191.556f DYNAMIC Fa0/1
1 0011.3206.25cb DYNAMIC Fa0/1
1 001a.4d55.2f7e DYNAMIC Fa0/1
1 001b.d40a.0538 DYNAMIC Fa0/1
1 001c.575e.c891 DYNAMIC Fa0/1
1 aaaa.bbbb.0ccc STATIC Fa0/7
Review Questions 453
3. In the diagram shown, what will the switch do if a frame with a destination MAC
address of 000a.f467.63b1 is received on Fa0/4? (Choose all that apply.)
Fa0/3
Fa0/4 Fa0/5
Fa0/6
Switch# show mac address-table
VLAN
1
Mac Address
0005.dccb.d74b
Ports
Fa0/4
1 000a.f467.9e80 Fa0/5
1 000a.f467.9e8b Fa0/6
ABCD
A. Drop the frame.
B. Send the frame out of Fa0/3.
C. Send the frame out of Fa0/4.
D. Send the frame out of Fa0/5.
E. Send the frame out of Fa0/6.
4. Write the command that generated the following output.
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
All 0100.0ccc.cccc STATIC CPU
[output cut]
1 000e.83b2.e34b DYNAMIC Fa0/1
1 0011.1191.556f DYNAMIC Fa0/1
1 0011.3206.25cb DYNAMIC Fa0/1
1 001a.2f55.c9e8 DYNAMIC Fa0/1
1 001a.4d55.2f7e DYNAMIC Fa0/1
1 001c.575e.c891 DYNAMIC Fa0/1
1 b414.89d9.1886 DYNAMIC Fa0/5
1 b414.89d9.1887 DYNAMIC Fa0/6
454 Chapter 10 u Layer 2 Switching
5. In the work area draw the functions of a switch from the list on the left to the right.
Address learning
Packet forwarding
Layer three security
Forward/filter decisions
Loop avoidance
Target 1
Target 2
Target 3
6. What statement(s) is/are true about the output shown below? (Choose all that apply.)
S3#sh port-security int f0/3
Port Security : Enabled
Port Status : Secure-shutdown
Violation Mode : Shutdown
Aging Time : 0 mins
Aging Type : Absolute
SecureStatic Address Aging : Disabled
Maximum MAC Addresses : 1
Total MAC Addresses : 2
Configured MAC Addresses : 0
Sticky MAC Addresses : 0
Last Source Address:Vlan : 0013:0ca69:00bb3:00ba8:1
Security Violation Count : 1
A. The port light for F0/3 will be amber in color.
B. The F0/3 port is forwarding frames.
C. This problem will resolve itself in a few minutes.
D. This port requires the shutdown command to function.
7. Write the command that would limit the number of MAC addresses allowed on a port
to 2. Write only the command and not the prompt.
Review Questions 455
8. Which of the following commands in the configuration, is a prerequisite for the other
commands to function?
S3#config t
S(config)#int fa0/3
S3(config-if#switchport port-security
S3(config-if#switchport port-security maximum 3
S3(config-if#switchport port-security violation restrict
S3(config-if#Switchport mode-security aging time 10
A. switchport mode-security aging time 10
B. switchport port-security
C. switchport port-security maximum 3
D. switchport port-security violation restrict
9. Which if the following is not an issue addressed by STP?
A. Broadcast storms
B. Gateway redundancy
C. A device receiving multiple copies of the same frame
D. Constant updating of the MAC filter table
10. What issue that arises when redundancy exists between switches is shown in the figure?
Segment 1
Segment 2
Switch BSwitch A
A. Broadcast storm
B. Routing loop
C. Port violation
D. Loss of gateway
456 Chapter 10 u Layer 2 Switching
11. Which two of the following switch port violation modes will alert you via SNMP that
a violation has occurred on a port?
A. Restrict
B. Protect
C. Shutdown
D. Err-disable
12. _______________ is the loop avoidance mechanism used by switches.
13. Write the command that must be present on any switch that you need to manage from
a different subnet.
14. On which interface have you configured an IP address for a switch?
A. int fa0/0
B. int vty 0 15
C. int vlan 1
D. int s/0/0
15. Which Cisco IOS command is used to verify the port security configuration of a
switch port?
A. show interfaces port-security
B. show port-security interface
C. show ip interface
D. show interfaces switchport
16. Write the command that will save a dynamically learned MAC address in the running-
configuration of a Cisco switch?
17. Which of the following methods will ensure that only one specific host can connect to
port F0/3 on a switch? (Choose two. Each correct answer is a separate solution.)
A. Configure port security on F0/3 to accept traffic other than that of the MAC
address of the host.
B. Configure the MAC address of the host as a static entry associated with port F0/3.
C. Configure an inbound access control list on port F0/3 limiting traffic to the IP
address of the host.
D. Configure port security on F0/3 to accept traffic only from the MAC address of
the host.
Review Questions 457
18. What will be the effect of executing the following command on port F0/1?
switch(config-if)# switchport port-security mac-address 00C0.35F0.8301
A. The command configures an inbound access control list on port F0/1, limiting
traffic to the IP address of the host.
B. The command expressly prohibits the MAC address of 00c0.35F0.8301 as an
allowed host on the switch port.
C. The command encrypts all traffic on the port from the MAC address of
00c0.35F0.8301.
D. The command statically defines the MAC address of 00c0.35F0.8301 as an
allowed host on the switch port.
19. The conference room has a switch port available for use by the presenter during classes,
and each presenter uses the same PC attached to the port. You would like to prevent
other PCs from using that port. You have completely removed the former configuration
in order to start anew. Which of the following steps is not required to prevent any other
PCs from using that port?
A. Enable port security.
B. Assign the MAC address of the PC to the port.
C. Make the port an access port.
D. Make the port a trunk port.
20. Write the command required to disable the port if a security violation occurs. Write
only the command and not the prompt.
Chapter
11
VLANs and InterVLAN
Routing
THE FOLLOWING ICND1 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 LAN Switching Technologies
■Describe how VLANs create logically separate networks and
the need for routing between them.
■Explain network segmentation and basic traffic
management concepts
■Configure and verify VLANs
■Configure and verify trunking on Cisco switches
■DTP
■Auto negotiation
1 IP Routing Technologies
■Configure and verify InterVLAN routing (Router on a stick)
■Sub interfaces
■Upstream routing
■Encapsulation
■Configure SVI interfaces
1 Troubleshooting
■Troubleshoot and Resolve VLAN problems
■Identify that VLANs are configured
■Port membership correct
■IP address configured
■Troubleshoot and Resolve trunking problems on Cisco switches
■Correct trunk states
■Correct encapsulation configured
■Correct VLANS allowed
I know I keep telling you this, but so you never forget it,
here I go, one last time: By default, switches break up col-
lision domains and routers break up broadcast domains.
Okay, I feel better! Now we can move on.
In contrast to the networks of yesterday that were based on collapsed backbones, today’s
network design is characterized by a atter architecture—thanks to switches. So now what?
How do we break up broadcast domains in a pure switched internetwork? By creating virtual
local area networks (VLANs). A VLAN is a logical grouping of network users and resources
connected to administratively dened ports on a switch. When you create VLANs, you’re
given the ability to create smaller broadcast domains within a layer 2 switched internetwork
by assigning different ports on the switch to service different subnetworks. A VLAN is treated
like its own subnet or broadcast domain, meaning that frames broadcast onto the network are
only switched between the ports logically grouped within the same VLAN.
So, does this mean we no longer need routers? Maybe yes; maybe no. It really depends on
what your particular networking needs and goals are. By default, hosts in a specic VLAN
can’t communicate with hosts that are members of another VLAN, so if you want interVLAN
communication, the answer is that you still need a router or Inter-VLAN Routing (IVR).
In this chapter, you’re going to comprehensively learn exactly what a VLAN is and how
VLAN memberships are used in a switched network. You’ll also become well-versed in
what a trunk link is and how to congure and verify them.
I’ll nish this chapter by demonstrating how you can make inter-VLAN communication
happen by introducing a router into a switched network. Of course, we’ll congure our
familiar switched network layout we used in the last chapter for creating VLANs and for
implementing trunking and Inter-VLAN routing on a layer 3 switch by creating Switched
Virtual Interfaces (SVIs).
To find up-to-the-minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
VLAN Basics
Figure11.1 illustrates the at network architecture that used to be so typical for layer 2
switched networks. With this conguration, every broadcast packet transmitted is seen by
every device on the network regardless of whether the device needs to receive that data or not.
VLAN Basics 461
FIGURE11.1 Flat network structure
Host A
By default, routers allow broadcasts to occur only within the originating network, while
switches forward broadcasts to all segments. Oh, and by the way, the reason it’s called a at
network is because it’s one broadcast domain, not because the actual design is physically at.
In Figure11.1 we see Host A sending out a broadcast and all ports on all switches forwarding
it—all except the port that originally received it.
Now check out Figure11.2. It pictures a switched network and shows Host A sending a
frame with Host D as its destination. Clearly, the important factor here is that the frame is
only forwarded out the port where Host D is located.
FIGURE11.2 The benefit of a switched network
Host A Host D
This is a huge improvement over the old hub networks, unless having one collision
domain by default is what you really want for some reason!
Okay—you already know that the biggest benet gained by having a layer 2 switched
network is that it creates individual collision domain segments for each device plugged into
each port on the switch. This scenario frees us from the old Ethernet density constraints
and makes us able to build larger networks. But too often, each new advance comes with
new issues. For instance, the more users and devices that populate and use a network, the
more broadcasts and packets each switch must handle.
462 Chapter 11 u VLANs and InterVLAN Routing
And there’s another big issue—security! This one is real trouble because within the
typical layer 2 switched internetwork, all users can see all devices by default. And you
can’t stop devices from broadcasting, plus you can’t stop users from trying to respond to
broadcasts. This means your security options are dismally limited to placing passwords
on your servers and other devices.
But wait—there’s hope if you create a virtual LAN (VLAN)! You can solve many of the
problems associated with layer 2 switching with VLANs, as you’ll soon see.
VLANs work like this: Figure11.3 shows all hosts in this very small company con-
nected to one switch, meaning all hosts will receive all frames, which is the default
behavior of all switches.
FIGURE11.3 One switch, one LAN: Before VLANs, there were no separations
between hosts.
Host A Host B Host C Host D
Sales Admin
If we want to separate the host’s data, we could either buy another switch or create
virtual LANs, as shown in Figure11.4.
FIGURE11.4 One switch, two virtual LANs (logical separation between hosts): Still
physically one switch, but this switch acts as many separate devices.
Host A Host B Host C Host D
Sales Admin
Still physically one switch;
Logically separate
In Figure11.4, I congured the switch to be two separate LANs, two subnets, two
broadcast domains, two VLANs—they all mean the same thing—without buying another
switch. We can do this 1,000 times on most Cisco switches, which saves thousands of dol-
lars and more!
VLAN Basics 463
Notice that even though the separation is virtual and the hosts are all still connected to
the same switch, the LANs can’t send data to each other by default. This is because they are
still separate networks, but no worries—we’ll get into inter-VLAN communication later in
this chapter.
Here’s a short list of ways VLANs simplify network management:
u Network adds, moves, and changes are achieved with ease by just configuring a port
into the appropriate VLAN.
u A group of users that need an unusually high level of security can be put into its own
VLAN so that users outside of that VLAN can’t communicate with it.
u As a logical grouping of users by function, VLANs can be considered independent
from their physical or geographic locations.
u VLANs greatly enhance network security if implemented correctly.
u VLANs increase the number of broadcast domains while decreasing their size.
Coming up, we’ll thoroughly explore the world of switching, and you learn exactly
how and why switches provide us with much better network services than hubs can in
our networks today.
Broadcast Control
Broadcasts occur in every protocol, but how often they occur depends upon three things:
u The type of protocol
u The application(s) running on the internetwork
u How these services are used
Some older applications have been rewritten to reduce their bandwidth consumption, but
there’s a new generation of applications that are so bandwidth greedy they’ll consume any
and all they can nd. These gluttons are the legion of multimedia applications that use both
broadcasts and multicasts extensively. As if they weren’t enough trouble, factors like faulty
equipment, inadequate segmentation, and poorly designed rewalls can seriously compound
the problems already caused by these broadcast-intensive applications. All of this has added a
major new dimension to network design and presents a bunch of new challenges for an admin-
istrator. Positively making sure your network is properly segmented so you can quickly isolate
a single segment’s problems to prevent them from propagating throughout your entire internet-
work is now imperative. And the most effective way to do that is through strategic switching
and routing!
Since switches have become more affordable, most everyone has replaced their at
hub networks with pure switched network and VLAN environments. All devices within
a VLAN are members of the same broadcast domain and receive all broadcasts relevant
to it. By default, these broadcasts are ltered from all ports on a switch that aren’t mem-
bers of the same VLAN. This is great because you get all the benets you would with
a switched design without getting hit with all the problems you’d have if all your users
were in the same broadcast domain—sweet!
464 Chapter 11 u VLANs and InterVLAN Routing
Security
But there’s always a catch, right? Time to get back to those security issues. A at internet-
work’s security used to be tackled by connecting hubs and switches together with routers.
So it was basically the router’s job to maintain security. This arrangement was pretty inef-
fective for several reasons. First, anyone connecting to the physical network could access the
network resources located on that particular physical LAN. Second, all anyone had to do
to observe any and all trafc traversing that network was to simply plug a network analyzer
into the hub. And similar to that last, scary, fact, users could easily join a workgroup by just
plugging their workstations into the existing hub. That’s about as secure as a barrel of honey
in a bear enclosure!
But that’s exactly what makes VLANs so cool. If you build them and create multiple broad-
cast groups, you can still have total control over each port and user! So the days when anyone
could just plug their workstations into any switch port and gain access to network resources
are history because now you get to control each port and any resources it can access.
And that’s not even all—VLANs can be created in harmony with a specic user’s need
for the network resources. Plus, switches can be congured to inform a network manage-
ment station about unauthorized access to those vital network resources. And if you need
inter-VLAN communication, you can implement restrictions on a router to make sure this
all happens securely. You can also place restrictions on hardware addresses, protocols, and
applications. Now we’re talking security—our honey barrel is now sealed tightly, made of
solid titanium and wrapped in razor wire!
Flexibility and Scalability
If you’ve been paying attention so far, you know that layer 2 switches only read frames
for ltering because they don’t look at the Network layer protocol. You also know that by
default, switches forward broadcasts to all ports. But if you create and implement VLANs,
you’re essentially creating smaller broadcast domains at layer 2.
As a result, broadcasts sent out from a node in one VLAN won’t be forwarded to ports
congured to belong to a different VLAN. But if we assign switch ports or users to VLAN
groups on a switch or on a group of connected switches, we gain the exibility to exclusively
add only the users we want to let into that broadcast domain regardless of their physical loca-
tion. This setup can also work to block broadcast storms caused by a faulty network interface
card (NIC) as well as prevent an intermediate device from propagating broadcast storms
throughout the entire internetwork. Those evils can still happen on the VLAN where the
problem originated, but the disease will be fully contained in that one ailing VLAN!
Another advantage is that when a VLAN gets too big, you can simply create more
VLANs to keep the broadcasts from consuming too much bandwidth. The fewer users in
a VLAN, the fewer users affected by broadcasts. This is all good, but you seriously need
to keep network services in mind and understand how the users connect to these services
when creating a VLAN. A good strategy is to try to keep all services, except for the email
and Internet access that everyone needs, local to all users whenever possible.
Identifying VLANs 465
Identifying VLANs
Switch ports are layer 2–only interfaces that are associated with a physical port that can
belong to only one VLAN if it’s an access port or all VLANs if it’s a trunk port.
Switches are denitely pretty busy devices. As myriad frames are switched throughout
the network, switches have to be able to keep track of all of them, plus understand what to
do with them depending on their associated hardware addresses. And remember—frames
are handled differently according to the type of link they’re traversing.
There are two different types of ports in a switched environment. Let’s take a look at the
rst type in Figure11.5.
FIGURE11.5 Access ports
Sales Admin
Access ports:
one for each VLAN
Access port
Sales Admin
Notice there are access ports for each host and an access port between switches—one
for each VLAN.
Access ports An access port belongs to and carries the trafc of only one VLAN. Trafc is
both received and sent in native formats with no VLAN information (tagging) whatsoever.
Anything arriving on an access port is simply assumed to belong to the VLAN assigned to the
port. Because an access port doesn’t look at the source address, tagged trafc—a frame with
added VLAN information—can be correctly forwarded and received only on trunk ports.
With an access link, this can be referred to as the congured VLAN of the port. Any device
attached to an access link is unaware of a VLAN membership—the device just assumes it’s
part of some broadcast domain. But it doesn’t have the big picture, so it doesn’t understand
the physical network topology at all.
466 Chapter 11 u VLANs and InterVLAN Routing
Another good bit of information to know is that switches remove any VLAN information
from the frame before it’s forwarded out to an access-link device. Remember that access-link
devices can’t communicate with devices outside their VLAN unless the packet is routed. Also,
you can only create a switch port to be either an access port or a trunk port—not both. So
you’ve got to choose one or the other and know that if you make it an access port, that port
can be assigned to one VLAN only. In Figure11.5, only the hosts in the Sales VLAN can talk
to other hosts in the same VLAN. This is the same with Admin VLAN, and they can both
communicate to hosts on the other switch because of an access link for each VLAN cong-
ured between switches.
Voice access ports Not to confuse you, but all that I just said about the fact that an
access port can be assigned to only one VLAN is really only sort of true. Nowadays,
most switches will allow you to add a second VLAN to an access port on a switch port
for your voice trafc, called the voice VLAN. The voice VLAN used to be called the aux-
iliary VLAN, which allowed it to be overlaid on top of the data VLAN, enabling both
types of trafc to travel through the same port. Even though this is technically considered
to be a different type of link, it’s still just an access port that can be congured for both
data and voice VLANs. This allows you to connect both a phone and a PC device to one
switch port but still have each device in a separate VLAN.
Trunk ports Believe it or not, the term trunk port was inspired by the telephone system
trunks, which carry multiple telephone conversations at a time. So it follows that trunk
ports can similarly carry multiple VLANs at a time as well.
A trunk link is a 100, 1,000, or 10,000 Mbps point-to-point link between two switches,
between a switch and router, or even between a switch and server, and it carries the trafc
of multiple VLANs—from 1 to 4,094 VLANs at a time. But the amount is really only up to
1,001 unless you’re going with something called extended VLANs.
Instead of an access link for each VLAN between switches, we’ll create a trunk link
demonstrated in Figure11.6.
Trunking can be a real advantage because with it, you get to make a single port part of
a whole bunch of different VLANs at the same time. This is a great feature because you
can actually set ports up to have a server in two separate broadcast domains simultane-
ously so your users won’t have to cross a layer 3 device (router) to log in and access it.
Another benet to trunking comes into play when you’re connecting switches. Trunk
links can carry the frames of various VLANs across them, but by default, if the links
between your switches aren’t trunked, only information from the congured access
VLAN will be switched across that link.
It’s also good to know that all VLANs send information on a trunked link unless you clear
each VLAN by hand, and no worries, I’ll show you how to clear individual VLANs from a
trunk in a bit.
Okay—it’s nally time to tell you about frame tagging and the VLAN identication
methods used in it across our trunk links.
Identifying VLANs 467
FIGURE11.6 VLANs can span across multiple switches by using trunk links, which
carry traffic for multiple VLANs.
Sales Admin
Access port
Sales Admin
Trunk link
Frame Tagging
As you now know, you can set up your VLANs to span more than one connected switch.
You can see that going on in Figure11.6, which depicts hosts from two VLANs spread
across two switches. This exible, power-packed capability is probably the main advantage
to implementing VLANs, and we can do this with up to a thousand VLANs and thousands
upon thousands of hosts!
All this can get kind of complicated—even for a switch—so there needs to be a way
for each one to keep track of all the users and frames as they travel the switch fabric and
VLANs. When I say, “switch fabric,” I’m just referring to a group of switches that share the
same VLAN information. And this just happens to be where frame tagging enters the scene.
This frame identication method uniquely assigns a user-dened VLAN ID to each frame.
Here’s how it works: Once within the switch fabric, each switch that the frame reaches
must rst identify the VLAN ID from the frame tag. It then nds out what to do with the
frame by looking at the information in what’s known as the lter table. If the frame reaches a
switch that has another trunked link, the frame will be forwarded out of the trunk-link port.
Once the frame reaches an exit that’s determined by the forward/lter table to be an
access link matching the frame’s VLAN ID, the switch will remove the VLAN identier.
This is so the destination device can receive the frames without being required to under-
stand their VLAN identication information.
468 Chapter 11 u VLANs and InterVLAN Routing
Another great thing about trunk ports is that they’ll support tagged and untagged trafc
simultaneously if you’re using 802.1q trunking, which we will talk about next. The trunk
port is assigned a default port VLAN ID (PVID) for a VLAN upon which all untagged trafc
will travel. This VLAN is also called the native VLAN and is always VLAN 1 by default, but
it can be changed to any VLAN number.
Similarly, any untagged or tagged trafc with a NULL (unassigned) VLAN ID is
assumed to belong to the VLAN with the port default PVID. Again, this would be
VLAN 1 by default. A packet with a VLAN ID equal to the outgoing port native VLAN
is sent untagged and can communicate to only hosts or devices in that same VLAN. All
other VLAN trafc has to be sent with a VLAN tag to communicate within a particular
VLAN that corresponds with that tag.
VLAN Identification Methods
VLAN identication is what switches use to keep track of all those frames as they’re tra-
versing a switch fabric. It’s how switches identify which frames belong to which VLANs,
and there’s more than one trunking method.
Inter-Switch Link (ISL)
Inter-Switch Link (ISL) is a way of explicitly tagging VLAN information onto an Ethernet
frame. This tagging information allows VLANs to be multiplexed over a trunk link through
an external encapsulation method. This allows the switch to identify the VLAN membership
of a frame received over the trunked link.
By running ISL, you can interconnect multiple switches and still maintain VLAN informa-
tion as trafc travels between switches on trunk links. ISL functions at layer 2 by encapsulat-
ing a data frame with a new header and by performing a new cyclic redundancy check (CRC).
Of note is that ISL is proprietary to Cisco switches and it’s used for Fast Ethernet and
Gigabit Ethernet links only. ISL routing is pretty versatile and can be used on a switch
port, router interfaces, and server interface cards to trunk a server. Although some Cisco
switches still support ISL frame tagging, Cisco is moving toward using only 802.1q.
IEEE 802.1q
Created by the IEEE as a standard method of frame tagging, IEEE 802.1q actually inserts a
eld into the frame to identify the VLAN. If you’re trunking between a Cisco switched link
and a different brand of switch, you’ve got to use 802.1q for the trunk to work.
Unlike ISL, which encapsulates the frame with control information, 802.1q inserts an
802.1q eld along with tag control information, as shown in Figure11.7.
For the Cisco exam objectives, it’s only the 12-bit VLAN ID that matters. This eld
identies the VLAN and can be 212, minus 2 for the 0 and 4,095 reserved VLANs, which
means an 802.1q tagged frame can carry information for 4,094 VLANs.
It works like this: You rst designate each port that’s going to be a trunk with 802.1q
encapsulation. The other ports must be assigned a specic VLAN ID in order for them to
communicate. VLAN 1 is the default native VLAN, and when using 802.1q, all trafc for
a native VLAN is untagged. The ports that populate the same trunk create a group with
Routing between VLANs 469
this native VLAN and each port gets tagged with an identication number reecting that.
Again the default is VLAN 1. The native VLAN allows the trunks to accept information
that was received without any VLAN identication or frame tag.
FIGURE11.7 IEEE 802.1q encapsulation with and without the 802.1q tag
Preamble
(7-bytes)
Start
Frame
Delimiter
(1-byte)
Destination
MAC Address
(6-bytes)
Source MAC
Address
(6-bytes)
Type/Length
(2-bytes)
Packet
(0 – n bytes)
Pad
(0 – p bytes)
Frame Check
Sequence
(4-bytes)
Preamble
(7-bytes)
Start
Frame
Delimiter
(1-byte)
Destination
MAC Address
(6-bytes)
Source MAC
Address
(6-bytes)
Type/Length
= 802.1Q Tag
Type
(2-bytes)
802.1q Field
inserted
3 bits = User priority field
1 bit = Canonical Format Identifier (CFI)
12 bits – VLAN Identifier (VLAN ID)
CRC must be recalculated
Tag Control
Information Length/Type
(2-bytes)
Packet
(0 – n bytes)
Pad
(0 – p bytes)
Frame Check
Sequence
(4-bytes)
Most 2960 model switches only support the IEEE 802.1q trunking protocol, but the
3560 will support both the ISL and IEEE methods, which you’ll see later in this chapter.
The basic purpose of ISL and 802.1q frame-tagging methods is to provide
inter-switch VLAN communication. Remember that any ISL or 802.1q frame
tagging is removed if a frame is forwarded out an access link—tagging is
used internally and across trunk links only!
Routing between VLANs
Hosts in a VLAN live in their own broadcast domain and can communicate freely. VLANs
create network partitioning and trafc separation at layer 2 of the OSI, and as I said when
I told you why we still need routers, if you want hosts or any other IP-addressable device to
communicate between VLANs, you must have a layer 3 device to provide routing.
For this, you can use a router that has an interface for each VLAN or a router that sup-
ports ISL or 802.1q routing. The least expensive router that supports ISL or 802.1q routing
is the 2600 series router. You’d have to buy that from a used-equipment reseller because they
are end-of-life, or EOL. I’d recommend at least a 2800 as a bare minimum, but even that
only supports 802.1q; Cisco is really moving away from ISL, so you probably should only
be using 802.1q anyway. Some 2800s may support both ISL and 802.1q; I’ve just have never
seen it supported.
470 Chapter 11 u VLANs and InterVLAN Routing
Anyway, as shown in Figure11.8, if you had two or three VLANs, you could get by
with a router equipped with two or three FastEthernet connections. And 10Base-T is okay
for home study purposes, and I mean only for your studies, but for anything else I’d highly
recommend Gigabit interfaces for real power under the hood!
What we see in Figure11.8 is that each router interface is plugged into an access link.
This means that each of the routers’ interface IP addresses would then become the default
gateway address for each host in each respective VLAN.
FIGURE11.8 Router connecting three VLANs together for inter-VLAN
communication, one router interface for each VLAN
Gi0/0 Gi0/2
G0/1
If you have more VLANs available than router interfaces, you can congure trunking
on one FastEthernet interface or buy a layer 3 switch, like the Cisco 3560 or a higher-end
switch like a 3750. You could even opt for a 6500 if you’ve got money to burn!
Instead of using a router interface for each VLAN, you can use one FastEthernet inter-
face and run ISL or 802.1q trunking. Figure11.9 shows how a FastEthernet interface on a
router will look when congured with ISL or 802.1q trunking. This allows all VLANs to
communicate through one interface. Cisco calls this a router on a stick (ROAS).
FIGURE11.9 Router on a stick: Single router interface connecting all three VLANs
together for inter-VLAN communication
Gi0/0
Routing between VLANs 471
I really want to point out that this creates a potential bottleneck, as well as a single point
of failure, so your host/VLAN count is limited. To how many? Well, that depends on your
trafc level. To really make things right, you’d be better off using a higher-end switch and
routing on the backplane. But if you just happen to have a router sitting around, conguring
this method is free, right?
Figure11.10 shows how we would create a router on a stick using a router’s physical
interface by creating logical interfaces—one for each VLAN.
FIGURE11.10 A router creates logical interfaces.
GigabitEthernet 0/0.1
GigabitEthernet 0/0.2
GigabitEthernet 0/0.3
GigabitEthernet 0/0
Here we see one physical interface divided into multiple subinterfaces, with one sub-
net assigned per VLAN, each subinterface being the default gateway address for each
VLAN/subnet. An encapsulation identier must be assigned to each subinterface to
dene the VLANID of that subinterface. In the next section where I’ll congure VLANs
and inter-VLAN routing, I’ll congure our switched network with a router on a stick
and demonstrate this conguration for you.
But wait, there’s still one more way to go about routing! Instead of using an external
router interface for each VLAN, or an external router on a stick, we can congure logical
interfaces on the backplane of the layer 3 switch; this is called inter-VLAN routing (IVR),
and is congured with a switched virtual interface (SVI). Figure11.11 shows how hosts see
these virtual interfaces.
FIGURE11.11 With IVR, routing runs on the backplane of the switch, and it
appears to the hosts that a router is present.
VLAN 10
VLAN 10
VLAN 20
3560
VLAN 20
I’m Virtual!
In Figure11.11, it appears there’s a router present, but there is no physical router present
as there was when we used router on a stick. The IVR process takes little effort and is easy
to implement, which makes it very cool! Plus, it’s a lot more efcient for inter-VLAN routing
than an external router is. To implement IVR on a multilayer switch, we just need to create
logical interfaces in the switch conguration for each VLAN. We’ll congure this method
in a minute, but rst let’s take our existing switched network from Chapter 10, “Layer 2
472 Chapter 11 u VLANs and InterVLAN Routing
Switching,” and add some VLANs, then congure VLAN memberships and trunk links
between our switches.
Configuring VLANs
Now this may come as a surprise to you, but conguring VLANs is actually pretty easy. It’s
just that guring out which users you want in each VLAN is not, and doing that can eat up
a lot of your time! But once you’ve decided on the number of VLANs you want to create and
established which users you want belonging to each one, it’s time to bring your rst VLAN
into the world.
To congure VLANs on a Cisco Catalyst switch, use the global cong vlan command.
In the following example, I’m going to demonstrate how to congure VLANs on the S1
switch by creating three VLANs for three different departments—again, remember that
VLAN 1 is the native and management VLAN by default:
S1(config)#vlan ?
WORD ISL VLAN IDs 1-4094
access-map Create vlan access-map or enter vlan access-map command mode
dot1q dot1q parameters
filter Apply a VLAN Map
group Create a vlan group
internal internal VLAN
S1(config)#vlan 2
S1(config-vlan)#name Sales
S1(config-vlan)#vlan 3
S1(config-vlan)#name Marketing
S1(config-vlan)#vlan 4
S1(config-vlan)#name Accounting
S1(config-vlan)#^Z
S1#
In this output, you can see that you can create VLANs from 1 to 4094. But this is only
mostly true. As I said, VLANs can really only be created up to 1001, and you can’t use,
change, rename, or delete VLANs 1 or 1002 through 1005 because they’re reserved. The
VLAN numbers above 1005 are called extended VLANs and won’t be saved in the data-
base unless your switch is set to what is called VLAN Trunk Protocol (VTP) transparent
mode. You won’t see these VLAN numbers used too often in production. Here’s an exam-
ple of me attempting to set my S1 switch to VLAN 4000 when my switch is set to VTP
server mode (the default VTP mode):
S1#config t
S1(config)#vlan 4000
S1(config-vlan)#^Z
Visit ccna
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for a
companion
MicroNugget
from CBT
Nuggets.
Configuring VLANs 473
% Failed to create VLANs 4000
Extended VLAN(s) not allowed in current VTP mode.
%Failed to commit extended VLAN(s) changes.
After you create the VLANs that you want, you can use the show vlan command to
check them out. But notice that, by default, all ports on the switch are in VLAN 1. To
change the VLAN associated with a port, you need to go to each interface and specically
tell it which VLAN to be a part of.
Remember that a created VLAN is unused until it is assigned to a switch
port or ports and that all ports are always assigned in VLAN 1 unless set
otherwise.
Once the VLANs are created, verify your conguration with the show vlan command
(sh vlan for short):
S1#sh vlan
VLAN Name Status Ports
---- ------------------------- --------- -------------------------------
1 default active Fa0/1, Fa0/2, Fa0/3, Fa0/4
Fa0/5, Fa0/6, Fa0/7, Fa0/8
Fa0/9, Fa0/10, Fa0/11, Fa0/12
Fa0/13, Fa0/14, Fa0/19, Fa0/20
Fa0/21, Fa0/22, Fa0/23, Gi0/1
Gi0/2
2 Sales active
3 Marketing active
4 Accounting active
[output cut]
This may seem repetitive, but it’s important, and I want you to remember it: You can’t
change, delete, or rename VLAN 1 because it’s the default VLAN and you just can’t change
that—period. It’s also the native VLAN of all switches by default, and Cisco recommends that
you use it as your management VLAN. If you’re worried about security issues, then change it!
Basically, any ports that aren’t specically assigned to a different VLAN will be sent down to
the native VLAN—VLAN 1.
In the preceding S1 output, you can see that ports Fa0/1 through Fa0/14, Fa0/19 through
23, and Gi0/1 and Gi02 uplinks are all in VLAN 1. But where are ports 15 through 18? First,
understand that the command show vlan only displays access ports, so now that you know
what you’re looking at with the show vlan command, where do you think ports Fa15–18
are? That’s right! They are trunked ports. Cisco switches run a proprietary protocol called
Dynamic Trunk Protocol (DTP), and if there is a compatible switch connected, they will
474 Chapter 11 u VLANs and InterVLAN Routing
start trunking automatically, which is precisely where my four ports are. You have to use the
show interfaces trunk command to see your trunked ports like this:
S1# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Fa0/15 desirable n-isl trunking 1
Fa0/16 desirable n-isl trunking 1
Fa0/17 desirable n-isl trunking 1
Fa0/18 desirable n-isl trunking 1
Port Vlans allowed on trunk
Fa0/15 1-4094
Fa0/16 1-4094
Fa0/17 1-4094
Fa0/18 1-4094
[output cut]
This output reveals that the VLANs from 1 to 4094 are allowed across the trunk by
default. Another helpful command, which is also part of the Cisco exam objectives, is the
show interfaces interface switchport command:
S1#sh interfaces fastEthernet 0/15 switchport
Name: Fa0/15
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: trunk
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: isl
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
[output cut]
The highlighted output shows us the administrative mode of dynamic desirable, that the
port is a trunk port, and that DTP was used to negotiate the frame tagging method of ISL. It
also predictably shows that the native VLAN is the default of 1.
Now that we can see the VLANs created, we can assign switch ports to specic ones.
Each port can be part of only one VLAN, with the exception of voice access ports. Using
trunking, you can make a port available to trafc from all VLANs. I’ll cover that next.
Configuring VLANs 475
Assigning Switch Ports to VLANs
You congure a port to belong to a VLAN by assigning a membership mode that species
the kind of trafc the port carries plus the number of VLANs it can belong to. You can also
congure each port on a switch to be in a specic VLAN (access port) by using the inter-
face switchport command. You can even congure multiple ports at the same time with
the interface range command.
In the next example, I’ll congure interface Fa0/3 to VLAN 3. This is the connection
from the S3 switch to the host device:
S3#config t
S3(config)#int fa0/3
S3(config-if)#switchport ?
access Set access mode characteristics of the interface
autostate Include or exclude this port from vlan link up calculation
backup Set backup for the interface
block Disable forwarding of unknown uni/multi cast addresses
host Set port host
mode Set trunking mode of the interface
nonegotiate Device will not engage in negotiation protocol on this
interface
port-security Security related command
priority Set appliance 802.1p priority
private-vlan Set the private VLAN configuration
protected Configure an interface to be a protected port
trunk Set trunking characteristics of the interface
voice Voice appliance attributes voice
Well now, what do we have here? There’s some new stuff showing up in our output now.
We can see various commands—some that I’ve already covered, but no worries because I’m
going to cover the access, mode, nonegotiate, and trunk commands very soon. Let’s start
with setting an access port on S1, which is probably the most widely used type of port you’ll
nd on production switches that have VLANs congured:
S3(config-if)#switchport mode ?
access Set trunking mode to ACCESS unconditionally
dot1q-tunnel set trunking mode to TUNNEL unconditionally
dynamic Set trunking mode to dynamically negotiate access or trunk mode
private-vlan Set private-vlan mode
trunk Set trunking mode to TRUNK unconditionally
S3(config-if)#switchport mode access
S3(config-if)#switchport access vlan 3
476 Chapter 11 u VLANs and InterVLAN Routing
By starting with the switchport mode access command, you’re telling the switch that this
is a nontrunking layer 2 port. You can then assign a VLAN to the port with the switchport
access command. Remember, you can choose many ports to congure simultaneously with
the interface range command.
Let’s take a look at our VLANs now:
S3#show vlan
VLAN Name Status Ports
---- ------------------------ --------- -------------------------------
1 default active Fa0/4, Fa0/5, Fa0/6, Fa0/7
Fa0/8, Fa0/9, Fa0/10, Fa0/11,
Fa0/12, Fa0/13, Fa0/14, Fa0/19,
Fa0/20, Fa0/21, Fa0/22, Fa0/23,
Gi0/1 ,Gi0/2
2 Sales active
3 Marketing active Fa0/3
Notice that port Fa0/3 is now a member of VLAN 3. But, can you tell me where ports 1
and 2 are? And why aren’t they showing up in the output of show vlan? That’s right, because
they are trunk ports!
We can also see this with the show interfaces interface switchport command:
S3#sh int fa0/3 switchport
Name: Fa0/3
Switchport: Enabled
Administrative Mode: static access
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Negotiation of Trunking: Off
Access Mode VLAN: 3 (Marketing)
The highlighted output shows that Fa0/3 is an access port and a member of VLAN 3
(Marketing).
That’s it. Well, sort of. If you plugged devices into each VLAN port, they can only talk
to other devices in the same VLAN. But as soon as you learn a bit more about trunking,
we’re going to enable inter-VLAN communication!
Configuring Trunk Ports
The 2960 switch only runs the IEEE 802.1q encapsulation method. To congure trunking
on a FastEthernet port, use the interface command switchport mode trunk. It’s a tad dif-
ferent on the 3560 switch.
Configuring VLANs 477
The following switch output shows the trunk conguration on interfaces Fa0/15–18 as
set to trunk:
S1(config)#int range f0/15-18
S1(config-if-range)#switchport trunk encapsulation dot1q
S1(config-if-range)#switchport mode trunk
If you have a switch that only runs the 802.1q encapsulation method, then you wouldn’t
use the encapsulation command as I did in the preceding output. Let’s check out our trunk
ports now:
S1(config-if-range)#do sh int f0/15 swi
Name: Fa0/15
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
Notice that port Fa0/15 is a trunk and running 802.1q. Let’s take another look:
S1(config-if-range)#do sh int trunk
Port Mode Encapsulation Status Native vlan
Fa0/15 on 802.1q trunking 1
Fa0/16 on 802.1q trunking 1
Fa0/17 on 802.1q trunking 1
Fa0/18 on 802.1q trunking 1
Port Vlans allowed on trunk
Fa0/15 1-4094
Fa0/16 1-4094
Fa0/17 1-4094
Fa0/18 1-4094
Take note of the fact that ports 15–18 are now in the trunk mode of on and the encap-
sulation is now 802.1q instead of the negotiated ISL. Here’s a description of the different
options available when conguring a switch interface:
switchport mode access I discussed this in the previous section, but this puts the inter-
face (access port) into permanent nontrunking mode and negotiates to convert the link into a
478 Chapter 11 u VLANs and InterVLAN Routing
nontrunk link. The interface becomes a nontrunk interface regardless of whether the neigh-
boring interface is a trunk interface. The port would be a dedicated layer 2 access port.
switchport mode dynamic auto This mode makes the interface able to convert the link
to a trunk link. The interface becomes a trunk interface if the neighboring interface is set
to trunk or desirable mode. The default is dynamic auto on a lot of Cisco switches, but that
default trunk method is changing to dynamic desirable on most new models.
switchport mode dynamic desirable This one makes the interface actively attempt to
convert the link to a trunk link. The interface becomes a trunk interface if the neighboring
interface is set to trunk, desirable, or auto mode. I used to see this mode as the default
on some switches, but not any longer. This is now the default switch port mode for all
Ethernet interfaces on all new Cisco switches.
switchport mode trunk Puts the interface into permanent trunking mode and negotiates
to convert the neighboring link into a trunk link. The interface becomes a trunk interface
even if the neighboring interface isn’t a trunk interface.
switchport nonegotiate Prevents the interface from generating DTP frames. You can
use this command only when the interface switchport mode is access or trunk. You must
manually congure the neighboring interface as a trunk interface to establish a trunk link.
Dynamic Trunking Protocol (DTP) is used for negotiating trunking on a link
between two devices as well as negotiating the encapsulation type of either
802.1q or ISL. I use the nonegotiate command when I want dedicated trunk
ports; no questions asked.
To disable trunking on an interface, use the switchport mode access command, which
sets the port back to a dedicated layer 2 access switch port.
Defining the Allowed VLANs on a Trunk
As I’ve mentioned, trunk ports send and receive information from all VLANs by default,
and if a frame is untagged, it’s sent to the management VLAN. Understand that this applies
to the extended range VLANs too.
But we can remove VLANs from the allowed list to prevent trafc from certain VLANs
from traversing a trunked link. I’ll show you how you’d do that, but rst let me again dem-
onstrate that all VLANs are allowed across the trunk link by default:
S1#sh int trunk
[output cut]
Port Vlans allowed on trunk
Fa0/15 1-4094
Fa0/16 1-4094
Fa0/17 1-4094
Fa0/18 1-4094
Configuring VLANs 479
S1(config)#int f0/15
S1(config-if)#switchport trunk allowed vlan 4,6,12,15
S1(config-if)#do show int trunk
[output cut]
Port Vlans allowed on trunk
Fa0/15 4,6,12,15
Fa0/16 1-4094
Fa0/17 1-4094
Fa0/18 1-4094
The preceding command affected the trunk link congured on S1 port F0/15, causing it to
drop all trafc sent and received for VLANs 4, 6, 12, and 15. You can try to remove VLAN1
on a trunk link, but it will still send and receive management like CDP, DTP, and VTP, so
what’s the point?
To remove a range of VLANs, just use the hyphen:
S1(config-if)#switchport trunk allowed vlan remove 4-8
If by chance someone has removed some VLANs from a trunk link and you want to set
the trunk back to default, just use this command:
S1(config-if)#switchport trunk allowed vlan all
Next, I want to show you how to congure a native VLAN for a trunk before we start
routing between VLANs.
Changing or Modifying the Trunk Native VLAN
You can change the trunk port native VLAN from VLAN 1, which many people do for
security reasons. To change the native VLAN, use the following command:
S1(config)#int f0/15
S1(config-if)#switchport trunk native vlan ?
<1-4094> VLAN ID of the native VLAN when this port is in trunking mode
S1(config-if)#switchport trunk native vlan 4
1w6d: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on
FastEthernet0/15 (4), with S3 FastEthernet0/1 (1).
So we’ve changed our native VLAN on our trunk link to 4, and by using the show
running-config command, I can see the conguration under the trunk link:
S1#sh run int f0/15
Building configuration...
480 Chapter 11 u VLANs and InterVLAN Routing
Current configuration : 202 bytes
!
interface FastEthernet0/15
description 1st connection to S3
switchport trunk encapsulation dot1q
switchport trunk native vlan 4
switchport trunk allowed vlan 4,6,12,15
switchport mode trunk
end
S1#!
Oops—wait a minute! You didn’t think it would be this easy and would just start work-
ing, did you? Of course not! Here’s the rub: If all switches don’t have the same native VLAN
congured on the given trunk links, then we’ll start to receive this error, which happened
immediately after I entered the command:
1w6d: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered
on FastEthernet0/15 (4), with S3 FastEthernet0/1 (1).
Actually, this is a good, noncryptic error, so either we can go to the other end of our
trunk link(s) and change the native VLAN or we set the native VLAN back to the default
to x it. Here’s how we’d do that:
S1(config-if)#no switchport trunk native vlan
1w6d: %SPANTREE-2-UNBLOCK_CONSIST_PORT: Unblocking FastEthernet0/15
on VLAN0004. Port consistency restored.
Now our trunk link is using the default VLAN 1 as the native VLAN. Just remember that
all switches on a given trunk must use the same native VLAN or you’ll have some serious
management problems. These issues won’t affect user data, just management trafc between
switches. Now, let’s mix it up by connecting a router into our switched network and congure
inter-VLAN communication.
Configuring Inter-VLAN Routing
By default, only hosts that are members of the same VLAN can communicate. To change
this and allow inter-VLAN communication, you need a router or a layer 3 switch. I’m going
to start with the router approach.
To support ISL or 802.1q routing on a FastEthernet interface, the router’s interface is
divided into logical interfaces—one for each VLAN—as was shown in Figure11.10. These
are called subinterfaces. From a FastEthernet or Gigabit interface, you can set the interface
to trunk with the encapsulation command:
ISR#config t
ISR(config)#int f0/0.1
Configuring VLANs 481
ISR(config-subif)#encapsulation ?
dot1Q IEEE 802.1Q Virtual LAN
ISR(config-subif)#encapsulation dot1Q ?
<1-4094> IEEE 802.1Q VLAN ID
Notice that my 2811 router (named ISR) only supports 802.1q. We’d need an older-
model router to run the ISL encapsulation, but why bother?
The subinterface number is only locally signicant, so it doesn’t matter which subinter-
face numbers are congured on the router. Most of the time, I’ll congure a subinterface
with the same number as the VLAN I want to route. It’s easy to remember that way since
the subinterface number is used only for administrative purposes.
It’s really important that you understand that each VLAN is actually a separate subnet.
True, I know—they don’t have to be. But it really is a good idea to congure your VLANs
as separate subnets, so just do that. Before we move on, I want to dene upstream routing.
This is a term used to dene the router on a stick. This router will provide inter-VLAN
routing, but it can also be used to forward trafc upstream from the switched network to
other parts of the corporate network or Internet.
Now, I need to make sure you’re fully prepared to congure inter-VLAN routing as well
as determine the IP addresses of hosts connected in a switched VLAN environment. And as
always, it’s also a good idea to be able to x any problems that may arise. To set you up for
success, let me give you few examples.
First, start by looking at Figure11.12 and read the router and switch conguration
within it. By this point in the book, you should be able to determine the IP address, masks,
and default gateways of each of the hosts in the VLANs.
FIGURE11.12 Configuring inter-VLAN example 1
Interface fastethernet 0/1
ip address 192.168.10.1 255.255.255.240
interface fastethernet 0/1.2
encapsulation dot1q 2
ip address 192.168.1.65 255.255.255.192
interface fastethernet 0/1.10
encapsulation dot1q 10
ip address 192.168.1.129 255.255.255.224
Fa0/1
Host A
1
234
Port 1: dot1q trunk
Ports 2,3: VLAN 2
Port 4: VLAN 10
Host B Host C
The next step is to gure out which subnets are being used. By looking at the router
conguration in the gure, you can see that we’re using 192.168.1.64/26 with VLAN 2,
192.168.1.128/27, and VLAN 10.
482 Chapter 11 u VLANs and InterVLAN Routing
By looking at the switch conguration, you can see that ports 2 and 3 are in VLAN 2 and
port 4 is in VLAN 10. This means that Host A and Host B are in VLAN 2, and Host C is in
VLAN 10.
But wait—what’s that IP address doing there under the physical interface? Can we even
do that? Sure we can! If we place an IP address under the physical interface, the result is that
frames sent from the IP address would be untagged. So what VLAN would those frames be a
member of? By default, they would belong to VLAN 1, our management VLAN. This means
the address 192.168.10.1/24 is my native VLAN IP address for this switch.
Here’s what the hosts’ IP addresses should be:
Host A: 192.168.1.66, 255.255.255.192, default gateway 192.168.1.65
Host B: 192.168.1.67, 255.255.255.192, default gateway 192.168.1.65
Host C: 192.168.1.130, 255.255.255.224, default gateway 192.168.1.129
The hosts could be any address in the range—I just chose the rst available IP address
after the default gateway address. That wasn’t so hard, was it?
Now, again using Figure11.12, let’s go through the commands necessary to congure
switch port 1 so it will establish a link with the router and provide inter-VLAN communi-
cation using the IEEE version for encapsulation. Keep in mind that the commands can vary
slightly depending on what type of switch you’re dealing with.
For a 2960 switch, use the following:
2960#config t
2960(config)#interface fa0/1
2960(config-if)#switchport mode trunk
That’s it! As you already know, the 2960 switch can only run the 802.1q encapsulation,
so there’s no need to specify it. You can’t anyway. For a 3560, it’s basically the same, but
because it can run ISL and 802.1q, you have to specify the trunking encapsulation protocol
you’re going to use.
Remember that when you create a trunked link, all VLANs are allowed to
pass data by default.
Let’s take a look at Figure11.13 and see what we can determine. This gure shows three
VLANs, with two hosts in each of them. The router in Figure11.13 is connected to the
Fa0/1 switch port, and VLAN 4 is congured on port F0/6.
When looking at this diagram, keep in mind that these three factors are what Cisco
expects you to know:
u The router is connected to the switch using subinterfaces.
u The switch port connecting to the router is a trunk port.
u The switch ports connecting to the clients and the hub are access ports, not trunk ports.
Configuring VLANs 483
FIGURE11.13 Inter-VLAN example 2
Host B
VLAN 2
Host A
Host D
VLAN 3
Host C
Fa0/3
Fa0/6
Fa0/1
Fa0/0
Fa0/2
VLAN 4
Fa0/5
Fa0/4
Host E
Host F
The conguration of the switch would look something like this:
2960#config t
2960(config)#int f0/1
2960(config-if)#switchport mode trunk
2960(config-if)#int f0/2
2960(config-if)#switchport access vlan 2
2960(config-if)#int f0/3
2960(config-if)#switchport access vlan 2
2960(config-if)#int f0/4
2960(config-if)#switchport access vlan 3
2960(config-if)#int f0/5
2960(config-if)#switchport access vlan 3
2960(config-if)#int f0/6
2960(config-if)#switchport access vlan 4
Before we congure the router, we need to design our logical network:
VLAN 1: 192.168.10.0/28
VLAN 2: 192.168.10.16/28
VLAN 3: 192.168.10.32/28
VLAN 4: 192.168.10.48/28
The conguration of the router would then look like this:
ISR#config t
ISR(config)#int fa0/0
484 Chapter 11 u VLANs and InterVLAN Routing
ISR(config-if)#ip address 192.168.10.1 255.255.255.240
ISR(config-if)#no shutdown
ISR(config-if)#int f0/0.2
ISR(config-subif)#encapsulation dot1q 2
ISR(config-subif)#ip address 192.168.10.17 255.255.255.240
ISR(config-subif)#int f0/0.3
ISR(config-subif)#encapsulation dot1q 3
ISR(config-subif)#ip address 192.168.10.33 255.255.255.240
ISR(config-subif)#int f0/0.4
ISR(config-subif)#encapsulation dot1q 4
ISR(config-subif)#ip address 192.168.10.49 255.255.255.240
Notice I didn’t tag VLAN 1. Even though I could have created a subinterface and
tagged VLAN 1, it’s not necessary with 802.1q because untagged frames are members
of the native VLAN.
The hosts in each VLAN would be assigned an address from their subnet range, and the
default gateway would be the IP address assigned to the router’s subinterface in that VLAN.
Now, let’s take a look at another gure and see if you can determine the switch and router
congurations without looking at the answer—no cheating! Figure11.14 shows a router con-
nected to a 2960 switch with two VLANs. One host in each VLAN is assigned an IP address.
What would your router and switch congurations be based on these IP addresses?
FIGURE11.14 Inter-VLAN example 3
Fa0/1
Fa0/2
Fa0/3
VLAN 2
85 Hosts
Host A
172.16.10.126
VLAN 3
115 Hosts
Host B
172.16.10.129
Okay—since the hosts don’t list a subnet mask, you have to look for the number of hosts
used in each VLAN to gure out the block size. VLAN 2 has 85 hosts and VLAN 3 has 115
hosts. Each of these will t in a block size of 128, which is a /25 mask, or 255.255.255.128.
Configuring VLANs 485
You should know by now that the subnets are 0 and 128; the 0 subnet (VLAN 2) has a
host range of 1–126, and the 128 subnet (VLAN 3) has a range of 129–254. You can almost
be fooled since Host A has an IP address of 126, which makes it almost seem that Host A
and B are in the same subnet. But they’re not, and you’re way too smart by now to be fooled
by this one!
Here is the switch conguration:
2960#config t
2960(config)#int f0/1
2960(config-if)#switchport mode trunk
2960(config-if)#int f0/2
2960(config-if)#switchport access vlan 2
2960(config-if)#int f0/3
2960(config-if)#switchport access vlan 3
Here is the router conguration:
ISR#config t
ISR(config)#int f0/0
ISR(config-if)#ip address 192.168.10.1 255.255.255.0
ISR(config-if)#no shutdown
ISR(config-if)#int f0/0.2
ISR(config-subif)#encapsulation dot1q 2
ISR(config-subif)#ip address 172.16.10.1 255.255.255.128
ISR(config-subif)#int f0/0.3
ISR(config-subif)#encapsulation dot1q 3
ISR(config-subif)#ip address 172.16.10.254 255.255.255.128
I used the rst address in the host range for VLAN 2 and the last address in the range
for VLAN 3, but any address in the range would work. You would just have to congure
the host’s default gateway to whatever you make the router’s address. Also, I used a differ-
ent subnet for my physical interface, which is my management VLAN router’s address.
Now, before we go on to the next example, I need to make sure you know how to set
the IP address on the switch. Since VLAN 1 is typically the administrative VLAN, we’ll
use an IP address from out of that pool of addresses. Here’s how to set the IP address of
the switch (not nagging, but you really should already know this!):
2960#config t
2960(config)#int vlan 1
2960(config-if)#ip address 192.168.10.2 255.255.255.0
2960(config-if)#no shutdown
2960(config-if)#exit
2960(config)#ip default-gateway 192.168.10.1
486 Chapter 11 u VLANs and InterVLAN Routing
Yes, you have to execute a no shutdown on the VLAN interface and set the ip default-
gateway address to the router.
One more example, and then we’ll move on to IVR using a multilayer switch—another
important subject that you denitely don’t want to miss! In Figure11.15 there are two
VLANs, plus the management VLAN 1. By looking at the router conguration: What’s
the IP address, subnet mask, and default gateway of Host A? Use the last IP address in
the range for Host A’s address.
If you really look carefully at the router conguration (the hostname in this congura-
tion is just Router), there’s a simple and quick answer. All subnets are using a /28, which
is a 255.255.255.240 mask. This is a block size of 16. The router’s address for VLAN 2 is
in subnet 128. The next subnet is 144, so the broadcast address of VLAN 2 is 143 and the
valid host range is 129–142. So the host address would be this:
IP address: 192.168.10.142
Mask: 255.255.255.240
Default gateway: 192.168.10.129
FIGURE11.15 Inter-VLAN example 4
Fa0/1
Fa0/2
Fa0/3
VLAN 2
Host A
VLAN 3
Host B
Router#config t
Router(config)#int fa0/0
Router(config-if)#ip address 192.168.10.1 255.255.255.240
Router(config-if)#no shutdown
Router(config-if)#int f0/0.2
Router(config-subif)#encapsulation dot1q 2
Router(config-subif)#ip address 192.168.10.129 255.255.255.240
Router(config-subif)#int fa0/0.3
Router(config-subif)#encapsulation dot1q 3
Router(config-subif)#ip address 192.168.10.46 255.255.255.240
This section was probably the hardest part of this entire book, and I honestly created the
simplest conguration you can possibly get away with using to help you through it!
I’ll use Figure11.16 to demonstrate conguring inter-VLAN routing (IVR) with a multi-
layer switch, which is often referred to as a switched virtual interface (SVI). I’m going to use
the same network that I used to discuss a multilayer switch back in Figure11.11, and I’ll use
this IP address scheme: 192.168.x.0/24, where x represents the VLAN subnet. In my example
this will be the same as the VLAN number.
Summary 487
FIGURE11.16 Inter-VLAN routing with a multilayer switch
VLAN 10
192.168.10.2/24
VLAN 10
VLAN 20
3560
192.168.20.2
VLAN 20
I’m Virtual!
The hosts are already congured with the IP address, subnet mask, and default gateway
address using the rst address in the range. Now I just need to congure the routing on the
switch, which is pretty simple actually:
S1(config)#ip routing
S1(config)#int vlan 10
S1(config-if)#ip address 192.168.10.1 255.255.255.0
S1(config-if)#int vlan 20
S1(config-if)#ip address 192.168.20.1 255.255.255.0
And that’s it! Enable IP routing and create one logical interface for each VLAN using
the interface vlan number command and voilà! You’ve now accomplished making inter-
VLAN routing work on the backplane of the switch!
Summary
In this chapter, I introduced you to the world of virtual LANs and described how Cisco
switches can use them. We talked about how VLANs break up broadcast domains in a
switched internetwork—a very important, necessary thing because layer 2 switches only
break up collision domains, and by default, all switches make up one large broadcast
domain. I also described access links to you, and we went over how trunked VLANs
work across a FastEthernet or faster link.
Trunking is a crucial technology to understand really well when you’re dealing with a
network populated by multiple switches that are running several VLANs.
You were also presented with some key troubleshooting and conguration examples for
access and trunk ports, conguring trunking options, and a huge section on IVR.
Visit ccna
.gg/ch11/b
for a
companion
MicroNugget
from CBT
Nuggets.
488 Chapter 11 u VLANs and InterVLAN Routing
Exam Essentials
Understand the term frame tagging. Frame tagging refers to VLAN identication; this is
what switches use to keep track of all those frames as they’re traversing a switch fabric. It’s
how switches identify which frames belong to which VLANs.
Understand the 802.1q VLAN identification method. This is a nonproprietary IEEE method
of frame tagging. If you’re trunking between a Cisco switched-link and a different brand of
switch, you have to use 802.1q for the trunk to work.
Remember how to set a trunk port on a 2960 switch. To set a port to trunking on a 2960,
use the switchport mode trunk command.
Remember to check a switch port’s VLAN assignment when plugging in a new host. If you
plug a new host into a switch, then you must verify the VLAN membership of that port. If
the membership is different than what is needed for that host, the host will not be able to
reach the needed network services, such as a workgroup server or printer.
Remember how to create a Cisco router on a stick to provide inter-VLAN communication.
You can use a Cisco FastEthernet or Gigabit Ethernet interface to provide inter-VLAN routing.
The switch port connected to the router must be a trunk port; then you must create virtual
interfaces (subinterfaces) on the router port for each VLAN connecting to it. The hosts in each
VLAN will use this subinterface address as their default gateway address.
Remember how to provide inter-VLAN routing with a layer 3 switch. You can use a layer 3
(multilayer) switch to provide IVR just as with a router on a stick, but using a layer 3 switch
is more efcient and faster. First you start the routing process with the command ip routing,
then create a virtual interface for each VLAN using the command interface vlan vlan, and
then apply the IP address for that VLAN under that logical interface.
Hands-on Labs 489
Written Lab 11
The answers to these labs can be found in Appendix A, “Answers to Written Labs.”
In this section, write the answers to the following questions:
1. True/False: To provide IVR with a layer 3 switch, you place an IP address on each
interface of the switch.
2. What protocol will stop loops in a layer 2 switched network?
3. VLANs break up ________ domains.
4. Switches, by default, only break up ________ domains.
5. If you have a switch that provides both ISL and 802.1q frame tagging, what command
under the trunk interface will make the trunk use 802.1q?
6. What does trunking provide?
7. What is frame tagging?
8. True/False: The 802.1q encapsulation is removed from the frame if the frame is for-
warded out an access link.
9. What type of link on a switch is a member of only one VLAN?
10. You want to change from the default of VLAN 1 to VLAN 4 for untagged traffic.
What command will you use?
Hands-on Labs
In these labs, you will use three switches and a router. To perform the last lab, you’ll need a
layer 3 switch.
Lab 11.1: Conguring and Verifying VLANs
Lab 11.2: Conguring and Verifying Trunk Links
Lab 11.3: Conguring Router on a Stick Routing
Lab 11.4: Conguring IVR with a Layer 3 Switch
490 Chapter 11 u VLANs and InterVLAN Routing
In these labs, I’ll use the following layout:
F0/1
F0/2
F0/16 F0/18
F0/15 F0/17
F0/5F0/5
F0/6F0/6
F0/2
F0/1
192.168.10.18/28
192.168.10.17/28
192.168.10.19/28
F0/0
F0/8
F0/4F0/3 F0/4F0/3
S1
S3 S2
Hands-on Lab 11.1: Configuring and Verifying VLANs
This lab will have you congure VLANs from global conguration mode, and then verify
the VLANs.
1. Configure two VLANs on each switch, VLAN 10 and VLAN 20.
S1(config)#vlan 10
S1(config-vlan)#vlan 20
S2(config)#vlan 10
S2(config-vlan)#vlan 20
S3(config)#vlan 10
S3(config-vlan)#vlan 20
2. Use the show vlan and show vlan brief command to verify your VLANs. Notice that
all interfaces are in VLAN 1 by default
S1#sh vlan
S1#sh vlan brief
Hands-on Labs 491
Hands-on Lab 11.2: Configuring and Verifying Trunk Links
This lab will have you congure trunk links and then verify them.
1. Connect to each switch and configure trunking on all switch links. If you are using a
switch that supports both 802.1q and ISL frame tagging, then use the encapsulation
command; if not, then skip that command.
S1#config t
S1(config)#interface fa0/15
S1(config-if)#switchport trunk encapsulation ?
dot1q Interface uses only 802.1q trunking encapsulation when trunking
isl Interface uses only ISL trunking encapsulation when trunking
negotiate Device will negotiate trunking encapsulation with peer on
interface
Again, if you typed the above and received an error, then your switch does not support
both encapsulation methods:
S1 (config-if)#switchport trunk encapsulation dot1q
S1 (config-if)#switchport mode trunk
S1 (config-if)#interface fa0/16
S1 (config-if)#switchport trunk encapsulation dot1q
S1 (config-if)#switchport mode trunk
S1 (config-if)#interface fa0/17
S1 (config-if)#switchport trunk encapsulation dot1q
S1 (config-if)#switchport mode trunk
S1 (config-f)#interface fa0/18
S1 (config-if)#switchport trunk encapsulation dot1q
S1 (config-if)#switchport mode trunk
2. Configure the trunk links on your other switches.
3. On each switch, verify your trunk ports with the show interface trunk command:
S1#show interface trunk
4. Verify the switchport configuration with the following:
S1#show interface interface switchport
The second interface in the command is a variable, such as Fa0/15.
492 Chapter 11 u VLANs and InterVLAN Routing
Hands-on Lab 11.3: Configuring Router on a Stick Routing
In this lab, you’ll use the router connected to port F0/8 of switch S1 to congure ROAS.
1. Configure the F0/0 of the router with two subinterfaces to provide inter-VLAN rout-
ing using 802.1q encapsulation. Use 172.16.10.0/24 for your management VLAN,
10.10.10.0/24 for VLAN 10, and 20.20.20.0/24 for VLAN 20.
Router#config t
Router (config)#int f0/0
Router (config-if)#ip address 172.16.10.1 255.255.255.0
Router (config-if)#interface f0/0.10
Router (config-subif)#encapsulation dot1q 10
Router (config-subif)#ip address 10.10.10.1 255.255.255.0
Router (config-subif)#interface f0/0.20
Router (config-subif)#encapsulation dot1q 20
Router (config-subif)#ip address 20.20.20.1 255.255.255.0
2. Verify the configuration with the show running-config command.
3. Configure trunking on interface F0/8 of the S1 switch connecting to your router.
4. Verify that your VLANs are still configured on your switches with the sh vlan command.
5. Configure your hosts to be in VLAN 10 and VLAN 20 with the switchport access
vlan x command.
6. Ping from your PC to the router’s subinterface configured for your VLAN.
7. Ping from your PC to your PC in the other VLAN. You are now routing through
the router!
Hands-on Lab 11.4: Configuring IVR with a Layer 3 Switch
In this lab, you will disable the router and use the S1 switch to provide inter VLAN routing
by creating SVI’s.
1. Connect to the S1 switch and make interface F0/8 an access port, which will make the
router stop providing inter-VLAN routing.
2. Enable IP routing on the S1 switch.
S1(config)#ip routing
3. Create two new interfaces on the S1 switch to provide IVR.
S1(config)#interface vlan 10
S1(config-if)#ip address 10.10.10.1 255.255.255.0
S1(config-if)#interface vlan 20
S1(config-if)#ip address 20.20.20.1 255.255.255.0
Hands-on Labs 493
4. Clear the ARP cache on the switch and hosts.
S1#clear arp
5. Ping from your PC to the router’s subinterface configured for your VLAN.
6. Ping from your PC to your PC in the other VLAN. You are now routing through the
S1 switch!
494 Chapter 11 u VLANs and InterVLAN Routing
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional ques-
tions, please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. Which of the following statements is true with regard to VLANs?
A. VLANs greatly reduce network security.
B. VLANs increase the number of collision domains while decreasing their size.
C. VLANs decrease the number of broadcast domains while decreasing their size.
D. Network adds, moves, and changes are achieved with ease by just configuring a
port into the appropriate VLAN.
2. Write the command that must be present for this layer 3 switch to provide inter-VLAN
routing between the two VLANs created with these commands:
S1(config)#int vlan 10
S1(config-if)#ip address 192.168.10.1 255.255.255.0
S1(config-if)#int vlan 20
S1(config-if)#ip address 192.168.20.1 255.255.255.0
Review Questions 495
3. In the diagram, how must the port on each end of the line be configured to carry traf-
fic between the two hosts in the Sales VLAN?
Sales Admin
Sales Admin
A. Access port
B. 10 GB
C. Trunk
D. Spanning
4. What is the only type of second VLAN of which an access port can be a member?
A. Secondary
B. Voice
C. Primary
D. Trunk
496 Chapter 11 u VLANs and InterVLAN Routing
5. In the following configuration, what command is missing in the creation of the VLAN
interface?
2960#config t
2960(config)#int vlan 1
2960(config-if)#ip address 192.168.10.2 255.255.255.0
2960(config-if)#exit
2960(config)#ip default-gateway 192.168.10.1
A. no shutdown under int vlan 1
B. encapsulation dot1q 1 under int vlan 1
C. switchport access vlan 1
D. passive-interface
6. Which of the following statements is true with regard to ISL and 802.1q?
A. 802.1q encapsulates the frame with control information; ISL inserts an ISL field
along with tag control information.
B. 802.1q is Cisco proprietary.
C. ISL encapsulates the frame with control information; 802.1q inserts an 802.1q
field along with tag control information.
D. ISL is a standard.
7. What concept is depicted in the diagram?
Gi0/0
A. Multiprotocol routing
B. Passive interface
C. Gateway redundancy
D. Router on a stick
8. Write the command that places an interface into VLAN 2. Write only the command
and not the prompt.
Review Questions 497
9. Write the command that generated the following output:
VLAN Name Status Ports
---- ------------------------- --------- ------------------------
1 default active Fa0/1, Fa0/2, Fa0/3, Fa0/4
Fa0/5, Fa0/6, Fa0/7, Fa0/8
Fa0/9, Fa0/10, Fa0/11, Fa0/12
Fa0/13, Fa0/14, Fa0/19, Fa0/20
Fa0/21, Fa0/22, Fa0/23, Gi0/1
Gi0/2
2 Sales active
3 Marketing active
4 Accounting active
[output cut]
10. In the configuration and diagram shown, what command is missing to enable inter-
VLAN routing between VLAN 2 and VLAN 3?
Fa0/1
Fa0/2
Fa0/3
VLAN 2
Host A
VLAN 3
Host B
Router#config t
Router(config)#int fa0/0
Router(config-if)#ip address 192.168.10.1 255.255.255.240
Router(config-if)#no shutdown
Router(config-if)#int f0/0.2
Router(config-subif)#ip address 192.168.10.129 255.255.255.240
Router(config-subif)#int fa0/0.3
Router(config-subif)#encapsulation dot1q 3
Router(config-subif)#ip address 192.168.10.46 255.255.255.240
A. encapsulation dot1q 3 under int f0/0.2
B. encapsulation dot1q 2 under int f0/0.2
C. no shutdown under int f0/0.2
D. no shutdown under int f0/0.3
498 Chapter 11 u VLANs and InterVLAN Routing
11. Based on the configuration shown below, what statement is true?
S1(config)#ip routing
S1(config)#int vlan 10
S1(config-if)#ip address 192.168.10.1 255.255.255.0
S1(config-if)#int vlan 20
S1(config-if)#ip address 192.168.20.1 255.255.255.0
A. This is a multilayer switch.
B. The two VLANs are in the same subnet.
C. Encapsulation must be configured.
D. VLAN 10 is the management VLAN.
12. What is true of the output shown below?
S1#sh vlan
VLAN Name Status Ports
---- ---------------------- --------- -------------------------------
1 default active Fa0/1, Fa0/2, Fa0/3, Fa0/4
Fa0/5, Fa0/6, Fa0/7, Fa0/8
Fa0/9, Fa0/10, Fa0/11, Fa0/12
Fa0/13, Fa0/14, Fa0/19, Fa0/20,
Fa0/22, Fa0/23, Gi0/1, Gi0/2
2 Sales active
3 Marketing Fa0/21
4 Accounting active
[output cut]
A. Interface F0/15 is a trunk port.
B. Interface F0/17 is an access port.
C. Interface F0/21 is a trunk port.
D. VLAN 1 was populated manually.
13. 802.1q untagged frames are members of the _________ VLAN.
A. Auxiliary
B. Voice
C. Native
D. Private
Review Questions 499
14. Write the command that generated the following output. Write only the command and
not the prompt:
Name: Fa0/15
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: trunk
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: isl
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
[output cut]
15. Which statement is true regarding virtual local area networks (VLANs)?
A. VLANs are location dependent.
B. VLANs are limited to a single switch.
C. VLANs may be subnets of major networks.
D. VLANs define collision domains.
16. In the diagram, what should be the default gateway address of Host B?
Interface fastethernet 0/1
ip address 192.168.10.1 255.255.255.240
interface fastethernet 0/1.2
encapsulation dot1q 2
ip address 192.168.1.65 255.255.255.192
interface fastethernet 0/1.10
encapsulation dot1q 10
ip address 192.168.1.129 255.255.255.224
Fa0/1
Host A
1
234
Port 1: dot1q trunk
Ports 2,3: VLAN 2
Port 4: VLAN 10
Host B Host C
A. 192.168.10.1
B. 192.168.1.65
C. 192.168.1.129
D. 192.168.1.2
500 Chapter 11 u VLANs and InterVLAN Routing
17. What is the purpose of frame tagging in virtual LAN (VLAN) configurations?
A. Inter-VLAN routing
B. Encryption of network packets
C. Frame identification over trunk links
D. Frame identification over access links
18. Write the command to create VLAN 2 on a layer 2 switch. Write only the command
and not the prompt.
19. Which statement is true regarding 802.1q frame tagging?
A. 802.1q adds a 26-byte trailer and 4-byte header.
B. 802.1q uses a native VLAN.
C. The original Ethernet frame is not modified.
D. 802.1q only works with Cisco switches.
20. Write the command that prevents an interface from generating DTP frames. Write only
the command and not the prompt.
Chapter
12
Security
THE FOLLOWING ICND1 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 IP Services
■Describe the types, features, and applications of ACLs
■Standard
■Sequence numbers
■Editing
■Extended
■Named
■Numbered
■Log option
■Configure and verify ACLs in a network environment
■Named
■Numbered
■Log option
1 Network Device Security
■Configure and verify ACLs to filter network traffic
■Configure and verify ACLs to limit telnet and SSH access to
the router
1 Troubleshooting
■Troubleshoot and Resolve ACL issues
■Statistics
■Permitted networks
■Direction
■Interface
If you’re a sys admin, it’s my guess that shielding sensitive,
critical data, as well as your network’s resources, from every
possible evil exploit is a top priority of yours, right? Good to
know you’re on the right page because Cisco has some really effective security solutions
to equip you with the tools you’ll need to make this happen in a very real way!
The rst power tool I’m going to hand you is known as the access control list (ACL). Being
able to execute an ACL prociently is an integral part of Cisco’s security solution, so I’m going
to begin by showing you how to create and implement simple ACLs. From there, I’ll move to
demonstrating more advanced ACLs and describe how to implement them strategically to pro-
vide serious armor for an internetwork in today’s challenging, high-risk environment.
In Appendix C, “Disabling and Conguring Network Services,” I’ll show you how to
mitigate most security-oriented network threats. Make sure you don’t skip this appendix
because it is chock full of great security information, and the information it contains is
part of the Cisco exam objectives as well!
The proper use and conguration of access lists is a vital part of router conguration
because access lists are such versatile networking accessories. Contributing mightily to the
efciency and operation of your network, access lists give network managers a huge amount
of control over trafc ow throughout the enterprise. With access lists, we can gather basic
statistics on packet ow and security policies can be implemented. These dynamic tools also
enable us to protect sensitive devices from the dangers of unauthorized access.
In this chapter, we’ll cover ACLs for TCP/IP as well as explore effective ways available
to us for testing and monitoring how well applied access lists are functioning. We’ll begin
now by discussing key security measures deployed using hardware devices and VLANs
and then I’ll introduce you to ACLs.
To find up-to-the-minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
Perimeter, Firewall, and Internal Routers
You see this a lot—typically, in medium to large enterprise networks—the various strategies
for security are based on some mix of internal and perimeter routers plus rewall devices.
Internal routers provide additional security by screening trafc to various parts of the pro-
tected corporate network, and they achieve this using access lists. You can see where each of
these types of devices would be found in Figure12.1.
Introduction to Access Lists 503
FIGURE12.1 A typical secured network
Untrusted
network Perimeter
(premises)
router Firewall
Internal
(local network)
router
Corporate
(trusted
network)
DMZ
Email
Server
Web
Server
Internet
I’ll use the terms trusted network and untrusted network throughout this chapter, as well
as in Chapter 13, “Network Address Translation (NAT),” so it’s important that you can see
where they’re found in a typical secured network. The demilitarized zone (DMZ) can be
global (real) Internet addresses or private addresses, depending on how you congure your
rewall, but this is typically where you’ll nd the HTTP, DNS, email, and other Internet-type
corporate servers.
As you now know, instead of using routers, we can create VLANs with switches on the
inside trusted network. Multilayer switches containing their own security features can some-
times replace internal (LAN) routers to provide higher performance in VLAN architectures.
Let’s look at some ways of protecting the internetwork using access lists.
Introduction to Access Lists
An access list is essentially a list of conditions that categorize packets, and they really come
in handy when you need to exercise control over network trafc. An ACL would be your
tool of choice for decision making in these situations.
One of the most common and easiest-to-understand uses of access lists is to lter
unwanted packets when implementing security policies. For example, you can set them up
to make very specic decisions about regulating trafc patterns so that they’ll allow only
certain hosts to access web resources on the Internet while restricting others. With the right
combination of access lists, network managers arm themselves with the power to enforce
nearly any security policy they can invent.
504 Chapter 12 u Security
Creating access lists is really a lot like programming a series of if-then statements—if a
given condition is met, then a given action is taken. If the specic condition isn’t met, nothing
happens and the next statement is evaluated. Access-list statements are basically packet lters
that packets are compared against, categorized by, and acted upon accordingly. Once the lists
are built, they can be applied to either inbound or outbound trafc on any interface. Applying
an access list causes the router to analyze every packet crossing that interface in the specied
direction and take the appropriate action.
There are three important rules that a packet follows when it’s being compared with an
access list:
u The packet is always compared with each line of the access list in sequential order—it
will always start with the first line of the access list, move on to line 2, then line 3, and
so on.
u The packet is compared with lines of the access list only until a match is made. Once it
matches the condition on a line of the access list, the packet is acted upon and no further
comparisons take place.
u There is an implicit “deny” at the end of each access list—this means that if a packet
doesn’t match the condition on any of the lines in the access list, the packet will be
discarded.
Each of these rules has some powerful implications when ltering IP packets with access
lists, so keep in mind that creating effective access lists denitely takes some practice.
There are two main types of access lists:
Standard access lists These ACLs use only the source IP address in an IP packet as the
condition test. All decisions are made based on the source IP address. This means that stan-
dard access lists basically permit or deny an entire suite of protocols. They don’t distinguish
between any of the many types of IP trafc such as Web, Telnet, UDP, and so on.
Extended access lists Extended access lists can evaluate many of the other elds in the layer3
and layer 4 headers of an IP packet. They can evaluate source and destination IP addresses, the
Protocol eld in the Network layer header, and the port number at the Transport layer header.
This gives extended access lists the ability to make much more granular decisions when con-
trolling trafc.
Named access lists Hey, wait a minute—I said there were only two types of access lists
but listed three! Well, technically there really are only two since named access lists are
either standard or extended and not actually a distinct type. I’m just distinguishing them
because they’re created and referred to differently than standard and extended access lists
are, but they’re still functionally the same.
We’ll cover these types of access lists in more depth later in the chapter.
Once you create an access list, it’s not really going to do anything until you apply it. Yes,
they’re there on the router, but they’re inactive until you tell that router what to do with them.
Introduction to Access Lists 505
To use an access list as a packet lter, you need to apply it to an interface on the router where
you want the trafc ltered. And you’ve got to specify which direction of trafc you want
the access list applied to. There’s a good reason for this—you may want different controls in
place for trafc leaving your enterprise destined for the Internet than you’d want for trafc
coming into your enterprise from the Internet. So, by specifying the direction of trafc, you
can and must use different access lists for inbound and outbound trafc on a single interface:
Inbound access lists When an access list is applied to inbound packets on an interface,
those packets are processed through the access list before being routed to the outbound
interface. Any packets that are denied won’t be routed because they’re discarded before
the routing process is invoked.
Outbound access lists When an access list is applied to outbound packets on an interface,
packets are routed to the outbound interface and then processed through the access list
before being queued.
There are some general access-list guidelines that you should keep in mind when creating
and implementing access lists on a router:
u You can assign only one access list per interface per protocol per direction. This means
that when applying IP access lists, you can have only one inbound access list and one
outbound access list per interface.
When you consider the implications of the implicit deny at the end of any
access list, it makes sense that you can’t have multiple access lists applied
on the same interface in the same direction for the same protocol. That’s
because any packets that don’t match some condition in the first access list
would be denied and there wouldn’t be any packets left over to compare
against a second access list!
u Organize your access lists so that the more specific tests are at the top.
u Anytime a new entry is added to the access list, it will be placed at the bottom of the
list, which is why I highly recommend using a text editor for access lists.
u You can’t remove one line from an access list. If you try to do this, you will remove the
entire list. This is why it’s best to copy the access list to a text editor before trying to
edit the list. The only exception is when you’re using named access lists.
You can edit, add, or delete a single line from a named access list. I’ll
show you how shortly.
u Unless your access list ends with a permit any command, all packets will be discarded
if they do not meet any of the list’s tests. This means every list should have at least one
permit statement or it will deny all traffic.
u Create access lists and then apply them to an interface. Any access list applied to an
interface without access-list test statements present will not filter traffic.
506 Chapter 12 u Security
u Access lists are designed to filter traffic going through the router. They will not filter
traffic that has originated from the router.
u Place IP standard access lists as close to the destination as possible. This is the reason we
don’t really want to use standard access lists in our networks. You can’t put a standard
access list close to the source host or network because you can only filter based on source
address and all destinations would be affected as a result.
u Place IP extended access lists as close to the source as possible. Since extended access
lists can filter on very specific addresses and protocols, you don’t want your traffic to
traverse the entire network just to be denied. By placing this list as close to the source
address as possible, you can filter traffic before it uses up precious bandwidth.
Before I move on to demonstrate how to congure basic and extended ACLs, let’s talk
about how they can be used to mitigate the security threats I mentioned earlier.
Mitigating Security Issues with ACLs
The most common attack is a denial of service (DoS) attack. Although ACLs can help with
a DoS, you really need an intrusion detection system (IDS) and intrusion prevention system
(IPS) to help prevent these common attacks. Cisco sells the Adaptive Security Appliance (ASA),
which has IDS/IPS modules, but lots of other companies sell IDS/IPS products too.
Here’s a list of the many security threats you can mitigate with ACLs:
u IP address spoofing, inbound
u IP address spoofing, outbound
u Denial of service (DoS) TCP SYN attacks, blocking external attacks
u DoS TCP SYN attacks, using TCP Intercept
u DoS smurf attacks
u Denying/filtering ICMP messages, inbound
u Denying/Filtering ICMP messages, outbound
u Denying/Filtering Traceroute
This is not an “introduction to security” book, so you may have to research
some of the preceding terms if you don’t understand them.
It’s generally a bad idea to allow into a private network any external IP packets that
contain the source address of any internal hosts or networks—just don’t do it!
Here’s a list of rules to live by when conguring ACLs from the Internet to your pro-
duction network to mitigate security problems:
u Deny any source addresses from your internal networks.
u Deny any local host addresses (127.0.0.0/8).
u Deny any reserved private addresses (RFC 1918).
u Deny any addresses in the IP multicast address range (224.0.0.0/4).
Standard Access Lists 507
None of these source addresses should be ever be allowed to enter your internetwork. Okay,
now nally, let’s get our hands dirty and congure some basic and advanced access lists!
Standard Access Lists
Standard IP access lists lter network trafc by examining the source IP address in a packet.
You create a standard IP access list by using the access-list numbers 1–99 or in the expanded
range of 1300–1999 because the type of ACL is generally differentiated using a number. Based
on the number used when the access list is created, the router knows which type of syntax to
expect as the list is entered. By using numbers 1–99 or 1300–1999, you’re telling the router
that you want to create a standard IP access list, so the router will expect syntax specifying
only the source IP address in the test lines.
The following output displays a good example of the many access-list number ranges
that you can use to lter trafc on your network. The IOS version delimits the protocols
you can specify access for:
Corp(config)#access-list ?
<1-99> IP standard access list
<100-199> IP extended access list
<1000-1099> IPX SAP access list
<1100-1199> Extended 48-bit MAC address access list
<1200-1299> IPX summary address access list
<1300-1999> IP standard access list (expanded range)
<200-299> Protocol type-code access list
<2000-2699> IP extended access list (expanded range)
<300-399> DECnet access list
<600-699> Appletalk access list
<700-799> 48-bit MAC address access list
<800-899> IPX standard access list
<900-999> IPX extended access list
dynamic-extended Extend the dynamic ACL absolute timer
rate-limit Simple rate-limit specific access list
Wow—there certainly are lot of old protocols listed in that output! IPX, AppleTalk, or
DECnet would no longer be used in any of today’s networks. Let’s take a look at the syntax
used when creating a standard IP access list:
Corp(config)#access-list 10 ?
deny Specify packets to reject
permit Specify packets to forward
remark Access list entry comment
508 Chapter 12 u Security
As I said, by using the access-list numbers 1–99 or 1300–1999, you’re telling the router
that you want to create a standard IP access list, which means you can only lter on source
IP address.
Once you’ve chosen the access-list number, you need to decide whether you’re creating a
permit or deny statement. I’m going to create a deny statement now:
Corp(config)#access-list 10 deny ?
Hostname or A.B.C.D Address to match
any Any source host
host A single host address
The next step is more detailed because there are three options available in it:
1. The first option is the any parameter is used to permit or deny any source host or
network.
2. The second choice is to use an IP address to specify either a single host or a range of them.
3. The last option is to use the host command to specify a specific host only.
The any command is pretty obvious—any source address matches the statement, so
every packet compared against this line will match. The host command is relatively simple
too, as you can see here:
Corp(config)#access-list 10 deny host ?
Hostname or A.B.C.D Host address
Corp(config)#access-list 10 deny host 172.16.30.2
This tells the list to deny any packets from host 172.16.30.2. The default parameter is
host. In other words, if you type access-list 10 deny 172.16.30.2, the router assumes
you mean host 172.16.30.2 and that’s exactly how it will show in your running-cong.
But there’s another way to specify either a particular host or a range of hosts, and it’s
known as wildcard masking. In fact, to specify any range of hosts, you must use wildcard
masking in the access list.
So exactly what is wildcard masking? Coming up, I’m going to show you using a stan-
dard access list example. I’ll also guide you through how to control access to a virtual ter-
minal. We’ll be using the same wildcard masks we used in the Conguring OSPF section
of Chapter9, “Open Shortest Path First (OSPF).”
Wildcard Masking
Wildcards are used with access lists to specify an individual host, a network, or a specic
range of a network or networks. The block sizes you learned about earlier used to specify a
range of addresses are key to understanding wildcards.
Let me pause here for a quick review of block sizes before we go any further. I’m sure you
remember that the different block sizes available are 64, 32, 16, 8, and 4. When you need to
specify a range of addresses, you choose the next-largest block size for your needs. So if you
Standard Access Lists 509
need to specify 34 networks, you need a block size of 64. If you want to specify 18 hosts,
you need a block size of 32. If you specify only 2 networks, then go with a block size of 4.
Wildcards are used with the host or network address to tell the router a range of available
addresses to lter. To specify a host, the address would look like this:
172.16.30.5 0.0.0.0
The four zeros represent each octet of the address. Whenever a zero is present, it indicates
that octet in the address must match the corresponding reference octet exactly. To specify that
an octet can be any value, use the value 255. Here’s an example of how a /24 subnet is speci-
ed with a wildcard mask:
172.16.30.0 0.0.0.255
This tells the router to match up the rst three octets exactly, but the fourth octet can be
any value.
Okay—that was the easy part. But what if you want to specify only a small range of
subnets? This is where block sizes come in. You have to specify the range of values in a
block size, so you can’t choose to specify 20 networks. You can only specify the exact
amount that the block size value allows. This means that the range would have to be
either 16 or 32, but not 20.
Let’s say that you want to block access to the part of the network that ranges from
172.16.8.0 through 172.16.15.0. To do that, you would go with a block size of 8, your
network number would be 172.16.8.0, and the wildcard would be 0.0.7.255. The 7.255
equals the value the router will use to determine the block size. So together, the network
number and the wildcard tell the router to begin at 172.16.8.0 and go up a block size of
eight addresses to network 172.16.15.0.
This really is easier than it looks! I could certainly go through the binary math for you,
but no one needs that kind of pain because all you have to do is remember that the wildcard
is always one number less than the block size. So, in our example, the wildcard would be 7
since our block size is 8. If you used a block size of 16, the wildcard would be 15. Easy, right?
Just to make you’ve got this, we’ll go through some examples that will denitely help you
nail it down. The following example tells the router to match the rst three octets exactly
but that the fourth octet can be anything:
Corp(config)#access-list 10 deny 172.16.10.0 0.0.0.255
The next example tells the router to match the rst two octets and that the last two octets
can be any value:
Corp(config)#access-list 10 deny 172.16.0.0 0.0.255.255
Now, try to gure out this next line:
Corp(config)#access-list 10 deny 172.16.16.0 0.0.3.255
510 Chapter 12 u Security
This conguration tells the router to start at network 172.16.16.0 and use a block size
of 4. The range would then be 172.16.16.0 through 172.16.19.255, and by the way, the
Cisco objectives seem to really like this one!
Okay, let’s keep practicing. What about this next one?
Corp(config)#access-list 10 deny 172.16.16.0 0.0.7.255
This example reveals an access list starting at 172.16.16.0 going up a block size of 8 to
172.16.23.255.
Let’s keep at it... What do you think the range of this one is?
Corp(config)#access-list 10 deny 172.16.32.0 0.0.15.255
This one begins at network 172.16.32.0 and goes up a block size of 16 to 172.16.47.255.
You’re almost done practicing! After a couple more, we’ll congure some real ACLs.
Corp(config)#access-list 10 deny 172.16.64.0 0.0.63.255
This example starts at network 172.16.64.0 and goes up a block size of 64 to 172.16.127.255.
What about this last example?
Corp(config)#access-list 10 deny 192.168.160.0 0.0.31.255
This one shows us that it begins at network 192.168.160.0 and goes up a block size of 32 to
192.168.191.255.
Here are two more things to keep in mind when working with block sizes and wildcards:
u Each block size must start at 0 or a multiple of the block size. For example, you can’t
say that you want a block size of 8 and then start at 12. You must use 0–7, 8–15,
16–23, etc. For a block size of 32, the ranges are 0–31, 32–63, 64–95, etc.
u The command any is the same thing as writing out the wildcard 0.0.0.0 255.255.255.255.
Wildcard masking is a crucial skill to master when creating IP access
lists, and it’s used identically when creating standard and extended
IP access lists.
Standard Access List Example
In this section, you’ll learn how to use a standard access list to stop specic users from
gaining access to the Finance department LAN.
In Figure12.2, a router has three LAN connections and one WAN connection to the
Internet. Users on the Sales LAN should not have access to the Finance LAN, but they should
be able to access the Internet and the marketing department les. The Marketing LAN needs
to access the Finance LAN for application services.
Standard Access Lists 511
FIGURE12.2 IP access list example with three LANs and a WAN connection
S0/0/0
Fa1/0
Fa0/0
Permitted!
Fa0/1
Denied!
Sales
172.16.40.0/24 Marketing
172.16.60.0/24
Finance
172.16.50.0/24
Internet
Lab A
We can see that the following standard IP access list is congured on the router:
Lab_A#config t
Lab_A(config)#access-list 10 deny 172.16.40.0 0.0.0.255
Lab_A(config)#access-list 10 permit any
It’s very important to remember that the any command is the same thing as saying the
following using wildcard masking:
Lab_A(config)#access-list 10 permit 0.0.0.0 255.255.255.255
Since the wildcard mask says that none of the octets are to be evaluated, every address
matches the test condition, so this is functionally doing the same as using the any keyword.
At this point, the access list is congured to deny source addresses from the Sales LAN
to the Finance LAN and to allow everyone else. But remember, no action will be taken until
the access list is applied on an interface in a specic direction!
But where should this access list be placed? If you place it as an incoming access list on
Fa0/0, you might as well shut down the FastEthernet interface because all of the Sales LAN
devices will be denied access to all networks attached to the router. The best place to apply
this access list is on the Fa0/1 interface as an outbound list:
Lab_A(config)#int fa0/1
Lab_A(config-if)#ip access-group 10 out
Doing this completely stops trafc from 172.16.40.0 from getting out FastEthernet0/1. It
has no effect on the hosts from the Sales LAN accessing the Marketing LAN and the Internet
because trafc to those destinations doesn’t go through interface Fa0/1. Any packet trying to
exit out Fa0/1 will have to go through the access list rst. If there were an inbound list placed
512 Chapter 12 u Security
on F0/0, then any packet trying to enter interface F0/0 would have to go through the access
list before being routed to an exit interface.
Now, let’s take a look at another standard access list example. Figure12.3 shows an
internetwork of two routers with four LANs.
FIGURE12.3 IP standard access list example 2
Lab BLab A
HR Data headed to server
192.168.10.161/27
E0
Accounting Data headed to server
ACL: OKAY!
ACL: DENIED!
Accounting
Put ACL here!
Apply to interface as
outbound list towards server
E0
E1
192.168.10.129/27
Human Resources
Human Resources server
192.168.10.222/27
Now we’re going to stop the Accounting users from accessing the Human Resources server
attached to the Lab_B router but allow all other users access to that LAN using a standard
ACL. What kind of standard access list would we need to create and where would we place it
to achieve our goals?
The real answer is that we should to use an extended access list and place it closest to
the source! But this question species using a standard access list, and as a rule, standard
ACLs are placed closest to the destination. In this example, Ethernet 0 is the outbound
interface on the Lab_B router and here’s the access list that should be placed on it:
Lab_B#config t
Lab_B(config)#access-list 10 deny 192.168.10.128 0.0.0.31
Lab_B(config)#access-list 10 permit any
Lab_B(config)#interface Ethernet 0
Lab_B(config-if)#ip access-group 10 out
Keep in mind that to be able to answer this question correctly, you really need to under-
stand subnetting, wildcard masks, and how to congure and implement ACLs. The account-
ing subnet is the 192.168.10.128/27, which is a 255.255.255.224, with a block size of 32 in
the fourth octet.
Standard Access Lists 513
With all this in mind and before we move on to restricting Telnet access on a router,
let’s take a look at one more standard access list example. This one is going to require
some thought. In Figure12.4, you have a router with four LAN connections and one
WAN connection to the Internet.
FIGURE12.4 IP standard access list example 3
172.16.92.10/21
172.16.144.17/19 172.16.198.94/18
172.16.50.173/20
E0
E3
E1 E2
S0
Okay—you need to write an access list that will stop access from each of the four LANs
shown in the diagram to the Internet. Each of the LANs reveals a single host’s IP address,
which you need to use to determine the subnet and wildcards of each LAN to congure the
access list.
Here is an example of what your answer should look like, beginning with the network
on E0 and working through to E3:
Router(config)#access-list 1 deny 172.16.128.0 0.0.31.255
Router(config)#access-list 1 deny 172.16.48.0 0.0.15.255
Router(config)#access-list 1 deny 172.16.192.0 0.0.63.255
Router(config)#access-list 1 deny 172.16.88.0 0.0.7.255
Router(config)#access-list 1 permit any
Router(config)#interface serial 0
Router(config-if)#ip access-group 1 out
Sure, you could have done this with one line:
Router(config)#access-list 1 deny 172.16.0.0 0.0.255.255
514 Chapter 12 u Security
But what fun is that?
And remember the reasons for creating this list. If you actually applied this ACL on the
router, you’d effectively shut down access to the Internet, so why even have an Internet con-
nection? I included this exercise so you can practice how to use block sizes with access lists,
which is vital for succeeding when you take the Cisco exam!
Controlling VTY (Telnet/SSH) Access
Trying to stop users from telnetting or trying to SSH to a router is really challenging
because any active interface on a router is fair game for VTY/SSH access. Creating an
extended IP ACL that limits access to every IP address on the router may sound like a
solution, but if you did that, you’d have to apply it inbound on every interface, which
really wouldn’t scale well if you happen to have dozens, even hundreds, of interfaces,
now would it? And think of all the latency dragging down your network as a result of
each and every router checking every packet just in case the packet was trying to access
your VTY lines—horrible!
Don’t give up—there’s always a solution! And in this case, a much better one, which
employs a standard IP access list to control access to the VTY lines themselves.
Why does this work so well? Because when you apply an access list to the VTY lines,
you don’t need to specify the protocol since access to the VTY already implies terminal
access via the Telnet or SSH protocols. You also don’t need to specify a destination address
because it really doesn’t matter which interface address the user used as a target for the
Telnet session. All you really need control of is where the user is coming from, which is
betrayed by their source IP address.
You need to do these two things to make this happen:
1. Create a standard IP access list that permits only the host or hosts you want to be able
to telnet into the routers.
2. Apply the access list to the VTY line with the access-class in command.
Here, I’m allowing only host 172.16.10.3 to telnet into a router:
Lab_A(config)#access-list 50 permit host 172.16.10.3
Lab_A(config)#line vty 0 4
Lab_A(config-line)#access-class 50 in
Because of the implied deny any at the end of the list, the ACL stops any host from
telnetting into the router except the host 172.16.10.3, regardless of the individual IP
address on the router being used as a target. It’s a good idea to include an admin subnet
address as the source instead of a single host, but the reason I demonstrated this was
to show you how to create security on your VTY lines without adding latency to
your router.
Extended Access Lists 515
Should You Secure Your VTY Lines on a Router?
You’re monitoring your network and notice that someone has telnetted into your core router
by using the show users command. You use the disconnect command and they’re discon-
nected from the router, but you notice that they’re right back in there a few minutes later.
You consider putting an ACL on the router interfaces, but you don’t want to add latency on
each interface since your router is already pushing a lot of packets. At this point, you think
about putting an access list on the VTY lines themselves, but not having done this before,
you’re not sure if this is a safe alternative to putting an ACL on each interface. Would placing
an ACL on the VTY lines be a good idea for this network?
Yes—absolutely! And the access-class command covered in this chapter is the way to
do it. Why? Because it doesn’t use an access list that just sits on an interface looking at
every packet, resulting in unnecessary overhead and latency.
When you put the access-class in command on the VTY lines, only packets trying to
telnet into the router will be checked and compared, providing easy-to-configure yet solid
security for your router!
Just a reminder—Cisco recommends using Secure Shell (SSH) instead of
Telnet on the VTY lines of a router, as we covered in Chapter 6, “Cisco’s Inter-
networking Operating System (IOS),” so review that chapter if you need a
refresher on SSH and how to configure it on your routers and switches.
Extended Access Lists
Let’s go back to the standard IP access list example where you had to block all access from
the Sales LAN to the nance department and add a new requirement. You now must allow
Sales to gain access to a certain server on the Finance LAN but not to other network services
for security reasons. What’s the solution? Applying a standard IP access list won’t allow users
to get to one network service but not another because a standard ACL won’t allow you to
make decisions based on both source and destination addresses. It makes decisions based
only on source address, so we need another way to achieve our new goal—but what is it?
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516 Chapter 12 u Security
Using an extended access list will save the day because extended ACLs allow us to spec-
ify source and destination addresses as well as the protocol and port number that identify
the upper-layer protocol or application. An extended ACL is just what we need to affec-
tively allow users access to a physical LAN while denying them access to specic hosts—
even specic services on those hosts!
We’re going to take a look at the commands we have in our arsenal, but rst, you need
to know that you must use the extended access-list range from 100 to 199. The 2000–2699
range is also available for extended IP access lists.
After choosing a number in the extended range, you need to decide what type of list
entry to make. For this example, I’m going with a deny list entry:
Corp(config)#access-list 110 ?
deny Specify packets to reject
dynamic Specify a DYNAMIC list of PERMITs or DENYs
permit Specify packets to forward
remark Access list entry comment
And once you’ve settled on the type of ACL, you then need to select a protocol eld entry:
Corp(config)#access-list 110 deny ?
<0-255> An IP protocol number
ahp Authentication Header Protocol
eigrp Cisco's EIGRP routing protocol
esp Encapsulation Security Payload
gre Cisco's GRE tunneling
icmp Internet Control Message Protocol
igmp Internet Gateway Message Protocol
ip Any Internet Protocol
ipinip IP in IP tunneling
nos KA9Q NOS compatible IP over IP tunneling
ospf OSPF routing protocol
pcp Payload Compression Protocol
pim Protocol Independent Multicast
tcp Transmission Control Protocol
udp User Datagram Protocol
If you want to filter by Application layer protocol, you have to choose the
appropriate layer 4 transport protocol after the permit or deny statement.
For example, to filter Telnet or FTP, choose TCP since both Telnet and FTP
use TCP at the Transport layer. Selecting IP wouldn’t allow you to specify
a particular application protocol later and only filter based on source and
destination addresses.
Extended Access Lists 517
So now, let’s lter an Application layer protocol that uses TCP by selecting TCP as the
protocol and indicating the specic destination TCP port at the end of the line. Next, we’ll
be prompted for the source IP address of the host or network and we’ll choose the any com-
mand to allow any source address:
Corp(config)#access-list 110 deny tcp ?
A.B.C.D Source address
any Any source host
host A single source host
After we’ve selected the source address, we can then choose the specic destination address:
Corp(config)#access-list 110 deny tcp any ?
A.B.C.D Destination address
any Any destination host
eq Match only packets on a given port number
gt Match only packets with a greater port number
host A single destination host
lt Match only packets with a lower port number
neq Match only packets not on a given port number
range Match only packets in the range of port numbers
In this output, you can see that any source IP address that has a destination IP address
of 172.16.30.2 has been denied:
Corp(config)#access-list 110 deny tcp any host 172.16.30.2 ?
ack Match on the ACK bit
dscp Match packets with given dscp value
eq Match only packets on a given port number
established Match established connections
fin Match on the FIN bit
fragments Check non-initial fragments
gt Match only packets with a greater port number
log Log matches against this entry
log-input Log matches against this entry, including input interface
lt Match only packets with a lower port number
neq Match only packets not on a given port number
precedence Match packets with given precedence value
psh Match on the PSH bit
range Match only packets in the range of port numbers
rst Match on the RST bit
syn Match on the SYN bit
518 Chapter 12 u Security
time-range Specify a time-range
tos Match packets with given TOS value
urg Match on the URG bit
<cr>
And once we have the destination host addresses in place, we just need to specify the type
of service to deny using the equal to command, entered as eq. The following help screen
reveals the options available now. You can choose a port number or use the application name:
Corp(config)#access-list 110 deny tcp any host 172.16.30.2 eq ?
<0-65535> Port number
bgp Border Gateway Protocol (179)
chargen Character generator (19)
cmd Remote commands (rcmd, 514)
daytime Daytime (13)
discard Discard (9)
domain Domain Name Service (53)
drip Dynamic Routing Information Protocol (3949)
echo Echo (7)
exec Exec (rsh, 512)
finger Finger (79)
ftp File Transfer Protocol (21)
ftp-data FTP data connections (20)
gopher Gopher (70)
hostname NIC hostname server (101)
ident Ident Protocol (113)
irc Internet Relay Chat (194)
klogin Kerberos login (543)
kshell Kerberos shell (544)
login Login (rlogin, 513)
lpd Printer service (515)
nntp Network News Transport Protocol (119)
pim-auto-rp PIM Auto-RP (496)
pop2 Post Office Protocol v2 (109)
pop3 Post Office Protocol v3 (110)
smtp Simple Mail Transport Protocol (25)
sunrpc Sun Remote Procedure Call (111)
syslog Syslog (514)
tacacs TAC Access Control System (49)
talk Talk (517)
telnet Telnet (23)
time Time (37)
Extended Access Lists 519
uucp Unix-to-Unix Copy Program (540)
whois Nicname (43)
www World Wide Web (HTTP, 80)
Okay—now let’s block Telnet (port 23) to host 172.16.30.2 only. If the users want
to use FTP, ne—that’s allowed. The log command is used to log messages every time
the access list entry is hit. This can be an extremely cool way to monitor inappropriate
access attempts, but be careful because in a large network, this command can overload
your console’s screen with messages!
Here’s our result:
Corp(config)#access-list 110 deny tcp any host 172.16.30.2 eq 23 log
This line says to deny any source host trying to telnet to destination host 172.16.30.2.
Keep in mind that the next line is an implicit deny by default. If you apply this access list
to an interface, you might as well just shut the interface down because by default, there’s
an implicit deny all at the end of every access list. So we’ve got to follow up the access list
with the following command:
Corp(config)#access-list 110 permit ip any any
The IP in this line is important because it will permit the IP stack. If TCP was used
instead of IP in this line, then UDP, etc. would all be denied. Remember, the 0.0.0.0
255.255.255.255 is the same command as any, so the command could also look like this:
Corp(config)#access-list 110 permit ip 0.0.0.0 255.255.255.255
0.0.0.0 255.255.255.255
But if you did this, when you looked at the running-cong, the commands would be
replaced with the any any. I like efciency so I’ll just use the any command because it
requires less typing.
As always, once our access list is created, we must apply it to an interface with the same
command used for the IP standard list:
Corp(config-if)#ip access-group 110 in
Or this:
Corp(config-if)#ip access-group 110 out
Next, we’ll check out some examples of how to use an extended access list.
Extended Access List Example 1
For our rst scenario, we’ll use Figure12.5. What do we need to do to deny access to a host
at 172.16.50.5 on the nance department LAN for both Telnet and FTP services? All other
services on this and all other hosts are acceptable for the sales and marketing departments
to access.
520 Chapter 12 u Security
FIGURE12.5 Extended ACL example 1
Sales
172.16.40.0/24
Marketing
172.16.60.0/24
Finance
172.16.50.5/24
S0/0/0
Fa1/0
Lab A
Fa0/0
Fa0/1
Denied!
Internet
ftp data
telnet data
Denied!
Here’s the ACL we must create:
Lab_A#config t
Lab_A(config)#access-list 110 deny tcp any host 172.16.50.5 eq 21
Lab_A(config)#access-list 110 deny tcp any host 172.16.50.5 eq 23
Lab_A(config)#access-list 110 permit ip any any
The access-list 110 tells the router we’re creating an extended IP ACL. The tcp is the
protocol eld in the Network layer header. If the list doesn’t say tcp here, you cannot lter by
TCP port numbers 21 and 23 as shown in the example. Remember that these values indicate
FTP and Telnet, which both use TCP for connection-oriented services. The any command is
the source, which means any source IP address, and the host is the destination IP address.
This ACL says that all IP trafc will be permitted from any host except FTP and Telnet to
host 172.16.50.5 from any source.
Remember that instead of the host 172.16.50.5 command when we
created the extended access list, we could have entered 172.16.50.5
0.0.0.0. There would be no difference in the result other than the router
would change the command to host 172.16.50.5 in the running-config.
After the list is created, it must be applied to the FastEthernet 0/1 interface outbound
because we want to block all trafc from getting to host 172.16.50.5 and performing FTP and
Telnet. If this list was created to block access only from the Sales LAN to host 172.16.50.5,
then we’d have put this list closer to the source, or on FastEthernet 0/0. In that situation, we’d
Extended Access Lists 521
apply the list to inbound trafc. This highlights the fact that you really need to analyze each
situation carefully before creating and applying ACLs!
Okay—now let’s go ahead and apply the list to interface Fa0/1 to block all outside FTP
and Telnet access to the host 172.16.50.5:
Lab_A(config)#int fa0/1
Lab_A(config-if)#ip access-group 110 out
Extended Access List Example 2
We’re going to use Figure12.4 again, which has four LANs and a serial connection. We need
to prevent Telnet access to the networks attached to the E1 and E2 interfaces.
The conguration on the router would look something like this, although the answer
can vary:
Router(config)#access-list 110 deny tcp any 172.16.48.0 0.0.15.255
eq 23
Router(config)#access-list 110 deny tcp any 172.16.192.0 0.0.63.255
eq 23
Router(config)#access-list 110 permit ip any any
Router(config)#interface Ethernet 1
Router(config-if)#ip access-group 110 out
Router(config-if)#interface Ethernet 2
Router(config-if)#ip access-group 110 out
Here are the key factors to understand from this list:
u First, you need to verify that the number range is correct for the type of access list you
are creating. In this example, it’s extended, so the range must be 100–199.
u Second, you must verify that the protocol field matches the upper-layer process or
application, which in this case, is TCP port 23 (Telnet).
The protocol parameter must be TCP since Telnet uses TCP. If it were
TFTP instead, then the protocol parameter would have to be UDP
because TFTP uses UDP at the Transport layer
u Third, verify that the destination port number matches the application you’re filtering
for. In this case, port 23 matches Telnet, which is correct, but know that you can also
type telnet at the end of the line instead of 23.
u Finally, the test statement permit ip any any is important to have there at the end of
the list because it means to enable all packets other than Telnet packets destined for the
LANs connected to Ethernet 1 and Ethernet 2.
522 Chapter 12 u Security
Extended Access List Example 3
I want to guide you through one more extended ACL example before we move on to named
ACLs. Figure12.6 displays the network we’re going to use for this last scenario.
FIGURE12.6 Extended ACL example 3
Fa0/1
Branch
office
Lab A
DNSFinance
172.22.89.26
WEB
192.168.177.1
Host A
192.168.177.2
Host B
192.168.177.3
Host C
Fa0/0
In this example, we’re going to allow HTTP access to the Finance server from source
Host B only. All other trafc will be permitted. We need to be able to congure this in only
three test statements, and then we’ll need to add the interface conguration.
Let’s take what we’ve learned and knock this one out:
Lab_A#config t
Lab_A(config)#access-list 110 permit tcp host 192.168.177.2 host 172.22.89.26 eq 80
Lab_A(config)#access-list 110 deny tcp any host 172.22.89.26 eq 80
Lab_A(config)#access-list 110 permit ip any any
This is really pretty simple! First we need to permit Host B HTTP access to the Finance
server. But since all other trafc must be allowed, we must detail who cannot HTTP to the
Finance server, so the second test statement is there to deny anyone else from using HTTP
on the Finance server. Finally, now that Host B can HTTP to the Finance server and every-
one else can’t, we’ll permit all other trafc with our third test statement.
Okay, not so bad—this just takes a little thought! But wait—we’re not done yet because
we still need to apply this to an interface. Since extended access lists are typically applied
closest to the source, we should simply place this inbound on F0/0, right? Well, this is one
time we’re not going to follow the rules. Our challenge required us to allow only HTTP
Extended Access Lists 523
trafc to the Finance server from Host B. If we apply the ACL inbound on Fa0/0, then the
branch ofce would be able to access the Finance server and perform HTTP. So in this
example, we need to place the ACL closest to the destination:
Lab_A(config)#interface fastethernet 0/1
Lab_A(config-if)#ip access-group 110 out
Perfect! Now let’s get into how to create ACLs using names.
Named ACLs
As I said earlier, named access lists are just another way to create standard and extended
access lists. In medium to large enterprises, managing ACLs can become a real hassle over
time! A handy way to make things easier is to copy the access list to a text editor, edit the
list, then paste the new list back into the router, which works pretty well if it weren’t for
the “pack rat” mentality. It’s really common to think things like, “What if I nd a problem
with the new list and need to back out of the change?” This and other factors cause people
to hoard unapplied ACLs, and over time, they can seriously build up on a router, leading to
more questions, like, “What were these ACLs for? Are they important? Do I need them?”
All good questions, and named access lists are the answer to this problem!
And of course, this kind of thing can also apply to access lists that are up and running.
Let’s say you come into an existing network and are looking at access lists on a router.
Suppose you nd an access list 177, which happens to be an extended access list that’s a
whopping 93 lines long. This leads to more of the same bunch of questions and can even
lead to needless existential despair! Instead, wouldn’t it be a whole lot easier to identify an
access with a name like “FinanceLAN” rather than one mysteriously dubbed “177”?
To our collective relief, named access lists allow us to use names for creating and applying
either standard or extended access lists. There’s really nothing new or different about these
ACLs aside from being readily identiable in a way that makes sense to humans, but there are
some subtle changes to the syntax. So let’s re-create the standard access list we created earlier
for our test network in Figure12.2 using a named access list:
Lab_A#config t
Lab_A(config)# ip access-list ?
extended Extended Access List
log-update Control access list log updates
logging Control access list logging
resequence Resequence Access List
standard Standard Access List
Notice that I started by typing ip access-list, not access-list. Doing this allows me
to enter a named access list. Next, I’ll need to specify it as a standard access list:
Lab_A(config)#ip access-list standard ?
<1-99> Standard IP access-list number
524 Chapter 12 u Security
<1300-1999> Standard IP access-list number (expanded range)
WORD Access-list name
Lab_A(config)#ip access-list standard BlockSales
Lab_A(config-std-nacl)#
I’ve specied a standard access list, then added the name, BlockSales. I denitely could’ve
used a number for a standard access list, but instead, I chose to use a nice, clear descriptive
name. And notice that after entering the name, I hit Enter and the router prompt changed.
This conrms that I’m now in named access list conguration mode and that I’m entering
the named access list:
Lab_A(config-std-nacl)#?
Standard Access List configuration commands:
default Set a command to its defaults
deny Specify packets to reject
exit Exit from access-list configuration mode
no Negate a command or set its defaults
permit Specify packets to forward
Lab_A(config-std-nacl)#deny 172.16.40.0 0.0.0.255
Lab_A(config-std-nacl)#permit any
Lab_A(config-std-nacl)#exit
Lab_A(config)#^Z
Lab_A#
Okay—so I’ve entered the access list and then exited conguration mode. Next, I’ll take
a look at the running conguration to verify that the access list is indeed in the router:
Lab_A#sh running-config | begin ip access
ip access-list standard BlockSales
deny 172.16.40.0 0.0.0.255
permit any
!
And there it is: the BlockSales access list has truly been created and is in the running-cong
of the router. Next, I’ll need to apply the access list to the correct interface:
Lab_A#config t
Lab_A(config)#int fa0/1
Lab_A(config-if)#ip access-group BlockSales out
Clear skies! At this point, we’ve re-created the work done earlier using a named access list.
But let’s take our IP extended example, shown in Figure12.6, and redo that list using a named
ACL instead as well.
Extended Access Lists 525
Same business requirements: Allow HTTP access to the Finance server from source Host B
only. All other trafc is permitted.
Lab_A#config t
Lab_A(config)#ip access-list extended 110
Lab_A(config-ext-nacl)#permit tcp host 192.168.177.2 host 172.22.89.26 eq 80
Lab_A(config-ext-nacl)#deny tcp any host 172.22.89.26 eq 80
Lab_A(config-ext-nacl)#permit ip any any
Lab_A(config-ext-nacl)#int fa0/1
Lab_A(config-if)#ip access-group 110 out
Okay—true—I named the extended list with a number, but sometimes it’s okay to do
that! I’m guessing that named ACLs don’t seem all that exciting or different to you, do they?
Maybe not in this conguration, except that I don’t need to start every line with access-list
110, which is nice. But where named ACLs really shine is that they allow us to insert, delete,
or edit a single line. That isn’t just nice, it’s wonderful! Numbered ACLs just can’t compare
with that, and I’ll demonstrate this in a minute.
Remarks
The remark keyword is really important because it arms you with the ability to include
comments—remarks—regarding the entries you’ve made in both your IP standard and
extended ACLs. Remarks are very cool because they efciently increase your ability to
examine and understand your ACLs to superhero level! Without them, you’d be caught in
a quagmire of potentially meaningless numbers without anything to help you recall what
all those numbers mean.
Even though you have the option of placing your remarks either before or after a permit
or deny statement, I totally recommend that you choose to position them consistently so you
don’t get confused about which remark is relevant to a specic permit or deny statement.
To get this going for both standard and extended ACLs, just use the access-list
access-list number remark remark global conguration command like this:
R2#config t
R2(config)#access-list 110 remark Permit Bob from Sales Only To Finance
R2(config)#access-list 110 permit ip host 172.16.40.1 172.16.50.0 0.0.0.255
R2(config)#access-list 110 deny ip 172.16.40.0 0.0.0.255 172.16.50.0 0.0.0.255
R2(config)#ip access-list extended No_Telnet
R2(config-ext-nacl)#remark Deny all of Sales from Telnetting to Marketing
R2(config-ext-nacl)#deny tcp 172.16.40.0 0.0.0.255 172.16.60.0 0.0.0.255 eq 23
R2(config-ext-nacl)#permit ip any any
R2(config-ext-nacl)#do show run
[output cut]
!
ip access-list extended No_Telnet
526 Chapter 12 u Security
remark Stop all of Sales from Telnetting to Marketing
deny tcp 172.16.40.0 0.0.0.255 172.16.60.0 0.0.0.255 eq telnet
permit ip any any
!
access-list 110 remark Permit Bob from Sales Only To Finance
access-list 110 permit ip host 172.16.40.1 172.16.50.0 0.0.0.255
access-list 110 deny ip 172.16.40.0 0.0.0.255 172.16.50.0 0.0.0.255
access-list 110 permit ip any any
!
Sweet—I was able to add a remark to both an extended list and a named access list.
Keep in mind that you cannot see these remarks in the output of the show access-list
command, which we’ll cover next because they only show up in the running-cong.
Speaking of ACLs, I still need to show you how to monitor and verify them. This is an
important topic, so pay attention!
Monitoring Access Lists
It’s always good to be able to verify a router’s conguration. Table12.1 lists the commands
that we can use to achieve that.
TABLE12.1 Commands used to verify access-list configuration
Command Effect
show access-list Displays all access lists and their parameters configured on
the router. Also shows statistics about how many times the
line either permitted or denied a packet. This command does
not show you which interface the list is applied on.
show access-list 110 Reveals only the parameters for access list 110. Again, this
command will not reveal the specific interface the list is
set on.
show ip access-list Shows only the IP access lists configured on the router.
show ip interface Displays which interfaces have access lists set on them.
show running-config Shows the access lists and the specific interfaces that have
ACLs applied on them.
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Monitoring Access Lists 527
We’ve already used the show running-config command to verify that a named access list
was in the router, so now let’s take a look at the output from some of the other commands.
The show access-list command will list all ACLs on the router, whether they’re
applied to an interface or not:
Lab_A#show access-list
Standard IP access list 10
10 deny 172.16.40.0, wildcard bits 0.0.0.255
20 permit any
Standard IP access list BlockSales
10 deny 172.16.40.0, wildcard bits 0.0.0.255
20 permit any
Extended IP access list 110
10 deny tcp any host 172.16.30.5 eq ftp
20 deny tcp any host 172.16.30.5 eq telnet
30 permit ip any any
40 permit tcp host 192.168.177.2 host 172.22.89.26 eq www
50 deny tcp any host 172.22.89.26 eq www
Lab_A#
First, notice that access list 10 as well as both of our named access lists appear on this
list—remember, my extended named ACL was named 110! Second, notice that even though
I entered actual numbers for TCP ports in access list 110, the show command gives us the
protocol names rather than TCP ports for serious clarity.
But wait! The best part is those numbers on the left side: 10, 20, 30, etc. Those are
called sequence numbers, and they allow us to edit our named ACL. Here’s an example
where I added a line into the named extended ACL 110:
Lab_A (config)#ip access-list extended 110
Lab_A (config-ext-nacl)#21 deny udp any host 172.16.30.5 eq 69
Lab_A#show access-list
[output cut]
Extended IP access list 110
10 deny tcp any host 172.16.30.5 eq ftp
20 deny tcp any host 172.16.30.5 eq telnet
21 deny udp any host 172.16.30.5 eq tftp
30 permit ip any any
40 permit tcp host 192.168.177.2 host 172.22.89.26 eq www
50 deny tcp any host 172.22.89.26 eq www
You can see that I added line 21. I could have deleted a line or edited an existing line as
well—very nice!
528 Chapter 12 u Security
Here’s the output of the show ip interface command:
Lab_A#show ip interface fa0/1
FastEthernet0/1 is up, line protocol is up
Internet address is 172.16.30.1/24
Broadcast address is 255.255.255.255
Address determined by non-volatile memory
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Outgoing access list is 110
Inbound access list is not set
Proxy ARP is enabled
Security level is default
Split horizon is enabled
[output cut]
Be sure to notice the bold line indicating that the outgoing list on this interface is 110, yet
the inbound access list isn’t set. What happened to BlockSales? I had congured that outbound
on Fa0/1! That’s true, I did, but I congured my extended named ACL 110 and applied it to
Fa0/1 as well. You can’t have two lists on the same interface, in the same direction, so what
happened here is that my last conguration overwrote the BlockSales conguration.
And as I’ve already mentioned, you can use the show running-config command to see
any and all access lists.
Summary
In this chapter you learned how to congure standard access lists to properly lter IP trafc.
You discovered what a standard access list is and how to apply it to a Cisco router to add secu-
rity to your network. You also learned how to congure extended access lists to further lter
IP trafc. We also covered the key differences between standard and extended access lists as
well as how to apply them to Cisco routers.
Moving on, you found out how to congure named access lists and apply them to
interfaces on the router and learned that named access lists offer the huge advantage of
being easily identiable and, therefore, a whole lot easier to manage than mysterious
access lists that are simply referred to by obscure numbers.
Appendix C, “Disabling and Conguring Network Services,” which takes off from this
chapter, had a fun section in it: turning off default services. I’ve always found performing
this administration task fun, and the auto secure command can help us congure basic,
much-needed security on our routers.
The chapter wrapped up by showing you how to monitor and verify selected access-list
congurations on a router.
Exam Essentials 529
Exam Essentials
Remember the standard and extended IP access-list number ranges. The number ranges
you can use to congure a standard IP access list are 1–99 and 1300–1999. The number
ranges for an extended IP access list are 100–199 and 2000–2699.
Understand the term implicit deny. At the end of every access list is an implicit deny.
What this means is that if a packet does not match any of the lines in the access list, it will
be discarded. Also, if you have nothing but deny statements in your list, the list will not
permit any packets.
Understand the standard IP access-list configuration command. To congure a standard
IP access list, use the access-list numbers 1–99 or 1300–1999 in global conguration mode.
Choose permit or deny, then choose the source IP address you want to lter on using one
of the three techniques covered in this chapter.
Understand the extended IP access-list configuration command. To congure an extended
IP access list, use the access-list numbers 100–199 or 2000–2699 in global conguration
mode. Choose permit or deny, the Network layer protocol eld, the source IP address you
want to lter on, the destination address you want to lter on, and nally, the Transport
layer port number if TCP or UDP has been specied as the protocol.
Remember the command to verify an access list on a router interface. To see whether
an access list is set on an interface and in which direction it is ltering, use the show ip
interface command. This command will not show you the contents of the access list,
merely which access lists are applied on the interface.
Remember the command to verify the access-list configuration. To see the congured
access lists on your router, use the show access-list command. This command will not
show you which interfaces have an access list set.
530 Chapter 12 u Security
Written Lab 12
The answers to this lab can be found in Appendix A, “Answers to Written Labs.”
In this section, write the answers to the following questions:
1. What command would you use to configure a standard IP access list to prevent all
machines on network 172.16.0.0/16 from accessing your Ethernet network?
2. What command would you use to apply the access list you created in question 1 to an
Ethernet interface outbound?
3. What command(s) would you use to create an access list that denies host 192.168.15.5
access to an Ethernet network?
4. Which command verifies that you’ve entered the access list correctly?
5. What two tools can help notify and prevent DoS attacks?
6. What command(s) would you use to create an extended access list that stops host
172.16.10.1 from telnetting to host 172.16.30.5?
7. What command would you use to set an access list on a VTY line?
8. Write the same standard IP access list you wrote in question 1 but this time as a named
access list.
9. Write the command to apply the named access list you created in question 8 to an
Ethernet interface outbound.
10. Which command verifies the placement and direction of an access list?
Hands-on Labs
In this section, you will complete two labs. To complete these labs, you will need at least
three routers. You can easily perform these labs with the Cisco Packet Tracer program. If
you are studying to take your Cisco exam, you really need to do these labs!
Lab 12.1: Standard IP Access Lists
Lab 12.2: Extended IP Access Lists
Hands-on Labs 531
All of the labs will use the following diagram for conguring the routers.
Fa0/0
192.168.10.2
SF
Fa0/0: 192.168.10.1/24
S0/0/0: 172.16.10.2/30
Fa0/0
Corp
Serial 0/0: 172.16.10.1/30
Serial 0/1: 172.16.10.5/30
Fa0/0: 10.10.10.1/24
SF
Fa0/0
192.168.20.2
LA
Fa0/0: 192.168.20.1/24
S0/0/0: 172.16.10.6/30
LA
Corp
S0/0
S0/0/0 S0/0/1
S0/1
Hands-on Lab 12.1: Standard IP Access Lists
In this lab, you will allow only packets from a single host on the SF LAN to enter the
LA LAN.
1. Go to LA router and enter global configuration mode by typing config t.
2. From global configuration mode, type access-list ? to get a list of all the different
access lists available.
3. Choose an access-list number that will allow you to create an IP standard access list.
This is a number between 1 and 99 or 1300 and 1399.
4. Choose to permit host 192.168.10.2, which is the host address:
LA(config)#access-list 10 permit 192.168.20.2 ?
A.B.C.D Wildcard bits
<cr>
To specify only host 192.168.20.2, use the wildcards 0.0.0.0:
LA(config)#access-list 10 permit 192.168.20.2
0.0.0.0
5. Now that the access list is created, you must apply it to an interface to make it work:
LA(config)#int f0/0
Lab_A(config-if)#ip access-group 10 out
532 Chapter 12 u Security
6. Verify your access list with the following commands:
LA#sh access-list
Standard IP access list 10
permit 172.16.30.2
LA#sh run
[output cut]
interface FastEthernet0/0
ip address 192.168.20.1 255.255.255.0
ip access-group 10 out
7. Test your access list by pinging from 192.168.10.2 to 192.168.20.2.
8. If you have another host on the LA LAN, ping that address, which should fail if your
ACL is working.
Hands-on Lab 12.2: Extended IP Access Lists
In this lab, you will use an extended IP access list to stop host 192.168.10.2 from creating a
Telnet session to router LA (172.16.10.6). However, the host still should be able to ping the
LA router. IP extended lists should be placed close to the source, so add the extended list on
router SF. Pay attention to the log command used in Step 6. It is a Cisco objective!
1. Remove any access lists on SF and add an extended list to SF.
2. Choose a number to create an extended IP list. The IP extended lists use 100–199 or
2000–2699.
3. Use a deny statement. (You’ll add a permit statement in step 7 to allow other traffic to
still work.)
SF(config)#access-list 110 deny ?
<0-255> An IP protocol number
ahp Authentication Header Protocol
eigrp Cisco's EIGRP routing protocol
esp Encapsulation Security Payload
gre Cisco's GRE tunneling
icmp Internet Control Message Protocol
igmp Internet Gateway Message Protocol
igrp Cisco's IGRP routing protocol
ip Any Internet Protocol
ipinip IP in IP tunneling
nos KA9Q NOS compatible IP over IP tunneling
ospf OSPF routing protocol
pcp Payload Compression Protocol
Hands-on Labs 533
tcp Transmission Control Protocol
udp User Datagram Protocol
4. Since you are going to deny Telnet, you must choose TCP as a Transport layer protocol:
SF(config)#access-list 110 deny tcp ?
A.B.C.D Source address
any Any source host
host A single source host
5. Add the source IP address you want to filter on, then add the destination host IP address.
Use the host command instead of wildcard bits.
SF(config)#access-list 110 deny tcp host
192.168.10.2 host 172.16.10.6 ?
ack Match on the ACK bit
eq Match only packets on a given port
number
established Match established connections
fin Match on the FIN bit
fragments Check fragments
gt Match only packets with a greater
port number
log Log matches against this entry
log-input Log matches against this entry,
including input interface
lt Match only packets with a lower port
number
neq Match only packets not on a given
port number
precedence Match packets with given precedence
value
psh Match on the PSH bit
range Match only packets in the range of
port numbers
rst Match on the RST bit
syn Match on the SYN bit
tos Match packets with given TOS value
urg Match on the URG bit
<cr>
534 Chapter 12 u Security
6. At this point, you can add the eq telnet command to filter host 192.168.10.2 from
telnetting to 172.16.10.6. The log command can also be used at the end of the com-
mand so that whenever the access-list line is hit, a log will be generated on the console.
SF(config)#access-list 110 deny tcp host
192.168.10.2 host 172.16.10.6 eq telnet log
7. It is important to add this line next to create a permit statement. (Remember that
0.0.0.0 255.255.255.255 is the same as the any command.)
SF(config)#access-list 110 permit ip any 0.0.0.0
255.255.255.255
You must create a permit statement; if you just add a deny statement, nothing will be
permitted at all. Please see the sections earlier in this chapter for more detailed infor-
mation on the deny any command implied at the end of every ACL.
8. Apply the access list to the FastEthernet0/0 on SF to stop the Telnet traffic as soon as it
hits the first router interface.
SF(config)#int f0/0
SF(config-if)#ip access-group 110 in
SF(config-if)#^Z
9. Try telnetting from host 192.168.10.2 to LA using the destination IP address of
172.16.10.6. This should fail, but the ping command should work.
10. On the console of SF, because of the log command, the output should appear as follows:
01:11:48: %SEC-6-IPACCESSLOGP: list 110 denied tcp
192.168.10.2(1030) -> 172.16.10.6(23), 1 packet
01:13:04: %SEC-6-IPACCESSLOGP: list 110 denied tcp
192.168.10.2(1030) -> 172.16.10.6(23), 3 packets
Review Questions 535
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional questions,
please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. Which of the following statements is false when a packet is being compared to an
access list?
A. It’s always compared with each line of the access list in sequential order.
B. Once the packet matches the condition on a line of the access list, the packet is
acted upon and no further comparisons take place.
C. There is an implicit “deny” at the end of each access list.
D. Until all lines have been analyzed, the comparison is not over.
2. You need to create an access list that will prevent hosts in the network range of
192.168.160.0 to 192.168.191.0. Which of the following lists will you use?
A. access-list 10 deny 192.168.160.0 255.255.224.0
B. access-list 10 deny 192.168.160.0 0.0.191.255
C. access-list 10 deny 192.168.160.0 0.0.31.255
D. access-list 10 deny 192.168.0.0 0.0.31.255
3. You have created a named access list called Blocksales. Which of the following is a valid
command for applying this to packets trying to enter interface Fa0/0 of your router?
A. (config)#ip access-group 110 in
B. (config-if)#ip access-group 110 in
C. (config-if)#ip access-group Blocksales in
D. (config-if)#Blocksales ip access-list in
4. Which access list statement will permit all HTTP sessions to network 192.168.144.0/24
containing web servers?
A. access-list 110 permit tcp 192.168.144.0 0.0.0.255 any eq 80
B. access-list 110 permit tcp any 192.168.144.0 0.0.0.255 eq 80
C. access-list 110 permit tcp 192.168.144.0 0.0.0.255 192.168.144.0
0.0.0.255 any eq 80
D. access-list 110 permit udp any 192.168.144.0 eq 80
536 Chapter 12 u Security
5. Which of the following access lists will allow only HTTP traffic into network
196.15.7.0?
A. access-list 100 permit tcp any 196.15.7.0 0.0.0.255 eq www
B. access-list 10 deny tcp any 196.15.7.0 eq www
C. access-list 100 permit 196.15.7.0 0.0.0.255 eq www
D. access-list 110 permit ip any 196.15.7.0 0.0.0.255
E. access-list 110 permit www 196.15.7.0 0.0.0.255
6. What router command allows you to determine whether an IP access list is enabled on
a particular interface?
A. show ip port
B. show access-lists
C. show ip interface
D. show access-lists interface
7. In the work area, connect the show command to its function on the right.
show access-list Shows only the parameters for the access list 110.
This command does not show you the interface the list
is set on.
show access-list 110 Shows only the IP access lists configured on the router.
show ip access-list Shows which interfaces have access lists set.
show ip interface Displays all access lists and their parameters configured
on the router. This command does not show you which
interface the list is set on.
8. If you wanted to deny all Telnet connections to only network 192.168.10.0, which
command could you use?
A. access-list 100 deny tcp 192.168.10.0 255.255.255.0 eq telnet
B. access-list 100 deny tcp 192.168.10.0 0.255.255.255 eq telnet
C. access-list 100 deny tcp any 192.168.10.0 0.0.0.255 eq 23
D. access-list 100 deny 192.168.10.0 0.0.0.255 any eq 23
Review Questions 537
9. If you wanted to deny FTP access from network 200.200.10.0 to network 200.199.11.0
but allow everything else, which of the following command strings is valid?
A. access-list 110 deny 200.200.10.0 to network 200.199.11.0 eq ftp
access-list 111 permit ip any 0.0.0.0 255.255.255.255
B. access-list 1 deny ftp 200.200.10.0 200.199.11.0 any any
C. access-list 100 deny tcp 200.200.10.0 0.0.0.255 200.199.11.0 0.0.0.255
eq ftp
D. access-list 198 deny tcp 200.200.10.0 0.0.0.255 200.199.11.0 0.0.0.255
eq ftp
access-list 198 permit ip any 0.0.0.0 255.255.255.255
10. You want to create an extended access list that denies the subnet of the following host:
172.16.50.172/20. Which of the following would you start your list with?
A. access-list 110 deny ip 172.16.48.0 255.255.240.0 any
B. access-list 110 udp deny 172.16.0.0 0.0.255.255 ip any
C. access-list 110 deny tcp 172.16.64.0 0.0.31.255 any eq 80
D. access-list 110 deny ip 172.16.48.0 0.0.15.255 any
11. Which of the following is the wildcard (inverse) version of a /27 mask?
A. 0.0.0.7
B. 0.0.0.31
C. 0.0.0.27
D. 0.0.31.255
12. You want to create an extended access list that denies the subnet of the following host:
172.16.198.94/19. Which of the following would you start your list with?
A. access-list 110 deny ip 172.16.192.0 0.0.31.255 any
B. access-list 110 deny ip 172.16.0.0 0.0.255.255 any
C. access-list 10 deny ip 172.16.172.0 0.0.31.255 any
D. access-list 110 deny ip 172.16.188.0 0.0.15.255 any
13. The following access list has been applied to an interface on a router:
access-list 101 deny tcp 199.111.16.32 0.0.0.31 host 199.168.5.60
Which of the following IP addresses will be blocked because of this single rule in the
list? (Choose all that apply.)
A. 199.111.16.67
B. 199.111.16.38
C. 199.111.16.65
D. 199.11.16.54
538 Chapter 12 u Security
14. Which of the following commands connects access list 110 inbound to interface
Ethernet0?
A. Router(config)#ip access-group 110 in
B. Router(config)#ip access-list 110 in
C. Router(config-if)#ip access-group 110 in
D. Router(config-if)#ip access-list 110 in
15. What is the effect of this single-line access list?
access-list 110 deny ip 172.16.10.0 0.0.0.255 host 1.1.1.1
A. Denies only the computer at 172.16.10
B. Denies all traffic
C. Denies the subnet 172.16.10.0/26
D. Denies the subnet 172.16.10.0/25
16. You configure the following access list. What will the result of this access list be?
access-list 110 deny tcp 10.1.1.128 0.0.0.63 any eq smtp
access-list 110 deny tcp any any eq 23
int ethernet 0
ip access-group 110 out
A. Email and Telnet will be allowed out E0.
B. Email and Telnet will be allowed in E0.
C. Everything but email and Telnet will be allowed out E0.
D. No IP traffic will be allowed out E0.
17. Which of the following series of commands will restrict Telnet access to the router?
A. Lab_A(config)#access-list 10 permit 172.16.1.1
Lab_A(config)#line con 0
Lab_A(config-line)#ip access-group 10 in
B. Lab_A(config)#access-list 10 permit 172.16.1.1
Lab_A(config)#line vty 0 4
Lab_A(config-line)#access-class 10 out
C. Lab_A(config)#access-list 10 permit 172.16.1.1
Lab_A(config)#line vty 0 4
Lab_A(config-line)#access-class 10 in
D. Lab_A(config)#access-list 10 permit 172.16.1.1
Lab_A(config)#line vty 0 4
Lab_A(config-line)#ip access-group 10 in
Review Questions 539
18. Which of the following is true regarding access lists applied to an interface?
A. You can place as many access lists as you want on any interface until you run out
of memory.
B. You can apply only one access list on any interface.
C. One access list may be configured, per direction, for each layer 3 protocol configured
on an interface.
D. You can apply two access lists to any interface.
19. What is the most common attack on a network today?
A. Lock picking
B. Naggle
C. DoS
D. auto secure
20. You need to stop DoS attacks in real time and have a log of anyone who has tried to
attack your network. What should you do your network?
A. Add more routers.
B. Use the auto secure command.
C. Implement IDS/IPS.
D. Configure Naggle.
Chapter
13
Network Address
Translation (NAT)
THE FOLLOWING ICND1 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 Operation of IP Data Networks
■Predict the data flow between two hosts across a network
1 IP Services
■Identify the basic operation of NAT
■Purpose
■Pool
■Static
■one-to-one
■Overloading
■Source addressing
■One-way NAT
■Configure and verify NAT for given network requirements
In this chapter, we’re going to dig into Network Address
Translation (NAT), Dynamic NAT, and Port Address Translation
(PAT), also known as NAT Overload. Of course, I’ll demonstrate
all the NAT commands. I also provided some fantastic hands-on labs for you to congure at the
end of this chapter, so be sure not to miss those!
It’s important to understand the Cisco objectives for this chapter. They are very straight-
forward: you have hosts on your inside Corporate network using RFC 1918 addresses and
you need to allow those hosts access to the Internet by conguring NAT translations. With
that objective in mind, that will be my direction with this chapter.
Because we’ll be using ACLs in our NAT congurations, it’s important that you’re
really comfortable with the skills you learned in the previous chapter before proceeding
with this one.
To find up-to-the-minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
When Do We Use NAT?
Network Address Translation (NAT) is similar to Classless Inter-Domain Routing (CIDR)
in that the original intention for NAT was to slow the depletion of available IP address space
by allowing multiple private IP addresses to be represented by a much smaller number of
public IP addresses.
Since then, it’s been discovered that NAT is also a useful tool for network migrations
and mergers, server load sharing, and creating “virtual servers.” So in this chapter, I’m
going to describe the basics of NAT functionality and the terminology common to NAT.
Because NAT really decreases the overwhelming amount of public IP addresses required
in a networking environment, it comes in really handy when two companies that have dupli-
cate internal addressing schemes merge. NAT is also a great tool to use when an organization
changes its Internet service provider (ISP) but the networking manager needs to avoid the
hassle of changing the internal address scheme.
Here’s a list of situations when NAT can be especially helpful:
u When you need to connect to the Internet and your hosts don’t have globally unique
IP addresses
When Do We Use NAT? 543
u When you’ve changed to a new ISP that requires you to renumber your network
u When you need to merge two intranets with duplicate addresses
You typically use NAT on a border router. For example, in Figure13.1, NAT is used on
the Corporate router connected to the Internet.
FIGURE13.1 Where to configure NAT
Global (real) Internet addresses
Private RFC 1918 addresses
Corporate
Internet
Engineering Sales
Now you may be thinking, “NAT’s totally cool and I just gotta have it!” But don’t get
too excited yet because there are some serious snags related to using NAT that you need to
understand rst. Don’t get me wrong—it can truly be a lifesaver sometimes, but NAT has a
bit of a dark side you need to know about too. For the pros and cons linked to using NAT,
check out Table13.1.
TABLE13.1 Advantages and disadvantages of implementing NAT
Advantages Disadvantages
Conserves legally registered addresses. Translation results in switching path delays.
Remedies address overlap events. Causes loss of end-to-end IP traceability
Increases flexibility when connecting
to the Internet.
Certain applications will not function with
NAT enabled
Eliminates address renumbering as a
network evolves.
544 Chapter 13 u Network Address Translation (NAT)
The most obvious advantage associated with NAT is that it allows you to
conserve your legally registered address scheme. But a version of it known
as PAT is also why we’ve only just recently run out of IPv4 addresses. With-
out NAT/PAT, we’d have run out of IPv4 addresses more than a decade ago!
Types of Network Address Translation
In this section, I’m going to go over the three types of NATs with you:
Static NAT This type of NAT is designed to allow one-to-one mapping between local and
global addresses. Keep in mind that the static version requires you to have one real Internet
IP address for every host on your network.
Dynamic NAT This version gives you the ability to map an unregistered IP address to a
registered IP address from out of a pool of registered IP addresses. You don’t have to stati-
cally congure your router to map each inside address to an individual outside address as
you would using static NAT, but you do have to have enough real, bona de IP addresses
for everyone who’s going to be sending packets to and receiving them from the Internet at
the same time.
Overloading This is the most popular type of NAT conguration. Understand that over-
loading really is a form of dynamic NAT that maps multiple unregistered IP addresses to a
single registered IP address (many-to-one) by using different source ports. Now, why is this
so special? Well, because it’s also known as Port Address Translation (PAT), which is also
commonly referred to as NAT Overload. Using PAT allows you to permit thousands of users
to connect to the Internet using only one real global IP address—pretty slick, right? Seriously,
NAT Overload is the real reason we haven’t run out of valid IP addresses on the Internet.
Really—I’m not joking!
I’ll show you how to configure all three types of NAT throughout this chapter
and at the end of this chapter with the hands-on labs.
NAT Names
The names we use to describe the addresses used with NAT are fairly straightforward.
Addresses used after NAT translations are called global addresses. These are usually the pub-
lic addresses used on the Internet, which you don’t need if you aren’t going on the Internet.
Visit ccna
.gg/ch13/a
for a
companion
MicroNugget
from CBT
Nuggets.
How NAT Works 545
Local addresses are the ones we use before NAT translation. This means that the inside
local address is actually the private address of the sending host that’s attempting to get to the
Internet. The outside local address would typically be the router interface connected to your
ISP and is also usually a public address used as the packet begins its journey.
After translation, the inside local address is then called the inside global address and the
outside global address then becomes the address of the destination host. Check out Table13.2,
which lists all this terminology and offers a clear picture of the various names used with NAT.
Keep in mind that these terms and their denitions can vary somewhat based on implementa-
tion. The table shows how they’re used according to the Cisco exam objectives.
TABLE13.2 NAT terms
Names Meaning
Inside local Source host inside address before translation—typically an RFC
1918 address.
Outside local Address from which source host is known on the Internet. This is
usually the address of the router interface connected to ISP—the
actual Internet address.
Inside global Source host address used after translation to get onto the Internet.
This is also the actual Internet address.
Outside global Address of outside destination host and, again, the real
Internet address.
How NAT Works
Okay, it’s time to look at how this whole NAT thing works. I’m going to start by using
Figure13.2 to describe basic NAT translation.
In this gure, we can see host 10.1.1.1 sending an Internet-bound packet to the border
router congured with NAT. The router identies the source IP address as an inside local
IP address destined for an outside network, translates the source IP address in the packet,
and documents the translation in the NAT table.
The packet is sent to the outside interface with the new translated source address. The
external host returns the packet to the destination host and the NAT router translates the
inside global IP address back to the inside local IP address using the NAT table. This is as
simple as it gets!
Let’s take a look at a more complex conguration using overloading, also referred to as
PAT. I’ll use Figure13.3 to demonstrate how PAT works by having an inside host HTTP to
a server on the Internet.
546 Chapter 13 u Network Address Translation (NAT)
FIGURE13.2 Basic NAT translation
Internet
Inside
NAT Table
Inside Local
IP Addresses
Inside Global
IP Addresses
10.1.1.3
10.1.1.2
10.1.1.1
170.168.2.4
170.168.2.3
170.168.2.2
SA
170.168.2.2
Host B
63.40.7.3
10.1.1.3
DA
10.1.1.1
DA
170.168.2.2
SA
10.1.1.1
10.1.1.2
10.1.1.1
FIGURE13.3 NAT overloading example (PAT)
Internet
Inside
NAT Table
Inside Local
IP Addresses
Inside Global
IP Addresses
10.1.1.3:1024
10.1.1.2:1025
10.1.1.1:1026
170.168.2.1:1024
170.168.2.1:1025
170.168.2.1:1026
DA
170.168.2.1:80
Host B
63.40.7.3
10.1.1.3
DA
10.1.1.1:1026
DA
170.168.2.1:1026
SA
10.1.1.1:1026
10.1.1.2
10.1.1.1
With PAT, all inside hosts get translated to one single IP address, hence the term
overloading. Again, the reason we’ve just run out of available global IP addresses on the
Internet is because of overloading (PAT).
How NAT Works 547
Take a look at the NAT table in Figure13.3 again. In addition to the inside local IP
address and inside global IP address, we now have port numbers. These port numbers help
the router identify which host should receive the return trafc. The router uses the source
port number from each host to differentiate the trafc from each of them. Notice the packet
has a destination port number of 80 when it leaves the router, and the HTTP server sends
back the data with a destination port number of 1026, in this example. This allows the NAT
translation router to differentiate between hosts in the NAT table and then translate the desti-
nation IP address back to the inside local address.
Port numbers are used at the Transport layer to identify the local host in this example. If
we had to use real global IP addresses to identify the source hosts, that’s called static NAT
and we would run out of addresses. PAT allows us to use the Transport layer to identify the
hosts, which in turn allows us to theoretically use up to about 65,000 hosts with only one
real IP address!
Static NAT Configuration
Let’s take a look at a simple example of a basic static NAT conguration:
ip nat inside source static 10.1.1.1 170.46.2.2
!
interface Ethernet0
ip address 10.1.1.10 255.255.255.0
ip nat inside
!
interface Serial0
ip address 170.46.2.1 255.255.255.0
ip nat outside
!
In the preceding router output, the ip nat inside source command identies which
IP addresses will be translated. In this conguration example, the ip nat inside source
command congures a static translation between the inside local IP address 10.1.1.1 to the
outside global IP address 170.46.2.2.
Scrolling farther down in the conguration, we nd an ip nat command under each
interface. The ip nat inside command identies that interface as the inside interface.
The ip nat outside command identies that interface as the outside interface. When
you look back at the ip nat inside source command, you can see that the command
is referencing the inside interface as the source or starting point of the translation. You
could also use the command like this: ip nat outside source. This option indicates the
interface that you designated as the outside interface should become the source or start-
ing point for the translation.
548 Chapter 13 u Network Address Translation (NAT)
Dynamic NAT Configuration
Basically, dynamic NAT really means we have a pool of addresses that we’ll use to provide
real IP addresses to a group of users on the inside. Because we don’t use port numbers, we
must have real IP addresses for every user who’s trying to get outside the local network
simultaneously.
Here is a sample output of a dynamic NAT conguration:
ip nat pool todd 170.168.2.3 170.168.2.254
netmask 255.255.255.0
ip nat inside source list 1 pool todd
!
interface Ethernet0
ip address 10.1.1.10 255.255.255.0
ip nat inside
!
interface Serial0
ip address 170.168.2.1 255.255.255.0
ip nat outside
!
access-list 1 permit 10.1.1.0 0.0.0.255
!
The ip nat inside source list 1 pool todd command tells the router to translate IP
addresses that match access-list 1 to an address found in the IP NAT pool named todd.
Here the ACL isn’t there to lter trafc for security reasons by permitting or denying trafc.
In this case, it’s there to select or designate what we often call interesting trafc. When inter-
esting trafc has been matched with the access list, it’s pulled into the NAT process to be
translated. This is actually a common use for access lists, which aren’t always just stuck with
the dull job of just blocking trafc at an interface!
The command ip nat pool todd 170.168.2.3 192.168.2.254 netmask
255.255.255.0 creates a pool of addresses that will be distributed to the specic hosts that
require global addresses. When troubleshooting NAT for the Cisco objectives, always check
this pool to conrm that there are enough addresses in it to provide translation for all the
inside hosts. Last, check to make sure the pool names match exactly on both lines, remem-
bering that they are case sensitive; if they don’t, the pool won’t work!
PAT (Overloading) Configuration
This last example shows how to congure inside global address overloading. This is the typi-
cal form of NAT that we would use today. It’s actually now rare to use static or dynamic NAT
unless it is for something like statically mapping a server, for example.
How NAT Works 549
Here is a sample output of a PAT conguration:
ip nat pool globalnet 170.168.2.1 170.168.2.1 netmask 255.255.255.0
ip nat inside source list 1 pool globalnet overload
!
interface Ethernet0/0
ip address 10.1.1.10 255.255.255.0
ip nat inside
!
interface Serial0/0
ip address 170.168.2.1 255.255.255.0
ip nat outside
!
access-list 1 permit 10.1.1.0 0.0.0.255
The nice thing about PAT is that these are the only differences between this conguration
and the previous dynamic NAT conguration:
u Our pool of addresses has shrunk to only one IP address.
u We included the overload keyword at the end of our ip nat inside source command.
A really key factor to see in the example is that the one IP address that’s in the pool for
us to use is the IP address of the outside interface. This is perfect if you are conguring
NAT Overload for yourself at home or for a small ofce that only has one IP address from
your ISP. You could, however, use an additional address such as 170.168.2.2 if you had
that address available to you as well, and doing that could prove very helpful in a very large
implementation where you’ve got such an abundance of simultaneously active internal users
that you need to have more than one overloaded IP address on the outside!
Simple Verification of NAT
Okay—as always, once you’ve chosen and congured the type of NAT you’re going to run,
which is typically PAT, you must be able to verify your conguration.
To see basic IP address translation information, use the following command:
Router#show ip nat translations
When looking at the IP NAT translations, you may see many translations from the same
host to the corresponding host at the destination. Understand that this is typical when there
are many connections to the same server.
You can also verify your NAT conguration via the debug ip nat command. This
output will show the sending address, the translation, and the destination address on each
debug line:
Router#debug ip nat
550 Chapter 13 u Network Address Translation (NAT)
But wait—how do you clear your NAT entries from the translation table? Just use the
clear ip nat translation command, and if you want to clear all entries from the NAT
table, just use an asterisk (*) at the end of the command.
Testing and Troubleshooting NAT
Cisco’s NAT gives you some serious power—and it does so without much effort, because
the congurations are really pretty simple. But we all know nothing’s perfect, so in case
something goes wrong, you can gure out some of the more common culprits by running
through this list of potential causes:
u Check the dynamic pools. Are they composed of the right scope of addresses?
u Check to see if any dynamic pools overlap.
u Check to see if the addresses used for static mapping and those in the dynamic
pools overlap.
u Ensure that your access lists specify the correct addresses for translation.
u Make sure there aren’t any addresses left out that need to be there, and ensure that
none are included that shouldn’t be.
u Check to make sure you’ve got both the inside and outside interfaces delimited properly.
A key thing to keep in mind is that one of the most common problems with a new NAT
conguration often isn’t specic to NAT at all—it usually involves a routing blooper. So,
because you’re changing a source or destination address in a packet, make sure your router
still knows what to do with the new address after the translation!
The rst command you should typically use is the show ip nat translations command:
Router#show ip nat trans
Pro Inside global Inside local Outside local Outside global
--- 192.2.2.1 10.1.1.1 --- ---
--- 192.2.2.2 10.1.1.2 --- ---
After checking out this output, can you tell me if the conguration on the router is static
or dynamic NAT? The answer is yes, either static or dynamic NAT is congured because
there’s a one-to-one translation from the inside local to the inside global. Basically, by look-
ing at the output, you can’t tell if it’s static or dynamic per se; but you absolutely can tell
that you’re not using PAT because there are no port numbers.
Let’s take a look at another output:
Router#sh ip nat trans
Pro Inside global Inside local Outside local Outside global
tcp 170.168.2.1:11003 10.1.1.1:11003 172.40.2.2:23 172.40.2.2:23
tcp 170.168.2.1:1067 10.1.1.1:1067 172.40.2.3:23 172.40.2.3:23
Testing and Troubleshooting NAT 551
Okay, you can easily see that the above output is using NAT Overload (PAT). The proto-
col in this output is TCP, and the inside global address is the same for both entries.
Supposedly the sky’s the limit regarding the number of mappings the NAT table can
hold. But this is reality, so things like memory and CPU, or even the boundaries set in place
by the scope of available addresses or ports, can cause limitations on the actual number
of entries. Consider that each NAT mapping devours about 160 bytes of memory. And
sometimes the amount of entries must be limited for the sake of performance or because of
policy restrictions, but this doesn’t happen very often. In situations like these, just go to the
ip nat translation max-entries command for help.
Another handy command for troubleshooting is show ip nat statistics. Deploying
this gives you a summary of the NAT conguration, and it will count the number of active
translation types too. Also counted are hits to an existing mapping as well any misses, with
the latter causing an attempt to create a mapping. This command will also reveal expired
translations. If you want to: check into dynamic pools, their types, the total available
addresses, how many addresses have been allocated and how many have failed, plus the
number of translations that have occurred, just use the pool keyword.
Here is an example of the basic NAT debugging command:
Router#debug ip nat
NAT: s=10.1.1.1->192.168.2.1, d=172.16.2.2 [0]
NAT: s=172.16.2.2, d=192.168.2.1->10.1.1.1 [0]
NAT: s=10.1.1.1->192.168.2.1, d=172.16.2.2 [1]
NAT: s=10.1.1.1->192.168.2.1, d=172.16.2.2 [2]
NAT: s=10.1.1.1->192.168.2.1, d=172.16.2.2 [3]
NAT*: s=172.16.2.2, d=192.168.2.1->10.1.1.1 [1]
Notice the last line in the output and how the NAT at the beginning of the line has an
asterisk (*). This means the packet was translated and fast-switched to the destination.
What’s fast-switched? Well in brief, fast-switching has gone by several aliases such as
cache-based switching and this nicely descriptive name, “route one switch many.” The fast-
switching process is used on Cisco routers to create a cache of layer 3 routing information
to be accessed at layer 2 so packets can be forwarded quickly through a router without the
routing table having to be parsed for every packet. As packets are packet switched (looked
up in the routing table), this information is stored in the cache for later use if needed for
faster routing processing.
Okay, let’s get back to verifying NAT. Did you know you can manually clear dynamic
NAT entries from the NAT table? You can, and doing this can come in seriously handy if
you need to get rid of a specic rotten entry without sitting around waiting for the timeout
to expire! A manual clear is also really useful when you want to clear the whole NAT table
to recongure a pool of addresses.
You also need to know that the Cisco IOS software just won’t allow you to change or
delete an address pool if any of that pool’s addresses are mapped in the NAT table. The
clear ip nat translations command clears entries—you can indicate a single entry
via the global and local address and through TCP and UDP translations, including ports,
552 Chapter 13 u Network Address Translation (NAT)
or you can just type in an asterisk (*) to wipe out the entire table. But know that if you
do that, only dynamic entries will be cleared because this command won’t remove static
entries.
Oh, and there’s more—any outside device’s packet destination address that happens to
be responding to any inside device is known as the inside global (IG) address. This means
that the initial mapping has to be held in the NAT table so that all packets arriving from a
specic connection get translated consistently. Holding entries in the NAT table also cuts
down on repeated translation operations happening each time the same inside machine
sends packets to the same outside destinations on a regular basis.
Let me clarify: when an entry is placed into the NAT table the rst time, a timer begins
ticking and its duration is known as the translation timeout. Each time a packet for a given
entry translates through the router, the timer gets reset. If the timer expires, the entry will
be unceremoniously removed from the NAT table and the dynamically assigned address
will then be returned to the pool. Cisco’s default translation timeout is 86,400 seconds (24
hours), but you can change that with the ip nat translation timeout command.
Before we move on to the conguration section and actually use the commands I just
talked about, let’s go through a couple of NAT examples and see if you can gure out the
best conguration to go with. To start, look at Figure13.4 and ask yourself two things:
Where would you implement NAT in this design? What type of NAT would you congure?
FIGURE13.4 NAT example
Global (real) Internet addresses
Private RFC 1918 addresses
Corporate
Internet
Engineering Sales
In Figure13.4, the NAT conguration would be placed on the corporate router, just as
I demonstrated with Figure13.1, and the conguration would be dynamic NAT with over-
load (PAT). In this next NAT example, what type of NAT is being used?
ip nat pool todd-nat 170.168.10.10 170.168.10.20 netmask 255.255.255.0
ip nat inside source list 1 pool todd-nat
Testing and Troubleshooting NAT 553
The preceding command uses dynamic NAT without PAT. The pool in the command
gives the answer away as dynamic, plus there’s more than one address in the pool and there
is no overload command at the end of our ip nat inside source command. This means
we are not using PAT!
In the next NAT example, refer to Figure13.5 and see if you can come up with the con-
guration needed.
FIGURE13.5 Another NAT example
Lab A
ISP
192.1.2.110
S0/1
Console
S0/0
192.1.2.109
Fa0/0
192.168.10.126
Figure13.5 shows a border router that needs to be congured with NAT and allow
the use of six public IP addresses to the inside locals, 192.1.2.109 through 192.1.2.114.
However, on the inside network, you have 62 hosts that use the private addresses of
192.168.10.65 through 192.168.10.126. What would your NAT conguration be on the
border router?
Actually, two different answers would both work here, but the following would be my
rst choice based on the exam objectives:
ip nat pool Todd 192.1.2.109 192.1.2.109 netmask 255.255.255.248
access-list 1 permit 192.168.10.64 0.0.0.63
ip nat inside source list 1 pool Todd overload
The command ip nat pool Todd 192.1.2.109 192.1.2.109 netmask
255.255.255.248 sets the pool name as Todd and creates a dynamic pool of only one
address using NAT address 192.1.2.109. Instead of the netmask command, you can use the
prefix-length 29 statement. Just in case you’re wondering, you cannot do this on router
interfaces as well!
The second answer would get you the exact same result of having only 192.1.2.109
as your inside global, but you can type this in and it will also work: ip nat pool Todd
554 Chapter 13 u Network Address Translation (NAT)
192.1.2.109 192.1.2.114 netmask 255.255.255.248. But this option really is a waste
because the second through sixth addresses would only be used if there was a conict with
a TCP port number. You would use something like what I’ve shown in this example if you
literally had about ten thousand hosts with one Internet connection! You would need it to
help with the TCP-Reset issue when two hosts are trying to use the same source port num-
ber and get a negative acknowledgment (NAK). But in our example, we’ve only got up to
62 hosts connecting to the Internet at the same time, so having more than one inside global
gets us nothing!
If you’re fuzzy on the second line where the access list is set in the NAT conguration,
do a quick review of Chapter 12, “Security.” But this isn’t difcult to grasp because it’s
easy to see in this access-list line that it’s just the network number and wildcard used with
that command. I always say, “Every question is a subnet question,” and this one is no
exception. The inside locals in this example were 192.168.10.65–126, which is a block of
64, or a 255.255.255.192 mask. As I’ve said in pretty much every chapter, you really need
to be able to subnet quickly!
The command ip nat inside source list 1 pool Todd overload sets the dynamic
pool to use PAT by using the overload command.
And be sure to add the ip nat inside and ip nat outside statements on the appropri-
ate interfaces.
If you’re planning on testing for any Cisco exam, configure the hands-on
labs at the end of this chapter until you’re really comfortable with doing that!
Okay, one more example, and then you are off to the written lab, hands-on labs, and
review questions.
The network in Figure13.6 is already congured with IP addresses as shown in the g-
ure, and there is only one congured host. However, you need to add 25 more hosts to the
LAN. Now, all 26 hosts must be able to get to the Internet at the same time.
FIGURE13.6 Last NAT example
S0/0
192.1.2.29/30 S0/1
192.1.2.30/30
Fa0/0
192.168.76.94
Corp
By looking at the congured network, use only the following inside addresses to cong-
ure NAT on the Corp router to allow all hosts to reach the Internet:
u Inside globals: 198.18.41.129 through 198.18.41.134
u Inside locals: 192.168.76.65 through 192.168.76.94
Summary 555
This one is a bit more challenging because all we have to help us gure out the con-
guration is the inside globals and the inside locals. But even meagerly armed with these
crumbs of information, plus the IP addresses of the router interfaces shown in the gure,
we can still congure this correctly.
To do that, we must rst determine what our block sizes are so we can get our
subnet mask for our NAT pool. This will also equip us to congure the wildcard for
the access list.
You should easily be able to see that the block size of the inside globals is 8 and the
block size of the inside locals is 32. Know that it’s critical not to stumble on this founda-
tional information!
Okay, so we can congure NAT now that we have our block sizes:
ip nat pool Corp 198.18.41.129 198.18.41.134 netmask 255.255.255.248
ip nat inside source list 1 pool Corp overload
access-list 1 permit 192.168.76.64 0.0.0.31
Since we had a block of only 8 for our pool, we had to use the overload command to
make sure all 26 hosts can get to the Internet at the same time.
There is one other simple way to congure NAT, and I use this command at my home
ofce to connect to my ISP. One command line and it’s done! Here it is:
ip nat inside source list 1 int s0/0/0 overload
I can’t say enough how much I love efciency, and being able to achieve something
cool using one measly line always makes me happy! My one little powerfully elegant line
essentially says, “Use my outside local as my inside global and overload it.” Nice! Of
course, I still had to create ACL 1 and add the inside and outside interface commands to
the conguration, but this is a really nice, fast way to congure NAT if you don’t have a
pool of addresses to use.
Summary
Now this really was a fun chapter. Come on—admit it! You learned a lot about Network
Address Translation (NAT) and how it’s congured as static and dynamic as well as with
Port Address Translation (PAT), also called NAT Overload.
I also described how each avor of NAT is used in a network as well as how each type
is congured.
We nished up by going through some verication and troubleshooting commands.
Now don’t forget to practice all the wonderfully helpful labs until you’ve got them nailed
down tight!
Visit ccna
.gg/ch13/b
for a
companion
MicroNugget
from CBT
Nuggets.
556 Chapter 13 u Network Address Translation (NAT)
Exam Essentials
Understand the term NAT. This may come as news to you, because I didn’t—okay, failed
to—mention it earlier, but NAT has a few nicknames. In the industry, it’s referred to as
network masquerading, IP-masquerading, and (for those who are besieged with OCD and
compelled to spell everything out) Network Address Translation. Whatever you want to
dub it, basically, they all refer to the process of rewriting the source/destination addresses
of IP packets when they go through a router or rewall. Just focus on the process that’s
occurring and your understanding of it (i.e., the important part) and you’re on it for sure!
Remember the three methods of NAT. The three methods are static, dynamic, and over-
loading; the latter is also called PAT.
Understand static NAT. This type of NAT is designed to allow one-to-one mapping
between local and global addresses.
Understand dynamic NAT. This version gives you the ability to map a range of unregis-
tered IP addresses to a registered IP address from out of a pool of registered IP addresses.
Understand overloading. Overloading really is a form of dynamic NAT that maps multiple
unregistered IP addresses to a single registered IP address (many-to-one) by using different
ports. It’s also known as PAT.
Hands-on Labs 557
Written Lab 13
The answers to this lab can be found in Appendix A, “Answers to Written Labs.”
In this section, write the answers to the following questions:
1. What type of address translation can use only one address to allow thousands of hosts
to be translated globally?
2. What command can you use to show the NAT translations as they occur on your router?
3. What command will show you the translation table?
4. What command will clear all your NAT entries from the translation table?
5. An inside local is before or after translation?
6. An inside global is before or after translation?
7. Which command can be used for troubleshooting and displays a summary of the NAT
configuration as well as counts of active translation types and hits to an existing mapping?
8. What commands must be used on your router interfaces before NAT will translate
addresses?
9. In the following output, what type of NAT is being used?
ip nat pool todd-nat 170.168.10.10 170.168.10.20 netmask 255.255.255.0
10. Instead of the netmask command, you can use the ____________ statement.
Hands-on Labs
I am going to use some basic routers for these labs, but really, almost any Cisco router will
work. Also, you can use the LammleSim IOS version to run through all the labs in this (and
every) chapter in this book.
Here is a list of the labs in this chapter:
Lab 13.1: Preparing for NAT
Lab 13.2: Conguring Dynamic NAT
Lab 13.3: Conguring PAT
I am going to use the network shown in the following diagram for our hands-on labs.
I highly recommend you connect up some routers and run through these labs. You will
congure NAT on router Lab_A to translate the private IP address of 192.168.10.0 to a
public address of 171.16.10.0.
S0/2
S0
ISPLab A
S0
E0 E0 S0/0
Lab BLab C
558 Chapter 13 u Network Address Translation (NAT)
Table13.3 shows the commands we will use and the purpose of each command.
TABLE13.3 Command summary for NAT/PAT hands-on labs
Command Purpose
ip nat inside source list acl pool name Translates IPs that match the ACL to
the pool
ip nat inside source static inside_addr
outside_addr
Statically maps an inside local address
to an outside global address
ip nat pool name Creates an address pool
ip nat inside Sets an interface to be an inside
interface
ip nat outside Sets an interface to be an outside
interface
show ip nat translations Shows current NAT translations
Lab 13.1: Preparing for NAT
In this lab, you’ll set up your routers with IP addresses and RIP routing.
1. Configure the routers with the IP addresses listed in the table below:
Router Interface IP Address
ISP S0 171.16.10.1/24
Lab_A S0/2 171.16.10.2/24
Lab_A S0/0 192.168.20.1/24
Lab_B S0 192.168.20.2/24
Lab_B E0 192.168.30.1/24
Lab_C E0 192.168.30.2/24
Hands-on Labs 559
After you congure IP addresses on the routers, you should be able to ping from router
to router, but since we do not have a routing protocol running until the next step, you
can verify only from one router to another but not through the network until RIP is set
up. You can use any routing protocol you wish; I am just using RIP for simplicity’s sake
to get this up and running.
2. On Lab_A, configure RIP routing, set a passive interface, and configure the default
network.
Lab_A#config t
Lab_A(config)#router rip
Lab_A(config-router)#network 192.168.20.0
Lab_A(config-router)#network 171.16.0.0
Lab_A(config-router)#passive-interface s0/2
Lab_A(config-router)#exit
Lab_A(config)#ip default-network 171.16.10.1
The passive-interface command stops RIP updates from being sent to the ISP and
the ip default-network command advertises a default network to the other routers so
they know how to get to the Internet.
3. On Lab_B, configure RIP routing:
Lab_B#config t
Lab_B(config)#router rip
Lab_B(config-router)#network 192.168.30.0
Lab_B(config-router)#network 192.168.20.0
4. On Lab_C, configure RIP routing:
Lab_C#config t
Lab_C(config)#router rip
Lab_C(config-router)#network 192.168.30.0
5. On the ISP router, configure a default route to the corporate network:
ISP#config t
ISP(config)#ip route 0.0.0.0 0.0.0.0 s0
6. Configure the ISP router so you can telnet into the router without being prompted for a
password:
ISP#config t
ISP(config)#line vty 0 4
ISP(config-line)#no login
7. Verify that you can ping from the ISP router to the Lab_C router and from the Lab_C
router to the ISP router. If you cannot, troubleshoot your network.
560 Chapter 13 u Network Address Translation (NAT)
Lab 13.2: Configuring Dynamic NAT
In this lab, you’ll congure dynamic NAT on the Lab_A router.
1. Create a pool of addresses called GlobalNet on the Lab_A router. The pool should
contain a range of addresses of 171.16.10.50 through 171.16.10.55.
Lab_A(config)#ip nat pool GlobalNet 171.16.10.50 171.16.10.55
net 255.255.255.0
2. Create access list 1. This list permits traffic from the 192.168.20.0 and 192.168.30.0
network to be translated.
Lab_A(config)#access-list 1 permit 192.168.20.0 0.0.0.255
Lab_A(config)#access-list 1 permit 192.168.30.0 0.0.0.255
3. Map the access list to the pool that was created.
Lab_A(config)#ip nat inside source list 1 pool GlobalNet
4. Configure serial 0/0 as an inside NAT interface.
Lab_A(config)#int s0/0
Lab_A(config-if)#ip nat inside
5. Configure serial 0/2 as an outside NAT interface.
Lab_A(config-if)#int s0/2
Lab_A(config-if)#ip nat outside
6. Move the console connection to the Lab_C router. Log in to the Lab_C router. Telnet
from the Lab_C router to the ISP router.
Lab_C#telnet 171.16.10.1
7. Move the console connection to the Lab_B router. Log in to the Lab_B router. Telnet
from the Lab_B router to the ISP router.
Lab_B#telnet 171.16.10.1
8. Execute the command show users from the ISP router. (This shows who is accessing
the VTY lines.)
ISP#show users
a. What does it show as your source IP address?________________
b. What is your real source IP address?__________________
Hands-on Labs 561
The show users output should look something like this:
ISP>sh users
Line User Host(s) Idle Location
0 con 0 idle 00:03:32
2 vty 0 idle 00:01:33 171.16.10.50
* 3 vty 1 idle 00:00:09 171.16.10.51
Interface User Mode Idle Peer Address
ISP>
Notice that there is a one-to-one translation. This means you must have a
real IP address for every host that wants to get to the Internet, which is not
typically possible.
9. Leave the session open on the ISP router and connect to Lab_A. (Use Ctrl+Shift+6, let
go, and then press X.)
10. Log in to your Lab_A router and view your current translations by entering the show
ip nat translations command. You should see something like this:
Lab_A#sh ip nat translations
Pro Inside global Inside local Outside local Outside global
--- 171.16.10.50 192.168.30.2 --- ---
--- 171.16.10.51 192.168.20.2 --- ---
Lab_A#
11. If you turn on debug ip nat on the Lab_A router and then ping through the router,
you will see the actual NAT process take place, which will look something like this:
00:32:47: NAT*: s=192.168.30.2->171.16.10.50, d=171.16.10.1 [5]
00:32:47: NAT*: s=171.16.10.1, d=171.16.10.50->192.168.30.2
Lab 13.3: Configuring PAT
In this lab, you’ll congure PAT on the Lab_A router. We will use PAT because we don’t
want a one-to-one translation, which uses just one IP address for every user on the network.
1. On the Lab_A router, delete the translation table and remove the dynamic NAT pool.
Lab_A#clear ip nat translations *
Lab_A#config t
Lab_A(config)#no ip nat pool GlobalNet 171.16.10.50
171.16.10.55 netmask 255.255.255.0
Lab_A(config)#no ip nat inside source list 1 pool GlobalNet
562 Chapter 13 u Network Address Translation (NAT)
2. On the Lab_A router, create a NAT pool with one address called Lammle. The pool
should contain a single address, 171.16.10.100. Enter the following command:
Lab_A#config t
Lab_A(config)#ip nat pool Lammle 171.16.10.100 171.16.10.100
net 255.255.255.0
3. Create access list 2. It should permit networks 192.168.20.0 and 192.168.30.0 to
be translated.
Lab_A(config)#access-list 2 permit 192.168.20.0 0.0.0.255
Lab_A(config)#access-list 2 permit 192.168.30.0 0.0.0.255
4. Map access list 2 to the new pool, allowing PAT to occur by using the overload
command.
Lab_A(config)#ip nat inside source list 2 pool Lammle overload
5. Log in to the Lab_C router and telnet to the ISP router; also, log in to the Lab_B router
and telnet to the ISP router.
6. From the ISP router, use the show users command. The output should look like this:
ISP>sh users
Line User Host(s) Idle Location
* 0 con 0 idle 00:00:00
2 vty 0 idle 00:00:39 171.16.10.100
4 vty 2 idle 00:00:37 171.16.10.100
Interface User Mode Idle Peer Address
ISP>
7. From the Lab_A router, use the show ip nat translations command.
Lab_A#sh ip nat translations
Pro Inside global Inside local Outside local Outside global
tcp 171.16.10.100:11001 192.168.20.2:11001 171.16.10.1:23 171.16.10.1:23
tcp 171.16.10.100:11002 192.168.30.2:11002 171.16.10.1:23 171.16.10.1:23
Hands-on Labs 563
8. Also make sure the debug ip nat command is on for the Lab_A router. If you ping
from the Lab_C router to the ISP router, the output will look like this:
01:12:36: NAT: s=192.168.30.2->171.16.10.100, d=171.16.10.1 [35]
01:12:36: NAT*: s=171.16.10.1, d=171.16.10.100->192.168.30.2 [35]
01:12:36: NAT*: s=192.168.30.2->171.16.10.100, d=171.16.10.1 [36]
01:12:36: NAT*: s=171.16.10.1, d=171.16.10.100->192.168.30.2 [36]
01:12:36: NAT*: s=192.168.30.2->171.16.10.100, d=171.16.10.1 [37]
01:12:36: NAT*: s=171.16.10.1, d=171.16.10.100->192.168.30.2 [37]
01:12:36: NAT*: s=192.168.30.2->171.16.10.100, d=171.16.10.1 [38]
01:12:36: NAT*: s=171.16.10.1, d=171.16.10.100->192.168.30.2 [38]
01:12:37: NAT*: s=192.168.30.2->171.16.10.100, d=171.16.10.1 [39]
01:12:37: NAT*: s=171.16.10.1, d=171.16.10.100->192.168.30.2 [39]
564 Chapter 13 u Network Address Translation (NAT)
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional ques-
tions, please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. Which of the following are disadvantages of using NAT? (Choose three.)
A. Translation introduces switching path delays.
B. NAT conserves legally registered addresses.
C. NAT causes loss of end-to-end IP traceability.
D. NAT increases flexibility when connecting to the Internet.
E. Certain applications will not function with NAT enabled.
F. NAT reduces address overlap occurrence.
2. Which of the following are advantages of using NAT? (Choose three.)
A. Translation introduces switching path delays.
B. NAT conserves legally registered addresses.
C. NAT causes loss of end-to-end IP traceability.
D. NAT increases flexibility when connecting to the Internet.
E. Certain applications will not function with NAT enabled.
F. NAT remedies address overlap occurrence.
3. Which command will allow you to see real-time translations on your router?
A. show ip nat translations
B. show ip nat statistics
C. debug ip nat
D. clear ip nat translations *
4. Which command will show you all the translations active on your router?
A. show ip nat translations
B. show ip nat statistics
C. debug ip nat
D. clear ip nat translations *
Review Questions 565
5. Which command will clear all the translations active on your router?
A. show ip nat translations
B. show ip nat statistics
C. debug ip nat
D. clear ip nat translations *
6. Which command will show you the summary of the NAT configuration?
A. show ip nat translations
B. show ip nat statistics
C. debug ip nat
D. clear ip nat translations *
7. Which command will create a dynamic pool named Todd that will provide you with 30
global addresses?
A. ip nat pool Todd 171.16.10.65 171.16.10.94 net 255.255.255.240
B. ip nat pool Todd 171.16.10.65 171.16.10.94 net 255.255.255.224
C. ip nat pool Todd 171.16.10.65 171.16.10.94 net 255.255.255.224
D. ip nat pool Todd 171.16.10.1 171.16.10.254 net 255.255.255.0
8. Which of the following are methods of NAT? (Choose three.)
A. Static
B. IP NAT pool
C. Dynamic
D. NAT double-translation
E. Overload
9. When creating a pool of global addresses, which of the following can be used instead
of the netmask command?
A. / (slash notation)
B. prefix-length
C. no mask
D. block-size
10. Which of the following would be a good starting point for troubleshooting if your
router is not translating?
A. Reboot
B. Call Cisco
C. Check your interfaces for the correct configuration
D. Run the debug all command
566 Chapter 13 u Network Address Translation (NAT)
11. Which of the following would be good reasons to run NAT? (Choose three.)
A. You need to connect to the Internet and your hosts don’t have globally unique
IP addresses.
B. You change to a new ISP that requires you to renumber your network.
C. You don’t want any hosts connecting to the Internet.
D. You require two intranets with duplicate addresses to merge.
12. Which of the following is considered to be the inside host’s address after translation?
A. Inside local
B. Outside local
C. Inside global
D. Outside global
13. Which of the following is considered to be the inside host’s address before translation?
A. Inside local
B. Outside local
C. Inside global
D. Outside global
14. By looking at the following output, which of the following commands would allow
dynamic translations?
Router#show ip nat trans
Pro Inside global Inside local Outside local Outside global
--- 1.1.128.1 10.1.1.1 --- ---
--- 1.1.130.178 10.1.1.2 --- ---
--- 1.1.129.174 10.1.1.10 --- ---
--- 1.1.130.101 10.1.1.89 --- ---
--- 1.1.134.169 10.1.1.100 --- ---
--- 1.1.135.174 10.1.1.200 --- ---
A. ip nat inside source pool todd 1.1.128.1 1.1.135.254 prefix-length 19
B. ip nat pool todd 1.1.128.1 1.1.135.254 prefix-length 19
C. ip nat pool todd 1.1.128.1 1.1.135.254 prefix-length 18
D. ip nat pool todd 1.1.128.1 1.1.135.254 prefix-length 21
Review Questions 567
15. Your inside locals are not being translated to the inside global addresses. Which of the fol-
lowing commands will show you if your inside globals are allowed to use the NAT pool?
ip nat pool Corp 198.18.41.129 198.18.41.134 netmask 255.255.255.248
ip nat inside source list 100 int pool Corp overload
A. debug ip nat
B. show access-list
C. show ip nat translation
D. show ip nat statistics
16. Which command would you place on the interface of a private network?
A. ip nat inside
B. ip nat outside
C. ip outside global
D. ip inside local
17. Which command would you place on an interface connected to the Internet?
A. ip nat inside
B. ip nat outside
C. ip outside global
D. ip inside local
18. Port Address Translation is also called what?
A. NAT Fast
B. NAT Static
C. NAT Overload
D. Overloading Static
19. What does the asterisk (*) represent in the following output?
NAT*: s=172.16.2.2, d=192.168.2.1->10.1.1.1 [1]
A. The packet was destined for a local interface on the router.
B. The packet was translated and fast-switched to the destination.
C. The packet attempted to be translated but failed.
D. The packet was translated but there was no response from the remote host.
568 Chapter 13 u Network Address Translation (NAT)
20. Which of the following needs to be added to the configuration to enable PAT?
ip nat pool Corp 198.18.41.129 198.18.41.134 netmask 255.255.255.248
access-list 1 permit 192.168.76.64 0.0.0.31
A. ip nat pool inside overload
B. ip nat inside source list 1 pool Corp overload
C. ip nat pool outside overload
D. ip nat pool Corp 198.41.129 net 255.255.255.0 overload
Chapter
14
Internet Protocol
Version 6 (IPv6)
THE FOLLOWING ICND1 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 IP addressing (IPv4 / IPv6)
■Identify the appropriate IPv6 addressing scheme to satisfy
addressing requirements in a LAN/WAN environment
■Describe the technological requirements for running IPv6 in
conjunction with IPv4 such as dual stack
■Describe IPv6 addresses
■Global unicast
■Multicast
■Link local
■Unique local
■EUI 64
■Autoconfiguration
1 IP Routing Technologies
■Configure and verify OSPF (single area)
■Configure OSPFv3
We’ve covered a lot of ground in this book, and though the
journey has been tough at times, it’s been well worth it! But
our networking expedition isn’t quite over yet because we still
have the vastly important frontier of IPv6 to explore. There’s still some expansive territory
to cover with this sweeping new subject, so gear up and get ready to discover all you need
to know about IPv6. Understanding IPv6 is vital now, so you’ll be much better equipped
and prepared to meet today’s real-world networking challenges as well as to ace the exam.
This nal chapter is packed and brimming with all the IPv6 information you’ll need to
complete your Cisco exam trek successfully, so get psyched—we’re in the home stretch!
I probably don’t need to say this, but I will anyway because I really want to go the
distance and do everything I can to ensure that you arrive and achieve… You absolutely
must have a solid hold on IPv4 by now, but if you’re still not condent with it, or feel
you could use a refresher, just page back to the chapters on TCP/IP and subnetting. And
if you’re not crystal clear on the address problems inherent to IPv4, you really need to
review Chapter 13, “Network Address Translation (NAT)”, before we decamp for this
chapter’s IPv6 summit push!
People refer to IPv6 as “the next-generation Internet protocol,” and it was originally
created as the solution to IPv4’s inevitable and impending address-exhaustion crisis. Though
you’ve probably heard a thing or two about IPv6 already, it has been improved even further
in the quest to bring us the exibility, efciency, capability, and optimized functionality that
can affectively meet our world’s seemingly insatiable thirst for ever-evolving technologies
and increasing access. The capacity of its predecessor, IPv4, pales wan and ghostly in com-
parison, which is why IPv4 is destined to fade into history completely, making way for IPv6
and the future.
The IPv6 header and address structure has been completely overhauled, and many of the
features that were basically just afterthoughts and addendums in IPv4 are now included as
full-blown standards in IPv6. It’s power-packed, well equipped with robust and elegant fea-
tures, poised and prepared to manage the mind-blowing demands of the Internet to come!
After an introduction like that, I understand if you’re a little apprehensive, but I promise—
really—to make this chapter and its VIP topic pretty painless for you. In fact, you might even
nd yourself actually enjoying it—I denitely did! Because IPv6 is so complex, while still being
so elegant, innovative, and powerful, it fascinates me like some weird combination of a sleek,
new Aston Martin and a riveting futuristic novel. Hopefully you’ll experience this chapter as
an awesome ride and enjoy reading it as much as I did writing it!
To find up-to-the-minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
The Benefits and Uses of IPv6 571
Why Do We Need IPv6?
Well, the short answer is because we need to communicate and our current system isn’t
really cutting it anymore. It’s kind of like the Pony Express trying to compete with
airmail! Consider how much time and effort we’ve been investing for years while we
scratch our heads to resourcefully come up with slick new ways to conserve bandwidth
and IP addresses. Sure, variable length subnet masks (VLSMs) are wonderful and cool,
but they’re really just another invention to help us cope while we desperately struggle to
overcome the worsening address drought.
I’m not exaggerating, at all, about how dire things are getting, because it’s simply reality.
The number of people and devices that connect to networks increases dramatically each and
every day, which is not a bad thing. We’re just nding new and exciting ways to communi-
cate to more people, more often, which is good thing. And it’s not likely to go away or even
decrease in the littlest bit, because communicating and making connections are, in fact, basic
human needs—they’re in our very nature. But with our numbers increasing along with the
rising tide of people joining the communications party increasing as well, the forecast for our
current system isn’t exactly clear skies and smooth sailing. IPv4, upon which our ability to
do all this connecting and communicating is presently dependent, is quickly running out of
addresses for us to use.
IPv4 has only about 4.3 billion addresses available—in theory—and we know that we don’t
even get to use most of those! Sure, the use of Classless Inter-Domain Routing (CIDR) and
Network Address Translation (NAT) has helped to extend the inevitable dearth of addresses,
but we will still run out of them, and it’s going to happen within a few years. China is barely
online, and we know there’s a huge population of people and corporations there that surely
want to be. There are myriad reports that give us all kinds of numbers, but all you really need
to think about to realize that I’m not just being an alarmist is this: there are about 7 billion
people in the world today, and it’s estimated that only just over 10 percent of that population is
currently connected to the Internet—wow!
That statistic is basically screaming at us the ugly truth that based on IPv4’s capacity,
every person can’t even have a computer, let alone all the other IP devices we use with them! I
have more than one computer, and it’s pretty likely that you do too, and I’m not even includ-
ing phones, laptops, game consoles, fax machines, routers, switches, and a mother lode of
other devices we use every day into the mix! So I think I’ve made it pretty clear that we’ve
got to do something before we run out of addresses and lose the ability to connect with each
other as we know it. And that “something” just happens to be implementing IPv6.
The Benefits and Uses of IPv6
So what’s so fabulous about IPv6? Is it really the answer to our coming dilemma? Is it
really worth it to upgrade from IPv4? All good questions—you may even think of a few
more. Of course, there’s going to be that group of people with the time-tested “resistance
572 Chapter 14 u Internet Protocol Version 6 (IPv6)
to change syndrome,” but don’t listen to them. If we had done that years ago, we’d still be
waiting weeks, even months for our mail to arrive via horseback. Instead, just know that
the answer is a resounding yes, it is really the answer, and it is worth the upgrade! Not only
does IPv6 give us lots of addresses (3.4 n 1038 = denitely enough), there are tons of other
features built into this version that make it well worth the cost, time, and effort required to
migrate to it.
Today’s networks, as well as the Internet, have a ton of unforeseen requirements that
simply weren’t even considerations when IPv4 was created. We’ve tried to compensate
with a collection of add-ons that can actually make implementing them more difcult
than they would be if they were required by a standard. By default, IPv6 has improved
upon and included many of those features as standard and mandatory. One of these
sweet new standards is IPsec—a feature that provides end-to-end security.
But it’s the efciency features that are really going to rock the house! For starters, the
headers in an IPv6 packet have half the elds, and they are aligned to 64 bits, which gives
us some seriously souped-up processing speed. Compared to IPv4, lookups happen at light
speed! Most of the information that used to be bound into the IPv4 header was taken out,
and now you can choose to put it, or parts of it, back into the header in the form of optional
extension headers that follow the basic header elds.
And of course there’s that whole new universe of addresses—the 3.4 n 1038 I just men-
tioned—but where did we get them? Did some genie just suddenly arrive and make them magi-
cally appear? That huge proliferation of addresses had to come from somewhere! Well it just
so happens that IPv6 gives us a substantially larger address space, meaning the address itself is
a whole lot bigger—four times bigger as a matter of fact! An IPv6 address is actually 128 bits
in length, and no worries—I’m going to break down the address piece by piece and show you
exactly what it looks like coming up in the section, “IPv6 Addressing and Expressions.” For
now, let me just say that all that additional room permits more levels of hierarchy inside the
address space and a more exible addressing architecture. It also makes routing much more
efcient and scalable because the addresses can be aggregated a lot more effectively. And IPv6
also allows multiple addresses for hosts and networks. This is especially important for enter-
prises veritably drooling for enhanced access and availability. Plus, the new version of IP now
includes an expanded use of multicast communication—one device sending to many hosts or
to a select group—that joins in to seriously boost efciency on networks because communica-
tions will be more specic.
IPv4 uses broadcasts quite prolically, causing a bunch of problems, the worst of which
is of course the dreaded broadcast storm. This is that uncontrolled deluge of forwarded
broadcast trafc that can bring an entire network to its knees and devour every last bit of
bandwidth! Another nasty thing about broadcast trafc is that it interrupts each and every
device on the network. When a broadcast is sent out, every machine has to stop what it’s
doing and respond to the trafc whether the broadcast is relevant to it or not.
But smile assuredly, everyone. There’s no such thing as a broadcast in IPv6 because it uses
multicast trafc instead. And there are two other types of communications as well: unicast,
which is the same as it is in IPv4, and a new type called anycast. Anycast communication
allows the same address to be placed on more than one device so that when trafc is sent to
IPv6 Addressing and Expressions 573
the device service addressed in this way, it’s routed to the nearest host that shares the same
address. And this is just the beginning—we’ll get into the various types of communication
later in the section called, “Address Types.”
IPv6 Addressing and Expressions
Just as understanding how IP addresses are structured and used is critical with IPv4 address-
ing, it’s also vital when it comes to IPv6. You’ve already read about the fact that at 128 bits, an
IPv6 address is much larger than an IPv4 address. Because of this, as well as the new ways the
addresses can be used, you’ve probably guessed that IPv6 will be more complicated to manage.
But no worries! As I said, I’ll break down the basics and show you what the address looks like
and how you can write it as well as many of its common uses. It’s going to be a little weird at
rst, but before you know it, you’ll have it nailed!
So let’s take a look at Figure14.1, which has a sample IPv6 address broken down into
sections.
FIGURE14.1 IPv6 address example
Global prefix Subnet Interface ID
2001:0db8:3c4d:0012:0000:0000:1234:56ab
48 bits 16 bits
64 bits 64 bits
As you can clearly see, the address is denitely much larger. But what else is different?
Well, rst, notice that it has eight groups of numbers instead of four and also that those
groups are separated by colons instead of periods. And hey, wait a second… there are let-
ters in that address! Yep, the address is expressed in hexadecimal just like a MAC address
is, so you could say this address has eight 16-bit hexadecimal colon-delimited blocks.
That’s already quite a mouthful, and you probably haven’t even tried to say the address
out loud yet!
One other thing I want to point out is an important thing to keep in mind for when
you set up your test network to actually experiment with IPv6 (because I know you’re
going to want to do that). When you use a web browser to make an HTTP connection to
an IPv6 device, you have to type the address into the browser with brackets around the
literal address. Why? Well, a colon is already being used by the browser for specifying a
port number, so basically, if you don’t enclose the address in brackets, the browser will
have no way to identify the information.
Here’s an example of how this looks:
http://[2001:0db8:3c4d:0012:0000:0000:1234:56ab]/default.html
574 Chapter 14 u Internet Protocol Version 6 (IPv6)
Now obviously if you can, you would rather use names to specify a destination (like
www.lammle.com), but even though it’s denitely going to be a pain in the rear, we just
have to accept the fact that sometimes we have to bite the bullet and type in the address
number. So it should be pretty clear that DNS is going to remain extremely important
when implementing IPv6.
There are four hexadecimal characters (16 bits) in each IPv6 field, sepa-
rated by colons.
Shortened Expression
The good news is there are a few tricks to help rescue us when writing these monster
addresses. For one thing, you can actually leave out parts of the address to abbreviate it,
but to get away with doing that you have to follow a couple of rules. First, you can drop
any leading zeros in each of the individual blocks. After you do that, the sample address
from earlier would then look like this:
2001:db8:3c4d:12:0:0:1234:56ab
Okay, that’s a denite improvement—at least we don’t have to write all of those extra
zeros! But what about whole blocks that don’t have anything in them except zeros? Well,
we can kind of lose those too—at least some of them. Again referring to our sample
address, we can remove the two consecutive blocks of zeros by replacing them with a
doubled colon, like this:
2001:db8:3c4d:12::1234:56ab
Cool—we replaced the blocks of all zeros with a doubled colon. The rule you have to
follow to get away with this is that you can replace only one contiguous block of such zeros
in an address. So if my address has four blocks of zeros and each of them were separated,
I just don’t get to replace them all because I can replace only one contiguous block with a
doubled colon. Check out this example:
2001:0000:0000:0012:0000:0000:1234:56ab
And just know that you can’t do this:
2001::12::1234:56ab
Instead, the best you can do is this:
2001::12:0:0:1234:56ab
The reason the preceding example is our best shot is that if we remove two sets of zeros,
the device looking at the address will have no way of knowing where the zeros go back in.
IPv6 Addressing and Expressions 575
Basically, the router would look at the incorrect address and say, “Well, do I place two blocks
into the rst set of doubled colons and two into the second set, or do I place three blocks into
the rst set and one block into the second set?” And on and on it would go because the infor-
mation the router needs just isn’t there.
Address Types
We’re all familiar with IPv4’s unicast, broadcast, and multicast addresses that basically
dene who or at least how many other devices we’re talking to. But as I mentioned, IPv6
modies that trio and introduces the anycast. Broadcasts, as we know them, have been
eliminated in IPv6 because of their cumbersome inefciency and basic tendency to drive
us insane!
So let’s nd out what each of these types of IPv6 addressing and communication methods
do for us:
Unicast Packets addressed to a unicast address are delivered to a single interface. For load
balancing, multiple interfaces across several devices can use the same address, but we’ll call
that an anycast address. There are a few different types of unicast addresses, but we don’t
need to get further into that here.
Global unicast addresses (2000::/3) These are your typical publicly routable addresses
and they’re the same as in IPv4. Global addresses start at 2000::/3. Figure14.2 shows how
a unicast address breaks down. The ISP can provide you with a minimum /48 network ID,
which in turn provides you 16-bits to create a unique 64-bit router interface address. The
last 64-bits are the unique host ID.
FIGURE14.2 IPv6 global unicast addresses
/23
Registry
ISP prefix
Company
Subnet
/32 /48 /64
Interface ID
Link-local addresses (FE80::/10) These are like the Automatic Private IP Address (APIPA)
addresses that Microsoft uses to automatically provide addresses in IPv4 in that they’re not
meant to be routed. In IPv6 they start with FE80::/10, as shown in Figure14.3. Think of
these addresses as handy tools that give you the ability to throw a temporary LAN together
for meetings or create a small LAN that’s not going to be routed but still needs to share and
access les and services locally.
576 Chapter 14 u Internet Protocol Version 6 (IPv6)
FIGURE14.3 IPv6 link local FE80::/10: The first 10 bits define the address type.
FE80::/10
1111 1110 10
64 bits 64 bits
Interface ID
Unique local addresses (FC00::/7) These addresses are also intended for nonrouting pur-
poses over the Internet, but they are nearly globally unique, so it’s unlikely you’ll ever have
one of them overlap. Unique local addresses were designed to replace site-local addresses,
so they basically do almost exactly what IPv4 private addresses do: allow communication
throughout a site while being routable to multiple local networks. Site-local addresses were
deprecated as of September 2004.
Multicast (FF00::/8) Again, as in IPv4, packets addressed to a multicast address are
delivered to all interfaces tuned into the multicast address. Sometimes people call them
“one-to-many” addresses. It’s really easy to spot a multicast address in IPv6 because they
always start with FF. We’ll get deeper into multicast operation coming up in, “How IPv6
Works in an Internetwork.”
Anycast Like multicast addresses, an anycast address identies multiple interfaces on
multiple devices. But there’s a big difference: the anycast packet is delivered to only one
device—actually, to the closest one it nds dened in terms of routing distance. And
again, this address is special because you can apply a single address to more than one
host. These are referred to as “one-to-nearest” addresses. Anycast addresses are typi-
cally only congured on routers, never hosts, and a source address could never be an
anycast address. Of note is that the IETF did reserve the top 128 addresses for each /64
for use with anycast addresses.
You’re probably wondering if there are any special, reserved addresses in IPv6 because
you know they’re there in IPv4. Well there are—plenty of them! Let’s go over those now.
Special Addresses
I’m going to list some of the addresses and address ranges (in Table14.1) that you should
denitely make sure to remember because you’ll eventually use them. They’re all special or
reserved for a specic use, but unlike IPv4, IPv6 gives us a galaxy of addresses, so reserving
a few here and there doesn’t hurt at all!
How IPv6 Works in an Internetwork 577
TABLE14.1 Special IPv6 addresses
Address Meaning
0:0:0:0:0:0:0:0 Equals ::. This is the equivalent of IPv4’s 0.0.0.0 and is typically
the source address of a host before the host receives an IP
address when you’re using DHCP-driven stateful configuration.
0:0:0:0:0:0:0:1 Equals ::1. The equivalent of 127.0.0.1 in IPv4.
0:0:0:0:0:0:192.168.100.1 This is how an IPv4 address would be written in a mixed IPv6/
IPv4 network environment.
2000::/3 The global unicast address range.
FC00::/7 The unique local unicast range.
FE80::/10 The link-local unicast range.
FF00::/8 The multicast range.
3FFF:FFFF::/32 Reserved for examples and documentation.
2001:0DB8::/32 Also reserved for examples and documentation.
2002::/16 Used with 6-to-4 tunneling, which is an IPv4-to-IPv6 transition
system. The structure allows IPv6 packets to be transmitted over
an IPv4 network without the need to configure explicit tunnels.
When you run IPv4 and IPv6 on a router, you have what is called
“dual-stack.”
Let me show you how IPv6 actually works in an internetwork. We all know how IPv4
works, so let’s see what’s new!
How IPv6 Works in an Internetwork
It’s time to explore the ner points of IPv6. A great place to start is by showing you how to
address a host and what gives it the ability to nd other hosts and resources on a network.
I’ll also demonstrate a device’s ability to automatically address itself—something called
stateless autoconguration—plus another type of autoconguration known as stateful.
Visit ccna
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for a
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578 Chapter 14 u Internet Protocol Version 6 (IPv6)
Keep in mind that stateful autoconguration uses a DHCP server in a very similar way to
how it’s used in an IPv4 conguration. I’ll also show you how Internet Control Message
Protocol (ICMP) and multicasting works for us in an IPv6 network environment.
Manual Address Assignment
In order to enable IPv6 on a router, you have to use the ipv6 unicast-routing global con-
guration command:
Corp(config)#ipv6 unicast-routing
By default, IPv6 trafc forwarding is disabled, so using this command enables it. Also,
as you’ve probably guessed, IPv6 isn’t enabled by default on any interfaces either, so we
have to go to each interface individually and enable it.
There are a few different ways to do this, but a really easy way is to just add an
address to the interface. You use the interface conguration command ipv6 address
<ipv6prefix>/<prefix-length> [eui-64]to get this done.
Here’s an example:
Corp(config-if)#ipv6 address 2001:db8:3c4d:1:0260:d6FF.FE73:1987/64
You can specify the entire 128-bit global IPv6 address as I just demonstrated with the
preceding command, or you can use the EUI-64 option. Remember, the EUI-64 (extended
unique identier) format allows the device to use its MAC address and pad it to make the
interface ID. Check it out:
Corp(config-if)#ipv6 address 2001:db8:3c4d:1::/64 eui-64
As an alternative to typing in an IPv6 address on a router, you can enable the interface
instead to permit the application of an automatic link-local address.
To congure a router so that it uses only link-local addresses, use the ipv6 enable
interface conguration command:
Corp(config-if)#ipv6 enable
Remember, if you have only a link-local address, you will be able to
communicate only on that local subnet.
Stateless Autoconfiguration (eui-64)
Autoconguration is an especially useful solution because it allows devices on a network to
address themselves with a link-local unicast address as well as with a global unicast address.
This process happens through rst learning the prex information from the router and then
appending the device’s own interface address as the interface ID. But where does it get that
How IPv6 Works in an Internetwork 579
interface ID? Well, you know every device on an Ethernet network has a physical MAC
address, which is exactly what’s used for the interface ID. But since the interface ID in an
IPv6 address is 64 bits in length and a MAC address is only 48 bits, where do the extra 16
bits come from? The MAC address is padded in the middle with the extra bits—it’s padded
with FFFE.
For example, let’s say I have a device with a MAC address that looks like this:
0060:d673:1987. After it’s been padded, it would look like this: 0260:d6FF:FE73:1987.
Figure14.4 illustrates what an EUI-64 address looks like.
FIGURE14.4 EUI-64 interface ID assignment
The 7th bit (U/L)
10= Universally unique
00= Locally unique
OUI
64-bit interface ID
Insert 16 bits
000000X0
Unique address
FF FE
So where did that 2 in the beginning of the address come from? Another good question.
You see that part of the process of padding, called modied EUI-64 format, changes a bit
to specify if the address is locally unique or globally unique. And the bit that gets changed
is the 7th bit in the address.
The reason for modifying the U/L bit is that, when using manually assigned addresses on
an interface, it means you can simply assign the address 2001:db8:1:9::1/64 instead of the
much longer 2001:db8:1:9:0200::1/64. Also, if you are going to manually assign a link-local
address, you can assign the short address fe80::1 instead of the long fe80::0200:0:0:1 or
fe80:0:0:0:0200::1. So, even though at rst glance it seems the IETF made this harder for you
to simply understand IPv6 addressing by ipping the 7th bit, in reality this made addressing
much simpler. Also, since most people don’t typically override the burned-in address, the U/L
bit is a 0, which means that you’ll see this inverted to a 1 most of the time. But because you’re
studying the Cisco exam objectives, you’ll need to look at inverting it both ways.
Here are a few examples:
u MAC address 0090:2716:fd0f
u IPv6 EUI-64 address: 2001:0db8:0:1:0290:27ff:fe16:fd0f
That one was easy! Too easy for the Cisco exam, so let’s do another:
u MAC address aa12:bcbc:1234
u IPv6 EUI-64 address: 2001:0db8:0:1:a812:bcff:febc:1234
580 Chapter 14 u Internet Protocol Version 6 (IPv6)
10101010 represents the rst 8 bits of the MAC address (aa), which when inverting the
7th bit becomes 10101000. The answer becomes A8. I can’t tell you how important this is
for you to understand, so bear with me and work through a couple more!
u MAC address 0c0c:dede:1234
u IPv6 EUI-64 address: 2001:0db8:0:1:0e0c:deff:fede:1234
0c is 00001100 in the rst 8 bits of the MAC address, which then becomes 00001110
when ipping the 7th bit. The answer is then 0e. Let’s practice one more:
u MAC address 0b34:ba12:1234
u IPv6 EUI-64 address: 2001:0db8:0:1:0934:baff:fe12:1234
0b in binary is 00001011, the rst 8 bits of the MAC address, which then becomes
00001001. The answer is 09.
Pay extra-special attention to this EUI-64 address assignment and be able
to convert the 7th bit based on the EUI-64 rules! Written Lab 14.2 will help
you practice this.
To perform autoconguration, a host goes through a basic two-step process:
1. First, the host needs the prefix information, similar to the network portion of an IPv4
address, to configure its interface, so it sends a router solicitation (RS) request for it.
This RS is then sent out as a multicast to all routers (FF02::2). The actual information
being sent is a type of ICMP message, and like everything in networking, this ICMP
message has a number that identifies it. The RS message is ICMP type 133.
2. The router answers back with the required prefix information via a router advertisement
(RA). An RA message also happens to be a multicast packet that’s sent to the all-nodes
multicast address (FF02::1) and is ICMP type 134. RA messages are sent on a periodic
basis, but the host sends the RS for an immediate response so it doesn’t have to wait until
the next scheduled RA to get what it needs.
These two steps are shown in Figure14.5.
FIGURE14.5 Two steps to IPv6 autoconfiguration
Step 1: Host sends
RS message
FF02::2
ICMP type 133
Step 2: Router sends
RA message
FF02::1
ICMP type 134
Host receives the RA and included
prefix, allowing it to autoconfigure
its interface.
How IPv6 Works in an Internetwork 581
By the way, this type of autoconguration is also known as stateless autoconguration
because it doesn’t contact or connect to and receive any further information from the other
device. We’ll get to stateful conguration when we talk about DHCPv6 next.
But before we do that, rst take a look at Figure14.6. In this gure, the Branch router
needs to be congured, but I just don’t feel like typing in an IPv6 address on the interface
connecting to the Corp router. I also don’t feel like typing in any routing commands, but I
need more than a link-local address on that interface, so I’m going to have to do something!
So basically, I want to have the Branch router work with IPv6 on the internetwork with the
least amount of effort from me. Let’s see if I can get away with that.
FIGURE14.6 IPv6 autoconfiguration example
Branch(config-if)#ipv6 address autoconfig default
Gi0/1
IPv6 Address
Gi0/0
Internet
Branch Corp
Default route
Ah ha—there is an easy way! I love IPv6 because it allows me to be relatively lazy when
dealing with some parts of my network, yet it still works really well. By using the command
ipv6 address autoconfig, the interface will listen for RAs and then, via the EUI-64 for-
mat, it will assign itself a global address—sweet!
Okay, this is all really great, but you’re hopefully wondering what that default is doing
there at the end of the command. If so, good catch! It happens to be a wonderful, optional
part of the command that smoothly delivers a default route received from the Corp router,
which will be automatically injected it into my routing table and set as the default route—
so easy!
DHCPv6 (Stateful)
DHCPv6 works pretty much the same way DHCP does in v4, with the obvious difference
that it supports IPv6’s new addressing scheme. And it might come as a surprise, but there are
a couple of other options that DHCP still provides for us that autoconguration doesn’t. And
no, I’m not kidding— in autoconguration, there’s absolutely no mention of DNS servers,
domain names, or many of the other options that DHCP has always generously provided for
us via IPv4. This is a big reason that the odds favor DHCP’s continued use into the future in
IPv6 at least partially—maybe even most of the time!
Upon booting up in IPv4, a client sends out a DHCP discover message looking for a server
to give it the information it needs. But remember, in IPv6, the RS and RA process happens
rst, so if there’s a DHCPv6 server on the network, the RA that comes back to the client will
tell it if DHCP is available for use. If a router isn’t found, the client will respond by sending
582 Chapter 14 u Internet Protocol Version 6 (IPv6)
out a DHCP solicit message, which is actually a multicast message addressed with a destina-
tion of ff02::1:2 that calls out, “All DHCP agents, both servers and relays.”
It’s good to know that there’s some support for DHCPv6 in the Cisco IOS even though
it’s limited. This rather miserly support is reserved for stateless DHCP servers and tells us it
doesn’t offer any address management of the pool or the options available for conguring
that address pool other than the DNS, domain name, default gateway, and SIP servers.
This means that you’re denitely going to need another server around to supply and
dispense all the additional, required information—maybe to even manage the address
assignment, if needed!
IPv6 Header
An IPv4 header is 20 bytes long, so since an IPv6 address is four times the size of IPv6 at 128
bits, then its header must then be 80 bytes long, right? That makes sense and is totally intui-
tive, but it’s also completely wrong! When IPv6 designers devised the header, they created
fewer, streamlined elds that would also result in a faster routed protocol at the same time.
Let’s take a look at the streamlined IPv6 header using Figure14.7.
FIGURE14.7 IPv6 header
4 bytes
Payload Length Next Header Hop Limit
Source Address
16 bytes, 128 bits
40 bytes
Version Class Flow Label
Destination Address
16 bytes, 128 bits
The basic IPv6 header contains eight elds, making it only twice as large as an IP header
at 40 bytes. Let’s zoom in on these elds:
Version This 4-bit eld contains the number 6, instead of the number 4 as in IPv4.
Traffic Class This 8-bit eld is like the Type of Service (ToS) eld in IPv4.
Flow Label This new eld, which is 24 bits long, is used to mark packets and trafc ows.
A ow is a sequence of packets from a single source to a single destination host, an anycast
or multicast address. The eld enables efcient IPv6 ow classication.
Payload Length IPv4 had a total length eld delimiting the length of the packet. IPv6’s
payload length describes the length of the payload only.
How IPv6 Works in an Internetwork 583
Next Header Since there are optional extension headers with IPv6, this eld denes the next
header to be read. This is in contrast to IPv4, which demands static headers with each packet.
Hop Limit This eld species the maximum number of hops that an IPv6 packet
can traverse.
Source Address This eld of 16 bytes or 128 bits identies the source of the packet.
Destination Address This eld of 16 bytes or 128 bits identies the destination of the packet.
There are also some optional extension headers following these eight elds, which carry
other Network layer information. These header lengths are not a xed number—they’re of
variable size.
It’s time to move on to talk about another IPv4 familiar face and nd out how a certain
very important, built-in protocol has evolved in IPv6.
ICMPv6
IPv4 used the ICMP workhorse for lots of tasks, including error messages like destination
unreachable and troubleshooting functions like Ping and Traceroute. ICMPv6 still does those
things for us, but unlike its predecessor, the v6 avor isn’t implemented as a separate layer
3 protocol. Instead, it’s an integrated part of IPv6 and is carried after the basic IPv6 header
information as an extension header. And ICMPv6 gives us another really cool feature—by
default, it prevents IPv6 from doing any fragmentation through an ICMPv6 process called
path MTU discovery. Figure14.8 shows how ICMPv6 has evolved to become part of the IPv6
packet itself.
FIGURE14.8 ICMPv6
IPv6 header
ICMPv6 Type ICMPv6 Code Checksum
ICMPv6 Data
Next header = 58
ICMPv6 packet
The ICMPv6 packet is identied by the value 58 in the Next Header eld, located inside
the ICMPv6 packet. The Type eld identies the particular kind of ICMP message that’s
being carried, and the Code eld further details the specics of the message. The Data eld
contains the ICMPv6 payload.
584 Chapter 14 u Internet Protocol Version 6 (IPv6)
And this is how it works: the source node of a connection sends a packet that’s equal to
the MTU size of its local link’s MTU. As this packet traverses the path toward its destina-
tion, any link that has an MTU smaller than the size of the current packet will force the
intermediate router to send a “packet too big” message back to the source machine. This
message tells the source node what the maximum size the restrictive link will allow and
asks the source to send a new scaled-down packet that can pass through. This process will
continue until the destination is nally reached, with the source node now sporting the new
path’s MTU. So now, when the rest of the data packets are transmitted, they’ll be protected
from fragmentation.
ICMPv6 is used for router solicitation and advertisement, for neighbor solicitation and
advertisement (i.e., nding the MAC data addresses for IPv6 neighbors), and for redirecting
the host to the best router (default gateway).
Neighbor Discovery (NDP)
ICMPv6 also takes over the task of nding the address of other devices on the local link.
The Address Resolution Protocol is used to perform this function for IPv4, but that’s been
renamed neighbor discovery (ND) in ICMPv6. This process is now achieved via a multicast
address called the solicited node address because all hosts join this multicast group upon
connecting to the network.
Neighbor discovery enables these functions:
u Determining the MAC address of neighbors
u Router solicitation (RS) FF02::2
u Router advertisements (RA) FF02::1
u Neighbor solicitation (NS)
u Neighbor advertisement (NA)
u Duplicate address detection (DAD)
The part of the IPv6 address designated by the 24 bits farthest to the right is added to the
end of the multicast address FF02:0:0:0:0:1:FF/104. When this address is queried, the cor-
responding host will send back its layer 2 address. Devices can nd and keep track of other
neighbor devices on the network in pretty much the same way. When I talked about RA and
RS messages earlier and told you that they use multicast trafc to request and send address
information, that too is actually a function of ICMPv6—specically, neighbor discovery.
In IPv4, the protocol IGMP was used to allow a host device to tell its local router that
it was joining a multicast group and would like to receive the trafc for that group. This
IGMP function has been replaced by ICMPv6, and the process has been renamed multicast
listener discovery.
With IPv4, our hosts could have only one default gateway congured, and if that
router went down we had to either x the router, change the default gateway, or run
some type of virtual default gateway with other protocols created as a solution for this
inadequacy in IPv4. Figure14.9 shows how IPv6 devices nd their default gateways using
neighbor discovery.
How IPv6 Works in an Internetwork 585
FIGURE14.9 Router solicitation (RS) and router advertisement (RA)
FF02::1 Use me! (RA)
Internet
Corp
Corp2
FF02::1 Use me! (RA)
FF02::2 All routers respond! (RS)
IPv6 hosts send a router solicitation (RS) onto their data link asking for all routers to
respond, and they use the multicast address FF02::2 to achieve this. Routers on the same
link respond with a unicast to the requesting host, or with a router advertisement (RA)
using FF02::1.
But that’s not all! Hosts also can send solicitations and advertisements between them-
selves using a neighbor solicitation (NS) and neighbor advertisement (NA), as shown in
Figure14.10. Remember that RA and RS gather or provide information about routers, and
NS and NA gather information about hosts. Remember that a “neighbor” is a host on the
same data link or VLAN.
FIGURE14.10 Neighbor solicitation (NS) and neighbor advertisement (NA)
NDP: NS
I need your MAC!
NDP: NA
Here is my MAC.
So what do you think are the odds that two hosts will assign themselves the same random
IPv6 address? Personally, I think you could probably win the lotto every day for a year and
still not come close to the odds against two hosts on the same data link duplicating an IPv6
address! Still, to make sure this doesn’t ever happen, duplicate address detection (DAD) was
created, which isn’t an actual protocol, but a function of the NS/NA messages. Figure14.11
shows how a host sends an NDP NS when it receives or creates an IPv6 address.
When hosts make up or receive an IPv6 address, they send three DADs out via NDP NS
asking if anyone has this same address. The odds are unlikely that this will ever happen,
but they ask anyway.
586 Chapter 14 u Internet Protocol Version 6 (IPv6)
FIGURE14.11 Duplicate address detection (DAD)
NDP: NS DAD
I just made up my IPv6
address. Does anyone
have this address?
NDP: NA
No! Seriously?
Do you know what
the odds are of that
happening? But thanks
for asking!
IPv6 Routing Protocols
All of the routing protocols we’ve already discussed have been tweaked and upgraded for
use in IPv6 networks, so it gures that many of the functions and congurations that you’ve
already learned will be used in almost the same way as they are now. Knowing that broad-
casts have been eliminated in IPv6, it’s safe to conclude that any protocols relying entirely on
broadcast trafc will go the way of the dodo. But unlike with the dodo, it’ll be really nice to
say goodbye to these bandwidth-hogging, performance-annihilating little gremlins!
The routing protocols we’ll still use in IPv6 have been renovated and given new names.
Even though this chapter’s focus is on the Cisco exam objectives, which cover only static
routing and OSPFv3, I want to discuss a few of the more important ones too.
First on the list is the IPv6 RIPng (next generation). Those of you who’ve been in IT for
a while know that RIP has worked pretty well for us on smaller networks. This happens to
be the very reason it didn’t get whacked and will still be around in IPv6. And we still have
EIGRPv6 because EIGRP already had protocol-dependent modules and all we had to do
was add a new one to it to t in nicely with the IPv6 protocol. Rounding out our group of
protocol survivors is OSPFv3—that’s not a typo, it really is v3! OSPF for IPv4 was actually
v2, so when it got its upgrade to IPv6, it became OSPFv3.
Static Routing with IPv6
Okay, now don’t let the heading of this section scare you into looking on Monster.com for
some job that has nothing to do with networking! I know that static routing has always run
a chill up our collective spines because it’s cumbersome, difcult, and really easy to screw
up. And I won’t lie to you—it’s certainly not any easier with IPv6’s longer addresses, but you
can do it!
Okay—we know that to make static routing work, whether in IP or IPv6, you need these
three tools:
u An accurate, up-to-date network map of your entire internetwork
u Next-hop address and exit interface for each neighbor connection
u All the remote subnet IDs
IPv6 Routing Protocols 587
Of course, we don’t need to have any of these for dynamic routing, which is why we
mostly use dynamic routing. It’s just so awesome to have the routing protocol do all that
work for us by nding all the remote subnets and automatically placing them into the
routing table!
Figure14.12 shows a really good example of how to use static routing with IPv6. It
really doesn’t have to be that hard, but just as with IPv4, you absolutely need an accurate
network map to make static routing work!
FIGURE14.12 IPv6 static and default routing
2001:DB8:43:91::1
Gi0/1
Static Route
Gi0/0
Internet
Branch Corp
2001:DB8:43:91::2
Default Route
2001:1234:4321:1::/64
ipv6 route ::/0 gi0/0
ipv6 route 2001:1234:4321:1::/64 2001:db8:43:91::1
So here’s what I did: First, I created a static route on the Corp router to the remote
network 2001:1234:4321:1::/64 using the next hop address. I could’ve just as easily used
the Corp router’s exit interface. Next, I just set up a default route for the Branch router
with ::/0 and the Branch exit interface of Gi0/0—not so bad!
OSPFv3
The new version of OSPF continues with the trend of routing protocols being quite similar to
their IPv4 versions. For instance, the foundation of OSPF remains the same—it’s still a link-
state routing protocol that divides an entire internetwork or AS into areas, creating a hier-
archy. And just trust me on this one—be really thankful that multi-area OSPF is beyond the
scope for the Cisco objectives covered in this chapter!
Anyway, in OSPF version 2, the router ID (RID) is determined by either the highest IP
addresses assigned to the router or one you manually assigned. In version 3, you assign the
RID and area ID, which are both still 32-bit values but aren’t found via the IP address any-
more because an IPv6 address is 128 bits. Changes in how these values are assigned, plus
the removal of the IP address information from OSPF packet headers, make the new version
of OSPF exible enough to be used over almost any Network layer protocol—sweet!
Adjacencies and next-hop attributes now use link-local addresses. OSPFv3 still uses
multicast trafc to send its updates and acknowledgments, with the addresses FF02::5
for OSPF routers and FF02::6 for OSPF-designated routers. These new addresses are the
replacements for 224.0.0.5 and 224.0.0.6, respectively.
588 Chapter 14 u Internet Protocol Version 6 (IPv6)
Other, less-exible IPv4 protocols just can’t compete with OSPFv2’s ability to assign
specic networks and interfaces into the OSPF process, but these are still congured during
router conguration. In OSPFv3, the interfaces and therefore the networks attached to them
are simply congured directly on the interface in interface conguration mode instead.
Here’s a sample of how the OSPFv3 conguration will look, starting with the optional
conguration of the RID in global conguration mode:
Router1(config)#ipv6 router osfp 10
Router1(config-rtr)#router-id 1.1.1.1
You get to execute some congurations from router conguration mode, like summari-
zation and redistribution, but we don’t even need to congure OSPFv3 from this prompt if
we congure OSPFv3 from the interface!
This is because if we go with the interface conguration option, the router conguration
process is added automatically. The interface conguration looks like this:
Router1(config-if)#ipv6 ospf 10 area 0
So, if we just go to each interface and assign a process ID and area—shazam, we’re done!
Let’ congure static, default, and OSPFv3 routing on our internetwork. It’s actually
pretty painless, and there’s still a lot for you to learn about OSPFv3!
Configuring IPv6 on Our Internetwork
We’re going to continue working on the same internetwork we’ve been conguring
throughout this book, as shown in Figure14.13. Let’s add IPv6 to the Corp, SF, and LA
routers by using a simple subnet scheme of 11, 12, 13, 14, and 15. After that, we’ll add the
OSPFv3 routing protocol. Notice in Figure14.13 how the subnet numbers are the same on
each end of the WAN links. Keep in mind that we’ll nish this chapter by running through
some verication commands.
As usual, I’ll start with the Corp router:
Corp#config t
Corp(config)#ipv6 unicast-routing
Corp(config)#int f0/0
Corp(config-if)#ipv6 address 2001:db8:3c4d:11::/64 eui-64
Corp(config-if)#int s0/0
Corp(config-if)#ipv6 address 2001:db8:3c4d:12::/64 eui-64
Corp(config-if)#int s0/1
Corp(config-if)#ipv6 address 2001:db8:3c4d:13::/64 eui-64
Corp(config-if)#^Z
Corp#copy run start
Configuring IPv6 on Our Internetwork 589
Destination filename [startup-config]?[enter]
Building configuration...
[OK]
FIGURE14.13 Our internetwork
SF
Corp
2001:db8:3c4d:11::/64Fa0/0
Fa0/02001:db8:3c4d:14::/64
2001:db8:3c4d:12::/64 S0/0/0
LA
Fa0/0 2001:db8:3c4d:15::/64
2001:db8:3c4d:13::/64
S0/0/1
2001:db8:3c4d:12::/64 S0/0 2001:db8:3c4d:13::/64S0/1
Pretty simple! In the above conguration, I only changed the subnet address for each
interface slightly. Let’s take a look at the routing table now:
Corp(config-if)#do sho ipv6 route
C 2001:DB8:3C4D:11::/64 [0/0]
via ::, FastEthernet0/0
L 2001:DB8:3C4D:11:20D:BDFF:FE3B:D80/128 [0/0]
via ::, FastEthernet0/0
C 2001:DB8:3C4D:12::/64 [0/0]
via ::, Serial0/0
L 2001:DB8:3C4D:12:20D:BDFF:FE3B:D80/128 [0/0]
via ::, Serial0/0
C 2001:DB8:3C4D:13::/64 [0/0]
via ::, Serial0/1
L 2001:DB8:3C4D:13:20D:BDFF:FE3B:D80/128 [0/0]
via ::, Serial0/1
L FE80::/10 [0/0]
via ::, Null0
L FF00::/8 [0/0]
via ::, Null0
Corp(config-if)#
590 Chapter 14 u Internet Protocol Version 6 (IPv6)
Alright, but what’s up with those two addresses for each interface? One shows C for
connected, one shows L. The connected address indicates the IPv6 address I congured on
each interface and the L is the link-local that’s been automatically assigned. Notice in the
link-local address that the FF:FE is inserted into the address to create the EUI-64 address.
Let’s congure the SF router now:
SF#config t
SF(config)#ipv6 unicast-routing
SF(config)#int s0/0/0
SF(config-if)#ipv6 address 2001:db8:3c4d:12::/64
% 2001:DB8:3C4D:12::/64 should not be configured on Serial0/0/0, a subnet router
anycast
SF(config-if)#ipv6 address 2001:db8:3c4d:12::/64 eui-64
SF(config-if)#int fa0/0
SF(config-if)#ipv6 address 2001:db8:3c4d:14::/64 eui-64
SF(config-if)#^Z
SF#show ipv6 route
C 2001:DB8:3C4D:12::/64 [0/0]
via ::, Serial0/0/0
L 2001:DB8:3C4D:12::/128 [0/0]
via ::, Serial0/0/0
L 2001:DB8:3C4D:12:21A:2FFF:FEE7:4398/128 [0/0]
via ::, Serial0/0/0
C 2001:DB8:3C4D:14::/64 [0/0]
via ::, FastEthernet0/0
L 2001:DB8:3C4D:14:21A:2FFF:FEE7:4398/128 [0/0]
via ::, FastEthernet0/0
L FE80::/10 [0/0]
via ::, Null0
L FF00::/8 [0/0]
via ::, Null0
Did you notice that I used the exact IPv6 subnet addresses on each side of the serial link?
Good... but wait—what’s with that anycast error I received when trying to congure the inter-
faces on the SF router? I didn’t meant to create that error; it happened because I forgot to add
the eui-64 at the end of the address. Still, what’s behind that error? An anycast address is a
host address of all 0s, meaning the last 64 bits are all off, but by typing in /64 without the
eui-64, I was telling the interface that the unique identier would be nothing but zero’s, and
that’s not allowed!
Let’s congure the LA router now, and then add OSPFv3:
SF#config t
SF(config)#ipv6 unicast-routing
Configuring Routing on Our Internetwork 591
SF(config)#int s0/0/1
SF(config-if)#ipv6 address 2001:db8:3c4d:13::/64 eui-64
SF(config-if)#int f0/0
SF(config-if)#ipv6 address 2001:db8:3c4d:15::/64 eui-64
SF(config-if)#do show ipv6 route
C 2001:DB8:3C4D:13::/64 [0/0]
via ::, Serial0/0/1
L 2001:DB8:3C4D:13:21A:6CFF:FEA1:1F48/128 [0/0]
via ::, Serial0/0/1
C 2001:DB8:3C4D:15::/64 [0/0]
via ::, FastEthernet0/0
L 2001:DB8:3C4D:15:21A:6CFF:FEA1:1F48/128 [0/0]
via ::, FastEthernet0/0
L FE80::/10 [0/0]
via ::, Null0
L FF00::/8 [0/0]
via ::, Null0
This looks good, but I want you to notice that I used the exact same IPv6 subnet addresses
on each side of the links from the Corp router to the SF router as well as from the Corp to the
LA router. Now let’s start adding OSPFv3!
Configuring Routing on Our Internetwork
We’re going to begin with default routing and then deal with OSPFv3. I’ll start at the Corp
router and add simple static routes. Check it out:
Corp(config)#ipv6 route 2001:db8:3c4d:14::/64 2001:DB8:3C4D:12:21A:2FFF:
FEE7:4398 150
Corp(config)#ipv6 route 2001:DB8:3C4D:15::/64 s0/1 150
Corp(config)#do sho ipv6 route static
[output cut]
S 2001:DB8:3C4D:14::/64 [150/0]
via 2001:DB8:3C4D:12:21A:2FFF:FEE7:4398
Okay—I agree that rst static route line was pretty long because I used the next-hop
address, but notice that I used the exit interface on the second entry. But it still wasn’t
really all that hard to create the longer static route entry. I just went to the SF router, used
the command show ipv6 int brief, and then copied and pasted the interface address
used for the next hop. You’ll get used to IPv6 addresses (You’ll get used to doing a lot of
copy/paste moves!).
592 Chapter 14 u Internet Protocol Version 6 (IPv6)
Okay, now since I put an AD of 150 on the static routes, once I congure OSPF they’ll
be replaced with an OSPF injected route. Let’s go to the SF and LA routers and put a single
entry in each router to get to remote subnet 11.
SF(config)#ipv6 route 2001:db8:3c4d:11::/64 s0/0/0 150
That’s it! I’m going to head over to LA and put a default route on that router now:
LA(config)#ipv6 route ::/0 s0/0/1
Let’s take a peek at the Corp router’s routing table and see if our static routes are in there.
Corp#sh ipv6 route static
[output cut]
S 2001:DB8:3C4D:14::/64 [150/0]
via 2001:DB8:3C4D:12:21A:2FFF:FEE7:4398
S 2001:DB8:3C4D:15::/64 [150/0]
via ::, Serial0/1
Voilà! I can see both of my static routes in the routing table, so IPv6 can now route to
those networks. But we’re not done because we still need to test our network! First I’m
going to go to the SF router and get the IPv6 address of the Fa0/0 interface:
SF#sh ipv6 int brief
FastEthernet0/0 [up/up]
FE80::21A:2FFF:FEE7:4398
2001:DB8:3C4D:14:21A:2FFF:FEE7:4398
FastEthernet0/1 [administratively down/down]
Serial0/0/0 [up/up]
FE80::21A:2FFF:FEE7:4398
2001:DB8:3C4D:12:21A:2FFF:FEE7:4398
Next, I’m going to go back to the Corporate router and ping that remote interface by copy-
ing and pasting in the address. No sense doing all that typing when copy/paste works great!
Corp#ping ipv6 2001:DB8:3C4D:14:21A:2FFF:FEE7:4398
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:DB8:3C4D:14:21A:2FFF:FEE7:4398, timeout is
2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 0/0/0 ms
Corp#
Configuring Routing on Our Internetwork 593
We can see that static route worked, so next, I’ll go get the IPv6 address of the LA router
and ping that remote interface as well:
LA#sh ipv6 int brief
FastEthernet0/0 [up/up]
FE80::21A:6CFF:FEA1:1F48
2001:DB8:3C4D:15:21A:6CFF:FEA1:1F48
Serial0/0/1 [up/up]
FE80::21A:6CFF:FEA1:1F48
2001:DB8:3C4D:13:21A:6CFF:FEA1:1F48
It’s time to head over to Corp and ping LA:
Corp#ping ipv6 2001:DB8:3C4D:15:21A:6CFF:FEA1:1F48
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:DB8:3C4D:15:21A:6CFF:FEA1:1F48, timeout is
2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/4 ms
Corp#
Static routing really isn’t so bad with IPv6! I’m not saying I’d like to do this in a ginormous
network—no way—I wouldn’t want to opt for doing that with IPv4 either! But you can see
that it can be done. Also, notice how easy it was to ping an IPv6 address. Copy/paste really is
your friend!
Okay, since I created my static and default routes with an AD of 150, I can see that the
OSPF routes have replaced the static ones in the routing table by turning on OSPF. All we
have to do in order to enable OSPF on the internetwork is to individually deal with each
interface we want to run OSPF on.
Here’s the Corp conguration:
Corp(config)#int f0/0
Corp(config-if)#ipv6 ospf 1 area 0
Corp(config-if)#int s0/0
Corp(config-if)#ipv6 ospf 1 area 0
Corp(config-if)#int s0/1
Corp(config-if)#ipv6 ospf 1 area 0
This is actually much easier than it is with IPv4! Let’s congure the other two routers now:
SF(config)#int f0/0
SF(config-if)#ipv6 ospf 1 area 0
SF(config-if)#int s0/0/0
SF(config-if)#ipv6 ospf 1 area 0
594 Chapter 14 u Internet Protocol Version 6 (IPv6)
*May 10 23:06:10.432: %OSPFv3-5-ADJCHG: Process 1, Nbr 172.131.1.1
on Serial0/0/0 from LOADING to FULL, Loading Done
Sweet! SF has become adjacent to the Corp router. One interesting output is that the
32-bit address RID is being used in the OSPFv3 adjacent change.
LA#config t
LA(config)#int fa0/0
LA(config-if)#ipv6 ospf 1 area 0
LA(config-if)#int s0/0/1
LA(config-if)#ipv6 ospf 1 area 0
*May 10 23:42:56.865: %OSPFv3-5-ADJCHG: Process 1, Nbr 172.31.1.2
on FastEthernet0/0 from LOADING to FULL, Loading Done
*May 10 23:42:57.197: %OSPFv3-5-ADJCHG: Process 1, Nbr 172.131.1.1
on Serial0/0/1 from LOADING to FULL, Loading Done
Presto—again, our adjacency popped up. This is great! Without even verifying our net-
work, it appears that our internetwork is up and running. But, we still have to verify!
Before we do that, remember that fourth router I used back in Chapter 9, “Open
Shortest Path First (OSPF),” to demonstrate the OSPF conguration and verication? I
want to use it again here. I named that router Boulder, and here is its simple conguration,
because having another router on the backbone area will really make a difference in our
verication commands:
Boulder#config t
Boulder(config)#int f0/0
Boulder(config-if)#ipv6 address autoconfig default
Boulder(config-if)#ipv6 ospf 1 area 0
*Apr 15 00:21:18.986: %OSPFv3-5-ADJCHG: Process 1, Nbr 172.131.1.1 on
FastEthernet0/0 from LOADING to FULL, Loading Done
I couldn’t even remember the IPv4 address that I assigned to the router’s Fa0/0 interface,
but it doesn’t matter because I just congured this router without adding even a single address!
And yet there it is—up and working! Yea IPv6! Since I had the Corp router congured, I just
told the Boulder router to autocongure the interface, then added Fa0/0 into OSPF area 0.
Verifying OSPFv3
Because verication is always important, I’ll run through the commands needed to verify
OSPFv3. I’ll start with the show ipv6 route command:
Corp#sh ipv6 route ospf
[output cut]
O 2001:DB8:3C4D:14::/64 [110/65]
Configuring Routing on Our Internetwork 595
via FE80::21A:2FFF:FEE7:4398, Serial0/0
O 2001:DB8:3C4D:15::/64 [110/65]
via FE80::21A:6CFF:FEA1:1F48, Serial0/1
Corp#
Perfect! I see both remote subnets 15 and 16 off the SF and LA routers, and the static
routes are no longer in the routing table because they had a higher AD. Let’s take a look at
what the show ipv6 protocols command reveals:
Corp#show ipv6 protocols
IPv6 Routing Protocol is "connected"
IPv6 Routing Protocol is "static"
IPv6 Routing Protocol is "ospf 1"
Interfaces (Area 0):
Serial0/1
Serial0/0
FastEthernet0/0
Redistribution:
None
This is an interesting command for sure, and it shows us the interfaces running the OSPF
process, but it’s very useful if you have multiple routing protocols buzzing around. A com-
mand for checking your RID is also a compelling prospect, so let’s see what doing that reveals:
Corp#sh ipv6 ospf
Routing Process "ospfv3 1" with ID 223.255.255.254
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
LSA group pacing timer 240 secs
Interface flood pacing timer 33 msecs
Retransmission pacing timer 66 msecs
Number of external LSA 0. Checksum Sum 0x000000
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Reference bandwidth unit is 100 mbps
Area BACKBONE(0) (Inactive)
Number of interfaces in this area is 1
SPF algorithm executed 1 times
Number of LSA 3. Checksum Sum 0x019C58
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0 --More--
596 Chapter 14 u Internet Protocol Version 6 (IPv6)
The rst line shows the process number and the router ID. Did you happen to notice that
it’s the same RID we set for our IPv4 OSPF RID back in Chapter 9? Let’s change the RID
under the OSPF process ID in the global conguration and see what happens:
Corp#config t
Corp(config)#ipv6 router ospf 1
Corp(config-rtr)#router-id 1.1.1.1
Reload or use "clear ip ospf process" command, for this to take effect
Corp(config-rtr)#do clear ip ospf process
Reset ALL OSPF processes? [no]: yes
Even though this is the rst time I’ve used global conguration mode to congure
options for OSPF, it’s working just ne. Let’s check our RID now:
Corp(config-router)#do sh ipv6 ospf
Routing Process "ospfv3 1" with ID 1.1.1.1
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
LSA group pacing timer 240 secs
[output cut]
Let’s check on the Corp router’s OSPF neighbors with the show ipv6 ospf neighbor
command:
Corp#sh ipv6 ospf neighbor
Neighbor ID Pri State Dead Time Interface ID Interface
192.168.20.1 1 FULL/ - 00:00:33 7 Serial0/1
172.31.1.2 1 FULL/ - 00:00:33 6 Serial0/0
10.10.10.2 1 FULL/BDR 00:00:39 4 FastEthernet0/0
Corp#
I know it is weird to look at the output of an IPv6 command and see IPv4 addresses,
because the listed neighbor ID is clearly the RID of our neighbors. Actually, I’m really glad
the IETF used a 32-bit address for the RID. Did we really need more 128-bit addresses
hanging around our networks to confuse us even more?
And of course, no verication would be complete without the show ipv6 interfaces
command:
Corp#sh ipv6 int
FastEthernet0/0 is up, line protocol is up
IPv6 is enabled, link-local address is FE80::20D:BDFF:FE3B:D80
No Virtual link-local address(es):
Global unicast address(es):
2001:DB8:3C4D:11:20D:BDFF:FE3B:D80, subnet is 2001:DB8:3C4D:11::/64 [EUI]
Joined group address(es):
FF02::1
Configuring Routing on Our Internetwork 597
FF02::2
FF02::5
FF02::6
FF02::1:FF3B:D80
MTU is 1500 bytes
ICMP error messages limited to one every 100 milliseconds
ICMP redirects are enabled
ICMP unreachables are sent
ND DAD is enabled, number of DAD attempts: 1
ND reachable time is 30000 milliseconds
ND advertised reachable time is 0 milliseconds
ND advertised retransmit interval is 0 milliseconds
ND router advertisements are sent every 200 seconds
ND router advertisements live for 1800 seconds
ND advertised default router preference is Medium
Hosts use stateless autoconfig for addresses.
Serial0/0 is up, line protocol is up
IPv6 is enabled, link-local address is FE80::20D:BDFF:FE3B:D80
No Virtual link-local address(es):
Global unicast address(es):
2001:DB8:3C4D:12:20D:BDFF:FE3B:D80, subnet is 2001:DB8:3C4D:12::/64 [EUI]
Joined group address(es):
[output cut]
This output reveals the interface IPv6 address, and even the subnet address is listed in
there. Now let’s use one of my favorite commands:
Corp#sh ipv6 int brief
FastEthernet0/0 [up/up]
FE80::20D:BDFF:FE3B:D80
2001:DB8:3C4D:11:20D:BDFF:FE3B:D80
Serial0/0 [up/up]
FE80::20D:BDFF:FE3B:D80
2001:DB8:3C4D:12:20D:BDFF:FE3B:D80
FastEthernet0/1 [administratively down/down]
unassigned
Serial0/1 [up/up]
FE80::20D:BDFF:FE3B:D80
2001:DB8:3C4D:13:20D:BDFF:FE3B:D80
Loopback0 [up/up]
unassigned
Corp#
598 Chapter 14 u Internet Protocol Version 6 (IPv6)
What a nice output! All our interfaces are up/up and we can see the link-local and
assigned global address.
But we still need to execute our debugging commands, and I’m going to use two of
them: the debug ipv6 ospf packet and debug ipv6 ospf hello. By the way, these are
the same commands I used with IPv4:
Corp#debug ipv6 ospf packet
OSPFv3 packet debugging is on
*Jan 26 14:49:40.738: OSPFv3: rcv. v:3 t:1 l:40 rid:10.10.10.2
aid:0.0.0.0 chk:AFF9 inst:0 from FastEthernet0/0
*Jan 26 14:49:43.982: OSPFv3: rcv. v:3 t:1 l:40 rid:172.31.1.2
aid:0.0.0.0 chk:2E42 inst:0 from Serial0/0
*Jan 26 14:49:44.306: OSPFv3: rcv. v:3 t:1 l:40 rid:192.168.20.1
aid:0.0.0.0 chk:EE4E inst:0 from Serial0/1
*Jan 26 14:49:50.738: OSPFv3: rcv. v:3 t:1 l:40 rid:10.10.10.2
aid:0.0.0.0 chk:AFF9 inst:0 from FastEthernet0/0
*Jan 26 14:49:53.982: OSPFv3: rcv. v:3 t:1 l:40 rid:172.31.1.2
aid:0.0.0.0 chk:2E42 inst:0 from Serial0/0
*Jan 26 14:49:54.306: OSPFv3: rcv. v:3 t:1 l:40 rid:192.168.20.1
aid:0.0.0.0 chk:EE4E inst:0 from Serial0/1
Corp#un all
All possible debugging has been turned off
Corp#debug ipv6 ospf hello
OSPFv3 hello events debugging is on
*Jan 26 14:50:10.738: OSPFv3: Rcv hello from 10.10.10.2 area 0
from FastEthernet0/0 FE80::20D:BDFF:FE3B:C20 interface ID 4
*Jan 26 14:50:10.738: OSPFv3: End of hello processing
*Jan 26 14:50:13.982: OSPFv3: Rcv hello from 172.31.1.2 area 0
from Serial0/0 FE80::21A:2FFF:FEE7:4398 interface ID 6
*Jan 26 14:50:13.982: OSPFv3: End of hello processing
*Jan 26 14:50:14.306: OSPFv3: Rcv hello from 192.168.20.1 area 0
from Serial0/1 FE80::21A:6CFF:FEA1:1F48 interface ID 7
*Jan 26 14:50:14.310: OSPFv3: End of hello processing
*Jan 26 14:50:20.738: OSPFv3: Rcv hello from 10.10.10.2 area 0
from FastEthernet0/0 FE80::20D:BDFF:FE3B:C20 interface ID 4
*Jan 26 14:50:20.738: OSPFv3: End of hello processing
*Jan 26 14:50:23.982: OSPFv3: Rcv hello from 172.31.1.2 area 0
from Serial0/0 FE80::21A:2FFF:FEE7:4398 interface ID 6
*Jan 26 14:50:23.982: OSPFv3: End of hello processing
*Jan 26 14:50:24.310: OSPFv3: Rcv hello from 192.168.20.1 area 0
from Serial0/1 FE80::21A:6CFF:FEA1:1F48 interface ID 7
*Jan 26 14:50:24.310: OSPFv3: End of hello processing
Exam Essentials 599
Holy output! That pretty much gives us the big picture! I hope you found this chapter as
rewarding as I did. The best thing you can do to learn IPv6 is to get some routers and just
go at it. Don’t give up because it’s seriously worth your time!
Summary
This last chapter introduced you to some very key IPv6 structural elements as well as how to
make IPv6 work within a Cisco internetwork. You now know that even when covering and
conguring IPv6 basics, there’s still a great deal to understand—and we just scratched the
surface! But you’re still well equipped with all you need to meet the Cisco exam objectives.
You learned the vital reasons why we need IPv6 and the benets associated with it. I cov-
ered IPv6 addressing and the importance of using the shortened expressions. As I covered
addressing with IPv6, I also showed you the different address types, plus the special addresses
reserved in IPv6.
IPv6 will mostly be deployed automatically, meaning hosts will employ autocongura-
tion. I demonstrated how IPv6 utilizes autoconguration and how it comes into play when
conguring a Cisco router. You also learned that in IPv6, we can and still should use a
DHCP server to the router to provide options to hosts just as we’ve been doing for years
with IPv4—not necessarily IPv6 addresses, but other mission-critical options like providing
a DNS server address.
From there, I discussed the evolution of some more integral and familiar protocols like
ICMP and OSPF. They’ve been upgraded to work in the IPv6 environment, but these net-
working workhorses are still vital and relevant to operations, and I detailed how ICMP
works with IPv6, followed by how to congure OSPFv3. I wrapped up this pivotal chapter
by demonstrating key methods to use when verifying that all is running correctly in your
IPv6 network. So take some time and work through all the essential study material, espe-
cially the written labs, to ensure that you meet your networking goals!
Exam Essentials
Understand why we need IPv6. Without IPv6, the world would be depleted of IP addresses.
Understand link-local. Link-local is like an IPv4 private IP address, but it can’t be routed
at all, not even in your organization.
Understand unique local. This, like link-local, is like a private IP address in IPv4 and
cannot be routed to the Internet. However, the difference between link-local and unique
local is that unique local can be routed within your organization or company.
Remember IPv6 addressing. IPv6 addressing is not like IPv4 addressing. IPv6 address-
ing has much more address space, is 128 bits long, and represented in hexadecimal, unlike
IPv4, which is only 32 bits long and represented in decimal.
Visit ccna
.gg/ch14/b
for a
companion
MicroNugget
from CBT
Nuggets.
600 Chapter 14 u Internet Protocol Version 6 (IPv6)
Understand and be able to read a EUI-64 address with the 7th bit inverted. Hosts can
use autoconguration to obtain an IPv6 address, and one of those ways is through what is
called EUI-64. This takes the unique MAC address of a host and inserts FF:FE in the mid-
dle of the address to change a 48-bit MAC address to a 64-bit interface ID. In addition to
inserting the 16-bits into the interface ID, the 7th bit of the rst byte is inverted, typically
from a 0 to a 1. Practice this with Written Lab 14.2.
Written Labs 601
Written Labs
The answers to these labs can be found in Appendix A, “Answers to Written Labs.”
Written Lab 14.1
In this section, write the answers to the following IPv6 questions:
1. Which type of packet is addressed and delivered to only a single interface?
2. Which type of address is used just like a regular public routable address in IPv4?
3. Which type of address is not meant to be routed?
4. Which type of address is not meant to be routed to the Internet but is still globally
unique?
5. Which type of address is meant to be delivered to multiple interfaces?
6. Which type of address identifies multiple interfaces, but packets are delivered only to
the first address it finds?
7. Which routing protocol uses multicast address FF02::5?
8. IPv4 had a loopback address of 127.0.0.1. What is the IPv6 loopback address?
9. What does a link-local address always start with?
10. What does a unique local unicast range start with?
Written Lab 14.2
In this section, you will practice inverting the 7th bit of a EUI-64 address. Use the prex
2001:db8:1:1/64 for each address.
1. Convert the following MAC address into a EUI-64 address: 0b0c:abcd:1234.
2. Convert the following MAC address into a EUI-64 address: 060c:32f1:a4d2.
3. Convert the following MAC address into a EUI-64 address: 10bc:abcd:1234.
4. Convert the following MAC address into a EUI-64 address: 0d01:3a2f:1234.
5. Convert the following MAC address into a EUI-64 address: 0a0c.abac.caba.
602 Chapter 14 u Internet Protocol Version 6 (IPv6)
Hands-on Labs
You’ll need at least three routers to complete these labs, ve would be better, but if you are
using the LammleSim IOS version, then these lab layouts are precongured for you. This
section will have you congure the following labs:
Lab 14.1: Manual and Stateful Autoconguration
Lab 14.2: Static and Default Routing
Lab 14.3: OSPFv3
Here is our network:
Fa0/0
E
Fa0/0
D
Fa0/0
B
Fa0/0
A
2001:db8:3c4d:1::/64 2001:db8:3c4d:2::/64Fa0/1Fa0/0 C
Hands-on Lab 14.1: Manual and Stateful
Autoconfiguration
In this lab, you will congure the C router with manual IPv6 addresses on the Fa0/0 and
Fa0/1 interfaces and then congure the other routers to automatically assign themselves an
IPv6 address.
1. Log in to the C router and configure IPv6 addresses on each interface based on the sub-
nets (1 and 2) shown in the graphic.
C(config)#ipv6 unicast-routing
C(config)#int fa0/0
C(config-if)#ipv6 address 2001:db8:3c4d:1::1/64
C(config-if)#int fa0/1
C(config-if)#ipv6 address 2001:db8:3c4d:2::1/64
2. Verify the interfaces with the show ipv6 route connected and sho ipv6 int brief
commands.
C(config-if)#do show ipv6 route connected
[output cut]
C 2001:DB8:3C4D:1::/64 [0/0]
via ::, FastEthernet0/0
C 2001:DB8:3C4D:2::/64 [0/0]
via ::, FastEthernet0/0
Hands-on Labs 603
C(config-if)#sh ipv6 int brief
FastEthernet0/0 [up/up]
FE80::20D:BDFF:FE3B:D80
2001:DB8:3C4D:1::1
FastEthernet0/1 [up/up]
FE80::20D:BDFF:FE3B:D81
2001:DB8:3C4D:2::1
Loopback0 [up/up]
Unassigned
3. Go to your other routers and configure the Fa0/0 on each router to autoconfigure an
IPv6 address.
A(config)#ipv6 unicast-routing
A(config)#int f0/0
A(config-if)#ipv6 address autoconfig
A(config-if)#no shut
B(config)#ipv6 unicast-routing
B(config)#int fa0/0
B(config-if)#ipv6 address autoconfig
B(config-if)#no shut
D(config)#ipv6 unicast-routing
D(config)#int fa0/0
D(config-if)#ipv6 address autoconfig
D(config-if)#no shut
E(config)#ipv6 unicast-routing
E(config)#int fa0/0
E(config-if)#ipv6 address autoconfig
E(config-if)#no shut
4. Verify that your routers received an IPv6 address.
A#sh ipv6 int brief
FastEthernet0/0 [up/up]
FE80::20D:BDFF:FE3B:C20
2001:DB8:3C4D:1:20D:BDFF:FE3B:C20
Continue to verify your addresses on all your other routers.
604 Chapter 14 u Internet Protocol Version 6 (IPv6)
Hands-on Lab 14.2: Static and Default Routing
Router C is directly connected to both subnets, so no routing of any type needs to be con-
gured. However, all the other routers are connected to only one subnet, so at least one
route needs to be congured on each router.
1. On the A router, configure a static route to the 2001:db8:3c4d:2::/64 subnet.
A(config)#ipv6 route 2001:db8:3c4d:2::/64 fa0/0
2. On the B router, configure a default route.
B(config)#ipv6 route ::/0 fa0/0
3. On the D router, create a static route to the remote subnet.
D(config)#ipv6 route 2001:db8:3c4d:1::/64 fa0/0
4. On the E router, create a static route to the remote subnet.
E(config)#ipv6 route 2001:db8:3c4d:1::/64 fa0/0
5. Verify your configurations with a show running-config and show ipv6 route.
6. Ping from router D to router A. First, you need to get router A’s IPv6 address with a
show ipv6 int brief command. Here is an example:
A#sh ipv6 int brief
FastEthernet0/0 [up/up]
FE80::20D:BDFF:FE3B:C20
2001:DB8:3C4D:1:20D:BDFF:FE3B:C20
7. Now go to router D and ping the IPv6 address from router A:
D#ping ipv6 2001:DB8:3C4D:1:20D:BDFF:FE3B:C20
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:DB8:3C4D:1:20D:BDFF:FE3B:C20, timeout
is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 0/2/4 ms
Our static and default routing is working. Now let’s add OSPFv3.
Hands-on Labs 605
Hands-on Lab 14.3: OSPFv3
In this lab, you’ll add OSPFv3 to your routers.
1. Before you start configuring OSPF, you need to remove any static and default routing
you have configured on your routers.
A(config)#no ipv6 route 2001:db8:3c4d:2::/64 fa0/0
B(config)#no ipv6 route ::/0 fa0/0
D(config)#no ipv6 route 2001:db8:3c4d:1::/64 fa0/0
E(config)#no ipv6 route 2001:db8:3c4d:1::/64 fa0/0
2. Go to each router and configure each interface with IPv6 OSPFv3. If you don’t have IPv4
configured on your routers, you’ll need to create a RID at global configuration mode.
A(config)#ipv6 router ospf 1
A(config-rtr)#router-id 1.1.1.1
A(config-rtr)#int f0/0
A(config-if)#ipv6 ospf 1 area 0
B(config)#ipv6 router ospf 1
B(config-rtr)#router-id 2.2.2.2
B(config-rtr)#int f0/0
B(config-if)#ipv6 ospf 1 area 0
C(config)#ipv6 router ospf 1
C(config-rtr)#router-id 3.3.3.3
C(config-rtr)#int f0/0
C(config-if)#ipv6 ospf 1 area 0
C(config-if)#int fa0/1
C(config-if)#ipv6 ospf 1 area 0
D(config)#ipv6 router ospf 1
D(config-rtr)#router-id 4.4.4.4
D(config-rtr)#int f0/0
D(config-if)#ipv6 ospf 1 area 0
E(config)#ipv6 router ospf 1
E(config-rtr)#router-id 5.5.5.5
E(config-rtr)#int f0/0
E(config-if)#ipv6 ospf 1 area 0
606 Chapter 14 u Internet Protocol Version 6 (IPv6)
3. From each router, use the following commands to verify your network.
A#sh ipv6 route
A#sh ipv6 route ospf
A#sh ipv6 int fa0/0
A#sh ipv6 neighbor
4. From each router, ping the other routers. First, you need to get the address of each
interface.
A#sh ipv6 int brief
B#sh ipv6 int brief
C#sh ipv6 int brief
D#sh ipv6 int brief
E#sh ipv6 int brief
5. Using the addresses you learned in step 4, ping to the neighbor routers, as in
this example:
A#ping ipv6 2001:DB8:3C4D:1:20D:BDFF:FE3B:C20
Review Questions 607
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional questions,
please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. Which of the following is true when describing a global unicast address?
A. Packets addressed to a unicast address are delivered to a single interface.
B. These are your typical publicly routable addresses, just like a regular publicly
routable address in IPv4.
C. These are like private addresses in IPv4 in that they are not meant to be routed
over the Internet.
D. These addresses are meant for nonrouting purposes, but they are almost globally
unique, so it is unlikely they will have an address overlap.
2. Which of the following is true when describing a unicast address?
A. Packets addressed to a unicast address are delivered to a single interface.
B. These are your typical publicly routable addresses, just like a regular publicly
routable address in IPv4.
C. These are like private addresses in IPv4 in that they are not meant to be routed.
D. These addresses are meant for nonrouting purposes, but they are almost globally
unique, so it is unlikely they will have an address overlap.
3. Which of the following is true when describing a link-local address?
A. Packets addressed to a broadcast address are delivered to a single interface.
B. These are your typical publicly routable addresses, just like a regular publicly
routable address in IPv4.
C. These are like private addresses in IPv4 in that they are not meant to be routed
over the Internet.
D. These addresses are meant for nonrouting purposes, but they are almost globally
unique, so it is unlikely they will have an address overlap.
608 Chapter 14 u Internet Protocol Version 6 (IPv6)
4. Which of the following is true when describing a unique local address?
A. Packets addressed to a unique local address are delivered to a single interface.
B. These are your typical publicly routable addresses, just like a regular publicly
routable address in IPv4.
C. These are like private addresses in IPv4 in that they are not meant to be routed.
D. These addresses are not meant for Internet routing purposes, but they are unique,
so it is unlikely they will have an address overlap.
5. Which of the following is true when describing a multicast address?
A. Packets addressed to a multicast address are delivered to a single interface.
B. Packets are delivered to all interfaces identified with the address. This is also called
a one-to-many address.
C. A multicast address identifies multiple interfaces and is delivered to only one
address. This address can also be called one-to-one-of-many.
D. These addresses are meant for nonrouting purposes, but they are almost globally
unique, so it is unlikely they will have an address overlap.
6. Which of the following is true when describing an anycast address?
A. Packets addressed to an anycast address are delivered to a single interface.
B. Packets are delivered to all interfaces identified by the address. This is also called a
one-to-many address.
C. This address identifies multiple interfaces and the anycast packet is only delivered
to one device. This address can also be called one-to-one-of-many.
D. These addresses are meant for nonrouting purposes, but they are almost globally
unique, so it is unlikely they will have an address overlap.
7. You want to ping the loopback address of your IPv6 local host. What will you type?
A. ping 127.0.0.1
B. ping 0.0.0.0
C. ping ::1
D. trace 0.0.::1
8. What two multicast addresses does OSPFv3 use? (Choose two.)
A. FF02::A
B. FF02::9
C. FF02::5
D. FF02::6
Review Questions 609
9. An IPv6 hostname Host A is trying to connect to a web page on a remote server.
Which of the following is true? (Choose two.)
Internet
R1
Remote server
2001:db8:c334:67::/64
Fa0/0
Gi0/1Fa0/0
Host A
2001:db8:c334:1::/64
A. A RA would be used by R1 to communicate its layer 2 MAC address to Host A.
B. OSPFv2 is used for the routers to share IPv6 routes.
C. IPv6 uses a two-part addressing scheme, similar to the way IPv4 uses a network
and host portion of an IPv4 address.
D. Host A would send the server’s link-local address to the router.
10. A host sends a router solicitation (RS) on the data link. What destination address is
sent with this request?
A. FF02::A
B. FF02::9
C. FF02::2
D. FF02::1
E. FF02::5
11. IPv6 unicast routing is running on the Corp router. Which of the following addresses
would show up with the show ipv6 int brief command?
Corp#sh int f0/0
FastEthernet0/0 is up, line protocol is up
Hardware is AmdFE, address is 000d.bd3b.0d80 (bia 000d.bd3b.0d80)
[output cut]
A. FF02::3c3d:0d:bdff:fe3b:0d80
B. FE80::3c3d:2d:bdff:fe3b:0d80
C. FE80::3c3d:0d:bdff:fe3b:0d80
D. FE80::3c3d:2d:ffbd:3bfe:0d80
610 Chapter 14 u Internet Protocol Version 6 (IPv6)
12. A host sends a type of NDP message providing the MAC address that was requested.
Which type of NDP was sent?
A. NA
B. RS
C. RA
D. NS
13. To enable OSPFv3, which of the following would you use?
A. Router(config-if)#ipv6 ospf 10 area 0.0.0.0
B. Router(config-if)#ipv6 router rip 1
C. Router(config)#ipv6 router eigrp 10
D. Router(config-rtr)#no shutdown
E. Router(config-if)#ospf ipv6 10 area 0
14. Which of the following statements about IPv6 addresses are true? (Choose two.)
A. Leading zeros are required.
B. Two colons (::) are used to represent successive hexadecimal fields of zeros.
C. Two colons (::) are used to separate fields.
D. A single interface will have multiple IPv6 addresses of different types.
15. What two statements about IPv4 and IPv6 addresses are true? (Choose two.)
A. An IPv6 address is 32 bits long, represented in hexadecimal.
B. An IPv6 address is 128 bits long, represented in decimal.
C. An IPv4 address is 32 bits long, represented in decimal.
D. An IPv6 address is 128 bits long, represented in hexadecimal.
16. Which of the following descriptions about IPv6 is correct?
A. Addresses are not hierarchical and are assigned at random.
B. Broadcasts have been eliminated and replaced with multicasts.
C. There are 2.7 billion addresses.
D. An interface can only be configured with one IPv6 address.
17. How many bits are in an IPv6 address field?
A. 24
B. 4
C. 3
D. 16
E. 32
F. 128
Review Questions 611
18. Which of the following correctly describe characteristics of IPv6 unicast addressing?
(Choose two.)
A. Global addresses start with 2000::/3.
B. Link-local addresses start with FF00::/10.
C. Link-local addresses start with FE00:/12.
D. There is only one loopback address and it is ::1.
19. Which of the following statements are true of IPv6 address representation? (Choose two.)
A. The first 64 bits represent the dynamically created interface ID.
B. A single interface may be assigned multiple IPv6 addresses of any type.
C. Every IPv6 interface contains at least one loopback address.
D. Leading zeroes in an IPv6 16-bit hexadecimal field are mandatory.
20. Which of the following is true regarding OSPFv3? (Choose three.)
A. Uses a wildcard to define interfaces
B. Uses a network command under global configuration mode
C. Uses a 32-bit router ID
D. Uses link-state advertisements
E. Uses an interface command to enable OSPF on an interface
ICND2 PART
II
Chapter
15
Enhanced Switched
Technologies
THE FOLLOWING ICND2 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 LAN Switching Technologies
■Identify enhanced switching technologies
■RSTP
■PVSTP
■EtherChannels
■Configure and verify PVSTP operation
■describe root bridge election
■spanning-tree mode
1 Troubleshooting
■Troubleshoot and resolve spanning-tree operation issues
■root switch
■priority
■mode is correct
■port states
■Troubleshoot EtherChannel problems
Long ago, a company called Digital Equipment Corporation
(DEC) created the original version of Spanning Tree Protocol
(STP). The IEEE later created its own version of STP called
802.1d. Cisco has moved toward another industry standard in its newer switches called 802.1w
.
We’ll explore both the old and new versions of STP in this chapter, but rst, I’ll dene some
important STP basics.
Routing protocols like RIP and OSPF have processes for preventing loops from occur-
ring at the Network layer, but if you have redundant physical links between your switches,
these protocols won’t do a thing to stop loops from occurring at the Data Link layer. That’s
exactly why STP was developed—to put an end to loop issues in a layer 2 switched network.
It’s also why we’ll be thoroughly exploring the key features of this vital protocol as well as
how it works within a switched network in this chapter.
After covering STP in detail, we’ll move on to explore EtherChannel.
To find up-to-the-minute updates for this chapter, please see www.lammle
.com/forum or the book’s web page at www.sybex.com.
Spanning Tree Protocol (STP)
Spanning Tree Protocol (STP) achieves its primary objective of preventing network loops on
layer 2 network bridges or switches by monitoring the network to track all links and shut
down the redundant ones. STP uses the spanning-tree algorithm (STA) to rst create a topol-
ogy database and then search out and disable redundant links. With STP running, frames
will be forwarded on only premium, STP-chosen links.
The Spanning Tree Protocol is a great protocol to use in networks like the one shown in
Figure15.1.
This is a switched network with a redundant topology that includes switching loops.
Without some type of layer 2 mechanism in place to prevent a network loop, this network
is vulnerable to nasty issues like broadcast storms, multiple frame copies, and MAC table
thrashing! Figure15.2 shows how this network would work with STP working on the
switches.
There a few types of spanning-tree protocols, but I’ll start with the IEEE version 802.1d,
which happens to be the default on all Cisco IOS switches.
Spanning Tree Protocol (STP) 617
FIGURE15.1 A switched network with switching loops
S1
S3 S2
Loop
FIGURE15.2 A switched network with STP
S1
No loop
S3 S2
Spanning-tree Terms
Now, before I get into describing the details of how STP works within a network, it would
be good for you to have these basic ideas and terms down rst:
Root bridge The root bridge is the bridge with the lowest and, therefore, the best bridge
ID. The switches within the STP network elect a root bridge, which becomes the focal
point in the network. All other decisions in the network, like which ports on the non root
bridges should be blocked or put in forwarding mode, are made from the perspective of the
root bridge, and once it has been elected, all other bridges must create a single path to it.
The port with the best path to the root bridge is called the root port.
618 Chapter 15 u Enhanced Switched Technologies
Non-root bridges These are all bridges that aren’t the root bridge. Non-root bridges
exchange BPDUs with all the other bridges and update the STP topology database on all
switches. This prevents loops and helps defend against link failures.
BPDU All switches exchange information to use for the subsequent conguration of the
network. Each switch compares the parameters in the Bridge Protocol Data Unit (BPDU)
that it sends to a neighbor with the parameters in the BPDU that it receives from other
neighbors. Inside the BPDU is the bridge ID.
Bridge ID The bridge ID is how STP keeps track of all the switches in the network. It’s
determined by a combination of the bridge priority, which is 32,768 by default on all Cisco
switches, and the base MAC address. The bridge with the lowest bridge ID becomes the
root bridge in the network. Once the root bridge is established, every other switch must
make a single path to it. Most networks benet by forcing a specic bridge or switch to be
on the Root Bridge by setting its bridge priority lower than the default value.
Port cost Port cost determines the best path when multiple links are used between two
switches. The cost of a link is determined by the bandwidth of a link, and this path cost is
the deciding factor used by every bridge to nd the most efcient path to the root bridge.
Path cost A switch may encounter one or more switches on its path to the Root Bridge, and
there may be more than one possible path. All unique paths are analyzed individually, and a
path cost is calculated for each unique path by adding the individual port costs encountered
on the way to the Root Bridge.
Bridge Port Roles
STP uses roles to determine how a port on a switch will act within the spanning-tree
algorithm.
Root port The root port is the link with the lowest path cost to the root bridge. If more
than one link connects to the root bridge, then a port cost is found by checking the band-
width of each link. The lowest-cost port becomes the root port. When multiple links con-
nect to the same device, the port connected to the lowest port number on the upstream
switch will be the one that’s used. The root bridge can never have a root port designation,
while every other switch in a network must have one and only one root port.
Designated port A designated port is one that’s been determined to have the best (lowest)
cost to get to on a given network segment, compared to other ports on that segment. A des-
ignated port will be marked as a forwarding port, and you can have only one forwarding
port per network segment.
Non-designated port A non-designated port is one with a higher cost than the designated
port. These are basically the ones left over after the root ports and designated ports have
been determined. Non-designated ports are put in blocking or discarding mode—they are
not forwarding ports!
Forwarding port A forwarding port forwards frames and will be either a root port or a
designated port.
Spanning Tree Protocol (STP) 619
Blocked port A blocked port won’t forward frames in order to prevent loops. A blocked
port will still always listen to BPDU frames from neighbor switches, but it will drop any
and all other frames received and will never transmit a frame.
Alternate port This corresponds to the blocking state of 802.1d, and is a term used with
the newer 802.1w (Cisco Rapid Spanning Tree Protocol). An alternative port is located on a
switch connected to a LAN segment with two or more switches connected, and one of the
other switches holds the designated port.
Backup port This corresponds to the blocking state of 802.1d, and is a term now used
with the newer 802.1w. A backup port is connected to a LAN segment where another port
on that switch is acting as the designated port.
Spanning-tree Port States
Okay, so you plug your host into a switch port and the light turns amber and your host doesn’t
get a DHCP address from the server. You wait and wait and nally the light goes green after
almost a full minute—that’s an eternity in today’s networks! This is the STA transitioning
through the different port states verifying that you didn’t just create a loop with the device you
just plugged in. STP would rather time out your new host than allow a loop into the network
because that would effectively bring your network to its knees. Let’s talk about the transition
states; then later in this chapter we’ll talk about how to speed this process up.
The ports on a bridge or switch running IEEE 802.1d STP can transition through ve
different states:
Disabled (technically, not a transition state) A port in the administratively disabled state
doesn’t participate in frame forwarding or STP. A port in the disabled state is virtually
nonoperational.
Blocking As I mentioned, a blocked port won’t forward frames; it just listens to BPDUs.
The purpose of the blocking state is to prevent the use of looped paths. All ports are in
blocking state by default when the switch is powered up.
Listening This port listens to BPDUs to make sure no loops occur on the network before
passing data frames. A port in listening state prepares to forward data frames without
populating the MAC address table.
Learning The switch port listens to BPDUs and learns all the paths in the switched net-
work. A port in learning state populates the MAC address table but still doesn’t forward
data frames. Forward delay refers to the time it takes to transition a port from listening to
learning mode, or from learning to forwarding mode, which is set to 15 seconds by default
and can be seen in the show spanning-tree output.
Forwarding This port sends and receives all data frames on the bridged port. If the port is
still a designated or root port at the end of the learning state, it will enter the forwarding state.
Switches populate the MAC address table in learning and forwarding
modes only.
620 Chapter 15 u Enhanced Switched Technologies
Switch ports are most often in either the blocking or forwarding state. A forwarding
port is typically the one that’s been determined to have the lowest (best) cost to the root
bridge. But when and if the network experiences a topology change due to a failed link or
because someone has added in a new switch, you’ll see the ports on a switch transitioning
through listening and learning states.
As I said earlier, blocking ports is a strategy for preventing network loops. Once a
switch determines the best path to the root bridge for its root port and any designated
ports, all other redundant ports will be in blocking mode. Blocked ports can still receive
BPDUs—they just don’t send out any frames.
If a switch determines that a blocked port should become the designated or root port
because of a topology change, it will go into listening mode and check all BPDUs it receives
to make sure it won’t create a loop once the port moves into forwarding mode.
Convergence
Convergence occurs when all ports on bridges and switches have transitioned to either for-
warding or blocking modes. No data will be forwarded until convergence is complete. Yes—
you read that right: When STP is converging, all host data stops transmitting through the
switches! So if you want to remain on speaking terms with your network’s users, or remain
employed for any length of time, you must make sure that your switched network is physi-
cally designed really well so that STP can converge quickly!
Convergence is vital because it ensures that all devices have a coherent database. And
making sure this happens efciently will denitely require your time and attention. The
original STP (802.1d) takes 50 seconds to go from blocking to forwarding mode by default
and I don’t recommend changing the default STP timers. You can adjust those timers for a
large network, but the better solution is simply to opt out of using 802.1d at all! We’ll get
to the various STP versions in a minute.
Link Costs
Now that you know about the different port roles and states, you need to really understand
all about path cost before we put this all together. Port cost is based on the speed of the link,
and Table15.1 breaks down the need-to-know path costs for you. Port cost is the cost of a
single link whereas path cost is the sum of the various port costs to the root bridge.
TABLE15.1 IEEE STP link costs
Speed Cost
10 Mb/s 100
100 Mb/s 19
1000 Mb/s 4
10,000 Mb/s 2
Spanning Tree Protocol (STP) 621
These costs will be used in the STP calculations to choose a single root port on each
bridge. You absolutely need to memorize this table, but no worries—I’ll guide you through
lots of examples in this chapter to help you do that quite easily! Now it’s time to take every-
thing we’ve learned so far and put it all together.
Spanning-tree Operations
Let’s start neatly summarizing what you’ve learned so far using the simple three-switch net-
work connected together as shown in Figure15.3.
FIGURE15.3 STP operations
Priority: 32768
0000.0CA7.A603
Root bridge
1 Gb/s
1 Gb/s 1 Gb/s
Priority: 32768
000A.41D5.7937
Priority: 32768
0030.F222.2794
S1
S3 S2
Basically, STP’s job is to nd all the links in the network and shut down any redundant
ones, thereby preventing network loops from occurring. It achieves this by rst electing a root
bridge that will have all ports forwarding and will also act as a point of reference for all other
devices within the STP domain. In Figure15.4, S1 has been elected the root bridge based on
bridge ID. Since the priorities are all equal to 32,768, we’ll compare MAC addresses and nd
the MAC address of S1 is lower than that of S2 and S3, meaning that S1 has a better bridge ID.
Once all switches agree on the root bridge, they must then determine their one and only
root port—the single path to the root bridge. It’s really important to remember that a bridge
can go through many other bridges to get to the root, so it’s not always the shortest path
that will be chosen. That role will be given to the port that happens to offer the fastest, high-
est bandwidth. Figure15.4 shows the root ports for both non-root bridges (the RP signies a
root port and the F signies a designated forwarding port).
Looking at the cost of each link, it’s clear why S2 and S3 are using their directly connected
links, because a gigabit link has a cost of 4. For example, if S3 chose the path through S2 as
its root port, we’d have to add up each port cost along the way to the root, which would be
4 + 4 for a total cost of 8.
Every port on the root bridge is a designated, or forwarding, port for a segment, and
after the dust settles on all other non-root bridges, any port connection between switches
622 Chapter 15 u Enhanced Switched Technologies
that isn’t either a root port or a designated port will predictably become a non-designated
port. These will again be put into the blocking state to prevent switching loops.
FIGURE15.4 STP operations
F
F
Priority: 32768
0000.0CA7.A603
Root bridge
1 Gb/s
Cost 4
1 Gb/s 1 Gb/s
RP Cost 4
Priority: 32768
000A.41D5.7937
RP
Cost 4
Priority: 32768
0030.F222.2794
S1
S3 S2
Okay—at this point, we have our root bridge with all ports in forwarding state and we’ve
found our root ports for each non-root bridge. Now the only thing left to do is to choose
the one forwarding port on the segment between S2 and S3. Both bridges can’t be forward-
ing on a segment because that’s exactly how we would end up with loops. So, based on the
bridge ID, the port with the best and lowest would become the only bridge forwarding on
that segment, with the one having the highest, worst bridge ID put into blocking mode.
Figure15.5 shows the network after STP has converged.
Since S3 had a lower bridge ID (better), S2’s port went into blocking mode. Let’s discuss
the root bridge election process more completely now.
FIGURE15.5 STP operations
F
F
Priority: 32768
0000.0CA7.A603
Root bridge
1 Gb/s
Cost 4
1 Gb/s 1 Gb/s
RP
F
Cost 4
Priority: 32768
000A.41D5.7937
RP
Cost 4
Priority: 32768
0030.F222.2794
S1
S3 S2
Types of Spanning-tree Protocols 623
Selecting the Root Bridge
The bridge ID is used to elect the root bridge in the STP domain and to determine the root
port for each of the remaining devices when there’s more than one potential root port avail-
able because they have equal-cost paths. This key bridge ID is 8 bytes long and includes both
the priority and the MAC address of the device, as illustrated in Figure15.6. Remember—the
default priority on all devices running the IEEE STP version is 32,768.
FIGURE15.6 STP operations
1 Gb/s
Priority: 32768
0000.0cab.3274
Priority: 32768
0000.0cf6.9370
A B
Bridge priority MAC address
0-65535 unique
2 bytes 6 bytes
So, to determine the root bridge, you combine the priority of each bridge with its MAC
address. If two switches or bridges happen to have the same priority value, the MAC address
becomes the tiebreaker for guring out which one has the lowest and, therefore, best ID.
This means that because the two switches in Figure15.6 are both using the default priority
of 32,768, the MAC address will be the determining factor instead. And because Switch A’s
MAC address is 0000.0cab.3274 and Switch B’s MAC address is 0000.0cf6.9370, Switch
A wins and will become the root bridge. A really easy way to gure out the lowest MAC
address is to just start reading from the left toward the right until you nd a lesser value. For
Switch A, I only needed to get to 0000.0ca before stopping. Switch A wins since switch B is
0000.0cf. Never forget that the lower value is always the better one when it comes to elect-
ing a root bridge!
I want to point out that prior to the election of the root bridge, BPDUs are sent every 2sec-
onds out all active ports on a bridge/switch by default, and they’re received and processed by
all bridges. The root bridge is elected based on this information. You can change the bridge’s
ID by lowering its priority so that it will become a root bridge automatically. Being able to do
that is important in a large switched network because it ensures that the best paths will actu-
ally be the ones chosen. Efciency is always awesome in networking!
Types of Spanning-tree Protocols
There are several varieties of spanning-tree protocols in use today:
IEEE 802.1d The original standard for bridging and STP, which is really slow but requires
very little bridge resources. It’s also referred to as Common Spanning Tree (CST).
624 Chapter 15 u Enhanced Switched Technologies
PVST+ The Cisco proprietary enhancement for STP that provides a separate 802.1d span-
ning-tree instance for each VLAN. Know that this is just as slow as the CST protocol, but
with it, we get to have multiple root bridges. This creates more efciency of the links in the
network, but it does use more bridge resources than CST does.
IEEE 802.1w Also called Rapid Spanning Tree Protocol (RSTP), this iteration enhanced
the BPDU exchange and paved the way for much faster network convergence, but it still
only allows for one root bridge per network like CST. The bridge resources used with RSTP
are higher than CST’s but less than PVST+.
Rapid PVST+ Cisco’s version of RSTP that also uses PVST+ and provides a separate
instance of 802.1w per VLAN. It gives us really fast convergence times and optimal trafc
ow but predictably requires the most CPU and memory of all.
Common Spanning Tree
If you’re running Common Spanning Tree (CST) in your switched network with redundant
links, there will be an election to choose what STP considers to be the best root bridge for
your network. That switch will also become the root for all VLANs in your network and
all bridges in your network will create a single path to it. You can manually override this
selection and pick whichever bridge you want if it makes sense for your particular network.
Figure15.7 shows how a typical root bridge would look on your switched network when
running CST.
Notice that switch A is the root bridge for all VLANs even though it’s really not the best
path for some VLANs because all switches must make a single path to it! This is where Per-
VLAN Spanning Tree+ (PVST+) comes into play. Because it allows for a separate instance
of STP for each VLAN, it frees up the individual selection of the most optimal path.
FIGURE15.7 Common STP example
CAD Marketing
Servers for:
• Sales
• Finance
• Marketing
Servers for:
• Management
• Engineering
• CAD
Engineering Finance Management Sales
Root bridge for all VLANs
SW-B SW-CSW-A
Types of Spanning-tree Protocols 625
Per-VLAN Spanning Tree+
PVST+ is a Cisco proprietary extension to 801.2d STP that provides a separate 802.1
spanning-tree instance for each VLAN congured on your switches. All of Cisco pro-
prietary extensions were created to improve convergence times, which is 50 seconds by
default. Cisco IOS switches run 802.1d PVST+ by default, which means you’ll have opti-
mal path selection, but the convergence time will still be slow.
Creating a per-VLAN STP instance for each VLAN is worth the increased CPU and
memory requirements, because it allows for per-VLAN root bridges. This feature allows
the STP tree to be optimized for the trafc of each VLAN by allowing you to congure the
root bridge in the center of each of them. Figure15.8 shows how PVST+ would look in an
optimized switched network with multiple redundant links.
FIGURE15.8 PVST+ provides efficient root bridge selection.
CAD Marketing
Servers for:
• Sales
• Finance
• Marketing
Servers for:
• Management
• Engineering
• CAD
Engineering Finance Management Sales
SW-B SW-CSW-A
Root bridge for
• Sales
• Finance
• Marketing
Root bridge for
• Management
• Engineering
• CAD
This root bridge placement clearly enables faster convergence as well as optimal path
determination. This version’s convergence is really similar to 802.1 CST’s, which has
one instance of STP no matter how many VLANs you have congured on your network.
The difference is that with PVST+, convergence happens on a per-VLAN basis, with each
VLAN running its own instance of STP. Figure15.8 shows us that we now have a nice, ef-
cient root bridge selection for each VLAN.
To allow for the PVST+ to operate, there’s a eld inserted into the BPDU to accommo-
date the extended system ID so that PVST+ can have a root bridge congured on a per-STP
instance, shown in Figure15.9. The bridge ID actually becomes smaller—only 4 bits—
which means that we would congure the bridge priority in blocks of 4,096 rather than in
increments of 1 as we did with CST. The extended system ID (VLAN ID) is a 12-bit eld,
and we can even see what this eld is carrying via show spanning-tree command output,
which I’ll show you soon.
626 Chapter 15 u Enhanced Switched Technologies
FIGURE15.9 PVST+ unique bridge ID
1 Gb/s
Priority: 32768
0000.0cab.3274
Priority: 32768
0000.0cf6.9370
A B
Bridge priority MAC address
0-65535 unique
4 bits 6 bytes
Sys-id-ext
12 bits
But still, isn’t there a way we can do better than a 50-second convergence time? That’s a
really long time in today’s world!
Rapid Spanning Tree Protocol 802.1w
Wouldn’t it be wonderful to have a solid STP conguration running on your switched net-
work, regardless of switch type, and still have all the features we just discussed built in and
enabled on every one of your switches too? Rapid Spanning Tree Protocol (RSTP) serves up
exactly this amazing capacity right to our networking table!
Cisco created proprietary extensions to “x” all the sinkholes and liabilities the IEEE
802.1d standard threw at us, with the main drawback to them being they require extra
conguration because they’re Cisco proprietary. But RSTP, the new 802.1w standard,
brings us most of the patches needed in one concise solution. Again, efciency is golden!
RSTP, or IEEE 802.1w, is essentially an evolution of STP that allows for much faster
convergence. But even though it does address all the convergence issues, it still only per-
mits a single STP instance, so it doesn’t help to take the edge off suboptimal trafc ow
issues. And as I mentioned, to support that faster convergence, the CPU usage and memory
demands are slightly higher than CST’s. The good news is that Cisco IOS can run the
Rapid PVST+ protocol—a Cisco enhancement of RSTP that provides a separate 802.1w
spanning -tree instance for each VLAN congured within the network. But all that power
needs fuel, and although this version addresses both convergence and trafc ow issues,
it also demands the most CPU and memory of all solutions. And it’s also good news that
Cisco’s newest switches don’t have a problem with this protocol running on them.
Keep in mind that Cisco documentation may say STP 802.1d and RSTP
802.1w, but it is referring to the PVST+ enhancement of each version.
Understand that RSTP wasn’t meant to be something completely new and different. The
protocol is more of an evolution than an innovation of the 802.1d standard, which offers
faster convergence whenever a topology change occurs. Backward compatibility was a must
when 802.1w was created.
So, RSTP helps with convergence issues that were the bane of traditional STP. Rapid
PVST+ is based on the 802.1w standard in the same way that PVST+ is based on 802.1d.
Types of Spanning-tree Protocols 627
The operation of Rapid PVST+ is simply a separate instance of 802.1w for each VLAN.
Here’s a list to clarify how this all breaks down:
u RSTP speeds the recalculation of the spanning tree when the layer 2 network topology
changes.
u It’s an IEEE standard that redefines STP port roles, states, and BPDUs.
u RSTP is extremely proactive and very quick, so it doesn’t need the 802.1d delay timers.
u RSTP (802.1w) supersedes 802.1d while remaining backward compatible.
u Much of the 802.1d terminology and most parameters remain unchanged.
u 802.1w is capable of reverting to 802.1d to interoperate with traditional switches on a
per-port basis.
And to clear up confusion, there are also ve terminology adjustments between 802.1d’s
ve port states to 802.1w’s, compared here, respectively:
802.1d State 802.1w State
Disabled =Discarding
Blocking =Discarding
Listening =Discarding
Learning =Learning
Forwarding =Forwarding
Make note of the fact that RSTP basically just goes from discarding to learning to for-
warding, whereas 802.1d requires ve states to transition.
The task of determining the root bridge, root ports, and designated ports hasn’t changed
from 802.1d to RSTP, and understanding the cost of each link is still key to making these
decisions well. Let’s take a look at an example of how to determine ports using the revised
IEEE cost specications in Figure15.10.
FIGURE15.10 RSTP example 1
SB SD
SA SC
0021.1bee.a700 0000.0c39.3127
0021.1c91.0d80 0030.F222.2794
Fa0/1
Fa0/2
Fa0/3 Gi0/1
Fa0/0 Gi0/1
Gi0/10
Gi0/9
628 Chapter 15 u Enhanced Switched Technologies
Can you gure out which is the root bridge? How about which port is the root and
which ones are designated? Well, because SC has the lowest MAC address, it becomes the
root bridge, and since all ports on a root bridge are forwarding designated ports, well,
that’s easy, right? Ports Gi0/1 and Gi0/10 become designated forwarding ports on SC.
But which one would be the root port for SA? To gure that out, we must rst nd the port
cost for the direct link between SA and SC. Even though the root bridge (SC) has a Gigabit
Ethernet port, it’s running at 100 Mbps because SA’s port is a 100-Mbps port, giving it a cost
of 19. If the paths between SA and SC were both Gigabit Ethernet, their costs would only be 4,
but because they’re running 100 Mbps links instead, the cost jumps to a whopping 19!
Can you nd SD’s root port? A quick glance at the link between SC and SD tells us that’s
a Gigabit Ethernet link with a cost of 4, so the root port for SD would be its GI0/9 port.
The cost of the link between SB and SD is also 19 because it’s also a Fast Ethernet link,
bringing the full cost from SB to SD to the root (SC) to a total cost of 19 + 4 = 23. If SB
were to go through SA to get to SC, then the cost would be 19 + 19, or 38, so the root port
of SB becomes the Fa0/3 port.
The root port for SA would be the Fa0/0 port since that’s a direct link with a cost of 19.
Going through SB to SD would be 19 + 19 + 4 = 42, so we’ll use that as a backup link for
SA to get to the root just in case we need to.
Now, all we need is a forwarding port on the link between SA and SB. Because SA has
the lowest bridge ID, Fa0/1 on SA wins that role. Also, the Gi0/1 port on SD would become
a designated forwarding port. This is because the SB Fa0/3 port is a designed root port and
you must have a forwarding port on a network segment! This leaves us with the Fa0/2 port
on SB. Since it isn’t a root port or designated forwarding port, it will be placed into block-
ing mode, which will prevent looks in our network.
Let’s take a look at this example network when it has converged in Figure15.11.
If this isn’t clear and still seems confusing, just remember to always tackle this process
following these three steps:
1. Find your root bridge by looking at bridge IDs.
2. Determine your root ports by finding the lowest path cost to the root bridge.
3. Find your designated ports by looking at bridge IDs.
FIGURE15.11 RSTP example 1 answer
Cost=19
SB SD
Cost=19
SA SC
0021.1bee.a700 0000.0c39.3127
Root
0021.1c91.0d80 0030.F222.2794
Fa0/1
Fa0/2
Fa0/3
Cost=19 Cost=4
Gi0/1
Gi0/1
Gi0/10
Gi0/9
RP
RP
RP
FF
F
F
Types of Spanning-tree Protocols 629
As usual, the best way to nail this down is to practice, so let’s explore another scenario,
shown in Figure15.12.
FIGURE15.12 RSTP example 2
SB SD
SA SC
0021.1bee.a700
Priority: 32768
0000.0c39.3127
Priority: 28672
0005.dccb.d740
Priority: 32768
0012.7f4b.6880
Priority: 32768
1 Gb/s
10 Gb/s
1 Gb/s
1 Gb/s
1 Gb/s
So which bridge is our root bridge? Checking priorities rst tells us that SC is the root
bridge, which means all ports on SC are designated forwarding ports. Now we need to nd
our root ports.
We can quickly see that SA has a 10-gigabit port to SC, so that would be a port cost
of 2, and it would be our root port. SD has a direct Gigabit Ethernet port to SC, so that
would be the root port for SD with a port cost of 4. SB’s best path would also be the
direct Gigabit Ethernet port to SC with a port cost of 4.
Now that we’ve determined our root bridge and found the three root ports we need, we’ve
got to nd our designated ports next. Whatever is left over simply goes into the discarding
role. Let’s take a look at Figure15.13 and see what we have.
FIGURE15.13 RSTP example 2, answer 1
SB SD
SA SC
0021.1bee.a700
32768
0000.0c39.3127
28672
0005.dccb.d740
32768
0012.7f4b.6880
32768
RP
RP
RP
F
FF
1 Gb/s1 Gb/s
10 Gb/s
1 Gb/s
1 Gb/s
Root
630 Chapter 15 u Enhanced Switched Technologies
All right, it looks like there are two links to choose between to nd one designated port
per segment. Let’s start with the link between SA and SD. Which one has the best bridge ID?
They’re both running the same default priority, so by looking at the MAC address, we can
see that SD has the better bridge ID (lower), so the SA port toward SD will go into a discard-
ing role, or will it? The SD port will go into discarding mode, because the link from SA to
the root has the lowest accumulated path costs to the root bridge, and that is used before the
bridge ID in this circumstance. It makes sense to let the bridge with the fastest path to the
root bridge be a designated forwarding port. Let’s talk about this a little more in depth.
As you know, once your root bridge and root ports have been chosen, you’re left with
nding your designated ports. Anything left over goes into discarding role. But how are the
designated ports chosen? Is it just bridge ID? Here are the rules:
1. To choose the switch that will forward on the segment, we select the switch with the low-
est accumulated path cost to the root bridge. We want the fast path to the root bridge.
2. If there is a tie on the accumulated path cost from both switches to the root bridge,
then we’ll use bridge ID, which was what we used in our previous example (but not
with this latest RSTP example; not with a 10-Gigabit Ethernet link to the root bridge
available!).
3. Port priorities can be set manually if we want a specific port chosen. The default prior-
ity is 32, but we can lower that if needed.
4. If there are two links between switches, and the bridge ID and priority are tied, the port
with the lowest number will be chosen—for example, Fa0/1 would be chosen over Fa0/2.
Let’s take a look at our answer now, but before we do, can you nd the forwarding port
between SA and SB? Take a look at Figure15.14 for the answer.
Again, to get the right answer to this question we’re going to let the switch on the network
segment with the lowest accumulated path cost to the root bridge forward on that segment.
This is denitely SA, meaning the SB port goes into discarding role—not so hard at all!
FIGURE15.14 RSTP example 2, answer 2
SB SD
SA SC
0021.1bee.a700
32768
0000.0c39.3127
28672
0005.dccb.d740
32768
0012.7f4b.6880
32768
RP
RP
RP
F
FFFF
1 Gb/s1 Gb/s
10 Gb/s
1 Gb/s
1 Gb/s
Root
Visit ccna
.gg/ch15/a
for a
companion
MicroNugget
from CBT
Nuggets.
Modifying and Verifying the Bridge ID 631
Modifying and Verifying the Bridge ID
To verify spanning tree on a Cisco switch, just use the command show spanning-tree.
From its output, we can determine our root bridge, priorities, root ports, and designated
and blocking/discarding ports.
Let’s use the same simple three-switch network we used earlier as the base to play
around with the conguration of STP. Figure15.15 shows the network we’ll work with in
this section.
FIGURE15.15 Our simple three-switch network
1 Gb/s
1 Gb/s 1 Gb/s
S1
S3 S2
Let’s start by taking a look at the output from S1:
S1#sh spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.42A7.A603
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0001.42A7.A603 him
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg FWD 4 128.25 P2p
Gi1/2 Desg FWD 4 128.26 P2p
First, we can see that we’re running the IEEE 802.1d STP version by default, and
don’t forget that this is really 802.1d PVST+! Looking at the output, we can see that
632 Chapter 15 u Enhanced Switched Technologies
S1 is the root bridge for VLAN 1. When you use this command, the top information is
about the root bridge, and the Bridge ID output refers to the bridge you’re looking at. In
this example, they are one and the same. Notice the sys-id-ext 1 (for VLAN 1). This
is the 12-bit PVST+ eld that is placed into the BPDU so it can carry multiple-VLAN
information. You add the priority and sys-id-ext to come up with the true priority for
the VLAN. We can also see from the output that both Gigabit Ethernet interfaces are
designated forwarding ports. You will not see a blocked/discarding port on a root bridge.
Now let’s take a look at S3’s output:
S3#sh spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.42A7.A603
Cost 4
Port 26(GigabitEthernet1/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000A.41D5.7937
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg FWD 4 128.25 P2p
Gi1/2 Root FWD 4 128.26 P2p
Looking at the Root ID, it’s easy to see that S3 isn’t the root bridge, but the output tells
us it’s a cost of 4 to get to the root bridge and also that it’s located out port 26 of the switch
(Gi1/2). This tells us that the root bridge is one Gigabit Ethernet link away, which we already
know is S1, but we can conrm this with the show cdp neighbors command:
Switch#sh cdp nei
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone
Device ID Local Intrfce Holdtme Capability Platform Port ID
S3 Gig 1/1 135 S 2960 Gig 1/1
S1 Gig 1/2 135 S 2960 Gig 1/1
Modifying and Verifying the Bridge ID 633
That’s how simple it is to nd your root bridge if you don’t have the nice gure as we
do. Use the show spanning-tree command, nd your root port, and then use the show cdp
neighbors command. Let’s see what S2’s output has to tell us now:
S2#sh spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.42A7.A603
Cost 4
Port 26(GigabitEthernet1/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0030.F222.2794
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/1 Altn BLK 4 128.25 P2p
Gi1/2 Root FWD 4 128.26 P2p
We’re certainly not looking at a root bridge since we’re seeing a blocked port, which is
S2’s connection to S3!
Let’s have some fun by making S2 the root bridge for VLAN 2 and for VLAN 3. Here’s
how easy that is to do:
S2#sh spanning-tree vlan 2
VLAN0002
Spanning tree enabled protocol ieee
Root ID Priority 32770
Address 0001.42A7.A603
Cost 4
Port 26(GigabitEthernet1/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32770 (priority 32768 sys-id-ext 2)
Address 0030.F222.2794
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
634 Chapter 15 u Enhanced Switched Technologies
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/1 Altn BLK 4 128.25 P2p
Gi1/2 Root FWD 4 128.26 P2p
We can see that the root bridge cost is 4, meaning that the root bridge is one-gigabit
link away. One more key factor I want to talk about before making S2 the root bridge for
VLANs 2 and 3, is the sys-id-ext, which shows up as 2 in this output because this output
is for VLAN 2. This sys-id-ext is added to the bridge priority, which in this case in 32768
+ 2, which makes the priority 32770. Okay, now that you understand what that output is
telling us, let’s make S2 the root bridge:
S2(config)#spanning-tree vlan 2 ?
priority Set the bridge priority for the spanning tree
root Configure switch as root
<cr>
S2(config)#spanning-tree vlan 2 priority ?
<0-61440> bridge priority in increments of 4096
S2(config)#spanning-tree vlan 2 priority 16384
You can set the priority to any value from 0 through 61440 in increments of 4096. Setting
it to zero (0) means that the switch will always be a root as long as it has a lower MAC
address than another switch that also has its bridge ID set to 0. If you want to set a switch
to be the root bridge for every VLAN in your network, then you have to change the priority
for each VLAN, with 0 being the lowest priority you can use. But trust me—it’s never a good
idea to set all switches to a priority of 0!
Furthermore, you don’t actually need to change priorities because there is yet another
way to congure the root bridge. Take a look:
S2(config)#spanning-tree vlan 3 root ?
primary Configure this switch as primary root for this spanning tree
secondary Configure switch as secondary root
S2(config)#spanning-tree vlan 3 root primary
Notice that you can set a bridge to either primary or secondary—very cool! Let’s check
to see if S2 is actually the root bridge for VLANs 2 and 3 now:
S2#sh spanning-tree vlan 2
VLAN0002
Spanning tree enabled protocol ieee
Root ID Priority 16386
Address 0030.F222.2794
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Modifying and Verifying the Bridge ID 635
Bridge ID Priority 16386 (priority 16384 sys-id-ext 2)
Address 0030.F222.2794
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/1 Desg FWD 4 128.25 P2p
Gi1/2 Desg FWD 4 128.26 P2p
Nice—S2 is the root bridge for VLAN 2, with a priority of 16386 (16384 + 2). Let’s take
a look to see the root bridge for VLAN 3. I’ll use a different command for that this time.
Check it out:
S2#sh spanning-tree summary
Switch is in pvst mode
Root bridge for: VLAN0002 VLAN0003
Extended system ID is enabled
Portfast Default is disabled
PortFast BPDU Guard Default is disabled
Portfast BPDU Filter Default is disabled
Loopguard Default is disabled
EtherChannel misconfig guard is disabled
UplinkFast is disabled
BackboneFast is disabled
Configured Pathcost method used is short
Name Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN0001 1 0 0 1 2
VLAN0002 0 0 0 2 2
VLAN0003 0 0 0 2 2
---------------------- -------- --------- -------- ---------- ----------
3 vlans 1 0 0 5 6
The preceding output tells us that S2 is the root for the two VLANs, but we can see
we have a blocked port for VLAN 1 on S2, so it’s not the root bridge for VLAN 1. This is
because there’s another bridge with a better bridge ID for VLAN 1 than S2’s.
One last burning question: How do you enable RSTP on a Cisco switch? Well, doing
that is actually the easiest part of this chapter! Take a look:
S2(config)#spanning-tree mode rapid-pvst
636 Chapter 15 u Enhanced Switched Technologies
Is that really all there is to it? Yes, because it’s a global command, not per VLAN. Let’s
verify we’re running RSTP now:
S2#sh spanning-tree
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32769
Address 0001.42A7.A603
Cost 4
Port 26(GigabitEthernet1/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
[output cut
S2#sh spanning-tree summary
Switch is in rapid-pvst mode
Root bridge for: VLAN0002 VLAN0003
Looks like we’re set! We’re running RSTP, S1 is our root bridge for VLAN 1, and S2 is the
root bridge for VLANs 2 and 3. I know this doesn’t seem hard, and it really isn’t, but you still
need to practice what we’ve covered so far in this chapter to really get your skills solid!
Spanning-tree Failure Consequences
Clearly, there will be consequences when a routing protocol fails on a single router, but
mainly, you’ll just lose connectivity to the networks directly connected to that router and it
usually does not affect the rest of your network. This denitely makes it easier to trouble-
shoot and x the issue!
There are two failure types with STP. One of them causes the same type of issue I
mentioned with a routing protocol, when certain ports have been placed in a blocking
state they should be forwarding on a network segment instead. This situation makes the
network segment unusable, but the rest of the network will still be working. But what
happens when blocked ports are placed into forwarding state when they should be block-
ing? Let’s work through this second failure issue now, using the same layout we used in
the last section. Let’s start with Figure15.16 and then nd out what happens when STP
fails. Squeamish readers be warned—this isn’t pretty!
Looking at Figure15.16, what do you think will happen if SD transitions its blocked port
to the forwarding state? Clearly, the consequences to the entire network will be pretty dev-
astating! Frames that already had a destination address recorded in the MAC address table
of the switches are forwarded to the port they’re associated with; however, any broadcast,
multicast, and unicasts not in the CAM are now in an endless loop. Figure15.17 shows us
the carnage—when you see all the lights on each port blinking super-fast amber/green, this
means serious errors are occurring, and lots of them!
Spanning-tree Failure Consequences 637
FIGURE15.16 STP stopping loops
SB SD
SA SC
0021.1bee.a700
32768
0000.0c39.3127
28672
0005.dccb.d740
32768
0012.7f4b.6880
32768
RP
RP
RP
F
FFFF
1 Gb/s1 Gb/s
10 Gb/s
1 Gb/s
1 Gb/s
Root
FIGURE15.17 STP failure
SB SD
SA SC
0021.1bee.a700
32768
0000.0c39.3127
28672
0005.dccb.d740
32768
0012.7f4b.6880
32768
RP
RP
RP
F
F
1 Gb/s1 Gb/s
10 Gb/s
1 Gb/s
1 Gb/s
Root
As frames begin building up on the network, the bandwidth starts getting saturated.
The CPU percentage goes way up on the switches until they’ll just give up and stop working
completely, and all this within a few seconds!
Here is a list of the problems that will occur in a failed STP network that you must be
aware of and you must be able to nd in your production network—and of course, you
must know them to meet the exam objectives:
u The load on all links begins increasing and more and more frames enter the loop. Remem-
ber, this loop affects all the other links in the network because these frames are always
flooded out all ports. This scenario is a little less dire if the loop occurs within a single
VLAN. In that case, the snag will be isolated to ports only in that VLAN membership,
plus all trunk links that carry information for that VLAN.
638 Chapter 15 u Enhanced Switched Technologies
u If you have more than one loop, traffic will increase on the switches because all the
circling frames actually get duplicated. Switches basically receive a frame, make a copy
of it, and send it out all ports. And they do this over and over and over again with the
same frame, as well as for any new ones!
u The MAC address table is now completely unstable. It no longer knows where any
source MAC address hosts are actually located because the same source address comes
in via multiple ports on the switch.
u With the overwhelmingly high load on the links and the CPUs, now possibly at 100%
or close to that, the devices become unresponsive, making it impossible to trouble-
shoot—it’s a terrible thing!
At this point your only option is to systematically remove every redundant link between
switches until you can nd the source of the problem. And don’t freak because, eventually,
your ravaged network will calm down and come back to life after STP converges. Your
fried switches will regain consciousness, but the network will need some serious therapy, so
you’re not out of the woods yet!
Now is when you start troubleshooting to nd out what caused the disaster in the rst
place. A good strategy is to place the redundant links back into your network one at a time
and wait to see when a problem begins to occur. You could have a failing switch port, or
even a dead switch. Once you’ve replaced all your redundant links, you need to carefully
monitor the network and have a back-out plan to quickly isolate the problem if it reoccurs.
You don’t want to go through this again!
You’re probably wondering how to prevent these STP problems from ever darkening
your doorstep in the rst place. Well, just hang on, because after the next section, I’ll
tell you all about EtherChannel, which can stop ports from being placed in the blocked/
discarding state on redundant links to save the day! But before we add more links to our
switches and then bundle them, let’s talk about PortFast.
PortFast and BPDU Guard
If you have a server or other devices connected into your switch that you’re totally sure
won’t create a switching loop if STP is disabled, you can use a Cisco proprietary extension
to the 802.1d standard called PortFast on these ports. With this tool, the port won’t spend
the usual 50 seconds to come up into forwarding mode while STP is converging, which is
what makes it so cool.
Since ports will transition from blocking to forwarding state immediately, PortFast can
prevent our hosts from being potentially unable to receive a DHCP address due to STP’s
slow convergence. If the host’s DHCP request times out, or if every time you plug a host
in you’re just tired of looking at the switch port being amber for almost a minute before it
transitions to forwarding state and turns green, PortFast can really help you out!
Figure15.18 illustrates a network with three switches, each with a trunk to each of the
others and a host and server off the S1 switch.
PortFast and BPDU Guard 639
FIGURE15.18 PortFast
Trunk
Trunk Trunk
S1
S3 S2
Gi0/1
Access Access
Gi0/2
We can use PortFast on the ports on S1 to help them transition to the STP forwarding
state immediately upon connecting to the switch.
Here are the commands, rst from global cong mode—they’re pretty simple:
S1(config)#spanning-tree portfast ?
bpdufilter Enable portfast bdpu filter on this switch
bpduguard Enable portfast bpdu guard on this switch
default Enable portfast by default on all access ports
If you were to type spanning-tree portfast default, you would enable all nontrunk-
ing ports with PortFast. From interface mode, you can be more specic, which is the better
way to go:
S1(config-if)#spanning-tree portfast ?
disable Disable portfast for this interface
trunk Enable portfast on the interface even in trunk mode
<cr>
From interface mode you can actually congure PortFast on a trunk port, but you would
do that only if the port connects to a server or router, not to another switch, so we won’t
use that here. So let’s take a look at the message I get when I turn on PortFast on an inter-
face Gi0/1:
S1#config t
S1#config)#int range gi0/1 - 2
S1(config-if)#spanning-tree portfast
%Warning: portfast should only be enabled on ports connected to a single
host. Connecting hubs, concentrators, switches, bridges, etc... to this
interface when portfast is enabled, can cause temporary bridging loops.
Use with CAUTION
%Portfast has been configured on GigabitEthernet0/1 but will only
have effect when the interface is in a non-trunking mode.
Switch(config-if)#
640 Chapter 15 u Enhanced Switched Technologies
PortFast is enabled on port Gi0/1 and Gi0/2, but notice that you get a pretty long mes-
sage that’s essentially telling you to be careful. This is because when using PortFast, you
denitely don’t want to create a network loop by plugging another switch or hub into a
port that’s also congured with PortFast! Why? Because if you let this happen, even though
the network may still sort of work, data will pass super slowly, and worse, it could take you
a really long time to nd the source of the problem, making you very unpopular. So pro-
ceed with caution!
At this juncture, you would be happy to know that there are some safeguard commands
to have handy when using PortFast just in case someone causes a loop in a port that’s con-
gured with PortFast enabled. Let’s talk a really key safeguard command now.
BPDU Guard
If you turn on PortFast for a switch port, it’s a really good idea to turn on BPDU Guard
as well. In fact, it’s such a great idea, I personally feel that it should be enabled by default
whenever a port is congured with PortFast!
This is because if a switch port that has PortFast enabled receives a BPDU on that port,
it will place the port into error disabled (shutdown) state, effectively preventing anyone
from accidentally connecting another switch or hub port into a switch port congured with
PortFast. Basically, you’re preventing (guarding) your network from being severely crippled
or even brought down. So let’s congure our S1 interface, which is already congured with
PortFast, with BPDU Guard now—it’s easy!
Here’s how to set it globally:
S1(config)# spanning-tree portfast bpduguard default
And specically on an interface:
S1(config-if)#spanning-tree bpduguard enable
It’s important to know that you would only congure this command on your access
layer switches—switches where users are directly connected.
Hedging My Bets Created Bad Switch Ports During the Super Bowl
A junior admin called me frantically telling me all switch ports have just gone bad on the
core switch, which was located at the data center where I was lead consultant for a data
center upgrade. Now these things happen, but keep in mind that I just happened to be at
a Super Bowl party having a great time watching my favorite team play in the “Big One”
when I received this call! So I took a deep breath to refocus. I needed to find out some key
information to determine just how bad the situation really was, and my client was in as
big of a hurry as I was to get to a solution!
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PortFast and BPDU Guard 641
First I asked the junior admin exactly what he did. Of course, he said, “Nothing, I swear!”
I figured that’s what he’d say, so I pressed him for more info and finally asked for stats on
the switch. The admin told me that all the ports on the 10/100/1000 line card went amber
at the same time—finally some information I could use! I confirmed that, as suspected,
these ports trunked to uplink distribution switches. Okay, wow—this was not good!
At this point, though, I found it hard to believe that all 24 ports would suddenly go bad,
but it’s possible, so I asked if he had a spare card to try. He told me that he had already
put in the new card but the same thing was still happening. Well, okay—it’s not the card,
or the ports, but maybe something happened with the other switches. I knew there were
a lot of switches involved, so someone must have screwed something up to make this
catastrophe happen! Or, maybe the fiber distribution closet went down somehow? If so,
how? Was there a fire in the closet or something? Some serious internal shenanigans
would be the only answer if that were the cause!
So remaining ever-patient (because, to quote Dr. House, “Patients lie”), I again had to ask
the admin exactly what he did, and sure enough, he finally admitted that he tried to plug his
personal laptop into the core switch so he could watch the Super Bowl, and quickly added,
“…but that’s it, I didn’t do anything else!” I’ll skip over the fact that this guy was about to
have the ugliest Monday ever, but something still didn’t make sense, and here’s why.
Knowing that the ports on that card would all connect to distribution switches, I config-
ured the ports with PortFast so they wouldn’t have to transition through the STP process.
And because I wanted to make sure no one plugged a switch into any of those ports, I
enabled BPDU Guard on the entire line card.
But a host would not bring down those ports, so I asked him if he had plugged in the lap-
top directly or used something in between. He admitted that he had indeed used another
switch because, turns out, there were lots of people from the office who wanted to plug
into the core switch and watch the game too. Was he kidding me? The security policy
wouldn’t allow connecting from their offices, so wouldn’t you think they’d consider the
core even more off-limits? Some people!
But wait… This doesn’t explain all ports turning amber, because only the one he plugged
into should be doing that. It took me a second, but I figured out what he did and finally
got him to confess. When he plugged the switch in, the port turned amber so he thought
it went bad. So what do think he did? Well, if at first you don’t succeed, try, try again, and
that’s just what he did—he actually kept trying ports—all 24 of them to be exact! Now
that’s what I call determined!
Sad to say, I got back to the party in time just to watch my team lose in the last few min-
utes! A dark day, indeed!
642 Chapter 15 u Enhanced Switched Technologies
EtherChannel
Know that almost all Ethernet networks today will typically have multiple links between
switches because this kind of design provides redundancy and resiliency. On a physical
design that includes multiple links between switches, STP will do its job and put a port
or ports into blocking mode. In addition to that, routing protocols like OSPF and EIGRP
could see all these redundant links as individual ones, depending on the conguration,
which can mean an increase in routing overhead.
We can gain the benets from multiple links between switches by using port channel-
ing. EtherChannel is a port channel technology that was originally developed by Cisco as
a switch-to-switch technique for grouping several Fast Ethernet or Gigabit Ethernet ports
into one logical channel.
Also important to note is that once your port channel (EtherXhannel) is up and work-
ing, layer 2 STP and layer 3 routing protocols will treat those bundled links as a single one,
which would stop STP from performing blocking. An additional nice result is that because
the routing protocols now only see this as a single link, a single adjacency across the link
can be formed—elegant!
Figure15.19 shows how a network would look if we had four connections between-
switches, before and after conguring port channels.
FIGURE15.19 Before and after port channels
Okay—now as usual, there’s the Cisco version and the IEEE version of port channel nego-
tiation protocols to choose from—take your pick. Cisco’s version is called Port Aggregation
Protocol (PAgP), and the IEEE 802.3ad standard is called Link Aggregation Control Protocol
(LACP). Both versions work equally well, but the way you congure each is slightly different.
Keep in mind that both PAgP and LACP are negotiation protocols and that EtherChannel can
actually be statically congured without PAgP or LACP. Still, it’s better to use one of these
protocols to help with compatibility issues as well as to manage link additions and failures
between two switches.
Cisco EtherChannel allows us to bundle up to 8 ports active between switches. The links
must have the same speed, duplex setting, and VLAN conguration—in other words you
can’t mix interface types and congurations into the same bundle.
EtherChannel 643
There are a few differences in conguring PAgP and LACP, but rst, let’s go over some
terms so you don’t get confused:
Port channeling Refers to combining two-to-eight Fast Ethernet or two-Gigabit Ethernet
ports together between two switches into one aggregated logical link to achieve more band-
width and resiliency.
EtherChannel Cisco’s proprietary term for port channeling.
PAgP This is a Cisco proprietary port channel negotiation protocol that aids in the auto-
matic creation for EtherChannel links. All links in the bundle must match the same
parameters (speed, duplex, VLAN info), and when PAgP identies matched links, it
groups the links into an EtherChannel. This is then added to STP as a single bridge port.
At this point, PAgP’s job is to send packets every 30 seconds to manage the link for consis-
tency, any link additions, and failures.
LACP (802.3ad) This has the exact same purpose as PAgP, but is nonproprietary so it can
work between multi-vendor networks.
Channel-group This is a command on Ethernet interfaces used to add the specied inter-
face to a single EtherChannel. The number following this command is the port channel ID.
Interface port-channel Here’s a command that creates the bundled interface. Ports can
be added to this interface with the channel-group command. Keep in mind that the inter-
face number must match the group number.
Okay, now let’s see if you can make some sense out of all these terms by actually cong-
uring something!
Configuring and Verifying Port Channels
Let’s use Figure15.20 for our simple example of how to congure port channels.
FIGURE15.20 EtherChannel example
Gi0/1 Gi0/13
Gi0/2 Gi0/14
S1 S2
You can enable your channel-group for each channel by setting the channel mode for
each interface to either active or passive if using LACP. When a port is congured in
passive mode, it will respond to the LACP packets it receives, but it won’t initiate an
LACP negotiation. When a port is congured for active mode, the port initiates negotia-
tions with other ports by sending LACP packets.
Let me show you a simple example of conguring port channels and then verifying
them. First I’ll go to global conguration mode and create a port channel interface, and
then I’ll add this port channel to the physical interfaces.
644 Chapter 15 u Enhanced Switched Technologies
Remember, all parameters and congurations of the ports must be the same, so I’ll start
by trunking the interfaces before I congure EtherChannel, like this:
S1(config)#int range g0/1 - 2
S1(config-if-range)#switchport trunk encapsulation dot1q
S1(config-if-range)#switchport mode trunk
All ports in your bundles must be congured the same, so I’ll congure both sides with
the same trunking conguration. Now I can assign these ports to a bundle:
S1(config-if-range)#channel-group 1 mode ?
active Enable LACP unconditionally
auto Enable PAgP only if a PAgP device is detected
desirable Enable PAgP unconditionally
on Enable Etherchannel only
passive Enable LACP only if a LACP device is detected
S1(config-if-range)#channel-group 1 mode active
S1(config-if-range)#exit
To congure the IEEE nonproprietary LACP, I’ll use the active or passive command; if
I wanted to use Cisco’s PAgP, I’d use the auto or desirable command. You can’t mix and
match these on either end of the bundle, and really, it doesn’t matter which one you use in a
pure Cisco environment, as long as you congure them the same on both ends (setting the
mode to on would be statically conguring your EtherChannel bundle). At this point in the
conguration, I’d have to set the mode to active on the S2 interfaces if I wanted the bundle
to come up with LACP because again, all parameters must be the same on both ends of
the link. Let’s create our port channel interface now with the interface port-channel
command:
S1(config)#int port-channel 1
S1(config-if)#switchport trunk encapsulation dot1q
S1(config-if)#switchport mode trunk
S1(config-if)#switchport trunk allowed vlan 1,2,3
Notice that I set the same trunking method under the port channel interface as I did the
physical interfaces, as well as VLAN information too.
Time to congure the interfaces, channel groups, and port channel interface on the S2
switch:
S2(config)#int range g0/13 - 14
S2(config-if-range)#switchport trunk encapsulation dot1q
S2(config-if-range)#switchport mode trunk
S2(config-if-range)#channel-group 1 mode active
S2(config-if-range)#exit
S2(config)#int port-channel 1
S2(config-if)#switchport trunk encapsulation dot1q
EtherChannel 645
S2(config-if)#switchport mode trunk
S2(config-if)#switchport trunk allowed vlan 1,2,3
On each switch, I congured the ports I wanted to bundle with the same conguration,
then created the port channel. After that, I added the ports into the port channel with the
channel-group command.
Let’s verify our EtherChannel with a few commands. We’ll start with the show
etherchannel port-channel command to see information about a specic port channel
interface:
S2#sh etherchannel port-channel
Channel-group listing:
----------------------
Group: 1
----------
Port-channels in the group:
---------------------------
Port-channel: Po1 (Primary Aggregator)
------------
Age of the Port-channel = 00d:00h:46m:49s
Logical slot/port = 2/1 Number of ports = 2
GC = 0x00000000 HotStandBy port = null
Port state = Port-channel
Protocol = LACP
Port Security = Disabled
Ports in the Port-channel:
Index Load Port EC state No of bits
------+------+------+------------------+-----------
0 00 Gig0/2 Active 0
0 00 Gig0/1 Active 0
Time since last port bundled: 00d:00h:46m:47s Gig0/1
S2#
Notice that we have one group and that we’re running the IEEE LACP version of port
channeling. We’re in Active mode, and that Port-channel: Po1 interface has two physical
646 Chapter 15 u Enhanced Switched Technologies
interfaces. The heading Load is not the load over the interfaces, it’s a hexadecimal value
that decides which interface will be chosen to specify the ow of trafc.
The show etherchannel summary command displays one line of information per port
channel:
S2#sh etherchannel summary
Flags: D - down P - in port-channel
I - stand-alone s - suspended
H - Hot-standby (LACP only)
R - Layer3 S - Layer2
U - in use f - failed to allocate aggregator
u - unsuitable for bundling
w - waiting to be aggregated
d - default port
Number of channel-groups in use: 1
Number of aggregators: 1
Group Port-channel Protocol Ports
------+-------------+-----------+----------------------------------------------
1 Po1(SU) LACP Gig0/1(P) Gig0/2(P)
This command shows that we have one group, that we’re running LACP, and Gig0/1
and Gig0/2 or (P), which means these ports are in port-channel mode. This command
isn’t really all that helpful unless you have multiple channel groups, but it does tell us our
group is working well!
Summary
This chapter was all about switching technologies, with a particular focus on the Spanning
Tree Protocol (STP) and its evolution to newer versions like RSTP and then Cisco’s PVST+.
You learned about the problems that can occur if you have multiple links between
bridges (switches) and the solutions attained with STP.
I also talked about and demonstrated issues that can occur if you have multiple links
between bridges (switches), plus how to solve these problems by using the Spanning Tree
Protocol (STP).
Exam Essentials 647
I covered a detailed conguration of Cisco’s Catalyst switches, including verifying the
conguration, setting the Cisco STP extensions, and changing the root bridge by setting a
bridge priority.
Finally, we discussed, congured, and veried the EtherChannel technology that helps
us bundle multiple links between switches.
Exam Essentials
Understand the main purpose of the Spanning Tree Protocol in a switched LAN. The main
purpose of STP is to prevent switching loops in a network with redundant switched paths.
Remember the states of STP. The purpose of the blocking state is to prevent the use of
looped paths. A port in listening state prepares to forward data frames without populat-
ing the MAC address table. A port in learning state populates the MAC address table but
doesn’t forward data frames. A port in forwarding state sends and receives all data frames
on the bridged port. Also, a port in the disabled state is virtually nonoperational.
Remember the command show spanning-tree. You must be familiar with the command
show spanning-tree and how to determine the root bridge of each VLAN. Also, you can
use the show spanning-tree summary command to help you get a quick glimpse of your
STP network and root bridges.
Understand what PortFast and BPDU Guard provides. PortFast allows a port to transi-
tion to the forwarding state immediately upon a connection. Because you don’t want other
switches connecting to this port, BPDU Guard will shut down a PortFast port if is receives
a BPDU.
Understand what EtherChannel is and how to configure it. EtherChannel allows you to
bundle links to get more bandwidth, instead of allowing STP to shut down redundant ports.
You can congure Cisco’s PAgP or the IEEE version, LACP, by creating a port channel inter-
face and assigning the port channel group number to the interfaces you are bundling.
648 Chapter 15 u Enhanced Switched Technologies
Written Lab 1
The answers to this lab can be found in Appendix A, “Answers to Written Labs.”
Write the answers to the following questions:
1. Which of the following is Cisco proprietary: LACP or PAgP?
2. What command will show you the STP root bridge for a VLAN?
3. What standard is RSTP PVST+ based on?
4. Which protocol is used in a layer 2 network to maintain a loop-free network?
5. Which proprietary Cisco STP extension would put a switch port into error-mode disabled
mode if a BPDU is received on this port?
6. You want to configure a switch port to not transition through the STP port states but to
go immediately to forwarding mode. What command will you use on a per-port basis?
7. What command will you use to see information about a specific port channel interface?
8. What command can you use to set a switch so that it will be the root bridge for VLAN 3
over any other switch?
9. You need to find the VLANs for which your switch is the root bridge. What two
commands can you use?
10. What are the two modes you can set with LACP?
Hands-on Labs
In this section, you will use congure and verify STP, as well as congure PortFast and
BPDU Guard, and nally bundle links together with EtherChannel.
Note that the labs in this chapter were written to be used with real equipment using
2960 switches, however, you can use the free LammleSim IOS version simulator or Cisco’s
Packet Tracer to run through these labs.
The labs in this chapter are as follows:
Lab 1.1: Verifying STP and Finding Your Root Bridge
Lab 1.2: Conguring and Verifying Your Root Bridge
Lab 1.3: Conguring PortFast and BPDU Guard
Lab 1.4: Conguring and Verifying EtherChannel
Hands-on Labs 649
We’ll use the following illustration for all four labs:
1 Gb/s
S1
S3 S2
100 Mbps 100 Mbps
Fa0/2 Fa0/2
Hands-on Lab 1.1: Verifying STP and Finding Your
RootBridge
This lab will assume that you have added VLANs 2 and 3 to each of your switches and all
of your links are trunked.
1. From one of your switches, use the show spanning-tree vlan 2 command. Verify
the output.
S3#sh spanning-tree vlan 2
VLAN0002
Spanning tree enabled protocol ieee
Root ID Priority 32770
Address 0001.C9A5.8748
Cost 19
Port 1(FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32770 (priority 32768 sys-id-ext 2)
Address 0004.9A04.ED97
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root FWD 19 128.1 P2p
Fa0/2 Desg FWD 19 128.2 P2p
650 Chapter 15 u Enhanced Switched Technologies
Gi1/1 Altn BLK 4 128.25 P2p
Gi1/2 Altn BLK 4 128.26 P2p
Notice that S3 is not the root bridge, so to nd your root bridge, just follow the root
port and see what bridge is connected to that port. Port Fa0/1 is the root port with a
cost of 19, which means the switch that is off the Fa0/1 port is the root port connecting
to the root bridge because it is a cost of 19, meaning one Fast Ethernet link away.
2. Find the bridge that is off of Fa0/1, which will be our root.
S3#sh cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone
Device ID Local Intrfce Holdtme Capability Platform Port ID
S1 Fas 0/1 158 S 2960 Fas 0/1
S2 Gig 1/1 151 S 2960 Gig 1/1
S2 Gig 1/2 151 S 2960 Gig 1/2
S3#
Notice that S1 is connected to the local interface Fa0/1, so let’s go to S1 and verify our
root bridge.
3. Verify the root bridge for each of the three VLANs. From S1 use the show spanning-
tree summary command.
S1#sh spanning-tree summary
Switch is in pvst mode
Root bridge for: default VLAN0002 VLAN0003
Extended system ID is enabled
Portfast Default is disabled
PortFast BPDU Guard Default is disabled
Portfast BPDU Filter Default is disabled
Loopguard Default is disabled
EtherChannel misconfig guard is disabled
UplinkFast is disabled
BackboneFast is disabled
Configured Pathcost method used is short
Name Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN0001 0 0 0 2 2
VLAN0002 0 0 0 2 2
VLAN0003 0 0 0 2 2
Hands-on Labs 651
---------------------- -------- --------- -------- ---------- ----------
3 vlans 0 0 0 6 6
S1#
Notice that S1 is the root bridge for all three VLANs.
4. Make note of all your root bridges, for all three VLANs, if you have more than one
root bridge.
Hands-on Lab 1.2: Configuring and Verifying Your
Root Bridge
This lab will assume you have performed Lab 1 and know who your root bridge is for each
VLAN.
1. Go to one of your non-root bridges and verify the bridge ID with the show spanning-
tree vlan command.
S3#sh spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.C9A5.8748
Cost 19
Port 1(FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0004.9A04.ED97
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Root FWD 19 128.1 P2p
Fa0/2 Desg FWD 19 128.2 P2p
Gi1/1 Altn BLK 4 128.25 P2p
Gi1/2 Altn BLK 4 128.26 P2p
Notice that this bridge is not the root bridge for VLAN 1, and the root port is Fa0/1
with a cost of 19, which means the root bridge is directly connected one Fast Ethernet
link away.
652 Chapter 15 u Enhanced Switched Technologies
2. Make one of your non-root bridges the root bridge for VLAN 1. Use priority 16,384,
which is lower than the 32,768 of the current root.
S3(config)#spanning-tree vlan 1 priority ?
<0-61440> bridge priority in increments of 4096
S3(config)#spanning-tree vlan 1 priority 16384
3. Verify the root bridge for VLAN 1.
S3#sh spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 16385
Address 0004.9A04.ED97
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 16385 (priority 16384 sys-id-ext 1)
Address 0004.9A04.ED97
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/2 Desg FWD 19 128.2 P2p
Gi1/1 Desg FWD 4 128.25 P2p
Gi1/2 Desg FWD 4 128.26 P2p
Notice that this bridge is indeed the root and all ports are in Desg FWD mode.
Hands-on Lab 1.3: Configuring PortFast and BPDU Guard
This lab will have you congure ports on switches S3 and S2 to allow the PC and server to
automatically go into forward mode when they connect into the port.
1. Connect to your switch that has a host connected and enable PortFast for the interface.
S3#config t
S3(config)#int fa0/2
S3(config-if)#spanning-tree portfast
%Warning: portfast should only be enabled on ports connected to a single
host. Connecting hubs, concentrators, switches, bridges, etc... to this
Hands-on Labs 653
interface when portfast is enabled, can cause temporary bridging loops.
Use with CAUTION
%Portfast has been configured on FastEthernet0/2 but will only
have effect when the interface is in a non-trunking mode.
2. Verify that that port will be shut down if another switch plugs into this port.
S3(config-if)#spanning-tree bpduguard enable
3. Verify your configuration with the show running-config command.
!
interface FastEthernet0/2
switchport mode trunk
spanning-tree portfast
spanning-tree bpduguard enable
!
Hands-on Lab 1.4: Configuring and Verifying
EtherChannel
This lab will have you congure the Cisco EtherChannel PAgP version on the switches used
in this lab. Because I have precongured the switches, I have set up the trunks on all inter-
switch ports. We’ll use the Gigabit Ethernet ports between switches S3 and S2.
1. Configure the S3 switch with EtherChannel by creating a port channel interface.
S3#config t
S3(config)#inter port-channel 1
2. Configure the ports to be in the bundle with the channel-group command.
S3(config-if)#int range g1/1 - 2
S3(config-if-range)#channel-group 1 mode ?
active Enable LACP unconditionally
auto Enable PAgP only if a PAgP device is detected
desirable Enable PAgP unconditionally
on Enable Etherchannel only
passive Enable LACP only if a LACP device is detected
S3(config-if-range)#channel-group 1 mode desirable
I chose the PAgP desirable mode for the S3 switch.
654 Chapter 15 u Enhanced Switched Technologies
3. Configure the S2 switch with EtherChannel, using the same parameters as S3.
S2#config t
S2(config)#interface port-channel 1
S2(config-if)#int rang g1/1 - 2
S2(config-if-range)#channel-group 1 mode desirable
%LINK-5-CHANGED: Interface Port-channel 1, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface Port-channel 1, changed state
to up
Pretty simple, really. Just a couple of commands.
4. Verify with the show etherchannel port-channel command.
S3#sh etherchannel port-channel
Channel-group listing:
----------------------
Group: 1
----------
Port-channels in the group:
---------------------------
Port-channel: Po1
------------
Age of the Port-channel = 00d:00h:06m:43s
Logical slot/port = 2/1 Number of ports = 2
GC = 0x00000000 HotStandBy port = null
Port state = Port-channel
Protocol = PAGP
Port Security = Disabled
Ports in the Port-channel:
Index Load Port EC state No of bits
------+------+------+------------------+-----------
0 00 Gig1/1 Desirable-Sl 0
Hands-on Labs 655
0 00 Gig1/2 Desirable-Sl 0
Time since last port bundled: 00d:00h:01m:30s Gig1/2
5. Verify with the show etherchannel summary command.
S3#sh etherchannel summary
Flags: D - down P - in port-channel
I - stand-alone s - suspended
H - Hot-standby (LACP only)
R - Layer3 S - Layer2
U - in use f - failed to allocate aggregator
u - unsuitable for bundling
w - waiting to be aggregated
d - default port
Number of channel-groups in use: 1
Number of aggregators: 1
Group Port-channel Protocol Ports
------+-------------+-----------+----------------------------------
1 Po1(SU) PAgP Gig1/1(P) Gig1/2(P)
S3#
656 Chapter 15 u Enhanced Switched Technologies
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional ques-
tions, please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Review
Questions.”
1. You receive the following output from a switch:
S2#sh spanning-tree
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32769
Address 0001.42A7.A603
Cost 4
Port 26(GigabitEthernet1/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
[output cut]
Which are true regarding this switch? (Choose two.)
A. The switch is a root bridge.
B. The switch is a non-root bridge.
C. The root bridge is four switches away.
D. The switch is running 802.1w.
E. The switch is running STP PVST+.
2. If you want to effectively disable STP on a port connected to a server, which command
would you use?
A. disable spanning-tree
B. spanning-tree off
C. spanning-tree security
D. spanning-tree portfast
Review Questions 657
3. Which of the following would you use to find the VLANs for which your switch is the
root bridge? (Choose two.)
A. show spanning-tree
B. show root all
C. show spanning-tree port root VLAN
D. show spanning-tree summary
4. You want to run the new 802.1w on your switches. Which of the following would
enable this protocol?
A. Switch(config)#spanning-tree mode rapid-pvst
B. Switch#spanning-tree mode rapid-pvst
C. Switch(config)#spanning-tree mode 802.1w
D. Switch#spanning-tree mode 802.1w
5. Which of the following is a layer 2 protocol used to maintain a loop-free network?
A. VTP
B. STP
C. RIP
D. CDP
6. Which statement describes a spanning-tree network that has converged?
A. All switch and bridge ports are in the forwarding state.
B. All switch and bridge ports are assigned as either root or designated ports.
C. All switch and bridge ports are in either the forwarding or blocking state.
D. All switch and bridge ports are either blocking or looping.
7. Which of the following modes enable LACP EtherChannel? (Choose two.)
A. On
B. Prevent
C. Passive
D. Auto
E. Active
F. Desirable
658 Chapter 15 u Enhanced Switched Technologies
8. Which of the following are true regarding RSTP? (Choose three.)
A. RSTP speeds the recalculation of the spanning tree when the layer 2 network
topology changes.
B. RSTP is an IEEE standard that redefines STP port roles, states, and BPDUs.
C. RSTP is extremely proactive and very quick, and therefore it absolutely needs the
802.1 delay timers.
D. RSTP (802.1w) supersedes 802.1d while remaining proprietary.
E. All of the 802.1d terminology and most parameters have been changed.
F. 802.1w is capable of reverting to 802.1d to interoperate with traditional switches
on a per-port basis.
9. What does BPDU Guard perform?
A. Makes sure the port is receiving BPDUs from the correct upstream switch.
B. Makes sure the port is not receiving BPDUs from the upstream switch, only the root.
C. If a BPDU is received on a BPDU Guard port, PortFast is used to shut down the port.
D. Shuts down a port if a BPDU is seen on that port.
10. How many bits is the sys-id-ext field in a BPDU?
A. 4
B. 8
C. 12
D. 16
11. There are four connections between two switches running RSTP PVST+ and you want
to figure out how to achieve higher bandwidth without sacrificing the resiliency that
RSTP provides. What can you configure between these two switches to achieve higher
bandwidth than the default configuration is already providing?
A. Set PortFast and BPDU Guard, which provides faster convergence.
B. Configure unequal cost load balancing with RSTP PVST+.
C. Place all four links into the same EtherChannel bundle.
D. Configure PPP and use multilink.
12. In which circumstance are multiple copies of the same unicast frame likely to be trans-
mitted in a switched LAN?
A. During high-traffic periods
B. After broken links are reestablished
C. When upper-layer protocols require high reliability
D. In an improperly implemented redundant topology
Review Questions 659
13. You want to configure LACP. Which do you need to make sure are configured exactly
the same on all switch interfaces you are using? (Choose three.)
A. Virtual MAC address
B. Port speeds
C. Duplex
D. PortFast enabled
E. VLAN information
14. Which of the following modes enable PAgP EtherChannel? (Choose two.)
A. On
B. Prevent
C. Passive
D. Auto
E. Active
F. Desirable
15. For this question, refer to the following illustration. SB’s RP to the root bridge has
failed.
SB SD
SA SC
0021.1bee.a700
Priority: 32768
0000.0c39.3127
Priority: 4096
0005.dccb.d740
Priority: 16384
0012.7f4b.6880
Priority: 32768
1 Gb/s1 Gb/s
100 Mbps
1 Gb/s
1 Gb/s
What is the new cost for SB to make a single path to the root bridge?
A. 4
B. 8
C. 23
D. 12
660 Chapter 15 u Enhanced Switched Technologies
16. Which of the following would put switch interfaces into EtherChannel port number 1,
using LACP? (Choose two.)
A. Switch(config)#interface port-channel 1
B. Switch(config)#channel-group 1 mode active
C. Switch#interface port-channel 1
D. Switch(config-if)#channel-group 1 mode active
17. Which two commands would guarantee your switch to be the root bridge for VLAN 30?
(Choose two.)
A. spanning-tree vlan 30 priority 0
B. spanning-tree vlan 30 priority 16384
C. spanning-tree vlan 30 root guarantee
D. spanning-tree vlan 30 root primary
18. Why does Cisco use its proprietary extension of PVST+ with STP and RSTP?
A. Root bridge placement enables faster convergence as well as optimal path
determination.
B. Non-root bridge placement clearly enables faster convergence as well as optimal
path determination.
C. PVST+ allows for faster discarding of non-IP frames.
D. PVST+ is actually an IEEE standard called 802.1w.
19. Which are states in 802.1d? (Choose all that apply.)
A. Blocking
B. Discarding
C. Listening
D. Learning
E. Forwarding
F. Alternate
20. Which of the following are roles in STP? (Choose all that apply.)
A. Blocking
B. Discarding
C. Root
D. Non-designated
E. Forwarding
F. Designated
Chapter
16
Managing Cisco
Devices
THE FOLLOWING ICND2 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 IP Routing Technologies
■Describe the boot process of Cisco IOS routers
■POST
■Router bootup process
■Manage Cisco IOS Files
■Boot preferences
■Cisco IOS image(s)
■Licensing
■Show license
■Change license
Here in Chapter 2, I’m going to show you how to manage Cisco
routers on an internetwork. The Internetwork Operating System
(IOS) and conguration les reside in different locations in a
Cisco device, so it’s really important to understand both where these les are located and how
they work.
You’ll be learning about the main components of a router, the router boot sequence, and the
conguration register, including how to use the conguration register for password recovery.
Finally, I’ll cover how to verify licenses on the ISRG2 routers as well as how to install a
permanent license and congure evaluation features in the latest universal images.
To find up-to-the-minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
The Internal Components of a
Cisco Router
To congure and troubleshoot a Cisco internetwork, you need to know the major compo-
nents of Cisco routers and understand what each one does. Table16.1 describes the major
Cisco router components.
TABLE16.1 Cisco router components
Component Description
Bootstrap Stored in the microcode of the ROM, the bootstrap is
used to bring a router up during initialization. It will boot
the router and then load the IOS.
POST (power-on self-test) Stored in the microcode of the ROM, the POST is used to
check the basic functionality of the router hardware and
determines which interfaces are present.
The Router Boot Sequence 663
Component Description
ROM monitor Stored in the microcode of the ROM, the ROM monitor is
used for manufacturing, testing, and troubleshooting. In
older routers it could load what was called a mini-IOS.
Mini-IOS Called the RXBOOT or bootloader by Cisco, the mini-
IOS is a small IOS in ROM that can be used to bring up
an interface and load a Cisco IOS into flash memory.
The mini-IOS can also perform a few other maintenance
operations, but not many.
RAM (random access memory) Used to hold packet buffers, ARP caches, routing
tables, and also the software and data structures that
allow the router to function. Running-config is stored
in RAM, and most routers expand the IOS from flash
into RAM upon boot.
ROM (read-only memory) Used to start and maintain the router. Holds the POST
and the bootstrap program as well as the mini-IOS.
Flash memory Stores the Cisco IOS by default. Flash memory is not
erased when the router is reloaded. It is EEPROM (elec-
tronically erasable programmable read-only memory)
created by Intel.
NVRAM (nonvolatile RAM) Used to hold the router and switch configuration.
NVRAM is not erased when the router or switch is
reloaded. Does not store an IOS. The configuration
register is stored in NVRAM.
Configuration register Used to control how the router boots up. This value can
be found as the last line of the show version command
output and by default is set to 0x2102, which tells the
router to load the IOS from flash memory as well as to
load the configuration from NVRAM.
The Router Boot Sequence
When a router boots up, it performs a series of steps, called the boot sequence, to test the
hardware and load the necessary software. The boot sequence consists of the following
steps, as shown in Figure16.1:
1. The router performs a POST. The POST tests the hardware to verify that all components
of the device are operational and present. For example, the POST checks for the different
interfaces on the router. The POST is stored in and run from ROM (read-only memory).
664 Chapter 16 u Managing Cisco Devices
FIGURE16.1 Router bootup process
• Major phases to the
router bootup process
• Test router hardware
• Power-on self-test (POST)
• Execute bootstrap loader
• Locate and load Cisco IOS
software
• Locate IOS
• Load IOS
• Locate and load startup
configuration file or enter
setup mode
• Bootstrap program looks
for configuration file
POSTROM1. Perform POST
BootstrapROM2. Load Bootstrap
Cisco
Internetwork
Operation
System
Flash
TFTP Server
3.
4.
Locate and load
Operating System
Configuration
NVRAM
TFTP
Server
5.
6.
Locate and load
configuration file
or
enter setup mode
Console7.
2. The bootstrap then looks for and loads the Cisco IOS software. The bootstrap is a pro-
gram in ROM that is used to execute programs. The bootstrap program is responsible
for finding where each IOS program is located and then loading the file. By default, the
IOS software is loaded from flash memory in all Cisco routers, but only after the con-
figuration register is read and NVRAM is checked for the boot sequence.
The default order of an IOS loading from a router is: flash, TFTP server,
then ROM.
3. After the IOS is expanded into RAM, the IOS software looks for a valid configuration
file stored in NVRAM. This file is called startup-config and is only there if an admin-
istrator copies the running-config file into NVRAM.
4. If a startup-config file is in NVRAM, the router will copy this file and place it in
RAM and call the file running-config. The router will use this file to run the router.
The router should now be operational. If a startup-config file is not in NVRAM, the
router will broadcast out any interface that detects carrier detect (CD) for a TFTP
host looking for a configuration, and when that fails (typically it will fail—most
people won’t even realize the router has attempted this process), it will start the setup
mode configuration process.
Managing Configuration Register
All Cisco routers have a 16-bit software register that’s written into NVRAM. By default,
the conguration register is set to load the Cisco IOS from ash memory and to look for
Managing Configuration Register 665
and load the startup-cong le from NVRAM. In the following sections, I am going to dis-
cuss the conguration register settings and how to use these settings to provide password
recovery on your routers.
Understanding the Configuration Register Bits
The 16 bits (2 bytes) of the conguration register are read from 15 to 0, from left to right. The
default conguration setting on Cisco routers is 0x2102. This means that bits 13, 8, and 1
are on, as shown in Table16.2. Notice that each set of 4 bits (called a nibble) is read in binary
with a value of 8, 4, 2, 1.
TABLE16.2 The configuration register bit numbers
Configuration
Register 2 1 0 2
Bit number 15 14 13 12 11 10 9876543210
Binary 0010000100000010
Add the prefix 0x to the configuration register address. The 0x means that
the digits that follow are in hexadecimal.
Table16.3 lists the software conguration bit meanings. Notice that bit 6 can be used
to ignore the NVRAM contents. This bit is used for password recovery—something I’ll go
over with you soon in the section “Recovering Passwords,” later in this chapter.
Remember that in hex, the scheme is 0–9 and A–F (A = 10, B = 11, C = 12,
D = 13, E = 14, and F = 15). This means that a 210F setting for the configu-
ration register is actually 210(15), or 1111 in binary.
TABLE16.3 Software configuration meanings
Bit Hex Description
0–3 0x0000–0x000F Boot field (see Table16.4).
60x0040 Ignore NVRAM contents.
70x0080 OEM bit enabled.
666 Chapter 16 u Managing Cisco Devices
Bit Hex Description
80x101 Break disabled.
10 0x0400 IP broadcast with all zeros.
5, 11–12 0x0800–0x1000 Console line speed.
13 0x2000 Boot default ROM software if network boot fails.
14 0x4000 IP broadcasts do not have net numbers.
15 0x8000 Enable diagnostic messages and ignore NVRAM contents.
The boot eld, which consists of bits 0–3 in the conguration register (the last four
bits), controls the router boot sequence and locates the Cisco IOS. Table16.4 describes
the boot eld bits.
TABLE16.4 The boot field (configuration register bits 00–03)
Boot Field Meaning Use
00 ROM monitor mode To boot to ROM monitor mode, set the configura-
tion register to 2100. You must manually boot the
router with the b command. The router will show
the rommon> prompt.
01 Boot image from ROM To boot the mini-IOS image stored in ROM, set the
configuration register to 2101. The router will show
the Router(boot)> prompt. The mini-IOS is not
available in all routers and is also referred to
as RXBOOT.
02–F Specifies a default boot
filename
Any value from 2102 through 210F tells the router
to use the boot commands specified in NVRAM.
Checking the Current Configuration Register Value
You can see the current value of the conguration register by using the show version
command (sh version or show ver for short), as demonstrated here:
Router>sh version
Cisco IOS Software, 2800 Software (C2800NM-ADVSECURITYK9-M),
TABLE16.3 Software configuration meanings
(continued)
Managing Configuration Register 667
Version 15.1(4)M6, RELEASE SOFTWARE (fc2)
[output cut]
Configuration register is 0x2102
The last information given from this command is the value of the conguration register.
In this example, the value is 0x2102—the default setting. The conguration register setting
of 0x2102 tells the router to look in NVRAM for the boot sequence.
Notice that the show version command also provides the IOS version, and in the preceding
example, it shows the IOS version as 15.1(4)M6.
The show version command will display system hardware configuration
information, the software version, and the names of the boot images on
a router.
To change the conguration register, use the config-register command from global
conguration mode:
Router(config)#config-register 0x2142
Router(config)#do sh ver
[output cut]
Configuration register is 0x2102 (will be 0x2142 at next reload)
It’s important that you are careful when you set the conguration register!
If you save your configuration and reload the router and it comes up in
setup mode, the configuration register setting is probably incorrect.
Boot System Commands
Did you know that you can congure your router to boot another IOS if the ash is corrupted?
Well, you can. You can boot all of your routers from a TFTP server, but it’s old school, and
people just don’t do it anymore; it’s just for backup in case of failure.
There are some boot commands you can play with that will help you manage the way
your router boots the Cisco IOS—but please remember, we’re talking about the router’s
IOS here, not the router’s conguration!
Router>en
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#boot ?
bootstrap Bootstrap image file
668 Chapter 16 u Managing Cisco Devices
config Configuration file
host Router-specific config file
network Network-wide config file
system System image file
The boot command truly gives you a wealth of options, but rst, I’ll show you the typical
settings that Cisco recommends. So let’s get started—the boot system command will allow you
to tell the router which system IOS le to boot from ash memory. Remember that the router,
by default, boots the rst system IOS le found in ash. You can change that with the following
commands as shown in the output:
Router(config)#boot system ?
WORD TFTP filename or URL
flash Boot from flash memory
ftp Boot from a server via ftp
mop Boot from a Decnet MOP server
rcp Boot from a server via rcp
rom Boot from rom
tftp Boot from a tftp server
Router(config)#boot system flash c2800nm-advsecurityk9-mz.151-4.M6.bin
Notice I could boot from FLASH, FTP, ROM, TFTP or another useless option. The
preceding command I used congures the router to boot the IOS listed in it. This is a help-
ful command for when you load a new IOS into ash and want to test it, or even when you
want to totally change which IOS is loading by default.
The next command is considered a fallback routine, but as I said, you can make it a per-
manent way to have your routers boot from a TFTP host. Personally, I wouldn’t necessarily
recommend doing this (single point of failure); I’m just showing you that it’s possible:
Router(config)#boot system tftp ?
WORD System image filename
Router(config)#boot system tftp c2800nm-advsecurityk9-mz.151-4.M6.bin?
Hostname or A.B.C.D Address from which to download the file
<cr>
Router(config)#boot system tftp c2800nm-advsecurityk9-mz.151-4.M6.bin 1.1.1.2
Router(config)#
As your last recommended fallback option—the one to go to if the IOS in ash doesn’t
load and the TFTP host does not produce the IOS—load the mini-IOS from ROM like this:
Router(config)#boot system rom
Router(config)#do show run | include boot system
boot system flash c2800nm-advsecurityk9-mz.151-4.M6.bin
Managing Configuration Register 669
boot system tftp c2800nm-advsecurityk9-mz.151-4.M6.bin 1.1.1.2
boot system rom
Router(config)#
If the preceding conguration is set, the router will try to boot from the TFTP server if
ash fails, and if the TFTP boot fails, the mini-IOS will load after six unsuccessful attempts
of trying to locate the TFTP server.
In the next section, I’ll show you how to load the router into ROM monitor mode so you
can perform password recovery.
Recovering Passwords
If you’re locked out of a router because you forgot the password, you can change the cong-
uration register to help you get back on your feet. As I said earlier, bit 6 in the conguration
register is used to tell the router whether to use the contents of NVRAM to load a router
conguration.
The default conguration register value is 0x2102, meaning that bit 6 is off. With
the default setting, the router will look for and load a router conguration stored in
NVRAM (startup-cong). To recover a password, you need to turn on bit 6. Doing this
will tell the router to ignore the NVRAM contents. The conguration register value to
turn on bit 6 is 0x2142.
Here are the main steps to password recovery:
1. Boot the router and interrupt the boot sequence by performing a break, which will
take the router into ROM monitor mode.
2. Change the configuration register to turn on bit 6 (with the value 0x2142).
3. Reload the router.
4. Say “no” to entering Setup mode, the enter privileged mode.
5. Copy the startup-config file to running-config, and don’t forget to verify that your
interfaces are re-enabled.
6. Change the password.
7. Reset the configuration register to the default value.
8. Save the router configuration.
9. Reload the router (optional).
I’m going to cover these steps in more detail in the following sections. I’ll also show you
the commands to restore access to ISR series routers.
You can enter ROM monitor mode by pressing Ctrl+Break or Ctrl+Shift+6 then b, during
router bootup. But if the IOS is corrupt or missing, if there’s no network connectivity available
to nd a TFTP host, or if the mini-IOS from ROM doesn’t load (meaning the default router
fallback failed), the router will enter ROM monitor mode by default.
670 Chapter 16 u Managing Cisco Devices
Interrupting the Router Boot Sequence
Your rst step is to boot the router and perform a break. This is usually done by pressing
the Ctrl+Break key combination when using HyperTerminal (personally, I use SecureCRT
or PuTTY) while the router rst reboots.
System Bootstrap, Version 15.1(4)M6, RELEASE SOFTWARE (fc2)
Copyright (c) 1999 by cisco Systems, Inc.
TAC:Home:SW:IOS:Specials for info
PC = 0xfff0a530, Vector = 0x500, SP = 0x680127b0
C2800 platform with 32768 Kbytes of main memory
PC = 0xfff0a530, Vector = 0x500, SP = 0x80004374
monitor: command “boot” aborted due to user interrupt
rommon 1 >
Notice the line monitor: command “boot” aborted due to user interrupt. At this
point, you will be at the rommon 1> prompt, which is called the ROM monitor mode.
Changing the Configuration Register
As I explained earlier, you can change the conguration register from within the IOS by
using the config-register command. To turn on bit 6, use the conguration register
value 0x2142.
Remember that if you change the configuration register to 0x2142, the
startup-config will be bypassed and the router will load into setup mode.
To change the bit value on a Cisco ISR series router, you just enter the following command
at the rommon 1> prompt:
rommon 1 >confreg 0x2142
You must reset or power cycle for new config to take effect
rommon 2 >reset
Reloading the Router and Entering Privileged Mode
At this point, you need to reset the router like this:
u From the ISR series router, type I (for initialize) or reset.
u From an older series router, type I.
The router will reload and ask if you want to use setup mode (because no startup-cong
is used). Answer no to entering setup mode, press Enter to go into user mode, and then type
enable to go into privileged mode.
Backing Up and Restoring the Cisco IOS 671
Viewing and Changing the Configuration
Now you’re past the point where you would need to enter the user-mode and privileged-mode
passwords in a router. Copy the startup-cong le to the running-cong le:
copy startup-config running-config
Or use the shortcut:
copy start run
The conguration is now running in random access memory (RAM), and you’re in
privileged mode, meaning that you can now view and change the conguration. But you
can’t view the enable-secret setting for the password since it is encrypted. To change the
password, do this:
config t
enable secret todd
Resetting the Configuration Register and Reloading the Router
After you’re nished changing passwords, set the conguration register back to the default
value with the config-register command:
config t
config-register 0x2102
It’s important to remember to enable your interfaces after copying the conguration
from NVRAM to RAM.
Finally, save the new conguration with a copy running-config startup-config and
use reload to reload the router.
If you save your configuration and reload the router and it comes up in
setup mode, the configuration register setting is probably incorrect.
To sum this up, we now have Cisco’s suggested IOS backup routine congured on our
router: ash, TFTP host, ROM.
Backing Up and Restoring the Cisco IOS
Before you upgrade or restore a Cisco IOS, you really should copy the existing le to a TFTP
host as a backup just in case the new image crashes and burns.
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672 Chapter 16 u Managing Cisco Devices
And you can use any TFTP host to accomplish this. By default, the ash memory in a router
is used to store the Cisco IOS. In the following sections, I’ll describe how to check the amount
of ash memory, how to copy the Cisco IOS from ash memory to a TFTP host, and how to
copy the IOS from a TFTP host to ash memory.
But before you back up an IOS image to a network server on your intranet, you’ve got to
do these three things:
u Make sure you can access the network server.
u Ensure that the network server has adequate space for the code image.
u Verify the file naming and path requirements.
You can connect your laptop or workstation’s Ethernet port directly to a router’s
Ethernet interface, as shown in Figure16.2.
FIGURE16.2 Copying an IOS from a router to a TFTP host
Copy the IOS to a TFTP host.
RouterX#copy flash tftp:
Source filename [] ?c2800nm-ipbase-mz.124-5a.bin
Address or name of remote host [] ? 10.1.1.1
Destination filename [c2800nm-ipbase-mz.124-5a.bin] [enter]
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!<output omitted>
12094416 bytes copied in 98.858 secs (122341 bytes/sec)
RouterX#
• TFTP server software must be running on the PC.
• The PC must be on the same subnet as the router’s E0 interface.
• The copy flash tftp command must be supplied the IP address of the PC.
Router# copy flash tftp
E0
Console
• IP address of the TFTP server
• IOS filename
You need to verify the following before attempting to copy the image to or from
the router:
u TFTP server software must be running on the laptop or workstation.
u The Ethernet connection between the router and the workstation must be made with a
crossover cable.
u The workstation must be on the same subnet as the router’s Ethernet interface.
u The copy flash tftp command must be supplied the IP address of the workstation if
you are copying from the router flash.
u And if you’re copying “into” flash, you need to verify that there’s enough room in flash
memory to accommodate the file to be copied.
Backing Up and Restoring the Cisco IOS 673
Verifying Flash Memory
Before you attempt to upgrade the Cisco IOS on your router with a new IOS le, it’s a good
idea to verify that your ash memory has enough room to hold the new image. You verify
the amount of ash memory and the le or les being stored in ash memory by using the
show flash command (sh flash for short):
Router#sh flash
-#- --length-- -----date/time------ path
1 45392400 Apr 14 2013 05:31:44 +00:00 c2800nm-advsecurityk9-mz.151-4.M6.bin
18620416 bytes available (45395968 bytes used)
There are about 45 MB of ash used, but there still about 18 MB available. If you
want to copy a le into ash that is more than 18 MB in size, the router will ask you if
you want to erase ash. Be careful here!
The show flash command will display the amount of memory consumed
by the current IOS image, as well as, tell you if there’s enough room avail-
able to hold both current and new images. You should know that if there’s
not enough room for both the old and new image you want to load, the old
image will be erased!
The amount of RAM and ash is actually easy to tally using the show version command
on routers:
Router#show version
[output cut]
System returned to ROM by power-on
System image file is "flash:c2800nm-advsecurityk9-mz.151-4.M6.bin"
[output cut]
Cisco 2811 (revision 1.0) with 249856K/12288K bytes of memory.
Processor board ID FTX1049A1AB
2 FastEthernet interfaces
2 Serial(sync/async) interfaces
1 Virtual Private Network (VPN) Module
DRAM configuration is 64 bits wide with parity enabled.
239K bytes of non-volatile configuration memory.
62720K bytes of ATA CompactFlash (Read/Write)
The rst highlighted line shows us that this router has about 256 MB of RAM, and you can
see that the amount of ash shows up on the last line. By estimating up, we get the amount of
ash to 64 MB.
674 Chapter 16 u Managing Cisco Devices
Notice that the lename in this example is c2800nm-advsecurityk9-mz.151-4.M6.bin.
The main difference in the output of the show flash and show version commands is that
the show flash command displays all les in ash memory and the show version command
shows the actual name of the le used to run the router and the location from which it was
loaded, which is ash memory.
Backing Up the Cisco IOS
To back up the Cisco IOS to a TFTP server, you use the copy flash tftp command. It’s a
straightforward command that requires only the source lename and the IP address of the
TFTP server.
The key to success in this backup routine is to make sure you’ve got good, solid connec-
tivity to the TFTP server. Check this by pinging the TFTP device from the router console
prompt like this:
Router#ping 1.1.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.2, timeout
is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max
= 4/4/8 ms
After you ping the TFTP server to make sure that IP is working, you can use the copy
flash tftp command to copy the IOS to the TFTP server as shown next:
Router#copy flash tftp
Source filename []?c2800nm-advsecurityk9-mz.151-4.M6.bin
Address or name of remote host []?1.1.1.2
Destination filename [c2800nm-advsecurityk9-mz.151-4.M6.bin]?[enter]
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
45395968 bytes copied in 123.724 secs (357532 bytes/sec)
Router#
Just copy the IOS lename from either the show flash or show version command and
then paste it when prompted for the source lename.
In the preceding example, the contents of ash memory were copied successfully to the
TFTP server. The address of the remote host is the IP address of the TFTP host, and the source
lename is the le in ash memory.
Many newer Cisco routers have removable memory. You may see names
for this memory such as flash0:, in which case the command in the preced-
ing example would be copy flash0: tftp:. Alternately, you may see it as
usbflash0:.
Backing Up and Restoring the Cisco IOS 675
Restoring or Upgrading the Cisco Router IOS
What happens if you need to restore the Cisco IOS to ash memory to replace an original le
that has been damaged or if you want to upgrade the IOS? You can download the le from
a TFTP server to ash memory by using the copy tftp flash command. This command
requires the IP address of the TFTP host and the name of the le you want to download.
But before you begin, make sure the le you want to place in ash memory is in the
default TFTP directory on your host. When you issue the command, TFTP won’t ask you
where the le is, so if the le you want to use isn’t in the default directory of the TFTP host,
this just won’t work.
Router#copy tftp flash
Address or name of remote host []?1.1.1.2
Source filename []?c2800nm-advsecurityk9-mz.151-4.M6.bin
Destination filename [c2800nm-advsecurityk9-mz.151-4.M6.bin]?[enter]
%Warning: There is a file already existing with this name
Do you want to over write? [confirm][enter]
Accessing tftp://1.1.1.2/ c2800nm-advsecurityk9-mz.151-4.M6.bin...
Loading c2800nm-advsecurityk9-mz.151-4.M6.bin from 1.1.1.2 (via
FastEthernet0/0): !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
[OK - 21710744 bytes]
45395968 bytes copied in 82.880 secs (261954 bytes/sec)
Router#
In the preceding example, I copied the same le into ash memory, so it asked me if I
wanted to overwrite it. Remember that we are “playing” with les in ash memory. If I had
just corrupted my le by overwriting it, I won’t know for sure until I reboot the router. Be
careful with this command! If the le is corrupted, you’ll need to do an IOS-restore from
ROM monitor mode.
If you are loading a new le and you don’t have enough room in ash memory to
store both the new and existing copies, the router will ask to erase the contents of ash
memory before writing the new le into ash memory, and if you are able to copy the IOS
without erasing the old version, then make sure you remember to use the boot system
flash:ios-file command.
As I demonstrated in the Real World Scenario, a Cisco router can become a
TFTP server host for a router system image that’s run in flash memory. The
global configuration command is tftp-server flash:ios-file.
676 Chapter 16 u Managing Cisco Devices
It’s Monday Morning and You Just Upgraded Your IOS
You came in early to work to upgrade the IOS on your router. After the upgrade, you
reload the router and the router now shows the rommon> prompt.
It seems that you’re about to have a bad day! This is what I call an RGE: a resume-generating
event! So, now what do you do? Just keep calm and chive on! Follow these steps to save
your job:
rommon 1 > tftpdnld
Missing or illegal ip address for variable IP_ADDRESS
Illegal IP address.
usage: tftpdnld [-hr]
Use this command for disaster recovery only to recover an image via TFTP.
Monitor variables are used to set up parameters for the transfer.
(Syntax: "VARIABLE_NAME=value" and use "set" to show current variables.)
"ctrl-c" or "break" stops the transfer before flash erase begins.
The following variables are REQUIRED to be set for tftpdnld:
IP_ADDRESS: The IP address for this unit
IP_SUBNET_MASK: The subnet mask for this unit
DEFAULT_GATEWAY: The default gateway for this unit
TFTP_SERVER: The IP address of the server to fetch from
TFTP_FILE: The filename to fetch
The following variables are OPTIONAL:
[unneeded output cut]
rommon 2 >set IP_Address:1.1.1.1
rommon 3 >set IP_SUBNET_MASK:255.0.0.0
rommon 4 >set DEFAULT_GATEWAY:1.1.1.2
rommon 5 >set TFTP_SERVER:1.1.1.2
rommon 6 >set TFTP_FILE: flash:c2800nm-advipservicesk9-mz.124-12.bin
rommon 7 >tftpdnld
Backing Up and Restoring the Cisco IOS 677
From here you can see the variables you need to configure using the set command; be
sure you use ALL_CAPS with these commands as well as underscore (_).From here, you
need to set the IP address, mask, and default gateway of your router, then the IP address
of the TFTP host, which in this example is a directly connected router that I made a TFTP
server with this command:
Router(config)#tftp-server flash:c2800nm-advipservicesk9-mz.124-12.bin
And finally, you set the IOS filename of the file on your TFTP server. Whew! Job saved.
There is one other way you can restore the IOS on a router, but it takes a while. You
can use what is called the Xmodem protocol to actually upload an IOS le into ash memory
through the console port. You’d use the Xmodem through the console port procedure if you
had no network connectivity to the router or switch.
Using the Cisco IOS File System (Cisco IFS)
Cisco has created a le system called Cisco IFS that allows you to work with les and directo-
ries just as you would from a Windows DOS prompt. The commands you use are dir, copy,
more, delete, erase or format, cd and pwd, and mkdir and rmdir.
Working with IFS gives you the ability to view all les, even those on remote servers.
And you denitely want to nd out if an image on one of your remote servers is valid before
you copy it, right? You also need to know how big it is—size matters here! It’s also a really
good idea to take a look at the remote server’s conguration and make sure it’s all good
before loading that le on your router.
It’s very cool that IFS makes the le system user interface universal—it’s not platform
specic anymore. You now get to use the same syntax for all your commands on all of your
routers, no matter the platform!
Sound too good to be true? Well, it kind of is because you’ll nd out that support for all
commands on each le system and platform just isn’t there. But it’s really no big deal since
various le systems differ in the actions they perform; the commands that aren’t relevant to
a particular le system are the very ones that aren’t supported on that le system. Be assured
that any le system or platform will fully support all the commands you need to manage it.
Another cool IFS feature is that it cuts down on all those obligatory prompts for a lot
of the commands. If you want to enter a command, all you have to do is type all the neces-
sary info straight into the command line—no more jumping through hoops of prompts! So,
if you want to copy a le to an FTP server, all you’d do is rst indicate where the desired
source le is on your router, pinpoint where the destination le is to be on the FTP server,
determine the username and password you’re going to use when you want to connect to
that server, and type it all in on one line—sleek! And for those of you resistant to change,
you can still have the router prompt you for all the information it needs and enjoy entering
a more elegantly minimized version of the command than you did before.
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But even in spite of all this, your router might still prompt you—even if you did everything
right in your command line. It comes down to how you’ve got the file prompt command con-
gured and which command you’re trying to use. But no worries—if that happens, the default
value will be entered right there in the command, and all you have to do is hit Enter to verify
the correct values.
IFS also lets you explore various directories and inventory les in any directory you want.
Plus, you can make subdirectories in ash memory or on a card, but you only get to do that
if you’re working on one of the more recent platforms.
And get this—the new le system interface uses URLs to determine the whereabouts
of a le. So just as they pinpoint places on the Web, URLs now indicate where les are
on your Cisco router, or even on a remote le server! You just type URLs right into your
commands to identify where the le or directory is. It’s really that easy—to copy a le
from one place to another, you simply enter the copy source-url destination-url
command—sweet! IFS URLs are a tad different than what you’re used to though, and
there’s an array of formats to use that vary depending on where, exactly, the le is that
you’re after.
We’re going to use Cisco IFS commands pretty much the same way that we used the
copy command in the IOS section earlier:
u For backing up the IOS
u For upgrading the IOS
u For viewing text files
Okay—with all that down, let’s take a look at the common IFS commands available to
us for managing the IOS. I’ll get into conguration les soon, but for now I’m going to get
you started with going over the basics used to manage the new Cisco IOS.
dir Same as with Windows, this command lets you view les in a directory. Type dir, hit
Enter, and by default you get the contents of the flash:/ directory output.
copy This is one popular command, often used to upgrade, restore, or back up an IOS.
But as I said, when you use it, it’s really important to focus on the details—what you’re
copying, where it’s coming from, and where it’s going to land.
more Same as with Unix, this will take a text le and let you look at it on a card. You can
use it to check out your conguration le or your backup conguration le. I’ll go over it
more when we get into actual conguration.
show file This command will give you the skinny on a specied le or le system, but
it’s kind of obscure because people don’t use it a lot.
delete Three guesses—yep, it deletes stuff. But with some types of routers, not as well
as you’d think. That’s because even though it whacks the le, it doesn’t always free up the
space it was using. To actually get the space back, you have to use something called the
squeeze command too.
Backing Up and Restoring the Cisco IOS 679
erase/format Use these with care—make sure that when you’re copying les, you say no
to the dialog that asks you if you want to erase the le system! The type of memory you’re
using determines if you can nix the ash drive or not.
cd/pwd Same as with Unix and DOS, cd is the command you use to change directories.
Use the pwd command to print (show) the working directory.
mkdir/rmdir Use these commands on certain routers and switches to create and delete
directories—the mkdir command for creation and the rmdir command for deletion. Use
the cd and pwd commands to change into these directories.
The Cisco IFS uses the alternate term system:running-config as well
as nvram:startup-config when copying the configurations on a router,
although it is not mandatory that you use this naming convention.
Using the Cisco IFS to Upgrade an IOS
Let’s take a look at some of these Cisco IFS commands on my ISR router (1841 series) with
a hostname of R1.
We’ll start with the pwd command to verify our default directory and then use the dir
command to verify its contents (flash:/):
R1#pwd
flash:
R1#dir
Directory of flash:/
1 -rw- 13937472 Dec 20 2006 19:58:18 +00:00 c1841-ipbase-
mz.124-1c.bin
2 -rw- 1821 Dec 20 2006 20:11:24 +00:00 sdmconfig-18xx.cfg
3 -rw- 4734464 Dec 20 2006 20:12:00 +00:00 sdm.tar
4 -rw- 833024 Dec 20 2006 20:12:24 +00:00 es.tar
5 -rw- 1052160 Dec 20 2006 20:12:50 +00:00 common.tar
6 -rw- 1038 Dec 20 2006 20:13:10 +00:00 home.shtml
7 -rw- 102400 Dec 20 2006 20:13:30 +00:00 home.tar
8 -rw- 491213 Dec 20 2006 20:13:56 +00:00 128MB.sdf
9 -rw- 1684577 Dec 20 2006 20:14:34 +00:00 securedesktop-
ios-3.1.1.27-k9.pkg
10 -rw- 398305 Dec 20 2006 20:15:04 +00:00 sslclient-win-1.1.0.154.pkg
32071680 bytes total (8818688 bytes free)
680 Chapter 16 u Managing Cisco Devices
What we can see here is that we have the basic IP IOS (c1841-ipbase-mz.124-1c.bin).
Looks like we need to upgrade our 1841. You’ve just got to love how Cisco puts the IOS type
in the lename now! First, let’s check the size of the le that’s in ash with the show file
command (show flash would also work):
R1#show file info flash:c1841-ipbase-mz.124-1c.bin
flash:c1841-ipbase-mz.124-1c.bin:
type is image (elf) []
file size is 13937472 bytes, run size is 14103140 bytes
Runnable image, entry point 0x8000F000, run from ram
With a le that size, the existing IOS will have to be erased before we can add our new
IOS le (c1841-advipservicesk9-mz.124-12.bin), which is over 21 MB. We’ll use the
delete command, but remember, we can play with any le in ash memory and nothing
serious will happen until we reboot—that is, if we made a mistake. So obviously, and as I
pointed out earlier, we need to be very careful here!
R1#delete flash:c1841-ipbase-mz.124-1c.bin
Delete filename [c1841-ipbase-mz.124-1c.bin]?[enter]
Delete flash:c1841-ipbase-mz.124-1c.bin? [confirm][enter]
R1#sh flash
-#- --length-- -----date/time------ path
1 1821 Dec 20 2006 20:11:24 +00:00 sdmconfig-18xx.cfg
2 4734464 Dec 20 2006 20:12:00 +00:00 sdm.tar
3 833024 Dec 20 2006 20:12:24 +00:00 es.tar
4 1052160 Dec 20 2006 20:12:50 +00:00 common.tar
5 1038 Dec 20 2006 20:13:10 +00:00 home.shtml
6 102400 Dec 20 2006 20:13:30 +00:00 home.tar
7 491213 Dec 20 2006 20:13:56 +00:00 128MB.sdf
8 1684577 Dec 20 2006 20:14:34 +00:00 securedesktop-ios-3.1.1.27-k9.pkg
9 398305 Dec 20 2006 20:15:04 +00:00 sslclient-win-1.1.0.154.pkg
22757376 bytes available (9314304 bytes used)
R1#sh file info flash:c1841-ipbase-mz.124-1c.bin
%Error opening flash:c1841-ipbase-mz.124-1c.bin (File not found)
R1#
So with the preceding commands, we deleted the existing le and then veried the dele-
tion by using both the show flash and show file commands. We’ll add the new le with
the copy command, but again, we need to make sure to be careful because this way isn’t
any safer than the rst method I showed you earlier:
R1#copy tftp://1.1.1.2/c1841-advipservicesk9-mz.124-12.bin/ flash:/
c1841-advipservicesk9-mz.124-12.bin
Backing Up and Restoring the Cisco IOS 681
Source filename [/c1841-advipservicesk9-mz.124-12.bin/]?[enter]
Destination filename [c1841-advipservicesk9-mz.124-12.bin]?[enter]
Loading /c1841-advipservicesk9-mz.124-12.bin/ from 1.1.1.2 (via
FastEthernet0/0): !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
[output cut]
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
[OK - 22103052 bytes]
22103052 bytes copied in 72.008 secs (306953 bytes/sec)
R1#sh flash
-#- --length-- -----date/time------ path
1 1821 Dec 20 2006 20:11:24 +00:00 sdmconfig-18xx.cfg
2 4734464 Dec 20 2006 20:12:00 +00:00 sdm.tar
3 833024 Dec 20 2006 20:12:24 +00:00 es.tar
4 1052160 Dec 20 2006 20:12:50 +00:00 common.tar
5 1038 Dec 20 2006 20:13:10 +00:00 home.shtml
6 102400 Dec 20 2006 20:13:30 +00:00 home.tar
7 491213 Dec 20 2006 20:13:56 +00:00 128MB.sdf
8 1684577 Dec 20 2006 20:14:34 +00:00 securedesktop-ios-3.1.1.27-k9.pkg
9 398305 Dec 20 2006 20:15:04 +00:00 sslclient-win-1.1.0.154.pkg
10 22103052 Mar 10 2007 19:40:50 +00:00 c1841-advipservicesk9-mz.124-12.bin
651264 bytes available (31420416 bytes used)
R1#
We can also check the le information with the show file command:
R1#sh file information flash:c1841-advipservicesk9-mz.124-12.bin
flash:c1841-advipservicesk9-mz.124-12.bin:
type is image (elf) []
file size is 22103052 bytes, run size is 22268736 bytes
Runnable image, entry point 0x8000F000, run from ram
Remember that the IOS is expanded into RAM when the router boots, so the new IOS
will not run until you reload the router.
I really recommend experimenting with the Cisco IFS commands on a router just to
get a good feel for them because, as I’ve said, they can denitely give you some grief if
not executed properly!
I mention “safer methods” a lot in this chapter. Clearly, I’ve caused
myself some serious pain by not being careful enough when working in
flash memory! I cannot stress this enough—pay attention when messing
around with flash memory!
682 Chapter 16 u Managing Cisco Devices
One of the brilliant features of the ISR routers is that they use the physical ash cards
that are accessible from the front or back of any router. These typically have a name like
usbflash0:, so to view the contents you’d type dir usbflash0:, for example. You can pull
these ash cards out, put them in an appropriate slot in your PC and the card will show up
as a drive. You can then add, change, and delete les. Just put the ash card back in your
router and power up—instant upgrade. Nice!
Licensing
IOS licensing is now done quite differently than it was with previous versions of the IOS.
Actually, there was no licensing before the new 15.0 IOS code, just your word and honor,
and we can only guess based on how all products are downloaded on the Internet daily
how well that has worked out for Cisco!
Starting with the IOS 15.0 code, things are much different—almost too different. I can
imagine that Cisco will come back toward the middle on its licensing issues, so that the
administration and management won’t be as detailed as the new 15.0 code license is now;
but you can be the judge of that after reading this section.
A new ISR router is pre-installed with the software images and licenses that you ordered,
so as long as you ordered and paid for everything you need, you’re set! If not, you can just
install another license, which can be a tad tedious at rst—enough so that installing a license
was made an objective on the Cisco exam! Of course, it can be done, but it denitely requires
some effort. As is typical with Cisco, if you spend enough money on their products, they tend
to make it easier on you and your administration, and the licensing for the newest IOS is no
exception, as you’ll soon see.
On a positive note, Cisco provides evaluation licenses for most software packages and
features that are supported on the hardware you purchased, and it’s always nice to be able
to try it out before you buy. Once the temporary license expires after 60 days, you need to
acquire a permanent license in order to continue to use the extended features that aren’t
available in your current version. This method of licensing allows you to enable a router to
use different parts of the IOS. So, what happens after 60 days? Well, nothing—back to the
honor system for now. This is now called Right-To-Use (RTU) licensing, and it probably
won’t always be available via your honor, but for now it is.
But that’s not the best part of the new licensing features. Prior to the 15.0 code release,
there were eight different software feature sets for each hardware router type. With the IOS
15.0 code, the packaging is now called a universal image, meaning all feature sets are available
in one le with all features packed neatly inside. So instead of the pre-15.0 IOS le packages of
one image per feature set, Cisco now just builds one universal image that includes all of them
in the le. Even so, we still need a different universal image per router model or series, just not
a different image for each feature set as we did with previous IOS versions.
To use the features in the IOS software, you must unlock them using the software
activation process. Since all features available are inside the universal image already,
you can just unlock the features you need as you need them, and of course pay for these
Licensing 683
features when you determine that they meet your business requirements. All routers
come with something called the IP Base licensing, which is the prerequisite for installing
all other features.
There are three different technology packages available for purchase that can be installed
as additional feature packs on top of the prerequisite IP Base (default), which provides entry-
level IOS functionality. These are as follows:
Data: MPLS, ATM, and multiprotocol support
Unied Communications: VoIP and IP telephony
Security: Cisco IOS Firewall, IPS, IPsec, 3DES, and VPN
For example, if you need MPLS and IPsec, you’ll need the default IP Base, Data, and
Security premium packages unlocked on your router.
To obtain the license, you’ll need the unique device identier (UDI), which has two
components: the product ID (PID) and the serial number of the router. The show license
UDI command provides this information in an output as shown:
Router#sh license udi
Device# PID SN UDI
-------------------------------------------------------------------------
*0 CISCO2901/K9 FTX1641Y07J CISCO2901/K9:FTX1641Y07J
After the time has expired for your 60-day evaluation period, you can either obtain the
license le from the Cisco License Manager (CLM), which is an automated process, or use
the manual process through the Cisco Product License Registration portal. Typically only
larger companies will use the CLM because you’d need to install software on a server, which
then keeps track of all your licenses for you. If you have just a few licenses that you use, you
can opt for the manual web browser process found on the Cisco Product License Registration
portal and then just add in a few CLI commands. After that, you just basically keep track of
putting all the different license features together for each device you manage. Although this
sounds like a lot of work, you don’t need to perform these steps often. But clearly, going with
the CLM makes a lot of sense if you have bunches of licenses to manage because it will put
together all the little pieces of licensing for each router in one easy process.
When you purchase the software package with the features that you want to install,
you need to permanently activate the software package using your UDI and the product
authorization key (PAK) that you received with your purchase. This is essentially your
receipt acknowledging that you purchased the license. You then need to connect the license
with a particular router by combining the PAK and the UDI, which you do online at the
Cisco Product License Registration portal (www.cisco.com/go/license). If you haven’t
already registered the license on a different router, and it is valid, Cisco will then email
you your permanent license, or you can download it from your account.
But wait! You’re still not done. You now need to activate the license on the router.
Whew… maybe it’s worthwhile to install the CLM on a server after all! Staying with the
manual method, you need to make the new license le available to the router either via a
USB port on the router or through a TFTP server. Once it’s available to the router, you’ll
use the license install command from privileged mode.
684 Chapter 16 u Managing Cisco Devices
Assuming that you copied the le into ash memory, the command would look like
something like this:
Router#license install ?
archive: Install from archive: file system
flash: Install from flash: file system
ftp: Install from ftp: file system
http: Install from http: file system
https: Install from https: file system
null: Install from null: file system
nvram: Install from nvram: file system
rcp: Install from rcp: file system
scp: Install from scp: file system
syslog: Install from syslog: file system
system: Install from system: file system
tftp: Install from tftp: file system
tmpsys: Install from tmpsys: file system
xmodem: Install from xmodem: file system
ymodem: Install from ymodem: file system
Router#license install flash:FTX1628838P_201302111432454180.lic
Installing licenses from "flash::FTX1628838P_201302111432454180.lic"
Installing...Feature:datak9...Successful:Supported
1/1 licenses were successfully installed
0/1 licenses were existing licenses
0/1 licenses were failed to install
April 12 2:31:19.786: %LICENSE-6-INSTALL: Feature datak9 1.0 was
installed in this device. UDI=CISCO2901/K9:FTX1628838P; StoreIndex=1:Primary
License Storage
April 12 2:31:20.078: %IOS_LICENSE_IMAGE_APPLICATION-6-LICENSE_LEVEL: Module name
=c2800 Next reboot level = datak9 and License = datak9
You need to reboot to have the new license take effect. Now that you have your license
installed and running, how do you use Right-To-Use licensing to check out new features on
your router? Let’s look into that now.
Right-To-Use Licenses (Evaluation Licenses)
Originally called evaluation licenses, Right-To-Use (RTU) licenses are what you need
when you want to update your IOS to load a new feature but either don’t want to wait
to get the license or just want to test if this feature will truly meet your business require-
ments. This makes sense because if Cisco made it complicated to load and check out a
Licensing 685
feature, they could potentially miss out on a sale! Of course if the feature does work for
you, they’ll want you to buy a permanent license, but again, this is on the honor system
at the time of this writing.
Cisco’s license model allows you to install the feature you want without a PAK. The
Right-To-Use license works for 60 days before you would need to install your permanent
license. To enable the Right-To-Use license you would use the license boot module com-
mand. The following demonstrates starting the Right-To-Use license on my 2900 series
router, enabling the security module named securityk9:
Router(config)#license boot module c2900 technology-package securityk9
PLEASE READ THE FOLLOWING TERMS CAREFULLY. INSTALLING THE LICENSE OR LICENSE KEY
PROVIDED FOR ANY CISCO PRODUCT FEATURE OR USING
SUCHPRODUCT FEATURE CONSTITUTES YOUR FULL ACCEPTANCE OF THE
FOLLOWING TERMS. YOU MUST NOT PROCEED FURTHER IF YOU ARE NOT WILLING
TO BE BOUND BY ALL THE TERMS SET FORTH HEREIN.
[output cut]
Activation of the software command line interface will be evidence of
your acceptance of this agreement.
ACCEPT? [yes/no]: yes
% use 'write' command to make license boot config take effect on next boot
Feb 12 01:35:45.060: %IOS_LICENSE_IMAGE_APPLICATION-6-LICENSE_LEVEL:
Module name =c2900 Next reboot level = securityk9 and License = securityk9
Feb 12 01:35:45.524: %LICENSE-6-EULA_ACCEPTED: EULA for feature
securityk9 1.0 has been accepted. UDI=CISCO2901/K9:FTX1628838P;
StoreIndex=0:Built-In License Storage
Once the router is reloaded, you can use the security feature set. And it is really nice that
you don’t need to reload the router again if you choose to install a permanent license for this
feature. The show license command shows the licenses installed on the router:
Router#show license
Index 1 Feature: ipbasek9
Period left: Life time
License Type: Permanent
License State: Active, In Use
License Count: Non-Counted
License Priority: Medium
Index 2 Feature: securityk9
Period left: 8 weeks 2 days
686 Chapter 16 u Managing Cisco Devices
Period Used: 0 minute 0 second
License Type: EvalRightToUse
License State: Active, In Use
License Count: Non-Counted
License Priority: None
Index 3 Feature: uck9
Period left: Life time
License Type: Permanent
License State: Active, In Use
License Count: Non-Counted
License Priority: Medium
Index 4 Feature: datak9
Period left: Not Activated
Period Used: 0 minute 0 second
License Type: EvalRightToUse
License State: Not in Use, EULA not accepted
License Count: Non-Counted
License Priority: None
Index 5 Feature: gatekeeper
[output cut]
You can see in the preceding output that the ipbasek9 is permanent and the securityk9
has a license type of EvalRightToUse. The show license feature command provides
the same information as show license, but it’s summarized into one line as shown in the
next output:
Router#sh license feature
Feature name Enforcement Evaluation Subscription Enabled RightToUse
ipbasek9 no no no yes no
securityk9 yes yes no no yes
uck9 yes yes no yes yes
datak9 yes yes no no yes
gatekeeper yes yes no no yes
SSL_VPN yes yes no no yes
ios-ips-update yes yes yes no yes
SNASw yes yes no no yes
hseck9 yes no no no no
cme-srst yes yes no yes yes
WAAS_Express yes yes no no yes
UCVideo yes yes no no yes
Licensing 687
The show version command also shows the license information at the end of the
command output:
Router#show version
[output cut]
License Info:
License UDI:
-------------------------------------------------
Device# PID SN
-------------------------------------------------
*0 CISCO2901/K9 FTX1641Y07J
Technology Package License Information for Module:'c2900'
-----------------------------------------------------------------
Technology Technology-package Technology-package
Current Type Next reboot
------------------------------------------------------------------
ipbase ipbasek9 Permanent ipbasek9
security None None None
uc uck9 Permanent uck9
data None None None
Configuration register is 0x2102
The show version command shows if the license was activated. Don’t forget, you’ll
need to reload the router to have the license features take effect if the license evaluation
is not already active.
Backing Up and Uninstalling the License
It would be a shame to lose your license if it has been stored in ash and your ash les
become corrupted. So always back up your IOS license!
If your license has been saved in a location other than ash, you can easily back it up to
ash memory via the license save command:
Router#license save flash:Todd_License.lic
688 Chapter 16 u Managing Cisco Devices
The above command will save your current license to ash. You can restore your license
with the license install command I demonstrated earlier.
There are two steps to uninstalling the license on a router. First, to uninstall the license
you need to disable the technology package, using the no license boot module command
with the keyword disable at the end of the command line:
Router#license boot module c2900 technology-package securityk9 disable
The second step is to clear the license. To achieve this from the router, use the license
clear command and then remove the license with the no license boot module command.
Router#license clear securityk9
Router#config t
Router(config)#no license boot module c2900 technology-package securityk9 disable
Router(config)#exit
Router#reload
After you run through the preceding commands, the license will be removed from
your router.
Here’s a summary of the license commands I used in this chapter. These are impor-
tant commands to have down and you really need to understand these to meet the Cisco
objectives:
u show license determines the licenses that are active on your system. It also displays
a group of lines for each feature in the currently running IOS image along with sev-
eral status variables related to software activation and licensing, both licensed and
unlicensed features.
u show license feature allows you to view the technology package licenses and feature
licenses that are supported on your router along with several status variables related to
software activation and licensing. This includes both licensed and unlicensed features.
u show license udi displays the unique device identifier (UDI) of the router, which
comprises the product ID (PID) and serial number of the router.
u show version displays various information about the current IOS version, including
the licensing details at the end of the command’s output.
u license install url installs a license key file into a router.
u license boot module installs a Right-To-Use license feature on a router.
Summary
You now know how Cisco routers are congured and how to manage those congurations.
This chapter covered the internal components of a router, which included ROM, RAM,
NVRAM, and ash.
Exam Essentials 689
In addition, I covered what happens when a router boots and which les are loaded at
that time. The conguration register tells the router how to boot and where to nd les. You
learned how to change and verify the conguration register settings for password recovery
purposes. I also showed you how to manage these les using the CLI and IFS.
Finally, the chapter covered licensing with the new 15.0 code, including how to install a
permanent license and Right-To-Use license to install features for 60 days. I also showed you
the verication commands used to see what licenses are installed and to verify their status.
Exam Essentials
Define the Cisco router components. Describe the functions of the bootstrap, POST, ROM
monitor, mini-IOS, RAM, ROM, ash memory, NVRAM, and the conguration register.
Identify the steps in the router boot sequence. The steps in the boot sequence are: POST,
loading the IOS, and copying the startup conguration from NVRAM to RAM.
Understand configuration register commands and settings. The 0x2102 setting is the
default on all Cisco routers and tells the router to look in NVRAM for the boot sequence.
0x2101 tells the router to boot from ROM, and 0x2142 tells the router not to load the
startup-cong in NVRAM to provide password recovery.
Perform password recovery. The steps in the password recovery process are: interrupt the
router boot sequence, change the conguration register, reload the router and enter privi-
leged mode, copy the startup-cong le to running-cong and verify that your interfaces
are re-enabled, change/set the password, save the new conguration, reset the conguration
register, and reload the router.
Back up an IOS image. By using the privileged-mode command copy flash tftp, you
can back up a le from ash memory to a TFTP (network) server.
Restore or upgrade an IOS image. By using the privileged-mode command copy tftp
flash, you can restore or upgrade a le from a TFTP (network) server to ash memory.
Describe best practices to prepare to back up an IOS image to a network server. Make
sure that you can access the network server, ensure that the network server has adequate
space for the code image, and verify the le naming and path requirement.
Understand and use Cisco IFS file system management commands. The commands to use
are dir, copy, more, delete, erase or format, cd and pwd, and mkdir and rmdir, as well as
system:running-config and nvram:startup-config.
Remember how to install a permanent and Right-To-Use license. To install a permanent
license on a router, use the install license url command. To install an evaluation feature,
use the license boot module command.
690 Chapter 16 u Managing Cisco Devices
Remember the verification commands used for licensing in the new ISR G2 routers. The
show license command determines the licenses that are active on your system. The show
license feature command allows you to view the technology package licenses and feature
licenses that are supported on your router. The show license udi command displays the
unique device identier (UDI) of the router, which comprises the product ID (PID) and serial
number of the router, and the show version command displays various information about
the current IOS version, including the licensing details at the end of the command’s output.
Written Labs 691
Written Labs
The answers to these labs can be found in Appendix A, “Answers to Written Labs.”
In this section, you’ll complete the following labs to make sure you’ve got the information
and concepts contained within them fully dialed in:
Lab 2.1: IOS Management
Lab 2.2: Router Memory
Written Lab 2.1: IOS Management
Write the answers to the following questions:
1. What is the command to copy a Cisco IOS to a TFTP server?
2. What do you set the configuration register setting to in order to boot the mini-IOS
in ROM?
3. What is the configuration register setting to tell the router to look in NVRAM for the
boot sequence?
4. What do you set the configuration register setting to in order to boot to ROM moni-
tor mode?
5. What is used with a PAK to generate a license file?
6. What is the configuration register setting for password recovery?
7. Which command can change the location from which the system loads the IOS?
8. What is the first step of the router boot sequence?
9. What command can you use to upgrade a Cisco IOS?
10. Which command determines the licenses that are active on your system?
Written Lab 2.2: Router Memory
Identify the location in a router where each of the following les is stored by default.
1. Cisco IOS
2. Bootstrap
3. Startup configuration
4. POST routine
5. Running configuration
6. ARP cache
7. Mini-IOS
8. ROM monitor
9. Routing tables
10. Packet buffers
692 Chapter 16 u Managing Cisco Devices
Hands-on Labs
To complete the labs in this section, you need at least one router (three would be best) and at
least one PC running as a TFTP server. TFTP server software must be installed and running
on the PC. For these labs, it is also assumed that your PC and the router(s) are connected
together with a switch or hub and that all interfaces (PC NIC and router interfaces) are in the
same subnet. You can alternately connect the PC directly to the router or connect the routers
directly to one another (use a crossover cable in that case). Remember that the labs listed here
were created for use with real routers but can easily be used with the LammleSim IOS version
or Cisco’s Packet Tracer program.
Here is a list of the labs in this chapter:
Lab 2.1: Backing Up Your Router IOS
Lab 2.2: Upgrading or Restoring Your Router IOS
Hands-on Lab 2.1: Backing Up Your Router IOS
In this lab, we’ll be backing up the IOS from ash to a TFTP host.
1. Log into your router and go into privileged mode by typing en or enable.
2. Make sure you can connect to the TFTP server that is on your network by pinging the
IP address from the router console.
3. Type show flash to see the contents of flash memory.
4. Type show version at the router privileged-mode prompt to get the name of the IOS cur-
rently running on the router. If there is only one file in flash memory, the show flash and
show version commands show the same file. Remember that the show version command
shows you the file that is currently running and the show flash command shows you all
of the files in flash memory.
5. Once you know you have good Ethernet connectivity to the TFTP server and you also
know the IOS filename, back up your IOS by typing copy flash tftp. This command
tells the router to copy a specified file from flash memory (this is where the IOS is
stored by default) to a TFTP server.
6. Enter the IP address of the TFTP server and the source IOS filename. The file is now
copied and stored in the TFTP server’s default directory.
Hands-on Lab 2.2: Upgrading or Restoring
Your Router IOS
In this lab, we’ll be copying an IOS from a TFTP host to ash memory.
1. Log into your router and go into privileged mode by typing en or enable.
2. Make sure you can connect to the TFTP server by pinging the IP address of the server
from the router console.
Hands-on Labs 693
3. Once you know you have good Ethernet connectivity to the TFTP server, type the
copy tftp flash command.
4. Confirm that the router will not function during the restore or upgrade by following
the prompts provided on the router console. It is possible this prompt may not occur.
5. Enter the IP address of the TFTP server.
6. Enter the name of the IOS file you want to restore or upgrade.
7. Confirm that you understand the contents of flash memory will be erased if there is not
enough room in flash to store the new image.
8. Watch in amazement as your IOS is deleted out of flash memory and your new IOS is
copied to flash memory.
If the le that was in ash memory is deleted but the new version wasn’t copied to ash
memory, the router will boot from ROM monitor mode. You’ll need to gure out why the
copy operation did not take place.
694 Chapter 16 u Managing Cisco Devices
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional ques-
tions, please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Review
Questions.”
1. What does the command confreg 0x2142 provide?
A. It is used to restart the router.
B. It is used to bypass the configuration in NVRAM.
C. It is used to enter ROM monitor mode.
D. It is used to view the lost password.
2. Which command will copy the IOS to a backup host on your network?
A. transfer IOS to 172.16.10.1
B. copy run start
C. copy tftp flash
D. copy start tftp
E. copy flash tftp
3. What command is used to permanently install a license on an ISR2 router?
A. install license
B. license install
C. boot system license
D. boot license module
4. You type the following into the router and reload. What will the router do?
Router(config)#boot system flash c2800nm-advsecurityk9-mz.151-4.M6.bin
Router(config)#config-register 0x2101
Router(config)#do sh ver
[output cut]
Configuration register is 0x2102 (will be 0x2101 at next reload)
A. The router will expand and run the c2800nm-advsecurityk9-mz.151-4.M6.bin
IOS from flash memory.
B. The router will go into setup mode.
C. The router will load the mini-IOS from ROM.
D. The router will enter ROM monitor mode.
Review Questions 695
5. A network administrator wants to upgrade the IOS of a router without removing the
image currently installed. What command will display the amount of memory con-
sumed by the current IOS image and indicate whether there is enough room available
to hold both the current and new images?
A. show version
B. show flash
C. show memory
D. show buffers
E. show running-config
6. The corporate office sends you a new router to connect, but upon connecting the con-
sole cable, you see that there is already a configuration on the router. What should be
done before a new configuration is entered in the router?
A. RAM should be erased and the router restarted.
B. Flash should be erased and the router restarted.
C. NVRAM should be erased and the router restarted.
D. The new configuration should be entered and saved.
7. Which command loads a new version of the Cisco IOS into a router?
A. copy flash ftp
B. copy nvram flash
C. copy flash tftp
D. copy tftp flash
8. Which command will show you the IOS version running on your router?
A. sh IOS
B. sh flash
C. sh version
D. sh protocols
9. What should the configuration register value be after you successfully complete the
password recovery procedure and return the router to normal operation?
A. 0x2100
B. 0x2101
C. 0x2102
D. 0x2142
696 Chapter 16 u Managing Cisco Devices
10. You save the configuration on a router with the copy running-config startup-con-
fig command and reboot the router. The router, however, comes up with a blank con-
figuration. What can the problem be?
A. You didn’t boot the router with the correct command.
B. NVRAM is corrupted.
C. The configuration register setting is incorrect.
D. The newly upgraded IOS is not compatible with the hardware of the router.
E. The configuration you saved is not compatible with the hardware.
11. Which command will install a Right-To-Use license so you can use an evaluation version
of a feature?
A. install Right-To-Use license feature feature
B. install temporary feature feature
C. license install feature
D. license boot module
12. Which command determines the licenses that are active on your system along with
several status variables?
A. show license
B. show license feature
C. show license udi
D. show version
13. Which command allows you to view the technology package licenses and feature
licenses that are supported on your router along with several status variables?
A. show license
B. show license feature
C. show license udi
D. show version
14. Which command displays the unique device identifier that comprises the product
ID and serial number of the router?
A. show license
B. show license feature
C. show license udi
D. show version
Review Questions 697
15. Which command displays various information about the current IOS version, including
the licensing details at the end of the command’s output?
A. show license
B. show license feature
C. show license udi
D. show version
16. Which command backs up your license to flash memory?
A. copy tftp flash
B. save license flash
C. license save flash
D. copy license flash
17. Which command displays the configuration register setting?
A. show ip route
B. show boot version
C. show version
D. show flash
18. What two steps are needed to remove a license from a router? (Choose two.)
A. Use the erase flash:license command.
B. Reload the system.
C. Use the license boot command with the disable variable at the end of the com-
mand line.
D. Clear the license with the license clear command.
19. You have your laptop directly connected into a router’s Ethernet port. Which of the
following are among the requirements for the copy flash tftp command to be suc-
cessful? (Choose three.)
A. TFTP server software must be running on the router.
B. TFTP server software must be running on your laptop.
C. The Ethernet cable connecting the laptop directly into the router’s Ethernet port
must be a straight-through cable.
D. The laptop must be on the same subnet as the router’s Ethernet interface.
E. The copy flash tftp command must be supplied the IP address of the laptop.
F. There must be enough room in the flash memory of the router to accommodate the
file to be copied.
698 Chapter 16 u Managing Cisco Devices
20. The configuration register setting of 0x2102 provides what function to a router?
A. Tells the router to boot into ROM monitor mode
B. Provides password recovery
C. Tells the router to look in NVRAM for the boot sequence
D. Boots the IOS from a TFTP server
E. Boots an IOS image stored in ROM
Chapter
17
IP Services
THE FOLLOWING ICND2 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 IP Services
■Recognize high availability (FHRP)
■VRRP
■HSRP
■GLBP
■Configure and verify Syslog
■Utilize Syslog Output
■Describe SNMP v2 & v3
1 Troubleshooting
■Utilize netflow data
■Monitor NetFlow statistics
In this chapter, I’m going to show you how to integrate redun-
dancy and load-balancing features into your network elegantly
with the routers that you likely have already. Acquiring some
overpriced load-balancing device just isn’t always necessary because knowing how to prop-
erly congure and use Hot Standby Router Protocol (HSRP), as well as the nonproprietary
Virtual Router Redundancy Protocol (VRRP), can often meet your needs instead. You’ll
soon see that while these technologies are very similar in function, they operate slightly
differently. I’ll also show you how to congure and use Gateway Load Balancing Protocol
(GLBP), which allows you to use up to four routers for an effective load-balancing solution
within your rst hop redundancy protocols (FHRPs).
I know we discussed syslog in earlier CCENT chapters, but I’m going to delve into it
more in-depth in this chapter, as well as look at Simple Network Management Protocol
(SNMP) and the type of alerts sent to the network management station (NMS). In addition,
we’ll cover Cisco NetFlow and how it works in an internetwork.
To find up-to-the-minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
Client Redundancy Issues
If you’re wondering how you can possibly congure a client to send data off its local link
when its default gateway router has gone down, you’ve targeted a key issue because the
answer is that usually, you can’t! Most host operating systems just don’t allow you to
change data routing. Sure, if a host’s default gateway router goes down, the rest of the
network will still converge, but it won’t share that information with the hosts. Take a look
at Figure17.1 to see what I am talking about. There are actually two routers available to
forward data for the local subnet, but the hosts know about only one of them. They learn
about this router when you provide them with the default gateway either statically or
through DHCP.
This begs the question: Is there another way to use the second active router? The answer
is a bit complicated, but bear with me. There is a feature that’s enabled by default on Cisco
routers called Proxy Address Resolution Protocol (Proxy ARP). Proxy ARP enables hosts,
which have no knowledge of routing options, to obtain the MAC address of a gateway
router that can forward packets for them.
Client Redundancy Issues 701
FIGURE17.1 Default gateway
Fa0/1
Fa0/0
10.0.0.1
Fa0/1
Fa0/0
10.0.0.2
IP: 10.0.0.100
Gateway: 10.0.0.1
You can see how this happens in Figure17.2. If a Proxy ARP–enabled router receives
an ARP request for an IP address that it knows isn’t on the same subnet as the request-
ing host, it will respond with an ARP reply packet to the host. The router will give its
own local MAC address—the MAC address of its interface on the host’s subnet—as the
destination MAC address for the IP address that the host is seeking to be resolved. After
receiving the destination MAC address, the host will then send all the packets to the
router, not knowing that what it sees as the destination host is really a router. The router
will then forward the packets toward the intended host.
So with Proxy ARP, the host device sends trafc as if the destination device were located
on its own network segment. If the router that responded to the ARP request fails, the
source host continues to send packets for that destination to the same MAC address. But
because they’re being sent to a failed router, the packets will be sent to the other router that
is also responding to ARP requests for remote hosts.
After the time-out period on the host, the proxy ARP MAC address ages out of the ARP
cache. The host can then make a new ARP request for the destination and get the address of
another proxy ARP router. Still, keep in mind that the host cannot send packets off of its sub-
net during the failover time. This isn’t exactly a perfect situation, so there has to be a better
way, right? Well, there is, and that’s precisely where redundancy protocols come to the rescue!
702 Chapter 17 u IP Services
FIGURE17.2 Proxy ARP
Fa0/1
Fa0/0
10.0.0.1
Fa0/1
Fa0/0
10.0.0.2
Proxy ARP
response
ARP request for remote host
IP: 10.0.0.100
Introducing First Hop Redundancy
Protocol (FHRP)
First hop redundancy protocols (FHRPs) work by giving you a way to congure more than
one physical router to appear as if they were only a single logical one. This makes client
conguration and communication easier because you can simply congure a single default
gateway and the host machine can use its standard protocols to communicate. First hop is
a reference to the default router being the rst router, or rst router hop, through which a
packet must pass.
So how does a redundancy protocol accomplish this? The protocols I’m going to describe
to you do this basically by presenting a virtual router to all of the clients. The virtual router
has its own IP and MAC addresses. The virtual IP address is the address that’s congured
on each of the host machines as the default gateway. The virtual MAC address is the address
that will be returned when an ARP request is sent by a host. The hosts don’t know or care
which physical router is actually forwarding the trafc, as you can see in Figure17.3.
Introducing First Hop Redundancy Protocol (FHRP) 703
FIGURE17.3 FHRPs use a virtual router with a virtual IP address and virtual MAC
address.
Fa0/1
Fa0/0
10.0.0.1
Fa0/1
Fa0/0
10.0.0.2
IP: 10.0.0.100
Gateway: 10.0.0.10
Virtual router
10.0.0.10
?
It’s the responsibility of the redundancy protocol to decide which physical router will
actively forward trafc and which one will be placed on standby in case the active router
fails. Even if the active router fails, the transition to the standby router will be transparent to
the hosts because the virtual router, which is identied by the virtual IP and MAC addresses,
is now used by the standby router. The hosts never change default gateway information, so
trafc keeps owing.
Fault-tolerant solutions provide continued operation in the event of
a device failure, and load-balancing solutions distribute the workload
over multiple devices.
Next we’ll explore these three important redundancy protocols:
Hot Standby Router Protocol (HSRP) is by far Cisco’s favorite protocol ever! Don’t buy just
one router; buy up to eight routers to provide the same service, and keep seven as backup in
case of failure! HSRP is a Cisco proprietary protocol that provides a redundant gateway for
hosts on a local subnet, but this isn’t a load-balanced solution. HSRP allows you to congure
704 Chapter 17 u IP Services
two or more routers into a standby group that shares an IP address and MAC address and
provides a default gateway. When the IP and MAC addresses are independent from the routers’
physical addresses (on a virtual interface, not tied to a specic interface), they can swap control
of an address if the current forwarding and active router fails. But there is actually a way you
can sort of achieve load balancing with HSRP—by using multiple VLANs and designating a
specic router active for one VLAN, then an alternate router as active for the other VLAN via
trunking. This still isn’t a true load-balancing solution and it’s not nearly as solid as what you
can achieve with GLBP!
Virtual Router Redundancy Protocol (VRRP) also provides a redundant—but again,
not load-balanced—gateway for hosts on a local subnet. It’s an open standard protocol
that functions almost identically to HSRP. I’ll comb through the ne differences that exist
between these protocols later in the chapter.
Gateway Load Balancing Protocol (GLBP) doesn’t just stop at providing us with a redun-
dant gateway; it’s a true load-balancing solution for routers. GLBP allows a maximum of
four routers in each forwarding group. By default, the active router directs the trafc from
hosts to each successive router in the group using a round-robin algorithm. The hosts are
directed to send their trafc toward a specic router by being given the MAC address of
the next router in line to be used.
Hot Standby Router Protocol (HSRP)
Again, HSRP is a Cisco proprietary protocol that can be run on most, but not all, of
Cisco’s router and multilayer switch models. It denes a standby group, and each standby
group that you dene includes the following routers:
u Active router
u Standby router
u Virtual router
u Any other routers that maybe attached to the subnet
The problem with HSRP is that with it, only one router is active and two or more routers
just sit there in standby mode and won’t be used unless a failure occurs—not very cost effec-
tive or efcient! Figure17.4 shows how only one router is used at a time in an HSRP group.
The standby group will always have at least two routers participating in it. The primary
players in the group are the one active router and one standby router that communicate to
each other using multicast Hello messages. The Hello messages provide all of the required
communication for the routers. The Hellos contain the information required to accomplish
the election that determines the active and standby router positions. They also hold the key
to the failover process. If the standby router stops receiving Hello packets from the active
router, it then takes over the active router role, as shown in Figure17.5.
As soon as the active router stops responding to Hellos, the standby router automatically
becomes the active router and starts responding to host requests.
Hot Standby Router Protocol (HSRP) 705
FIGURE17.4 HSRP active and standby routers
Fa0/1
Fa0/0
Fa0/1
Fa0/0
IP: 10.0.0.100
Gateway: 10.0.0.10
Virtual router
10.0.0.10
10.0.0.1
Active
10.0.0.2
Standby
Virtual MAC Address
A virtual router in an HSRP group has a virtual IP address and a virtual MAC address. So
where does that virtual MAC come from? The virtual IP address isn’t that hard to gure
out; it just has to be a unique IP address on the same subnet as the hosts dened in the
conguration. But MAC addresses are a little different, right? Or are they? The answer is
yes—sort of. With HSRP, you create a totally new, made-up MAC address in addition to
the IP address.
The HSRP MAC address has only one variable piece in it. The rst 24 bits still identify
the vendor who manufactured the device (the organizationally unique identier, or OUI).
The next 16 bits in the address tell us that the MAC address is a well-known HSRP MAC
address. Finally, the last 8 bits of the address are the hexadecimal representation of the
HSRP group number.
Let me clarify all this with an example of what an HSRP MAC address would look like:
0000.0c07.ac0a
706 Chapter 17 u IP Services
u The first 24 bits (0000.0c) are the vendor ID of the address; in the case of HSRP being
a Cisco protocol, the ID is assigned to Cisco.
u The next 16 bits (07.ac) are the well-known HSRP ID. This part of the address was
assigned by Cisco in the protocol, so it’s always easy to recognize that this address is
for use with HSRP.
u The last 8 bits (0a) are the only variable bits and represent the HSRP group number
that you assign. In this case, the group number is 10 and converted to hexadecimal
when placed in the MAC address, where it becomes the 0a that you see.
FIGURE17.5 Example of HSRP active and standby routers swapping interfaces
Fa0/1
Fa0/0
Fa0/1
Fa0/0
IP: 10.0.0.100
Gateway: 10.0.0.10
Virtual router
10.0.0.10
10.0.0.1
Standby
10.0.0.2
Active
You can see this displayed with every MAC address added to the ARP cache of every
router in the HSRP group. There will be the translation from the IP address to the MAC
address, as well as the interface on which it’s located.
HSRP Timers
Before we get deeper into the roles that each of the routers can have in an HSRP group, I
want to dene the HSRP timers. The timers are very important to HSRP function because
Hot Standby Router Protocol (HSRP) 707
they ensure communication between the routers, and if something goes wrong, they allow
the standby router to take over. The HSRP timers include hello, hold, active, and standby.
Hello timer The hello timer is the dened interval during which each of the routers send
out Hello messages. Their default interval is 3 seconds and they identify the state that each
router is in. This is important because the particular state determines the specic role of
each router and, as a result, the actions each will take within the group. Figure17.6 shows
the Hello messages being sent and the router using the hello timer to keep the network
owing in case of a failure.
FIGURE17.6 HSRP Hellos
Fa0/1
Fa0/0
Fa0/1
Fa0/0
IP: 10.0.0.100
Gateway: 10.0.0.10
Virtual router
10.0.0.10
Hello?
10.0.0.1
Active
10.0.0.2
Standby
This timer can be changed and people used to avoid doing so because it was thought that low-
ering the hello value would place an unnecessary load on the routers. That isn’t true with most
of the routers today; in fact, you can congure the timers in milliseconds, meaning the failover
time can be in milliseconds! Still, keep in mind that increasing the value will make the standby
router wait longer before taking over for the active router when it fails or can’t communicate.
Hold timer The hold timer species the interval the standby router uses to determine whether
the active router is ofine or out of communication. By default, the hold timer is 10 seconds,
roughly three times the default for the hello timer. If one timer is changed for some reason,
708 Chapter 17 u IP Services
I recommend using this multiplier to adjust the other timers too. By setting the hold timer at
three times the hello timer, you ensure that the standby router doesn’t take over the active role
every time there’s a short break in communication.
Active timer The active timer monitors the state of the active router. The timer resets
each time a router in the standby group receives a Hello packet from the active router.
This timer expires based on the hold time value that’s set in the corresponding eld of
the HSRP hello message.
Standby timer The standby timer is used to monitor the state of the standby router. The
timer resets anytime a router in the standby group receives a Hello packet from the standby
router and expires based on the hold time value that’s set in the respective Hello packet.
Large Enterprise Network Outages with FHRPs
Years ago when HSRP was all the rage, and before VRRP and GLBP, enterprises used hun-
dreds of HSRP groups. With the hello timer set to 3 seconds and a hold time of 10 seconds,
these timers worked just fine and we had great redundancy with our core routers.
However, in the last few years, and certainly in the future, 10 seconds is now a lifetime!
Some of my customers have been complaining with the failover time and loss of connec-
tivity to their virtual server farm.
So lately I’ve been changing the timers to well below the defaults. Cisco had changed the
timers so you could use sub-second times for failover. Because these are multicast packets,
the overhead that is seen on a current high-speed network is almost nothing.
The hello timer is typically set to 200 msec and the hold time is 700 msec. The command
is as follows:
(config-if)#Standby 1 timers msec 200 msec 700
This almost ensures that not even a single packet is lost when there is an outage.
Group Roles
Each of the routers in the standby group has a specic function and role to fulll. The three
main roles are as virtual router, active router, and standby router. Additional routers can
also be included in the group.
Virtual router As its name implies, the virtual router is not a physical entity. It really
just denes the role that’s held by one of the physical routers. The physical router that
Hot Standby Router Protocol (HSRP) 709
communicates as the virtual router is the current active router. The virtual router is
nothing more than a separate IP address and MAC address to which packets are sent.
Active router The active router is the physical router that receives data sent to the virtual
router address and routes it onward to its various destinations. As I mentioned, this router
accepts all the data sent to the MAC address of the virtual router in addition to the data
that’s been sent to its own physical MAC address. The active router processes the data
that’s being forwarded and will also answer any ARP requests destined for the virtual
router’s IP address.
Standby router The standby router is the backup to the active router. Its job is to monitor
the status of the HSRP group and quickly take over packet-forwarding responsibilities if
the active router fails or loses communication. Both the active and standby routers transmit
Hello messages to inform all other routers in the group of their role and status.
Other routers An HSRP group can include additional routers, which are members of the
group but that don’t take the primary roles of either active or standby states. These routers
monitor the Hello messages sent by the active and standby routers to ensure that an active
and standby router exists for the HSRP group that they belong to. They will forward data
that’s specically addressed to their own IP addresses, but they will never forward data
addressed to the virtual router unless elected to the active or standby state. These routers
send “speak” messages based on the hello timer interval that informs other routers of their
position in an election.
Interface Tracking
By now, you probably understand why having a virtual router on a LAN is a great idea.
You also know why it’s a very good thing that the active router can change dynamically,
giving us much needed redundancy on our inside network. But what about the links to the
upstream network or the Internet connection off of those HSRP-enabled routers? And how
will the inside hosts know if an outside interface goes down or if they are sending packets
to an active router that can’t route to a remote network? Key questions and HSRP do pro-
vide a solution for them called interface tracking.
Figure17.7 shows how HSRP-enabled routers can keep track of the interface status of the
outside interfaces and how they can switch the inside active router as needed to keep the inside
hosts from losing connectivity upstream.
If the outside link of the active router goes down, the standby router will take over and
become the active router. There is a default priority of 100 on routers congured with an
HSRP interface, and if you raise this priority (we’ll do this in a minute), it means your router
has a higher priority to become the active router. The reason I am bringing this up now is
because when a tracked interface goes down, it decrements the priority of this router.
Configuring and Verifying HSRP
Conguring and verifying the different FHRPs can be pretty simple, especially regard-
ing the Cisco objectives, but as with most technologies, you can quickly get into advanced
congurations and territory with the different FHRPs. The Cisco objectives don’t cover
710 Chapter 17 u IP Services
much about the conguration of FHRPs, but verication is important, so I’ll use a simple
conguration on two routers here, starting with HSRP, which I’ll also refer to for a simple
GLBP conguration and verication later in this chapter. Figure17.8 shows the network I’ll
use to demonstrate the FHRPs.
FIGURE17.7 Interface tracking setup
Tracked interfaces
Virtual router
10.0.0.10
Fa0/1
Fa0/0
Fa0/1
Fa0/0
10.0.0.1
Standby
10.0.0.2
Active
Tracked interfaces
Virtual router
10.0.0.10
Fa0/1
Fa0/0
Fa0/1
Fa0/0
10.0.0.1
Active
10.0.0.2
Standby
This is a simple conguration that you really need only one command for: standby group
ip virtual_ip. After using this single mandatory command, I’ll name the group and set the
interface on router HSRP1 so it wins the election and becomes the active router by default.
HSRP1#config t
HSRP1(config)#int fa0/0
HSRP1(config-if)#standby ?
<0-255> group number
authentication Authentication
delay HSRP initialisation delay
ip Enable HSRP and set the virtual IP address
mac-address Virtual MAC address
name Redundancy name string
preempt Overthrow lower priority Active routers
priority Priority level
redirect Configure sending of ICMP Redirect messages with an HSRP
virtual IP address as the gateway IP address
Hot Standby Router Protocol (HSRP) 711
timers Hello and hold timers
track Priority tracking
use-bia HSRP uses interface's burned in address
version HSRP version
HSRP1(config-if)#standby 1 ip 10.1.1.10
HSRP1(config-if)#standby 1 name HSRP_Test
HSRP1(config-if)#standby 1 priority ?
<0-255> Priority value
HSRP1(config-if)#standby 1 priority 110
000047: %HSRP-5-STATECHANGE: FastEthernet0/0 Grp 1 state Speak -> Standby
000048: %HSRP-5-STATECHANGE: FastEthernet0/0 Grp 1 state Standby -> Active110
FIGURE17.8 HSRP configuration and verification
Virtual router
10.1.1.10
Fa0/1
Fa0/0
Fa0/1
Fa0/0
10.1.1.1 10.1.1.2
HSRP1 HSRP2
Gateway: 10.1.1.10
712 Chapter 17 u IP Services
There are quite a few commands available to use in an advanced setting with the standby
command, but we’ll stick with the simple commands that follow the Cisco objectives. First,
I numbered the group (1), which must be the same on all routers sharing HSRP duties; then
I added the virtual IP address shared by all routers in the HSRP group. Optionally, I named
the group and then set the priority of HSRP1 to 110, and I left HSRP2 to a default of 100.
The router with the highest priority will win the election to become the active router. Let’s
congure the HSRP2 router now:
HSRP2#config t
HSRP2(config)#int fa0/0
HSRP2(config-if)#standby 1 ip 10.1.1.10
HSRP2(config-if)#standby 1 name HSRP_Test
*Jun 23 21:40:10.699:%HSRP-5-STATECHANGE:FastEthernet0/0 Grp 1 state
Speak -> Standby
I really only needed the rst command—naming it was for administrative purposes
only. Notice that the link came up and HSRP2 became the standby router because it had
the lower priority of 100 (the default). Make a note that this priority comes into play only
if both routers were to come up at the same time. This means that HSRP2 would be the
active router, regardless of the priority, if it comes up rst.
Let’s take a look at the congurations with the show standby and show standby brief
commands:
HSRP1(config-if)#do show standby
FastEthernet0/0 - Group 1
State is Active
2 state changes, last state change 00:03:40
Virtual IP address is 10.1.1.10
Active virtual MAC address is 0000.0c07.ac01
Local virtual MAC address is 0000.0c07.ac01 (v1 default)
Hello time 3 sec, hold time 10 sec
Next hello sent in 1.076 secs
Preemption disabled
Active router is local
Standby router is 10.1.1.2, priority 100 (expires in 7.448 sec)
Priority 110 (configured 110)
IP redundancy name is "HSRP_Test" (cfgd)
HSRP1(config-if)#do show standby brief
P indicates configured to preempt.
|
Hot Standby Router Protocol (HSRP) 713
Interface Grp Prio P State Active Standby Virtual IP
Fa0/0 1 110 Active local 10.1.1.2 10.1.1.10
Notice the group number in each output—it’s a key troubleshooting spot! Each router must
be congured in the same group or they won’t work. Also, you can see the virtual MAC and
congured virtual IP address, as well as the hello time of 3 seconds. The standby and virtual
IP addresses are also displayed.
HSRP2’s output tells us that it’s in standby mode:
HSRP2(config-if)#do show standby brief
P indicates configured to preempt.
|
Interface Grp Prio P State Active Standby Virtual IP
Fa0/0 1 100 Standby 10.1.1.1 local 10.1.1.10
HRSP2(config-if)#
There is one other command that I want to cover. If you’re studying and want to under-
stand HSRP, you should learn to use this debug command and have your active and standby
routers move. You’ll really get to see what is going on.
HSRP2#debug standby
*Sep 15 00:07:32.344:HSRP:Fa0/0 Interface UP
*Sep 15 00:07:32.344:HSRP:Fa0/0 Initialize swsb, Intf state Up
*Sep 15 00:07:32.344:HSRP:Fa0/0 Starting minimum intf delay (1 secs)
*Sep 15 00:07:32.344:HSRP:Fa0/0 Grp 1 Set virtual MAC 0000.0c07.ac01
type: v1 default
*Sep 15 00:07:32.344:HSRP:Fa0/0 MAC hash entry 0000.0c07.ac01, Added
Fa0/0 Grp 1 to list
*Sep 15 00:07:32.348:HSRP:Fa0/0 Added 10.1.1.10 to hash table
*Sep 15 00:07:32.348:HSRP:Fa0/0 Grp 1 Has mac changed? cur 0000.0c07.ac01
new 0000.0c07.ac01
*Sep 15 00:07:32.348:HSRP:Fa0/0 Grp 1 Disabled -> Init
*Sep 15 00:07:32.348:HSRP:Fa0/0 Grp 1 Redundancy "hsrp-Fa0/0-1" state Disabled ->
Init
*Sep 15 00:07:32.348:HSRP:Fa0/0 IP Redundancy "hsrp-Fa0/0-1" added
*Sep 15 00:07:32.348:HSRP:Fa0/0 IP Redundancy "hsrp-Fa0/0-1" update,
Disabled -> Init
*Sep 15 00:07:33.352:HSRP:Fa0/0 Intf min delay expired
*Sep 15 00:07:39.936:HSRP:Fa0/0 Grp 1 MAC addr update Delete from SMF 0000.0c07.
ac01
*Sep 15 00:07:39.936:HSRP:Fa0/0 Grp 1 MAC addr update Delete from SMF 0000.0c07.
ac01
*Sep 15 00:07:39.940:HSRP:Fa0/0 ARP reload
714 Chapter 17 u IP Services
HSRP Load Balancing
As you know, HSRP doesn’t really perform true load balancing, but it can be congured
to use more than one router at a time for use with different VLANs. This is different from
the true load balancing that’s possible with GLBP, which I’ll demonstrate in a minute, but
HSRP still performs a load-balancing act of sorts. Figure17.9 shows how load balancing
would look with HSRP.
FIGURE17.9 HSRP load balancing per VLAN
Fa0/1
Fa0/0.10
Ip address for VLAN 10
Fa0/0.20
Ip address for VLAN 20
Fa0/1
Fa0/0.10
Ip address for VLAN 10
Fa0.0.20
Ip address for VLAN 20
VLAN 20 VLAN 10
Trunk Trunk
HSRP active group 10 HSRP active group 20
How can you get two HSRP routers active at the same time? Well for the same subnet
with this simple conguration you can’t, but by trunking the links to each router, they’ll
run and be congured with a “router on a stick” (ROAS) conguration. This means that
each router can be the default gateway for different VLANs, but you still can have only
one active router per VLAN. Typically, in a more advanced setting you won’t use HSRP for
load balancing; you’ll use GLBP, but you can do load-sharing with HSRP, and that is the
topic of an objective, so we’ll remember that, right? It comes in handy because it prevents
situations where a single point of failure causes trafc interruptions. This HSRP feature
improves network resilience by allowing for load-balancing and redundancy capabilities
between subnets and VLANs.
Virtual Router Redundancy Protocol 715
Virtual Router Redundancy Protocol
Like HSRP, Virtual Router Redundancy Protocol (VRRP) allows a group of routers to
form a single virtual router. In an HSRP or VRRP group, one router is elected to handle
all requests sent to the virtual IP address. With HSRP, this is the active router. An HSRP
group has one active router, at least one standby router, and many listening routers. A
VRRP group has one master router and one or more backup routers and is the open stan-
dard implementation of HSRP.
Comparing VRRP and HSRP
The LAN workstations are congured with the address of the virtual router as their default
gateway, just as they are with HSRP, but VRRP differs from HSRP in these important ways:
u VRRP is an IEEE standard (RFC 2338) for router redundancy; HSRP is a Cisco
proprietary protocol.
u The virtual router that represents a group of routers is known as a VRRP group.
u The active router is referred to as the master virtual router.
u The master virtual router may have the same IP address as the virtual router group.
u Multiple routers can function as backup routers.
u VRRP is supported on Ethernet, Fast Ethernet, and Gigabit Ethernet interfaces, as well as
on Multiprotocol Label Switching (MPLS) virtual private networks (VPNs) and VLANs.
VRRP Redundancy Characteristics
VRRP has some unique features:
u VRRP provides redundancy for the real IP address of a router or for a virtual IP address
shared among the VRRP group members.
u If a real IP address is used, the router with that address becomes the master.
u If a virtual IP address is used, the master is the router with the highest priority.
u A VRRP group has one master router and one or more backup routers.
u The master router uses VRRP messages to inform group members of its status.
u VRRP allows load sharing across more than one virtual router.
716 Chapter 17 u IP Services
Gateway Load Balancing Protocol
Although HSRP and VRRP provide gateway resiliency with per-subnet load balancing, the
upstream bandwidth of the standby members of the redundancy group isn’t used while the
devices are in standby mode.
Only the active routers in HSRP and VRRP groups forward trafc for the virtual MAC.
Resources associated with the standby router are not fully utilized. Some load-balancing can
be accomplished with these protocols through the creation of multiple groups and through
the assignment of multiple default gateways, but be warned—these congurations create an
administrative burden and are inefcient for today’s networks!
Cisco designed a proprietary load-balancing protocol, Gateway Load Balancing Protocol
(GLBP), to allow automatic selection and simultaneous use of multiple available gateways,
as well as permit automatic failover between those gateways. GLBP takes an active/active
approach on a per-subnet basis to support rst-hop (default router) trafc when implemented
with two routers on the same LAN. Multiple routers share the load of frames that, from a
client perspective, are sent to a single default gateway address, as shown in Figure17.10.
FIGURE17.10 Gateway Load Balancing Protocol (GLBP)
Fa0/1
Fa0/0.0
10.0.0.1
Forwarding
Fa0/1
Fa0/0.10
10.0.0.2
Forwarding
IP: 10.0.0.101
Gateway: 10.0.0.10
IP: 10.0.0.100
Gateway: 10.0.0.10
IP: 10.0.0.102
Gateway: 10.0.0.10
IP: 10.0.0.103
Gateway: 10.0.0.10
Virtual router
10.0.0.10
Gateway Load Balancing Protocol 717
With GLBP, resources can be fully utilized without the administrative hassle of cong-
uring multiple groups and managing multiple default gateway congurations as required
when working with HSRP and VRRP.
GLBP Functions
GLBP essentially provides clients with the following:
u An active virtual gateway (AVG)
u An active virtual forwarder (AVF)
It also allows members of the group to communicate with each other through Hello mes-
sages sent every 3 seconds to the multicast address 224.0.0.102, User Datagram Protocol
(UDP) port 3222.
GLBP AVG Members of a GLBP group elect one gateway to be the AVG for that group.
Other group members provide backup for the AVG in the event that the AVG becomes unavail-
able. The AVG assigns a different virtual MAC address to each member of the GLBP group.
GLBP AVF Each gateway assumes responsibility for forwarding packets that are sent to
the virtual MAC address assigned to that gateway by the AVG. These gateways are known
as AVFs for their virtual MAC address.
GLBP Features
GLBP provides upstream load-sharing by utilizing the redundant uplinks simultaneously. It
uses link capacity efciently, thus providing solid peak-load trafc coverage. By making use
of multiple available paths upstream from the routers or layer 3 switches running GLBP,
you can also reduce output queues. GLBP supports the following features:
Load sharing You can congure GLBP so that trafc from LAN clients is shared by
multiple routers. As the name suggests, load sharing distributes the trafc load more
evenly among available routers.
Multiple virtual routers GLBP supports up to 1,024 virtual routers as GLBP groups on
each router’s physical interface and up to four virtual forwarders per group.
Preemption According to the dictionary, preempt means “to replace with something consid-
ered to be of greater value or priority.” The redundancy scheme of GLBP allows us to preempt
an AVG with a higher-priority backup virtual gateway that has become available. Forwarder
preemption works in a similar way, except that it’s based upon weighting instead of priority
and is enabled by default. One router can take over another router only during an election, and
preemption is the only way to force an election when a device hasn’t gone down.
718 Chapter 17 u IP Services
Efficient resource utilization GLBP makes it possible for any router in a group to serve
as a backup, which eliminates the need for a dedicated backup router because all available
routers can support network trafc.
Remember that only a single path is used with HSRP or VRRP, while other resources are
idle unless you’ve got multiple groups and gateways congured. This means that a single path
can be subjected to higher output queue rates during peak times, leading to lower performance
caused by higher jitter rates. The good news is that we can mitigate the impact of jitter with
GLBP because with it, more upstream bandwidth is available and additional upstream paths
are used.
Further, GLBP permits automatic selection and simultaneous use of all available gate-
ways in the group. The members of a GLBP group elect one gateway to be the AVG for
that group, and other members of the group provide backup for the AVG if it becomes
unavailable. The AVG assigns a virtual MAC address to each member of the GLBP group.
All routers become AVFs for frames addressed to that specic virtual MAC address. And
as clients send ARP requests for the address of the default gateway, the AVG sends these
virtual MAC addresses in the ARP replies. Don’t forget that a GLBP group can have up to
four group members.
GLBP automatically manages the virtual MAC address assignment, determines who
handles the forwarding, and ensures that each host has a reliable forwarding path if failures
to gateways or tracked interfaces occur. Also, when failures do occur, the load-balancing
ratio is adjusted among the remaining AVFs so that resources are used in the most efcient
way—nice!
GLBP Per-host Traffic Balancing
These two steps will really help clarify how GLBP balances trafc using the round-robin
algorithm:
1. When a client sends an ARP message for the gateway IP address, the AVG returns the
virtual MAC address of one of the AVFs.
2. When a second client sends an ARP message, the AVG returns the next virtual MAC
address from the list.
So having resolved a different MAC address for the default gateway, each client will send
its routed trafc to separate routers even though they both have the same default gateway
address congured. Remember that each GLBP router will be the designated AVF for the
specic virtual MAC address that’s been assigned to it.
Configuring GLBP
Let’s go ahead and congure GLBP on our little internetwork now, as shown in Figure17.11.
I’ll use the same minimal conguration, with GLBP1 getting a higher priority:
Gateway Load Balancing Protocol 719
FIGURE17.11 Configuring GLBP
Virtual router
10.1.1.20
Fa0/1
Fa0/0
10.1.1.1
Fa0/1
Fa0/0
10.1.1.2
GLBP1 GLBP2
GLBP1(config)#int fa0/0
GLBP1(config-if)#glbp 1 ip 10.1.1.20
GLBP1(config-if)#glbp 1 name GLBP_Test
GLBP1(config-if)#GLBP 1 priority 110
GLBP1(config-if)#
000050:%GLBP-6-STATECHANGE:FastEthernet0/0 Grp 1 state Standby -> Active
000051:%GLBP-6-FWDSTATECHANGE:FastEthernet0/0 Grp 1 Fwd 1 state Listen -> Active
Next, I’ll congure GLBP2:
GLBP2(config)#int fa0/0
GLBP2(config-if)#glbp 1 ip 10.1.1.20
GLBP2(config-if)#glbp 1 name GLBP_Test
GLBP2(config-if)#
*Jun 23 21:49:16.059:%GLBP-6-FWDSTATECHANGE:FastEthernet0/0 Grp 1 Fwd 2
state Listen -> Active
720 Chapter 17 u IP Services
Okay—this is pretty much the same conguration as HSRP, except that I used glbp
instead of the standby command. Let’s see which router is active now:
GLBP1(config-if)#do sh glbp
FastEthernet0/0 - Group 1
State is Active
2 state changes, last state change 00:02:29
Virtual IP address is 10.1.1.20
Hello time 3 sec, hold time 10 sec
Next hello sent in 0.140 secs
Redirect time 600 sec, forwarder time-out 14400 sec
Preemption disabled
Active is local
Standby is 10.1.1.2, priority 100 (expires in 8.444 sec)
Priority 110 (configured)
Weighting 100 (default 100), thresholds: lower 1, upper 100
Load balancing: round-robin
IP redundancy name is "GLBP_Test"
Group members:
001a.2fe7.4398 (10.1.1.1) local
001a.6ca1.1f48 (10.1.1.2)
There are 2 forwarders (1 active)
Forwarder 1
State is Active
1 state change, last state change 00:02:19
MAC address is 0007.b400.0101 (default)
Owner ID is 001a.2fe7.4398
Redirection enabled
Preemption enabled, min delay 30 sec
Active is local, weighting 100
Forwarder 2
State is Listen
MAC address is 0007.b400.0102 (learnt)
Owner ID is 001a.6ca1.1f48
Redirection enabled, 599.788 sec remaining (maximum 600 sec)
Time to live: 14399.788 sec (maximum 14400 sec)
Preemption enabled, min delay 30 sec
Active is 10.1.1.2 (primary), weighting 100 (expires in 9.788 sec)
GLBP1(config-if)#do sh glbp brief
Interface Grp Fwd Pri State Address Active router Standby route
Fa0/0 1 - 110 Active 10.1.1.20 local 10.1.1.2
Syslog 721
Fa0/0 1 1 7 Active 0007.b400.0101 local -
Fa0/0 1 2 7 Listen 0007.b400.0102 10.1.1.2 -
Wow—the show glbp command gave us a lot more output than the show standby com-
mand did because there’s more to GLBP than there is to HSRP! For example, you can see
that there are two forwarders listed under the Fwd heading.
And there are three lines associated with the show glbp brief command. The rst is the
AVG, the dash (-) that shows up under the Fwd column. Don’t get confused about the active
and listen states in the output because they just indicate which router will respond to ARP
requests for the virtual IP address. This example highlights GLBP1 as the active router. Make
sure to note the two MAC addresses under the Address column because they reveal the virtual
MAC addresses used by the routers.
Syslog
Reading system messages from a switch’s or router’s internal buffer is the most popular
and efcient method of seeing what’s going on with your network at a particular time. But
the best way is to log messages to a syslog server, which stores messages from you and can
even time-stamp and sequence them for you, and it’s easy to set up and congure!
Syslog allows you to display, sort, and even search messages, all of which makes it a
really great troubleshooting tool. The search feature is especially powerful because you
can use keywords and even severity levels. Plus, the server can email admins based on
the severity level of the message.
Network devices can be congured to generate a syslog message and forward it to various
destinations. These four examples are popular ways to gather messages from Cisco devices:
u Logging buffer (on by default)
u Console line (on by default)
u Terminal lines (using the terminal monitor command)
u Syslog server
As you already know, all system messages and debug output generated by the IOS go out
only the console port by default and are also logged in buffers in RAM. And you also know
that Cisco routers aren’t exactly shy about sending messages! To send message to the VTY
lines, use the terminal monitor command. We’ll also add a small conguration needed for
syslog, which I’ll show you soon in the conguration section.
Okay, so by default, we’d see something like this on our console line:
*Oct 21 17:33:50.565:%LINK-5-CHANGED:Interface FastEthernet0/0, changed
state to administratively down
*Oct 21 17:33:51.565:%LINEPROTO-5-UPDOWN:Line protocol on Interface
FastEthernet0/0, changed state to down
722 Chapter 17 u IP Services
And the Cisco router would send a general version of the message to the syslog server
that would be formatted into something like this:
Seq no:timestamp: %facility-severity-MNEMONIC:description
The system message format can be broken down in this way:
seq no This stamp logs messages with a sequence number, but not by default. If you want
this output, you’ve got to congure it.
Timestamp Data and time of the message or event, which again will show up only if
congured.
Facility The facility to which the message refers.
Severity A single-digit code from 0 to 7 that indicates the severity of the message.
MNEMONIC Text string that uniquely describes the message.
Description Text string containing detailed information about the event being reported.
The severity levels, from the most severe level to the least severe, are explained in
Table17.1. Informational is the default and will result in all messages being sent to the
buffers and console.
TABLE17.1 Severity levels
Severity Level Explanation
Emergency (severity 0) System is unusable.
Alert (severity 1) Immediate action is needed.
Critical (severity 2) Critical condition.
Error (severity 3) Error condition.
Warning (severity 4) Warning condition.
Notification (severity 5) Normal but significant condition.
Information (severity 6) Normal information message.
Debugging (severity 7) Debugging message.
If you are studying for your Cisco exam, you need to memorize Table17.1.
Syslog 723
Understand that only emergency-level messages will be displayed if you’ve congured
severity level 0. But if, for example, you opt for level 4 instead, level 0 through 4 will be
displayed, giving you emergency, alert, critical, error, and warning messages too. Level 7
is the highest-level security option and displays everything, but be warned that going with
it could have a serious impact on the performance of your device. So always use debugging
commands carefully with an eye on the messages you really need to meet your specic busi-
ness requirements!
Configuring and Verifying Syslog
As I said, Cisco devices send all log messages of the severity level you’ve chosen to the con-
sole. They’ll also go to the buffer, and both happen by default. Because of this, it’s good to
know that you can disable and enable these features with the following commands:
Router(config)#logging ?
Hostname or A.B.C.D IP address of the logging host
buffered Set buffered logging parameters
buginf Enable buginf logging for debugging
cns-events Set CNS Event logging level
console Set console logging parameters
count Count every log message and timestamp last occurrence
esm Set ESM filter restrictions
exception Limit size of exception flush output
facility Facility parameter for syslog messages
filter Specify logging filter
history Configure syslog history table
host Set syslog server IP address and parameters
monitor Set terminal line (monitor) logging parameters
on Enable logging to all enabled destinations
origin-id Add origin ID to syslog messages
queue-limit Set logger message queue size
rate-limit Set messages per second limit
reload Set reload logging level
server-arp Enable sending ARP requests for syslog servers when
first configured
source-interface Specify interface for source address in logging
transactions
trap Set syslog server logging level
userinfo Enable logging of user info on privileged mode enabling
Router(config)#logging console
Router(config)#logging buffered
724 Chapter 17 u IP Services
Wow—as you can see in this output, there are plenty of options you can use with the
logging command! The preceding conguration enabled the console and buffer to receive
all log message of all severities, and don’t forget that this is the default setting for all Cisco
IOS devices. If you want to disable the defaults, use the following commands:
Router(config)#no logging console
Router(config)#no logging buffered
I like leaving the console and buffers commands on in order to receive the logging info,
but that’s up to you. You can see the buffers with the show logging command here:
Router#sh logging
Syslog logging: enabled (11 messages dropped, 1 messages rate-limited,
0 flushes, 0 overruns, xml disabled, filtering disabled)
Console logging: level debugging, 29 messages logged, xml disabled,
filtering disabled
Monitor logging: level debugging, 0 messages logged, xml disabled,
filtering disabled
Buffer logging: level debugging, 1 messages logged, xml disabled,
filtering disabled
Logging Exception size (4096 bytes)
Count and timestamp logging messages: disabled
No active filter modules.
Trap logging: level informational, 33 message lines logged
Log Buffer (4096 bytes):
*Jun 21 23:09:37.822: %SYS-5-CONFIG_I: Configured from console by console
Router#
Notice that the default trap (message from device to NMS) level is informational, but
you can change this too. And now that you’ve seen the default system message format on
a Cisco device, I want to show you how you can also control the format of your messages
via sequence numbers and time stamps, which aren’t enabled by default. We’ll begin with a
basic, very simple example of how to congure a device to send messages to a syslog server,
demonstrated in Figure17.12.
FIGURE17.12 Messages sent to a syslog server
Syslog server
I want to look at the console messages
of the SF router from last night.
SF
Syslog 725
A syslog server saves copies of console messages and can time-stamp them for viewing at
a later time. This is actually pretty easy to congure and here’s how doing that would look
on the SF router:
SF(config)#logging 172.16.10.1
SF(config)#service timestamps log datetime msec
This is awesome—now all the console messages will be stored in one location to be
viewed at your convenience! I typically use the logging host ip_address command, but
logging IP_ a d dre ss command without the host keyword gets the same result.
I want to point out that even though I had the messages time-stamped in the congura-
tion associated with Figure17.12, the command service timestamps log datetime msec
doesn’t mean that I’ll know the messages’ exact time if I’m using default clock sources. To
make sure all devices are synchronized with the same time information, make sure you use
an NTP server.
We can limit the amount of messages sent to the syslog server, based on severity with the
following command:
SF(config)#logging trap ?
<0-7> Logging severity level
alerts Immediate action needed (severity=1)
critical Critical conditions (severity=2)
debugging Debugging messages (severity=7)
emergencies System is unusable (severity=0)
errors Error conditions (severity=3)
informational Informational messages (severity=6)
notifications Normal but significant conditions (severity=5)
warnings Warning conditions (severity=4)
<cr>
SF(config)#logging trap warnings
Notice that we can use either the number or the actual severity level name—and they are
in alphabetical order, not severity order, which makes it even harder to memorize the order!
(Thanks, Cisco!) Since I went with severity level 4, I’ll receive messages for levels 0 through 4.
Now let’s congure the router to use sequence numbers:
SF(config)#no service timestamps
SF(config)#service sequence-numbers
SF(config)#^Z
000038: %SYS-5-CONFIG_I: Configured from console by console
When you exit conguration mode, the router will send a message like the one shown in
the preceding code lines. Without the time stamps enabled, we’ll no longer see a time and
date, but we will see a sequence number.
726 Chapter 17 u IP Services
So we now have the following:
u Sequence number: 000038
u Facility: %SYS
u Severity level: 5
u MNEMONIC: CONFIG_I
u Description: Configured from console by console
I want to stress that of all of these, the security level is what you need to pay attention to
the most for the Cisco exams as well as for a means to control the amount of messages sent
to the syslog server!
SNMP
Although Simple Network Management Protocol (SNMP) certainly isn’t the oldest protocol
ever, it’s still pretty old, considering it was created way back in 1988 (RFC 1065)!
SNMP is an Application layer protocol that provides a message format for agents on
a variety of devices to communicate with network management stations (NMSs)—for
example, Cisco Prime or HP Openview. These agents send messages to the NMS station
which then either reads or writes information in the database stored on the NMS that’s
called a Management Information Base (MIB).
The NMS periodically queries or polls the SNMP agent on a device to gather and analyze
statistics via GET messages. End devices running SNMP agents would send an SNMP trap
to the NMS if a problem occurs. This is demonstrated in Figure17.13.
FIGURE17.13 SNMP GET and TRAP messages
NMS
My interface Gi0/1 went down!
It’s the end of the world!
HELP!
Gi0/1
TRAP!
Router
NMS
Check interface status!
Gi0/1
GET
Router
reply
SNMP 727
Admins can also use SNMP to provide some congurations to agents as well, called SET
messages. In addition to polling to obtain statistics, SNMP can be used for analyzing informa-
tion and compiling the results in a report or even a graph. Thresholds can be used to trigger a
notication process when exceeded. Graphing tools are used to monitor the CPU statistics of
Cisco devices like a core router. The CPU should be monitored continuously and the NMS can
graph the statistics. Notication will be sent when any threshold you’ve set has been exceeded.
SNMP has three versions, with version 1 being rarely, if ever implemented today. Here’s
a summary of these three versions:
SNMPv1 Supports plaintext authentication with community strings and uses only UDP.
SNMPv2c Supports plaintext authentication with MD5 or SHA with no encryption but
provides GET BULK, which is a way to gather many types of information at once and mini-
mize the number of GET requests. It offers a more detailed error message reporting method,
but it’s not more secure than v1. It uses UDP even though it can be congured to use TCP.
SNMPv3 Supports strong authentication with MD5 or SHA, providing condentiality
(encryption) and data integrity of messages via DES or DES-256 encryption between agents
and managers. GET BULK is a supported feature of SNMPv3, and this version also uses TCP.
Management Information Base (MIB)
With so many kinds of devices and so much data that can be accessed, there needed to be
a standard way to organize this plethora of data, so MIB to the rescue! A management
information base (MIB) is a collection of information that’s organized hierarchically and
can be accessed by protocols like SNMP. RFCs dene some common public variables, but
most organizations dene their own private branches along with basic SNMP standards.
Organizational IDs (OIDs) are laid out as a tree with different levels assigned by different
organizations, with top-level MIB OIDs belonging to various standards organizations.
Vendors assign private branches in their own products. Let’s take a look at Cisco’s
OIDs, which are described in words or numbers to locate a particular variable in the tree,
as shown in Figure17.14.
Luckily, you don’t need to memorize the OIDs in Figure17.14 for the Cisco exams!
I’ll use CPU as an example of a key thing to check at least every 5 minutes. We’ll examine
output from an SNMP application. It’s called snmpget and it comes from an NMS station.
Here’s the command from an NMS prompt on a Linux box running the SNMP
application:
[14:11][admin@nms]$~snmpget -v2c -c community 192.168.10.12
.1.3.6.1.4.1.9.2.1.58.0
SNMPv2-SMI::enterprises.9.2.1.58.0=INTEGER: 19
You must specify the version, the correct community string, the IP address of the network
device you’re querying, plus the OID number. The community string will authenticate your
access to the MIB database; and so the NMS can access the switch, the community string
denition on the NMS must match at least one of the three community string denitions on
the network devices.
728 Chapter 17 u IP Services
FIGURE17.14 Cisco’s MIB OIDs
.iso (1)
.org (3)
.dod (6)
.internet (1)
.private (4)
.enterprise (1)
.cisco (9)
.local variables (2)
.interface goup (2)
.cisco mgmt (9)
.cisco flash group (10)
Configuring SNMP
Conguring SNMP is a pretty straightforward process for which you only need a few
commands. These four steps are all you need to run through to congure a Cisco device
for SNMP access:
1. Enable SNMP read-write access to the router.
2. Configure SNMP contact information.
3. Configure SNMP location.
4. Configure an ACL to restrict SNMP access to the NMS hosts.
The only required conguration is the community string because the other three are
optional. Here’s an example of a typical SNMP router conguration:
Router(config)#snmp-server ?
chassis-id String to uniquely identify this chassis
community Enable SNMP; set community string and access privs
contact Text for mib object sysContact
context Create/Delete a context apart from default
drop Silently drop SNMP packets
enable Enable SNMP Traps or Informs
engineID Configure a local or remote SNMPv3 engineID
SNMP 729
group Define a User Security Model group
host Specify hosts to receive SNMP notifications
ifindex Enable ifindex persistence
inform Configure SNMP Informs options
location Text for mib object sysLocation
manager Modify SNMP manager parameters
packetsize Largest SNMP packet size
queue-length Message queue length for each TRAP host
source-interface Assign an source interface
system-shutdown Enable use of the SNMP reload command
tftp-server-list Limit TFTP servers used via SNMP
trap SNMP trap options
trap-source Assign an interface for the source address of all traps
trap-timeout Set timeout for TRAP message retransmissions
user Define a user who can access the SNMP engine
view Define an SNMP MIB view
Router(config)#snmp-server community ?
WORD SNMP community string
Router(config)#snmp-server community Todd ?
<1-99> Std IP accesslist allowing access with this community string
<1300-1999> Expanded IP accesslist allowing access with this community
string
WORD Access-list name
ipv6 Specify IPv6 Named Access-List
ro Read-only access with this community string
rw Read-write access with this community string
view Restrict this community to a named MIB view
<cr>
Router(config)#snmp-server community Todd rw
Router(config)#snmp-server location Boulder
Router(config)#snmp-server contact Todd Lammle
Router(config)#ip access-list standard Protect_NMS_Station
Router(config-std-nacl)#permit host 192.168.10.254
Entering the snmp-server command enables SNMPv1 on the Cisco device.
730 Chapter 17 u IP Services
You can enter the ACL directly in the SNMP conguration to provide security, using
either a number or a name. Here is an example:
Router(config)#snmp-server community Todd Protect_NMS_Station rw
Notice that even though there’s a boatload of conguration options under SNMP, you
only really need to work with a few of them to congure a basic SNMP trap setup on a
router. First, I chose the community name of Todd with RW access (read-write), which means
the NMS will be able to retrieve and modify MIB objects from the router. Location and con-
tact information comes in really handy for troubleshooting the conguration. Make sure you
understand that the ACL protects the NMS from access, not the devices with the agents!
Let’s dene the SNMP read and write options.
Read-only Gives authorized management stations read-access to all objects in the MIB
except the community strings and doesn’t allow write-access
Read-write Gives authorized management stations read-and write-access to all objects in
the MIB but doesn’t allow access to the community strings
There are still more ways to gather information from Cisco devices, and next, we’ll
explore a Cisco proprietary method of gathering statistics on internetwork devices.
NetFlow
SNMP can be a powerful tool to help you manage and troubleshoot your network, but
Cisco knew it would be very helpful for engineers to be able to track TCP/IP ows within
the network as well.
That’s why we have NetFlow as an application for collecting IP trafc information.
Cisco compares NetFlow informational reports to receiving a phone bill with detailed call
information to track calls, call frequency, and even calls that shouldn’t have been made at
all! A more current analogy would be the IRS and certain additional government “alphabet
agencies” watching who has talked to whom, when, and for how long!
Cisco IOS NetFlow efciently provides a key set of services for IP applications, including net-
work trafc accounting for baselining, usage-based network billing for consumers of network
services, network design and planning, general network security, and DoS and DDoS monitoring
capabilities as well as general network monitoring. Figure17.15 shows basic ow monitoring via
Cisco NetFlow with the latest version, version 9, which is called Flexible NetFlow.
FIGURE17.15 Basic Flexible NetFlow
172.16.10.254/24 172.16.20.254/24
(listens for NetFlow on port 9996)
NetFlow Collector
Egress
Ingress
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NetFlow 731
In Figure17.15, let’s assume that a host has connected to a server located in the ctitious
Sales VLAN using Telnet. NetFlow can monitor the application by counting packets, bytes
sent and received, and so on, and then send this information to a NetFlow collector.
NetFlow Overview and Flows
Understand that NetFlow is completely transparent to the users in the network, including
all end stations and applications, and you don’t need to run it on all your routers. Actually,
you shouldn’t because there’s denitely overhead when using NetFlow because it requires
memory for storing information in cache on the device. NetFlow enables near real-time
visualization and analysis of recorded and aggregated ow data. You can specify the router,
the aggregation scheme, and the time interval for when you want to view and then retrieve
the relevant data and sort it into bar charts, pie charts, and so on. The components used
with NetFlow include a router enabled with NetFlow and a NetFlow collector.
Service providers use NetFlow to do the following:
u Efficiently measuring who is using network service and for which purpose
u Accounting and charging back according to the resource utilizing level
u Using the measured information for more effective network planning so that resource
allocation and deployment are well aligned with customer requirements
u Using the information to better structure and customize the set of available applica-
tions and services to meet user needs and customer service requirements
Moreover, there are different types of analyzers available to gather NetFlow statistics
and analyze the trafc on your network by showing the following:
u Major users of the network, meaning top talkers, top listeners, top protocols, and so on
u Websites that are routinely visited, plus what’s been downloaded
u Who’s generating the most traffic and using excessive bandwidth
u Descriptions of bandwidth needs for an application as well as your available bandwidth
NetFlow is built around TCP/IP communication for statistical record-keeping using the
concept of a ow. A ow is a unidirectional stream of packets between a source and desti-
nation host or system. With an understanding of TCP/IP, you can gure out that NetFlow
is using socket information, meaning source and destination IP addresses and source and
destination port numbers. But there are a few more elds that NetFlow uses. Here is a list
of commonly used NetFlow ows:
u Source IP address
u Destination IP address
u Source port number
u Destination port number
u Layer 3 protocol field
u Type of Service (ToS) marking
u Input logical interface
732 Chapter 17 u IP Services
As mentioned, the rst four listings are the sockets used between the source and destina-
tion host, which identify the application. The protocol eld identies the data the packet is
carrying, and ToS in the IPv4 header describes how QoS rules are applied to the packets in
the ow. If a packet has a key eld that’s different from another packet, it’s considered to
belong to another ow. You congure NetFlow on the router’s interfaces, and that’s exactly
what I’ll show you next—how to congure and then verify NetFlow.
Configuring NetFlow
These four factors must be completed to properly implement NetFlow on a router:
u Configure NetFlow data capture by configuring ingress (incoming) and egress (outgo-
ing) packets.
u Configure NetFlow data export by specifying the IP address of the NetFlow collector
and the UDP port the collector listens for.
u Configure the NetFlow data export version by specifying the version of NetFlow, with
version 9 being the most current.
u Verify NetFlow by analyzing the exported data on a host running a NetFlow collection
engine or by using show command on the NetFlow router.
Here’s an example of conguring NetFlow on the SF router:
SF(config)#int fa0/0
SF(config-if)#ip flow ingress
SF(config-if)#ip flow egress
SF(config-if)#exit
SF(config)#ip flow-export destination 172.16.20.254 9996
SF(config)#ip flow-export version ?
1
5
9
SF(config)#ip flow-export version 9
SF(config)#ip flow-export source loopback 0
First I congured the Fast Ethernet 0/0 interface of the Corp router as both my ingress
and egress interface, which tells the router to capture NetFlow data for ows on the interface.
After that, I congured the NetFlow collector’s IP address, as well as the version. Notice that
I could opt to congure only versions 1, 5, and 9. Version 9 includes all the elds I mentioned
already, plus MPLS and IPv6 information and ports. The loopback interface denes the source
IP address of packets sent to the collector.
To verify NetFlow, you need to verify that the correct interfaces in the correct direction
have been congured, starting with the show ip flow interface command like this:
SF#sh ip flow interface
FastEthernet0/0
NetFlow 733
ip flow ingress
ip flow egress
Sure enough! The correct interface of Fast Ethernet 0/0 is congured with the ingress
and egress command. Now I’ll check that I’ve correctly congured the export parameters
via the show ip flow export command:
SF#sh ip flow export
Flow export v9 is enabled for main cache
Exporting flows to 172.16.20.254 (9996) 172.16.20.254 (9996)
Exporting using source interface Loopback0
Version 9 flow records
43 flows exported in 15 udp datagrams
[output cut]
Notice that the destination port is 9996. This is the Cisco default port number on which
the NetFlow collectors listen for NetFlow packets. I can use the sh ip cache flow command
to verify my ows by examining the information stored on a router directly, which will show
that I’m actually collecting packets:
SF#sh ip cache flow
IP packet size distribution (161 total packets):
[output cut]
IP Flow Switching Cache, 278544 bytes
1 active, 4095 inactive, 1 added
215 ager polls, 0 flow alloc failures
Active flows timeout in 30 minutes
Inactive flows timeout in 15 seconds
IP Sub Flow Cache, 21640 bytes
1 active, 1023 inactive, 1 added, 1 added to flow
0 alloc failures, 0 force free
1 chunk, 1 chunk added
last clearing of statistics never
Protocol Total Flows Packets Bytes Packets Active(Sec) Idle(Sec)
-------- Flows /Sec /Flow /Pkt /Sec /Flow /Flow
TCP-Telnet 14 0.0 19 58 0.1 6.5 11.7
TCP-WWW 8 0.0 9 108 0.1 2.5 1.7
SrcIf SrcIPaddress DstIf DstIPaddress Pr SrcP DstP Pkts
Fa0/0 172.16.10.1 gig0/1 255.255.255.255 11 0044 0050 1161
Nice: You can see that packets are truly being received—1161 so far—and the bottom
lines show that the router is collecting ow for Telnet and HTTP. You can also see the
source interface, source IP, destination interface, and source and destination ports in hex
(50 is 80 in hex). It’s important to remember that the show ip cache flow command pro-
vides a summary of the NetFlow statistics, including which protocols are in use.
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734 Chapter 17 u IP Services
Summary
Now you can implement and congure three different redundancy protocols! You learned
that HSRP and VRRP are very similar but that VRRP is the open standard and can be used
on any vendor’s equipment. HSRP is Cisco proprietary and has a couple of features and con-
gurations not available in VRRP—specically, the preempt and interface tracking features.
You were then introduced to GLBP, which is also a Cisco proprietary protocol that can
provide true load balancing for all of your clients, even if they are in the same subnet.
SNMP is an Application layer protocol that provides a message format for agents on a
variety of devices to communicate to network management stations (NMSs). I discussed the
basic information you need to use syslog and SNMP, that is, conguration and verication.
Last, I discussed Cisco NetFlow. Cisco IOS NetFlow efciently provides a key set of
services for IP applications, including network trafc accounting for baselining, usage-
based network billing for consumers of network services, network design and planning,
general network security, and DoS and DDoS monitoring capabilities, as well as general
network monitoring.
Exam Essentials
Remember the three FHRPs. HSRP, VVRP, and GLBP are all FHRPs, with HSRP and
GLBP being Cisco proprietary protocols.
Remember how load balancing works with HSRP and GLBP. HSRP load balance per
VLAN’s trunk links and GLBP can perform per-host load balancing.
Remember how to verify HSRP and GLBP. Use the show standby command with HSRP
and show glbp with GLBP.
Remember the eight severity levels you can configure with the logging trap command. They
are not listed in severity, but in alphabetical order, which makes it slightly more difcult
to remember: 1=alerts, 2=critical, 7=debugging, 0=emergencies, 3=errors, 6=information,
5=notications, 4=warnings.
Remember the differences between SNMPv2 and SNMPv3. SNMPv2 uses UDP but can
use TCP, however, v2 still sends data to the NMS station in clear text. SNMPv3 uses TCP
and authenticates users, plus can use ACLs in the SNMP strings to protect the NMS station
from unauthorized use.
Understand what Cisco’s NetFlow is used for. Cisco IOS NetFlow efciently provides a
key set of services for IP applications, including network trafc accounting for baselining,
usage-based network billing for consumers of network services, network design and plan-
ning, general network security, and DoS and DDoS monitoring capabilities as well as gen-
eral network monitoring.
Written Lab 3 735
Written Lab 3
The answers to this lab can be found in Appendix A, “Answers to Written Labs.”
1. What command is used to verify your active router with HSRP?
2. Which command is used to verify your forwarding routers with GLBP?
3. Which FHRP can perform true load-balancing within the same subnet?
4. Which Cisco protocol can efficiently provide a key set of services for IP applications,
including network traffic accounting?
5. Which syslog severity level results in notification-level messages?
6. Which protocol can request and receive information from a monitored device on the
network?
7. Which syslog severity level results in warning-level messages?
8. Which command provides a summary of the NetFlow statistics, including which
protocols are in use?
9. What command is used to configure a host to send messages to a syslog server?
10. Which router assigns a virtual MAC address to each member of the GLBP group?
736 Chapter 17 u IP Services
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional questions,
please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. How can you efficiently restrict the read-only function of a requesting SNMP manage-
ment station based on the IP address?
A. Place an ACL on the logical control plane.
B. Place an ACL on the line when configuring the RO community string.
C. Place an ACL on the VTY line.
D. Place an ACL on all router interfaces.
2. Why would you use GLBP over HSRP and VRRP?
A. GLBP is an open standard protocol.
B. GLBP uses a virtual IP address, whereas HSRP and VRRP must have a static
default gateway on each host.
C. GLBP provides true load-balancing within a single subnet.
D. GLBP is easily configured and propagated by DHCP to clients.
E. There is no reason to choose GLBP.
3. What is the default priority setting on an HSRP router?
A. 25
B. 50
C. 100
D. 125
4. You want to add a sequence number on your console message on a Cisco router. Which
command will you use?
A. service sequence-numbers
B. service timestamps
C. service number-sequence
D. sequence service messages
Review Questions 737
5. You want to collect details about network traffic patterns on your network, including
source and destination addresses and protocols used. Which of the following will you use?
A. SNMPv3
B. Syslogv2
C. NetFlow 9
D. logging host ip_address
6. You want to send a console message to a syslog server, but you only want to send status
messages of 4 and lower. Which of the following commands will you use?
A. logging trap emergencies
B. logging trap errors
C. logging trap debugging
D. logging trap notifications
E. logging trap critical
F. logging trap warnings
G. logging trap alerts
7. In an FHRP network, which feature allows you to keep track of outside interfaces of
an FHRP configured router?
A. Interface up/down status from show ip int brief command
B. show nvram:show running-config in RAM
C. Interface tracking within the HSRP configuration
D. HSRP’s inability to verify or understand the status of outside interfaces
8. Which command enables you to view a summary of the NetFlow statistics of the pro-
tocols on a router?
A. show ip flow
B. show ip cache flow
C. show ip netflow
D. show ip flow interface gi0/1
9. Why should you use NetFlow over SNMP or syslog? (Choose three.)
A. NetFlow allows you to send configuration information to a remote network device.
B. NetFlow allows you to learn who is using which network resources.
C. NetFlow allows you to perform network capacity planning.
D. NetFlow allows you to retrieve accounting of network resource usage.
E. NetFlow receives alerts from remote routers if an interface fails.
738 Chapter 17 u IP Services
10. You want to send a console message to a syslog server, but you only want to send status
messages of 5 and lower. Which of the following commands will you use?
A. logging trap emergencies
B. logging trap errors
C. logging trap debugging
D. logging trap notifications
E. logging trap critical
F. logging trap warnings
G. logging trap alerts
11. Which two of the following are true regarding the output of the show log command?
(Choose two.)
Corp#sh log
Syslog logging: enabled (11 messages dropped, 0 messages rate-limited,
0 flushes, 0 overrunds, xml disabled, filtering disabled)
Console logging: level debugging, 3013 messages logged, xml disabled,
filtering disabled
Monitor logging: level debugging, 0 messages logged, xml disabled,
filtering disabled
Buffer logging: level debugging, 582 messages logged, xml disabled,
filtering disabled
Logging Exception size (4096 bytes)
Count and timestamp logging messages: disabled
Trap logging: level debugging, 2523 message lines logged
Logging to 172.16.10.1
A. The router is configured for trap level 6.
B. The router is configured for trap level 7.
C. Debugging messages will be sent only to 172.16.10.1.
D. Debugging messages will not be sent to the buffers.
12. What are the two pieces needed for NetFlow to gather data? (Choose two.)
A. An SNMP NMS station
B. Collector
C. Syslog configured
D. NetFlow-configured router
Review Questions 739
13. You want to send a console message to a syslog server, but you only want to send status
messages of 3 and lower. Which of the following commands will you use?
A. logging trap emergencies
B. logging trap errors
C. logging trap debugging
D. logging trap notifications
E. logging trap critical
F. logging trap warnings
G. logging trap alerts
14. Which device will send the ARP replies to clients with GLBP?
A. The HSRP active router
B. The router with the highest IP address
C. The AVR router
D. The VRRP router
E. The passive listening router
15. Which command will you type in to see the active and listening router with GLBP?
A. show standby
B. sh glbp
C. show active
D. show glbp infromation
16. You want to send a console message to a syslog server, but you only want to send status
messages of 7 and lower. Which of the following commands will you use?
A. logging trap emergencies
B. logging trap errors
C. logging trap debugging
D. logging trap notifications
E. logging trap critical
F. logging trap warnings
G. logging trap alerts
740 Chapter 17 u IP Services
17. HSRP is load-balancing with three routers and three VLANs. Each router is configured
with subinterfaces, one for each VLAN. How many routers in the group will forward
traffic for each VLAN?
A. All the routers in the same group
B. Up to two routers per VLAN
C. One router per VLAN
D. HSRP must be configured with the GLBP command in order to do any load-
balancing.
18. You want to send a console message to a syslog server, but you only want to send status
messages of 6 and lower. Which of the following commands will you use?
A. logging trap emergencies
B. logging trap errors
C. logging trap debugging
D. logging trap notifications
E. logging trap critical
F. logging trap warnings
G. logging trap informational
19. Which SNMP version provides authentication, data integrity, and encryption?
A. SNMPv1
B. SNMPv2
C. SNMPv3
D. SNMPv4
E. SNMPv6
20. Which statements are true about both HSRP and GLBP? (Choose two.)
A. VRRP should always be used if possible instead of HSRP and GLBP.
B. HSRP is proprietary, and GLBP should be used with multiple vendors.
C. HSRP can load-balance based on VLANs for which they are active.
D. GLBP can have multiple forwarders with the same subnet/VLAN.
E. HSRP can load-balance on the same LAN.
Chapter
18
Troubleshooting IP,
IPv6, and VLANs
THE FOLLOWING ICND2 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 Troubleshooting
■Identify and correct common network problems
■Troubleshoot and Resolve routing issues
■Routing is enabled
■Routing table is correct
■Correct path selection
■Troubleshoot and resolve interVLAN routing problems
■Connectivity
■Encapsulation
■Subnet
■Native VLAN
■Port mode trunk status
In this chapter, especially at rst, it’s going to seem like we’re
going over lot of the same ground and concepts already cov-
ered in other chapters. The reason for this is that troubleshoot-
ing is such a major focus of the Cisco ICND1 and ICND2 objectives that I’ve got to make
sure I’ve guided you through this vital topic in depth. If not, then I just haven’t done all
I can to really set you up for success! So to make that happen, we’re going to thoroughly
examine troubleshooting with IP, IPv6, and virtual LANs (VLANs) now. And I can’t stress
the point enough that you absolutely must have a solid, fundamental understanding of IP
and IPv6 routing as well as a complete understanding of VLANs and trunking nailed down
tight if you’re going to win at this!
To help you do that, I’ll be using different scenarios to walk you through the Cisco
trouble shooting steps to correctly solve the problems you’re likely to be faced with. Although
it’s hard to tell exactly what the ICND1 and ICND2 exams will throw at you, you can
read and completely understand the objectives so that no matter what, you’ll be prepared,
equipped, and up to the challenge. The way to do this is by building upon a really strong
foundation, including being skilled at troubleshooting. This chapter is precisely designed,
and exactly what you need, to seriously help solidify your troubleshooting foundation.
The chapters following this one will focus on EIGRP and OSPF, and each has its own
troubleshooting section. Troubleshooting WAN protocols will be thoroughly covered in
Chapter 7. In this chapter we’ll concentrate solely on IP, IPv6, and VLAN troubleshooting.
To find up-to-the-minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
Troubleshooting IP Network Connectivity
Let’s start out by taking a moment for a short and sweet review of IP routing. Always remem-
ber that when a host wants to transmit a packet, IP looks at the destination address and deter-
mines if it’s a local or remote request. If it’s determined to be a local request, IP just broadcasts
a frame out on the local network looking for the local host using an ARP request. If it’s a
remote request, the host sends an ARP request to the default gateway to discover the MAC
address of the router.
Once the hosts have the default gateway address, they’ll send each packet that needs to
be transmitted to the Data Link layer for framing, and newly framed packets are then sent
Troubleshooting IP Network Connectivity 743
out on the local collision domain. The router will receive the frame and remove the packet
from the frame, and IP will then parse the routing table looking for the exit interface on the
router. If the destination is found in the routing table, it will packet-switch the packet to
the exit interface. At this point, the packet will be framed with new source and destination
MAC addresses.
Okay, with that short review in mind, what would you say to someone who called you say-
ing they weren’t able to get to a server on a remote network? What’s the rst thing you would
have this user do (besides reboot Windows) or that you would do yourself to test network con-
nectivity? If you came up with using the Ping program, that’s a great place to start. The Ping
program is a great tool for nding out if a host is alive on the network with a simple ICMP
echo request and echo reply. But being able to ping the host as well as the server doesn’t guar-
antee that all is well in the network! Keep in mind that there’s more to the Ping program than
just being used as a quick and simple testing protocol.
To be prepared for the exam objectives, it’s a great idea to get used to connecting to
various routers and pinging from them. Of course, pinging from a router is not as good
as pinging from the host reporting the problem, but that doesn’t mean we can’t isolate
problems from the routers themselves.
Let’s use Figure18.1 as a basis to run through some troubleshooting scenarios.
FIGURE18.1 Troubleshooting scenario
10.1.1.1 Fa0/0 Fa0/0
Fa0/1
.1
Fa0/1
.254
172.16.20.1
10.1.1.254 172.16.20.2
10.1.1.10 172.16.20.254
PC1 Server1
192.168.10.0/24 Switch
S1 S2
R1 R2
In this rst scenario, a manager calls you and says that he cannot log in to Server1 from
PC1. Your job is to nd out why and x it. The Cisco objectives are clear on the trouble-
shooting steps you need to take when a problem has been reported, and here they are:
1. Check the cables to find out if there’s a faulty cable or interface in the mix and verify
the interface’s statistics.
2. Make sure that devices are determining the correct path from the source to the destina-
tion. Manipulate the routing information if needed.
3. Verify that the default gateway is correct.
744 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
4. Verify that name resolution settings are correct.
5. Verify that there are no access control lists (ACLs) blocking traffic.
In order to effectively troubleshoot this problem, we’ll narrow down the possibilities by
process of elimination. We’ll start with PC1 and verify that it’s congured correctly and
also that IP is working correctly.
There are four steps for checking the PC1 conguration:
1. Test that the local IP stack is working by pinging the loopback address.
2. Test that the local IP stack is talking to the Data Link layer (LAN driver) by pinging
the local IP address.
3. Test that the host is working on the LAN by pinging the default gateway.
4. Test that the host can get to remote networks by pinging remote Server1.
Let’s check out the PC1 conguration by using the ipconfig command, or ifconfig on
a Mac:
C:\Users\Todd Lammle>ipconfig
Windows IP Configuration
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . : localdomain
Link-local IPv6 Address . . . . . : fe80::64e3:76a2:541f:ebcb%11
IPv4 Address. . . . . . . . . . . : 10.1.1.10
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 10.1.1.1
We can also check the route table on the host with the route print command to see if it
truly does know the default gateway:
C:\Users\Todd Lammle>route print
[output cut]
IPv4 Route Table
=======================================================================
Active Routes:
Network Destination Netmask Gateway Interface Metric
0.0.0.0 0.0.0.0 10.1.1.10 10.1.1.1 10
[output cut]
Between the output of the ipconfig command and the route print command, we can
be assured that the hosts are aware of the correct default gateway.
Troubleshooting IP Network Connectivity 745
For the Cisco objectives, it’s extremely important to be able to check and
verify the default gateway on a host and also that this address matches the
router’s interface!
So, let’s verify that the local IP stack is initialized by pinging the loopback address now:
C:\Users\Todd Lammle>ping 127.0.0.1
Pinging 127.0.0.1 with 32 bytes of data:
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Ping statistics for 127.0.0.1:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
This rst output conrms the IP address and congured default gateway of the host, and
then I veried the fact that the local IP stack is working. Our next move is to verify that the
IP stack is talking to the LAN driver by pinging the loopback address.
C:\Users\Todd Lammle>ping 10.1.1.10
Pinging 10.1.1.10 with 32 bytes of data:
Reply from 10.1.1.10: bytes=32 time<1ms TTL=128
Reply from 10.1.1.10: bytes=32 time<1ms TTL=128
Reply from 10.1.1.10: bytes=32 time<1ms TTL=128
Reply from 10.1.1.10: bytes=32 time<1ms TTL=128
Ping statistics for 10.1.1.10:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
And now that we know the local stack is solid and the IP stack is communicating to the
LAN driver, it’s time to check our local LAN connectivity by pinging the default gateway:
C:\Users\Todd Lammle>ping 10.1.1.1
746 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
Pinging 10.1.1.1 with 32 bytes of data:
Reply from 10.1.1.1: bytes=32 time<1ms TTL=128
Reply from 10.1.1.1: bytes=32 time<1ms TTL=128
Reply from 10.1.1.1: bytes=32 time<1ms TTL=128
Reply from 10.1.1.1: bytes=32 time<1ms TTL=128
Ping statistics for 10.1.1.1:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
Looking good! I’d say our host is in good shape. Let’s try to ping the remote server next
to see if our host is actually getting off the local LAN to communicate remotely:
C:\Users\Todd Lammle>ping 172.16.20.254
Pinging 172.16.20.254 with 32 bytes of data:
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Ping statistics for 172.16.20.254:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),
Well, looks like we’ve conrmed local connectivity but not remote connectivity, so we’re
going to have to dig deeper to isolate our problem. But rst, and just as important, it’s key
to make note of what we can rule out at this point:
1. The PC is configured with the correct IP address and the local IP stack is working.
2. The default gateway is configured correctly and the PC’s default gateway configuration
matches the router interface IP address.
3. The local switch is working because we can ping through the switch to the router.
4. We don’t have a local LAN issue, meaning our Physical layer is good because we can
ping the router. If we couldn’t ping the router, we would need to verify our physical
cables and interfaces.
Let’s see if we can narrow the problem down further using the traceroute command:
C:\Users\Todd Lammle>tracert 172.16.20.254
Troubleshooting IP Network Connectivity 747
Tracing route to 172.16.20.254 over a maximum of 30 hops
1 1 ms 1 ms <1 ms 10.1.1.1
2 * * * Request timed out.
3 * * * Request timed out.
Well, we didn’t get beyond our default gateway, so let’s go over to R2 and see if we can
talk locally to the server:
R2#ping 172.16.20.254
Pinging 172.16.20.254 with 32 bytes of data:
Reply from 172.16.20.254: bytes=32 time<1ms TTL=128
Reply from 172.16.20.254: bytes=32 time<1ms TTL=128
Reply from 172.16.20.254: bytes=32 time<1ms TTL=128
Reply from 172.16.20.254: bytes=32 time<1ms TTL=128
Ping statistics for 172.16.20.254:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),
Okay, we just eliminated a local LAN problem by connecting to Server1 from the R2
router, so we’re good there. Let’s summarize what we know so far:
1. PC1 is configured correctly.
2. The switch located on the 10.1.1.0 LAN is working.
3. PC1’s default gateway is configured correctly.
4. R2 can communicate to the Server1 so we don’t have a remote LAN issue.
But something is still clearly wrong, so what should we check now? Now would be a
great time to verify the Server1 IP conguration and make sure the default gateway is con-
gured correctly. Let’s take a look:
C:\Users\Server1>ipconfig
Windows IP Configuration
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . : localdomain
Link-local IPv6 Address . . . . . : fe80::7723:76a2:e73c:2acb%11
748 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
IPv4 Address. . . . . . . . . . . : 172.16.20.254
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 172.16.20.1
Okay—the Server1 conguration looks good and the R2 router can ping the server, so
it seems that the server’s local LAN is solid, the local switch is working, and there are no
cable or interface issues. But let’s zoom in on interface Fa0/0 on R2 and talk about what to
expect if there were errors on this interface:
R2#sh int fa0/0
FastEthernet0/0 is up, line protocol is up
[output cut]
Full-duplex, 100Mb/s, 100BaseTX/FX
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:05, output 00:00:01, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
1325 packets input, 157823 bytes
Received 1157 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog
0 input packets with dribble condition detected
2294 packets output, 244630 bytes, 0 underruns
0 output errors, 0 collisions, 3 interface resets
347 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
4 lost carrier, 0 no carrier
0 output buffer failures, 0 output buffers swapped out
You’ve got to be able to analyze interface statistics to nd problems there if they exist, so
let’s pick out the important factors relevant to meeting that challenge effectively now.
Speed and duplex settings Good to know that the most common cause of interface errors
is a mismatched duplex mode between two ends of an Ethernet link. This is why it’s so
important to make sure that the switch and its hosts (PCs, router interfaces, etc.) have the
same speed setting. If not, they just won’t connect. And if they have mismatched duplex
settings, you’ll receive a legion of errors, which cause nasty performance issues, intermit-
tent connectivity—even total loss of communication!
Troubleshooting IP Network Connectivity 749
Using autonegotiation for speed and duplex is a very common practice, and it’s enabled by
default. But if this fails for some reason, you’ll have to set the conguration manually like this:
Switch(config)#int gi0/1
Switch(config-if)#speed ?
10 Force 10 Mbps operation
100 Force 100 Mbps operation
1000 Force 1000 Mbps operation
auto Enable AUTO speed configuration
Switch(config-if)#speed 1000
Switch(config-if)#duplex ?
auto Enable AUTO duplex configuration
full Force full duplex operation
half Force half-duplex operation
Switch(config-if)#duplex full
If you have a duplex mismatch, a telling sign is that the late collision counter will increment.
Input queue drops If the input queue drops counter increments, this signies that more
trafc is being delivered to the router that it can process. If this is consistently high, try to
determine exactly when these counters are increasing and how the events relate to CPU
usage. You’ll see the ignored and throttle counters increment as well.
Output queue drops This counter indicates that packets were dropped due to interface
congestion, leading to packet drops and queuing delays. When this occurs, applications
like VoIP will experience performance issues. If you observe this constantly incrementing,
consider QoS.
Input errors Input errors often indicate high errors such as CRCs. This can point to cabling
problems, hardware issues, or duplex mismatches.
Output errors This is the total number of frames that the port tried to transmit when an
issue such as a collision occurred.
We’re going to move on in our troubleshooting process of elimination by analyzing the
routers’ actual congurations. Here’s R1’s routing table:
R1>sh ip route
[output cut]
Gateway of last resort is 192.168.10.254 to network 0.0.0.0
S* 0.0.0.0/0 [1/0] via 192.168.10.254
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.1.1.0/24 is directly connected, FastEthernet0/0
L 10.1.1.1/32 is directly connected, FastEthernet0/0
750 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.10.0/24 is directly connected, FastEthernet0/1
L 192.168.10.1/32 is directly connected, FastEthernet0/1
This actually looks pretty good! Both of our directly connected networks are in the table
and we can conrm that we have a default route going to the R2 router. So now let’s verify
the connectivity to R2 from R1:
R1>sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.1.1.1 YES manual up up
FastEthernet0/1 192.168.10.1 YES manual up up
Serial0/0/0 unassigned YES unset administratively down down
Serial0/1/0 unassigned YES unset administratively down down
R1>ping 192.168.10.254
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.10.254, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
This looks great too! Our interfaces are correctly congured with the right IP address
and the Physical and Data Link layers are up. By the way, I also tested layer 3 connectivity
by pinging the R2 Fa0/1 interface.
Since everything looks good so far, our next step is to check into the status of R2’s
interfaces:
R2>sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 172.16.20.1 YES manual up up
FastEthernet0/1 192.168.10.254 YES manual up up
R2>ping 192.168.10.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.10.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
Well, everything still checks out at this point. The IP addresses are correct and the Physical
and Data Link layers are up. I also tested the layer 3 connectivity with a ping to R1, so we’re
all good so far. We’ll examine the routing table next:
R2>sh ip route
[output cut]
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 1 subnets
Troubleshooting IP Network Connectivity 751
S 10.1.1.0 is directly connected, FastEthernet0/0
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.16.20.0/24 is directly connected, FastEthernet0/0
L 172.16.20.1/32 is directly connected, FastEthernet0/0
192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.10.0/24 is directly connected, FastEthernet0/1
L 192.168.10.254/32 is directly connected, FastEthernet0/1
Okay—we can see that all our local interfaces are in the table, as well as a static route
to the 10.1.1.0 network. But do you see the problem? Look closely at the static route. The
route was entered with an exit interface of Fa0/0, and the path to the 10.1.1.0 network is
out Fa0/1! Aha! We’ve found our problem! Let’s x R2:
R2#config t
R2(config)#no ip route 10.1.1.0 255.255.255.0 fa0/0
R2(config)#ip route 10.1.1.0 255.255.255.0 192.168.10.1
That should do it. Let’s verify from PC1:
C:\Users\Todd Lammle>ping 172.16.20.254
Pinging 172.16.20.254 with 32 bytes of data:
Reply from 172.16.20.254: bytes=32 time<1ms TTL=128
Reply from 172.16.20.254: bytes=32 time<1ms TTL=128
Reply from 172.16.20.254: bytes=32 time<1ms TTL=128
Reply from 172.16.20.254: bytes=32 time<1ms TTL=128
Ping statistics for 172.16.20.254
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
Our snag appears to be solved, but just to make sure, we really need to verify with a
higher-level protocol like Telnet:
C:\Users\Todd Lammle>telnet 172.16.20.254
Connecting To 172.16.20.254...Could not open connection to the host, on
port 23: Connect failed
Okay, that’s not good! We can ping to the Server1, but we can’t telnet to it. In the past,
I’ve veried that telnetting to this server worked, but it’s still possible that we have a failure
on the server side. To nd out, let’s verify our network rst, starting at R1:
R1>ping 172.16.20.254
Type escape sequence to abort.
752 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
Sending 5, 100-byte ICMP Echos to 172.16.20.254, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms
R1>telnet 172.16.20.254
Trying 172.16.20.254 ...
% Destination unreachable; gateway or host down
This is some pretty ominous output! Let’s try from R2 and see what happens:
R2#telnet 172.16.20.254
Trying 172.16.20.254 ... Open
User Access Verification
Password:
Oh my—I can ping the server from a remote network, but I can’t telnet to it, but the
local router R2 can! These factors eliminate the server being a problem since I can telnet to
the server when I’m on the local LAN.
And we know we don’t’ have a routing problem because we xed that already. So what’s
next? Let’s check to see if there’s an ACL on R2:
R2>sh access-lists
Extended IP access list 110
10 permit icmp any any (25 matches)
Seriously? What a loopy access list to have on a router! This ridiculous list permits
ICMP, but that’s it. It denies everything except ICMP due to the implicit deny ip any any
at the end of every ACL. But before we uncork the champagne, we need to see if this foolish
list has been applied to our interfaces on R2 to conrm that this is really our problem:
R2>sh ip int fa0/0
FastEthernet0/0 is up, line protocol is up
Internet address is 172.16.20.1/24
Broadcast address is 255.255.255.255
Address determined by setup command
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Outgoing access list is 110
Inbound access list is not set
Troubleshooting IP Network Connectivity 753
There it is—that’s our problem all right! In case you’re wondering why R2 could telnet
to Server1, it’s because an ACL lters only packets trying to go through the router—not
packets generated at the router. Let’s get to work and x this:
R2#config t
R2(config)#no access-list 110
I just veried that I can telnet from PC1 to Server1, but let’s try telnetting from R1 again:
R1#telnet 172.16.20.254
Trying 172.16.20.254 ... Open
User Access Verification
Password:
Nice—looks like we’re set, but what about using the name?
R1#telnet Server1
Translating "Server1"...domain server (255.255.255.255)
% Bad IP address or host name
Well, we’re not all set just yet. Let’s x R1 so that it can provide name resolution:
R1(config)#ip host Server1 172.16.20.254
R1(config)#^Z
R1#telnet Server1
Trying Server1 (172.16.20.254)... Open
User Access Verification
Password:
Great—things are looking good from the router, but if the customer can’t telnet to the
remote host using the name, we’ve got to check the DNS server to conrm connectivity and
for the correct entry to the server. Another option would be to congure the local host table
manually on PC1.
The last thing to do is to check the server to see if it’s responding to HTTP requests via
the telnet command, believe it or not! Here’s an example:
R1#telnet 172.16.20.254 80
Trying 172.16.20.254, 80 ... Open
754 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
Yes—nally! Server1 is responding to requests on port 80, so we’re in the clear. Now, let’s
mix things up a little by adding IPv6 to our network and work through the same trouble-
shooting steps.
Troubleshooting IPv6 Network
Connectivity
I’m going to be straight with you: there isn’t a lot that’s going to be much different
between this section and the process you just went through with the IPv4 troubleshooting
steps. Except regarding the addressing of course! So other than that key factor, we’ll take
the same approach, using Figure18.2, specically because I really want to highlight the
differences associated with IPv6. So the problem scenario I’m going to use will also stay
the same: PC1 cannot communicate to Server1.
I want to point out that this is not an “introduction to IPv6” chapter, so I’m assuming
you’ve got some IPv6 fundamentals down.
FIGURE18.2 IPv6 troubleshooting scenario
Fa0/0 Fa0/0
Fa0/1 Fa0/1
2001:db8:3c4d:1:21a:6dff:fe37:a443
Fe80::21a:6dff:fe64:9b2
PC1 Server1
2001:db8:3c4d:3:ac3b:2ef:1823:8938 2001:db8:3c4d:1:a14c:8c33:2d1:be3d
2001:db8:3c4d:2::64 Switch
S1 S2
R1 R2
2001:db8:3c4d:2:21a:6dff:fe37:a44f
Fe80::21a:6dff:fe37:a44f
2001:db8:3c4d:2:21a:6dff:fe64:9b3
Fe80::21a:6dff:fe64:9b3
2001:db8:3c4d:3:21a:6dff:fe37:a44e
Fe80::21a:6dff:fe37:a44e
Notice that I documented both the link-local and global addresses assigned to each
router interface in Figure18.2. We need both in order to troubleshoot, so right away, you
can see that things get a bit more complicated because of the longer addresses and the fact
that there are multiple addresses per interface involved!
But before we start troubleshooting the IPv6 network in Figure18.2, I want to
refresh your memory on the ICMPv6 protocol, which is an important protocol in our
troubleshooting arsenal.
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Troubleshooting IPv6 Network Connectivity 755
ICMPv6
IPv4 used the ICMP workhorse for lots of tasks, including error messages like destination
unreachable and troubleshooting functions like Ping and Traceroute. ICMPv6 still does
those things for us, but unlike its predecessor, the v6 avor isn’t implemented as a separate
layer 3 protocol. Instead, it’s an integrated part of IPv6 and is carried after the basic IPv6
header information as an extension header.
ICMPv6 is used for router solicitation and advertisement, for neighbor solicitation and
advertisement (i.e., nding the MAC addresses for IPv6 neighbors), and for redirecting the
host to the best router (default gateway).
Neighbor Discovery (NDP)
ICMPv6 also takes over the task of nding the address of other devices on the local link.
The Address Resolution Protocol is used to perform this function for IPv4, but that’s been
renamed Neighbor Discovery (ND or NDP) in ICMPv6. This process is now achieved via
a multicast address called the solicited node address because all hosts join this multicast
group upon connecting to the network.
Neighbor discovery enables these functions:
u Determining the MAC address of neighbors
u Router solicitation (RS) FF02::2
u Router advertisements (RA) FF02::1
u Neighbor solicitation (NS)
u Neighbor advertisement (NA)
u Duplicate address detection (DAD)
The part of the IPv6 address designated by the 24 bits farthest to the right is added
to the end of the multicast address FF02:0:0:0:0:1:FF/104. When this address is queried,
the corresponding host will send back its layer 2 address. Devices can nd and keep track
of other neighbor devices on the network in pretty much the same way. When I talked
about RA and RS messages earlier in the CCENT chapters, and told you that they use
multicast trafc to request and send address information, that too is actually a function
of ICMPv6—specically, neighbor discovery.
In IPv4, the protocol IGMP was used to allow a host device to tell its local router that
it was joining a multicast group and would like to receive the trafc for that group. This
IGMP function has been replaced by ICMPv6, and the process has been renamed multicast
listener discovery.
With IPv4, our hosts could have only one default gateway congured, and if that router
went down we had to x the router, change the default gateway, or run some type of vir-
tual default gateway with other protocols created as a solution for this inadequacy in IPv4.
Figure18.3 shows how IPv6 devices nd their default gateways using neighbor discovery.
756 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
FIGURE18.3 Router solicitation (RS) and router advertisement (RA)
FF02::1 Use me! (RA)
Internet
Corp
Corp2
FF02::1 Use me! (RA)
FF02::2 All routers respond! (RS)
IPv6 hosts send a router solicitation (RS) onto their data link asking for all routers to
respond, and they use the multicast address FF02::2 to achieve this. Routers on the same
link respond with a unicast to the requesting host, or with a router advertisement (RA)
using FF02::1.
But that’s not all! Hosts also can send solicitations and advertisements between them-
selves using a neighbor solicitation (NS) and neighbor advertisement (NA), as shown in
Figure18.4.
FIGURE18.4 Neighbor solicitation (NS) and neighbor advertisement (NA)
NDP: NS
I need your MAC!
NDP: NA
Here is my MAC
Remember that RA and RS gather or provide information about routers and NS and NA
gather information about hosts. Also, remember that a “neighbor” is a host on the same
data link or VLAN.
With that foundation review in mind, here are the troubleshooting steps we’ll progress
through in our investigation:
1. Check the cables because there might be a faulty cable or interface. Verify interfacs
statistics.
2. Make sure that devices are determining the correct path from the source to the destina-
tion. Manipulate the routing information if needed.
3. Verify that the default gateway is correct.
Troubleshooting IPv6 Network Connectivity 757
4. Verify that name resolution settings are correct, and especially for IPv6, make sure the
DNS server is reachable via IPv4 and IPv6.
5. Verify that there are no ACLs that are blocking traffic.
In order to troubleshoot this problem, we’ll use the same process of elimination, beginning
with PC1. We must verify that it’s congured correctly and that IP is working properly. Let’s
start by pinging the loopback address to verify the IPv6 stack:
C:\Users\Todd Lammle>ping ::1
Pinging ::1 with 32 bytes of data:
Reply from ::1: time<1ms
Reply from ::1: time<1ms
Reply from ::1: time<1ms
Reply from ::1: time<1ms
Well, the IPv6 stack checks out, so let’s ping the Fa0/0 of R1, which PC1 is directly
connected to on the same LAN, starting with the link-local address:
C:\Users\Todd Lammle>ping fe80::21a:6dff:fe37:a44e
Pinging fe80:21a:6dff:fe37:a44e with 32 bytes of data:
Reply from fe80::21a:6dff:fe37:a44e: time<1ms
Reply from fe80::21a:6dff:fe37:a44e: time<1ms
Reply from fe80::21a:6dff:fe37:a44e: time<1ms
Reply from fe80::21a:6dff:fe37:a44e: time<1ms
Next, we’ll ping the global address on Fa0/0:
C:\Users\Todd Lammle>ping 2001:db8:3c4d:3:21a:6dff:fe37:a44e
Pinging 2001:db8:3c4d:3:21a:6dff:fe37:a44e with 32 bytes of data:
Reply from 2001:db8:3c4d:3:21a:6dff:fe37:a44e: time<1ms
Reply from 2001:db8:3c4d:3:21a:6dff:fe37:a44e: time<1ms
Reply from 2001:db8:3c4d:3:21a:6dff:fe37:a44e: time<1ms
Reply from 2001:db8:3c4d:3:21a:6dff:fe37:a44e: time<1ms
Okay—looks like PC1 is congured and working on the local LAN to the R1 router, so
we’ve conrmed the Physical, Data Link, and Network layers between the PC1 and the R1
router Fa0/0 interface.
Our next move is to check the local connection on Server1 to the R2 router to verify that
LAN. First we’ll ping the link-local address of the router from Server1:
C:\Users\Server1>ping fe80::21a:6dff:fe64:9b2
758 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
Pinging fe80::21a:6dff:fe64:9b2 with 32 bytes of data:
Reply from fe80::21a:6dff:fe64:9b2: time<1ms
Reply from fe80::21a:6dff:fe64:9b2: time<1ms
Reply from fe80::21a:6dff:fe64:9b2: time<1ms
Reply from fe80::21a:6dff:fe64:9b2: time<1ms
And next, we’ll ping the global address of Fa0/0 on R2:
C:\Users\Server1>ping 2001:db8:3c4d:1:21a:6dff:fe37:a443
Pinging 2001:db8:3c4d:1:21a:6dff:fe37:a443 with 32 bytes of data:
Reply from 2001:db8:3c4d:1:21a:6dff:fe37:a443: time<1ms
Reply from 2001:db8:3c4d:1:21a:6dff:fe37:a443: time<1ms
Reply from 2001:db8:3c4d:1:21a:6dff:fe37:a443: time<1ms
Reply from 2001:db8:3c4d:1:21a:6dff:fe37:a443: time<1ms
Let’s quickly summarize what we know at this point:
1. By using the ipconfig /all command on PC1 and Server1, I was able to document
their global and link-local IPv6 addresses.
2. We know the IPv6 link-local addresses of each router interface.
3. We know the IPv6 global address of each router interface.
4. We can ping from PC1 to router R1’s Fa0/0 interface.
5. We can ping from Server1 to router R2’s Fa0/0 interface.
6. We can eliminate a local problem on both LANs.
From here, we’ll go to PC1 and see if we can route to Server1:
C:\Users\Todd Lammle>tracert 2001:db8:3c4d:1:a14c:8c33:2d1:be3d
Tracing route to 2001:db8:3c4d:1:a14c:8c33:2d1:be3d over a maximum of 30 hops
1 Destination host unreachable.
Okay, now that’s not good. Looks like we might have a routing problem. And on this little
network, we’re doing static IPv6 routing, so getting to the bottom of things will denitely take
a little effort! But before we start looking into our potential routing issue, let’s check the link
between R1 and R2. We’ll ping R2 from R1 to test the directly connected link.
The rst thing you need to do before attempting to ping between routers is verify your
addresses—yes, verify them again! Let’s check out both routers, then try pinging from R1
to R2:
R1#sh ipv6 int brief
FastEthernet0/0 [up/up]
Troubleshooting IPv6 Network Connectivity 759
FE80::21A:6DFF:FE37:A44E
2001:DB8:3C4D:3:21A:6DFF:FE37:A44E
FastEthernet0/1 [up/up]
FE80::21A:6DFF:FE37:A44F
2001:DB8:3C4D:2:21A:6DFF:FE37:A44F
R2#sh ipv6 int brief
FastEthernet0/0 [up/up]
FE80::21A:6DFF:FE64:9B2
2001:DB8:3C4D:1:21A:6DFF:FE37:A443
FastEthernet0/1 [up/up]
FE80::21A:6DFF:FE64:9B3
2001:DB8:3C4D:2:21A:6DFF:FE64:9B3
R1#ping 2001:DB8:3C4D:2:21A:6DFF:FE64:9B3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to ping 2001:DB8:3C4D:2:21A:6DFF:FE64:9B3, timeout
is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 0/2/8 ms
In the preceding output, you can see that I now have the IPv6 addresses for both the R1 and
R2 directly connected interfaces. The output also shows that I used the Ping program to verify
layer 3 connectivity. Just as with IPv4, we need to resolve the logical (IPv6) address to a MAC
address in order to communicate on the local LAN. But unlike IPv4, IPv6 doesn’t use ARP—it
uses ICMPv6 neighbor solicitations instead—so after the successful ping, we can now see the
neighbor resolution table on R1:
R1#sh ipv6 neighbors
IPv6 Address Age Link-layer Addr State Interface
FE80::21A:6DFF:FE64:9B3 0 001a.6c46.9b09 DELAY Fa0/1
2001:DB8:3C4D:2:21A:6DFF:FE64:9B3 0 001a.6c46.9b09 REACH Fa0/1
Let’s take a minute to talk about the possible states that a resolved address shows us:
INCMP (incomplete) Address resolution is being performed on the entry. A neighbor
solicitation message has been sent, but the neighbor message has not yet been received.
REACH (reachable) Positive conrmation has been received conrming that the path to
the neighbor is functioning correctly. REACH is good!
STALE The state is STALE when the interface has not communicated within the neighbor
reachable time frame. The next time the neighbor communicates, the state will change back
to REACH.
760 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
DELAY Occurs after the STALE state, when no reachability conrmation has been received
within what’s known as the DELAY_FIRST_PROBE_TIME. This means that the path was
functioning but it hasn’t had communication within the neighbor reachable time frame.
PROBE When in PROBE state, the congured interface is resending a neighbor solicita-
tion and waiting for a reachability conrmation from a neighbor.
We can verify our default gateway with IPv6 with the ipconfig command like this:
C:\Users\Todd Lammle>ipconfig
Connection-specific DNS Suffix . : localdomain
IPv6 Address. . . . . . . . . . . : 2001:db8:3c4d:3:ac3b:2ef:1823:8938
Temporary IPv6 Address. . . . . . : 2001:db8:3c4d:3:2f33:44dd:211:1c3d
Link-local IPv6 Address . . . . . : fe80::ac3b:2ef:1823:8938%11
IPv4 Address. . . . . . . . . . . : 10.1.1.10
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : Fe80::21a:6dff:fe37:a44e%11
10.1.1.1
It’s important to understand that the default gateway will be the link-local address of the
router, and in this case, we can see that the address the host learned is truly the link-local
address of the Fa0/0 interface of R1. The %11 is just used to identify an interface and isn’t
used as part of the IPv6 address.
Temporary IPv6 Addresses
The temporary IPv6 address, listed under the unicast IPv6 address as, “2001:db8:3c4d:3:2f3
3:44dd:211:1c3d,” was created by Windows to provide privacy from the EUI-64 format. This
creates a global address from your host without using your MAC address by generating a
random number for the interface and hashing it, which is then appended to the /64 prefix
from the router. You can disable this feature with the following commands:
netsh interface ipv6 set global randomizeidentifiers=disabled
netsh interface ipv6 set privacy state-disabled
In addition to the ipconfig command, we can use the command netsh interface ipv6
show neighbor to verify our default gateway address:
C:\Users\Todd Lammle>netsh interface ipv6 show neighbor
[output cut]
Troubleshooting IPv6 Network Connectivity 761
Interface 11: Local Area Connection
Internet Address Physical Address Type
-------------------------------------------- ----------------- -----------
2001:db8:3c4d:3:21a:6dff:fe37:a44e 00-1a-6d-37-a4-4e (Router)
Fe80::21a:6dff:fe37:a44e 00-1a-6d-37-a4-4e (Router)
ff02::1 33-33-00-00-00-01 Permanent
ff02::2 33-33-00-00-00-02 Permanent
ff02::c 33-33-00-00-00-0c Permanent
ff02::16 33-33-00-00-00-16 Permanent
ff02::fb 33-33-00-00-00-fb Permanent
ff02::1:2 33-33-00-01-00-02 Permanent
ff02::1:3 33-33-00-01-00-03 Permanent
ff02::1:ff1f:ebcb 33-33-ff-1f-eb-cb Permanent
I’ve checked the default gateway addresses on Server1 and they are cor-
rect. They should be, because this is provided directly from the router with
an ICMPv6 RA (router advertisement) message. The output for that verifi-
cation is not shown.
Let’s establish the information we have right now:
1. Our PC1 and Server1 configurations are working and have been verified.
2. The LANs are working and verified, so there is no Physical layer issue.
3. The default gateways are correct.
4. The link between the R1 and R2 routers is working and verified.
So all this tells us is that it’s now time to check our routing tables! We’ll start with the
R1 router:
R1#sh ipv6 route
C 2001:DB8:3C4D:2::/64 [0/0]
via FastEthernet0/1, directly connected
L 2001:DB8:3C4D:2:21A:6DFF:FE37:A44F/128 [0/0]
via FastEthernet0/1, receive
C 2001:DB8:3C4D:3::/64 [0/0]
via FastEthernet0/0, directly connected
L 2001:DB8:3C4D:3:21A:6DFF:FE37:A44E/128 [0/0]
via FastEthernet0/0, receive
L FF00::/8 [0/0]
via Null0, receive
762 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
All we can see in the output is the two directly connected interfaces congured on the
router, and that won’t help us send IPv6 packets to the 2001:db8:3c4d:1::/64 subnet off of
Fa0/0 on R2. So let’s nd out what R2 can tell us:
R2#sh ipv6 route
C 2001:DB8:3C4D:1::/64 [0/0]
via FastEthernet0/0, directly connected
L 2001:DB8:3C4D:1:21A:6DFF:FE37:A443/128 [0/0]
via FastEthernet0/0, receive
C 2001:DB8:3C4D:2::/64 [0/0]
via FastEthernet0/1, directly connected
L 2001:DB8:3C4D:2:21A:6DFF:FE64:9B3/128 [0/0]
via FastEthernet0/1, receive
S 2001:DB8:3C4D:3::/64 [1/0]
via 2001:DB8:3C4D:2:21B:D4FF:FE0A:539
L FF00::/8 [0/0]
via Null0, receive
Now we’re talking—that tells us a lot more than R1’s table did! We have both of our
directly connected congured LANs, Fa0/0 and Fa0/1, right there in the routing table, as
well as a static route to 2001:DB8:3C4D:3::/64, which is the remote LAN Fa0/0 off of R1,
which is good. Now let’s x the route problem on R1 by adding a route that gives us access
to the Server1 network and then move on to VLANs and trunking:
R1(config)#ipv6 route ::/0 fastethernet 0/1 FE80::21A:6DFF:FE64:9B3
I want to point out that I didn’t need to make the default route as difcult as I did. I
entered both the exit interface and next-hop link-local address when just the exit interface
or next-hop global addresses would be mandatory, but not the link-local.
Next, we’ll verify that we can now ping from PC1 to Server1:
C:\Users\Todd Lammle>ping 2001:db8:3c4d:1:a14c:8c33:2d1:be3d
Pinging 2001:db8:3c4d:1:a14c:8c33:2d1:be3d with 32 bytes of data:
Reply from 2001:db8:3c4d:1:a14c:8c33:2d1:be3d: time<1ms
Reply from 2001:db8:3c4d:1:a14c:8c33:2d1:be3d: time<1ms
Reply from 2001:db8:3c4d:1:a14c:8c33:2d1:be3d: time<1ms
Reply from 2001:db8:3c4d:1:a14c:8c33:2d1:be3d: time<1ms
Sweet—we’re looking golden with this particular scenario! But know that it is still
possible to have name resolution issues. If that were the case, you would just need to
check your DNS server or local host table.
Moving on in the same way we did in the IPv4 troubleshooting section, it’s a good time
to check into your ACLs, especially if you’re still having a problem after troubleshooting
Troubleshooting VLAN Connectivity 763
all your local LANs and all other potential routing issues. To do that, just use the show
ipv6 access-lists command to verify all congured ACLs on a router and the show ipv6
interface command to verify if an ACL is attached to an interface. Once you’ve conrmed
that your ACLs all make sense, you’re good to go!
Troubleshooting VLAN Connectivity
You know by now that VLANs are used to break up broadcast domains in a layer 2
switched network. You’ve also learned that we assign ports on a switch into a VLAN
broadcast domain by using the switchport access vlan command.
The access port carries trafc for a single VLAN that the port is a member of. If mem-
bers of one VLAN want to communicate to members in the same VLAN that are located
on a different switch, then a port between the two switches needs to be either congured
to be a member of this single VLAN or congured as a trunk link, which passes informa-
tion on all VLANs by default.
We’re going to use Figure18.5 to reference as we go through the procedures for trouble-
shooting VLAN and trunking.
FIGURE18.5 VLAN connectivity
Gi0/14
Gi0/2Gi0/1
PC2
VLAN 10
192.168.10.2
PC1
VLAN 10
192.168.10.1
Gi0/3
Gi0/14
Gi0/13 Gi0/13
VLAN 10
192.168.10.3
S1 S2
PC3
I’m going to begin with VLAN troubleshooting and then move on to trunk troubleshooting.
VLAN Troubleshooting
A couple of key times to troubleshoot VLANs are when and if you lose connectivity between
hosts and when you’re conguring new hosts into a VLAN but they’re not working.
Here are the steps we’ll follow to troubleshoot VLANs:
1. Verify the VLAN database on all your switches.
2. Verify your content addressable memory (CAM) table.
3. Verify that your port VLAN assignments are configured correctly.
764 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
And here’s a list of the commands we’ll be using in the following sections:
Show vlan
Show mac address-table
Show interfaces interface switchport
switchport access vlan vlan
VLAN Troubleshooting Scenario
A manager calls and says they can’t communicate to the new sales team member that just
connected to the network. How would you proceed to solve this issue? Well, because the
sales hosts are in VLAN 10, we’ll begin with step 1 and verify that our databases on both
switches are correct.
First, I’ll use the show vlan or show vlan brief command to check if the expected
VLAN is actually in the database. Here’s a look at the VLAN database on S1:
S1#sh vlan
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Gi0/3, Gi0/4, Gi0/5, Gi0/6
Gi0/7, Gi0/8, Gi0/9, Gi0/10
Gi0/11, Gi0/12, Gi0/13, Gi0/14
Gi0/15, Gi0/16, Gi0/17, Gi0/18
Gi0/19, Gi0/20, Gi0/21, Gi0/22
Gi0/23, Gi0/24, Gi0/25, Gi0/26
Gi0/27, Gi0/28
10 Sales active Gi0/1, Gi0/2
20 Accounting active
26 Automation10 active
27 VLAN0027 active
30 Engineering active
170 VLAN0170 active
501 Private501 active
502 Private500 active
[output cut]
This output shows that VLAN 10 is in the local database and that Gi0/1 and Gi0/2 are
associated to VLAN 10.
So next, we’ll go to step 2 and verify the CAM with the show mac address-table
command:
S1#sh mac address-table
Troubleshooting VLAN Connectivity 765
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
All 0100.0ccc.cccc STATIC CPU
[output cut]
1 000d.2830.2f00 DYNAMIC Gi0/24
1 0021.1c91.0d8d DYNAMIC Gi0/13
1 0021.1c91.0d8e DYNAMIC Gi0/14
1 b414.89d9.1882 DYNAMIC Gi0/17
1 b414.89d9.1883 DYNAMIC Gi0/18
1 ecc8.8202.8282 DYNAMIC Gi0/15
1 ecc8.8202.8283 DYNAMIC Gi0/16
10 001a.2f55.c9e8 DYNAMIC Gi0/1
10 001b.d40a.0538 DYNAMIC Gi0/2
Total Mac Addresses for this criterion: 29
Okay—know that your switch will show quite a few MAC addresses assigned to the
CPU at the top of the output; those MAC addresses are used by the switch to manage the
ports. The very rst MAC address listed is the base MAC address of the switch and used
by STP in the bridge ID. In the preceding output, we can see that there there are two MAC
addresses associated with VLAN 10 and that it was dynamically learned. We can also
establish that this MAC address is associated to Gi0/1. S1 looks really good!
Let’s take a look at S2 now. First, let’s conrm that port PC3 is connected and check its
conguration. I’ll use the command show interfaces interface switchport command to
do that:
S2#sh interfaces gi0/3 switchport
Name: Gi0/3
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: On
Access Mode VLAN: 10 (Inactive)
Trunking Native Mode VLAN: 1 (default)
[output cut]
Okay—we can see that the port is enabled and that it’s set to dynamic desirable. This
means that if it connects to another Cisco switch, it will desire to trunk on that link. But
766 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
keep in mind that we’re using it as an access port, which is conrmed by the operational
mode of static access. At the end of the output, the text shows Access Mode VLAN: 10
(Inactive). This is not a good thing! Let’s examine S2’s CAM and see what we nd out:
S2#sh mac address-table
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
All 0100.0ccc.cccc STATIC CPU
[output cut]
1 001b.d40a.0538 DYNAMIC Gi0/13
1 0021.1bee.a70d DYNAMIC Gi0/13
1 b414.89d9.1884 DYNAMIC Gi0/17
1 b414.89d9.1885 DYNAMIC Gi0/18
1 ecc8.8202.8285 DYNAMIC Gi0/16
Total Mac Addresses for this criterion: 26
Referring back to Figure18.5, we can see that the host is connected to Gi0/3. The problem
here is that we don’t see a MAC address dynamically associated to Gi0/3 in the MAC address
table. So what do we know so far that can help us? Well rst, we can see that Gi0/3 is cong-
ured into VLAN 10, but that VLAN is inactive. Second, the host off of Gi0/3 doesn’t appear
in the CAM table. Now would be a good time to take a look at the VLAN database like this:
S2#sh vlan brief
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Gi0/1, Gi0/2, Gi0/4, Gi0/5
Gi0/6, Gi0/7, Gi0/8, Gi0/9
Gi0/10, Gi0/11, Gi0/12, Gi0/13
Gi0/14, Gi0/15, Gi0/16, Gi0/17
Gi0/18, Gi0/19, Gi0/20, Gi0/21
Gi0/22, Gi0/23, Gi0/24, Gi0/25
Gi0/26, Gi0/27, Gi0/28
26 Automation10 active
27 VLAN0027 active
30 Engineering active
170 VLAN0170 active
[output cut]
Troubleshooting VLAN Connectivity 767
Look at that: there is no VLAN 10 in the database! Clearly the problem, but also an
easy one to x by simply creating the VLAN in the database:
S2#config t
S2(config)#vlan 10
S2(config-vlan)#name Sales
That’s all there is to it. Now let’s check the CAM again:
S2#sh mac address-table
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
All 0100.0ccc.cccc STATIC CPU
[output cut]
1 0021.1bee.a70d DYNAMIC Gi0/13
10 001a.6c46.9b09 DYNAMIC Gi0/3
Total Mac Addresses for this criterion: 22
We’re good to go—the MAC address off of Gi0/3 shows in the MAC address table
congured into VLAN 10.
That was pretty straightforward, but if the port had been assigned to the wrong VLAN,
I would have used the switch access vlan command to correct the VLAN membership.
Here’s an example of how to do that:
S2#config t
S2(config)#int gi0/3
S2(config-if)#switchport access vlan 10
S2(config-if)#do sh vlan
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Gi0/1, Gi0/2, Gi0/4, Gi0/5
Gi0/6, Gi0/7, Gi0/8, Gi0/9
Gi0/10, Gi0/11, Gi0/12, Gi0/13
Gi0/14, Gi0/15, Gi0/16, Gi0/17
Gi0/18, Gi0/19, Gi0/20, Gi0/21
Gi0/22, Gi0/23, Gi0/24, Gi0/25
Gi0/26, Gi0/27, Gi0/28
10 Sales active Gi0/3
768 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
Okay, great—we can see that our port Gi0/3 is in the VLAN 10 membership. Now let’s
try to ping from PC1 to PC3:
PC1#ping 192.168.10.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.10.3, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
No luck, so let’s see if PC1 can ping PC2:
PC1#ping 192.168.10.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.10.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
PC1#
That worked! I can ping a host that’s a member of the same VLAN connected to the same
switch, but I can’t ping to a host on another switch that’s a member of the same VLAN, which
is VLAN 10. To get to the bottom of this, let’s quickly summarize what we’ve learned so far:
1. We know that the VLAN database is now correct on each switch.
2. The MAC address table shows the ARP entries for each host as well as a connection to
each switch.
3. We’ve verified that our VLAN memberships are now correct on all the ports we’re using.
But since we still can’t ping to a host on another switch, we need to start checking out
the connections between our switches.
Trunk Troubleshooting
You’ll need to troubleshoot trunk links when you lose connectivity between hosts that are
in the same VLAN but are located on different switches. Cisco refers to this as “VLAN
leaking.” Seems to me we are leaking VLAN 10 between switches somehow.
These are the steps we’ll take to troubleshoot VLANs:
1. Verify that the interface configuration is set to the correct trunk parameters.
2. Verify that the ports are configured correctly.
3. Verify the native VLAN on each switch.
And here are the commands we’ll use to perform trunk troubleshooting:
Show interfaces trunk
Show vlan
Troubleshooting VLAN Connectivity 769
Show interfaces interface trunk
Show interfaces interface switchport
Show dtp interface interface
switchport mode
switchport mode dynamic
switchport trunk native vlan vlan
Okay, let’s get started by checking ports Gi0/13 and Gi0/14 on each switch because
these are what the gure is showing as forming the connection between our switches. We’ll
start with the show interfaces trunk command:
S1>sh interfaces trunk
S2>sh interfaces trunk
Not a scrap of output—that’s denitely a bad sign! Let’s take another look at the show vlan
output on S1 and see what we can nd out:
S1>sh vlan brief
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Gi0/3, Gi0/4, Gi0/5, Gi0/6
Gi0/7, Gi0/8, Gi0/9, Gi0/10
Gi0/11, Gi0/12, Gi0/13, Gi0/14
Gi0/15, Gi0/16, Gi0/17, Gi0/18
Gi0/19, Gi0/20, Gi0/21, Gi0/22
Gi0/23, Gi0/24, Gi0/25, Gi0/26
Gi0/27, Gi0/28
10 Sales active Gi0/1, Gi0/2
20 Accounting active
[output cut]
Nothing new from when we checked it a few minutes ago, but look there under
VLAN 1—we can see interfaces Gi/013 and Gi0/14. This means that our ports between
switches are members of VLAN 1 and will pass only VLAN 1 frames!
Typically I’ll tell my students that if you type the show vlan command, you’re really
typing the nonexistent “show access ports” command since this output shows inter-
faces in access mode but doesn’t show the trunk interfaces. This means that our ports
between switches are access ports instead of trunk ports, so they’ll pass information
about only VLAN 1.
770 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
Let’s go back over to the S2 switch to verify and see which port interfaces Gi0/13 and
Gi0/14 are members of:
S2>sh vlan brief
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Gi0/1, Gi0/2, Gi0/4, Gi0/5
Gi0/6, Gi0/7, Gi0/8, Gi0/9
Gi0/10, Gi0/11, Gi0/12, Gi0/13
Gi0/14, Gi0/15, Gi0/16, Gi0/17
Gi0/18, Gi0/19, Gi0/20, Gi0/21
Gi0/22, Gi0/23, Gi0/24, Gi0/25
Gi0/26, Gi0/27, Gi0/28
10 Sales active Gi0/3
Again, as with S1, the links between switches are showing in the output of the show vlan
command, which means that they are not trunk ports. We can use the show interfaces
interface switchport command to verify this as well:
S1#sho interfaces gi0/13 switchport
Name: Gi0/13
Switchport: Enabled
Administrative Mode: dynamic auto
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
This output tells us that interface Gi0/13 is in dynamic auto mode. But its operational
mode is static access, meaning it’s not a trunk port. We can look closer at its trunking
capabilities with the show interfaces interface trunk command:
S1#sh interfaces gi0/1 trunk
Port Mode Encapsulation Status Native vlan
Gi0/1 auto negotiate not-trunking 1
[output cut]
Sure enough—the port is not trunking, but we already knew that. Now we know it again.
Notice that we can see that native VLAN is set to VLAN 1, which is the default native VLAN.
This means that VLAN 1 is the default VLAN for untagged trafc.
Troubleshooting VLAN Connectivity 771
Now, before we check the native VLAN on S2 to verify that there isn’t a mismatch, I want
to point out a key fact about trunking and how we would get these ports between switches to
do that.
Many Cisco switches support the Cisco proprietary Dynamic Trunking Protocol
(DTP), which is used to manage automatic trunk negotiation between switches. Cisco
recommends that you don’t allow this and to congure your switch ports manually
instead. I agree with them!
Okay, with that in mind, let’s check out our switch port Gi0/13 on S1 and view its DTP
status. I’ll use the show dtp interface interface command to view the DTP statistics:
S1#sh dtp interface gi0/13
DTP information for GigabitEthernet0/13:
TOS/TAS/TNS: ACCESS/AUTO/ACCESS
TOT/TAT/TNT: NATIVE/NEGOTIATE/NATIVE
Neighbor address 1: 00211C910D8D
Neighbor address 2: 000000000000
Hello timer expiration (sec/state): 12/RUNNING
Access timer expiration (sec/state): never/STOPPED
Did you notice that our port GI0/13 from S1 to S2 is an access port congured to auto-
negotiate using DTP? That’s interesting, and I want to delve a bit deeper into the different
port congurations and how they affect trunking capabilities to clarify why.
Access Trunking is not allowed on a port set to access mode.
Auto Will trunk to neighbor switch only if the remote port is set to on or to desirable mode.
This creates the trunk based on the DTP request from the neighboring switch.
Desirable This will trunk with all port modes except access. Ports set to dynamic desirable
will communicate via DTP that the interface is attempting to become a trunk if the neighbor-
ing switch interface is able to become a trunk.
Nonegotiate No DTP frames are generated from the interface. Can only be used if the
neighbor interface is manually set as trunk or access.
Trunk (on) Trunks with all switch port modes except access. Automatically enables trunk-
ing regardless of the state of the neighboring switch and regardless of any DTP requests.
Let’s check out the different options available on the S1 switch with the switchport mode
dynamic command:
S1(config-if)#switchport mode ?
access Set trunking mode to ACCESS unconditionally
dot1q-tunnel set trunking mode to TUNNEL unconditionally
dynamic Set trunking mode to dynamically negotiate access or trunk mode
private-vlan Set private-vlan mode
trunk Set trunking mode to TRUNK unconditionally
772 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
S1(config-if)#switchport mode dynamic ?
auto Set trunking mode dynamic negotiation parameter to AUTO
desirable Set trunking mode dynamic negotiation parameter to DESIRABLE
From interface mode, use the switch mode trunk command to turn trunking on. You
can also use the switch mode dynamic command to set the port to auto or desirable trunk-
ing modes. To turn off DTP and any type of negotiation, use the switchport nonegotiate
command.
Let’s take a look at S2 and see if we can gure out why our two switches didn’t create
a trunk:
S2#sh int gi0/13 switchport
Name: Gi0/13
Switchport: Enabled
Administrative Mode: dynamic auto
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: On
Okay—we can see that the port is in dynamic auto and that it’s operating as an access
port. Let’s look into this further:
S2#sh dtp interface gi0/13
DTP information for GigabitEthernet0/3:
DTP information for GigabitEthernet0/13:
TOS/TAS/TNS: ACCESS/AUTO/ACCESS
TOT/TAT/TNT: NATIVE/NEGOTIATE/NATIVE
Neighbor address 1: 000000000000
Neighbor address 2: 000000000000
Hello timer expiration (sec/state): 17/RUNNING
Access timer expiration (sec/state): never/STOPPED
Do you see the problem? Don’t be fooled—it’s not that they’re running in access mode; it’s
because two ports in dynamic auto will not form a trunk! This is a really common problem to
look for since most Cisco switches ship in dynamic auto. The other issue you need to be aware
of, as well as check for, is the frame-tagging method. Some switches run 802.1q, some run
both 802.1q and Inter-Switch Link (ISL) routing, so be sure the tagging method is compatible
between all of your switches!
It’s time to x our problem on the trunk ports between S1 and S2. All we need to do is to
just x one side of each link since dynamic auto will trunk with a port set to desirable or on:
S2(config)#int gi0/13
S2(config-if)#switchport mode dynamic desirable
23:11:37:%LINEPROTO-5-UPDOWN:Line protocol on Interface GigabitEthernet0/13,
changed state to down
Troubleshooting VLAN Connectivity 773
23:11:37:%LINEPROTO-5-UPDOWN:Line protocol on Interface Vlan1, changed state to
down
23:11:40:%LINEPROTO-5-UPDOWN:Line protocol on Interface GigabitEthernet0/13,
changed state to up
23:12:10:%LINEPROTO-5-UPDOWN:Line protocol on Interface Vlan1, changed state to up
S2(config-if)#do show int trunk
Port Mode Encapsulation Status Native vlan
Gi0/13 desirable n-isl trunking 1
[output cut]
Nice—it worked! With one side in Auto and the other now in Desirable, DTPs will be
exchanged and they will trunk. Notice in the preceding output that the mode of S2’s Gi0/13
link is desirable and that the switches actually negotiated ISL as a trunk encapsulation—
go gure! But don’t forget to notice the native VLAN. We’ll work on the frame-tagging
method and native VLAN in a minute, but rst, let’s congure our other link:
S2(config-if)#int gi0/14
S2(config-if)#switchport mode dynamic desirable
23:12:%LINEPROTO-5-UPDOWN:Line protocol on Interface GigabitEthernet0/14, changed
state to down
23:12:%LINEPROTO-5-UPDOWN:Line protocol on Interface GigabitEthernet0/14, changed
state to up
S2(config-if)#do show int trunk
Port Mode Encapsulation Status Native vlan
Gi0/13 desirable n-isl trunking 1
Gi0/14 desirable n-isl trunking 1
Port Vlans allowed on trunk
Gi0/13 1-4094
Gi0/14 1-4094
[output cut]
Great, we now have two trunked links between switches. But I’ve got to say, I really
don’t like the ISL method of frame tagging since it can’t send untagged frames across the
link. So let’s change our native VLAN from the default of 1 to 392. The number 392 just
randomly sounded good at the moment. Here’s what I entered on S1:
S1(config-if)#switchport trunk native vlan 392
S1(config-if)#
23:17:40: Port is not 802.1Q trunk, no action
774 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
See what I mean? I tried to change the native VLAN and ISL basically responded with,
“What’s a native VLAN?” Very annoying, so I’m going to take care of that now!
S1(config-if)#int range gi0/13 - 14
S1(config-if-range)#switchport trunk encapsulation ?
dot1q Interface uses only 802.1q trunking encapsulation when trunking
isl Interface uses only ISL trunking encapsulation when trunking
negotiate Device will negotiate trunking encapsulation with peer on
interface
S1(config-if-range)#switchport trunk encapsulation dot1q
23:23:%LINEPROTO-5-UPDOWN:Line protocol on Interface GigabitEthernet0/13, changed
state to down
23:23:%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/14,
changed state to down
23:23:%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on
GigabitEthernet0/13 (392), with S2 GigabitEthernet0/13 (1).
23:23:%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/14,
changed state to up
23:23:%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/13,
changed state to up
23:23:%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on
GigabitEthernet0/13 (392), with S2 GigabitEthernet0/13 (1).
Okay, that’s more like it! As soon as I changed the encapsulation type on S1, DTP
frames changed the frame-tagging method between S2 to 802.1q. Since I had already
changed the native VLAN on port Gi0/13 on S1, the switch lets us know, via CDP, that
we now have a native VLAN mismatch. Let’s proceed to deal with this by verifying our
interfaces with the show interface trunk command:
S1#sh int trunk
Port Mode Encapsulation Status Native vlan
Gi0/13 auto 802.1q trunking 392
Gi0/14 auto 802.1q trunking 1
S2#sh int trunk
Port Mode Encapsulation Status Native vlan
Gi0/13 desirable n-802.1q trunking 1
Gi0/14 desirable n-802.1q trunking 1
Troubleshooting VLAN Connectivity 775
Now notice that both links are running 802.1q and that S1 is in auto mode and S2 is in
desirable mode. And we can see a native VLAN mismatch on port Gi0/13. We can also see
the mismatched native VLAN with the show interfaces interface switchport command
by looking at both sides of the link like this:
S2#sh interfaces gi0/13 switchport
Name: Gi0/13
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: trunk
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
S1#sh int gi0/13 switchport
Name: Gi0/13
Switchport: Enabled
Administrative Mode: dynamic auto
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 392 (Inactive)
So this has got to be bad, right? I mean really—are we sending any frames down that
link or not? Let’s see if we solved our little problem of not being able to ping to hosts from
S1 to S2 and nd out:
PC1#ping 192.168.10.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.10.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms
Yes, it works! Not so bad after all. We’ve solved our problem, or at least most of it. Having
a native VLAN mismatch only means you can’t send untagged frames down the link, which
are essentially management frames like CDP, for example. So although it’s not the end of the
world, it will prevent us from being able to remotely manage the switch, or even sending any
other types of trafc down just that one VLAN.
776 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
So am I saying you can just leave this issue the way it is? Well, you could, but you
won’t. No, you’ll x it because if you don’t, CDP will send you a message every minute
telling you that there’s a mismatch, which will drive you mad! So, this is how we’ll stop
that from happening:
S2(config)#int gi0/13
S2(config-if)#switchport trunk native vlan 392
S2(config-if)#^Z
S2#sh int trunk
Port Mode Encapsulation Status Native vlan
Gi0/13 desirable n-802.1q trunking 392
Gi0/14 desirable n-802.1q trunking 1
[output cut]
All better! Both sides of the same link between switches are now using native VLAN 392
on Gigabit Ethernet 0/13. I want you to know that it’s ne to have different native VLANs
for each link if that’s what works best for you. Each network is different and you have to
make choices between options that will end up meeting your particular business require-
ments the most optimal way.
Summary
This chapter covered troubleshooting techniques from basic to advanced. Although most
chapters in this book cover troubleshooting, this chapter focused purely on IPv4, IPv6, and
VLAN/trunk troubleshooting.
You learned how to troubleshoot step-by-step from a host to a remote device. Starting
with IPv4, you learned the steps to test the host and the local connectivity and then how to
troubleshoot remote connectivity.
We then moved on to IPv6 and proceeded to troubleshoot using the same techniques
that you learned with IPv4. It’s important that you can use the verication commands that
I used in each step of this chapter.
Last, I covered VLAN and trunk troubleshooting and how to go step-by-step through a
switched network using verication commands and narrowing down the problem.
Visit ccna
.gg/ch18/b
for a
companion
MicroNugget
from CBT
Nuggets.
Exam Essentials 777
Exam Essentials
Remember the Cisco steps in troubleshooting an IPv4 and IPv6 network.
1. Check the cables to find out if there’s a faulty cable or interface in the mix and verify
the interface’s statistics.
2. Make sure that devices are determining the correct path from the source to the destina-
tion. Manipulate the routing information if needed.
3. Verify that the default gateway is correct.
4. Verify that name resolution settings are correct.
5. Verify that there are no ACLs blocking traffic.
Remember the commands to verify and troubleshoot IPv4 and IPv6. You need to remember
and practice the commands used in this chapter, especially ping and traceroute (tracert on
Windows). But we also used the Windows commands ipconfig and route print and Cisco’s
commands show ip int brief, show interface, and show route.
Remember how to verify an ARP cache with IPv6. The command show ipv6 neighbors
shows the IP-to-MAC-address resolution table on a Cisco router.
Remember to look at the statistics on a router and switch interface to determine problems.
You’ve got to be able to analyze interface statistics to nd problems if they exist, and this
includes speed and duplex settings, input queue drops, output queue drops, and input and
output errors.
Understand what a native VLAN is and how to change it. A native VLAN works with
only 802.1q trunks and allows untagged trafc to traverse the trunk link. This is VLAN1
by default on all Cisco switches, but it can be changed for security reasons with the
switchport native vlan vlan command.
778 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
Written Lab 4
The answers to this lab can be found in Appendix A, “Answers to Written Labs.”
Write the answers to the following questions:
1. If your IPv6 ARP cache shows an entry of INCMP, what does this mean?
2. You want traffic from VLAN 66 to traverse a trunked link untagged. Which command
will you use?
3. What are the five modes you can set a switch port to?
4. You are having a network problem and have checked the cables to find out if there’s a
faulty cable or interface in the mix and also verified the interface’s statistics, made sure
that devices are determining the correct path from the source to the destination, and
verified that you don’t need to manipulate the routing. What are your next trouble-
shooting steps?
5. You need to find out if the local IPv6 stack is working on a host. What command will
you use?
Hands-on Labs for Troubleshooting
Please check www.lammle.com, www.lammlesim.com, and www.lammle.com/forum for the
latest information and downloads available for studying when using my books. Precon-
figured hands-on troubleshooting labs are available for download, with the answers to
the troubleshooting problems found on my forum. The troubleshooting hands-on lab
simulator will be updated often.
Review Questions 779
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional questions,
please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. You need to verify the IPv6 ARP cache on a router and see that the state of an entry is
REACH. What does REACH mean?
A. The router is reaching out to get the address.
B. The entry is incomplete.
C. The entry has reached the end of life and will be discarded from the table.
D. A positive confirmation has been received by the neighbor and the path to it is
functioning correctly.
2. What is the most common cause of interface errors?
A. Speed mismatch
B. Duplex mismatch
C. Buffer overflows
D. Collisions between a dedicated switch port and an NIC
3. Which command will verify the DTP status on a switch interface?
A. sh dtp status
B. sh dtp status interface interface
C. sh interface interface dtp
D. sh dtp interface interface
4. What mode will not allow DTP frames generated from a switch port?
A. Nonegotiate
B. Trunk
C. Access
D. Auto
780 Chapter 18 u Troubleshooting IP, IPv6, and VLANs
5. The following output was generated by which command?
IPv6 Address Age Link-layer Addr State Interface
FE80::21A:6DFF:FE64:9B3 0 001a.6c46.9b09 DELAY Fa0/1
2001:DB8:3C4D:2:21A:6DFF:FE64:9B3 0 001a.6c46.9b09 REACH Fa0/1
A. show ip arp
B. show ipv6 arp
C. show ip neighbors
D. show ipv6 neighbors
6. Which of the following states tells you that an interface has not communicated within
the neighbor-reachable time frame?
A. REACH
B. STALE
C. TIMEOUT
D. CLEARED
7. You receive a call from a user who says that they cannot log in to a remote server,
which only runs IPv6. Based on the output, what could the problem be?
C:\Users\Todd Lammle>ipconfig
Connection-specific DNS Suffix . : localdomain
IPv6 Address. . . . . . . . . . . : 2001:db8:3c4d:3:ac3b:2ef:1823:8938
Temporary IPv6 Address. . . . . . : 2001:db8:3c4d:3:2f33:44dd:211:1c3d
Link-local IPv6 Address . . . . . : fe80::ac3b:2ef:1823:8938%11
IPv4 Address. . . . . . . . . . . : 10.1.1.10
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 10.1.1.1
A. The global address is in the wrong subnet.
B. The IPv6 default gateway has not been configured or received from the router.
C. The link-local address has not been resolved, so the host cannot communicate to
the router.
D. There are two IPv6 global addresses configured. One must be removed from the
configuration.
Review Questions 781
8. Your host cannot reach remote networks. Based on the output, what is the problem?
C:\Users\Server1>ipconfig
Windows IP Configuration
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . : localdomain
Link-local IPv6 Address . . . . . : fe80::7723:76a2:e73c:2acb%11
IPv4 Address. . . . . . . . . . . : 172.16.20.254
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 172.16.2.1
A. The link-local IPv6 address is wrong.
B. The IPv6 global address is missing.
C. There is no DNS server configuration.
D. The IPv4 default gateway address is misconfigured.
9. Which two commands will show you if you have a native VLAN mismatch?
A. show interface native vlan
B. show interface trunk
C. show interface interface switchport
D. show switchport interface
10. You connect two new Cisco 3560 switches together and expect them to use DTP and
create a trunk. However, when you check statistics, you find that they are access ports
and didn’t negotiate. Why didn’t DTP work on these Cisco switches?
A. The ports on each side of the link are set to auto trunking.
B. The ports on each side of the link are set to on.
C. The ports on each side of the link are set to dynamic.
D. The ports on each side of the link are set to desirable.
Chapter
19
Enhanced IGRP
THE FOLLOWING ICND2 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 IP Routing Technologies
■Differentiate methods of routing and routing protocols
■Administrative distance
■Split horizon
■Metric
■Next hop
■Configure and verify EIGRP (single AS)
■Feasible Distance/Feasible Successors/
Administrative distance
■Feasibility condition
■Metric composition
■Router ID
■Auto-summary
■Path selection
■Load balancing
■Equal
■Unequal
■Passive interface
1 Troubleshooting
■Troubleshoot and resolve routing issues
■Routing is enabled
■Routing table is correct
■Correct path selection
■Troubleshoot and resolve EIGRP problems
■Neighbor adjacencies
■AS number
■Load balancing
■Split horizon
Enhanced Interior Gateway Routing Protocol (EIGRP)
is a Cisco protocol that runs on Cisco routers and on some
Cisco switches. In this chapter, I’ll cover the many features
and functions of EIGRP, with an added focus on the unique way that it discovers,
selects, and advertises routes.
EIGRP has a number of features that make it especially useful within large, complex
networks. A real standout among these is its support of VLSM, which is crucial to its ultra-
efcient scalability. EIGRP even includes benets gained through other common protocols like
OSPF and RIPv2, such as the ability to create route summaries at any location you choose.
I’ll also cover key EIGRP conguration details and give you examples of each, as well
as, demonstrate the various commands required to verify that EIGRP is working properly.
Finally, I’ll wrap up the chapter by showing you how to congure and verify EIGRPv6. I
promise that after you get through it, you’ll agree that EIGRPv6 is truly the easiest part of
this chapter!
To find up-to-the-minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
EIGRP Features and Operations
EIGRP is a classless, distance-vector protocol that uses the concept of an autonomous system
to describe a set of contiguous routers that run the same routing protocol and share routing
information, which also includes the subnet mask in its route updates. This is a very big deal
because by advertising subnet information, this robust protocol enables us to use VLSM and
permits summarization to be included within the design of EIGRP networks.
EIGRP is sometimes referred to as a hybrid routing protocol or an advanced distance-
vector protocol because it has characteristics of both distance-vector and some link-state
protocols. For example, EIGRP doesn’t send link-state packets like OSPF does. Instead, it
sends traditional distance-vector updates that include information about networks plus the
cost of reaching them from the perspective of the advertising router.
EIGRP has link-state characteristics as well—it synchronizes network topology infor-
mation between neighbors at startup and then sends specic updates only when topology
changes occur (bounded updates). This particular feature is a huge advancement over RIP
and is a big reason that EIGRP works so well in very large networks.
EIGRP Features and Operations 785
EIGRP has a default hop count of 100, with a maximum of 255, but don’t let this confuse
you because EIGRP doesn’t rely on hop count as a metric like RIP does. In EIGRP-speak, hop
count refers to how many routers an EIGRP route update packet can go through before it will
be discarded, which limits the size of the autonomous system (AS). So don’t forget that this
isn’t how metrics are calculated with EIGRP!
There are a bunch of powerful features that make EIGRP a real standout from other
protocols. Here’s a list of some of the major ones:
u Support for IP and IPv6 (and some other useless routed protocols) via protocol-
dependent modules
u Considered classless (same as RIPv2 and OSPF)
u Support for VLSM/CIDR
u Support for summaries and discontiguous networks
u Efficient neighbor discovery
u Communication via Reliable Transport Protocol (RTP)
u Best path selection via Diffusing Update Algorithm (DUAL)
u Reduced bandwidth usage with bounded updates
u No broadcasts
Cisco refers to EIGRP as a distance-vector routing protocol but also as an
advanced distance-vector or even a hybrid routing protocol.
Neighbor Discovery
Before EIGRP routers can exchange routes with each other, they must become neighbors, and
there are three conditions that must be met before this can happen, as shown in Figure19.1.
FIGURE19.1 EIGRP neighbor discovery
gi0/0 10.1.1.2
10.1.1.1 gi0/1
224.0.0.10
AS
Metrics ACK
Neighbor table Neighbor table
10.1.1.2 gi0/0 10.1.1.1 gi0/1
Hello?
Hello!
786 Chapter 19 u Enhanced IGRP
And these three things will be exchanged with directly connected neighbors:
u Hello or ACK received
u AS numbers match
u Identical metrics (K values)
Link-state protocols often use Hello messages to establish who their neighbors are because
they usually don’t send out periodic route updates but still need a way to help neighbors know
when a new peer has arrived or an old one has gone down. And because Hellos are also used
to maintain neighbor relationships, it follows that EIGRP routers must also continuously
receive Hellos from their neighbors.
But EIGRP routers that belong to different ASs don’t automatically share routing informa-
tion and, therefore, don’t become neighbors. This factor is really helpful operating in larger
networks because it reduces the amount of route information propagated through a specic
AS. But it also means that manual redistribution can sometimes be required between different
ASs as a result. Because metrics play a big role in choosing between the ve possible factors to
be evaluated when choosing the best possible route, it’s important that all EIGRP neighbors
agree on how a specic route is chosen. This is vital because the calculations on one router
depend upon the calculations of its neighbors.
Hellos between EIGRP routers are set to 5 seconds by default. Another timer that’s related
to the hello timer is the hold timer. The hold timer determines the amount of time a router
is willing to wait to get a Hello from a neighbor before declaring it dead. Once a neighbor is
declared dead, it’s removed from the neighbor table and all routes that depended upon it are
recalculated. Interestingly, the hold timer conguration doesn’t determine how long a router
waits before it declares neighbors dead; it establishes how long the router will tell others to
wait before they can declare it dead. This means that the hold timers on neighboring routers
don’t need to match because they only tell the others how long to wait.
The only time EIGRP advertises its entire information is when it discovers a new neighbor
and forms a relationship or adjacency with it by exchanging Hello packets. When this hap-
pens, both neighbors then advertise their complete information to one another. After each has
learned its neighbor’s routes, only changes to the routing table will be propagated.
During each EIGRP session running on a router, a neighbor table is created in which the
router stores information about all routers known to be directly connected neighbors. Each
neighboring router’s IP address, hold time interval, smooth round-trip timer (SRTT), and
queue information are all kept in this table. It’s an important reference used to establish
that topology changes have occurred that neighboring routers need to know about.
To sum this all up, remember that EIGRP routers receive their neighbors’ updates and
store them in a local topology table that contains all known routes from all known neighbors
and serves as the raw material from which the best routes are selected.
Let’s dene some terms before we move on:
Reported/advertised distance (AD) This is the metric of a remote network, as reported by
a neighbor. It’s also the routing table metric of the neighbor and is the same as the second
number in parentheses as displayed in the topology table. The rst number is the adminis-
trative distance, and I’ll discuss more about these values in a minute. In Figure19.2, routers
SF and NY are both advertising the path to network 10.0.0.0 to the Corp router, but the
cost through SF to network 10.0.0.0 is less than NY.
EIGRP Features and Operations 787
FIGURE19.2 Advertised distance
S0/1
S0/0
Corp
SF
NY
To
get to network 10.0.0.0, it will cost you 1000.
To get to network 10.0.0.0, it will cost you 1500.
10.0.0.0/8
Hmm...two advertisements
to network 10.0.0.0.
I wonder what my true cost
to network 10.0.0.0 is?
We’re not done yet because the Corp router still needs to calculate its cost to each
neighbor.
Feasible distance (FD) This is the best metric among all paths to a remote network, including
the metric to the neighbor that’s advertising the remote network. The route with the lowest FD
is the route that you’ll nd in the routing table because it’s considered the best path. The metric
of a feasible distance is calculated using the metric reported by the neighbor that’s referred to
as the reported or advertised distance plus the metric to the neighbor reporting the route. In
Figure19.3, the Corp router will have the path through router SF to network 10.0.0.0 in the
routing table since its the lowest feasible distance. It’s the lowest true cost from end to end.
Take a look at an EIGRP route that’s been injected into a routing table and nd the FD listed
in the entry.
D 10.0.0.0/8 [90/2195456] via 172.16.10.2, 00:27:06,Serial0/0
First, the D means Dual, and is an EIGRP injected route and is the route used by EIGRP to
forward trafc to the 10.0.0.0 network via its neighbor, 172.16.10.2. But that’s not what I
want to focus on right now. See the [90/2195456] entry in the line? The rst number (90) is
the administrative distance (AD), which is not to be confused with advertised distance (AD),
which is why a lot of people call it the reported distance! The second number, is the feasible
distance (FD), or the entire cost for this router to get to network 10.0.0.0. To sum this up,
the neighbor router sends a reported, or advertised, distance (RD/AD) for network 10.0.0.0,
and EIGRP calculates the cost to get to that neighbor and then adds those two numbers
together to get the FD, or total cost.
788 Chapter 19 u Enhanced IGRP
FIGURE19.3 Feasible distance
S0/1
S0/0
Branch
SF
NY
To get to network 10.0.0.0, it will cost you 1000.
To get to network 10.0.0.0, it will cost you 1500.
10.0.0.0/8
It will cost me 500 to get to SF
and 500 to get to NY, so my total cost
to get to network 10.0.0.0 through SF
is 1500 and through NY is 2000
Neighbor table Each router keeps state information about adjacent neighbors. When a
newly discovered neighbor is found, its address and interface are recorded and the infor-
mation is held in the neighbor table, stored in RAM. Sequence numbers are used to match
acknowledgments with update packets. The last sequence number received from the neigh-
bor is recorded so that out-of-order packets can be detected. We’ll get into this more, later
in the chapter, when we look at the neighbor table and nd out how it’s useful for trouble-
shooting links between neighbor routers.
Topology table The topology table is populated by the neighbor table and the Diffusing
Update Algorithm (DUAL) calculates the best loop-free path to each remote network. It con-
tains all destinations advertised by neighboring routers, holding each destination address and
a list of neighbors that have advertised the destination. For each neighbor, the advertised met-
ric (distance), which comes only from the neighbor’s routing table, is recorded, as well as, the
FD. The best path to each remote network is copied and placed in the routing table and then
IP will use this route to forward trafc to the remote network. The path copied to the routing
table is called a successor router—think “successful” to help you remember. The path with a
good, but less desirable, cost will be entered in the topology table as a backup link and called
the feasible successor. Let’s talk more about these terms now.
The neighbor and topology tables are stored in RAM and maintained
through the use of Hello and update packets. While the routing table is
also stored in RAM, the information stored in the routing table is gathered
only from the topology table.
EIGRP Features and Operations 789
Feasible successor (FS) So a feasible successor is basically an entry in the topology table that
represents a path that’s inferior to the successor route(s). An FS is dened as a path whose
advertised distance is less than the feasible distance of the current successor and considered a
backup route. EIGRP will keep up to 32 feasible successors in the topology table in 15.0 code
but only up to 16 in previous IOS versions, which is still a lot! Only the path with the best
metric—the successor—is copied and placed in the routing table. The show ip eigrp topol-
ogy command will display all the EIGRP feasible successor routes known to the router.
A feasible successor is a backup route and is stored in the topology table. A
successor route is stored in the topology table and is copied and placed in
the routing table.
Successor A successor route—again, think “successful”—is the best route to a remote net-
work. A successor route is the lowest cost to a destination and stored in the topology table
along with everything else. However, this particular best route is copied and placed in the
routing table so IP can use it to get to the remote network. The successor route is backed up
by a feasible successor route, which is also stored in the topology table, if there’s one avail-
able. The routing table contains only successor routes; the topology table contains successor
and feasible successor routes.
Figure19.4 illustrates that the SF and NY routers each have subnets of the 10.0.0.0 network
and the Corp router has two paths to get to this network.
FIGURE19.4 The tables used by EIGRP
S0/1
S0/0
Corp
SF
NY
10.0.0.0/8
Topology Table—IP
Destination 10.0.0.0
Destination 10.0.0.0
SF
NY
Successor
Feasible Successor
S0/0
S0/1
Neighbor Table—IP
Router SF
Router NY
Serial0/0
Serial0/1
Routing Table—IP
SF Router/Exit S0/0 Successor
790 Chapter 19 u Enhanced IGRP
As shown in Figure19.4, there are two paths to network 10.0.0.0 that can be used by the
Corp router. EIGRP picks the best path and places it in the routing table, but if both links have
equal-cost paths, EIGRP would load-balance between them—up to four links, by default. By
using the successor, and having feasible successors in the topology table as backup links, the
network can converge instantly and updates to any neighbor make up the only trafc sent
from EIGRP—very clean!
Reliable Transport Protocol (RTP)
EIGRP depends on a proprietary protocol, called Reliable Transport Protocol (RTP), to
manage the communication of messages between EIGRP-speaking routers. As the name
suggests, reliability is a key concern of this protocol, so Cisco designed this mechanism,
which leverages multicasts and unicasts, to ensure that updates are delivered quickly and
that data reception is tracked accurately.
But how does this really work? Well, when EIGRP sends multicast trafc, it uses the
Class D address 224.0.0.10, and each EIGRP router knows who its neighbors are. For each
multicast it sends out, a list is built and maintained that includes all the neighbors who have
replied. If a router doesn’t get a reply from a neighbor via the multicast, EIGRP will then try
using unicasts to resend the same data. If there’s no reply from a neighbor after 16 unicast
attempts, that neighbor will then be declared dead. This process is often referred to as reliable
multicast.
Routers keep track of the information they send by assigning a sequence number to
each packet that enables them to identify old, redundant information and data that’s out
of sequence. You’ll get to actually see this information in the neighbor table coming up
when we get into conguring EIGRP.
Remember, EIGRP is all about topology changes and updates, making it the quiet,
performance-optimizing protocol it is. Its ability to synchronize routing databases at
startup time, while maintaining the consistency of databases over time, is achieved quietly
by communicating only necessary changes. The downside here is that you can end up with
a corrupted routing database if any packets have been permanently lost or if packets have
been mishandled out of order!
Here’s a description of the ve different types of packets used by EIGRP:
Update An Update packet contains route information. When these are sent in response to
metric or topology changes, they use reliable multicasts. In the event that only one router
needs an update, like when a new neighbor is discovered, it’s sent via unicasts. Keep in mind
that the unicast method still requires an acknowledgment, so updates are always reliable
regardless of their underlying delivery mechanism.
Query A Query packet is a request for specic routes and always uses the reliable multicast
method. Routers send queries when they realize they’ve lost the path to a particular network
and are searching for alternatives.
Reply A Reply packet is sent in response to a query via the unicast method. Replies either
include a specic route to the queried destination or declare that there’s no known route.
EIGRP Features and Operations 791
Hello A Hello packet is used to discover EIGRP neighbors and is sent via unreliable multi-
cast, meaning it doesn’t require an acknowledgment.
ACK An ACK packet is sent in response to an update and is always unicast. ACKs are never
sent reliably because this would require another ACK sent for acknowledgment, which would
just create a ton of useless trafc!
It’s helpful to think of all these different packet types like envelopes. They’re really just
types of containers that EIGRP routers use to communicate with their neighbors. What’s
really interesting is the actual content envelopes these communications and the procedures
that guide their conversations, and that’s what we’ll be exploring next!
Diffusing Update Algorithm (DUAL)
I mentioned that EIGRP uses Diffusing Update Algorithm (DUAL) for selecting and
maintaining the best path to each remote network. DUAL allows EIGRP to carry out
these vital tasks:
u Figure out a backup route if there’s one available.
u Support variable length subnet masks (VLSMs).
u Perform dynamic route recoveries.
u Query neighbors for unknown alternate routes.
u Send out queries for an alternate route.
Quite an impressive list, but what really makes DUAL so great is that it enables EIGRP
to converge amazingly fast! The key to the speed is twofold: First, EIGRP routers maintain a
copy of all of their neighbors’ routes to refer to for calculating their own cost to each remote
network. So if the best path goes down, all it often takes to nd another one is a quick scan of
the topology table looking for a feasible successor. Second, if that quick table survey doesn’t
work out, EIGRP routers immediately ask their neighbors for help nding the best path. It’s
exactly this, ahem, DUAL strategy of reliance upon, and the leveraging of, other routers’ infor-
mation that accounts for the algorithm’s “diffusing” character. Unlike other routing protocols
where the change is propagated through the entire network, EIGRP bounded updates are
propagated only as far as needed.
Three critical conditions must be met for DUAL to work properly:
u Neighbors are discovered or noted as dead within a finite time.
u All transmitted messages are received correctly.
u All changes and messages are processed in the order in which they’re detected.
As you already know, the Hello protocol ensures the rapid detection of new or dead
neighbors, and RTP provides a reliable method of conveying and sequencing messages.
Based upon this solid foundation, DUAL can then select and maintain information
about the best paths. Let’s check further into the process of route discovery and mainte-
nance next.
792 Chapter 19 u Enhanced IGRP
Route Discovery and Maintenance
The hybrid nature of EIGRP is fully revealed in its approach to route discovery and main-
tenance. Like many link-state protocols, EIGRP supports the concept of neighbors that are
formally discovered via a Hello process and whose state is monitored thereafter. And like
many distance-vector protocols, EIGRP uses the routing-by-rumor approach, which implies
that many routers within an AS never actually hear about a route update rsthand. Instead,
these devices rely on “network gossip” to hear about neighbors and their respective status
via another router that may have also gotten the info from yet another router and so on.
Given all of the information that EIGRP routers have to collect, it follows that they must
have a place to store it, and they do this in the tables I referred to earlier in this chapter. As
you know, EIGRP doesn’t depend on just one table—it actually uses three of them to store
important information about its environment:
Neighbor table Contains information about the specic routers with whom neighbor rela-
tionships have been formed. It also displays information about the Hello transmit interval
and queue counts for unaccounted Hello acknowledgment.
Topology table Stores the route advertisements received from each neighbor. All routes in
the AS are stored in the topology table, both successors, and feasible successors.
Route table Stores the routes that are currently in use to make local routing decisions.
Anything in the routing table is considered a successor route.
We’ll explore more of EIGRP’s features in greater detail soon, beginning with a look
at the metrics associated with particular routes. After that, I’ll cover the decision-making
process that’s used to select the best routes, and then we’ll review the procedures followed
when routes change.
Configuring EIGRP
I know what you’re thinking! “We’re going to jump in to conguring EIGRP already when
I’ve heard how complex it is?” No worries here—what I’m about to show is basic, and I know
you won’t have a problem with it at all! We’re going to start with the easy part of EIGRP, and
by conguring it on our little internetwork, you’ll learn a lot more this way than you would if
I just continued explaining more at this point. After we’ve completed the initial conguration,
we’ll ne-tune it and have fun experimenting with it throughout this chapter!
But rst, keep in mind that even though EIGRP can be congured for IP, IPv6, IPX, and
AppleTalk, as a future Cisco Certied Network Associate R/S your focus needs to zero in
on conguring EIGRP for IP and IPv6.
Okay, there are two modes for entering EIGRP commands: router conguration mode
and interface conguration mode. In router conguration mode, we’ll enable the protocol,
determine which networks will run EIGRP, and set global factors. When in interface con-
guration mode, we’ll customize summaries and bandwidth.
Configuring EIGRP 793
To initiate an EIGRP session on a router, I’ll use the router eigrp command followed
by our network’s AS number. After that, we’ll enter the specic numbers of the networks
that we want to connect to the router using the network command followed by the network
number. This is pretty straightforward stuff—if you can congure RIP, then you can con-
gure EIGRP!
Just so you know, we’ll use the same network I used in the previous CCENT routing
chapters, but I’m going to connect more networks so we can look deeper into EIGRP. With
that, I’m going to enable EIGRP for autonomous system 20 on our Corp router connected
to four networks.
Figure19.5 shows the network we’ll be conguring throughout this chapter and the next
chapter. Here’s the Corp conguration:
FIGURE19.5 Configuring our little internetwork with EIGRP
S0/1
S0/0
Corp
SF
NY
10.10.10.0/24
g0/0
172.16.10.0/30 .2
10.10.20.0/24
10.10.30.0/24
10.10.11.0/24
g0/1
172.16.10.4/30 .6 10.10.40.0/24
10.10.50.0/24
Corp#config t
Corp(config)#router eigrp 20
Corp(config-router)#network 172.16.0.0
Corp(config-router)#network 10.0.0.0
Remember, just as we would when conguring RIP, we need to use the classful network
address, which is all subnet and host bits turned off. This is another thing that makes EIGRP
so great: it has the complexity of a link-state protocol running in the background and the
same easy conguration process used for RIP!
Understand that the AS number is irrelevant—that is, as long as all routers
use the same number! You can use any number from 1 to 65,535.
But wait, the EIGRP conguration can’t be that easy, can it? A few simple EIGRP
commands and my network just works? Well, it can be and usually is, but not always.
794 Chapter 19 u Enhanced IGRP
Remember the wildcards you learned about in your OSPF and access list congurations
in your preparation for the Cisco exam? We can go with the same type of conguration
as we did with OSPF. Let’s say, for example, that we wanted to advertise all the directly
connected networks with EIGRP off the Corp router. By using the command network
10.0.0.0, we can effectively advertise to all subnets within that classful network; how-
ever, take a look at this conguration now:
Corp#config t
Corp(config)#router eigrp 20
Corp(config-router)#network 10.10.11.0 0.0.0.255
Corp(config-router)#network 172.16.10.0 0.0.0.3
Corp(config-router)#network 172.16.10.4 0.0.0.3
This conguration should look pretty familiar to you because by now you should have a
solid understanding of how wildcards are congured. This conguration will advertise the
network connected to g0/1 on the Corp router as well as the two WAN links. Still, all we
accomplished with this conguration was to stop the g0/0 interface from being placed into
the EIGRP process, and unless you have tens of thousands of networks worldwide, then
there is really no need to use wildcards because they don’t provide any other administrative
purpose other than what I’ve already described.
Now let’s take a look at the simple conguration needed for the SF and NY routers in
our internetwork:
SF(config)#router eigrp 20
SF(config-router)#network 172.16.0.0
SF(config-router)#network 10.0.0.0
000060:%DUAL-5-NBRCHANGE:IP-EIGRP(0) 20:Neighbor 172.16.10.1 (Serial0/0/0) is up:
new adjacency
NY(config)#router eigrp 20
NY(config-router)#network 172.16.0.0
NY(config-router)#network 10.0.0.0
*Jun 26 02:41:36:%DUAL-5-NBRCHANGE:IP-EIGRP(0) 20:Neighbor 172.16.10.5
(Serial0/0/1) is up: new adjacency
Nice and easy—or is it? We can see that the SF and NY router created an adjacency to the
Corp router, but are they actually sharing routing information? To nd out, let’s take a look
at the number that I pointed out as the autonomous system (AS) number in the conguration.
EIGRP uses ASs to identify the group of routers that will share route information. Only
routers that have the same AS share routes. The range of values we can use to create an AS
with EIGRP is 1–65535:
Corp(config)#router eigrp ?
<1-65535> Autonomous System
Configuring EIGRP 795
WORD EIGRP Virtual-Instance Name
Corp(config)#router eigrp 20
Notice that I could have used any number from 1 to 65,535, but I chose to use 20 because
it just felt good at the time. As long as all routers use the same number, they’ll create an adja-
cency. Okay, now the AS makes sense, but it looks like I can type a word in the place of the
AS number, and I can! Let’s take a look at the conguration:
Corp(config)#router eigrp Todd
Corp(config-router)#address-family ipv4 autonomous-system 20
Corp(config-router-af)#network 10.0.0.0
Corp(config-router-af)#network 172.16.0.0
What I just showed you is not part of the Cisco exam objectives, but it’s also not really
necessary for any IPv4 routing conguration in your network. The previous conguration
examples I’ve gone through so far in this chapter covers the objectives and works just ne,
but I included this last conguration example because it’s now an option in IOS 15.0 code.
VLSM Support and Summarization
Being one of the more sophisticated classless routing protocols, EIGRP supports using vari-
able length subnet masks. This is good because it allows us to conserve address space by using
subnet masks that map to specic host requirements in a much better way. Being able to use
30-bit subnet masks for the point-to-point networks that I congured in our internetwork is a
great example. Plus, because the subnet mask is propagated with every route update, EIGRP
also supports the use of discontiguous subnets, giving us greater administrative exibility
when designing a network IP address scheme. Another versatile feature is that EIGRP allows
us to use and place route summaries at strategically optimal locations throughout the EIGRP
network to reduce the size of the route table.
Keep in mind that EIGRP automatically summarizes networks at their classful boundaries
and supports the manual creation of summaries at any and all EIGRP routers. This is usually
a good thing, but by checking out the routing table in the Corp router, you can see the possible
complications that auto-summarization can cause:
Corp#sh ip route
[output cut]
172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
C 172.16.10.4/30 is directly connected, Serial0/1
C 172.16.10.0/30 is directly connected, Serial0/0
D 172.16.0.0/16 is a summary, 00:01:37, Null0
10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C 10.10.10.0/24 is directly connected, GigabitEthernet0/0
D 10.0.0.0/8 is a summary, 00:01:19, Null0
C 10.10.11.0/24 is directly connected, GigabitEthernet0/1
796 Chapter 19 u Enhanced IGRP
Now this just doesn’t look so good—both 172.16.0.0 and 10.0.0.0/8 are being advertised
as summary routes injected by EIGRP, but we have multiple subnets in the 10.0.0.0/8 classful
network address, so how would the Corp router know how to route to a specic network like
10.10.20.0? The answer is, it wouldn’t. Let’s see why in Figure19.6.
FIGURE19.6 Discontiguous networks
S0/1
S0/0
Corp
SF
NY
10.10.10.0/24
g0/0
172.16.10.0/30 .2
10.10.20.0/24
10.10.30.0/24
10.10.11.0/24
g0/1
172.16.10.4/30 .6 10.10.40.0/24
10.10.50.0/24
Network 10.0.0.0 is over here!
Network 10.0.0.0 is over here!
The networks we’re using are considered a discontiguous network because we have the
10.0.0.0/8 network subnetted across a different class of address; the 172.16.0.0 network,
with 10.0.0.0/8 subnets on both sides of the WAN links.
You can see that the SF and NY routers will both create an automatic summary of
10.0.0.0/8 and then inject it into their routing tables. This is a common problem, and an
important one that Cisco really wants you to understand (by including it in the objectives)!
With this type of topology, disabling automatic summarization is denitely the better
option. Actually, it’s the only option if we want this network to work.
Let’s take a look at the routing tables on the NY and SF routers to nd out what
they’re seeing:
SF>sh ip route
[output cut]
172.16.0.0/16 is variably subnetted, 3 subnets, 3 masks
C 172.16.10.0/30 is directly connected, Serial0/0/0
D 172.16.10.0/24 [90/2681856] via 172.16.10.1, 00:54:58, Serial0/0/0
D 172.16.0.0/16 is a summary, 00:55:12, Null0
10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
D 10.0.0.0/8 is a summary, 00:54:58, Null0
C 10.10.20.0/24 is directly connected, FastEthernet0/0
C 10.10.30.0/24 is directly connected, Loopback0
SF>
Configuring EIGRP 797
NY>sh ip route
[output cut]
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.16.10.4/30 is directly connected, Serial0/0/1
D 172.16.0.0/16 is a summary, 00:55:56, Null0
10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
D 10.0.0.0/8 is a summary, 00:55:26, Null0
C 10.10.40.0/24 is directly connected, FastEthernet0/0
C 10.10.50.0/24 is directly connected, Loopback0
NY>ping 10.10.10.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.10.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
NY>
The conrmed answer is that our network isn’t working because we’re discontiguous
and our classful boundaries are auto-summarizing. We can see that EIGRP is injecting
summary routes into both the SF and NY routing tables.
We need to advertise our subnets in order to make this work, and here’s how we make
that happen, starting with the Corp router:
Corp#config t
Corp(config)#router eigrp 20
Corp(config-router)#no auto-summary
Corp(config-router)#
*Feb 25 18:29:30%DUAL-5-NBRCHANGE:IP-EIGRP(0) 20:Neighbor 172.16.10.6 (Serial0/1)
is resync: summary configured
*Feb 25 18:29:30%DUAL-5-NBRCHANGE:IP-EIGRP(0) 20:Neighbor 172.16.10.2 (Serial0/0)
is resync: summary configured
Corp(config-router)#
Okay—our network still isn’t working because the other routers are still sending a
summary. So let’s congure the SF and NY routers to advertise subnets:
SF#config t
SF(config)#router eigrp 20
SF(config-router)#no auto-summary
SF(config-router)#
000090:%DUAL-5-NBRCHANGE:IP-EIGRP(0) 20:Neighbor 172.16.10.1 (Serial0/0/0) is
resync: summary configured
798 Chapter 19 u Enhanced IGRP
NY#config t
NY(config)#router eigrp 20
NY(config-router)#no auto-summary
NY(config-router)#
*Jun 26 21:31:08%DUAL-5-NBRCHANGE:IP-EIGRP(0) 20:Neighbor 172.16.10.5
(Serial0/0/1)
is resync: summary configured
Let’s take a look at the Corp router’s output now:
Corp(config-router)#do show ip route
[output cut]
172.16.0.0/30 is subnetted, 2 subnets
C 172.16.10.4 is directly connected, Serial0/1
C 172.16.10.0 is directly connected, Serial0/0
10.0.0.0/24 is subnetted, 6 subnets
C 10.10.10.0 is directly connected, GigabitEthernet0/0
C 10.10.11.0 is directly connected, GigabitEthernet0/1
D 10.10.20.0 [90/3200000] via 172.16.10.2, 00:00:27, Serial0/0
D 10.10.30.0 [90/3200000] via 172.16.10.2, 00:00:27, Serial0/0
D 10.10.40.0 [90/2297856] via 172.16.10.6, 00:00:29, Serial0/1
D 10.10.50.0 [90/2297856] via 172.16.10.6, 00:00:30, Serial0/1
Corp# ping 10.10.20.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.20.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
Wow, what a difference compared to the previous routing table output! We can see all
the subnets now. It would be hard to justify using auto-summarization today. If you want
to summarize, it should denitely be done manually. Always typing in no auto-summary
under RIPv2 and EIGRP is common practice today.
The new 15.x code auto-summarization feature is disabled by default, as it
should be. But don’t think that discontiguous networks and disabling auto-
summary are no longer topics in the Cisco exam objectives, because they
most certainly are!
Controlling EIGRP Traffic
But what if you need to stop EIGRP from working on a specic interface? Maybe it’s a
connection to your ISP, or where we didn’t want to have the g0/0 interface be part of the
Configuring EIGRP 799
EIGRP process as in our earlier example. All you need to do is to ag the interface as pas-
sive, and to do this from an EIGRP session, just use this command:
passive-interface interface-type interface-number
This works because the interface-type portion denes the type of interface and the
interface-number portion denes the number of the interface. The following command
makes interface serial 0/0 into a passive interface:
Corp(config)#router eigrp 20
Corp(config-router)#passive-interface g0/0
What we’ve accomplished here is to prevent this interface from sending or reading received
Hello packets so that it will no longer form adjacencies or send or receive route information.
But this still won’t stop EIGRP from advertising the subnet of this interface out all other inter-
faces without using wildcards. This really illustrates the reason you must understand why and
when to use wildcards, as well as, what the passive-interface command does. This knowl-
edge really helps you to make an informed decision on which command you need to use to
meet your specic business requirements!
The impact of the passive-interface command depends upon the routing
protocol under which the command is issued. For example, on an interface
running RIP, the passive-interface command will prohibit sending route
updates but will permit receiving them. An RIP router with a passive inter-
face will still learn about the networks advertised by other routers. This
is different from EIGRP, where an interface configured with the passive-
interface command will neither send nor read received Hellos.
Typically, EIGRP neighbors use multicast to exchange routing updates. You can change
this by specically telling the router about a particular neighbor, which will ensure that
unicast packets will only be used for the routing updates with that specic neighbor. To
take advantage of this feature, apply the neighbor command and execute it under the
EIGRP process.
I’m going to congure the Corp router with information about routers SF and NY:
Corp(config)#router eigrp 20
Corp(config-router)#neighbor 172.16.10.2
Corp(config-router)#neighbor 172.16.10.6
Understand that you don’t need to use the preceding commands to create neighbor
relationships, but they’re available if you need them.
EIGRP Metrics
Unlike many other protocols that use a single element to compare routes and select the best
possible path, EIGRP uses a combination of these four factors:
u Bandwidth
u Delay
800 Chapter 19 u Enhanced IGRP
u Load
u Reliability
It’s worth noting that there’s a fth element, maximum transmission unit (MTU),
which has never been used in EIGRP metrics calculations though it’s still a required
parameter in some EIGRP-related commands—especially those involving redistribution.
The value of the MTU element represents the smallest MTU value encountered along the
path to the destination network.
Also good to know is that there’s a mathematical formula that combines the four main
elements to create a single value representing just how good a given route actually is. The
higher the metric associated with it, the less desirable the route. Here’s that formula:
metric = [K1 n Bandwidth + (K2 n Bandwidth) / (256 – Load) + K3 n Delay] n [K5 /
(Reliability + K4)]
The formula’s components break down like this:
u By default, K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0.
u Delay equals the sum of all the delays of the links along the path.
u Delay = [Delay in 10s of microseconds] n 256.
u Bandwidth is the lowest bandwidth of the links along the path.
u Bandwidth = [10000000 / (bandwidth in Kbps)] n 256.
u By default, metric = lowest bandwidth along path + sum of all delays along path.
If necessary, you can adjust the constant K values on a per-interface basis, but I would rec-
ommend that you only do this under the direction of the Cisco Technical Assistance Center
(TAC). Metrics are tuned to change the manner in which routes are calculated. The K values
can be seen with a show ip protocols output:
Corp#sh ip protocols
*** IP Routing is NSF aware ***
Routing Protocol is "eigrp 1"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP-IPv4 Protocol for AS(1)
Metric weight K1=1, K2=0, K3=1, K4=0, K5=0
Notice that that the K1 and K3 values are enabled by default—for example, K1 = 1.
Table19.1 shows the relationship between each constant and the metric it affects.
Configuring EIGRP 801
TABLE19.1 Metric association of K values
Constant Metric
K1 Bandwidth (Be)
K2 Load (utilization on path)
K3 Delay (Dc)
K4 Reliability (r)
K5 MTU
Each constant is used to assign a weight to a specic variable, meaning that when the met-
ric is calculated, the algorithm will assign a greater importance to the specied metric. This is
very cool because it means that by assigning a weight, you get to specify the factor that’s most
important to you. For example, if bandwidth is your priority, you would assign K1 to weight it
accordingly, but if delay is totally unacceptable, then K3 would be assigned a greater weight. A
word of caution though: Always remember that any changes to the default values could result
in instability and convergence problems, particularly if delay or reliability values are constantly
changing! But if you’re looking for something to do on a rainy Saturday, it’s an interesting
experiment to pass some time and gain some nice networking insight!
Maximum Paths and Hop Count
By default, EIGRP can provide equal-cost load balancing across up to 4 links. RIP and
OSPF do this too. But you can have EIGRP actually load-balance across up to 32 links
with 15.0 code (equal or unequal) by using the following command:
Corp(config)#router eigrp 10
Corp(config-router)#maximum-paths ?
<1-32> Number of paths
As I mentioned, pre–15.0 code routers allowed up to 16 paths to remote networks,
which is still a lot!
EIGRP has a default maximum hop count of 100 for route update packets, but it can
be set up to 255. Chances are you wouldn’t want to ever change this, but if you did, here
is how you would do it:
Corp(config)#router eigrp 10
Corp(config-router)#metric maximum-hops ?
<1-255> Hop count
802 Chapter 19 u Enhanced IGRP
As you can see from this router output, EIGRP can be set to a maximum of 255 hops.
Even though it doesn’t use hop count in the path metric calculation, it still uses the maximum
hop count to limit the scope of the AS.
Route Selection
Now that you’ve got a good idea how EIGRP works and also how easy it actually is to
congure, it’s probably clear that determining the best path simply comes down to seeing
which one gets awarded the lowest metric. But it’s not the winning path that really sets
EIGRP apart from other protocols. You know that EIGRP stores route information from
its neighbors in its topology table and that as long as a given neighbor remains alive, it will
rarely throw out anything it has learned from that neighbor. This makes EIGRP able to ag
the best routes in its topology table for positioning in its local routing table, enabling it to
ag the next-best routes as alternatives if the best route goes down.
In Figure19.7 you can see that I added another Fast Ethernet link between the SF and NY
routers. This will give us a great opportunity to play with the topology and routing tables!
FIGURE19.7 EIGRP route selection process
Fa0/1
s0/1
s0/0
Corp
SF
NY
10.10.10.0/24
g0/0
172.16.10.0/30 .2
10.10.20.0/24
10.10.30.0/24
10.10.11.0/24
g0/1
172.16.10.4/30 .6
10.10.40.0/24
10.10.50.0/24
Fa0/1
192.168.10.0/24
First, let’s take another look at the routing table on the Corp router before I bring up the
new interfaces:
172.16.0.0/30 is subnetted, 2 subnets
C 172.16.10.4 is directly connected, Serial0/1
C 172.16.10.0 is directly connected, Serial0/0
10.0.0.0/24 is subnetted, 6 subnets
C 10.10.10.0 is directly connected, GigabitEthernet0/0
C 10.10.11.0 is directly connected, GigabitEthernet0/1
D 10.10.20.0 [90/3200000] via 172.16.10.2, 00:00:27, Serial0/0
D 10.10.30.0 [90/3200000] via 172.16.10.2, 00:00:27, Serial0/0
Configuring EIGRP 803
D 10.10.40.0 [90/2297856] via 172.16.10.6, 00:00:29, Serial0/1
D 10.10.50.0 [90/2297856] via 172.16.10.6, 00:00:30, Serial0/1
We can see the three directly connected interfaces, as well as, the other four networks
injected into the routing table by EIGRP. Now, I’ll add the network 192.168.10.0/24 between
the SF and NY routers, then enable the interfaces.
And let’s check out the routing table of the Corp router now that I’ve congured that link:
D 192.168.10.0/24 [90/2172416] via 172.16.10.6, 00:04:27, Serial0/1
172.16.0.0/30 is subnetted, 2 subnets
C 172.16.10.4 is directly connected, Serial0/1
C 172.16.10.0 is directly connected, Serial0/0
10.0.0.0/24 is subnetted, 6 subnets
C 10.10.10.0 is directly connected, GigabitEthernet0/0
C 10.10.11.0 is directly connected, GigabitEthernet0/1
D 10.10.20.0 [90/3200000] via 172.16.10.2, 00:00:27, Serial0/0
D 10.10.30.0 [90/3200000] via 172.16.10.2, 00:00:27, Serial0/0
D 10.10.40.0 [90/2297856] via 172.16.10.6, 00:00:29, Serial0/1
D 10.10.50.0 [90/2297856] via 172.16.10.6, 00:00:30, Serial0/1
Okay—that’s weird. The only thing different I see is one path to the 192.168.10.0/24
network listed rst. Glad it is there, which means that we can route to that network. Notice
that we can reach the network from the Serial0/1 interface, but what happened to my link to
the SF router—shouldn’t we have an advertisement from that router and be load-balancing?
Let’s take a look the topology table to nd out what’s going on:
Corp#sh ip eigrp topology
IP-EIGRP Topology Table for AS(20)/ID(10.10.11.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.10.10.0/24, 1 successors, FD is 128256
via Connected, GigbitEthernet0/0
P 10.10.11.0/24, 1 successors, FD is 128256
via Connected, GigbitEthernet0/1
P 10.10.20.0/24, 1 successors, FD is 2300416
via 172.16.10.6 (2300416/156160), Serial0/1
via 172.16.10.2 (3200000/128256), Serial0/0
P 10.10.30.0/24, 1 successors, FD is 2300416
via 172.16.10.6 (2300416/156160), Serial0/1
via 172.16.10.2 (3200000/128256), Serial0/0
804 Chapter 19 u Enhanced IGRP
P 10.10.40.0/24, 1 successors, FD is 2297856
via 172.16.10.6 (2297856/128256), Serial0/1
via 172.16.10.2 (3202560/156160), Serial0/0
P 10.10.50.0/24, 1 successors, FD is 2297856
via 172.16.10.6 (2297856/128256), Serial0/1
via 172.16.10.2 (3202560/156160), Serial0/0
P 192.168.10.0/24, 1 successors, FD is 2172416
via 172.16.10.6 (2172416/28160), Serial0/1
via 172.16.10.2 (3074560/28160), Serial0/0
P 172.16.10.4/30, 1 successors, FD is 2169856
via Connected, Serial0/1
P 172.16.10.0/30, 1 successors, FD is 3072000
via Connected, Serial0/0
Okay, we can see there are two paths to the 192.168.10.0/24 network, but it’s using the
next hop of 172.16.10.6 (NY) because the feasible distance (FD) is less! The advertised dis-
tance from both routers is 28160, but the cost to get to each router via the WAN links is the
same. This means the FD is not the same, meaning we’re not load-balancing by default.
Both WAN links are a T1, so this should have load-balanced by default, but EIGRP has
determined that it costs more to go through SF than through NY. Since EIGRP uses band-
width and delay of the line to determine the best path, we can use the show interfaces
command to verify our stats like this:
Corp#sh int s0/0
Serial0/0 is up, line protocol is up
Hardware is PowerQUICC Serial
Description: <<Connection to CR1>>
Internet address is 172.16.10.1/30
MTU 1500 bytes, BW 1000 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set Keepalive set (10 sec)
Corp#sh int s0/1
Serial0/1 is up, line protocol is up
Hardware is PowerQUICC Serial
Internet address is 172.16.10.5/30
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set Keepalive set (10 sec)
I highlighted the statistic that EIGRP uses to determine the metrics to a next-hop router:
MTU, bandwidth, delay, reliability, and load, with bandwidth and delay enabled by default.
Configuring EIGRP 805
We can see that the bandwidth on the Serial0/0 interface is set to 1000 Kbit, which is not the
default bandwidth. Serial0/1 is set to the default bandwidth of 1544 Kbit.
Let’s set the bandwidth back to the default on the s0/0 interface and we should start load-
balancing to the 192.168.10.0 network. I’ll just use the no bandwidth command, which will
set it back to its default of 1544 Mbps:
Corp#config t
Corp(config)#int s0/0
Corp(config-if)#no bandwidth
Corp(config-if)#^Z
Now let’s take a look at the topology table and see if we’re equal.
Corp#sh ip eigrp topo | section 192.168.10.0
P 192.168.10.0/24, 2 successors, FD is 2172416
via 172.16.10.2 (2172416/28160), Serial0/0
via 172.16.10.6 (2172416/28160), Serial0/1
Since the topology tables can get really huge in most networks, the show ip eigrp
topology | section network command comes in handy because it allows us to see infor-
mation about the network we want to look into in a couple of lines.
Let’s use the show ip route network command and check out what is going on there:
Corp#sh ip route 192.168.10.0
Routing entry for 192.168.10.0/24
Known via "eigrp 20", distance 90, metric 2172416, type internal
Redistributing via eigrp 20
Last update from 172.16.10.2 on Serial0/0, 00:05:18 ago
Routing Descriptor Blocks:
* 172.16.10.6, from 172.16.10.6, 00:05:18 ago, via Serial0/1
Route metric is 2172416, traffic share count is 1
Total delay is 20100 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
172.16.10.2, from 172.16.10.2, 00:05:18 ago, via Serial0/0
Route metric is 2172416, traffic share count is 1
Total delay is 20100 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
Lots of detail about our routes to the 192.168.10.0 network! The Corp route has two
equal-cost links to the 192.168.10.0 network. And to reveal load balancing even better,
we’ll just use the plain, ever useful show ip route command:
Corp#sh ip route
[output cut]
806 Chapter 19 u Enhanced IGRP
D 192.168.10.0/24 [90/2172416] via 172.16.10.6, 00:05:35, Serial0/1
[90/2172416] via 172.16.10.2, 00:05:35, Serial0/0
Now we can see that there are two successor routes to the 192.168.10.0 net-
work. Pretty sweet! But in the routing table, there’s one path to 192.168.20.0 and
192.168.30.0, with the link between the SF and NY routers being feasible successors.
And it’s the same with the 192.168.40.0 and 192.168.50.0 networks. Let’s take a look
at the topology table to examine this more closely:
Corp#sh ip eigrp topology
IP-EIGRP Topology Table for AS(20)/ID(10.10.11.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.10.10.0/24, 1 successors, FD is 128256
via Connected, GigabitEthernet0/0
P 10.10.11.0/24, 1 successors, FD is 128256
via Connected, GigabitEthernet0/1
P 10.10.20.0/24, 1 successors, FD is 2297856
via 172.16.10.2 (2297856/128256), Serial0/0
via 172.16.10.6 (2300416/156160), Serial0/1
P 10.10.30.0/24, 1 successors, FD is 2297856
via 172.16.10.2 (2297856/128256), Serial0/0
via 172.16.10.6 (2300416/156160), Serial0/1
P 10.10.40.0/24, 1 successors, FD is 2297856
via 172.16.10.6 (2297856/128256), Serial0/1
via 172.16.10.2 (2300416/156160), Serial0/0
P 10.10.50.0/24, 1 successors, FD is 2297856
via 172.16.10.6 (2297856/128256), Serial0/1
via 172.16.10.2 (2300416/156160), Serial0/0
P 192.168.10.0/24, 2 successors, FD is 2172416
via 172.16.10.2 (2172416/28160), Serial0/0
via 172.16.10.6 (2172416/28160), Serial0/1
P 172.16.10.4/30, 1 successors, FD is 2169856
via Connected, Serial0/1
P 172.16.10.0/30, 1 successors, FD is 2169856
via Connected, Serial0/0
Configuring EIGRP 807
It is nice that we can see that we have a successor and a feasible successor to each network,
so we know that EIGRP is doing its job. Let’s take a close look at the links to 10.10.20.0 now
and dissect what it’s telling us:
P 10.10.20.0/24, 1 successors, FD is 2297856
via 172.16.10.2 (2297856/128256), Serial0/0
via 172.16.10.6 (2300416/156160), Serial0/1
Okay—rst, we can see that it’s passive (P), which means that it has found all the
usable paths to the network 10.10.20.0 and is happy! If we see active (A), that means that
EIGRP is not happy at all and is querying its neighbors for a new path to that network. The
(2297856/128256) is the FD/AD, meaning that the SF router is advertising the 10.10.20.0
network as a cost of 128256, which is the AD. The Corp router adds the bandwidth and
delay of the line to get to the SF router and then adds that number to the AD (128256) to
come up with a total cost (FD) of 2297856 to get to network 10.10.20.0.
To become a CCNA R/S, you must understand how to read a topology table!
Unequal-Cost Load Balancing
As with all routing protocols running on Cisco routers, EIGRP automatically supports load
balancing over four equal-cost routes and can be congured to support up to 32 equal-cost
paths with IOS 15.0 code. As you know, previous IOS versions supported up to 16. I’ve
mentioned this a few times in this chapter already, but I want to show you how to congure
unequal-cost load balancing with EIGRP. First let’s take a look at the Corp router by typing
in the show ip protocols command:
Corp#sh ip protocols
Routing Protocol is "eigrp 20"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 20
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is not in effect
Maximum path: 4
808 Chapter 19 u Enhanced IGRP
Routing for Networks:
10.0.0.0
172.16.0.0
Routing Information Sources:
Gateway Distance Last Update
(this router) 90 19:15:10
172.16.10.6 90 00:25:38
172.16.10.2 90 00:25:38
Distance: internal 90 external 170
The variance 1 means equal path load balancing with the maximum paths set to 4 by
default. Unlike most other protocols, EIGRP also supports unequal-cost load balancing
through the use of the variance parameter.
To clarify, let’s say the parameter has been set to a variance of 2. This would effectively
load-balance trafc across the best route plus any route with a feasible distance of up to twice
as large. But still keep in mind that load balancing occurs in proportion with and relative to
the cost of the route, meaning that more trafc would travel across the best route than the
suboptimal one.
Let’s congure the variance on the Corp router and see if we can load-balance across
our feasible successors now:
Corp# config t
Corp(config)#router eigrp 20
Corp(config-router)#variance 2
Corp(config-router)#
*Feb 26 22:24:24:IP-EIGRP(Default-IP-Routing-Table:20):route installed for
10.10.20.0
*Feb 26 22:24:24:IP-EIGRP(Default-IP-Routing-Table:20):route installed for
10.10.20.0
*Feb 26 22:24:24:IP-EIGRP(Default-IP-Routing-Table:20):route installed for
10.10.30.0
*Feb 26 22:24:24:IP-EIGRP(Default-IP-Routing-Table:20):route installed for
10.10.30.0
*Feb 26 22:24:24:IP-EIGRP(Default-IP-Routing-Table:20):route installed for
10.10.40.0
*Feb 26 22:24:24:IP-EIGRP(Default-IP-Routing-Table:20):route installed for
10.10.40.0
*Feb 26 22:24:24:IP-EIGRP(Default-IP-Routing-Table:20):route installed for
10.10.50.0
*Feb 26 22:24:24:IP-EIGRP(Default-IP-Routing-Table:20):route installed for
10.10.50.0
*Feb 26 22:24:24:IP-EIGRP(Default-IP-Routing-Table:20):route installed for
192.168.10.0
*Feb 26 22:24:24:IP-EIGRP(Default-IP-Routing-Table:20):route installed for
192.168.10.0
Configuring EIGRP 809
Corp(config-router)#do show ip route
[output cut]
D 192.168.10.0/24 [90/2172416] via 172.16.10.6, 00:00:18, Serial0/1
[90/2172416] via 172.16.10.2, 00:00:18, Serial0/0
172.16.0.0/30 is subnetted, 2 subnets
C 172.16.10.4 is directly connected, Serial0/1
C 172.16.10.0 is directly connected, Serial0/0
10.0.0.0/24 is subnetted, 6 subnets
C 10.10.10.0 is directly connected, GigabitEthernet0/0
C 10.10.11.0 is directly connected, GigabitEthernet0/1
D 10.10.20.0 [90/2300416] via 172.16.10.6, 00:00:18, Serial0/1
[90/2297856] via 172.16.10.2, 00:00:19, Serial0/0
D 10.10.30.0 [90/2300416] via 172.16.10.6, 00:00:19, Serial0/1
[90/2297856] via 172.16.10.2, 00:00:19, Serial0/0
D 10.10.40.0 [90/2297856] via 172.16.10.6, 00:00:19, Serial0/1
[90/2300416] via 172.16.10.2, 00:00:19, Serial0/0
D 10.10.50.0 [90/2297856] via 172.16.10.6, 00:00:20, Serial0/1
[90/2300416] via 172.16.10.2, 00:00:20, Serial0/0
Corp(config-router)#
Nice—it worked! Now we have two paths to each remote network in the routing table,
even though the feasible distances to each route aren’t equal. Don’t forget that unequal load
balancing is not enabled by default and that you can perform load-balancing through paths
that have up to 128 times worse metrics than the successor route!
Split Horizon
Split horizon is enabled on interfaces by default, which means that if a route update is received
on an interface from a neighbor router, this interface will not advertise those networks back
out to the neighbor router who sent them. Let’s take a look at an interface and then go through
an example:
Corp#sh ip int s0/0
Serial0/0 is up, line protocol is up
Internet address is 172.16.10.1/24
Broadcast address is 255.255.255.255
Address determined by setup command
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Multicast reserved groups joined: 224.0.0.10
Outgoing access list is not set
810 Chapter 19 u Enhanced IGRP
Inbound access list is not set
Proxy ARP is enabled
Local Proxy ARP is disabled
Security level is default
Split horizon is enabled
[output cut]
Okay—we can see that split horizon is enabled by default. But what does this really
mean? Most of the time it’s more helpful than harmful, but let’s check out our internet-
work in Figure19.8 so I can really explain what split horizon is doing.
FIGURE19.8 Split horizon in action, part 1
s0/1
s0/0
Corp
SF
NY
10.10.10.0/24
g0/0
.2
10.10.20.0/24
10.10.30.0/24
10.10.11.0/24
g0/1
.6 10.10.40.0/24
10.10.50.0/24
Here is a route update for networks:
10.10.40.0/24
10.10.50.0/24
Here is a route update for the networks:
10.10.20.0/24
10.10.30.0/24
Notice that the SF and NY routers are each advertising their routes to the Corp router.
Now, let’s see what the Corp router sends back to each router in Figure19.9.
Verifying and Troubleshooting EIGRP 811
Can you see that the Corp router is not advertising back out the advertised networks that it
received on each interface? This is saving the SF and NY routers from receiving the incorrect
route information that they could possibly get to their own network through the Corp router,
which we know is wrong.
FIGURE19.9 Split horizon in action, part 2
s0/1
s0/0
Corp
SF
NY
10.10.10.0/24
g0/0
0.2
10.10.20.0/24
10.10.30.0/24
10.10.11.0/24
g0/1
0.6 10.10.40.0/24
10.10.50.0/24
Here is a route update for the networks:
10.10.10.0/24
10.10.11.0/24
10.10.40.0/24
10.10.50.0/24
Here is a route update for networks:
10.10.10.0/24
10.10.11.0/24
10.10.20.0/24
10.10.30.0/24
So how can this cause a problem? After all, it seems reasonable not to send misinfor-
mation back to an originating router, right? You’ll see this create a problem on point-to-
multipoint links, such as Frame Relay, when multiple remote routers connect to a single
interface at the Corp location. We can use logical interfaces, called subinterfaces, which
I’ll tell you all about in Chapter 7, “Wide Area Networks,” to solve the split horizon issue
on a point-to-multipoint interface.
Verifying and Troubleshooting EIGRP
Even though EIGRP usually runs smoothly and is relatively low maintenance, there are
several commands you need to memorize for using on a router that can be super helpful
when troubleshooting EIGRP! I’ve already showed you a few of them, but I’m going to
demonstrate all the tools you’ll need to verify and troubleshoot EIGRP now. Table19.2
contains all of the commands you need to know for verifying that EIGRP is functioning
well and offers a brief description of what each command does.
Visit ccna
.gg/ch19/a
for a
companion
MicroNugget
from CBT
Nuggets.
812 Chapter 19 u Enhanced IGRP
TABLE19.2 EIGRP troubleshooting commands
Command Description/Function
show ip eigrp neighbors Shows all EIGRP neighbors
show ip eigrp interfaces Lists the interfaces on which the router has actually
enabled EIGRP
show ip route eigrp Shows EIGRP entries in the routing table
show ip eigrp topology Shows entries in the EIGRP topology table
show ip eigrp traffic Shows the packet count for EIGRP packets sent
and received
show ip protocols Shows information about the active protocol sessions
When troubleshooting an EIGRP problem, it’s always a good idea to start by getting an
accurate map of the network, and the best way to do that is by using the show ip eigrp
neighbors command to nd out who your directly connected neighbors are. This command
shows all adjacent routers that share route information within a given AS. If neighbors are
missing, check the conguration, AS number, and link status on both routers to verify that
the protocol has been congured correctly.
Let’s execute the command on the Corp router:
Corp#sh ip eigrp neighbors
IP-EIGRP neighbors for process 20
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
1 172.16.10.2 Se0/0 11 03:54:25 1 200 0 127
0 172.16.10.6 Se0/1 11 04:14:47 1 200 0 2010
Here’s a breakdown of the important information we can see in the preceding output:
u H indicates the order in which the neighbor was discovered.
u Hold time in seconds is how long this router will wait for a Hello packet to arrive from
a specific neighbor.
u The Uptime value indicates how long the neighbor relationship has been established.
u The SRTT field is the smooth round-trip timer and represents how long it takes to com-
plete a round-trip from this router to its neighbor and back. This value delimits how
long to wait after a multicast for a reply from this neighbor. As mentioned earlier, the
router will attempt to establish communication via unicasts if it doesn’t receive a reply.
u The time between multicast attempts is specified by the Retransmission Time Out
(RTO) field, which is based upon the SRTT values.
Verifying and Troubleshooting EIGRP 813
u The Q value tells us if there are any outstanding messages in the queue. We can make a
mental note that there’s a problem if we see consistently large values here!
u Finally, the Seq field shows the sequence number of the last update from that neighbor,
which is used to maintain synchronization and avoid duplicate messages or their out-
of-sequence processing.
The neighbors command is a great command, but we can get local status of our router
by also using the show ip eigrp interface command like this:
Corp#sh ip eigrp interfaces
IP-EIGRP interfaces for process 20
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Gi0/0 0 0/0 0 0/1 0 0
Se0/1 1 0/0 1 0/15 50 0
Se0/0 1 0/0 1 0/15 50 0
Gi0/1 0 0/0 0 0/1 0 0
Corp#sh ip eigrp interface detail s0/0
IP-EIGRP interfaces for process 20
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Se0/0 1 0/0 1 0/15 50 0
Hello interval is 5 sec
Next xmit serial <none>
Un/reliable mcasts: 0/0 Un/reliable ucasts: 21/26
Mcast exceptions: 0 CR packets: 0 ACKs suppressed: 9
Retransmissions sent: 0 Out-of-sequence rcvd: 0
Authentication mode is not set
The rst command, show ip eigrp interfaces, lists all interfaces for which EIGRP is
enabled, as well as, those the router is currently sending Hello messages to in an attempt to
nd new EIGRP neighbors. The show ip eigrp interface detail interface command
lists more details per interface, including the local router’s own Hello interval. Understand
that you can use these commands to verify that all your interfaces are within the AS process
used by EIGRP, but also note that the passive interfaces won’t show up in these outputs. So
be sure to also check to see if an interface has been congured as passive if is not present in
the outputs.
Okay, if all neighbors are present, then verify the routes learned. By executing the show
ip route eigrp command, you’re given a quick picture of the routes in the routing table.
814 Chapter 19 u Enhanced IGRP
If a certain route doesn’t appear in the routing table, you need to verify its source. If the
source is functioning properly, then check the topology table.
The routing table according to Corp looks like this:
D 192.168.10.0/24 [90/2172416] via 172.16.10.6, 02:29:09, Serial0/1
[90/2172416] via 172.16.10.2, 02:29:09, Serial0/0
172.16.0.0/30 is subnetted, 2 subnets
C 172.16.10.4 is directly connected, Serial0/1
C 172.16.10.0 is directly connected, Serial0/0
10.0.0.0/24 is subnetted, 6 subnets
C 10.10.10.0 is directly connected, Loopback0
C 10.10.11.0 is directly connected, Loopback1
D 10.10.20.0 [90/2300416] via 172.16.10.6, 02:29:09, Serial0/1
[90/2297856] via 172.16.10.2, 02:29:10, Serial0/0
D 10.10.30.0 [90/2300416] via 172.16.10.6, 02:29:10, Serial0/1
[90/2297856] via 172.16.10.2, 02:29:10, Serial0/0
D 10.10.40.0 [90/2297856] via 172.16.10.6, 02:29:10, Serial0/1
[90/2300416] via 172.16.10.2, 02:29:10, Serial0/0
D 10.10.50.0 [90/2297856] via 172.16.10.6, 02:29:11, Serial0/1
[90/2300416] via 172.16.10.2, 02:29:11, Serial0/0
You can see here that most EIGRP routes are referenced with a D and that their admin-
istrative distance is 90. Remember that the [90/2300416] represents AD/FD, and in the
preceding output, EIGRP is performing equal- and unequal-cost load balancing between
two links to our remote networks.
We can see this by looking closer at two different networks. Pay special attention to the
FD of each output:
Corp#sh ip route | section 192.168.10.0
D 192.168.10.0/24 [90/2172416] via 172.16.10.6, 01:15:44, Serial0/1
[90/2172416] via 172.16.10.2, 01:15:44, Serial0/0
The preceding output show equal-cost load balancing, and here’s our unequal-cost load
balancing in action:
Corp#sh ip route | section 10.10.50.0
D 10.10.50.0 [90/2297856] via 172.16.10.6, 01:16:16, Serial0/1
[90/2300416] via 172.16.10.2, 01:16:16, Serial0/0
We can get the topology table displayed for us via the show ip eigrp topology command.
If the route is in the topology table but not in the routing table, it’s a pretty safe assumption
that there’s a problem between the topology database and the routing table. After all, there
must be a good reason the topology database isn’t adding the route into the routing table,
right? We discussed this issue in detail earlier in the chapter, and it’s oh so important!
Verifying and Troubleshooting EIGRP 815
Corp’s topology table looks like this:
P 10.10.10.0/24, 1 successors, FD is 128256
via Connected, GigabitEthernet0/0
P 10.10.11.0/24, 1 successors, FD is 128256
via Connected, GigabitEthernet0/1
P 10.10.20.0/24, 1 successors, FD is 2297856
via 172.16.10.2 (2297856/128256), Serial0/0
via 172.16.10.6 (2300416/156160), Serial0/1
P 10.10.30.0/24, 1 successors, FD is 2297856
via 172.16.10.2 (2297856/128256), Serial0/0
via 172.16.10.6 (2300416/156160), Serial0/1
P 10.10.40.0/24, 1 successors, FD is 2297856
via 172.16.10.6 (2297856/128256), Serial0/1
via 172.16.10.2 (2300416/156160), Serial0/0
P 10.10.50.0/24, 1 successors, FD is 2297856
via 172.16.10.6 (2297856/128256), Serial0/1
via 172.16.10.2 (2300416/156160), Serial0/0
P 192.168.10.0/24, 2 successors, FD is 2172416
via 172.16.10.2 (2172416/28160), Serial0/0
via 172.16.10.6 (2172416/28160), Serial0/1
P 172.16.10.4/30, 1 successors, FD is 2169856
via Connected, Serial0/1
P 172.16.10.0/30, 1 successors, FD is 2169856
via Connected, Serial0/0
Notice that every route in this output is preceded by a P, which shows that these routes are
in a passive state. This is good because routes in the active state indicate that the router has
lost its path to this network and is searching for a replacement. Each entry also reveals the
feasible distance, or FD, to each remote network, as well as, the next-hop neighbor through
which packets will travel to this destination. Each entry also has two numbers in brackets,
with the rst indicating the feasible distance and the second, the advertised distance to a
remote network.
Again, here’s our equal- and unequal-cost load-balancing output shown in the topol-
ogy table:
Corp#sh ip eigrp top | section 192.168.10.0
P 192.168.10.0/24, 2 successors, FD is 2172416
via 172.16.10.2 (2172416/28160), Serial0/0
via 172.16.10.6 (2172416/28160), Serial0/1
816 Chapter 19 u Enhanced IGRP
The preceding output shows equal-cost load balancing, and here is our unequal-cost
load balancing in action:
Corp#sh ip eigrp top | section 10.10.50.0
P 10.10.50.0/24, 1 successors, FD is 2297856
via 172.16.10.6 (2297856/128256), Serial0/1
via 172.16.10.2 (2300416/156160), Serial0/0
The command show ip eigrp traffic enables us to see if updates are being sent. If the
counters for EIGRP input and output packets don’t increase, it means that no EIGRP infor-
mation is being sent between peers. The following output indicates that the Corp router is
experiencing normal trafc:
Corp#show ip eigrp traffic
IP-EIGRP Traffic Statistics for process 200
Hellos sent/received: 2208/2310
Updates sent/received: 184/183
Queries sent/received: 17/4
Replies sent/received: 4/18
Acks sent/received: 62/65
Input queue high water mark 2, 0 drops
All of the packet types I talked about in the section on RTP are represented in the output
of this command. And we can’t forget the always useful troubleshooting command show ip
protocols. Here’s the output the Corp router gives us after using it:
Routing Protocol is "eigrp 20"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 2
Redistributing: eigrp 20
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is not in effect
Maximum path: 4
Routing for Networks:
10.0.0.0
172.16.0.0
Routing Information Sources:
Gateway Distance Last Update
Verifying and Troubleshooting EIGRP 817
(this router) 90 04:23:51
172.16.10.6 90 02:30:48
172.16.10.2 90 02:30:48
Distance: internal 90 external 170
In this output, we can see that EIGRP is enabled for autonomous system 20 and that the K
values are set to their defaults. The variance is 2, so both equal- and unequal-cost load balanc-
ing is happening here. Automatic summarization has been turned off. We can also see that
EIGRP is advertising two classful networks and that it sees two neighbors.
The show ip eigrp events command displays a log of every EIGRP event: when routes
are injected and removed from the routing table and when EIGRP adjacencies are reset or
fail. This information is so helpful in determining if there are routing instabilities in the
network! Be advised that this command can result in quite a ood of information even for
really simple congurations like ours. To demonstrate, here’s the output the Corp router
divulged after I used it:
Corp#show ip eigrp events
Event information for AS 20:
1 22:24:24.258 Metric set: 172.16.10.0/30 2169856
2 22:24:24.258 FC sat rdbmet/succmet: 2169856 0
3 22:24:24.258 FC sat nh/ndbmet: 0.0.0.0 2169856
4 22:24:24.258 Find FS: 172.16.10.0/30 2169856
5 22:24:24.258 Metric set: 172.16.10.4/30 2169856
6 22:24:24.258 FC sat rdbmet/succmet: 2169856 0
7 22:24:24.258 FC sat nh/ndbmet: 0.0.0.0 2169856
8 22:24:24.258 Find FS: 172.16.10.4/30 2169856
9 22:24:24.258 Metric set: 192.168.10.0/24 2172416
10 22:24:24.258 Route install: 192.168.10.0/24 172.16.10.2
11 22:24:24.258 Route install: 192.168.10.0/24 172.16.10.6
12 22:24:24.254 FC sat rdbmet/succmet: 2172416 28160
13 22:24:24.254 FC sat nh/ndbmet: 172.16.10.6 2172416
14 22:24:24.254 Find FS: 192.168.10.0/24 2172416
15 22:24:24.254 Metric set: 10.10.50.0/24 2297856
16 22:24:24.254 Route install: 10.10.50.0/24 172.16.10.6
17 22:24:24.254 FC sat rdbmet/succmet: 2297856 128256
18 22:24:24.254 FC sat nh/ndbmet: 172.16.10.6 2297856
19 22:24:24.254 Find FS: 10.10.50.0/24 2297856
20 22:24:24.254 Metric set: 10.10.40.0/24 2297856
21 22:24:24.254 Route install: 10.10.40.0/24 172.16.10.6
22 22:24:24.250 FC sat rdbmet/succmet: 2297856 128256
--More--
818 Chapter 19 u Enhanced IGRP
Troubleshooting Example with EIGRP
Throughout this chapter I’ve covered many of the problems that commonly occur with
EIGRP and how to verify and troubleshoot these issues. Make sure you clearly understand
what I have shown you so far in this chapter so you’re prepared to answer any question the
Cisco exam could possibly throw at you!
Just to make sure you’re solidly armed with all the skills you need to ace the exam, as
well as, successfully administer a network, I’m going to provide even more examples about
verifying EIGRP. We’ll be dealing with mostly the same commands and problems we’ve
already covered, but this is so important and the best way to get this all nailed down is to
practice troubleshooting an EIGRP network as much as possible!
With that, after you’ve congured EIGRP, you would rst test connectivity to the remote
network by using the Ping program. If that fails, you need to check whether the directly
connected router is in the neighbor table.
Here are some key things to look for if neighbors haven’t formed an adjacency:
u Interfaces between the devices are down.
u The two routers have mismatching EIGRP autonomous system numbers.
u Proper interfaces are not enabled for the EIGRP process.
u An interface is configured as passive.
u The K values are mismatched.
u EIGRP authentication is misconfigured.
Also, if the adjacency is up, but you’re not receiving remote network updates, there may
be a routing problem, likely caused by these issues:
u The proper networks aren’t being advertised under the EIGRP process.
u An access list is blocking the advertisements from remote networks.
u Automatic summary is causing confusion in your discontiguous network.
Let’s use Figure19.10 as our example network and run through some troubleshooting
scenarios. I’ve precongured the routers with IP addresses, and without having to try too
hard, I also snuck in a few snags for us to nd and x. Let’s see what we’re facing.
FIGURE19.10 Troubleshooting scenario
s0/0/0
192.168.1.2
192.168.1.1
s0/0
Corp Branch
10.1.1.0/24 10.2.2.0/24
A good place to start is by checking to see if we have an adjacency with show ip eigrp
neighbors and show ip eigrp interfaces. It’s also smart to see what information the
show ip eigrp topology command reveals:
Corp#sh ip eigrp neighbors
IP-EIGRP neighbors for process 20
Verifying and Troubleshooting EIGRP 819
Corp#
Corp#sh ip eigrp interfaces
IP-EIGRP interfaces for process 20
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Se0/1 0 0/0 0 0/15 50 0
Fa0/0 0 0/0 0 0/1 0 0
Se0/0 0 0/0 0 0/15 50 0
Corp#sh ip eigrp top
IP-EIGRP Topology Table for AS(20)/ID(10.10.11.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.1.1.0/24, 1 successors, FD is 28160
via Connected, FastEthernet0/0
Alright—we can see by looking at the neighbor and the interface as well as the topol-
ogy table command that our LAN is up on the Corp router but the serial link isn’t working
between routers because we don’t have an adjacency. From the show ip eigrp interfaces
command, we can establish that EIGRP is running on all interfaces, so that means our net-
work statements under the EIGRP process are probably correct, but we’ll verify that later.
Let’s move on by checking into our Physical and Data Link status with the show ip int
brief command because maybe there’s a physical problem between routers:
Corp#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.1.1.1 YES manual up up
Serial0/0 192.168.1.1 YES manual up up
FastEthernet0/1 unassigned YES manual administratively down down
Serial0/1 172.16.10.5 YES manual administratively down down
Corp#
Corp#sh protocols s0/0
Serial0/0 is up, line protocol is up
Internet address is 192.168.1.1/30
Corp#
820 Chapter 19 u Enhanced IGRP
Well, since the Serial0/0 interface shows the correct IP address and the status is up/up,
it means we have a good Data Link connection between routers, so it’s not a physical link
issue between the routers, which is good! Notice I also used the show protocols command,
which gave me the subnet mask for the link. Remember, the information obtained via the
two commands gives us only layer 1 and layer 2 status and doesn’t mean we can ping across
the link. In other words, we might have a layer 3 issue, so let’s check the Branch router with
the same commands:
Branch#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.2.2.2 YES manual up up
FastEthernet0/1 unassigned YES manual administratively down down
Serial0/0/0 192.168.1.2 YES manual up up
Serial0/0/1 unassigned YES unset administratively down down
Branch#
Branch#sh proto s0/0/0
Serial0/0/0 is up, line protocol is up
Internet address is 192.168.1.2/30
Branch#
Okay, well, we can see that our IP address and mask are correct, and that the link shows
up/up, so we’re looking pretty good! Let’s try to ping from the Corp router to the Branch
router now:
Corp#ping 192.168.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms
Corp#
Now because that was successful, we’ve ruled out layer 1, 2, or 3 issues between
routers at this point! Since everything seems to be working between the routers, except
EIGRP, checking our EIGRP congurations is our next move. Let’s start with the show
ip protocols command:
Corp#sh ip protocols
Routing Protocol is "eigrp 20"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
Verifying and Troubleshooting EIGRP 821
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 2
Redistributing: eigrp 20
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
10.0.0.0
172.16.0.0
192.168.1.0
Passive Interface(s):
FastEthernet0/1
Routing Information Sources:
Gateway Distance Last Update
(this router) 90 20:51:48
192.168.1.2 90 00:22:58
172.16.10.6 90 01:58:46
172.16.10.2 90 01:59:52
Distance: internal 90 external 170
This output shows us we’re using AS 20, that we don’t have an access-list lter list set on
the routing tables, and that our K values are set to default. We can see that we’re routing for
the 10.0.0.0, 172.16.0.0, and 192.168.1.0 networks and that we have a passive interface on
interface FastEthernet0/1. We don’t have an interface congured for the 172.16.0.0 network,
which means that this entry is an extra network statement under EIGRP. But that won’t hurt
anything, so this is not causing our issue. Last, the passive interface is not causing a problem
with this network either, because we’re not using interface Fa0/1. Still, keep in mind that
when troubleshooting, it’s always good to see if there are any interfaces set to passive.
Let’s see what the show interface command will tell us:
Corp#sh interfaces s0/0
Serial0/0 is up, line protocol is up
Hardware is PowerQUICC Serial
Description: <<Connection to Branch>>
Internet address is 192.168.1.1/30
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set
[output cut]
822 Chapter 19 u Enhanced IGRP
Looks like our statistics are set to defaults, so nothing really pops as a problem here. But
remember when I covered the steps to check if there is no adjacency back at the beginning
of this section? In case you forgot, here’s a list of things to investigate:
u The interface between the devices are down.
u The two routers have mismatching EIGRP autonomous system numbers.
u The proper interfaces aren’t enabled for the EIGRP process.
u An interface is configured as passive.
u K values are mismatched.
u EIGRP authentication is misconfigured.
Okay, our interfaces are not down, our AS number matches, layer 3 is working between
routers, all the interfaces show up under the EIGRP process, and none of our needed inter-
faces are passive, so now we’ll have to look even deeper into the EIGRP conguration to
uncover the problem.
Since the Corp router has the basic default congurations, we need to check the Branch
router’s EIGRP conguration:
Branch#sh ip protocols
Routing Protocol is "eigrp 20"
Outgoing update filter list for all interfaces is 10
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=0, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 20
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is not in effect
Maximum path: 4
Routing for Networks:
10.0.0.0
192.168.1.0
Routing Information Sources:
Gateway Distance Last Update
192.168.1.1 90 00:27:09
Distance: internal 90 external 170
This router has the correct AS—always check this rst—and we’re routing for the correct
networks. But I see two possible snags here, do you? First, the outgoing ACL lter list is set,
but the metrics are not set to default. Remember, just because an ACL is set doesn’t mean it’s
automatically giving you grief. Second, the K values must match, and we know these values
are not matching the Corp router!
Verifying and Troubleshooting EIGRP 823
Let’s take a look at the Branch interface statistics to see what else might be wrong:
Branch>sh int s0/0/0
Serial0/0/0 is up, line protocol is up
Hardware is GT96K Serial
Internet address is 192.168.1.2/30
MTU 1500 bytes, BW 512 Kbit, DLY 30000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set
[output cut]
Aha! The bandwidth and delay are not set to their defaults and don’t match the directly
connected Corp router. Let’s start by changing those back to the default and see if that xes
our problem:
Branch#config t
Branch(config)#int s0/0/0
Branch(config-if)#no bandwidth
Branch(config-if)#no delay
And let’s check out our stats now to see if we’re back to defaults:
Branch#sh int s0/0/0
Serial0/0/0 is up, line protocol is up
Hardware is GT96K Serial
Internet address is 192.168.1.2/30
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set
[output cut]
The bandwidth and delay are now at the defaults, so let’s check our adjacencies next:
Corp#sh ip eigrp neighbors
IP-EIGRP neighbors for process 20
Corp#
Okay, so it wasn’t the bandwidth and delay settings because our adjacency didn’t come
up, so let’s set our K values back to default like this:
Branch#config t
Branch(config)#router eigrp 20
Branch(config-router)#metric weights 0 1 0 1 0 0
Branch(config-router)#do sho ip proto
Routing Protocol is "eigrp 20"
824 Chapter 19 u Enhanced IGRP
Outgoing update filter list for all interfaces is 10
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
[output cut]
I know this probably seems a little complicated at rst, but it’s something you shouldn’t
have to do much, if ever. Remember, there are ve K values, so why 6 numbers? The rst
number listed is type of service (ToS), so always just set that to 0, which means you must type
in six numbers as shown in my conguration example. After we chose the default of 0 rst,
the default K values are then 1 0 1 0 0, which is bandwidth and delay enabled. Let’s check
our adjacency now:
Corp#sh ip eigrp neighbors
IP-EIGRP neighbors for process 20
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 192.168.1.2 Se0/0 14 00:02:09 7 200 0 18
Bam! There we go! Looks like mismatched K values were our problem. Now let’s just
check to make sure we can ping from end to end and we’re done:
Corp#ping 10.2.2.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.2.2.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
Corp#
Rats! It looks like even though we have our adjacency, we still can’t reach our remote
network. Next step? Let’s see what the routing table shows us:
Corp#sh ip route
[output cut]
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.1.1.0/24 is directly connected, FastEthernet0/0
D 10.0.0.0/8 is a summary, 00:18:55, Null0
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.1.0/30 is directly connected, Serial0/0
D 192.168.1.0/24 is a summary, 00:18:55, Null0
Verifying and Troubleshooting EIGRP 825
The problem is screamingly clear now because I went through this in detail throughout
this chapter. But just in case you still can’t nd it, let’s look at the show ip protocols
command output:
Routing Protocol is "eigrp 20"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 2
Redistributing: eigrp 20
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Automatic address summarization:
192.168.1.0/24 for FastEthernet0/0
Summarizing with metric 2169856
10.0.0.0/8 for Serial0/0
Summarizing with metric 28160
[output cut]
By looking at the Figure19.10 you should have noticed right away that we had a discon-
tiguous network. This means that unless they are running 15.0 IOS code, the routers will
auto-summarize, so we need to disable auto-summary:
Branch(config)#router eigrp 20
Branch(config-router)#no auto-summary
008412:%DUAL-5-NBRCHANGE:IP-EIGRP(0) 20:Neighbor 192.168.1.1 (Serial0/0/0) is
resync:
peer graceful-restart
Corp(config)#router eigrp 20
Corp(config-router)#no auto-summary
Corp(config-router)#
*Feb 27 19:52:54:%DUAL-5-NBRCHANGE: IP-EIGRP(0) 20:Neighbor 192.168.1.2
(Serial0/0)
is resync: summary configured
*Feb 27 19:52:54.177:IP-EIGRP(Default-IP-Routing-Table:20):10.1.1.0/24 - do
advertise
out Serial0/0
*Feb 27 19:52:54:IP-EIGRP(Default-IP-Routing-Table:20):Int 10.1.1.0/24 metric
2816
826 Chapter 19 u Enhanced IGRP
0 - 25600 2560
*Feb 27 19:52:54:IP-EIGRP(Default-IP-Routing-Table:20):192.168.1.0/30 - do
advertise out Serial0/0
*Feb 27 19:52:54:IP-EIGRP(Default-IP-Routing-Table:20):192.168.1.0/24 - do
advertise out Serial0/0
*Feb 27 19:52:54:IP-EIGRP(Default-IP-Routing-Table:20):Int 192.168.1.0/24 metric
4294967295 - 0 4294967295
*Feb 27 19:52:54:IP-EIGRP(Default-IP-Routing-Table:20):10.0.0.0/8 - do advertise
out Serial0/0
Corp(config-router)#
*Feb 27 19:52:54:IP-EIGRP(Default-IP-Routing-Table:20):Int 10.0.0.0/8 metric
4294967295 - 0 4294967295
*Feb 27 19:52:54:IP-EIGRP(Default-IP-Routing-Table:20):Processing incoming REPLY
packet
*Feb 27 19:52:54:IP-EIGRP(Default-IP-Routing-Table:20):Int 192.168.1.0/24 M
4294967295 - 1657856 4294967295 SM 4294967295 - 1657856 4294967295
*Feb 27 19:52:54:IP-EIGRP(Default-IP-Routing-Table:20):Int 10.0.0.0/8 M
4294967295 - 25600 4294967295 SM 4294967295 - 25600 4294967295
*Feb 27 19:52:54:IP-EIGRP(Default-IP-Routing-Table:20):Processing incoming UPDATE
packet
Finally the Corp looks happy, so it looks like we’re good to go! Let’s just check our routing
table to be sure:
Corp#sh ip route
[output cut]
10.0.0.0/24 is subnetted, 1 subnets
C 10.1.1.0 is directly connected, FastEthernet0/0
192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.0 is directly connected, Serial0/0
What the heck? How can this be! We saw all those updates on the Corp console, right?
Let’s check the conguration of EIGRP by looking at the active conguration on the
Branch router:
Branch#sh run
[output cut]
!
router eigrp 20
network 10.0.0.0
network 192.168.1.0
distribute-list 10 out
no auto-summary
!
Verifying and Troubleshooting EIGRP 827
We can see that the access list is set outbound on the routing table of the Branch router.
This may be preventing us from receiving the updates from remote networks! Let’s see what
the ACL 10 list is doing:
Branch#sh access-lists
Standard IP access list 10
10 deny any (40 matches)
20 permit any
Now who in the world would stick an access list like this on a router? This ACL says
to deny every packet, which makes the second line of the ACL irrelevant since every single
packet will match the rst line! This has got to be the source of our troubles, so let’s remove
that list and see if the Corp router starts working:
Branch#config t
Branch(config)#router eigrp 20
Branch(config-router)#no distribute-list 10 out
Okay, with that ugly thing gone, let’s check to see if we’re receiving our remote net-
works now:
Corp#sh ip route
[output cut]
10.0.0.0/24 is subnetted, 2 subnets
D 10.2.2.0 [90/2172416] via 192.168.1.2, 00:00:24, Serial0/0
C 10.1.1.0 is directly connected, FastEthernet0/0
192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.0 is directly connected, Serial0/0
Corp#
Corp#ping 10.2.2.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.2.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms
Corp#
Clear skies! We’re up and running. We had mismatched K values, discontiguous network-
ing, and a nasty ACL on our routing table. For the CCNA R/S objectives, always check for
an ACL on the actual interface as well, not just in the routing table. It could be set on the
interface or routing table, either one, or both! And never forget to check for passive interfaces
when troubleshooting a routing protocol issue!
828 Chapter 19 u Enhanced IGRP
All of these commands are seriously powerful tools in the hands of a savvy professional
faced with the task of troubleshooting myriad network issues. I could go on and on about
the profusion of information these commands can generate and how well they can equip us
to solve virtually every networking ill, but that would be way outside the scope of this book.
Even so, I have no doubt that the foundation I’ve given you here will prove practical and
valuable for certication purposes, as well as, when working in the real networking world.
Now it’s time to relax a bit as we move into the easiest part of this chapter, seriously—
not joking! You still need to pay attention though.
EIGRPv6
As I just was saying, welcome to the easiest part of the chapter! Of course, I only mostly
mean that, and here’s why: I talked about IPv6 in the earlier ICND1 chapters, and in order
to continue on with this section of the chapter, you need to have that vital, foundational
part of IPv6 down solidly before you dare to dwell here! If you do, you’re pretty much set
and this will all be pretty simple for you.
EIGRPv6 works much the same way as its IPv4 predecessor does—most of the features
that EIGRP provided before EIGRPv6 will still be available.
EIGRPv6 is still an advanced distance-vector protocol that has some link-state features.
The neighbor discovery process using Hellos still happens, and it still provides reliable com-
munication with Reliable Transport Protocol that gives us loop-free fast convergence using
the Diffusing Update Algorithm (DUAL).
Hello packets and updates are sent using multicast transmission, and as with RIPng,
EIGRPv6’s multicast address stayed almost the same. In IPv4 it was 224.0.0.10; in IPv6,
it’s FF02::A (A = 10 in hexadecimal notation).
But clearly, there are key differences between the two versions. Most notably the use of
the pesky network command is gone, so it’s hard to make a mistake with EIGRPv6. Also,
the network and interface to be advertised must be enabled from interface conguration
mode with one simple command.
But you still have to use the router conguration mode to enable the routing protocol in
EIGRPv6 because the routing process must be literally enabled like an interface with the
no shutdown command—interesting! However, the 15.0 code does enable this by default,
so this command actually may or may not be needed.
Here’s an example of enabling EIGRPv6 on the Corp router:
Corp(config)#ipv6 unicast-routing
Corp(config)#ipv6 router eigrp 10
The 10 in this case is still the AS number. The prompt changes to (config-rtr), and
from here, just initiate a no shutdown if needed:
Corp(config-rtr)#no shutdown
Visit ccna
.gg/ch19/b
for a
companion
MicroNugget
from CBT
Nuggets.
EIGRPv6 829
Other options also can be congured in this mode, like redistribution and router ID
(RID). So now, let’s go to the interface and enable IPv6:
Corp(config-if)#ipv6 eigrp 10
The 10 in the interface command again references the AS number that was enabled in
the conguration mode.
Figure19.11 shows the layout we’ve been using throughout this chapter, only with IPv6
addresses now assigned to interfaces. I used the EUI-64 option on each interface so each router
assigned themselves an IPv6 address after I typed in the 64-bit network/subnet address.
FIGURE19.11 Configuring EIGRPv6 on our Internetwork
Corp
SF
NY
g0/0
g0/1
2001:db8:3c4d:13::/64
2001:db8:3c4d:14::/64
g0/0
g0/1
2001:db8:3c4d:17::/64
2001:db8:3c4d:18::/64
g0/0
g0/1
2001:db8:3c4d:15::/64
2001:db8:3c4d:16::/64
2001:db8:3c4d:11::/64
2001:db8:3c4d:12::/64
We’ll start with the Corp router. Really, all we need to know in order to enable EIGRPv6
are which interfaces we’re using and want to advertise our networks.
Corp#config t
Corp(config)#ipv6 router eigrp 10
Corp(config-rtr)#no shut
Corp(config-rtr)#router-id 1.1.1.1
Corp(config-rtr)#int s0/0/0
Corp(config-if)#ipv6 eigrp 10
Corp(config-if)#int s0/0/1
Corp(config-if)#ipv6 eigrp 10
Corp(config-if)#int g0/0
Corp(config-if)#ipv6 eigrp 10
Corp(config-if)#int g0/1
Corp(config-if)#ipv6 eigrp 10
I had erased and reloaded the routers before I started this EIGRPv6 section of the chapter.
What this means is that there were no 32-bit addresses on the router in order to create the
830 Chapter 19 u Enhanced IGRP
RID for EIGRP, so I had to set it under the IPv6 router global command, which is the same
command used with EIGRP and EIGRPv6. Unlike OSPF, the RID isn’t that important, and it
can actually be the same address on every router. You just can’t get away with doing this with
OSPF! The conguration for EIGRPv6 was pretty straightforward because unless you type
the AS number wrong, it’s pretty hard to screw this up!
Okay, let’s congure the SF and NY routers now, and then we’ll verify our networks:
SF#config t
SF(config)#ipv6 router eigrp 10
SF(config-rtr)#no shut
SF(config-rtr)#router-id 2.2.2.2
SF(config-rtr)#int s0/0/0
SF(config-if)#ipv6 eigrp 10
SF(config-if)#int g0/0
SF(config-if)#ipv6 eigrp 10
SF(config-if)#int g0/1
SF(config-if)#ipv6 eigrp 10
NY#config t
NY(config)#ipv6 router eigrp 10
NY(config-rtr)#no shut
NY(config-rtr)#router-id 3.3.3.3
NY(config-rtr)#int s0/0/0
NY(config-if)#ipv6 eigrp 10
NY(config-if)#int g0/0
NY(config-if)#ipv6 eigrp 10
NY(config-if)#int g0/1
Since we congured EIGRPv6 on a per-interface basis, no worries about having to use
the passive-interface command. This is because if we don’t enable the routing protocol
on an interface, it’s just not part of the EIGRPv6 process. We can see which interfaces are
part of the EIGRPv6 process with the show ipv6 eigrp interfaces command like this:
Corp#sh ipv6 eigrp interfaces
IPv6-EIGRP interfaces for process 10
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Se0/0/0 1 0/0 1236 0/10 0 0
Se0/0/1 1 0/0 1236 0/10 0 0
Gig0/1 0 0/0 1236 0/10 0 0
Gig0/0 0 0/0 1236 0/10 0 0
Corp#
EIGRPv6 831
Looks great so far—all the interfaces we want in our AS are listed, so we’re looking good
for our Corp’s local conguration. Now it’s time to check if our adjacencies came up with
the show ipv6 eigrp neighbors command:
Corp#sh ipv6 eigrp neighbors
IPv6-EIGRP neighbors for process 10
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 Link-local address: Se0/0/0 10 00:01:40 40 1000 0 11
FE80::201:C9FF:FED0:3301
1 Link-local address: Se0/0/1 14 00:01:24 40 1000 0 11
FE80::209:7CFF:FE51:B401
It’s great that we can see neighbors listed off of each serial interface, but do you notice
something missing from the preceding output? That’s right, the actual IPv6 network/subnet
addresses of the links aren’t listed in the neighbor table! Only the link-local addresses are
used for forming EIGRP neighbor adjacencies. With IPv6, neighbor interfaces and next-hop
addresses are always link-local.
We can verify our conguration with the show ip protocols command:
Corp#sh ipv6 protocols
IPv6 Routing Protocol is "connected"
IPv6 Routing Protocol is "static
IPv6 Routing Protocol is "eigrp 10 "
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Interfaces:
Serial0/0/0
Serial0/0/1
GigabitEthernet0/0
GigabitEthernet0/1
Redistributing: eigrp 10
Maximum path: 16
Distance: internal 90 external 170
You can verify the AS number from this output, but be sure to verify your K values, vari-
ance, and interfaces, too. Remember that the AS number and interfaces are the rst factors
to check when troubleshooting.
The topology table lists all feasible routes in the network, so this output can be rather long,
but let’s see what this shows us:
Corp#sh ipv6 eigrp topology
IPv6-EIGRP Topology Table for AS 10/ID(1.1.1.1)
832 Chapter 19 u Enhanced IGRP
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - Reply status
P 2001:DB8:C34D:11::/64, 1 successors, FD is 2169856
via Connected, Serial0/0/0
P 2001:DB8:C34D:12::/64, 1 successors, FD is 2169856
via Connected, Serial0/0/1
P 2001:DB8:C34D:14::/64, 1 successors, FD is 2816
via Connected, GigabitEthernet0/1
P 2001:DB8:C34D:13::/64, 1 successors, FD is 2816
via Connected, GigabitEthernet0/0
P 2001:DB8:C34D:17::/64, 1 successors, FD is 2170112
via FE80::201:C9FF:FED0:3301 (2170112/2816), Serial0/0/0
P 2001:DB8:C34D:18::/64, 1 successors, FD is 2170112
via FE80::201:C9FF:FED0:3301 (2170112/2816), Serial0/0/0
P 2001:DB8:C34D:15::/64, 1 successors, FD is 2170112
via FE80::209:7CFF:FE51:B401 (2170112/2816), Serial0/0/1
P 2001:DB8:C34D:16::/64, 1 successors, FD is 2170112
via FE80::209:7CFF:FE51:B401 (2170112/2816), Serial0/0/1
Since we only have eight networks in our internetwork, we can see all eight networks
in the topology table, which clearly is as it should be. I’ve highlighted a couple of things I
want to discuss, and the rst is that you need to be able to read and understand a topology
table. This includes understanding which routes are directly connected and which are being
advertised via neighbors. The via Connected shows us our directly connected networks.
The second item I want to show you is (2170112/2816), which is the FD/AD, and by the
way, it’s no different than if you’re working with IPv4.
So let’s wrap up this chapter by taking a look at a routing table:
Corp#sh ipv6 route eigrp
IPv6 Routing Table - 13 entries
Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP
U - Per-user Static route, M - MIPv6
I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary
O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2
ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2
D - EIGRP, EX - EIGRP external
C 2001:DB8:C34D:11::/64 [0/0]
via ::, Serial0/0/0
L 2001:DB8:C34D:11:230:A3FF:FE36:B101/128 [0/0]
Summary 833
via ::, Serial0/0/0
C 2001:DB8:C34D:12::/64 [0/0]
via ::, Serial0/0/1
L 2001:DB8:C34D:12:230:A3FF:FE36:B102/128 [0/0]
via ::, Serial0/0/1
C 2001:DB8:C34D:13::/64 [0/0]
via ::, GigabitEthernet0/0
L 2001:DB8:C34D:13:2E0:F7FF:FEDA:7501/128 [0/0]
via ::, GigabitEthernet0/0
C 2001:DB8:C34D:14::/64 [0/0]
via ::, GigabitEthernet0/1
L 2001:DB8:C34D:14:2E0:F7FF:FEDA:7502/128 [0/0]
via ::, GigabitEthernet0/1
D 2001:DB8:C34D:15::/64 [90/2170112]
via FE80::209:7CFF:FE51:B401, Serial0/0/1
D 2001:DB8:C34D:16::/64 [90/2170112]
via FE80::209:7CFF:FE51:B401, Serial0/0/1
D 2001:DB8:C34D:17::/64 [90/2170112]
via FE80::201:C9FF:FED0:3301, Serial0/0/0
D 2001:DB8:C34D:18::/64 [90/2170112]
via FE80::201:C9FF:FED0:3301, Serial0/0/0
L FF00::/8 [0/0]
via ::, Null0
I highlighted the EIGRPv6 injected routes that were injected into the routing table.
It’s important to notice that in order for IPv6 to get to a remote network, the router
uses the next hop link-local address. Do you see that in the table? For example, via
FE80::209:7CFF:FE51:B401, Serial0/0/1 is the link-local address of the NY router.
See? I told you it was easy!
Summary
It’s true that this chapter has been pretty extensive, so let’s briey recap what we covered in it.
EIGRP, the main focus of the chapter, is a hybrid of link-state routing and typically referred to
as an advanced distance-vector protocol. It allows for unequal-cost load balancing, controlled
routing updates, and formal neighbor adjacencies called relationships to be formed.
EIGRP uses the capabilities of the Reliable Transport Protocol (RTP) to communicate
between neighbors and utilizes the Diffusing Update Algorithm (DUAL) to compute the
best path to each remote network.
834 Chapter 19 u Enhanced IGRP
We also covered the conguration of EIGRP and explored a number of troubleshooting
commands plus key ways and means to help solve some common networking issues.
Moving on, EIGRP facilitates unequal-cost load balancing, controlled routing updates,
and formal neighbor adjacencies.
I also went over the conguration of EIGRP and explored a number of troubleshooting
commands, as well as, taking you through a highly informative scenario that will not only
help you to ace the exam, it will help you confront and overcome many troubleshooting
issues common to today’s internetworks!
Finally, I went over the easiest section at the end of this long chapter: EIGRPv6. Easy to
understand, congure, and verify!
Exam Essentials
Know EIGRP features. EIGRP is a classless, advanced distance-vector protocol that sup-
ports IP and now IPv6. EIGRP uses a unique algorithm, called DUAL, to maintain route
information and uses RTP to communicate with other EIGRP routers reliably.
Know how to configure EIGRP. Be able to congure basic EIGRP. This is congured the
same as RIP with classful addresses.
Know how to verify EIGRP operation. Know all of the EIGRP show commands and be
familiar with their output and the interpretation of the main components of their output.
Be able to read an EIGRP topology table. Understand which are successors, which are
feasible successors, and which routes will become successors if the main successor fails.
You must be able to troubleshoot EIGRP. Go through the EIGRP troubleshooting sce-
nario and make sure you understand to look for the AS number, ACLs, passive interfaces,
variance, and other factors.
Be able to read an EIGRP neighbor table. Understand the output of the show ip eigrp
neighbor command.
Understand how to configure EIGRPv6. To congure EIGRPv6, rst create the autono-
mous system from global conguration mode and perform a no shutdown. Then enable
EIGRPv6 on each interface individually.
Hands-on Labs 835
Written Lab 5
The answers to this lab can be found in Appendix A, “Answers to Written Labs.”
1. What is the command to enable EIGRPv6 from global configuration mode?
2. What is the EIGRPv6 multicast address?
3. True/False: Each router within an EIGRP domain must use different AS numbers.
4. If you have two routers with various K-values assigned, what will this do to the link?
5. What type of EIGRP interface will neither send nor receive Hello packets?
Hands-on Labs
In this section, you will use the following network and add EIGRP and EIGRPv6 routing.
S0/0/0
192.168.1.2
192.168.1.1
S0/0
RouterA RouterB
10.1.1.0/24 10.2.2.0/24
2001:db8:3c4d:1::/64
g0/0 g0/0
2001:db8:3c4d:3::/642001:db8:3c4d:2::/64
The rst lab requires you to congure two routers for EIGRP and then view the congu-
ration. In the last lab, you will be asked to enable EIGRPv6 routing on the same network.
Note that the labs in this chapter were written to be used with real equipment—real cheap
equipment that is. I wrote these labs with the cheapest, oldest routers I had lying around
so you can see that you don’t need expensive gear to get through some of the hardest labs
in this book. However, you can use the free LammleSim IOS version simulator or Cisco’s
Packet Tracer to run through these labs.
The labs in this chapter are as follows:
Lab 5.1: Conguring and Verifying EIGRP
Lab 5.2: Conguring and Verifying EIGRPv6
Hands-on Lab 5.1: Configuring and Verifying EIGRP
This lab will assume you have congured the IP addresses on the interfaces as shown in the
preceding diagram.
1. Implement EIGRP on RouterA.
RouterA#conf t
Enter configuration commands, one per line.
End with CNTL/Z.
836 Chapter 19 u Enhanced IGRP
RouterA(config)#router eigrp 100
RouterA(config-router)#network 192.168.1.0
RouterA(config-router)#network 10.0.0.0
RouterA(config-router)#^Z
RouterA#
2. Implement EIGRP on RouterB.
RouterB#conf t
Enter configuration commands, one per line.
End with CNTL/Z.
RouterB(config)#router eigrp 100
RouterB(config-router)#network 192.168.1.0
RouterA(config-router)#network 10.0.0.0
RouterB(config-router)#exit
RouterB#
3. Display the topology table RouterA.
RouterA#show ip eigrp topology
4. Display the routing table for RouterA.
RouterA #show ip route
5. Display the neighbor table for RouterA.
RouterA show ip eigrp neighbor
6. Type the command on each router to fix the routing problem. You did see a problem
didn’t you? Yes, the network is discontiguous.
RouterA#config t
RouterA(config)#router eigrp 100
RouterA(config-router)#no auto-summary
RouterB#config t
RouterA(config)#router eigrp 100
RouterA(config-router)#no auto-summary
7. Verify your routes with the show ip route command.
Hands-on Labs 837
Hands-on Lab 5.2: Configuring and Verifying EIGRPv6
This lab will assume you congured the IPv6 address as shown in the diagram preceding
Lab 5.1.
1. Implement EIGRPv6 on RouterA with AS 100.
RouterA#config t
RouterA (config)#ipv6 router eigrp 100
RouterA (config-rtr)#no shut
RouterA (config-rtr)#router-id 2.2.2.2
RouterA (config-rtr)#int s0/0
RouterA (config-if)#ipv6 eigrp 100
RouterA (config-if)#int g0/0
RouterA (config-if)#ipv6 eigrp 100
2. Implement EIGRP on RouterB.
RouterA#config t
RouterB(config)#ipv6 router eigrp 100
RouterB(config-rtr)#no shut
RouterB(config-rtr)#router-id 2.2.2.2
RouterB(config-rtr)#int s0/0
RouterB(config-if)#ipv6 eigrp 100
RouterB(config-if)#int g0/0
RouterB(config-if)#ipv6 eigrp 100
3. Display the topology table RouterA.
RouterA#show ipv6 eigrp topology
4. Display the routing table for RouterA.
RouterA #show ipv6 route
5. Display the neighbor table for RouterA.
RouterA show ipv6 eigrp neighbor
838 Chapter 19 u Enhanced IGRP
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional questions,
please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. There are three possible routes for a router to reach a destination network. The first
route is from OSPF with a metric of 782. The second route is from RIPv2 with a metric
of 4. The third is from EIGRP with a composite metric of 20514560. Which route will
be installed by the router in its routing table?
A. RIPv2
B. EIGRP
C. OSPF
D. All three
2. Which EIGRP information is held in RAM and maintained through the use of Hello
and update packets? (Choose two.)
A. Neighbor table
B. STP table
C. Topology table
D. DUAL table
3. What will be the reported distance to a downstream neighbor router for the 10.10.30.0
network, with the neighbor adding the cost to this neighbor to find the true FD?
P 10.10.30.0/24, 1 successors, FD is 2297856
via 172.16.10.2 (2297856/128256), Serial0/0
A. Four hops
B. 2297856
C. 128256
D. EIGRP doesn’t use reported distances.
Review Questions 839
4. Where are EIGRP successor routes stored?
A. In the routing table only
B. In the neighbor table only
C. In the topology table only
D. In the routing table and the neighbor table
E. In the routing table and the topology table
F. In the topology table and the neighbor table
5. Which command will display all the EIGRP feasible successor routes known to a router?
A. show ip routes *
B. show ip eigrp summary
C. show ip eigrp topology
D. show ip eigrp adjacencies
E. show ip eigrp neighbors detail
6. Which of the following commands are used when routing with EIGRP or EIGRPv6?
(Choose three.)
A. network 10.0.0.0
B. eigrp router-id
C. variance
D. router eigrp
E. maximum-paths
7. Serial0/0 goes down. How will EIGRP send packets to the 10.1.1.0 network?
Corp#show ip eigrp topology
[output cut]
P 10.1.1.0/24, 2 successors, FD is 2681842
via 10.1.2.2 (2681842/2169856), Serial0/0
via 10.1.3.1 (2973467/2579243), Serial0/2
via 10.1.3.3 (2681842/2169856), Serial0/1
A. EIGRP will put the 10.1.1.0 network into active mode.
B. EIGRP will drop all packets destined for 10.1.1.0.
C. EIGRP will just keep sending packets out s0/1.
E. EIGRP will use s0/2 as the successor and keep routing to 10.1.1.0.
840 Chapter 19 u Enhanced IGRP
8. What command do you use to enable EIGRPv6 on an interface?
A. router eigrp as
B. ip router eigrp as
C. router eigrpv6 as
D. ipv6 eigrp as
9. What command was typed in to have these two paths to network 10.10.50.0 in the
routing table?
D 10.10.50.0 [90/2297856] via 172.16.10.6, 00:00:20, Serial0/1
[90/6893568] via 172.16.10.2, 00:00:20, Serial0/0
A. maximum-paths 2
B. variance 2
C. variance 3
D. maximum-hops 2
10. A route to network 10.10.10.0 goes down. How does EIGRP respond in the local
routing table? (Choose two.)
A. It sends a poison reverse with a maximum hop of 16.
B. If there is a feasible successor, that is copied and placed into the routing table.
C. If a feasible successor is not found, a query will be sent to all neighbors asking for
a path to network 10.10.10.0.
D. EIGRP will broadcast out all interfaces that the link to network 10.10.10.0 is
down and that it is looking for a feasible successor.
11. You need the IP address of the devices with which the router has established an adja-
cency. Also, the retransmit interval and the queue counts for the adjacent routers need
to be checked. What command will display the required information?
A. show ip eigrp adjacency
B. show ip eigrp topology
C. show ip eigrp interfaces
D. show ip eigrp neighbors
Review Questions 841
12. For some reason, you cannot establish an adjacency relationship on a common Ether-
net link between two routers. Looking at the output shown here, what are the causes
of the problem? (Choose two.)
RouterA##show ip protocols
Routing Protocol is "eigrp 20"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
RouterB##show ip protocols
Routing Protocol is "eigrp 220"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=1, K3=1, K4=0, K5=0
A. EIGRP is running on RouterA and OSPF is running RouterB.
B. There is an ACL set on the routing protocol.
C. The AS numbers don’t match.
D. There is no default network accepted from incoming updates.
E. The K-values don’t match.
F. There is a passive interface set.
13. Which are true regarding EIGRP successor routes? (Choose two.)
A. A successor route is used by EIGRP to forward traffic to a destination.
B. Successor routes are saved in the topology table to be used if the primary route fails.
C. Successor routes are flagged as “active” in the routing table.
D. A successor route may be backed up by a feasible successor route.
E. Successor routes are stored in the neighbor table following the discovery process.
14. The remote RouterB router has a directly connected network of 10.255.255.64/27.
Which two of the following EIGRP network statements could you use so this directly
connected network will be advertised under the EIGRP process? (Choose two.)
A. network 10.255.255.64
B. network 10.255.255.64 0.0.0.31
C. network 10.255.255.64 0.0.0.0
D. network 10.255.255.64 0.0.0.15
842 Chapter 19 u Enhanced IGRP
15. RouterA and RouterB are connected via their Serial 0/0 interfaces, but they have not
formed an adjacency. Based on the following output, what could be the problem?
RouterA#sh ip protocols
Routing Protocol is "eigrp 220"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 2
Redistributing: eigrp 220
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
10.0.0.0
172.16.0.0
192.168.1.0
Routing Information Sources:
Gateway Distance Last Update
(this router) 90 20:51:48
192.168.1.2 90 00:22:58
172.16.10.6 90 01:58:46
172.16.10.2 90 01:59:52
Distance: internal 90 external 170
RouterB#sh ip protocols
Routing Protocol is "eigrp 220"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 2
Redistributing: eigrp 220
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
10.0.0.0
172.16.0.0
192.168.1.0
Passive Interface(s):
Serial0/0
Routing Information Sources:
Gateway Distance Last Update
(this router) 90 20:51:48
192.168.1.2 90 00:22:58
172.16.10.6 90 01:58:46
172.16.10.2 90 01:59:52
Distance: internal 90 external 170
A. The metric K values don’t match.
B. The AS numbers don’t match.
C. There is a passive interface on RouterB.
D. There is an ACL set on RouterA.
Review Questions 843
16. How many paths will EIGRPv6 load-balance by default?
A. 16
B. 32
C. 4
D. None
17. What would your configurations be on RouterB based on the illustration? (Choose two.)
RouterB
g0/0
2001:db8:3c4d:15::/64
A. (config)#router eigrp 10
B. (config)#ipv6 router eigrp 10
C. (config)#ipv6 router 2001:db8:3c4d:15::/64
D. (config-if)#ip eigrp 10
E. (config-if)#ipv6 eigrp 10
F. (config-if)#ipv6 router eigrp 10
18. RouterA has a feasible successor not shown in the following output. Based on what
you can learn from the output, which one of the following will be the successor for
2001:db8:c34d:18::/64 if the current successor fails?
via FE80::201:C9FF:FED0:3301 (29110112/33316), Serial0/0/0
via FE80::209:7CFF:FE51:B401 (4470112/42216), Serial0/0/1
via FE80::209:7CFF:FE51:B401 (2170112/2816), Serial0/0/2
A. Serial0/0/0
B. Serial0/0/1
C. Serial0/0/2
D. There is no feasible successor.
844 Chapter 19 u Enhanced IGRP
19. You have an internetwork as shown in the following illustration with routers running
IOS 12.4. However, the two networks are not sharing routing table route entries. What
is the problem?
RouterA#sh ip protocols
Routing Protocol is "eigrp 930"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 2
Redistributing: eigrp 930
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Automatic address summarization:
192.168.1.0/24 for FastEthernet0/0
Summarizing with metric 2169856
10.0.0.0/8 for Serial0/0
Summarizing with metric 28160
[output cut]
RouterB#sh ip protocols
Routing Protocol is "eigrp 930"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 3
Redistributing: eigrp 930
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
10.0.0.0
192.168.1.0
Passive Interface(s):
Serial0/0
Routing Information Sources:
Gateway Distance Last Update
(this router) 90 20:51:48
192.168.1.2 90 00:22:58
172.16.10.6 90 01:58:46
172.16.10.2 90 01:59:52
Distance: internal 90 external 170
S0/0/0
192.168.1.2
192.168.1.1
S0/0
RouterA RouterB
10.1.1.0/24 10.2.2.0/24
Review Questions 845
A. The variances don’t match between routers.
B. The metrics are not valid between neighbors.
C. There is a discontiguous network.
D. There is a passive interface on RouterB.
E. An ACL is set on the router.
20. Which should you look for when troubleshooting an adjacency? (Choose four.)
A. Verify the AS numbers.
B. Verify that you have the proper interfaces enabled for EIGRP.
C. Make sure there are no mismatched K values.
D. Check your passive interface settings.
E. Make sure your remote routers are not connected to the Internet.
F. If authentication is configured, make sure all routers use different passwords.
Chapter
20
Multi-Area OSPF
THE FOLLOWING ICND2 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 IP Routing Technologies
■Configure and verify OSPF (single area)
■Neighbor adjacencies
■OSPF states
■Discuss Multi-area
■Configure OSPF v2
■Configure OSPF v3
■Router ID
■LSA types
1 Troubleshooting
■Troubleshoot and resolve routing issues
■Routing is enabled
■Routing table is correct
■Correct path selection
■Troubleshoot and resolve OSPF problems
■Neighbor adjacencies
■Hello and Dead timers
■OSPF area
■Interface MTU
■Network types
■Neighbor states
■OSPF topology database
We’ll begin this chapter by focusing on the scalability constraints
of an Open Shortest Path First (OSPF) network with a single
area and move on from there to explore the concept of multi-
area OSPF as a solution to these scalability limitations.
I’ll also identify and introduce you to the various categories of routers used in multi-area
congurations, including backbone routers, internal routers, area border routers (ABRs),
and autonomous system boundary routers (ASBRs).
The functions of different OSPF Link-State Advertisements (LSAs) are absolutely crucial
for you to understand for success in taking the Cisco exam, so I’ll go into detail about the
types of LSAs used by OSPF, as well as, the Hello protocol and different neighbor states
when an adjacency is taking place.
And because troubleshooting is always a vital skill to have, I’ll guide you through the pro-
cess with a collection of show commands that can be effectively used to monitor and trouble-
shoot a multi-area OSPF implementation. Finally, I’ll end the chapter with the easiest part:
conguring and verifying OSPFv3.
To find up-to-the-minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
OSPF Scalability
At this point, and before you read this chapter, be sure that you have the foundation of single-
area OSPF down pat. I’m sure you remember OSPF’s signicant advantage over distance-
vector protocols like RIP, due to OSPF’s ability to represent an entire network within its
link-state database, which dramatically reduces the time required for convergence!
But what does a router actually go through to give us this great performance? Each router
recalculates its database every time there’s a topology change. If you have numerous routers
in an area, they’ll clearly have lots of links. Every time a link goes up or down, an LSA Type 1
packet is advertised, forcing all of the routers in the same area to recalculate their shortest path
rst (SPF) tree. Predictably, this kind of heavy lifting requires a ton of CPU overhead. On top
of that, each router must hold the entire link-state database that represents the topology of the
entire network, which results in considerable memory overhead. As if all that weren’t enough,
each router also holds a complete copy of the routing table, adding more to the already heavy
overhead burden on memory. And keep in mind that the number of entries in the routing table
OSPF Scalability 849
can be much greater than the number of networks in the routing table because there are typi-
cally multiple routes to to the same remote networks!
Considering these OSPF factors, it’s easy to imagine that in a really large network, single-
area OSPF presents some serious scalability challenges, as shown in Figure20.1. We’ll move
on in a bit to compare the single-area OSPF network in that illustration to our multi-area
networks.
FIGURE20.1 OSPF single-area network: All routers flood the network with link-
state information to all other routers within the same area.
Area 0
Single-area OSPF design places all routers into a single OSPF area, which results in many
LSAs being processed on every router.
Fortunately, OSPF allows us to take a large OSPF topology and break it down into
multiple, more manageable areas, as illustrated in Figure20.2.
FIGURE20.2 OSPF multi-area network: All routers flood the network only within
their area.
Area 2Area 1
Area 0
850 Chapter 20 u Multi-Area OSPF
Just take a minute to think about the advantages of this hierarchical approach. First, rout-
ers that are internal to a dened area don’t need to worry about having a link-state database
for the entire network because they need one for only their own areas. This factor seriously
reduces memory overhead! Second, routers that are internal to a dened area now have to
recalculate their link-state database only when there’s a topology change within their given
area. Topology changes in one area won’t cause global OSPF recalculations, further reducing
processor overhead. Finally, because routes can be summarized at area boundaries, the rout-
ing tables on each router just don’t need to be nearly as huge as they would be in a single-area
environment!
But of course there’s a catch: As you start subdividing your OSPF topology into multiple
areas, the conguration gets more complex, so we’ll explore some strategic ways to nesse
the conguration plus look at some cool tricks for effectively troubleshooting multi-area
OSPF networks.
Categories of Multi-area Components
In the following sections, I’m going to cover the various roles that routers play in a multi-
area OSPF network. You’ll nd routers serving as backbone routers, internal routers, area
border routers, and autonomous system boundary routers. I’ll also introduce you to the
different types of advertisements used in an OSPF network.
Link-State Advertisements (LSAs) describe a router and the networks that are connected
to it by sending the LSAs to neighbor routers. Routers exchange LSAs and learn the com-
plete topology of the network until all routers have the exact same topology database. After
the topology database is built, OSPF uses the Dijkstra algorithm to nd the best path to
each remote network and places only the best routes into the routing table.
Adjacency Requirements
Once neighbors have been identied, adjacencies must be established so that routing (LSA)
information can be exchanged. There are two steps required to change a neighboring OSPF
router into an adjacent OSPF router:
1. Two-way communication (achieved via the Hello protocol)
2. Database synchronization, which consists of three packet types being exchanged
between routers:
u Database Description (DD) packets
u Link-State Request (LSR) packets
u Link-State Update (LSU) packets
Once database synchronization is complete, the two routers are considered adjacent.
This is how adjacency is achieved, but you need to know when an adjacency will occur.
Categories of Multi-area Components 851
It’s important to remember that neighbors will not form an adjacency if the following do
not match:
u Area ID
u Subnet
u Hello and dead timers
u Authentication (if configured)
When adjacencies form, depends on the network type. If the link is point-to-point, the
two neighbors will become adjacent if the Hello packet information for both routers is con-
gured properly. On broadcast multi-access networks, adjacencies are formed only between
the OSPF routers on the network and the DR and BDR.
OSPF Router Roles
Routers within a multi-area OSPF network fall into different categories. Check out Figure20.3
to see the various roles that routers can play.
FIGURE20.3 Router roles: Routers within an area are called internal routers.
Area 2
NorfolkBoston
To a different AS
ABR ABR
San Jose
Area 1
SF
Area 0
ASBR Corp
NY
Oakland
Notice that there are four routers that are part of area 0: the Corp router, SF and NY,
and the autonomous system border router (ASBR). When conguring multi-area OSPF, one
area must be called area 0, referred to as the backbone area. All other areas must connect
to area 0. The four routers are referred to as the backbone routers, which are any routers
that exist either partially or completely in OSPF area 0.
Another key distinction about the SF and NY routers connecting to other areas is that
they have interfaces in more than one area. This makes them area border routers (ABRs)
because in addition to having an interface in area 0, SF has an interface in area 1 and NY
has an interface in area 2.
852 Chapter 20 u Multi-Area OSPF
An ABR is a router that belongs to more than one OSPF area. It maintains information
from all directly connected areas in its topology table but doesn’t share the topological
details from one area with the other. But it will forward routing information from one area
to the other. The key concept here is that an ABR separates the LSA ooding zone, is a pri-
mary point for area address summarization, and typically has the source default route, all
while maintaining the link-state database (LSDB) for each area it’s connected to.
Remember that a router can play more than one role. In Figure20.3, SF and
NY are both backbone routers and area border routers.
Let’s turn our focus to the San Jose and Oakland routers. You can see that all interfaces
on both of these routers reside only in area 1. Because all of San Jose’s and Oakland’s inter-
faces are internal to a single area, they’re called internal routers. An internal router is any
router with all of its interfaces included as members of the same area. This also applies to
the Boston and Norfolk routers and their relationship to area 2. The Corp router is internal
to area 0.
Finally, the ASBR is unique among all routers in our example because of its connection to
an external autonomous system (AS). When an OSPF network is connected to an EIGRP net-
work, a Border Gateway Protocol (BGP) network, or a network running any other external
routing process, it’s referred to as an AS.
An autonomous system boundary router (ASBR) is an OSPF router with at least one
interface connected to an external network or different AS. A network is considered exter-
nal if it’s either running a routing protocol other than OSPF. An ASBR is responsible for
injecting route information learned via the external network into OSPF.
I want to point out that an ASBR doesn’t automatically exchange routing information
between its OSPF routing process and the external routing process that it’s connected to.
These routes are exchanged through a method called route redistribution, which is beyond
the scope of this book.
Link-state Advertisements
You know that a router’s link-state database is made up of Link-State Advertisements (LSAs).
But just as there are several OSPF router categories to remember, there are also various types
of LSAs to keep in mind—ve of them, to be exact. These LSA classications may not seem
important at rst, but you’ll see why they are when we cover how the various types of OSPF
areas operate. Let’s start by exploring the different types of LSAs that Cisco uses:
Type 1 LSA Referred to as a router link advertisement (RLA), or just router LSA, a Type 1
LSA is sent by every router to other routers in its area. This advertisement contains the status
of a router’s link in the area to which it is connected. If a router is connected to multiple areas,
then it will send separate Type 1 LSAs for each of the areas it’s connected to. Type 1 LSAs con-
tain the router ID (RID), interfaces, IP information, and current interface state. For example,
Categories of Multi-area Components 853
in the network in Figure20.4, router SF will send an LSA Type 1 advertisement for its inter-
face into area 0 and a separate LSA Type 1 advertisement for its interfaces into area 1 describ-
ing the state of its links. The same will happen with the other routers in Figure20.4.
FIGURE20.4 Type 1 Link-state Advertisements
Area 2
NorfolkBoston
To a different AS
ABR ABR
San Jose
Area 1
SF
Area 0
ASBR Corp
NY
Oakland
Type 1
Type 1
Type 1
Type 1
Type 1
Type 1
Type 1: Here is the status of my links!
Type 2 LSA Referred to as a network link advertisement (NLA), a Type 2 LSA is gener-
ated by designated routers (DRs). Remember that a designated router is elected to represent
other routers in its network, and it establishes adjacencies with them. The DR uses a Type 2
LSA to send out information about the state of other routers that are part of the same net-
work. Note that the Type 2 LSA is ooded to all routers that are in the same area as the one
containing the specic network but not to any outside of that area. These updates contain
the DR and BDR IP information.
Type 3 LSA Referred to as a summary link advertisement (SLA), a Type 3 LSA is generated
by area border routers. These ABRs send Type 3 LSAs toward the area external to the one
where they were generated. The Type 3 LSA advertises networks, and these LSAs advertise
inter-area routes to the backbone area (area 0). Advertisements contain the IP information and
RID of the ABR that is advertising an LSA Type 3.
The word summary often invokes images of a summarized network address
that hides the details of many small subnets within the advertisement of a
single large one. But in OSPF, Summary Link Advertisements don’t necessar-
ily contain network summaries. Unless the administrator manually creates a
summary, the full list of individual networks available within an area will be
advertised by the SLAs.
854 Chapter 20 u Multi-Area OSPF
Type 4 LSA Type 4 LSAs are generated by area border routers. These ABRs send a Type4
LSA toward the area external to the one in which they were generated. These are also sum-
mary LSAs like Type 3, but Type 4 are specically used to inform the rest of the OSPF areas
how to get to the ASBR.
Type 5 LSA Referred to as AS external link advertisements, a Type 5 LSA is sent by autono-
mous system boundary routers to advertise routes that are external to the OSPF autonomous
system and are ooded everywhere. A Type 5 LSA is generated for each individual external
network advertised by the ASBR.
Figure20.5 shows how each LSA type would be used in a multi-area OSPF network.
FIGURE20.5 Basic LSA types
LSA Type Description
1Router LSA
2Network LSA
3 Summary LSA
4 ASBR Summary LSA
5Autonomous system LSA
Area 0Area 1
ASBR ABR
Corp
Type 4
Type 1 or 2 Type 1 or 2
Type 3
Type 5
It’s important to understand the different LSA types and how they work. Looking at
Figure20.5, you can see that Type 1 and 2 are ooded between routers in their same area.
Type 3 LSAs from the Corp router (which is an ABR and maintains the LSDB for each area
it is connected to) will summarize information learned from area 1 into area 0 and vice
versa. The ASBR will ood Type 5 LSAs into area 1, and the Corp router will then ood
Type 4 LSAs into area 0, telling all routers how to get to the ASBR, basically becoming a
proxy ASBR.
OSPF Hello Protocol
The Hello protocol provides a lot of information to neighbors. The following is communicated
between neighbors, by default, every 10 seconds:
Router ID (RID) This is the highest active IP address on the router. The highest loopback
IP addresses are used rst. If no loopback interfaces are congured, OSPF will choose from
physical interfaces instead.
Categories of Multi-area Components 855
Hello/Dead interval The period between Hello packets is the Hello time, which is 10 seconds
by default. The dead time is the length of time allotted for a Hello packet to be received before
a neighbor is considered down—four times the Hello interval, unless otherwise congured.
Neighbors The information includes a list of the router IDs for all the originating router’s
neighbors; neighbors being dened as routers that are attached to a common IP subnet and
use identical subnet masks.
Area ID This represents the area that the originating router interface belongs to.
Router priority The priority is an 8-bit value used to aid in the election of the DR and
BDR. This isn’t set on point-to-point links!
DR IP address This is the router ID of the current DR.
BDR IP address This is the router ID of the current BDR.
Authentication data This is the authentication type and corresponding information
(if congured).
The mandatory information within the Hello update that must match exactly are the
hello and dead timer values intervals, area ID, OSPF area type, subnet, and authentication
data if used. If any of those don’t match perfectly, no adjacency will occur!
Neighbor States
Before we move on to conguration, verication, and troubleshooting OSPF, it’s important for
you to grasp how OSPF routers traverse different states when adjacencies are being established.
When OSPF routers are initialized, they rst start exchanging information using the
Hello protocol via the multicast address 224.0.0.5. After the neighbor relationship is
established between routers, the routers synchronize their link-state database (LSDB) by
reliably exchanging LSAs. They actually exchange quite a bit of vital information when
they start up.
The relationship that one router has with another consists of eight possible states. All
OSPF routers begin in the DOWN state, and if all is well, they’ll progress to either the 2WAY
or FULL state with their neighbors. Figure20.6 shows this neighbor state progression.
FIGURE20.6 OSPF neighbor states, part 1
Corp Branch
DOWN state
INIT state
2WAY state
224.0.0.5
Hello?
856 Chapter 20 u Multi-Area OSPF
The process starts by sending out Hello packets. Every listening router will then add
the originating router to the neighbor database. The responding routers will reply with all
of their Hello information so that the originating router can add them to its own neigh-
bor table. At this point, we will have reached the 2WAY state—only certain routers will
advance beyond this to establish adjacencies.
Here’s a denition of the eight possible relationship states:
DOWN In the DOW N state, no Hello packets have been received on the interface. Bear
in mind that this does not imply that the interface itself is physically down.
ATTEMPT In the ATTEMPT state, neighbors must be congured manually. It applies
only to nonbroadcast multi-access (NBMA) network connections.
INIT In the INIT state, Hello packets have been received from another router. Still,
the absence of the Router ID for the receiving router in the Neighbor eld indicates that
bidirectional communication hasn’t been established yet.
2WAY In the 2WAY state, Hello packets that include their own router ID in the Neighbor
eld have been received. Bidirectional communication has been established. In broadcast
multi-access networks, an election can occur after this point.
After the DR and BDR have been selected, the routers will enter into the EXSTART state
and the routers are ready to discover the link-state information about the internetwork and
create their LSDB. This process is illustrated in Figure20.7.
FIGURE20.7 OSPF router neighbor states, part 2
Corp Branch
LSDB summary
EXSTART state
EXCHANGE state
LOADING state
FULL state
LSAck
I need info on a network!
Here is that info!
LSAck
LSU
LSU
EXSTART In the EXSTART state, the DR and BDR establish adjacencies with each router
in the network. A master-slave relationship is created between each router and its adjacent
DR and DBR. The router with the highest RID becomes the master, and the master-slave
election dictates which router will start the exchange. Once routers exchange DBD packets,
the routers will move into the EXCHANGE state.
Basic Multi-area Configuration 857
EXCHANGE In the EXCHANGE state, routing information is exchanged using
Database Description (DBD or DD) packets and Link-State Request (LSR) and Link-State
Update packets may also be sent. When routers start sending LSRs, they’re considered to
be in the LOADING state.
LOADING In the LOADING state, Link-State Request (LSR) packets are sent to
neighbors to request any Link-State Advertisements (LSAs) that may have been missed
or corrupted while the routers were in the EXCHANGE state. Neighbors respond with
Link-State Update(LSU) packets, which are in turn acknowledged with Link-State
Acknowledgement (LSAck) packets. When all LSRs have been satised for a given
router, the adjacent routers are considered synchronized and enter the FULL state.
FULL In the FULL state, all LSA information is synchronized among neighbors and
adjacency has been established. OSPF routing can begin only after the FULL state has
been reached!
It’s important to understand that routers should be in the 2WAY and FULL states and the
others are considered transitory. Routers shouldn’t remain in any other state for extended
period of times. Let’s congure OSPF now to see what we’ve covered so far in action.
Basic Multi-area Configuration
Basic multi-area conguration isn’t all that hard. Understanding your design, layout, types of
LSAs, DRs and conguring the elections, troubleshooting, and fully comprehending what’s
happening in the background are really the most complicated aspects of OSPF.
As I was saying, conguring OSPF is pretty simple, and you’ll see toward the end of this
chapter that conguring OSPFv3 is even easier! After I show you the basic OSPF multi-area
conguration in this section, we’ll work on the verication of OSPF and then go through a
detailed troubleshooting scenario just as we did with EIGRP. Let’s get the ball rolling with
the multi-area conguration shown in Figure20.8.
We’ll use the same routers we’ve been working with throughout all the chapters, but
we’re going to create three areas. The routers are still congured with the IPv6 addresses
from my last EIGRPv6 section in the previous chapter, and I’ve also veried that the IPv4
addresses are on the interfaces and working as well since then, so we’re all set to rock the
congs for this chapter! Here’s the Corp conguration:
Corp#config t
Corp(config)#router ospf 1
Corp(config-router)#router-id 1.1.1.1
Corp(config-router)#Reload or use "clear ip ospf process" command, for this to
take effect
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Corp(config-router)#network 10.10.0.0 0.0.255.255 area 0
Corp(config-router)#network 172.16.10.0 0.0.0.3 area 1
Corp(config-router)#network 172.16.10.4 0.0.0.3 area 2
FIGURE20.8 Our internetwork
s0/1
s0/0
Corp
SF
NY
10.10.10.0/24
g0/0
.2
10.10.11.0/24
g0/1
.6
Area 0
Area 1
Area 2
172.16.10.0/30
172.16.10.4/30
10.10.20.0/24
10.10.30.0/24
10.10.40.0/24
10.10.50.0/24
Pretty straightforward, but let’s talk about it anyway. First I started the OSPF process
with the router ospf process-id command, using any number from 1–65,535 because
they’re only locally signicant so they don’t need to match my neighbor routers. I set the
RID of the router only to remind you that this can be congured under the router process,
but with our small network it wouldn’t really be necessary to mess with RIDs if this was
an actual production network. The one thing that you need to keep in mind here is that
in OSPF, the RID must be different on each router. With EIGRP, they can all be the same
because they are not as important in that process. Still, as I showed you in the EIGRPv6
section, we still need them!
Anyway, at this point in the congurations I needed to choose my network statements
for the OSPF process to use, which allowed me to place my four interfaces on the Corp
router into three different areas. In the rst network statement, 10.10.0.0 0.0.255.255,
placed the gi0/0 and gi0/1 interfaces into area 0. The second and third statements needed to
be more exact since there are /30 networks. 172.16.10.0 0.0.0.3 tells OSPF process 1 to go
nd an active interface that’s congured with 172.16.10.1 or .2 and to place that interface
into area 1. The last line tells the OSPF process to go nd any active interface congured
with 172.16.10.5 or .6 and place that interface into area 2. The wildcard of 0.0.0.3 means
the rst three octets can match any value, but the last octet is a block size of 4.
The only thing different about these congurations from those in the single-area OSPF is
the different areas at the end of the command—that’s it!
Verifying and Troubleshooting Multi-area OSPF Networks 859
Here is the conguration for the SF and NY routers:
SF(config)#router ospf 1
SF(config-router)#network 10.10.0.0 0.0.255.255 area 1
SF(config-router)#network 172.16.0.0 0.0.255.255 area 1
NY(config)#router ospf 1
NY(config-router)#network 0.0.0.0 255.255.255.255 area 2
00:01:07: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on Serial0/0/0 from LOADING to
FULL,
Loading Done
I congured each one slightly different from the Corp router, but since they didn’t have
an interface in more than area 1, I had more leeway in conguring them. For the NY router
I just congured a network statement (0.0.0.0 255.255.255.255) that says “go nd any
active interface and place it into area 2!” I’m not recommending that you congure your
routers in such a broad manner; I just wanted to show you your options.
Now that our three routers are congured, let’s verify our internetwork.
Verifying and Troubleshooting
Multi-area OSPF Networks
Cisco’s IOS has several show and debug commands that can help you monitor and trouble-
shoot OSPF networks. A sampling of these commands, which can be used to gain informa-
tion about various OSPF characteristics, is included in Table20.1.
TABLE20.1 OSPF verification commands
Command Provides the following
show ip ospf neighbor Verifies your OSPF-enabled interfaces
show ip ospf interface Displays OSPF-related information on an OSPF-enabled
interface
show ip protocols Verifies the OSPF process ID and that OSPF is enabled on
the router
show ip route Verifies the routing table, and displays any OSPF injected routes
show ip ospf database Lists a summary of the LSAs in the database, with one line of
output per LSA, organized by type
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Let’s go through some verication commands—the same commands we used to verify
our single-area OSPF network—then we’ll move onto the OSPF troubleshooting scenario
section.
Okay, once you’ve checked the link between your neighbors and can use the Ping program,
the best command when verifying a routing protocol is to always check the status of your
neighbor’s connection rst. The show ip ospf neighbor command is super useful because it
summarizes the pertinent OSPF information regarding neighbors and their adjacency state. If
a DR or BDR exists, that information will also be displayed. Here’s a sample:
Corp#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
172.16.10.2 0 FULL/ - 00:00:34 172.16.10.2 Serial0/0/0
172.16.10.6 0 FULL/ - 00:00:31 172.16.10.6 Serial0/0/1
SF#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 0 FULL/ - 00:00:39 172.16.10.1 Serial0/0/0
NY#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 0 FULL/ - 00:00:34 172.16.10.5 Serial0/0/0
The reason that the Corp connections to SF and LA don’t have a DR or BDR listed in the
output is that by default, elections don’t happen on point-to-point links and they show FULL/-.
But we can see that the Corp router is fully adjacent to all three routers from its output.
The output of this command shows the neighbor ID, which is the RID of the router.
Notice in the output of the Corp router that the RIDs for the SF and NY routers were chosen
based on highest IP address of any active interface when I started the OSPF process on those
routers. Both the SF and NY routers see the Corp router RID as 1.1.1.1 because I set that
manually under the router ospf process command.
Next we see the Prield, which is the priority eld that’s set to 1 by default. Don’t
forget that on point-to-point links, elections don’t happen, so the interfaces are all set to 0
in this example because none of these routers will have elections on these interfaces with
each other over this serial WAN network. The state eld shows Full/-, which means all
routers are synchronized with their LSDB, and the /- means there is no election on this
type of interface. The dead time is counting down, and if the router does not hear from
this neighbor before this expires, the link will be considered down. The address is the
actual address of the neighbor’s interface connecting to the router, and the interface is the
neighbor’s interface—not your interface!
Verifying and Troubleshooting Multi-area OSPF Networks 861
The show ip ospf Command
We use the show ip ospf command to display OSPF information for one or all OSPF
processes running on the router. Information contained therein includes the router ID,
area information, SPF statistics, and LSA timer information. Let’s check out the output
from the Corp router:
Corp#sh ip ospf
Routing Process "ospf 1" with ID 1.1.1.1
Supports only single TOS(TOS0) routes
Supports opaque LSA
It is an area border router
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
Number of external LSA 0. Checksum Sum 0x000000
Number of opaque AS LSA 0. Checksum Sum 0x000000
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 3. 3 normal 0 stub 0 nssa
External flood list length 0
Area BACKBONE(0)
Number of interfaces in this area is 2
Area has no authentication
SPF algorithm executed 19 times
Area ranges are
Number of LSA 7. Checksum Sum 0x0384d5
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
Area 1
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm executed 43 times
Area ranges are
Number of LSA 7. Checksum Sum 0x0435f8
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
862 Chapter 20 u Multi-Area OSPF
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
Area 2
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm executed 38 times
Area ranges are
Number of LSA 7. Checksum Sum 0x0319ed
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
You’ll notice that most of the preceding information wasn’t displayed with this command
output in single-area OSPF. We have more displayed here because it’s providing information
about each area we’ve congured on this router.
The show ip ospf interface Command
The show ip ospf interface command displays all interface-related OSPF information.
Data is displayed for all OSPF-enabled interfaces or for specied interfaces. I’ll highlight
some important portions I want you to pay special attention to.
Corp#sh ip ospf interface gi0/0
GigabitEthernet0/0 is up, line protocol is up
Internet address is 10.10.10.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 1.1.1.1, Interface address 10.10.10.1
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:05
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 0, Adjacent neighbor count is 0
Suppress hello for 0 neighbor(s)
Verifying and Troubleshooting Multi-area OSPF Networks 863
Let’s take a look at a serial interface so we can compare it to the Gigabit Ethernet inter-
face just shown. The Ethernet network is a broadcast multi-access network by default, and
the serial interface is a point-to-point nonbroadcast multi-access network, so they will act
differently with OSPF:
Corp#sh ip ospf interface s0/0/0
Serial0/0/0 is up, line protocol is up
Internet address is 172.16.10.1/30, Area 1
Process ID 1, Router ID 1.1.1.1, Network Type POINT-TO-POINT, Cost: 64
Transmit Delay is 1 sec, State POINT-TO-POINT, Priority 0
No designated router on this network
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:02
Index 3/3, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1 , Adjacent neighbor count is 1
Adjacent with neighbor 172.16.10.2
Suppress hello for 0 neighbor(s)
The following information is displayed via this command:
u Interface IP address
u Area assignment
u Process ID
u Router ID
u Network type
u Cost
u Priority
u DR/BDR election information (if applicable)
u Hello and dead timer intervals
u Adjacent neighbor information
I used the show ip ospf interface gi0/0 command rst because I knew that there
would be a designated router elected on the Ethernet broadcast multi-access network on
the Corp router, even though it has no one to run against, which means the Corp router
automatically wins. The information that I bolded is all very important! What are the hello
and dead timers set to by default? Even though I haven’t talked much about the cost out-
put on an interface, it can also be very important. Two OSPF routers still could create an
adjacency if the costs don’t match, but it could lead to certain links not being utilized. We’ll
discuss this more at the end of the verication section.
864 Chapter 20 u Multi-Area OSPF
Neighbor Routers Don’t Form an Adjacency
I’d like to talk more about the adjacency issue and how the show ip ospf interface
command can help you solve problems, especially in multi-vendor networks.
Years ago I was consulting with the folks at a large PC/laptop manufacturer and was help-
ing them build out their large internetwork. They were using OSPF because their company
was a worldwide company and used many types of routers from all manufacturers.
I received a call from a remote branch informing me that they installed a new router but it
was not seeing the Cisco router off their Ethernet interface. Of course it was an emergency
because this new router was holding some important WAN links to a new remote location
that needed to be up yesterday!
After calming down the person on the phone, I simply had the admin use the show ip
ospf interface fa0/0 command and verify the hello and dead timers and the area
configured for that interface and then had him verify that the IP addresses were correct
between routers and that there was no passive interface set.
Then I had him verify that same information on the neighbor, and sure enough the
neighbors’ hello and dead timers didn’t match. Quick and easy fix on the interface of
the Cisco router with the ip ospf dead 30 command, and they were up!
Always remember that OSPF can work with multi-vendor routers, but no one ever said it
works out of the box between various vendors!
The show ip protocols Command
The show ip protocols command is also useful, whether you’re running OSPF, EIGRP,
RIP, BGP, IS-IS, or any other routing protocol that can be congured on your router. It
provides an excellent overview of the actual operation of all currently running protocols.
Check the output from the Corp router:
Corp#sh ip protocols
Routing Protocol is "ospf 1"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Router ID 1.1.1.1
Number of areas in this router is 3. 3 normal 0 stub 0 nssa
Verifying and Troubleshooting Multi-area OSPF Networks 865
Maximum path: 4
Routing for Networks:
10.10.0.0 0.0.255.255 area 0
172.16.10.0 0.0.0.3 area 1
172.16.10.4 0.0.0.3 area 2
Routing Information Sources:
Gateway Distance Last Update
1.1.1.1 110 00:17:42
172.16.10.2 110 00:17:42
172.16.10.6 110 00:17:42
Distance: (default is 110)
Here we can determine the OSPF process ID, OSPF router ID, type of OSPF area,
networks, and the three areas congured for OSPF, as well as, the OSPF router IDs of
neighbors—that’s a lot. Read efcient!
The show ip route Command
Now would be a great time to issue a show ip route command on the Corp router. The
Corp router shows only four dynamic routes for our internetwork, with the O representing
OSPF internal routes. The Cs clearly represent our directly connected networks, but our
four remote networks are also showing up—nice! Notice the 110/65, which is the adminis-
trative distance/metric:
Corp#sh ip route
[output cut]
10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks
C 10.10.10.0/24 is directly connected, GigabitEthernet0/0
L 10.10.10.1/32 is directly connected, GigabitEthernet0/0
C 10.10.11.0/24 is directly connected, GigabitEthernet0/1
L 10.10.11.1/32 is directly connected, GigabitEthernet0/1
O 10.10.20.0/24 [110/65] via 172.16.10.2, 02:18:27, Serial0/0/0
O 10.10.30.0/24 [110/65] via 172.16.10.2, 02:18:27, Serial0/0/0
O 10.10.40.0/24 [110/65] via 172.16.10.6, 03:37:24, Serial0/0/1
O 10.10.50.0/24 [110/65] via 172.16.10.6, 03:37:24, Serial0/0/1
172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks
C 172.16.10.0/30 is directly connected, Serial0/0/0
L 172.16.10.1/32 is directly connected, Serial0/0/0
C 172.16.10.4/30 is directly connected, Serial0/0/1
L 172.16.10.5/32 is directly connected, Serial0/0/1
866 Chapter 20 u Multi-Area OSPF
In addition, you can use the show ip route ospf command to get only OSPF-injected
routes in your routing table. I can’t stress enough how useful this is when dealing with
large networks!
Corp#sh ip route ospf
10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks
O 10.10.20.0 [110/65] via 172.16.10.2, 02:18:33, Serial0/0/0
O 10.10.30.0 [110/65] via 172.16.10.2, 02:18:33, Serial0/0/0
O 10.10.40.0 [110/65] via 172.16.10.6, 03:37:30, Serial0/0/1
O 10.10.50.0 [110/65] via 172.16.10.6, 03:37:30, Serial0/0/1
Now that’s a really nice-looking OSPF routing table! Troubleshooting and xing an OSPF
network is as vital a skill to have as it is in any other networking environment, which is why I
always use the show ip int brief command when conguring my routing protocols. It’s very
easy to make little mistakes with OSPF, so pay very close attention to the details—especially
when troubleshooting!
The show ip ospf database Command
Using the show ip ospf database command will give you information about the number
of routers in the internetwork (AS), plus the neighboring router’s ID. This is the topology
database I referred to earlier.
The output is broken down by area. Here’s a sample, again from Corp:
Corp#sh ip ospf database
OSPF Router with ID (1.1.1.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 196 0x8000001a 0x006d76 2
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
172.16.10.0 1.1.1.1 182 0x80000095 0x00be04
172.16.10.4 1.1.1.1 177 0x80000096 0x009429
10.10.40.0 1.1.1.1 1166 0x80000091 0x00222b
10.10.50.0 1.1.1.1 1166 0x80000092 0x00b190
10.10.20.0 1.1.1.1 1114 0x80000093 0x00fa64
10.10.30.0 1.1.1.1 1114 0x80000094 0x008ac9
Verifying and Troubleshooting Multi-area OSPF Networks 867
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 1118 0x8000002a 0x00a59a 2
172.16.10.2 172.16.10.2 1119 0x80000031 0x00af47 4
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
10.10.10.0 1.1.1.1 178 0x80000076 0x0021a5
10.10.11.0 1.1.1.1 178 0x80000077 0x0014b0
172.16.10.4 1.1.1.1 173 0x80000078 0x00d00b
10.10.40.0 1.1.1.1 1164 0x80000074 0x005c0e
10.10.50.0 1.1.1.1 1164 0x80000075 0x00eb73
Router Link States (Area 2)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 1119 0x8000002b 0x005cd6 2
172.16.10.6 172.16.10.6 1119 0x8000002d 0x0020a3 4
Summary Net Link States (Area 2)
Link ID ADV Router Age Seq# Checksum
10.10.10.0 1.1.1.1 179 0x8000007a 0x0019a9
10.10.11.0 1.1.1.1 179 0x8000007b 0x000cb4
172.16.10.0 1.1.1.1 179 0x8000007c 0x00f0ea
10.10.20.0 1.1.1.1 1104 0x80000078 0x003149
10.10.30.0 1.1.1.1 1104 0x80000079 0x00c0ae
Corp#
Considering we only have eight networks congured in our internetwork, there’s a huge
amount of information in this database! You can see all the routers and the RID of each—
the highest IP address related to individual routers. And each output under each area repre-
sents LSA Type 1, indicating the area they’re connected to.
The router output also shows the link ID. Remember that an interface is also a link, as is
the RID of the router on that link under the ADV router—the advertising router.
So far, this has been a great chapter, brimming with detailed OSPF information, a whole
lot more than what was needed to meet past Cisco objectives, for sure! Next, we’ll use the
same sample network that I built in Chapter 5 on EIGRP and run through a troubleshooting
scenario using multi-area OSPF.
868 Chapter 20 u Multi-Area OSPF
Troubleshooting OSPF Scenario
When you notice problems with your OSPF network, it’s wise to rst test your layer 3 con-
nectivity with Ping and the traceroute command to see if your issue is a local one. If all
looks good locally, then follow these Cisco-provided guidelines:
1. Verify your adjacency with your neighbor routers using the show ip ospf neighbors
command. If you are not seeing your neighbor adjacencies, then you need to verify that
the interfaces are operational and enabled for OSPF. If all is well with the interfaces,
verify the hello and dead timers next, and establish that the interfaces are in the same
area and that you don’t have a passive interface configured.
2. Once you’ve determined that your adjacencies to all neighbors are working, use the
show ip route to verify your layer 3 routes to all remote networks. If you see no OSPF
routes in the routing table, you need to verify that you don’t have another routing pro-
tocol running with a lower administrative distance. You can use show ip protocols
to see all routing protocols running on your router. If no other protocols are running,
then verify your network statements under the OSPF process. In a multi-area network,
make sure all non-backbone area routers are directly connected to area 0 through an
ABR or they won’t be able to send and receive updates.
3. If you can see all the remote networks in the routing table, move on to verify the path
for each network and that each path for specific networks is correct. If not, you need
to verify the cost on your interfaces with the show ip ospf interface command. You
may need to adjust the cost on an interface either higher or lower, depending on which
path you want OSPF to use for sending packets to a remote network. Remember—the
path with the lowest cost is the preferred path!
Okay, with our marching orders for troubleshooting OSPF in hand, let’s take a look at
Figure20.9, which we’ll use to verify our network now.
FIGURE20.9 Our internetwork
Area 0
10.1.1.0/24
192.168.1.1
s0/0
192.168.1.2
s0/0/0 Area 1 10.2.2.0/24
Internal Corp
Area 1
Branch
Here’s the OSPF conguration on the three routers:
Corp(config-if)#router ospf 1
Corp(config-router)#network 10.1.1.0 0.0.0.255 area 0
Corp(config-router)#network 192.168.1.0 0.0.0.3 area 1
Troubleshooting OSPF Scenario 869
Internal(config)#router ospf 3
Internal(config-router)#network 10.1.1.2 0.0.0.0 area 0
Branch(config-if)#router ospf 2
Branch(config-router)#network 192.168.1.2 0.0.0.0 area 0
Branch(config-router)#network 10.2.2.1 0.0.0.0 area 0
Let’s check out our network now, beginning by checking the layer 1 and layer 2 status
between routers:
Corp#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.1.1.1 YES manual up up
Serial0/0 192.168.1.1 YES manual up up
The IP addresses look correct and the layer 1 and 2 status is up/up, so next we’ll use the
Ping program to check connectivity like this:
Corp#ping 192.168.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
Corp#ping 10.1.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
Nice—I can ping both directly connected neighbors, so this means layers 1, 2, and 3 are
working between neighbor routers. This is a great start, but it still doesn’t mean OSPF is
actually working yet. If any of the preceding commands had failed, I rst would’ve veried
layers 1 and 2 to make sure my data link was working between neighbors and then moved
on to verify my layer 3 IP conguration.
Since our data link appears to be working between each neighbor, our next move is to
check the OSPF conguration and status of the routing protocol. I’ll start with the interfaces:
Corp#sh ip ospf interface s0/0
Serial0/0 is up, line protocol is up
Internet Address 192.168.1.1/30, Area 1
Process ID 1, Router ID 192.168.1.1, Network Type POINT_TO_POINT, Cost: 100
Transmit Delay is 1 sec, State POINT_TO_POINT
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:03
870 Chapter 20 u Multi-Area OSPF
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/2, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 192.168.1.2
Suppress hello for 0 neighbor(s)
I’ve highlighted the important statistics that you should always check rst on an OSPF
interface. You need to verify that the interface is congured in the same area as the neigh-
bor and that the hello and dead timers match. A cost mismatch won’t stop an adjacency
from forming, but it could cause ugly routing issues. We’ll explore that more in a minute.
For now let’s take a look at the LAN interface that’s connecting to the Internal router:
Corp#sh ip ospf int f0/0
FastEthernet0/0 is up, line protocol is up
Internet Address 10.1.1.1/24, Area 0
Process ID 1, Router ID 192.168.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 192.168.1.1, Interface address 10.1.1.1
Backup Designated router (ID) 10.1.1.2, Interface address 10.1.1.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:00
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 10.1.1.2 (Backup Designated Router)
Suppress hello for 0 neighbor(s)
We’ll focus on the same key factors on a LAN interface that we did on our serial interface:
the area ID and hello and dead timers. Notice that the cost is 1. According to Cisco’s method
of calculating cost, anything 100 Mbps or higher will always be a cost of 1 and serial links
with the default bandwidth are always 64. This can cause problems in a large network with
Troubleshooting OSPF Scenario 871
lots of high-bandwidth links. One thing to take special note of is that there’s a designated and
backup designated router on a broadcast multi-access network. DRs and BDRs won’t cause
a routing problem between neighbors, but it’s still a consideration when designing and con-
guring in a really large internetwork environment. But we won’t be focusing on that for our
purposes here. It’s just something to keep in mind.
Staying with the troubleshooting step of checking our interfaces, look at the error I received
when I tried to verify OSPF on the fa0/1 interface of the Corp router (which we’re not using):
Corp#sh ip ospf int fa0/1
%OSPF: OSPF not enabled on FastEthernet0/1
Corp#
I got this error because the network statements under the OSPF process are not enabled
for the network on the fa0/1 interface. If you receive this error, immediately check your
network statements!
Next, let’s check out the networks we’re routing for with the show ip protocols
command:
Corp#sh ip protocols
Routing Protocol is "ospf 1"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Router ID 192.168.1.1
It is an area border router
Number of areas in this router is 2. 2 normal 0 stub 0 nssa
Maximum path: 4
Routing for Networks:
10.1.1.0 0.0.0.255 area 0
192.168.1.0 0.0.0.3 area 1
Reference bandwidth unit is 100 mbps
Routing Information Sources:
Gateway Distance Last Update
192.168.1.2 110 00:28:40
Distance: (default is 110)
From this output we can check our process ID, as well as, reveal if we have an ACL set on
our routing protocol, just as we found when troubleshooting EIGRP in the last chapter. But
this time, we’ll rst examine the network statements and the area they’re congured for—
most important, the specic areas that each interface is congured for. This is key because
if your neighbor’s interface isn’t in the same area, you won’t be able to form an adjacency!
This command’s output provides a great view of what exactly we typed in for the network
statements under the OSPF process. Also, notice that the default reference bandwidth is set
to 100 Mbps. I’ll talk about this factor at the end of this section.
872 Chapter 20 u Multi-Area OSPF
I want to point out that the neighbor IP address and administrative distance is listed.
OSPF uses 110 by default, so remember that if EIGRP were running here, we wouldn’t
see OSPF routes in the routing table because EIGRP has an AD of 90!
Next, we’ll look at our neighbor table on the Corp router to nd out if OSPF has formed
an adjacency with the Branch router:
Corp#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.1.1.2 1 FULL/BDR 00:00:39 10.1.1.2 FastEthernet0/0
Okay, we’ve nally zeroed in on our problem—the Corp router can see the Internal
router in area 0 but not the Branch router in area 1! What now?
First, let’s review what we know so far about the Corp and Branch router. The data link
is good, and we can use Ping successfully between the routers. This shouts out that we have
a routing protocol issue, so we’ll look further into the details of the OSPF conguration on
each router. Let’s run a show ip protocols on the Branch router:
Branch#sh ip protocols
Routing Protocol is "eigrp 20"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 20
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is not in effect
Maximum path: 4
Routing for Networks:
10.0.0.0
192.168.1.0
Routing Information Sources:
Gateway Distance Last Update
(this router) 90 3d22h
192.168.1.1 90 00:00:07
Distance: internal 90 external 170
Routing Protocol is "ospf 2"
Outgoing update filter list for all interfaces is not set
Troubleshooting OSPF Scenario 873
Incoming update filter list for all interfaces is not set
Router ID 192.168.1.2
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Maximum path: 4
Routing for Networks:
10.2.2.1 0.0.0.0 area 1
192.168.1.2 0.0.0.0 area 1
Reference bandwidth unit is 100 mbps
Passive Interface(s):
Serial0/0/0
Routing Information Sources:
Gateway Distance Last Update
192.168.1.1 110 03:29:07
Distance: (default is 110)
Do you see two routing protocols running on the Branch router? Both EIGRP and
OSPF are running, but that’s not necessarily our problem. The Corp router would need
to be running EIGRP, and if so, we would have only EIGRP routes in our routing table
because EIGRPs have the lower AD of 90 versus OSPF’s AD of 110.
Let’s check the routing table of the Branch router and see if the Corp router is also running
EIGRP. This will be easy to determine if we discover EIGRP-injected routes in the table:
Branch#sh ip route
[output cut]
10.0.0.0/24 is subnetted, 2 subnets
C 10.2.2.0 is directly connected, FastEthernet0/0
D 10.1.1.0 [90/2172416] via 192.168.1.1, 00:02:35, Serial0/0/0
192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.0 is directly connected, Serial0/0/0
Okay—so yes, the Corp router is clearly running EIGRP. This is a leftover conguration
from Chapter 5. All I need to do to x this issue is to disable EIGRP on the Branch router.
After that, we should see OSPF in the routing table:
Branch#config t
Branch(config)#no router eigrp 10
Branch(config)#do sh ip route
[output cut]
10.0.0.0/24 is subnetted, 1 subnets
C 10.2.2.0 is directly connected, FastEthernet0/0
192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.0 is directly connected, Serial0/0/0
874 Chapter 20 u Multi-Area OSPF
That’s not so good—I disabled the EIGRP protocol on the Branch router, but we still
didn’t receive OSPF updates! Let investigate further using the show ip protocols command
on the Branch router:
Branch#sh ip protocols
Routing Protocol is "ospf 2"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Router ID 192.168.1.2
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Maximum path: 4
Routing for Networks:
10.2.2.1 0.0.0.0 area 1
192.168.1.2 0.0.0.0 area 1
Reference bandwidth unit is 100 mbps
Passive Interface(s):
Serial0/0/0
Routing Information Sources:
Gateway Distance Last Update
192.168.1.1 110 03:34:19
Distance: (default is 110)
Do you see the problem? There’s no ACL, the networks are congured correctly, but
see the passive interface for Serial0/0/0? That will denitely prevent an adjacency from
happening between the Corp and Branch routers! Let’s x that:
Branach#show run
[output cut]
!
router ospf 2
log-adjacency-changes
passive-interface Serial0/0/0
network 10.2.2.1 0.0.0.0 area 1
network 192.168.1.2 0.0.0.0 area 1
!
[output cut]
Branch#config t
Branch(config)#router ospf 2
Branch(config-router)#no passive-interface serial 0/0/0
Let’s see what our neighbor table and routing table look like now:
Branch#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.1.1 0 FULL/ - 00:00:32 192.168.1.1 Serial0/0/0
Troubleshooting OSPF Scenario 875
Branch#sh ip route
10.0.0.0/24 is subnetted, 2 subnets
C 10.2.2.0 is directly connected, FastEthernet0/0
O IA 10.1.1.0 [110/65] via 192.168.1.1, 00:01:21, Serial0/0/0
192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.0 is directly connected, Serial0/0/0
Awesome—our little internetwork is nally happy! That was actually pretty fun and
really not all that hard once you know what to look for.
But there’s one more thing we need to cover before moving onto OSPFv3—load balancing
with OSPF. To explore that, we’ll use Figure20.10, wherein I added another link between
the Corp and Branch routers.
FIGURE20.10 Our internetwork with dual links
Area 0
10.1.1.0/24
192.168.1.1/30
s0/0
Area 1 10.2.2.0/24
Internal Corp
Area 1
Branch
192.168.1.1/30
s0/0/0
g0/0 g0/0
First, it’s clear that having a Gigabit Ethernet interface between our two routers is way
better than any serial link we could possibly have, which means we want the routers to use
the LAN link. We can either disconnect the serial link or use it as a backup link.
Let’s start by looking at the routing table and seeing what OSPF found:
Corp#sh ip route ospf
10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O 10.2.2.0 [110/2] via 192.168.1.6, 00:00:13, GigabitEthernet0/1
Look at that! OSPF wisely went with the Gigabit Ethernet link because it has the lowest
cost. Although it’s possible you’ll have to mess with the links to help OSPF choose the best
paths, it’s likely best to just leave it alone at this point.
But that wouldn’t be very much fun, now would it? Instead, let’s congure OSPF to
fool it into thinking the links are equal so it will use both of them by setting the cost on
the interfaces to the same value:
Corp#config t
Corp(config)#int g0/1
Corp(config-if)#ip ospf cost 10
Corp(config-if)#int s0/0/0
Corp(config-if)#ip ospf cost 10
876 Chapter 20 u Multi-Area OSPF
Obviously you need to deploy this conguration on both sides of the link, and I’ve already
congured the Branch router as well. Now that both sides are congured with the same cost,
let’s check out our routing table now:
Corp#sh ip route ospf
10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O 10.2.2.0 [110/11] via 192.168.1.2, 00:01:23, Serial0/0/0
[110/11] via 192.168.1.6, 00:01:23, GigabitEthernet0/1
I’m not saying you should congure a serial link and Gigabit Ethernet link as equal costs
as I just demonstrated, but there are times when you need to adjust the cost for OSPF. If
you don’t have multiple links to any remote networks, you really don’t need to worry about
this, but with regard to the objectives, you absolutely must understand the cost, how it
works, and how to set it so OSPF can choose a preferred path. And there’s still one more
thing about cost I want to cover with you.
It’s possible to change the reference bandwidth of the router, but you need to make sure
all the routers within the OSPF AS have the same reference bandwidth. The default reference
bandwidth is 108, which is 100,000,000, or the equivalent of the bandwidth of Fast Ethernet,
which is 100 Mbps, as demonstrated via show ip ospf and the show ip protocols command:
Routing for Networks:
10.2.2.1 0.0.0.0 area 1
192.168.1.2 0.0.0.0 area 1
Reference bandwidth unit is 100 mbps
This will basically make any interface running 100 Mbps or higher have a cost of 1.
The default is 100, and if you change it to 1,000, it will increase the cost by a factor of 10.
Again, if you do want to change this, you must make sure to congure the change on all
routers in your AS! Here is how you would do that:
Corp(route)#router ospf 1
Corp(config-router)#auto-cost reference-bandwidth ?
<1-4294967> The reference bandwidth in terms of Mbits per second
Now, nally, let’s get to the easy section of the chapter!
OSPFv3
The new version of OSPF continues the trend of routing protocols having a lot in common
with their IPv4 versions. The foundation of OSPF remains the same—it’s still a link-state
routing protocol that divides an entire internetwork or autonomous system into areas,
establishing a hierarchy.
In OSPF version 2, the router ID (RID) is determined by the highest IP addresses assigned
to the router. And as you now know, the RID can be assigned. In version 3, nothing has
OSPFv3 877
really changed because you can still assign the RID, area ID, and link-state ID, which remain
32-bit values.
Adjacencies and next-hop attributes now use link-local addresses, but OSPFv3 still uses
multi-cast trafc to send its updates and acknowledgements. It uses the addresses FF02::5
for OSPF routers and FF02::6 for OSPF-designated routers. These new addresses are the
replacements for 224.0.0.5 and 224.0.0.6, respectively.
Other, less exible IPv4 protocols don’t give us the ability that OSPFv2 does to assign
specic networks and interfaces into the OSPF process, but this is still congured under
the router conguration process. And with OSPFv3, just as with the EIGRPv6 routing pro-
tocols we’ve talked about, the interfaces and therefore the networks attached to them are
congured directly on the interface in interface conguration mode.
The conguration of OSPFv3 is going to look like this: First, optionally start by assigning
the RID, but if you have IPv4 addresses assigned to your interface, you can let OSPF pick the
RID just as we did with OSPFv2:
Router(config)#ipv6 router osfp 10
Router(config-rtr)#router-id 1.1.1.1
You get to perform some other congurations from router conguration mode, like
summarization and redistribution, but again, we don’t even need to congure OSPFv3
from this prompt if we congure it from the interface!
A simple interface conguration looks like this:
Router(config-if)#ipv6 ospf 10 area 0.0.0.0
So, if we just go to each interface and assign a process ID and area—poof, we’re done!
See? Easy! As the conguration shows, I congured the area as 0.0.0.0, which is the same
thing as just typing area 0. We’ll use Figure20.11, which is the same network and IPv6-
addressing we used in the EIGRPv6 section in Chapter 5.
FIGURE20.11 Configuring OSPFv3
2001:db8:3c4d:13::/64
2001:db8:3c4d:11::/64
2001:db8:3c4d:12::/64
2001:db8:3c4d:14::/64
Corp
g0/0
g0/1
g0/0
g0/1
g0/0
g0/1
SF
NY
2001:db8:3c4d:17::/64
2001:db8:3c4d:18::/64
2001:db8:3c4d:15::/64
2001:db8:3c4d:16::/64
878 Chapter 20 u Multi-Area OSPF
Okay, so all we have to do to enable OSPF on the internetwork is go to each interface
that we want to run it on. Here’s the Corp conguration:
Corp#config t
Corp(config)#int g0/0
Corp(config-if)#ipv6 ospf 1 area 0
Corp(config-if)#int g0/1
Corp(config-if)#ipv6 ospf 1 area 0
Corp(config-if)#int s0/0/0
Corp(config-if)#ipv6 ospf 1 area 0
Corp(config-if)#int s0/0/1
Corp(config-if)#ipv6 ospf 1 area 0
That wasn’t so bad—much easier than it was with IPv4! To congure OSPFv3, you
just need to establish the specic interfaces you’ll be using! Let’s congure the other two
routers now:
SF#config t
SF(config)#int g0/0
SF(config-if)#ipv6 ospf 1 area 1
SF(config-if)#int g0/1
SF(config-if)#ipv6 ospf 1 area 1
SF(config-if)#int s0/0/0
SF(config-if)#ipv6 ospf 1 area 0
01:03:55: %OSPFv3-5-ADJCHG: Process 1, Nbr 192.168.1.5 on Serial0/0/0 from
LOADING to
FULL, Loading Done
Sweet—the SF has become adjacent to the Corp router! One interesting output line I want
to point out is that the IPv4 RID is being used in the OSPFv3 adjacent change. I didn’t set
the RIDs manually because I knew I had interfaces with IPv4 addresses already on them,
which the OSPF process would use for a RID.
Now let’s congure the NY router:
NY(config)#int g0/0
NY(config-if)#ipv6 ospf 1 area 2
%OSPFv3-4-NORTRID:OSPFv3 process 1 could not pick a router-id,please configure
manually
NY(config-if)#ipv6 router ospf 1
NY(config-rtr)#router-id 1.1.1.1
NY(config-if)#int g0/0
NY(config-if)#ipv6 ospf 1 area 2
NY(config-if)#int g0/1
NY(config-if)#ipv6 ospf 1 area 2
OSPFv3 879
NY(config-if)#int s0/0/0
NY(config-if)#ipv6 ospf 1 area 0
00:09:00: %OSPFv3-5-ADJCHG: Process 1, Nbr 192.168.1.5 on Serial0/0/0 from
LOADING to
FULL, Loading Done
Our adjacency popped up—this is great. But did you notice that I had to set the RID?
That’s because there wasn’t an IPv4 32-bit address already on an interface for the router to
use as the RID, so it was mandatory to set the RID manually!
Without even verifying our network, it appears it’s up and running. Even so, it’s always
important to verify!
Verifying OSPFv3
I’ll start as usual with the show ipv6 route ospf command:
Corp#sh ipv6 route ospf
OI 2001:DB8:3C4D:15::/64 [110/65]
via FE80::201:C9FF:FED2:5E01, Serial0/0/1
OI 2001:DB8:3C4D:16::/64 [110/65]
via FE80::201:C9FF:FED2:5E01, Serial0/0/1
O 2001:DB8:C34D:11::/64 [110/128]
via FE80::2E0:F7FF:FE13:5E01, Serial0/0/0
OI 2001:DB8:C34D:17::/64 [110/65]
via FE80::2E0:F7FF:FE13:5E01, Serial0/0/0
OI 2001:DB8:C34D:18::/64 [110/65]
via FE80::2E0:F7FF:FE13:5E01, Serial0/0/0
Corp#
Perfect. I see all six subnets. Notice the O and OI? The O is intra-area and the OI is inter-
area, meaning it’s a route from a different area. You can’t simply distinguish the area by look-
ing at the routing table though. Plus, don’t forget that the routers communicate with their
neighbor via link-local addresses: via FE80::2E0:F7FF:FE13:5E01, Serial0/0/0, for example.
Let’s take a look at the show ipv6 protocols command:
Corp#sh ipv6 protocols
IPv6 Routing Protocol is "connected"
IPv6 Routing Protocol is "static
IPv6 Routing Protocol is "ospf 1"
Interfaces (Area 0)
GigabitEthernet0/0
GigabitEthernet0/1
Serial0/0/0
Serial0/0/1
880 Chapter 20 u Multi-Area OSPF
This just tells us which interfaces are part of OSPF process 1, area 0. To congure OSPFv3,
you absolutely have to know which interfaces are in use. Sho ip int brief can really help you
if you’re having a problem nding your active interfaces.
Let’s take a look at the Gigabit Ethernet OSPFv3 active interface on the Corp router:
Corp#sh ipv6 ospf int g0/0
GigabitEthernet0/0 is up, line protocol is up
Link Local Address FE80::2E0:F7FF:FE0A:3301 , Interface ID 1
Area 0, Process ID 1, Instance ID 0, Router ID 192.168.1.5
Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 192.168.1.5, local address FE80::2E0:F7FF:FE0A:3301
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:09
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 0, Adjacent neighbor count is 0
Suppress hello for 0 neighbor(s)
This is basically the same information we saw earlier in the verication and trouble-
shooting section. Let’s take a look at the neighbor table on the Corp router via show ipv6
ospf neighbor:
Corp#sh ipv6 ospf neighbor
Neighbor ID Pri State Dead Time Interface ID Interface
2.2.2.2 0 FULL/ - 00:00:36 4 Serial0/0/1
192.168.1.6 0 FULL/ - 00:00:39 4 Serial0/0/0
Okay, we can see our two neighbors, and there’s also a slight difference in this version’s
command from OSPFv2. We still see the RID on the left and that we’re also fully adjacent
with both our neighbors—the dash is there because there are no elections on serial point-
to-point links. But we don’t see the neighbor’s IPv6 address listed as we did with OSPFv2’s
IPv4 addreses, which were listed in the interface ID eld.
There’s one other command I want to nish with—the show ipv6 ospf command:
Corp#sh ipv6 ospf
Routing Process "ospfv3 1" with ID 192.168.1.5
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
LSA group pacing timer 240 secs
Interface flood pacing timer 33 msecs
Retransmission pacing timer 66 msecs
Exam Essentials 881
Number of external LSA 0. Checksum Sum 0x000000
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Reference bandwidth unit is 100 mbps
Area BACKBONE(0)
Number of interfaces in this area is 4
SPF algorithm executed 10 times
Number of LSA 10. Checksum Sum 0x05aebb
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
This shows the process ID and RID, our reference bandwidth for this interface, and how
many interfaces we have in each area, which in our example is only area 0.
Holy output! Now that’s what I call a fun chapter. The best thing you can do to get a
solid grasp of OSPF and OSPv3 multi-area networks is to gather up some routers and spend
some quality time with them, practicing everything we’ve covered!
Summary
In this chapter, you learned about the scalability constraints of a single-area OSPF net-
work, and you were introduced to the concept of multi-area OSPF as a solution to these
scalability limitations.
You’re now able to identify the different categories of routers used in multi-area congu-
rations, including the backbone router, internal router, area border router, and autonomous
system boundary router.
I detailed the function of different OSPF Link-State Advertisements (LSAs) and you dis-
covered how these LSAs can be minimized through the effective implementation of specic
OSPF area types. I discussed the Hello protocols and the different neighbor states experi-
enced when an adjacency is taking place.
Verication and troubleshooting are very large parts of the objectives, and I covered
everything you need to know in order to verify and troubleshoot OSPFv2 and meet those
requirements.
Finally, we ended the chapter with the easiest part: conguring and verifying OSPFv3.
Exam Essentials
Know the scalability issues multi-area OSPF addresses. The primary problems in single-area
OSPF networks are the large size of the topology and routing tables, as well as, the excessive
computation of the SPF algorithm due to the large number of link-state updates that occur in
this single area.
882 Chapter 20 u Multi-Area OSPF
Know the different types of OSPF routers. Backbone routers have at least one interface
in area 0. Area border routers (ABRs) belong to two or more OSPF areas simultaneously.
Internal routers have all of their interfaces within the same area. Autonomous system
boundary routers (ASBRs) have at least one interface connected to an external network.
Know the different types of LSA packets. There are seven different types of LSA pack-
ets that Cisco uses, but here are the ones you need to remember: Type 1 LSAs (router link
advertisements), Type 2 LSAs (network link advertisements), Type 3 and 4 LSAs (summary
LSAs), and Type 5 LSAs (AS external link advertisements). Know how each functions.
Be able to monitor multi-area OSPF. There are a number of commands that provide
information useful in a multi-area OSPF environment: show ip route ospf, show ip ospf
neighbor, show ip ospf, and show ip ospf database. It’s important to understand what
each provides.
Be able to troubleshoot OSPF networks. It’s important that you can work your way
through the troubleshooting scenario that I presented in this chapter. Be able to look for
neighbor adjacencies, and if they are not there, look for ACLs set on the routing protocol,
passive interfaces, and wrong network statements.
Understand how to configure OSPFv3. OSPFv3 uses the same basic mechanisms that
OSPFv2 uses, but OSPFv3 is more easily congured by placing the conguring OSPFv3
on a per-interface basis with the ipv6 ospf process-ID area area.
Hands-on Labs 883
Written Lab 6
The answers to this lab can be found in Appendix A, “Answers to Written Labs.”
1. What type of LSAs are sent by an ASBR?
2. What state would a router adjacency be in after the INIT state has finished?
3. What LSA types are sent by ABR toward the area external to the one in which they
were generated?
4. When would you see an adjacency show this: FULL/-?
5. True/False: OSPFv3 is configured per area, per interface.
6. Which OSPF state uses DBD packets and LSRs?
7. Which LSA type is referred to as a router link advertisement (RLA)?
8. What is the command to configure OSPFv3 on an interface with process ID 1 into area 0?
9. What must match exactly between two routers to form an adjacency when using OSPFv3?
10. How can you see all the routing protocols configured and running on your router from
user mode?
Hands-on Labs
In this section, you will use the following network and add OSPF and OSPFv3 routing.
Area 0
10.1.1.0/24
192.168.1.1
s0/0
192.168.1.2
s0/0 Area 1 10.2.2.0/24
RouterA
Area 1
RouterB
g0/0 g0/0
The rst lab requires you to congure two routers with OSPF and then verify the con-
guration. In the second, you will be asked to enable OSPFv3 routing on the same network.
Note that the labs in this chapter were written to be used with real equipment—real cheap
equipment, that is. As with Chapter 5 on EIGRP, I wrote these labs with the cheapest, oldest
routers I had lying around so you can see that you don’t need expensive gear to get through
some of the hardest labs in this book. However, you can use the free LammleSim IOS version
simulator or Cisco’s Packet Tracer to run through these labs.
884 Chapter 20 u Multi-Area OSPF
The labs in this chapter are as follows:
Lab 6.1: Conguring and Verifying Multi-Area OSPF
Lab 6.2: Conguring and Verifying OSPFv3
Hands-on Lab 6.1: Configuring and Verifying OSPF
Multi-Area
In this lab, you’ll congure and verify multi-area OSPF:
1. Implement OSPFv2 on RouterA based on the information in the diagram.
RouterA#conf t
RouterA(config)#router ospf 10
RouterA(config-router)#network 10.0.0.0 0.255.255.255 area 0
RouterA(config-router)#network 192.168.1.0 0.0.0.255 area 0
2. Implement OSPF on RouterB based on the diagram.
RouterB#conf t
RouterB(config)#router ospf 1
RouterB(config-router)#network 192.168.1.2 0.0.0.0 area 0
RouterB(config-router)#network 10.2.2.0 0.0.0.255 area 1
3. Display all the LSAs received on RouterA.
RouterA#sh ip ospf database
OSPF Router with ID (192.168.1.1) (Process ID 10)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
10.1.1.2 10.1.1.2 380 0x80000035 0x0012AB 1
192.168.1.1 192.168.1.1 13 0x8000000A 0x00729F 3
192.168.1.2 192.168.1.2 10 0x80000002 0x0090F9 2
Net Link States (Area 0)
Hands-on Labs 885
Link ID ADV Router Age Seq# Checksum
10.1.1.2 10.1.1.2 381 0x80000001 0x003371
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.2.2.0 192.168.1.2 8 0x80000001 0x00C3FD
RouterA#
4. Display the routing table for RouterA.
RouterA#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia -IS-IS inter area,* - candidate default,U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 2 subnets
O IA 10.2.2.0 [110/101] via 192.168.1.2, 00:00:29, Serial0/0
C 10.1.1.0 is directly connected, FastEthernet0/0
192.168.1.0/30 is subnetted, 1 subnets
C 192.168.1.0 is directly connected, Serial0/0
RouterA#
5. Display the neighbor table for RouterA.
RouterA#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.1.2 0 FULL/ - 00:00:35 192.168.1.2 Serial0/0
10.1.1.2 1 FULL/DR 00:00:34 10.1.1.2 FastEthernet0/0
RouterA#
886 Chapter 20 u Multi-Area OSPF
6. Use the show ip ospf command on RouterB to see that it is an ABR.
RouterB#sh ip ospf
Routing Process "ospf 1" with ID 192.168.1.2
Start time: 1w4d, Time elapsed: 00:07:04.100
Supports only single TOS(TOS0) routes
Supports opaque LSA
Supports Link-local Signaling (LLS)
Supports area transit capability
It is an area border router
Router is not originating router-LSAs with maximum metric
Initial SPF schedule delay 5000 msecs
Minimum hold time between two consecutive SPFs 10000 msecs
Maximum wait time between two consecutive SPFs 10000 msecs
Incremental-SPF disabled
Minimum LSA interval 5 secs
Minimum LSA arrival 1000 msecs
LSA group pacing timer 240 secs
Interface flood pacing timer 33 msecs
Retransmission pacing timer 66 msecs
Number of external LSA 0. Checksum Sum 0x000000
Number of opaque AS LSA 0. Checksum Sum 0x000000
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 2. 2 normal 0 stub 0 nssa
Number of areas transit capable is 0
External flood list length 0
Area BACKBONE(0)
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm last executed 00:06:44.492 ago
SPF algorithm executed 3 times
Area ranges are
Number of LSA 5. Checksum Sum 0x020DB1
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
Area 1
Number of interfaces in this area is 1
Hands-on Labs 887
Area has no authentication
SPF algorithm last executed 00:06:45.640 ago
SPF algorithm executed 2 times
Area ranges are
Number of LSA 3. Checksum Sum 0x00F204
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
Hands-on Lab 6.2: Configuring and Verifying OSPFv3
In this lab, you will congure and verify OSPFv3:
1. Implement OSPFv3 on RouterA. Since the routers have IPv4 addresses, we don’t need
to set the RID of the router.
RouterA#config t
RouterA(config)#int g0/0
RouterA(config-if)#ipv6 ospf 1 area 0
RouterA(config-if)#int s0/0
RouterA(config-if)#ipv6 ospf 1 area 0
That’s all there is to it! Nice.
2. Implement OSPFv3 on RouterB.
RouterB#config t
RouterB(config)#int s0/0/0
RouterB(config-if)#ipv6 ospf 1 area 0
RouterB(config-if)#int f0/0
RouterB(config-if)#ipv6 ospf 1 area 1
Again, that’s all there is to it!
3. Display the routing table for RouterA.
RouterA#sh ipv6 route ospf
IPv6 Routing Table - 11 entries
Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP
U - Per-user Static route
I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary
O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2
888 Chapter 20 u Multi-Area OSPF
ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2
D - EIGRP, EX - EIGRP external
OI 2001:DB8:3C4D:15::/64 [110/65]
via FE80::21A:2FFF:FEE7:4398, Serial0/0
RouterA#
Notice that the one route OSPFv3 found is an inter-area route, meaning the network is
another area.
4. Display the neighbor table for RouterA.
RouterA#sh ipv6 ospf neighbor
Neighbor ID Pri State Dead Time Interface ID Interface
192.168.1.2 1 FULL/ - 00:00:32 6 Serial0/0
RouterA#
5. Display the show ipv6 ospf command on RouterB.
RouterB#sh ipv6 ospf
Routing Process "ospfv3 1" with ID 192.168.1.2
It is an area border router
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
LSA group pacing timer 240 secs
Interface flood pacing timer 33 msecs
Retransmission pacing timer 66 msecs
Number of external LSA 0. Checksum Sum 0x000000
Number of areas in this router is 2. 2 normal 0 stub 0 nssa
Reference bandwidth unit is 100 mbps
Area BACKBONE(0)
Number of interfaces in this area is 1
SPF algorithm executed 3 times
Number of LSA 7. Checksum Sum 0x041C1B
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
Area 1
Number of interfaces in this area is 1
SPF algorithm executed 2 times
Number of LSA 5. Checksum Sum 0x02C608
Hands-on Labs 889
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
RouterB#
890 Chapter 20 u Multi-Area OSPF
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional ques-
tions, please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. Which of the following are scalability issues with single-area OSPF networks? (Choose
all that apply.)
A. Size of the routing table
B. Size of the OSPF database
C. Maximum hop-count limitation
D. Recalculation of the OSPF database
2. Which of the following describes a router that connects to an external routing process
(e.g., EIGRP)?
A. ABR
B. ASBR
C. Type 2 LSA
D. Stub router
3. Which of the following must match in order for an adjacency to occur between routers?
(Choose three.)
A. Process ID
B. Hello and dead timers
C. Link cost
D. Area
E. IP address/subnet mask
4. Which OSPF state do two routers forming an adjacency appear as in the show ip ospf
neighbor output after adding neighbors into the table and exchanging hello information?
A. ATTEMPT
B. INIT
C. 2WAY
D. EXSTART
E. FULL
Review Questions 891
5. You need to set up a preferred link that OSPF will use to route information to a
remote network. Which command will allow you to set the interface link as preferred
over another?
A. ip ospf preferred 10
B. ip ospf priority 10
C. ospf bandwidth 10
D. ip ospf cost 10
6. When would a router’s neighbor table show the FULL/DR state?
A. After the first Hello packets are received by a neighbor
B. When all information is synchronized among adjacent neighbors
C. When the router’s neighbor table is too full of information and is discarding
neighbor information
D. After the EXSTART state
7. Which is/are true regarding OSPFv3? (Choose all that apply.)
A. You must add network statements under the OSPF process.
B. There are no network statements in OSPFv3 configurations.
C. OSPFv3 uses a 128-bit RID.
D. If you have IPv4 configured on the router, it is not mandatory that you configure
the RID.
E. If you don’t have IPv4 configured on the router, it is mandatory that you configure
the RID.
F. OSPFv3 doesn’t use LSAs like OSPFv2 does.
8. What is the default cost of a Fast Ethernet interface configured with OSPF?
A. 1
B. 10
C. 100
D. 1,000
9. Which type of LSA is generated by DRs and referred to as a network link advertise-
ment (NLA)?
A. Type 1
B. Type 2
C. Type 3
D. Type 4
E. Type 5
892 Chapter 20 u Multi-Area OSPF
10. Which type of LSA is generated by ABRs and refers to a summary link advertise-
ment (SLA)?
A. Type 1
B. Type 2
C. Type 3
D. Type 4
E. Type 5
11. Which command will show all the LSAs known by a router?
A. show ip ospf
B. show ip ospf neighbor
C. show ip ospf interface
D. show ip ospf database
12. Using the following illustration, what is the cost from R1’s routing table to reach the
network with Server 1? Each Gigabit Ethernet link has a cost of 4, and each serial link
has a cost of 15.
R2R1
R3 R5
R4
g0/2
g0/0
HostA
Server 1
g0/0
g0/0
g0/1
EoMPLS
A. 100
B. 23
C. 64
D. 19
E. 27
Review Questions 893
13. Using the following illustration, which of the following are true? (Choose all that apply.)
R2R1
R3 R5
R4
g0/2
g0/0
HostA
Area 2 Area 0 Area 1
Server 1
g0/0
g0/0
g0/1
EoMPLS
A. R1 is an internal router.
B. R3 would see the networks connected to the R1 router as an inter-area route.
C. R2 is an ASBR.
D. R3 and R4 would receive information from R2 about the backbone area, and the
same LSA information would be in both LSDBs.
E. R4 is an ABR.
14. Which of the following could cause two routers to not form an adjacency? (Choose all
that apply.)
A. They are configured in different areas.
B. Each router sees the directly connected link as different costs.
C. Two different Process ID’s configured.
D. ACL is configured on the routing protocol.
E. IP address/mask.
F. Passive interface is configured.
15. Which of the following IOS commands shows the state of an adjacency with directly
connected routers?
A. debug ospf events
B. show ip ospf border-routers
C. show ip ospf neighbor
D. show ip ospf database
894 Chapter 20 u Multi-Area OSPF
16. What command will show you the DR and DBR address of the area you are connected
to directly with an interface?
A. show interface s0/0/0
B. show interface fa0/0
C. show ip ospf interface s0/0/0
D. show ip ospf interface fa0/0
17. Which of the following could be causing a problem with the Corp router not forming
an adjacency with its neighbor router? (Choose all that apply.)
Corp#sh ip protocols
Routing Protocol is "ospf 1"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is 10
Router ID 1.1.1.1
Number of areas in this router is 3. 3 normal 0 stub 0 nssa
Maximum path: 4
Routing for Networks:
10.10.0.0 0.0.255.255 area 0
172.16.10.0 0.0.0.3 area 1
172.16.10.4 0.0.0.3 area 2
Reference bandwidth unit is 100 mbps
Passive Interface(s):
Serial0/0/0
Routing Information Sources:
Gateway Distance Last Update
1.1.1.1 110 00:17:42
172.16.10.2 110 00:17:42
172.16.10.6 110 00:17:42
Distance: (default is 110)
A. The routers are configured with the wrong network statements.
B. They have different maximum paths configured.
C. There is a passive interface configured.
D. There is an ACL set stopping Hellos.
E. The costs of the links between the routers are configured differently.
F. They are in different areas.
Review Questions 895
18. Which of the following is/are true? (Choose all that apply.)
A. The reference bandwidth for OSPF and OSPFv3 is 1.
B. The reference bandwidth for OSPF and OSPFv3 is 100.
C. You change the reference bandwidth from global config with the command auto-cost
reference bandwidth number.
D. You change the reference bandwidth under the OSPF router process with the
command auto-cost reference bandwidth number.
E. Only one router needs to set the reference bandwidth if it is changed from its default.
F. All routers in a single area must set the reference bandwidth if it is changed from
its default.
G. All routers in the AS must set the reference bandwidth if it is changed from its default.
19. Which of the following interfaces would have the same default cost? (Choose two.)
A. Fast Ethernet
B. Ethernet
C. Serial
D. Gigabit Ethernet
20. What is the default cost of a serial interface with OSPF?
A. 1
B. 10
C. 32
D. 64
E. 100
Chapter
21
Wide Area Networks
THE FOLLOWING ICND2 EXAM TOPICS
ARE COVERED IN THIS CHAPTER:
1 IP Routing Technologies
■Configure and verify operation status of a Serial interface
1 WAN Technologies
■Identify different WAN Technologies
■Metro Ethernet
■VSAT
■Cellular 3G/4G
■MPLS
■T1/E1
■ISDN
■DSL
■Frame relay
■Cable
■VPN
■Configure and verify a basic WAN serial connection
■Configure and verify a PPP connection between Cisco routers
■Configure and verify Frame Relay on Cisco routers
■Implement and troubleshoot PPPoE
1 Troubleshooting
■Troubleshoot and resolve WAN implementation issues
■Serial interfaces
■PPP
■Frame Relay
The Cisco IOS supports a ton of different wide area network
(WAN) protocols that help you extend your local LANs to other
LANs at remote sites. And I don’t think I have to tell you how
essential information exchange between disparate sites is these days—it’s absolutely vital! But
even so, it wouldn’t exactly be cost effective or efcient to install your own cable and connect
all of your company’s remote locations yourself, would it? A much better way to get this done
is to just lease the existing installations that service providers already have in place.
This is exactly why I’m going to devote most of this chapter to covering the various types
of connections, technologies, and devices used in today’s WANs. We’ll also delve into how
to implement and congure High-Level Data-Link Control (HDLC), Point-to-Point Protocol
(PPP), and Frame Relay. I’ll describe Point-to-Point Protocol over Ethernet (PPPoE), cable,
digital subscriber line (DSL), MultiProtocol Label Switching (MPLS), Metro Ethernet, plus
last mile and long-range WAN technologies. I’ll also introduce you to WAN security con-
cepts, tunneling, and virtual private network basics.
Just so you know, I’m not going to cover each and every type of Cisco WAN support
here because the focus of this book is to equip you with everything you need to successfully
meet the Cisco exam objectives. This chapter’s emphasis will be on cable, DSL, HDLC,
PPP, PPPoE, Metro Ethernet, MPLS, and Frame Relay, but I’ll wrap it up by giving you a
solid grounding in virtual private networks (VPNs) and how to create a tunnel using GRE
(Generic Routing Encapsulation).
To find up-to-the-minute updates for this chapter, please see
www.lammle.com/forum or the book’s web page at www.sybex.com.
Introduction to Wide Area Networks
Let’s begin exploring WAN basics by asking, What’s the difference between a wide area
network (WAN) and a local area network (LAN)? Clearly there’s the distance factor, but
modern wireless LANs can cover some serious turf, so there’s more to it than that. What
about bandwidth? Here again, some really big pipes can be had for a price in many places,
so that’s not it either. What’s the answer we’re looking for?
A major distinction between a WAN and a LAN is that while you generally own a LAN
infrastructure, you usually lease a WAN infrastructure from a service provider. And to be
honest, modern technologies even blur this characteristic somewhat, but it still ts neatly
into the context of Cisco’s exam objectives!
Introduction to Wide Area Networks 899
I’ve already talked about the Data Link that you usually own when we covered Ethernet,
so I’m going to focus on the type you usually don’t own—the kind you typically lease from
a service provider.
And as usual, the rst key to understanding WAN technologies is to be familiar with
the different WAN terms and connection types commonly used by service providers to join
your LAN networks together. We’ll begin covering these topics now.
Defining WAN Terms
Before you run out and order a WAN service type from a provider, you really need to
understand the following terms that service providers typically use. Take a look at these
in Figure21.1:
FIGURE21.1 WAN terms
Provider
toll network
CO
Corporate
Local loop
CSU/DSU
Demarc
CPE
Customer premises equipment (CPE) Customer premises equipment (CPE) is equipment
that’s typically owned by the subscriber and located on the subscriber’s premises.
Demarcation point The demarcation point (demarc for short) is the precise spot where
the service provider’s responsibility ends and the CPE begins. It’s generally a device in a
telecommunications closet owned and installed by the telecommunications company (telco).
It’s your responsibility to cable (extended demarc) from this box to the CPE, which is usu-
ally a connection to a CSU/DSU, although more recently we see the provider giving us an
Ethernet connection. Nice!
Local loop The local loop connects the demarc to the closest switching ofce, referred to
as the central ofce.
Central office (CO) This point connects the customer’s network to the provider’s switch-
ing network. Make a mental note that a central ofce (CO) is sometimes also referred to as
a point of presence (POP).
Toll network The toll network is a trunk line inside a WAN provider’s network. This net-
work is a collection of switches and facilities owned by the Internet service provider (ISP).
Denitely familiarize yourself with these terms, what they represent, and where they’re
located, as shown in Figure21.1, because they’re key to understanding WAN technologies.
900 Chapter 21 u Wide Area Networks
WAN Connection Bandwidth
Next, I want you to know these basic but very important bandwidth terms used when refer-
ring to WAN connections:
Digital Signal 0 (DS0) This is the basic digital signaling rate of 64 Kbps, equivalent to
one channel. Europe uses the E0 and Japan uses the J0 to reference the same channel speed.
Typical to T-carrier transmission, this is the generic term used by several multiplexed digital
carrier systems and is also the smallest-capacity digital circuit. 1 DS0 = 1 voice/data line.
T1 Also referred to as a DS1, a T1 comprises 24 DS0 circuits bundled together for a total
bandwidth of 1.544 Mbps.
E1 This is the European equivalent of a T1 and comprises 30 DS0 circuits bundled together
for a bandwidth of 2.048 Mbps.
T3 Referred to as a DS3, a T3 comprises 28 DS1s bundled together, or 672 DS0s, for a
bandwidth of 44.736 Mbps.
OC-3 Optical Carrier (OC) 3 uses ber and is made up of three DS3s bundled together.
It’s made up of 2,016 DS0s and avails a total bandwidth of 155.52 Mbps.
OC-12 Optical Carrier 12 is made up of four OC-3s bundled together and contains
8,064 DS0s for a total bandwidth of 622.08 Mbps.
OC-48 Optical Carrier 48 is made up of four OC-12s bundled together and contains
32,256 DS0s for a total bandwidth of 2488.32 Mbps.
WAN Connection Types
You’re probably aware that a WAN can use a number of different connection types available
on the market today. Figure21.2 shows the different WAN connection types that can be
used to connect your LANs (made up of data terminal equipment, or DTE) together over the
data communication equipment (DCE) network.
Let me explain the different WAN connection types in detail now:
Dedicated (leased lines) These are usually referred to as a point-to-point or dedicated
connections. A leased line is a pre-established WAN communications path that goes from
the CPE through the DCE switch, then over to the CPE of the remote site. The CPE enables
DTE networks to communicate at any time with no cumbersome setup procedures to mud-
dle through before transmitting data. When you’ve got plenty of cash, this is denitely the
way to go because it uses synchronous serial lines up to 45 Mbps. HDLC and PPP encapsu-
lations are frequently used on leased lines, and I’ll go over these with you soon.
Circuit switching When you hear the term circuit switching, think phone call. The big
advantage is cost; most plain old telephone service (POTS) and ISDN dial-up connections
are not at rate, which is their advantage over dedicated lines. No data can transfer before
an end-to-end connection is established. Circuit switching uses dial-up modems or ISDN and
Introduction to Wide Area Networks 901
is used for low-bandwidth data transfers. Okay, I know what you’re thinking: “Modems?
Did he say modems? Aren’t those found only in museums now?” After all, with all the wire-
less technologies available, who would use a modem these days? Well, some people do have
ISDN; it’s still viable and there are a few who still use a modem now and then. And circuit
switching can be used in some of the newer WAN technologies as well.
FIGURE21.2 WAN connection types
Dedicated
Circuit-switched
Packet-switched
Telephone
company
Service
provider
Synchronous serial
Asynchronous serial, ISDN
Synchronous serial
Packet switching This is a WAN switching method that allows you to share band-
width with other companies to save money, just like a super old party line, where homes
shared the same phone number and line to save money. Packet switching can be thought
of as a network that’s designed to look like a leased line yet it charges you more, like
circuit switching does. As usual, you get what you pay for, and there’s denitely a seri-
ous downside to this technology. If you need to transfer data constantly, well, just forget
about this option and get a leased line instead! Packet switching will only really work
for you if your data transfers are bursty, not continuous; think of a highway, where you
can only go as fast as the trafc—packet switching is the same thing. Frame Relay and
X.25 are packet-switching technologies with speeds that can range from 56 Kbps up to
T3 (45 Mbps).
MultiProtocol Label Switching (MPLS) uses a combination of both circuit
switching and packet switching, but it’s not within the scope of topics
this book covers. Still, because I would highly recommend checking
into MPLS after you pass your Cisco exam exam, I’ll talk about it briefly
in a minute.
902 Chapter 21 u Wide Area Networks
WAN Support
Cisco supports many layer 2 WAN encapsulations on its serial interfaces, including HDLC,
PPP, and Frame Relay, which map to the Cisco exam objectives. You can view them via the
encapsulation ? command from any serial interface, but understand that the output you’ll
get can vary based upon the specic IOS version you’re running:
Corp#config t
Corp(config)#int s0/0/0
Corp(config-if)#encapsulation ?
atm-dxi ATM-DXI encapsulation
frame-relay Frame Relay networks
hdlc Serial HDLC synchronous
lapb LAPB (X.25 Level 2)
ppp Point-to-Point protocol
smds Switched Megabit Data Service (SMDS)
x25 X.25
I also want to point out that if I had other types of interfaces on my router, I would
have a different set of encapsulation options. And never forget that you can’t congure an
Ethernet encapsulation on a serial interface or vice versa!
Next, I’m going to dene the most prominently known WAN protocols used in the latest
Cisco exam objectives: Frame Relay, ISDN, HDLC, PPP, PPPoE, cable, DSL, MPLS, ATM,
3G/4G, VSAT, and Metro Ethernet. Just so you know, the only WAN protocols you’ll usually
nd congured on a serial interface are HDLC, PPP, and Frame Relay, but who said you’re
stuck with using only serial interfaces for wide area connections? Actually, we’re beginning
to see fewer and fewer serial connections because they’re not as scalable or cost effective as a
Fast Ethernet connection to your ISP.
Frame Relay A packet-switched technology that made its debut in the early 1990s,
Frame Relay is a high-performance Data Link and Physical layer specication. It’s pretty
much a successor to X.25, except that much of the technology in X.25 used to compen-
sate for physical errors like noisy lines has been eliminated. An upside to Frame Relay is
that it can be more cost effective than point-to-point links, plus it typically runs at speeds
of 64 Kbps up to 45 Mbps (T3). Another Frame Relay benet is that it provides features
for dynamic bandwidth allocation and congestion control.
ISDN Integrated Services Digital Network (ISDN) is a set of digital services that transmit
voice and data over existing phone lines. ISDN offers a cost-effective solution for remote
users who need a higher-speed connection than analog POTS dial-up links can give them,
and it’s also a good choice to use as a backup link for other types of links, such as Frame
Relay or T1 connections.
HDLC High-Level Data-Link Control (HDLC) was derived from Synchronous Data Link
Control (SDLC), which was created by IBM as a Data Link connection protocol. HDLC works
Introduction to Wide Area Networks 903
at the Data Link layer and creates very little overhead compared to Link Access Procedure,
Balanced (LAPB).
Generic HDLC wasn’t intended to encapsulate multiple Network layer protocols across the
same link—the HDLC header doesn’t contain any identication about the type of protocol
being carried inside the HDLC encapsulation. Because of this, each vendor that uses HDLC
has its own way of identifying the Network layer protocol, meaning each vendor’s HDLC is
proprietary with regard to its specic equipment.
PPP Point-to-Point Protocol (PPP) is a pretty famous, industry-standard protocol. Because
all multiprotocol versions of HDLC are proprietary, PPP can be used to create point-to-point
links between different vendors’ equipment. It uses a Network Control Protocol eld in the
Data Link header to identify the Network layer protocol being carried and allows authenti-
cation and multilink connections to be run over asynchronous and synchronous links.
PPPoE Point-to-Point Protocol over Ethernet encapsulates PPP frames in Ethernet frames
and is usually used in conjunction with xDSL services. It gives you a lot of the familiar PPP
features like authentication, encryption, and compression, but there’s a downside—it has
a lower maximum transmission unit (MTU) than standard Ethernet does. If your rewall
isn’t solidly congured, this little factor can really give you some grief!
Still somewhat popular in the United States, PPPoE’s main feature is that it adds a direct
connection to Ethernet interfaces while also providing DSL support. It’s often used by many
hosts on a shared Ethernet interface for opening PPP sessions to various destinations via at
least one bridging modem.
Cable In a modern hybrid ber-coaxial (HFC) network, typically 500 to 2,000 active data
subscribers are connected to a certain cable network segment, all sharing the upstream and
downstream bandwidth. HFC is a telecommunications industry term for a network that
incorporates both optical ber and coaxial cables to create a broadband network. The actual
bandwidth for Internet service over a cable TV (CATV) line can be up to about 27Mbps on
the download path to the subscriber, with about 2.5 Mbps of bandwidth on the upload path.
Typically users get an access speed from 256 Kbps to 6 Mbps. This data rate varies greatly
throughout the United States and can be much, much higher today.
DSL Digital subscriber line is a technology used by traditional telephone companies to
deliver advanced services such as high-speed data and sometimes video over twisted-pair
copper telephone wires. It typically has lower data-carrying capacity than HFC networks,
and data speeds can be limited in range by line lengths and quality. Digital subscriber
line is not a complete end-to-end solution but rather a Physical layer transmission tech-
nology like dial-up, cable, or wireless. DSL connections are deployed in the last mile of a
local telephone network—the local loop. The connection is set up between a pair of DSL
modems on either end of a copper wire located between the customer premises equip-
ment (CPE) and the Digital Subscriber Line Access Multiplexer (DSLAM). A DSLAM is
the device located at the provider’s central ofce (CO) and concentrates connections from
multiple DSL subscribers.
904 Chapter 21 u Wide Area Networks
MPLS MultiProtocol Label Switching (MPLS) is a data-carrying mechanism that emulates
some properties of a circuit-switched network over a packet-switched network. MPLS is a
switching mechanism that imposes labels (numbers) to packets and then uses them to forward
packets. The labels are assigned on the edge of the MPLS network, and forwarding inside
the MPLS network is carried out solely based on the labels. The labels usually correspond to
a path to layer 3 destination addresses, which is on par with IP destination-based routing.
MPLS was designed to support the forwarding of protocols other than TCP/IP. Because of
this, label switching within the network is achieved the same way irrespective of the layer 3
protocol. In larger networks, the result of MPLS labeling is that only the edge routers per-
form a routing lookup. All the core routers forward packets based on the labels, which makes
forwarding the packets through the service provider network faster. This is a big reason most
companies have replaced their Frame Relay networks with MPLS service today. Last, you
can use Ethernet with MPLS to connect a WAN, and this is called Ethernet over MPLS,
or EoMPLS.
ATM Asynchronous Transfer Mode (ATM) was created for time-sensitive trafc, provid-
ing simultaneous transmission of voice, video, and data. ATM uses cells that are a xed
53-bytes long instead of packets. It also can use isochronous clocking (external clocking) to
help the data move faster. Typically, if you’re running Frame Relay today, you will be run-
ning Frame Relay over ATM.
Cellular 3G/4G Having a wireless hot spot in your pocket is pretty normal these days. If
you have a pretty current cellular phone, then you can probably can gain access through
your phone to the Internet. You can even get a 3G/4G card for an ISR router that’s useful
for a small remote ofce that’s in the coverage area.
VSAT Very Small Aperture Terminal (VSAT) can be used if you have many locations
geographically spread out in a large area. VSAT uses a two-way satellite ground station
with dishes available through many companies like Dish Network or Hughes and con-
nects to satellites in geosynchronous orbit. A good example of where VSATs are a useful,
cost-effective solution would be companies that use satellite communications to VSATs,
like gasoline stations that have hundreds or thousands of locations spread out over the
entire country. How could you connect them otherwise? Using leased lines would be cost
prohibitive and dial-ups would be way too slow and hard to manage. Instead, the signal
from the satellite connects to many remote locations at once, which is much more cost
effective and efcient!
Metro Ethernet Metropolitan-area Ethernet is a metropolitan area network (MAN)
that’s based on Ethernet standards and can connect a customer to a larger network and
the Internet. If available, businesses can use Metro Ethernet to connect their own ofces
together, which is another very cost-effective connection option. MPLS-based Metro Ethernet
networks use MPLS in the ISP by providing an Ethernet or ber cable to the customer as a
connection. From the customer, it leaves the Ethernet cable, jumps onto MPLS, and then
Ethernet again on the remote side. This is a smart and thrifty solution that’s very popular if
you can get it in your area.
Cable and DSL 905
Cable and DSL
Okay, before we get into talking about the principal serial encapsulation connections used
on Cisco routers, like HDLC, PPP, and Frame Relay, I’m going to discuss cable modems
and DSL, including ADSL and PPPoE, as solutions for connections to wide area networks.
I think this approach will really help you understand the practical differences between DSL
and cable modem networking.
DSL and cable Internet services actually do have a lot in common, but they still have
some basic, essential differences that are important for you to understand:
Speed Most would say that cable is faster than DSL Internet, but cable doesn’t always win
the race in real-world use.
Security DSL and cable are based on different network security models, and until recently,
cable has been the reputed loser in this contest. But now, it’s pretty much a toss-up, and both
offer adequate security that meets the needs of most users. But when I say adequate, I mean
that there are still some very real security issues relating to both alternatives, no matter what
your ISP says!
Popularity Cable Internet is denitely “best in show” in the United States, but DSL is
beginning to catch up.
Customer satisfaction Here, the reverse is true—in the United States, DSL is top dog. But
still, do you know anyone who’s really totally satised with their ISP?
Figure21.3 shows how a connection can terminate from modems to either a PC directly
or to a router. Typically, your router would run DHCP on that interface as well as PPPoE.
Both DSL and cable high-speed Internet services are available to millions of residential and
business consumers worldwide, but in some areas, only one and sometimes even none of
these services are available.
FIGURE21.3 Broadband access using cable or DSL
Always-on Voice, Video, and Data Services
Cable or DSL
Cable or DSL
Ethernet
Underlying
network is
transparent
to the user.
Surprisingly, some of the differences between DSL and cable modem have nothing to do
with the actual technologies—it comes down to the individual ISP. All other things being
equal issues like cost, reliability, and quality of customer support for both installation and
maintenance issues vary signicantly from one provider to the next.
906 Chapter 21 u Wide Area Networks
Cable
Cable is a great cost-effective connection for a small ofce or home ofce that’s commonly
referred to as SOHO—yes, there really is an acronym for everything! And even in larger
organizations, cable or even DSL can be great to have as a backup link.
Here are a few cable network terms:
Headend This is where all cable signals are received, processed, and formatted. The sig-
nals are then transmitted over the distribution network from the headend.
Distribution network These are relatively small service areas that usually range in size
from 100 to 2,000 customers. They’re typically made up of a mixed, ber-coaxial, or HFC
architecture, with optical ber substituting for the distribution network’s trunk portion.
The ber forms both the connection from the headend and an optical node that changes
light to radio frequency (RF) signals that are then distributed through a coaxial cable
throughout the specic area being serviced.
Data Over Cable Service Interface Specification (DOCSIS) All cable modems and like
devices have to measure up to this standard.
Figure21.4 shows where you would nd the various types of networks and how most of
the terms I just listed would be used in a network diagram.
FIGURE21.4 Cable network and terms
Headend
HFC
DOCSIS standard
Coaxial cable serving area
Fiber Node
Let me explain the tangle here: ISPs often use a ber-optic network that extends from
the cable operator’s master headend, sometimes even to regional headends, and proceeds
out to a neighborhood’s hub site before nally arriving at a ber-optic node, which serves
anywhere from 25 to 2,000 or more homes. Not exactly an elegant process, but don’t get me
wrong, all types of links have their own, specic problems—I’m not just picking on cable!
As if that wasn’t enough, here’s another issue: if you have cable, open your PC’s command
prompt and type ipconfig and then check out your subnet mask. It’s probably a /20 or /21
Class B address. Oh my! You already know that means there are either 4,094 or 2,046 hosts
per cable network connection, right? This level of population density makes Manhattan look
rural and empty!
When we say “cable,” we’re really referring to the act of using coax (coaxial) cable for
transmission. And CATV, or community antenna television, is now used as a means to
offer cost-effective broadcasting to subscribers. So cable can provide voice and data, plus
analog and digital video, all without devouring your whole paycheck.
Cable and DSL 907
Your average cable connection gives you a maximum download speed of 20 Mbps or more.
And remember—you have to share that bandwidth with all those other subscribers. As if that
weren’t enough, there are other things like overloaded web servers and plain old Net conges-
tion that factor in as well. But your email-checking neighbors really aren’t making that much
of a difference. So who or what is? Well, if you’re an online gamer, you would likely notice a
bit more lag during peak periods, which can be a matter of virtual life and death! And if some-
body in your neighborhood is uploading a large amount of data like, say, an entire collection
of Star Wars movies, well, that could denitely max out the entire connection, bringing every-
one’s browser to a crawl.
Cable modem access may or may not be faster or easier to install than DSL, and your
mileage will vary, depending on where you live plus a variety of other factors. But it’s usually
more available and a tad less pricey, making it a winner by a nose. But no worries, if cable
access isn’t available in your neighborhood, DSL is okay—anything is better than dial-up!
Digital Subscriber Line (DSL)
Coming in second in our subscriber-based popularity contest is digital subscriber line (DSL),
a technology that uses your garden-variety copper phone wires to give you high-speed data
transmission. DSL requires a phone line, a DSL modem that’s usually included with the ser-
vice, either an Ethernet card or a router that has an Ethernet connection, and someone that
can provide service wherever you happen to be located.
The acronym DSL originally meant “digital subscriber loop,” but now its meaning has
morphed to “line.” DSL group types fall into two categories based upon upstream or down-
stream speed connections:
Symmetrical DSL The speed for both downstream and upstream connections are equal,
or symmetrical. SDSL has a service distance of 12,000 feet at high speeds.
Asymmetrical DSL Different transmission speeds occur between two ends of a network—
downstream speed is always faster. ADSL has a service distance of 18,000 feet at high speeds.
Figure21.5 shows an average home user with xDSL, which is a transmission technology
that moves data over copper pairs.
FIGURE21.5 xDSL connection from home user to central office. All types of DSL
are layer 1 technologies.
Subscriber
End User
ATU-R
ATU-R = ASDL Transmission Unit – Remote
ATU-C = ASDL Transmission Unit – Central
Copper loop
Local loop
ATM
Ethernet
xDSL
ATU-C
CO
908 Chapter 21 u Wide Area Networks
The term xDSL covers a number of DSL variations, such as Asymmetrical DSL (ADSL),
high-bit-rate DSL (HDSL), Rate Adaptive DSL (RADSL), Synchronous DSL (SDSL), ISDN
DSL (IDSL), and very-high-data-rate DSL (VDSL).
DSL avors that don’t use the voice frequencies band, like ADSL and VDSL, allow DSL
lines to carry both data and voice signals simultaneously. Others, like SDSL and IDSL, which
occupy the complete frequency range, can carry only data. And by the way, the data service
that the DSL connection gives you is always on.
The speed that DSL service can offer depends on how far you are from the central ofce
(CO)—the closer the better. In fact, you can blaze at rates up to around 6.1 Mbps if you’re
physically close enough!
ADSL
Asymmetrical DSL (ADSL) supports both voice and data at the same time, but it was cre-
ated to allot more bandwidth downstream than upstream because it’s best for residential
subscribers who usually need more downstream bandwidth for activities like downloading
video, movies, music, online gaming, general surng, and getting emails—some of which
include sizeable attachments. ADSL will give you a downstream rate from 256 Kbps to
8Mbps, but anything going upstream is only going to reach around 1.5 Mbps max.
Plain old telephone service (POTS) provides a channel for analog voice transmission
and can transmit without a problem with ADSL over the same twisted-pair telephone line.
Actually, depending on the type of ADSL, not just two but three information channels com-
monly utilize the same wiring simultaneously. This is why people can use a phone line and
an ADSL connection at the same time and not affect either service.
ATM is the Data Link layer protocol typically used over the DSL layer 1 connection
from the CPE and is terminated at what’s known as the DSLAM—an ATM switch that
contains DSL interface cards, or ATU-Cs. After ADSL connections meet their end at the
DSLAM, it switches the data over an ATM network to something called an aggregation
router—a layer 3 device where the subscriber’s IP connection then expires.
You know by now how important encapsulation is, so as you’ve probably guessed, any
IP packets over an ATM and DSL connection must provide it. This happens in one of three
ways, depending on your interface type and the service provider’s switch:
PPPoE I’ll discuss this in detail in the next section.
RFC1483 routing RFC1483 describes two different methods for carrying connectionless
network trafc over an ATM network: routed protocols and bridged protocols.
PPPoA Point-to-Point Protocol (PPP) over ATM is used to encapsulate PPP frames in ATM
AAL5 (ATM Adaptation Layer 5). It’s typically used with cable modems, DSL, and ADSL
services and offers the usual PPP features of authentication, encryption, and compression. It
actually has less overhead compared to PPPoE.
PPPoE
Used with ADSL services, PPPoE (Point-to-Point Protocol over Ethernet) encapsulates PPP
frames in Ethernet frames and uses common PPP features like authentication, encryption,
Cable and DSL 909
and compression. But as I said earlier, it’s trouble if you’ve got a badly congured rewall.
This is a tunneling protocol that layers IP and other protocols that run over PPP with the
attributes of a PPP link so they can then be used to contact other Ethernet devices and initi-
ate a point-to-point connection to transport IP packets.
Figure21.6 displays typical usage of PPPoE over ADSL. As you can see, a PPP session
is connected from the PC of the end user to the router and the subscriber PC IP address is
assigned by the router via IPCP.
FIGURE21.6 PPPoE with ADSL
CPE
(bridging) DSLAM
ATM
Aggregation
router
ISP/Corp
router
AAA
OC3 ATMIP
PPPoE is used to equip custom PPP-based software with the ability to deal with a connec-
tion that’s not using a serial line and to be at home in a packet-oriented network environment
like Ethernet. It also allows for a custom connection with login and password for Internet
connection accounting. Another factor is that the opposite side of the link’s IP address is
given to it and it’s available only for the specic period that the PPPoE connection is open.
This means that reusing IP addresses is dynamically permitted.
PPPoE has a discovery stage and a PPP session stage (see RFC 2516) that works like this:
First, a host begins a PPPoE session, during which it has to execute a discovery process so
it can choose the best server to meet the needs of the client machine’s request. After that, it
has to discover the Ethernet MAC address of the peer device and create a PPPoE session
ID. So even though PPP delimits a peer-to-peer relationship, the discovery part is innately
a client-server relationship.
Okay, before getting into serial connections, there’s one last thing I want to cover—
Cisco LRE.
Cisco Long Range Ethernet (LRE)
The Cisco Long Range Ethernet solution employs VDSL (very-high-data-rate digital sub-
scriber line) technology to signicantly expand Ethernet service capacity. And LRE can
achieve impressive speeds from 5 to 15 Mbps (full-duplex) at distances up to 5,000 feet,
traveling over existing twisted-pair wiring!
910 Chapter 21 u Wide Area Networks
So basically, Cisco LRE technology can give us broadband service on POTS, digital
telephone, and ISDN trafc lines, and it can also operate in modes compatible with ADSL
technologies. This exibility is important because it makes it possible for service providers
to make LRE available in structures and/or buildings that have broadband services already
in place but need it enhanced—very cool indeed!
Cabling the Serial Wide Area Network
As you can imagine, there are a few things that you need to know before connecting your
WAN to ensure that everything goes well. For starters, you have to understand the kind
of WAN Physical layer implementation that Cisco provides and be familiar with the vari-
ous types of WAN serial connectors involved.
The good news is that Cisco serial connections support almost any type of WAN service.
Your typical WAN connection is a dedicated leased line using HDLC, PPP, and Frame Relay
with speeds that can kick it up to 45 Mbps (T3).
HDLC, PPP, and Frame Relay can use the same Physical layer specications. I’ll go over
the various types of connections and then move on to tell you all about the WAN protocols
specied in the Cisco exam objectives.
Serial Transmission
WAN serial connectors use serial transmission, something that takes place 1 bit at a time
over a single channel.
Older Cisco routers have used a proprietary 60-pin serial connector that you have to
get from Cisco or a provider of Cisco equipment. Cisco also has a new, smaller proprietary
serial connection that’s about one-tenth the size of the 60-pin basic serial cable called the
smart-serial. You have to verify that you have the right type of interface in your router
before using this cable connector.
The type of connector you have on the other end of the cable depends on your service
provider and its particular end-device requirements. There are several different types of
ends you’ll run into:
u EIA/TIA-232—Allowed speed up to 64 Kbps on 24-pin connector
u EIA/TIA-449
u V.35—Standard used to connect to a CSU/DSU, with speeds up to 2.048 Mbps using a
34-pin rectangular connector
u EIA-530
Make sure you’re clear on these things: serial links are described in frequency, or
cycles per second (hertz). The amount of data that can be carried within these frequencies
is called bandwidth. Bandwidth is the amount of data in bits per second that the serial
channel can carry.
Cabling the Serial Wide Area Network 911
Data Terminal Equipment and Data
Communication Equipment
By default, router interfaces are typically data terminal equipment (DTE), and they con-
nect into data communication equipment (DCE) like a channel service unit/data service
unit (CSU/DSU) using a V.35 connector. CSU/DSU then plugs into a demarcation location
(demarc) and is the service provider’s last responsibility. Most of the time, the demarc is a jack
that has an RJ45 (8-pin modular) female connector located in a telecommunications closet.
Actually, you may already have heard of demarcs. If you’ve ever had the glorious experi-
ence of reporting a problem to your service provider, they’ll usually tell you everything tests
out ne up to the demarc, so the problem must be the CPE, or customer premises equipment.
In other words, it’s your problem, not theirs!
Figure21.7 shows a typical DTE-DCE-DTE connection and the devices used in the network.
FIGURE21.7 DTE-DCE-DTE WAN connection: Clocking is typically provided by
the DCE network to routers. In nonproduction environments, a DCE network is not
always present.
DTE
DTE
DCE
CSU/DSU CSU/DSU
The idea behind a WAN is to be able to connect two DTE networks through a DCE net-
work. The DCE network includes the CSU/DSU, through the provider’s wiring and switches,
all the way to the CSU/DSU at the other end. The network’s DCE device (CSU/DSU) provides
clocking to the DTE-connected interface (the router’s serial interface).
As mentioned, the DCE network provides clocking to the router; this is the CSU/DSU. If
you have a nonproduction network and you’re using a WAN crossover type of cable and do
not have a CSU/DSU, then you need to provide clocking on the DCE end of the cable by using
the clock rate command. To nd out which interface needs the clock rate command, use
the show controllers int command:
Corp#sh controllers s0/0/0
Interface Serial0/0/0
912 Chapter 21 u Wide Area Networks
Hardware is PowerQUICC MPC860
DCE V.35, clock rate 2000000
The preceding output shows a DCE interface that has the clock rate set to 2000000,
which is the default for ISR routers. This next output shows a DTE connector, so you don’t
need enter the clock rate command on this interface:
SF#sh controllers s0/0/0
Interface Serial0/0/0
Hardware is PowerQUICC MPC860
DTE V.35 TX and RX clocks detected
Terms such as EIA/TIA-232, V.35, X.21, and HSSI (High-Speed Serial
Interface) describe the Physical layer between the DTE (router) and DCE
device (CSU/DSU).
High-Level Data-Link Control
(HDLC) Protocol
The High-Level Data-Link Control (HDLC) protocol is a popular ISO-standard, bit-oriented,
Data Link layer protocol. It species an encapsulation method for data on synchronous serial
data links using frame characters and checksums. HDLC is a point-to-point protocol used on
leased lines. No authentication is provided by HDLC.
In byte-oriented protocols, control information is encoded using entire bytes. On the other
hand, bit-oriented protocols use single bits to represent the control information. Some com-
mon bit-oriented protocols are SDLC and HDLC; TCP and IP are byte-oriented protocols.
HDLC is the default encapsulation used by Cisco routers over synchronous serial links.
And Cisco’s HDLC is proprietary, meaning it won’t communicate with any other vendor’s
HDLC implementation. But don’t give Cisco grief for it—everyone’s HDLC implementa-
tion is proprietary. Figure21.8 shows the Cisco HDLC format.
The reason every vendor has a proprietary HDLC encapsulation method is that each
vendor has a different way for the HDLC protocol to encapsulate multiple Network layer
protocols. If the vendors didn’t have a way for HDLC to communicate the different layer
3 protocols, then HDLC would be able to operate in only a single layer 3 protocol environ-
ment. This proprietary header is placed in the data eld of the HDLC encapsulation.
It’s pretty simple to congure a serial interface if you’re just going to connect two
Cisco routers across a T1, for example. Figure21.9 shows a point-to-point connection
between two cities.
High-Level Data-Link Control (HDLC) Protocol 913
FIGURE21.8 Cisco’s HDLC frame format: Each vendor’s HDLC has a proprietary
data field to support multiprotocol environments.
Cisco HDLC
Flag Address Control Proprietary Data FCS Flag
Supports only single-protocol environments
HDLC
Flag Address Control Data FCS Flag
FIGURE21.9 Configuring Cisco’s HDLC proprietary WAN encapsulation
DCE
Clocking provided by CSU/DSU to DTE device
Corp S0/0
CSU/DSU CSU/DSU
CSU/DSU CSU/DSU SF
We can easily congure the routers with a basic IP address and then enable the interface.
Assuming the link to the ISP is up, the routers will start communicating using the default
HDLC encapsulation. Let’s take a look at the Corp router conguration so you can see just
how easy this can be:
Corp(config)#int s0/0
Corp(config-if)#ip address 172.16.10.1 255.255.255.252
Corp(config-if)#no shut
Corp#sh int s0/0
Serial0/0 is up, line protocol is up
Hardware is PowerQUICC Serial
Internet address is 172.16.10.1/30
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set
Keepalive set (10 sec)
914 Chapter 21 u Wide Area Networks
Corp#sh run | begin interface Serial0/0
interface Serial0/0
ip address 172.16.10.1 255.255.255.252
!
Note that all I did was add an IP address before I then enabled the interface—pretty simple!
Now, as long as the SF router is running the default serial encapsulation, this link will come
up. Notice in the preceding output that the show interface command does show the encap-
sulation type of HDLC, but the output of show running-config does not. This is important—
remember that if you don’t see an encapsulation type listed under a serial interface in the active
conguration le, you know it’s running the default encapsulation of HDLC.
So let’s say you have only one Cisco router and you need to connect to a non-Cisco router
because your other Cisco router is on order or something. What would you do? You couldn’t
use the default HDLC serial encapsulation because it wouldn’t work. Instead, you would need
to go with an option like PPP, an ISO-standard way of identifying the upper-layer protocols.
Now is a great time to get into more detail about PPP as well as how to connect to routers
using the PPP encapsulation. You can check out RFC 1661 for more information on the origins
and standards of PPP.
Point-to-Point Protocol (PPP)
Point-to-Point Protocol (PPP) is a Data Link layer protocol that can be used over either
asynchronous serial (dial-up) or synchronous serial (ISDN) media. It relies on Link Control
Protocol (LCP) to build and maintain data-link connections. Network Control Protocol
(NCP) enables multiple Network layer protocols (routed protocols) to be used on a point-
to-point connection.
Because HDLC is the default serial encapsulation on Cisco serial links and it works
great, why in the world would you choose to use PPP? Well, the basic purpose of PPP is to
transport layer 3 packets across a Data Link layer point-to-point link, and it’s nonpropri-
etary. So unless you have all Cisco routers, you need PPP on your serial interfaces because
the HDLC encapsulation is Cisco proprietary, remember? Plus, since PPP can encapsulate
several layer 3 routed protocols and provide authentication, dynamic addressing, and call-
back, PPP could actually be the best encapsulation solution for you over HDLC anyway.
Figure21.10 shows the PPP protocol stack compared to the OSI reference model.
PPP contains four main components:
EIA/TIA-232-C, V.24, V.35, and ISDN A Physical layer international standard for serial
communication.
HDLC A method for encapsulating datagrams over serial links.
LCP A method of establishing, conguring, maintaining, and terminating the point-to-
point connection. It also provides features such as authentication. I’ll give you a complete
list of these features in the next section.
Point-to-Point Protocol (PPP) 915
FIGURE21.10 Point-to-Point Protocol stack
OSI layer
3
2
1
Upper-layer protocols
(such as IP and IPv6)
Network Control Protocol (NCP)
(specific to each Network layer protocol)
Link Control Protocol (LCP)
High-Level Data Link Control (HDLC)
Physical layer
(such as EIA/TIA-232, V.24, V.35, ISDN)
NCP A method of establishing and conguring different Network layer protocols for
transport across the PPP link. NCP is designed to allow the simultaneous use of multiple
Network layer protocols. Two examples of protocols here are Internet Protocol Control
Protocol (IPCP) and Cisco Discovery Protocol Control Protocol (CDPCP).
Burn it into your mind that the PPP protocol stack is specied at the Physical and Data
Link layers only. NCP is used to allow communication of multiple Network layer protocols
by identifying and encapsulating the protocols across a PPP data link.
Remember that if you have a Cisco router and a non-Cisco router connected
with a serial connection, you must configure PPP or another encapsulation
method like Frame Relay because the HDLC default just won’t work!
Next, we’ll cover the options for LCP and PPP session establishment.
Link Control Protocol (LCP) Configuration Options
Link Control Protocol (LCP) offers different PPP encapsulation options, including the
following:
Authentication This option tells the calling side of the link to send information that can
identify the user. The two methods for this task are PAP and CHAP.
Compression This is used to increase the throughput of PPP connections by compressing the
data or payload prior to transmission. PPP decompresses the data frame on the receiving end.
Error detection PPP uses Quality and Magic Number options to ensure a reliable, loop-
free data link.
916 Chapter 21 u Wide Area Networks
Multilink Starting with IOS version 11.1, multilink is supported on PPP links with Cisco
routers. This option makes several separate physical paths appear to be one logical path at
layer 3. For example, two T1s running multilink PPP would show up as a single 3 Mbps
path to a layer 3 routing protocol.
PPP callback On a dial-up connection, PPP can be congured to call back after successful
authentication. PPP callback can be a very good thing because it allows us to keep track of
usage based upon access charges for accounting records and a bunch of other reasons. With
callback enabled, a calling router (client) will contact a remote router (server) and authenticate.
Predictably, both routers have to be congured for the callback feature for this to work. Once
authentication is completed, the remote router will terminate the connection and then reiniti-
ate a connection to the calling router from the remote router.
PPP Session Establishment
When PPP connections are started, the links go through three phases of session establish-
ment, as shown in Figure21.11:
FIGURE21.11 PPP session establishment
PPP Session Establishment
1. Link establishment phase
2. Authentication phase (optional)
3. Network layer protocol phase
PPP
Link-establishment phase LCP packets are sent by each PPP device to congure and test
the link. These packets contain a eld called Conguration Option that allows each device
to see the size of the data, the compression, and authentication. If no Conguration Option
eld is present, then the default congurations will be used.
Authentication phase If required, either CHAP or PAP can be used to authenticate a link.
Authentication takes place before Network layer protocol information is read, and it’s also
possible that link-quality determination will occur simultaneously.
Network layer protocol phase PPP uses the Network Control Protocol (NCP) to allow
multiple Network layer protocols to be encapsulated and sent over a PPP data link. Each
Network layer protocol (e.g., IP, IPv6, which are routed protocols) establishes a service
with NCP.
Point-to-Point Protocol (PPP) 917
PPP Authentication Methods
There are two methods of authentication that can be used with PPP links:
Password Authentication Protocol (PAP) The Password Authentication Protocol (PAP)
is the less secure of the two methods. Passwords are sent in clear text and PAP is performed
only upon the initial link establishment. When the PPP link is rst established, the remote
node sends the username and password back to the originating target router until authenti-
cation is acknowledged. Not exactly Fort Knox!
Challenge Handshake Authentication Protocol (CHAP) The Challenge Handshake
Authentication Protocol (CHAP) is used at the initial startup of a link and at periodic
checkups on the link to ensure that the router is still communicating with the same host.
After PPP nishes its initial link-establishment phase, the local router sends a challenge
request to the remote device. The remote device sends a value calculated using a one-way
hash function called MD5. The local router checks this hash value to make sure it matches.
If the values don’t match, the link is immediately terminated.
CHAP authenticates at the beginning of the session and periodically
throughout the session.
Configuring PPP on Cisco Routers
Conguring PPP encapsulation on an interface is really pretty straightforward. To congure
it from the CLI, use these simple router commands:
Router#config t
Router(config)#int s0
Router(config-if)#encapsulation ppp
Router(config-if)#^Z
Router#
Of course, PPP encapsulation has to be enabled on both interfaces connected to a serial
line in order to work and there are several additional conguration options available to you
via the ppp ? command.
Configuring PPP Authentication
After you congure your serial interface to support PPP encapsulation, you can then congure
authentication using PPP between routers. But rst, you must set the hostname of the router if
918 Chapter 21 u Wide Area Networks
it hasn’t been set already. After that, you set the username and password for the remote router
that will be connecting to your router, like this:
Router#config t
Router(config)#hostname RouterA
RouterA(config)#username RouterB password cisco
When using the username command, remember that the username is the hostname of
the remote router that’s connecting to your router. And it’s case sensitive too. Also, the
password on both routers must be the same. It’s a plain-text password that you can see
with a show run command, and you can encrypt the password by using the command
service password-encryption. You must have a username and password congured for
each remote system you plan to connect to. The remote routers must also be similarly
congured with usernames and passwords.
Now, after you’ve set the hostname, usernames, and passwords, choose either CHAP or
PAP as the authentication method:
RouterA#config t
RouterA(config)#int s0
RouterA(config-if)#ppp authentication chap pap
RouterA(config-if)#^Z
RouterA#
If both methods are congured on the same line as I’ve demonstrated here, then only the
rst method will be used during link negotiation. The second acts as a backup just in case
the rst method fails.
Verifying and Troubleshooting Serial Links
Okay—now that PPP encapsulation is enabled, you need to verify that it’s up and running.
First, let’s take a look at a gure of a sample nonproduction network serial link. Figure21.12
shows two routers connected with a point-to-point serial connection, with the DCE side on
the Pod1R1 router.
FIGURE21.12 PPP authentication example
Pod1R1 Pod1R2
hostname Pod1R1
username Pod1R2 password cisco
interface serial 0
ip address 10.0.1.1 255.255.255.0
encapsulation ppp
clock rate 64000
bandwidth 512
ppp authentication chap
hostname Pod1R2
username Pod1R1 password cisco
interface serial 0
ip address 10.0.1.2 255.255.255.0
encapsulation ppp
bandwidth 512
ppp authentication chap
Point-to-Point Protocol (PPP) 919
You can start verifying the conguration with the show interface command like this:
Pod1R1#sh int s0/0
Serial0/0 is up, line protocol is up
Hardware is PowerQUICC Serial
Internet address is 10.0.1.1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 239/255, txload 1/255, rxload 1/255
Encapsulation PPP
loopback not set
Keepalive set (10 sec)
LCP Open
Open: IPCP, CDPCP
[output cut]
The rst line of output is important because it tells us that serial 0/0 is up/up. Notice
that the interface encapsulation is PPP and that LCP is open. This means that it has negoti-
ated the session establishment and all is well. The last line tells us that NCP is listening for
the protocols IP and CDP, shown with the NCP headers IPCP and CDPCP.
But what would you see if everything isn’t so perfect? I’m going to type in the congura-
tion shown in Figure21.13 to nd out.
FIGURE21.13 Failed PPP authentication
Pod1R1 Pod1R2
hostname Pod1R1
username Pod1R2 password Cisco
interface serial 0
ip address 10.0.1.1 255.255.255.0
clock rate 64000
bandwidth 512
encapsulation ppp
ppp authentication chap
hostname Pod1R2
username Pod1R1 password cisco
interface serial 0
ip address 10.0.1.2 255.255.255.0
bandwidth 512
encapsulation ppp
ppp authentication chap
Okay—what’s wrong here? Take a look at the usernames and passwords. Do you see the
problem now? That’s right, the C is capitalized on the Pod1R2 username command found in
the conguration of router Pod1R1. This is wrong because the usernames and passwords are
case sensitive. Now let’s take a look at the show interface command and see what happens:
Pod1R1#sh int s0/0
Serial0/0 is up, line protocol is down
Hardware is PowerQUICC Serial
Internet address is 10.0.1.1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
920 Chapter 21 u Wide Area Networks
reliability 243/255, txload 1/255, rxload 1/255
Encapsulation PPP, loopback not set
Keepalive set (10 sec)
LCP Closed
Closed: IPCP, CDPCP
First, notice that the rst line of output shows us that Serial0/0 is up and line
protocol is down. This is because there are no keepalives coming from the remote
router. The next thing I want you to notice is that the LCP and NCP are closed because
the authentication failed.
Debugging PPP Authentication
To display the CHAP authentication process as it occurs between two routers in the network,
just use the command debug ppp authentication.
If your PPP encapsulation and authentication are set up correctly on both routers and
your usernames and passwords are all good, then the debug ppp authentication command
will display an output that looks like the following output, which is called the three-way
handshake:
d16h: Se0/0 PPP: Using default call direction
1d16h: Se0/0 PPP: Treating connection as a dedicated line
1d16h: Se0/0 CHAP: O CHALLENGE id 219 len 27 from "Pod1R1"
1d16h: Se0/0 CHAP: I CHALLENGE id 208 len 27 from "Pod1R2"
1d16h: Se0/0 CHAP: O RESPONSE id 208 len 27 from "Pod1R1"
1d16h: Se0/0 CHAP: I RESPONSE id 219 len 27 from "Pod1R2"
1d16h: Se0/0 CHAP: O SUCCESS id 219 len 4
1d16h: Se0/0 CHAP: I SUCCESS id 208 len 4
But if you have the password wrong as they were previously in the PPP authentication
failure example back in Figure21.13, the output would look something like this:
1d16h: Se0/0 PPP: Using default call direction
1d16h: Se0/0 PPP: Treating connection as a dedicated line
1d16h: %SYS-5-CONFIG_I: Configured from console by console
1d16h: Se0/0 CHAP: O CHALLENGE id 220 len 27 from "Pod1R1"
1d16h: Se0/0 CHAP: I CHALLENGE id 209 len 27 from "Pod1R2"
1d16h: Se0/0 CHAP: O RESPONSE id 209 len 27 from "Pod1R1"
1d16h: Se0/0 CHAP: I RESPONSE id 220 len 27 from "Pod1R2"
1d16h: Se0/0 CHAP: O FAILURE id 220 len 25 msg is "MD/DES compare failed"
PPP with CHAP authentication is a three-way authentication, and if the username and
passwords aren’t congured exactly the way they should be, then the authentication will
fail and the link will go down.
Point-to-Point Protocol (PPP) 921
Mismatched WAN Encapsulations
If you have a point-to-point link but the encapsulations aren’t the same, the link will never
come up. Figure21.14 shows one link with PPP and one with HDLC.
FIGURE21.14 Mismatched WAN encapsulations
Pod1R1 Pod1R2
hostname Pod1R1
username Pod1R2 password cisco
interface serial 0
ip address 10.0.1.1 255.255.255.0
clock rate 64000
bandwidth 512
encapsulation ppp
hostname Pod1R2
username Pod1R1 password cisco
interface serial 0
ip address 10.0.1.2 255.255.255.0
bandwidth 512
encapsulation hdlc
Look at router Pod1R1 in this output:
Pod1R1#sh int s0/0
Serial0/0 is up, line protocol is down
Hardware is PowerQUICC Serial
Internet address is 10.0.1.1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 254/255, txload 1/255, rxload 1/255
Encapsulation PPP, loopback not set
Keepalive set (10 sec)
LCP REQsent
Closed: IPCP, CDPCP
The serial interface is up/down and LCP is sending requests but will never receive any
responses because router Pod1R2 is using the HDLC encapsulation. To x this problem,
you would have to go to router Pod1R2 and congure the PPP encapsulation on the serial
interface. One more thing: Even though the usernames are congured incorrectly, it doesn’t
matter because the command ppp authentication chap isn’t used under the serial inter-
face conguration. This means that the username command isn’t relevant in this example.
You can set a Cisco serial interface back to the default of HDLC with the no
encapsulation command like this:
Router(config)#int s0/0
Router(config-if)#no encapsulation
*Feb 7 16:00:18.678:%LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0,
changed state to up
922 Chapter 21 u Wide Area Networks
Notice the link came up because it now matches the encapsulation on the other end of
the link!
Always remember that you just can’t have PPP on one side and HDLC on
the other—they don’t get along!
Mismatched IP Addresses
A tricky problem to spot is if you have HDLC or PPP congured on your serial interface
but your IP addresses are wrong. Things seem to be just ne because the interfaces will
show that they are up. Take a look at Figure21.15 and see if you can see what I mean—the
two routers are connected with different subnets—router Pod1R1 with 10.0.1.1/24 and
router Pod1R2 with 10.2.1.2/24.
FIGURE21.15 Mismatched IP addresses
Pod1R1 Pod1R2
hostname Pod1R1
username Pod1R2 password cisco
interface serial 0
ip address 10.0.1.1 255.255.255.0
clock rate 64000
bandwidth 512
encapsulation ppp
ppp authentication chap
hostname Pod1R2
username Pod1R1 password cisco
interface serial 0
ip address 10.2.1.2 255.255.255.0
bandwidth 512
encapsulation ppp
ppp authentication chap
This will never work. Let’s take a look at the output:
Pod1R1#sh int s0/0
Serial0/0 is up, line protocol is up
Hardware is PowerQUICC Serial
Internet address is 10.0.1.1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation PPP, loopback not set
Keepalive set (10 sec)
LCP Open
Open: IPCP, CDPCP
See that? The IP addresses between the routers are wrong but the link appears to be work-
ing just ne. This is because PPP, like HDLC and Frame Relay, is a layer 2 WAN encapsula-
tion, so it doesn’t care about IP addresses at all. So yes, the link is up, but you can’t use IP
Frame Relay 923
across this link since it’s miscongured, or can you? Well, yes and no. If you try to ping you’ll
see that this actually works! This is a feature of PPP, but not HDLC or Frame Relay. But just
because you can ping to an IP address that’s not in the same subnet doesn’t mean your net-
work trafc and routing protocols will work. So be careful with this issue, especially when
troubleshooting PPP links!
Take a look at the routing table of Pod1R1 and see if you can nd the mismatched
IP address problem:
[output cut]
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.2.1.2/32 is directly connected, Serial0/0
C 10.0.1.0/24 is directly connected, Serial0/0
Interesting! We can see our serial interface S0/0 address of 10.0.1.0/24, but what is
that other address on interface S0/0— 10.2.1.2/32? That’s our remote router’s interface IP
address! PPP determines and places the neighbor’s IP address in the routing table as a con-
nected interface, which then allows you to ping it even though it’s actually congured on a
separate IP subnet.
For the Cisco objectives, you need to be able to troubleshoot PPP from the
routing table as I just described.
To nd and x this problem, you can also use the show running-config, show
interfaces, or show ip interfaces brief commands on each router, or you can use
the show cdp neighbors detail command:
Pod1R1#sh cdp neighbors detail
-------------------------
Device ID: Pod1R2
Entry address(es):
IP address: 10.2.1.2
Since the layer 1 Physical and layer 2 Data Link is up/up, you can view and verify the
directly connected neighbor’s IP address and then solve your problem.
Frame Relay
Frame Relay was one of the most popular WAN services deployed over the past two decades,
and there’s a good reason for this—cost. And it’s a rare network design or designer that has
the privilege to ignore that all-important cost factor!
By default, Frame Relay is classied as a nonbroadcast multi-access (NBMA) network,
meaning it doesn’t send any broadcasts such as RIP updates across the network.
924 Chapter 21 u Wide Area Networks
In addition, Frame Relay is considerably more complex than the simple leased-line networks
you learned about when we covered the HDLC and PPP protocols. The leased-line networks
are easy to conceptualize, but not so much when it comes to Frame Relay. But with complexity
comes versatility, which is why it’s often represented as a “cloud” in networking graphics. I’ll
get to that more in a minute—for right now, I’m going to introduce Frame Relay in concept
and explain how it differs from simpler leased-line technologies.
I’m going to begin your introduction to this technology by giving you a solid reference of
all the new terms you’ll need to really grasp the basics of Frame Relay. After that, I’ll guide
you through some simple Frame Relay implementations.
Introduction to Frame Relay Technology
As a Cisco exam, you’ll be expected to understand the basics of the Frame Relay technology
and also to be able to congure it in simple scenarios. First, understand that Frame Relay
is a packet-switched technology. From everything you’ve learned so far, just telling you this
should make you immediately realize several things about it:
u You won’t be using the encapsulation hdlc or encapsulation ppp command to
configure it.
u Frame Relay doesn’t work like a point-to-point leased line even though it can be made
to look and act like one.
u Frame Relay is usually less expensive than leased lines are, but there are some sacrifices
required to net that savings.
So why do we use Frame Relay? Figure21.16 will help you form a snapshot of what a
network looked like before Frame Relay as a rst step to answering this question.
FIGURE21.16 Before Frame Relay
Dedicated/leased lines
to each location
$$$
Now check out Figure21.17. You can see that there’s now only one connection between
the Corporate router and the Frame Relay switch. That right there saves some major cash!
Let’s say you needed to add seven remote sites to be accessed from the corporate ofce
but you only have one free serial port on your router—it’s Frame Relay to the rescue! Of
Frame Relay 925
course, this also means that you now have a single point of failure, which is not so good.
But Frame Relay was typically used to save money, not to make a network more resilient.
Coming up, I’m going to cover the Frame Relay technology information you need to
effectively meet the Cisco objectives.
FIGURE21.17 After Frame Relay
Statistically multiplexing
multiple logical circuits over a
single physical connection
Frame Relay
Committed Information Rate (CIR)
Frame Relay provides a packet-switched network to lots of different customers at the same
time, which is good because it spreads out the cost of the switches among many customers.
But remember, Frame Relay is based on the assumption that those dependent on it will never
need to transmit data constantly or simultaneously.
Frame Relay works by providing a portion of dedicated bandwidth to each user, and it
also allows the user to exceed their guaranteed bandwidth if resources on the telco network
happen to be available. So basically, Frame Relay providers allow customers to buy a lower
amount of bandwidth than what they really use. There are two separate bandwidth speci-
cations with Frame Relay:
Access rate The maximum speed at which the Frame Relay interface can transmit.
CIR The maximum bandwidth of data guaranteed to be delivered. It’s the average amount
that the service provider will allow you to transmit.
If these two values are equal, the Frame Relay connection will operate pretty much as a
leased line would. But these values can also be set differently. To clarify this, suppose I’ve
bought a T1 access rate, giving me 1.544 Mbps and a CIR of 256 Kbps. With this option,
the rst 256 Kbps of trafc I send is guaranteed to be delivered. Anything beyond that is
called a “burst”—a transmission that exceeds my guaranteed 256 Kbps rate, which can be
any amount up to my contracted T1 access rate. But what would happen if I were to send
a combined committed burst that exceeds the MBR, or maximum burst rate? Most likely,
my excess trafc will be dropped, but whether or not it’s to be toast really depends on the
subscription level of my particular service provider.
926 Chapter 21 u Wide Area Networks
Of course, in a perfect world, this always works beautifully. But remember that little word
guaranteed—as in guaranteed rate of 256 Kbps, to be exact? This means that any burst of
data you send that exceeds your guaranteed 256 Kbps rate will be delivered on something
called a “best effort” basis of delivery. Or maybe it won’t—if your telco’s equipment doesn’t
have the capacity to deliver it at the time you transmitted, then your frames will be discarded
and the DTE will be notied. Timing is everything—you can scream data out at six times
your guaranteed rate of 256 Kbps (T1) only if your telco has the capacity available on its
equipment at the exact time you try to send. This is “oversubscription” in action!
The CIR is the rate, in bits per second, at which the Frame Relay switch
agrees to transfer data.
Frame Relay Encapsulation Types
When conguring Frame Relay on Cisco routers, you need to specify it as an encapsulation
on serial interfaces. And as I said earlier, you can’t use HDLC or PPP with Frame Relay.
When you congure Frame Relay, you must choose one of two encapsulation types—Cisco
and IETF (Internet Engineering Task Force), as shown in the following output:
RouterA(config)#int s0
RouterA(config-if)#encapsulation frame-relay ?
ietf Use RFC1490 encapsulation
<cr>
Unless you manually type in ietf, the default encapsulation option is Cisco, and predict-
ably, it’s what you want to go with when connecting two Cisco devices. You’d opt for the
IETF-type encapsulation if you needed to connect a Cisco device to a non-Cisco device with
Frame Relay. Whichever you choose, make sure the Frame Relay encapsulation is the same
on both ends.
Virtual Circuits
Frame Relay operates using virtual circuits as opposed to the physical circuits that leased
lines use. These virtual circuits are what link together the thousands of devices connected
to the provider’s “cloud.” Frame Relay provides a virtual circuit between your two DTE
devices, which makes them appear to be connected via an actual circuit. In reality, they’re
dumping their frames into a large, shared infrastructure. You never see the complexity of
what’s actually happening inside the cloud because you only have a virtual circuit.
And on top of all that, there are two types of virtual circuits—permanent and switched.
Permanent virtual circuits (PVCs) are by far the most common type in use today. What
“permanent” means here is that the telco creates the mappings inside its gear, and as long
as you pay the bill, they’ll remain in place.
Switched virtual circuits (SVCs) are more like a phone call. The virtual circuit is estab-
lished when data needs to be transmitted. The virtual circuit is dismantled when the data
transfer is complete.
Frame Relay 927
Data Link Connection Identifiers (DLCIs)
Frame Relay PVCs are identied to DTE end devices by Data Link Connection Identiers
(DLCIs). A Frame Relay service provider typically assigns DLCI values, which are used on
Frame Relay interfaces to distinguish between different virtual circuits. Because many virtual
circuits can be terminated on one multipoint Frame Relay interface, many DLCIs are often
afliated with it.
To clarify this, let’s say you have a central HQ with three branch ofces. If you were to
connect each branch ofce to HQ using a T1, you would need three serial interfaces on your
router at HQ, one for each T1. Simple, right? But suppose you use Frame Relay PVCs instead.
You could have a T1 at each branch connected to a service provider and only a single T1 at
HQ. There would be three PVCs on the single T1 at HQ, one going to each branch. And even
though there’s only a single interface and a single CSU/DSU, the three PVCs function as three
separate circuits. Remember what I said about saving money? How much for two additional
T1 interfaces and a pair of CSU/DSUs? Answer: A lot! So, why not just go ahead and ask for
a percentage of the savings in your bonus?
Okay, before we go on, I want to dene Inverse ARP (IARP) and discuss how it’s used
with DLCIs in a Frame Relay network. Yes, it is somewhat similar to ARP in the fact that it
maps a DLCI to an IP address—kind of like ARP does with MAC addresses to IP addresses.
And even though you can’t congure IARP, you can disable it. It runs on a Frame Relay
router and maps the DLCI to an IP address for Frame Relay so it knows how to get to the
IP address at the other end of the PVC. You can see IP-to-DLCI mappings with the show
frame-relay map command.
But if you have a non-Cisco router living in your network and it doesn’t support IARP, then
you’re stuck with having to statically provide IP-to-DLCI mappings with the frame-relay map
command—something I’ll demonstrate in a bit.
Inverse ARP (IARP) is used to map a known DLCI to an IP address.
Let’s talk about DLCIs a bit more. They’re locally signicant—global signicance requires
the entire network to use the Local Management Interface (LMI) extensions that offer global
signicance. This is why you’ll mostly nd global DLCIs only in private networks.
But the DLCI doesn’t have to be globally signicant for it to be functional in getting a
frame across the network. Let me explain: when RouterA wants to send a frame to RouterB,
it looks up the IARP or manual mapping of the DLCI to the IP address it’s trying to get to.
Equipped with the DLCI, it then sends the frame out with the DLCI value it found in the
DLCI eld of the FR header. The provider’s ingress switch gets this frame and does a lookup
on the DLCI/physical-port combination it observes. Associated with that combination, it
nds a new “locally signicant” (meaning, between itself and the next-hop switch) DLCI to
use in the header, and in the same entry in its table, it nds an outgoing physical port. This
happens repeatedly all the way to RouterB. So basically, you could actually say that the DLCI
for RouterA uses the entire virtual circuit to RouterB, even though each DLCI between every
pair of devices could be completely different. The big point here is that RouterA is unaware of
these differences. That’s what makes the DLCI locally signicant. So make a mental note that
DLCIs really are used by the telco to “nd” the other end of your PVC.
928 Chapter 21 u Wide Area Networks
For a picture of why DLCIs are considered locally signicant, take a look at Figure21.18.
In the gure, DLCI 100 is considered locally signicant to RouterA and identies the circuit
to RouterB between RouterA and its ingress Frame Relay switch. DLCI 200 would identify
this same circuit to RouterA between RouterB and its ingress Frame Relay switch.
FIGURE21.18 DLCIs are local to your router. RouterA uses DLCI 100 to send data
to RouterB.
RouterA DLCI 100 DLCI 200 RouterB
To get to RouterB use DLCI 100. To get to RouterA use DLCI 200.
DLCI numbers that are used to identify a PVC are typically assigned by the provider and
start at 16.
You congure a DLCI number to be applied to a subinterface like this:
RouterA(config-if)#frame-relay interface-dlci ?
<16-1007> Define a DLCI as part of the current subinterface
RouterA(config-if)#frame-relay interface-dlci 16
DLCIs identify the logical circuit between the local router and a Frame
Relay switch.
Local Management Interface (LMI)
Local Management Interface (LMI) is a signaling standard used between your router and
the rst Frame Relay switch it’s connected to. It allows for passing information about the
operation and status of the virtual circuit between the provider’s network and the DTE
(your router). It communicates information about the following:
Keepalives These verify that data is owing.
Multicasting This is an optional extension of the LMI specication that permits the ef-
cient distribution of routing information and ARP requests over a Frame Relay network.
Multicasting uses the reserved DLCIs from 1019 through 1022.
Global addressing This provides global signicance to DLCIs, allowing the Frame
Relay cloud to work exactly like a LAN. This has never been run in a production net-
work to this day.
Status of virtual circuits This provides DLCI status. The status inquiries and messages are
used as keepalives when there is no regular LMI trafc to send.
Frame Relay 929
But remember, LMI is not communication between your routers; it’s communication
between your router and the nearest Frame Relay switch. So it’s entirely possible that the
router on one end of a PVC is actively receiving LMI while the router on the other end of
the PVC is not. And of course, PVCs won’t work with one end down, which claries the
local nature of LMI communications.
There are three different types of LMI message formats: Cisco, ANSI, and Q.933A. The
different kinds in use depend on both the type and conguration of the telco’s switching
gear, so it’s imperative that you congure your router for the correct format, which should
be provided by the telco.
Beginning with IOS version 11.2, the LMI type is autosensed. This enables
the interface to determine the LMI type supported by the switch. If you’re
not going to use the autosense feature, you’ll need to check with your
Frame Relay provider to find out which type to use instead.
On Cisco equipment, the default type is, surprise, Cisco, but you still might have to change
to ANSI or Q.933A, depending on what your service provider tells you. The three different
LMI types are shown in the following router output:
RouterA(config-if)#frame-relay lmi-type ?
cisco
ansi
q933a
As seen in the output, all three standard LMI signaling formats are supported. Here’s a
description of each one:
Cisco LMI dened by the Gang of Four (default). The Local Management Interface (LMI)
was developed in 1990 by Cisco Systems, StrataCom, Northern Telecom, and Digital
Equipment Corporation and became known as the Gang-of-Four LMI, or Cisco LMI.
ANSI Annex D included with ANSI standard T1.617.
ITU-T (Q.933A) Annex A included in the ITU-T standard, dened by using the q933a
command keyword.
Routers receive LMI information from the service provider’s Frame Relay switch on a
frame-relay encapsulated interface and update the virtual circuit status to one of three dif-
ferent states:
Active state Everything is up, and routers can exchange information.
Inactive state The router’s interface is up and working with a connection to the switching
ofce, but the remote router isn’t up.
Deleted state No LMI information is being received on the interface from the switch, which
could be due to a mapping problem or a line failure.
930 Chapter 21 u Wide Area Networks
Frame Relay Congestion Control
Remember back to our talk about CIR? From that, it should be obvious that the lower your
CIR is set, the greater the risk that your data will be lost. This can be easily avoided if you
have just one key piece of information—the most optimal time to transmit that huge burst!
This begs the question: Is there any way for us to nd out when our telco’s shared infrastruc-
ture is free and clear and when it’s crammed and jammed, just as we check the freeway before
we leave work? And if so, how do we nd out? Well, that’s exactly what I’m going to talk
about next—how the Frame Relay switch noties the DTE of congestion problems!
Here are the three congestion bits and their meanings, as shown in the show frame-
relay map output:
R1#sh frame map
[output cut]
input pkts 14055 output pkts 32795 in bytes 1096228
out bytes 6216155 dropped pkts 0 in FECN pkts 0
in BECN pkts 0 out FECN pkts 0 out BECN pkts 0
in DE pkts 0 out DE pkts 0
out bcast pkts 32795 out bcast bytes 6216155
Discard Eligibility (DE) As you know, when you transmit a burst of packets beyond the
CIR of a given PVC, any packets exceeding the CIR could very well be discarded if the pro-
vider’s network is congested at the time. Because of this, the excessive bits are marked with
a Discard Eligibility (DE) bit in the Frame Relay header. And if the provider’s network
happens to be congested, the Frame Relay switch will discard the packets with the rst DE
bit set. So if your bandwidth is congured with a CIR of zero, the DE will always be on.
Forward Explicit Congestion Notification (FECN) When the Frame Relay network
recognizes congestion in the cloud, the switch will set the Forward Explicit Congestion
Notication (FECN) bit to 1 in a Frame Relay packet header. This will indicate to the
destination DTE that the path the frame just traversed is congested.
Backward Explicit Congestion Notification (BECN) When the switch detects congestion in
the Frame Relay network, it’ll set the Backward Explicit Congestion Notication (BECN)
bit in a Frame Relay frame that’s destined for the source router. This noties the originating
router that congestion is ahead. But Cisco routers won’t take action on this congestion infor-
mation unless you tell them to!
If your in FECN count is incrementing, the local PVC is congested. If your
in BECN count is increasing, then the remote PVC is congested.
Troubleshooting Using Frame Relay Congestion Control
Now let’s say all your users are whining about the fact that their Frame Relay connection
to the corporate site is super slow. Because you strongly suspect that the link is overloaded,
Frame Relay 931
you verify the Frame Relay congestion control information with the show frame-relay pvc
command and get this output:
RouterA#sh frame-relay pvc
PVC Statistics for interface Serial0/0 (Frame Relay DTE)
Active Inactive Deleted Static
Local 1 0 0 0
Switched 0 0 0 0
Unused 0 0 0 0
DLCI = 100, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0
input pkts 1300 output pkts 1270 in bytes 21212000
out bytes 21802000 dropped pkts 4 in pkts dropped 147
out pkts dropped 0 out bytes dropped 0 in FECN pkts 147
in BECN pkts 192 out FECN pkts 147
out BECN pkts 259 in DE pkts 0 out DE pkts 214
out bcast pkts 0 out bcast bytes 0
pvc create time 00:00:06, last time pvc status changed 00:00:06
Pod1R1#
What you want to look for in this output is the in BECN pkts 192 output because this is
what’s telling the local router that trafc sent to the corporate site is experiencing congestion.
BECN means that the path that a frame took to get back to you is congested.
Frame Relay Implementation and Monitoring
As I’ve said, there are a ton of Frame Relay commands and conguration options, but I’m
going to zero in on the ones you really need to know when studying for the Cisco exam
exam objectives. I’m going to start with one of the simplest conguration options—two
routers with a single PVC between them. Next, I’ll show you a more complex congura-
tion using subinterfaces and demonstrate some of the monitoring commands available to
verify the conguration.
Single Interface
Let’s get started by looking at a simple example where we just want to connect two routers
with a single PVC. Here’s how that conguration would look:
RouterA#config t
RouterA(config)#int s0/0
RouterA(config-if)#encapsulation frame-relay
RouterA(config-if)#ip address 172.16.20.1 255.255.255.0
RouterA(config-if)#frame-relay lmi-type ansi
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RouterA(config-if)#frame-relay interface-dlci 101
RouterA(config-if)#^Z
RouterA#
The rst step is to specify the encapsulation as Frame Relay. Notice that since I didn’t
specify a particular encapsulation type—either Cisco or IETF—the Cisco default type was
used. If the other router were non-Cisco, I would’ve specied IETF. Next, I assigned an IP
address to the interface and then specied the LMI type of ANSI based on information pro-
vided by the telecommunications provider. Again, the default is Cisco. Finally, I added the
DLCI of 101, which indicates the PVC we want to use and has been given to me by my ISP,
assuming there’s only one PVC on this physical interface.
That’s all there is to it—if both sides are congured correctly, and the switch is working,
the circuit will come up.
Check out Hands-on Lab 7.3 for a complete example of this type of con-
figuration, including instructions on creating your own Frame Relay
switch from a router.
Subinterfaces
As I mentioned earlier, you can have multiple virtual circuits on a single serial interface and
yet treat each as a separate interface. We can make this happen by creating subinterfaces.
Think of a subinterface as a logical interface that’s dened by the IOS software. Several sub-
interfaces will share a single hardware interface, yet for conguration purposes, they operate
as if they were separate physical interfaces. This is known as multiplexing.
To congure a router in a Frame Relay network so it will avoid split horizon issues that
will not permit certain routing updates, just congure a separate subinterface for each PVC
with a unique DLCI and subnet assigned to the subinterface.
You dene subinterfaces using a command like int s0/0.subinterface number. First,
you have to set the encapsulation on the physical serial interface, and then you can dene the
subinterfaces—generally one subinterface per PVC. Here’s an example, using Figure21.19:
FIGURE21.19 Configuring subinterfaces
To NY use DLCI 103
To SF use DLCI 102
Frame Relay
DLCI 200
DLCI 400
Corp
SF
NY
Frame Relay 933
Corp(config)#int s0/0
Corp(config-if)#no shut
Corp(config-if)#encapsulation frame-relay
Corp(config-if)#int s0/0.?
<0-4294967295> Serial interface number
Corp(config-if)#int s0/0.102 ?
multipoint Treat as a multipoint link
point-to-point Treat as a point-to-point link
Corp(config-if)#int s0/0.102 point-to-point
Corp(config-subif)#ip address 10.1.12.1 255.255.255.0
Corp(config-subif)#frame-relay interface-dlci 102
Corp(config-subif)#int s0/0.103
Corp(config-subif)#ip address 10.1.13.1 255.255.255.0
Corp(config-subif)#frame-relay interface-dlci 103
Make sure you don’t have an IP address under the physical interface if you
have configured subinterfaces!
You can dene a legion of subinterfaces on any given physical interface, but keep in mind
that there are only about a thousand available DLCIs. In the preceding example, I chose to
use subinterface 102 and 103 because they represent the DLCI number assigned to that PVC
by the carrier. There are two types of subinterfaces:
Point-to-point Used when a single virtual circuit connects one router to another. Each
point-to-point subinterface requires its own subnet.
A point-to-point subinterface maps a single IP subnet per DLCI and addresses
and resolves NBMA split horizon issues.
Multipoint This is when the router is the center of a star of virtual circuits that are using
a single subnet for all routers’ serial interfaces connected to the Frame Relay cloud. You’ll
usually nd this implemented with the hub router in this mode with the spoke routers in
physical interface (always point-to-point) or point-to-point subinterface mode.
In the following output, notice that the subinterface number matches the DLCI number—
not a requirement, but it seriously helps you administer the interfaces:
interface Serial0/0
no ip address (notice there is no IP address on the physical interface!)
no ip directed-broadcast
encapsulation frame-relay
!
interface Serial0/0.102 point-to-point
934 Chapter 21 u Wide Area Networks
ip address 10.1.12.1 255.255.255.0
no ip directed-broadcast
frame-relay interface-dlci 102
!
interface Serial0/0.103 point-to-point
ip address 10.1.13.1 255.255.255.0
no ip directed-broadcast
frame-relay interface-dlci 103
!
Notice that there’s no LMI type dened. This means that the routers are either running
the Cisco default or using autodetect if you’re running Cisco IOS version 11.2 or newer. I
also want to point out that each interface maps to a single DLCI and is dened as a separate
subnet. And remember—point-to-point subinterfaces solve split horizon issues as well!
Monitoring Frame Relay
Several commands are used frequently to check the status of your interfaces and PVCs once
you have Frame Relay encapsulation set up and running. To list them, use the show frame
? command, as seen here:
Corp>sho frame ?
end-to-end Frame-relay end-to-end VC information
fragment show frame relay fragmentation information
ip show frame relay IP statistics
lapf show frame relay lapf status/statistics
lmi show frame relay lmi statistics
map Frame-Relay map table
pvc show frame relay pvc statistics
qos-autosense show frame relay qos-autosense information
route show frame relay route
svc show frame relay SVC stuff
traffic Frame-Relay protocol statistics
vofr Show frame-relay VoFR statistics
The most common parameters that you view with the show frame-relay command are
lmi, pvc, and map.
Now, let’s take a look at the most frequently used commands and the information they
provide.
The show frame-relay lmi Command
The show frame-relay lmi command will give you the LMI trafc statistics exchanged
between the local router and the Frame Relay switch. Here’s an example:
Corp#sh frame lmi
Frame Relay 935
LMI Statistics for interface Serial0/0 (Frame Relay DTE)LMI TYPE = CISCO
Invalid Unnumbered info 0 Invalid Prot Disc 0
Invalid dummy Call Ref 0 Invalid Msg Type 0
Invalid Status Message 0 Invalid Lock Shift 0
Invalid Information ID 0 Invalid Report IE Len 0
Invalid Report Request 0 Invalid Keep IE Len 0
Num Status Enq. Sent 61 Num Status msgs Rcvd 0
Num Update Status Rcvd 0 Num Status Timeouts 60
The router output from the show frame-relay lmi command shows you any LMI errors,
plus the LMI type. So, I have a question based on the output of the command. Is this Frame-
Relay network working? The answer is no because the router has sent 60 inquiries and has
not received even one reply from the Frame Relay switch. If you see this, you need to call the
provider because this is a Frame Relay switch conguration issue.
The show frame pvc Command
The show frame pvc command will present you with a list of all congured PVCs and
DLCI numbers. It provides the status of each PVC connection and trafc statistics too. It
will also give you the number of BECN, FECN, and DE packets sent and received on the
router per PVC.
Here is an example:
Corp#sho frame pvc
PVC Statistics for interface Serial0/0 (Frame Relay DTE)
DLCI = 102,DLCI USAGE = LOCAL,PVC STATUS =ACTIVE,
INTERFACE = Serial0/0.102
input pkts 50977876 output pkts 41822892
in bytes 3137403144
out bytes 3408047602 dropped pkts 5
in FECN pkts 0
in BECN pkts 0 out FECN pkts 0 out BECN pkts 0
in DE pkts 9393 out DE pkts 0
pvc create time 7w3d, last time pvc status changed 7w3d
DLCI = 103,DLCI USAGE =LOCAL,PVC STATUS =ACTIVE,
INTERFACE = Serial0/0.103
input pkts 30572401 output pkts 31139837
in bytes 1797291100
out bytes 3227181474 dropped pkts 5
in FECN pkts 0
936 Chapter 21 u Wide Area Networks
in BECN pkts 0 out FECN pkts 0 out BECN pkts 0
in DE pkts 28 out DE pkts 0
pvc create time 7w3d, last time pvc status changed 7w3d
If you want to see information about only PVC 102, you can type the command show
frame-relay pvc 102. Let’s take a closer look at the output of this one line:
DLCI = 102,DLCI USAGE = LOCAL,PVC STATUS =ACTIVE,
INTERFACE = Serial0/0.102
The PVC status eld in the output of the show frame-relay pvc command reports the sta-
tus of the PVC between the router and the Frame-Relay switch. The switch (DCE) reports the
status to the router (DTE) using the LMI protocol. There are three types of reported statuses:
ACTIVE The switch is correctly programmed with the DLCI and there is a successful
DTE-to-DTE circuit (router to router).
INACTIVE The router is connected to the switch (DTE to DCE), but there’s not a con-
nection to the far-end router (DTE). This can be a router or switch conguration issue.
DELETED The router (DTE) is congured for a DLCI that the switch (DCE) does not
recognize or is not congured correctly.
The three LMI reported statuses are Cisco exam objectives! Understand
why you’d see each status.
The show interface Command
You can use the show interface command to check for LMI trafc. The show interface
command displays information about the encapsulation as well as layer 2 and layer 3 infor-
mation. It also displays line, protocol, DLCI, and LMI information. Check it out:
Corp#sho int s0/0
Serial0/0 is up, line protocol is up
Hardware is HD64570
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely
255/255, load 2/255
Encapsulation FRAME-RELAY, loopback not set, keepalive
set (10 sec)
LMI enq sent 451751,LMI stat recvd 451750,LMI upd recvd
164,DTE LMI up
LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0
LMI DLCI 1023 LMI type is CISCO frame relay DTE
Broadcast queue 0/64, broadcasts sent/dropped 0/0,
interface broadcasts 839294
Frame Relay 937
The LMI DLCI is used to dene the type of LMI being used. If it happens to be 1023,
it’s the default LMI type for Cisco routers. If the LMI DLCI is zero, then it’s the ANSI
LMI type (Q.933A uses 0 as well). If the LMI DLCI is anything other than 0 or 1023, it’s
a 911—call your provider; they’ve got major issues!
The show frame map Command
The show frame map command displays the mappings from the Network layer to DLCI.
Here’s how that looks, using Figure21.19:
Corp#show frame map
Serial0/0.102 (up): ip 10.1.12.2 dlci 102(0x66,0x400),
dynamic, broadcast,, status defined, active
Serial0/0.103 (up): ip 10.1.13.2 dlci 103(0x67,0x410),
dynamic, broadcast,, status defined, active
Notice that the Network layer addresses are marked dynamic, which means they were
resolved with the dynamic protocol Inverse ARP (IARP). After the DLCI number is listed,
you can see some numbers in parentheses. The rst one is 0x66, which is the hex equivalent
for the DLCI number 102, used on serial 0/0.102. And the 0x67 is the hex for DLCI 103
used on serial 0/0.103. The second numbers, 0x400 and 0x410, are the DLCI numbers con-
gured in the Frame Relay frame. They’re different because of the way the bits are spread
out in the frame.
Again, looking at Figure21.19, the preceding output is telling the Corp router that to get
SF using 10.1.12.2, use DLCI 102. To get to the NY router with IP address 10.1.13.2, use
DLCI 103. The Corp router would never use a remote DLCI.
You must be able to find the DLCI number used to get to a remote site by
using the show frame-relay map command.
The debug frame lmi Command
The debug frame lmi command will show real-time output on the router consoles by default
(as with any debug command). The information this command gives you will enable you to
verify and troubleshoot the Frame Relay connection by helping you determine whether the
router and switch are exchanging the correct LMI information. Here’s an example:
Corp#debug frame-relay lmi
Serial3/1(in): Status, myseq 214
RT IE 1, length 1, type 0
KA IE 3, length 2, yourseq 214, myseq 214
PVC IE 0x7 , length 0x6 , dlci 130, status 0x2 , bw 0
Serial3/1(out): StEnq, myseq 215, yourseen 214, DTE up
datagramstart = 0x1959DF4, datagramsize = 13
938 Chapter 21 u Wide Area Networks
FR encap = 0xFCF10309
00 75 01 01 01 03 02 D7 D6
Serial3/1(in): Status, myseq 215
RT IE 1, length 1, type 1
KA IE 3, length 2, yourseq 215, myseq 215
Serial3/1(out): StEnq, myseq 216, yourseen 215, DTE up
datagramstart = 0x1959DF4, datagramsize = 13
FR encap = 0xFCF10309
00 75 01 01 01 03 02 D8 D7
Troubleshooting Frame Relay Networks
Troubleshooting Frame Relay networks isn’t any harder than troubleshooting any other
type of network as long as you know what to look for, which is what I’m going to cover
now. I’ll go over some basic problems that commonly occur in Frame Relay conguration
and how to solve them.
First on the list are serial encapsulation problems. As you learned recently, there are two
Frame Relay encapsulations: Cisco and IETF. Cisco is the default, and it means that you have
a Cisco router on each end of the Frame Relay network. Using Figure21.20, if you don’t
have a Cisco router on the remote end of your Frame Relay network, then you need to run
the IETF encapsulation as shown here:
RouterA(config)#int s0
RouterA(config-if)#encapsulation frame-relay ?
ietf Use RFC1490 encapsulation
<cr>
RouterA(config-if)#encapsulation frame-relay ietf
Once you verify that you’re using the correct encapsulation, you then need to check out
your Frame Relay mappings. For an example, take a look at Figure21.20.
FIGURE21.20 Frame Relay mappings
RouterA DLCI 100 DLCI 200 RouterB
RouterA#show running-config
interface s0/0
ip address 172.168.100.2 255.255.0.0
encapsulation frame-relay
frame-relay map ip 172.16.100.1 200 broadcast
Frame Relay
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Frame Relay 939
So why can’t RouterA talk to RouterB across the Frame Relay network in Figure21.20?
To nd that out, take a close look at the frame-relay map statement. See the problem now?
You cannot use a remote DLCI to communicate to the Frame Relay switch; you must use
your DLCI number! The mapping should have included DLCI 100 instead of DLCI 200.
Now that you know how to ensure that you have the correct Frame Relay encapsulation,
and that DLCIs are only locally signicant, let’s look into some routing protocol problems
that are often associated with Frame Relay. See if you can nd a problem with the two con-
gurations in Figure21.21.
FIGURE21.21 Frame Relay routing problems
RouterA DLCI 100 DLCI 200 RouterB
RouterA#show running-config
interface s0/0
ip address 172.16.100.2 255.255.0.0
encapsulation frame-relay
frame-relay map ip 172.16.100.1 100
router rip
network 172.16.0.0
RouterB#show running-config
interface s0/0
ip address 172.16.100.1 255.255.0.0
encapsulation frame-relay
frame-relay map ip 172.16.100.2 200
router rip
network 172.16.0.0
Frame Relay
Hmmmm, the congs look pretty good. Actually, they look great, so what’s the problem?
Well, remember that Frame Relay is a nonbroadcast multi-access (NBMA) network by default,
meaning that it doesn’t allow any broadcasts across the PVC. So, because the mapping state-
ments do not have the broadcast argument at the end of the line, broadcasts such as RIP
updates or multicasts to neighbors such as Hello packets, won’t be sent across the PVC. The
correct line for RouterA would look like this:
frame-relay map ip 172.16.100.1 100 broadcast
But wait, do we even use RIP in our internetworks today? Maybe there are problems
with the routing protocols EIGRP and OSPF over Frame Relay as well? Let’s take a look
at Figure21.22 and see if you can spot a problem with the OSPF conguration. After this,
we’ll move on to troubleshooting EIGRP.
Since Frame Relay NBMA networks won’t allow broadcasts or multicasts, an OSPF router
will not attempt to dynamically discover any OSPF neighbors on the Frame Relay interface.
Also, since this means that elections won’t be allowed, you’d have to statically congure OSPF
neighbors, plus the Corp router would need to be congured as a DR. Even though these are
serial links, an NBMA network behaves like Ethernet and a DR is needed to exchange rout-
ing information. Only the Corp router can act as a DR because it would have the PVCs for all
other routers. But the easiest way to x this problem is to use the command ip ospf network
point-to-multipoint on all router Frame Relay interfaces—not just the Corp router, but all
branches too!
940 Chapter 21 u Wide Area Networks
FIGURE21.22 Frame Relay OSPF routing problems
Frame Relay
DLCI 200
Corp
SF
To SF use DLCI 101
Corp#show running-config
interface s0/0
ip address 172.16.100.2 255.255.0.0
encapsulation frame-relay
frame-relay interface-dlci 101
frame-relay map ip 172.16.100.1 101
router ospf 1
network 172.16.0.0 0.0.255.255 area 0
SF#show running-config
interface s0/0
ip address 172.16.100.1 255.255.0.0
encapsulation frame-relay
frame-relay map ip 172.16.100.2 200
router ospf 1
network 172.16.0.0 0.0.255.255 area 0
Okay, this would solve the problem if you’re running OSPF, but what if you’re running
EIGRP? In Figure21.23 you can see three remote connection sites to the Corp router with
all routers running EIGRP. The hosts behind the Corp router can communicate to all hosts
in all remote networks, but hosts in SF, LA, and NY cannot communicate to each other.
FIGURE21.23 Frame Relay EIGRP routing problems
Frame Relay
EIGRP 100
DLCI 200
Corp
SF
DLCI 400
DLCI 300
To NY use DLCI 103
To SF use DLCI 101
To LA use DLCI 102
LA
NY
Frame Relay 941
Let’s take a look at the conguration of the Corp router now:
interface Serial0/0
ip address 192.168.10.1 255.255.255.0
encapsulation frame-relay
frame-relay interface-dlci 101
frame-relay interface-dlci 102
frame-relay interface-dlci 103
!
The Frame Relay network is all on one subnet, and the conguration looks good, so why
can’t hosts on the remote networks communicate to each other? Here’s your answer: The SF
router sends an EIGRP route update to the Corp router and the Corp router updates the local
routing table with a route to SF’s network. LA and NY do the same thing and then each site’s
remote networks can communicate to the hosts behind the Corp router. However, when the
Corp router sends route updates to the SF, LA, and NY routers, the updates never included
each other’s remote network because of the split horizon rule. This rules says you cannot
advertise a network back out the same interface you received it on, which is the default cong-
uration of all Cisco serial interfaces. This prevents the threat of network loops from occurring.
We can solve this problem with subinterfaces. Take a look at the Corp router’s new con-
guration with subinterfaces, which solves the split horizon issues. Here are the steps to take:
1. Remove the IP address and interface-dlci commands from under the physical interface.
2. Create a subinterface (logical interface) for each PVC.
3. Design and implement a separate subnet (address space) for each subinterface.
4. Add the command frame-relay interface-dlci dlci under each subinterface.
interface Serial0/0
no ip address (notice there is no IP address on the physical interface!)
encapsulation frame-relay
!
interface Serial0/0.101 point-to-point
ip address 192.168.10.1 255.255.255.252
frame-relay interface-dlci 101
!
interface Serial0/0.102 point-to-point
ip address 192.168.10.5 255.255.255.252
frame-relay interface-dlci 102
!
interface Serial0/0.103 point-to-point
ip address 192.168.10.9 255.255.255.252
frame-relay interface-dlci 103
!
942 Chapter 21 u Wide Area Networks
Notice that there is no IP address under the physical interface, that each subinterface is a
separate subnet or address space, and that I needed to add the frame-relay interface-dlci
command under each subinterface. Our split horizon issue is now resolved.
Virtual Private Networks
I’d be pretty willing to bet you’ve heard the term VPN more than once before. Maybe you
even know what one is, but just in case, a virtual private network (VPN) allows the creation
of private networks across the Internet, enabling privacy and tunneling of non-TCP/IP pro-
tocols. VPNs are used daily to give remote users and disjointed networks connectivity over a
public medium like the Internet instead of using more expensive permanent means.
No worries—VPNs aren’t really that hard to understand. A VPN ts somewhere between
a LAN and WAN, with the WAN often simulating a LAN link because your computer, on
one LAN, connects to a different, remote LAN and uses its resources remotely. The key
drawback to using VPNs is a big one—security! So the denition of connecting a LAN (or
VLAN) to a WAN may sound the same as using a VPN, but a VPN is actually much more.
Here’s the difference: A typical WAN connects two or more remote LANs together using
a router and someone else’s network, like, say, your Internet service provider (ISP). Your local
host and router see these networks as remote networks and not as local networks or local
resources. This would be a WAN in its most general denition. A VPN actually makes your
local host part of the remote network by using the WAN link that connects you to the remote
LAN. The VPN will make your host appear as though it’s actually local on the remote net-
work. This means that we now have access to the remote LAN’s resources, and that access is
also very secure!
This may sound a lot like a VLAN denition, and really, the concept is the same:
“Take my host and make it appear local to the remote resources.” Just remember this key
distinction: For networks that are physically local, using VLANs is a good solution, but
for physically remote networks that span a WAN, opt for using VPNs instead.
For a simple VPN example, let’s use my home ofce in Boulder, Colorado. Here, I
have my personal host, but I want it to appear as if it’s on a LAN in my corporate ofce
in Dallas, Texas, so I can get to my remote servers. VPN is the solution I would opt for
to achieve my goal.
Figure21.24 shows this example of my host using a VPN connection from Boulder to
Dallas, which allows me to access the remote network services and servers as if my host
were right there on the same VLAN as my servers.
Why is this so important? If you answered, “Because my servers in Dallas are secure, and
only the hosts on the same VLAN are allowed to connect to them and use the resources of
these servers,” you nailed it! A VPN allows me to connect to these resources by locally attach-
ing to the VLAN through a VPN across the WAN. The other option is to open up my network
and servers to everyone on the Internet or another WAN service, in which case my security
goes “poof.” So clearly, it’s imperative I have a VPN!
Virtual Private Networks 943
FIGURE21.24 Example of using a VPN
My host in Colorado
My host appears
local to the servers.
Secure VLAN at
Dallas corporate office
Secure server room
Benefits of VPNs
There are many benets to using VPNs on your corporate and even home network. The
benets covered in the Cisco exam objectives are as follows:
Security VPNs can provide very good security by using advanced encryption and authen-
tication protocols, which will help protect your network from unauthorized access. IPsec
and SSL fall into this category. Secure Sockets Layer (SSL) is an encryption technology used
with web browsers, which has native SSL encryption, and is known as Web VPN. You can
also use the Cisco AnyConnect SSL VPN client installed on your PC to provide an SSL VPN
solution, as well as the Clientless Cisco SSL VPN.
Cost Savings By connecting the corporate remote ofces to their closest Internet provider,
and then creating a VPN tunnel with encryption and authentication, I gain a huge savings
over opting for traditional leased point-to-point lines. This also permits higher bandwidth
links and security, all for far less money than traditional connections.
Scalability VPNs scale very well to quickly bring up new ofces or have mobile users
connect securely while traveling or when connecting from home.
Compatibility with broadband technology For remote and traveling users and remote
ofces, any Internet access can provide a connection to the corporate VPN. This allows
users to take advantage of the high-speed Internet access of DSL or cable modems.
VPNs are categorized based upon the role they play in a business. There are three different
categories of VPNs:
Remote access VPNs allow remote users such as telecommuters to securely access the
corporate network wherever and whenever they need to.
944 Chapter 21 u Wide Area Networks
Site-to-site VPNs, or intranet VPNs, allow a company to connect its remote sites to the
corporate backbone securely over a public medium like the Internet instead of requir-
ing more expensive WAN connections like Frame Relay.
Extranet VPNs allow an organization’s suppliers, partners, and customers to be
connected to the corporate network in a limited way for business-to-business (B2B)
communications.
Now you’re interested, huh? And since VPNs are inexpensive and secure, I’m guessing
you just can’t wait to nd out how to create VPNs now! There’s more than one way to bring
a VPN into being. The rst approach uses IPsec to create authentication and encryption ser-
vices between endpoints on an IP network. The second way is via tunneling protocols, which
allow you to establish a tunnel between endpoints on a network. And understand that the
tunnel itself is a means for data or protocols to be encapsulated inside another protocol—
pretty clean!
I’m going to go over IPsec in a minute, but rst I really want to describe four of the most
common tunneling protocols in use today:
Layer 2 Forwarding (L2F) is a Cisco-proprietary tunneling protocol, and it was Cisco’s
rst tunneling protocol created for virtual private dial-up networks (VPDNs). A VPDN
allows a device to use a dial-up connection to create a secure connection to a corporate
network. L2F was later replaced by L2TP, which is backward compatible with L2F.
Point-to-Point Tunneling Protocol (PPTP) was created by Microsoft and others to
allow the secure transfer of data from remote networks to the corporate network.
Layer 2 Tunneling Protocol (L2TP) was created by Cisco and Microsoft to replace
L2F and PPTP. L2TP merged the capabilities of both L2F and PPTP into one tunnel-
ing protocol.
Generic Routing Encapsulation (GRE) is another Cisco-proprietary tunneling pro-
tocol. It forms virtual point-to-point links, allowing for a variety of protocols to be
encapsulated in IP tunnels. I’ll cover GRE in more detail, including how to congure
it, at the end of this chapter.
Okay—now that you’re clear on both exactly what a VPN is and the various types of
VPNs available, it’s time to dive into IPsec.
Introduction to Cisco IOS IPsec
Simply put, IPsec is an industry-wide standard framework of protocols and algorithms that
allows for secure data transmission over an IP-based network and functions at the layer 3
Network layer of the OSI model.
Did you notice I said IP-based network? That’s really important because by itself, IPsec
can’t be used to encrypt non-IP trafc. This means that if you run into a situation where
you have to encrypt non-IP trafc, you’ll need to create a Generic Routing Encapsulation
(GRE) tunnel for it (which I explain later) and then use IPsec to encrypt that tunnel!
Virtual Private Networks 945
IPsec Transforms
An IPsec transform species a single security protocol with its corresponding security algo-
rithm; without these transforms, IPsec wouldn’t be able to give us its glory. It’s important to be
familiar with these technologies, so let me take a second to dene the security protocols and
briey introduce the supporting encryption and hashing algorithms that IPsec relies upon.
Security Protocols
The two primary security protocols used by IPsec are Authentication Header (AH) and
Encapsulating Security Payload (ESP).
Authentication Header (AH)
The AH protocol provides authentication for the data and the IP header of a packet using a
one-way hash for packet authentication. It works like this: The sender generates a one-way
hash; then the receiver generates the same one-way hash. If the packet has changed in any way,
it won’t be authenticated and will be dropped. So basically, IPsec relies upon AH to guarantee
authenticity. AH checks the entire packet, but it doesn’t offer any encryption services.
This is unlike ESP, which only provides an integrity check on the data of a packet.
Encapsulating Security Payload (ESP)
It won’t tell you when or how the NASDAQ’s gonna bounce up and down like a superball,
but ESP will provide condentiality, data origin authentication, connectionless integrity,
anti-replay service, and limited trafc-ow condentiality by defeating trafc ow analysis—
which is almost as good! Anyway, there are ve components of ESP:
Confidentiality (encryption) This allows the sending device to encrypt the packets before
transmitting in order to prevent eavesdropping. Condentiality is provided through the
use of symmetric encryption algorithms like DES or 3DES. Condentiality can be selected
separately from all other services, but the condentiality selected must be the same on both
endpoints of your VPN.
Data integrity Data integrity allows the receiver to verify that the data received was not
altered in any way along the way. IPsec uses checksums as a simple check of the data.
Authentication Authentication ensures that the connection is made with the correct part-
ner. The receiver can authenticate the source of the packet by guaranteeing and certifying
the source of the information.
Anti-replay service Anti-replay election is based upon the receiver, meaning the service
is effective only if the receiver checks the sequence number. In case you were wondering, a
replay attack is when a hacker nicks a copy of an authenticated packet and later transmits
it to the intended destination. When the duplicate, authenticated IP packet gets to the des-
tination, it can disrupt services and generally wreak havoc. The Sequence Number eld is
designed to foil this type of attack.
946 Chapter 21 u Wide Area Networks
Traffic flow For trafc ow condentiality to work, you have to have at least tunnel
mode selected. It’s most effective if it’s implemented at a security gateway where tons
of trafc amasses because it’s precisely the kind of environment that can mask the true
source-destination patterns to bad guys who are trying to breach your network’s security.
Encryption
VPNs create a private network over a public network infrastructure, but to maintain
condentiality and security, we really need to use IPsec with our VPNs. IPsec uses vari-
ous types of protocols to perform encryption. The types of encryption algorithms used
today are as follows:
Symmetric encryption This encryption requires a shared secret to encrypt and decrypt.
Each computer encrypts the data before sending info across the network, with this same
key being used to both encrypt and decrypt the data. Examples of symmetric key encryp-
tion are Data Encryption Standard (DES), Triple DES (3DES), and Advanced Encryption
Standard (AES).
Asymmetric encryption Devices that use asymmetric encryption use different keys for
encryption than they do for decryption. These keys are called private and public keys.
Private keys encrypt a hash from the message to create a digital signature, which is then veri-
ed via decryption using the public key. Public keys encrypt a symmetric key for secure distri-
bution to the receiving host, which then decrypts that symmetric key using its exclusively held
private key. It’s not possible to encrypt and decrypt using the same key. This is a variant of
public key encryption that uses a combination of both a public and private keys. An example
of an asymmetric encryption is Rivest, Shamir, and Adleman (RSA).
As you can see from the amount of information I’ve thrown at you so far, establishing a
VPN connection between two sites takes study, time, and practice. And I am just scratching
the surface here! I know it can be difcult at times, and it can take quite a bit of patience.
Cisco does have some GUI interfaces to help with this process, and they can be very helpful
for conguring VPNs with IPsec. Though highly useful and very interesting, they are just
beyond the scope of this book, so I’m not going to delve further into this topic here.
To read about the latest buzz on encryption techniques, take a look at this Cisco URL:
http://www.cisco.com/web/about/security/intelligence/nextgen_crypto.html
GRE Tunnels
Generic Routing Encapsulation (GRE) is a tunneling protocol that can encapsulate many pro-
tocols inside IP tunnels. Some examples would be routing protocols such as EIGRP and OSFP
and the routed protocol IPv6. Figure21.25 shows the different pieces of a GRE header.
A GRE tunnel interface supports a header for each of the following:
u A passenger protocol or encapsulated protocols like IP or IPv6, which is the protocol
being encapsulated by GRE
GRE Tunnels 947
u GRE encapsulation protocol
u A Transport delivery protocol, typically IP
GRE tunnels have the following characteristics:
u GRE uses a protocol-type field in the GRE header so any layer 3 protocol can be used
through the tunnel.
u GRE is stateless and has no flow control.
u GRE offers no security.
u GRE creates additional overhead for tunneled packets—at least 24 bytes.
Now let’s take a look at how to congure a GRE tunnel. It’s actually pretty simple.
FIGURE21.25 Generic Routing Encapsulation (GRE) tunnel structure
IP Network
(Transportation Protocol)
GRE tunnel
(Carrier Protocol)
IP VPN
site
(passenger protocol)
IP VPN
site
(passenger protocol)
Transport
IP header
GRE
header Passenger (IP) packet
Configuring GRE Tunnels
Before you attempt to congure a GRE tunnel, you need to create an implementation plan.
Here’s a checklist for what you need to congure and implement a GRE:
u Use IP addressing.
u Create the logical tunnel interfaces.
u Specify that you’re using GRE tunnel mode under the tunnel interface (this is optional
since this is the default tunnel mode).
u Specify the tunnel source and destination IP addresses.
u Configure an IP address for the tunnel interface.
Let’s take a look at how to bring up a simple GRE tunnel. Figure21.26 shows the net-
work with two routers.
First, we need to make the logical tunnel with the interface tunnel number command.
We can use any number up to 2.14 billion.
Corp(config)#int s0/0/0
Corp(config-if)#ip address 63.1.1.1 255.255.255.252
948 Chapter 21 u Wide Area Networks
Corp(config)#int tunnel ?
<0-2147483647> Tunnel interface number
Corp(config)#int tunnel 0
*Jan 5 16:58:22.719:%LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0,
changed state to down
FIGURE21.26 Example of GRE configuration
Internet
GRE tunnel
192.168.10.1/24
Tunnel 0
S0/0/0
63.1.1.2/30
S0/0
63.1.1.1/30
Corp
192.168.10.2/24
Tunnel 0
SF
Once we have congured our interface and created the logical tunnel, we need to cong-
ure the mode and then transport protocol.
Corp(config-if)#tunnel mode ?
aurp AURP TunnelTalk AppleTalk encapsulation
cayman Cayman TunnelTalk AppleTalk encapsulation
dvmrp DVMRP multicast tunnel
eon EON compatible CLNS tunnel
gre generic route encapsulation protocol
ipip IP over IP encapsulation
ipsec IPSec tunnel encapsulation
iptalk Apple IPTalk encapsulation
ipv6 Generic packet tunneling in IPv6
ipv6ip IPv6 over IP encapsulation
nos IP over IP encapsulation (KA9Q/NOS compatible)
rbscp RBSCP in IP tunnel
Corp(config-if)#tunnel mode gre ?
ip over IP
ipv6 over IPv6
multipoint over IP (multipoint)
Corp(config-if)#tunnel mode gre ip
Okay, now that we’ve created the tunnel interface, the type, and the transport protocol,
we must congure our IP addresses for use inside of the tunnel. Of course, you need to use
your actual physical interface IP for the tunnel to send trafc across the Internet, but you
also need to congure the tunnel source and tunnel destination addresses.
Corp(config-if)#ip address 192.168.10.1 255.255.255.0
Corp(config-if)#tunnel source 63.1.1.1
GRE Tunnels 949
Corp(config-if)#tunnel destination 63.1.1.2
Corp#sho run interface tunnel 0
Building configuration...
Current configuration : 117 bytes
!
interface Tunnel0
ip address 192.168.10.1 255.255.255.0
tunnel source 63.1.1.1
tunnel destination 63.1.1.2
end
Now let’s congure the other end of the serial link and watch the tunnel pop up!
SF(config)#int s0/0/0
SF(config-if)#ip address 63.1.1.2 255.255.255.252
SF(config-if)#int t0
SF(config-if)#ip address 192.168.10.2 255.255.255.0
SF(config-if)#tunnel source 63.1.1.2
SF(config-if)#tun destination 63.1.1.1
*May 19 22:46:37.099: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0,
changed state to up
Oops—did I forget to set my tunnel mode and transport to GRE and IP on the SF
router? No, I didn’t need to because it’s the default tunnel mode on Cisco IOS. Nice!
So, rst I set the physical interface IP address (which used a global address even though
I didn’t have to), then I created the tunnel interface and set the IP address of the tunnel
interface. It’s really important that you remember to congure the tunnel interface with
the actual source and destination IP addresses to use or the tunnel won’t come up. In my
example, the 63.1.1.2 was the source and 63.1.1.1 was the destination.
Verifying GRP Tunnels
As usual I’ll start with my favorite troubleshooting command, show ip interface brief.
Corp#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.10.10.5 YES manual up up
Serial0/0 63.1.1.1 YES manual up up
FastEthernet0/1 unassigned YES unset administratively down down
Serial0/1 unassigned YES unset administratively down down
Tunnel0 192.168.10.1 YES manual up up
950 Chapter 21 u Wide Area Networks
In this output, you can see that the tunnel interface is now showing as an interface on
my router. You can see the IP address of the tunnel interface, and the Physical and Data
Link status show as up/up. So far so good, let’s take a look at the interface with the show
interfaces tunnel 0 command.
Corp#sh int tun 0
Tunnel0 is up, line protocol is up
Hardware is Tunnel
Internet address is 192.168.10.1/24
MTU 1514 bytes, BW 9 Kbit, DLY 500000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation TUNNEL, loopback not set
Keepalive not set
Tunnel source 63.1.1.1, destination 63.1.1.2
Tunnel protocol/transport GRE/IP
Key disabled, sequencing disabled
Checksumming of packets disabled
Tunnel TTL 255
Fast tunneling enabled
Tunnel transmit bandwidth 8000 (kbps)
Tunnel receive bandwidth 8000 (kbps)
The show interfaces command shows the conguration settings and the interface status
as well as the IP address, tunnel source, and destination address. The output also shows the
tunnel protocol, which is GRE/IP. Last, let’s take a look at the routing table with the show
ip route command.
Corp#sh ip route
[output cut]
192.168.10.0/24 is subnetted, 2 subnets
C 192.168.10.0/24 is directly connected, Tunnel0
L 192.168.10.1/32 is directly connected, Tunnel0
63.0.0.0/30 is subnetted, 2 subnets
C 63.1.1.0 is directly connected, Serial0/0
L 63.1.1.1/32 is directly connected, Serial0/0
The tunnel0 interface shows up as a directly connected interface, and although it’s
a logical interface, the router treats it as a physical interface, just like serial 0/0 in the
routing table.
Corp#ping 192.168.10.2
Exam Essentials 951
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.10.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5)
Did you notice that I just pinged 192.168.10.2 across the Internet?
Summary
In this chapter, you learned the difference between the following WAN services: cable, DSL,
HDLC, PPP, PPPoE, and Frame Relay. You also learned that you can use a VPN once any of
those services are up and running, as well as, create and verify a tunnel interface.
It’s so important for you to understand High-Level Data-Link Control (HDLC) and how
to verify with the show interface command that HDLC is enabled! You’ve been provided
with some really important HDLC information as well as information on how the Point-to-
Point Protocol (PPP) is used if you need more features than HDLC offers or if you’re using
two different brands of routers. You now know that this is because various versions of
HDLC are proprietary and won’t work between two different vendors’ routers.
When we went through the section on PPP, I discussed the various LCP options as well
as the two types of authentication that can be used: PAP and CHAP.
And we talked about Frame Relay and the two different encapsulation methods used
with it in detail. We also discussed LMI options, Frame Relay maps, and subinterface con-
gurations. In addition to the Frame Relay terms and features we covered, I demonstrated
Frame Relay conguration and verication in depth.
We nished up the chapter with a discussion on virtual private networks, IPsec, and
encryption, and I explained GRE and how to congure the tunnel and then verify it.
Exam Essentials
Remember the default serial encapsulation on Cisco routers. Cisco routers use a proprietary
High-Level Data-Link Control (HDLC) encapsulation on all their serial links by default.
Understand the different Frame Relay encapsulations. Cisco uses two different Frame
Relay encapsulation methods on its routers: Cisco and IETF. If you are using the Cisco
encapsulation method, you are telling your router that a Cisco router is installed on the
other side of the PVC. If you are using the IETF encapsulation, you are telling your router
that a non-Cisco router is installed on the other side of the PVC.
Remember what the CIR is in Frame Relay. The CIR is the average rate, in bits per second,
at which the Frame Relay switch agrees to transfer data.
952 Chapter 21 u Wide Area Networks
Remember the commands for verifying and troubleshooting Frame Relay. The show
frame-relay lmi command will give you the LMI trafc statistics regarding LMI traf-
c exchanged between the local router and the Frame Relay switch. The show frame pvc
command will list all congured PVCs and DLCI numbers.
Remember the PPP Data Link layer protocols. The three Data Link layer protocols are
Network Control Protocol (NCP), which denes the Network layer protocols; Link Control
Protocol (LCP), a method of establishing, conguring, maintaining, and terminating the
point-to-point connection; and High-Level Data-Link Control (HDLC), the MAC layer
protocol that encapsulates the packets.
Be able to troubleshoot a PPP link. Understand that a PPP link between two routers will
show up and a ping would even work between the router if the layer 3 addresses are wrong.
Remember the various types of serial WAN connections. The serial WAN connections
that are most widely used are HDLC, PPP, and Frame Relay.
Understand the term virtual private network. You need to understand why and how to
use a VPN between two sites and the purpose that IPsec serves with VPNs.
Understand how to configure and verify a GRE tunnel. To congure GRE, rst congure
the logical tunnel with the interface tunnel number command. Congure the mode and
transport, if needed, with the tunnel mode mode protocol command, then congure the
IP addresses on the tunnel interfaces, the tunnel source and tunnel destination addresses,
and your physical interfaces with global addresses. Verify with the show interface tunnel
command as well as the ping protocol.
Hands-on Labs 953
Written Lab 7
The answers to this lab can be found in Appendix A, “Answers to Written Labs.”
Write the answers to the following WAN questions:
1. Write the command to see the encapsulation method on serial 0/0 of a Cisco router.
2. Write the commands to configure s0/0 to PPP encapsulation.
3. Write the commands to configure a username of todd and password of cisco that is
used on a Cisco router for PPP authentication.
4. Write the commands to enable CHAP authentication on a Cisco serial interface.
(Assume PPP is the encapsulation type.)
5. Write the commands to configure the DLCI numbers for two serial interfaces, 0/0 and
0/1. Use 16 for s0/0 and 17 for s0/1.
6. Write the commands to configure a remote office using a point-to-point subinterface.
Use DLCI 16 and IP address 172.16.60.1/24.
7. What protocol would you use if you were running xDSL and needed authentication?
8. What are the three protocols specified in PPP?
9. To provide security in your VPN tunnel, what protocol suite would you use?
10. What are the typical three different categories of VPNs?
Hands-on Labs
In this section, you will congure Cisco routers in three different WAN labs using the g-
ure supplied in each lab. (These labs are included for use with real Cisco routers but work
perfectly with the LammleSim IOS simulator and with Cisco’s Packet Tracer program.)
Lab 7.1: Conguring PPP Encapsulation and Authentication
Lab 7.2: Conguring and Monitoring HDLC
Lab 7.3: Conguring Frame Relay and Subinterfaces
Lab 7.4: Conguring a GRE Tunnel
Hands-on Lab 7.1: Configuring PPP Encapsulation
and Authentication
By default, Cisco routers use High-Level Data-Link Control (HDLC) as a point-to-point
encapsulation method on serial links. If you are connecting to non-Cisco equipment, then
you can use the PPP encapsulation method to communicate.
954 Chapter 21 u Wide Area Networks
Labs 7.1 and 7.2 will have you congure the following diagram.
S0/1
DCE
RouterA RouterB
S0/0
Fa0/0 Fa0/0
S0/0
DCE
RouterC
Fa0/0
S0/0
1. Ty pe sh int s0/0 on RouterA and RouterB to see the encapsulation method.
2. Make sure each router has the hostname assigned.
RouterA#config t
RouterA(config)#hostname RouterA
RouterB#config t
RouterB(config)#hostname RouterB
3. To change the default HDLC encapsulation method to PPP on both routers, use the
encapsulation command at interface configuration. Both ends of the link must run
the same encapsulation method.
RouterA#Config t
RouterA(config)#int s0
RouterA(config-if)#encap ppp
4. Now go to RouterB and set serial 0/0 to PPP encapsulation.
RouterB#config t
RouterB(config)#int s0
RouterB(config-if)#encap ppp
5. Verify the configuration by typing sh int s0/0 on both routers.
6. Notice the IPCP and CDPCP (assuming the interface is up). This is the information
used to transmit the upper-layer (Network layer) information across the HDLC at the
MAC sublayer.
Hands-on Labs 955
7. Define a username and password on each router. Notice that the username is the name
of the remote router. Also, the password must be the same.
RouterA#config t
RouterA(config)#username RouterB password todd
RouterB#config t
RouterB(config)#username RouterA password todd
8. Enable CHAP or PAP authentication on each interface.
RouterA(config)#int s0
RouterA(config-if)#ppp authentication chap
RouterB(config)#int s0
RouterB(config-if)#ppp authentication chap
9. Verify the PPP configuration on each router by using these commands.
RouterB(config-if)#shut
RouterB(config-if)#debug ppp authentication
RouterB(config-if)#no shut
Hands-on Lab 7.2: Configuring and Monitoring HDLC
There really is no conguration required for HDLC (as it is the default conguration on
Cisco serial interfaces), but if you completed Lab 7.1, then the PPP encapsulation would be
set on both routers. This is why I put the PPP lab rst. This lab allows you to actually con-
gure HDLC encapsulation on a router.
For this second lab, you will use the same configuration you used for Lab 7.1.
1. Set the encapsulation for each serial interface by using the encapsulation hdlc
command.
RouterA#config t
RouterA(config)#int s0
RouterA(config-if)#encapsulation hdlc
956 Chapter 21 u Wide Area Networks
RouterB#config t
RouterB(config)#int s0
RouterB(config-if)#encapsulation hdlc
2. Verify the HDLC encapsulation by using the show interface s0 command on
each router.
Hands-on Lab 7.3: Configuring Frame Relay
and Subinterfaces
In this lab, you will use the following diagram to congure Frame Relay, which can easily
be created in LammleSim as well as Packet Tracer.
S0/0
Lab_A DLCI 100 DLCI 200
S0/0
Lab_C
Lab_B
S0/0 S0/1
You will congure the Lab_B router to be a Frame Relay switch (this information was not
covered in this chapter and is not included in the exam objectives; it will be precongured
in LammleSim). You will then congure the Lab_A and Lab_C routers to use the switch to
bring up the PVC.
1. Set the hostname, using the frame-relay switching command, and the encapsulation
of each serial interface on the Frame Relay switch.
Router#config t
Router(config)#hostname Lab_B
Lab_B(config)#frame-relay switching [makes the router an
FR switch]
Lab_B(config)#int s0/0
Lab_B(config-if)#encapsulation frame-relay
Lab_B(config-if)#int s0/1
Lab_B(config-if)#encapsulation frame-relay
2. Configure the Frame Relay mappings on each interface. You do not have to have IP
addresses on these interfaces because they are only switching one interface to another
with Frame Relay frames.
Lab_B(config-if)#int s0/0
Lab_B(config-if)#frame intf-type dce
[The above command makes this an FR DCE interface, which
is different than a router's interface being DCE]
Hands-on Labs 957
Lab_B(config-if)#frame-relay route 100 interface
Serial0/1 200
Lab_B(config-if)#clock rate 64000
[The above command is used if you have this as DCE, which
is different than an FR DCE]
Lab_B(config-if)#int s0/1
Lab_B(config-if)#frame intf-type dce
Lab_B(config-if)#frame-relay route 200 interface
Serial0/0 100
Lab_B(config-if)#clock rate 64000 [if you have this as DCE]
This is not as hard as it looks. The route command just says that if you receive frames
from PVC 102, send them out int s0/1 using PVC 201. The second mapping on serial
0/1 is just the opposite. Anything that comes in int s0/1 is routed out serial0/0 using
PVC 102.
3. Configure Lab_A with a point-to-point subinterface.
Router#config t
Router(config)#hostname Lab_A
Lab_A(config)#int s0/0
Lab_A(config-if)#encapsulation frame-relay
Lab_A(config-if)#int s0/0.102 point-to-point
Lab_A(config-if)#ip address 172.16.10.1
255.255.255.0
Lab_A(config-if)#frame-relay interface-dlci 102
4. Configure Lab_C with a point-to-point subinterface.
Router#config t
Router(config)#hostname Lab_C
Lab_C(config)#int s0/0
Lab_C(config-if)#encapsulation frame-relay
Lab_C(config-if)#int s0/0.201 point-to-point
Lab_C(config-if)#ip address 172.16.10.2
255.255.255.0
Lab_C(config-if)#frame-relay interface-dlci 201
5. Verify your configurations with the following highlighted commands.
Lab_A>sho frame ?
ip show frame relay IP statistics
lmi show frame relay lmi statistics
map Frame-Relay map table
958 Chapter 21 u Wide Area Networks
pvc show frame relay pvc statistics
route show frame relay route
traffic Frame-Relay protocol statistics
6. Also, use Ping and Telnet to verify connectivity.
Hands-on Lab 7.4: Configuring a GRE Tunnel
In this lab you will congure two point-to-point routers with a simple IP GRE tunnel. You
can use a real router, LammleSim, or Packet Tracer to do this lab.
1. First, configure the logical tunnel with the interface tunnel number command.
Corp(config)#int s0/0/0
Corp(config-if)#ip address 63.1.1.2 255.255.255.252
Corp(config)#int tunnel ?
<0-2147483647> Tunnel interface number
Corp(config)#int tunnel 0
*Jan 5 16:58:22.719: %LINEPROTO-5-UPDOWN: Line protocol
on Interface Tunnel0, changed state to down
2. Once you have configured your interface and created the logical tunnel, you need to
configure the mode and then the transport protocol.
Corp(config-if)#tunnel mode ?
aurp AURP TunnelTalk AppleTalk encapsulation
cayman Cayman TunnelTalk AppleTalk encapsulation
dvmrp DVMRP multicast tunnel
eon EON compatible CLNS tunnel
gre generic route encapsulation protocol
ipip IP over IP encapsulation
ipsec IPSec tunnel encapsulation
iptalk Apple IPTalk encapsulation
ipv6 Generic packet tunneling in IPv6
ipv6ip IPv6 over IP encapsulation
nos IP over IP encapsulation (KA9Q/NOS compatible)
rbscp RBSCP in IP tunnel
Corp(config-if)#tunnel mode gre ?
ip over IP
ipv6 over IPv6
multipoint over IP (multipoint)
Corp(config-if)#tunnel mode gre ip
Hands-on Labs 959
3. Okay, now that you have created the tunnel interface, the type, and the transport pro-
tocol, you need to configure your IP addresses. Of course, you need to use your actual
interface IP for the tunnel, but you also need to configure the tunnel source and tunnel
destination addresses.
Corp(config-if)#int t0
Corp(config-if)#ip address 192.168.10.1 255.255.255.0
Corp(config-if)#tunnel source 63.1.1.1
Corp(config-if)#tunnel destination 63.1.1.2
Corp#sho run interface tunnel 0
Building configuration...
Current configuration : 117 bytes
!
interface Tunnel0
ip address 192.168.10.1 255.255.255.0
tunnel source 63.1.1.1
tunnel destination 63.1.1.2
end
4. Now configure the other end of the serial link and watch the tunnel pop up!
SF(config)#int s0/0/0
SF(config-if)#ip address 63.1.1.2 255.255.255.252
SF(config-if)#int t0
SF(config-if)#ip address 192.168.10.2 255.255.255.0
SF(config-if)#tunnel source 63.1.1.2
SF(config-if)#tun destination 63.1.1.1
*May 19 22:46:37.099: %LINEPROTO-5-UPDOWN: Line protocol on Interface
Tunnel0, changed state to up
Remember, you don’t need to congure your tunnel mode and transport protocol because
GRE and IP are the defaults. It’s really important that you remember to congure the tun-
nel interface with the actual source and destination IP addresses to use or the tunnel won’t
come up. In my example, 63.1.1.2 was the source and 63.1.1.1 was the destination.
5. Verify with the following commands:
Corp#sh ip int brief
960 Chapter 21 u Wide Area Networks
You should see that the tunnel interface is now showing as an interface on your
router. The IP address of the tunnel interface and the physical and data link status
shows as up/up.
Corp#sh int tun 0
The show interfaces command shows the conguration settings and the interface
status as well as the IP address and tunnel source and destination address.
Corp#sh ip route
The tunnel0 interface shows up as a directly connected interface, and although it’s
a logical interface, the router treats it as a physical interface just like serial0/0 in the
routing table.
Review Questions 961
Review Questions
The following questions are designed to test your understanding of this
chapter’s material. For more information on how to get additional questions,
please see this book’s introduction.
The answers to these questions can be found in Appendix B, “Answers to Chapter
Review Questions.”
1. Which command will display the CHAP authentication process as it occurs between
two routers in the network?
A. show chap authentication
B. show interface serial 0
C. debug ppp authentication
D. debug chap authentication
2. Which command is required for connectivity in a Frame Relay network if Inverse ARP
is not operational?
A. frame-relay arp
B. frame-relay map
C. frame-relay interface-dci
D. frame-relay lmi-type
3. Suppose you have a customer who has a central HQ and six branch offices. The cus-
tomer anticipates adding six more branches in the near future. It wishes to implement a
WAN technology that will allow the branches to economically connect to HQ and you
have no free ports on the HQ router. Which of the following would you recommend?
A. PPP
B. HDLC
C. Frame Relay
D. ISDN
4. Which of the following command options are displayed when you use the Router#show
frame-relay ? command? (Choose three.)
A. dlci
B. neighbors
C. lmi
D. pvc
E. map
962 Chapter 21 u Wide Area Networks
5. How should a router that is being used in a Frame Relay network be configured to
keep split horizon issues from preventing routing updates?
A. Configure a separate subinterface for each PVC with a unique DLCI and subnet
assigned to the subinterface.
B. Combine multiple Frame Relay circuits as a point-to-point line to support multi-
cast and broadcast traffic.
C. Configure many subinterfaces in the same subnet.
D. Configure a single subinterface to establish multiple PVC connections to multiple
remote router interfaces.
6. Which encapsulations can be configured on a serial interface? (Choose three.)
A. Ethernet
B. Token Ring
C. HDLC
D. Frame Relay
E. PPP
7. When setting up Frame Relay for point-to-point subinterfaces, which of the following
must not be configured?
A. The Frame Relay encapsulation on the physical interface
B. The local DLCI on each subinterface
C. An IP address on the physical interface
D. The subinterface type as point-to-point
8. When a router is connected to a Frame Relay WAN link using a serial DTE interface,
how is the clock rate determined?
A. By the CSU/DSU
B. By the far end router
C. By the clock rate command
D. By the Physical layer bit stream timing
9. A default Frame Relay WAN is classified as what type of physical network?
A. Point-to-point
B. Broadcast multi-access
C. Nonbroadcast multi-access
D. Nonbroadcast multipoint
Review Questions 963
10. Which of the following encapsulates PPP frames in Ethernet frames and uses common
PPP features like authentication, encryption, and compression?
A. PPP
B. PPPoA
C. PPPoE
D. Token Ring
11. You need to configure a router for a Frame Relay connection to a non-Cisco router.
Which of the following commands will prepare the WAN interface of the router for
this connection?
A. Router(config-if)#encapsulation frame-relay q933a
B. Router(config-if)#encapsulation frame-relay ansi
C. Router(config-if)#encapsulation frame-relay ietf
D. Router(config-if)#encapsulation frame-relay cisco
12. You have configured a serial interface with GRE IP commands on a corporate router
with a point-to-point link to a remote office. What command will show you the IP
addresses and tunnel source and destination addresses of the interfaces?
A. show int serial 0/0
B. show ip int brief
C. show interface tunnel 0
D. show tunnel ip status
E. debug ip interface tunnel
13. Which of the following is true regarding WAN technologies? (Choose three.)
A. You must use PPP on a link connecting two routers using a point-to-point lease line.
B. You can use a T1 to connect a customer site to the ISP.
C. You can use a T1 to connect a Frame Relay connection to the ISP.
D. You can use Ethernet as a WAN service by using EoMPLS.
E. When using an Ethernet WAN, you must configure the DLCI.
14. You want to allow remote users to send protected packets to the corporate site, but you
don’t want to install software on the remote client machines. What is the best solution
that you could implement?
A. GRE tunnel
B. Web VPN
C. VPN Anywhere
D. IPsec
964 Chapter 21 u Wide Area Networks
15. Why won’t the serial link between the Corp router and the Remote router come up?
Corp#sh int s0/0
Serial0/0 is up, line protocol is down
Hardware is PowerQUICC Serial
Internet address is 10.0.1.1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 254/255, txload 1/255, rxload 1/255
Encapsulation PPP, loopback not set
Remote#sh int s0/0
Serial0/0 is up, line protocol is down
Hardware is PowerQUICC Serial
Internet address is 10.0.1.2/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 254/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set
A. The serial cable is faulty.
B. The IP addresses are not in the same subnet.
C. The subnet masks are not correct.
D. The keepalive settings are not correct.
E. The layer 2 frame types are not compatible.
16. Which of the following are benefits of using a VPN in your internetwork? (Choose three)
A. Security
B. Private high-bandwidth links
C. Cost savings
D. Incompatibility with broadband technologies
E. Scalability
Review Questions 965
17. A remote site has just been connected to the central office, named Lab_A. However,
remote users cannot access applications at the central office. The remote router can be
pinged from the Lab_A office router. After reviewing the following command output,
which do you think is the most likely reason for the problem?
S0/0
Lab_A
Lab_B
DLCI 100 DLCI 200
S0/0
Lab_C
S0/0 S0/1
Lab A#show running-config
!
interface Serial 0/0
ip address 10.0.8.1 255.255.248.0
encapsulation frame-relay
frame-relay map ip 10.0.15.2 200
!
Router rip
Network 10.0.0.0
Lab C#show running-config
!
interface Serial 0/0
ip address 10.0.15.2 255.255.248.0
encapsulation frame-relay
frame-relay map ip 10.0.8.1 100
!
Router rip
Network 10.0.0.0
A. The Frame Relay PVC is down.
B. The IP addressing on the central/remote router link is incorrect.
C. RIP routing information is not being forwarded.
D. Frame Relay Inverse ARP is not properly configured.
18. Which of the following is an industry-wide standard suite of protocols and algorithms
that allows for secure data transmission over an IP-based network that functions at the
layer 3 Network layer of the OSI model?
A. HDLC
B. Cable
C. VPN
D. IPsec
E. xDSL
966 Chapter 21 u Wide Area Networks
19. Which of the following describes the creation of private networks across the Internet,
enabling privacy and tunneling of TCP/IP protocols?
A. HDLC
B. Cable
C. VPN
D. IPsec
E. xDSL
20. Referring to the following diagram, what functions does the Frame Relay DLCI provide
with respect to router Lab_A?
S0/0
Lab_A DLCI 100 DLCI 200
S0/0
Lab_C
Lab_B
S0/0 S0/1
A. Identifies the signaling standard between Lab_A and the frame switch
B. Identifies a portion of the virtual circuit between Lab_A and the frame switch
C. Identifies the encapsulation used between Lab_A and Lab_B
D. Defines the signaling standard between Lab_B and the frame switch
Appendix
A
Answers to
Written Labs
Chapter 1: Internetworking
Written Lab 1.1: OSI Questions
1. The Application layer is responsible for finding the network resources broadcast from a
server and adding flow control and error control (if the application developer chooses).
2. The Physical layer takes frames from the Data Link layer and encodes the 1s and 0s
into a digital signal for transmission on the network medium.
3. The Network layer provides routing through an internetwork and logical addressing.
4. The Presentation layer makes sure that data is in a readable format for the Applica-
tion layer.
5. The Session layer sets up, maintains, and terminates sessions between applications.
6. PDUs at the Data Link layer are called frames and provide physical addressing plus
other options to place packets on the network medium.
7. The Transport layer uses virtual circuits to create a reliable connection between two hosts.
8. The Network layer provides logical addressing, typically IP addressing and routing.
9. The Physical layer is responsible for the electrical and mechanical connections
between devices.
10. The Data Link layer is responsible for the framing of data packets.
11. The Session layer creates sessions between different hosts’ applications.
12. The Data Link layer frames packets received from the Network layer.
13. The Transport layer segments user data.
14. The Network layer creates packets out of segments handed down from the
Transport layer.
Chapter 1: Internetworking 969
15. The Physical layer is responsible for transporting 1s and 0s (bits) in a digital signal.
16. Segments, packets, frames, bits
17. Transport
18. Data Link
19. Network
20. 48 bits (6 bytes) expressed as a hexadecimal number
Written Lab 1.2: Defining the OSI Layers and Devices
Description Device or OSI Layer
This device sends and receives information about the Net-
work layer.
Router
This layer creates a virtual circuit before transmitting
between two end stations.
Transport
This device uses hardware addresses to filter a network. Bridge or switch
Ethernet is defined at these layers. Data Link and Physical
This layer supports flow control, sequencing, and acknowl-
edgments.
Transport
This device can measure the distance to a remote network. Router
Logical addressing is used at this layer. Network
Hardware addresses are defined at this layer. Data Link (MAC sublayer)
This device creates one big collision domain and one large
broadcast domain.
Hub
This device creates many smaller collision domains, but the
network is still one large broadcast domain.
Switch or bridge
This device can never run full-duplex. Hub
This device breaks up collision domains and broadcast
domains.
Router
970 Appendix A u Answers to Written Labs
Written Lab 1.3: Identifying Collision and
Broadcast Domains
A. Hub: One collision domain, one broadcast domain
B. Bridge: Two collision domains, one broadcast domain
C. Switch: Four collision domains, one broadcast domain
D. Router: Three collision domains, three broadcast domains
Chapter 2: Ethernet Networking
and Data Encapsulation
Written Lab 2.1: Binary/Decimal/Hexadecimal Conversion
1.
Decimal 128 64 32 16 8421Binary
192 1100000011000000
168 1010100010101000
10 0000101000001010
15 0000111100001111
Chapter 2: Ethernet Networking and Data Encapsulation 971
Decimal 128 64 32 16 8421Binary
172 1010110010101100
16 0001000000010000
20 0001010000010100
55 0011011100110111
Decimal 128 64 32 16 8421Binary
10 0000101000001010
11 0000101100001011
12 0000110000001100
99 0110001101100011
2.
Binary 128 64 32 16 8421Decimal
11001100 11001100204
00110011 0011001151
10101010 10101010170
01010101 0101010185
972 Appendix A u Answers to Written Labs
Binary 128 64 32 16 8421Decimal
11000110 11000110198
11010011 11010011211
00111001 0011100157
11010001 11010001209
Binary 128 64 32 16 8421Decimal
10000100 10000100132
11010010 11010010210
10111000 10111000184
10100110 10100110166
3.
Binary 128 64 32 16 8421Hexadecimal
11011000 11011000D8
00011011 000110111B
00111101 001111013D
01110110 0111011076
Chapter 2: Ethernet Networking and Data Encapsulation 973
Binary 128 632 16 8421Hexadecimal
11001010 11001010CA
11110101 11110101F5
10000011 1000001183
11101011 11101011EB
Binary 128 64 32 16 8421Hexadecimal
10000100 1000010084
11010010 11010010D2
01000011 0100001143
10110011 10110011B3
Written Lab 2.2: CSMA/CD Operations
When a collision occurs on an Ethernet LAN, the following happens:
1. A jam signal informs all devices that a collision occurred.
2. The collision invokes a random back-off algorithm.
3. Each device on the Ethernet segment stops transmitting for a short time until the
timers expire.
4. All hosts have equal priority to transmit after the timers have expired.
Written Lab 2.3: Cabling
1. Crossover
2. Straight-through
3. Crossover
4. Crossover
5. Straight-through
6. Crossover
7. Crossover
8. Rolled
974 Appendix A u Answers to Written Labs
Written Lab 2.4: Encapsulation
At a transmitting device, the data encapsulation method works like this:
1. User information is converted to data for transmission on the network.
2. Data is converted to segments, and a reliable connection is set up between the trans-
mitting and receiving hosts.
3. Segments are converted to packets or datagrams, and a logical address is placed in the
header so each packet can be routed through an internetwork.
4. Packets or datagrams are converted to frames for transmission on the local network.
Hardware (Ethernet) addresses are used to uniquely identify hosts on a local net-
work segment.
5. Frames are converted to bits, and a digital encoding and clocking scheme is used.
Chapter 3: Introduction to TCP/IP
Written Lab 3.1: TCP/IP
1. 192 through 223, 110xxxxx
2. Host-to-host
3. 1 through 126
4. Loopback or diagnostics
5. Turn all host bits off.
6. Turn all host bits on.
7. 10.0.0.0 through 10.255.255.255
8. 172.16.0.0 through 172.31.255.255
9. 192.168.0.0 through 192.168.255.255
10. 0 through 9 and A, B, C, D, E, and F
Written Lab 3.2: Mapping Applications to the DoD Model
1. Internet
2. Process/Application
3. Process/Application
4. Process/Application
5. Process/Application
Chapter 4: Easy Subnetting 975
6. Internet
7. Process/Application
8. Host-to-host
9. Process/Application
10. Host-to-host
11. Process/Application
12. Internet
13. Internet
14. Internet
15. Process/Application
16. Process/Application
17. Process/Application
Chapter 4: Easy Subnetting
Written Lab 4.1: Written Subnet Practice #1
1. 192.168.100.25/30. A /30 is 255.255.255.252. The valid subnet is 192.168.100.24,
broadcast is 192.168.100.27, and valid hosts are 192.168.100.25 and 26.
2. 192.168.100.37/28. A /28 is 255.255.255.240. The fourth octet is a block size of 16.
Just count by 16s until you pass 37. 0, 16, 32, 48. The host is in the 32 subnet, with a
broadcast address of 47. Valid hosts 33–46.
3. A /27 is 255.255.255.224. The fourth octet is a block size of 32. Count by 32s until
you pass the host address of 66. 0, 32, 64, 96. The host is in the 64 subnet, and the
broadcast address is 95. Valid host range is 65–94.
4. 192.168.100.17/29. A /29 is 255.255.255.248. The fourth octet is a block size of 8. 0,
8, 16, 24. The host is in the 16 subnet, broadcast of 23. Valid hosts 17–22.
5. 192.168.100.99/26. A /26 is 255.255.255.192. The fourth octet has a block size of 64.
0, 64, 128. The host is in the 64 subnet, broadcast of 127. Valid hosts 65–126.
6. 192.168.100.99/25. A /25 is 255.255.255.128. The fourth octet is a block size of 128.
0, 128. The host is in the 0 subnet, broadcast of 127. Valid hosts 1–126.
7. A default Class B is 255.255.0.0. A Class B 255.255.255.0 mask is 256 subnets, each
with 254 hosts. We need fewer subnets. If we used 255.255.240.0, this provides 16
subnets. Let’s add one more subnet bit. 255.255.248.0. This is 5 bits of subnetting,
which provides 32 subnets. This is our best answer, a /21.
976 Appendix A u Answers to Written Labs
8. A /29 is 255.255.255.248. This is a block size of 8 in the fourth octet. 0, 8, 16. The
host is in the 8 subnet, broadcast is 15.
9. A /29 is 255.255.255.248, which is 5 subnet bits and 3 host bits. This is only 6 hosts
per subnet.
10. A /23 is 255.255.254.0. The third octet is a block size of 2. 0, 2, 4. The subnet is in the
16.2.0 subnet; the broadcast address is 16.3.255.
Written Lab 4.2: Written Subnet Practice #2
Written Lab 4.3: Written Subnet Practice #3
Classful Address Subnet Mask Number of Hosts per Subnet (2x – 2)
/16 255.255.0.0 65,534
/17 255.255.128.0 32,766
/18 255.255.192.0 16,382
/19 255.255.224.0 8,190
/20 255.255.240.0 4,094
/21 255.255.248.0 2,046
/22 255.255.252.0 1,022
/23 255.255.254.0 510
/24 255.255.255.0 254
/25 255.255.255.128 126
/26 255.255.255.192 62
/27 255.255.255.224 30
/28 255.255.255.240 14
/29 255.255.255.248 6
/30 255.255.255.252 2
Chapter 5: VLSMs, Summarization and Troubleshooting TCP/IP 977
Written Lab 4.3: Written Subnet Practice #3
Decimal IP Address
Address
Class
Number of Subnet
and Host Bits
Number of Sub-
nets (2x)
Number of Hosts
(2x – 2)
10.25.66.154/23 A15/9 32,768 510
172.31.254.12/24 B8/8 256 254
192.168.20.123/28 C4/4 16 14
63.24.89.21/18 A10/14 1,024 16,382
128.1.1.254/20 B4/12 16 4,094
208.100.54.209/30 C6/2 64 2
Chapter 5: VLSMs, Summarization and
Troubleshooting TCP/IP
1. 192.168.0.0/20
2. 172.144.0.0 255.240.0.0
3. 192.168.32.0 255.255.224.0
4. 192.168.96.0 255.255.240.0
5. 66.66.0.0 255.255.240.0
6. 192.168.0.0/17
7. 172.16.1.0 255.255.248.0
8. 192.168.128.0 255.255.192.0
9. 53.60.96.0 255.255.224.0
10. 172.16.0.0 255.255.192.0
978 Appendix A u Answers to Written Labs
Chapter 6: Cisco’s Internetworking
Operating System (IOS)
Written Lab 6
1. Router(config)#clock rate 1000000
2. Switch#config t
switch config)# line vty 0 15
switch(config-line)# no login
3. Switch#config t
Switch(config)# int f0/1
Switch(config-if)# no shutdown
4. Switch#erase startup-config
5. Switch#config t
Switch(config)# line console 0
Switch(config)# password todd
Switch(config)# login
6. Switch#config t
Switch(config)# enable secret cisco
7. Router#show controllers serial 0/2
8. Switch#show terminal
9. Switch#reload
10. Switch#config t
Switch(config)#hostname Sales
Chapter 7: Managing a Cisco Internetwork
Written Lab 7.1: IOS Management
1. copy start run
2. show cdp neighbor detail or show cdp entry *
3. show cdp neighbor
Chapter 8: IP Routing 979
4. Ctrl+Shift+6, then X
5. show sessions
6. Either copy tftp run or copy start run
7. NTP
8. ip helper-address
9. ntp server ip_address version 4
10. show ntp status
Written Lab 7.2: Router Memory
1. Flash memory
2. ROM
3. NVRAM
4. ROM
5. RAM
6. RAM
7. ROM
8. ROM
9. RAM
10. RAM
Chapter 8: IP Routing
1. router(config)#ip route 172.16.10.0 255.255.255.0 172.16.20.1 150
2. It will use the gateway interface MAC at L2 and the actual destination IP at L3.
3. router(config)#ip route 0.0.0.0 0.0.0.0 172.16.40.1
4. Stub network
5. Router#show ip route
6. Exit interface
7. False. The MAC address would be the router interface, not the remote host.
8. True
9. router(config)#router rip
router(config-router)#passive-interface S1
10. True
980 Appendix A u Answers to Written Labs
Chapter 9: Open Shortest Path
First (OSPF)
1. router ospf 101
2. show ip ospf
3. show ip ospf interface
4. show ip ospf neighbor
5. show ip route ospf
Chapter 10: Layer 2 Switching
1. show mac address-table
2. Flood the frame out all ports except the port on which it was received
3. Address learning, forward/filter decisions, and loop avoidance
4. It will add the source MAC address in the forward/filter table and associate it with the
port on which the frame was received.
5. Maximum 1, violation shutdown
Chapter 11: VLANs and
InterVLAN Routing
1. False! You do not provide an IP address under any physical port.
2. STP
3. Broadcast
4. Collision
5. switchport trunk encapsulation dot1q
6. Trunking allows you to make a single port part of multiple VLANs at the same time.
7. Frame identification (frame tagging) uniquely assigns a user-defined ID to each frame.
This is sometimes referred to as a VLAN ID or color.
8. True
9. Access link
10. switchport trunk native vlan 4
Chapter 13: Network Address Translation 981
Chapter 12: Security
1. access-list 10 deny 172.16.0.0 0.0.255.255
access-list 10 permit any
2. ip access-group 10 out
3. access-list 10 deny host 192.168.15.5
access-list 10 permit any
4. show access-lists
5. IDS, IPS
6. access-list 110 deny tcp host
172.16.10.1 host 172.16.30.5 eq 23
access-list 110 permit ip any any
7. line vty 0 4
access-class 110 in
8. ip access-list standard No172Net
deny 172.16.0.0 0.0.255.255
permit any
9. ip access-group No172Net out
10. show ip interfaces
Chapter 13: Network Address Translation
1. Port Address Translation (PAT), also called NAT Overload
2. debug ip nat
3. show ip nat translations
4. clear ip nat translations *
5. Before
6. After
7. show ip nat statistics
8. The ip nat inside and ip nat outside commands
9. Dynamic NAT
10. prefix-length
982 Appendix A u Answers to Written Labs
Chapter 14: Internet Protocol
Version 6 (IPv6)
Written Lab 14.1
1. Unicast
2. Global unicast
3. Link-local
4. Unique local (used to be called site-local)
5. Multicast
6. Anycast
7. OSPFv3
8. ::1
9. FE80::/10
10. FC00:: /7
Written Lab 14.2
1. 2001:db8:1:1:090c:abff:fecd:1234
2. 2001:db8:1:1: 040c:32ff:fef1:a4d2
3. 2001:db8:1:1:12:abff:fecd:1234
4. 2001:db8:1:1:0f01:3aff:fe2f:1234
5. 2001:db8:1:1:080c:abff:feac:caba
Chapter 15: Enhanced Switch
Technologies
1. PAgP
2. show spanning-tree summary
3. 802.1w
4. STP
5. BPDU Guard
Chapter 16: Managing Cisco Devices 983
6. (config-if)#spanning-tree portfast
7. Switch#show etherchannel port-channel
8. Switch(config)#spanning-tree vlan 3 root primary
9. show spanning-tree, then follow the root port that connects to the root bridge using
CDP, or show spanning-tree summary.
10. Active and Passive
Chapter 16: Managing Cisco Devices
Written Lab 16.1: IOS Management
1. copy flash tftp
2. 0x2101
3. 0x2102
4. 0x2100
5. UDI
6. 0x2142
7. boot system
8. POST test
9. copy tftp flash
10. show license
Written Lab 16.2: Router Memory
1. Flash memory
2. ROM
3. NVRAM
4. ROM
5. RAM
6. RAM
7. ROM
8. ROM
9. RAM
10. RAM
984 Appendix A u Answers to Written Labs
Chapter 17: IP Services
1. show standby
2. show glbp
3. GLBP
4. NetFlow
5. 5
6. SNMP
7. 4
8. show ip cache flow
9. logging ip_address
10. T he AVG
Chapter 18: Troubleshooting IP, IPv6,
and VLANs
1. The INCMP is an incomplete message, which means a neighbor solicitation message
has been sent but the neighbor message has not yet been received.
2. switchport trunk native vlan 66
3. Access, auto, desirable, nonegotiate, and trunk (on)
4. Verify that the default gateway is correct. Verify that name resolution settings are
correct. Verify that there are no ACLs blocking traffic.
5. ping ::1
Chapter 19: Enhanced IGRP
1. ipv6 router eigrpv as
2. FF02::A
3. False
4. The routers will not form an adjacency
5. Passive interface
Chapter 21: Wide Area Networks 985
Chapter 20: Multi-Area OSPF
1. Type 5 or type 7
2. 2WAY
3. Type 3, and possibly Type 4 and 5
4. When all LSAs have synchronized with a neighbor on a point-to-point link
5. True
6. EXCHANGE
7. Ty pe 1
8. ipv6 ospf 1 area 0
9. OSPFv2 and v3 use the same items when forming an adjacency: hello and dead timers,
subnet info, and area ID all must match; authentication must also match if configured.
10. and show ipv6 protocols
Chapter 21: Wide Area Networks
1. sh int s0/0
2. config t
int s0/0
encap ppp
3. config t
username todd password cisco
4. config t
int serial0/0
ppp authentication chap
5. config t
int s0/0
frame interface-dlci 16
int s0/1
frame interface-dlci 17
986 Appendix A u Answers to Written Labs
6. config t
int s0/0
no ip address
encap frame
int s0/0.16 point-to-point
ip address 172.16.60.1 255.255.255.0
frame interface-dlci 16
7. PPPoE or PPPoA
8. HDLC, LCP, and NCP
9. IPsec
10. Remote access VPNs, site-to-site VPNs, and extranet VPNs
Appendix
B
Answers to Review
Questions
Chapter 1: Internetworking
1. A. The device shown is a hub and hubs place all ports in the same broadcast domain
and the same collision domain.
2. B. The contents of a protocol data unit (PDU) depend on the PDU as they are created
in a specic order and their contents are based on that order. A packet will contain
IP addresses but not MAC addresses as MAC addresses are not present until the PDU
becomes a frame.
3. C. You should select a router to connect the two groups. When computers are in differ-
ent subnets, as these two groups are, you will require a device that can make decisions
based on IP addresses. Routers operate at layer 3 of the Open Systems Interconnect
(OSI) model and make data-forwarding decisions based on layer 3 networking infor-
mation, which are IP addresses. They create routing tables that guide them in forward-
ing trafc out of the proper interface to the proper subnet.
4. C. Replacing the hub with a switch would reduce collisions and retransmissions which
would have the most impact on reducing congestion.
5. Answer:
Layer Description
Transport Bits
Data Link Segment
Physical Packet
Network Frame
The given layers of the OSI model use the PDUs shown in the above diagram.
6. C. The diagram depicts the three steps taken to create a TCP connection.
7. B. You should use a switch to accomplish the task in this scenario. A switch is used to
provide dedicated bandwidth to each node by eliminating the possibility of collisions
on the switch port where the node resides. Switches work at layer 2 in the Open System
Interconnection (OSI) model and perform the function of separating collision domains.
Chapter 1: Internetworking 989
8.
Transport
Physical
Data Link
Network
End-to-end connection
Conversion to bits
Framing
Routing
The listed layers of the OSI model have the functions shown in the diagram above.
9. D. Reliable data delivery ensures the integrity of a stream of data sent from one
machine to the other through a fully functional data link. It guarantees that the
data won’t be duplicated or lost. This is achieved through something called positive
acknowledgment with retransmission—a technique that requires a receiving machine
to communicate with the transmitting source by sending an acknowledgment message,
based on the window size, back to the sender when it receives data.
10. B. Protocols used to support data trafc are called routed protocols and some key
examples of them include IP and IPv6.
11. B. Data translation and code formatting occur on the Presentation layer of the
OSI model.
12. C. The IEEE Ethernet Data Link layer has two sublayers, the Media Access Control
(MAC) and the Logical Link Control (LLC).
13. B. Windows are used to control the amount of outstanding, unacknowledged data seg-
ments. In the diagram, a windows size of three has been agreed upon by the stations.
14. A. Hubs operate on the Physical Layer as they have no intelligence and send all trafc
in all directions.
15. C. While it is true that the OSI model’s primary purpose is to allow different vendors’
networks to interoperate, there is no requirement that any vendor follows the model.
16. A. Routers by default do NOT forward broadcasts.
17. C. Switches create separate collision domains within a single broadcast domain. Routers
provide a separate broadcast domain for each interface.
18. B. The all-hub network at the bottom is one collision domain; the bridge network
on top equals three collision domains. Add in the switch network of ve collision
domains—one for each switch port—and you get a total of nine.
19. A. The top three layers dene how the applications within the end stations will com-
municate with each other as well as with users.
20. A. The following network devices operate at all seven layers of the OSI model: network
management stations (NMSs), gateways (not default gateways), servers and network hosts.
990 Appendix B u Answers to Review Questions
Chapter 2: Ethernet Networking and
Data Encapsulation
1. D. The organizationally unique identier (OUI) is assigned by the IEEE to an organi-
zation composed of 24 bits, or 3 bytes, which in turn assigns a globally administered
address also comprised of 24 bits, or 3 bytes, that’s supposedly unique to each and
every adapter it manufactures.
2. A. Back-off on an Ethernet network is the retransmission delay that’s enforced when
a collision occurs. When that happens, a host will only resume transmission after the
forced time delay has expired. Keep in mind that after the backoff has elapsed, all
stations have equal priority to transmit data.
3. A. When using a hub all ports are in the same collision domain which will introduce
collisions as shown between devices connected to the same hub.
4. B. FCS is a eld at the end of the frame that’s used to store the cyclic redundancy check
(CRC) answer. The CRC is a mathematical algorithm that’s run when each frame is
built based on the data in the frame. When a receiving host receives the frame and
runs the CRC, the answer should be the same. If not, the frame is discarded, assuming
errors have occurred.
5. B. Ethernet networking uses a protocol called Carrier Sense Multiple Access with Colli-
sion Detection (CSMA/CD), which helps devices share the bandwidth evenly, while pre-
venting two devices from transmitting simultaneously on the same network medium
6. D. Hubs are not capable of providing a full-duplex connection.
7. D. The cable shown is a straight-through cable which is used between dissimilar
devices.
8. B, C. The high-order bit is the Individual/Group (I/G) bit. When it has a value of 0,
we can assume that the address is the MAC address of a device and that it may well
appear in the source portion of the MAC header. When it’s a 1, we can assume that the
address represents either a broadcast or multicast address in Ethernet.
9. B. In creating the gigabit crossover cable, you’d still cross 1 to 3 and 2 to 6, but you
would add 4 to 7 and 5 to 8.
10. D. When you set up the connection, use these settings:
u Bits per sec: 9600
u Data bits: 8
u Parity: none
u Stop bits: 1
u Flow control: none
Chapter 3: Introduction to TCP/IP 991
11. D. When set to 0, this bit represents a globally administered address, as by the IEEE,
but when it’s a 1, it represents a locally governed and administered address.
12. B. You can use a rolled Ethernet cable to connect a host EIA-TIA 232 interface to a
router console serial communication (COM) port.
13. B. The collision will invoke a back-off algorithm on all systems, not just the ones
involve in the collision.
14. A. There are no collisions in full-duplex mode.
15. B. The connection between the two switches requires a crossover and the connection
from the hosts to the switches required a straight through.
16. The given cable types are matched with their standards in the diagram below.
IEEE 802.3u 100Base-Tx
IEEE 802.3 10Base-T
IEEE 802.3ab 1000Base-T
IEEE 802.3z 1000Base-SX
The given cable types are matched with their standards in the diagram above.
17. B. Although rolled cable isn’t used to connect any Ethernet connections together, you
can use a rolled Ethernet cable to connect a host EIA-TIA 232 interface to a router
console serial communication (COM) port.
18. B. If you’re using TCP, the virtual circuit is dened by the source and destination port
number plus the source and destination IP address and called a socket.
19. A. The hex value 1C is converted as 28 in decimal.
20. A. Fiber optic cables are the only ones that have a core surrounded by a material called
cladding.
Chapter 3: Introduction to TCP/IP
1. C. If a DHCP conict is detected, either by the server sending a ping and getting a
response or by a host using a gratuitous ARP (arp’ing for its own IP address and seeing
if a host responds), then the server will hold that address and not use it again until it is
xed by an administrator.
2. B. Secure Shell (SSH) protocol sets up a secure session that’s similar to Telnet over a stan-
dard TCP/IP connection and is employed for doing things like logging into systems, run-
ning programs on remote systems, and moving les from one system to another.
992 Appendix B u Answers to Review Questions
3. C. A host uses something called a gratuitous ARP to help avoid a possible duplicate
address. The DHCP client sends an ARP broadcast out on the local LAN or VLAN
using its newly assigned address to solve conicts before they occur.
4. B. Address Resolution Protocol (ARP) is used to nd the hardware address from a
known IP address.
5. A, C, D. The listed answers are from the OSI model and the question asked about the
TCP/IP protocol stack (DoD model). Yes, it is normal for the objectives to have this
type of question. However, let’s just look for what is wrong. First, the Session layer is
not in the TCP/IP model; neither are the Data Link and Physical layers. This leaves us
with the Transport layer (Host-to-host in the DoD model), Internet layer (Network
layer in the OSI), and Application layer (Application/Process in the DoD). Remem-
ber, the CCENT objectives can list the layers as OSI layers or DoD layer at any time,
regardless of what the question is asking.
6. C. A Class C network address has only 8 bits for dening hosts: 28 – 2 = 256.
7. A, B. A client that sends out a DHCP Discover message in order to receive an IP
address sends out a broadcast at both layer 2 and layer 3. The layer 2 broadcast is all
Fs in hex, or FF:FF:FF:FF:FF:FF. The layer 3 broadcast is 255.255.255.255, which
means any networks and all hosts. DHCP is connectionless, which means it uses User
Datagram Protocol (UDP) at the Transport layer, also called the Host-to-host layer.
8. B. Although Telnet does use TCP and IP (TCP/IP), the question specically asks about
layer 4, and IP works at layer 3. Telnet uses TCP at layer 4.
9. RFC 1918 These addresses can be used on a private network, but they’re not routable
through the Internet.
10. B, D, E. SMTP, FTP, and HTTP use TCP.
11. C. Class C addresses devote 24 bits to the network portion and 8 bits to the host portion.
12. C. The range of multicast addresses starts with 224.0.0.0 and goes through
239.255.255.255.
13. C. First, you should know easily that only TCP and UDP work at the Transport layer,
so now you have a 50/50 shot. However, since the header has sequencing, acknowledg-
ment, and window numbers, the answer can only be TCP.
14. A. Both FTP and Telnet use TCP at the Transport layer; however, they both are Appli-
cation layer protocols, so the Application layer is the best answer for this question.
15. C. The four layers of the DoD model are Application/Process, Host-to-Host, Internet, and
Network Access. The Internet layer is equivalent to the Network layer of the OSI model.
Chapter 4: Easy Subnetting 993
16. C, E. The Class A private address range is 10.0.0.0 through 10.255.255.255. The Class
B private address range is 172.16.0.0 through 172.31.255.255, and the Class C private
address range is 192.168.0.0 through 192.168.255.255.
17. B. The four layers of the TCP/IP stack (also called the DoD model) are Application/
Process, Host-to-host, Internet, and Network Access. The Host-to-host layer is
equivalent to the Transport layer of the OSI model.
18. B, C. ICMP is used for diagnostics and destination unreachable messages. ICMP is
encapsulated within IP datagrams, and because it is used for diagnostics, it will pro-
vide hosts with information about network problems.
19. C. The range of a Class B network address is 128–191. This makes our binary range
10xxxxxx.
20.
DHCPOffer TargetB
DHCPDiscover TargetA
DHCPAck TargetD
DHCPRequest TargetC
The steps are as shown in the answer diagram.
Chapter 4: Easy Subnetting
1. D. A /27 (255.255.255.224) is 3 bits on and 5 bits off. This provides 8 subnets, each
with 30 hosts. Does it matter if this mask is used with a Class A, B, or C network
address? Not at all. The number of host bits would never change.
2. D. A 240 mask is 4 subnet bits and provides 16 subnets, each with 14 hosts. We need
more subnets, so let’s add subnet bits. One more subnet bit would be a 248 mask. This
provides 5 subnet bits (32 subnets) with 3 host bits (6 hosts per subnet). This is the
best answer.
3. C. This is a pretty simple question. A /28 is 255.255.255.240, which means that
our block size is 16 in the fourth octet. 0, 16, 32, 48, 64, 80, etc. The host is in the
64 subnet.
4. F. A CIDR address of /19 is 255.255.224.0. This is a Class B address, so that is only
3 subnet bits, but it provides 13 host bits, or 8 subnets, each with 8,190 hosts.
994 Appendix B u Answers to Review Questions
5. B,D The mask 255.255.254.0 (/23) used with a Class A address means that there are
15 subnet bits and 9 host bits. The block size in the third octet is 2 (256 – 254). So this
makes the subnets in the interesting octet 0, 2, 4, 6, etc., all the way to 254. The host
10.16.3.65 is in the 2.0 subnet. The next subnet is 4.0, so the broadcast address for the
2.0 subnet is 3.255. The valid host addresses are 2.1 through 3.254.
6. D. A /30, regardless of the class of address, has a 252 in the fourth octet. This means
we have a block size of 4 and our subnets are 0, 4, 8, 12, 16, etc. Address 14 is obvi-
ously in the 12 subnet.
7. D. A point-to-point link uses only two hosts. A /30, or 255.255.255.252, mask pro-
vides two hosts per subnet.
8. C. A /21 is 255.255.248.0, which means we have a block size of 8 in the third octet, so
we just count by 8 until we reach 66. The subnet in this question is 64.0. The next sub-
net is 72.0, so the broadcast address of the 64 subnet is 71.255.
9. A. A /29 (255.255.255.248), regardless of the class of address, has only 3 host bits.
Six hosts are the maximum number of hosts on this LAN, including the router
interface.
10. C. A /29 is 255.255.255.248, which is a block size of 8 in the fourth octet. The subnets
are 0, 8, 16, 24, 32, 40, etc. 192.168.19.24 is the 24 subnet, and since 32 is the next
subnet, the broadcast address for the 24 subnet is 31. 192.168.19.26 is the only correct
answer.
11. A. A /29 (255.255.255.248) has a block size of 8 in the fourth octet. This means the
subnets are 0, 8, 16, 24, etc. 10 is in the 8 subnet. The next subnet is 16, so 15 is the
broadcast address.
12. B. You need 5 subnets, each with at least 16 hosts. The mask 255.255.255.240 pro-
vides 16 subnets with 14 hosts—this will not work. The mask 255.255.255.224 pro-
vides 8 subnets, each with 30 hosts. This is the best answer.
13. C. First, you cannot answer this question if you can’t subnet. The 192.168.10.62
with a mask of 255.255.255.192 is a block size of 64 in the fourth octet. The host
192.168.10.62 is in the zero subnet, and the error occurred because ip subnet-zero
is not enabled on the router.
14. A. A /25 mask is 255.255.255.128. Used with a Class B network, the third and fourth
octets are used for subnetting with a total of 9 subnet bits, 8 bits in the third octet and
1 bit in the fourth octet. Since there is only 1 bit in the fourth octet, the bit is either off
or on—which is a value of 0 or 128. The host in the question is in the 0 subnet, which
has a broadcast address of 127 since 112.128 is the next subnet.
Chapter 5: VLSMs, Summarization, and Troubleshooting TCP/IP 995
15. A. A /28 is a 255.255.255.240 mask. Let’s count to the ninth subnet (we need to nd
the broadcast address of the eighth subnet, so we need to count to the ninth subnet).
Starting at 16 (remember, the question stated that we will not use subnet zero, so we
start at 16, not 0), 16, 32, 48, 64, 80, 96, 112, 128, 144 etc.. The eighth subnet is 128
and the next subnet is 144, so our broadcast address of the 128 subnet is 143. This
makes the host range 129–142. 142 is the last valid host.
16. C. A /28 is a 255.255.255.240 mask. The rst subnet is 16 (remember that the question
stated not to use subnet zero) and the next subnet is 32, so our broadcast address is 31.
This makes our host range 17–30. 30 is the last valid host.
17. E. A Class C subnet mask of 255.255.255.224 is 3 bits on and 5 bits off (11100000)
and provides 8 subnets, each with 30 hosts. However, if the command ip subnet-zero
is not used, then only 6 subnets would be available for use.
18. E. A Class B network ID with a /22 mask is 255.255.252.0, with a block size of 4 in
the third octet. The network address in the question is in subnet 172.16.16.0 with a
broadcast address of 172.16.19.255. Only option E has the correct subnet mask listed,
and 172.16.18.255 is a valid host.
19. D,E The router’s IP address on the E0 interface is 172.16.2.1/23, which is
255.255.254.0. This makes the third octet a block size of 2. The router’s interface is in
the 2.0 subnet, and the broadcast address is 3.255 because the next subnet is 4.0. The
valid host range is 2.1 through 3.254. The router is using the rst valid host address in
the range.
20. C. To test the local stack on your host, ping the loopback interface of 127.0.0.1.
Chapter 5: VLSMs, Summarization,
and Troubleshooting TCP/IP
1. D. A point-to-point link uses only two hosts. A /30, or 255.255.255.252, mask pro-
vides two hosts per subnet.
2. C. Using a /28 mask, there are 4 bits available for hosts. Two to the fourth power
minus 2 = 14.
3. D. For 6 hosts we need to leave 3 bits in the host portion since 2 to the third power = 8
and 8 less 2 is 6.With 3 bits for the host portion, that leaves 29 bits for the mask or /29.
4. C. To use VLSM, the routing protocols in use possess the capability to transmit subnet
mask information.
996 Appendix B u Answers to Review Questions
5. D. In a question like this, you need to look for an interesting octet where you can
combine networks. In this example, the third octet has all our subnets so we just
need to nd our block size now. If we used a block of 8 starting at 172.16.0.0/19,
then we cover 172.16.0.0 through 172.16.7.255. However, if we used 172.16.0.0/20,
then we’d cover a block of 16 which would be from 172.16.0.0 through
172.16.15.255, which is the best answer.
6. C. The IP address of the station and the gateway are not in the same network. Since
the address of the gateway is correct on the station, it is most likely the IP address
of the station is incorrect.
7. B. With an incorrect gateway, Host A will not be able to communicate with the router
or beyond the router but will be able to communicate within the subnet.
8. A. Pinging the remote computer would fail if any of the other tests fail.
9. C. When a ping to the local host IP address fails, you can assume the NIC is not
functional.
10. C, D. If a ping to the local host succeeds, you can rule out IP stack or NIC failure.
11. E. A /29 mask yields only 6 addresses, so none of the networks could use it.
12. A. The most likely problem if you can ping a computer by IP address but not by name
is a failure of DNS.
13. D. When you issue the ping command, you are using the ICMP protocol.
14. B. The traceroute command displays the networks traversed on a path to a network
destination.
15. C. The ping command tests connectivity to another station. The full command is
shown below.
C:\>ping 172.16.10.2
Pinging 172.16.10.2 with 32 bytes of data:
Reply from 172.16.10.2: bytes=32 time<1ms TTL=128
Reply from 172.16.10.2: bytes=32 time<1ms TTL=128
Reply from 172.16.10.2: bytes=32 time<1ms TTL=128
Reply from 172.16.10.2: bytes=32 time<1ms TTL=128
Ping statistics for 172.16.10.2:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
Chapter 6: Cisco’s Internetworking Operating System (IOS) 997
16.
traceroute
arp -a
show ip arp
ipconfig /all
Displays the list of routers on a path to a network destination
Displays IP-to-MAC-address mappings on a Windows PC
Displays the ARP table on a Cisco router
Shows you the PC network configuration
The command uses the functions described in the answer table.
17. C. The interesting octet in this example is the second octet, and it is a block size of four
starting at 10.0.0.0. By using a 255.252.0.0 mask, we are telling the summary to use a
block size of four in the 2nd octet. This will cover 10.0.0.0 through 10.3.255.255. This
is the best answer.
18. A. The command that displays the ARP table on a Cisco router is show ip arp.
19. C. The /all switch must be added to the ipconfig command on a PC to verify DNS
conguration.
20. C. If you start at 192.168.128.0 and go through 192.168.159.0, you can see this is a
block of 32 in the third octet. Since the network address is always the rst one in the
range, the summary address is 192.168.128.0. What mask provides a block of 32 in
the third octet? The answer is 255.255.224.0, or /19.
Chapter 6: Cisco’s Internetworking
Operating System (IOS)
1. D. Typically we’d see the input errors and CRC statistics increase with a duplex error,
but it could be another Physical layer issue such as the cable might be receiving exces-
sive interference or the network interface cards might have a failure. Typically you can
tell if it is interference when the CRC and input errors output grow but the collision
counters do not, which is the case with this question.
2. C. Once the IOS is loaded and up and running, the startup-cong will be copied from
NVRAM into RAM and from then on, referred to as the running-cong.
3. C, D. To congure SSH on your router, you need to set the username command, the ip
domain-name, login local, and the transport input ssh under the VTY lines and the
crypto key command. However, SSH version 2 is suggested but not required.
4. C. The show controllers serial 0/0 command will show you whether either a DTE
or DCE cable is connected to the interface. If it is a DCE connection, you need to add
clocking with the clock rate command.
998 Appendix B u Answers to Review Questions
5.
Mode Definition
User EXEC mode Commands that affect the entire system
Privileged EXEC mode Commands that affect interfaces/processes only
Global configuration mode Interactive configuration dialog
Specific configuration modes Provides access to all other router commands
Setup mode Limited to basic monitoring commands
User exec mode is limited to basic monitoring commands; privileged exec mode pro-
vides access to all other router commands. Specific configuration modes include the
commands that affect a specific interface or process, while global configuration mode
allows commands that affect the entire system. Setup mode is where you access the
interactive configuration dialog.
6. B. We can see that the bandwidth is 100000 Kbit, which is 100,000,000. Kbit means
to add three zeros, which is 100 Mbits per second, or FastEthernet.
7. B. From global conguration mode, use the line vty 0 4 command to set all ve default
VTY lines. However, you would typically always set all lines, not just the defaults.
8. C. The enable secret password is case sensitive, so the second option is wrong. To set
the enable secret password, use the enable secret password command from global
conguration mode. This password is automatically encrypted.
9. C. The typical banner is a message of the day (MOTD) and is set by using the global
conguration mode command banner MOTD.
10. C. The prompts offered as options indicate the following modes:
Switch(config)# is global conguration mode.
Switch> is user mode.
Switch# is privileged mode.
Switch(config-if)# is interface conguration mode.
11. D. To copy the running-cong to NVRAM so that it will be used if the router is
restarted, use the copy running-config startup-config command in privileged
mode (copy run start for short).
12. D. To allow a VTY (Telnet) session into your router, you must set the VTY password.
Option C is wrong because it is setting the password on the wrong router. Notice that
you have to set the password before you set the login command. Remember, Cisco may
have you set the password before the login command.
Chapter 7: Managing a Cisco Internetwork 999
13. D. The erase startup-config command erases the contents of NVRAM and will put
you in setup mode if the switch is restarted. Option E is wrong because you must type
in the full command!
14. B. If an interface is shut down, the show interface command will show the interface
as administratively down. (It is possible that no cable is attached, but you can’t tell that
from this message.)
15. C. With the show interfaces command, you can view the congurable parameters,
get statistics for the interfaces on the switch, check for input and CRC errors, and
verify if the interfaces are shut down.
16. C. If you delete the startup-cong and reload the switch, the device will automatically
enter setup mode. You can also type setup from privileged mode at any time.
17. D. You can view the interface statistics from user mode, but the command is show
interface fastethernet 0/0.
18. B. The % ambiguous command error means that there is more than one possible show
command that starts with r. Use a question mark to nd the correct command.
19. B, D. The commands show interfaces and show ip interface will show you the
layer 1 and 2 status and the IP addresses of your router’s interfaces.
20. A. If you see that a serial interface and the protocol are both down, then you have a
Physical layer problem. If you see serial1 is up, line protocol is down, then you
are not receiving (Data Link) keepalives from the remote end.
Chapter 7: Managing a Cisco Internetwork
1. B. The IEEE created a new standardized discovery protocol called 802.1AB for Station
and Media Access Control Connectivity Discovery. We’ll just call it Link Layer Discov-
ery Protocol (LLDP).
2. C. The show processes (or show processes cpu) is a good tool for determining
a given router’s CPU utilization. When it is high, it is not a good time to execute a
debug command.
3. B. The command traceroute (trace for short), which can be issued from user mode or
privileged mode, is used to nd the path a packet takes through an internetwork and
will also show you where the packet stops because of an error on a router.
4. C. Since the conguration looks correct, you probably didn’t screw up the copy job.
However, when you perform a copy from a network host to a router, the interfaces
are automatically shut down and need to be manually enabled with the no shutdown
command.
1000 Appendix B u Answers to Review Questions
5. D. Specifying the address of the DHCP server allows the router to relay broadcast traf-
c destined for a DHCP server to that server.
6. C. Before you start to congure the router, you should erase the NVRAM with the erase
startup-config command and then reload the router using the reload command.
7. C. This command can be run on both routers and switches and it displays detailed
information about each device connected to the device you’re running the command
on, including the IP address.
8. C. The Port ID column describes the interfaces on the remote device end of the
connection.
9. C. This command can be run on both routers and switches, and it displays detailed
information about each device connected to the device you’re running the command
on, including the IP address.
10. C. If you save a conguration and reload the router and it comes up either in setup
mode or as a blank conguration, chances are you have the conguration register set-
ting incorrect.
11. D. To keep open one or more Telnet sessions, use the Ctrl+Shift+6 and then
X keystroke combination.
12. B, D. The best answers, the ones you need to remember, are that either an access con-
trol list is ltering the Telnet session or the VTY password is not set on the remote
device.
13. A, D. The show hosts command provides information on temporary DNS entries and
permanent name-to-address mappings created using the ip host command.
14. A, B, D. The tracert command is a Windows command and will not work on a router
or switch! IOS uses the traceroute command.
15. D. Since the question never mentioned anything about a suspended session, you can
assume that the Telnet session is still open, and you would just type exit to close
the session.
16. C. To see console messages through your Telnet session, you must enter the terminal
monitor command.
17. terminal monitor When you telnet into a remote device, you will not see console mes-
sages by default. For example, you will not see debugging output. To allow console
messages to be sent to your Telnet session, use the terminal monitor command.
18. E. Although option A is certainly the “best” answer, unfortunately option E will work
just ne and your boss would probably prefer you to use the show cdp neighbors
detail command.
Chapter 8: IP Routing 1001
19. D. To enable a device to be an NTP client, use the ntp server IP_ a d dre ss version
number command at global conguration mode. That’s all there is to it! Assuming your
NTP server is working of course.
20. B, D. You can verify your NTP client with the show ntp status and show ntp
associations commands.
Chapter 8: IP Routing
1. show ip route The ip route command is used to display the routing table of a router.
2. B. In the new 15 IOS code, Cisco denes a different route called a local route. Each has
a /32 prex dening a route just for the one address.
3. A, B. Although option D almost seems right, it is not; the mask is the mask used on
the remote network, not the source network. Since there is no number at the end of the
static route, it is using the default administrative distance of 1.
4. C, F. The switches are not used as either a default gateway or other destination.
Switches have nothing to do with routing. It is very important to remember that the
destination MAC address will always be the router’s interface. The destination address
of a frame, from HostA, will be the MAC address of the Fa0/0 interface of RouterA.
The destination address of a packet will be the IP address of the network interface card
(NIC) of the HTTPS server. The destination port number in the segment header will
have a value of 443 (HTTPS).
5. B. This mapping was learned dynamically which means it was learned
through ARP.
6. B. Hybrid protocols use aspects of both distance vector and link state—for example,
EIGRP. Although be advised that Cisco typically just calls EIGRP an advanced distance
vector routing protocol. Do not be mislead by the way the question is worded. Yes, I know
that MAC addresses are not in a packet. You must read the question for understanding of
what they are really asking.
7. A. Since the destination MAC address is different at each hop, it must keep changing.
The IP address which is used for the routing process does not.
8. B, E. Classful routing means that all hosts in the internetwork use the same mask and
that only default masks are in use. Classless routing means that you can use variable
length subnet masks (VLSMs).
9. B, C. The distance-vector routing protocol sends its complete routing table out of all
active interfaces at periodic time intervals. Link-state routing protocols send updates
containing the state of their own links to all routers in the internetwork.
1002 Appendix B u Answers to Review Questions
10. C. This is how most people see routers, and certainly they could do this type of plain
ol’ packet switching in 1990 when Cisco released their very rst router and trafc was
seriously slow, but not in today’s networks! This process involves looking up every des-
tination in the routing table and nding the exit interface for every packet.
11. A, C. The S* shows that this is a candidate for default route and that it was congured
manually.
12. B. RIP has an administrative distance (AD) of 120, while EIGRP has an administrative
distance of 90, so the router will discard any route with a higher
AD than 90 to that same network.
13. D. Recovery from a lost route requires manual intervention by a human to replace the
lost route.
14. A. RIPv1 and RIPv2 only use the lowest hop count to determine the best path to a
remote network.
15. A. Since the routing table shows no route to the 192.168.22.0 network, the router will
discard the packet and send an ICMP destination unreachable message out of interface
FastEthernet 0/0, which is the source LAN from which the packet originated.
16. C. Static routes have an administrative distance of 1 by default. Unless you change
this, a static route will always be used over any other dynamically learned route.
EIGRP has an administrative distance of 90, and RIP has an administrative distance
of 120, by default.
17. C. BGP is the only EGP listed.
18. D. Recovery from a lost route requires manual intervention by a human to replace
the lost route. The advantages are less overhead on the router and network, as well
as more security.
19. C. The show ip interface brief command displays a concise summary of the interfaces.
20. B. The 150 at the end changes the default administrative distance (AD) of 1 to 150.
Chapter 9: Open Shortest
Path First (OSPF)
1. B. Only the EIGRP routes will be placed in the routing table because it has the lowest
administrative distance (AD), and that is always used before metrics.
2. A, B, C. Any router that is member of two areas must be an area border router or ABR.
Chapter 9: Open Shortest Path First (OSPF) 1003
3. A, C. The process ID for OSPF on a router is only locally signicant and you can use
the same number on each router, or each router can have a different number—it just
doesn’t matter. The numbers you can use are from 1 to 65,535. Don’t get this confused
with area numbers, which can be from 0 to 4.2 billion.
4. B. The router ID (RID) is an IP address used to identify the router. It need not and
should not match.
5. C. The router ID (RID) is an IP address used to identify the router. Cisco chooses the
router ID by using the highest IP address of all congured loopback interfaces. If no
loopback interfaces are congured with addresses, OSPF will choose the highest IP
address of all active physical interfaces.
6. A. The administrator typed in the wrong wildcard mask conguration. The wildcard
should have been 0.0.0.255 or even 0.255.255.255.
7. A. A dash (-) in the State column indicates no DR election, because they are not
required on a point-to-point link such as a serial connection.
8. D. By default the administrative distance of OSPF is 110.
9. A. Hello packets are addressed to multicast address 224.0.0.5.
10. A. The show ip ospf neighbor command displays all interface-related neighbor infor-
mation. This output shows the DR and BDR (unless your router is the DR or BDR), the
RID of all directly connected neighbors and the IP address and name of the directly con-
nected interface.
11. A. 224.0.0.6 is used on broadcast networks to reach the DR and BDR.
12. D. The Hello and Dead timers must be set the same on two routers on the same link or
they will not form an adjacency (relationship). The default timers for OSPF are 10 sec-
onds for the Hello timer and 40 seconds for the Dead timer.
13.
Designated router contains only the best routes
Topological database elected on broadcast networks
Hello protocol contains all routes learned
Routing table provides dynamic neighbor discovery
A designated router is elected on broadcast networks. Each OSPF router maintains
an identical database describing the AS topology. A Hello protocol provides dynamic
neighbor discovery. A routing table contains only the best routes.
1004 Appendix B u Answers to Review Questions
14. passive-interface fastEthernet 0/1 The command passive-interface
fastEthernet 0/1 will disable OSPF on the specied interface only.
15. B, G. To enable OSPF, you must rst start OSPF using a process ID. The number is
irrelevant; just choose a number from 1 to 65,535 and you’re good to go. After you
start the OSPF process, you must congure interfaces on which to activate OSPF using
the network command with wildcards and specication of an area. Option F is wrong
because there must be a space after the parameter area and before you list the area
number.
16. A. The default OSPF interface priority is 1, and the highest interface priority determines
the designated router (DR) for a subnet. The output indicates that the router with a
router ID of 192.168.45.2 is currently the backup designated router (BDR) for the seg-
ment, which indicates that another router became the DR. It can be then be assumed that
the DR router has an interface priority higher than 2. (The router serving the DR func-
tion is not present in the truncated sample output.)
17. A, B, C. OSPF is created in a hierarchical design, not a at design like RIP. This
decreases routing overhead, speeds up convergence, and connes network instability to
a single area of the network.
18. show ip ospf interface The show ip ospf interface command displays all inter-
face-related OSPF information. Data is displayed about OSPF information for all
OSPF-enabled interfaces or for specied interfaces.
19. A. LSA packets are used to update and maintain the topological database.
20. B. At the moment of OSPF process startup, the highest IP address on any active inter-
face will be the router ID (RID) of the router. If you have a loopback interface cong-
ured (logical interface), then that will override the interface IP address and become the
RID of the router automatically.
Chapter 10: Layer 2 Switching
1. A. Layer 2 switches and bridges are faster than routers because they don’t take up time
looking at the Network Layer header information. They do make use of the Data Link
layer information.
2. mac address-table static aaaa.bbbb.cccc vlan 1 int fa0/7 You can set a static
MAC address in the MAC address table and when done it will appear as a static entry
in the table.
3. B, D, E. Since the MAC address is not present in the table, it will send the frame out of
all ports in the same VLAN with the exception of the port on which it was received.
Chapter 10: Layer 2 Switching 1005
4. show mac address-table This command displays the forward lter table, also called a
content addressable memory (CAM) table.
5.
Address learning
Packet forwarding
Layer three security
Forward/filter decisions
Loop avoidance
Address learning
Forward/filter decisions
Loop avoidance
The three functions are address learning, forward/filter decisions, and loop avoidance.
6. A, D. In the above output, you can see that the port is in Secure-shutdown mode and
the light for the port would be amber. To enable the port again you’d need to do the
following:
S3(config-if)#shutdown
S3(config-if)#no shutdown
7. switchport port-security maximum 2 The maximum setting of 2 means only two
MAC addresses can be used on that port; if the user tries to add another host on
that segment, the switch port will take the action specied. In the port-security
violation command.
8. B. The switchport port-security command enables port security, which is a prereq-
uisite for the other commands to function.
9. B. Gateway redundancy is not an issue addressed by STP.
10. A. If no loop avoidance schemes are put in place, the switches will ood broadcasts
endlessly throughout the internetwork. This is sometimes referred to
as a broadcast storm.
11. B, C. Shutdown and protect mode will alert you via SNMP that a violation has
occurred on a port.
12. Spanning tree protocol (STP) STP is a switching loop avoidance scheme use by
switches.
13. ip default-gateway If you want to manage your switches from outside your LAN,
you need to set a default gateway on the switches, just as you would with a host.
14. C. The IP address is congured under a logical interface, called a management domain
or VLAN 1.
1006 Appendix B u Answers to Review Questions
15. B. The show port-security interface command displays the current port security and
status of a switch port, as in this sample output:
Switch# show port-security interface fastethernet0/1
Port Security: Enabled
Port status: SecureUp
Violation mode: Shutdown
Maximum MAC Addresses: 2
Total MAC Addresses: 2
Configured MAC Addresses: 2
Aging Time: 30 mins
Aging Type: Inactivity
SecureStatic address aging: Enabled
Security Violation count: 0
16. switchport port-security mac-address sticky Issuing the switchport port-security
mac-address sticky command will allow a switch to save a dynamically learned MAC
address in the running-conguration of the switch, which prevents the administrator from
having to document or congure specic MAC addresses.
17. B, D. To limit connections to a specic host, you should congure the MAC address of
the host as a static entry associated with the port, although be aware that this host can
still connect to any other port, but no other port can connect to f0/3, in this example.
Another solution would be to congure port security to accept trafc only from the
MAC address of the host. By default, an unlimited number of MAC addresses can be
learned on a single switch port, whether it is congured as an access port or a trunk
port. Switch ports can be secured by dening one or more specic MAC addresses that
should be allowed to connect and by dening violation policies (such as disabling the
port) to be enacted if additional hosts try to gain a connection.
18. D. The command statically denes the MAC address of 00c0.35F0.8301 as an allowed
host on the switch port. By default, an unlimited number of MAC addresses can be
learned on a single switch port, whether it is congured as an access port or a trunk
port. Switch ports can be secured by dening one or more specic MAC addresses that
should be allowed to connect and violation policies (such as disabling the port) if addi-
tional hosts try to gain a connection.
19. D. You would not make the port a trunk. In this example, this switchport is a member
of one VLAN. However, you can congure port security on a trunk port, but again,
not valid for this question.
20. switchport port-security violation shutdown This command is used to set the
reaction of the switch to a port violation of shutdown.
Chapter 11: VLANs and InterVLAN Routing 1007
Chapter 11: VLANs and
InterVLAN Routing
1. D. Here’s a list of ways VLANs simplify network management:
u Network adds, moves, and changes are achieved with ease by just configuring a
port into the appropriate VLAN.
u A group of users that need an unusually high level of security can be put into its
own VLAN so that users outside of the VLAN can’t communicate with them.
u As a logical grouping of users by function, VLANs can be considered independent
from their physical or geographic locations.
u VLANs greatly enhance network security if implemented correctly.
u VLANs increase the number of broadcast domains while decreasing their size.
2. ip routing Routing must be enabled on the layer 3 switch.
3. C. VLANs can span across multiple switches by using trunk links, which carry trafc
for multiple VLANs.
4. B. While in all other cases access ports can be a member of only one VLAN, most
switches will allow you to add a second VLAN to an access port on a switch port for
your voice trafc; it’s called the voice VLAN. The voice VLAN used to be called the
auxiliary VLAN, which allowed it to be overlaid on top of the data VLAN, enabling
both types of trafc through the same port.
5. A. Yes, you have to do a no shutdown on the VLAN interface.
6. C. Unlike ISL which encapsulates the frame with control information, 802.1q inserts
an 802.1q eld along with tag control information.
7. D. Instead of using a router interface for each VLAN, you can use one FastEthernet
interface and run ISL or 802.1q trunking. This allows all VLANs to communicate
through one interface. Cisco calls this a “router on a stick.”
8. switchport access vlan 2 This command is executed under the interface (switch
port) that is being placed in the VLAN.
9. show vlan After you create the VLANs that you want, you can use the show vlan
command to check them out.
10. B. The encapsulation command specifying the VLAN for the subinterface must be
present under both subinterfaces.
1008 Appendix B u Answers to Review Questions
11. A. With a multilayer switch, enable IP routing and create one logical interface for each
VLAN using the interface vlan number command and you’re now doing inter-VLAN
routing on the backplane of the switch!
12. A. Ports Fa0/15–18 are not present in any VLANs. They are trunk ports.
13. C. Untagged frames are members of the native VLAN, which by default is VLAN 1.
14. sh interfaces fastEthernet 0/15 switchport This show interfaces interface
switchport command shows us the administrative mode of dynamic desirable and that
the port is a trunk port, DTP was used to negotiate the frame tagging method of ISL,
and the native VLAN is the default of 1.
15. C. VLANs are not location dependent and can span to multiple switches using trunk
links. Moreover, they can be subnets of major networks.
16. B. The host’s default gateway should be set to the IP address of the subinterface that is
associated with the VLAN of which the host is a member, in this case VLAN 2.
17. C. Frame tagging is used when VLAN trafc travels over a trunk link. Trunk links
carry frames for multiple VLANs. Therefore, frame tags are used for identication of
frames from different VLANs.
18. vlan 2 To congure VLANs on a Cisco Catalyst switch, use the global cong vlan
command.
19. B. 802.1q uses the native VLAN.
20. switchport nonegotiate You can use this command only when the interface switch-
port mode is access or trunk. You must manually congure the neighboring interface
as a trunk interface to establish a trunk link.
Chapter 12: Security
1. D. It’s compared with lines of the access list only until a match is made. Once the
packet matches the condition on a line of the access list, the packet is acted upon and
no further comparisons take place.
2. C. The range of 192.168.160.0 to 192.168.191.0 is a block size of 32. The network
address is 192.168.160.0 and the mask would be 255.255.224.0, which for an access
list must be a wildcard format of 0.0.31.255. The 31 is used for a block size of 32.
The wildcard is always one less than the block size.
3. C. Using a named access list just replaces the number used when applying the list to the
router’s interface. ip access-group Blocksales in is correct.
Chapter 12: Security 1009
4. B. The list must specify TCP as the Transport layer protocol and use a correct wild-
card mask (in this case 0.0.0.255), and it must specify the destination port (80). It also
should specify all as the set of computers allowed to have this access.
5. A. The rst thing to check in a question like this is the access-list number. Right away,
you can see that the second option is wrong because it is using a standard IP access-list
number. The second thing to check is the protocol. If you are ltering by upper-layer
protocol, then you must be using either UDP or TCP; this eliminates the fourth option.
The third and last answers have the wrong syntax.
6. C. Of the available choices, only the show ip interface command will tell you which
interfaces have access lists applied. show access-lists will not show you which inter-
faces have an access list applied.
7.
show access-list Shows only the parameters for the access list 110.
This command does not show you the interface the list
is set on.
show access-list 110 Shows only the IP access lists configured on the router.
show ip access-list Shows which interfaces have acces lists set.
show ip interface Displays all access lists and their parameters configured
on the router. This command does not show you which
interface the list is set on.
The command show access-list displays all access lists and their parameters congured
on the router; it does not show you which interface the list is set on. show access-list
110 shows only the parameters for the access list 110 and, again, does not tell you which
interface the list is set on. show ip access-list reveals only the IP access lists congured
on the router. Finally, show ip interface shows which interfaces have access lists set.
The functions of each command are as shown in the solution graphic.
8. C. The extended access list ranges are 100–199 and 2000–2699, so the access-list
number of 100 is valid. Telnet uses TCP, so the protocol TCP is valid. Now you just
need to look for the source and destination address. Only the third option has the cor-
rect sequence of parameters. Option B may work, but the question specically states
“only” to network 192.168.10.0, and the wildcard in option B is too broad.
9. D. Extended IP access lists use numbers 100–199 and 2000–2699 and lter based on
source and destination IP address, protocol number, and port number. The last option
is correct because of the second line that species permit ip any any. (I used 0.0.0.0
255.255.255.255, which is the same as the any option.) The third option does not have
this, so it would deny access but not allow everything else.
1010 Appendix B u Answers to Review Questions
10. D. First, you must know that a /20 is 255.255.240.0, which is a block size of 16 in
the third octet. Counting by 16s, this makes our subnet 48 in the third octet, and the
wildcard for the third octet would be 15 since the wildcard is always one less than the
block size.
11. B. To nd the wildcard (inverse) version of this mask, the zero and one bits are simply
reversed as follows:
11111111.11111111.11111111.11100000 (27 one bits, or /27)
00000000.00000000.00000000.00011111 (wildcard/inverse mask)
12. A. First, you must know that a /19 is 255.255.224.0, which is a block size of 32 in the
third octet. Counting by 32s, this makes our subnet 192 in the third octet, and the
wildcard for the third octet would be 31 since the wildcard is always one less than the
block size.
13. B. The scope of an access list is determined by the wildcard mask and the network
address to which it is applied. For example, in this case the starting point of the
list of addresses affected by the mask is the network ID 192.111.16.32. The wild-
card mask is 0.0.0.31. Adding the value of the last octet in the mask to the network
address (32 + 31 = 63) tells you where the effects of the access list ends, which is
192.111.16.63. Therefore, all addresses in the range 192.111.16.32–192.111.16.63
will be denied by this list.
14. C. To place an access list on an interface, use the ip access-group command in inter-
face conguration mode.
15. B. With no permit statement, the ACL will deny all trafc.
16. D. If you add an access list to an interface and you do not have at least one permit state-
ment, then you will affectively shut down the interface because of the implicit deny any
at the end of every list.
17. C. Telnet access to the router is restricted by using either a standard or extended IP
access list inbound on the VTY lines of the router. The command access-class is used
to apply the access list to the VTY lines.
18. C. A Cisco router has rules regarding the placement of access lists on a router interface.
You can place one access list per direction for each layer 3 protocol congured on an
interface.
19. C. The most common attack on a network today is a denial of service (DoS) because
they are the easiest attack to achieve.
20. C. Implementing intrusion detection services and intrusion prevention services will
help notify you and stop attacks in real time.
Chapter 13: Network Address Translation 1011
Chapter 13: Network Address Translation
1. A, C, E. NAT is not perfect and can cause some issues in some networks, but most net-
works work just ne. NAT can cause delays and troubleshooting problems, and some
applications just won’t work.
2. B, D, F. NAT is not perfect, but there are some advantages. It conserves global
addresses, which allow us to add millions of hosts to the Internet without “real” IP
addresses. This provides exibility in our corporate networks. NAT can also allow
you to use the same subnet more than once in the same network without overlapping
networks.
3. C. The command debug ip nat will show you in real time the translations occurring
on your router.
4. A. The command show ip nat translations will show you the translation table con-
taining all the active NAT entries.
5. D. The command clear ip nat translations * will clear all the active NAT entries
in your translation table.
6. B. The show ip nat statistics command displays a summary of the NAT congura-
tion as well as counts of active translation types, hits to an existing mapping, misses
(causing an attempt to create a mapping), and expired translations.
7. B. The command ip nat pool name creates the pool that hosts can use to get onto the
global Internet. What makes option B correct is that the range 171.16.10.65 through
171.16.10.94 includes 30 hosts, but the mask has to match 30 hosts as well, and that
mask is 255.255.255.224. Option C is wrong because there is a lowercase t
in the pool name. Pool names are case sensitive.
8. A, C, E. You can congure NAT three ways on a Cisco router: static, dynamic, and
NAT Overload (PAT).
9. B. Instead of the netmask command, you can use the prefix-length length statement.
10. C. In order for NAT to provide translation services, you must have ip nat inside and
ip nat outside congured on your router’s interfaces.
11. A, B, D. The most popular use of NAT is if you want to connect to the Internet and
you don’t want hosts to have global (real) IP addresses, but options B and D are correct
as well.
12. C. An inside global address is considered to be the IP address of the host on the private
network after translation.
1012 Appendix B u Answers to Review Questions
13. A. An inside local address is considered to be the IP address of the host on the private
network before translation.
14. D. What we need to gure out for this question is only the inside global pool. Basically
we start at 1.1.128.1 and end at 1.1.135.174; our block size is 8 in the third octet, or
/21. Always look for your block size and the interesting octet and you can nd your
answer every time.
15. B. Once you create your pool, the command ip nat inside source must be used to
say which inside locals are allowed to use the pool. In this question we need to see if
access-list 100 is congured correctly, if at all, so show access-list is the best answer.
16. A. You must congure your interfaces before NAT will provide any translations. On the
inside network interfaces, you would use the command ip nat inside. On the outside
network interfaces, you will use the command ip nat outside.
17. B. You must congure your interfaces before NAT will provide any translations. On
the inside networks you would use the command ip nat inside. On the outside net-
work interfaces, you will use the command ip nat outside.
18 . C. Another term for Port Address Translation is NAT Overloa d because that is the keyword
used to enable port address translation.
19. B. Fast-switching is used on Cisco routers to create a type of route cache in order to
quickly forward packets through a router without having to parse the routing table for
every packet. As packets are processed-switched (looked up in the routing table), this
information is stored in the cache for later use if needed for faster routing processing.
20. B. Once you create a pool for the inside locals to use to get out to the global Internet,
you must congure the command to allow them access to the pool. The ip nat inside
source list number pool-name overload command has the correct sequence for this
question.
Chapter 14: Internet Protocol
Version 6 (IPv6)
1. B. Unlike unicast addresses, global unicast addresses are meant to be routed.
2. A. Packets addressed to a unicast address are delivered to a single interface. For load
balancing, multiple interfaces can use the same address.
3. C. Link-local addresses are meant for throwing together a temporary LAN for meet-
ings or a small LAN that is not going to be routed but needs to share and access les
and services locally.
Chapter 14: Internet Protocol Version 6 (IPv6) 1013
4. D. These addresses are meant for nonrouting purposes like link-local, but they are
almost globally unique, so it is unlikely they will have an address overlap. Unique local
addresses were designed as a replacement for site-local addresses.
5. B. Packets addressed to a multicast address are delivered to all interfaces identied with
the multicast address, the same as in IPv4. It is also called a one-to-many address. You
can always tell a multicast address in IPv6 because multicast addresses always start
with FF.
6. C. Anycast addresses identify multiple interfaces, which is somewhat similar to multicast
addresses; however, the big difference is that the anycast packet is only delivered to one
address, the rst one it nds dened in the terms of routing distance. This address can
also be called one-to-one-of-many, or one-to-nearest.
7. C. The loopback address with IPv4 is 127.0.0.1. With IPv6, that address is ::1.
8. C, D. Adjacencies and next-hop attributes now use link-local addresses, and OSPFv3
still uses multicast trafc to send its updates and acknowledgments with the addresses
FF02::5 for OSPF routers and FF02::6 for OSPF designated routers. These are the
replacements for 224.0.0.5 and 224.0.0.6, respectively.
9. A, C. Host A would send an RS to R1, which would respond with an RA, if the host
hasn’t already learned this information previously. The host now has the default gate-
way information it needs to send packets to a remote network. IPv6, like IPv4, has
both a network portion and host portion in the IPv6 packet.
10. C. A router solicitation is sent out using the all-routers multicast address of FF02::2.
The router can send a router advertisement to all hosts using the FF02::1 multicast
address.
11. B. This can be a hard question if you don’t remember to invert the 7th bit! Always
look for the 7th bit when studying for the Cisco exams. The EUI-64 autocongura-
tion inserts an FF:FE in the middle of the 48-bit MAC address to create a unique IPv6
address.
12. A. The NDP neighbor advertisement (NA) contains the MAC address. A neighbor
solicitation (NS) was initially sent asking for the MAC address.
13. A. To enable OSPFv3, you enable the protocol at the interface level as with RIPng. The
command string is ipv6 ospf process-id area a re a -id .
14. B, D. To shorten the written length of an IPv6 address, successive elds of zeros may be
replaced by double colons. In trying to shorten the address further, leading zeros may
also be removed. Just as with IPv4, a single device’s interface can have more than one
address; with IPv6 there are more types of addresses and the same rule applies. There
can be link-local, global unicast, multicast, and anycast addresses all assigned to the
same interface.
1014 Appendix B u Answers to Review Questions
15. C, D. IPv4 addresses are 32 bits long and are represented in decimal format. IPv6
addresses are 128 bits long and represented in hexadecimal format.
16. B. There are no broadcasts with IPv6. Unicast, multicast, anycast, global, and link-
local unicast are used.
17. D. There are 16 bits (four hex characters) in an IPv6 eld.
18. A, D. Global addresses start with 2000::/3, link-locals start with FE80::/10, loopback
is ::1, and unspecied is just two colons (::). Each interface will have a loopback address
automatically congured.
19. B, C. If you verify your IP conguration on your host, you’ll see that you have mul-
tiple IPv6 addresses, including a loopback address. The last 64 bits represent the
dynamically created interface ID, and leading zeros are not mandatory in a 16-bit
IPv6 eld.
20. C, D, E. OSPFv2 does not use the network command under global conguration mode,
nor does it use wildcard masks as IPv4 does. However, they can both use the interface
command to congure OSPF, use a 32-bit RID, and both use LSAs.
Chapter 15: Enhanced Switched
Technologies
1. B, D. The switch is not the root bridge for VLAN 1 or the output would tell us exactly
that. We can see that the root bridge for VLAN 1 is off of interface G1/2 with a cost of
4, meaning it is directly connected. Use the command show cdp nei to nd your root
bridge at this point. Also, the switch is running RSTP (802.1d), not STP.
2. D. If you have a server or other devices connected into your switch that you’re totally
sure won’t create a switching loop if STP is disabled, you can use something called
PortFast on these ports. Using it means the port won’t spend the usual 50 seconds to
come up while STP is converging. Understand this doesn’t completely disable STP, just
stops any port congured with PortFast from transitioning the different port states,
and immediately puts the port into forwarding state.
3. A, D. It is important that you can nd your root bridge, and the show spanning-tree
command will help you do this. To quickly nd out which VLANs your switch is the
root bridge for, use the show spanning-tree summary command.
4. A. 802.1w is the also called Rapid Spanning Tree Protocol. It is not enabled by default
on Cisco switches, but it is a better STP to run because it has all the xes that the Cisco
extensions provide with 802.1d. Remember, Cisco runs RSTP PVST+, not just RSTP.
Chapter 15: Enhanced Switched Technologies 1015
5. B. The Spanning Tree Protocol is used to stop switching loops in a layer 2 switched
network with redundant paths.
6. C. Convergence occurs when all ports on bridges and switches have transitioned to
either the forwarding or blocking states. No data is forwarded until convergence is
complete. Before data can be forwarded again, all devices must be updated.
7. C, E. There are two types of EtherChannel: Cisco’s PAgP and the IEEE’s LACP. They
are basically the same, and there is little difference to conguring them. For PAgP,
use auto or desirable mode, and with LACP use passive or active. These modes decide
which method you are using, and they must be congured the same on both sides of
the EtherChannel bundle.
8. A, B, F. RSTP helps with convergence issues that plague traditional STP. Rapid PVST+
is based on the 802.1w standard in the same way that PVST+ is based on 802.1d. The
operation of Rapid PVST+ is simply a separate instance of 802.1w for each VLAN.
9. D. BPDU Guard is used when a port is congured for PortFast, or it should be used,
because if that port receives a BPDU from another switch, BPDU Guard will shut that
port down to stop a loop from occurring.
10. C. To allow for the PVST+ to operate, there’s a eld inserted into the BPDU to accom-
modate the extended system ID so that PVST+ can have a root bridge congured on
a per-STP instance. The extended system ID (VLAN ID) is a 12-bit eld, and we can
even see what this eld is carrying via the show spanning-tree command output.
11. C. PortFast and BPDU Guard allow a port to transition to the forwarding state quickly,
which is great for a switch port but not for load balancing. You can somewhat load bal-
ance with RSTP, but that is out of the scope of our objectives, and although you can use
PPP to congure multilink (bundle links), this is performed on asynchronous or synchro-
nous serial links. Cisco’s EtherChannel can bundle up to eight ports between switches.
12. D. If the Spanning Tree Protocol is not running on your switches and you connect them
together with redundant links, you will have broadcast storms and multiple frame copies;
you will have broadcast storms and multiple frame copies being received by the same
destination device..
13. B, C, E. All the ports on both sides of every link must be congured exactly the same
or it will not work. Speed, duplex, and allowed VLANs must match.
14. D, F. There are two types of EtherChannel: Cisco’s PAgP and the IEEE’s LACP. They
are basically the same, and there is little difference to congure them. For PAgP, use the
auto or desirable mode, and with LACP use the passive or active mode. These modes
decide which method you are using, and they must be congured the same on both sides
of the EtherChannel bundle.
15. D. You can’t answer this question if you don’t know who the root bridge is. SC has
a bridge priority of 4,096, so that is the root bridge. The cost for SB was 4, with the
1016 Appendix B u Answers to Review Questions
direct link, but that link went down. If SB goes through SA to SC, the cost would be
4 + 19, or 23. If SB goes to SA to SD to SC, the cost is 4 + 4 + 4 = 12.
16. A, D. To congure EtherChannel, create the port channel from global conguration
mode, and then assign the group number on each interface using the active mode to
enable LACP, although just conguring the channel-group command under your inter-
faces will enable the bundle, but options A and D are the best Cisco objective answers.
17. A, D. You can set the priority to any value from 0 through 61,440 in increments of
4,096. Setting it to zero (0) means that the switch will always be a root as long as it has
a lower MAC than another switch with its bridge ID also set to 0. You can also force a
switch to be a root for a VLAN with the spanning-tree vlan vlan primary command.
18. A. By using per-VLAN spanning tree, the root bridge can be placed in the center
of where all the resources are for a particular VLAN, which enables optimal path
determination.
19. A, C, D, E. Each 802.1d port transitions through blocking, listening, learning, and
nally forwarding after 50 seconds, by default. RSTP uses discarding, learning,
and forwarding only.
20. A, C, D, E, F. The roles a switch port can play in STP are root, non-root, designated,
non-designated, forwarding, and blocking. Discarding is used in RSTP, and disabled
could be a role, but it’s not listed as a possible answer.
Chapter 16: Managing Cisco Devices
1. B. The default conguration setting is 0x2102, which tells the router to load the IOS
from ash and the conguration from NVRAM. 0x2142 tells the router to bypass the
conguration in NVRAM so that you can perform password recovery.
2. E. To copy the IOS to a backup host, which is stored in ash memory by default, use
the copy flash tftp command.
3. B. To install a new license on an ISR G2 router, use the license install url command.
4. C. The conguration register provides the boot commands, and 0x2101 tells the router to
boot the mini-IOS, if found, and not to load a le from ash memory. Many newer rout-
ers do not have a mini-IOS, so as an alternative, the router would end up in ROM monitor
mode if the mini-IOS is not found. However, option C is the best answer for this question.
5. B. The show flash command will provide you with the current IOS name and size and
the size of ash memory.
6. C. Before you start to congure the router, you should erase the NVRAM with the erase
startup-config command and then reload the router using the reload command.
Chapter 16: Managing Cisco Devices 1017
7. D. The command copy tftp flash will allow you to copy a new IOS into ash memory
on your router.
8. C. The best answer is show version, which shows you the IOS le running currently
on your router. The show flash command shows you the contents of ash memory, not
which le is running.
9. C. All Cisco routers have a default conguration register setting of 0x2102, which tells
the router to load the IOS from ash memory and the conguration from NVRAM.
10. C. If you save a conguration and reload the router and it comes up either in setup
mode or as a blank conguration, chances are the conguration register setting is
incorrect.
11. D. The License boot module command installs a Right-To-Use license feature on
a router.
12. A. The show license command determines the licenses that are active on your system.
It also displays a group of lines for each feature in the currently running IOS image
along with several status variables related to software activation and licensing, both
licensed and unlicensed features.
13. B. The show license feature command allows you to view the technology package
licenses and feature licenses that are supported on your router along with several status vari-
ables related to software activation and licensing, both licensed and unlicensed features.
14. C. The show license udi command displays the unique device identier (UDI) of the
router, which comprises the product ID (PID) and serial number of the router.
15. D. The show version command displays various information about the current IOS
version, including the licensing details at the end of the command’s output.
16. C. The license save flash command allows you to back up your license to
ash memory.
17. C. The show version command provides you with the current conguration
register setting.
18. C, D. The two steps to remove a license are to rst disable the technology package and
then clear the license.
19. B, D, E. Before you back up an IOS image to a laptop directly connected to a router’s
Ethernet port, make sure that the TFTP server software is running on your laptop, that
the Ethernet cable is a “crossover,” and that the laptop is in the same subnet as the router’s
Ethernet port, and then you can use the copy flash tftp command from your laptop.
20. C. The default conguration setting of 0x2102 tells the router to look in NVRAM for
the boot sequence.
1018 Appendix B u Answers to Review Questions
Chapter 17: IP Services
1. B. You can create and then enter the ACL into the snmp-server command when setting
your SNMP conguration on a router.
2. C. GLBP is a Cisco proprietary FHRP that can provide true load-balancing. Instead of
having active and passive routers, all routers on a LAN can be forwarders with GLBP.
3. C. The default priority is 100, and if you change the priority to a higher setting, you
provide the router with a better chance of being active, but only if the FHRP congured
routers come up at the same time.
4. A. The command service sequence-numbers adds a sequence number to each system
message.
5. C. With NetFlow, you can collect information about the ow of trafc on the network,
including address and protocols in use. A recent version of NetFlow is version 9, referred
to as Flexible NetFlow.
6. F. There are eight different trap levels. If you choose level 4, level 0 through 4 will be
displayed, giving you emergency, alert, critical, and error messages as well as warning
messages. Here are the levels and what they perform.
Router (config) #logging trap ?
<0-7>
alerts
critical
debugging
emergencies
errors
informational
notifications
warnings
Logging severity level
Immediate action needed
Critical conditions
Debugging messages
System is unusable
Error conditions
Informational messages
Normal but significant conditions
Warning conditions
(severity=1)
(severity=2)
(severity=7)
(severity=0)
(severity=3)
(severity=6)
(severity=5)
(severity=4)
7. C. HSRP can keep track of outside interfaces to switch routers from passive to active if
the WAN on one router goes down, for example.
8. B. The show ip cache flow command provide a summary of the NetFlow statistics,
including which protocols are in use.
9. B, C, D. NetFlow, SNMP, and syslog are all different applications that provide dif-
ferent features for administrating your network. NetFlow is a very powerful Cisco
tool that allows you to see who is doing what, understand network capacity, and even
gather accounting information.
Chapter 17: IP Services 1019
10. D. There are eight different trap levels. If you choose level 5, level 0 through 5 will be
displayed, giving you emergency, alert, critical, error, and warning messages as well as
notication messages. Here are the levels and what they perform.
Router (config) #logging trap ?
<0-7>
alerts
critical
debugging
emergencies
errors
informational
notifications
warnings
Logging severity level
Immediate action needed
Critical conditions
Debugging messages
System is unusable
Error conditions
Informational messages
Normal but significant conditions
Warning conditions
(severity=1)
(severity=2)
(severity=7)
(severity=0)
(severity=3)
(severity=6)
(severity=5)
(severity=4)
11. B, C. There is a logging host command congured to 172.16.10.1, so all messages will
be sent there, but don’t forget that all messages are always sent to the memory buffers
on a router as well. Debugging is trap level 7.
12. B, D. You need to have a Cisco router to run the proprietary NetFlow application, plus
you need a server to send the information to, and this is called a collector.
13. B. There are eight different trap levels. If you choose level 3, level 0 through 3 will be
displayed, giving you emergency, alert, and critical messages as well as error messages.
Here are the levels and what they perform.
Router (config) #logging trap ?
<0-7>
alerts
critical
debugging
emergencies
errors
informational
notifications
warnings
Logging severity level
Immediate action needed
Critical conditions
Debugging messages
System is unusable
Error conditions
Informational messages
Normal but significant conditions
Warning conditions
(severity=1)
(severity=2)
(severity=7)
(severity=0)
(severity=3)
(severity=6)
(severity=5)
(severity=4)
14. C. The active virtual router (AVR) responds with the virtual MAC addresses to the clients.
15. B. With HSRP, the command is show standby; with GLBP it is simply show glbp, or
even show glbp brief.
1020 Appendix B u Answers to Review Questions
16. C. There are eight different trap levels. If you choose level 7, level 0 through 7 will be
displayed, giving you emergency, alert, critical, error, warning, notication, and informa-
tional messages as well as debugging messages. Here are the levels and what they perform.
Router (config) #logging trap ?
<0-7>
alerts
critical
debugging
emergencies
errors
informational
notifications
warnings
Logging severity level
Immediate action needed
Critical conditions
Debugging messages
System is unusable
Error conditions
Informational messages
Normal but significant conditions
Warning conditions
(severity=1)
(severity=2)
(severity=7)
(severity=0)
(severity=3)
(severity=6)
(severity=5)
(severity=4)
17. C. HSRP is not true load balancing, but it does provide more than one active router in
only one situation; that is, if you have your router interface connected to a trunk link
and then congured with subinterfaces, no different than router on a stick. However,
only one router can be active for a LAN, unlike with GLBP.
18. G. There are eight different trap levels. If you choose level 6, level 0 through 6 will be
displayed, giving you emergency, alert, critical, error, warning, and notication messages
as well as informational messages. Here are the levels and what they perform.
Router (config) #logging trap ?
<0-7>
alerts
critical
debugging
emergencies
errors
informational
notifications
warnings
Logging severity level
Immediate action needed
Critical conditions
Debugging messages
System is unusable
Error conditions
Informational messages
Normal but significant conditions
Warning conditions
(severity=1)
(severity=2)
(severity=7)
(severity=0)
(severity=3)
(severity=6)
(severity=5)
(severity=4)
19. C. SNMPv3 supports strong authentication with MD5 or SHA, providing condential-
ity (encryption) and data integrity of messages via DES or DES-256 encryption between
agents and managers.
20. C, D. Both HSRP and GLBP are Cisco proprietary FHRPs, but only GLBP can load-
balance between multiple routers on the same LAN.
Chapter 18: Troubleshooting IP, IPv6, andVLANs 1021
Chapter 18: Troubleshooting IP, IPv6,
andVLANs
1. D. Positive conrmation has been received conrming that the path to the neighbor is
functioning correctly. REACH is good!
2. B. The most common cause of interface errors is a mismatched duplex mode between
two ends of an Ethernet link. If they have mismatched duplex settings, you’ll receive a
legion of errors, which cause nasty slow performance issues, intermittent connectivity,
and massive collisions—even total loss of communication!
3. D. You can verify the DTP status of an interface with the sh dtp interface interface
command.
4. A. No DTP frames are generated from the interface. Nonegotiate can be used only if
the neighbor interface is manually set as trunk or access.
5. D. The command show ipv6 neighbors provides the ARP cache on a router.
6. B. The state is STALE when the interface has not communicated within the neighbor
reachable time frame. The next time the neighbor communicates, the state will change
back to REACH.
7. B. There is no IPv6 default gateway, which will be the link-local address of the
router interface, sent to the host as a router advertisement. Until this host receives
the router address, the host will communicate with IPv6 only on the local subnet.
8. D. This host is using IPv4 to communicate on the network, and without an IPv6 global
address, the host will be able to communicate to only remote networks with IPv4. The
IPv4 address and default gateway are not congured into the same subnet.
9. B, C. The commands show interface trunk and show interface interface switch-
port will show you statistics of ports, which includes native VLAN information.
10. A. Most Cisco switches ship with a default port mode of auto, meaning that they will
automatically trunk if they connect to a port that is on or desirable. Remember that
not all switches are shipped as mode auto, but many are, and you need to set one side
to either on or desirable in order to trunk between switches.
1022 Appendix B u Answers to Review Questions
Chapter 19: Enhanced IGRP
1. B. Only the EIGRP routes will be placed in the routing table because it has the lowest
administrative distance (AD), and that is always used before metrics.
2. A, C. EIGRP maintains three tables in RAM: neighbor, topology, and routing. The neigh-
bor and topology tables are built and maintained with the use of Hello and update packets.
3. B. EIGRP does use reported distance, or advertised distance (AD), to tell neighbor
routers the cost to get to a remote network. This router will send the FD to the neigh-
bor router and the neighbor router will add the cost to get to this router plus the AD
to nd the true FD.
4. E. Successor routes are going to be in the routing table since they are the best path to a
remote network. However, the topology table has a link to each and every network, so the
best answer is topology table and routing table. Any secondary route to a remote network
is considered a feasible successor, and those routes are found only in the topology table
and used as backup routes in case of primary route failure.
5. C. Any secondary route to a remote network is considered a feasible successor, and
those routes are found only in the topology table and used as backup routes in case
of primary route failure. You can see the topology table with the show ip eigrp
topology command.
6. B, C, E. EIGRP and EIGRPv6 routers can use the same RID, unlike OSPF, and this
can be set with the eigrp router-id command. Also a variance can be set to provide
unequal-cost load balancing, along with the maximum-paths command to set the amount
of load-balanced paths.
7. C. There were two successor routes, so by default, EIGRP was load-balancing out s0/0
and s0/1. When s0/0 goes down, EIGRP will just keep forwarding trafc out the second
link s0/1. s0/0 will be removed from the routing table.
8. D. To enable EIGRPv6 on a router interface, use the command ipv6 eigrp as on
individual interfaces that will be part of the EIGRPv6 process.
9. C. The path to network 10.10.50.0 out serial0/0 is more than two times the current FD, so
I used a variance 3 command to load-balance unequal-cost links three times the FD.
10. B, C. First, a maximum hop count of 16 only is associated with RIP, and EIGRP never
broadcasts, so we can eliminate A and D as options. Feasible successors are backup
routes and stored in the topology table, so that is correct, and if no feasible successor is
located, the EIGRP will ood its neighbors asking for a new path to network 10.10.10.0.
11. D. The show ip eigrp neighbors command allows you to check the IP addresses as
well as the retransmit interval and queue counts for the neighbors that have established
an adjacency.
Chapter 19: Enhanced IGRP 1023
12. C, E. For EIGRP to form an adjacency with a neighbor, the AS numbers must match,
and the metric K-values must match as well. Also, option F could cause the problem;
we can see if it is causing a problem from the output given.
13. A, D. Successor routes are the routes picked from the topology table as the best route
to a remote network, so these are the routes that IP uses in the routing table to forward
trafc to a remote destination. The topology table contains any route that is not as good
as the successor route and is considered a feasible successor, or backup route. Remember
that all routes are in the topology table, even successor routes.
14. A, B. Option A will work because the router will change the network statement to
10.0.0.0 since EIGRP uses classful addresses by default. Therefore, it isn’t techni-
cally a wrong answer, but please understand why it is correct for this question. The
10.255.255.64/27 subnet address can be congured with wildcards just as we use with
OSPF and ACLs. The /27 is a block of 32, so the wildcard in the fourth octet will be 31.
The wildcard of 0.0.0.0 is wrong because this is a network address, not a host address,
and the 0.0.0.15 is wrong because that is only a block of 16 and would only work if the
mask was a /28.
15. C. To troubleshoot adjacencies, you need to check the AS numbers, the K-values,
networks, passive interfaces, and ACLs.
16. C. EIGRP and EIGRPv6 will load-balance across 4 equal cost-paths by default but can
be congured to load-balance across equal- and unequal-cost paths, up to 32 with IOS
15.0 code.
17. B, E. EIGRP must be enabled with an AS number from global conguration mode
with the ipv6 router eigrp as command if you need to set the RID or other global
parameters. Instead of conguring EIGRP with the network command as with EIGRP,
EIGRPv6 is congured on a per-interface basis with the ipv6 eigrp as command.
18. C. There isn’t a lot to go on from with the output, but that might make this easier than
if there were a whole page of output. Since s0/0/2 has lowest FD and AD, that would
become the successor route. For a route to become a feasible successor, its reported dis-
tance must be lower than the feasible distance of the current successor route, so C
is our best answer based on what we can see.
19. C. The network in the diagram is considered a discontiguous network because you have
one classful address subnetted and separated by another classful address. Only RIPv2,
OSPF, and EIGRP can work with discontiguous networks, but RIPv2 and EIGRP won’t
work by default (except for routers running the new 15.0 code). You must use the no
auto-summary command under the routing protocol conguration. There is a passive
interface on RouterB, but this is not on an interface between RouterA and RouterB and
won’t stop an adjacency.
1024 Appendix B u Answers to Review Questions
20. A, B, C, D. Here are the documented steps that Cisco says to check when you have an
adjacency issue:
u Interfaces between the devices are down.
u The two routers have mismatching EIGRP autonomous system numbers.
u Proper interfaces are not enabled for the EIGRP process.
u An interface is configured as passive.
u K values are mismatched.
u EIGRP authentication is misconfigured.
Chapter 20: Multi-Area OSPF
1. A, B, D. As the size of a single-area OSPF network grows, so does the size of the routing
table and OSPF database that have to be maintained. Also, if there is a change in network
topology, the OSPF algorithm has to be rerun for the entire network.
2. B. An autonomous system boundary router (ASBR) is any OSPF router that is connected
to an external routing process (another AS). An ABR, on the other hand, connects one
(or more) OSPF areas together to area 0.
3. B, D, E. In order for two OSPF routers to create an adjacency, the hello and dead tim-
ers must match, and they must both be congured into the same area as well as being
in the same subnet. Also, if authentication is congured, that info must match as well.
4. C. The process starts by sending out Hello packets. Every listening router will then add
the originating router to the neighbor database. The responding routers will reply with
all of their Hello information so that the originating router can add them to its own
neighbor table. At this point, we will have reached the 2WAY state—only certain routers
will advance beyond this to establish adjacencies.
5. D. If you have multiple links to the same network, you can change the default cost of a
link so OSPF will prefer that link over another with the ip ospf cost cost command.
6. B. In the FULL state, all LSA information is synchronized among adjacent neighbors.
OSPF routing can begin only after the FULL state has been reached. The FULL state
occurs after the LOADING state nishes.
7. B, D, E. Conguring OSPFv3 is pretty simple, as long as you know what interfaces you
are using on your router. There are no network statements, OSPFv3 is congured on a
per-interface basis. OSPFv2 and OSPFv3 both use a 32-bit RID, have an IPv4 address
congured on at least one interface, you do not need to manually set a RID when con-
guring EIGRPv3.
Chapter 20: Multi-Area OSPF 1025
8. A. Cisco uses a simple calculation of 108/bandwidth of a link to determine the cost.
This means anything that is 100 Mbps or higher will have a default cost of 1.
9. B. Referred to as a network link advertisement (NLA), Type 2 LSAs are generated by
designated routers (DRs). Remember that a designated router is elected to represent
other routers in its network, and it establishes adjacencies with them. The DR uses a
Type 2 LSA to send out information about the state of other routers that are part of
the same network.
10. C. Referred to as summary link advertisements (SLAs), Type 3 LSAs are generated by
area border routers. These ABRs send Type 3 LSAs toward the area external to the
one where they were generated. The Type 3 LSA advertises networks, and these LSAs
advertise inter-area routes to the backbone area (area 0).
11. D. To see all LSAs a router has learned from its neighbors, you need to see the OSPF
LSDB, and you can see this with the show ip ospf database command.
12. B. Based on the information in the question, the cost from R1 to R2 is 4, the cost from
R2 to R3 is 15, and the cost from R3 to R5 is 4. 15 + 4 + 4 = 23. Pretty simple.
13. B, D. Since R3 is connected to area 1 and R1 is connected to area 2 and area 0, the
routes advertised from R3 would show as OI, or inter-area routes.
14. A, D, E, F. For two OSPF routers to form an adjacency, they must be in the same area,
must be in the same subnet, and must have authentication information match, if con-
gured. You need to also check if an ACL is set and if a passive interface is congured.
15. C. The IOS command show ip ospf neighbor shows neighbor router information,
such as neighbor ID and the state of adjacency with the neighboring router.
16. D. The command show ip ospf interface on a default broadcast multi-access network
will show you DRs and BDRs on that network.
17. A, C, D, F. It’s hard to tell from this single output what is causing the problem with
the adjacency, but we need to check the ACL 10 to see what that is doing, verify
that the routers are in the same area and in the same subnet, and see if that passive
interface is congured with the interface we’re using.
18. B, D, G. The default reference bandwidth is 100 by default, and you can change it
under the OSPF process with the auto-cost reference bandwidth number command,
but if you do, you need to congure this command on all routers in your AS.
19. A, D. Anything Fast Ethernet or above would have a default cost of 1, and serial interfaces
have a default cost of 64.
20. D. Anything Fast Ethernet or above would have a default cost of 1, and serial interfaces
have a default cost of 64.
1026 Appendix B u Answers to Review Questions
Chapter 21: Wide Area Networks
1. C. The command debug ppp authentication will show you the authentication process
that PPP uses across point-to-point connections.
2. B. If you have a router in your Frame Relay network that does not support IARP,
you must create Frame Relay maps on your router, which provide known DLCI-to-
IP-address mappings.
3. C. The key is “there are no free ports” on your router. Only Frame Relay can provide a
connection to multiple locations with one interface, and in an economical manner no less.
4. C, D, E. The show frame-relay ? command provides many options, but the options
available in this question are lmi, pvc, and map.
5. A. If you have a serial port congured with multiple DLCIs connected to multiple remote
sites, split horizon rules (discussed in Chapter 5) stop route updates received on an inter-
face from being sent out the same interface. By creating subinterfaces for each PVC, you
can avoid the split horizon issues when using Frame Relay.
6. C, D, E. Ethernet and Token Ring are LAN technologies and cannot be congured on
a serial interface. PPP, HDLC, and Frame Relay are layer 2 WAN technologies that are
typically congured on a serial interface.
7. C. It is very important to remember when studying the CCNA R/S exam objectives,
and when conguring Frame Relay with point-to-point subinterfaces, that you do not
put an IP address on the physical interface.
8. A. Clocking on a serial interface is always provided by the CSU/DSU (DCE device).
However, if you do not have a CSU/DSU in your nonproduction test environment, then
you need to supply clocking with the clock rate command on the serial interface of
the router with the DCE cable attached.
9. C. Frame Relay, by default, is a nonbroadcast multi-access (NBMA) network, which
means that broadcasts, such as RIP updates, will not be forwarded across the link
by default.
10. C. PPPoE encapsulates PPP frames in Ethernet frames and uses common PPP features
like authentication, encryption, and compression. PPPoA is used for ATM.
11. C. If you have a Cisco router on one side of a Frame Relay network and a non-Cisco
router on the other side, you would need to use the Frame Relay encapsulation type of
IETF. The default is Cisco encapsulation, which means that a Cisco router must be on
both sides of the Frame Relay PVC.
12. C. The show interfaces command shows the conguration settings and the interface
status as well as the IP address and tunnel source and destination address.
Chapter 21: Wide Area Networks 1027
13. B, C, D. This is just a basic WAN question to test your understanding of connections.
PPP does not need to be used, so option A is not valid. You can use any type of connec-
tion to connect to a customer site, so option B is a valid answer. You can also use any
type of connection to get to the Frame Relay switch, as long as the ISP supports it, and
T1 is valid, so option C is okay. Ethernet as a WAN can be used with Ethernet over
MPLS (EoMPLS); however, you don’t need to congure a DLCI unless you’re using
Frame Relay, so E is not a valid answer for this question.
14. B. All web browsers support Secure Sockets Layer (SSL), and SSL VPNs are known as
Web VPNs. Remote users can use their browser to create an encrypted connection and
they don’t need to install any software. GRE doesn’t encrypt the data.
15. E. This is an easy question because the Remote router is using the default HDLC serial
encapsulation and the Corp router is using the PPP serial encapsulation. You should go to
the Remote router and set that encapsulation to PPP or change the Corp router back to the
default of HDLC by typing no encapsulation under the interface.
16. A, C, E. VPNs can provide very good security by using advanced encryption and
authentication protocols, which will help protect your network from unauthorized
access. By connecting the corporate remote ofces to their closest Internet provider
and then creating a VPN tunnel with encryption and authentication, you’ll gain a huge
savings over opting for traditional leased point-to-point lines. VPNs scale very well to
quickly bring up new ofces or have mobile users connect securely while traveling or
when connecting from home. VPNs are very compatible with broadband technologies.
17. C. Even though the IP addresses don’t look correct, they are in the same subnet, so
option B is not correct. The question states that you can ping the other side, so the PVC
must be up— option A can’t be correct. You cannot congure IARP, so only option C
can be correct. Since a Frame Relay network is a nonbroadcast multi-access network
by default, broadcasts such as RIP updates cannot be sent across the PVC unless you
use the broadcast statement at the end of the frame-relay map command.
18. D. IPsec is an industry-wide standard suite of protocols and algorithms that allows for
secure data transmission over an IP-based network that functions at the layer 3 Network
layer of the OSI model.
19. C. A VPN allows or describes the creation of private networks across the Internet,
enabling privacy and tunneling of TCP/IP protocols. A VPN can be set up across any
type of link.
20. B. As I mentioned many times in this chapter, and you need to remember this, DLCIs
are locally signicant only and dene the circuit from the router to the switch only.
They do not reference a remote router or DLCI. Lab_A would use DLCI 100 to get to
the Lab_B networks. RouterB would use DLCI 200 to get to the Lab_A networks.
Appendix
C
Disabling and
Configuring Network
Services
By default, the Cisco IOS runs some services that are unnec-
essary to its normal operation, and if you don’t disable
them, they can be easy targets for denial-of-service (DoS)
attacks and break-in attempts.
DoS attacks are the most common attacks because they are the easiest to perform. Using
software and/or hardware tools such as an intrusion detection system (IDS) and intru-
sion prevention system (IPS) can both warn and stop these simple, but harmful, attacks.
However, if we can’t implement IDS/IPS, there are some basic commands we can use on
our router to make them more safe. Keep in mind, though, that nothing will make you
completely safe in today’s networks.
Let’s take a look at the basic services we should disable on our routers.
Blocking SNMP Packets
The Cisco IOS default congurations permit remote access from any source, so unless
you’re either way too trusting or insane, it should be totally obvious to you that those con-
gurations need a bit of attention. You’ve got to restrict them. If you don’t, the router will
be a pretty easy target for an attacker who wants to log in to it. This is where access lists
come into the game—they can really protect you.
If you place the following command on the serial0/0 interface of the perimeter router,
it’ll stop any SNMP packets from entering the router or the DMZ. (You’d also need to have
a permit command along with this list to really make it work, but this is just an example.)
Lab_B(config)#access-list 110 deny udp any any eq snmp
Lab_B(config)#interface s0/0
Lab_B(config-if)#access-group 110 in
Disabling Echo
In case you don’t know this already, small services are servers (daemons) running in the router
that are quite useful for diagnostics. And here we go again—by default, the Cisco router has a
series of diagnostic ports enabled for certain UDP and TCP services, including echo, chargen,
and discard.
Turning off BootP and Auto-Config 1031
When a host attaches to those ports, a small amount of CPU is consumed to service
these requests. All a single attacking device needs to do is send a whole slew of requests
with different, random, phony source IP addresses to overwhelm the router, making it slow
down or even fail. You can use the no version of these commands to stop a chargen attack:
Lab_B(config)#no service tcp-small-servers
Lab_B(config)#no service udp-small-servers
Finger is a utility program designed to allow users of Unix hosts on the Internet to get
information about each other:
Lab_B(config)#no service finger
This matters because the finger command can be used to nd information about all
users on the network and/or the router. It’s also why you should disable it. The finger
command is the remote equivalent to issuing the show users command on the router.
Here are the TCP small services:
Echo Echoes back whatever you type. Type the command telnet x.x.x.x echo ? to see
the options.
Chargen Generates a stream of ASCII data. Type the command telnet x.x.x.x chargen ?
to see the options.
Discard Throws away whatever you type. Type the command telnet x.x.x.x discard ? to
see the options.
Daytime Returns the system date and time, if correct. It is correct if you are running
NTP or have set the date and time manually from the EXEC level. Type the command
telnet x.x.x.x daytime ? to see the options.
The UDP small services are as follows:
Echo Echoes the payload of the datagram you send.
Discard Silently pitches the datagram you send.
Chargen Pitches the datagram you send and responds with a 72-character string of ASCII
characters terminated with a CR+LF.
Turning off BootP and Auto-Config
Again, by default, the Cisco router also offers BootP service as well as remote
auto-conguration. To disable these functions on your Cisco router, use the following
commands:
Lab_B(config)#no ip boot server
Lab_B(config)#no service config
1032 Appendix C u Disabling and Configuring Network Services
Disabling the HTTP Interface
The ip http server command may be useful for conguring and monitoring the router,
but the cleartext nature of HTTP can obviously be a security risk. To disable the HTTP
process on your router, use the following command:
Lab_B(config)#no ip http server
To enable an HTTP server on a router for AAA, use the global conguration command
ip http server.
Disabling IP Source Routing
The IP header source-route option allows the source IP host to set a packet’s route through
the IP network. With IP source routing enabled, packets containing the source-route option
are forwarded to the router addresses specied in the header. Use the following command
to disable any processing of packets with source-routing header options:
Lab_B(config)#no ip source-route
Disabling Proxy ARP
Proxy ARP is the technique in which one host—usually a router—answers ARP requests
intended for another machine. By “faking” its identity, the router accepts responsibility for get-
ting those packets to the “real” destination. Proxy ARP can help machines on a subnet reach
remote subnets without conguring routing or a default gateway. The following command
disables proxy ARP:
Lab_B(config)#interface fa0/0
Lab_B(config-if)#no ip proxy-arp
Apply this command to all your router’s LAN interfaces.
Disabling Redirect Messages
ICMP redirect messages are used by routers to notify hosts on the data link that a
better route is available for a particular destination. To disable the redirect messages so
Disabling the Maintenance Operation Protocol (MOP) 1033
bad people can’t draw out your network topology with this information, use the follow-
ing command:
Lab_B(config)#interface s0/0
Lab_B(config-if)#no ip redirects
Apply this command to all your router’s interfaces. However, just understand that if this
is congured, legitimate user trafc may end up taking a suboptimal route. Use caution
when disabling this command.
Disabling the Generation of ICMP
Unreachable Messages
The no ip unreachables command prevents the perimeter router from divulging topology
information by telling external hosts which subnets are not congured. This command is
used on a router’s interface that is connected to an outside network:
Lab_B(config)#interface s0/0
Lab_B(config-if)#no ip unreachables
Again, apply this to all the interfaces of your router that connect to the outside world.
Disabling Multicast Route Caching
The multicast route cache lists multicast routing cache entries. These packets can be
read, and so they create a security problem. To disable the multicast route caching, use
the following command:
Lab_B(config)#interface s0/0
Lab_B(config-if)#no ip mroute-cache
Apply this command to all the interfaces of the router. However, use caution when
disabling this command because it may slow legitimate multicast trafc.
Disabling the Maintenance Operation
Protocol (MOP)
The Maintenance Operation Protocol (MOP) works at the Data Link and Network layers
in the DECnet protocol suite and is used for utility services like uploading and downloading
1034 Appendix C u Disabling and Configuring Network Services
system software, remote testing, and problem diagnosis. So, who uses DECnet? Anyone with
their hands up? I didn’t think so. To disable this service, use the following command:
Lab_B(config)#interface s0/0
Lab_B(config-if)#no mop enabled
Apply this command to all the interfaces of the router.
Turning Off the X.25 PAD Service
Packet assembler/disassembler (PAD) connects asynchronous devices like terminals and
computers to public/private X.25 networks. Since every computer in the world is pretty
much IP savvy, and X.25 has gone the way of the dodo bird, there is no reason to leave
this service running. Use the following command to disable the PAD service:
Lab_B(config)#no service pad
Enabling the Nagle TCP
Congestion Algorithm
The Nagle TCP congestion algorithm is useful for small packet congestion, but if you’re
using a higher setting than the default MTU of 1,500 bytes, it can create an above-average
trafc load. To enable this service, use the following command:
Lab_B(config)#service nagle
It is important to understand that the Nagle congestion service can break X Window
connections to an X server, so don’t use it if you’re using X Window.
Logging Every Event
Used as a syslog server, the Cisco ACS server can log events for you to verify. Use the logging
trap debugging or logging trap level command and the logging ip_address command
to turn this feature on:
Lab_B(config)#logging trap debugging
Lab_B(config)#logging 192.168.254.251
Lab_B(config)#exit
Disabling the Default Forwarded UDP Protocols 1035
Lab_B#sh logging
Syslog logging: enabled (0 messages dropped, 0 flushes, 0 overruns)
Console logging: level debugging, 15 messages logged
Monitor logging: level debugging, 0 messages logged
Buffer logging: disabled
Trap logging: level debugging, 19 message lines logged
Logging to 192.168.254.251, 1 message lines logged
The show logging command provides you with statistics of the logging conguration on
the router.
Disabling Cisco Discovery Protocol
Cisco Discovery Protocol (CDP) does just that—it’s a Cisco proprietary protocol that discovers
directly connected Cisco devices on the network. But because it’s a Data Link layer protocol, it
can’t nd Cisco devices on the other side of a router. Plus, by default, Cisco switches don’t for-
ward CDP packets, so you can’t see Cisco devices attached to any other port on a switch.
When you are bringing up your network for the rst time, CDP can be a really helpful
protocol for verifying it. But since you’re going to be thorough and document your net-
work, you don’t need the CDP after that. And because CDP does discover Cisco routers
and switches on your network, you should disable it. You do that in global conguration
mode, which turns off CDP completely for your router or switch:
Lab_B(config)#no cdp run
Or, you can turn off CDP on each individual interface using the following command:
Lab_B(config-if)#no cdp enable
Disabling the Default Forwarded
UDP Protocols
When you use the ip helper-address command as follows on an interface, your router
will forward UDP broadcasts to the listed server or servers:
Lab_B(config)#interface f0/0
Lab_B(config-if)#ip helper-address 192.168.254.251
You would generally use the ip helper-address command when you want to for-
ward DHCP client requests to a DHCP server. The problem is that not only does this
1036 Appendix C u Disabling and Configuring Network Services
forward port 67 (BootP server request), it forwards seven other ports by default as well.
To disable the unused ports, use the following commands:
Lab_B(config)#no ip forward-protocol udp 69
Lab_B(config)#no ip forward-protocol udp 53
Lab_B(config)#no ip forward-protocol udp 37
Lab_B(config)#no ip forward-protocol udp 137
Lab_B(config)#no ip forward-protocol udp 138
Lab_B(config)#no ip forward-protocol udp 68
Lab_B(config)#no ip forward-protocol udp 49
Now, only the BOOTP server request (67) will be forwarded to the DHCP server. If you
want to forward a certain port—say, TACACS+, for example—use the following command:
Lab_B(config)#ip forward-protocol udp 49
Cisco’s Auto Secure
Okay, so ACLs seem like a lot of work and so does turning off all those services I just dis-
cussed. But you do want to secure your router with ACLs, especially on your interface con-
nected to the Internet. However, you are just not sure what the best approach should be, or
maybe you just don’t want to miss happy hour with your buddies because you’re creating
ACLs and turning off default services all night long.
Either way, Cisco has a solution that is a good start, and it’s darn easy to implement.
The command is called auto secure, and you just run it from privileged mode as shown:
R1#auto secure
--- AutoSecure Configuration ---
*** AutoSecure configuration enhances the security of
the router, but it will not make it absolutely resistant
to all security attacks ***
AutoSecure will modify the configuration of your device.
All configuration changes will be shown. For a detailed
explanation of how the configuration changes enhance
security and any possible side effects, please refer to Cisco.com
for Autosecure documentation.
At any prompt you may enter '?' for help.
Use ctrl-c to abort this session at any prompt.
Cisco’s Auto Secure 1037
Gathering information about the router for AutoSecure
Is this router connected to internet? [no]: yes
Enter the number of interfaces facing the internet [1]: [enter]
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.10.10.1 YES NVRAM up up
Serial0/0 1.1.1.1 YES NVRAM down down
FastEthernet0/1 unassigned YES NVRAM administratively down down
Serial0/1 unassigned YES NVRAM administratively down down
Enter the interface name that is facing the internet: serial0/0
Securing Management plane services...
Disabling service finger
Disabling service pad
Disabling udp & tcp small servers
Enabling service password encryption
Enabling service tcp-keepalives-in
Enabling service tcp-keepalives-out
Disabling the cdp protocol
Disabling the bootp server
Disabling the http server
Disabling the finger service
Disabling source routing
Disabling gratuitous arp
Here is a sample Security Banner to be shown
at every access to device. Modify it to suit your
enterprise requirements.
Authorized Access only
This system is the property of So-&-So-Enterprise.
UNAUTHORIZED ACCESS TO THIS DEVICE IS PROHIBITED.
You must have explicit permission to access this
device. All activities performed on this device
are logged. Any violations of access policy will result
in disciplinary action.
1038 Appendix C u Disabling and Configuring Network Services
Enter the security banner {Put the banner between
k and k, where k is any character}:
#
If you are not part of the www.globalnettc.com domain, disconnect now!
#
Enable secret is either not configured or
is the same as enable password
Enter the new enable secret: [password not shown]
% Password too short - must be at least 6 characters. Password configuration
failed
Enter the new enable secret: [password not shown]
Confirm the enable secret : [password not shown]
Enter the new enable password: [password not shown]
Confirm the enable password: [password not shown]
Configuration of local user database
Enter the username: Todd
Enter the password: [password not shown]
Confirm the password: [password not shown]
Configuring AAA local authentication
Configuring Console, Aux and VTY lines for
local authentication, exec-timeout, and transport
Securing device against Login Attacks
Configure the following parameters
Blocking Period when Login Attack detected: ?
% A decimal number between 1 and 32767.
Blocking Period when Login Attack detected: 100
Maximum Login failures with the device: 5
Maximum time period for crossing the failed login attempts: 10
Configure SSH server? [yes]: [enter to take default of yes]
Enter the domain-name: lammle.com
Configuring interface specific AutoSecure services
Disabling the following ip services on all interfaces:
no ip redirects
no ip proxy-arp
no ip unreachables
no ip directed-broadcast
no ip mask-reply
Disabling mop on Ethernet interfaces
Cisco’s Auto Secure 1039
Securing Forwarding plane services...
Enabling CEF (This might impact the memory requirements for your platform)
Enabling unicast rpf on all interfaces connected
to internet
Configure CBAC Firewall feature? [yes/no]:
Configure CBAC Firewall feature? [yes/no]: no
Tcp intercept feature is used prevent tcp syn attack
on the servers in the network. Create autosec_tcp_intercept_list
to form the list of servers to which the tcp traffic is to
be observed
Enable tcp intercept feature? [yes/no]: yes
And that’s it—all the services I mentioned earlier are disabled, plus some! By saving the
conguration that the auto secure command created, you can then take a look at your
running-cong to see your new conguration. It’s a long one!
Although it is tempting to run out to happy hour right now, you still need to verify your
security and add your internal access-list congurations to your intranet.
Appendix
D
About the Additional
Study Tools
IN THIS APPENDIX:
1 Additional study tools
1 System requirements
1 Using the study tools
1 Troubleshooting
Additional Study Tools
The following sections are arranged by category and summarize the software and other
goodies you’ll nd on the book’s companion website. If you need help with installing the
items, refer to the installation instructions in the section “Using the Study Tools,” later in
this appendix.
The additional study tools can be found at www.sybex.com/go/ccnarssg.
Here, you will get instructions on how to download the files to your
hard drive.
Test Engine
The les contain a test engine, which includes practice test questions, as well as the assess-
ment questions and the chapter review questions that were also included in the book itself.
Electronic Flashcards
These handy electronic ashcards are just what they sound like. One side contains a question,
and the other side shows the answer.
Videos
Author Todd Lammle has provided approximately 60 minutes of companion videos to walk
through some of the more difcult concepts of the book.
Minimum System Requirements 1043
Network Simulator
Readers can get access to a LammleSim IOS network simulator that can be used to perform
all of the hands-on labs in the book.
PDF of Glossary of Terms
We have included an electronic version of the glossary in PDF format. You can view the
electronic version of the glossary with Adobe Reader.
Adobe Reader
We’ve also included a link to download the latest version of Adobe Reader. For more informa-
tion on Adobe Reader or to check for a newer version, visit Adobe’s website at www.adobe.com/
products/reader/.
Minimum System Requirements
Windows
u 2.33 GHz or faster x86-compatible processor, or Intel Atom 1.6 GHz or faster proces-
sor for netbooks
u Microsoft Windows XP (32-bit), Windows Server 2008 (32-bit), Windows Vista®
(32-bit), Windows7 (32-bit and 64-bit), Windows8 (32-bit and 64-bit), or Windows
Server 2012 (64-bit)
u Internet Explorer 7.0 or later, Mozilla Firefox 17 or later, Google Chrome, Safari 5.0
or later, or Opera 11
u 512 MB of RAM (1 GB of RAM recommended for netbooks); 128 MB of graphics
memory
Mac OS
u Intel Core™ Duo 1.83 GHz or faster processor
u Mac OS X v10.6, v10.7, or v10.8
u Safari 5.0 or later, Mozilla Firefox 17, Google Chrome, or Opera 11
u 512 MB of RAM; 128 MB of graphics memory
For the latest information about the software and files, please refer to the
ReadMe file.
1044 Appendix D u About the Additional Study Tools
Using the Study Tools
To install the items, follow these steps:
1. Download the ZIP file to your hard drive, and unzip to an appropriate location. Instruc-
tions on where to download this file can be found here: www.sybex.com/go/ccnarssg.
2. Click the Start.EXE file to open up the study tools file.
3. Read the license agreement, and then click the Accept button if you want to use the
study tools.
The main interface appears. The interface allows you to access the content with just one
or two clicks.
Troubleshooting
Wiley has attempted to provide programs that work on most computers with the minimum
system requirements. Alas, your computer may differ, and some programs may not work
properly for some reason.
The two likeliest problems are that you don’t have enough memory (RAM) for the pro-
grams you want to use or you have other programs running that are affecting installation
or running of a program. If you get an error message, such as, “Not enough memory” or
“Setup cannot continue,” try one or more of the following suggestions and then try using
the software again:
Turn off any antivirus software running on your computer. Installation programs some-
times mimic virus activity and may make your computer incorrectly believe that it’s being
infected by a virus.
Close all running programs. The more programs you have running, the less memory is
available to other programs. Installation programs typically update les and programs, so
if you keep other programs running, installation may not work properly.
Have your local computer store add more RAM to your computer. This is, admittedly,
a drastic and somewhat expensive step. However, adding more memory can really help the
speed of your computer and allow more programs to run at the same time.
Be sure to email todd@lammle.com with the receipt of your purchase in
order to get even more bonus material and labs provided by Todd Lammle!
You can find out more information at www.lammle.com/forum.
Customer Care
If you have trouble with the book’s companion study tools, please call the Wiley
Product Technical Support phone number at (800) 762-2974, or contact them at
http://sybex.custhelp.com/.
SYMBOLS
?, 223–224
A
AAL5. See ATM Adaptation Layer 5
ABR. See area border router
access control list (ACL), 502,
1018,1025
EIGRP troubleshooting, 827
named, 523–525
OSPF troubleshooting, 871
security issue mitigation, 506–507
SNMP configuration, 730
troubleshooting, 744, 752–753, 984
access layer, 72
access link, 465–466
access lists, 8, 503–507, 1008
applying, 504–505, 520
block size, 508–510, 1010
configurations, 222
deny entries, 516
extended, 504, 512, 515–526,
529, 1009
configuration commands, 529
named, 523
number ranges, 516, 529
guidelines, 505–506
inbound, 505, 511
interfaces, 505, 1010
monitoring, 526–528
named, 504, 523–525, 527, 1008
outbound, 505
packet filtering, 505, 507–508
placement, 506, 511, 512
scope, 1010
show ip interface, 528
show running-config, 527
SSH access control, 514
standard, 504, 506, 507–515,
529, 1009
configuration commands, 529
example, 510–514
named, 523
number ranges, 507, 529
syntax, 507–508, 523
wildcard masking, 508–510
statements, 504
TCP control, 519
Telnet access control, 514–515
traffic direction, 505
types, 504
UDP control, 519
verifying, 526, 529
VTY access control, 514–515
wildcard masking, 508–510, 1010
access point (AP), 44
access ports, 465, 465–466
access protocols, 236
access rate, 925
access-class, 1010
access-list, 222, 225, 981
ACK packet, 791
Index
Note to the reader: Throughout this index boldfaced page numbers indicate pri-
mary discussions of a topic. Italicized page numbers indicate illustrations.
1046 acknowledgments (ACK) – ARP
acknowledgments (ACK), 19, 19,
22–23, 30, 786
ACL. See access control list
active configuration, 826
active router, 704, 706, 708, 709
active timer, 708
active virtual forwarder (AVF), 717
active virtual gateway (AVG), 717, 983
active virtual router (AVR), 1019
AD. See administrative distance;
advertised distance
Adaptive Security Appliance
(ASA), 506
address learning, 427–429, 428,
438, 447
Address Resolution Protocol (ARP),
69, 110, 117–118, 130, 1001.
See also neighbor discovery
default gateway lookup, 336
reverse, 130
table, 198
address resolution states, 759–760
address-family ipv4 autonomous-
system, 795
adjacencies, 389, 412, 823, 824, 1013
multi-area OSPF network
requirements, 850–851, 1024
OSPF troubleshooting, 868
OSPFv3, 877, 879
troubleshooting, 1022, 1023, 1024
verifying, 868
administrative configurations,
228–241
banners, 229–231
descriptions, 239–240
encrypting passwords, 237–239
hostnames, 228–229
setting passwords, 231–237
administrative distance (AD), 358,
359, 365–366, 374, 1002, 1022
default, 366, 366
static routes, 368
advanced distance-vector protocols,
784, 785
Advanced Encryption Standard
(AES), 946
Advanced Research Projects Agency
(ARPA), 89
advertised distance (AD), 786,
787, 1022
AES. See Advanced Encryption
Standard
aggregate rate, 48
AH. See Authentication Header
all-nodes multicast address, 580
all-routers multicast address, 1013
alternate port, 619
American Standard Code for
Information Interchange
(ASCII), 17
ANSI LMI format, 929, 937
anti-replay service, 945
anycast, 572–573, 575, 576, 982,
1013, 1014
AP. See access point
APIPA. See Automatic Private IP
Addressing
AppleTalk, 507, 792
Application layer, 14–15, 16, 16 –17,
30, 968
filtering, 516
application-specific integrated circuit
(ASIC), 26, 426
area border router (ABR), 388, 848,
851–852, 854, 882, 1024
AR P. See Address
Resolution Protocol
arp -a – auto-summarization 1047
arp -a, 198, 204, 336–337, 997
ARP broadcasts
congestion from, 7
DHCP conflicts, 100
routing, 337, 338
ARP cache, 336–337, 340, 341,
777, 1021
Ethernet buffer, 338
ARP reply, 344, 345
ARP requests, 344, 345,
364–365, 742
ARPA. See Advanced Research
Projects Agency
AS. See autonomous system
AS external link advertisements, 854
ASA. See Adaptive Security Appliance
ASBRs. See autonomous system
boundary routers
ASCII. See American Standard Code
for Information Interchange
ASIC. See application-specific
integrated circuit
asymmetric encryption, 946
asymmetrical DSL, 907, 908
Asynchronous Transfer Mode
(ATM), 390, 904, 1026
DSL connections, 908
PPP over, 908
ATM. See Asynchronous
Transfer Mode
ATM Adaptation Layer 5
(AAL5), 908
ATTEMPT state, 856
ATU-Cs, 908
authentication, 916
adjacency requirements, 851
EIGRP troubleshooting,
818, 822
Hello protocol, 855
IPsec, 944, 945
PPP, 918, 985, 1026
configuring, 917–918
debugging, 920
failed, 919
methods, 917
SNMP, 727
Authentication Header (AH), 945
autoconfiguration
ICM P, 580
stateless, 578–581, 580, 581
auto-cost reference bandwidth, 1025
auto-detect mechanism, 48
Automatic Private IP Addressing
(APIPA), 100–101, 575
automatic scrolling, 226
autonegotiation, 749
autonomous system (AS), 365, 407,
786, 817, 831
adjacency troubleshooting,
1023, 1024
EIGRP configuration, 786,
793–795
external, 852
external link advertisements, 854
hop count limits, 785
mismatched numbers, 818, 822
OSPFv3, 876
route advertising, 854
autonomous system boundary
routers (ASBRs), 388, 848, 851,
882, 1024
external AS connections, 852
LSAs, 854
auto-summarization, 368, 375,
795–798, 825
disabling, 797–798, 825
discontiguous network confusion,
797, 818
1048 auxiliary password – boot field
auxiliary password, 235
auxiliary port, 216, 265, 439
AV F. See active virtual forwarder
AVG. See active virtual gateway
AV R . See active virtual router
B
B2B communications. See business-
to-business communications
backbone area, 851, 853
backbone routers, 848, 851, 882
backoff, 47, 990
backoff algorithms, 46, 973
backup designated router (BDR),
390, 409, 860
election, 402, 410
RID, 402
backup port, 619
Backward Explicit Congestion
Notification (BECN), 930,
931, 935
bad mask error, 200
bandwidth, 257, 910, 998
dynamic allocation, 902
Frame Relay, 925
link cost, 393, 410
reference, 876, 1025
serial link, 249, 258
WAN connection, 900
bandwidth, 250
banners, 229–231, 266, 998
base MAC addresses, 445
BDR. See backup designated router
BECN. See Backward Explicit
Congestion Notification
Berkeley Software Distribution
(BSD), 89
BGP. See Border Gateway Protocol
big process problem, 341
binary conversion, 50–53, 73,
971–973
binary digit, 50
binding, 13
bit, 50
bit-oriented protocols, 912
bits, 969
boot field, 666
bridge ID, 625, 626
classful network address, 793
configuration register,
665–666, 666
load from NVRAM, 669
password recovery, 669, 670
congestion, 930
EIGRP configuration, 793
HSRP MAC address, 705, 706
IPv6 addresses, 755
bit-timing rules, 69
block sizes, 171, 201, 996
access lists, 508–510, 1010
CIDR, 144, 203
NAT, 555
subnet masks, 204
summarization, 193–194
VLSM table, 185, 185, 187
wildcard configuration, 396
wildcard masking, 508–510
blocked port, 619
blocking state, 619–620, 628,
642, 1014
boot, 667–668
boot field, 666
boot sequence – byte-oriented protocols 1049
boot sequence, 265, 285–286, 317,
663–664, 688
interrupting, 670
boot system, 667–669, 983
boot system commands, 667–669, 675
boot system flash, 668, 675
boot system rom, 668–669
boot system tftp, 668–669
bootloader, 663
BootP. See Bootstrap Protocol
bootstrap, 664
Bootstrap Protocol (BootP), 98–100
port numbers, 109
Border Gateway Protocol (BGP),
365, 852
border routers, 553
area, 388
NAT, 543, 543
bounded updates, 785
BPDU. See Bridge Protocol
Data Unit
BPDU Guard, 640, 647, 982, 1015
bridge ID, 622, 630
bits, 625, 626
designated port selection, 630
modifying and verifying, 631–636
roles in STP, 618
root bridge election, 623
VLAN troubleshooting, 765
bridge port
PAgP, 643
roles, 618–619
Bridge Protocol Data Unit (BPDU),
618, 625
bridges, 6, 10, 30, 426, 969
collision and broadcast
domains, 970
collision domain breakup with, 9
at Data Link layer, 26–28
multiport, 9
bridging, 9
transparent, 27
broadcast address, 119,
126–127, 131
LSA update multicast, 392
subnets, 148, 150
broadcast control, 463
broadcast domains, 30, 461
breaking up, 7–8, 969
congestion, 5–7
controlling size, 45
defining, 44
identifying, 970
large, 10
layer 2 switches and, 8
network design, 43, 44–45, 45
routers and, 9
simple networks, 5
switches and, 9
VLANs, 11, 44–45
broadcast (multi-access)
networks, 390
broadcast packets, 10
broadcast storms, 7, 434, 435, 435,
464, 572, 616
BSD. See Berkeley Software
Distribution
buffers, 20, 338
business-to-business communications
(B2B communications), 944
byte, 51
byte-oriented protocols, 912
1050 cable – Cisco 2800/2900 series routers
C
cable, 898, 903, 905, 906–907
cable TV (CATV), 903, 906
cabling, 973
autodetecting type, 438
crossover, 59, 60 – 61, 437, 672,
692, 990, 1017
demarc to CPE, 899
Ethernet, 59–66, 74
faulty, 743, 777
fiber-optic, 64–66, 65, 991
input errors, 749
IP network connectivity
troubleshooting, 743, 746,
748, 777
IPv6 network connectivity
troubleshooting, 756
serial WAN, 910–912
DTEs and DCEs, 911–912, 1026
serial transmission, 910
types, 991
cache-based switching, 551
call setup, 18
CAM. See Content
Addressable Memory
carrier detect (CD), 286, 664
Carrier Sense Multiple Access with
Collision Detection (CSMA/CD),
45–47, 46, 73, 990
Catalyst 2960 series, 216, 469
Catalyst 3560 series, 216, 437
Catalyst switches, 437
configuring, 436–447
console port, 436
POST, 437
verifying, 444–447
category 5 Enhanced UTP cable,
59, 59
category 6 Enhanced UTP cable, 59
CAT V. See cable TV
CD. See carrier detect
CDP. See Cisco Discovery Protocol
CDP holdtime, 295–296
CDP timers, 295–296
CDPCP. See Cisco Discovery
Protocol Control Protocol
CEF. See Cisco Express Forwarding
cellular 3G/4G, 904
central office (CO), 899, 903, 908
Challenge Handshake Authentication
Protocol (CHAP), 916, 917, 920
channel groups, 642–646, 647
channel service unit (CSU), 28,
247, 264
channel-group, 643–645
CHAP. See Challenge Handshake
Authentication Protocol
CIDR. See Classless
Inter-Domain Routing
CIR. See Committed
Information Rate
circuit switching, 900–901
Cisco, 4
configuration, 286–290
packet-forwarding techniques,
341–342
routers, 7, 284–286
switches, 284–286
troubleshooting methodology,
196–198, 204
Cisco 1800/1900 series routers, 217
Cisco 1900 router, 217
Cisco 2800/2900 series routers, 217
Cisco 2960 switch – Class B subnets 1051
Cisco 2960 switch, 469, 488
console connections, 63, 63
interfaces, 217
Cisco 3560 switch, 217, 295,
296–297
interfaces, 242
Cisco AnyConnect SSL VPN
client, 943
Cisco Discovery Protocol (CDP),
199, 284, 317, 774–776
documenting network topology,
300–303, 301, 303
neighbor information, 296–299
using, 295–303
Cisco Discovery Protocol Control
Protocol (CDPCP), 915
Cisco encapsulation, 926, 938,
951, 1026
Cisco Express Forwarding
(CEF), 342
Cisco hierarchical model, 70–72,
71, 74
Cisco IFS. See Cisco IOS File System
Cisco Internetworking Operating
System (IOS), 63, 146, 147, 978,
997–999
administrative configurations,
228–241
boot sequence, 285–286, 317
connecting to devices, 216–217
console messages, 293–294
DHCP configuration, 290–293
overview, 216
responsibilities, 265
user interface, 215–219
verifying DHCP, 292–293
Cisco IOS File System (Cisco IFS),
677–682
commands, 678–679, 688
IOS upgrade, 679–682
Cisco License Manager (CLM), 683
Cisco LMI format, 929, 937
Cisco Long Range Ethernet (LRE),
909–910
Cisco Nexus, 147
Cisco Prime, 726
Cisco Product License Registration
portal, 683
Cisco routers
boot sequence, 663–664, 688
internal components,
662–663, 688
PPP configuration on, 917
Cisco Technical Assistance Center
(TAC), 800
Class A network addresses, 120,
120–121, 122–123, 131, 993
subnet masks, 144–145
subnetting, 167–170
Class A subnets
/16, 168
/20, 168–169
/26, 169
calculating in head, 169–170
Class B network addresses, 120,
120–121, 123–124, 131
subnet masks, 145
subnetting, 158–167
Class B subnets
/17, 159 –160
/18, 160 –161
/20, 161
/21, 161–162
1052 Class C network addresses – command history
/22, 162
/23, 162–163
/24, 163
/25, 164
/26, 164–165
/27, 165–166
calculating in head, 166–167
Class C network addresses, 120,
120–121, 124, 131, 203, 992
subnet masks, 145
subnetting, 146 –158
Class C subnets
/25, 148–150, 149, 156, 157
/26, 150–151, 151, 156, 157
/27, 151–152, 152, 156–157, 157
/28, 153, 157, 158
/29, 153–154, 157, 158
/30, 154, 157, 158
calculating in head, 155–156
Class D network addresses, 120,
120, 121
Class E network addresses, 120,
120, 121
classful boundaries, 797
classful network, 171
classful network address, 368,
369, 793
classful routing, 142, 182–183,
367, 375
Classless Inter-Domain Routing
(CIDR), 144 –146
block sizes, 144, 203
IP address availability, 571
NAT similarity, 542
slash notation, 144, 156–157,
157–158
subnet masks, 144, 144–145,
171, 993
classless routing, 367, 368
classless routing protocols, 182,
785, 795
clear counters, 259–260
clear ip nat translation, 550
clear ip nat translations, 551, 1011
clear ip nat translations *, 981
clear ip ospf process, 596
CLI. See command-line interface
client redundancy, 700–701
Clientless Cisco SSL VPN, 943
CLM. See Cisco License Manager
clock, 223–224
clock rate, 248–249, 353, 911–912,
978, 997
clocking, 247, 247–248, 263–264,
294, 353, 911–912, 1026
CO. See central office
code formatting, 989
collision domains, 743
breaking up, 6, 6, 9–10, 426,
461, 969
bridges and, 9, 970
congestion, 7
defining, 8, 30
identifying, 970
internetworks, 11
IP routing, 337
network design, 43, 43–44, 44
simple networks, 5
switches, 9, 11, 45, 970, 988
collisions, 46–47, 973
full-duplex avoiding, 48, 991
hubs, 8
COM port. See serial
communication port
command history, 226, 226–227
command-line interface (CLI) – configuration 1053
command-line interface (CLI), 146,
215, 219–227
automatic scrolling, 226
editing and help features,
223–228, 266
entering, 219
prompts, 220–221, 265–266
static route configuration, 374
switch configuration, 436
Committed Information Rate (CIR),
925–926, 951
Common Spanning Tree (CST),
623, 624
community antenna television, 906
config, 220
config net, 289
config t, 355–356, 823, 978
config-register, 667, 670
configuration, 266
active, 826
backing up, 286, 317
backing up and restoring, 286–290
Catalyst switches, 436–447
clocking, 263–264
copying to NVRAM, 287–288
copying to TFTP server, 288
current, 287
deleting, 252–253
DHCP, 290–293, 355–356
DHCP relay, 291–292
dynamic NAT, 548
EIGR P, 792–811, 834
auto-summarization, 795–798
command modes, 792
discontiguous networks,
796–797
hop count, 785, 801–802
maximum paths, 801–802
metrics, 799–801
session initiation, 793
split horizon, 809–811
subnets, 796–799
summarization, 795–798
traffic control, 798–799
unequal-cost load balancing,
807–809
VLSMs, 795–798
wildcards, 794
EIGRPv6, 829–831, 834
erasing, 250–264, 267,
289–290, 317
GLBP, 718–721
GRE tunnels, 947–949, 952
HDLC, 912–914, 913
HSR P, 709–713
IPv6 network, 588–591
IPv6 routing, 591–599
IVR, 480–487, 481, 483, 484,
486, 487
layer 3, 260
LCP, 915–916
loopback interfaces, 403–405
multi-area networks, 857–859
NetFlow, 732–733
OSPFv3, 877–879, 882
PAT, 548–549
port channels, 643–646
PPP
authentication methods,
917–918
on Cisco routers, 917
prompts, 998
restoring, 288–289
saving, 250–264, 267
SNMP, 728–730
static NAT, 547
1054 configuration modes – cost
static routes, 357–362
stored, 287
subinterfaces, 932–933
syslog, 723–726
trunk ports, 476– 480
verifying, 253–264, 287, 363–365
verifying DHCP, 292–293,
363–365
viewing, 250–264, 251, 267
viewing and changing, 671
VLANs, 472–487
configuration modes
configuration register, 667
EIGRP, 792, 1022
EIGRPv6, 828–829
global, 639, 643, 667, 1016, 1023
interface, 828, 877
OSPFv3, 877
port channels, 643
router, 792, 877
syslog, 725
configuration register, 663, 688,
1016, 1017
changing, 670
checking current value, 666–667
load from NVRAM, 669
managing, 664–671
password recovery, 669, 670
resetting, 671
understanding bits, 665–666, 666
configure terminal, 219–220
congestion control, 902
Frame Relay, 930–931
connectionless network services,
18, 30
connectionless protocols, 104, 130
connection-oriented communication,
18–20, 19, 30, 101, 130
console, 232
console emulation programs, 63,
63, 74
console line, 721
console messages, sending to syslog
server, 293–294
console password, 232–234, 266
console port, 62–63, 74, 216, 233,
265, 991
Catalyst switches, 436
restoring IOS, 677
syslog messages, 721
Content Addressable Memory
(CAM), 426, 427, 428, 438, 445,
763, 1005
convergence, 1015
EIGRPv6, 828
STP, 620, 1014–1015
copy flash tftp, 672, 674, 983
copy run start, 250–251, 286
copy running-config, 251
options, 288
copy running-config startup-config,
250, 287, 288, 317, 998
copy running-config tftp, 286, 288
copy source-url destination-url, 678
copy start run, 288, 978, 979
copy startup-config
running-config, 671
copy startup-config tftp, 286
copy tftp flash, 675, 680, 983, 1017
copy tftp run, 979
copy tftp running-config, 288–289
core layer, 70–71
cost, 393, 410
CPE – datagrams 1055
CPE. See customer premises
equipment
CRC. See cyclic redundancy check
crossover cable, 59, 60 – 61, 437, 672,
692, 990, 1017
crosstalk, 56
crypto key, 997
crypto key generate rsa, 236
CSMA/CD. See Carrier Sense
Multiple Access with Collision
Detection
CST. See Common Spanning Tree
CSU. See channel service unit
CSU/DSU, 899, 1026
customer premises equipment (CPE),
899, 900, 903
DSL, 908
cyclic redundancy check (CRC)
de-encapsulation, 67
duplex error, 260, 997
error checking, 54
FCS field, 55, 67, 69, 990
interface troubleshooting, 260
IP routing, 337–339
TCP, 102
D
DA. See Destination Address
DAD. See duplicate address detection
data communication equipment
(DCE), 28, 900, 911–912,
936, 997
clocking, 247, 247–248, 353
data encapsulation, 66–69, 74
Data Encryption Standard (DES),
945, 946
data frame, 25, 27
Data license, 683
Data Link Connection Identifiers
(DLCIs), 927–928
debug frame lmi information, 937
Frame Relay maps, 939, 1026
locally significant, 1027
show frame map information, 937
show frame pvc information,
935–936, 952
show interface information,
936–937
subinterface configuration,
933–934
WAN connections, 1027
Data Link layer, 15, 16, 25–28, 26,
30, 742, 968, 969
encapsulation, 67, 67
Ethernet, 49–55
Ethernet frames, 53–55
frames, 69, 73, 336, 339
show interface output, 258
switches and bridges, 26–28
Data Over Cable Service Interface
Specification (DOCSIS), 906
data packets, 23, 968
data service unit (DSU), 28, 247,
263, 264
data terminal equipment (DTE), 28,
263, 900, 911–912, 926,
936, 997
serial interfaces, 248
data translation, 989
Database Description packets (DD),
850, 857
database synchronization, 850
datagrams, 66
1056 DCE – dial-up modems
DCE. See data communication
equipment
DD. See Database
Description packets
DE. See Discard Eligibility
dead timer, 410, 985, 1003
debug, 314–326
debug frame lmi, 937–938
debug ip nat, 549, 981, 1011
debug ppp authentication, 920, 1026
debug standby, 713
debugging, 314–316
DEC. See Digital Equipment
Corporation
decimal, binary conversion, 50–53,
73, 971–973
DECnet, 12, 507
dedicated connections, 900
de-encapsulation, 67
default administrative distances,
366, 366
default clock sources, 294
default gateway, 339, 700, 701, 984,
1001, 1008, 1021
ARP request for, 742
confirming, 745
FHRPs, 702–704
HSRP load balancing, 714
IP network connectivity
troubleshooting, 742–747
IPv6 network connectivity
troubleshooting, 755–756,
760–761
NDP, 584
pinging, 197
routing, 336
virtual routers, 715
default routing, 356, 362–365, 374
OSPF advertising, 401
RIP advertising, 372–373
verifying configuration, 363–365
default trap level, 724
default-information originate,
372–373
delete, 680
demarcation point (demarc),
899, 911
demilitarized zone (DMZ), 503
denial of service (DoS), 506, 1010
deny, 516, 529, 752
Department of Defense (DoD), 88, 89
DES. See Data Encryption Standard
descriptions, 239–240
designated forwarding port,
628, 632
designated port, 618, 629–630
RSTP, 627
designated router (DR), 389–390,
409, 853, 860, 863, 1003, 1025
backup, 860
election, 402, 410
NBMA networks, 939
RID, 402
desktop layer, 72
Destination Address (DA), 54
destination unreachable message,
114, 337, 340, 1002
DHCP. See Dynamic Host
Configuration Protocol
DHCP Discover message, 992
DHCP relay, 291–292
DHCPv6, 581–582
diagnostic address, 196
dial-up modems, 900–901, 914
Diffusing Update Algorithm (DUAL) – dynamic desirable mode 1057
Diffusing Update Algorithm (DUAL),
785, 791, 833, 834
EIGRPv6, 828
path calculation, 788
digital carrier systems, 900
Digital Equipment Corporation
(DEC), 616
Digital Signal 0 (DS0), 900
digital subscriber line (DSL), 898,
903, 905, 907–908
Digital Subscriber Line Access
Multiplexer (DSLAM), 903, 908
Dijkstra algorithm, 386, 390, 850
directly connected networks, 335,
338, 358
administrative distance, 366
disabled ports, 619
Discard Eligibility (DE), 930, 935
discarding state, 627
disconnect, 306–307
discontiguous networks, 798, 825,
827, 1023
auto-summarizing confusing,
797, 818
EIGRP support, 785, 795–797, 796
troubleshooting EIGRP, 818, 827
Dish Network, 904
distance-vector protocols, 366, 375,
785, 1001
distribution layer, 72
distribution network, 906
DIX, 55
DLCIs. See Data Link
Connection Identifiers
DMZ. See demilitarized zone
DNS. See Domain Name Service
do, 240–241
DOCSIS. See Data Over Cable
Service Interface Specification
DoD. See Department of Defense
DoD model, 89–118, 130,
974–975, 993
layers, 89–90
Domain Name Service (DNS), 97,
97–98, 130, 307
port numbers, 109
resolving names with, 309–310
TCP/IP and UDP, 110
troubleshooting, 198
domain names, SSH setup, 235
DoS smurf attacks, 506
DoS TCP SYN attacks, 506
DOWN state, 855–856
DR. See designated router
DRAM. See dynamic RAM
DS0. See Digital Signal 0
DS1, 900
DSL. See digital subscriber line
DSL interface cards, 908
DSLAM. See Digital Subscriber Line
Access Multiplexer
DSU. See data service unit
DTE. See data terminal equipment
DT P. See Dynamic
Trunking Protocol
DUAL. See Diffusing Update
Algorithm
duplex error, 260, 997
duplex settings, 748–749, 1021
duplicate address detection (DAD),
584–585, 586, 755
dynamic auto ports, 772
dynamic bandwidth allocation, 902
dynamic desirable mode, 474
1058 Dynamic Host Configuration Protocol (DHCP) – enable
Dynamic Host Configuration
Protocol (DHCP), 98–100,
99, 130
configuring, 290–293, 355–356
conflicts, 100
IPv6, 581–582
network protocols, 110
protocols, 992
server address, 1000
verifying, 292–293, 363–365
dynamic NAT, 542, 544, 556,
981, 1011
configuration, 548
IP address pools, 548, 551
port numbers, 548
testing and troubleshooting, 550
dynamic RAM (DRAM), 220
dynamic route recoveries, 791
dynamic routing, 333–334, 356,
365–367, 374
Dynamic Trunking Protocol (DTP),
473– 474, 478, 771–774, 1021
disabling, 772
frame tagging, 774
viewing statistics, 771
E
E0, 900
EBCDIC. See Extended Binary
Coded Decimal Interchange Code
echo requests
IP routing, 336, 339, 340
Ping, 114, 115, 198, 253, 318, 743
editing commands, 225–226,
225–228, 266
EEPROM. See electronically erasable
programmable read-only memory
EGPs. See exterior gateway protocols
EIA-530, 910
EIA-TIA 232 interface, 62, 991
EIA/TIA-232, 910
EIA/TIA-232-C, 914
EIA/TIA-449, 910
802.1d, 616, 620, 623, 625
802.1q, 772, 774–775, 777
802.1w, 616, 619, 624, 1014
EIGR P. See Enhanced Interior
Gateway Protocol
EIGRP multicast packets, 128, 129
eigrp router-id, 1022
EIGRPv6, 784, 828–833,
1022, 1023
configuring, 829–831, 834
RIDs, 858
verifying, 831–833
elections
GLBP preemption, 717
HSRP positions, 704, 709,
710, 712
multi-area OSPF, 855–857, 860,
863, 880
point-to-point links, 860
root bridge, 622–624
serial point-to-point links, 880
electromagnetic interference
(EMI), 57
electronically erasable programmable
read-only memory (EEPROM),
218, 663
EMI. See electromagnetic
interference
enable, 231–232, 266
enable secret – Enhanced Interior Gateway Protocol (EIGRP) 1059
enable secret, 232, 266, 998
enable secret password, 232, 266
enable-secret setting, 671
encap frame, 986
Encapsulating Security Payload
(ESP), 945–946
encapsulation, 42, 974, 1007.
See also Generic Routing
Encapsulation
ADSL, 908
Cisco, 926, 938, 951, 1026
802.1q, 482, 772, 774–775, 777
EtherChannel, 644
Ethernet, 902
Frame Relay, 951
implementation, 932
monitoring, 936
types, 926
HDLC, 903, 912–914, 1027
IETF, 926, 938, 951, 1026
inter-VLAN communication, 482
layer 2 WAN, 902
mismatched, 258, 921–922
OSI stack, 66–69, 74
PPP, 914–920
PPPoA, 908
PPPoE, 903, 908, 1026
serial, 902, 914, 951, 1027
troubleshooting, 938–939,
939, 940
trunk, 773–776
tunneling, 944
VLAN identification, 468, 469
encapsulation, 480–481
encapsulation ?, 270, 902
encapsulation frame-relay, 926, 931
encapsulation frame-relay ?, 938
encapsulation frame-relay ietf, 938
encapsulation identifier, 471
encapsulation ppp, 917
encrypting passwords, 237–239
encryption
IPsec, 944, 945
passwords, 918
PPPoA, 908
PPPoE, 903, 1026
SNMPv3, 727, 1020
SSL, 943, 1027
VPNs, 943, 946, 1027
Enhanced Interior Gateway Protocol
(EIGRP), 332, 367, 742, 1001,
1022–1024. See also EIGRPv6
administrative distance, 366, 1002
advertisement, 786
ASs, 786, 793–795
auto-summarization, 368
configuring, 792–811, 834
auto-summarization, 795–798
command modes, 792
discontiguous networks,
796–797
hop count, 785, 801–802
maximum paths, 801–802
metrics, 799–801
session initiation, 793
split horizon, 809–811
subnets, 796–799
summarization, 795–798
traffic control, 798–799
unequal-cost load balancing,
807–809
VLSMs, 795–798
wildcards, 794
default hop count, 785
DUAL, 791
events, 817
1060 EoMPLS – Ethernet
features and operations,
784–792, 834
Frame Relay routing problems,
939–941, 940
Hellos, 786
metrics calculation, 785
multicast traffic, 790
multiple link networks, 642
neighbor discovery, 785–790
neighbor table, 834
OSPF troubleshooting, 872–874
RIDs, 858
route discovery and
maintenance, 792
route selection, 802–807
load balancing, 803,
805–806, 1023
routing table, 802–803
topology table, 803–804, 805
routing databases, 790
routing updates, 799
RTP, 790–791
serial link bandwidth, 249, 258
tables used by, 788, 789
verifying and troubleshooting,
811–828, 834
ACLs, 827
discontiguous networks,
818, 827
event logs, 817
K values, 818, 821–824, 827
load-balancing output, 815–816
local router status, 813
mapping network, 812
passive state routes, 815
routing protocols, 816–817
routing table, 814
topology table, 814–815
traffic data, 816
troubleshooting commands, 812
VLSM, 182
EoMPLS. See Ethernet over MPLS
equal-cost load balancing, 814–815,
1022, 1023
erase startup-config, 252–253,
289–290, 317, 978, 1016
IP routing setup, 348, 350
router setup, 1000
setup mode, 999
error disabled state, 640
errors
bad mask, 200
detecting, 54
duplex, 260, 997
ICMP messages, 198, 312
input, 997
IP address configuration, 198
known, 340
logging, 293
unknown, 340
err-shutdown mode, 434
ESP. See Encapsulating
Security Payload
EtherChannel, 638, 642–646, 647,
1014, 1015–1016
Ethernet, 8, 25, 42–57, 990–991
addressing, 49–50
broadcast domain, 44–45
buffers, 338
cabling, 59–66, 74
collision domains, 43–44
collisions, 46–47
CSMA/CD, 45–47
Data Link layer, 49–55
Ethernet encapsulation – flash memory 1061
frames, 53–55, 73
full-duplex, 47–49, 48
half-duplex, 47–49
Physical layer, 55–57
power over, 437
standards, 55–56
Ethernet encapsulation, 902
Ethernet over MPLS (EoMPLS),
904, 1027
Ethernet_II frames, 54–55
Ether-Type field, 116
Ether-Type field, 116
EUI-64, 578–581, 600, 1013
evaluation licenses. See Right-To-Use
licensing
EXCHANGE state, 857
exec modes
privileged, 219, 252, 998
user, 219, 998
exec process creation banner,
229, 230
EXEC processes, 230
EXEC session, 216
time-out, 233
exec-timeout 0 0, 233–234
exit, 305, 306–307
exit interface, 358, 359
exit interface caching, 341
EXSTART state, 856
extended access lists, 504, 512,
515–526, 1009
configuration commands, 529
named, 523
number ranges, 516, 529
Extended Binary Coded Decimal
Interchange Code (EBCDIC), 17
extended system ID, 625, 1015
exterior gateway protocols
(EGPs), 365
extranet VPNs, 944, 986
F
fast switching, 341, 551
faulty cables, 743, 777
FCS. See Frame Check Sequence
feasible distance (FD), 787, 788, 804,
814, 815, 1022
feasible successor (FS), 789, 1022
FECN. See Forward Explicit
Congestion Notification
FHRPs. See first hop
redundancy protocols
fiber cabling, 56, 57, 991
fiber-optic cables, 64–66, 65
File Transfer Protocol (FTP), 92–93,
93, 94, 130, 255
port numbers, 109
filter tables, 27, 426, 435
firewall routers, 502–503
first hop, 702
first hop redundancy protocols
(FHRPs), 700, 702–704, 703,
734, 1018, 1020. See also
Gateway Load Balancing
Protocol; Hot Standby Router
Protocol; Virtual Router
Redundancy Protocol
flash memory, 218, 663, 664,
983, 1017
backing up, 674
removable, 674, 682
verifying, 673–674
working with, 681
1062 flat networks – FS
flat networks, 460–461, 461, 464
Flexible NetFlow, 730, 1018
flow control, 20–21, 21, 30
Forward Explicit Congestion
Notification (FECN), 930,
931, 935
forward/filter decisions, 427,
429– 431
forward/filter table, 427–428, 428,
429–431, 430
forwarding port, 618
forwarding state, 619–620,
627, 1014
FQDN. See fully qualified
domain name
Frame Check Sequence (FCS), 55, 67,
69, 337, 339, 990
frame interface-dlci, 986
Frame Relay, 390, 898, 902,
923–942, 1026–1027
ATM, 904
bandwidth specifications, 925
CIR, 925–926, 951
congestion control, 930–931
encapsulation
implementation, 932
monitoring, 936
types, 926, 951
IARP, 927
implementation and monitoring,
931–942
single interface, 931–932
subinterfaces, 932–934, 1026
mappings, 938, 938–939, 1026
monitoring, 934–938
show interface, 936–937
MPLS replacing, 904
packet switching, 901
Physical layer connections, 910
routing problems, 939,
939–941, 940
EIGR P, 940, 940–941
OSPF, 939–940, 940
subinterfaces, 941–942
serial interfaces, 927
split horizon problems, 811
troubleshooting, 938–942, 952
routing, 939, 939–941, 940
subinterfaces, 941–942
subnets, 941
verification, 952
frame tagging
changing method, 773–774
switch compatibility, 772
frame-relay interface-dlci, 928, 932
frame-relay interface-dlci dlci,
941–942
frame-relay lmi-type, 929
frame-relay lmi-type ansi, 931
frame-relay map ip, 939
frames, 66, 69, 742–743, 969
address learning, 427–428
data, 25, 27
Ethernet, 53–55, 73
fields, 54–55, 73, 336, 339
filtering, 429
flooding, 427
HDLC format, 913
MAC address field, 337
multiple copies, 616
at Physical layer, 69
ping, 337, 338, 339
PPPoE, 903
tagging, 467–468, 468–469,
488, 1008
unicast, 434
untagged, 773, 777, 1008
FS. See feasible successor
FTP – groups 1063
FT P. See File Transfer Protocol
FULL state, 855–856, 857, 1024
full-duplex, 18, 47–49, 48, 73,
257, 991
fully qualified domain name
(FQDN), 98, 310
G
Gateway Load Balancing
Protocol (GLBP), 700, 704,
716–721, 734, 984, 1018,
1019–1020
AV F, 717
AVG , 717
configuring, 718–721
features, 717–718
functions, 717
per-host traffic balancing, 718
gateway routers, 27
gateways, 15, 336, 989. See also
default gateway
AVG, 717, 984
default, 700, 701, 984
ARP request for, 742
confirming, 745
FHRPs, 702–704
HSRP load balancing, 714
IP network connectivity
troubleshooting,
742–747
IPv6, 1020
IPv6 network connectivity
troubleshooting, 755–756,
760–761
virtual routers, 715
redundancy, 1005
GLBP, 704
HSRP, 703–704
VRRP, 704
security, 946
virtual, 717
Generic Routing Encapsulation
(GRE), 898, 944
headers, 946–947
tunnels, 946 –951, 952
configuring, 947–949, 952
structure, 947
verifying, 949–951, 952
GET BULK, 727
GET messages, 726
GETBULK, 95
gigabit crossover cables, 990
Gigabit Ethernet, 57
G/L bit. See global/local bit
glbp, 719–720
GLBP. See Gateway Load
Balancing Protocol
global addresses, 544, 754,
1014, 1020
LMI, 928
global configuration mode, 220, 265,
639, 643, 667, 998, 1016, 1023
static routes, 357
global unicast addresses, 575, 575,
982, 1012
global/local bit (G/L bit/U/L bit),
50, 579
GRE. See Generic Routing
Encapsulation
groups
channel, 642–646, 647
GLBP, 717–718
1064 half-duplex – hostnames
hello timer identifying roles, 707
HSRP roles, 704, 708–709
multicast, 755
VRRP, 715
H
half-duplex, 18, 46, 47–49,
73, 257
hardware address, 111, 969
HDLC. See High-Level
Data-Link Control
HDSL. See high-bit-rate DSL
headend, 906
Hello packet, 791, 812, 1024
EIGRPv6, 828
neighbor states, 856
Hello protocol, 390, 391, 391, 791,
848, 850, 854–855, 1003
hello timer, 410, 707, 786,
985, 1003
Hello/Dead interval, 855
Hellos, 786
help commands, 223–227, 266
hexadecimal
binary conversion, 50–53, 73,
971–973
IPv6 addresses, 573, 573
hexadecimal addressing, 52
HFC. See hybrid fiber-coaxial
hierarchical architecture, 13
hierarchical IP addressing, 119–124
High Speed Serial Data Connector
(HSSDC), 56
high-bit-rate DSL (HDSL), 908
High-Level Data-Link Control
(HDLC), 898, 902–903,
912–914, 951, 952, 986, 1026
configuring, 912–914, 913
encapsulation, 903,
912–914, 1027
frame format, 913
leased lines, 900
mismatched encapsulation,
921–922
mismatched IP addresses, 922–923
Network layer protocol
identification, 903, 912
Physical layer specifications, 910
history, 226–227
hold timer, 707–708, 786
holdtime information, 295–296
hop count, 365, 370–371
EIGR P, 801–802
default, 785
maximum, 1022
AS size limits, 785
host configurations, IP addressing
problems, 202
host distance, 57–59
host IDs
Class A, 123
Class B, 123–124
Class C, 124
pinging, 318
host table, 318
building, 307–309
hostname, 235, 354, 918
hostnames, 97, 266
authentication, 228
DNS resolving, 309–310
host table, 307–309
Host-to-Host layer – ifconfig 1065
resolving, 307–311
router, 228–229
SSH setup, 235
Host-to-Host layer, 89, 90. See also
Transport layer
DHCP, 99
protocols, 101–110, 130
Hot Standby Router Protocol
(HSRP), 700, 703–704, 704–714,
734, 1018, 1019, 1020, 1021
configuring and verifying,
709–713
group roles, 708–709
interface swapping, 706
interface tracking, 709, 710
load balancing, 714
position elections, 704, 709,
710, 712
timers, 706–708
virtual MAC address, 705–706
VRRP comparison, 715
HP Openview, 726
HSR P. See Hot Standby Router
Protocol
HSSDC. See High Speed Serial
Data Connector
HTML, 16
HT TP. See Hypertext
Transfer Protocol
HTTPS. See Hypertext Transfer
Protocol Secure
hubs, 5, 10, 30, 969
collision and broadcast domains,
970, 988
collisions in, 8
network topology, 29
at Physical layer, 28–29, 989
traffic congestion and, 7, 9
Hughes, 904
hybrid fiber-coaxial (HFC), 903, 906
hybrid routing protocols, 367, 375,
784, 785, 1001
HyperTerminal, 670
Hypertext Transfer Protocol
(HTTP), 95–96, 96, 255, 265
port numbers, 109
Hypertext Transfer Protocol Secure
(HTTPS), 96
port numbers, 109
I
IA RP. See Inverse ARP
IBM, 12, 902
ICM P. See Internet Control
Message Protocol
ICMP echo, 198, 318, 743
ICMPv6, 583, 583–584, 755
IDS. See intrusion detection system
IDSL. See ISDN DSL
IE. See Internet Explorer
IEEE 802.1AB standard. See Link
Layer Discovery Protocol
IEEE 802.1q, 468–469, 469,
476–477, 480
IEEE 802.1q trunking, 436
IEEE 802.2 standard, 25
IEEE 802.3 standard, 47, 55
IEEE 802.3ad, 642
IEEE 802.11 standard, 44
IEEE 802.11q, 488
ietf, 926
IETF encapsulation, 926, 938,
951, 1026
ifconfig, 744
1066 IFS – interfaces
IFS. See Cisco IOS File System
IG. See inside global address
I/G bit. See Individual/Group bit
IGPs. See interior gateway protocols
IMAP4, port numbers, 109
implicit deny, 505, 514, 529,
752, 1010
in-band communication, 265
in-band configuration, 216
in-band management, 438
INCMP message, 984
incomplete message, 984
Individual/Group bit (I/G bit),
50, 990
INIT state, 856
input errors, 749, 997
input queue drops, 749
inside global address (IG), 545, 545,
552, 1011
inside global pool, 1012
inside local address, 545, 545, 1012
install license, 688
int loopback, 403
int s0/0.subinterface number, 932
Integrated Services Digital Network
(ISDN), 900–901, 902, 910, 914
Integrated Services Router (ISR),
217, 350
interactive configuration dialog, 998
inter-area routes, 853
interface, 220–221, 767, 913, 985
GRE tunnels, 947, 949
interface configuration mode, 221,
243, 265, 792, 828, 877, 998
interface errors, 748, 1021
interface mode, 639
interface port=channel, 643, 644
interface tracking, 709, 710
interface tunnel number, 952
interface vlan vlan, 488
interface-dlci, 941
interfaces, 24, 25, 241–250
access lists, 505, 1010
bringing up, 244–245
clearing counters, 259–260
descriptions, 239–240
disabling, 244
EIGRP metrics, 799–801
EIGRP traffic control, 798–799
Frame Relay
single interface, 931–932
subinterfaces, 932–934
HSRP
swapping, 706
tracking, 709, 710
IP address configuration,
245–246
layer 3 configurations, 260
logical, 256, 471, 480, 811
loopback, 402–405
passive, 799, 821, 822, 874, 984,
1023, 1024
pipe, 246–247
preparing, 267
prompts, 265
serial, 243, 247–250, 258, 263,
353, 902, 912, 921
Frame Relay, 927
HDLC, 912
LAN technologies, 1026
OSPF load balancing, 875–876
passive, 874
WAN encapsulations,
902, 921
interior gateway protocols (IGPs) – Internet Protocol version 6 (IPv6) 1067
showing, 242
split horizon, 809–811
switched virtual, 471
trunk, 769
tunnel, 947–948
interior gateway protocols
(IGPs), 365
internal routers, 502–503, 848,
852, 882
International Organization for
Standardization (ISO), 12, 15
Internet Control Message
Protocol (ICMP), 110,
114–116, 116
autoconfiguration, 580
common events and
messages, 114
denying/filtering
messages, 506
echo requests, 115, 198,
336, 339
error messages, 198, 312
IPv6, 578, 580, 583, 583–584
ping, 198, 253, 336, 996
role, 130, 131, 993
routing table entries, 337–338
Internet Explorer (IE), 16
Internet layer, 89, 90
protocols, 110–118, 130
Internet names, 97, 130
Internet Protocol (IP), 110 –113,
130, 974
header, 111, 111–112, 130,
582–583, 583
ping packets, 336, 338–339
Internet Protocol Control Protocol
(IPCP), 915
Internet Protocol version 6 (IPv6),
88, 792, 982, 1012–1014
address space, 572
address types, 575–576
addresses, 573, 573, 599, 755,
1013–1014
addressing and expressions,
573–577
anycast, 572–573
ARP cache verification, 777
benefits and uses, 571–573
configuring network, 588–591
configuring routing, 591–599
default gateway, 1020
DHCPv6, 581–582
efficiency features, 572
EIGRPv6, 828–833
headers, 582–583, 583
ICMPv6, 583, 583–584
in internetwork, 577–585
MAC addresses, 578–581
manual address
assignment, 578
multicast, 572, 580
multicast address, 575, 576,
584–585, 1013, 1014
NDP, 584–585
OSPF, 586, 587–588,
594–599
packet headers, 572
reasons for, 571, 599
routing protocols, 586–588
routing tables, 589, 592
shortened expression, 574–575
special addresses, 576–577, 577
stateless autoconfiguration,
578–581, 580, 581
static routing, 586–587, 593
1068 Internet service provider (ISP) – IP addresses
troubleshooting network
connectivity, 754–763, 777
ACLs, 762–763
address resolution states,
759–760
cables, 756
default gateway, 755–756,
760–761
ICMPv6, 755
local host connectivity, 757
local LAN, 757–758
NDP, 755–756
routing tables, 761–762
steps, 756 –757
verifying OSPFv3, 594–599
Internet service provider (ISP), 542,
899, 942
internetwork communication, 8
Internetwork Operating System
(IOS), 662, 983
backing up, 674, 688, 1017
backing up and restoring,
671–682
backup to TFTP, 671–672, 674
boot sequence, 664
licensing, 682–688
mini-IOS, 663, 1016
restoring, 675, 677, 688
upgrading, 675, 677, 688
upgrading with IFS, 679–682
WAN support, 898,
902–904
internetworking
basics, 4–12
models, 12–14
summarization, 193, 193
troubleshooting, 253
Internetworking Operating System.
See Cisco Internetworking
Operating System
internetworks, 7, 10, 11, 25, 27
Inter-Switch Link (ISL), 436, 468,
469–470, 480, 772, 773
inter-VLAN communication, 463,
470, 482
ROAS, 488
inter-VLAN routing (IVR), 460, 471,
471, 980, 1007–1008
configuring, 480–487, 481, 483,
484, 486, 487
multilayer switches, 486–487,
487, 488
router on a stick, 470, 470–471
intrusion detection system (IDS),
506, 1010
intrusion prevention system (IPS),
506, 1010
Inverse ARP (IARP), 927,
1026, 1027
IOS. See Cisco Internetworking
Operating System
IP. See Internet Protocol
ip access-group, 981,
1008, 1010
ip access-list, 523–525,
729, 981
ip access-list standard, 981
ip address, 913, 931
GRE tunnels, 947, 948, 949
IP address spoofing, 506
IP addresses, 8, 30, 69, 88, 111, 996
block sizes, 171
broadcast, 126–127, 127, 131
configuration errors, 198
IP addressing – ip routing 1069
DNS resolution, 97
dynamic NAT pools, 548, 551
global, 754, 1020
LMI, 928
global unicast, 1012
interface configuration, 245–246
IPv4 types, 126–129
layer 2 broadcast, 126, 127
layer 3 broadcast, 126–127, 127
mismatched, 922–923
multicast, 128–129, 131
problems, 198–203, 204
reserved, 121, 122
resolution states, 759–760
shortage, 571, 599
switches, 438, 444
unicast, 127–128, 128,
131, 1012
IP addressing, 118–125
APIPA, 100–101, 575
drawing out scheme, 198–199
hierarchical scheme, 119–124
private addresses, 124–125, 131
static, 100
subnet mask problems,
199–201, 203
terminology, 118–119
troubleshooting, 196–203
IP Base licensing, 683
IP datagrams, 114
ip default-gateway, 1005
ip dhcp, 291
ip domain-lookup, 310
ip domain-name, 235, 310, 997
ip flow egress, 732
ip flow ingress, 732
ip flow-export, 732
IP header, 111
fields, 111–113
IPv6, 582–583, 583
Protocol field, 112–113, 113
protocol numbers, 113
ip helper-address, 292, 979
ip host, 307–308, 753
ip name-server, 310
ip nat inside, 547, 981, 1011
ip nat inside source, 552–553, 1012
ip nat inside source list, 548, 554,
555, 1012
ip nat outside, 547, 981, 1011, 1012
ip nat pool, 548, 552–553, 1011
ip nat translation max-entries, 551
ip nat translation timeout, 552
IP network connectivity
troubleshooting, 742–754,
777, 1020
ACLs, 752–753
default gateway, 742–747
HTTP requests, 753–754
interface statistics, 748–749
local host connectivity,
744–746
local LAN, 746 –747
pinging, 743
router configurations, 749–751
server configuration, 747–748
steps for, 743–744
telnet, 751–752, 753
VPNs, 942
ip ospf cost, 393
ip ospf network
point-to-multipoint, 939
ip route, 357–358, 751, 979
ip routing, 488, 1007
1070 IP routing – keepalives
IP routing, 742–743, 979, 1001–1002
configuring, 346–356, 356–365
process, 335–346, 374
ip ssh version 2, 236
IP stack failure, 196
ip subnet-zero, 146, 170, 185
IP time to live (TTL), 254
iPad, 63
ipconfig, 744, 747, 760, 906
ipconfig /all, 198, 204, 997
IPC P. See Internet Protocol
Control Protocol
iPhone, 63
IP-masquerading, 556
IPS. See intrusion prevention system
IPsec, 944, 986, 1027
transforms, 945–946
IPv4, 88
address shortage, 571, 599
address types, 126–129
broadcasts, 572
IPv6. See Internet Protocol
version 6
ipv6 address, 578
ipv6 eigrp, 829
ipv6 enable, 578
ipv6 int brief, 592–593
ipv6 ospf, 878, 985
ipv6 ospf process-ID area area, 882
ipv6 ospf process-id area
area-id, 1013
ipv6 route, 591–592, 762
ipv6 router eigrp, 828, 830
ipv6 router eigrp as, 984, 1022
ipv6 router ospf, 877
ipv6 unicast-routing, 578,
590, 828
IPX, 507, 792
ISDN. See Integrated Services
Digital Network
ISDN DSL (IDSL), 908
ISL. See Inter-Switch Link
ISL routing, 468
ISO. See International Organization
for Standardization
ISP. See Internet service provider
ISR. See Integrated
Services Router
ISR series routers
password recovery, 669, 670
removable memory, 682
software licenses, 682, 1016
ISRG2 routers, 662
IT U-T, 929
IVR. See inter-VLAN routing
J
J0, 900
jam signal, 46, 46, 973
K
K values, 786, 817, 1023, 1024
EIGRP troubleshooting, 818,
821–824, 827
EIGRPv6 troubleshooting, 831
formula for uses, 800
metric association,
800–801, 801
mismatched, 818, 822, 827
keepalives, 928
L2F – Link layer 1071
L
L2F. See Layer 2 Forwarding
L2T P. See Layer 2
Tunneling Protocol
LACP. See Link Aggregation
Control Protocol
LAN. See local area network
LAN driver, 197
LAPB. See Link Access
Procedure, Balanced
last-resort, 231–232
layer 2 broadcasts, 126, 127
Layer 2 Forwarding (L2F), 944
layer 2 switches, 8, 980, 1004–1006
advantages, 426–427
functions of, 427–431
Layer 2 Tunneling Protocol
(L2TP), 944
layer 2 WAN encapsulation, 902
layer 2 WAN technologies, 1026
layer 3 broadcasts, 126–127, 127
layer 3 device. See routers
layer 3 switches, 460
layer addressing, 68
Layered approach, 13
layered architecture, 13
layers, 13
LCP. See Link Control Protocol
learning state, 619, 627
leased lines, 900, 910
least significant bit (LSB), 54
license boot module, 685, 688
License boot module, 1017
license clear, 688
license install, 683–684, 688
license install url, 1016
license save, 687
license save flash, 1017
licenses, 662, 688, 1016
backing up and uninstalling,
687–688
Data, 683
installing, 688
IP Base, 683
ISR series routers, 682, 1016
permanent, 685, 688
removing, 1017
RTU, 682, 684–687, 1017
Security, 683
Unified Communications, 683
verifying, 690
licensing, 682–688
line, 221, 232
line commands, 221
line configuration mode, 221,
265–266
line console 0, 221, 232–233
line sty, 234, 981, 998
link, 389
Link Access Procedure, Balanced
(LAPB), 903
Link Aggregation Control Protocol
(LACP), 642–644, 1015, 1016
Link Control Protocol (LCP),
914, 952, 986
configuration options,
915–916
link cost, 393, 627, 1024
OSPF troubleshooting, 870
STP, 620–621
Link layer. See Network
Access layer
1072 Link Layer Discovery Protocol (LLDP) – logging synchronous
Link Layer Discovery Protocol
(LLDP), 303, 999
Link Layer Discovery Protocol
Media Endpoint Discovery
(LLDP-MED), 303
link-establishment phase, 916
link-local, 754, 833, 877
link-local addresses, 575, 578, 599,
982, 1012, 1013
link-local unicast addresses, 1014
Link-State Advertisements (LSAs),
390, 402, 848, 850, 852–854,
985, 1004, 1024
ASBRs, 854
flooding zones, 852
neighbor states, 857
Type 1, 852–853, 853
Type 2, 853
Type 3, 853
Type 4, 854
Type 5, 854
types, 852–854, 854, 882
link-state database (LSDB), 848,
852, 855, 860
information exchange, 857
synchronization, 850
link-state packets, 784
link-state protocols, 367, 375,
784, 1001
Link-State Request (LSR), 850, 857
Link-State Update (LSU), 850, 857
listening state, 619
LLC. See Logical Link Control
LLDP. See Link Layer
Discovery Protocol
LLDP-MED. See Link Layer
Discovery Protocol Media
Endpoint Discovery
LMI. See Local Management
Interface
LMI DLCI, 936–937
load balancing, 734, 1012, 1022.
See also Gateway Load
Balancing Protocol
EIGRP route selection, 803,
805–806, 1022, 1023
equal-cost, 814–815, 1022, 1023
HSR P, 714
OSPF troubleshooting, 875–876
port state transitioning, 1015
unequal-cost, 807–809, 815,
1022, 1023
WANs, 804
load sharing, 717
LOADING state, 857, 1024
local addresses, 545
local area network (LAN), 5,
5, 898
bridges and, 9
MAC addresses, 336, 341, 345
traffic congestion causes, 7, 30
local loop, 899
Local Management Interface (LMI),
927, 928–929, 932
DLCIs, 937
information communicated, 928
message formats, 929, 937
traffic statistics, 934–935
local route, 335, 1001
local host, 126
logging, 723, 725, 984
logging buffer, 721
logging buffered, 723
logging console, 723
logging host ip_address, 725, 1019
logging synchronous, 233–234, 349
logging trap – MD5 1073
logging trap, 725, 734, 1018
logging trap warnings, 725
logical address, 111
logical addressing, 66, 69, 969
subnets, 149, 149
logical interfaces, 256, 471, 480, 811
Logical Link Control (LLC), 26, 989
logical separation, 462, 462
login, 304
login banner, 229–231
login local, 997
longest match rule, 334–335
loop avoidance, 427, 434–436
loopback, 126, 196, 204
loopback address, 745, 1013, 1014
loopback interfaces, 402–405
configuring, 403–405
LRE. See Cisco Long Range Ethernet
LSA advertisements, 394
LSA flooding, 391, 392
LSA timer information, 406
LSA update multicast addresses,
392, 392
LSAs. See Link-State Advertisements
LSB. See least significant bit
LSDB. See link-state database
LSR. See Link-State Request
LSU. See Link-State Update
M
MAC. See Media Access Control
MAC address table, 445–446, 447,
636, 764–767, 980, 1005
thrashing, 435, 616
MAC addresses, 26, 27, 30, 42, 69,
342, 743
ARP resolution, 117
base, 445
frame fields, 337
HSRP, 705, 706
IPv6 addresses, 578–581
local LAN, 336, 341, 345
PDUs, 988
port security, 431
Proxy ARP, 700–701
routing, 374
sections of, 49–50, 73
static, 446–447, 1004
virtual, 703, 705–706
VLAN troubleshooting,
764–767
mac address-table static, 1004
MAC database, 428, 429
MAC forward/filter table, 427, 435
mac-address, 433
MAN. See metropolitan
area network
management connections, 265
management domain, 1005
management information base
(MIB), 727–728
management VLAN, 473, 482
maximum burst rate (MBR), 925
maximum hop count, 1022
maximum paths
EIGR P, 801–802
unequal-cost load balancing,
808, 1022
maximum transmission unit (MTU),
254, 257, 800, 903
maximum-paths, 801
MBR. See maximum burst rate
MD5, 917
1074 Media Access Control (MAC) – multicast address
Media Access Control (MAC), 26,
42, 49–50, 989
local LAN, 336
message of the day banner (MOTD
banner), 229–230, 998
metric, 24
metric maximum-hops, 801
metric weights, 823
MetroEthernet, 898, 904
metropolitan area network
(MAN), 904
MIB. See management
information base
Mills, David, 96
MILNET, 89
mini USB ports, 63
mini-IOS, 663, 1016
MMF. See multimode fiber
mode, 475
modems, 216
modified EUI-64, 579
modular routers, 242
MOTD banner. See message of the
day banner
MPLS. See MultiProtocol
Label Switching
MTU. See maximum
transmission unit
multi-area configurations, 848
areas, 851–852
basic, 857–859
router roles, 851–852, 882
multi-area OSPF networks, 849,
849–850, 985, 1024–1025
adjacency requirements,
850–851, 1024
component categories, 850–857
configuration, 857–859
Hello protocol, 854–855
LSAs, 852–854
monitoring, 882
neighbor states, 855–857
OSPFv3, 876–881
router roles, 851–852
scalability issues, 881
troubleshooting, 859–876, 882
access control list, 871
adjacencies, 868
EIGRP, 872–874
link costs, 870
load balancing, 875–876
ping, 869
Ping, 868–869, 872
routing protocols, 871
show ip interface, 869–871
show ip interface brief, 869
show ip ospf interface, 868
show ip ospf neighbor, 868, 872
show ip protocols, 871,
872, 874
show ip route, 868, 873
verifying, 859, 859–867
Ping, 860
multicast address, 128–129, 131, 982
all-nodes, 580
all-routers, 1013
Hello packets, 390, 1003
I/G bit, 990
interfaces, 1013
IPv6, 575, 576, 584–585,
1013, 1014
LSA update, 392, 392
OSPF, 387, 390–392
range, 992
multicast group addresses – network 1075
multicast group addresses, 128
multicast groups, 755
multicast packets, 10
EIGRP, 128, 129
multicast traffic
congestion from, 7
EIGR P, 790
EIGRPv6, 828
IPv6, 572, 580
OSPFv3, 587, 877, 1013
reliable, 790
multicasting, 928
multilayer switches, 471, 486–488,
487, 1008
security features, 503
multimode fiber (MMF), 57, 65,
65–66
multiple frame copies, 434, 435, 616
multiplexing, 932
multipoint subinterfaces, 933–934
multiport bridges, 9
MultiProtocol Label Switching
(MPLS), 715, 898, 901, 904
multi-vendor networks, 410
N
NA. See neighbor advertisement
NAK. See negative acknowledgment
name, 472, 767
name resolution, 984
named access list configuration
mode, 524
named access lists, 504,
523–525, 1008
NAT. See Network Address
Translation
NAT Overload, 542, 544, 546,
1011, 1012
configuration, 548–549
native VLAN, 473, 479–480, 482,
773–776, 777
NBMA. See nonbroadcast multi-
access
NCP. See Network Control Protocol
NDP. See neighbor discovery
negative acknowledgment
(NAK), 554
neighbor, 799
neighbor advertisement (NA),
584 –585, 755, 756
neighbor and adjacency
initialization, 391
neighbor database, 1024
neighbor discovery (NDP),
584–585, 1013
EIGR P, 785–790
troubleshooting network
connectivity, 755–756
neighbor resolution table, 759
neighbor solicitation (NS), 584–585,
755, 756, 759, 984, 1013
neighbor states, 855–857
neighbor table, 788, 792, 834
neighbors, 389, 412
neighborship database, 390
NetFlow, 700, 730–733, 734, 984,
1018, 1019
configuring, 732–733
flows, 731–732
overview, 731
netsh interface ipv6 show neighbor,
760–761
network, 394, 794
1076 Network Access layer – networks
Network Access layer, 89, 90
Network Address Translation
(NAT), 124, 503, 556, 571, 981,
1011–1012
advantages, 543–544, 544, 1011
block sizes, 555
common problem causes, 550
location, 543, 543
memory use, 551
names, 544–545
operation, 545–550, 546
terms, 545, 545
testing and troubleshooting,
550–555
translation timeout, 552
types, 544, 556, 1011
verification, 549–550
when to use, 542–544
network addresses, 24, 119, 120–121
reserved, 121, 122
network classes, 120, 120–121
network connectivity, checking,
311–317
Network Control Protocol (NCP),
89, 903, 914–916, 952, 986
serial link troubleshooting,
919–920
Network File System (NFS), 104
network hosts, 15, 989
network interface card (NIC), 16, 49,
118, 342, 1001
broadcast storms from faulty, 464
troubleshooting, 197
Network layer, 15, 16, 23–25, 30,
968, 969
encapsulation, 67, 67
logical addressing, 69
network layer protocol phase, 916
network link advertisement (NLA),
853, 1024
network loops, 616, 620
PortFast configuration, 640
network management station (NMS),
15, 94–95, 700, 726, 734, 989
network maps, 300
network masquerading, 556
network number, 554
network overhead, 101
network segmentation, 6
Network Time Protocol (NTP), 96,
97, 293–295, 1001
error logs using, 293
port numbers, 109
network topology
CDP documenting, 300–303,
301, 303
hubs, 29
star, 56
network topology information
ABR storing, 852
DUAL, 791
EIGRP route discovery and
maintenance, 792
EIGRP troubleshooting, 818
synchronization, 784
networks. See also internetworks
administrative distance, 365–366
directly connected, 335, 338,
358, 366
drawing out, 198–199
flat, 460–461, 461, 464
multi-vendor, 410
simple, 5
switched, 437
next-hop address – OC-48 1077
trusted, 503
untrusted, 503
next-hop address, 357–358,
359, 1013
next-hop attributes
EIGRP configuration, 804
EIGRP troubleshooting, 815
IPv6, 762, 831
OSPFv3, 877
NFS. See Network File System
NIC. See network interface card
NLA. See network link advertisement
NMS. See network
management station
no auto-summary, 797–798, 825
no bandwidth, 805
no distribute-list, 827
no encapsulation, 921
no ip address, 986
no ip domain-lookup, 310, 349
no ip host, 309
no ip route, 751
no license boot module, 688
no logging buffered, 724
no logging console, 724
no login, 304
no passive-interface, 874
no router eigrp, 873
no router rip, 398, 399
no service timestamps, 725
no shutdown, 244–245, 258, 267,
999, 1007
administratively down
interfaces, 262
copying and merging
configurations, 289
EIGRPv6 configuration,
828–830, 834
HDLC configuration, 913
port security, 432
router configuration, 347, 349
VLAN configuration, 484–486
node address, 120
nonbroadcast multi-access (NBMA),
390, 856, 923, 939, 1026, 1027
non-designated port, 618
nonegotiate, 475
nonhierarchical addressing, 119
non-root bridges, 618
nonvolatile RAM (NVRAM), 285,
317, 663, 983, 1016, 1017
configuration register bits,
665–667
copying current configuration,
287–288
erasing contents, 289–290, 350
load configuration register
from, 669
saving configuration, 250
startup-config, 218, 265, 286,
664–665
usage, 252
NS. See neighbor solicitation
N T P. See Network Time Protocol
ntp master, 294
ntp server ip_address version 4,
979, 1001
NVRAM. See nonvolatile RAM
O
OC. See Optical Carrier
OC-3, 900
OC-12, 900
OC-48, 900
1078 OIDs – Open Systems Interconnection reference model (OSI model)
OIDs. See organizational IDs
1 Gbps switches, 12
100Base-FX, 56
100Base-T, 49, 56
1000Base-CX, 56
1000Base-LX, 57
1000Base-SX, 57
1000Base-T, 56
wiring for, 61– 62
1000Base-ZX, 57
one-to-many addresses. See
multicast address
Open Shortest Path First (OSPF),
332, 333, 616, 742, 784, 794,
980, 1002–1004
administrative distance, 366
basics, 386–393
configuring, 393–402
configuring areas, 394–397, 412
cost, 393, 410
default interface priority, 1004
enabling, 393–394, 1004
Frame Relay routing problems,
939–940, 940
Hello protocol, 854–855
hierarchical design, 388, 388
IPv6, 586, 587–588, 594–599
link-state, 367
loopback interfaces, 402–405
metrics, 393
multi-area networks, 849,
849–850, 985, 1024–1025
adjacency requirements,
850–851, 1024
component categories, 850–857
configuration, 857–859
Hello protocol, 854–855
LSAs, 852–854
monitoring, 882
neighbor states, 855–857
router roles, 851–852, 882
scalability issues, 881
troubleshooting, 859–876, 882
verifying, 859, 859–867
multicast address, 387, 390–392
multiple link networks, 642
multi-vendor networks, 410
network configuration, 397–402
network layout, 398
non-OSPF layers, 400, 400
operation, 391–393
reasons to use, 388
RID, 1024
RIP comparison, 387, 412
routing table, 866
scalability, 848–850, 881
serial link bandwidth, 249, 258
single-area, 412, 849, 849
subnets, 395–397
terminology, 389–391
topology changes, 850
troubleshooting, 882
verification of configuration,
405– 411, 412
VLSMs, 182
wildcard configuration,
395–397
Open Systems Interconnection
reference model (OSI model), 4,
14–29, 968–969, 988
Application layer, 14–15, 16,
16–17, 30, 968
filtering, 516
creation, 12
Optical Carrier (OC) – Packet Internet Groper (Ping) 1079
Data Link layer, 15, 16, 25–28,
26, 30, 968, 969
encapsulation, 67, 67
Ethernet, 49–55
Ethernet frames, 53–55
frames, 69, 73, 336, 339
show interface output, 258
switches and bridges, 26–28
DoD model similarities,
89–90, 130
encapsulation in, 66, 67
layers, 14, 14–16, 15, 16, 30, 969
Network layer, 15, 16, 23–25, 30,
968, 969
encapsulation, 67, 67
logical addressing, 69
Physical layer, 15, 16, 28–29, 30,
968–969, 989
Ethernet, 55–57
frames at, 69
hubs, 28–29, 989
show interface output, 258
Presentation layer, 15, 16, 17, 30,
968, 989
reference model, 12–13
Session layer, 15, 16, 17–18,
30, 968
Transport layer, 15, 16, 18–23, 30,
255, 969
acknowledgments, 22–23, 23
connection-oriented
communication, 18–20
connection-oriented protocol
role, 69
DHCP, 99
encapsulation, 67, 67
flow control, 20, 21
port numbers, 68, 68–69
reliable delivery, 23
windowing, 21, 22
Optical Carrier (OC), 900
organizational IDs (OIDs), 727
organizationally unique identifier
(OUI), 49, 73, 705, 990
OSI model. See Open Systems
Interconnection reference model
OSPF. See Open Shortest Path First
OSPFv2, 985
OSPFv3, 586, 587–588, 982, 985,
1013, 1024
configuring, 877–879, 882
multi-area, 876 –881
RID, 1024
verifying, 594–599, 879–881
OUI. See organizationally
unique identifier
out-of-band configuration, 216
output errors, 749
output queue drops, 749
outside global address, 545
outside local address, 545
overload, 549, 554
overloading, 544, 545–547, 546,
556, 1011, 1012
configuration, 548–549
P
packet filtering, 8
access lists, 505, 507–508
source address, 508
Packet Internet Groper (Ping),
114–116, 253, 318
configuration verification, 363
connectivity testing, 743, 744, 755
1080 packet switching – ping ?
multi-area OSPF
troubleshooting, 868–869, 872
verification, 860
OSPFv3, 877
routing, 336–340
packet switching, 8, 901, 924
packet-forwarding techniques,
341–342
packets, 66, 969
PAgP. See Port Aggregation Protocol
PAK. See product authorization key
PA P. See Password
Authentication Protocol
passive interfaces, 799, 821, 822,
874, 984, 1023, 1024
passive state, 815
passive-interface, 371, 979, 1004
passive-interface interface-type
interface-number, 799
password, 231–232
Password Authentication Protocol
(PAP), 916, 917
passwords, 266
auxiliary, 235
console, 232–234, 266
enable, 231–232, 266
enable secret, 232, 266
enabling, 231–232
encrypting, 237–239, 918
recovering, 669–671, 688, 1016
setting, 231–237
SSH setup, 235–237
Telnet, 234–235, 265, 267
types, 231
user-mode, 232
VTY, 234–235, 303–305
PAT. See Port Address Translation
path cost, 618
path MTU discovery, 583
path selection, 8
PDUs. See protocol data units
perimeter routers, 502–503
permanent, 358, 360
permanent licenses, 685, 688
permanent virtual circuits (PVCs),
926, 927, 935–936, 951, 1026
permit, 505, 516, 529, 1010
permit ip any any, 521
Per-VLAN Spanning Tree+ (PVST+),
624, 625–626, 646, 1015
Physical layer, 15, 16, 28–29, 30,
968–969, 989
Ethernet, 55–57
frames at, 69
hubs, 28–29, 989
show interface output, 258
PID. See product ID
ping, 198, 204, 311, 312, 318, 353,
984, 996
EIGRP
configuration, 797–798
troubleshooting and verifying,
818, 820, 824, 827
GRE tunnel verification, 950–951
IP connectivity troubleshooting,
743–748, 750 –752
IPv6 connectivity troubleshooting,
757–759, 762
OSPF troubleshooting, 869
TFTP verification, 674
VLAN connectivity
troubleshooting, 768, 775
Ping. See Packet Internet Groper
ping ?, 253
pipe – ports 1081
pipe, 246–247
plain old telephone service (POTS),
900, 902, 908
PoE. See Power over Ethernet
point of presence (POP), 899
point-to-multipoint, 391, 811
LSA update multicast, 392, 392
point-to-point, 391, 902
address learning, 428
elections, 860
LSA update multicast, 392
Point-to-Point Protocol (PPP), 898,
900, 903, 914 –923, 1026, 1027
authentication, 918, 985, 1026
configuring, 917–918
debugging, 920
failed, 919
methods, 917
components, 914–915, 915
configuring on Cisco routers, 917
Data Link layer protocols, 952
encapsulation, 914–920
mismatched, 921–922
LCP configuration options,
915–916
mismatched IP addresses, 922–923
Physical layer specifications, 910
serial encapsulation, 1027
serial link verification and
troubleshooting, 918–920, 952
session establishment, 916
verification, 952
Point-to-Point Protocol over ATM
(PPPoA), 908
Point-to-Point Protocol over Ethernet
(PPPoE), 898, 903, 986, 1026
ADSL, 908–909
point-to-point subinterfaces, 933
Point-to-Point Tunneling Protocol
(PPTP), 944
POP. See point of presence
POP3, port numbers, 109
Port Address Translation (PAT),
542, 544, 545–547, 546, 556,
1011, 1012
configuration, 548–549
testing and troubleshooting, 551
Port Aggregation Protocol (PAgP),
642, 643, 982, 1015
port channeling, 642, 643, 1016
port channels, 642, 647
configuring and verifying,
643–646
port cost, 618
port modes, 771–773, 1021
port numbers, 68, 68–69, 106,
106–110, 109, 131, 342
dynamic NAT, 548
NAT, 547
port security, 431–433, 447,
1005–1006
MAC addresses, 431
switch configuration, 442–444
port states, 647
BPDU Guard, 640
load balancing, 1015
spanning-tree, 619 – 620
STP failure, 636
port VLAN ID (PVID), 468
PortFast, 638–640, 647, 1014, 1015
ports
alternate, 619
backup, 619
blocked, 619
1082 positive acknowledgment with retransmission – Query packet
bridge
PAgP, 643
roles, 618–619
console
restoring IOS, 677
syslog messages, 721
designated, 618, 629–630
RSTP, 627
designated forwarding, 628, 632
disabled, 619
dynamic auto, 772
forwarding, 618
non-designated, 618
root, 618, 983
RSTP, 627, 628
trunk, 769, 770
trunking configurations, 771
positive acknowledgment with
retransmission, 22
POST. See power-on self test
POST test, 983
POTS. See plain old
telephone service
Power over Ethernet (PoE), 437
power-on self test (POST), 218, 265
Catalyst switches, 437
powers of 2, 143–144
PPP. See Point-to-Point Protocol
ppp authentication chap, 985
ppp authentication chap pap, 918
PPPoA. See Point-to-Point Protocol
over ATM
PPPoE. See Point-to-Point Protocol
over Ethernet
PP TP. See Point-to-Point
Tunneling Protocol
preamble, 54
preemption, 717
prefix routing, 367
prefix-length, 553, 981, 1011
Presentation layer, 15, 16, 17, 30,
968, 989
priority value, 623
private IP addresses, 124–125, 131
privileged exec mode, 219, 252, 998
privileged mode, 218, 265, 670
passwords, 671
process ID, 871
process switching, 341
Process/Application layer, 89, 90
protocols, 91–101, 130
product authorization key
(PAK), 683
product ID (PID), 683, 688, 690
prompts, 265–266, 998
protocol data units (PDUs), 66,
68, 968
contents, 988
Proxy Address Resolution Protocol
(Proxy ARP), 700–701
PuTTY, 670
PVC status field, 936
PVCs. See permanent virtual circuits
PVID. See port VLAN ID
PVST+. See Per-VLAN
Spanning Tree+
pwd, 679
Q
Q value, 813
Q.933A LMI format, 929, 937
quality of service (QoS), 25
Query packet, 790
RA – RJ connector 1083
R
RA. See router advertisement
RADSL. See Rate Adaptive DSL
random access memory (RAM), 663,
664, 671, 673, 983
neighbor and topology tables
in, 788
random back-off algorithm, 973
Rapid PVST+, 624, 1015
Rapid Spanning Tree Protocol
(RSTP), 624, 626–630, 646,
1014, 1015. See also 802.1w
enabling, 635–636
R A RP. See reverse ARP
Rate Adaptive DSL (RADSL), 908
REACH state, 1021
read-only memory (ROM), 285–286,
663, 983
redistribution, 372, 829
reference bandwidth, 876, 1025
reference models
advantages, 14
defining, 13
OSI, 12–13, 14
registered jack connector (RJ
connector), 56
reliable delivery, 23, 989
reliable multicast, 790
reliable networking, 18, 20
Reliable Transport Protocol (RTP),
785, 790–791
reload, 252, 350, 1000, 1016
reloading device, 252–253
remark, 525–526
remarks, 525–526
remote access VPNs, 943, 986
remote configuration, 216
remote mask, 359
remote network, 359
advertising, 370
removable memory, 674
repeaters, 58
Reply packet, 790
reported distance. See
advertised distance
request timed-out message, 340
Requests for Comments (RFCs), 106
reserved IP addresses, 121, 122
resolving hostnames, 307–311
Retransmission Time Out
(RTO), 812
retransmissions, 988
delays, 990
positive acknowledgment
with, 22
reverse ARP (RARP), 130
RFC 791, 111
RFC 1256, 114
RFC 1661, 914
RFC 1918, 124–125
RFC 2338, 393
RFC1483 routing, 908
RFCs. See Requests for Comments
RID. See router ID
Right-To-Use licensing (RTU), 682,
684–687, 688, 1017
RI P. See Routing
Information Protocol
RIPng, 586, 828
RIPv2. See Routing Information
Protocol version 2
Rivest, Shamir, and Adleman
encryption (RSA), 946
RJ connector. See registered
jack connector
1084 RJ45 connector – routers
RJ45 connector, 56, 63, 216
RLA. See router link advertisement
rlogin, 92
ROAS. See router on a stick
rolled cable, 59, 62–64
ROM. See read-only memory
ROM monitor, 663
ROM monitor mode, 666
root bridge, 617, 983, 1015
configuring, 634
elections, 622–624
per-VLAN, 625, 1014, 1015
RSTP, 627–629
selecting, 623, 625
root port, 618, 983
RSTP, 627, 628
route aggregation. See
summarization
route discovery, EIGRP, 792
route maintenance, EIGRP, 792
route print, 744
route redistribution, 852
route selection
EIGR P, 802–807
load balancing, 803,
805–806, 1023
routing table, 802–803
topology table, 803–804, 805
route table, 792
route update packets, 24
routed protocol, 332, 989
router advertisement (RA), 580,
584–585, 755, 756, 1013
router configuration mode,
792, 877
router eigrp, 793–795, 797, 801, 823,
825, 827
router ID (RID), 389, 402, 406–407,
829, 853, 1004, 1023
changing, 404–405
designated router election, 402
EIGR P, 858
loopback interface, 403–405
OSPF, 858, 1024
OSPF Hello protocol, 854
OSPFv2, 876
OSPFv3, 587, 876–877, 878
router link advertisement (RLA), 852
router on a stick (ROAS), 470,
470–471, 481, 488, 714, 1020
router ospf, 393–397, 857, 859,
868–869, 980
router ospf process-id, 858
router ospf process-id, 404
router rip, 367, 368, 375, 979
router simulators, 346
router solicitation (RS), 580,
584–585, 755, 756, 1013
router-command history, 226,
226–227
router-id, 404–405, 830, 857, 877
routers, 6, 7, 7, 10, 25, 30, 969
ABRs, 848, 851–852, 854,
882, 1024
active, 704, 708, 709
advantages, 8, 30
ASBRs, 848, 851, 852, 854,
882, 1024
AV R , 1019
backbone, 848, 851, 882
boot sequence, 265, 285–286, 317,
663–664, 688
broadcast domains, 9, 45
broadcast forwarding, 989
routing – routing database 1085
characteristics, 25
collision and broadcast
domains, 970
configuring, 347–355
connecting to, 216–217
CPU utilization, 316–317
data-forwarding decisions, 988
debugging, 314–316
designated, 853, 860, 863,
939, 1025
DHCP configuration, 355–356
error logging, 293
firewall, 502–503
functions, 8, 30
hostnames, 228–229
interfaces, 241–250
internal, 502–503, 848, 852, 882
internal components, 284–285,
284–286, 317, 662–663, 688
internal process, 341–342
IP addressing problems, 201
memory, 979, 983
modes, 219–220
modular, 242
multi-area configurations, 848
roles, 851–852, 882
password recovery, 669–671
password types, 231
perimeter, 502–503
pinging, 197
priority, 855
QoS, 25
reloading, 252–253, 348, 670, 671
routing tables in, 24
serial connectors, 910
standby, 704, 707–708, 709
statistics, 777
stub, 362
successor, 788
terms for, 222, 222
troubleshooting tools, 204
types of, 217
virtual, 703, 704, 708–709,
717, 1019
gateways, 715
GLBP, 717
HRSP, 704, 708–709
WAN connections, 7
routing
basics, 333–335
classful, 367
classless, 367, 368
default, 356, 362–365,
372–373, 374, 401
defining, 333
dynamic, 333–334, 356,
365–367, 374
Frame Relay
troubleshooting, 939,
939–941, 940
information requirements, 374
IPv6 protocols, 586–588
ISL, 772
prefix, 367
static, 333–334, 356, 357–362,
374, 586–587, 1002
switches, 343
types of, 374
upstream, 481
routing database
EIGRP
corrupted, 790
synchronizing, 790
troubleshooting, 814
1086 Routing Information Protocol (RIP) – RXBOOT
LSDB, 848, 852, 855, 860
information exchange, 857
synchronization, 850
multi-area OSPF, 848
monitoring, 882
synchronization, 850
Routing Information Protocol (RIP),
182–183, 249, 332, 365, 366,
367–373, 616
administrative distance, 1002
auto-summarization, 368
classful addresses, 368, 369
configuring, 367–371, 375
default route advertising, 372–373
maximum hop count, 1022
next generation, 586
OSPF comparison, 387, 412
propagations, 371–373
remote network advertising, 370
removing, 398–399
routing tables, 370
Routing Information Protocol
version 2 (RIPv2), 332, 365, 367,
372, 784
routing protocol, 332, 1001
basics, 365–367
classes, 366–367
configurations, 222, 266
IPv6, 586–588
metrics, 365–366
routing protocols, 616
advanced distance-vector, 784, 785
classless, 785, 795
distance-vector, 785
hybrid, 784, 785
link-state, 784
OSPF troubleshooting, 871
routing table, 826
ACL problems, 827
EIGRP route selection,
802–803
EIGRP troubleshooting, 814
EIGRPv6, 832–833
mismatched IP addresses, 923
OSPF, 866
topology table population, 788
troubleshooting, 749–751
routing tables, 24, 24, 111, 343, 374,
988, 1003
default routing, 362
directly connected networks, 358
IPv6, 589, 592
longest match rule, 334–335
packet forwarding, 341
ping routing, 337–338, 340
RIP, 370
static routes, 357–362
summarization, 193
RS. See router solicitation
RSA. See Rivest, Shamir, and
Adleman encryption
rsh, 92
RST P. See Rapid Spanning
Tree Protocol
RTO. See Retransmission
Time Out
RT P. See Reliable
Transport Protocol
RTU. See Right-To-Use licensing
running-config file, 286. See also
copy running-config; show
running-config
verifying, 253
RXBOOT, 663
SA – show cdp entry * 1087
S
SA. See Source Address
satellite, 904
SC connector, 56
SDLC. See Synchronous Data
Link Control
SDSL. See Synchronous DSL
secret, 231–232
Secure Shell (SSH), 92, 93, 216, 265,
267, 997
controlling access, 514
port numbers, 109
setup, 235–237
Telnet, 236–237
Secure Sockets Layer (SSL), 96,
943, 1027
SecureCRT, 74, 670
Secure-down, 443
Secure-shutdown, 443, 1005
Secure-up, 443
security gateway, 946
Security license, 683
segments, 969
Sequence Number field, 945
sequence numbers, 725, 788
serial communication port (COM
port), 62, 74, 991
serial encapsulation, 914, 951, 1027
troubleshooting, 938–939,
939, 940
serial interfaces, 243, 263, 353
clocking, 248
commands, 247–250
DTE devices, 248
Frame Relay, 927
HDLC, 912
LAN technologies, 1026
OSPF load balancing, 875–876
passive, 874
troubleshooting, 258, 267
WAN encapsulations, 902
mismatched, 921
serial lines, 900
NBMA networks, 939
serial links
point-to-point, 880
verifying and troubleshooting,
918–920, 952
serial transmission, 910
serial1 is up, line protocol is
down, 999
servers, 15, 989
service password-encryption,
238–239, 918
service sequence-numbers, 725
service timestamps log datetime
msec, 294, 725
Session layer, 15, 16, 17–18, 30, 968
session multiplexing, 22, 110
SET messages, 727
setup, 218
setup mode, 250, 265, 348,
998, 1016
severity levels, 722–723, 725, 734
SFD. See Start Frame Delimiter
sh run, 251, 287
sh start, 252, 287
shortest path first (SPF), 392,
406, 848
show access-list, 527, 529,
1009, 1012
show access-lists, 752, 827,
981, 1009
show cdp entry *, 978
1088 show cdp entry * protocol – show ip eigrp neighbors
show cdp entry * protocol, 253
show cdp nei, 1014
show cdp neighbor, 979
show cdp neighbor detail, 978
show cdp neighbors, 296–297, 297,
302, 318, 632
show cdp neighbors detail, 298–299,
298–299, 923, 1000
show controllers, 249, 263–264, 353
show controllers interface, 911–912
show controllers serial, 263,
978, 997
show dtp interface, 1021
show dtp interface interface, 769,
771, 772, 1021
show etherchannel port-channel,
645, 983
show etherchannel summary, 646
show file, 680
show file information, 681
show flash, 673–674, 680, 1017
show frame ?, 934
show frame map, 937
show frame pvc, 935–936
show frame-relay ?, 1026
show frame-relay lmi, 934–935
show frame-relay map, 930
show frame-relay pvc, 931
show glbp, 720–721, 734, 984, 1019
show glbp brief, 720–721, 1019
show history, 227
show hosts, 308, 318, 1000
show interface, 748, 823, 913, 914,
985, 999
configuration verification,
255–260
Frame Relay monitoring, 936–937
mismatched encapsulations, 921
mismatched IP addresses, 922
PPP troubleshooting, 919
show interface description, 239–240
show interface fastethernet 0/0,
256, 999
show interface interface switchport,
772, 1021
show interface serial 0/0/0, 258, 267
show interface trunk, 773, 774,
776, 1021
show interface tunnel, 952
show interfaces, 244, 255, 257, 311,
804, 821, 999, 1026
troubleshooting, 260–261
show interfaces interface switchport,
474, 764, 765, 769, 770,
775, 1008
show interfaces interface trunk,
769, 770
show interfaces trunk, 474, 768, 769
show interfaces tunnel 0, 950
show ip access-list, 1009
show ip arp, 198, 338, 997
show ip cache flow, 733, 984, 1018
show ip dhcp binding, 292, 364
show ip dhcp conflict, 293, 364
show ip dhcp pool [poolname],
292, 364
show ip dhcp server statistics, 293
show ip eigrp events, 817
show ip eigrp interface detail
interface, 813
show ip eigrp interfaces, 812, 813,
818, 819
show ip eigrp neighbors, 812, 812,
818, 823, 824, 834, 1022
show ip eigrp topology – show ipv6 ospf hello 1089
show ip eigrp topology, 803, 806,
812, 814–815, 816, 818, 819
show ip eigrp topology | section
network, 805
show ip eigrp traffic, 812, 816
show ip flow export, 733
show ip flow interface, 732–733
show ip interface, 264, 752, 809,
999, 1009
access lists, 528
configuration verification, 261
OSPF troubleshooting, 869–871
show ip interface brief, 242, 243,
262, 352, 750, 758–759, 819, 820
GRE tunnel verification, 949
interface IP address, 245
OSPF troubleshooting, 869
show ip interfaces, 981
show ip interfaces brief, 923
show ip nat, 1011
show ip nat statistics, 551, 981, 1011
show ip nat translations, 549, 981
show ip ospf, 406–407, 412,
861–862, 882, 980
show ip ospf database, 407–408,
412, 859, 866–867, 882
show ip ospf interface, 408–409,
410, 412, 859, 862–863, 868,
980, 1004, 1025
OSPF troubleshooting, 868
show ip ospf neighbor, 409– 410,
410, 412, 859, 860, 882, 980,
1003, 1025
OSPF troubleshooting, 868, 872
show ip protocols, 411, 412
EIGRP
configuring, 800, 807
troubleshooting, 812, 816, 820,
822, 825
EIGRPv6, 831
K values, 800
OSPF troubleshooting, 859,
864–865, 868, 871, 872,
874, 876, 985
unequal-cost load balancing, 807
show ip route, 311, 979, 1001
EIGRP auto-summarization,
795–798
EIGRP route selection, 805
EIGRP troubleshooting, 814, 824,
826, 827
forwarding decisions, 334–335
GRE tunnel verification, 950
IP network connectivity
troubleshooting, 749–751
multi-area OSPF troubleshooting,
859, 865–866
OSPF configuration verification,
405–406
OSPF troubleshooting, 868, 873
router configuration, 349,
352–354
source LAN, 343
static routes, 359–360, 361
unequal-cost load balancing, 809
show ip route eigrp, 812, 813
show ip route network, 805
show ip route ospf, 866, 875–876,
882, 980
show ipv6 access-lists, 763
show ipv6 eigrp interfaces, 830
show ipv6 eigrp neighbors, 831
show ipv6 interface brief, 759
show ipv6 interfaces, 596–597
show ipv6 interfaces brief, 597
show ipv6 neighbors, 759, 777, 1021
show ipv6 ospf, 595, 596, 880–881
show ipv6 ospf hello, 598
1090 show ipv6 ospf interface – smurf attacks
show ipv6 ospf interface, 880
show ipv6 ospf neighbor, 596, 880
show ipv6 ospf packet, 598
show ipv6 protocols, 595, 831, 879
show ipv6 route, 589, 590, 594–595,
761–762
show ipv6 route eigrp, 832–833
show ipv6 route ospf, 879
show ipv6 topology, 831–832
show license, 685, 688, 690,
983, 1017
show license feature, 686, 688,
690, 1017
show license udi, 683, 688,
690, 1017
show logging, 724
show mac address-table, 445–446,
447, 764–767, 980, 1005
show ntp status, 979
show port-security, 442–443, 447
show port-security interface, 1006
show port-security interface
interface, 447
show processes, 316–317, 999
show protocols, 262, 819, 820
show run, 826, 918
show run | begin interface, 914
show running-config, 914, 923
documenting network topology,
300–302
interface configuration, 243
interface IP address, 245
interface status, 245
named access lists, 527
passwords, 237, 239
verifying configurations, 253, 287,
444, 447
viewing configurations, 251
show sessions, 309, 318, 979
show spanning-tree, 631, 633, 647,
982, 983, 1014
show spanning-tree summary,
635–636, 983
show standby, 712, 734, 984
show standby brief, 712–713
show startup-config, 253, 287
show terminal, 227, 978
show version, 266, 666–667,
673–674, 687, 688, 1017
show vlan, 473, 764, 768, 769, 1007
show vlan brief, 764, 766, 769, 770
shutdown, 244, 432
Simple Mail Transfer Protocol
(SMTP), 130
port numbers, 109
Simple Network Management
Protocol (SNMP), 94–95, 700,
726–730, 734, 984, 1018
ACL configuration, 730
configuring, 728–730
MIB, 727–728
port numbers, 109
versions, 727
simplex, 18
single-area OSPF, 412, 849, 849
single-mode fiber, 65, 65–66
site-to-site VPNs, 944, 986
SLA. See summary link
advertisement
slash notation, 144, 146, 156–158
smart-serial, 910
smooth round-trip timer (SRTT),
786, 812
SM T P. See Simple Mail
Transfer Protocol
smurf attacks, 506
SNMP – Start Frame Delimiter (SFD) 1091
SNMP. See Simple Network
Management Protocol
snmp-server, 728–730, 1018
snmp-server community, 729–730
SNMPv3, 727, 1020
software address, 111
software configuration bit meanings,
665–666, 666
Source Address (SA), 54
source address filtering, 508
Spanning Tree Protocol (STP),
616 – 623, 646–647, 982, 1005,
1015–1016. See also Rapid
Spanning Tree Protocol
base MAC address, 765
bridge port roles, 618 – 619
convergence, 620, 1014
failure consequences, 636–638
failure types, 636
link costs, 620–621
loop avoidance, 427, 436
operations, 621–623
port states, 619– 620
root bridge selection, 623
states, 647
terms, 617– 621
types, 623–630
verifying, 631
spanning-tree, 634
spanning-tree algorithm (STA),
616, 618
spanning-tree bpduguard enable, 640
spanning-tree converging, 438
spanning-tree mode rapid-pvst,
635–636
spanning-tree port states, 619–620
spanning-tree portfast, 639, 983
spanning-tree portfast
bpduguard, 640
spanning-tree vlan number root
primary, 983
spanning-tree vlan vlan
primary, 1016
special purpose network addresses,
121, 122
speed settings, 748–749
SPF. See shortest path first
SPF tree calculation, 391, 392
split horizon, 809–811, 810,
932, 1026
SRTT. See smooth
round-trip timer
SSH. See Secure Shell
SSL. See Secure Sockets Layer
ST connector, 56
STA. See spanning-tree algorithm
STALE state, 1021
standard access lists, 504, 506,
507–515, 1009
configuration commands, 529
example, 510–514
named, 523
number ranges, 507, 529
syntax, 507–508, 523
wildcard masking, 508–510
standard IP access list, 507
standby group ip virtual_ip,
710–712
standby groups, 704
standby router, 704, 706,
707–708, 709
standby timer, 708
star topology, 56
Start Frame Delimiter (SFD), 54
1092 startup-config file – switch fabric
startup-config file, 265, 286. See
also erase startup-config; show
startup-config
deleting, 252–253, 289–290
state transitions, 28
stateless autoconfiguration, 578–581,
580, 581
static IP addressing, 100
static MAC addresses, 446–447, 1004
static NAT, 544, 556, 1011
configuration, 547
testing and troubleshooting, 550
static routing, 333–334, 356,
357–362, 374, 1002
administrative distance, 368
IPv6, 586–587, 593
Station and Media Access Control
Connectivity Discovery, 303, 999
sticky, 433
ST P. See Spanning Tree Protocol
straight-through cable, 59, 60, 60, 64
stub routers, 362
subinterfaces, 471, 480–481, 484,
811, 932–934, 1007, 1026
configuring, 932–933
Frame Relay troubleshooting,
941–942
types, 933–934
subnet masks, 142–143, 143,
144 –145, 171, 784, 994–995. See
also variable length subnet masks
block size, 204
CIDR, 144, 144–145, 171, 993
classful routing, 182–183
IP addressing problems,
199–201, 203
IP routing configuration, 346
subnets
advertisement of information, 182
broadcast addresses, 148, 150
creating, 142
EIGRP, 796, 941
advertising, 797–798, 799
configuring, 796–798
Frame Relay troubleshooting, 941
hosts per, 147, 148, 150
interface configuration, 245
IP addressing problems,
199–201, 203
logical addressing, 149, 149
OSPF, 395–397
VLSMs, 784, 791
EIGR P, 795–798
subnetting, 140 –141, 141, 156–158,
975–977, 993–995
advantages, 170
Class A addresses, 167–170
Class B addresses, 158–167
Class C addresses, 146 –158, 203
summarization, 204
successor route, 789, 1022, 1023
successor router, 788
summaries, 785
summarization, 193, 193–195, 204,
368, 375, 784, 977
EIGR P, 795–798
summary address, 193, 194
summary link advertisement (SLA),
853, 1025
summary mask, 204
SVCs. See switched virtual circuits
SVIs. See Switched Virtual Interfaces
switch access vlan, 767
switch fabric, 467
switch native vlan – switchport trunk encapsulation dot1q 1093
switch native vlan, 984
switched network, 437, 461, 461
switched networks
bridge ID, 623
CST, 624
layer 2, 763
PVST+, 625
RSTP, 626
STP, 616, 617, 647
switched virtual circuits (SVCs), 926
Switched Virtual Interfaces (SVIs),
460, 471
switches, 6, 6, 10, 30, 969. See also
Catalyst switches
advantages, 426–427
boot sequence, 285–286
bridging, 9
bringing up, 218–219
broadcast domains, 8, 9, 970
collision domains, 9, 11, 45,
970, 988
configuring, 436–447
connecting to, 216–217
CPU load, 637–638
at Data Link layer, 26–28
error logging, 293
functions of, 427–431, 447
interfaces, 241–250
internal components, 284–285,
284–286
internetworks, 11, 27
IP address, 438, 444
layer 2, 8, 980, 1004–1006
advantages, 426–427
functions of, 427–431
layer 3, 460
multilayer, 471, 486–487, 487,
503, 1008
port assignment to VLAN,
475–476, 488
port security, 431–433, 442–444
routing, 343
statistics, 777
trunk port configuration, 476 – 480
types of, 12, 217
upgrading, 12
switching, 426–436
switching loops, 616, 617, 1015
switchport ?, 475
switchport access vlan, 763, 764,
767, 1007
switchport mode, 769, 771
switchport mode access, 432, 442,
475–478
switchport mode dynamic, 769, 772
switchport mode dynamic
auto, 478
switchport mode dynamic desirable,
478, 772, 773
switchport mode trunk, 476–478,
488, 644, 645
switchport native vlan vlan, 777
switchport nonegotiate, 478, 1008
switchport port-security, 432–434,
442–443, 447, 1005
switchport port-security mac-address
sticky, 1006
switchport port-security violation
shutdown, 1006
switchport trunk allowed, 479
switchport trunk allowed vlan, 645
switchport trunk encapsulation,
477, 980
switchport trunk encapsulation ?, 774
switchport trunk encapsulation
dot1q, 644, 774
1094 switchport trunk native vlan – timers
switchport trunk native vlan,
479–480, 980
switchport trunk native vlan vlan,
769, 773, 776
symmetric encryption, 946
symmetrical DSL, 907
synchronization (SYN), 19, 19
TCP session acknowledgment,
108–109
Synchronous Data Link Control
(SDLC), 902, 912
Synchronous DSL (SDSL), 908
syslog, 700, 721–726
configuring and verifying,
723–726
message format, 722
message gathering, 721
severity levels, 722, 722–723, 725,
734, 1018
syslog server, 721, 725
syslog servers, 293
T
T1, 900, 927
T3, 900, 902
TAC. See Cisco Technical
Assistance Center
TACACS servers, 231–232
T-carrier transmission, 900
TCP Intercept, 506
TCP SYN attacks, 506
TCP/ IP. See Transmission Control
Protocol/Internet Protocol
TCP-Reset, 554
telnet, 235, 303–304, 751–753
Telnet, 91–92, 92, 216, 253, 255,
265, 731, 998
banners, 229
blocking, 519
checking connections, 306
checking users, 306
closing sessions, 306–307
controlling access, 514–515
hostnames, 308
multiple devices simultaneously,
305–306, 318
multiple sessions, 1000
password, 234–235, 265, 267
port numbers, 109
protocols, 992
sessions, 318
SSH, 236–237
using, 303–307
10GBase-T, 57
10Gbps switches, 12
10/100 Mbps switches, 12
10Base-T, 49, 56
terminal lines, 721
terminal monitor, 305, 721, 1000
test environments, 1026
TF TP. See trivial file
transfer protocol
tftp-server flash:ios_name, 675
thin protocols, 103
thrashing the MAC table, 435
3DES. See Triple DES
three-way handshake, 18
time to live (TTL), 312
time-outs, 254, 312
timers
active, 708
dead, 985
Token Ring – Transport layer 1095
hello, 707, 786, 985
hold, 707–708, 786
HSR P, 706–708
SRTT, 786, 812
standby, 708
Token Ring, 1026
toll network, 899
topological database, 390, 402, 1003
topology database, 866
topology table, 788, 792, 814, 834
EIGRP route selection,
803–804, 805
FS, 789, 1022
successor routes in, 1022
ToS. See type of service
trace, 204
Trace, 115
traceroute, 996, 997, 999
IOS configuration
verification, 254
IP addressing troubleshooting,
198, 204
IP network connectivity
troubleshooting, 746
IPv6 network connectivity
troubleshooting, 758
network connectivity testing, 311,
312–313
OSPF troubleshooting, 868
Traceroute, 114, 115, 254, 583
denying/filtering, 506
tracert, 198, 204, 313, 1000
traffic balancing, 717
per-host, 718
traffic congestion, 7, 9, 30
traffic flow confidentiality, 945, 946
translation timeout, 552
Transmission Control Protocol/
Internet Protocol (TCP/IP),
16, 88–89, 101–103, 130,
974, 991–993. See also
Internet Protocol
access list control, 519
DNS, 110
DoD model, 89–118
headers, 102, 102–103, 130
history of, 89
key features, 105, 105–106, 130
key protocols using, 109
network overhead, 101
port numbers, 106, 106–110
segment format, 102, 102–103
session multiplexing, 110
sessions
destination port, 108
source port, 107
syn packet acknowledgment,
108–109
stack failure, 196
stack layers, 993
Transport layer role, 18, 69
transparent bridging, 27
transport input all, 236
transport input ssh, 997
transport input ssh ?, 236
Transport layer, 15, 16, 18–23, 30,
255, 969
acknowledgments, 22–23, 23
connection-oriented
communication, 18–20
connection-oriented protocol
role, 69
DHCP, 99
encapsulation, 67, 67
1096 trap levels – type of service (ToS)
flow control, 20, 21
port numbers, 68, 68–69
reliable delivery, 23
windowing, 21, 22
trap levels, 1018, 1019, 1020
TRAP messages, 726
Triple DES (3DES), 945, 946
trivial file transfer protocol (TFTP),
93–94, 94, 130, 286, 664
booting from, 667–669
copying configuration, 288
IOS backup, 671–672, 674
port numbers, 109
troubleshooting, 204, 742
adjacencies, 1022, 1023, 1024
Cisco methodology, 196–198, 204
debugging, 314–316
DNS, 198
dynamic NAT, 550
EIGR P, 811–828
Frame Relay networks,
938–942, 952
congestion control, 930–931
interface, 260
internetworking, 253
IP addressing, 196–203
IP network connectivity,
742–754, 777
IPv6 network connectivity,
754–763, 777
multi-area OSPF networks,
859–876, 882
network connectivity, 311–317
NICs, 197
OSPF, 882
PAT, 551
PPP, 918–920, 952
serial links, 918–920, 952
show interfaces, 260–261
show processes, 316–317
static NAT, 550
steps for, 743–744
tools, 198, 204
trunk, 768–776
VLANs, 763–776
scenario, 764–768
steps for, 763–764
trunk, 768–776
trunk, 475
trunk encapsulation, 773–776
trunk interfaces, 769
trunk lines, 899
trunk link, 466, 467
trunk port, 466, 468, 488,
769, 770
allowed VLANs, 478–479
configuring, 476–480
native VLAN, 479–480
trunk troubleshooting, 768–776
trunked link, 482
trunking, 466, 469, 644, 770. See
also Dynamic Trunking Protocol
port configurations, 771
port modes, 771–773
trusted network, 503
TTL. See IP time to live; time to live
tunnel destination, 949
tunnel mode, 948, 952
tunnel mode gre, 948
tunnel mode gre ip, 948
tunnel source, 948, 949
tunneling, 54
tunneling protocols, 944, 1027
GRE, 946 –951
interface, 947–948
verifying, 950
2WAY state, 855–856
type of service (ToS), 824
UDI – VDSL 1097
U
UDI. See unique device identifier
UDP. See User Datagram Protocol
U/L bit. See global/local bit
unequal-cost load balancing,
807–809, 815, 1022, 1023
unicast address, 127–128, 128, 131,
982, 1012, 1014
global, 575, 575, 982, 1012
IPv6, 575
local-link, 1014
unicast frames, 434
Unified Communications
license, 683
Uniform Resource Locator
(URL), 96
unique device identifier (UDI), 683,
688, 690, 983
unique local addresses, 576, 599,
982, 1013
universal image, 682
UNIX, 89
unknown errors, 340
unshielded twisted-pair (UTP), 56,
57, 59, 64
untagged frames, 773, 777, 1008
untrusted network, 503
Update packet, 790
upstream routing, 481
URL. See Uniform Resource Locator
User Datagram Protocol (UDP), 18,
69, 103–105, 130, 717, 992
access list control, 519
advantages, 104
DHCP over, 99
DNS, 110
headers, 104, 104–105, 130
key features, 105, 105–106, 130
key protocols using, 109
overhead, 101
port numbers, 106, 106–107
segment format, 104–105
session multiplexing, 110
user exec mode, 219, 998
user mode, 265
user-mode passwords, 232, 671
username, 235, 918, 997
username username password
password, 985
use-TACACS, 231–232
U T P. See unshielded twisted-pair
UTP gigabit wiring, 61– 62
V
V.24, 914
V.35 interface, 7, 64, 217, 263, 910,
911, 914
variable length subnet masks
(VLSMs), 88, 142, 182–192, 571,
977, 995
benefits, 204
block sizes, 185, 185, 187
classless routing, 1001
design, 183, 187, 189, 191, 192,
192–193
DUAL, 791
EIGRP, 784, 791, 795–798
implementing networks, 183–192
tables, 185, 186, 187, 188, 190
variance parameter, 808
VDSL. See very-high-data-rate DSL
1098 verification – virtual LANs (VLANs)
verification
access lists, 526, 529
adjacency, 868
bridge ID, 631–636
Catalyst switches, 444–447
configuration, 253–264, 287,
363–365
default routing configuration,
363–365
DHCP, 292–293, 363–365
EIGR P, 811–828, 834
ACLs, 827
discontiguous networks,
818, 827
event logs, 817
K values, 818, 821–824, 827
load-balancing output, 815–816
local router status, 813
mapping network, 812
passive state routes, 815
routing protocols, 816–817
routing table, 814
topology table, 814–815
traffic data, 816
troubleshooting commands, 812
EIGRPv6, 831–833
flash memory, 673–674
Frame Relay, 952
GRE tunnels, 949–951, 952
HSR P, 709–713
multi-area OSPF networks, 859,
859–867
NAT, 549–550
OSPF configuration, 405–411, 412
OSPFv3, 594–599, 879–881
ping configuration, 363
port channels, 643–646
PPP, 952
running-config file, 253
serial links, 918–920, 952
spanning trees, 631
syslog, 723–726
Very Small Aperture Terminal
(VSAT), 904
very-high-data-rate DSL (VDSL), 908
via Connected, 832
violation restrict, 433
violation shutdown, 433, 443
virtual circuits, 101, 926
advantages, 463
DLCI status, 928
flexibility and scalability, 464
LMI information, 928–929
port numbers, 106
PVCs, 926, 927, 935–936,
951, 1026
security, 464
SVCs, 926
virtual gateway, 717
virtual LANs (VLANs), 11,
436, 980, 1007–1008. See
also inter-VLAN communication;
inter-VLAN routing; Per-VLAN
Spanning Tree+
advantages, 1007
allowed on trunk, 478–479
basics, 460–464
broadcast control, 463
broadcast domains, 11, 44–45
configuring, 472–487
creating, 472–473
frame tagging, 467–468
host separation, 462, 462–463
HSRP load balancing, 714
virtual MAC address – Web VPNs 1099
identifying, 465–469
layer 3 devices connecting, 25
management, 473, 482
native, 473, 479–480, 482,
773–776, 777
root bridges, 625, 1014,
1015, 1016
routing between, 469–472
switch port assignment,
475–476, 488
troubleshooting connectivity, 742,
763–776, 984
scenario, 764–768
steps for, 763–764
trunk, 768–776
unused port, 432
voice, 466
VPN similarities, 942
virtual MAC address, 703,
705–706
virtual private dial-up networks
(VPDNs), 944
virtual private networks (VPNs),
715, 898, 942–946, 986, 1027
benefits, 943–944
encryption, 946
extranet, 944, 986
IP network connectivity, 942
IPsec, 945
IPsec transforms, 945–946
remote access, 943, 986
site-to-site, 944, 986
VLAN similarities, 942
virtual router, 703, 1019
gateways, 715
GLBP, 717
HRSP, 704, 708–709
Virtual Router Redundancy Protocol
(VRRP), 700, 704, 715, 734
virtual terminal, 91
vlan, 472, 767
VLAN 1, 473, 479–480, 482,
484, 1005
VLAN database, 764
VLAN ID, 467–468, 471,
625, 1015
VLAN leaking, 768
VLANs. See virtual LANs
VLSMs. See variable length
subnet masks
voice access ports, 466
voice VLAN, 466
VPDNs. See virtual private
dial-up networks
VPNs. See virtual
private networks
VRRP. See Virtual Router
Redundancy Protocol
VSAT. See Very Small
Aperture Terminal
VTP clients, 294
vty, 232
VTY lines, 1010
controlling access, 514–515
passwords, 234–235, 303–305
VTY ports, 304, 306
VTY sessions, 998
W
WAN. See wide area network
Web VPN, 943
Web VPNs, 1027
1100 wide area network (WAN) – Yeager, William
wide area network (WAN), 152,
342, 898, 985–986, 1026–1027.
See also Frame Relay
clocking, 247, 247
connection bandwidth, 900
connection types, 900–901, 901
DTE/DCE, 911–912
encapsulation
layer 2, 902
mismatched, 921–922
hostname authentication, 228
IOS support, 902–904
load balancing, 804
router connections to, 7
serial connections, 910–912
serial connectors, 910
serial transmission, 910
terms, 899
troubleshooting, 742
wildcard mask, 394–395
access lists, 1010
standard access lists, 508–510
wildcards, 554, 794
window, 21, 989
windowing, 21–22, 22
wireless access points, 44
workgroup layer, 72
X
X.25, 390, 901, 902
xDSL, 903, 907, 908
Xmodem, 63, 677
Y
Yeager, William, 216
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