Configuring BGP On Cisco RoutersStudent Guide V2708129e9 8940 407c 899c 88cd8f0d2962 Config V4 Student

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Credits
Copyrightnotices:
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www.cisco.com/go/trademarks.Thirdpartytrademarksmentioned
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CopyrightDate:
©2015CiscoSystems,Inc.
Students,thisletterdescribesimportantcourseevaluation
accessinformation.
WelcometoCiscoSystemsLearning.ThroughtheCiscoLearningPartnerProgram,
Ciscoiscommittedtobringingyouthehighest-qualitytrainingintheindustry.Cisco
learningproductsaredesignedtoadvanceyourprofessionalgoalsandgiveyouthe
expertisethatyouneedtobuildandmaintainstrategicnetworks.
Ciscoreliesoncustomerfeedbacktoguidebusinessdecisions.Therefore,your
valuableinputwillhelpshapefutureCiscocoursecurricula,products,andtraining
offerings.PleasecompleteabriefCiscoonlinecourseevaluationofyourinstructor
andthecoursematerialsinthisstudentkit.Onthefinaldayofclass,yourinstructor
willprovideyouwithaURL,directingyoutoashortpost-courseevaluation.Ifthere
isnoInternetaccessintheclassroom,pleasecompletetheevaluationwithinthe
next48hoursorassoonasyoucanaccesstheweb.
OnbehalfofCisco,thankyouforchoosingCiscoLearningPartnersforyourInternet
technologytraining.
Sincerely,
CiscoSystemsLearning
WelcomeStudents
CourseIntroduction
Inthiscourseyouwilllearnhowtodesign,implement,andverifyaBGPinthe
customerandserviceprovidernetworks.
Uponcompletingthiscourse,youwillbeabletomeettheseobjectives:
Configure,monitor,andtroubleshootbasicBGPtoenableinterdomainroutingin
anetworkscenariowithmultipledomains
UseBGPpolicycontrolstoinfluencetherouteselectionprocesswithminimal
impactonBGProuteprocessinginanetworkscenariowhereyoumustsupport
connectionstomultipleISPs
UseBGPattributestoinfluencetherouteselectionprocessinanetwork
scenariowhereyoumustsupportmultipleconnections
CompletethecorrectBGPconfigurationtosuccessfullyconnectthecustomer
networktotheInternetinanetworkscenarioinwhichmultipleconnectionsmust
beimplemented
ConfiguretheserviceprovidernetworktobehaveasatransitASinatypical
implementationwithmultipleBGPconnectionstootherautonomoussystems
EnableroutereflectionaspossiblesolutiontoBGPscalingissuesinatypical
serviceprovidernetworkwithmultipleBGPconnectionstootherautonomous
systems
UseavailableBGPtoolsandfeaturestooptimizethescalabilityoftheBGP
routingprotocolinatypicalBGPnetwork
LearnerSkillsandKnowledge
Thissubtopicliststheskillsandknowledgethatlearnersmustpossesstobenefit
fullyfromthecourse.ThesubtopicalsoincludesrecommendedCiscolearning
offeringsthatlearnersshouldfirstcompletetobenefitfullyfromthiscourse.
Intermediate to advanced knowledge of Cisco IOS Software configuration
Configuring and troubleshooting RIP, EIGRP, OSPF and IS-IS
Skills and knowledge equivalent to those learned in:
Interconnecting Cisco Networking Devices v2.0, Part 1 (ICND1 v2.0) and
Part 2 (ICND2 v2.0), or
Interconnecting Cisco Networking Devices: Accelerated Version 2.0
(CCNAX v2.0)
Implementing Cisco IP Routing (ROUTE v2.0)
Building Cisco Service Provider Next-Generation Networks Part 1
(SPNGN1) v1.2
Building Cisco Service Provider Next-Generation Networks Part 2
(SPNGN2) v1.2
Overview
Thistopicdescribesthecoursegoalandobjectives.
Upon completing this course, you will be able to meet these objectives:
Describe how to configure, monitor, and troubleshoot basic BGP to enable
interdomain routing in a network scenario with multiple domains
Describe how to use BGP policy controls to influence the BGP route selection
process in a network scenario in which you must support connections to multiple
ISPs
Describe how to use BGP attributes to influence the route selection process in a
network scenario where you must support multiple connections.
Describe how to successfully connect the customer network to the Internet in a
network scenario in which multiple connections must be implemented
Upon completing this course, you will be able to meet these objectives:
Describe how to configure the service provider network to behave as a transit
AS in a typical implementation with multiple BGP connections to other
autonomous systems.
Enable route reflection as possible solution to BGP scaling issues in a typical
service provider network with multiple BGP connections to other autonomous
systems
Describe the available BGP tools and features to optimize the scalability of the
BGP routing protocol in a typical BGP network
Thegoalofthecourseistoprovidelearnerswithin-depthknowledgeofBGP.
CourseGoalandObjectives
Thistopicpresentsthesuggestedflowofthecoursematerials.
AM PM
Day1 CourseIntro
Module1:BGPOverview
Module1(Cont.)
Day2 Module2:BGPTransitAutonomousSystems Module3:RouteSelectionUsingPolicy
Controls
Day3 Module3(Cont.) Module4:RouteSelectionUsingAttributes
Day4 Module4(Cont.)
Module5:Customer-to-ProviderConnectivity
withBGP
Module5(Cont.)
Day5 Module6:ScalingServiceProviderNetworks
Module7:OptimizingBGPScalability
Module7(Cont.)
Theschedulereflectstherecommendedstructureforthiscourse.Thisstructure
allowsenoughtimefortheinstructortopresentthecourseinformationandforyouto
workthroughthelabactivities.Theexacttimingofthesubjectmaterialsandlabs
dependsonthepaceofyourspecificclass.
CourseFlow
Thistopicpresentsthetrainingcurriculumforthiscourse.
YouareencouragedtojointheCiscoCertificationCommunity,adiscussionforum
opentoanyoneholdingavalidCiscoCareerCertification(suchasCiscoCCIE®,
CCNA®,CCDA®,CCNP®,CCDP®,CCIP®,CCNP®SecurityandCCNP®Voice,or
CCSP™).ItprovidesagatheringplaceforCiscocertifiedprofessionalstoshare
questions,suggestions,andinformationaboutCiscoCareerCertificationprograms
andothercertification-relatedtopics.Formoreinformation,visit
http://www.cisco.com/web/learning/training-index.html.
YourTrainingCurriculum
Thissubtopicpresentsthelearningcenterinformation.
Additionalinformationisavailableat:
https://learningnetwork.cisco.com/community/learning_center
https://learningnetwork.cisco.com/community/certifications/ccna.
Your name
Your company
Job responsibilities
Skills and knowledge
Brief history
Objective
TrainingCurriculum
ThistopicpresentstheCiscoiconsandsymbolsthatareusedinthiscourse,aswell
asinformationonwheretofindadditionaltechnicalreferences.
CiscoGlossaryofTerms
ForadditionalinformationonCiscoterminology,refertotheCiscoInternetworking
TermsandAcronymsglossaryoftermsat
http://docwiki.cisco.com/wiki/Category:Internetworking_Terms_and_Acronyms_(ITA)
.
AdditionalReferences
Introduction
BGPisaninterdomain(interautonomoussystem)routingprotocolthatisusedto
exchangeroutinginformationfortheInternet.BGP,bydesign,isaveryrobustand
scalableroutingprotocol.BecauseBGPisdeployedasaninterdomainrouting
protocol,ithasmanyrichfeaturesthatallowyoutoimplementavarietyofrouting
policies.
InthismoduleyouwilllearnthebasicBGPtechnology,detailsaboutBGPsession
establishmentandroutinginformationexchange.YouwillalsolearnthebasicCisco
IOSBGPconfigurationandtroubleshootingtasks.
Uponcompletingthismodule,youwillbeableto:
DefineshowtoidentifyappropriateBGPuseandlimitations.
DescribeBGPpathattributesandthefunctionalityofeachattribute.
DescribeBGPneighborsandneighborsessionestablishmentprocedures.
DescribeBGProuteprocessingandBGProutingupdates
DescribehowtoconfigurebasicBGP
DescribehowtoperformthestepstocorrectbasicBGPconfigurationand
sessionerrors
BGPOverview
Overview
TheBGPisaveryrobustandscalableroutingprotocol.Youcanseethatfromthe
factthatitistheroutingprotocolthatisusedontheInternet.Serviceprovidersand
customernetworks,suchasuniversitiesandcorporations,usuallyuseanIGPsuch
asEIGRP,OSPF,orIS-ISfortheexchangeofroutinginformationwithintheir
networks.AnycommunicationbetweentheseIGPsandtheInternetorbetween
serviceproviderswillbeaccomplishedthroughBGP.Thislessonintroducesbasic
BGPcharacteristicsandfeatures.
Uponcompletingthislesson,youwillbeableto:
Describeinterdomainroutinginrelationtothedesigngoalsofinterdomain
routingprotocols
Explaintheneedforaninterdomainroutingprotocol
ListthebasiccharacteristicsofBGP
ListBGPdevelopmentconsiderations
Identifywhenasingle-homedcustomershoulduseBGPasaninterdomain
routingprotocol
DescribewhenBGPisappropriateforthemultihomedcustomer
DescribetheuseofBGPinatransitautonomousbackbone
ListsomeofthelimitationsofBGP
IntroducingBGP
WhentalkingtopeoplewhoareinvolvedwithInternetrouting,network
administratorscommonlyusetheterms"autonomoussystem,""interdomainrouting,"
"interiorroutingprotocol,"and"exteriorroutingprotocol."
An AS is a collection of networks under a single technical administration.
An IGP is run inside an AS, resulting in optimum intra-AS routing.
An EGP is run between autonomous systems to enable routing policies and
improve security.
Theseterms,whichmaybeconfusingforanovice,aredefinedasfollows:
AnASisacollectionofnetworksunderasingletechnicaladministration.Other
definitionsrefertoacollectionofroutersorIPprefixes,butintheendtheyare
allessentiallythesamething.Theimportantprincipleisthetechnical
administration,whichmeanssharingthesameroutingprotocolandrouting
policy.Legalandadministrativeownershipoftheroutersdoesnotmatterwith
autonomoussystems.AutonomoussystemsareidentifiedbyASnumbers.AS
numbersare16-bit,unsignedintegersrangingfrom1to65,535.PublicAS
numbers(1to64,511)areassignedandmanagedbyanInternetregistry.A
rangeofprivateASnumbers(64,512to65,535)hasbeenreservedfor
customersthatneedanASnumbertorunBGPintheirprivatenetworks.New
32-bitASnumberswerecreatedwhentheASnumberpoolfromIANA
approachedexhaustion.
Interdomainroutingisroutingbetweenautonomoussystems.Itisusuallybased
onasetofpolicies,notjustthetechnicalcharacteristicsoftheunderlying
infrastructure.
Exteriorroutingprotocols(BGPbeingtheonlyexteriorroutingprotocolthatis
usedtoday)areprotocolsthathavetherightsetoffunctionstosupportvarious
interdomainroutingpolicies.Suchprotocolsarecontrarytointeriorrouting
protocols(forexample,OSPF,IS-IS,orEIGRP).Interiorroutingprotocolsfocus
onlyonfindingtheoptimum(usuallyfastest)routebetweentwopoints,without
respecttoroutingpolicies.
DesignGoalsforInterdomainRouting
Scalability
The Internet has more than 600,000 aggregated routes and is still growing.
Secure routing information exchange
Routers from another AS cannot be trusted.
Tight filters are required; authentication is desirable.
Support for routing policies
Routing between autonomous systems might not always follow the optimum
path.
Exterior routing protocols have to support a wide range of customer
requirements.
InterdomainRouting
Theexampleillustratestheneedforaninterdomainroutingprotocol.Itdepictstwo
companiesthatareconnectedtotheInternetvialeasedlinesofdifferingspeeds.
Assuming standard IGP route selection rules, how will the traffic between AS 1
and AS 20 flow?
Will AS 2 allow this traffic?
How would you solve this problem with OSPF or EIGRP?
InroutingprotocolsotherthanBGP,routingdecisionsarenormallymadetotake
advantageofthehighestbandwidthavailable.Doingsowouldmaketrafficbetween
AS1andAS20flowviaAS2.ThissituationwouldallowtheusersinCompanyAto
generatetrafficontheInternetaccesslinethatwaspurchasedandpaidforby
CompanyB.So,thissituationisnotdesirableforAS2.
CompanyBisunlikelytoallowtrafficfromCompanyAtoreachtheInternetusing
theCompanyBaccessline.CompanyB,infact,couldcreateanaccess-listblocking
allIPpacketsfromAS1frombeingtransmittedonthe2-Mbpsseriallinefrom
CompanyBtotheInternet.ThatactionwouldcreateablackholebecauseCompany
AwouldsenditspacketstoCompanyBandthenCompanyBwoulddropthem.
Toavoidthissituation,CompanyBmustmakesurethatthepacketsfromCompany
AthataredestinedfortheInternetareneversenttoCompanyB.Also,CompanyB
mustmakesurethatpacketsfromtheInternetthataredestinedforCompanyAare
neversentusingtheInternetaccesslinetoCompanyB.CompanyBcould
implementaroutingpolicythatindicatesthatAS2willreceivereachability
informationfromAS1foritsownusebutthatAS2willnotforwardthatparticular
informationtotheInternet.Also,AS2willreceivereachabilityinformationaboutthe
InternetfromitsISPbutwillneverforwardthatinformationtoAS1.Onlynetworks
localtoAS2willbesenttoAS1.
TheresultofthisroutingpolicywouldbethatAS1seesallnetworkswithinAS2as
reachableoverthe2-MbpslinkthatdirectlyconnectsAS1withAS2.Theroutersin
AS1willnotseetherestoftheInternetasreachablethroughAS2.Therefore,AS1
forwardspacketstowardtheInternetdirectlyoverthe64-kbpslink.
Also,theIPnetworksinAS1willappearreachablebyAS2overthe2-Mbpslink,
whichdirectlyconnectsAS1withAS2.However,theISPwillnotreceivethat
reachabilityinformationfromAS2;itwillreceiveitonlyfromAS1.Therefore,traffic
fromtheInternettoCompanyAwillbetransmittedoverthe64-kbpslink.
ThisroutingpolicyiseasytoimplementwhennetworkadministratorsareusingBGP
butimpossibletoimplementwithanyotherroutingprotocol.EIGRP,forexample,
candoroutefilteringonlyonindividualIPsubnets,notonallprefixesbelongingtoan
AS.Link-stateprotocols,suchasOSPF,cannotdopowerfulroutefilteringatall.
BGPusesASnumberstodoroutefiltering.Thisapproachmakesitpossibletoscale
BGPoverInternet.
WhyExternalRoutingProtocols?
BGPisadistancevectorprotocol.So,BGPannouncestoitsneighborsthoseIP
networksthatitcanreachitself.Thereceiversofthatinformationsay,"IfthatAScan
reachthosenetworks,thenIcanreachthemviatheAS."
BGP is a distance vector protocol with enhancements:
Reliable updates
Triggered updates only
Rich metrics (called path attributes)
BGP is designed to scale to huge internetworks
IftwodifferentpathsareavailabletoreachthesameIPsubnet,thentheshortest
pathisused.Thisdeterminationrequiresamechanismcapableofmeasuringthe
distance.Alldistancevectorprotocolshavesuchmechanisms,called"metrics."BGP
containsaverysophisticatedmethodofcomputingtheshortestpathbyusing
attributesthatareattachedtothereachableIPsubnet.
BGPsendsroutingupdatestoitsneighborsbyusingareliabletransport.This
techniquemeansthatthesenderoftheinformationalwaysknowsthatthereceiver
hasactuallyreceivedtheinformation.Asaresult,thereisnoneedforperiodic
updatesorroutinginformationrefreshes.InBGP,onlyinformationthathaschanged
istransmitted.
BGPenablesreliableinformationexchangeandiscapableofbatchingtherouting
updates.ThesetwocharacteristicsallowBGPtoscaletolarge,Internet-sized
networks.
Reliable updates:
TCP used as transport protocol
No periodic updates
Periodic keepalives to verify TCP connectivity
Triggered updates are batched and rate-limited
Every 5 seconds for internal peer
Every 30 seconds for external peer
ThereliabletransportmechanismthatBGPusesisstandardTCP.BGPisan
applicationprotocolthatusesboththeTCPandIPprotocolsforreliableconnections.
BecauseBGPusesareliabletransport,thesenderknowsthatthereceiverhas
actuallyreceivedthetransmittedinformation.Thiscapabilitymakesperiodicupdates
unnecessary.
ArouterthathasreceivedreachabilityinformationfromaBGPpeermustbesure
thatthepeerrouterisstillthere.Otherwise,theroutercouldroutetraffictowarda
next-hoprouterthatisnolongeravailable,causingtheIPpacketstobelostina
blackhole.TCPdoesnotprovidetheservicetosignalthattheTCPpeerhasbeen
lost,unlesssomeapplicationdataisactuallytransmittedbetweenthepeers.Inan
idlestate,wherethereisnoneedforBGPtoupdateitspeer,thepeercouldbe
unreachablewithoutTCPdetectingit.Therefore,BGPtakescareofdetectingthe
presenceofneighborsbyperiodicallysendingsmallBGPkeepalivepacketstothem.
ThesepacketsareconsideredapplicationdatabyTCPandthereforemustbe
transmittedreliably.AccordingtotheBGPspecification,thepeerrouteralsomust
replywithaBGPkeepalivepacket.
WhenBGPwascreated,akeydesigngoalwastobeabletohandleenormous
amountsofroutinginformationinverylargeandcomplexnetworks.Inthis
environment,manylinkscouldgoupanddown(flapping),causingtopology
changes.Theroutingprotocolmustconsiderallthesechanges.Butlow
convergencetimeandquickresponsestotopologychangesrequirefastupdates
andhighCPUpowertoprocessbothincomingandoutgoingupdates.Thelargerthe
network,themoreupdatespersecondcanbeexpectedifimmediateresponseis
required.Thepresenceoftoomanyupdatesinlargenetworkscanjeopardize
networkscalability.
ThedesignersofBGPdecidedthatscalabilitywasamoreimportantissuethanlow
convergencetime,soBGPwasdesignedtobatchupdates.Anychangesthatare
receivedwithinthebatchintervaltimearesaved.Attheendoftheinterval,onlythe
remainingresultisforwardedinanoutgoingupdate.Ifanetworkflapsseveraltimes
duringthebatchinterval,onlythestateattheendoftheintervalissentinanupdate.
BGPCharacteristics
ThebatchingfeatureavoidsanuncontrolledfloodofupdatesallovertheInternet
becausethebatchingprocedurelimitsthenumberofupdates.
Common BGP uses:
Customers that are connected to more than one service provider
Service provider networks (transit autonomous systems)
Service providers exchanging traffic at an exchange point (CIX, GIX, NAP, ...).
NAP defines the exchange points between region and core.
CIX or GIX points define international exchange points.
Network cores of large-enterprise customers
NAP—Network Access Point, CIX—Commercial Internet eXchange, GIX—Global
Internet eXchange
ThedesignersoftheBGPprotocolhavesucceededincreatingahighlyscalable
routingprotocol,whichcanforwardreachabilityinformationbetweenautonomous
systems(alsoknownasroutingdomains).Thedesignershadtoconsideran
environmentwithanenormousnumberofreachablenetworksandcomplexrouting
policiesthatweredrivenbycommercialratherthantechnicalconsiderations.
BGP was designed to perform well in the following areas:
Interdomain routing applications
Huge internetworks with large routing tables
Environments that require complex routing policies
Some design tradeoffs were made:
BGP uses TCP for reliable transport—CPU-intensive
Scalability is the top priority—slower convergence
TCP,awell-knownandwidelyprovenprotocol,waschosenasthetransport
mechanism.ThatdecisionkeptBGPsimple,butitincreasedtheCPUresource
requirementsforroutersrunningBGP.Thepoint-to-pointnatureofTCPalso
introducesaslightincreaseinnetworktraffic.Thisincreaseoccursbecauseany
updatethatshouldbesenttomanyreceivershastobemultipliedintoseveral
copies.ThosecopiesarethentransmittedonindividualTCPsessionstothe
receivers.
Whenevertherewasadesignchoicebetweenfastconvergenceandscalability,
scalabilitywasthetoppriority.Thebatchingofupdatesandtherelativelylow
frequencyofkeepalivepacketsareexamplesofdesignersplacingconvergencetime
secondtoscalability.
BGPconvergencetimescanbemodifiedwiththe
configurationofnondefaultvaluesforBGPscanand
advertisementtimers.
BGPDevelopmentConsiderations
NormalInternetaccesstoasingleISPdoesnotrequireBGP;staticroutesaremore
commonlyusedtohandlethissituation.SmallISPsthatbuyInternetconnectivity
fromotherISPsusethistypeofconnectivitymoreoften.Especiallyiftheywantto
starttheirbusinesstheproperway—byusingtheirownASnumberandhavingtheir
ownaddressspace.
Large customer or small ISP connecting to the Internet
ThefigureshowsacustomernetworkthatisconnectedtotheInternetusingasingle
ISP,butsuchascenarioisgenerallynotthecasewhenBGPisused.
UseGuidelines—Single-HomedCustomers
Use BGP between the customer and the service provider in these situations:
Customers multihomed to the same service provider
Customers that need dynamic routing protocol with the service provider to detect
failures
Hint: Use private AS number for these customers.
Smaller ISPs that need to originate their routes in the Internet
Use static routes in all other cases:
Static routes always simpler than BGP
Undercertainconditions,BGPmustbeconfiguredbetweenthecustomerandthe
serviceprovider.Forexample,BGPisneededwhencustomersaremultihomedto
thesameserviceprovider,thatis,thecustomernetworkshavemultiplelinks
connectingthemwiththeserviceprovidernetwork.Suchcustomersrequiredynamic
routingprotocolinteractionwiththeserviceprovidertodetectlinkfailures.PrivateAS
numbers(ASnumbersbetween64,512and65,534orbetween4,200,000,000—
4,294,967,294)areusuallyimplementedinBGPconfigurationsforthesecustomers.
CustomersthatplantoconnecttomorethanoneISP,andsmallISPsthatplanto
havemultipleInternetconnectionsinthefuture,usuallyuseBGPwiththeirservice
provider.Theyusethisoptionevenwhentheyhaveasinglelinkwiththeservice
providerinordertobepreparedforfutureupgrades.
Inallothercases,usingstaticroutesfromtheserviceprovidertowardthecustomer
andusingadefaultstaticroutefromthecustomertowardtheserviceprovideristhe
preferredmethodofprovider-to-customerroutingintheInternet.
Single-HomedCustomers
TheBGPisappropriateforthemultihomedcustomer.Thecustomermusthaveits
ownofficiallyassignedASnumber.Thecustomerisalsoresponsibleforannouncing
itsownIPnetworkstobothISPs.BothISPsforwardallroutesthattheyreceivefrom
theInternettothecustomernetwork.Thecustomershouldavoidforwardingany
routinginformationthattheyreceivefromoneISPtotheother.Otherwise,the
customerbecomesatransitproviderbetweenthetwoISPs.Mostcustomersliketo
avoidthissituationbecauseitcreatesaresourcedrainonroutersandnetworklinks.
Customer connecting to more than one service provider
ThefigureillustratesacustomernetworkthatisconnectedtotwodifferentISPs,
requiringtheuseofBGPforfullredundancy.
Fullredundancyisachievedinthissetup.Ifeitherofthetwoaccesslinksfails,the
reachabilityinformationthatwaspreviouslytransmittedonthenow-failedlinkis
withdrawn.ButBGPreachabilityinformationisstillannouncedbythecustomer
routerovertheremaininglink.Thus,theISPstillseesallnetworkswithinthe
customerASasreachablebutonlyovertheremainingpath.Also,receivedroutes
fromtheInternetarewithdrawnwhenthelinkfailsbutroutesthatarereceivedover
theremaininglinkarenotaffected.Thus,theInternet,includingtheISPtowhichthe
directconnectionhasfailed,isstillreachableovertheremaininglink.
Thisdesigncanalsohandleotherproblems.Acasewherebothaccesslinksare
available,buttheconnectionbetweenoneoftheISPsandtherestoftheInternetis
lost,worksasfollows.TheISPthathasaproblemreachingtherestoftheInternet
withdrawsallthoseroutesandtellsthecustomerASthatitcannolongerreachthe
Internet.ButthenetworkslocaltotheISPwiththeInternetreachabilityproblemare
stillreachablebythecustomer,sothoseroutesarenotwithdrawn.Thenetworksin
thecustomerASarestillreachablebytheISPintrouble,butthatISPcannolonger
forwardtheannouncementtotherestoftheInternet.TherestoftheInternetwill,
however,seethecustomernetworksasreachableoverthepathtotheotherISP,
whichisfullyfunctional.
UserGuidelines—MultihomedCustomers
BGP is almost mandatory for multihomed customers.
Multihomed customers have to use public AS numbers.
Multihomed customers should use a provider-independent address space.
Thefollowinguseguidelinesapplytomultihomedcustomers:
AlthoughtherearedesignswhereBGPcouldbeavoided,mostmultihomed
customersneedtouseBGPwiththeirserviceproviders.
MultihomedcustomersmusthavetheirownASnumbers.Itisrecommendedto
useapublicASnumber(between1and23.455,between23.457and64.534,or
between131.072and4.199.999.999).
Multihomedcustomersshoulduseaprovider-independentaddressspace,which
isallocatedtothemdirectlybyanInternetregistry.
MultihomedCustomers
BGPismostcommonlyimplementedinserviceprovidernetworkstoensure
connectivitybetweencustomersandtherestoftheInternet.AnISPmightexchange
BGPupdateswiththecustomersorusestaticroutingtowardthem.ThatISPalso
connectstootherISPsandisrequiredtoforwardtheroutesthatarereceivedfrom
customerstotherestoftheInternet,aswellasintheotherdirection.Asaresult,
userdatatrafficstartstoflowbetweenthecustomersandtherestoftheInternet.
Suchanetwork,providingtransitservicestotrafficthatisoriginatedinother
networks,iscalleda"transitautonomoussystem,"or"transitAS."AtransitASisan
ASthatexchangesBGProutinginformationwithotherautonomoussystemsand
forwardsinformationthatisreceivedfromoneAStoanotherAS.
Using BGP to exchange routes is mandatory for transit autonomous systems
(provider networks carrying customer traffic).
Whenroutinginformationisforwarded,thereceiverwillseeanavailablepathtoa
destinationandstarttransmittinguserdatatowardthedestinationusingthatpath.
ThetransitASmustbepreparedtorelaytheuserdata.
ISPnetworkscansometimeshavededicatedpeer-to-peerconnections.These
connectionsaresometimescalledprivatepeering.ISPsalsointerconnectat
exchangepoints.Technically,anexchangepointisjustamultiaccesssubnet:aLAN
(forexample,aGigabitEthernetorFastEthernetswitch),aDPTring,oranATM
switch.ManyISPscanconnecttoanexchangepointandestablishBGPsessions.
Thebenefitofanexchangepointisthatitishighlyscalable.Thereisnoneedfor
morephysicalinterfacesintheISPborderrouterwhenanewISPislaunched.Ifthe
alreadyestablishedISPswantto,theycanopenaBGPsessionwiththenewISP.
Whenthissessionisopened,theystarttoexchangeroutinginformationandthen
userdatatrafficovertheexchangepoint.
TransitAutonomousSystems
BGP-enabledroutersmakeforwardingdecisionsbasedonthedestination
IPaddressonly.ThesourceIPaddressdoesnotaffecttheBGPdecision.
BGP and associated tools cannot express all routing policies.
You cannot influence the routing policies of downstream autonomous systems.
“BGP does not enable one AS to send traffic to a neighbor AS intending that the
traffic take a different route from that taken by traffic originating in the neighbor AS.”
(RFC 1771)
IfanASactsasatransitASforotherautonomoussystems,theIPpacketsthatare
createdandtransmittedfromtheotherautonomoussystemsarenottreated
differentlyfromtheIPpacketsthatarecreatedandtransmittedfromthelocalAS.If
thelocalAShasdecidedthatthebestpathtoreachacertaindestinationisviaa
specificnext-hoprouter,thenitwillroutealluserdatatraffictowardthefinal
destinationviathatspecificnext-hoprouter.ThelocalASmakesitsdecisionbased
ondestinationaddressonly,regardlessofwhichIPhosthassourcedtheIPpackets.
BGPLimitations
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
BGP is an enhanced distance vector protocol with reliable transport.
Customers that plan to connect to more than one ISP, and small ISPs that plan
to have multiple Internet connections in the future, usually use BGP with their
service provider. Most multihomed customers use BGP with their service
providers.
A transit AS is an AS that exchanges BGP routing information with other
autonomous systems and forwards information that is received from one AS to
another AS.
In the BGP, you cannot influence the routing policies of downstream
autonomous systems.
Summary
Overview
Toaidroutersincalculatingthebestroutetoselectwhenmultiplepathstoa
particulardestinationexist,routesthatarelearnedviaBGPhavepropertiesthatare
associatedwiththem.ThesepropertiesarereferredtoasBGPpathattributes.An
understandingofhowBGPpathattributesinfluencerouteselectionisrequiredto
designrobustBGPnetworks.
Uponcompletingthislesson,youwillbeableto:
DescribetheconceptofBGPpathattributes
Explainthedifferencebetweenmandatoryanddiscretionarywell-knownBGP
attributes
ExplainthedifferencebetweennontransitiveandtransitiveoptionalBGP
attributes
DescribethefunctionalityoftheAS-pathattribute
Describethefunctionalityofthenext-hopattribute
UnderstandingBGPPathAttributes
EachBGPupdateconsistsofoneormoreIPsubnetsandasetofattributesthatare
attachedtothem.
BGP metrics are called path attributes.
BGP attributes are categorized as "well-known" and "optional."
All compliant implementations must recognize well-known attributes.
You should not expect that all implementations can recognize optional attributes.
Only some implementations (could be private) are able to recognize optional
attributes.
AllBGPimplementationsarerequiredtorecognizecertainattributes.Those
attributesarecalled"well-knownBGPattributes."
Attributesthatarenotwell-knownarecalled"optional."Optionalattributesareeither
specifiedinalaterextensionofBGPoreveninprivatevendorextensionsthatare
notdocumentedinastandarddocument.
BGPPathAttributes
Well-knownattributesaredividedintomandatoryanddiscretionary.
All well-known attributes are propagated to other neighbors.
The difference between mandatory and discretionary well-known attributes:
Mandatory well-known attributes must be present in all update messages.
Discretionary well-known attributes are optional; they could be present in
update messages.
Thereisasmallsetofthreespecificwell-knownattributesthatarerequiredtobe
presentoneveryupdate.Theseattributesarethenext-hop,AS-path,andorigin
attributesandarereferredtoas"mandatorywell-knownattributes."
Otherwell-knownattributesmayormaynotbepresent,dependingonthe
circumstancesunderwhichtheupdatesaresentandthedesiredroutingpolicy.The
well-knownattributesthatcouldbepresent,butarenotrequired,arecalled
"discretionarywell-knownattributes."
WhenarouterreceivesaBGPupdate,itanalyzestheattachedattributesand
comparesthemwiththeattributesthatwereattachedtothesameIPsubnetwhenit
wasreceivedfromadifferentsource.Therouterthenmakesadecisionaboutwhich
sourceindicatesthebestpathtotheparticularIPsubnet.Thebestrouteis
propagated,alongwithitswell-knownattributes,tootherBGP-speakingneighbors.
MandatoryWell-KnownBGPAttributes
Origin
The origin of a BGP route
i RouteoriginatedinanIGP
e RouteoriginatedinEGP
? RoutewasredistributedintoBGP
AS-path
Sequence of AS numbers through which the network is accessible
Next-hop
IP address of the next-hop router
Thethreemandatorywell-knownattributesareorigin,AS-path,andnext-hop:
Origin:WhenarouterfirstoriginatesarouteinBGP,itsetstheoriginattribute.
IfinformationaboutanIPsubnetisinjectedusingthenetworkcommandorvia
aggregation(routesummarizationwithinBGP),theoriginattributeissetto"i"for
IGP.IfinformationaboutanIPsubnetisinjectedusingredistribution,theorigin
attributeissetto"?"forunknownorincompleteinformation(thesetwowords
havethesamemeaning).Theorigincode"e"wasusedwhentheInternetwas
migratingfromEGPtoBGPandisnowobsolete.
AS-path:TheegressroutermodifiestheAS-pathattributeeverytime
informationaboutaparticularIPsubnetpassesoveranASborder.Whena
routerfirstoriginatesarouteinBGP,theAS-pathattributeisempty.Eachtime
thattheroutecrossesanASboundary,thetransmittingASprependsitsownAS
numbertoappearfirstintheASpath.Youcantrackthesequenceof
autonomoussystemsthroughwhichtheroutehaspassedbyusingtheAS-path
attribute.
Next-hop:Therouteralsomodifiesthenext-hopattributeastheroutepasses
throughthenetwork.ThisattributeindicatestheIPaddressofthenext-hop
router.Thenext-hoprouteristheroutertowhichthereceivingroutershould
forwardtheIPpacketstoreachthedestinationthatisadvertisedintherouting
update.
DiscretionaryWell-KnownBGPAttributes
Local preference
Used for consistent routing policy within AS
Atomic aggregate
Informs the neighbor AS that the originating router aggregated routes
AllBGPimplementationsmustsupportdiscretionarywell-knownattributes.However,
Well-KnownBGPAttributes
discretionarywell-knownattributesdonothavetobepresentinallBGPupdates.
Routersusediscretionarywell-knownattributesonlywhenthosefunctionsare
required.
Thefollowingaredescriptionsofthesetwoattributes:
Localpreference:Localpreferenceisusedintherouteselectionprocess.This
attributeiscarriedwithinanASonly.Therouterprefersaroutewithahighlocal
preferencevaluetoaroutewithalowvalue.Bydefault,routesthatarereceived
fromapeerASaretaggedwiththelocalpreferencesettoavalueof100before
theyareenteredintothelocalAS.IfthisvalueischangedthroughBGP
configuration,theBGPselectionprocessisinfluenced.Becauseallrouters
withintheASgettheattributealongwiththeroute,aconsistentroutingdecision
ismadethroughouttheAS.
Atomicaggregate:Theatomicaggregateattributeisattachedtoaroutethatis
createdasaresultofroutesummarization(called"aggregation"inBGP).This
attributesignalsthatinformationthatwaspresentintheoriginalroutingupdates
mayhavebeenlostwhentheupdatesweresummarizedintoasingleentry.
OptionalBGPattributesaretransitiveornontransitive.
The difference between transitive or nontransitive optional BGP attributes:
Transitive optional attributes: Propagated to other neighbors if not
recognized; partial bit set to indicate that the attribute was not recognized
Nontransitive optional attributes: Discarded if not recognized
Recognized optional attributes are propagated to other neighbors based on their
meaning (not constrained by transitive bit).
Whenarouterreceivesanupdatethatcontainsanoptionalattribute,therouter
checkstoseewhetheritsimplementationrecognizestheparticularattribute.Ifit
does,thentheroutershouldknowhowtohandleitandwhethertopropagateit.
Iftherouterdoesnotrecognizetheattribute,theBGPimplementationshouldlook
forthetransitivebitintheattributecode.Someattributes,althoughnotrecognized
bytherouter,mightstillbehelpfultoupstreamroutersandshouldbepropagated.
Theseattributes(called"transitiveoptionalattributes")arepropagatedevenwhen
theyarenotrecognized.Ifarouterpropagatesanunknowntransitiveoptional
attribute,itsetsanextrabitintheattributeheader.Thisbitiscalledthe"partialbit."
The"partialbit"indicatesthatatleastoneoftheroutersinthepathdidnotrecognize
themeaningofatransitiveoptionalattribute.
Otherattributes,called"nontransitiveoptionalattributes,"mightbeofnovalueto
upstreamroutersifarouterearlierinthepathdoesnotrecognizethem.Routersthat
donotrecognizetheseattributesdropthem.
Nontransitive attributes
MED:
Used to discriminate between multiple entry points to a single AS
Transitive attributes
Aggregator
Specifies IP address and AS number of the router that performed route
aggregation
Community
Used for route tagging
OneofthenontransitiveoptionalattributesistheMEDattribute,whichalso
influencestheBGProuteselectionprocess.Wheneverthereareseverallinks
betweentwoadjacentautonomoussystems,oneAScanusetheMEDattributeto
tellanotherAStopreferoneofthelinksforspecificdestinations.
Transitiveoptionalattributesincludethefollowing:
Aggregator:IdentifiestheASandtherouterwithinthatASthatcreatedaroute
summarization,oraggregate.
Community:Anumericalvaluethatcanbeattachedtocertainroutesasthey
passaspecificpointinthenetwork.Forfilteringorrouteselectionpurposes,
otherrouterscanexaminethecommunityvalueatdifferentpointsinthenetwork
.BGPconfigurationmaycauserouteswithaspecificcommunityvaluetobe
treateddifferentlythanothers.
OptionalBGPAttributes
AnedgeroutermodifiestheAS-pathattributeeverytimeinformationabouta
particularIPsubnetpassesoveranASborder.Whenarouterfirstoriginatesaroute
inBGP,theAS-pathattributeisempty.ThelocalASnumberisprependedtotheAS
patheachtimethattheroutecrossesanASboundary.
The AS-path attribute is empty when a local route is inserted in the BGP table.
The AS number of the sender is prepended to the AS-path attribute when the
routing update crosses AS boundary.
The receiver of BGP routing information can use the AS-path attribute to
determine through which AS the information has passed.
An AS that receives routing information with its own AS number in the AS path
silently ignores the information.
ThereareseveralconsequencesofAS-pathattributebehavior:
WhenyouexamineBGProutes,theASpathcanbeinterpretedasthe
sequenceofautonomoussystemsthatmustbepassedthroughtoreachthe
indicatednetwork.TheASthatoriginallyinjectedtherouteintoBGPisalways
foundattherightmostendoftheASpath.
Itiseasytodistinguishlocalroutesfromroutesthathavebeenreceivedfrom
otherautonomoussystems.BGProuteswithanemptyASpathwereinjected
intoBGPfromwithinthelocalAS.
TheAS-pathattributeisalsousedtoavoidroutingloops.Whenarouterreceivesa
BGPupdate,itcheckstheAS-pathattributeandlooksforitsownASnumber.Ifthat
numberisfoundintheASpath,thentheroutehasalreadycrossedthelocalASand
therouterisnowfacedwitharoutinginformationloop.Toavoidthissituation,the
routeissilentlyignored.
AS-PathAttributeExample
ThefigureshowshowBGPlooppreventionworks.
Thenetwork10.0.0.0/8islocaltoAS123.TherouterinAS123injectstheroute
10.0.0.0/8intoBGPwithanemptyAS-pathattribute.
WhentheedgerouterinAS123sendsaroutingupdateaboutnetwork10.0.0.0/8to
AS2,theASnumber123isprependedtotheemptyASpath,resultinginanAS
pathconsistingofonly123.Thesendingrouterdoestheprependingaspartofthe
outgoingBGPupdateprocessing.WhiletherouteisstillwithinAS123,theAS-path
entryforAS123doesnotappearintheASpath.
TherouterinAS21propagatestheinformationaboutthenetwork10.0.0.0/8toAS
37.BecauseitissendingtheBGPupdatetoAS37,itprependsitsownASnumber
totheASpath,resultinginanASpathconsistingofthesequenceof21123.
AS37alsopropagatesthereceivedroutetoAS123.Toavoidaroutingloop,where
AS123mighttrytoreachitsownnetwork(10.0.0.0/8)viaAS37,BGPhasabuilt-in
mechanismthatcausestherouterinAS123todroptheincomingupdateassoonas
itfindsitsownAS123intheASpath.Noerrorwillbesignaled,becausenothingis
reallywrong.ItismerelytheprocedurethatBGPusestoavoidaroutinginformation
AS-PathAttribute
loop.
1.
2.
3.
TheBGPnext-hopattributeidentifiestheIPaddressthataroutershoulduseto
forwardpacketstowardthedestinationthatisannouncedinaBGProutingupdate.
The next-hop attribute indicates the next-hop IP address that is used for packet
forwarding.
It is usually set to the IP address of the sending EBGP router.
Can be set to a third-party IP address to optimize routing.
Inmostcases,thesendingroutersetsthenext-hopattributetoitsownIPaddress.
Therearecases,however,wherethenext-hopIPaddresspointstoathirdrouter.
Next-HopProcessing
Thefigureshowstheusualnext-hopprocessing.
Next-hop attribute is usually set to the IP address of the sending router.
Theprocessingisasfollows:
R2announcesnetwork21.0.0.0/8toR1.TheoutgoingIPaddressofR2(the
addressthatisusedtoestablishtheBGPTCPsession)isusedastheBGPnext
hop.
R1receivestheroutingupdateandinstallsitinitsBGPtableandroutingtable.
ShouldR1needtoforwardpacketstowardnetwork21.0.0.0/8,itwouldsend
thosepacketstowardtheIPaddress10.0.0.1(R2).
WhenR1propagatestheinformationabout21.0.0.0/8toR3,itsetstheBGP
next-hopattributetoitsownIPaddress.
Next-HopProcessingonSharedMedia
If the receiving BGP router is in the same subnet as the current next-hop
address, the next-hop address remains unchanged to optimize packet
forwarding.
Thenext-hopprocessingchangesiftheBGProutersconnecttoasharedsubnet.In
thefigurehere,ifR1announcesthenetwork21.0.0.0/8toR3withtheBGPnext-hop
addresssettoR1,thepacketsfromAS37towardnetwork21.0.0.0/8willhaveto
crossthesharedLANtwice.R1thussendstheroutingupdatetowardR3withthe
BGPnext-hopaddressunchanged(stillpointingtowardR2),allowingoptimaldata
transferacrossthesharedLAN.
Moreformally,theBGPnext-hoprulestatesthatifthe
Next-HopAttribute
currentBGPnexthopisinthesameIPsubnetasthe
receivingrouter,thenext-hopaddressisnotchanged;
otherwise,thenext-hopattributeischangedtotheIP
addressofthesendingrouter.
Next-HopProcessingonNBMANetwork
BGP next-hop processing can break connectivity with improper network designs
over partially meshed WAN networks.
BGPnext-hopprocessingresultsinoptimumdatatransferoversharedmedia(for
example,aLANsubnet).Inpartiallymeshednetworks(suchasDMVPN),BGPnext-
hopprocessingcanbreakIPconnectivity.
Consider,forexample,thenetworkdiagraminthefigure.R1sendsaroutingupdate
aboutnetwork21.0.0.0/8toR3withR2settothenext-hopaddress(astheyareall
inthesamesubnet).Becausethereisnodirectconnection(virtualcircuit)between
R3andR2butR3stilltriestosendpacketsdirectlytowardR2,theconnectivity
betweenAS37andAS21isbroken.
Therearetwowaystosolvethisconnectivityloss:
UsethesubinterfacesonR1tomakesurethatR2andR3areindifferent
subnets.BGPnext-hopprocessingwillthenensurethatR1istheBGPnext-hop
intheoutgoingBGPupdates.
DisabletheBGPnext-hopprocessingonR1inanexistingmultipointDMVPN
designthatsharesacommonsubnet.(Thisoptionisstronglydiscouragedin
normalBGPdesignbecauseroutingproblemsshouldbesolvedwithaproper
networkdesignofpoint-to-pointsubinterfaces.)
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
BGP metrics that are attached to a BGP route are called "path attributes." Some
path attributes are well-known. Every BGP implementation should be able to
recognize the well-known attributes.
Some of the well-known attributes are mandatory and have to be present in
every BGP update. Mandatory well-known attributes are the AS-path, next-hop,
and origin. Other well-known attributes are discretionary.
Attributes that are not required to be recognized by every BGP implementation
are called “optional.” These attributes could be transitive (propagated if not
recognized) or nontransitive (dropped).
The AS-path attribute lists the autonomous systems that the routing update has
already crossed. This attribute is used for BGP loop detection and BGP route
selection.
The next-hop attribute specifies the IP address that should be used for packet
forwarding. The next hop is usually set to the IP address of the BGP router
sending the update.
Summary
Overview
UnderstandingtheBGPneighborsessionestablishmentprocessisakeycomponent
tounderstandingthefundamentaloperationoftheBGPprotocol.
BGPisanEGPthathasbeendesignedforscalabilityandpolicycontrol.Asaresult,
BGPrequiresneighboringrouterstobeexplicitlyconfiguredbeforeBGProuting
updatescanbesentbetweenthem.ThissituationdiffersfromIGPssuchasEIGRP,
IS-IS,andOSPF,thatdiscoverneighborsbyusingabroadcastpacketorahello
protocol.Inthislesson,BGPneighborsessionestablishmentproceduresare
discussed.
Uponcompletingthislesson,youwillbeableto:
ExplainhowBGPdiscoversneighbors
DescribetheBGPsessionestablishmentprocess
DescribetheroleoftheBGPkeepaliveinsessionestablishmentand
maintenance
ExplainhowoptionalMD5authenticationcanprotectsessionsbetweenBGP
peers
EstablishingBGPSessions
Unlikeotherroutingprotocols,BGPhasnomeansofautomaticallydetecting
neighbors.TheBGPprotocoliscarriedinaTCPsession,whichmustbeopened
fromoneroutertotheother.Todoso,therouterattemptingtoopenthesession
mustbemanuallyconfiguredwithneighborinformationindicatingtowhich
IPaddresstodirectitsconnectionattempts.
BGP neighbors are not discovered; they must be configured manually.
Configuration must be done on both sides of the connection.
Both routers will attempt to connect to the other with a TCP session on port
number 179.
Only the session with the higher router-ID remains after the connection attempt.
The source IP address of incoming connection attempts is verified against a list
of configured neighbors.
Therouterthatreceivestheincomingconnectionattemptsdoesnotanswerthemif
theattemptsarenotfromoneoftheconfiguredneighbors.TheIPsourceaddressof
theconnectionattemptpacket(TCPSYNpacket)isverifiedagainstthelistofIP
addressesthattherouteritselfwoulddirectitsconnectionattemptsto.
Tosucceedintheconnectionattempts,bothroutersmustbeconfiguredtoreach
eachother.Asideeffectofthissituationisthattheywillbothattempttoconnect.
Thissideeffectaddsrobustnesstothesessionestablishmentprocess,butitalso
introducestheriskthattwoBGPsessionswillbeestablishedbetweenapairofBGP
routers.
TworoutersshouldhaveonlyasingleBGPsessionbetweenthem.Therouter-ID
valuesthatareexchangedwhentheBGPsessionisestablishedallowtheBGP
routerstodetectwhentwoparallelsessionsexist.Onlythesessionthatwasinitiated
bytherouterwiththenumericallyhigherrouter-IDwillberetained.Theothersession
isdropped.
AroutermaynotopenaBGPsessiontoitself.IftheconfiguredneighborIPaddress
is,infact,anIPaddressofthelocalrouter,therouterrecognizestheproblemand
tearsdownthesession.Therouter-IDisalsousedforthisverification.
BGPNeighborDiscoveryExample
ThisexampleillustratesasmallBGPnetwork.
Small BGP Network
Thenetworkthatisdisplayedinthefigureservesasthesamplenetworktogenerate
printoutsinthefollowingexamples.
Initially, all BGP sessions to the neighbors are idle.
R2# show ip bgp summary
BGP router identifier 172.16.22.2, local AS number 1
BGP table version is 31, main routing table version 31
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/P
fxRcd
172.16.12.11 4 100 0 0 1 0 0 00:00:37 Idle
BGPNeighborDiscovery
172.16.22.22 4 200 0 0 1 0 0 00:00:14 Idle
TheshowipbgpsummarycommandgivesanoverviewoftheBGPstatus.Each
configuredneighborislistedintheoutputofthecommand.TheIPaddresstowhich
theconnectionattemptsaredirectedisalsodisplayed,alongwiththeBGPversion
number,theremoteASnumber,somecountervalues,thestatusofthesession,and
howlongagothesessionchangedstate.
The"Idle"stateindicatesthattherouteriscurrentlynotattemptinganyconnection
establishments.
ThevariousstatesforaBGPconnectionareIdle,Active,OpenSent,OpenConfirm,
andEstablished.
Beforeanyconnectionattemptismade,theBGPpeerrelationmusthaveleftthe
IdlestateandenteredtheActivestate.ForaBGPsessionbetweentworoutersin
differentautonomoussystems,thisstatusresultswhentheIPaddressoftheremote
routerbecomesreachableonadirectlyconnectedinterface.
A TCP session is established when the neighbor becomes reachable.
BGP Open messages are exchanged.
R2# debug ip tcp transactions
R2# debug ip bgp
R2# debug ip bgp events
BGP: 172.16.12.11 active went from Idle to Active
BGP: 172.16.12.11 open active, local address 172.16.12.2
TCP0: state was LISTEN -> SYNRCVD [179 -> 172.16.12.11(11374)]
TCP0: state was SYNRCVD -> ESTAB [179 -> 172.16.12.11(11374)]
TCBEFF01278 accepting EC3FFBE8 from 172.16.12.11.11374
BGP: 172.16.12.11 passive open to 172.16.12.2
BGP: 172.16.12.11 passive went from Idle to Connect
BGP: 172.16.12.11 passive rcv message type 1, length (excl. header) 38
BGP: ses global 172.16.12.11 (0xEFF0F1B8:0) pas Receive OPEN
BGP: 172.16.12.11 passive rcvd OPEN w/ remote AS 100, 4-byte remote AS 100
BGP: 172.16.12.11 passive went from Connect to OpenSent
BGP: 172.16.12.11 passive went from OpenSent to OpenConfirm
BGP: 172.16.12.11 passive went from OpenConfirm to Established
BGP(0): 172.16.12.11 was the first peer to be established for IPv4 Unicast
%BGP-5-ADJCHANGE: neighbor 172.16.12.11 Up
Thedebugoutputshowshowtheroutercreatesasocketdatastructureandbindsit
toitslocalIPaddress172.16.12.2andthewell-knownport179.Thentherouter
sendsaTCPSYNpackettotheconfiguredpeerrouterIPaddressof172.16.12.11
andahighportnumber11374.Theconnectionattemptsucceeds,andtheTCP
sessionisthenreadytotransfertheBGPinformation.
ThefirstBGPinformationthatissentistheBGPOpenmessage.TheBGPsession
thengoesfromtheActivestatetotheOpenSentstatewhilewaitingfortheother
routertorespond.IfthepeerrouteracceptstheparametersintheOpenmessage,it
respondswithitsownOpenmessage.Whenthelocalrouterreceivesthismessage,
thestategoesfromOpenSenttoOpenConfirm.Thelocalrouterthenverifiesthe
peerrouterparametersinitsOpenmessage.Iftheyareaccepted,akeepalive
packetissenttosignalthisacceptance.Thestateisthen"Established."
The BGP Open message contains the following:
BGP version number
AS number of the local router
Holdtime
BGP router identifier
Optional parameters
TheparametersintheBGPOpenmessageareasfollows:
Versionnumber:Thesuggestedversionnumber.Thehighestcommonversion
thatbothrouterssupportisused.MostBGPimplementationstodayuseBGP4.
ASnumber:TheASnumberofthelocalrouter.Thepeerrouterverifiesthis
information.IfitisnottheASnumberthatisexpected,theBGPsessionistorn
down.
Holdtime:Thenumberofsecondsthatmayelapsebetweenreceptionof
successiveBGPmessages.Ifthetimeisexceeded,thepeerisconsidered
dead.Thetworoutersagreetousethelowestsuggestedvalue.Whenthe
sessionisestablished,bothroutersusekeepalivemessagestomakesurethat
theholdtimerdoesnotexpire.Asuggestedhold-timervalueof0indicatesthat
thetimerneverexpiresandnokeepalivesshouldbesent.
BGPidentifier:Anumberuniquelyidentifyingtherouter.TheCiscorouteruses
oneofitsIPaddressesforthisnumber,therouter-ID.Therouter-IDisselected
asthenumericallyhighestIPaddressofanyloopbackinterface.Ifthereisno
loopbackinterface,therouterusesthehighestIPaddressofanyinterfacethatis
upatthetimeofthestartoftheBGPprocess.
Optionalparameters:TLVencoded.Anexampleofoptionalparametersis
sessionauthentication.
EstablishingaBGPSession
BGP neighbors—steady state
All neighbors shall be up (no state information).
R2# show ip bgp summary
BGP router identifier 172.16.22.2, local AS number 1
BGP table version is 16, main routing table version 16
15 network entries using 2220 bytes of memory
15 path entries using 960 bytes of memory
2/2 BGP path/bestpath attribute entries using 272 bytes of memory
2 BGP AS-PATH entries using 48 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 3500 total bytes of memory
BGP activity 78/63 prefixes, 84/69 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/P
fxRcd
172.16.12.11 4 100 18 15 16 0 0 00:11:46 6
172.16.22.22 4 200 5 8 16 0 0 00:00:27 9
WhentheBGPsessionsareintheEstablishedstate,routinginformationexchange
cantakeplace.Theshowipbgpsummarycommandoutputhereindicatesthata
sessionisestablishedbynotdisplayinganyinformationatallinthe"State"column.
Thecountervaluesshowhowmanymessageshavebeenreceivedandsentinthe
session."InQ"showshowmanymessageshavebeenreceivedbutnotyet
processed.AhighInQnumberindicateslackofCPUresourcestoprocesstheinput.
"OutQ"showshowmanyoutgoingmessagesarequeued.AhighOutQnumber
indicateslackofbandwidthtotransmittheoutgoingmessagesorCPUoverloadof
theotherrouter.
TheBGProuteruses"TblVer"(tableversion)totrackthechangesthatneedtobe
senttotheneighbors.ThereisamajortableversionnumberforthelocalBGPtable.
Thetableversionnumberisdisplayedonthefirstlineofoutputfromthisshow
command.Thereisalsoonetableversionnumberthatismaintainedforeachofthe
neighborsoftheBGProuter;thisnumberisdisplayedontheinformationlineofthe
neighbors.
WheneveraBGProuterentersachangeintoitsBGPtable,themajortableversion
numberisincrementedandthechangedrouteistaggedwiththisnumber.Whenthe
timecomestoupdateaspecificneighbor,therouterscanstheBGPtable.All
changeswheretheversionnumberisbetweentheneighborversionandthecurrent
tableversionaresenttotheBGPneighborinasingleBGProutingupdate.Afterthe
entiretableisscannedandallchangeshavebeensenttotheneighbor,thetable
versionnumberoftheneighborissettothehighestvalueoftheroutesbeingsent.
Atableversionofaneighborthatislowerthanthemajortableversionindicatesthat
theneighborisnotyetfullyupdated.Theupdateintervalforaneighborinanother
ASisnormally30seconds(thedefaultvalueoftheBGPadvertisementtimer).
Inadditiontotheinformationaboutallsessionstoallneighbors,theoutputalso
showstheamountofmemorythatisbeingusedfortheBGPdatastructures.
1.
2.
3.
TCP-basedBGPsessionsdonotprovideanymeansofverifyingthepresenceofa
BGPneighbor.AfterBGPhasestablishedtheTCPsession,theonlymethodof
verifyingneighborpresenceistoactuallysendBGPtraffic.BGPtrafficissentover
theTCPsessionwithACKs,andisthereforereliable.SuccessfullysendingBGP
trafficconfirmstheexistenceofaBGPneighbor.
A TCP-based BGP session does not provide any means of verifying BGP
neighbor presence:
Except when sending BGP traffic
BGP needs an extra mechanism:
Keepalive BGP messages provide verification of neighbor existence.
Keepalive messages are sent every 60 seconds.
However,thereareoftenlongperiodsoftimewhennoBGPtrafficissentbetween
neighbors.Duringthoseperiods,TCPimplementsnomechanismtocheckforthe
existenceoftheconfiguredneighbor.BGPneighborscouldthereforeeasilybe
disconnectedduringtimesofsessioninactivity.Thissituationwouldleadtoincorrect
routinginformationontheothersideoftheBGPsession.
Toavoidroutingpacketstoarouterthatisnolongerthere,BGPneedsanextra
mechanismtomakesurethataneighborexists.BGPsendsspecialkeepalive
messagesduringeverykeepaliveintervaltoinformitspeerofitspresence.By
default,thisintervalisevery60seconds.IfnoBGPtrafficisreceivedwithinthe
selectedholdtimeinterval,theBGProutersendsaBGPnotificationmessagetothe
inactivepeerandtearsdowntheirBGPsession.ThedefaultBGPholdtimevalueis
180seconds.
Whenchangingthedefaultvaluesofkeepaliveandholdtimeintervals,youmusttake
carenottoconfiguretoobigakeepaliveintervalincomparisontotheholdtime.Too
bigadifferencecouldresultinresettingoftheBGPsessionafteronlyonekeepalive
messagehasbeenmissed,makinganetworkunstable.Thesuggestedratioof
keepalive-to-holdtimeintervalis1:3.
Keepalive interval value is not communicated in the BGP Open message.
Keepalive value is selected as follows:
Configured value, if local holdtime is used
Configured value, if holdtime of neighbor is used and keepalive < (holdtime /
3)
Smaller integer in relation to (holdtime / 3), if holdtime of neighbor is used
and keepalive > (holdtime / 3)
Asopposedtotheholdtimeinterval,BGPpeersdonotcommunicatethekeepalive
intervalintheOpenmessage.Theselectionofakeepaliveintervalistherefore
basedontheselectedholdtimevalue.Theselectedholdtimevalue,whichisusedby
bothpeers,isthesmallerofbothconfiguredvalues.
TheBGPprocessselectsthekeepaliveintervalvalueaccordingtotheseconditions:
Ifthelocallyconfiguredvalueofholdtimeisselected(beingtheloweroftwo),
thepeersusethelocallyusedkeepaliveinterval.
Iftheholdtimeintervaloftheneighborisselected,andthelocallyconfigured
keepaliveintervalislessthanathirdoftheholdtimeinterval,thepeersusethe
locallyconfiguredkeepalive.
Iftheholdtimeintervaloftheneighborisselected,andthelocallyconfigured
keepaliveintervalismorethanathirdoftheholdtimeinterval,thepeersusethe
smallerintegervalueinrelationto(holdtime/3).
KeepaliveValueExample
Iftheselectedholdtimeequals17secondsandtheconfiguredkeepaliveequals10
seconds,the(holdtime/3)rulewillbeusedtoselectthekeepalivevalue.Therefore,
(17/3)=5.67,andthekeepalivevaluethatisusedbyBGPwillequal5seconds.
BGPKeepalives
AuthenticationbetweenBGPneighborscanbenegotiatedbetweenBGP-speaking
routersusingoptionalparametersintheOpenmessage.
BGP peers may optionally use MD5 TCP authentication using a shared secret.
Both routers must be configured with the same password (MD5 shared secret).
Each TCP segment is verified.
IfyouareusingMD5authentication,everyTCPsegmentontheBGPsessionwillbe
transmittedtotheconfiguredneighboralongwithachecksum.Thechecksumis
calculatedtogetherwithasecretknownbythetworoutersusingtheMD5algorithm.
Thecommonsecretisnevertransmittedonthenetwork.Ifthereceiver,whichis
usingthesamecommonsecret,calculatesthesamechecksumfromtheTCP
segment,thenthereceivercanbeprettysurethattheinformationistransmittedfrom
thecorrectsourceandtheinformationhasnotbeenaltered.
AuthenticationofBGPsessionsisavitaltooltoavoidDoSattacks.
CiscorouterssupportkeychaintoconfigureBGP
authentication.BGPsupportsonlyHMAC-MD5and
HMAC-SHA1-12cryptographicalgorithms.
MD5Authentication
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
With interior routing protocols, adjacent routers are usually discovered through a
dedicated hello protocol. In BGP, neighbors must be manually configured to
increase routing protocol security.
BGP neighbors, once configured, establish a TCP session and exchange the
BGP Open message, which contains the parameters that each BGP router
proposes to use.
The router uses BGP keepalives to provide a verification of the existence of a
configured BGP neighbor.
MD5 authentication can be configured on a BGP session to help prevent
spoofing, DoS attacks, or man-in-the-middle attacks.
Summary
Overview
RouteprocessingisfundamentaltotheoperationofBGP.KnowledgeoftheBGP
routeselectionprocess,routepropagation,andhowtheBGPandIProutingtables
arebuiltiskeytoproperlyconfiguringBGPandtroubleshootingBGProutingissues.
Uponcompletingthislesson,youwillbeableto:
DescribeBGProutingupdates
ExplainhowarouterbuildsBGPtables
DescribetherouteselectionprocessinBGP
ExplainhowarouterpropagatesBGProutestootherBGPneighbors
ExplainhowarouterbuildsanIProutingtablewhenitisusingBGP
ExplainhowBGPadvertiseslocalnetworks
DescribetheroleofautomaticsummarizationinBGProuteprocessing
ProcessingBGPRoutes
AfteraBGPsessionisestablished,routingupdatesstarttoarrive.EachBGProuting
updateconsistsofoneormoreentries(routes).Eachrouteisdescribedaccordingto
theIPaddressandsubnetmask,alongwithanynumberofattributes.Thenext-hop,
AS-path,andoriginattributesmustalwaysbepresent.OtherBGPattributesare
optionallypresent.
Small BGP Network
Thenetworkinthefigureservesasthesamplenetworkforgeneratingprintoutsin
thefollowingexamples.
Information from the BGP tables is exchanged after adjacency establishment.
R2# debug ip bgp updates
%BGP-5-ADJCHANGE: neighbor 172.16.22.22 Up
BGP(0): 172.16.22.22 rcvd UPDATE w/ attr: nexthop 172.16.22.22, origin i,
metric 0, merged path 200, AS_PATH
BGP(0): 172.16.22.22 rcv UPDATE about 10.0.2.128/28 -- DENIED due to: AS-
PATH contains our own AS;
BGP(0): 172.16.22.22 rcvd 10.0.2.16/28
BGP(0): Revise route installing 1 of 1 routes for 10.0.2.16/28 -
> 172.16.22.22(global) to main IP table
Thedebugoutputshowshowinformationaboutnetwork10.0.2.16/28isreceived
fromneighbor172.16.22.22.TheneighborindicatesthatIPpacketstodestinationIP
addressesinnetwork10.0.2.16/28canbeforwardedtothenext-hopaddress
172.16.22.22.TheASpath200indicatesthatthefinaldestinationisinAS200.The
metricistheMEDvalue.
Network10.0.2.128/28isdenied.ThereasonisASpathloopdetection.The
receivingrouterdetectsitsownASnumberintheASpathandsilentlydiscards
(denies)theroute.
ReceivingRoutingUpdates
Allroutesthatarereceivedfromaneighboraresavedintheroutermemory.
Therefore,thereisnoneedtoretransmitorrefreshanyunchangedinformation.
All inbound updates are placed into the BGP table.
R2# show ip bgp
BGP table version is 11, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - inte
rnal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-
Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
* 1.0.0.0/24 172.16.22.22 0 0 200 99 i
*> 172.16.12.11 0 0 100 99 i
*> 10.0.1.0/24 172.16.12.11 0 0 100 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
*> 10.0.2.16/28 172.16.22.22 0 0 200 i
*> 10.0.2.32/28 172.16.22.22 0 0 200 i
*> 10.0.2.48/28 172.16.22.22 0 0 200 i
*> 10.0.2.64/28 172.16.22.22 0 0 200 i
*> 10.0.2.80/28 172.16.22.22 0 0 200 i
*> 10.0.2.96/28 172.16.22.22 0 0 200 i
*> 10.0.2.112/28 172.16.22.22 0 0 200 i
Whenthereismorethanonewaytoreachaparticularnetwork,thelocalrouter
selectsoneofthemasthebest.Thebestalternativemightbelaterlostbecausethe
neighboringrouterwithdrawstherouteortheneighboringrouterisnolonger
reachable.However,theremainingalternativesarestillstoredinmemoryandanew
alternativeisselectedasthebestwithoutinvolvingotherBGProuters.
Theshowipbgproutercommandgivesanoverviewofallroutinginformationthatis
receivedfromallneighbors.Thecommanddisplaysbasicinformationabouteach
routeonasingleline.Theoutputissorted—alternativestoreachthesamenetwork
aredisplayedonconsecutivelines.Thenetworknumberisdisplayedonlyonthefirst
linesindicatingthesamenetwork.Thenetworkcolumnisleftblankonthe
consecutivelinesindicatingalternativestoreachthesamenetwork.
Therouterselectsonlyoneofthealternativesasthebestpathtowardthe
destination.Thisalternativeisindicatedwiththe">"sign.
BuildingBGPTable
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
WhenarouterhasmorethanonealternativeroutetoreachthesameIPsubnet
(networkandmask),therouterhastoselectoneoftheroutesasbestinitsdefault
modeofoperation.Tomakethisselection,therouterusestheBGPattributesthat
areattachedtothevariousupdates.
Exclude routes with inaccessible next hop
Prefer highest weight (local to router)
Prefer highest local preference (global within AS)
Prefer routes that the router originated
Prefer shortest AS path (only length is compared)
Prefer lowest origin code (IGP < EGP < Incomplete)
Prefer lowest MED
Prefer external (EBGP) paths over internal (IBGP)
For IBGP paths, prefer path through closest IGP neighbor
For EBGP paths, prefer oldest (most stable) path
Prefer paths from router with the lowest BGP router-ID
Theselectioncriteriaarecheckedintheorderthatisindicatedinthefollowingsteps.
Thefirstcheckthatindicatesadifferenceisusedforrouteselection,andnofurther
testingisdone.
Theroutercheckswhetherthenext-hopattributeindicatesanIPaddressthatis
reachableaccordingtothecurrentroutingtable.Itisnotnecessarytohavea
directconnectiontothenexthop.Itcanverywellbeseveralrouterhopsaway
andtheroutetoitlearnedbytheIGP.Ifthenexthopisnotreachable,therouter
doesnotconsidertheBGProuteasacandidatetobecomeselectedasthe
best.
Therouterpreferstheroutewiththehigherweight.Theweightisnotcarried
withtheupdates;itisavaluethatisassignedtotheroutebythelocalrouterand
consideredonlywithintherouteritself.
Ifthelocalpreferenceattributesaredifferent,theroutewiththehighestvalueis
selectedasbest.
IfoneoftheroutesisinjectedintotheBGPtablebythelocalrouter,thelocal
routerprefersittoanyroutesthatitreceivesfromotherBGProuters.
Atthispoint,thelengthsoftheASpathsarecompared(thecontentisnot
checked;onlythenumberofautonomoussystemsineachASpathiscounted).
Theroutewiththeshortestlengthisselected.
IftheAS-pathlengthsarethesame,theorigincodeischecked.BGPwillprefer
thepathwiththelowestorigintype:IGPislowerthanEGP,andEGPislower
thanIncomplete.
TherouternextcomparesMEDvaluesbutonlyifitreceivestheupdatesfrom
thesameneighboringAS.RouteswithalowerMEDarepreferred.
Atthispoint,itisclearthatthedestinationnetworkisoutsidethelocalASand
thatthereisnotmuchdifferenceamongthealternatives.BecausetheIP
packetstothedestinationnetworkmustleavetheAS,itisbetterthattheydoso
asquicklyaspossible.IfanyofthealternativesarereceivedfromaBGPpeerin
anotherAS,thatalternativeispreferred.
IftherouterreceivesallalternativesfrompeerroutersinthelocalAS,eachof
themwillindicateanexitpoint,andtheclosestexitisused.Distancetotheexit
pointiscalculatedbycomparingtheIGPcostsagainsttheBGPnexthops,as
indicatedintheroutingtable.
IftherouterreceivesallalternativesfromEBGPneighbors,themoststable
path(theoldestpath)ispreferred.
Iftherouterstillcannotdifferentiateamongtheroutes,itneverthelesshasto
makeadecisionandselectthebestroute.ItcheckstheBGPsessionson
whichitreceivedtheupdatesandchoosestheroutethatwasreceivedonthe
sessionforwhichthepeerrouterhasthelowestBGProuter-ID.
Theroutermakesthefinaltestonlyafterithasmadeallotherchecksand
determinedthatallalternativeroutesareequallygood.
BGPRouteSelectionCriteria
Alocalrouterpropagatesonlytheroutethatitselectedasbesttotheneighbors.
However,therouterneversendsaroutebackonthesameBGPsessionuponwhich
itwasreceived.Onthecontrary,whenitselectsaneighborasthebestnexthop,the
routermakessurethattheneighborisnotpointingbacktothelocalrouter.The
routeraccomplishesthistaskby"poisoning"theroute(markingtheroute
unreachable)andsendingawithdrawmessagetothatneighbor.
The best BGP routes are propagated to BGP neighbors.
R2# debug ip bgp updates
BGP(0): (base) 172.16.12.11 send UPDATE (format) 10.0.2.0/28, next 172.16.
12.2, metric 0, path 200
BGP(0): 172.16.12.11 NEXT_HOP is on same subnet as the bgp peer and set to
172.16.12.11 for net 10.0.1.0/24, flags 200, sb: AC100C00, mask: FFFFFF00
BGP(0): (base) 172.16.12.11 send UPDATE (format) 10.0.1.0/24, next 172.16.
12.11, metric 0, path 100
BGP(0): (base) 172.16.12.11 send UPDATE (format) 10.0.0.0/28, next 172.16.
12.2, metric 0, path Local
Therouterconductsroutepoisoningtoavoidapotentialroutingloopproblemin
whichaneighborrouterselectedasthebestnexthopmightrelyonthelocalrouter
asthebestnexthop.
Theprocessofpreventingroutinginformationfrombeingsentbacktothesourceof
informationiscalled"splithorizon."
BGPRoutePropagation
TherouteintheBGPtablethatBGPselectsasthebestisacandidatefor
installationintheIPforwardingtableorroutingtable.
The best BGP routes are copied into the IP routing table based on administrative
distance.
R2# show ip route
<... output omittted ...>
Gateway of last resort is not set
1.0.0.0/24 is subnetted, 1 subnets
B 1.0.0.0 [20/0] via 172.16.22.22, 00:03:07
10.0.0.0/8 is variably subnetted, 20 subnets, 3 masks
<... output omitted ...>
B 10.0.1.0/24 [20/0] via 172.16.12.11, 00:03:06
B 10.0.2.0/28 [20/0] via 172.16.22.22, 00:06:46
B 10.0.2.16/28 [20/0] via 172.16.22.22, 00:06:46
B 10.0.2.32/28 [20/0] via 172.16.22.22, 00:06:46
B 10.0.2.48/28 [20/0] via 172.16.22.22, 00:06:46
B 10.0.2.64/28 [20/0] via 172.16.22.22, 00:06:46
B 10.0.2.80/28 [20/0] via 172.16.22.22, 00:06:46
B 10.0.2.96/28 [20/0] via 172.16.22.22, 00:06:46
B 10.0.2.112/28 [20/0] via 172.16.22.22, 00:06:46
<... output omitted ...>
Beforearoutecanbeinstalled,therouterhastocheckwhetherthereisanyother
routingprotocolthathasinformationaboutthesamesubnet(networkandmask).If
thesubnetisknownviadifferentsources,therouterusestheADtodeterminewhich
sourcetouse.ADisaratingofthetrustworthinessofaroutinginformationsource.
ADisoftenexpressedasanumericalvaluebetween0and255.Thehigherthe
value,thelowerthetrustworthinessrating.Inthiscase,therouterwillinstallthe
routewiththelowestAD.
Theoutputfromtheshowiproutecommandindicateswhichroutesintherouting
tablewereinstalledusingtheBGPinformation.Thoseroutesaredenotedwiththe
letter"B."TheADisshowninthecommandoutputasthefirstnumberwithinthe
brackets.
Inthisexample,network1.0.0.0/24isreachablevia172.16.22.22.Aftertherouter
hasinstalledtherouteintheroutingtable,userdatatrafficstartstobeforwarded.
BuildingIPRoutingTable
TheBGProutingprocesscaninjectnewroutesintotheBGPtable.Arouterwill
propagatenewlyinjectedroutestoneighboringBGPpeersifitselectsthemasbest.
Thiswaytherouterprovidestheneighboringautonomoussystemswithinformation
aboutnetworksthatarereachableinthelocalAS.Thisprocessiscalledadvertising,
originating,orannouncinglocalroutes.
The BGP router process keeps a list of local networks (defined with the network
command or through redistribution).
The BGP process periodically scans the IP routing table and inserts or revokes
routes from the BGP routing table based on their presence in the IP routing
table.
TheBGPprocesscaninjectlocalroutesintwodifferentways:
AlistofnetworksisconfiguredontherouterundertheBGProuterprocessusing
thenetworkconfigurationcommand.Thelistednetworksarecandidatesfor
beinginjected.Networksareinjectedonlyiftheyappearintheroutingtable.In
thecasewheretheIGPthatisusedwithintheASfindsavalidpathtothem,the
routeswillbeintheroutingtable.
Routesthatarelearnedbyanotherroutingprotocolareredistributed.TheIGP
thatisusedwiththeAScanalsoactasasourceofroutinginformationabout
localnetworks.
AdvertisingLocalNetworksExample
Inthisexample,network10.0.0.0/28isdirectlyconnectedtointerfaceLoopback1.
The BGP route is revoked after the network is removed from the routing table.
R2# debug ip routing
R2# debug ip bgp updates
is_up: Loopback1 0 state: 6 sub state: 1 line: 0
RT: interface Loopback1 removed from routing table
RT: del 10.0.0.0 via 0.0.0.0, connected metric [0/0]
RT: delete subnet route to 10.0.0.0/28
RT: del 10.0.0.1 via 0.0.0.0, connected metric [0/0]
RT: delete subnet route to 10.0.0.1/32
BGP(0): route 10.0.0.0/28 down
BGP(0): no valid path for 10.0.0.0/28
BGP: topo global:IPv4 Unicast:base Remove_fwdroute for 10.0.0.0/28
BGP(0): (base) 172.16.12.11 send unreachable (format) 10.0.0.0/28
Therouteto10.0.0.0/28waspreviouslyinstalledintheBGPtablebecauseitwas
listedwithanetworkstatementanditwasintheroutingtableasdirectlyconnected.
WhentheLoopback1interfacegoesdown,therouterremovesthedirectly
connectedroutefromitsroutingtable.Becausetheroutenolongerexistsinthe
routingtable,itmustalsoberemovedfromtheBGPtable.
BecausetherehasbeenachangeintheBGPtable,theBGPneighborsmustbe
informed.TheroutersendsaBGPupdatemessagetoitsneighborindicatingthat
network10.0.0.0/28isnowunreachable.
The BGP route is advertised after the network appears in the routing table.
RT: add 10.0.0.0/28 via 0.0.0.0, connected metric [0/0]
RT: interface Loopback1 added to routing table
RT: updating connected 10.0.0.1/32 (0x0):
via 0.0.0.0 Lo1
RT: add 10.0.0.1/32 via 0.0.0.0, connected metric [0/0]
BGP(0): route 10.0.0.0/28 up
BGP: topo global:IPv4 Unicast:base Remove_fwdroute for 10.0.0.0/28
BGP(0): redistributedlocal route 10.0.0.0/28 modified
BGP(0): (base) 172.16.12.11 send UPDATE (format) 10.0.0.0/28, next 172.16.
12.2, metric 0, path Local
Inthisexample,network10.0.0.0/28islistedwithanetworkstatementintheBGP
process.However,thenetworkwasnotintheroutingtableoftherouter,sothe
networkwasnotinjectedintoitsBGPtable.
Later,theLoopback1interfacecomesbackupagain.Thisreappearancemeansthat
thenetwork10.0.0.0/28isnowintheroutingtableasadirectlyconnectedroute.As
aresult,therouteronceagaininjectsthe10.0.0.0/28networkintoitsBGPtableand
AdvertisingLocalNetworks
thenupdatesitsconfiguredneighbor.
WhenaBGProuterisconfiguredtolocallyannounceroutesintoBGP,thebehavior
ofthenetworkcommandvariesdependingonwhetherautomaticsummarizationis
enabledordisabled.Whenautomaticsummarizationisenabled,BGPsummarizes
thelocallyoriginatedBGPnetworks(networkx.x.x.x)totheirclassfulboundaries.
VerifyifBGPautomaticsummarizationisdisabledorenabled.Thefollowing
exampleswillhaveBGPautomaticsummarizationenabled.
Verify BGP automatic summarization.
Enable automatic summarization when:
Summarization of IGP-to-BGP redistributed routes to major network
boundary required
Using classful network command to summarize subnets to a major network
boundary
Disable automatic summarization when:
Summarization on IGP-to-BGP redistribution not desired
Using classless variant of the network command
Whenasubnetexistsintheroutingtableandthefollowingthreeconditionsare
satisfied,thenanysubnet(componentroute)ofthatclassfulnetworkinthelocal
routingtablewillpromptBGPtoinstalltheclassfulnetworkintotheBGPtable:
Aclassfulnetworkstatementforthatnetworkexistsintheroutingtable.
Aclassfulmaskhasbeenconfiguredonthatnetworkstatement.
Automaticsummarizationisenabled.
Whenautomaticsummarizationisdisabled,theroutesthatareintroducedlocally
intotheBGPtablearenotsummarizedtotheirclassfulboundaries.
ThebehavioroftheredistributionprocedureinBGPisalsoinfluencedbythe
configurationofautomaticsummarizationontherouter.Whenenabled,all
redistributedsubnetswillbesummarizedtotheirclassfulboundariesintheBGP
table.Whendisabled,allredistributedsubnetswillbepresentintheiroriginalformin
theBGPtable.
EnableautomaticsummarizationinBGPwhenthesummarizationofsubnetstotheir
classfulboundarieswillnotintroduceflawedinformationintotheBGPtable.Inother
words,leaveautomaticsummarizationenabledonlywhenyouareusingafully
assignedclassfulnetworkmatchingthenetworkthatwassummarizedinBGP.
Wheneverpossible,usetheclasslessvariantofthenetworkcommand,specifying
thesubnetmasklengthofthenetwork.Whenyouareredistributingnetworksinto
BGP,thepreferredmethodistodisableautomaticsummarization.Disabling
automaticsummarizationensuresthatcorrectinformationisinsertedintotheBGP
tableoftherouter.
AutomaticSummarizationExample
Inthisexample,fivesubnetsofthemajorclassAnetwork10.0.0.0/8(10.0.0.0/28,
10.0.0.16/28,10.0.0.32/28,10.0.0.48/28,and10.0.0.64/28)existintheroutingtable.
R2# show ip route
10.0.0.0/8 is variably subnetted, 11 subnets, 3 masks
<... output omitted ...>
C 10.0.0.0/28 is directly connected, Loopback1
C 10.0.0.16/28 is directly connected, Loopback2
C 10.0.0.32/28 is directly connected, Loopback3
C 10.0.0.48/28 is directly connected, Loopback4
C 10.0.0.64/28 is directly connected, Loopback5
<... output omitted ...>
Five subnets for 10.0.0.0 exist in the routing table.
Automatic summarization is enabled for BGP.
BGP has been configured to locally announce 10.0.0.0.
router bgp 1
network 10.0.0.0
auto-summary
AutomaticSummarization
R2# show ip bgp
BGP table version is 12, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - inte
rnal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-
Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 1.0.0.0/24 172.16.12.11 0 0 100 99 i
*> 10.0.0.0 0.0.0.0 0 32768 i
*> 10.0.1.0/24 172.16.12.11 0 0 100 i
Classful network summary is inserted into BGP table.
WhenyouareinsertingnetworksintotheBGPtablewiththeclassfulnetwork
commandandautomaticsummarizationisdisabled,noinsertionintotheBGPtable
willoccurunlessanexactmatchexistsintheIProutingtable(meaningthata
classfulnetworkhastobepresentintheIProutingtable).
Whenautomaticsummarizationisenabled,themajornetworkcommandwill
summarizeallsubnetsintheIProutingtabletotheirmajornetworkboundary.
Thereisaclassfulnetworkcommand,andautomaticsummarizationisenabledfor
BGP.ThissetupresultsintheinsertionofaclassfulnetworksummaryintotheBGP
table,insteadofseparatesubnets.
Subnets10.0.0.0/28,10.0.0.16/28,10.0.0.32/28,10.0.0.48/28,and10.0.0.64/28
weresummarizedduringinsertionintotheBGPtabletotheclassfulnetwork
10.0.0.0/8.Thisactionoccurredbecauseaclassfulnetworkcommandand
automaticsummarizationwereconfiguredontherouter.Ifautomaticsummarization
isdisabled,noinsertionintotheBGPtableoccursatall.
ThelocallysourcedsummaryhasalltheattributesofalocallysourcedBGProute
(nexthop=0.0.0.0,weight=32768,emptyAS-pathlist),andismarkedashaving
anIGPorigin(beingsourcedwiththenetworkcommand).
R2# show ip route
<... output omitted ...>
10.0.0.0/8 is variably subnetted, 11 subnets, 3 masks
C 10.0.0.0/28 is directly connected, Loopback1
C 10.0.0.16/28 is directly connected, Loopback2
C 10.0.0.32/28 is directly connected, Loopback3
C 10.0.0.48/28 is directly connected, Loopback4
C 10.0.0.64/28 is directly connected, Loopback5
<... output omitted ...>
Five subnets 10.0.0.0 exist in the routing table.
Automatic summarization is enabled for BGP.
BGP has been configured to redistribute connected routes into BGP.
router bgp 1
redistribute connected
auto-summary
R2# show ip bgp
BGP table version is 15, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - inte
rnal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-
Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 1.0.0.0/24 172.16.12.11 0 0 100 99 i
*> 10.0.0.0 0.0.0.0 0 32768 ?
*> 10.0.1.0/24 172.16.12.11 0 0 100 i
*> 172.16.0.0 0.0.0.0 0 32768 ?
Classful network summary is inserted into BGP table.
Inthisexample,automaticsummarizationisenabled,resultinginthesummarization
ofredistributedsubnetstotheirclassfulboundaries.Subnets10.0.0.0/28,
10.0.0.16/28,10.0.0.32/28,10.0.0.48/28,and10.0.0.64/28willbesummarizedinto
thesingleclassAnetwork10.0.0.0/8.Thenetwork10.0.0.0/8isalocallysourced
summarywithalltheattributesofalocallysourcedBGProute(nexthop=0.0.0.0,
weight=32768,emptyAS-pathlist).Theoriginoftherouteismarkedasincomplete
becausetherouteissourcedthroughredistribution.
Ifautomaticsummarizationwasdisabled,morespecificrouteswouldbepresentin
theBGPtableinsteadofthesummaryprefix10.0.0.0/8.
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
After BGP sessions are established between BGP routers, they can start
exchanging routing updates.
All updates that are received from BGP neighbors are stored in the BGP table,
regardless of whether they are used.
The route selection process takes into account various BGP attributes that are
attached to the route, as well as local decisions (indicated with weights).
Only the best BGP routes are propagated to other BGP routers.
Only the best BGP routes are installed in the local IP routing table.
Every BGP router can also originate the routes in BGP. The routes to be
originated are entered manually in the BGP routing process or redistributed into
BGP from an IGP.
Summary
Overview
BasicBGPconfigurationiscriticaltoanysuccessfulBGPimplementation.Network
administratorsuseCiscoIOScommandsthatareincludedinthislessoninallBGP
implementations.Thoroughknowledgeofthecommandsinthislessonistherefore
crucialtoensuringasuccessfulimplementationusingBGP.
Uponcompletingthislesson,youwillbeableto:
IdentifytheCiscoIOScommandthatisrequiredtoconfiguretheBGProuting
process
IdentifytheCiscoIOScommandsthatarerequiredtoconfigureexternalBGP
neighbors
IdentifytheCiscoIOScommandsthatarerequiredtoconfigurebasictimersand
MD5authenticationinBGP
IdentifythecommandsthatarerequiredtoannouncelocalnetworksinBGP
DescribeBGProuteredistribution,includingthecommandsthatarerequiredto
configureBGProuteredistribution
DescribetheclasslessbehaviorofBGPandidentifytheCiscoIOScommand
thatisrequiredtoconfigureBGPforclasslessoperation
DescribeBGProuteaggregation,includingtheCiscoIOScommandsthatare
requiredtoconfigurebasicBGProuteaggregation
DescribetheBGPConditionalRouteInjectionfeature
DescribetheBGPSupportforTTLSecurityCheckfeature
DeterminewhenBGProuteaggregationisnotappropriateinmultihomed
topologies
ConfiguringBasicBGP
YouwillusetherouterbgpcommandtostarttheBGProutingprocessintherouter.
Therecanbe,atmost,oneBGPprocessinarouter.Itmustbeassignedthelocal
ASnumber.
router(config)# router bgp as-number
Starts BGP routing.
Get your AS number from American Registry for Internet Numbers
(http://www.arin.net) or Réseaux IP Européens (http://www.ripe.net).
Use private AS numbers (between 64,512 and 65,534 or between
4,200,000,000 and 4,294,967,294) if you run BGP in a private network.
Only one BGP routing process per router is allowed.
TheASnumberisa16-bitor32-bitinteger.New32-bitASnumberswerecreated
whentheASnumberpoolfromIANAapproachedexhaustion.TheASnumberpool
wasextendedfrom16to32bits.ItmustuniquelyidentifytheASamongallrouters
thatareexchangingBGProutinginformation,eitherdirectlyorindirectly.This
requirementmeansthattheASnumbersmustbeuniquewhenBGPinformationis
exchangedwiththeInternet.
TheASnumbercanbeapublicASnumberoraprivateASnumber.PublicAS
numbersrangefrom1to64,534or131,072to4,199,999,999.AnInternetregistry(
ARIN:http://www.arin.netorRIPE:http://www.ripe.net)assignspublicASnumbers.
PrivateASnumbersrangefrom64,512to65,534or4,200,000,000—
4,294,967,294.PrivateASnumbersareneverpropagatedontothepublicInternet.
BGPRoutingProcess
BGPdoesnotautomaticallydiscoverneighbors.Youwillneedtoexplicitlyconfigure
BGPneighbors.ThelocalrouterwilltrytoconnecttotheindicatedIPaddressand
alsoacceptincomingconnectionattemptsfromtheindicatedIPaddress.
router(config-router)# neighbor ip-address remote-as as-number
Defines an external neighbor
External neighbor has to be reachable over directly connected subnet
router(config-router)# neighbor ip-
address description neighbor description
Assigns a description to an external neighbor.
router(config-router)# neighbor ip-address shutdown
Disables a BGP neighbor
ThefirstattributethatyoumustconfigurewithanewneighboristheremoteAS
numberinwhichtheneighboristakingpart.WhentheTCPsessionisestablished
betweenBGProuters,eachrouterverifiestheconfiguredremoteASnumberwith
theexchangeofBGPOpenmessages.
Youmayoptionallyconfigureotherattributeswiththeneighboronsuccessive
configurationlines,referringtothesameneighborIPaddressbutindicatingdifferent
attributes.
Withtheneighbordescriptioncommand,adescription(textstring)canbeentered
thatdescribestheneighbor.
Whenyouaredebuggingortroubleshooting,youcanusetheneighborshutdown
commandtotemporarilydisableBGPneighborwithoutdeletingBGPneighbor
configuration.YoucanshutdowntheBGPneighborduringextensivemodificationof
routingpoliciestopreventinconsistentroutingdata.
ConfiguringExternalNeighbors
Overview
Throughthisdiscovery,youwilllearnhowtoconfigureexternalBGP,BGPtimers,
andMD5authentication.
Discovery1:ConfigureBasicBGP
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
R2 Ethernet0/0 172.16.22.2/24 ConnectiontoISP2
R2 Ethernet0/1 172.16.12.2/24 ConnectiontoISP1
ISP1 Ethernet0/1 172.16.12.11/24 ConnectiontoR2
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
ISP2 Ethernet0/0 172.16.22.22/24 ConnectiontoR2
ISP2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
Loopback7
Loopback8
Loopback9
10.0.2.1/28
10.0.2.17/28
10.0.2.33/28
10.0.2.49/28
10.0.2.65/28
10.0.2.81/28
10.0.2.97/28
10.0.2.113/28
10.0.2.129/28
Loopbackssimulate
LANnetworks
JobAids
Step1
YouwillconfigureanexternalBGPsessionbetweenR2andISP1routers.On
theR2router,configureBGPexternalneighborISP1.
ISP1hasIPaddress172.16.12.11andBGPAS100.R2routerisalready
configuredintheBGPAS1.
R2(config)# router bgp 1
R2(config-router)# neighbor 172.16.12.11 remote-as 100
Step2
OntheR2router,verifythestateoftheBGPsession.
ToverifytheBGPsession,usetheshowipbgpsummarycommand.
R2# show ip bgp summary
BGP router identifier 172.16.22.2, local AS number 1
BGP table version is 34, main routing table version 34
9 network entries using 1332 bytes of memory
9 path entries using 576 bytes of memory
1/1 BGP path/bestpath attribute entries using 136 bytes of memory
1 BGP AS-PATH entries using 24 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 2068 total bytes of memory
BGP activity 15/6 prefixes, 21/12 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
172.16.12.11 4 100 0 0 1 0 0 never Idle
172.16.22.22 4 200 1359 1357 34 0 0 20:29:01 9
Youshouldsee"Idle"statenexttotheneighbor172.16.12.11.
Step3
OntheISP1router,configureBGPexternalneighborR2.
R2hasIPaddress172.16.12.2andBGPAS1.ISP1routerisalreadyconfigured
intheBGPAS100.
ISP1(config)# router bgp 100
ISP1(config-router)# neighbor 172.16.12.2 remote-as 1
Shortly,afterconfiguringexternalBGPneighborontheISP1router,youwillsee
thefollowingmessageontheISP1router'sconsole:
*Feb 5 01:13:18.858: %BGP-5-ADJCHANGE: neighbor 172.16.12.2 Up
ExternalBGPsessionbetweenR2andISP1issuccessfullyestablished.
Step4
OntheR2router,verifythestateoftheBGPsession.
ToverifytheBGPsession,usetheshowipbgpsummarycommand.
R2# show ip bgp summary
BGP router identifier 172.16.22.2, local AS number 1
BGP table version is 40, main routing table version 40
15 network entries using 2220 bytes of memory
15 path entries using 960 bytes of memory
2/2 BGP path/bestpath attribute entries using 272 bytes of memory
2 BGP AS-PATH entries using 48 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 3500 total bytes of memory
BGP activity 21/6 prefixes, 27/12 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
ConfigureBasicBGP
DiscoverySteps
172.16.12.11 4 100 32 36 40 0 0 00:25:36 6
172.16.22.22 4 200 1398 1398 40 0 0 21:04:31 9
Insteadofstateinformation,youshouldseethenumberofprefixesrouterreceivedfromits
neighbor.NowyouknowthatthestateisEstablishedandthatrouterR2received6prefixesfrom
theISP1router.
Step5
OntheR2routerconfirmthatBGPstateisestablished.
ToconfirmBGPstate,usetheshowipbgpneighborcommand.
R2# show ip bgp neighbors 172.16.12.11
BGP neighbor is 172.16.12.11, remote AS 100, external link
BGP version 4, remote router ID 10.0.1.81
BGP state = Established, up for 00:32:50
Last read 00:00:46, last write 00:00:11, hold time is 180, keepalive interval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable (disabled)
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN Capability: advertised and received
Address family IPv4 Unicast: advertised and received
Enhanced Refresh Capability: advertised and received
Multisession Capability:
Stateful switchover support enabled: NO for session 1
Fromtheoutput,alsoobservethedefaultholdtimeandkeepaliveinterval.
ConfiguringBGPTimers
ChangingtheBGPdefaultholdtimeandkeepalivetimersisusuallynot
recommended.Thedefaults(keepalive:60seconds;holdtime:180seconds)should
workfineinmostsituations.IfforanyreasonafasterBGPresponsetoapeerdown
eventisneeded,theneighbortimersontheroutercanbereduced.Forexample,in
scenarioswheremultiplepathstowarddestinationsareavailable.Thisreductionwill
resultinafasterdetectionofalostpeerandfasterswitchingtothealternatepathin
theBGPtable,thusimprovingconvergence.
router(config-router)# timers bgp keepalive holdtime
Changes the default values of BGP timers per BGP process.
Only the holdtime value is communicated in the BGP Open message.
Both peers use the smallest configured holdtime value on BGP peers.
router(config-router)# neighbor ip-address timers keepalive holdtime
Changes the default values of BGP timers per specific neighbor.
Overrides the bgp settings of the timers.
SupposethatthereisaBGProuterwithanexpiredholdtime,whichmeansthatno
BGPtrafficwasreceivedwithintheholdtimeinterval.ThisBGProuterwillsenda
notificationtoitsBGPpeer,notifyingitastothereasonforclosingthesession.The
BGProuteronwhichtheholdtimehasexpiredmovestheinactivepeerintotheIdle
state.Afteracertaintimeinterval,aBGProuteragaintriestoreconnecttothe
previouslydisconnectedBGPpeerandwillalsoacceptconnectionattemptsfrom
thatpeer.Thistimeintervalisdeterminedbyauto-enableandconnectiontimers.
TosetthetimersforaspecificBGPpeerorpeergroup,usetheneighbortimers
routerconfigurationcommand.Thiscommandoverridesthevaluesthatyouhaveset
usingthetimersbgpcommand.
Step6
YouwillchangeBGPtimersforsessionbetweenR2andISP1routers.Onthe
R2router,changethedefaultholdtimeandkeepaliveintervalforISP1neighbor
only.
Useaholdtimeof30secondsandkeepaliveinterval10seconds.
R2(config)# router bgp 1
R2(config-router)# neighbor 172.16.12.11 timers 10 30
Step7
OntheISP1router,changethedefaultholdtimeandkeepaliveintervalforBGPprocess.
Useholdtimeof30secondsandkeepaliveinterval10seconds.
ISP1(config)# router bgp 100
ISP1(config-router)# timers bgp 10 30
TimersarenegotiatedatBGPneighborestablishment.OntheISP1router,clearBGPsessionsby
usingthefollowingcommand:
ISP1# clear ip bgp *
*Feb 5 02:41:15.321: %BGP-5-ADJCHANGE: neighbor 172.16.12.2 Down User reset
*Feb 5 02:41:15.321: %BGP_SESSION-5-
ADJCHANGE: neighbor 172.16.12.2 IPv4 Unicast topology base removed from session User reset
*Feb 5 02:41:15.382: %BGP-5-ADJCHANGE: neighbor 172.16.12.2 Up
Step8
OntheR2router,verifytheBGPholdtimeandkeepaliveinterval.
BGPtimershavechangedforISP1neighbor:
R2# show ip bgp neighbors 172.16.12.11
BGP neighbor is 172.16.12.11, remote AS 100, external link
BGP version 4, remote router ID 10.0.1.81
BGP state = Established, up for 00:01:03
Last read 00:00:08, last write 00:00:00, hold time is 30, keepalive interval is 10 seconds
Neighbor sessions:
1 active, is not multisession capable (disabled)
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN Capability: advertised and received
Address family IPv4 Unicast: advertised and received
Enhanced Refresh Capability: advertised and received
BGPtimersaredefaultforISP2neighbor:
R2# show ip bgp neighbors 172.16.22.22
BGP neighbor is 172.16.22.22, remote AS 200, external link
BGP version 4, remote router ID 10.0.2.129
BGP state = Established, up for 00:11:01
Last read 00:00:57, last write 00:00:06, hold time is 180, keepalive interval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable (disabled)
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN Capability: advertised and received
Address family IPv4 Unicast: advertised and received
Enhanced Refresh Capability: advertised and received
ConfiguringMD5Authentication
ToenableMD5authenticationonaTCPconnectionbetweentwoBGPpeers,use
theneighborpasswordrouterconfigurationcommand.
router(config-router)# neighbor ip-address password string
Enables MD5 authentication on a specific BGP session.
Password string on both routers must match.
Step9
YouwillenableMD5authenticationforBGPsessionbetweenR2andISP1
routers.OntheR2router,enableMD5authenticationontheBGPsessiontothe
ISP1neighbor.
UseMD5authenticationpasswordcisco.
R2(config)# router bgp 1
R2(config-router)# neighbor 172.16.12.11 password cisco
Step10
OntheISP1router,enableMD5authenticationontheBGPsessiontotheR2neighbor.
UseMD5authenticationpasswordcisco.
ISP1(config)# router bgp 100
ISP1(config-router)# neighbor 172.16.12.2 password cisco
MakesurethepasswordthatisusedonISP1issameaspasswordusedonR2.Spacecharacter
afterpasswordwillbecomepartofthepassword.
Ifpasswordismisconfigured,youwillseethefollowingmessageontherouterconsole:
%TCP-6-
BADAUTH: Invalid MD5 digest from 172.16.12.2(60123) to 172.16.12.11(179) (RST) tableid - 0
Step11
OntheISP1router,verifythatBGPsessiontotheR2routerisestablished.
ISP1# show ip bgp neighbors 172.16.12.2
BGP neighbor is 172.16.12.2, remote AS 1, external link
BGP version 4, remote router ID 172.16.22.2
BGP state = Established, up for 00:08:19
Last read 00:00:02, last write 00:00:02, hold time is 30, keepalive interval is 10 seconds
Configured hold time is 30, keepalive interval is 10 seconds
Minimum holdtime from neighbor is 0 seconds
Neighbor sessions:
1 active, is not multisession capable (disabled)
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN Capability: advertised and received
Address family IPv4 Unicast: advertised and received
Enhanced Refresh Capability: advertised and received
Step12
OntheR2router,verifythatBGPprefixesarereceivedfromISP1router.
YoushouldseesixBGPprefixesreceivedfromISP1router.
R2# show ip bgp summary
BGP router identifier 172.16.22.2, local AS number 1
BGP table version is 28, main routing table version 28
15 network entries using 2220 bytes of memory
15 path entries using 960 bytes of memory
2/2 BGP path/bestpath attribute entries using 272 bytes of memory
2 BGP AS-PATH entries using 48 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 3500 total bytes of memory
BGP activity 63/48 prefixes, 75/60 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
172.16.12.11 4 100 69 73 28 0 0 00:10:15 6
172.16.22.22 4 200 19 24 28 0 0 00:13:06 9
BeforeyouinjectanylocalroutinginformationintoBGPtableforadvertisingtoother
BGProuters,somebasicconfigurationisrequired.
Only administratively defined networks are announced in BGP.
Manually configure networks to be announced.
Use redistribution from IGP.
Use aggregation to announce summary prefixes.
Therearetwowaystodothisconfiguration:
Listthenetworknumbersthatarecandidatestobeadvertisedusingthe
networkconfigurationcommand.Ifanyofthelistednetworksarereachableby
thelocalrouter,accordingtoitsroutingtable,thenthenetworkisinjectedasa
routeintotheBGPtable.
Redistributeroutinginformationthathasbeenlearnedbyotherroutingprotocols
intotheBGPtable.YoucanusetheIGPthatisusedwithintheAS.Anyroute
thatisknownbythelocalIGPcanbeinjectedintotheBGPtableusingroute
redistributionbetweentheIGPandBGPonthelocalrouter.
AroutercanalsointroducenewroutinginformationintotheBGPtableby
summarizingroutesalreadythere.Thisactivityiscalledrouteaggregationandalso
requiresconfiguration.
AnyroutethattherouterintroducesintotheBGPtablewillappearasanewroute.
TheAS-pathattributeforsucharoutewillbeempty,indicatingalocalroute.TheAS
pathchangeslaterastheroutepassesASboundaries.
router(config-router)# (no) auto-summary
Enables or disables summarization of networks before insertion into the BGP
table:
Locally inserted networks (using the network command)
Redistributed routes
Enabled by default
WhentherouterisconfiguredtolocallyannounceroutesintoBGP,thebehaviorof
thenetworkcommandvariesdependingonwhetherautomaticsummarizationis
enabledordisabled.Whenautomaticsummarizationisenabled,thecommand
summarizeslocallyoriginatedBGPnetworkstotheirclassfulboundaries.Bydefault,
automaticsummarizationisenabledforBGP.
Whenasubnetexistsintheroutingtableandthefollowingthreeconditionsare
satisfied,thenanysubnet(componentroute)ofthatclassfulnetworkinthelocal
routingtablewillpromptBGPtoinstalltheclassfulnetworkintotheBGPtable:
Aclassfulnetworkstatementforthatnetworkexistsintheroutingtable.
Aclassfulmaskhasbeenconfiguredonthatnetworkstatement.
Automaticsummarizationisenabled.
Whenautomaticsummarizationisdisabled,theroutesthatareintroducedlocally
intotheBGPtablearenotsummarizedtotheirclassfulboundaries.
TheBGPauto-summarycommandisalsoresponsibleforthebehaviorofthe
redistributionprocedureinBGP.Whenthecommandisenabled,allredistributed
subnetswillbesummarizedtotheirclassfulboundariesintheBGPtable.Whenitis
disabled,allredistributedsubnetswillbepresentintheiroriginalformintheBGP
table.
router(config-router)# network major-network-number
To manually define a major network
Allows advertising of major networks into BGP.
At least one of the subnets must be present in the routing table.
Behavior depends on the presence of the auto-summary command.
The meaning of the network command in BGP is completely different from any
other routing protocol.
AnnouncingNetworksinBGP
TospecifythenetworkstobeadvertisedbytheBGProutingprocess,usethe
networkrouterconfigurationcommand.Toremoveanentry,usethenoformofthis
command.
ThemeaningofthenetworkcommandinBGPisradically
differentfromthemeaningofthecommandinother
routingprotocols.Inallotherroutingprotocols,the
networkcommandindicatesinterfacesoverwhichthe
routingprotocolwillberun.InBGP,itindicatesonlywhich
routesshouldbeinjectedintotheBGPtableonthelocal
router.Also,BGPneverrunsoverindividualinterfaces;it
isrunoverTCPsessionswithmanuallyconfigured
neighbors.
Thenetworkcommandwithnomaskoptionusestheclassfulapproachtoinserta
majornetworkintotheBGPtable.Atleastonesubnetofthespecifiedmajornetwork
needstobepresentintheIProutingtabletoallowBGPtostartannouncingthe
majornetworkasaBGProute.Ifautomaticsummarizationisdisabled,anexact
matchisrequired.
router(config-router)# network major-network-number route-map route-map-
name
The addition of the route-map option allows network parameters to be modified
before you enter them into the BGP table.
The route-map option can be used for the following:
Changing the weight value of a locally sourced route
Tagging sourced routes with BGP communities
Setting the local preference for a specific network
Changing the value of the MED for a specific network
WhentherouterisconfiguredtoinsertroutesintotheBGPtable,thedefault
attributesoflocallysourcedroutescanbemodifiedwiththeinclusionoftheroute-
mapoptioninthebasicnetworkcommand.
Theattachedroute-mapcanchangethefollowingattributesoflocallysourced
networkswiththenetworkcommand:
Weight(defaultvalue=32768):TheweightattributeisaspecialCiscoattribute
thatisusedinthepathselectionprocesswhenthereismorethanonerouteto
thesamedestination.Becauseweightisconsideredbeforelocalpreferencein
BGProuteselection,locallysourcedroutesarealwayspreferred,unlessthe
weightvalueismodified.
Community(defaultvalue=nonexistent):Usedfortaggingroutesattheir
source.
Localpreference(defaultvalue=100):UsedforAS-wideBGPbest-path
selection.
MED(defaultvalue=0):Usedforreturn-pathselectionintopologieswhere
multipleexitpointstothesameneighborASexist.
AnnouncingNetworksinBGPExample
Ifasubnetexistingintheroutingtableis75.75.75.0/24,andnetwork75.0.0.0is
configuredundertherouterbgpcommand(assumingthatautomaticsummarization
isenabled),BGPwillintroducetheclassfulnetwork75.0.0.0/8intheBGPtable.If
thefollowingthreeconditionsarenotallmet,thenBGPwillnotinstallanyentryin
theBGPtableunlessthereisanexactmatchintheIProutingtable:
Aclassfulnetworkstatementforthenetworkexistsintheroutingtable.
Aclassfulmaskhasbeenconfiguredonthatnetworkstatement.
Automaticsummarizationisenabled.
TherearetwoalternativesforinjectinglocalroutesintotheBGPtable:listthem
usingthenetworkcommandorredistributethem.Listingtheroutesgivesyoutotal
controlovernetworksthatcouldpossiblybeadvertisedbyBGP.Thisoptionisvery
desirableformultihomedcustomersorISPs.Ontheotherhand,thisapproach
requiresmanyconfigurationcommandsthatcouldbedifficulttomaintain.
Easier than listing networks in BGP process in large networks.
Redistributed routes carry origin attribute "incomplete."
Always filter redistributed routes to prevent route leaking.
Avoid in service provider environments.
Iftherearealotofnetworkstobeadvertised,andBGPisusedprimarilytoachieve
scalability,notroutingsecurity(forexample,inenterprisenetworks),itcouldbe
easiertoletthelocalIGPfindtheroutesandthenredistributethemintoBGP.
However,thisapproachintroducestheriskthattheIGPmayfindsomenetworksthat
arenotsupposedtobeadvertised.Privatenetworknumbers,suchasnetwork
10.0.0.0/8,areoftenusedwithinanASforvariousreasonsbutmustneverbe
advertisedouttotheInternet.Carefulfilteringmustbedonetopreventunintentional
advertising.
WhentherouterinjectsaroutethatislistedwithanetworkcommandintoitsBGP
table,theorigincodeissetto"IGP."IftherouteisinjectedintotheBGPtable
throughredistribution,theorigincodeissetto"unknown/incomplete."
RedistributingRoutesintoBGP
Overview
Throughthisdiscovery,youwilllearnhowtoconfigureBGPredistributionandBGP
routeaggregation.
Discovery2:AnnouncingNetworksinBGP
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
R2 Ethernet0/0 172.16.22.2/24 ConnectiontoISP2
R2 Ethernet0/1 172.16.12.2/24 ConnectiontoISP1
R2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
10.0.0.1/28
10.0.0.17/28
10.0.0.33/28
10.0.0.49/28
10.0.0.65/28
Loopbackssimulate
LANnetworks
ISP1 Ethernet0/1 172.16.12.11/24 ConnectiontoR2
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
ISP2 Ethernet0/0 172.16.22.22/24 ConnectiontoR2
ISP2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
Loopback7
Loopback8
Loopback9
10.0.2.1/28
10.0.2.17/28
10.0.2.33/28
10.0.2.49/28
10.0.2.65/28
10.0.2.81/28
10.0.2.97/28
10.0.2.113/28
10.0.2.129/28
Loopbackssimulate
LANnetworks
JobAids
Step1
YouwillredistributeconnectedroutesintoBGP.R2routerispreconfiguredwith
fiveLoopbackinterfacesandisalreadyrunningBGPintheAS1.
Initially,verifywhichLoopbackinterfacesareontheR2router.ReadLoopback
interface'sIPaddressesandsubnetmasks.Youwillneedthisinformationwhen
configuringaccesslist.
R2# show ip interface | include Loopback|Internet address
< text omitted >
Loopback1 is up, line protocol is up
Internet address is 10.0.0.1/28
Loopback2 is up, line protocol is up
Internet address is 10.0.0.17/28
Loopback3 is up, line protocol is up
Internet address is 10.0.0.33/28
Loopback4 is up, line protocol is up
Internet address is 10.0.0.49/28
Loopback5 is up, line protocol is up
Internet address is 10.0.0.65/28
OntheR2router,redistributeconnectedroutesintoBGP:
R2(config)# router bgp 1
R2(config-router)# redistribute connected
Thisway,youwillincludeallconnectedinterfaces,includingLoopback
interfaces,intotheBGProutingtable.
Step2
OntheR2router,configureaccesslistthatpermitseven-numberedLoopback
interfaces(Loopback2andLoopback4).
Useaccesslistnumber1.
R2(config)# access-list 1 permit 10.0.0.16
R2(config)# access-list 1 permit 10.0.0.48
Step3
OntheR2router,makesurethatonlyeven-numberedLoopbackinterfacesare
redistributedintoBGP.
R2(config)# router bgp 1
R2(config-router)# distribute-list 1 out connected
Step4
ClearallBGPsessionsontheR2router.
R2# clear ip bgp *
Step5
OntheR2router,verifythattworoutesareaddedintoBGProutingtable.
R2# show ip bgp
BGP table version is 18, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.16/28 0.0.0.0 0 32768 ?
AnnouncingNetworksinBGP
DiscoverySteps
*> 10.0.0.48/28 0.0.0.0 0 32768 ?
*> 10.0.1.0/28 172.16.12.11 0 0 100 i
*> 10.0.1.16/28 172.16.12.11 0 0 100 i
*> 10.0.1.32/28 172.16.12.11 0 0 100 i
*> 10.0.1.48/28 172.16.12.11 0 0 100 i
*> 10.0.1.64/28 172.16.12.11 0 0 100 i
*> 10.0.1.80/28 172.16.12.11 0 0 100 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
*> 10.0.2.16/28 172.16.22.22 0 0 200 i
*> 10.0.2.32/28 172.16.22.22 0 0 200 i
*> 10.0.2.48/28 172.16.22.22 0 0 200 i
*> 10.0.2.64/28 172.16.22.22 0 0 200 i
*> 10.0.2.80/28 172.16.22.22 0 0 200 i
*> 10.0.2.96/28 172.16.22.22 0 0 200 i
*> 10.0.2.112/28 172.16.22.22 0 0 200 i
*> 10.0.2.128/28 172.16.22.22 0 0 200 i
Whenyouseenexthopis0.0.0.0,youknowthattheseroutesarelocallyinsertedintoBGP
routing.
AlsoobservethatorigincodeisincompleteforroutesthatareinsertedbyR2router.
RedistributionUsingRouteMaps
RoutesthatareredistributedintoBGPwillcarrytheoriginattributeincomplete.In
mostcases,thissituationdoesnotjeopardizeBGPfunctionality.Itcouldposea
problemiftherouteselectionprocesshastodecideonthebestroutetowarda
particulardestinationbasedontheMEDattribute.Inthecaseofreceivingtwo
routes,onewiththeIGPorigin(insertedwiththenetworkcommand),andanother
onewiththeincompleteorigin,thefirstroutewouldalwaysbeselected,nomatter
whatvaluetheMEDattributeissetto(accordingtotheBGProuteselectionrules).
Origin can be set to "IGP" with a route-map.
Other BGP path attributes can also be set:
Metric
Next-hop
Community
Youcanconfigureroute-mapsontheroutertofilterupdatesandmodifyvarious
attributes.Aconfiguredroute-mapcanbeappliedtoroutesbeingredistributedfrom
theIGP.
Onlytheroutesthatarepermittedbytheroute-mapwillberedistributed.Whenyou
usethesetcommandintheroute-map,youcanmodifyspecificpathattributesthat
areattachedtotheredistributedroutes.Thus,onlyselectedrouteswillbe
advertised,andtheywillhavethedesiredattributevalues.
Whenyouconfiguretheroute-map,useaname.Thisnameisacase-sensitive
string,whichisusedwhenyouarereferringtotheroute-map.Anystringcouldbe
used,butameaningfulnameissuggested.
Usetheroute-mapglobalconfigurationcommandandthematchandsetroute-
mapconfigurationcommandstodefinetheconditionsforredistributingroutes.Each
repetitionoftheroute-mapcommandhasalistofmatchandsetcommandsthat
areassociatedwithit.Thematchcommandsspecifythematchcriteria—the
conditionsunderwhichredistributionisallowedforthecurrentroute-mapcommand.
Thesetcommandsspecifythesetactions—theparticularredistributionactionsto
performifthecriteriaenforcedbythematchcommandsaremet.
Whenyouarepassingroutesthrougharoute-map,itcanhaveseveralparts.Any
routethatdoesnotmatchatleastonematchclauserelatingtoaroute-map
commandwillbeignored;thatis,theroutewillnotbeadvertised.Ifyouwantto
modifyonlysomedata,youmustconfigureasecondroute-mapsectionwithan
explicitmatchspecified.
Step6
OntheR2router,configureroutemap,whereyouwillmatchbasedon
previouslyconfiguredaccesslistandsetorigincodetoIGP.
R2(config)# route-map evenLoop permit 10
R2(config-route-map)# match ip address 1
R2(config-route-map)# set origin igp
Step7
OntheR2router,redistributeconnectedroutesintoBGPbyusingpreviously
configuredroutemap.
R2(config)# router bgp 1
R2(config-router)# redistribute connected route-map evenLoop
Step8
OntheR2router,verifythatorigincodeoftwoaddedroutesintheBGProuting
tablehaschangedintoIGP.
R2# show ip bgp | include 0.0.0.0
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
ConfiguringClasslessBGP
BGPversion4isaclasslessprotocol,meaningthatitsroutingupdatesincludethe
IPaddressandthesubnetmask.ThecombinationoftheIPaddressandthesubnet
maskiscalledanIPprefix.AnIPprefixcanbeasubnet,amajornetwork,ora
supernet.
BGPusesprefixnotation(address/numberofbits)todisplayIPprefixes.The
numberfollowingtheslash(/)inthe192.168.0.0/16notationinthefigurerefersto
thenumberofbitsinthesubnetmaskbeingsetto1.Thesubnetmask255.255.0.0
startswith16consecutivebitssetto1,andtherestofthebitssetto0.
Asanotherexample,thesubnet172.16.1.0withmask255.255.255.0canbewritten
usingtheprefixnotationas172.16.1.0/24.
Whenclasslessprefixnotationisused,anoldclassAnetwork,forexample,
10.0.0.0,withthenaturalmask,iswrittenas10.0.0.0/8.AclassBnetwork,
172.17.0.0withnaturalmask,iswrittenas172.17.0.0/16,andaclassCnetwork,
192.168.1.0withnaturalmask,iswrittenas192.168.1.0/24.
BGP supports CIDR.
Any BGP router can advertise individual networks or supernets (prefixes).
Prefix notation is used with BGP instead of subnet masks.
192.168.0.0/16 = 192.168.0.0 255.255.0.0
Step9
OntheR2router,configureLoopback1interfacesubnettobeadvertisedinto
BGP.TheLoopback1interfacehasaclasslessprefixpreconfigured
(10.0.0.0/28).
ToadvertiseclasslessnetworksintoBGP(asubnetorasupernet),youcanuse
thenetworkcommandwiththemaskkeywordandthesubnetmaskspecified.
SupposethatanexactmatchisnotfoundintheIProutingtable.Thissituation
canoccurforexample,whenyouarecreatingasummaryorwhenyouare
advertisingonlyapartofyouraddressspace.Fortheadvertisementtosucceed,
youwillneedtomanuallyconfigureamatchingprefixontherouterintheformof
astaticroutepointingtothenull0interface.
R2(config)# router bgp 1
R2(config-router)# network 10.0.0.0 mask 255.255.255.240
Step10
OntheR2router,verifythatLoopback1interfacesubnetispresentintheBGProutingtable.
R2# show ip bgp
BGP table version is 21, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.1.0/28 172.16.12.11 0 0 100 i
*> 10.0.1.16/28 172.16.12.11 0 0 100 i
*> 10.0.1.32/28 172.16.12.11 0 0 100 i
*> 10.0.1.48/28 172.16.12.11 0 0 100 i
*> 10.0.1.64/28 172.16.12.11 0 0 100 i
*> 10.0.1.80/28 172.16.12.11 0 0 100 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
*> 10.0.2.16/28 172.16.22.22 0 0 200 i
*> 10.0.2.32/28 172.16.22.22 0 0 200 i
*> 10.0.2.48/28 172.16.22.22 0 0 200 i
*> 10.0.2.64/28 172.16.22.22 0 0 200 i
*> 10.0.2.80/28 172.16.22.22 0 0 200 i
*> 10.0.2.96/28 172.16.22.22 0 0 200 i
*> 10.0.2.112/28 172.16.22.22 0 0 200 i
*> 10.0.2.128/28 172.16.22.22 0 0 200 i
R2# show ip bgp 10.0.0.0/28
BGP routing table entry for 10.0.0.0/28, version 21
Paths: (1 available, best #1, table default)
Advertised to update-groups:
3
Refresh Epoch 1
Local
0.0.0.0 from 0.0.0.0 (10.0.0.65)
Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced, local, best
Ifthekeywordmaskandthesubnetmaskareomitted,thenetworkisassumedtohaveits
naturalmaskaccordingtothenetworkclass.Inthiscase,theroutewillstillbeinjectedintothe
BGPtableontherouterifthereisanysubnetofthemajornetworkthatisreachableaccording
totheroutingtable.
Step11
OntheR2router,advertisesubnet10.0.0.0/24intoBGP.Subnet10.0.0.0/24is
representingallLoopbackinterfacesthatareconfiguredonR2router.
R2(config)# router bgp 1
R2(config-router)# network 10.0.0.0 mask 255.255.255.0
Step12
OntheR2router,verifythatsubnet10.0.0.0/24ispresentintheBGProuting
table.
R2# show ip bgp 10.0.0.0/24
% Network not in table
RouteisnotpresentintheBGProutingtable.
ThenetworkcommandwiththemaskoptiontellsBGPthat10.0.0.0/24isa
candidateforbeingadvertised.Themaskkeywordandthemask255.255.255.0
arerequiredbecausethemaskisnotthenaturalone.However,beforethe
candidaterouteisactuallyadvertised,theroutercheckstheroutingtableforan
exactmatch(bothnetworknumberandmask).Itwillalwaysbefoundbecause
thereisastaticrouteforit.Thisstaticroutepointstothenullinterface,whichis
alwaysavailable.
Theconclusionisthat10.0.0.0/24willalwaysbeadvertisedbythisrouter.All
otherBGProuterswillusethisinformationandforwardanyIPpacketswiththe
destinationIPaddressintheinterval10.0.0.0to10.0.0.255(inclusive)inthe
directionofthisrouter.Whenthosepacketsarrive,therouter,inthisexample,
musthavemoreexplicitroutestothedifferentpartsofthe10.0.0.0/24address
range.IGPcouldanswerthisneedbutitisnotshowninthisconfiguration
example.
Step13
OntheR2router,configureexplicitstaticroutefor10.0.0.0/24pointingtothenull
interface.
R2(config)# ip route 10.0.0.0 255.255.255.0 Null 0
If,however,anIPpacketarriveswithadestinationaddresstowhichthisrouter
doesnothaveamoreexplicitroute,thestaticroutewillroutethepackettothe
nullinterface,whereitisdropped.Thisroutingisasafetyprecautionthatwill
preventaroutingloop,whichmightoccurwhenroutesummariesareusedin
combinationwithdefaultrouting.SupposethatapacketarrivesfromtheInternet
toasubnetof10.0.0.0/24,whichiscurrentlynotreachable.Inthiscase,the
packetmightfollowthedefaultroutetowardtheInternetbecausethereisno
moreexplicitroute.Ofcourse,thepacketwouldimmediatelyberoutedback
again,andaroutingloopwouldoccur.
Step14
OntheR2router,verifythatsubnet10.0.0.0/24isnowpresentintheBGProutingtable.
R2# show ip bgp 10.0.0.0/24
BGP routing table entry for 10.0.0.0/24, version 22
Paths: (1 available, best #1, table default)
Advertised to update-groups:
3
Refresh Epoch 1
Local
0.0.0.0 from 0.0.0.0 (10.0.0.65)
Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced, local, best
Route10.0.0.0/24ispresentintheBGProutingtable.
AggregatingBGPNetworks
WhentheBGPtableisalreadypopulatedwithroutesthatshouldbesummarized,
youmustconfigurearoutertodoso.ThesummarizationofBGProutesiscalled
"aggregation."
Useaggregationwhenagroupofmorespecificrouteshasbeeninjectedintothe
BGPtableatonestagebutcanbesummarizedatalaterstage.Theroutestobe
summarizedcouldbeIGProutesthathavebeenredistributedintoBGP.BeforeBGP
advertisestheseroutestotherestofthenetwork,anaggregationofthesubnetsinto
alargerannouncementwouldbeappropriate.
Insomenetworks,morespecificroutesareinjectedintotheBGPtablebysome
routers,andaggregationisdoneinanotherrouteroreveninanotherAS.This
approachiscalled"proxyaggregation."
Whenarouterisconfiguredtodoaggregation,youmustconfiguretheroute
summary.IfanyroutethatisalreadyintheBGPtableiswithintherangethatis
indicatedbythesummary,thenthesummaryrouteisalsoinjectedintotheBGP
tableontherouteandadvertisedtootherrouters.Thisactioncreatesmore
informationintheBGPtable.Togetanybenefitsfromtheaggregation,youmust
suppressthemorespecificroutesthatarecoveredbytheroutesummary.This
suppressionisanoptiontotheaggregateconfigurationcommand.
Whenyousuppressthemorespecificroutesthroughconfiguration,theyarestill
presentintheBGPtableoftherouterdoingtheaggregation.However,becausethe
routesaremarkedassuppressed,theyareneveradvertisedtoanyotherrouter.
Summarization is called "aggregation" in BGP.
Aggregation creates summary routes (called "aggregates") from networks
already in BGP table.
Individual networks can be announced or suppressed.
Step15
OntheISP1router,configureBGPaggregationonthesubnet10.0.1.0/24.
Donotusesummary-onlykeyword,toobserveboththeroutesummary,and
themorespecificroutesthatwillbeadvertised.
ISP1(config)# router bgp 100
ISP1(config-router)# aggregate-address 10.0.1.0 255.255.255.0
Step16
OntheISP1router,verifypresenceofaggregatedroute10.0.1.0/24intheBGProuting
table.
ISP1# show ip bgp
BGP table version is 73, local router ID is 10.0.1.81
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.12.2 0 0 1 i
*> 10.0.0.0/24 172.16.12.2 0 0 1 i
*> 10.0.0.16/28 172.16.12.2 0 0 1 i
*> 10.0.0.48/28 172.16.12.2 0 0 1 i
*> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.0/24 0.0.0.0 32768 i
*> 10.0.1.16/28 0.0.0.0 0 32768 i
*> 10.0.1.32/28 0.0.0.0 0 32768 i
*> 10.0.1.48/28 0.0.0.0 0 32768 i
*> 10.0.1.64/28 0.0.0.0 0 32768 i
*> 10.0.1.80/28 0.0.0.0 0 32768 i
*> 10.0.2.0/28 172.16.12.2 0 1 200 i
*> 10.0.2.16/28 172.16.12.2 0 1 200 i
*> 10.0.2.32/28 172.16.12.2 0 1 200 i
*> 10.0.2.48/28 172.16.12.2 0 1 200 i
*> 10.0.2.64/28 172.16.12.2 0 1 200 i
*> 10.0.2.80/28 172.16.12.2 0 1 200 i
*> 10.0.2.96/28 172.16.12.2 0 1 200 i
*> 10.0.2.112/28 172.16.12.2 0 1 200 i
*> 10.0.2.128/28 172.16.12.2 0 1 200 i
Step17
OntheR2router,verifypresenceofaggregatedroute10.0.1.0/24andmorespecificroutes
intheBGProutingtable.
R2# show ip bgp
BGP table version is 37, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 0.0.0.0 0 32768 i
*> 10.0.0.0/24 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.1.0/28 172.16.12.11 0 0 100 i
*> 10.0.1.0/24 172.16.12.11 0 0 100 i
*> 10.0.1.16/28 172.16.12.11 0 0 100 i
*> 10.0.1.32/28 172.16.12.11 0 0 100 i
*> 10.0.1.48/28 172.16.12.11 0 0 100 i
*> 10.0.1.64/28 172.16.12.11 0 0 100 i
*> 10.0.1.80/28 172.16.12.11 0 0 100 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
*> 10.0.2.16/28 172.16.22.22 0 0 200 i
*> 10.0.2.32/28 172.16.22.22 0 0 200 i
*> 10.0.2.48/28 172.16.22.22 0 0 200 i
*> 10.0.2.64/28 172.16.22.22 0 0 200 i
*> 10.0.2.80/28 172.16.22.22 0 0 200 i
*> 10.0.2.96/28 172.16.22.22 0 0 200 i
*> 10.0.2.112/28 172.16.22.22 0 0 200 i
*> 10.0.2.128/28 172.16.22.22 0 0 200 i
Thisapproachisgenerallynotdesirable.Therefore,suppressionofindividualroutes,
describednext,isusedinmostcases.
Step18
OntheISP1router,configureBGPaggregationonthesubnet10.0.1.0/24by
usingkeywordsummary-only.
ISP1(config)# router bgp 100
ISP1(config-router)# aggregate-address 10.0.1.0 255.255.255.0 summary-
only
Step19
OntheISP1router,verifypresenceofaggregatedroute10.0.1.0/24intheBGProuting
table.
ISP1# show ip bgp
BGP table version is 79, local router ID is 10.0.1.81
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.12.2 0 0 1 i
*> 10.0.0.0/24 172.16.12.2 0 0 1 i
*> 10.0.0.16/28 172.16.12.2 0 0 1 i
*> 10.0.0.48/28 172.16.12.2 0 0 1 i
s> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.0/24 0.0.0.0 32768 i
s> 10.0.1.16/28 0.0.0.0 0 32768 i
s> 10.0.1.32/28 0.0.0.0 0 32768 i
s> 10.0.1.48/28 0.0.0.0 0 32768 i
s> 10.0.1.64/28 0.0.0.0 0 32768 i
s> 10.0.1.80/28 0.0.0.0 0 32768 i
*> 10.0.2.0/28 172.16.12.2 0 1 200 i
*> 10.0.2.16/28 172.16.12.2 0 1 200 i
*> 10.0.2.32/28 172.16.12.2 0 1 200 i
*> 10.0.2.48/28 172.16.12.2 0 1 200 i
*> 10.0.2.64/28 172.16.12.2 0 1 200 i
*> 10.0.2.80/28 172.16.12.2 0 1 200 i
*> 10.0.2.96/28 172.16.12.2 0 1 200 i
*> 10.0.2.112/28 172.16.12.2 0 1 200 i
*> 10.0.2.128/28 172.16.12.2 0 1 200 i
Theprefix10.0.1.0/24isinjectedbecausethereisatleastonemorespecificroutewithin
thesummaryrange.Inthiscase10.0.1.0/28,10.0.1.16/28,10.0.1.32/28,10.0.1.48/28,
10.0.1.64/28,and10.0.1.80/28arewithintherange.Themorespecificroutesaremarked
assuppressedusingthelowercaseletter"s."The"s"meansthattheyarestillpresentand
availableintheBGPtableoftherouter,buttheyarenotadvertisedonanyBGPsession.
Whenthesummary-onlyoptionisused,onlytheroutesummarywillbeadvertised,notthe
morespecificroutes.
Oneofthebenefitsofthisapproachisthattherestoftherouterswillreceiveonlyone
routeinsteadofmanymorespecificroutes.Iteasestheburdenontheotherroutersby
reducingtheamountofmemorythatisrequiredtoholdtheBGPtable.
Anotherbenefitisthatrouteflappingisreduced.Therouterdoingtheaggregation
continuesadvertisingtheaggregateaslongasthereisatleastonespecificroutewithin
therangestillavailable.Ifoneofthemorespecificroutesislostbutatleastoneremains,
theaggregateitselfisnotlost.Theflapofthemorespecificrouteisnotvisibletotherest
ofthenetwork.ThisapproachreducesthenumberofupdatesnecessaryandtheCPU
powerthatisrequiredtoprocessthem.
However,allroutesummarizationinanyroutingprotocolcausesalossofgranularity(that
is,lackofmoredetailednetworkorsubnetvisibility).Suboptimalroutingcouldbe
introducedwhenredundantpathsareavailabletoreachagroupofnetworksthatare
advertisedbyasingleroutesummary.Someofthenetworkscouldbemorereachablevia
oneofthepaths,whileothersmaybemorereachableanotherway.Fromoutsidethe
immediatenetwork,multiplepathsmaynotbevisiblebecauseonlysummaryroutesare
advertised.Therefore,thereisariskthattheleastoptimalpathwillbechosen.
Step20
OntheR2router,verifythatonlyBGPaggregatedrouteisreceivedfromISP1router.
Youwillnotseemorespecificroutes(10.0.1.0/28,10.0.1.16/28,10.0.1.32/28,
10.0.1.48/28,10.0.1.64/28,or10.0.1.80/28)advertisedfromISP1router.
R2# show ip bgp
BGP table version is 29, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 0.0.0.0 0 32768 i
*> 10.0.0.0/24 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.1.0/24 172.16.12.11 0 0 100 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
*> 10.0.2.16/28 172.16.22.22 0 0 200 i
*> 10.0.2.32/28 172.16.22.22 0 0 200 i
*> 10.0.2.48/28 172.16.22.22 0 0 200 i
*> 10.0.2.64/28 172.16.22.22 0 0 200 i
*> 10.0.2.80/28 172.16.22.22 0 0 200 i
*> 10.0.2.96/28 172.16.22.22 0 0 200 i
*> 10.0.2.112/28 172.16.22.22 0 0 200 i
*> 10.0.2.128/28 172.16.22.22 0 0 200 i
RoutesthatareadvertisedthroughtheBGParecommonlyaggregatedtominimize
thenumberofroutesthatareusedandreducethesizeofglobalroutingtables.
However,commonrouteaggregationcanobscuremorespecificroutinginformation
thatismoreaccuratebutnotnecessarytoforwardpacketstotheirdestinations.
Commonrouteaggregationobscuresroutingaccuracybecauseaprefixthat
representsmultipleaddressesorhostsoveralargetopologicalareacannotbe
accuratelyreflectedinasingleroute.CiscoIOSSoftwareprovidesseveralmethods
bywhichyoucanoriginateaprefixintoBGP.Themethodsincluderedistributionand
usingthenetworkoraggregate-addresscommand.Thesemethodsassumethe
existenceofmorespecificroutinginformation(matchingtheroutetobeoriginated)
ineithertheroutingtableortheBGPtable.
The BGP Conditional Route Injection feature has the following functions:
It provides means to originate a prefix into a BGP routing table without the
corresponding match.
It allows more specific routes to be generated based on administrative policy or
traffic engineering information to provide more specific control over the
forwarding of packets to these more specific routes, which are injected into the
BGP routing table only if the configured conditions are met.
It improves accuracy of common route aggregation by conditionally injecting or
replacing less specific prefixes with more specific prefixes.
TheBGPConditionalRouteInjectionfeatureallowsyoutooriginateaprefixintoa
BGProutingtablewithoutthecorrespondingmatch.Thisfeatureallowsmore
specificroutestobegeneratedbasedonadministrativepolicyortrafficengineering
information.Thisfeatureprovidesmorespecificcontrolovertheforwardingof
packetstothesemorespecificroutes.TheseroutesareinjectedintotheBGP
routingtableonlyiftheconfiguredconditionsaremet.Enablingthisfeatureallows
youtoimprovetheaccuracyofcommonrouteaggregationbyconditionallyinjecting
orreplacinglessspecificprefixeswithmorespecificprefixes.Onlyprefixesthatare
equaltoormorespecificthantheoriginalprefixmaybeinjected.
TheBGPConditionalRouteInjectionfeatureisenabledwiththebgpinject-map
exist-mapcommand.Thiscommandusestworoutemaps(inject-mapandexist-
map)toinstallone(ormorethanone)morespecificprefixintoaBGProutingtable.
Theexist-mapspecifiestheprefixesthattheBGPspeakerwilltrack.Theinject-map
definestheprefixesthatwillbecreatedandinstalledintothelocalBGPtable.
BGPConditionalRouteInjectionExample
Thisconfigurationexampleconfiguresconditionalrouteinjectionfortheinject-map
thatisnamedORIGINATEandtheexist-mapthatisnamedLEARNED_PATH:
router bgp 109
bgp inject-map ORIGINATE exist-map LEARNED_PATH
!
route-map LEARNED_PATH permit 10
match ip address prefix-list ROUTE
match ip route-source prefix-list ROUTE_SOURCE
!
route-map ORIGINATE permit 10
set ip address prefix-list ORIGINATED_ROUTES
set community 14616:555 additive
!
ip prefix-list ROUTE permit 10.1.1.0/24
!
ip prefix-list ORIGINATED_ROUTES permit 10.1.1.0/25
ip prefix-list ORIGINATED_ROUTES permit 10.1.1.128/25
!
ip prefix-list ROUTE_SOURCE permit 10.2.1.1/32
BGPConditionalRouteInjection
TheBGPSupportforTTLSecurityCheckfeatureintroducesalightweightsecurity
mechanismtoprotectEBGPpeeringsessionsfromCPUutilization-basedattacks.
Thesetypesofattacksaretypicallybrute-forceDoSattacksthatattempttodisable
thenetworkbyfloodingthenetworkwithIPpacketsthatcontainforgedsourceand
destinationIPaddresses.ThisfeatureprotectstheEBGPpeeringsessionby
comparingthevalueintheTTLfieldofreceivedIPpacketsagainstahopcount.The
hopcountisconfiguredlocallyforeachEBGPpeeringsession.Ifthevalueinthe
TTLfieldoftheincomingIPpacketisgreaterthanorequaltothelocallyconfigured
value,theIPpacketisacceptedandprocessednormally.IftheTTLvalueintheIP
packetislessthanthelocallyconfiguredvalue,thepacketissilentlydiscardedand
noICMPmessageisgenerated.Notrespondingtoaforgedpacketispartofa
designedbehaviorthathelpspreventCPUutilization-basedattacks.
BGP Support for TTL Security Check feature has the following characteristics:
Lightweight security mechanism to protect EBGP peering sessions from CPU
utilization-based attacks.
Protects the EBGP peering session by comparing the value in the TTL field of
received IP packets against a hop count that is configured locally for each EBGP
peering session.
Supports both directly connected peering sessions and multihop EBGP peering
sessions.
AccuratelyforgingtheTTLcountinanIPpacketisgenerallyconsideredtobe
impossible.ItispossibletoforgetheTTLfieldinanIPpacketheader.However,
accuratelyforgingapackettomatchtheTTLcountfromatrustedpeerisnot
possibleunlessthenetworktowhichthetrustedpeerbelongshasbeen
compromised.
ThisfeaturesupportsbothdirectlyconnectedpeeringsessionsandmultihopEBGP
peeringsessions.TheincomingpacketsthatcontaininvalidTTLvaluesdonotaffect
theBGPpeeringsession.TheBGPpeeringsessionremainsopen,andtherouter
silentlydiscardstheinvalidpacket.TheBGPsession,however,canstillexpireif
keepalivepacketsarenotreceivedbeforethesessiontimerexpires.
TheBGPSupportforTTLSecurityCheckfeatureshouldbeconfiguredoneach
participatingrouter.Itprovidesaneffectiveandeasy-to-deploysolutiontoprotect
EBGPpeeringsessionsfromCPUutilization-basedattacks.Whenthisfeatureis
enabled,ahostcannotattackaBGPsessionifthehostisnotamemberofthelocal
orremoteBGPnetworkorifthehostisnotdirectlyconnectedtoanetworksegment
betweenthelocalandremoteBGPnetworks.Thissolutiongreatlyreducesthe
effectivenessofDoSattacksagainstaBGPAS.
BGPSupportforTTLSecurityCheck
Overview
Throughthisdiscovery,youwilllearnhowtoenableBGPTTLsecuritycheck.
Discovery3:ImplementBGPTTLSecurityCheck
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
R2 Ethernet0/0 172.16.22.2/24 ConnectiontoISP2
R2 Ethernet0/1 172.16.12.2/24 ConnectiontoISP1
R2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
10.0.0.1/28
10.0.0.17/28
10.0.0.33/28
10.0.0.49/28
10.0.0.65/28
Loopbackssimulate
LANnetworks
ISP1 Ethernet0/1 172.16.12.11/24 ConnectiontoR2
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
ISP2 Ethernet0/0 172.16.22.22/24 ConnectiontoR2
ISP2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
Loopback7
Loopback8
Loopback9
10.0.2.1/28
10.0.2.17/28
10.0.2.33/28
10.0.2.49/28
10.0.2.65/28
10.0.2.81/28
10.0.2.97/28
10.0.2.113/28
10.0.2.129/28
Loopbackssimulate
LANnetworks
JobAids
Step1
TosecureaBGPpeeringsessionbetweenR2andISP2routersconfigureBGP
TTLsecuritycheck.
OntheR2router,configureBGPTTLsecuritychecktotheISP2withhopcount
10.
R2(config)# router bgp 1
R2(config-router)# neighbor 172.16.22.22 ttl-security hops 10
BGPsessionbetweenR2andISP2routerswillgodown.
RouterR2willstartsendingBGPpacketswithTTL255andwillacceptBGP
packetswith245ormore.
Step2
OntheISP2router,configureBGPTTLsecuritychecktotheR2withhopcount
10.
ISP2(config)# router bgp 200
ISP2(config-router)# neighbor 172.16.22.2 ttl-security hops 10
BGPsessionbetweenR2andISP2routerswillgoup.
%BGP-5-ADJCHANGE: neighbor 172.16.22.2 Up
Step3
OntheISP2router,verifywhatTTLvaluesareusedforBGPneighbor.
ISP2# show ip bgp neighbors 172.16.22.2 | include TTL
Connection is ECN Disabled, Mininum incoming TTL 245, Outgoing TTL 255
YoucanseeBGPpacketsaresentwithTTLnumber255andareacceptedwith
TTLvalue245ormore.
Step4
OntheR2router,verifywhatTTLvaluesareusedforBGPneighbor.
R2# show ip bgp neighbors 172.16.22.22 | include TTL
Connection is ECN Disabled, Mininum incoming TTL 245, Outgoing TTL 255
YoucanseeBGPpacketsaresentwithTTLnumber255andareacceptedwith
TTLvalue245ormore.
Step5
OntheR2router,verifythatBGPsessiontotheISP2isestablished.
R2# show ip bgp summary
BGP router identifier 10.0.0.65, local AS number 1
BGP table version is 63, main routing table version 63
14 network entries using 2072 bytes of memory
14 path entries using 896 bytes of memory
3/3 BGP path/bestpath attribute entries using 408 bytes of memory
2 BGP AS-PATH entries using 48 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 3424 total bytes of memory
BGP activity 38/24 prefixes, 38/24 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
ImplementBGPTTLSecurityCheck
DiscoverySteps
172.16.12.11 4 100 241 247 63 0 0 00:37:51 1
172.16.22.22 4 200 19 23 63 0 0 00:12:47 9
BGPsessionbetweenR2andISP2routersissecuredwithBGPTTLsecuritycheck.
Inthisexample,theprimaryproviderisdoingaggregationof192.1.0.0/16before
sendingittotherestofthenetwork.Thissituationmeansthattheprimaryprovideris
alsodoingproxyaggregationfortheroute192.1.1.0/24thatisadvertisedbythe
multihomedcustomer.TherestoftheInternetwillnotseetheroute192.1.1.0/24via
theprimaryprovider.
Customer prefers primary provider, using alternate only as backup.
Primary provider advertises the aggregate.
Alternate provider advertises individual network.
Themultihomedcustomeralsoadvertises192.1.1.0/24tothealternateprovider.In
thiscase,theproviderdoesnotdoanyaggregationofanyroutesstartingwith192.1
(andshouldnotdoso).Thissituationmeansthatthealternateproviderwill
propagate192.1.1.0/24totherestoftheInternet.
Customer prefers primary provider, using alternate only as backup.
Primary provider advertises the aggregate.
Alternate provider advertises individual network.
TherestoftheInternetnowseesoverlappingroutes.Itsees192.1.1.0/24as
reachableviathealternateproviderand192.1.0.0/16asreachableviatheprimary
provider.Thesetworoutesaretreatedasdifferentroutes.Theyarenotcompared
witheachotherinarouteselectionprocessbecausetheyindicatedifferent
destinations.Becausetherouterviewsthemasdifferentdestinations,bothroutes
willbeinjectedintotheroutingtable.
Ifapacketarriveswithadestinationaddressinthe192.1.1.0/24network,therestof
theInternetwillfollowthe"longestmatchingprefix"ruleandforwardthepacketto
thealternateprovider.
Toavoidthisissue,theprimaryprovidermustturnoffaggregation.Iftheprimary
providerdoesso,therestoftheInternetwillsee192.1.1.0/24bothways.And,
becausethesameroute(networkandmask)isreachableintwoways,route
selectionprocessingstarts.Dependingontheattributevalues,therestofthe
Internetcouldbeadvisedtousetheprimaryproviderinsteadofthealternateone.
However,turningoffaggregationwillalsocausetheprimaryprovidertoadvertiseall
routeswithintheaggregate,andallbenefitsofaggregationwillbelost.
MultihomedCustomerProblem
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
The BGP process in a Cisco router is started with the router bgp command and
BGP neighbors are added with neighbor command.
MD5 authentication can be used to secure a connection between two BGP
neighbors. BGP also supports TTL security check feature, which protects EBGP
sessions from CPU utilization-based attacks.
Local networks are announced in BGP by listing them with the network
command or by redistributing them.
BGP supports aggregation but BGP route aggregation is not appropriate in
multihomed topologies.
Summary
Overview
ToensurethatbasicBGPconfigurationsareoperatingcorrectly,youneedtobe
familiarwithBGPmonitoringcommands.IfbasicBGPconfigurationsarenot
functioningasexpected,BGPtroubleshootingskillsarecriticaltosuccessful
problemresolution.
ThislessonintroducestheCiscoIOScommandsthatareavailableformonitoring
andtroubleshootingbasicBGPconfigurations.Thecommandsthatarerequiredto
monitorthestatusofBGP,neighborconnections,andtheBGPtablearediscussed.
ThelessonalsodiscussestechniquesfortroubleshootingthemostcommonBGP
sessionstartupissues.
Uponcompletingthislesson,youwillbeableto:
IdentifytheCiscoIOScommandthatisrequiredtomonitortheoverallstatusof
theBGProutingprocess
IdentifytheCiscoIOScommandthatisrequiredtomonitorBGPneighbors
IdentifytheCiscoIOScommandsthatarerequiredtomonitortheBGPtable
IdentifytheCiscoIOScommandsthatarerequiredtoperformbasicBGP
debugging
ListcommonBGPsessionstartupproblems
TroubleshootbasicBGPsessionstartupproblemswhentheneighborisnot
reachable
TroubleshootbasicBGPsessionstartupproblemswhentheneighborisnot
configured
TroubleshootbasicBGPsessionstartupproblemswhenanASnumber
mismatchexists
MonitoringandTroubleshootingBGP
TodisplaythestatusofallBGPconnections,usetheshowipbgpsummaryEXEC
command.
Thiscommandhasnoargumentsorkeywords.
router> show ip bgp summary
Displays BGP memory use, and displays BGP neighbors and the state of
communication with them
R2# show ip bgp summary
BGP router identifier 172.16.22.2, local AS number 1
BGP table version is 10, main routing table version 10
9 network entries using 1332 bytes of memory
9 path entries using 576 bytes of memory
1/1 BGP path/bestpath attribute entries using 136 bytes of memory
1 BGP AS-PATH entries using 24 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 2068 total bytes of memory
BGP activity 9/0 prefixes, 9/0 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/P
fxRcd
172.16.12.11 4 100 0 0 1 0 0 never Idle
172.16.22.22 4 200 63 62 10 0 0 00:54:24
9
ThiscommandisveryusefulwhenyouaretroubleshootingBGP.Theoutputinthe
figureprovidesashortsummaryofthestatusoftheBGPprocessintherouter.
ThefirstsectionoftheoutputdescribestheBGPtableanditscontent:
TheBGPtableversionistheversionnumberofthelocalBGPtable.This
numberisincreasedeverytimethatthetableischanged.
ThemainroutingtableversionshowsthelastversionoftheBGPdatabasethat
wasinjectedintothemainroutingtable.
Thesubsequentlinesoftextindicatetheamountofmemorythathasbeen
allocatedtoholdthetable.Theselinesoftextdisplayhowmanynetworksare
knownandhowmanydifferentpathsandattributevaluesareassociatedwith
them.
Thesecondsectionoftheoutputisatableinwhichthecurrentneighborstatuses
areshown.Thereisonelineoftextforeachneighborthathasbeenconfigured.The
columnsareasfollows:
IPaddressoftheneighborasconfiguredinthelocalrouter
BGPversionnumberthattherouteruseswhencommunicatingwiththe
neighbor
ASnumberoftheremoteneighbor
Numberofmessagesandupdatesthathavebeenreceivedfromtheneighbor
sincethesessionwasestablished
Numberofmessagesandupdatesthathavebeensenttotheneighborsincethe
sessionwasestablished
VersionnumberofthelocalBGPtablethathasbeenincludedinthemostrecent
updatetotheneighbor
Numberofmessagesthatarewaitingtobeprocessedintheincomingqueue
fromthisneighbor
Numberofmessagesthatarewaitingintheoutgoingqueuefortransmissionto
theneighbor
Howlongtheneighborhasbeeninthecurrentstateandthenameofthecurrent
state(thestate"Established"isnotprintedout,sonostatenameindicates
"Established")
YoucanusethisinformationtoverifythatBGPsessionsareupandestablished.If
theyarenot,youwillhavetofurtherinvestigatetheBGPconfigurationtolocatethe
problem.YoucanalsoverifytheIPaddressandASnumberoftheconfiguredBGP
neighborwiththeshowipbgpsummarycommand.
Ifthesessionstateis"Established,"thenumberofmessagesthathavebeensent
MonitoringOverallBGPRouting
andreceived,asdisplayedintheoutputoftheshowipbgpsummarycommand,
canindicateBGPstability.Usethecommandafewtimes,withatimeinterval
betweentheprintouts,andcalculatehowmanymessageshavebeenexchanged
duringthatperiod.
ManymessagesintheincomingqueueindicatealackofCPUresourcesinthelocal
router.Manymessagesintheoutgoingqueueindicatealackofbandwidthtothe
remoterouteroralackofCPUresourcesintheremoterouter.
TodisplayinformationabouttheTCPandBGPconnectionstoneighbors,usethe
showipbgpneighborsEXECcommand.
router> show ip bgp neighbors ip-address
Displays detailed neighbor information
R2# show ip bgp neighbors 172.16.12.11
BGP neighbor is 172.16.12.11, remote AS 100, external link
BGP version 4, remote router ID 10.0.1.81
BGP state = Established, up for 00:03:10
Last read 00:00:26, last write 00:00:31, hold time is 180, keepalive int
erval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable (disabled)
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN Capability: advertised and received
Address family IPv4 Unicast: advertised and received
Enhanced Refresh Capability: advertised and received
Multisession Capability:
Stateful switchover support enabled: NO for session 1
Youcanusethiscommandfortwodifferentpurposes.Thegeneralpurpose,as
showninthefigure,istogetinformationabouttheTCPsessionandtheBGP
parametersofthesession.AllBGPsessionparametersaredisplayed.Inaddition,
TCPtimersandcountersarealsodisplayed.
Theotheruseisnotshowninthisexample.Ifanyoftheoptionalqualifiersreferring
toroutesorpathsaregiven,theBGProutinginformationthatwassentorreceived
onthissessionisdisplayed.Thisfeatureisusefulwhenyouaretroubleshootingpath
selection.
MonitoringBGPNeighbors
TodisplayentriesintheBGProutingtable,usetheshowipbgpEXECcommand.
router> show ip bgp
Displays all routes in the BGP table in summary format
R2# show ip bgp
BGP table version is 18, local router ID is 172.16.22.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - inte
rnal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-
Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 1.0.0.0/24 172.16.22.22 0 0 200 i
* 172.16.12.11 0 0 100 99 i
*> 10.0.1.0/28 172.16.12.11 0 0 100 i
*> 10.0.1.16/28 172.16.12.11 0 0 100 i
*> 10.0.1.32/28 172.16.12.11 0 0 100 i
*> 10.0.1.48/28 172.16.12.11 0 0 100 i
*> 10.0.1.64/28 172.16.12.11 0 0 100 i
*> 10.0.1.80/28 172.16.12.11 0 0 100 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
<... output omitted ...>
Inmostcases,whentheshowipbgpcommandisgivenwithoutoptionalqualifiers,
theentireBGPtableisdisplayed.Anabbreviatedlistofinformationabouteachroute
isdisplayed,onelineperprefix.Theoutputissortedinnetworknumberorder.
Therefore,iftheBGPtablecontainsmorethanoneroutetothesamenetwork,the
routesaredisplayedonsuccessivelines.Thenetworknumberisprintedonthefirst
oftheselinesonly.Thefollowinglines,whichrefertothesamenetwork,havethe
networknumberfieldleftblank.
Some,butnotall,oftheBGPattributesthatareassociatedwiththerouteare
displayedontheline.Next-hop,MED(displayedas"Metric"),localpreference,and
weighteachhavetheirowncolumns.TheAS-pathattributeisdisplayedasthe
sequenceofASnumbersinthe"Path"column.ImmediatelyfollowingtheASpath,
butnotpartoftheAS-pathattribute,theoriginattributeisdisplayed.Thelowercase
letter"i"meansIGP,"e"meansEGP,and"?"meansincompleteorunknown.
TheBGPpathselectionprocessselectsoneoftheavailableroutestoeachofthe
networksasthebest.Theseroutesarepointedoutbythecharacter">"intheleft
column.
router> show ip bgp ip-prefix [mask subnet-mask]
Displays detailed information about all paths for a single prefix
R2# show ip bgp 1.0.0.1
BGP routing table entry for 1.0.0.0/24, version 18
Paths: (2 available, best #1, table default)
Advertised to update-groups:
1
Refresh Epoch 1
200
172.16.22.22 from 172.16.22.22 (10.0.2.129)
Origin IGP, metric 0, localpref 100, valid, external, best
Refresh Epoch 1
100 99
172.16.12.11 from 172.16.12.11 (10.0.1.81)
Origin IGP, metric 0, localpref 100, valid, external
BGP attributes are listed for each route: AS-Path, Next-Hop, advertising router
IP address, advertising router Router-ID.
The bottom row lists other BGP attributes.
IfmoreinformationandthecompletesetofBGPattributesarerequired,theshowip
bgpcommandshouldbeenteredwiththenetworknumberonthecommandline.
ThiscommanddisplaysallrelevantBGPinformationaboutthatspecificnetwork.
MonitoringBGPTable
Inthisexample,theinformationaboutnetwork1.0.0.1isdisplayed.Therearetwo
routesto1.0.0.1.Oneisreceivedfromneighbor172.16.12.11andtheotherfrom
172.16.22.22.
TheBGProuteselectionprocesshasselectedthisrouteasthebest.So,BGPwill
trytoinstallthisrouteintotheroutingtable.Installationofroutesintheroutingtable
ismadebasedontheAD.
ThereareseveralCiscoIOScommandsthatyoucanusetoperformdebuggingof
basicBGPconfigurations.
router# debug ip tcp transactions
Displays all TCP transactions (start of session, session errors, etc.)
router# debug ip bgp events
Displays significant BGP events (neighbor state transitions, update runs)
IfaBGPsessionstaysintheActivestate,whereitisactivelysendingconnection
attemptstotheneighbor,debugiptcptransactionscanprovidevaluable
informationaboutfailedconnectionattempts.AllTCPtransactionsintherouterare
displayedontheconsoleastheyhappen.Thenetworkadministratorcanthen
determinewhethertheTCPsessionisbeingestablished,and,ifnot,whatthe
probablecauseoftheproblemmightbe.
IftheTCPsessionsucceedsbutistorndownwithinashorttime,youmightfindthe
reasonifyouusedebugipbgpevents.AllBGPeventswillbedisplayedonthe
consoleastheyhappenifthisdebugcommandisenabled.
router# debug ip bgp keepalives
Debugs BGP keepalive packets
router# debug ip bgp updates
Displays all incoming or outgoing BGP updates
Use with caution
Inastablestatewithnonetworktopologychanges,noBGPupdatesaresent
betweenneighboringrouters.WhenaBGPsessionhasbeenidleforsometime,the
BGPprotocolwillexchangekeepalivepacketsbetweenBGPneighbors.The
keepalivetimerhasadefaultvalueof60seconds.
Usethedebugipbgpkeepalivescommandtogetaprintoutontheconsolefor
everykeepalivepacketthatissentorreceived.Successfulkeepaliveexchanges
indicatethatthesessionisworkingandisinastablestate.
Ifnokeepaliveshavebeensentorreceived,thesessionmightstillbeworking.The
reasonfornotseeinganykeepaliveswouldbethatthesessionisneveridlelong
enough.
Usethedebugipbgpupdatescommandtogetaprintoutontheconsoleforevery
updatemessagethatissentorreceived.Thesuccessfulexchangeofupdates
indicatesthatthesessionisworkingandisnotintheIdlestate.
Inalargenetwork,updatesaresentandreceivedinlargevolumes.Startingthe
debugipbgpupdatescommandmightcauseextensiveoutputontheconsole.In
somecases,theCPUresourcesthatareusedtogeneratethoseoutputsareso
greatthatfewCPUresourcesremaintoactuallyforwardtraffic.Inacasewithvery
busyBGPsessions,itisactuallypossibletosettherouterinaconditionwhereall
CPUresourcesareconsumedwiththedebuggingprintouts.
router# debug ip bgp updates acl
Displays all incoming or outgoing BGP updates for routes matching an IP
access-list
router# debug ip bgp ip-address updates [acl]
Displays all BGP updates that are received from or sent to a BGP neighbor
(optionally matching an IP access-list)
Toavoiddebugprintoutsforeveryupdatethatissentorreceived,youcancreate
andassociateanaccess-listwiththedebugcommand.Whenyouusethis
command,theconsoledisplaysonlytheupdatesthatrefertoanetworknumberthat
DebuggingBGP
ispermittedbytheaccess-list.Thecommandisextremelyusefulinalivenetwork
withbusyBGPsessionswherethetroubleshooterisinterestedonlyinupdatesfor
specificnetworks.
Indicatingaspecificneighborcanevenfurtherrestrictthedebugging.Theconsole
displaysonlytheupdatesonthesessionwiththeindicatedneighbor.Optionally,you
cancombinethisdebugcommandwithanaccess-list.
Thereareseveralcommonsessionstartupissuesthatyoucanexperiencewhen
configuringbasicBGP.
Common BGP session startup symptoms:
BGP neighbors do not become active.
BGP neighbor is active, but the session is never established.
BGP neighbor oscillates between idle and active.
BGPSessionStartupProblems
BGPsessionstoarouterinanotherASshouldnormallyrunacrossdirectly
connectedinterfaces(routersthatshareacommonIPsubnet).Youmustconfigure
neighboringrouterstoreacheachotherusingtheIPaddressbelongingtothis
sharedsubnetsothatnootherroutingprotocolisrequiredtosetuptheBGP
session.
IfarouterisconfiguredwithaBGPneighborthatisinanotherASbutnotdirectly
connected,thesessionwillstayintheIdlestate.Therouterwillnotevenattemptto
setupthesession.
Symptom:
BGP neighbors do not become active.
show ip bgp neighbors displays the neighbor state as Idle for several
minutes.
Diagnosis:
Neighbor is not directly connected.
Verification:
Verify with show ip route.
Thenormalwaytofixthisproblemistochangetheneighborreferencesothatitis
referredbyanIPaddressthatisdirectlyconnected.However,insomeoddcases,
theneighborisintentionallyreachableusinganinterfacethatisnotdirectly
connected.Inthatrarecase,thelocalroutermusthaveroutinginformationonhow
toreachthataddress.Also,youmustconfiguretheBGPsessionwiththeebgp-
multihopoption.
IfthesessiongoesintotheActivestate,therouterwillattempttoestablishthe
session.Ifsessionestablishmentisunsuccessful,youwillhavetotroubleshootthe
problem.Thedebugiptcptransactionscommandwilldisplaytheconnect
attempts.
Symptom:
BGP neighbor is active; session is not established.
debug ip tcp transactions display shows that the TCP SYN packet is not
answered with a SYN-ACK packet.ex
Diagnosis:
Neighbor is not reachable.
Verification:
Verify connectivity with ping.
Check for the presence of an access-list.
TCPsessionestablishmentstartswiththeroutersendingaTCPSYNpacket.Ifthe
TCPSYNpacketisneveranswered,theremoteroutermightbedeadornot
reachable.Trytousethepingcommandandverifytheexistenceoftheremote
routerandtheIPpacketexchangebetweenthelocalandremoterouter.
BGPNeighborNotReachableExample
Inthisexample,theremoteBGProuterisnotavailable.
ISP1# debug ip tcp transactions
TCP special event debugging is on
<... output omitted ...>
*Feb 6 05:04:01: TCBEFE6AF18 created
*Feb 6 05:04:01: TCBEFE6AF18 setting property TCP_VRFTABLEID (20) EC3C8C6
C
<... output omitted ...>
*Feb 6 05:04:01: TCP: Random local port generated 43645, network 1
*Feb 6 05:04:01: TCBEFE6AF18 bound to 172.16.12.11.43645
*Feb 6 05:04:01: Reserved port 43645 in Transport Port Agent for TCP IP t
ype 1
*Feb 6 05:04:01: TCP: sending SYN, seq 2161553223, ack 0
*Feb 6 05:04:01: TCP0: Connection to 172.16.12.22:179, advertising MSS 14
60
*Feb 6 05:04:01: TCP0: state was CLOSED -> SYNSENT [43645 -
> 172.16.12.22(179)]
<... ouput omitted ...>
BGPNeighborNotReachable
*Feb 6 05:04:31: TCP0: state was SYNSENT -> CLOSED [43645 -
> 172.16.12.22(179)]
*Feb 6 05:04:31: TCB 0xEFE6AF18 destroyed
SYN packet is sent.
SYN-ACK reply never comes back. TCP session is closed.
ThesendingrouterneverreceivesthereplytotheSYNpacketandabortstheTCP
sessioninapproximately45seconds(changingthestatefromsynsenttoclosed).
IftheTCPSYNpacketisansweredwithaTCPRSTpacket,theremoterouteris
aliveandreachablebutisnotwillingtogranttheconnectionattempt.Thereasonfor
thisrefusalmaybethatBGPhasnotbeenfullyconfiguredontheremoterouteror
thatthesourceIPaddressthatisusedbythelocalrouterintheconnectionattempt
isnotinthelistofvalidneighborsfortheremoterouter.
Symptom:
BGP neighbor is active; session is not established.
debug ip tcp transactions display shows that the TCP SYN packet is
answered with an RST packet.
Diagnosis:
This router is not configured as the BGP neighbor on the neighboring router.
Verification:
Check IP addresses of BGP neighbors with show ip bgp summary on the
neighboring router.
BGPNeighborNotConfiguredExample
Inthisexample,theremoterouterisnotconfiguredforBGPortherewasa
mismatchintheneighborIPaddresses.
ISP2# debug ip tcp transactions
*Feb 9 00:35:12.675: TCBEFA28EF0 created
<... output omitted ...>
*Feb 9 00:35:12: TCBEFA28EF0 bound to 172.16.22.22.45207
*Feb 9 00:35:12: Reserved port 45207 in Transport Port Agent for TCP IP t
ype 1
*Feb 9 00:35:12: TCP: sending SYN, seq 4270019552, ack 0
*Feb 9 00:35:12: TCP0: Connection to 172.16.22.2:179, advertising MSS 146
0
*Feb 9 00:35:12: TCP0: state was CLOSED -> SYNSENT [45207 -
> 172.16.22.2(179)]
*Feb 9 00:35:12: Released port 45207 in Transport Port Agent for TCP IP t
ype 1 delay 240000
*Feb 9 00:35:12: TCP0: state was SYNSENT -> CLOSED [45207 -
> 172.16.22.2(179)]
*Feb 9 00:35:12: TCP0: bad seg from 172.16.22.2 -
- closing connection: port 45207 seq 0 ack 4270019553 rcvnxt 0 rcvwnd 0 le
n 0
*Feb 9 00:35:12: TCP0: connection closed - remote sent RST
*Feb 9 00:35:12: TCB 0xEFA28EF0 destroyed
SYN packet is sent.
Neighbor replies with an RST packet. TCP session is closed.
TheremoterouterrespondswithanRSTpacketassoonasitreceivestheinitial
SYNpacket,terminatingtheBGPsession.
BGPNeighborNotConfigured
IftheTCPsessionisestablishedusingthespecifiedthree-wayhandshakeofSYN,
SYN-ACK,andACK,buttherouterdropsthesessionafterashortpacketexchange,
theBGPparametersaremismatched.MakesurethattheremoteASthatis
configuredontheroutermatchesthelocalASthatisconfiguredontheneighbor.If
theASnumbersdonotmatch,therouterdropsthesessionafterexchangingBGP
Openmessages.
Symptom:
BGP neighbor oscillates between Active and Idle.
debug ip tcp transactions displays the TCP session being established and
torn down immediately.
Diagnosis:
There is an AS number mismatch between BGP neighbors.
Verification:
Verify the AS numbers that are configured for neighboring routers using the
show ip bgp summary on both routers.
BGPASNumberMismatchExample
ThisexampleillustratesamismatchinanASnumber.
R2# debug ip tcp transaction
R2# debug ip bgp event
*Feb 6 06:37:44: TCBEE9C5A38 created
*Feb 6 06:37:44: TCP0: state was LISTEN -> SYNRCVD [179 -
> 172.16.12.11(60291)]
*Feb 6 06:37:44: TCP: tcb EE9C5A38 connection to 172.16.12.11:60291, peer
MSS 1460, MSS is 516
*Feb 6 06:37:44: TCP: sending SYN, seq 2355663235, ack 879693158
*Feb 6 06:37:44: TCP0: Connection to 172.16.12.11:60291, advertising MSS
1460
*Feb 6 06:37:44: TCP0: state was SYNRCVD -> ESTAB [179 -
> 172.16.12.11(60291)]
*Feb 6 06:37:44: TCBEE9C4B20 accepting EE9C5A38 from 172.16.12.11.60291
*<... output omitted ...>
*Feb 6 06:37:44: %BGP-3-
NOTIFICATION: sent to neighbor 172.16.12.11 passive 2/2 (peer in wrong AS)
2 bytes 0064
*<... output omitted ...>
*Feb 6 06:37:44: TCP0: FIN processed
*Feb 6 06:37:44: TCP0: state was ESTAB -> CLOSEWAIT [179 -
> 172.16.12.11(60291)]
*<... output omitted ...>
*Feb 6 06:37:49: TCP0: sending FIN
First, TCP session is established.
BGP notification is sent because of AS number mismatch.
WheneverthereisamismatchinASnumbers(oranyotherBGPparametersthat
arenecessaryforproperBGPoperation),theBGPsessionisterminatedwithaBGP
notification.TheTCPsessionisterminatedaswell.
BGPASNumberMismatch
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
The show ip bgp summary command displays the overall status of BGP and
shows configured neighbors and their state.
You can use the show ip bgp neighbors command to get more in-depth
information about a specific BGP neighbor.
All entries in the BGP table can be displayed with the show ip bgp command.
You can also use show ip bgp to display an extended printout about a specific
route in the BGP table.
You can use the debug ip tcp transactions command to troubleshoot BGP
session establishment problems. The command debug ip bgp events displays
significant BGP events, while debug ip bgp updates displays the routing
information being exchanged between BGP neighbors.
Summary
Overview
Thistopicsummarizesthekeypointsthatwerediscussedinthismodule.
BGP has reliable transport provided by TCP, a rich set of metrics called BGP
path attributes, and scalability features such as batched updates that make it
suitable for very large networks.
Configured BGP neighbors establish a TCP session and exchange the BGP
Open message, which contains the parameters that each BGP router proposes
to use.
Some path attributes are well-known and should be recognized by every BGP
implementation. Some of the well-known attributes, such as AS-path, next-hop,
and origin, are mandatory and have to be present in every BGP update.
The route selection process takes into account various BGP attributes that are
attached to the route, as well as local decisions.
When you are configuring BGP neighbors, you will enable the BGP routing
protocol process, establish neighbors, and advertise local routes.
ModuleSummary
Foradditionalinformation,refertotheseresources:
CiscoSystems,Inc.BorderGatewayProtocol.
http://docwiki.cisco.com/wiki/Border_Gateway_Protocol
CiscoSystems,Inc.ConfiguringBGP.
http://www.cisco.com/c/en/us/td/docs/ios/12_2/ip/configuration/guide/fipr_c/1cfbgp.html
CiscoSystems,Inc.UsingtheBorderGatewayProtocolforInterdomainRouting
http://docwiki.cisco.com/wiki/Internetworking_Case_Studies
References
Usethequestionsheretoreviewwhatyoulearnedinthismodule.Thecorrect
answersandsolutionsarefoundintheModuleSelf-CheckAnswerKey.
ModuleSelf-Check
1. WhichstatementabouttheASistrue?(Source:"IntroducingBGP")
TheASisacollectionofnetworksunderasingleadministrativedomain.
TheASisacollectionofnetworksthatbelongtodifferententerprise
networks.
TheASrequiresIGPprotocoltoexchangeroutinginformationbetween
autonomoussystems.
TheASnumbercanbeanynumberbetween0and255.
1. WhichroutingmethodbestdescribesBGP?(Source:"Introducing
BGP")
distancevector
linkstate
pathvector
hybridoflinkstateanddistancevector
1. AtransitASisanASthatexchangesBGProutinginformationwith
otherautonomoussystemsandforwardsinformationthatisreceived
fromoneAStoanotherAS.Trueorfalse?(Source:"Introducing
BGP")
true
false
1. WhichthreeitemsareBGPenhancementstotraditionaldistance
vectorroutingprotocols?(Choosethree.)(Source:"Introducing
BGP")
reliableupdates
useoftriggeredupdatesonly
enhancedsecurity
richmetrics
routesummarization
snapshotupdates
1. WhichprotocolfacilitatesreliableupdatecapabilitiesinBGP?
(Source:"IntroducingBGP")
TCP
UDP
HSRP
ICMP
1. WhatarethreecharacteristicsofanAS?(Choosethree.)(Source:
"IntroducingBGP")
ASusesIGPsforintradomainrouting.
ASusesEGPsforinterdomainrouting.
ASisacollectionofnetworksunderacommonadministrativeauthority.
ASconsistsofagroupofnetworkdomains.
ASautomaticallysummarizesaddresses
ASisregulatedbytheIETF.
1. WhichthreescenariosarecommonscenarioswhereBGPisused?
(Choosethree.)(Source:"IntroducingBGP")
acustomerwithaconnectiontomultipleserviceproviders
serviceprovidernetworksactingastransitsystemsandforwarding
externaltrafficthroughtheirnetwork
asingle-sitecustomerintranetwithcomplexadministrativepolicies
betweendepartments
asthecoreroutingprotocolinverylargeenterprisenetworks
astheroutingprotocolinanIS-ISbackbonearea
asthecoreroutingprotocolinanSNAnetwork
1. WhatarethreerecommendedBGPuseguidelinesformultihomed
customernetworks?(Choosethree.)(Source:"IntroducingBGP")
MostmultihomedcustomersshoulduseBGPwiththeirserviceproviders.
Mostmultihomedcustomersshouldforwardroutinginformationthatis
receivedfromoneprovidertotheotherprovider.
ThemultihomedcustomermusthaveitsownpublicASnumber.
Multihomedcustomersshoulduseaprovider-independent,publicaddress
space.
ThemultihomedcustomermayuseandadvertiseRFC1918addresses.
MultihomedcustomersshouldusetheASnumberoftheirprimaryISP.
1. WhatisalimitationoftheBGProutingprotocol?(Source:
"IntroducingBGP")
YoucannotuseBGPtoimplementhop-by-hoproutingpolicycontrols.
YoucannotuseBGPtoinfluencetheroutingpolicyinadownstreamAS.
BGPcannotcontrolforwardingofpacketsbasedontheirdestination
address.
BGPcannotscaletoverylargenetworkswithmorethan110,000routes.
1. WhichthreestatementsaretrueofBGPmandatorywell-known
attributes?(Choosethree.)(Source:"UnderstandingBGPPath
Attributes")
TheymustbepresentinallBGPupdates.
AllBGP-compliantimplementationsmustrecognizethem.
AllBGP-compliantroutersmustadheretopoliciesspecifiedinmandatory
attributes.
Allwell-knownattributesarepropagatedtootherneighbors.
TheymustbepresentinsomeBGPupdates.
1. WhichthreeattributesareBGPmandatorywell-knownattributes?
(Choosethree.)(Source:"UnderstandingBGPPathAttributes")
next-hop
weight
AS-path
origin
MED
localpreference
1. WhichthreepossiblevaluesareassignedtotheBGPorigin
attribute?(Choosethree.)(Source:"UnderstandingBGPPath
Attributes")
IGP
EGP
unknown
internal
external
MED
1. WhichnontransitiveoptionalBGPattributeisusefulinassistingwith
therouteselectionprocesswhenmultiplelinkstoanotherASexist?
(Source:"UnderstandingBGPPathAttributes")
next-hop
localpreference
MED
AS-path
1. InwhichtwowayscantheBGPnext-hopattributebemodified?
(Choosetwo.)(Source:"UnderstandingBGPPathAttributes")
Ifthenext-hopattributeisinthesameIPsubnetasthereceivingrouter,
theattributeisunchanged;otherwise,itissettotheIPaddressofthe
sendingrouter.
Thenext-hopattributeisalwayssettotheIPaddressofthesending
router.
Thenext-hopattributeismodifiedonlywhenBGPpacketsexitanAS.
TheBGPnext-hopattributeismodifiedonlywhenBGPpacketstraverse
point-to-pointlinks.
1. WhichthreestatementsregardingtheBGPAS-pathattributeare
true?(Choosethree.)(Source:"UnderstandingBGPPath
Attributes")
ThelocalASnumberisprependedtotheASpatheachtimethattheroute
crossesanASboundary.
TheASthatoriginallyinjectedtherouteintoBGPisalwaysfoundatthe
rightmostendoftheASpath.
TheAS-pathattributecanbeusedtoavoidroutingloops.
BGProuteswithanemptyASpathwereinjectedintoBGPfromoutside
thelocalAS.
ThelocalASnumberisappendedtotheendoftheASpatheachtimethat
theroutecrossesanASboundary.
TheASthatoriginallyinjectedtherouteintoBGPisalwaysfoundatthe
leftmostendoftheASpath.
1. Whichdescriptionappliestothelocalpreferenceattribute?(Source:
"UnderstandingBGPPathAttributes")
well-knownmandatory
well-knowndiscretionary
optionaltransitive
optionalnontransitive
1. WhichtwoattributesareBGPoptionaltransitiveattributes?(Choose
two.)(Source:"UnderstandingBGPPathAttributes")
MED
weight
aggregator
atomicaggregate
community
1. Whatisindicatedbyastateof"Idle"intheoutputofthe showip
bgpsummarycommand?(Source:"EstablishingBGPSessions")
Therouteriscurrentlynotattemptingtoestablishaconnectionwitha
neighbor.
Theconnectiontotheconfiguredneighborhastimedout.
TheconnectiontoaBGPneighborhasbeenestablished,andnoerrors
havebeenreceivedontheconnection.
TheconnectiontoaBGPneighborhasbeenestablished,andnopackets
havebeensent.
1. HowareneighborsdetectedinBGP?(Source:"EstablishingBGP
Sessions")
Theyarediscoveredautomatically
Theymustbeconfiguredmanually.
1. Router-IDisanumberthatuniquelyidentifiestherouter.Howisit
selected?(Source:"EstablishingBGPSessions")
Router-IDisthehighestIPaddressofanyloopbackinterface.Ifthereisno
loopbackinterface,therouter-IDmustbeconfiguredmanually.
Router-IDmustbeconfiguredmanually.
Router-IDisthehighestIPaddressofanyloopbackinterface.Ifthereisno
loopbackinterface,therouterusesthehighestIPaddressofanyinterface.
Router-IDisthelowestIPaddressofanyloopbackinterface.Ifthereisno
loopbackinterface,therouterusesthelowestIPaddressofanyinterface.
1. WhathappensiftwoTCPconnectionattemptsbetweenconfigured
BGPneighborssucceed?(Source:"EstablishingBGPSessions")
Bothconnectionswillbeterminated,andtheneighborswillre-establisha
neighborrelationship.
Oneconnectionwillbemaintainedasprimaryandtheotherasbackup.
Oneofthetwoconnectionswillbetorndown
Therouterwiththelowerrouter-IDwilldetermineifthesecondconnection
istorndownorusedasabackupTCPconnection.
1. GiventhefollowingBGPsessionstates,inwhichstateistherouter
whilewaitingfortheotherroutertorespondtotheBGPOpen
message?(Source:"EstablishingBGPSessions")
OpenConfirm
Established
Idle
OpenSent
Active
1. GiventhefollowingBGPsessionstates,whatistheirorderof
progressionduringthecreationofasuccessfulneighborsession?
(Source:"EstablishingBGPSessions")
OpenConfirm 3
OpenSent 5
Established 4
Active 2
Idle 1
1. BGPsessionsareestablishedoverTCPusingportnumber179.
Trueorfalse?(Source:"EstablishingBGPSessions")
true
false
1. Whatdoesthefield"TblVer"indicateintheoutputofthe showip
bgpsummarycommand?(Source:"EstablishingBGPSessions")
thecurrentversionofBGPinusebytherouter
thenumberofrouteprefixesthatarecontainedintheBGPupdateofthe
router
BGPmessagesthathavebeenreceivedfromthatneighbor
thelastversionoftheBGPdatabasethatwassenttothatneighbor
1. WhatoccurswhenyouuseMD5betweentwoBGPneighbors?
(Source:"EstablishingBGPSessions")
EverypacketisencryptedwithMD5.
TheIPheaderisencryptedusingMD5.
AnMD5checksumiscalculatedandsentwitheachpacketsothatits
sourcecanbeverified.
AusernameandpasswordareembeddedinanIPdatagramthatis
matchedtoausernameandpasswordontheremoteneighbor.
1. WhatdoesarouterthatisrunningBGPdowithaBGPupdatethat
containsitsownASpath?(Source:"ProcessingBGPRoutes")
Theroutercheckstoseewhethertheinformationthatiscontainedinthe
updateisbetterthanitscurrentinformation.Ifitis,itwillupdateitsBGP
table.
Therouteracceptstherouteupdate.
Theroutersilentlydiscards(denies)theroute.
Therouterreturnsanerrortotherouterthatsenttheupdate.
1. Whichthreeattributesmustalwaysbepresentintherouting
update?(Choosethree.)(Source:"ProcessingBGPRoutes")
MED
AS-path
local-preference
next-hop
origin
1. Whenarouterhasmorethanonealternativeroutetoreachthe
samedestination,localpreferenceischeckedbeforetheweight
attribute.Trueorfalse?(Source:"ProcessingBGPRoutes")
true
false
1. HowmanyalternatepathstoasingledestinationwillaBGProuter
maintainintheBGPtable?(Source:"ProcessingBGPRoutes")
Therouterwillmaintainonlythebestpathtothedestination.
Therouterwillmaintaintwopaths,thebestpathandabackuproute.
TheBGPtablewillholduptofourroutesbydefaultandamaximumofsix
configurableroutes.
TheBGPtablewillstoreallvalid,advertisedroutestothedestinationin
theBGPtable.
1. Whenarouterhasmorethanonealternativeroutetoreachthe
sameIPsubnet(networkandmask),therouterhastoselectoneof
themasbestinitsdefaultmodeofoperationMatchthefollowing
stepstothecorrectstepintheprocess.(Source:"ProcessingBGP
Routes")
TheroutercomparesMEDvalues,butonlyif
itreceivestheupdatesfromthesame
neighboringAS.RouteswithalowerMED
arepreferred.
Step1
ThelengthsoftheASpathsarecompared
(thecontentisnotchecked;onlythenumber
ofautonomoussystemsineachASpathis
counted).Theroutewiththeshortestlength
isselected.
Step5
IftheAS-pathlengthsarethesame,the
origincodeischecked.BGPprefersthepath
withthelowestorigintype:IGPislowerthan
EGP,andEGPislowerthanIncomplete.
Step4
Ifthelocalpreferenceattributesaredifferent,
theroutewiththehighestvalueisselected
asbest.
Step2
Therouterpreferstheroutewiththehigher
weight. Step6
Theroutercheckswhetherthenext-hop
attributeindicatesanIPaddressthatis
reachableaccordingtothecurrentrouting
table.Ifthenexthopisnotreachable,the
routerdoesnotconsidertheBGProuteasa
candidatetobecomeselectedasthebest.
Step3
IfoneoftheroutesisinjectedintotheBGP
tablebythelocalrouter,thelocalrouter
prefersittoanyroutesthatitreceivesfrom
otherBGProuters.
Step7
1. Whataretwowaysinwhichlocalnetworksareadvertisedintothe
BGProutingprotocolprocess?(Choosetwo.)(Source:"Processing
BGPRoutes")
automatically,afteraBGPneighborsessionisestablished
manually,withthenetworkcommand
throughredistributionintotheBGPprocess
byadvertisingthemtotheBGPtableontherouterafterCiscoDiscovery
Protocoldiscoversconnectednetworks
1. Whataretwosituationswhenisitappropriatetodisableautomatic
summarizationinBGP?(Choosetwo.)(Source:"ProcessingBGP
Routes")
whenBGPneighborsarenotconfiguredtoadvertiseaggregateroutesto
upstreamproviders
whentheclasslessvariantofthenetworkcommandisused
whenyouareusingaclasslessIGPintheAS
whentheeffectsofautomaticsummarizationofIGP-to-BGPredistribution
arenotdesired
1. WhatistheADofBGProutesintheIProutingtablethatwere
learnedfromBGPneighborsinadifferentAS?(Source:"Processing
BGPRoutes")
1
20
90
120
1. WhichthreeBGPattributesaredisplayedforeachrouteintheBGP
tablewhenyouareusingtheshowipbgpcommand?(Choose
three.)(Source:"ProcessingBGPRoutes")
weight
communities
origin
AS-path
1. WhatisthevalidASnumberrangeforaBGPprocessonaCisco
router?(Source:"ConfiguringBasicBGP")
1to256
1to32768
1to65535
1to131072
1. Whichtwoparametersmustyouconfigurewiththeneighbor
commandtoestablishaBGPsessionwithanexternalneighbor?
(Choosetwo.)(Source:"ConfiguringBasicBGP")
neighborIPaddress
subnetmaskoftheIPnetwork
remoteASnumber
localASnumber
descriptionoftheneighbor
1. WhatisthebestmethodtotemporarilydisableaBGPneighbor
session?(Source:"ConfiguringBasicBGP")
RemovetheneighborcommandfromtheBGProuterprocess.
RemovetheBGProuterprocessfromtheconfiguration.
Terminatetheneighborconnectionwiththeneighborshutdown
command
Disconnecttheneighborbyinitiatingarouterreload.
1. WhichtwoofthefollowingstatementsaboutconfiguringBGPtimers
areaccurate?(Choosetwo.)(Source:"ConfiguringBasicBGP")
ChangingtheBGPdefaultholdtimeandkeepalivetimersisusuallynot
recommended.
TheneighbortimerscommandsetsthetimersforaspecificBGPpeeror
peergroup.
ThetimersbgpcommandsetsthetimersforaspecificBGPpeerorpeer
group.
Holdtimeindicatesthefrequency(inseconds)withwhichtheCiscoIOS
softwaresendsmessagestoitspeer.
1. Whichtwoofthefollowingarecharacteristicsofthestring
componentoftheneighbor{ip-address|peer-group-name}
passwordstringcommand?(Choosetwo.)(Source:"Configuring
BasicBGP")
cancontainanyalphanumericcharacters,includingspaces
case-sensitivepasswordofupto100characters
firstcharactercanbeanumber
cannotspecifyapasswordintheformat"number-space-anything"
1. Whichthreestepsmustyoucompletetoadvertiseaclasslessprefix
intoBGP?(Choosethree.)(Source:"ConfiguringBasicBGP")
Configuretheprefixwiththenetworkcommand.
Specifythemaskkeywordwiththelocallyadvertisedroute.
ConfiguretheredistributeconnectedcommandundertheBGProuter
process.
Useastaticroutepointingtonull0thatmatchestheprefix.
1. Whichorigincodeiscarriedwithroutesthatareredistributedinto
BGP?(Source:"ConfiguringBasicBGP")
internal
external
unknown
incomplete
1. Whichtwoofthefollowingstatementsabouttheclasslessbehavior
ofBGParecorrect?(Choosetwo.)(Source:"ConfiguringBasic
BGP")
WhenanexactmatchisnotfoundintheIProutingtableamatchingprefix
isautomaticallyconfiguredontherouter.
Inthenetworkip-prefix-addressmasksubnet-maskcommand,theprefix
doesnothavetomatchanentryintheIProutingtable.
ToadvertiseclasslessnetworksintoBGP(asubnetorasupernet),you
canusethenetworkcommandwiththemaskkeywordandthesubnet
maskspecified.
Ifthekeywordmaskandthesubnetmaskareomitted,thenetworkis
assumedtohaveitsnaturalmaskaccordingtothenetworkclass.
1. WhataretwobenefitsofusingrouteaggregationinBGP?(Choose
two.)(Source:"ConfiguringBasicBGP")
Itensuresthatevenifaggregatenetworksaredown,theaggregateis
advertised,whicheliminatesblackholes.
Itreducestheamountofmemorythatisusedintheroutertostorethe
BGPtable.
ItreducesrouteflappinganditseffectsonrouterCPUresources.
BGPattributegranularityismaintained,whichensuresoptimalpath
selection.
1. WhichtwoofthefollowingarecharacteristicsoftheBGPConditional
RouteInjectionfeature?(Choosetwo.)(Source:"ConfiguringBasic
BGP")
allowsyoutooriginateaprefixintoaBGProutingtablewithoutthe
correspondingmatch
enabledwiththebgpinject-mapexist-mapcommand
allowsconditionalinjectingorreplacingmorespecificprefixeswithless
specificprefixes
allowsoriginationofaprefixintoaBGProutingtableonlywiththe
correspondingmatch
1. WhichtwoofthefollowingarecharacteristicsoftheBGPSupportfor
TTLSecurityCheckfeature?(Choosetwo.)(Source:"Configuring
BasicBGP")
shouldbeconfiguredononlyoneparticipatingrouter
preventsBGPsessionsfromexpiringevenifkeepalivepacketsarenot
receivedbeforethesessiontimerexpires
protectstheEBGPpeeringsessionbycomparingthevalueintheTTL
fieldofreceivedIPpacketsagainstahopcountthatisconfiguredlocally
foreachEBGPpeeringsession
supportsbothdirectlyconnectedpeeringsessionsandmultihopEBGP
peeringsessions
1. Whichtwoofthefollowingarefunctionsoftheshowipbgp
summarycommand?(Choosetwo.)
displaysBGPmemoryuse
displaysBGPneighborsandstatusofcommunicationwiththem
locatesproblemsinBGPsessionsthatareupandestablished
displaystheBGProutingtable
1. WhichcommanddoyouusetodisplaydetailedBGPneighbor
information?(Source:"MonitoringandTroubleshootingBGP")
showipbgpsummary
showipbgp
showipbgpneighborsaddress
showipbgpdetail
1. TogetinformationabouttheTCPsessionandBGPparametersof
thesession,youcanissueshowipbgpsummarycommand.True
orfalse?(Source:"MonitoringandTroubleshootingBGP")
true
false
1. Whichtwoofthefollowingstatementsabouttheshowipbgp
commandthatisusedtomonitortheBGProutingtableare
accurate?(Choosetwo.)(Source:"MonitoringandTroubleshooting
BGP")
Theshowipbgpcommandshowsanabbreviatedlistofinformation
abouteachroute,displayingonelineperprefix.
Theshowipbgpcommandshowsafulllistofinformationabouteach
route,displayingonelineperprefix.
AlloftheBGPattributesthatareassociatedwiththeroutearedisplayed
ontheline.
IftheBGPtablecontainsmorethanoneroutetothesamenetwork,the
routesaredisplayedonsuccessivelinesofthecommandoutput.
1. WhichdebugcommandshouldyouenabletotroubleshootBGP
sessionstartupissueswheretheTCPconnectionneversucceeds?
(Source:"MonitoringandTroubleshootingBGP")
ipbgpupdates
ippackets
ipbgpkeepalives
iptcptransactions
1. Whendebugginganexchangeofupdates,youshouldusedebugip
bgpkeepalivecommand.TrueorFalse?
true
false
1. Whatarethethreemostcommonsessionstartupissuesthatyou
canexperiencewhenconfiguringbasicBGP?(Choosethree.)
(Source:"MonitoringandTroubleshootingBGP")
BGPneighborsdonotbecomeactive.
BGProutingloopscauseblackholes.
ABGPneighborisactive,buttheBGPsessionisnotestablished.
TheBGPneighborstateoscillatesbetweenIdleandActive.
TheBGPsessionisactive,buttheneighborcannotbereached.
BGPkeepalivesexperienceintermittentfailures.
1. WhatisthemostcommonreasonforaBGPsessionnotleavingthe
Idlestate?(Source:"MonitoringandTroubleshootingBGP")
TheTCPportfortheconnectionisnotconfigured.
Theexternalneighborisnotdirectlyconnected.
TheTCPSYNpacketisansweredwithanRSTpacket.
TheneighborshavebeenconfiguredwiththesameASnumber.
1. WhatwillresultfromattemptingtoopenaBGPconnectionwitha
neighborthathasnotbeenproperlyconfiguredforBGP?(Source:
"MonitoringandTroubleshootingBGP")
TheBGPsessionwillremainintheIdlestate.
Theneighborsessionwillbeestablished,andthesessionstartup
parameterswillbenegotiatedovertheTCPsession.
TheBGPsessionwillbeimmediatelyterminatedwithaTCPRSTpacket.
TheBGPsessionwillbecome"stuckinActivestate."
1. WhenaBGPneighboroscillatesbetweenActiveandIdle,whatis
thelikelydiagnosis?(Source:"MonitoringandTroubleshooting
BGP")
Therearemismatchedkeepaliveintervals.
ThereisanASnumbermismatchbetweenBGPneighbors.
OnerouterisnotconfiguredastheBGPneighborontheneighboring
router.
TheBGPneighborisnotreachable.
1. WhichstatementabouttheASistrue?(Source:"IntroducingBGP")
TheASisacollectionofnetworksunderasingleadministrativedomain.
TheASisacollectionofnetworksthatbelongtodifferententerprise
networks.
TheASrequiresIGPprotocoltoexchangeroutinginformationbetween
autonomoussystems.
TheASnumbercanbeanynumberbetween0and255.
AnswerKey
1. WhichroutingmethodbestdescribesBGP?(Source:"Introducing
BGP")
distancevector
linkstate
pathvector
hybridoflinkstateanddistancevector
1. AtransitASisanASthatexchangesBGProutinginformationwith
otherautonomoussystemsandforwardsinformationthatisreceived
fromoneAStoanotherAS.Trueorfalse?(Source:"Introducing
BGP")
true
false
1. WhichthreeitemsareBGPenhancementstotraditionaldistance
vectorroutingprotocols?(Choosethree.)(Source:"Introducing
BGP")
reliableupdates
useoftriggeredupdatesonly
enhancedsecurity
richmetrics
routesummarization
snapshotupdates
1. WhichprotocolfacilitatesreliableupdatecapabilitiesinBGP?
(Source:"IntroducingBGP")
TCP
UDP
HSRP
ICMP
1. WhatarethreecharacteristicsofanAS?(Choosethree.)(Source:
"IntroducingBGP")
ASusesIGPsforintradomainrouting.
ASusesEGPsforinterdomainrouting.
ASisacollectionofnetworksunderacommonadministrativeauthority.
ASconsistsofagroupofnetworkdomains.
ASautomaticallysummarizesaddresses
ASisregulatedbytheIETF.
1. WhichthreescenariosarecommonscenarioswhereBGPisused?
(Choosethree.)(Source:"IntroducingBGP")
acustomerwithaconnectiontomultipleserviceproviders
serviceprovidernetworksactingastransitsystemsandforwarding
externaltrafficthroughtheirnetwork
asingle-sitecustomerintranetwithcomplexadministrativepolicies
betweendepartments
asthecoreroutingprotocolinverylargeenterprisenetworks
astheroutingprotocolinanIS-ISbackbonearea
asthecoreroutingprotocolinanSNAnetwork
1. WhatarethreerecommendedBGPuseguidelinesformultihomed
customernetworks?(Choosethree.)(Source:"IntroducingBGP")
MostmultihomedcustomersshoulduseBGPwiththeirserviceproviders.
Mostmultihomedcustomersshouldforwardroutinginformationthatis
receivedfromoneprovidertotheotherprovider.
ThemultihomedcustomermusthaveitsownpublicASnumber.
Multihomedcustomersshoulduseaprovider-independent,publicaddress
space.
ThemultihomedcustomermayuseandadvertiseRFC1918addresses.
MultihomedcustomersshouldusetheASnumberoftheirprimaryISP.
1. WhatisalimitationoftheBGProutingprotocol?(Source:
"IntroducingBGP")
YoucannotuseBGPtoimplementhop-by-hoproutingpolicycontrols.
YoucannotuseBGPtoinfluencetheroutingpolicyinadownstreamAS.
BGPcannotcontrolforwardingofpacketsbasedontheirdestination
address.
BGPcannotscaletoverylargenetworkswithmorethan110,000routes.
1. WhichthreestatementsaretrueofBGPmandatorywell-known
attributes?(Choosethree.)(Source:"UnderstandingBGPPath
Attributes")
TheymustbepresentinallBGPupdates.
AllBGP-compliantimplementationsmustrecognizethem.
AllBGP-compliantroutersmustadheretopoliciesspecifiedinmandatory
attributes.
Allwell-knownattributesarepropagatedtootherneighbors.
TheymustbepresentinsomeBGPupdates.
1. WhichthreeattributesareBGPmandatorywell-knownattributes?
(Choosethree.)(Source:"UnderstandingBGPPathAttributes")
next-hop
weight
AS-path
origin
MED
localpreference
1. WhichthreepossiblevaluesareassignedtotheBGPorigin
attribute?(Choosethree.)(Source:"UnderstandingBGPPath
Attributes")
IGP
EGP
unknown
internal
external
MED
1. WhichnontransitiveoptionalBGPattributeisusefulinassistingwith
therouteselectionprocesswhenmultiplelinkstoanotherASexist?
(Source:"UnderstandingBGPPathAttributes")
next-hop
localpreference
MED
AS-path
1. InwhichtwowayscantheBGPnext-hopattributebemodified?
(Choosetwo.)(Source:"UnderstandingBGPPathAttributes")
Ifthenext-hopattributeisinthesameIPsubnetasthereceivingrouter,
theattributeisunchanged;otherwise,itissettotheIPaddressofthe
sendingrouter.
Thenext-hopattributeisalwayssettotheIPaddressofthesending
router.
Thenext-hopattributeismodifiedonlywhenBGPpacketsexitanAS.
TheBGPnext-hopattributeismodifiedonlywhenBGPpacketstraverse
point-to-pointlinks.
1. WhichthreestatementsregardingtheBGPAS-pathattributeare
true?(Choosethree.)(Source:"UnderstandingBGPPath
Attributes")
ThelocalASnumberisprependedtotheASpatheachtimethattheroute
crossesanASboundary.
TheASthatoriginallyinjectedtherouteintoBGPisalwaysfoundatthe
rightmostendoftheASpath.
TheAS-pathattributecanbeusedtoavoidroutingloops.
BGProuteswithanemptyASpathwereinjectedintoBGPfromoutside
thelocalAS.
ThelocalASnumberisappendedtotheendoftheASpatheachtimethat
theroutecrossesanASboundary.
TheASthatoriginallyinjectedtherouteintoBGPisalwaysfoundatthe
leftmostendoftheASpath.
1. Whichdescriptionappliestothelocalpreferenceattribute?(Source:
"UnderstandingBGPPathAttributes")
well-knownmandatory
well-knowndiscretionary
optionaltransitive
optionalnontransitive
1. WhichtwoattributesareBGPoptionaltransitiveattributes?(Choose
two.)(Source:"UnderstandingBGPPathAttributes")
MED
weight
aggregator
atomicaggregate
community
1. Whatisindicatedbyastateof"Idle"intheoutputofthe showip
bgpsummarycommand?(Source:"EstablishingBGPSessions")
Therouteriscurrentlynotattemptingtoestablishaconnectionwitha
neighbor.
Theconnectiontotheconfiguredneighborhastimedout.
TheconnectiontoaBGPneighborhasbeenestablished,andnoerrors
havebeenreceivedontheconnection.
TheconnectiontoaBGPneighborhasbeenestablished,andnopackets
havebeensent.
1. HowareneighborsdetectedinBGP?(Source:"EstablishingBGP
Sessions")
Theyarediscoveredautomatically
Theymustbeconfiguredmanually.
1. Router-IDisanumberthatuniquelyidentifiestherouter.Howisit
selected?(Source:"EstablishingBGPSessions")
Router-IDisthehighestIPaddressofanyloopbackinterface.Ifthereisno
loopbackinterface,therouter-IDmustbeconfiguredmanually.
Router-IDmustbeconfiguredmanually.
Router-IDisthehighestIPaddressofanyloopbackinterface.Ifthereisno
loopbackinterface,therouterusesthehighestIPaddressofanyinterface.
Router-IDisthelowestIPaddressofanyloopbackinterface.Ifthereisno
loopbackinterface,therouterusesthelowestIPaddressofanyinterface.
1. WhathappensiftwoTCPconnectionattemptsbetweenconfigured
BGPneighborssucceed?(Source:"EstablishingBGPSessions")
Bothconnectionswillbeterminated,andtheneighborswillre-establisha
neighborrelationship.
Oneconnectionwillbemaintainedasprimaryandtheotherasbackup.
Oneofthetwoconnectionswillbetorndown
Therouterwiththelowerrouter-IDwilldetermineifthesecondconnection
istorndownorusedasabackupTCPconnection.
1. GiventhefollowingBGPsessionstates,inwhichstateistherouter
whilewaitingfortheotherroutertorespondtotheBGPOpen
message?(Source:"EstablishingBGPSessions")
OpenConfirm
Established
Idle
OpenSent
Active
1. GiventhefollowingBGPsessionstates,whatistheirorderof
progressionduringthecreationofasuccessfulneighborsession?
(Source:"EstablishingBGPSessions")
Idle 1
Active 2
OpenSent 3
OpenConfirm 4
Established 5
1. BGPsessionsareestablishedoverTCPusingportnumber179.
Trueorfalse?(Source:"EstablishingBGPSessions")
true
false
1. Whatdoesthefield"TblVer"indicateintheoutputofthe showip
bgpsummarycommand?(Source:"EstablishingBGPSessions")
thecurrentversionofBGPinusebytherouter
thenumberofrouteprefixesthatarecontainedintheBGPupdateofthe
router
BGPmessagesthathavebeenreceivedfromthatneighbor
thelastversionoftheBGPdatabasethatwassenttothatneighbor
1. WhatoccurswhenyouuseMD5betweentwoBGPneighbors?
(Source:"EstablishingBGPSessions")
EverypacketisencryptedwithMD5.
TheIPheaderisencryptedusingMD5.
AnMD5checksumiscalculatedandsentwitheachpacketsothatits
sourcecanbeverified.
AusernameandpasswordareembeddedinanIPdatagramthatis
matchedtoausernameandpasswordontheremoteneighbor.
1. WhatdoesarouterthatisrunningBGPdowithaBGPupdatethat
containsitsownASpath?(Source:"ProcessingBGPRoutes")
Theroutercheckstoseewhethertheinformationthatiscontainedinthe
updateisbetterthanitscurrentinformation.Ifitis,itwillupdateitsBGP
table.
Therouteracceptstherouteupdate.
Theroutersilentlydiscards(denies)theroute.
Therouterreturnsanerrortotherouterthatsenttheupdate.
1. Whichthreeattributesmustalwaysbepresentintherouting
update?(Choosethree.)(Source:"ProcessingBGPRoutes")
MED
AS-path
local-preference
next-hop
origin
1. Whenarouterhasmorethanonealternativeroutetoreachthe
samedestination,localpreferenceischeckedbeforetheweight
attribute.Trueorfalse?(Source:"ProcessingBGPRoutes")
true
false
1. HowmanyalternatepathstoasingledestinationwillaBGProuter
maintainintheBGPtable?(Source:"ProcessingBGPRoutes")
Therouterwillmaintainonlythebestpathtothedestination.
Therouterwillmaintaintwopaths,thebestpathandabackuproute.
TheBGPtablewillholduptofourroutesbydefaultandamaximumofsix
configurableroutes.
TheBGPtablewillstoreallvalid,advertisedroutestothedestinationin
theBGPtable.
1. Whenarouterhasmorethanonealternativeroutetoreachthe
sameIPsubnet(networkandmask),therouterhastoselectoneof
themasbestinitsdefaultmodeofoperationMatchthefollowing
stepstothecorrectstepintheprocess.(Source:"ProcessingBGP
Routes")
TheroutercomparesMEDvalues,butonlyif
itreceivestheupdatesfromthesame
neighboringAS.RouteswithalowerMED
arepreferred.
Step7
Theroutercheckswhetherthenext-hop
attributeindicatesanIPaddressthatis
reachableaccordingtothecurrentrouting
table.Ifthenexthopisnotreachable,the
routerdoesnotconsidertheBGProuteasa
candidatetobecomeselectedasthebest.
Step1
Therouterpreferstheroutewiththehigher
weight. Step2
Ifthelocalpreferenceattributesaredifferent,
theroutewiththehighestvalueisselected
asbest.
Step3
ThelengthsoftheASpathsarecompared
(thecontentisnotchecked;onlythenumber
ofautonomoussystemsineachASpathis
counted).Theroutewiththeshortestlength
isselected.
Step5
IfoneoftheroutesisinjectedintotheBGP
tablebythelocalrouter,thelocalrouter
prefersittoanyroutesthatitreceivesfrom
otherBGProuters.
Step4
IftheAS-pathlengthsarethesame,the
origincodeischecked.BGPprefersthepath
withthelowestorigintype:IGPislowerthan
EGP,andEGPislowerthanIncomplete.
Step6
1. Whataretwowaysinwhichlocalnetworksareadvertisedintothe
BGProutingprotocolprocess?(Choosetwo.)(Source:"Processing
BGPRoutes")
automatically,afteraBGPneighborsessionisestablished
manually,withthenetworkcommand
throughredistributionintotheBGPprocess
byadvertisingthemtotheBGPtableontherouterafterCiscoDiscovery
Protocoldiscoversconnectednetworks
1. Whataretwosituationswhenisitappropriatetodisableautomatic
summarizationinBGP?(Choosetwo.)(Source:"ProcessingBGP
Routes")
whenBGPneighborsarenotconfiguredtoadvertiseaggregateroutesto
upstreamproviders
whentheclasslessvariantofthenetworkcommandisused
whenyouareusingaclasslessIGPintheAS
whentheeffectsofautomaticsummarizationofIGP-to-BGPredistribution
arenotdesired
1. WhatistheADofBGProutesintheIProutingtablethatwere
learnedfromBGPneighborsinadifferentAS?(Source:"Processing
BGPRoutes")
1
20
90
120
1. WhichthreeBGPattributesaredisplayedforeachrouteintheBGP
tablewhenyouareusingtheshowipbgpcommand?(Choose
three.)(Source:"ProcessingBGPRoutes")
weight
communities
origin
AS-path
1. WhatisthevalidASnumberrangeforaBGPprocessonaCisco
router?(Source:"ConfiguringBasicBGP")
1to256
1to32768
1to65535
1to131072
1. Whichtwoparametersmustyouconfigurewiththeneighbor
commandtoestablishaBGPsessionwithanexternalneighbor?
(Choosetwo.)(Source:"ConfiguringBasicBGP")
neighborIPaddress
subnetmaskoftheIPnetwork
remoteASnumber
localASnumber
descriptionoftheneighbor
1. WhatisthebestmethodtotemporarilydisableaBGPneighbor
session?(Source:"ConfiguringBasicBGP")
RemovetheneighborcommandfromtheBGProuterprocess.
RemovetheBGProuterprocessfromtheconfiguration.
Terminatetheneighborconnectionwiththeneighborshutdown
command
Disconnecttheneighborbyinitiatingarouterreload.
1. WhichtwoofthefollowingstatementsaboutconfiguringBGPtimers
areaccurate?(Choosetwo.)(Source:"ConfiguringBasicBGP")
ChangingtheBGPdefaultholdtimeandkeepalivetimersisusuallynot
recommended.
TheneighbortimerscommandsetsthetimersforaspecificBGPpeeror
peergroup.
ThetimersbgpcommandsetsthetimersforaspecificBGPpeerorpeer
group.
Holdtimeindicatesthefrequency(inseconds)withwhichtheCiscoIOS
softwaresendsmessagestoitspeer.
1. Whichtwoofthefollowingarecharacteristicsofthestring
componentoftheneighbor{ip-address|peer-group-name}
passwordstringcommand?(Choosetwo.)(Source:"Configuring
BasicBGP")
cancontainanyalphanumericcharacters,includingspaces
case-sensitivepasswordofupto100characters
firstcharactercanbeanumber
cannotspecifyapasswordintheformat"number-space-anything"
1. Whichthreestepsmustyoucompletetoadvertiseaclasslessprefix
intoBGP?(Choosethree.)(Source:"ConfiguringBasicBGP")
Configuretheprefixwiththenetworkcommand.
Specifythemaskkeywordwiththelocallyadvertisedroute.
ConfiguretheredistributeconnectedcommandundertheBGProuter
process.
Useastaticroutepointingtonull0thatmatchestheprefix.
1. Whichorigincodeiscarriedwithroutesthatareredistributedinto
BGP?(Source:"ConfiguringBasicBGP")
internal
external
unknown
incomplete
1. Whichtwoofthefollowingstatementsabouttheclasslessbehavior
ofBGParecorrect?(Choosetwo.)(Source:"ConfiguringBasic
BGP")
WhenanexactmatchisnotfoundintheIProutingtableamatchingprefix
isautomaticallyconfiguredontherouter.
Inthenetworkip-prefix-addressmasksubnet-maskcommand,theprefix
doesnothavetomatchanentryintheIProutingtable.
ToadvertiseclasslessnetworksintoBGP(asubnetorasupernet),you
canusethenetworkcommandwiththemaskkeywordandthesubnet
maskspecified.
Ifthekeywordmaskandthesubnetmaskareomitted,thenetworkis
assumedtohaveitsnaturalmaskaccordingtothenetworkclass.
1. WhataretwobenefitsofusingrouteaggregationinBGP?(Choose
two.)(Source:"ConfiguringBasicBGP")
Itensuresthatevenifaggregatenetworksaredown,theaggregateis
advertised,whicheliminatesblackholes.
Itreducestheamountofmemorythatisusedintheroutertostorethe
BGPtable.
ItreducesrouteflappinganditseffectsonrouterCPUresources.
BGPattributegranularityismaintained,whichensuresoptimalpath
selection.
1. WhichtwoofthefollowingarecharacteristicsoftheBGPConditional
RouteInjectionfeature?(Choosetwo.)(Source:"ConfiguringBasic
BGP")
allowsyoutooriginateaprefixintoaBGProutingtablewithoutthe
correspondingmatch
enabledwiththebgpinject-mapexist-mapcommand
allowsconditionalinjectingorreplacingmorespecificprefixeswithless
specificprefixes
allowsoriginationofaprefixintoaBGProutingtableonlywiththe
correspondingmatch
1. WhichtwoofthefollowingarecharacteristicsoftheBGPSupportfor
TTLSecurityCheckfeature?(Choosetwo.)(Source:"Configuring
BasicBGP")
shouldbeconfiguredononlyoneparticipatingrouter
preventsBGPsessionsfromexpiringevenifkeepalivepacketsarenot
receivedbeforethesessiontimerexpires
protectstheEBGPpeeringsessionbycomparingthevalueintheTTL
fieldofreceivedIPpacketsagainstahopcountthatisconfiguredlocally
foreachEBGPpeeringsession
supportsbothdirectlyconnectedpeeringsessionsandmultihopEBGP
peeringsessions
1. Whichtwoofthefollowingarefunctionsoftheshowipbgp
summarycommand?(Choosetwo.)
displaysBGPmemoryuse
displaysBGPneighborsandstatusofcommunicationwiththem
locatesproblemsinBGPsessionsthatareupandestablished
displaystheBGProutingtable
1. WhichcommanddoyouusetodisplaydetailedBGPneighbor
information?(Source:"MonitoringandTroubleshootingBGP")
showipbgpsummary
showipbgp
showipbgpneighborsaddress
showipbgpdetail
1. TogetinformationabouttheTCPsessionandBGPparametersof
thesession,youcanissueshowipbgpsummarycommand.True
orfalse?(Source:"MonitoringandTroubleshootingBGP")
true
false
1. Whichtwoofthefollowingstatementsabouttheshowipbgp
commandthatisusedtomonitortheBGProutingtableare
accurate?(Choosetwo.)(Source:"MonitoringandTroubleshooting
BGP")
Theshowipbgpcommandshowsanabbreviatedlistofinformation
abouteachroute,displayingonelineperprefix.
Theshowipbgpcommandshowsafulllistofinformationabouteach
route,displayingonelineperprefix.
AlloftheBGPattributesthatareassociatedwiththeroutearedisplayed
ontheline.
IftheBGPtablecontainsmorethanoneroutetothesamenetwork,the
routesaredisplayedonsuccessivelinesofthecommandoutput.
1. WhichdebugcommandshouldyouenabletotroubleshootBGP
sessionstartupissueswheretheTCPconnectionneversucceeds?
(Source:"MonitoringandTroubleshootingBGP")
ipbgpupdates
ippackets
ipbgpkeepalives
iptcptransactions
1. Whendebugginganexchangeofupdates,youshouldusedebugip
bgpkeepalivecommand.TrueorFalse?
true
false
1. Whatarethethreemostcommonsessionstartupissuesthatyou
canexperiencewhenconfiguringbasicBGP?(Choosethree.)
(Source:"MonitoringandTroubleshootingBGP")
BGPneighborsdonotbecomeactive.
BGProutingloopscauseblackholes.
ABGPneighborisactive,buttheBGPsessionisnotestablished.
TheBGPneighborstateoscillatesbetweenIdleandActive.
TheBGPsessionisactive,buttheneighborcannotbereached.
BGPkeepalivesexperienceintermittentfailures.
1. WhatisthemostcommonreasonforaBGPsessionnotleavingthe
Idlestate?(Source:"MonitoringandTroubleshootingBGP")
TheTCPportfortheconnectionisnotconfigured.
Theexternalneighborisnotdirectlyconnected.
TheTCPSYNpacketisansweredwithanRSTpacket.
TheneighborshavebeenconfiguredwiththesameASnumber.
1. WhatwillresultfromattemptingtoopenaBGPconnectionwitha
neighborthathasnotbeenproperlyconfiguredforBGP?(Source:
"MonitoringandTroubleshootingBGP")
TheBGPsessionwillremainintheIdlestate.
Theneighborsessionwillbeestablished,andthesessionstartup
parameterswillbenegotiatedovertheTCPsession.
TheBGPsessionwillbeimmediatelyterminatedwithaTCPRSTpacket.
TheBGPsessionwillbecome"stuckinActivestate."
1. WhenaBGPneighboroscillatesbetweenActiveandIdle,whatis
thelikelydiagnosis?(Source:"MonitoringandTroubleshooting
BGP")
Therearemismatchedkeepaliveintervals.
ThereisanASnumbermismatchbetweenBGPneighbors.
OnerouterisnotconfiguredastheBGPneighborontheneighboring
router.
TheBGPneighborisnotreachable.
Introduction
ThismoduleisoneofthefocalpointsoftheBGPcurriculum:adiscussionofBGP
issuesinatransitAS.InthismoduleyouwilllearnbasicBGPtransitASissues,
rangingfromsynchronizationbetweenanIGP,andBGPtoIBGPfull-meshandnext-
hoprequirements.
Uponcompletingthismodule,youwillbeableto:
DescribethefunctionofatransitASandtheneedforIBGP
DescribetheinteractioninatransitASbetweenEBGPandIBGP
DescribethefunctionofanIGPinforwardingpacketsthroughanAS
VerifyproperoperationofaconfiguredBGPtransitnetworkbyperformingthe
stepsnecessarytocorrectbasicIBGPconfigurationerrors
BGPTransitAutonomousSystems
Overview
Alltransitautonomoussystemsarerequiredtocarrytrafficoriginatingfromor
destinedforlocationsoutsideofthatAS.ForthetransitAStomeetthisrequirement,
adegreeofinteractionandcoordinationbetweenBGPandtheIGPthatthis
particularASusesisnecessary.Suchaconfigurationrequiresspecialcareto
ensureconsistencyofroutinginformationthroughouttheAS.
ThetopologyoftheInternetcanbeviewedasaseriesofconnectionsbetweenstub
networks,multihomednetworks,andtransitautonomoussystems.AmultihomedAS
containingmorethanoneconnectiontotheoutsideworldandallowingtrafficnot
originatinginthatAStotravelthroughitisatransitAS.Thislessonintroducesthe
conceptofthemultihomedtransitASandhowBGPexchangesroutinginformation
insidetheASandbetweenneighboringautonomoussystems.Italsoexplainsthe
requirementforIBGPwithinthemultihomedtransitAS.
Uponcompletingthislesson,youwillbeableto:
ListthefunctionsofatransitAS
DescribeexternalroutepropagationbetweenautonomoussystemsinaBGP
network
DescribeinternalroutepropagationwithinaBGPAS
ExplainhowtransitingpacketsareforwardedinsideatransitAS
ExplaintheneedfordeployingIBGPonallcorerouters
WorkingwithTransitAS
AtransitASisrequiredtocarrytrafficthatoriginatesfromorisdestinedforlocations
outsideofthatAS.AtransitAShastwobasicfunctions.
Propagate routes between remote autonomous systems
Route packets between remote networks
RoutersinatransitAShavetoperformtwotasks:
Receiveroutinginformationupdatesaboutreachablenetworksfromneighboring
autonomoussystems,propagatetheinformationthroughtheirownAS,andsend
ittootherneighboringautonomoussystems.
ForwardIPpacketsthattheyhavereceivedfromaneighboringASthroughtheir
ownAStoadownstreamneighboringAS.TheroutersinthetransitASperform
thistaskusingtheroutinginformationthattheyhavereceivedaspartofthefirst
task.
TransitASTasks
Twoautonomoussystemsusuallyexchangeroutinginformationaboutreachable
networksusingBGP.Thereiscurrentlynoalternativeroutingprotocolthathasthe
scalabilityandsecuritycharacteristicsofBGP.
Inthefigure,theBGPsessionbetweenISP1andR1iscalledanEBGPsession
becauseISP1andR1areindifferentautonomoussystems.
BGProutinginformationupdatesconsistofthenetworkaddress,subnetmask,and
anynumberofBGPattributes.Nootherroutingprotocolprovidesthesamerichness
ofrouteattributesasBGP.TranslatingBGProuteattributeinformationintoanyother
protocolwouldlikelycausealossofinformation.Therefore,theEBGPinformation
thatR1receivesisnottranslated;itisjustforwardedtootherBGP-speakingrouters
(R2,R3,andR4inthefigure)withintheAS.
Likewise,R4hasBGPinformationandcanpropagateittoISP3BinAS300overthe
EBGPsession.
EBGPsessionsare,ingeneral,establishedbetweendirectlyconnectedneighbors.
BGP-speakingrouters,therefore,neednoadditionalroutinginformationtoestablish
asession.
ExternalRoutePropagation
Inthisexample,theBGPsessionbetweenR1andR4,whicharebothinthesame
AS,isanIBGPsession.
The only protocol that can transport all BGP attributes across the backbone is BGP
inside an AS, called Internal BGP (IBGP).
IBGPsessionsare,ingeneral,establishedbetweendistantroutersinthesameAS.
Theseroutersneedextraroutinginformationtoestablishthesession,becausethere
isnorequirementthatIBGPneighborsbedirectlyconnected.Thisinformation
typicallycomesfromtheIGP,whichisrunningwithintheASindependentlyofBGP.
InternalRoutePropagation
Inthisexample,afterAS300hasreceivedtheroutinginformationaboutreachable
networksinsideAS100,IPpacketscanstarttoflow.Inthefigure,IPpacketsflow
fromAS300towardAS100.ISP3B,theegressrouterinAS300,forwardsIP
packetswithdestinationsinAS100towardR4,accordingtoinformationreceived
throughEBGP.
Conclusions:
R2 needs external routes for proper packet forwarding. R2 must receive BGP
routes.
R4nowusestheIBGPinformationthatitreceivedfromR1andforwardsthepackets
inthedirectionofR1,whichinthiscasemeansviaR2.
WhentheIPpacketsreachR2,therouterchecksitsroutingtableforamatching
entry,butitfailstofindone.Thepacketisdroppedbecausethedestinationnetwork
isunreachablefromtheperspectiveofR2.
Thissituationis,ofcourse,unacceptable.Topreventdroppedpacketsresultingfrom
unreachablenetworks,R2mustalsohaveroutinginformationaboutthenetworks
reachableinsideAS100.ThesameinformationthatR4receivedfromR1overthe
IBGPsessionmustbepropagatedtoR2.
R3hasthesamenetworkreachabilityrequirementsasR2,
becauseR4couldforwardthepacketsviaR3aswellas
viaR2.
PacketForwardinginAS
WithinatransitAS,allroutersthatareinatheoreticaltransitpathbetweenexternal
destinationsshouldhaveinformationaboutallexternalroutesthatarereceivedfrom
anyneighboringAS.Ifasinglerouteronatransitpathdoesnothavethis
information,thereisalwaysapossibilitythatanIPpacketthatisreceivedfroma
neighboringASwillnotbeabletobeforwardedbythatrouterthroughthetransitAS.
TherouterlackingroutinginformationaboutthefinaldestinationoftheIPpacket
dropsitintowhateffectivelybecomesablackhole.
All core routers must have all external routes.
Core routers must receive BGP routes.
Redistribution of BGP routes into IGP is not scalable.
Default routing is not applicable in transit AS core.
Theonlyfeasiblewayfortheroutertodistributeallexternalroutinginformationisby
usingIBGP.RedistributionoftheEBGProutesintoanIGPisnotviablebecauseno
IGPcancarrythevolumeofinformationthatBGPcurrentlycarriesintheInternet.
Theriskoflosinginformationduringredistributionof
EBGProutesintoanIGPisnotthereasonwhyBGPis
usedtoupdateintermediateroutersinthetransitpath
insteadofanIGP.RedistributionintoanIGPisnotused
becauseofthescalabilityissuesthatwouldarisefrom
doingso.
Defaultroutingoragatewayoflastresortcannotbeusedbyrouterswithinthe
transitpathwhentransitservicesareprovidedtootherautonomoussystems.If
someroutesarefilteredoutandthedefaultrouteisusedinstead,fullrouting
flexibilityislost.Inthiscase,thetransitASisnotabletoforwardpacketstoall
destinationsatalltimes.Infact,routingloopsandblackholesmightbeeasily
introduced.
CoreRouterIBGPRequirementsinTransitAS
Overview
Throughthisdiscovery,youwilllearnhowtheBGProutesarepropagatedbetween
differentautonomoussystemsthatareconnecteddirectlyandneedforfullmesh
withinanAS.
Discovery4:BGPRoutePropagation
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
ISP1 Ethernet0/0 172.16.11.11/24 ConnectiontoR1
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
R1 Ethernet0/0 172.16.11.1/24 ConnectiontoISP1
R1 Ethernet0/2 192.168.12.1/24 ConnectiontoR2
R1 Ethernet0/3 192.168.13.1/24 ConnectiontoR3
R1 Loopback1 10.0.0.1/28 Loopbacksimulates
LANnetwork
R2 Ethernet0/2 192.168.12.2/24 ConnectiontoR1
R2 Ethernet0/3 192.168.24.2/24 ConnectiontoR4
R2 Loopback1 10.0.0.33/28 Loopbacksimulates
LANnetwork
R3 Ethernet0/2 192.168.34.3/24 ConnectiontoR4
R3 Ethernet0/3 192.168.13.3/24 ConnectiontoR1
R3 Loopback1 10.0.0.17/28 Loopbacksimulates
LANnetwork
R4 Ethernet0/0 172.16.34.4/24 ConnectiontoISP3B
R4 Ethernet0/2 192.168.34.4/24 ConnectiontoR3
R4 Ethernet0/3 192.168.24.4/24 ConnectiontoR2
R4 Loopback1 10.0.0.49/28 Loopbacksimulates
LANnetwork
ISP3B Ethernet0/0 172.16.34.34/24 ConnectiontoR4
ISP3B Loopback1
Loopback2
Loopback3
Loopback4
10.0.3.1/28
10.0.3.17/28
10.0.3.33/28
10.0.3.49/28
Loopbackssimulate
LANnetworks
JobAids
Step1
Youwillverifyroute10.0.1.0/28ispropagatedfromAS100intoAS300.
IPaddresses,IGPandBGParepreconfiguredasshowninthetopologybelow:
OntheISP1router,verifyroute10.0.1.0/28ispresentintheIProutingtable.
ISP1# show ip route
< output omitted >
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 20 subnets, 2 masks
B 10.0.0.0/28 [20/0] via 172.16.11.1, 01:49:30
B 10.0.0.16/28 [20/0] via 172.16.11.1, 01:49:30
B 10.0.0.32/28 [20/0] via 172.16.11.1, 01:49:30
B 10.0.0.48/28 [20/0] via 172.16.11.1, 01:49:30
C 10.0.1.0/28 is directly connected, Loopback1
L 10.0.1.1/32 is directly connected, Loopback1
C 10.0.1.16/28 is directly connected, Loopback2
L 10.0.1.17/32 is directly connected, Loopback2
C 10.0.1.32/28 is directly connected, Loopback3
<... output omitted ...>
IPaddress10.0.1.1/28isconfiguredontheLoopback1interface.
Step2
OntheISP1router,verifyroute10.0.1.0/28ispresentintheBGProutingtable
aslocallyoriginated.
ISP1# show ip bgp
< ouput omitted >
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.11.1 0 0 1 i
*> 10.0.0.16/28 172.16.11.1 0 1 i
*> 10.0.0.32/28 172.16.11.1 0 1 i
*> 10.0.0.48/28 172.16.11.1 0 1 i
*> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.16/28 0.0.0.0 0 32768 i
*> 10.0.1.32/28 0.0.0.0 0 32768 i
*> 10.0.1.48/28 0.0.0.0 0 32768 i
*> 10.0.1.64/28 0.0.0.0 0 32768 i
*> 10.0.1.80/28 0.0.0.0 0 32768 i
*> 10.0.3.0/28 172.16.11.1 0 1 300 i
*> 10.0.3.16/28 172.16.11.1 0 1 300 i
*> 10.0.3.32/28 172.16.11.1 0 1 300 i
*> 10.0.3.48/28 172.16.11.1 0 1 300 i
Theroute10.0.1.0/28islocallyoriginated,becausenexthopIPaddressis
0.0.0.0andASpathattributeisempty.
Step3
OntheR1router,verifyroute10.0.1.0/28ispresentintheBGProutingtable.
BGPRoutePropagation
DiscoverySteps
R1# show ip bgp
< output omitted >
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 0.0.0.0 0 32768 i
*>i 10.0.0.16/28 10.0.0.17 0 100 0 i
*>i 10.0.0.32/28 10.0.0.33 0 100 0 i
*>i 10.0.0.48/28 10.0.0.49 0 100 0 i
*> 10.0.1.0/28 172.16.11.11 0 0 100 i
*> 10.0.1.16/28 172.16.11.11 0 0 100 i
*> 10.0.1.32/28 172.16.11.11 0 0 100 i
*> 10.0.1.48/28 172.16.11.11 0 0 100 i
*> 10.0.1.64/28 172.16.11.11 0 0 100 i
*> 10.0.1.80/28 172.16.11.11 0 0 100 i
*>i 10.0.3.0/28 172.16.34.34 0 100 0 300 i
*>i 10.0.3.16/28 172.16.34.34 0 100 0 300 i
*>i 10.0.3.32/28 172.16.34.34 0 100 0 300 i
*>i 10.0.3.48/28 172.16.34.34 0 100 0 300 i
Theroute10.0.1.0/28hasnexthopIPaddress172.16.11.11andASpath
attribute100.
Step4
OntheR4router,verifyroute10.0.1.0/28ispresentintheBGProutingtable.
R4# show ip bgp
< text omitted >
Network Next Hop Metric LocPrf Weight Path
*>i 10.0.0.0/28 10.0.0.1 0 100 0 i
*>i 10.0.0.16/28 10.0.0.17 0 100 0 i
*>i 10.0.0.32/28 10.0.0.33 0 100 0 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*>i 10.0.1.0/28 172.16.11.11 0 100 0 100 i
*>i 10.0.1.16/28 172.16.11.11 0 100 0 100 i
*>i 10.0.1.32/28 172.16.11.11 0 100 0 100 i
*>i 10.0.1.48/28 172.16.11.11 0 100 0 100 i
*>i 10.0.1.64/28 172.16.11.11 0 100 0 100 i
*>i 10.0.1.80/28 172.16.11.11 0 100 0 100 i
*> 10.0.3.0/28 172.16.34.34 0 0 300 i
*> 10.0.3.16/28 172.16.34.34 0 0 300 i
*> 10.0.3.32/28 172.16.34.34 0 0 300 i
*> 10.0.3.48/28 172.16.34.34 0 0 300 i
Theroute10.0.1.0/28hasnexthopIPaddress172.16.11.11andASpath
attribute100.YoucanseerouteislearnedviainternalBGP.
Step5
OntheR4router,verifywhatisthebestpathtoreachroutenexthopIPaddress
172.16.11.11.
R4# show ip route
< output omitted >
172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
O 172.16.11.0/24 [110/30] via 192.168.34.3, 02:13:15, Ethernet0/2
[110/30] via 192.168.24.2, 02:13:15, Ethernet0/3
<... output omitted ...>
OutputshowsthatnexthopIPaddress172.16.11.11isreachableviatwoOSPF
routes.RouterR4willloadsharebetweentwoOSPFroutes.Somepacketswill
besenttotheR2routerandsometotheR3router.
So,R2andR3routersneedIProutinginformationtoreachnexthop
172.16.11.11.
Step6
OntheISP3Brouter,verifyroute10.0.1.0/28ispresentintheBGProutingtable.
ISP3B# show ip bgp
< output omitted >
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.34.4 0 1 i
*> 10.0.0.16/28 172.16.34.4 0 1 i
*> 10.0.0.32/28 172.16.34.4 0 1 i
*> 10.0.0.48/28 172.16.34.4 0 0 1 i
*> 10.0.1.0/28 172.16.34.4 0 1 100 i
*> 10.0.1.16/28 172.16.34.4 0 1 100 i
*> 10.0.1.32/28 172.16.34.4 0 1 100 i
*> 10.0.1.48/28 172.16.34.4 0 1 100 i
*> 10.0.1.64/28 172.16.34.4 0 1 100 i
*> 10.0.1.80/28 172.16.34.4 0 1 100 i
*> 10.0.3.0/28 0.0.0.0 0 32768 i
*> 10.0.3.16/28 0.0.0.0 0 32768 i
*> 10.0.3.32/28 0.0.0.0 0 32768 i
*> 10.0.3.48/28 0.0.0.0 0 32768 i
Theroute10.0.1.0/28hasnexthopIPaddress172.16.34.4andASpath
attribute1100.
Step7
Youwillverify,howpacketsareforwardedfromAS300totheIP10.0.1.1inthe
AS100.OntheISP3Brouter,pingnexthopIPaddressoftheBGProute
10.0.1.0/24.
Inthepreviousstep,youhavelearnednexthopIPaddressis172.16.34.4.
SourcepingfromtheISP3Brouter'sLoopback1interface.
ISP3B# ping 172.16.34.4 source Loopback 1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.34.4, timeout is 2 seconds:
Packet sent with a source address of 10.0.3.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms
FromtheISP3Brouter,youcansuccessfullyreachnexthop172.16.34.4(router
R4).
Step8
FromtheR4router,toreachIPaddress10.0.1.1,nexthopIPaddressis
172.16.11.11.FromtheR4router,pingnexthopIPaddressfromtheBGProute
10.0.1.0/24.
SourcepingfromtheR4router'sLoopback1interface.
R4# ping 172.16.11.11 source Loopback 1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.11.11, timeout is 2 seconds:
Packet sent with a source address of 10.0.0.49
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms
FromtheR4router,youcansuccessfullyreachnexthop172.16.11.11(ISP1
router).
Step9
FromtheISP3Brouter,pingdirectlytotheIPaddress10.0.1.1.Sourcepingfrom
theISP3Brouter'sLoopback1interface.
Pingshouldbesuccessful.ThesuccessfulpingdemonstratesIPconnectivity
betweenAS100andAS300viaAS1.
ISP3B# ping 10.0.1.1 source Loopback 1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.1.1, timeout is 2 seconds:
Packet sent with a source address of 10.0.3.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
Routers in a transit AS receive routing information updates from neighboring
autonomous systems, propagate the information through their own AS, and send
it to other neighboring autonomous systems.
Two autonomous systems usually exchange routing information over an EBGP
session.
A BGP session between two routers in the same AS is called an IBGP session.
For packets to be properly forwarded in a transit AS, all routers must have
external routing information.
The only feasible method of distributing external routing information to all routers
in the transit AS is through IBGP.
Summary
Overview
ConfiguringaBGPnetworkinatransitservicesconfigurationrequiresspecialcare
toensureconsistencyofroutinginformationthroughouttheAS.Understandingthe
interactionbetweenEBGPandIBGPiscrucialtosuccessfullyconfiguringand
troubleshootingthetransitautonomousnetwork.
Uponcompletingthislesson,youwillbeableto:
DescribeAS-pathprocessinginIBGP
DescribeBGPmultipathloadsharing
ExplaintheneedforBGPsplithorizon
Explaintheneedforafull-meshtopologybetweenIBGProutersandthe
implicationsofthatneed
ListthebenefitsofestablishingIBGPneighborsessionsusingloopback
interfaces
Describenext-hopprocessinginIBGP
ExplainwhyallEBGPpeersmustbereachablebyallBGP-speakingrouters
withintheAS
Describehowtoconfigureedgerouterstoannouncethemselvesasthenext
hopinIBGPupdates
DescribethedifferencesbetweenEBGPandIBGPsessions
ListthescalabilitylimitationsofIBGP-basedbackbones
InteractingwithIBGPandEBGPinTransit
AS
AllBGProutingupdatescarrythemandatorywell-knownattributeAS-path,which
liststheautonomoussystemsthattheroutingupdatehasalreadycrossed.
WhenarouteroriginatesaBGPprefix(networkXinthisexample),theASpathis
empty.WheneveraBGPprefixisannouncedoveranEBGPsession,theAS
numberoftherouterthatissendingtheinformationisprependedtotheASpath.In
theexample,ISP1inserts"100"intheASpathbeforeforwardingtheroutingupdate
toR1.
TheASpathisnotchangedwhentheBGPprefixispropagatedacrossIBGP
sessionsbecausetheroutingupdatehasnotcrossedanASboundary.
Inthefigure,R1forwardstheinformationoveranIBGPsessiontoR4withtheAS-
pathunchanged.TheAS-pathinformationaboutnetworkXwillbethesameinall
routerswithinAS1,becausealltheroutersareupdatedusingIBGPsessionsfrom
R1.
WhenR4forwardstheinformationaboutnetworkXtoISP3B,R4prependsitsown
ASnumber(1)totheASpath.Thus,ISP3Breceivestheroutinginformationabout
networkXwithanAS-pathattributeof"1100."
AS-PathProcessinginIBGP
WhenaBGP-speakingrouterwithnolocalpolicyconfiguredreceivesNLRIfrom
multipleIBGPsourcesforthesamedestination,therouterchoosesoneIBGPpath
asthebestpath.ThebestpathistheninstalledintheIProutingtableoftherouter.
ThefigureillustratesthatwiththreepathstoAS200,theR2routerdeterminesthat
oneofthepathstoAS200isthebestpath.So,itusesonlythispathtoreachAS
200.
TheIBGPmultipathload-sharingfeatureenablestheBGP-speakingroutertoselect
multipleIBGPpathsasthebestpathstoadestination.Thebestpaths,ormultipaths,
aretheninstalledintheIProutingtableoftherouter.
OntheR2routerinthefigure,thepathstotheR3,R4,andR5routersare
configuredasmultipathsandcanbeusedtoreachAS200.TheloadtoAS200is
equallysharedbetweenthepaths.
Formultiplepathstothesamedestinationtobeconsideredasmultipaths,the
followingcriteriamustbemet:
Allattributesmustbethesame.Theattributesincludeweight,localpreference,
ASpath(entireattributeandnotjustlength),origincode,MED,andIGP
distance.
Thenexthoprouterforeachmultipathmustbedifferent.
Evenifthecriteriaaremetandmultiplepathsareconsideredmultipaths,theBGP-
speakingrouterstilldesignatesoneofthemultipathsasthebestpathandadvertises
thisbestpathtoitsneighbors.
ConfiguringmultipleIBGPbestpathsenablesaroutertoevenlysharethetrafficthat
isdestinedforaparticularsite.
Usethemaximum-pathsibgpcommandinrouterconfigurationmode,tocontrolthe
maximumnumberofparallelinternalBGProutesthatcanbeinstalledinarouting
table.
MultipathLoadSharinginBGP
AllrouterswithinanASmustmakeroutingdecisionsinaconsistentway.Theymust
haveaccesstothesameroutinginformationwiththesameattributesinorderto
cometothesameconclusionaboutwhichexitpointoftheAStouse.Inotherwords,
theBGPattributesshouldnotbechangedwithintheAS.
Result: Full mesh of IBGP sessions is required for proper IBGP update propagation.
TheAS-pathattributeisnotchangedoveranIBGPsession,becausetheBGP
updatehasnotcrossedtheASboundary.However,theAS-pathattributeisthe
primarymeansofdetectingroutinginformationloops.ABGProuterthatencounters
itsownASintheASpathofanincomingBGPupdatesilentlyignoresthe
information.BecausetheBGP-speakingroutersmodifyASpathonlyonEBGP
sessions,thisloop-preventingmechanismisonlyusefulbetweenautonomous
systems,notwithinthem.
IBGPsplithorizonpreventsroutinginformationloopswithintheAS.Routing
informationthatisreceivedthroughanIBGPsessionisneverforwardedtoanother
IBGPneighbor,onlytowardEBGPneighbors.BecauseofBGPsplithorizon,no
routercanrelayIBGPinformationwithintheAS—allroutersmustbedirectlyupdated
fromtheborderrouterthatreceivedtheEBGPupdate.
BGPSplitHorizon
Full mesh of IBGP sessions has to be established between all BGP-speaking
routers in the AS for proper IBGP route propagation.
The IBGP full mesh is a logical mesh of TCP sessions only; physical full mesh is
not required.
EveryrouteronthetransitpathwithintheASmusthaveroutinginformationaboutall
externalnetworksthatarereceivedbyanyoftheborderrouters.So,R2andR3
musthaveIBGPsessionstoallborderrouters.
Thislevelofcommunicationisnotenough,though,becauseanyoftheinternal
routerscouldalsocreatenewBGProutinginformation(forexample,originatea
customernetwork).TheseupdatesmustalsoreachalltherouterswithintheAS.The
conclusionisthatallBGProuterswithinanASmusthaveIBGPsessionswithevery
otherBGProuterintheAS.ThisrequirementresultsinafullmeshofBGPsessions
betweenBGP-speakingroutersinanAS.
Inthenetworkinthefigure,R1musthaveIBGPsessionswithR2,R3,andR4to
propagateroutesthatarereceivedfromAS100toallrouterswithinAS1.Similarly,
R4musthaveIBGPsessionswithR1,R2,andR3tobeabletopropagateroutes
thatarereceivedfromAS300toallrouterswithinAS1.
TheIBGPsessionbetweenR2andR3isnotstrictly
necessaryforproperforwardingofIPpacketsbetween
externaldestinations.ItdoesbecomemandatoryifR2or
R3startstooriginateBGPnetworks.Topreventpotential
futureconnectivityissues,itisagoodpracticetoestablish
afullmeshofIBGPsessionsregardlessofwhetherthey
areneededatthetimeofnetworkdeploymentornot.
TheIGPthatrunswithinAS1providesenoughinformationtoanyBGProuterwithin
AS1tosendIPpacketstoanyotherrouterintheAS.Havingenoughrouter
reachabilityinformationmakesitpossibletoestablishIBGPsessionsbetween
routerseventhoughtheyarenotphysicallyconnected.TheIBGPfullmeshisa
logicalfullmeshofTCPsessionsandwillrunonanarbitraryphysicaltopology.
IBGPFullMeshExample
ThefigureillustratesIBGPsplit-horizonandIBGPfull-meshprinciplesinasample
network.
IBGPFullMesh
ISP1issendinganupdatetoR1overanEBGPsession.Updatesthatarereceived
onanEBGPsessionshouldbeforwardedonallotherIBGPsessions,soR1
updatesR2,R3,andR4.AllrouterswithinAS1areupdateddirectlybyR1.
R2andR3arepreventedfromforwardingtheupdatethattheyreceivedfromR1
becauseofBGPsplithorizon.
R4,whichreceivedtheinformationonanIBGPsession,ispreventedfromupdating
R2andR3becauseofthesamesplit-horizonrule.ButR4willupdateISP3Boveran
EBGPsession.
Inthefigure,thetransitAS1hasaredundantphysicaltopology.TheIGPprovides
reachabilityinformationforallroutersandnetworkswithinAS1.Thisway,IGP
enablesallroutersintheAStoestablishIBGPsessionstoallotherrouters,evenif
theroutersarenotdirectlyconnected.
Because of IBGP full-mesh requirements, IBGP neighbors are usually not directly
connected. Which interfaces should be chosen as the source and destination
addresses of IBGP TCP sessions?
IftheIBGPsessionbetweenR1andR4wasestablishedusingIPaddressesthat
belongtothephysicalinterfaces,theIBGPsessionwouldgodownifeitherofthe
physicalinterfaceswentdown.TheIPaddressofaninterfacethatisinthedown
stateisinvalid.Asaresult,therouterwouldteardowntheTCPsessionthatisused
forBGPbetweentherouters.Then,allIPpacketsthatarereceivedwitha
destinationaddresspointingtothatinterfacewillalsobedropped.
Duringthenetworkdesignandimplementationphase,thenetworkdesignersmust
becarefulthatthoseIBGPsessionsremainestablishedforaslongasthetwoBGP
routershaveanyusablepathbetweenthem.
Always run IBGP sessions between loopback interfaces.
IBGP sessions can always be established, even if some physical interfaces are
down.
IBGP sessions are stable—physical interface failure will not tear down IBGP
sessions.
There is no BGP recovery after a failure inside the transit AS.
The configured IGP will re-establish the path between loopback interfaces.
IBGP sessions are not affected.
ThebestchoicewhenyouareconfiguringIBGPsessionsistoestablisheach
sessionbetweenloopbackinterfacesoneachBGProuter.
ToestablishBGPconnectivitybetweentheloopbackinterfaces,theIPaddressesof
theseinterfaceshavetobereachablebybothrouters.ItisimportantthattheIGP
carryinformationaboutthesubnetsthatareassignedtoeachloopbackinterfaceso
thattheinterfacesarereachablebyallBGProutersintheAS.
TheIBGPsessionsthatareestablishedbetweenloopbackinterfaceshave
increasedstability.Thesesessionswillnotgodownifasinglephysicalinterface
goesdown.AslongastheIGPcanfindanypathbetweenthetworouters,theIBGP
sessionwillremainup.BGPwillnotnoticethattheIGPhaschangedthetrafficpath
betweenthetworouters.
BecauseBGPsessionsrunoverTCP,theycansurvive
evenashortlossofconnectivitybetweenBGProuters
withnoimpacttotheBGProutingprotocol.Theonly
requirementplacedontheIGPisthatthenetworkmust
convergebeforetheBGPkeepalivetimerexpires.
IBGPNeighbors
EveryBGPupdatecarriesthemandatorywell-knownattributenexthop.Thenext-
hopattributespecifiestheIPaddressthattheroutershoulduseastheforwarding
nexthopforpacketsthataresenttowardtheannounceddestinationaddress.In
mostcases,thenexthopissettotheIPaddressthatthesendingrouterisusingas
itssourceIPaddressforEBGPsessions.ThereceivingBGProuterwillusethe
informationandrouteIPpacketstowardtheannounceddestinationviatheindicated
nexthop,whichisnormallydirectlyconnected.
Thenext-hopattributeisnotchangedonIBGPupdates.So,whentheborderrouter
forwardstheBGPupdateonIBGPsessions,thenext-hopaddressisstillsettothe
IPaddressofthefarendoftheEBGPsession.Therefore,thereceiverofIBGP
updateswillseethenext-hopinformationindicatingadestinationthatisnotdirectly
connected.Toresolvethisproblem,therouterwillcheckitsroutingtableandseeif
andhowitcanreachthenext-hopaddress.TheroutercanthenrouteIPpackets
withdestinationaddressesmatchingthenetworkintheBGPupdateinthesame
directionasitwouldhaveroutedanIPpacketwithadestinationaddressequalto
theIPaddressstatedinthenext-hopattribute.Thisprocessisknownasrecursive
routing.
Inthefigure,ISP1sendsaBGPupdateaboutnetworkX.Becauseitissendingthis
updateoveranEBGPsessiontoR1,thenext-hopattributeissettotheIPaddress
thatisusedattheISP1sideoftheEBGPsession,172.16.11.11.
R1canusethisinformationandroutepacketstonetworkXbyforwardingthemto
ISP1.
R1alsoforwardstheBGPupdateoverallitsIBGPsessions.Itdoesnotchangethe
next-hopattribute,soR2,R3,andR4getinformationthattheycanreachnetworkX
byforwardingpacketsto172.16.11.11.ButthatIPaddressisnotdirectlyconnected,
sotheroutersmustlookintheirroutingtablestoseeifandhowtheycanreach
172.16.11.11.Iftherecursiveroutelookupissuccessful,eachroutercanthenroute
packetstonetworkXinthesamedirectionasitwouldroutepacketsto
172.16.11.11.
R4alsoforwardstheBGPupdateaboutnetworkXtoISP3B.Theconnection
betweentheseroutersisanEBGPsession.So,R4setsthenext-hopattributetoits
ownIPaddress,172.16.34.4,whichisusedbyR4ontheEBGPsessiontoward
ISP3B.
IBGPNext-HopProcessing
Overview
Throughthisdiscovery,youwilllearnhowtheBGPsplithorizonworksandhowit
impactstheroutingtable.YouwilllearnwhyfullmeshIBGPsessionswithinanAS
arerequiredandhownext-hopisprocessedduringroutepropagationwithinanAS.
Discovery5:IBGPFullMesh
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
ISP1 Ethernet0/0 172.16.11.11/24 ConnectiontoR1
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
R1 Ethernet0/0 172.16.11.1/24 ConnectiontoISP1
R1 Ethernet0/2 192.168.12.1/24 ConnectiontoR2
R1 Ethernet0/3 192.168.13.1/24 ConnectiontoR3
R1 Loopback1 10.0.0.1/28 Loopbacksimulates
LANnetwork
R2 Ethernet0/2 192.168.12.2/24 ConnectiontoR1
R2 Ethernet0/3 192.168.24.2/24 ConnectiontoR4
R2 Loopback1 10.0.0.33/28 Loopbacksimulates
LANnetwork
R3 Ethernet0/2 192.168.34.3/24 ConnectiontoR4
R3 Ethernet0/3 192.168.13.3/24 ConnectiontoR1
R3 Loopback1 10.0.0.17/28 Loopbacksimulates
LANnetwork
R4 Ethernet0/0 172.16.34.4/24 ConnectiontoISP3B
R4 Ethernet0/2 192.168.34.4/24 ConnectiontoR3
R4 Ethernet0/3 192.168.24.4/24 ConnectiontoR2
R4 Loopback1 10.0.0.49/28 Loopbacksimulates
LANnetwork
ISP3B Ethernet0/0 172.16.34.34/24 ConnectiontoR4
ISP3B Loopback1
Loopback2
Loopback3
Loopback4
10.0.3.1/28
10.0.3.17/28
10.0.3.33/28
10.0.3.49/28
Loopbackssimulate
LANnetworks
IPaddressesandIGParepreconfiguredasshowninthetopologybelow:
JobAids
BGPisalsoconfiguredasEBGP(ISP1toR1andISP3BtoR4)andasIBGP(R1to
R2,R1toR3,R3toR4,andR2toR4).IBGPisnotconfiguredbetweenR1toR4or
betweenR2toR3.
Step1
OntheR1router,verifywhatBGProutesarereceivedfromISP1router.
AnexternalBGPsessionisestablishedbetweenR1andISP1routers.
R1# show ip bgp
BGP table version is 10, local router ID is 10.0.0.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 0.0.0.0 0 32768 i
*>i 10.0.0.16/28 10.0.0.17 0 100 0 i
*>i 10.0.0.32/28 10.0.0.33 0 100 0 i
*> 10.0.1.0/28 172.16.11.11 0 0 100 i
*> 10.0.1.16/28 172.16.11.11 0 0 100 i
*> 10.0.1.32/28 172.16.11.11 0 0 100 i
*> 10.0.1.48/28 172.16.11.11 0 0 100 i
*> 10.0.1.64/28 172.16.11.11 0 0 100 i
*> 10.0.1.80/28 172.16.11.11 0 0 100 i
TheR1routerreceivedsixBGPprefixesfromtheISP1router.Fortheseprefixes,thenext
hopIPaddressis172.16.11.11.
Step2
OntheR1router,verifywhatBGPprefixesaresenttointernalBGPpeerR2(10.0.0.33).
R1# show ip bgp neighbors 10.0.0.33 advertised-routes
BGP table version is 10, local router ID is 10.0.0.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 0.0.0.0 0 32768 i
*> 10.0.1.0/28 172.16.11.11 0 0 100 i
*> 10.0.1.16/28 172.16.11.11 0 0 100 i
*> 10.0.1.32/28 172.16.11.11 0 0 100 i
*> 10.0.1.48/28 172.16.11.11 0 0 100 i
*> 10.0.1.64/28 172.16.11.11 0 0 100 i
*> 10.0.1.80/28 172.16.11.11 0 0 100 i
Total number of prefixes 7
YoucanseethatalltheprefixesthatarereceivedfromtheexternalBGPpeeraresentto
theinternalBGPpeer.
Step3
OntheR2router,verifyifallsixprefixesthatareoriginatedinAS100arereceivedfromR1
router.
R2# show ip bgp
BGP table version is 16, local router ID is 10.0.0.33
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*>i 10.0.0.0/28 10.0.0.1 0 100 0 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*>i 10.0.0.48/28 10.0.0.49 0 100 0 i
* i 10.0.1.0/28 172.16.11.11 0 100 0 100 i
* i 10.0.1.16/28 172.16.11.11 0 100 0 100 i
IBGPFullMesh
DiscoverySteps
* i 10.0.1.32/28 172.16.11.11 0 100 0 100 i
* i 10.0.1.48/28 172.16.11.11 0 100 0 100 i
* i 10.0.1.64/28 172.16.11.11 0 100 0 100 i
* i 10.0.1.80/28 172.16.11.11 0 100 0 100 i
* i 10.0.3.0/28 172.16.34.34 0 100 0 300 i
* i 10.0.3.16/28 172.16.34.34 0 100 0 300 i
* i 10.0.3.32/28 172.16.34.34 0 100 0 300 i
* i 10.0.3.48/28 172.16.34.34 0 100 0 300 i
OntheR2router,youshouldseeallsixprefixesfromAS100intheBGProutingtable.
Step4
OntheR1router,verifywhatBGPprefixesaresenttointernalBGPpeerR3(10.0.0.17).
R1# show ip bgp neighbors 10.0.0.17 advertised-routes
BGP table version is 10, local router ID is 10.0.0.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 0.0.0.0 0 32768 i
*> 10.0.1.0/28 172.16.11.11 0 0 100 i
*> 10.0.1.16/28 172.16.11.11 0 0 100 i
*> 10.0.1.32/28 172.16.11.11 0 0 100 i
*> 10.0.1.48/28 172.16.11.11 0 0 100 i
*> 10.0.1.64/28 172.16.11.11 0 0 100 i
*> 10.0.1.80/28 172.16.11.11 0 0 100 i
Total number of prefixes 7
YoucanseeallprefixesreceivedfromexternalBGPpeeraresenttointernalBGPpeer.
Step5
OntheR3router,verifyifallsixprefixesthatareoriginatedinAS100arereceivedfromR1
router.
R3# show ip bgp
BGP table version is 16, local router ID is 10.0.0.17
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*>i 10.0.0.0/28 10.0.0.1 0 100 0 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*>i 10.0.0.48/28 10.0.0.49 0 100 0 i
* i 10.0.1.0/28 172.16.11.11 0 100 0 100 i
* i 10.0.1.16/28 172.16.11.11 0 100 0 100 i
* i 10.0.1.32/28 172.16.11.11 0 100 0 100 i
* i 10.0.1.48/28 172.16.11.11 0 100 0 100 i
* i 10.0.1.64/28 172.16.11.11 0 100 0 100 i
* i 10.0.1.80/28 172.16.11.11 0 100 0 100 i
* i 10.0.3.0/28 172.16.34.34 0 100 0 300 i
* i 10.0.3.16/28 172.16.34.34 0 100 0 300 i
* i 10.0.3.32/28 172.16.34.34 0 100 0 300 i
* i 10.0.3.48/28 172.16.34.34 0 100 0 300 i
OntheR3router,youshouldseeallsixprefixesfromAS100intheBGProutingtable.
Step6
OntheR2router,verifywhatBGPprefixesaresenttointernalBGPpeerR4(10.0.0.49).
R2# show ip bgp neighbors 10.0.0.49 advertised-routes
BGP table version is 16, local router ID is 10.0.0.33
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.32/28 0.0.0.0 0 32768 i
Total number of prefixes 1
YoucanseethatnoprefixesarereceivedfrominternalBGPpeerandsenttointernalBGP
peer.Thereisasingleprefix,locallyoriginated,thatissenttointernalBGPpeer.
Step7
OntheR3router,verifywhatBGPprefixesaresenttointernalBGPpeerR4(10.0.0.49).
R3# show ip bgp neighbors 10.0.0.49 advertised-routes
BGP table version is 16, local router ID is 10.0.0.17
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.16/28 0.0.0.0 0 32768 i
Total number of prefixes 1
YoucanseethatnoprefixesarereceivedfrominternalBGPpeerandsenttointernalBGP
peer.Thereisasingleprefix,locallyoriginated,thatissenttointernalBGPpeer.
R4routerhasnointernalBGPsessiontotheR1router.
Step8
OntheR4router,youwillnotfindanyprefixesfromAS100.
R4# show ip bgp
BGP table version is 8, local router ID is 10.0.0.49
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*>i 10.0.0.16/28 10.0.0.17 0 100 0 i
*>i 10.0.0.32/28 10.0.0.33 0 100 0 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.3.0/28 172.16.34.34 0 0 300 i
*> 10.0.3.16/28 172.16.34.34 0 0 300 i
*> 10.0.3.32/28 172.16.34.34 0 0 300 i
*> 10.0.3.48/28 172.16.34.34 0 0 300 i
Step9
ConfigureBGPsessionbetweenR1andR4routers.
BGPsessionissourcedfromLoopback1interface.RouterR1Loopback1
interfacehasIPaddress10.0.0.1androuterR4Loopback1interfacehasIP
address10.0.0.49.BothroutersareintheBGPAS1.
R1(config)# router bgp 1
R1(config-router)# neighbor 10.0.0.49 remote-as 1
R1(config-router)# neighbor 10.0.0.49 update-source Loopback 1
R4(config)# router bgp 1
R4(config-router)# neighbor 10.0.0.1 remote-as 1
R4(config-router)# neighbor 10.0.0.1 update-source Loopback 1
Step10
OntheR1router,verifywhatBGPprefixesaresenttointernalBGPpeerR4(10.0.0.49).
R1# show ip bgp neighbors 10.0.0.49 advertised-routes
BGP table version is 11, local router ID is 10.0.0.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 0.0.0.0 0 32768 i
*> 10.0.1.0/28 172.16.11.11 0 0 100 i
*> 10.0.1.16/28 172.16.11.11 0 0 100 i
*> 10.0.1.32/28 172.16.11.11 0 0 100 i
*> 10.0.1.48/28 172.16.11.11 0 0 100 i
*> 10.0.1.64/28 172.16.11.11 0 0 100 i
*> 10.0.1.80/28 172.16.11.11 0 0 100 i
Total number of prefixes 7
YoucanseeallprefixesreceivedfromexternalBGPpeeraresenttointernalBGPpeer.
Step11
OntheR4router,verifyifallsixprefixesthatareoriginatedinAS100,arereceivedfrom
R1router.
OntheR4router,youshouldseeallsixprefixesfromAS100intheBGProutingtable.
R4# show ip bgp
BGP table version is 9, local router ID is 10.0.0.49
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*>i 10.0.0.0/28 10.0.0.1 0 100 0 i
*>i 10.0.0.16/28 10.0.0.17 0 100 0 i
*>i 10.0.0.32/28 10.0.0.33 0 100 0 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
* i 10.0.1.0/28 172.16.11.11 0 100 0 100 i
* i 10.0.1.16/28 172.16.11.11 0 100 0 100 i
* i 10.0.1.32/28 172.16.11.11 0 100 0 100 i
* i 10.0.1.48/28 172.16.11.11 0 100 0 100 i
* i 10.0.1.64/28 172.16.11.11 0 100 0 100 i
* i 10.0.1.80/28 172.16.11.11 0 100 0 100 i
*> 10.0.3.0/28 172.16.34.34 0 0 300 i
*> 10.0.3.16/28 172.16.34.34 0 0 300 i
*> 10.0.3.32/28 172.16.34.34 0 0 300 i
*> 10.0.3.48/28 172.16.34.34 0 0 300 i
NowallroutersintheAS1havesameBGProutes.
Step12
BGPsessionbetweenR2andR3routersisnotneededinthislabsetup,
becauseR2andR3havenoexternalBGPsessions.Butinordertobe
consistentandprepareBGPtoaccommodatefutureexpansions,configureBGP
sessionbetweenR2andR3routers.
BGPsessionissourcedfromLoopback1interface.RouterR2Loopback1
interfacehasIPaddress10.0.0.33androuterR3Loopback1interfacehasIP
address10.0.0.17.BothroutersareintheBGPAS1.
R2(config)# router bgp 1
R2(config-router)# neighbor 10.0.0.17 remote-as 1
R2(config-router)# neighbor 10.0.0.17 update-source Loopback 1
R3(config)# router bgp 1
R3(config-router)# neighbor 10.0.0.33 remote-as 1
R3(config-router)# neighbor 10.0.0.33 update-source Loopback 1
Step13
OntheR4router,verifywhatBGPprefixesaresenttoexternalBGPpeerISP3B
(172.16.34.34).
R4# show ip bgp neighbors 172.16.34.34 advertised-routes
BGP table version is 9, local router ID is 10.0.0.49
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*>i 10.0.0.0/28 10.0.0.1 0 100 0 i
*>i 10.0.0.16/28 10.0.0.17 0 100 0 i
*>i 10.0.0.32/28 10.0.0.33 0 100 0 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
Total number of prefixes 4
YoucanseetherearenoprefixesfromAS100.
Step14
OntheR4router,verifywhyroutesfromAS100arenotsenttoexternalBGP
peerISP3B(172.16.34.34).
ExaminesingleBGProutefromAS100.
R4# show ip bgp 10.0.1.0/28
BGP routing table entry for 10.0.1.0/28, version 0
Paths: (1 available, no best path)
Not advertised to any peer
Refresh Epoch 2
100
172.16.11.11 (inaccessible) from 10.0.0.1 (10.0.0.1)
Origin IGP, metric 0, localpref 100, valid, internal
Theoutputshowsthatroute10.0.1.0/28hasinaccessiblenexthop
172.16.11.11.Thisinaccessibilityisareasonthattherouteisnotmarkedasa
goodrouteandisnotadvertisedtowardexternalBGPpeer.
R4# show ip route 172.16.11.11
% Subnet not in table
OntheR4router,thereisnoroutetothenexthop172.16.11.11intheIProuting
table.
Step15
OntheR1router,verifywhatisnexthopIPaddressoftheBGProute
10.0.1.0/28.
R1# show ip bgp 10.0.1.0/28
BGP routing table entry for 10.0.1.0/28, version 3
Paths: (1 available, best #1, table default)
Advertised to update-groups:
2
Refresh Epoch 1
100
172.16.11.11 from 172.16.11.11 (10.0.1.81)
Origin IGP, metric 0, localpref 100, valid, external, best
Outputshowsthatroute10.0.1.0/28hasvalidnexthop172.16.11.11.
TransitNetworkUsingExternalNextHops
AllBGP-speakingrouterswithintheASgetinformationaboutexternalnetworkswith
thenext-hopattribute.Thenext-hopattributeissettothefarendoftheEBGP
sessionsreachingtheborderroutersoftheAS.
All EBGP peers must be reachable by all BGP-speaking routers within the AS.
EBGP next hops shall be announced using the IGP.
Redistribute connected interfaces into the IGP at the edge routers.
Include links to EBGP neighbors into the IGP and configure them as passive
interfaces.
RoutersusearecursiveroutingmechanismwhentheydeterminehowtoforwardIP
packetstowardexternaldestinations.WhenBGProutesareusedintherouting
table,theroutercheckshowitwouldhavereachedthenext-hopaddress,andit
installstheBGProutewiththesameforwardingindicationasfortheroutethatis
usedtoreachthenext-hopIPaddress.
Togettherecursiveroutingtowork,theroutermustresolveallpossiblenext-hop
referencesthatuseinformationintheroutingtable,whichisalreadythere.TheIGP
thatisusedwithintheASmustcarrythisinformation.
OnewayofmakingtheIGPcarrytheinformationthatisnecessarytoresolvethe
BGPnext-hopaddressesistomakesurethatalltheborderrouters,whichcontain
theEBGPsessions,redistributeconnectedsubnetsintotheIGPusingthe
redistributeconnectedroutingprotocolconfigurationcommand.BecauseEBGP
sessionsareestablishedbetweenroutersusingadirectlyconnectedinterface,the
farendoftheEBGPsessionsisanIPaddresswithinthedirectlyconnectedsubnet.
ByredistributingtheconnectedinterfacesintotheIGP,theborderroutersallownext-
hopreferencestoberesolvablebyallrouterswithintheAS.
ExternalsubnetsthatareredistributedintotheIGPmightappearasexternalIGP
routes,dependingonwhatIGPisconfiguredwithintheAS.Thereareseveral
scalabilityissuesthatareassociatedwithexternalroutesinsomeroutingprotocols.
Forexample,OSPFcarrieseachexternalsubnetinaseparateLSAobject.Ifroute
redistributionisnotdesirableforanyreason,analternativemethodistoincludethe
subnetonwhichtheEBGPsessionisrunningintheIGPconfigurationusingthe
networkcommand.TopreventtheborderrouterfromexchangingIGProutingwith
theborderrouteroftheotherAS,youmustconfiguretheinterfaceasapassive
interface.Failuretodosocouldcausethetwoautonomoussystemstoexchange
routesusingtheIGP.Inthatcase,allbenefitsofhavingseparateautonomous
systemswouldbelost.
Step16
OntheR1router,redistributesubnetfromtheEthernet0/0,facingtotheISP1
router,intoOSPFroutingprotocol.
BetweenroutersintheAS1(R1,R2,R3,andR4)thereisOSPFwithprocess
ID1inthearea0preconfigured.
R1(config)# route-map toISP1 permit 10
R1(config-route-map)# match interface Ethernet 0/0
R1(config-route-map)# exit
R1(config)# router ospf 1
R1(config-router)# redistribute connected subnets route-map toISP1
Youneedtousesubnetskeyword,becausesubnetofthenetwork
172.16.0.0./16isusedbetweenR1andISP1routers.
Step17
OntheR4router,examineBGProute10.0.1.0/28fromAS100again.
R4# show ip bgp 10.0.1.0/28
BGP routing table entry for 10.0.1.0/28, version 10
Paths: (1 available, best #1, table default)
Advertised to update-groups:
1
Refresh Epoch 3
100
172.16.11.11 (metric 20) from 10.0.0.1 (10.0.0.1)
Origin IGP, metric 0, localpref 100, valid, internal, best
Outputshowsthatroute10.0.1.0/28hasvalidnexthop172.16.11.11andisselectedas
bestroute.
R4# show ip route 172.16.11.11
Routing entry for 172.16.11.0/24
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 20
Last update from 192.168.34.3 on Ethernet0/2, 00:10:23 ago
Routing Descriptor Blocks:
192.168.34.3, from 10.0.0.1, 00:10:23 ago, via Ethernet0/2
Route metric is 20, traffic share count is 1
* 192.168.24.2, from 10.0.0.1, 00:10:23 ago, via Ethernet0/3
Route metric is 20, traffic share count is 1
OntheR4router,thereisalsoroutinginformationtoreachthenexthop172.16.11.11.
Step18
OntheR4router,verifywhatBGPprefixesaresenttoexternalBGPpeerISP3B
(172.16.34.34).
R4# show ip bgp neighbors 172.16.34.34 advertised-routes
BGP table version is 15, local router ID is 10.0.0.49
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*>i 10.0.0.0/28 10.0.0.1 0 100 0 i
*>i 10.0.0.16/28 10.0.0.17 0 100 0 i
*>i 10.0.0.32/28 10.0.0.33 0 100 0 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*>i 10.0.1.0/28 172.16.11.11 0 100 0 100 i
*>i 10.0.1.16/28 172.16.11.11 0 100 0 100 i
*>i 10.0.1.32/28 172.16.11.11 0 100 0 100 i
*>i 10.0.1.48/28 172.16.11.11 0 100 0 100 i
*>i 10.0.1.64/28 172.16.11.11 0 100 0 100 i
*>i 10.0.1.80/28 172.16.11.11 0 100 0 100 i
Total number of prefixes 10
YoucanseetherearealsoprefixesfromAS100advertisedtoexternalBGPpeerISP3B.
Step19
OntheISP3Brouter,verifywhatisnexthopIPaddressoftheBGProute
10.0.1.0/28.
ISP3B# show ip bgp 10.0.1.0/28
BGP routing table entry for 10.0.1.0/28, version 10
Paths: (1 available, best #1, table default)
Not advertised to any peer
Refresh Epoch 1
1 100
172.16.34.4 from 172.16.34.4 (10.0.0.49)
Origin IGP, localpref 100, valid, external, best
Outputshowsthatroute10.0.1.0/28hasdifferentnexthopIPaddress
(172.16.34.4),asseenontheR4router(172.16.11.11).WhenBGProuteissent
toexternalBGPpeer,thenext-hopIPaddresschanges.
TransitNetworkUsingEdgeRoutersasNextHops
AnIBGPpeerusuallydoesnotmodifythenext-hopattributewhentheBGPupdate
ispropagatedacrossIBGPsessions.However,youcouldconfiguretheBGProuter
tohaveadifferentbehaviorandsetitsIPaddressasthenext-hopaddresseven
whentheBGPupdatesaresentacrossIBGPsessions(emulatingbehavioron
EBGPsessions).IfyoudoconfigureanIBGProutertoemulatethebehaviorof
EBGPsessionsontheIBGPsessionsoftheborderrouters,theBGPupdatesthat
arereceivedontheEBGPsessionswillbeforwardedontheIBGPsessionsandthe
next-hopattributewillbesettotheIPaddressthatisusedonthelocalsideofthe
IBGPsession.Theoriginalnexthop,setbythefarendoftheEBGPsession,willbe
lost.
Alternate design: Next-hop processing is modified at the edge routers.
Edge routers announce themselves as the next hop in IBGP updates.
No redistribution of external subnets is necessary.
This design might result in suboptimal routing if multiple paths to a
neighboring AS exist.
Use default next-hop processing if at all possible.
ThereceiveroftheIBGPinformationwilldorecursiveroutinginthenormalway.But
thenext-hopaddressthatisusedwillbetheIPaddressofthefarendoftheIBGP
session,becausetheborderrouterhaschangedit.TheIPaddressofthefar-end
IBGPpeerisalwaysknownintheroutingtable;otherwise,theIBGPsessionwould
nothavebeenestablished.ThereisnoneedforthereceiveroftheIBGPinformation
tohaveknowledgeofhowtoreachthefarendoftheEBGPsession,becausethat
IPaddressisnolongersetasthenexthop.
ToconfiguretherouterasthenexthopforaBGP-speakingneighbor,usethe
neighbornext-hop-selfrouterconfigurationcommand.
Step20
OntheR1router,removethepreviouslyconfiguredredistribution.
R1(config)# router ospf 1
R1(config-router)# no redistribute connected
OntheR4router,theBGProute(10.0.1.0/28)nexthopIPaddress172.16.11.11
willagainbecomeinaccessible.
R4# show ip bgp 10.0.1.0/28
BGP routing table entry for 10.0.1.0/28, version 16
Paths: (1 available, no best path)
Not advertised to any peer
Refresh Epoch 3
100
172.16.11.11 (inaccessible) from 10.0.0.1 (10.0.0.1)
Origin IGP, metric 0, localpref 100, valid, internal
Step21
ConfigureR1routertobenexthopforallIBGPspeakingrouters.
TheIBGPspeakingroutersoftheR1routerareR2(10.0.0.33),R3(10.0.0.17),
andR4(10.0.0.49).
R1(config)# router bgp 1
R1(config-router)# neighbor 10.0.0.17 next-hop-self
R1(config-router)# neighbor 10.0.0.33 next-hop-self
R1(config-router)# neighbor 10.0.0.49 next-hop-self
Step22
OntheR4router,examineBGProute10.0.1.0/28fromAS100again.
R4# show ip bgp 10.0.1.0/28
BGP routing table entry for 10.0.1.0/28, version 22
Paths: (1 available, best #1, table default)
Advertised to update-groups:
1
Refresh Epoch 3
100
10.0.0.1 (metric 21) from 10.0.0.1 (10.0.0.1)
Origin IGP, metric 0, localpref 100, valid, internal, best
Outputshowsthatroute10.0.1.0/28hasvalidnexthop10.0.0.1andisselected
asbestroute.Nexthop10.0.0.1isR1Loopback1interface.
R4# show ip route 10.0.0.1
Routing entry for 10.0.0.1/32
Known via "ospf 1", distance 110, metric 21, type intra area
Last update from 192.168.34.3 on Ethernet0/2, 00:46:42 ago
Routing Descriptor Blocks:
192.168.34.3, from 10.0.0.1, 00:46:42 ago, via Ethernet0/2
Route metric is 21, traffic share count is 1
* 192.168.24.2, from 10.0.0.1, 00:46:42 ago, via Ethernet0/3
Route metric is 21, traffic share count is 1
OntheR4router,thereisroutinginformationtoreachthenexthop10.0.0.1.
Step23
OntheISP3Brouter,verifywhatisnexthopIPaddressoftheBGProute
10.0.1.0/28.
ISP3B# show ip bgp 10.0.1.0/28
BGP routing table entry for 10.0.1.0/28, version 22
Paths: (1 available, best #1, table default)
Not advertised to any peer
Refresh Epoch 1
1 100
172.16.34.4 from 172.16.34.4 (10.0.0.49)
Origin IGP, localpref 100, valid, external, best
Theoutputshowsthatroute10.0.1.0/28hasdifferentnexthopIPaddress
(172.16.34.4),asseenontheR4router(10.0.0.1).
Thenext-hopattributeisnormallynotchangedonIBGPupdates.Whentheborder
routerforwardstheincomingEBGPupdateoveranoutgoingIBGPsession,the
borderrouterchangesthenext-hopaddresstotheIPaddressthatisusedasthe
sourceaddressoftheIBGPsession.
ThereceiverofIBGPupdateswillseenext-hopinformationthatindicatesa
destination,whichmightnotbedirectlyconnected.Toresolvethisproblem,itwill
checkitsroutingtableandseeifandhowthenext-hopaddresscanbereached.
ThenitwillrouteIPpacketswithdestinationaddressesthatmatchthenetworkinthe
BGPupdateinthesamedirectionasitwouldhaveroutedanIPpacketwiththe
destinationaddressequaltotheIPaddressinthenext-hopattribute.Inthiscase,it
isobviousthatthenext-hopaddresscanbereached,becausetheIBGPsession
wouldnothavebeenestablishedotherwise.
Inthefigure,ISP1sendsaBGPupdateaboutnetworkX.Becauseitissendinga
BGPupdateoveranEBGPsessiontoR1,thenext-hopattributeissettotheIP
addressthatisusedattheISP1sideoftheEBGPsession,172.16.11.11.R1can
usethisinformationandroutepacketstonetworkXbyforwardingthemtoISP1.
R1alsoforwardstheBGPupdateonallitsIBGPsessions.Itchangesthenext-hop
attributetotheIPaddressofitsownloopbackinterface,soR2,R3,andR4willget
informationthattheycanreachnetworkXbyforwardingpacketsto10.0.0.1.Butthat
addressisnotdirectlyconnected.Therouterswillinspecttheroutingtabletoseeif
andhowtheycanreach10.0.0.1.TheycanthenroutepacketstonetworkXinthe
samedirectionthattheywouldusetoroutepacketsto10.0.0.1.
R4alsoforwardstheBGPupdateaboutnetworkXtoISP3B.Thissessionisan
EBGPsession,whichmeansthatR4willsetthenext-hopattributetoitsownIP
addressthatisusedonthatEBGPsession,172.16.34.4.
TransitNetworkUsingEdgeRoutersasNextHops
Example
BothEBGPandIBGPsessionsforwardBGPupdates;however,theydoitinslightly
differentways.
No BGP attributes are changed in IBGP updates.
Because of BGP split horizon, routes that are learned from an IBGP peer are not
advertised to other IBGP peers.
Local preference is propagated only over IBGP sessions.
EBGP peers are directly connected; IBGP peers are usually distant.
Route selection rules slightly prefer EBGP routes.
HerearethedifferencesbetweenEBGPandIBGPsessionsinforwardingBGP
updates:
TherouterdoesnotchangeBGPattributeswhenanupdateissentacrossan
IBGPsession,unlessnext-hop-selfisconfigured.WhenaBGP-speakingrouter
sendsanupdateacrossanEBGPsession,thenext-hopattributeisalwaysset
andtheASnumberoftherouterisprependedtotheAS-pathattribute.
IBGPusessplithorizontopreventroutinginformationloops.EBGPdoesnot
usesplithorizonandinsteadusestheASpathtodetectloops.Inbothcases,a
routerforwardsonlythebestrouteandneversendsaroutebackonthesession
fromwhichitwasreceived.ButIBGPsplit-horizonrulesalsoprohibitarouter
fromforwardinganyinformationthatisreceivedonanIBGPsessiontoanother
IBGPsession.
IBGPborderroutersremovethelocalpreferenceattributefromaBGProute
beforetheBGPupdateissentoveranEBGPsession.Thisdifferencemeans
thatthelocalpreferenceattributeisdistributedonIBGPsessionsonly.
TworouterswithanEBGPsessionbetweenthemnormallyestablishthesession
usingtheIPaddressesfromacommon,sharedsubnet.Usingtheshared
subnettoestablishthesessionguaranteesthatthetworouterscanexchangeIP
packetswithoutanyIGPrunningbetweenthem.Also,recursiveroutingwill
alwayssucceedbecausethenext-hopaddressisreachableusingadirectly
connectedroute.
IBGPsessionsarenormallyestablishedbetweenallroutersintheASinafull
mesh.ButallroutersinanASmightnothavephysicalconnectionstoevery
otherrouterwithintheAS.BecauseIBGPsessionsareestablishedbetween
routersusingIPaddressesofdifferentsubnets,anIGPmustberunningwithin
theASinordertoestablishIBGPsessions.
BGProuteselectionrulesslightlyfavorEBGProutesoverequivalentIBGP
routes.
DifferencesBetweenEBGPandIBGPSessionsExample
ThisexampleillustratesthepreferenceoftheEBGProute.
Whenever identical routes are received from IBGP and EBGP peers, the route
from the EBGP peer is preferred.
DifferencesBetweenEBGPandIBGPSessions
Oneofthedefaultgoalsoftransitpacketforwardingistopropagatethetransit
packettowardthedownstreamASassoonaspossible.Aborderrouterthatreceives
otherwiseequivalentroutestothesamedestinationoverbothanEBGPsessionand
anIBGPsessionwillprefertheinformationthatisreceivedthroughtheEBGP
session.
EquivalentroutesareroutesthathaveequalBGPpath
attributesusedintheBGProuteselectionrules(weight,
localpreference,AS-pathlength,origin,MED).
Inthefigure,theupperrouterinAS300receivesBGPupdatesaboutnetwork
10.0.0.0/24overtwodifferentpaths.OneupdateisreceivedovertheEBGPsession
toAS1.TheotherupdateisreceivedovertheIBGPsessiontothelowerrouterin
AS300.Allessentialattributesarethesame,sorouteselectioncannotbemade
easily.
TheupperrouterinAS300realizesthatIPpacketswithdestinationaddresses
withinnetwork10.0.0.0/24shouldsoonerratherthanlaterleaveAS300.Itisbetter
tomakethemleavetheASrightaway.Sotheupdatethatwasreceivedonthe
EBGPsessionispreferredovertheupdatethatwasreceivedontheIBGPsession.
TherequirementsthatareassociatedwithIBGP-basedtransitbackbonescanresult
inlimitedscalability.
Transit backbone requires IBGP full mesh between all core routers.
Large number of TCP sessions
Unnecessary, duplicate routing traffic
There are two scalability solutions:
Route reflectors
BGP confederations
IBGPsplit-horizonrulesmandateanIBGPconnectionbetweeneveryborderrouter
andeveryotherBGProuterinanAS.
ThegeneraldesignruleinIBGPdesignistohaveafullmeshofIBGPsessions.But,
afullmeshofIBGPsessionsamongnnumberofrouterswouldrequire(n*(n-1))/
2IBGPsessions.Forexample,afullmeshbetween10routerswouldrequire(10*
9)/2=45IBGPsessions.
BecauseeveryIBGPsessiononarouterusesaseparateTCPsession,anupdate
thatmustbesentbytheroutertoallIBGPpeersmustbesentoneachoftheTCP
sessions.Ifarouterisattachedtotherestofthenetworkoverjustasinglelink,this
singlelinkhastocarryallTCP/IPpacketsforallIBGPsessions.Thissituation
resultsinduplicationoftheupdateoverthesinglelink.
Twosolutionsareavailable:
TheroutereflectorsolutionmodifiestheIBGPsplit-horizonrulesandallowsa
particularroutertoforward(undercertainconditions)incomingIBGPupdatesto
aselectgroupofIBGPneighbors.Therouterperformingthisfunctionisthe
"routereflector."
TheBGPconfederationssolutionintroducestheconceptofanumberofsmaller
autonomoussystemswithintheoriginalAS.Thesesmallerautonomoussystems
exchangeBGPupdatesamongthemselvesusingintraconfederationEBGP
sessions.
ScalabilityLimitationsofIBGP-BasedTransitBackbones
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
All BGP routing updates carry the mandatory well-known attribute AS-path. The
AS-path attribute is not changed when the BGP prefix is propagated across
IBGP sessions.
The IBGP multipath load sharing feature enables the BGP speaking router to
select multiple IBGP paths as the best paths to a destination.
IBGP split horizon prevents routing information loops within the AS.
All BGP routers within an AS must have IBGP sessions with every other BGP
router in the AS.
For recursive routing to work, a router must resolve all possible next-hop
references that use information in the routing table.
You can configure an edge router to set its IP address as the next-hop address
even when the BGP updates are sent across IBGP sessions.
BGP attributes are not changed when an update is sent across an IBGP session
unless next-hop-self is configured.
The full-mesh IBGP requirement in the transit AS creates scalability issues in
the number of TCP sessions and unnecessary, duplicate routing traffic. IBGP
scalability solutions to these issues exist.
Summary
Overview
AtransitASrequiresinteractionbetweenEBGPandIBGPandbetweenIBGPand
anIGPinthetransitAS.Thislessondescribespacketforwardingthroughatransit
ASanddiscussestherequirementsforsuccessfulpacketforwarding,suchas
recursiveroutelookupandanIGPinthetransitAS.Thislessonconcludeswitha
discussionoftheinteractionbetweenIBGPandanIGPrunningwithinthetransitAS.
Uponcompletingthislesson,youwillbeableto:
DescribepacketforwardinginatransitAS
ExplainhowrecursivelookupfunctionsinCiscoIOSsoftware
ExplaintheneedforanIGPinatransitbackbonethatisrunningBGPonall
routers
DescribeinteractionsbetweenBGPandIGPinatransitAS
DescribehowtochangeBGPadministrativedistances
ExplainthepotentialproblemsthatmightarisefromBGPandIGPinteraction
ForwardingPacketsinTransitAS
WhenBGPupdateshavepropagatedthroughthetransitAStoallneighboring
autonomoussystems,theIPtrafficcanstarttoflow.
All core routers need external routers for proper packet forwarding.
Redistributing can overload IGP resources.
IBGP is preferred for scalability.
Inthefigure,ISP3BrouterforwardstoR4IPpacketswiththedestinationaddress
matchinganetworkinAS100.R4checksitsroutingtableandfindsthatthereisa
BGProuteforthatdestination.TheBGProutehasanext-hopreference,which
pointstothefarendoftheEBGPsessionbetweenISP1andR1.SoR4onceagain
checkstheroutingtableandfindsthatitshouldforwardthepackettoR2inthis
case.
Thus,R2receivestheIPpacketwithadestinationaddressindicatingahostwithin
AS100.Tobeabletoforwardthispacket,R2musthaveamatchingrouteinits
routingtable.Adefaultrouteorgatewayoflastresortisnotappropriatebecausein
thenextinstantR2couldreceiveanotherpacket,comingfromtheotherdirection
anddestinedforAS300.
TheconclusionisthatbothR2andR3,tohandleallpossiblecases,musthave
routinginformationtoalltheexternalnetworksthatR1andR4have.Theonly
scalablewayofprovidingrouterswiththisinformationistoupdateR2andR3with
IBGPfrombothR1andR4.
Intheory,theexternalinformationthatR1andR4receivecouldberedistributedby
theseingressroutersintotheIGPinusewithinthetransitAS.However,noIGPcan
handlethevolumeofinformationthatBGPcan.Sotherewouldalwaysbeariskthat
theIGPwouldbreakbecauseofinformationoverload,causingatotalnetwork
meltdownintheAS.ThevolumeofroutinginformationthattheBGPcarriesinthe
contemporaryInternetlongagopassedthelimitsofwhatitispossibletocarryinany
IGP.
Routes that are learned via BGP do not have an outgoing interface associated
with them in the routing table.
Recursive lookup is performed to forward IP packets toward external
destinations.
R2# show ip route 10.0.1.0 255.255.255.240
Routing entry for 10.0.1.0/28
Known via "bgp 1", distance 200, metric 0
Tag 100, type internal
Last update from 10.0.0.1 16:37:59 ago
Routing Descriptor Blocks:
* 10.0.0.1, from 10.0.0.1, 16:37:59 ago
Route metric is 0, traffic share count is 1
AS Hops 1
Route tag 100
MPLS label: none
Observe that the route that is known via BGP has no outgoing interface, but it
has a BGP next hop.
R2# show ip route 10.0.0.1
Routing entry for 10.0.0.1/32
Known via "ospf 1", distance 110, metric 11, type intra area
Last update from 192.168.12.1 on Ethernet0/2, 17:23:10 ago
PacketForwardinginTransitAS
Routing Descriptor Blocks:
* 192.168.12.1, from 10.0.0.1, 17:23:10 ago, via Ethernet0/2
Route metric is 11, traffic share count is 1
The route toward BGP next hop is known via OSPF and has an outgoing
interface.
ABGProuteisinstalledintheIProutingtableofarouteronlyiftheIPaddressinthe
next-hopattributeisreachableaccordingtotheinformationalreadyintherouting
table.TheinstalledBGProutecontainsareferencetothatnext-hopaddress.So,the
networkwillbereachableviaanIPaddress,whichmayormaynotbedirectly
connected.Becausethereisnoclearreferencetoaphysicalinterface,theBGP
routeisinstalledintheIProutingtablewithoutanyinformationaboutoutgoing
interface.
TheroutermustevaluatetherecursivereferencetotheBGPnexthopsooneror
laterinordertoallowpacketforwardingtowardexternaldestinations.Thepointin
timewhentherecursivereferenceisresolveddependsontheIPswitching
mechanismthattherouteruses.Atthelatest,therouterperformstherecursiveroute
lookupwhenanIPpacketwithadestinationaddressthatmatchestheBGProute
shouldbeforwarded.Therouterdetermineswhichoutgoinginterfaceshouldbe
usedandwhichLayer2addresstoassign(ifapplicable).Theroutercreatesacache
entrysothatsuccessiveIPpacketstothesamedestinationcanberoutedusingthe
sameoutgoinginterfaceandLayer2address.
RecursivelookupreferstotheBGProutesforwhichanotherroutinglookupis
requiredinordertoresolvetheBGProutenexthop.
Entries in routing table are built from BGP table.
Outgoing interface is never associated with a BGP route.
ARP cache lookup is performed to build Layer 2 header.
ThefigurepresentsthestepsintherecursivelookupprocessinCiscoIOSsoftware.
TherouterhasreceivedaBGPupdateaboutnetwork10.0.1.0/28.Itwasassociated
withanAS-pathattributesetto100,anext-hopattributesettotheIPaddress
10.0.0.1.Someotherattributeswerealsocarriedwiththeupdate.
Becausethenext-hopaddress10.0.0.1isreachableaccordingtotheroutingtable,
theBGProuteisalsoinstalledintheroutingtable.Networknumber,subnetmask,
andnext-hopattributesareinheritedfromtheBGPtable.Nooutgoinginterfaceis
assigned.
WhenanIPpacketwithadestinationinnetwork10.0.1.0/28isreceived,therouter
searchestheroutingtableandfindstheinstalledBGProute.Theroutertakesthe
indicatednext-hopaddress10.0.0.1andsearchestheroutingtableagain.Itnow
findsamatchwiththeOSPFroutetosubnet10.0.0.0/24.The10.0.0.0/24routehas
anoutgoinginterfacesettointerfaceEthernet0/2andanexthopsetto
192.168.12.1.So,thepacketsthataredestinedfornetwork10.0.1.0/28shouldbe
forwardedvia192.168.12.1,whichisdirectlyreachableoverEthernet0/2.TheARP
tableisusedtofindtheMACaddressforIPaddress192.168.12.1.TheMAC
addressisusedtoforwardtheIPpackettonetwork10.0.1.0/28outtheEthernet0/2
interface.TheMACheaderisstoredinthecacheforsuccessivepacketstonetwork
10.0.1.0/28.
Theexampleillustratestherecursivelookupperformed
whentherouterusescache-basedIPswitching
mechanisms(forexample,fastswitchingoroptimum
switching).
Traditional Cisco IOS software switching mechanisms perform recursive lookup
when forwarding the first packet.
Fast switching, optimum switching.
CEF precomputes the routing table.
All recursive lookups are performed while the routing table is built.
TraditionalCiscoIOSswitchingmechanismsusedthetraffic-driven,cache-based
switchingapproach.BothfastswitchingandoptimumswitchingpopulatetheIP
switchingcacheondemand,meaningthatbeforeanyIPpacketsareforwarded,the
cacheisempty.Afterthefirstpackettoaspecificdestinationarrives,allroutingtable
lookupsaredone,includingrecursivelookupinthecaseofaBGProute.Theresult
ofthelookupiscachedforlaterusewhensuccessivepacketsforthesame
destinationarrive.Theprocessisrepeatedforeveryspecificdestination.
CiscoExpressForwarding(CEF)prebuildsacompleteIPforwardingtable,called
theFIB,thatisbasedontheIProutingtable.Aftertherouterinstallsaroutingentry
intoitsroutingtable,incomingroutinginformationupdatestriggertherecursive
lookup.Withtherecursivelookup,theoutgoinginterfaceandtheactualphysicalnext
hopoftheroutearedetermined.MACaddressresolutionandMACheader
RecursiveLookupinCiscoIOSSoftware
generationarestilltraffic-drivenandstoredinthecache.
Somenetworkdesignersbasetheirnetworkdesignonthewrongassumptionthatan
internalroutingprotocolisnotneededinatransitASwhereallroutersrunBGP.
However,theinternalroutingprotocolisstillneededinsideanASfortworeasons:
ToprovideroutinginformationthatisneededtoestablishtheIBGPsessions.
Toresolvenext-hopreferences(recursiverouting).
With BGP running on all core routers, is an IGP still needed in the core? An IGP is
needed to resolve BGP next hops and perform fast convergence after a failure in the
core network.
WhenR4inthefigurereceivesanIPpacketwiththedestinationinAS100,itdoesa
recursivelookuptofindtheoutgoinginterfacetobeusedforpacketforwarding.It
performstherecursivelookupthatisbasedontheIGPinformation.Ifthereis
suddenlyaninternalproblemwithinAS1,andthenext-hopaddressisreachablea
differentway,theIGPdeterminesthisfact.TherouterchangestheIGProutetothe
next-hopnetworkbecauseofthenewlyreceivedIGProuteinformation,andall
cacheentriesthatrelyontheoldinformationareinvalidated.Thenextrecursive
lookupthatR4performswillindicateadifferentoutgoinginterfacethanbeforethe
problemoccurred.
DuringtheIGPconvergenceprocess,theBGProutingisnotaffected.Theonly
routingupdatesthatareexchangedduringthetransitASconvergenceareIGP
updatesdescribinghowtoreachinternaldestinations(includingthefarendsofthe
EBGPsessions).
Thepacketforwardingtoexternaldestinationsthusbenefitsfromthehigh-speed
convergencethattheIGPoffers.ThefastertheIGPdeterminesthatitshouldusean
alternatepathwithintheAStoreachthenext-hopaddress,thefasteritwillre-
establishIPconnectivitytowardexternaldestinations.
TheconclusionisthatanIGPisstillneededinsideatransitAS,andthenetworkwill
workbetterifitisanIGPwithfastconvergence.
Core routers need to run BGP and an IGP.
BGP carries all external routes.
The IGP propagates BGP next hops and other core subnets only.
All customer routes are also carried in BGP.
Reduces IGP topology database.
Removes customer-caused route flaps from IGP; IGP becomes more stable.
BothBGPandtheconfiguredIGPshouldbeconfiguredonallcoreroutersinsidethe
transitAS.TheIGPshouldcarryaslittleinformationaspossible.Ideally,this
informationincludesonlythelinkswithinthecorenetwork,theloopbackinterfaces,
andtheexternalsubnetsthatareusedinEBGPsessionswiththeneighboring
autonomoussystems.ThisinformationisenoughtoestablishIBGPsessionsand
resolvenext-hopaddresses.TheIGPwillalsoworkbetterifitcarrieslessrouting
information.
NoroutesexternaltothetransitASshouldeverberedistributedbyanyrouterfrom
BGPintotheIGP.AllexternalroutesshouldbeinBGPonly.
Inautonomoussystemsthatprovidecustomerconnectivity(notonlytransitservice),
itisalsohighlyrecommendedthatthecustomernetworksbecarriedinBGP.This
RoutingProtocolsinTransitAS
approachreducestheamountofinformationintheIGPandincreasesIGPstability.
Ideally,BGPandtheIGPcarrytwodifferentsetsofroutinginformation.BGPcarries
theroutesthatarereceivedfromotherautonomoussystemsandtheroutesthat
belongtothelocalASandshouldbeannouncedtootherautonomoussystems.The
IGPcarriesonlyenoughinformationtoestablishIBGPsessionsandresolvethe
EBGPnext-hopaddressesviatheIGProutingtables.
Ideally, there will be no interaction between BGP and the IGP.
BGP carries external and customer routes.
The IGP carries only core subnets.
External route flaps do not affect IGP.
Failures internal to the network do not affect BGP as long as the BGP next hop
remains reachable.
The only link between BGP and the IGP should be the recursive lookup.
TheIGPwillprovidereachabilitytowardtheBGPnext-hopaddressesonlyifitisnot
disturbedbyexternalupdatesfromotherautonomoussystems.
BGPshouldtakecareoftheexternalinformation.AslongastheIGPfindsausable
waytotheBGPnexthops,theBGPdoesnotneedtodoanyrecalculationbecause
ofinternalproblemswithintheAS.
Sometimes, BGP and the IGP will propagate the same route.
Usually stems from bad network design.
In this case, routes are determined in EBGP/IGP/IBGP order based on
administrative distances of the routes.
Routing Protocol Default Administrative Distance
RoutingProtocol DefaultAdministrativeDistance
EBGP 20
IGP 90—170
IBGP 200
SometimestheinteractionbetweenBGPandtheIGPisnotideal,foranumberof
reasons,includingbadnetworkdesign.Intheworstcase,thesamenetworksmight
becarriedinboththeIGPandBGP.Forexample,thesubnetsconnectingtheAS
withneighboringautonomoussystemshavetobeannouncedviatheIGPtoenable
next-hopresolution.ThesesubnetsmayalsobeannouncedviaBGPbytheremote
ASorthelocalAS.Inanycase,informationaboutthesameIPprefixwillappearin
boththeIGPandtheBGPdatastructures.
Whentherouterinstallsroutinginformationintotheroutingtable,itcheckstosee
whetherthereareseveralsourcesofinformationforaparticularIPprefix.Ifso,the
routerinstallstheinformationthatitdeterminesismostreliable.TheADdetermines
whichsourcetouse.
BGPconsidersbothEBGPandIBGProutesintheBGPselectionprocess.BGPwill
thereforenevertrytoinstallbothanEBGProuteandanIBGProuteforthesame
destination.ComparisonbetweenADswillthusoccuronlywhentwodifferent
protocolscarrythesamedestinationnetwork.
IfBGPselectsanEBGProuteasthebestrouteforagivendestinationnetwork,it
willtrytoinstallthatroutewithaverylowAD.So,theroutesthatarelearnedvia
EBGPhaveahighlikelihoodofbeinginstalledintheroutingtable.
IfBGPselectsanIBGProuteasthebest,itwilltrytoinstallitwithahighAD.Thus,
theroutesthatarelearnedviaIBGPhavealowlikelihoodofbeinginstalledinthe
routingtable.
AllIGPs,suchasEIGRP,OSPF,IS-IS,andsoon,haveamediumlikelihoodof
beinginstalled.TheADsforIGPsfallbetweentheADsofEBGPandIBGP.
ThereasonforgivingEBGPalowdefaultADisbecause
EBGPindicatesroutesexternaltothelocalAS.IPpackets
withdestinationaddressestothosenetworksshouldleave
theASsoonerratherthanlater.Itis,inmostcases,better
thattheyleavetheASrightaway.
BGPandIGPInteraction
Overview
Throughthisdiscovery,youwilllearnhowtochangeBGPadministrativedistances.
Discovery6:BGPAdministrativeDistance
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
ISP1 Ethernet0/0 172.16.11.11/24 ConnectiontoR1
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
R1 Ethernet0/0 172.16.11.1/24 ConnectiontoISP1
R1 Ethernet0/2 192.168.12.1/24 ConnectiontoR2
R1 Ethernet0/3 192.168.13.1/24 ConnectiontoR3
R1 Loopback1 10.0.0.1/28 Loopbacksimulates
LANnetwork
R2 Ethernet0/2 192.168.12.2/24 ConnectiontoR1
R2 Ethernet0/3 192.168.24.2/24 ConnectiontoR4
R2 Loopback1 10.0.0.33/28 Loopbacksimulates
LANnetwork
R3 Ethernet0/2 192.168.34.3/24 ConnectiontoR4
R3 Ethernet0/3 192.168.13.3/24 ConnectiontoR1
R3 Loopback1 10.0.0.17/28 Loopbacksimulates
LANnetwork
R4 Ethernet0/0 172.16.34.4/24 ConnectiontoISP3B
R4 Ethernet0/2 192.168.34.4/24 ConnectiontoR3
R4 Ethernet0/3 192.168.24.4/24 ConnectiontoR2
R4 Loopback1 10.0.0.49/28 Loopbacksimulates
LANnetwork
ISP3B Ethernet0/0 172.16.34.34/24 ConnectiontoR4
ISP3B Loopback1
Loopback2
Loopback3
Loopback4
10.0.3.1/28
10.0.3.17/28
10.0.3.33/28
10.0.3.49/28
Loopbackssimulate
LANnetworks
IPaddressesandIGParepreconfiguredasshowninthetopologybelow:
JobAids
BGPisalsoconfiguredasEBGP(ISP1toR1andISP3BtoR4)andasIBGP(full
meshbetweenR1,R2,R3,andR4).
Step1
OntheR1router,verifywhatareBGPadministrativedistances.
R1# show ip protocols
*** IP Routing is NSF aware ***
Routing Protocol is "bgp 1"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
IGP synchronization is disabled
Automatic route summarization is disabled
Neighbor(s):
Address FiltIn FiltOut DistIn DistOut Weight RouteMap
10.0.0.17
10.0.0.33
10.0.0.49
172.16.11.11
Maximum path: 1
Routing Information Sources:
Gateway Distance Last Update
10.0.0.17 200 00:04:14
10.0.0.33 200 00:04:33
10.0.0.49 200 00:04:09
172.16.11.11 20 00:04:33
Distance: external 20 internal 200 local 200
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 10.0.0.1
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Maximum path: 4
Routing for Networks:
10.0.0.0 0.0.0.255 area 0
192.168.0.0 0.0.255.255 area 0
Routing Information Sources:
Gateway Distance Last Update
10.0.0.17 110 00:09:56
10.0.0.33 110 00:10:11
10.0.0.49 110 00:09:32
Distance: (default is 110)
TheBGPexternalADis20,internalandlocalADare200.Youcanalsolearn
OSPFADvalue.
ChangingAdministrativeDistanceofBGPRoutes
YoucanchangeBGPadministrativedistances,toallowtheuseofBGProute,that
couldbeabetterroutetoanode.
router(config-router)# distance bgp external-distance internal-
distance local-distance
This command sets the AD for EBGP, IBGP, and local routes.
This change applies only to routes received after the command has been
entered (similar to filters).
Defaults: EBGP routes have a distance of 20; IBGP and local routes have a
distance of 200.
The defaults are usually correct; do not change them.
Step2
OntheR1router,changeEBGPADinto100andIBGPADinto190.
LeavedefaultBGPlocalAD.
R1(config)# router bgp 1
R1(config-router)# distance bgp 100 190 200
Step3
BGPAdministrativeDistance
DiscoverySteps
OntheR1router,verifywhatareBGPadministrativedistancesagain.
R1# show ip protocols
*** IP Routing is NSF aware ***
Routing Protocol is "bgp 1"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
IGP synchronization is disabled
Automatic route summarization is disabled
Neighbor(s):
Address FiltIn FiltOut DistIn DistOut Weight RouteMap
10.0.0.17
10.0.0.33
10.0.0.49
172.16.11.11
Maximum path: 1
Routing Information Sources:
Gateway Distance Last Update
10.0.0.17 200 00:19:41
10.0.0.33 200 00:20:00
10.0.0.49 200 00:19:35
172.16.11.11 20 00:19:59
Distance: external 100 internal 190 local 200
<... output omitted ...>
TheBGPEBGPandIBGPADschanged.
Step4
OntheR1router,verifyIProutingtable.
R1# show 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
10.0.0.0/8 is variably subnetted, 18 subnets, 2 masks
C 10.0.0.0/28 is directly connected, Loopback1
L 10.0.0.1/32 is directly connected, Loopback1
B 10.0.0.16/28 [200/0] via 10.0.0.17, 01:21:20
O 10.0.0.17/32 [110/11] via 192.168.13.3, 01:22:35, Ethernet0/3
B 10.0.0.32/28 [200/0] via 10.0.0.33, 01:21:39
O 10.0.0.33/32 [110/11] via 192.168.12.2, 01:22:50, Ethernet0/2
B 10.0.0.48/28 [200/0] via 10.0.0.49, 01:21:14
O 10.0.0.49/32 [110/21] via 192.168.13.3, 01:22:11, Ethernet0/3
[110/21] via 192.168.12.2, 01:22:11, Ethernet0/2
B 10.0.1.0/28 [20/0] via 172.16.11.11, 01:21:39
B 10.0.1.16/28 [20/0] via 172.16.11.11, 01:21:39
B 10.0.1.32/28 [20/0] via 172.16.11.11, 01:21:39
B 10.0.1.48/28 [20/0] via 172.16.11.11, 01:21:39
B 10.0.1.64/28 [20/0] via 172.16.11.11, 01:21:39
B 10.0.1.80/28 [20/0] via 172.16.11.11, 01:21:39
B 10.0.3.0/28 [200/0] via 10.0.0.49, 01:21:14
B 10.0.3.16/28 [200/0] via 10.0.0.49, 01:21:14
B 10.0.3.32/28 [200/0] via 10.0.0.49, 01:21:14
B 10.0.3.48/28 [200/0] via 10.0.0.49, 01:21:14
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.16.11.0/24 is directly connected, Ethernet0/0
L 172.16.11.1/32 is directly connected, Ethernet0/0
192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.12.0/24 is directly connected, Ethernet0/2
L 192.168.12.1/32 is directly connected, Ethernet0/2
192.168.13.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.13.0/24 is directly connected, Ethernet0/3
L 192.168.13.1/32 is directly connected, Ethernet0/3
O 192.168.24.0/24 [110/20] via 192.168.12.2, 01:22:21, Ethernet0/2
O 192.168.34.0/24 [110/20] via 192.168.13.3, 01:22:21, Ethernet0/3
YoucanstillseeBGProutesareusingdefaultADvalues.
Step5
OntheR1router,clearBGPsessions.
R1# clear ip bgp *
R1#
%BGP-5-ADJCHANGE: neighbor 10.0.0.17 Down User reset
%BGP_SESSION-5-
ADJCHANGE: neighbor 10.0.0.17 IPv4 Unicast topology base removed from session User reset
%BGP-5-ADJCHANGE: neighbor 10.0.0.33 Down User reset
%BGP_SESSION-5-
ADJCHANGE: neighbor 10.0.0.33 IPv4 Unicast topology base removed from session User reset
%BGP-5-ADJCHANGE: neighbor 10.0.0.49 Down User reset
%BGP_SESSION-5-
ADJCHANGE: neighbor 10.0.0.49 IPv4 Unicast topology base removed from session User reset
%BGP-5-ADJCHANGE: neighbor 172.16.11.11 Down User reset
%BGP_SESSION-5-
ADJCHANGE: neighbor 172.16.11.11 IPv4 Unicast topology base removed from session User reset
%BGP-5-ADJCHANGE: neighbor 10.0.0.33 Up
%BGP-5-ADJCHANGE: neighbor 10.0.0.17 Up
%BGP-5-ADJCHANGE: neighbor 172.16.11.11 Up
%BGP-5-ADJCHANGE: neighbor 10.0.0.49 Up
AfterallBGPneighborswillcome"Up,"proceedtothenextstep.
Step6
OntheR1router,verifyIProutingtableagain.
R1# show 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
10.0.0.0/8 is variably subnetted, 18 subnets, 2 masks
C 10.0.0.0/28 is directly connected, Loopback1
L 10.0.0.1/32 is directly connected, Loopback1
B 10.0.0.16/28 [190/0] via 10.0.0.17, 00:01:28
O 10.0.0.17/32 [110/11] via 192.168.13.3, 01:29:04, Ethernet0/3
B 10.0.0.32/28 [190/0] via 10.0.0.33, 00:01:28
O 10.0.0.33/32 [110/11] via 192.168.12.2, 01:29:19, Ethernet0/2
B 10.0.0.48/28 [190/0] via 10.0.0.49, 00:01:28
O 10.0.0.49/32 [110/21] via 192.168.13.3, 01:28:40, Ethernet0/3
[110/21] via 192.168.12.2, 01:28:40, Ethernet0/2
B 10.0.1.0/28 [100/0] via 172.16.11.11, 00:01:28
B 10.0.1.16/28 [100/0] via 172.16.11.11, 00:01:28
B 10.0.1.32/28 [100/0] via 172.16.11.11, 00:01:28
B 10.0.1.48/28 [100/0] via 172.16.11.11, 00:01:28
B 10.0.1.64/28 [100/0] via 172.16.11.11, 00:01:28
B 10.0.1.80/28 [100/0] via 172.16.11.11, 00:01:28
B 10.0.3.0/28 [190/0] via 10.0.0.49, 00:01:28
B 10.0.3.16/28 [190/0] via 10.0.0.49, 00:01:28
B 10.0.3.32/28 [190/0] via 10.0.0.49, 00:01:28
B 10.0.3.48/28 [190/0] via 10.0.0.49, 00:01:28
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.16.11.0/24 is directly connected, Ethernet0/0
L 172.16.11.1/32 is directly connected, Ethernet0/0
192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.12.0/24 is directly connected, Ethernet0/2
L 192.168.12.1/32 is directly connected, Ethernet0/2
192.168.13.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.13.0/24 is directly connected, Ethernet0/3
L 192.168.13.1/32 is directly connected, Ethernet0/3
O 192.168.24.0/24 [110/20] via 192.168.12.2, 01:28:50, Ethernet0/2
O 192.168.34.0/24 [110/20] via 192.168.13.3, 01:28:50, Ethernet0/3
Now,youcanseethatBGProutesADvalueshavechangedperconfiguration.
ThereareseveralpotentialproblemsthatmightarisefromBGPandIGPinteraction.
If an IGP route is learned through EBGP, the EBGP route will take precedence.
Potential causes include bad network design, routing problems, or denial-of-
service attack.
Protect IGP routes with inbound prefix-list filters at AS edges.
Routers should never accept information about local subnets from an external
source.
IfroutinginformationaboutthesameIPprefixislearnedviabothEBGPandanIGP,
therouterwillusetheEBGPinformation.IfanexternalASisfeedingthelocalAS
withEBGProutesthatactuallyshouldbelocal,routerswithintheASwillerroneously
forwardIPpacketsthataredestinedtothoselocalnetworksoutofthelocalAS.
Thereareseveralpotentialreasonsforthisbehavior;themostcommonisthatthe
remoteASisimproperlyconfiguredorthereisaDoSattack.ToprotectalocalAS
fromthisundesiredbehavior,networkadministratorsshouldinstallinboundfilterson
allEBGPsessionstofilterincomingroutesandrejectroutinginformationabout
networksthatareactuallylocaltotheAS.
ProblemswithBGPandIGPInteraction
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
All core routers need external routers for proper packet forwarding.
A recursive lookup is performed in BGP to resolve the forwarding path reference
of the next-hop attribute.
Packet forwarding to external destinations benefits from the high-speed
convergence that IGP offers.
The IGP should provide reachability toward BGP next-hop addresses only.
IP packets could be erroneously forwarded out of the local AS if an external AS
accidentally (or by intent: DoS) feeds the local AS with EBGP routes that should
be local.
Summary
Overview
IntroductionofatransitbackboneintoaBGPnetworkcancreateunique
troubleshootingchallenges.ThislessonintroducesIBGPmonitoringcommandsand
troubleshootingtechniquesforsolvingthemostcommonIBGPproblemsthatyou
mightencounterinatransitbackbone.CommonproblemswithIBGP,asdiscussed
inthislesson,occurwhenIBGPsessionsdonotreachtheEstablishedstate,when
routinginformationthatisreceivedviaIBGPisneverselected,andwhenthebest
BGProuteisneverinstalledintheroutingtable.
Uponcompletingthislesson,youwillbeableto:
IdentifytheCiscoIOScommandsthatarerequiredtomonitorIBGPoperation
DescribecommonIBGPconfigurationproblems
ExplainhowtotroubleshootIBGPsessionstartupissues
ExplainhowtotroubleshootIBGProuteselectionissues
ExplainhowtotroubleshootIBGPsynchronizationissues
MonitoringandTroubleshootingIBGPin
TransitAS
TodisplayinformationabouttheTCPandBGPconnectionstoneighbors,usethe
showipbgpneighborsEXECcommand.TodisplayentriesintheBGProuting
table,usetheshowipbgpEXECcommand.
router> show ip bgp neighbors
Displays whether a neighbor is an IBGP neighbor
router> show ip bgp
Uses a special marker (i) for IBGP routes
router> show ip bgp prefix
Displays whether the prefix is an IBGP route
MonitoringIBGPExample
ThisexampleshowsthecommandstomonitorIBGP.
R1# show ip bgp neighbors 10.0.0.49
BGP neighbor is 10.0.0.49, remote AS 1, internal link
BGP version 4, remote router ID 10.0.0.49
BGP state = Established, up for 00:47:57
Last read 00:00:00, last write 00:00:37, hold time is 180, keepalive int
erval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable (disabled)
<... output omitted ...>
Internal BGP neighbor
R1# show ip bgp neighbors 172.16.11.11
BGP neighbor is 172.16.11.11, remote AS 100, external link
BGP version 4, remote router ID 10.0.1.81
BGP state = Established, up for 00:49:24
Last read 00:00:37, last write 00:00:51, hold time is 180, keepalive int
erval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable (disabled)
<... output omitted ...>
External BGP neighbor
Theshowipbgpneighborscommanddisplayswhetherarouterisrunningan
IBGP(internal)orEBGP(external)sessionwithaBGPneighbor.The"internallink"
or"externallink"phrasesprovideyouwiththisinformation.
R1# show ip bgp 10.0.3.0
BGP routing table entry for 10.0.3.0/28, version 12
Paths: (1 available, best #1, table default)
Advertised to update-groups:
2
Refresh Epoch 1
300
10.0.0.49 (metric 21) from 10.0.0.49 (10.0.0.49)
Origin IGP, metric 0, localpref 100, valid, internal, best
Route that is received from internal BGP neighbor.
R1# show ip bgp 10.0.1.0
BGP routing table entry for 10.0.1.0/28, version 4
Paths: (1 available, best #1, table default)
Advertised to update-groups:
1
Refresh Epoch 1
100
172.16.11.11 from 172.16.11.11 (10.0.1.81)
Origin IGP, metric 0, localpref 100, valid, external, best
Route that is received from external BGP neighbor.
MonitoringIBGP
TheshowipbgpprefixcommanddisplayswhetheraBGProutewasreceivedfrom
anIBGP(internal)orEBGP(external)neighbor.Theword"internal"or"external"
providesyouwiththisinformation.
TroubleshootingtheBGPconfigurationofatransitAScanbecumbersome,because
thereareanumberofcommonpitfallsthatyoumightencounter.
Three of the most common problems are the following:
IBGP sessions do not start (do not reach the Established state).
IBGP route is in the BGP table but is not selected.
IBGP route is selected but is not entered in the routing table.
CommonIBGPProblems
ThereareseveralapproachestoIBGPsessionstartupissuestroubleshooting.
Symptom:
IBGP session does not start.
Diagnosis:
IBGP session is run between loopbacks, and update-source keyword is
missing.
Verification:
Use debug ip tcp transactions. You should see BGP sessions coming from
unexpected IP addresses.
AcommonmistakewhenyouareconfiguringIBGPsessionsistoforgetthe
neighborupdate-sourceconfigurationcommand.
WhenyouareconfiguringIBGPneighborsontherouter,itiseasytorememberto
makeacorrectreferencetotheloopbackinterfaceoftheremoterouter.Butitis
equallyimportanttomakesurethatthecorrectsourceIPaddressoftheoutgoing
TCPsessionisset.Thepeerrouterwillnotacceptthesessioniftheincoming
sourceaddressdoesnotmatchthepeerrouterlistofIBGPneighbors.
Toverifythatthisissueiscausingtheproblem,usethedebugiptcptransactions
command.Theoutputofthedebugiptcptransactionscommandshoulddisplay
TCPSYNpacketscomingfromunexpectedIPaddressesonthereceivingrouter
andTCPsessionsbeingresetwithTCPRSTpacketsonthesending
(misconfigured)router.
Symptom:
IBGP session does not start.
Diagnosis:
Loopback interfaces are not reachable.
Verification:
Do extended ping between loopback addresses to verify reachability.
AnIBGPsessionbetweentworouterscanbeestablishedfromtheloopback
interfaceofoneroutertotheloopbackinterfaceoftheotherrouteronlyifthetwo
routerscanexchangeIPpacketsusingthoseaddressesassourceanddestination.
ThisexchangeispossibleonlyiftheIGPcarriesthesubnetsthatareassignedto
eachoftheloopbackinterfaces.
Whenyouareverifyingreachabilitywiththepingcommand,makesurethattheping
packetsaresourcedfromtheloopbackinterface.Useanextendedpingand
explicitlyrefertotheIPaddressoftheloopbackinterfacetoensurethatpacketsare
sourcedfromtheloopbackinterface.
Symptom:
IBGP session does not start.
Diagnosis:
Packet filters prevent establishment of BGP sessions.
Verification:
Use debug ip tcp transactions and debug ip icmp to see whether the initial
TCP SYN packets are rejected.
PacketfilterscanstoptheBGPsessions.ThepathbetweenthetwoBGPpeer
routersmustbefreefromfiltersblockingtheBGPtraffic.
BGPrunsonthewell-knownTCPport179.Bothrouterswillmakeconnection
attemptstothatdestinationport.Theywilluseahigh-numberedTCPportassource.
Itisenoughthatoneoftheconnectionattemptssucceeds.Butforbetter
performanceduringrecoveryfromnetworkfailure,bothattemptsshouldhavethe
possibilityofsucceeding.Ifbothattemptsdosucceed,oneoftheconnectionswillbe
broughtdown.
TroubleshootingIBGPSessionStartupIssues
AroutercanuseaBGPupdatetoreachnetworkdestinationsonlyifthenext-hop
addressspecifiedintheBGPupdateisreachable.ABGPupdatethatreferstoa
nexthopthatiscurrentlynotreachableaccordingtotheroutingtablewillbesavedin
theBGPtable.However,theroutercannotinstallthisupdateinitsroutingtable.If
thenext-hopaddresslaterbecomesreachable,theBGProutewillbecomea
candidateroutethattheroutercoulduseforpacketforwardingtothatdestination.
Symptom:
An IBGP route is in the BGP table but is never selected as the best route.
Diagnosis:
The BGP next hop is not reachable.
Verification:
Use show ip bgp prefix to find the BGP next hop.
Use show ip route to verify next-hop reachability.
Toverifythenext-hopreachability,checktheBGProuteintheBGPtablebyusing
theshowipbgpprefixcommand.Thenexthopisreferredtoas"inaccessible"ifitis
notcurrentlyreachableaccordingtotheroutingtable.
AcommonmistakeistoforgettolettheIGPannouncethereachabilityofsubnets
thatphysicallyconnectthelocalASwithaneighboringAS.Therouterusesthese
subnetstoestablishtheEBGPsession.Thenexthopthatisreceivedinanincoming
BGPupdatewillbethefarendoftheEBGPsession.IfallroutersinthelocalASdo
nothaveapathtothatsubnet,thenext-hopaddresswillbeinaccessible.
YoucanpreventthisproblembyincludingthesubnetthatlinksthetransitASto
neighboringautonomoussystemsintheIGP.Toperformthisaction,useeitherthe
redistributeconnectedcommandorthenetworkandpassive-interface
configurationcommands.
TroubleshootingIBGPRouteSelectionIssues
InoldBGPdesigns,redistributionbetweenBGPandanIGPwasacommon
practice,andtheseprotocolshadtobesynchronizedtoensureproperpacket
forwarding.Inmoderndesigns,redistributionisnolongerusedandsynchronization
hastobeturnedoff.Thedefaultvalueisdisabledsynchronization.
Symptom:
An IBGP route is selected as the best route but not entered into the IP routing
table.
Diagnosis:
BGP synchronization is not disabled.
Verification:
Disable BGP synchronization, clear the BGP sessions, and re-examine the IP
routing table after the BGP table becomes stable.
RouterswithBGPsynchronizationenabledwillnotbeabletoinstallIBGProutesin
theroutingtableorpropagatethemtootherEBGPneighbors.
YoucanfixthisproblembyconfiguringnosynchronizationintherouterBGP
configuration.
TroubleshootingIBGPSynchronizationIssues
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
You can use the show ip bgp neighbors and show ip bgp prefix commands
to monitor IBGP operation.
There are a number of problems that can occur during IBGP session startup.
You can use debug ip tcp transactions to see BGP sessions coming from
unexpected IP addresses.
It is important to include the subnet linking the transit AS to an external AS in the
IGP to prevent the BGP next hop from being unreachable.
Routers with BGP synchronization enabled will not be able to install IBGP routes
in the routing table or propagate them to other EBGP neighbors. The BGP
synchronization is disabled by default.
Summary
Overview
Thistopicsummarizesthekeypointsthatwerediscussedinthismodule.
Because all transit autonomous systems are required to carry traffic originating
from or destined to locations outside of that AS, a degree of interaction and
coordination between BGP and the IGP is necessary. You should take a special
care to ensure consistency of routing information throughout the AS.
Both EBGP and IBGP sessions forward BGP updates, but they do it in slightly
different ways.
An IGP is still needed inside a transit AS. The high-speed convergence offered
by an IGP helps in the packet forwarding to external destinations.
Configuring in a transit AS involves configuring IBGP neighbors, BGP
synchronization, and IBGP sessions between loopback interfaces.
The three common IBGP configuration problems concern session startup, route
selection, and synchronization, and there are specific commands and
procedures that you can use to solve those problems.
ModuleSummary
Foradditionalinformation,refertotheseresources:
CiscoSystems,Inc.BGPCaseStudies
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/26634-bgp-toc.html
CiscoSystems,Inc.UsingtheBorderGatewayProtocolforInterdomainRouting
http://docwiki.cisco.com/wiki/Internetworking_Case_Studies
CiscoSystems,Inc.TroubleshootingTCP/IP."TroubleshootingIPConnectivityandRouting
Problems."
http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1907.html#wp1021148
CiscoSystems,Inc.ConfiguringBGP.
http://www.cisco.com/c/en/us/td/docs/ios/12_2/ip/configuration/guide/fipr_c/1cfbgp.html
CiscoSystems,Inc.CiscoIOSIPRoutingConfigurationGuide,Release15M&T
http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/15-mt/irg-15-mt-
book.html
References
Usethequestionsheretoreviewwhatyoulearnedinthismodule.Thecorrect
answersandsolutionsarefoundintheModuleSelf-CheckAnswerKey.
ModuleSelf-Check
1. WhyisIBGPamandatorycomponentofatransitAS?(Source:
"WorkingwithaTransitAS")
ItistheonlyfeasiblewaytoensurethatallroutersintheAShave
consistentexternalroutinginformation.
IteliminatesthescalabilityissuesofrunninganIGPwithinthetransitAS.
RunningIBGPonallroutersistheonlywaytosatisfythefiltering
requirementsofthetransitAS.
AnIGPisnotcapableofhandlingthepotentialroutingloopsinthetransit
AS.
1. IBGPneighborsmustbedirectlyconnected.Trueorfalse?(Source:
"WorkingwithaTransitAS")
true
false
1. WhyisredistributingBGProutesintoanIGPforuseinatransit
backboneNOTrecommended?(Source:"WorkingwithaTransit
AS")
RedistributionremovesallBGPattributesthatareneededtoensure
optimalroutingwithinthetransitAS.
AnIGPcannotenforcecomplexadministrativepoliciesandrouteselection
rules.
IGPscannotscaletothedemandsthatarepresentedbythenumberof
routesontheInternet.
IGPsarenotstablewhenfacedwithaflappingnetwork.
1. HowisEBGPusedinatransitAS?(Source:"WorkingwithaTransit
AS")
asameansoftransportingcustomerroutesacrossthetransitbackbone
toexchangeroutesbetweendifferentautonomoussystemsandthetransit
AS
toenhancescalabilitybytransportingIGProutesforthetransitAS
asameansofinjectinglocalroutesintothetransitbackbone
1. ThereceivedEBGPinformationhastobetranslatedandthen
forwardedtootherBGP-speakingrouterswithinthetransitAS.True
orfalse?(Source:"WorkingwithaTransitAS")
true
false
1. AllrouterswithinatransitASmusthaveallexternalroutes,received
fromanyneighboringAS.Trueorfalse?(Source:"Workingwitha
TransitAS")
true
false
1. WhatarethetwokeyfunctionsofatransitAS?(Choosetwo.)
(Source:"WorkingwithaTransitAS")
tofilteroutroutesthatdonotbelongtocustomersoftheserviceprovider
toprovideInternetconnectivitytocustomersoftheserviceprovider
topropagateroutesbetweenremoteautonomoussystems
toroutepacketsbetweenremotenetworks
1. HowareBGProutessentacrossthetransitbackbone?(Source:
"WorkingwithaTransitAS")
byredistributingBGPintoanIGPandthenbackintoBGP
byusingIBGP
byestablishingEBGPsessionsbetweenallroutersinthetransitbackbone
byredistributingconnectedroutesattheedgeofthetransitbackbone
1. WhichtwostatementsaretrueregardingtheAS-pathattributeasit
relatestoIBGP?(Choosetwo.)(Source:"InteractingwithIBGPand
EBGPinaTransitAS")
EachrouterintheASappendsitsASnumbertotheASpathonoutgoing
BGPupdates.
TheASpathinsideanASwillbeemptyforroutesoriginatinginsidea
neighboringAS.
TheAS-pathattributeisnotusedtodetectroutingloopsinsideanAS.
TheAS-pathattributeisnotmodifiedwithintheAS.
1. WhatistheprimaryfunctionoftheIBGPmultipathload-sharing
feature?(Source:"InteractingwithIBGPandEBGPinaTransitAS")
tochooseoneIBGPpathasthebestpathtoadestination
tochoosemultipleIBGPpathsasthebestpathstoadestination
todesignateonepathasthebestpathandadvertisethisbestpathtoits
neighbors
toenablearoutertohandleallthetrafficdestinedforaparticularsite
1. WhichofthefollowingstatementsaboutaBGPsplithorizonis
accurate?(Source:"InteractingwithIBGPandEBGPinaTransit
AS")
BecauseoftheBGPsplithorizon,anyroutercanrelayIBGPinformation
withintheAS.
BGPsplithorizonisusefulonlywithinautonomoussystems.
RoutinginformationloopswithintheASarepreventedbyanIBGPsplit
horizon.
WithaBGPsplithorizon,routinginformationthatisreceivedthroughan
IBGPsessionisforwardedtoanotherIBGPneighbor.
1. Whyisitrecommendedthatloopbackinterfacesbeusedtoform
IBGPneighborsessions?(Source:"InteractingwithIBGPandEBGP
inaTransitAS")
reducesroutermemoryresourcerequirements
reducesrouterCPUresourcerequirements
ensuresIBGPsessionstability
ismoresecurethanusingthephysicalinterface
1. HowistheBGPnext-hopattributeprocessedoveranIBGP
connection?(Source:"InteractingwithIBGPandEBGPinaTransit
AS")
Thenext-hopaddressissettotheaddressofthereceivingrouter.
Thenext-hopaddressisnotmodifiedovertheIBGPsession.
Thenext-hopaddressissettotheIPaddressofthenearestEBGPpeer.
Thenext-hopattributeissettotheIPaddressofthenearestEBGPpeer;if
noexternalASconnectionhasbeenconfigured,thenexthopissettothe
defaultgatewaythatisconfiguredontherouter.
1. Whichtwostatementsaretrueofthefull-meshrequirementin
IBGP?(Choosetwo.)(Source:"InteractingwithIBGPandEBGPina
TransitAS")
TheIBGPmeshmustbealogicalfullmesh.
AphysicalfullmeshmustbemaintainedwithintheIBGPAS.
BecauseofBGPsplithorizon,noroutercanrelayIBGPinformationwithin
theAS.
AllrouterswithintheASmustbedirectlyconnectedtoensurecorrect
deliveryofBGProutinginformation.
1. Whichthreestatementsregardingthenext-hop-selfconfigurationin
BGParetrue?(Choosethree.)(Source:"InteractingwithIBGPand
EBGPinaTransitAS")
Changingthenext-hopattributemightcausesuboptimalrouting.
Theconfigurationchangeshowthenext-hopattributeisprocessedat
edgerouters.
TheconfigurationannouncesthelocalIPaddressastheBGPnexthopin
outgoingupdatesthataresenttothespecifiedneighbor.
TheconfigurationremovestherequirementfortheIGPtocarry
reachabilityinformationforintra-ASdestinations.
1. WhymustallEBGPpeersbereachablebyallBGP-speakingrouters
withinthetransitAS?(Source:"InteractingwithIBGPandEBGPina
TransitAS")
BGP-speakingroutersinatransitASusethenext-hop-selfattributetofind
theirEBGPneighbors.
EBGPpeersinatransitASusethelengthoftheASpathtodecidewhich
BGProutetoinstallintheroutingtable.
WhenBGProutesareusedintheroutingtable,theroutercheckshowit
wouldhavereachedthenext-hopaddress,anditinstallstheBGProute
withthesameforwardingindicationasfortheroutethatisusedtoreach
thenext-hopIPaddress.
AllBGPpeersdonotneedtospeaktoeachotherwithinatransitAS.
1. Whichcommandisusedtoconfiguretherouterasthenexthopfora
BGP-speakingneighbororpeergroup?(Source:"Interactingwith
IBGPandEBGPinaTransitAS")
neighbordescription
neighborremoteas
maximum-pathsibgp
neighbornext-hop-sf
1. WhatarethreedifferencesbetweenIBGPandEBGPsessions?
(Choosethree.)(Source:"InteractingwithIBGPandEBGPina
TransitAS")
RouteselectionrulesslightlypreferIBGProutes.
RoutesthatarelearnedfromIBGPpeersarenotadvertisedtootherIBGP
peers.
EBGPpeersaredirectlyconnected,andIBGPpeersareusuallydistant.
Bydefault,noBGPattributesarechangedinIBGPupdates.
1. WhenyouareconfiguringtheBGPneighborsession,what
differentiatesanEBGPneighborfromanIBGPneighbor?(Source:
"InteractingwithIBGPandEBGPinTransitAS")
Thekeywordinternalisattheendoftheneighborcommand.
IBGPneighborswillhavethesameASnumberspecified.
Adescriptionfortheinternalorexternalneighbormustbeattachedwith
theneighbordescriptioncommand.
DirectlyconnectedneighborswillautomaticallyformanEBGPsession.
1. Whataretwoscalabilitytoolsthatyoucanusetoovercomethefull-
meshrequirementforIBGPsessions?(Choosetwo.)(Source:
"InteractingwithIBGPandEBGPinTransitAS")
confederations
floatingstaticroutes
routereflectors
disablingBGPsynchronization
1. Whataretwonegativeramificationsofthefull-meshrequirement
thatisimposedbyIBGP?(Choosetwo.)(Source:"Interactingwith
IBGPandEBGPinTransitAS")
administrativedifficultyofapplyinganAS-wideroutingpolicy
requirementtousenext-hop-selfforproperroutingtoexternaldestinations
largenumberofTCPsessions
unnecessaryduplicationofroutingtraffic
1. Itisnotrecommendedtoredistributetheinformationfromexternal
routersintotheIGPwhichisusedwithinthetransitAS.Trueor
false?(Source:"ForwardingPacketsinaTransitAS")
true
false
1. WhataretworeasonswhyyoumustrunIBGPonallrouterswithina
transitbackbone?(Choosetwo.)(Source:"ForwardingPacketsina
TransitAS")
sorouterscanproperlyforwardpacketstowardallexternaldestinations
toensurethatafullmeshexistsamongallroutersintheAS
toallowrouterstoproperlyprocesstheBGPnext-hopattribute
becauseIGPscannotscalelargeenoughtohandleredistributionofBGP
routes
1. RouteslearnedviaBGPdonothaveanoutgoinginterface
associatedwiththemintheroutingtable,sorecursivelookupis
performedtoforwardIPpacketstowardexternaldestinations.True
orfalse?(Source:"ForwardingPacketsinaTransitAS")
true
false
1. IfatransitbackbonehasIBGPrunningonallrouters,whataretwo
reasonswhyitisstillnecessarytouseanIGP?(Choosetwo.)
(Source:"ForwardingPacketsinaTransitAS")
toprovideroutinginformationthatisneededtoestablishtheIBGP
sessions
toresolvenext-hopreferencesthatareusedinrecursiverouting
sothatBGProutescanbeproperlytransportedthroughtheAS
toprovideuserworkstationswithanetworkdefaultgateway
1. Maproutingprotocoltoitsfunction.(Source:"ForwardingPacketsin
aTransitAS")
IBGP exchanginginformationabouthowtoreach
internaldestinations
IGP exchanginginformationabouthowtoreach
externaldestinations
1. WhatistheADofthefollowingprotocols?(Source:"Forwarding
PacketsinaTransitAS")
OSPF 120
RIP 115
IS-IS 110
IBGP 20
EBGP 200
1. WhataretworeasonswhytheADisanimportantconsiderationfor
BGPnetworkdesign?(Choosetwo.)(Source:"ForwardingPackets
inaTransitAS")
TheADaffectshowroutesareselectedforuseintheIProutingtable.
TheADcontrolshowroutinginformationisenteredintotheBGPtable.
IfarouteisadvertisedbybothanIGPandthroughEBGP,therouterwill
prefertheexternalroute.
TheADisnotalargeconcerntoBGPdesign,becausetherouterwill
alwayschoosetheroutethatisadvertisedbytheprotocolthatisbest
suitedtoreachthedestination.
1. IfarouterlearnsthesameinformationviaEBGPandIGP,which
onewillituse?(Source:"ForwardingPacketsinaTransitAS")
theonefromIGP
theonefromEBGP
1. Withregardtorecursiveroutelookups,whataretwowaysinwhich
CEFisdifferentfromtraditionalCiscoIOSswitchingmechanisms
suchasroutecaching?(Choosetwo.)(Source:"ForwardingPackets
inaTransitAS")
TraditionalCiscoIOSswitchingmechanismswaitforthefirstpacketto
arrivebeforerecursivelookupcantakeplace.
NewentriesintheIProutingtablewilltriggerarecursivelookupin
traditionalCiscoIOSswitchingmechanisms.
CEFprebuildsacompleteIPforwardingtablebasedontheIProuting
table.
CEFwillbuildaFIBdirectlyfromtheentriesintheBGPtablebeforeany
BGPpacketsarriveattherouter.
1. WhichadministrativedistanceofBGProutecannotbechanged?
(Source:"ForwardingPacketsinaTransitAS")
externaldistance
internaldistance
localdistance
remotedistance
1. WhyisitimportanttodisableBGPsynchronizationinatransit
backbone?(Source:"MonitoringandTroubleshootingIBGPin
TransitAS")
IGPscansupporttheroutingrequirementsoffullInternetrouting,and
hencesynchronizationisnolongernecessary.
BecauseBGPredistributionintoanIGPisnolongerpractical,enablingthe
synchronizationfeatureisnolongerapplicable.
SynchronizationrequiresallBGPtransitroutestobeexplicitlymappedto
anexitpoint,creatingtoomuchadministrativeoverhead.
SynchronizationrequiresBGPattributestobeproperlymappedtoIGP
metricsforBGProutingacrossthetransitbackbonetofunctionproperly,
creatingtoomuchoverhead.
1. Whichtwostepsarerequiredwhenyouusealoopbackinterfacefor
IBGPpeeringsessions?(Choosetwo.)(Source:"Monitoringand
TroubleshootingIBGPinTransitAS")
EnsurethattheloopbackinterfacesarereachablethroughanIGP.
Ensurethatthetwoneighborsaredirectlyattached.
Verifythateachrouterhasmultiplephysicallyredundantpaths.
Configureaneighborstatementwiththeupdate-sourcecommand.
1. WhichCiscoIOSshowcommandindicatesthataBGProuteisan
IBGProute?(Source:"MonitoringandTroubleshootingIBGPina
TransitAS")
showiproute
showiproutebgp
showipbgp
showipbgpinternal
1. WhichCiscoIOSshowcommanddisplaystheinforbationaboutthe
TCPandBGPconnectiontoneighbors?(Source:"Monitoringand
TroubleshootingIBGPinaTransitAS")
showipbgpprefix
showipbgpneighbors
showipbgp
showipbgpinternal
1. BGPrunsonthewell-knownTCPport169.TrueorFalse?(Source:
"MonitoringandTroubleshootingIBGPinaTransitAS")
true
false
1. WhichthreeofthefollowingarethemostcommonBGP
implementationproblems?(Choosethree.)(Source:"Monitoringand
TroubleshootingIBGPinaTransitAS")
IBGPsessionsdonotreachtheEstablishedstate.
TCPwindowsizeissetincorrectly.
RoutinginformationthatisreceivedviaIBGPisneverselected.
ThebestBGProuteisneverinstalledintheroutingtable.
1. WhatarethreecommonsituationsthatpreventIBGPsessionsfrom
starting?(Choosethree.)(Source:"MonitoringandTroubleshooting
IBGPinaTransitAS")
TheIBGPsessionhasbeenconfiguredtopeertoaloopbackinterface,but
update-sourcehasnotbeenconfiguredontheneighbor.
Anaccess-listfilterisblockingaccesstoTCPport179.
TheIBGPsessionhasbeenconfiguredtopeertoaloopbackinterface,but
theloopbackinterfacehasnotbeenadministrativelyenabledwiththeno
shutdowncommand.
TheIBGPsessionhasbeenconfiguredtopeertoaloopbackinterface,but
theinterfacesarenotreachableviatheIGP.
1. WhichcommonissuecouldpreventIBGPbestroutesfrombeing
insertedintotheIProutingtable?(Source:"Monitoringand
TroubleshootingIBGPinaTransitAS")
failuretodisableBGPsynchronization
failuretodisableBGPsplithorizon
lackofaroutetotheBGPnexthopfortheIGP
failuretoinjectadefaultrouteintotheIGP
1. WhichtwoofthefollowingstatementsaboutsolvingIBGP
synchronizationproblemsareaccurate?(Choosetwo.)(Source:
"MonitoringandTroubleshootingIBGPinaTransitAS")
RouterswithBGPsynchronizationenabledwillbeabletoinstallIBGP
routesintheroutingtableandpropagatethemtootherEBGPneighbors.
RouterswithBGPsynchronizationenabledwillnotbeabletoinstallIBGP
routesintheroutingtableorpropagatethemtootherEBGPneighbors.
Thedefaultvalueistohavesynchronizationenabled.
Thedefaultvalueistohavesynchronizationdisabled.
1. WhyisIBGPamandatorycomponentofatransitAS?(Source:
"WorkingwithaTransitAS")
ItistheonlyfeasiblewaytoensurethatallroutersintheAShave
consistentexternalroutinginformation.
IteliminatesthescalabilityissuesofrunninganIGPwithinthetransitAS.
RunningIBGPonallroutersistheonlywaytosatisfythefiltering
requirementsofthetransitAS.
AnIGPisnotcapableofhandlingthepotentialroutingloopsinthetransit
AS.
AnswerKey
1. IBGPneighborsmustbedirectlyconnected.Trueorfalse?(Source:
"WorkingwithaTransitAS")
true
false
1. WhyisredistributingBGProutesintoanIGPforuseinatransit
backboneNOTrecommended?(Source:"WorkingwithaTransit
AS")
RedistributionremovesallBGPattributesthatareneededtoensure
optimalroutingwithinthetransitAS.
AnIGPcannotenforcecomplexadministrativepoliciesandrouteselection
rules.
IGPscannotscaletothedemandsthatarepresentedbythenumberof
routesontheInternet.
IGPsarenotstablewhenfacedwithaflappingnetwork.
1. HowisEBGPusedinatransitAS?(Source:"WorkingwithaTransit
AS")
asameansoftransportingcustomerroutesacrossthetransitbackbone
toexchangeroutesbetweendifferentautonomoussystemsandthetransit
AS
toenhancescalabilitybytransportingIGProutesforthetransitAS
asameansofinjectinglocalroutesintothetransitbackbone
1. ThereceivedEBGPinformationhastobetranslatedandthen
forwardedtootherBGP-speakingrouterswithinthetransitAS.True
orfalse?(Source:"WorkingwithaTransitAS")
true
false
1. AllrouterswithinatransitASmusthaveallexternalroutes,received
fromanyneighboringAS.Trueorfalse?(Source:"Workingwitha
TransitAS")
true
false
1. WhatarethetwokeyfunctionsofatransitAS?(Choosetwo.)
(Source:"WorkingwithaTransitAS")
tofilteroutroutesthatdonotbelongtocustomersoftheserviceprovider
toprovideInternetconnectivitytocustomersoftheserviceprovider
topropagateroutesbetweenremoteautonomoussystems
toroutepacketsbetweenremotenetworks
1. HowareBGProutessentacrossthetransitbackbone?(Source:
"WorkingwithaTransitAS")
byredistributingBGPintoanIGPandthenbackintoBGP
byusingIBGP
byestablishingEBGPsessionsbetweenallroutersinthetransitbackbone
byredistributingconnectedroutesattheedgeofthetransitbackbone
1. WhichtwostatementsaretrueregardingtheAS-pathattributeasit
relatestoIBGP?(Choosetwo.)(Source:"InteractingwithIBGPand
EBGPinaTransitAS")
EachrouterintheASappendsitsASnumbertotheASpathonoutgoing
BGPupdates.
TheASpathinsideanASwillbeemptyforroutesoriginatinginsidea
neighboringAS.
TheAS-pathattributeisnotusedtodetectroutingloopsinsideanAS.
TheAS-pathattributeisnotmodifiedwithintheAS.
1. WhatistheprimaryfunctionoftheIBGPmultipathload-sharing
feature?(Source:"InteractingwithIBGPandEBGPinaTransitAS")
tochooseoneIBGPpathasthebestpathtoadestination
tochoosemultipleIBGPpathsasthebestpathstoadestination
todesignateonepathasthebestpathandadvertisethisbestpathtoits
neighbors
toenablearoutertohandleallthetrafficdestinedforaparticularsite
1. WhichofthefollowingstatementsaboutaBGPsplithorizonis
accurate?(Source:"InteractingwithIBGPandEBGPinaTransit
AS")
BecauseoftheBGPsplithorizon,anyroutercanrelayIBGPinformation
withintheAS.
BGPsplithorizonisusefulonlywithinautonomoussystems.
RoutinginformationloopswithintheASarepreventedbyanIBGPsplit
horizon.
WithaBGPsplithorizon,routinginformationthatisreceivedthroughan
IBGPsessionisforwardedtoanotherIBGPneighbor.
1. Whyisitrecommendedthatloopbackinterfacesbeusedtoform
IBGPneighborsessions?(Source:"InteractingwithIBGPandEBGP
inaTransitAS")
reducesroutermemoryresourcerequirements
reducesrouterCPUresourcerequirements
ensuresIBGPsessionstability
ismoresecurethanusingthephysicalinterface
1. HowistheBGPnext-hopattributeprocessedoveranIBGP
connection?(Source:"InteractingwithIBGPandEBGPinaTransit
AS")
Thenext-hopaddressissettotheaddressofthereceivingrouter.
Thenext-hopaddressisnotmodifiedovertheIBGPsession.
Thenext-hopaddressissettotheIPaddressofthenearestEBGPpeer.
Thenext-hopattributeissettotheIPaddressofthenearestEBGPpeer;if
noexternalASconnectionhasbeenconfigured,thenexthopissettothe
defaultgatewaythatisconfiguredontherouter.
1. Whichtwostatementsaretrueofthefull-meshrequirementin
IBGP?(Choosetwo.)(Source:"InteractingwithIBGPandEBGPina
TransitAS")
TheIBGPmeshmustbealogicalfullmesh.
AphysicalfullmeshmustbemaintainedwithintheIBGPAS.
BecauseofBGPsplithorizon,noroutercanrelayIBGPinformationwithin
theAS.
AllrouterswithintheASmustbedirectlyconnectedtoensurecorrect
deliveryofBGProutinginformation.
1. Whichthreestatementsregardingthenext-hop-selfconfigurationin
BGParetrue?(Choosethree.)(Source:"InteractingwithIBGPand
EBGPinaTransitAS")
Changingthenext-hopattributemightcausesuboptimalrouting.
Theconfigurationchangeshowthenext-hopattributeisprocessedat
edgerouters.
TheconfigurationannouncesthelocalIPaddressastheBGPnexthopin
outgoingupdatesthataresenttothespecifiedneighbor.
TheconfigurationremovestherequirementfortheIGPtocarry
reachabilityinformationforintra-ASdestinations.
1. WhymustallEBGPpeersbereachablebyallBGP-speakingrouters
withinthetransitAS?(Source:"InteractingwithIBGPandEBGPina
TransitAS")
BGP-speakingroutersinatransitASusethenext-hop-selfattributetofind
theirEBGPneighbors.
EBGPpeersinatransitASusethelengthoftheASpathtodecidewhich
BGProutetoinstallintheroutingtable.
WhenBGProutesareusedintheroutingtable,theroutercheckshowit
wouldhavereachedthenext-hopaddress,anditinstallstheBGProute
withthesameforwardingindicationasfortheroutethatisusedtoreach
thenext-hopIPaddress.
AllBGPpeersdonotneedtospeaktoeachotherwithinatransitAS.
1. Whichcommandisusedtoconfiguretherouterasthenexthopfora
BGP-speakingneighbororpeergroup?(Source:"Interactingwith
IBGPandEBGPinaTransitAS")
neighbordescription
neighborremoteas
maximum-pathsibgp
neighbornext-hop-sf
1. WhatarethreedifferencesbetweenIBGPandEBGPsessions?
(Choosethree.)(Source:"InteractingwithIBGPandEBGPina
TransitAS")
RouteselectionrulesslightlypreferIBGProutes.
RoutesthatarelearnedfromIBGPpeersarenotadvertisedtootherIBGP
peers.
EBGPpeersaredirectlyconnected,andIBGPpeersareusuallydistant.
Bydefault,noBGPattributesarechangedinIBGPupdates.
1. WhenyouareconfiguringtheBGPneighborsession,what
differentiatesanEBGPneighborfromanIBGPneighbor?(Source:
"InteractingwithIBGPandEBGPinTransitAS")
Thekeywordinternalisattheendoftheneighborcommand.
IBGPneighborswillhavethesameASnumberspecified.
Adescriptionfortheinternalorexternalneighbormustbeattachedwith
theneighbordescriptioncommand.
DirectlyconnectedneighborswillautomaticallyformanEBGPsession.
1. Whataretwoscalabilitytoolsthatyoucanusetoovercomethefull-
meshrequirementforIBGPsessions?(Choosetwo.)(Source:
"InteractingwithIBGPandEBGPinTransitAS")
confederations
floatingstaticroutes
routereflectors
disablingBGPsynchronization
1. Whataretwonegativeramificationsofthefull-meshrequirement
thatisimposedbyIBGP?(Choosetwo.)(Source:"Interactingwith
IBGPandEBGPinTransitAS")
administrativedifficultyofapplyinganAS-wideroutingpolicy
requirementtousenext-hop-selfforproperroutingtoexternaldestinations
largenumberofTCPsessions
unnecessaryduplicationofroutingtraffic
1. Itisnotrecommendedtoredistributetheinformationfromexternal
routersintotheIGPwhichisusedwithinthetransitAS.Trueor
false?(Source:"ForwardingPacketsinaTransitAS")
true
false
1. WhataretworeasonswhyyoumustrunIBGPonallrouterswithina
transitbackbone?(Choosetwo.)(Source:"ForwardingPacketsina
TransitAS")
sorouterscanproperlyforwardpacketstowardallexternaldestinations
toensurethatafullmeshexistsamongallroutersintheAS
toallowrouterstoproperlyprocesstheBGPnext-hopattribute
becauseIGPscannotscalelargeenoughtohandleredistributionofBGP
routes
1. RouteslearnedviaBGPdonothaveanoutgoinginterface
associatedwiththemintheroutingtable,sorecursivelookupis
performedtoforwardIPpacketstowardexternaldestinations.True
orfalse?(Source:"ForwardingPacketsinaTransitAS")
true
false
1. IfatransitbackbonehasIBGPrunningonallrouters,whataretwo
reasonswhyitisstillnecessarytouseanIGP?(Choosetwo.)
(Source:"ForwardingPacketsinaTransitAS")
toprovideroutinginformationthatisneededtoestablishtheIBGP
sessions
toresolvenext-hopreferencesthatareusedinrecursiverouting
sothatBGProutescanbeproperlytransportedthroughtheAS
toprovideuserworkstationswithanetworkdefaultgateway
1. Maproutingprotocoltoitsfunction.(Source:"ForwardingPacketsin
aTransitAS")
IGP exchanginginformationabouthowtoreach
internaldestinations
IBGP exchanginginformationabouthowtoreach
externaldestinations
1. WhatistheADofthefollowingprotocols?(Source:"Forwarding
PacketsinaTransitAS")
IBGP 200
EBGP 20
OSPF 110
IS-IS 115
RIP 120
1. WhataretworeasonswhytheADisanimportantconsiderationfor
BGPnetworkdesign?(Choosetwo.)(Source:"ForwardingPackets
inaTransitAS")
TheADaffectshowroutesareselectedforuseintheIProutingtable.
TheADcontrolshowroutinginformationisenteredintotheBGPtable.
IfarouteisadvertisedbybothanIGPandthroughEBGP,therouterwill
prefertheexternalroute.
TheADisnotalargeconcerntoBGPdesign,becausetherouterwill
alwayschoosetheroutethatisadvertisedbytheprotocolthatisbest
suitedtoreachthedestination.
1. IfarouterlearnsthesameinformationviaEBGPandIGP,which
onewillituse?(Source:"ForwardingPacketsinaTransitAS")
theonefromIGP
theonefromEBGP
1. Withregardtorecursiveroutelookups,whataretwowaysinwhich
CEFisdifferentfromtraditionalCiscoIOSswitchingmechanisms
suchasroutecaching?(Choosetwo.)(Source:"ForwardingPackets
inaTransitAS")
TraditionalCiscoIOSswitchingmechanismswaitforthefirstpacketto
arrivebeforerecursivelookupcantakeplace.
NewentriesintheIProutingtablewilltriggerarecursivelookupin
traditionalCiscoIOSswitchingmechanisms.
CEFprebuildsacompleteIPforwardingtablebasedontheIProuting
table.
CEFwillbuildaFIBdirectlyfromtheentriesintheBGPtablebeforeany
BGPpacketsarriveattherouter.
1. WhichadministrativedistanceofBGProutecannotbechanged?
(Source:"ForwardingPacketsinaTransitAS")
externaldistance
internaldistance
localdistance
remotedistance
1. WhyisitimportanttodisableBGPsynchronizationinatransit
backbone?(Source:"MonitoringandTroubleshootingIBGPin
TransitAS")
IGPscansupporttheroutingrequirementsoffullInternetrouting,and
hencesynchronizationisnolongernecessary.
BecauseBGPredistributionintoanIGPisnolongerpractical,enablingthe
synchronizationfeatureisnolongerapplicable.
SynchronizationrequiresallBGPtransitroutestobeexplicitlymappedto
anexitpoint,creatingtoomuchadministrativeoverhead.
SynchronizationrequiresBGPattributestobeproperlymappedtoIGP
metricsforBGProutingacrossthetransitbackbonetofunctionproperly,
creatingtoomuchoverhead.
1. Whichtwostepsarerequiredwhenyouusealoopbackinterfacefor
IBGPpeeringsessions?(Choosetwo.)(Source:"Monitoringand
TroubleshootingIBGPinTransitAS")
EnsurethattheloopbackinterfacesarereachablethroughanIGP.
Ensurethatthetwoneighborsaredirectlyattached.
Verifythateachrouterhasmultiplephysicallyredundantpaths.
Configureaneighborstatementwiththeupdate-sourcecommand.
1. WhichCiscoIOSshowcommandindicatesthataBGProuteisan
IBGProute?(Source:"MonitoringandTroubleshootingIBGPina
TransitAS")
showiproute
showiproutebgp
showipbgp
showipbgpinternal
1. WhichCiscoIOSshowcommanddisplaystheinforbationaboutthe
TCPandBGPconnectiontoneighbors?(Source:"Monitoringand
TroubleshootingIBGPinaTransitAS")
showipbgpprefix
showipbgpneighbors
showipbgp
showipbgpinternal
1. BGPrunsonthewell-knownTCPport169.TrueorFalse?(Source:
"MonitoringandTroubleshootingIBGPinaTransitAS")
true
false
1. WhichthreeofthefollowingarethemostcommonBGP
implementationproblems?(Choosethree.)(Source:"Monitoringand
TroubleshootingIBGPinaTransitAS")
IBGPsessionsdonotreachtheEstablishedstate.
TCPwindowsizeissetincorrectly.
RoutinginformationthatisreceivedviaIBGPisneverselected.
ThebestBGProuteisneverinstalledintheroutingtable.
1. WhatarethreecommonsituationsthatpreventIBGPsessionsfrom
starting?(Choosethree.)(Source:"MonitoringandTroubleshooting
IBGPinaTransitAS")
TheIBGPsessionhasbeenconfiguredtopeertoaloopbackinterface,but
update-sourcehasnotbeenconfiguredontheneighbor.
Anaccess-listfilterisblockingaccesstoTCPport179.
TheIBGPsessionhasbeenconfiguredtopeertoaloopbackinterface,but
theloopbackinterfacehasnotbeenadministrativelyenabledwiththeno
shutdowncommand.
TheIBGPsessionhasbeenconfiguredtopeertoaloopbackinterface,but
theinterfacesarenotreachableviatheIGP.
1. WhichcommonissuecouldpreventIBGPbestroutesfrombeing
insertedintotheIProutingtable?(Source:"Monitoringand
TroubleshootingIBGPinaTransitAS")
failuretodisableBGPsynchronization
failuretodisableBGPsplithorizon
lackofaroutetotheBGPnexthopfortheIGP
failuretoinjectadefaultrouteintotheIGP
1. WhichtwoofthefollowingstatementsaboutsolvingIBGP
synchronizationproblemsareaccurate?(Choosetwo.)(Source:
"MonitoringandTroubleshootingIBGPinaTransitAS")
RouterswithBGPsynchronizationenabledwillbeabletoinstallIBGP
routesintheroutingtableandpropagatethemtootherEBGPneighbors.
RouterswithBGPsynchronizationenabledwillnotbeabletoinstallIBGP
routesintheroutingtableorpropagatethemtootherEBGPneighbors.
Thedefaultvalueistohavesynchronizationenabled.
Thedefaultvalueistohavesynchronizationdisabled.
Introduction
BGPenablestrafficinInternetbackbonestodetermineanoptimalpathtoits
destinationacrossnetworkscomprisingmorethanoneAS.Routesthatarelearned
viaBGPhavepropertiesthatareassociatedwiththemthataidBGPindetermining
thebestroutetoaparticulardestination.Therearemanyinstancesinwhichthe
defaultBGProuteselectiondoesnotmatchadministrativeorbusinesspolicies.
Likewise,redundantnetworkdesignsoftenrequireenterprisestorunBGPwhen
theyareconnectedtomorethanoneISP.Inthesesituations,fullBGProutingtables
anddefaultBGProuteselectionarenotdesirable.
ThismoduleprovidesinformationonhowtoconnectInternetcustomerstomultiple
serviceproviders.ItintroducestheneedforfilteringBGPupdatesandchanging
BGProuteselectionpolicies.Inaddition,thismoduledescribesdifferentCiscoIOS
mechanisms(ASpathfilters,prefix-lists,route-maps)availableforBGProute
filtering.
Uponcompletingthismodule,youwillbeableto:
InfluencingBGProuteselectioninacustomerscenariowhereconnectionsto
multipleISPsmustbesupported
ConfigureBGPtoinfluencerouteselectionusingASpathfilters
DefinehowtosuccessfullyconfigureBGPtoinfluencerouteselectionusing
prefix-listfiltersinacustomerscenarioinwhichconnectionstomultipleISPs
mustbesupported
DefineshowtouseoutboundroutefilteringtominimizetheimpactofBGP
routingupdatesonrouterresourcesinanoperationalBGPnetwork
Describeroute-mapsandhowyoucanusethemforBGPfiltering.
Definehowtoconfigurethesoftreconfigurationfeaturetominimizetheimpact
ofexpeditingBGPpolicyupdatesinatypicalBGPnetwork
RouteSelectionUsingPolicyControls
Overview
Insomecircumstances,itisimportanttohavemultiplepathstoanISP.Thereare
businessandtechnicalreasonstoconfigureaBGPnetworkinamultihomed
configuration.Mission-criticalapplicationsoftencallforredundantnetworkdesigns.
WhenaccesstoapplicationsisprovidedovertheInternet,enterprisestypicallyuse
multihomedBGPnetworkstoachievetheirgoalsofhighavailability.
FullBGProutingtablesanddefaultBGProuteselectionmightordinarilybe
considereddesirablecharacteristicsofanetwork.However,theoverheadoffull
BGProutingtablesisnotwarrantedinthesesituations.Furthermore,thedefault
routeselectioninBGPoftendoesnotmatchthebusinessandtechnical
requirementsformultihomedenterprisenetworksthatuseBGP.
Uponcompletingthislesson,youwillbeableto:
ListthebusinessrequirementsformultihomedBGPnetworksinserviceprovider
environments
DescribethetechnicalrequirementsformultihomedBGPnetworksinservice
providerenvironments
ExplaintheneedforBGPpoliciesthatinfluencerouteselectioninamultihomed
BGPnetwork
DescribetypicalroutingpoliciesformultihomedBGPcustomers
ExplaintheneedtoinfluenceBGProuteselectioninaserviceprovider
environment
ExplaintheneedforBGPfiltersinamultihomedcustomer
ExplaintheneedforBGPfiltersinaserviceprovider
Explainthereturntrafficissue
UsingMultihomedBGPNetworks
Companieswhousewebservers(orsimilarservers)andoffermission-critical
businessservicesovertheInternetoftenliketohavetheirnetworksredundantly
connectedtotheInternet.Whenthecompaniescalculatetheexpectedlossof
businessbecauseofanunexpecteddisconnectionfromtheInternet,theymay
concludethathavingtwoconnectionstotheInternetisprofitable.Insuchcases,the
companymayconsiderbeingacustomertotwodifferentISPsorhavingtwo
separateconnectionstooneISP.
Some customers need redundant Internet access for their mission-critical
applications.
Full redundancy is achieved only by connecting to two independent service
providers.
WithtwoconnectionstoonesingleISP,BGPisusuallynotrequired.Thissolution
providesabackupforalinkfailureandrouterfailure.However,thissolutiondoesnot
provideabackupforproblemsinthenetworkoftheISPortheconnectionoftheISP
totherestoftheInternet.
FullredundancyisachievedonlybyconnectingtotwoindependentISPs.Ifoneof
theISPnetworkslosesitsconnectiontotherestoftheInternet,thecustomerwillstill
reachtherestoftheInternetviatheotherserviceprovider.Atthesametime,the
customerwillstillreachthoseuserswhoaredirectlyconnectedtothefailingISPvia
itsdirectconnection.
BusinessRequirementsforMultihomedBGPNetworks
ThemultihomedcustomernetworkmustexchangeBGPinformationwithbothISP
networks.Dynamicroutingisrequiredforfullredundancy,andBGPistheonly
protocolavailablethatcanbeusedinthisscenario.
Multihomed customers have to run BGP with their ISPs.
Multihomed customers usually need a public AS number and provider-
independent address space.
Thecustomermust,inmostcases,haveitsownpublicASnumberandannounceits
ownIPnetworkstobothISPs.TheISPswillpropagatecustomerannouncementsto
therestoftheInternet,andthecustomerwillbeseenasreachableviabothISP
networks.ThecustomernetworkalsoreceivesfullInternetroutingfrombothISPs.
Thiscapabilitygivesthecustomernetworktheopportunitytochoosethebest
connectionatthattimetoreachanydestinationontheInternet.
Mostcustomersarenotmultihomed.TheydonotexchangeBGPwiththeirISP.
Instead,theyusedefaultroutingtotheISP,andtheISPdoesstaticroutingtothe
customer.ISPsusethisfacttooptimizethenumberofprefixesthattheyannounce
intotheInternet.IPnetworknumbersareusuallyassignedtocustomersfroma
rangeofIPnetworksthataredelegatedtotheISP.Thissituationmeans,intheideal
case,thatallcustomersthatareconnectedtoonesingleISPcanhavetheirIP
networkssummarizedinafewBGPupdates.
Inthemultihomedscenario,however,theISPcannotbenefitfromIPnetwork
numberassignmentfromthedelegatedrange.Thecustomerisconnectedtotwo
differentISPs,anditisnotobviousfromwhichprovider-assignedaddressspaceit
shouldgettheIPaddresses.Thebestsolutionistodotheassignmentfromarange
completelyindependentoftheproviders,aprovider-independentaddressspace.
TechnicalRequirementsforMultihomedBGPNetworks
Thesimpleapproach,whichisillustratedinthefigure,maybethesourceofmany
problems.BysimplystartingBGPsessionswithbothISPs,andannouncingthe
customernetworkstobothISPs,thecustomercouldexperiencedifficultiesasa
resultofthedefaultbehaviorsofBGP.Thefollowingexampleillustratesproblems
thatmayoccurinthisenvironment.
router bgp 1
network 10.0.0.0 mask 255.255.255.0
neighbor 172.16.12.11 remote-as 100
neighbor 172.16.22.22 remote-as 200
Customer configures two BGP sessions and announces its address space.
BGPRouteSelectionWithoutBGPPoliciesExample
The BGP routes are selected based on AS-path length.
The default BGP route selection does not always result in optimum routing.
R2# show ip bgp
BGP table version is 30, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - inte
rnal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-
Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/24 0.0.0.0 0 32768 i
*> 10.0.1.0/28 172.16.12.11 0 0 100 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
*> 10.0.21.0/24 172.16.22.22 0 0 200 21 i
* 172.16.12.11 0 0 100 37 40 2
1 i
* 10.0.37.0/24 172.16.22.22 0 0 200 21 40 3
7 i
*> 172.16.12.11 0 0 100 37 i
*> 10.0.40.0/24 172.16.22.22 0 0 200 21 40 i
* 172.16.12.11 0 0 100 37 40 i
ThemultihomedcustomerisconnectedtotwoISPs:AS100andAS200.Thetwo
ISPsareconnectedtotheupstreamISPs:AS21,AS40,orAS37.
Thecustomerreceivesallroutesfrombothserviceproviders,givingredundancy.
ThedefaultBGProuteselectionpreferstheshortestASpath.IftheASpathlengths
areequal,BGPprefersthemoststableroute,ortheroutethatisreceivedfromthe
peerwiththelowerrouter-ID.
Inmanycases,however,thisrouteisnotthemostoptimalwaytoreachall
destinations.Forexample,thebandwidththatisavailabletoreachtheISPshasnot
beentakenintoconsideration.Tochangetherouteselectionbehavior,someBGP
parametersmustbeconfiguredtosupportmorecomplexroutingpolicies.
BGPRouteSelectionWithoutBGPPolicies
Multihomed customers could require a number of routing policies, for example:
One provider is primary; the other is backup.
Traffic to direct customers of the ISPs goes direct; all other traffic goes through
the primary provider.
All traffic to a particular part of the world goes through one ISP.
Traffic toward a specific destination goes through only one of the ISPs.
Dependingonthecircumstances,herearethedifferentpolicesthatamultihomed
customermightrequire:
OneofthetwoISPscanbeconsideredtheprimaryconnection.Thisdistinction
canbetheresultofavailablebandwidthorcommercialagreements.However,
althoughoneoftheISPsisconsideredtheprimaryconnection,someusersmay
havedirectconnectionstothesecondaryISP.Therefore,goingviatheprimary
ISPtoreachusersthatareconnectedtothesecondaryISPmaybesuboptimal.
Destinationsinonepartoftheworldmaybereachedmoreoptimallyviaoneof
theISPs,ratherthanviatheother.ThissituationoccursbecausethetwoISPs
mayhavedifferentinfrastructuresandpeeringagreementswithotherISPs.
Itisvirtuallyimpossibletoestablisharoutingpolicythatgivesoptimalroutingto
everydestinationontheInternet.Optimizationcanbedoneonlywiththemost
commondestinationsinmind.Thissituationcanresultinspecificrulesonhowto
reachspecificdestinationnetworksortheAS.
MultihomedCustomerRoutingPolicies
WhenoneofthetwoISPsisdesignatedasaprimaryISPandtheotherasa
backup,BGPattributesmustbeconfiguredtoinfluenceBGProuteselection.
Internet traffic always flows over primary ISP.
Routes that are received from primary ISP should be preferred over routes that
are received from backup ISP.
A route selection tool is needed in BGP weights or local preference.
IfbothISPconnectionsterminateinonesinglecustomerrouter,allroutesthatare
receivedfromtheprimaryISPcanbeassignedaBGPweight.Ahigherweight
indicatesamorepreferredpath.
However,theweightvalueislocaltoonerouter.Theweightvalueisnotshared
betweenrouters.IfoneISPconnectionterminatesinoneofthecustomerrouters
andtheotherISPconnectionterminatesinanother,thetwocustomerroutersmust
agreeonwhichlinktouse.Usinglocalpreferenceinsteadofweightcancausethis
situation.AllroutesthatarereceivedfromtheprimaryISPovertheprimarylinkare
assignedalocalpreferencevalue,whichishigherthanthedefaultvalueof100.The
customerrouterthatreceivestheroutesfromtheprimaryISPcompletesthe
assignmentandcommunicatestheinformationtotheotherrouterswithintheASof
thecustomer.
Whenusingeitherweightorlocalpreference,thecustomerASreachesall
destinationsontheInternetviatheprimarylinkaslongasitisavailable.Itreaches
thosedestinationswithintheASofthesecondaryISP.Ifthereisalinkfailureor
failureswithinthenetworkoftheprimaryISP,someoftheroutes,oralloftheroutes,
willnolongerbereceivedovertheprimarylink.Inthatcase,theASofthecustomer
nolongerseesthosedestinationsasreachableovertheprimarylink.Theonly
remainingchoiceisthebackuplink.Therefore,thecustomernetworkusesthe
backuplinkonlytoreachdestinationsthatarenotreachableovertheprimarylink.
Internet traffic flows over primary ISP; traffic to customers of backup ISP goes
direct.
Route selection has to be performed based on AS numbers in the AS path.
Inmostcases,itisoptimaltoreachothercustomerswhoareconnectedtothe
backupISPviathebackuplink,comparedwithreachingthemviatheprimarylink.
Theroutingpolicy,whereroutesareblindlypreferrediftheyarereceivedonthe
primarylink,caneasilybemodifiedtousethebackuplinkfordestinationsintheAS
InfluencingBGPRouteSelection
ofthebackupISP.Ontherouter,filteringtoolscanbeconfiguredtoselectrouting
informationthatisbasedonthecontentintheASpathattribute.Thoseroutes,with
anASpathattributematchingspecificselectioncriteria,canbeassignedaneven
higherweightorlocalpreference.
Thisapproachresultsinaroutingpolicythatgivesprecedencetoreaching
destinationswithintheASoftheprimaryISPandwithinallASsupstreamofthe
primaryISPovertheprimarylink.DestinationswithintheASofthebackupISP
receiveprecedenceoverthebackuplink.
WhenBGPhasselectedthebestpath,therouteradvertisestheinformationtoall
neighboringautonomoussystems,exceptforthesessionthatthepathwasreceived
on.Thisfunctionalityiscalled"BGPsplit-horizon,"whichpreventsnear-rangerouting
loops.ThissituationcausesthecustomerAStobecomeatransitASbetweenthe
twoISPs,andshouldbeavoided.
Customers could become a transit AS for the service providers.
Requirement: Do not propagate provider routes to other providers.
MostcustomersdonotintendtocreatetransittrafficbetweenISPnetworks.The
accesslinestotheISPsarenotsuitedtocarrythisvolumeoftraffic,andthe
customercertainlydoesnotwantthetransittraffictoconsumethebandwidth.
ThesolutiontothisproblemistofiltertheoutgoinginformationtobothISPs.Filtering
ofroutinginformationisperformedbasedonthecontentoftheASpathattributethat
isassignedtoeveryBGProute.OnlyrouteshavinganASpathattributethat
indicatesthattheyaresourcedbytheASofthecustomerareallowedtobesentto
eitherofthetwoISPs.
TransitTrafficIssue
Withoutsomesortoffiltering,BGProutinginformationthattheASofthecustomer
createscanpotentiallybepropagatedallovertheInternet.Inthisway,thecustomer
caninjecterroneousinformationintotheInternetroutingtables.
Customers running BGP could announce any route to the SPs.
Requirement: Service providers have to filter IP prefixes in incoming updates.
Customersaremuchlessexperiencedinavoidingthesekindsofproblemsthanare
serviceproviders.Thereismuchmoreriskoferrorsbeingintroducedwhena
customerisassigneditsownASandusesBGPwiththeISP,ascomparedwiththe
single-homedscenarioinwhichtheISPhassoleresponsibilitytoannounceBGP
routestotherestoftheInternet.
TheISPcanstopalmostalloftheInternetproblemsthatacustomercancauseby
improperlyconfiguringitsBGP.TheISPshouldalsofilterallincominginformation
fromthecustomerandacceptonlywhatissupposedtoarrive.TheISPshould
discardanythingoutsidestrictlimits.Inthisway,theISPpreventsthepropagationof
erroneousinformationtotherestoftheInternet.
TheISPcanmaintainalistoftheIPnetworknumbersthatthecustomeris
announcingandfilteroutanyotherroute.Ifthisapproachisnotpossiblebecauseof
thevolumeofthoselists,theISPshouldatleastbeabletofilteroutthemost
obviouserroneousannouncements.
Privateaddresses,accordingtoRFC1918,shouldnever
beannouncedtotheInternet.
RoutingUpdateReliabilityIssue
ThecustomercaneasilydefineapolicyabouthowtosendoutgoingIPpacketson
thecorrectlink.ItismuchhardertoinfluencetheneighboringASabouthowtodirect
theIPpacketscomingintothecustomernetwork.
Customers can influence only their outgoing traffic, not the return traffic.
Return traffic can take any path—backup ISP must also perform proper route
selection.
AcustomerthatcreatesaroutingpolicyinwhichoneofthetwoISPsisalways
preferredmayseethereturntrafficarrivingonthebackuplink.Thissituationmeans
thatthecustomerhasconfiguredtheweightorlocalpreferencetomakesurethatall
outgoingtrafficisleavingthecustomerASovertheprimarylink.Howeverthe
backupISPdoesnothaveanysuchconfiguration.Therefore,returntrafficentersthe
customerASbyusingtheshortestASpathasitsselectioncriterion.
ThebestwaytosolvethisproblemisforthecustomertoaskthebackupISPto
changeitsroutingpolicy.ThechangeshouldcausethebackupISPtoprefer
reachingthecustomerASviatheASoftheprimaryISP.ThebackupISPmust
implementthischangeinitsownAS.
SometimesthebackupISPadministratormightbe
reluctanttochangetheconfigurationforasingle
customer.Inthiscase,thecustomershoulduseanother
BGPfeature,theASpathprependingtool,toinfluencethe
selectionoftheprimaryorbackuplinkbylengtheningthe
ASpathofroutesthataresenttothebackupprovider.
ReturnTrafficIssue
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
Some customers need redundant Internet access for their mission-critical
applications and address this need by having two separate connections to one
ISP or implementing a multihomed configuration (connecting to two different
Internet service providers).
The multihomed customer network must exchange BGP information with both
ISP networks. Dynamic routing is required for full redundancy, and BGP is the
only protocol available that can be used in this scenario.
An approach to multihoming that is too simple can be a source of problems.
Starting BGP sessions and announcing customer networks to multiple ISPs by
using the default behavior of BGP may not result in optimal routing.
Depending on the circumstances, a multihomed customer may require different
polices. For example, one of the two ISPs being considered the primary
connection or reaching destinations in one part of the world more optimally via
one of the ISPs. Optimization should be done with the most common
destinations in mind, resulting in specific rules on how to reach specific
destination networks or the AS.
In BGP route selection, a routing policy may be created that gives precedence to
reaching destinations within the AS of the primary ISP and all upstream ASs
over the primary link. All destinations within the AS of the backup ISP can be
reached over the backup link.
When BGP has selected the best path and the information has been propagated
to all neighboring autonomous systems, the customer AS may become a transit
AS between the two ISPs. The customer must avoid this situation by using BGP
filters.
Summary
Overview
InnetworkimplementationsthatrequireconnectionstomultipleISPs,youwill
typicallyuseASpathfilterstoinfluenceBGProuteselection.Itisimportantforyou
asanetworkadministratortounderstandthesyntaxofanASpathregular
expression.Itisalsoimportanttounderstandhowstring-matchingoperatorsfunction
whentheyareusingASpathregularexpressionstomatchBGProutes.
BGPallowsconnectivitybetweenmultipleISPsforredundancyandscalability.You
canemployASpathfilterstoremedytheproblemsthatareassociatedwiththe
variousconnectivitymethodsthatareusedwithinBGP.Thislessonexplainsthe
methodsthatareusedtoimplementBGPASpathfilters.
Uponcompletingthislesson,youwillbeableto:
Identifynetworkscenariosinwhichyoumustsupportconnectionstomultiple
ISPsandinwhichASpathfilterscanbeusedtoinfluencerouteselection
DescribethefunctionofanASpathregularexpression
Explainhowstring-matchingoperatorsfunctionwhenyouareusingASpath
regularexpressionstomatchBGProutes
IdentifywhereyoucanapplyanASpathfilterwhenconfiguringarouterto
influencerouteselection
IdentifytheCiscoIOScommandsthatarerequiredtoconfigureASpathfiltersto
influencerouteselection
IdentifytheCiscoIOScommandsthatarerequiredtomonitortheoperationof
configuredASpathfilters
EmployingASPathFilters
Severalscenariosrequirefilteringandselectionofroutinginformation,whichis
basedonthecontentoftheASpathattribute.EachBGProutemusthaveanAS
pathattribute.Itisawell-knownmandatoryattributeandmustthereforebepresent
ineachBGPupdate.
Several scenarios require BGP route-filtering which is based on the AS path:
Announce only local routes to the ISP—AS path needs to be empty
Select routes that are based on a specific AS number in the AS path
Accept routes for specific AS only from some BGP neighbors
AS path filters use regular expressions.
UsingselectioncriteriathatarebasedontheASpathattribute,aroutercanidentify
asetofspecificroutesfromthetotalsetofroutesthatitreceives.Thoseroutes
wheretheASpathcontentsmatchthecriteriaareselected.Routesthatdonot
matchthecriteriaarenotselected.
TheASpathisasequenceofnumbers.EachnumberindicatesanAS.Whena
routeissourcedbyanetworkcommandinaBGPprocessorredistributionintoa
BGPprocess,theASpathattributeiscreatedandleftempty.Eachtimetheegress
routeradvertisestheroutetoanotherAS,itmodifiestheASpathattribute.Egress
routerprependsitsASnumbertotheASpathattribute.
WhileanewlysourcedrouteisstillwithintheASinwhichitwascreated,theAS
pathisempty.AssumethattheAShasarequirementtofilteroutallbuttheroutes
thatarelocaltoitselfbeforesendingthemtoaneighboringAS.TheASwillpermit
sendingoftherouteswiththeemptyASpathandwilldenyallothers.
RouterscanalsofilterincomingroutesthatarebasedontheirASpathattributes.
Somedestinationautonomoussystemsshouldnotbereceivedfromacertain
neighbor.Therefore,theroutesmatchingthatASintheASpathcanbefilteredon
thereceivingrouterincasetheyareaccidentallysent.
SelectionthatisbasedontheASpathisalsoatoolthatyoucanusewhenchanging
theweightorlocalpreferenceattributesonlyforsomedestinationASs.
WhenroutersfilterBGPupdatesbasedonthecontentoftheASpathattribute,they
useregularexpressions.RegularexpressionsarecommonlyfoundintheUNIX
environmentandalsoinsomeMicrosoftWindows-basedapplications.Regular
expressionsareastring-matchingtool.Aregularexpressionconsistsofastringof
characters.Someofthesecharactershavespecialmeanings,suchasfunctioningas
wildcardsandoperators.Someofthesecharacterssimplymeanthemselves,for
example,AtoZ,atoz,or0to9.Aregularexpressionissaidtomatchastringifthe
ordinarycharactersandtheappliedmeaningofthespecialoperatorcharacterscan
betranslatedintothematchedstring.Whenaregularexpressionmatches,the
selectiontestissaidtobetrue.Ifitdoesnotmatch,thetestisfalse.
ASPathFilteringScenarios
TheASpathattributethatiscarriedwithallBGProutesinaBGPupdateisavery
compactbinaryencodingofasequenceofintegernumbers.Itisnotasequencethat
canbetestedbyusingaregularexpression.
CiscoIOSsoftwareinternallytranslatesthebinaryencodingintoacharacterstring.
EachASnumberinthesequenceisconvertedintoastringusingdecimal
representation.ThespacecharacterseparateseachASnumberintheASpath
attribute.Therouterappliestheregularexpressiontesttothisinternallycreated
characterstring.
Charactersinaregularexpressionthatarenotassignedaspecificoperationmatch
themselves.Theregularexpression"31"matchesalloccurrencesofthecharacter
"3"followedbythecharacter"1"intheASpath.Inthisexample,"31"matchesattwo
occurrences.Oneoccurrenceissufficienttomakethetesttrue.Nooccurrence
meansthatthetestfailed.
ASPathRegularExpressions
HowdoesstringmatchingfunctionswhenyouareusingASpathregular
expressionstomatchBGProutes?
RegularExpressions
A string of characters in a regular expression matches any equivalent substring in
the AS path.
How many times does "31" match?
|213 317 2316 31|
Answer: Three times
|213 317 2316 31|
Theregularexpression"31"willmatchanyoccurrenceof"3"followedby"1"
regardlessofthecharactersimmediatelyprecedingthe"3"andimmediately
followingthe"1."Therefore,"31"willmatchanoccurrenceof"3"and"1"inthe
middleofanASnumber.
Theregularexpression"31"matchestheASpathstring"213317231631"three
times,because"31"matchesapartof"317,""2316,"and"31."
Alternatives
Expression expr1|expr2 matches the string if either subexpression matches the
string.
How many times does "21|31" match?
|213 317 2316 31|
Answer: Four times
|213 317 2316 31|
Thecharacter"|"(verticalbar)hasaspecialmeaning.Itisanoperatorthatmeans
"or."Theregularexpression"21|31"matchesthesequenceof"2"followedby"1"or
thesequenceof"3"followedby"1."Therefore,thissampleregularexpressionwill
matchatwo-charactersequence:the"21"orthe"31."
Theregularexpression"21|31"matchestheASpathstring"213317231631"four
times,because"21"matchesapartof"213"and"31"matchesapartofboth"317"
and"2316"andalso"31."
RangesandWildcardCharacters
A range of characters matches any single character in the range.
Example: "[1234]" or "[1-4]"
Dot "." matches any single character
How many times does "[1-3].[34]" match?
|213 317 2316 31|
Answer: Twice
|213 317 2316 31|
|213 317 2316 31|
Thepairofbrackets"["and"]"hasaspecialmeaning.Bracketssurroundasetof
charactersofwhichanyonematches.Thesetofcharactersiseitherexpressedas
thelistofthematches(forexample,"[1234]")orthesequencewiththestarting
StringMatching
character,ahyphen,andtheendingcharacter(forexample,"[1-4]").Bothexamples
matchonesinglecharacter,whichmustbeanyoneinthesetofthefourcharacters
"1,""2,""3,"and"4."
Thecharacter"."(adot)matchesanysinglecharacter.Smallregularexpressions
canbecombinedintoalargerexpression.Suchacombinationismatchingifallof
thepartsmatchoneaftertheother.Thesampleregularexpression"[1-3].[34]"
matchesasequenceofthreecharacters,ofwhichthefirstmustbeeither"1,""2,"or
"3."Thesecondcharactercanbeanycharacter,andthethirdmustbeeither"3"or
"4."
ThespacecharacterdelimitingtwoASnumbersisjusta
character.Thedot(".")forexample,matchesthis
character.
Theregularexpression"[1-3].[34]"matchestheASpathstring"213317231631"
twice.Initially,itmatches"213."Theleading"[1-3]"matchestheleading"2."Thedot,
whichmatchesanycharacter,matchesthe"1,"and"[34]"matchesthetrailing"3."
Theregularexpressionalsomatchesin"213317231631."Thematchisalittle
hardertosee,becausethedot(".")matchesthespacecharacterbetween"213"and
"317."
MatchingDelimiters
^ Matchesbeginningofstring
$ Matchesendofstring
_ Matchesanydelimiter(beginning,end,whitespace,tab,comma)
|213 317 218 31 731|
Does "^21" match?
Answer: Yes
|213 317 218 31 731|
|213 317 218 31 731|
Does "31$" match?
Answer: Yes
|213 317 218 31 731|
Does "_31_" match?
Answer: Yes
|213 317 218 31 731|
Acharacterstringmusthaveastartandanend.Thecharacterwiththespecial
meaning"^"matchesthebeginningofastring.Becauseallstringshaveabeginning,
the"^"charactermatchesallstrings.However,the"^"characterisusedtoposition
thepartoftheregularexpressionthatfollows.Thecharacterfollowingthe"^"
charactermustbethefirstcharacterofthestring;otherwise,thatcharacterwouldnot
matchthebeginningofthestring.
Thespecialcharacter"$"isusedanalogously,butitmeanstheendofthestring.
Thecharacterprecedingthe"$"mustbethelastcharacterinthestring;otherwise,
the"$"doesnotmatchtheendofthestring.
Theunderscore("_")matchesanydelimiter.ThespacecharacterbetweentwoAS
numbersisanexampleofadelimiter.Thebeginningofthestringandtheendofthe
stringarealsoconsidereddelimiters.Otherdelimitersarethetabandthecomma
(",").Theunderscore("_")isusedtoensurethatthedesiredASnumberisfoundin
anASpathstringbutnotaspartofsomeotherASnumber.Forexample,theregular
expression"31"willmatchtheASnumberstring"317,"buttheregularexpression
"_31_"willnot.Both"31"and"_31_"willmatchtheASnumberstring"31."
Theregularexpression"^21"canmatchtheASpathstring"21331721831731"
onlyonetimebecausethereisonlyonebeginningofthestring.Theregular
expression"^21"matchesonlyifthestringstartswiththesequence"21,"whichit
does.
Theregularexpression"31$"canmatchtheASpathstring"21331721831731"
onlyonetimebecausethereisonlyoneendofthestring.Theregularexpression
"31$"matchesonlyifthestringendswiththesequence"31,"whichitdoes.
Theregularexpression"_31_"can,intheory,matchanASpathstringseveraltimes.
However,inthiscase,whenmatchedagainstthestring"21331721831731,"the
regularexpression"_31_"matchesonlytheASnumber"31"intheASpath.
Grouping
Parentheses can be used to group smaller regular expressions intro larger
expressions.
How many times does "(213|218)_31" match?
|213 317 1218 316 31|
Answer: Twice
|213 317 1218 316 31|
Complicatedexpressionsmustsometimesbegroupedwithparentheses,"("and")."
Thisfeaturecanbeusefulinthefollowingexample.Youaresearchingfora
sequenceoftwoormoreASnumbersofwhichthefirstcanmatchanyofthespecific
AS(inthisexample,"213"or"218").HoweverthelastmustbeaspecificAS("31"in
thisexample).Iftheparentheseswerenotusedhere,theexpressionwouldmatch
eitherthesingleAS"213"orthesequenceofthetwo(21831).
Theregularexpression"(213|218)_31"matchestheASpathstring"2133171218
31631"twice.Thefirstmatchis"213317121831631."Thesecondmatchis"213
317121831631."
SpecialCharacters
To use the special character as single-character patterns, remove the special
meaning by preceding each character with backslash "\"
How do you match "AS 213" in the beginning of the string?
| (213 317) 1218 316 31|
Answer:
^\(213_
Sometimesthetargetstringthatyouaretryingtomatchwitharegularexpression
containssomeofthecharactersthatalsohavespecialmeaningsintheregular
expression.Tomatchthesecharactersinthetargetstring,usethebackslash"\"
togetherwiththecharacterintheregularexpression.
Thistypeofregularexpressionsyntaxisusedfor
matchingASpathstringsinsideaBGPconfederation.A
confederationisusedtoeliminatethescalingproblemof
full-meshIBGPbysplittingtheASintosmallerregional
autonomoussystems.Theexampleshowsthat213and
317werepartofaconfederationbyitsuseof"("and")."
RepeatingOperators
* Matcheszeroormoreatoms
?Matcheszerooroneatom
+ Matchesoneormoreatoms
An atom is a single character or a grouping.
How do you match AS sequences "23 45" and "23 78 45" in a single regular
expression?
Answer:
_23(_78)?_45_
Thespecialcharacters,star(asterisk),"*,"questionmark,"?,"andplus,"+,"allapply
repetitionoftheexpressionthatimmediatelyprecedesthem.
Thestar(asterisk),"*,"meansthattheexpressionthatimmediatelyprecedesitis
repeatedzeroormoretimes.Theexpressionmaynotbethere,butitmayalsobe
thereanynumberoftimes.Theexpression"1*"willmatchasequenceofno
charactersorasequenceofanynumberofthecharacter"1."
Aquestionmarkmeans,thattheexpressionthatimmediatelyprecedesitisrepeated
zerooronetime.Theexpressionmaynotbethere,butitmayalsobethereonce.
Theexpression"1?"willmatchasequenceofnocharactersorthesinglecharacter
"1."
Theplussign("+")meansthattheexpressionthatimmediatelyprecedesitis
repeatedoneormoretimes.Theexpressionmustbethereatleastonce.The
expression"1+"willmatchasequenceofoneormoreofthecharacters"1."
StringMatchingExample
_100_ GoingthroughAS100
^100$ DirectlyconnectedtoAS100
_100$ OriginatedinAS100
^100_. NetworksbehindAS100
^[0-9]+$ ASpathsoneASlong
^([0-9]+)(_\1)*$ PrependingperformedinneighboringoriginatingAS
^$ NetworksthatoriginateinlocalAS
.* Matcheseverything
Regularexpressionscanbearbitrarilycomplex.However,mostsearchingis
accomplishedusingregularexpressionssimilartotheregularexpressionsfoundin
thefollowingexamples:
IfyouaresearchingforallroutesthathaveAS100intheirASpaths,theregular
expressiontouseis"_100_."
Ifyouaresearchingforallroutesthataresourcedinyourdirectlyconnected
neighboringAS100,theregularexpressiontouseis"^100$."Usean
expressionwithbothacarat("^")anddollarsign("$")presentwhenyouare
searchingforanexactmatch.
IfyouaresearchingforallroutesthataresourcedinAS100,whichisnot
necessarilyadirectlyconnectedneighboringAS,theregularexpressiontouse
is"_100$."Thedollarsign("$")indicatesthattheASpathmustendwithAS
100.MeaningthattheroutewassourcedinAS100.Theunderscore("_")is
usedtomakesurethatitisAS100attheendofthestringandnot,forexample,
AS2100.
IfyouaresearchingforalltheroutesthatarereachablebehindAS100,the
regularexpressiontouseis"^100_."Thecarat("^")indicatesthattheASpath
muststartwithAS100.Theunderscore("_")isusedtomakesurethatthis
numberisnotmatchingwith,forexample,AS1001.Thedot(".")isusedto
indicatethattheASpathdoesnotendwithAS100,andthattheremustbe
somethingfollowingAS100.
IfyouaresearchingforallroutesthataresourcedinanyASdirectlyneighboring
yourAS,theregularexpressiontouseis"^[0-9]+$."The"[0-9]"partmeansany
digit.Theplussign("+")repetitioncharactermeansoneormoretimes.
Therefore,thecombination"[0-9]+"meansasequenceofoneormoreofdigits.
Thecarat("^")anddollarsign("$")meanthebeginningandtheendofthe
string.Therefore,thestringmayconsistonlyofasequenceofoneormore
digits.
IfyouaresearchingforallroutesthataresourcedinanyASdirectlyneighboring
yourAS,andpossiblyperformingASpathprepending(multiplicationofadirectly
connectedASnumber),theregularexpressiontouseis"^([0-9]+)(_\1)*$."The
expressioninthefirstsetofparenthesesmatchesanyASnumber.The
parenthesesstorethevalueofthematchedAS,andthesecondpartofthe
regularexpression,includingavariable,thenrecallsit.Thevariable"\1"isput
intoparenthesesforthepurposeofthemultiplieroperator"*."Imeaningthatthis
partcanmatchanynumberofsuccessiveoccurrencesofthesameASnumber
thatthe"[0-9]+"expressionmatched.Forexample,thisregularexpression
matchesASpaths"999999,""200,""101101,"or"55555,"butitdoesnot
matchtheASpath"10199."
Thecombination"^$"meansanemptystringandisusedwhenyouare
searchingforallroutesthataresourcedinthelocalAS.
Sometimesasearchismadetoselectafewspecificroutesanddosomething
specialwiththem,whiletherestoftherouteswillbehandledinadifferentway.
Tosearchforallroutes,regardlessofthecontentoftheirASpathattribute,use
theregularexpression".*".Thedot(".")matchesanysinglecharacter.The
repetitioncharacter,star(asterisk),"*,"meansthatthematchshouldbe
repeatedzeroormoretimes.Thus,thecombination(".*")matchesanystring.
CommonlyUsedCharactersinExpressionsExample
. Anysinglecharacter,includingaspace
*Zeroormoresequencesofpattern
+Oneormoresequencesofpattern
?Zerooroneoccurrenceofpattern
^Beginningofstring
$Endofstring
_Matchanydelimiter(includingbeginning,end,space,tab,comma)
\Removespecialmeaningofcharacterthatfollows
[] Matchonecharacterinarange
|LogicalOR
Thefigureliststhemostcommonlyusedcharactersinexpressions.
ASpathfiltersthatareconfiguredonarouterselectthoseroutesthatareallowed.
Routesthatareselectedbehaveasdescribedhere:
TheselectedroutesenterthelocalBGPtablewhentheselectionisappliedon
theincomingroutesfromaneighbor;routesthatarenotselectedaresilently
dropped.
Theselectedroutesaretransmittedtotheneighborwhentheselectionis
appliedontheoutgoingroutestotheneighbor.Routesthatarenotselectedare
usedlocallybutareneversenttotheneighbor.
ApplyingASPathFilters
ASpathaccesslistcreatesanASpathfilter.Thisaccesslistisappliedtoasetof
routesfromwhichtoselectasubset.Routesthattheaccesslistpermitsareincluded
inthesubset,andthoseroutesthataredeniedarenotincluded.Asinallaccess
lists,thecandidatetobepermittedordeniedmembershipinthesubsetistested
againstallthelinesintheaccesslist.Theorderoftestingistheorderinwhichthe
listisconfigured.Thefirstmatchindicates"permit"or"deny,"asspecified.Iftheend
oftheaccesslistisreachedwithoutanyexplicitmatch,thecandidateisimplicitly
denied.
router(config)# ip as-path access-list number {permit|deny} regexp
Configures AS path access-list
router(config-router)# neighbor ip-address filter-list as-path-
filter {in|out}
Configures inbound or outbound AS path filter for specified BGP neighbor
ThetestbytheASpathaccesslistisperformedbyusingregularexpressionsthat
areappliedontheASpathattributeoftheroute.
Theaccesslistcan,forexample,beappliedontheroutesreceivedfrom,orsentto,
aspecificBGPneighbor.
ipas-pathaccess-list
TodefineaBGPASpathaccess-list,usetheipas-pathaccess-listglobal
configurationcommand:
ip as-path access-list access-list-number {permit | deny} as-regular-
expression
Todisableuseoftheaccess-list,usethenoformofthiscommand.
no ip as-path access-list access-list-number {permit | deny} as-regular-
expression
SyntaxDescription
Parameter Description
access-list-number Integerfrom1to199thatindicatestheregularexpression
access-listnumber.
permit Permitsaccessformatchingconditions.
deny Deniesaccesstomatchingconditions.
as-regular-expression ASintheaccess-listusingaregularexpression(Seethe"Regular
Expressions"appendixintheCiscoIOSDialServicesCommand
Referenceforinformationaboutformingregularexpressions.)
neighborfilter-list
TosetupaBGPfilter,usetheneighborfilter-listrouterconfigurationcommand:
neighbor {ip-address | peer-group-name} filter-list access-list-
number {in | out}
Todisablethisfunction,usethenoformofthiscommand:
no neighbor {ip-address | peer-group-name} filter-list access-list-
number {in | out}
SyntaxDescription
Parameter Description
ip-address IPaddressoftheneighbor.
ConfiguringBGPASPathFilters
peer-group-name NameofaBGPpeergroup.
access-list-number NumberofanASpathaccess-list.Definethisaccess-listwiththe
ipas-pathaccess-listcommand.
in Access-listtoincomingroutes.
out Access-listtooutgoingroutes.
Parameter Description
MultihomedcustomersdonotwanttoactasatransitASbetweentheirservice
providers.Thecustomeravoidsthissituationbynottransmittingallitsroutestoits
serviceproviders.TheserviceprovidersendsIPpacketstothecustomeronlyifthe
destinationaddressesmatchoneoftheroutesthatthecustomerhassentbyBGPto
theserviceprovider.Bymakingsurethatonlylocallysourcedroutesaresent,the
customeravoidsreceivingIPpacketsfordestinationsoutsideitsownAS.
WithinthecustomerAS,thelocallysourcedrouteshaveemptyASpaths.The
regularexpression"^$"matchestheemptystrin.Thecommandipas-pathaccess-
list1permitsonlytheroutesthatarelocallysourcedandimplicitlydeniestherest.
Byapplyingthisfilter-listonoutgoinginformationtoallneighbors,thecustomerwill
announcelocalroutesonly.
Overview
Throughthisdiscovery,youwilllearnhowtoconfigureBGPASpathfiltersasa
routefilteringmechanism.Youwillpreventtransmittingroutes,thatarereceivedon
routerR2,fromoneISProutertootherISProuter.Thisway,youmakesurethat
customer'sautonomoussystemAS1doesnotbecometransitAS.
Discovery7:ConfigureNon-TransitAutonomousSystem
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
R2 Ethernet0/0 172.16.22.2/24 ConnectiontoISP2
R2 Ethernet0/1 172.16.12.2/24 ConnectiontoISP1
R2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
10.0.0.1/28
10.0.0.17/28
10.0.0.33/28
10.0.0.49/28
10.0.0.65/28
Loopbackssimulate
LANnetworks
ISP1 Ethernet0/1 172.16.12.11/24 ConnectiontoR2
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
ISP1 Loopback21
Loopback37
Loopback40
10.0.21.1
10.0.37.1
10.0.40.1
Loopbackssimulate
morenetworksin
differentautonomous
systems.
ISP2 Ethernet0/0 172.16.22.22/24 ConnectiontoR2
ISP2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
Loopback7
Loopback8
Loopback9
10.0.2.1/28
10.0.2.17/28
10.0.2.33/28
10.0.2.49/28
10.0.2.65/28
10.0.2.81/28
10.0.2.97/28
10.0.2.113/28
10.0.2.129/28
Loopbackssimulate
LANnetworks
ISP2 Loopback21
Loopback37
Loopback40
10.0.21.1
10.0.37.1
10.0.40.1
Loopbackssimulate
morenetworksin
differentautonomous
systems.
IPaddressesandadvertisednetworksinBGParepreconfiguredasshownbelow:
JobAids
BGPisalsopreconfiguredasEBGP(R2toISP1andR2toISP2).
Step1
OntheISP2router,verifythatyoureceiveroutesthattherouterISP1advertises.
ISP2# show ip bgp
BGP table version is 32, local router ID is 10.0.40.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/24 172.16.22.2 0 0 1 i
*> 10.0.1.0/28 172.16.22.2 0 1 100 i
*> 10.0.1.16/28 172.16.22.2 0 1 100 i
*> 10.0.1.32/28 172.16.22.2 0 1 100 i
*> 10.0.1.48/28 172.16.22.2 0 1 100 i
*> 10.0.1.64/28 172.16.22.2 0 1 100 i
*> 10.0.1.80/28 172.16.22.2 0 1 100 i
*> 10.0.2.0/28 0.0.0.0 0 32768 i
*> 10.0.2.16/28 0.0.0.0 0 32768 i
*> 10.0.2.32/28 0.0.0.0 0 32768 i
*> 10.0.2.48/28 0.0.0.0 0 32768 i
*> 10.0.2.64/28 0.0.0.0 0 32768 i
*> 10.0.2.80/28 0.0.0.0 0 32768 i
*> 10.0.2.96/28 0.0.0.0 0 32768 i
Network Next Hop Metric LocPrf Weight Path
*> 10.0.2.112/28 0.0.0.0 0 32768 i
*> 10.0.2.128/28 0.0.0.0 0 32768 i
*> 10.0.21.0/24 0.0.0.0 0 32768 i
* 10.0.37.0/24 172.16.22.2 0 1 100 37 i
*> 0.0.0.0 0 32768 i
* 10.0.40.0/24 172.16.22.2 0 1 100 37 40 i
*> 0.0.0.0 0 32768 i
YoucanseethatISP1routerinAS100eitheroriginatesoradvertisesallhighlighted
routestotherouterR2:
Networks10.0.1.0/28,10.0.1.16/28,10.0.1.32/28,10.0.1.48/28,10.0.1.64/28and
10.0.1.80/28wereoriginatedbyISP1routerinAS100andadvertisedtotherouter
R2.
RouterISP1advertisesnetworks10.0.37.0/24and10.0.40.0/24totherouterR2.
ThecustomerrouterR2originatesroute10.0.0.0/24.
AlltheserouteshavenexthopsettoIPaddress172.16.22.2,whichistheIPaddressof
theR2router.
Step2
OntheR2router,verifywhichroutesarebeingadvertisedtoEBGPneighborrouterISP2.
R2# show ip bgp neighbors 172.16.22.22 advertised-routes
BGP table version is 22, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/24 0.0.0.0 0 32768 i
*> 10.0.1.0/28 172.16.12.11 0 0 100 i
*> 10.0.1.16/28 172.16.12.11 0 0 100 i
*> 10.0.1.32/28 172.16.12.11 0 0 100 i
*> 10.0.1.48/28 172.16.12.11 0 0 100 i
*> 10.0.1.64/28 172.16.12.11 0 0 100 i
*> 10.0.1.80/28 172.16.12.11 0 0 100 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
*> 10.0.2.16/28 172.16.22.22 0 0 200 i
*> 10.0.2.32/28 172.16.22.22 0 0 200 i
*> 10.0.2.48/28 172.16.22.22 0 0 200 i
*> 10.0.2.64/28 172.16.22.22 0 0 200 i
*> 10.0.2.80/28 172.16.22.22 0 0 200 i
ConfigureASPathFilters
DiscoverySteps
*> 10.0.2.96/28 172.16.22.22 0 0 200 i
Network Next Hop Metric LocPrf Weight Path
*> 10.0.2.112/28 172.16.22.22 0 0 200 i
*> 10.0.2.128/28 172.16.22.22 0 0 200 i
*> 10.0.21.0/24 172.16.22.22 0 0 200 21 i
*> 10.0.37.0/24 172.16.12.11 0 0 100 37 i
*> 10.0.40.0/24 172.16.12.11 0 0 100 37 40 i
Total number of prefixes 19
YoucanseethatISP1routerinAS100eitheroriginatesoradvertisesallhighlighted
routestotherouterR2:
ThecustomerrouterR2advertisesthelocallyoriginatedroute10.0.0.0/24tothe
routerISP2.
ThecustomerrouterR2advertisesnetworks10.0.1.0/28,10.0.1.16/28,10.0.1.32/28,
10.0.1.48/28,10.0.1.64/28and10.0.1.80/28totherouterISP2.
ThecustomerrouterR2advertisesnetworks10.0.37.0/24and10.0.40.0/24tothe
routerISP2.
Step3
ConfigureASpathfilteringontheR2router,sothatonlylocallyoriginated
networkwillbeadvertisedtobothISP1andISP2routers.
R2(config)# ip as-path access-list 1 permit ^$
R2(config)# router bgp 1
R2(config-router)# neighbor 172.16.12.11 filter-list 1 out
R2(config-router)# neighbor 172.16.22.22 filter-list 1 out
Step4
ClearallBGPsessionsontheR2router.
R2# clear ip bgp *
Step5
OntheISP2router,verifythatyoudonotreceiveroutesthattheISP1routeradvertises.
ISP2# show ip bgp
BGP table version is 26, local router ID is 10.0.40.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/24 172.16.22.2 0 0 1 i
*> 10.0.2.0/28 0.0.0.0 0 32768 i
*> 10.0.2.16/28 0.0.0.0 0 32768 i
*> 10.0.2.32/28 0.0.0.0 0 32768 i
*> 10.0.2.48/28 0.0.0.0 0 32768 i
*> 10.0.2.64/28 0.0.0.0 0 32768 i
*> 10.0.2.80/28 0.0.0.0 0 32768 i
*> 10.0.2.96/28 0.0.0.0 0 32768 i
*> 10.0.2.112/28 0.0.0.0 0 32768 i
*> 10.0.2.128/28 0.0.0.0 0 32768 i
*> 10.0.21.0/24 0.0.0.0 0 32768 i
*> 10.0.37.0/24 0.0.0.0 0 32768 i
*> 10.0.40.0/24 0.0.0.0 0 32768 i
TheonlyroutethatisreceivedfromtherouterR2withnexthopIPaddress172.16.22.2is
10.0.0.0/24,whichislocallyoriginatednetworkoncustomer'srouterR2.
Step6
OntheR2router,verifywhichroutesarebeingadvertisedtoEBGPneighborrouterISP2.
R2# show ip bgp neighbors 172.16.22.22 advertised-routes
BGP table version is 22, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/24 0.0.0.0 0 32768 i
Total number of prefixes 1
Youshouldonlyseethenetwork10.0.0.0/24beingadvertisedtotheEBGPneighbor
ISP2.ThisnetworkislocallyoriginatedontheR2router.YoumadethecustomerAS1a
non-transitautonomoussystem.
TheCiscoIOScommandsthataremostfrequentlyusedtomonitortheoperationof
configuredASpathfiltersincludeshowipas-path-access-list,showipbgp
regexp,andshowipbgpfilter-list.
router# show ip as-path-access-list [filter list]
Displays all routes in the BGP table matching regular-expression in one or all
filter-lists
router# show ip bgp regexp regular-expression
Displays one or all filter-lists
router# show ip bgp filter-list access-list-number
Displays all routes in the BGP table that the specified AS path access-list
permits
showipas-path-access-listCommand
Displaying configured filters:
R2# show ip as-path-access-list
AS path access list 1
permit ^$
AS path access list 7
deny _21_
permit .*
AS path access list 25
permit 40$
Theshowipas-path-access-listcommanddisplaysaspecificaccess-listorallAS
pathaccess-listsintherouter.
showipbgpregexpCommand
Routes that an expression matches:
R2# show ip bgp regexp _21_
BGP table version is 20, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - inte
rnal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-
Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.21.0/24 172.16.22.22 0 0 200 21 i
* 172.16.12.11 0 0 100 37 40 2
1 i
* 10.0.37.0/24 172.16.22.22 0 0 200 21 40 3
7 i
* 10.0.40.0/24 172.16.22.22 0 0 200 21 40 i
Becauseregularexpressionssometimesgetcomplex,thoroughtestingofthemis
required.Theshowipbgpregexpcommanddisplaysallroutescurrentlyinthe
BGPtablethathaveanASpathattributethatmatchesthetyped-inregular
expression.Usetheshowipbgpregexpcommandtotestaregularexpressionthat
istypedinonthecommandline.Theresultisaprintoutonthescreenofallthose
routescurrentlyintheBGPtablethathadanASpathattributematchingthetyped-in
regularexpression.
Intheexample,youwishtofindallBGProutespassingthroughAS21.
showipbgpregexpCommand
TodisplayroutesthatmatchanASpathregularexpression,usetheshowipbgp
regexpprivilegedEXECcommand:
MonitoringASPathFilters
show ip bgp regexp regular-expression
SyntaxDescription
Parameter Description
regular-expression RegularexpressiontomatchtheBGPASpaths
Routes that a filter-list matches:
R2# show ip as-path-access-list 25
AS path access list 25
permit 40$
R2# show ip bgp filter-list 25
BGP table version is 20, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - inte
rnal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-
Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
* 10.0.40.0/24 172.16.22.22 0 0 200 21 40 i
*> 172.16.12.11 0 0 100 37 40 i
AnASpathaccess-listisevenmorecomplexbecauseitisacombinationofseveral
regularexpressions.Thereisoneexpressiononeachaccess-listline.Usetheshow
ipbgpfilter-listcommandtotesttheentireASpathaccess-list.Theresultisa
printoutonthescreenofalltheroutescurrentlyintheBGPtablethattheaccess-list
permitstheirASpathattribute.
TheASpathaccess-listnumber25intheexamplefigureconsistsofonesingleline.
ItpermitstheroutesthatoriginateinAS40.Theshowipbgpfilter-listcommand
displaysalltheroutescurrentlyintheBGPtablethattheASpathaccess-list25
permits.
showipbgpfilter-listCommand
Todisplayroutesthatconformtoaspecifiedfilter-list,usetheshowipbgpfilter-list
privilegedEXECcommand:
show ip bgp filter-list access-list-number
SyntaxDescription
Parameter Description
access-list-number Integerfrom1to199thatindicatestheregularexpression
access-listnumber
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
Several scenarios require BGP route-filtering that is based on AS path,
including:
Announcing only local routes to the ISP (AS path needs to be empty).
Selecting routes that are based on a specific AS number in the AS path.
Accepting routes for a specific AS only from some BGP neighbors
By applying specific selection criteria to the contents of the AS path attribute,
routers can select a subset of routes from the total set of routes that are
received.
Cisco IOS software internally translates the AS path encoding, which is carried
with all BGP routes into a character string. This string is then tested against the
regular expression.
String matching operates when you are using AS path regular expressions to
match BGP routes.
You can use AS path filters to select those routes that will be allowed.
AS path access-list creates an AS path filter, which is applied to a set of routes
from which to select a subset. The ip as-path access-list global configuration
command defines a BGP AS path access-list, and the neighbor filter-list router
configuration command sets up a BGP filter.
There are a number of Cisco IOS commands that are required to monitor the
operation of configured AS path filters, including show ip as-path-access-list,
show ip bgp regexp, and show ip bgp filter-list.
Summary
Overview
WheremultiplepathsbetweenacustomerandanISPexist,thereisarequirement
tofiltercertaininformationduringBGPupdates.Youfiltertheinformationto
influencetherouteselectionortoenforceanadministrativepolicy.Tomeetthis
requirement,youmustusefilters.Usingprefixlistsistypicallyeasierthanusing
standardIPaccesslistsandprovidesperformancebenefits.Itisimportantforyouto
understandthecommandstoapplyfilteringofinboundoroutboundupdateswith
prefixlistsandwheretheyshouldbeapplied.
Inthislesson,youwilllearntherequirementsforusingprefix-basedfiltersin
customerimplementationswhereconnectionstomultipleISPsmustbesupported.
YouwillalsolearntheadvantagesofprefixlistsoverIPaccesslists.Thecommands
toapplyfilteringofinboundoroutboundupdateswithprefixlistsandtoconfigure
prefixlistfiltersarediscussed,andalsowherenetworkadministratorsshouldapply
them.
Uponcompletingthislesson,youwillbeableto:
Identifytherequirementforprefix-basedfiltersinnetworkimplementations
wheremultipleconnectionsbetweenacustomerandISPsexist
ListtheadvantagesofprefixlistsversusIPaccesslists
IdentifytheCiscoIOScommandthatisrequiredtoconfigureprefixlistfilters
DescribewhereyoucanimplementprefixlistsinaBGPnetwork
IdentifytheCiscoIOScommandsthatarerequiredtoapplyfilteringofinbound
oroutboundupdateswithprefixlists
IdentifytheCiscoIOScommandsthatarerequiredtomodifyconfiguredprefix-
listfilters
IdentifytheCiscoIOScommandsthatarerequiredtomonitortheoperationof
configuredprefixlistfilters
FilteringwithPrefixLists
Customerswithmultihomednetworksareresponsibleforannouncingtheirown
networksusingBGP.Typically,customersarenotasexperiencedwithBGPas
serviceproviders,andthereforeproblemsaremorelikelytooccur.Aservice
providerwithamultihomedcustomermusttakeprecautionsnottoaccept,use,or
forwardanyerroneousroutinginformationthatisreceivedfromthecustomer.
Service providers have to filter customer updates to ensure that the customers
announce only their assigned address space.
ThecustomerisassignedasetofIPnetworknumbersthatitshouldannounce.Ifthe
customerannouncesanyadditionalnetworks,somethingiswrong.Thecustomer
mayhaveforgottennottoactasatransitASandmayhavestartedpropagating
routesthatithasreceivedfromtheotherserviceprovider.Or,thecustomermay
haveaccidentallystartedtoannounceitsprivateaddressspace.Thecustomermay
usethisprivateaddressspaceforaddresslinks,loopbackinterfaces,orother
devicesthatshouldneveraccesstheInternet.
Toavoidproblems,theserviceprovidercanapplyanIPprefixfilterontheincoming
informationfromthecustomer.Theserviceproviderwillacceptonlynetwork
numbersthattheaccesslistorprefixlistpermits.
RequirementsforPrefix-BasedFilters
Traditionally,thefilteringofIPnetworknumbershasbeenaccomplishedusingan
accesslist.Theaccesslististhenboundtoeithertheincomingoroutgoing
informationofaneighborbyusingtheneighbordistribute-listcommand.ABGP
updateaboutanetworknumberthattheaccesslistpermitswillbeaccepted.An
updateaboutanetworknumberthattheaccesslistdenieswillbedropped.
Traditional prefix filters
Traditional IP prefix filters were implemented with IP access lists configured with
the distribute-list command.
IP access lists used as route filters have several drawbacks:
Subnet mask cannot be easily matched.
Access lists are evaluated sequentially for every IP prefix in the routing
update.
Extended access lists can be cumbersome to configure.
However,standardaccesslistsdonotsupportthetestingofthesubnetmasks.Ifthe
accesslistpermits10.0.0.0/16,itwouldalsopermit10.0.0.0/8.
ExtendedaccesslistscandotestingonbothanIPnetworknumberandasubnet
mask,butthesyntaxiscumbersome.
Prefix lists
Route-filtering mechanism
Significant performance improvement on long filters
Inside Cisco IOS software, the prefix list is a tree structure and is not
scanned sequentially.
Support for incremental updates
Individual entries in prefix lists can be inserted or deleted.
More user-friendly CLI
The CLI for using access lists to filter BGP updates is difficult to understand
and use, because it uses the packet-filtering format.
Greater flexibility; can match on subnet masks
Theipprefix-listconfigurationcommandhasseveralbenefitsifyoucompareitto
theaccess-listcommand.Theintendeduseofprefixlistsislimitedtoroutefiltering,
whereasaccesslistswereinitiallyintendedforpacketfiltering,whichwasthen
extendedtofilterroutes.
Theprefixlistisinternallytransformedintoatreestructure,witheachbranchingof
thetreeservingasatest.CiscoIOSsoftwaredeterminesaverdictofeither"permit"
or"deny"muchfasterthisway,comparedtosequentiallyinterpretinganaccesslist.
TheconfigurationCLIthatyouusewhenconfiguringtheipprefix-listcommand,
allowsyoutoassignalinenumbertoeachlineoftheprefixlist.Therouterusesthis
numbertosorttheentriesintheprefixlist.Ifthelinesareinitiallyassignedline
numbers,withsomespacinginbetweenthem,administratorscanlaterinsertextra
lines.Individuallinescanalsoberemovedwithoutremovingtheentirelist.
Routersmatchnetworknumbersinaroutingupdateagainsttheprefixlist,usingas
manybitsasindicated.Forexample,aprefixlistcanbespecifiedtobe10.0.0.0/16,
whichwillmatch10.0.0.0routesbutnot10.1.0.0routes.
Optionally,theprefixlistcanalsospecifythesizeofthesubnetmask.Inaddition,
theprefixlistcanindicatethatthesubnetmaskmustbeinaspecifiedrange.
Key access list features are preserved.
Filtering using "permit" or "deny"
Order dependency (first match wins)
Security-focused: no match means "deny"
The matching mechanism has changed.
Matches routes in a part of address space with subnet mask longer or
shorter than a set number
Theprefixlistsharesseveralsimilaritieswiththeaccesslist.Itcanconsistofany
numberoflines,eachofwhichindicatesatestandaresult.Theroutercaninterpret
thelinesinthespecifiedorder,althoughthisprocessisoptimizedintheCiscoIOS
software.Whenarouterevaluatesarouteagainsttheprefixlist,thefirstlinethat
PrefixListsvs.IPAccessLists
matchesresultsineithera"permit"or"deny."Ifnoneofthelinesinthelistmatch,
theresultis"implicitlydeny."
Testingisdoneusingprefixes.Theindicatednumberofbitsintheprefixis
comparedwiththesamenumberofbitsinthenetworknumberintheupdate.Ifthe
bitsmatch,testingcontinueswithanexaminationofthenumberofbitssetinthe
subnetmask.Theprefixlistlinecanindicatearangewithinwhichthenumbermust
falltopassthetest.Ifnorangeisindicated,thesubnetmaskmustmatchtheprefix
size.
YoucanfiltercertaininformationduringBGPupdatesusingprefixlistfiltersto
influencerouteselectionortoenforceanadministrativepolicy.
router(config)# ip prefix-list list-
name [seq seq] {permit|deny} network/len [ge value] [le value]
Prefix lists have names and sequence numbers (like route-maps).
An entry with no le or ge parameter matches exactly the specified prefix.
An entry with an le or ge parameter matches any route within the address space
of address/prefix. But the prefix must be longer or equal to ge value and shorter
than or equal to le value.
ipprefix-listConfigurationCommand
Tocreateanentryinaprefixlist,usetheipprefix-listglobalconfiguration
command.
ip prefix-list list-name [seq seq-
value] {permit | deny} network/len [ge ge-value] [le le-value]
SyntaxDescription
Parameter Description
list-name Nameofaprefix
seq (Optional)Appliesthesequencenumbertotheprefix
seq-value (Optional)Specifiesthesequencenumberfortheprefix
permit Permitsaccessformatchingconditions
deny Deniesaccesstomatchingconditions
network/len (Mandatory)Thenetworknumberandlength(inbits)ofthe
subnetmask
ge (Optional)Appliesthege-valuetotherangespecified
ge-value (Optional)Specifiesthelesservalueofarange(the"from"portion
oftherangedescription)
le (Optional)Appliesthele-valuetotherangespecified
le-value (Optional)Specifiesthegreatervalueofarange(the"to"portion
oftherangedescription)
Whenmultipleentriesofaprefixlistmatchagivenprefix,thesequencenumberofa
prefix-listentryidentifiestheentrywiththelowestsequencenumber.Inthiscase,
theentrywiththesmallestsequencenumberisconsideredtobethe"real"match.
Youcanspecifysequencevaluesforprefixlistentriesin
anyincrementsthatyouwant(theautomaticallygenerated
numbersareincreasedinunitsof5).Ifyouspecifythe
sequencevaluesinincrementsof1,youwillnotbeableto
insertmoreentriesintotheprefixlist.Ifyouchoosevery
largeincrements,youcouldrunoutofsequencevalues.
Youcanusetheparametersgeandletospecifytherangeoftheprefixlengthtobe
matchedforprefixesthataremorespecificthannetwork/len.Theexactmatchis
assumedwhenneithergenorleisspecified.Therangeisassumedtobefromge-
valueto32onlyifthegeattributeisspecified.Therangeisassumedtobefromlen
tole-valueonlyiftheleattributeisspecified.
Prefix list matching rules
Prefix list entries with no ge or le option match only the specified route.
Similar to IP access lists with no wildcard bits
Matching also considers the subnet mask
ConfiguringPrefixLists
a)
b)
c)
d)
e)
a)
b)
c)
d)
e)
Prefixlistentrieswithoutthegeorleoptionmatchonlytheroutewiththespecified
IPaddressandsubnetmask.Intheexamplehere,theprefixlistentrypermit
192.168.0.0/16willnotmatchtheroute192.168.2.0/24becauseofthemismatchin
theIPaddress.Itwillalsonotmatchtheroute192.168.0.0/20becauseofthe
mismatchinthesubnetmask.
A prefix list entry with ge or le option matches any prefix within specified
address space where the subnet mask falls within the specified limits.
Prefixlistentrieswiththegeorleoptionspecifiedmatchanyprefixwithinthe
addressspacethatthenetwork/lenparameterspecifies,underonecondition.The
subnetmasklengthoftheroutemustfallwithintherangethattheleandge
parametersspecify.
Inthefirstexampleinthefigure,theroute192.168.2.0/24isnotmatchedbythe
prefixlistentrypermit192.168.0.0/16eventhoughtheIPaddressfallswithinthe
specifiedaddressrange.Itisbecausethesubnetmaskistoolong.
Inthesecondexample,theroute192.168.0.0/16isnotmatchedbyprefixlistentry
permit192.168.0.0/18becausethesubnetmaskistooshort.
ConfiguringPrefixListsExample
What will be matched by:
ipprefix-listApermit0.0.0.0/0ge32
ipprefix-listBpermit128.0.0.0/2ge17
ipprefix-listCpermit0.0.0.0/0le32
ipprefix-listDpermit0.0.0.0/0
ipprefix-listEpermit0.0.0.0/1le24
Allhostroutes
AnysubnetinclassBaddressspace
Allroutes
Justthedefaultroute
AnyprefixinclassAaddressspacecoveringatleast256addresses
Thelistcontainssomecommonlyusedprefixlistexamples.
Inthelist,prefixlistApermit0.0.0./0ge32willmatchallhostroutes.PrefixlistB
permit128.0.0.0/2ge17willmatchanysubnetinaclassBaddressspace.Prefix
listCpermit0.0.0.0/0le32willmatchallroutes,butonlythedefaultroutewillbe
matchedbyprefixlistDpermit0.0.0.0/0.Finally,anyprefixinaclassAaddress
spacethatcoversatleast256addresseswillbematchedtoprefixlistEpermit
0.0.0.0/1le24.
Youcanoptionallyapplyfilterlistsandprefixlistsoneitherincomingoroutgoing
neighborsinanycombination.Boththeincomingprefixlistandtheincomingfilterlist
mustpermittheroutesthatarereceivedfromaneighborbeforetheyareaccepted
intotheBGPtable.Outgoingroutesmustpassboththeoutgoingfilterlistandthe
outgoingprefixlistbeforebeingtransmittedtotheneighbor.
WhenarouterisconfiguredtoredistributeroutinginformationfromanIGPintoBGP,
thefollowingmustbetruebeforearouteisinjectedintotheBGPtable:
Theroutesmustsuccessfullypassanyprefixlistoraccesslistthatisappliedto
theredistribution.
BGPFiltersImplementation
YoucanuseprefixliststofilterincomingoroutgoingBGPupdatestoneighbors.You
canalsouseprefixliststofilterroutesthatarebeingredistributedintotheBGP
processfromotherroutingprotocols.
router(config-router)# neighbor {ip-address|peer-group-name} prefix-
list prefix-listname {in|out}
Filters inbound or outbound BGP routing updates for a configured neighbor
session
router(config-router)# distribute-list prefix-list prefix-
list out routing-process
Filters routes redistributed from specified routing process into BGP
neighborprefix-listCommand
TodistributeBGPneighborinformationasspecifiedinaprefixlist,usetheneighbor
prefix-listrouterconfigurationcommand:
neighbor {ip-address | peer-group-name} prefix-list prefix-
listname {in | out}
Toremoveanentry,usethenoformofthiscommand:
no neighbor {ip-address | peer-group-name} prefix-list prefix-
listname {in | out}
SyntaxDescription
Parameter Description
ip-address IPaddressofneighbor
peer-group-name NameofaBGPpeergroup
prefix-listname Nameofaprefixlist
in Accesslistisappliedtoincomingadvertisementstothatneighbor
out Accesslistisappliedtooutgoingadvertisementsfromthat
neighbor
ABGPpeergroupisagroupofBGPneighborswiththe
sameupdatepolicies.Routemaps,distributelists,filter
lists,andsoonusuallysetupdatepolicies.Insteadof
definingthesamepoliciesforeachseparateneighbor,a
peergroupnameisconfiguredontherouter,andthese
policiesareassignedtothepeergroup.
distribute-listoutCommand
Tosuppressnetworksfrombeingadvertisedinupdates,usethedistribute-listout
routerconfigurationcommand:
distribute-list {access-list-number | name | prefix-list prefix-
listname} out [interface-name | routing-process | autonomous-system-
number]
Todisablethisfunction,usethenoformofthiscommand:
no distribute-list {access-list-number | name | prefix-list prefix-
listname} out [interface-name| routing-process | autonomous-system-number]
SyntaxDescription
Parameter Description
access-list-number|name StandardIPaccesslistnumberorname.
ImplementingPrefixListsintheBGPProcess
Thelistdefineswhichnetworksshouldbereceivedandwhich
shouldbesuppressedinroutingupdates.
prefix-listname Nameofaprefixlist.
Thelistdefineswhichnetworksshouldbereceivedandwhich
shouldbesuppressedinroutingupdates.Thisdecisionisbased
onmatchingofthenetworkprefixtotheprefixesinthelist.
out Appliestheaccesslisttooutgoingroutingupdates.
interface-name (Optional)Nameofaparticularinterface.
routing-process (Optional)Nameofaparticularroutingprocess,orthekeyword
staticorconnected.
autonomous-system-number (Optional)ASnumber.
Parameter Description
Althoughyoucanusetheneighborprefix-listrouter
configurationcommandasanalternativetotheneighbor
distribute-listcommand,donotuseboththeneighbor
prefix-listandneighbordistribute-listcommandfiltering
forthesameneighborinanygivendirection.Thesetwo
commandsaremutuallyexclusive,andonlyonecommand
(neighborprefix-listorneighbordistribute-list)canbe
appliedforeachinboundoroutbounddirection.
PrefixListExample:FilteringCustomerPrefixes
Thefigureillustratesfilteringcustomerprefixes.
Requirement: The customer will announce prefixes only from assigned address
space (10.0.0.0/16), with subnet masks no longer than /24.
router bgp Primary-ISP-AS
neighbor Customer prefix-list Cust-A in
!
ip prefix-list Cust-A permit 10.0.0.0/16 le 24
Inthisexample,amultihomedcustomerhasbeenassignedtheaddressspace
10.0.0.0/16.Thecustomermaysubnetthisaddressspacebutmaynotannounce
subnetssmallerthanasubnetmaskof255.255.255.0.Largersubnetsareaccepted.
Ifthecustomerhassubnettedthenetworkintosmallersubnets,itmustsummarize
theroutinginformationaboutthosesubnetsintoatleast/24prefixesbefore
announcingthem.
TheprimaryISPimplementsaprefixlistwithaCust-Aname,toperformthefiltering
ofincominginformationfromthemultihomedcustomer.Theprefixlistpermitsall
routesthatarereceivedfromthecustomerthathave10.0inthefirst16bitsand
haveasubnetmaskof24bitsorless.Anyotherroutesfromthecustomerare
deniedandsilentlyignored.
PrefixListExample:FilteringPeerPrefixes
Requirement: The ISP will not accept routes with subnet masks longer than /24;
subnet masks from class B address space will be no longer than /20.
router bgp Primary-ISP-AS
neighbor Backup-ISP prefix-list Peer in
!
ip prefix-list Peer seq 5 permit 128.0.0.0/2 le 20
ip prefix-list Peer seq 10 permit 0.0.0.0/0 le 24
Inthisexample,theprimaryISPwillnotacceptanyroutefromthecustomerthat
indicatesasubnetsmallerthana255.255.255.0subnetmask.TheclassBnetwork,
however,mustnotbesubnettedintosubnetssmallerthana255.255.240.0subnet
mask.
TheprimaryISPimplementsthisroutebyusingaprefixlistnamedPeer.Thefirst
lineintheprefixlistcheckswhetheritisaclassBnetwork.RememberthataclassB
addressalwayshasthebinarysequence10asthefirst2bitsinthefirstbyte.The
secondlinematchesanyprefix.
WhentheprimaryISPreceivesaroutefromthecustomer,itcomparestheroutewith
bothlines.IftherouteisaclassBnetwork,bothlinesmatch.Testingcontinueswith
checkingthesubnetmask.Anupperboundisexplicitlyindicated,givingamaximum
prefixlengthof20bits.
IfthereceivedrouteisnotaclassBnetwork,onlythesecondlinematches.Inthis
case,thesubnetmasklengthmustbegreaterthanorequalto0andlessthanor
equalto24.Thisruleprovidesaroutelessexplicitthana/24prefix.
Overview
Throughthisdiscovery,youwilllearnhowtoconfigureaprefixlisttofiltercustomer
prefixes.YouasanISP1serviceproviderwillallowthecustomerontherouterR2to
announceprefixesonlyfromtheassignedaddressspace(10.0.0.0/24).Youwillnot
allowthecustomertofurthersubnettheassignedaddressspace,thussubnetmask
shouldbe24.
Discovery8:FilteringCustomerPrefixes
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
R2 Ethernet0/0 172.16.22.2/24 ConnectiontoISP2
R2 Ethernet0/1 172.16.12.2/24 ConnectiontoISP1
R2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
10.0.0.1/28
10.0.0.17/28
10.0.0.33/28
10.0.0.49/28
10.0.0.65/28
Loopbackssimulate
LANnetworks
ISP1 Ethernet0/1 172.16.12.11/24 ConnectiontoR2
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
ISP1 Loopback21
Loopback37
Loopback40
10.0.21.1
10.0.37.1
10.0.40.1
Loopbackssimulate
additionalnetworksin
differentautonomous
systems.
ISP2 Ethernet0/0 172.16.22.22/24 ConnectiontoR2
ISP2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.2.1/28
10.0.2.17/28
10.0.2.33/28
10.0.2.49/28
10.0.2.65/28
10.0.2.81/28
Loopbackssimulate
LANnetworks
ISP2 Loopback21
Loopback37
Loopback40
10.0.21.1
10.0.37.1
10.0.40.1
Loopbackssimulate
additionalnetworksin
differentautonomous
systems.
IPaddressesandadvertisednetworksinBGParepreconfiguredasshownbelow:
JobAids
BGPisalsopreconfiguredasEBGP(R2toISP1andR2toISP2).
TheR2routerannouncesthesenetworks:
10.0.0.0/24(assignedaddressspacetotheCustomer)
10.0.0.16/28
10.0.0.32/28
10.0.0.48/28
10.0.0.64/28
Step1
OntheISP1router,verifythatyoureceiverouteswithmasklargerthan/24fromthe
customerrouterR2.
ISP1# show ip bgp
BGP table version is 15, local router ID is 10.0.40.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/24 172.16.12.2 0 0 1 i
*> 10.0.0.16/28 172.16.12.2 0 0 1 i
*> 10.0.0.32/28 172.16.12.2 0 0 1 i
*> 10.0.0.48/28 172.16.12.2 0 0 1 i
*> 10.0.0.64/28 172.16.12.2 0 0 1 i
*> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.16/28 0.0.0.0 0 32768 i
*> 10.0.1.32/28 0.0.0.0 0 32768 i
*> 10.0.1.48/28 0.0.0.0 0 32768 i
*> 10.0.1.64/28 0.0.0.0 0 32768 i
*> 10.0.1.80/28 0.0.0.0 0 32768 i
*> 10.0.21.0/24 0.0.0.0 0 32768 i
*> 10.0.37.0/24 0.0.0.0 0 32768 i
*> 10.0.40.0/24 0.0.0.0 0 32768 i
Youcanseethatcustomerannouncesfournetworkswithmask/28.
Step2
OntheR2router,verifythatcustomerrouterR2originatesassignednetwork10.0.0.0/24
andfoursubnetsfromthisassignedaddressspacewithmask/28.
R2# show ip bgp neighbors 172.16.12.11 advertised-routes
BGP table version is 58, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/24 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.0.64/28 0.0.0.0 0 32768 i
Total number of prefixes 5
Customerassignedaddressspaceis10.0.0.0/24,whichisfurthersubnettedtofour/28
subnets,whicharealsoannouncedinBGPupdates.
Step3
ConfigureprefixlistfilteringontheISP1routertoallowthecustomerontheR2
routertoannounceprefixesonlyfromtheassignedaddressspace(10.0.0.0/24).
Youshouldnotallowthecustomertofurthersubnettheassignedaddress
space,thussubnetmaskshouldbe24.
ISP1(config)# ip prefix-list Customer permit 10.0.0.0/24
ISP1(config)# router bgp 100
ISP1(config-router)# neighbor 172.16.12.2 prefix-list Customer in
Step4
ClearallBGPsessionsontheR2router.
FilteringCustomerPrefixes
DiscoverySteps
R2# clear ip bgp *
Step5
OntheISP1router,verifythatyoureceiverouteswithmask/24onlyfromthecustomer
routerR2.
ISP1# show ip bgp
BGP table version is 21, local router ID is 10.0.40.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/24 172.16.12.2 0 0 1 i
*> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.16/28 0.0.0.0 0 32768 i
*> 10.0.1.32/28 0.0.0.0 0 32768 i
*> 10.0.1.48/28 0.0.0.0 0 32768 i
*> 10.0.1.64/28 0.0.0.0 0 32768 i
*> 10.0.1.80/28 0.0.0.0 0 32768 i
*> 10.0.21.0/24 0.0.0.0 0 32768 i
*> 10.0.37.0/24 0.0.0.0 0 32768 i
*> 10.0.40.0/24 0.0.0.0 0 32768 i
Youcanseethatonlycustomernetwork10.0.0.0withmask/24isputintotheBGPtable
becauseoftheprefixlistfilteringconfigured.
Step6
OntheR2router,verifythatcustomerrouterR2stilloriginatesassignednetwork
10.0.0.0/24andfoursubnetsfromthisassignedaddressspacewithmask/28.
R2# show ip bgp neighbors 172.16.12.11 advertised-routes
BGP table version is 58, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/24 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.0.64/28 0.0.0.0 0 32768 i
Total number of prefixes 5
Thisway,youconfirmthatfilteringofBGPupdatesoccursontheISP1router.
Linesinaprefixlistareassignedsequencenumbers.Theseassignments
significantlyimprovethemanageabilityofthelist.Thesesequencenumber
assignmentsprovidetheopportunitytoremoveaspecificline.Ifthespacing
betweenthesequencenumbersallows,theyalsogiveyoutheabilitytoinsertaline
betweentwoexistinglines.
router# show ip prefix-list list-name [detail|summary]
Displays the prefix list and the sequence numbers
router(config)# no ip prefix-list seq seq condition
Erases the line with the specified sequence number from the prefix list
router(config)# ip prefix-list seq seq condition
Inserts the line into the prefix list at the specified point
Todisplayacurrentlyconfiguredprefixlistanditssequencenumbers,usetheshow
ipprefix-listcommandwiththedetailkeyword.
Youcanspecifysequencevaluesforprefixlistentriesinanyincrementsthatyou
want(theautomaticallygeneratednumbersareincreasedinunitsof5).Ifyou
specifythesequencevaluesinincrementsof1,youwillnotbeabletoinsertmore
entriesintotheprefixlist.Ifyouchooseverylargeincrements,youcouldrunoutof
sequencevalues.
ModifyingPrefixLists
1.
2.
Youcanmonitorprefixlistsbydisplayinginformationaboutaprefixlistorbyusing
prefixlistsforselectivefilteringofBGPtableoutputonCiscoIOSdevices.
router# show ip prefix-list [detail | summary] prefix-list-
name [network/length] [seq sequence-number] [longer] [first-match]
To display information about a prefix list or prefix-list entries
router# show ip bgp prefix-list prefix-list-name
Displays all routes in the BGP table matching the prefix list.
Used for easier monitoring of a desired network prefix group in the BGP table.
showipprefix-listCommand
Todisplayinformationaboutaprefixlistorprefixlistentries,usetheshowipprefix-
listEXECcommand:
show ip prefix-list [detail | summary] name [network/len] [seq seq-
num] [longer] [first-match]
SyntaxDescription
Parameter Description
detail|summary (Optional)Displaysdetailedorsummarizedinformationaboutall
prefixlists
name (Optional)Thenameofaspecificprefixlist
network/len (Optional)Thenetworknumberandlength(inbits)ofthenetwork
mask
seq (Optional)Appliesthesequencenumbertotheprefix-listentry
seq-num Thesequencenumberoftheprefix
longer Displaysallentriesofaprefixthataremorespecificthanthe
givennetwork/len
first-match Displaystheentryofaprefixthatmatchesthegivennetwork/len
Youcanuseshowipbgpprefix-listcommandtodisplayselectedroutesfroma
BGProutingtablebasedonthecontentsofaprefixlist.Usethiscommandfor
selectivefilteringofBGPtableoutputonCiscoIOSdevicesbasedonnetworkprefix
groups.
Toperformprefixlist-basedBGPtablefiltering,followthesesteps:
Configureaprefixlistthatpermitsrangesofnetworksthataremeanttobe
displayedintheBGPtableoutput.
Includeareferencetoaconfiguredprefixlistintheshowipbgpprefix-list
command.
ThesupportforprefixlistBGPtablefilteringwasaddedin
CiscoIOSSoftwareRelease12.2(11)Tand12.0(14)ST.
ISP1# show ip prefix-list detail InFilter
ip prefix-list InFilter:
count: 4, range entries: 3, sequences: 5 - 20, refcount: 2
seq 5 deny 128.0.0.0/2 le 15 (hit count: 0, refcount: 1)
seq 10 deny 192.0.0.0/3 ge 25 (hit count: 0, refcount: 2)
seq 15 deny 193.0.0.0/8 ge 21 (hit count: 0, refcount: 1)
seq 20 permit 0.0.0.0/0 (hit count: 0, refcount: 1)
Displays details about InFilter prefix list
Inthisexample,theshowipprefix-listcommandhasbeenissuedwiththedetail
keyword.Theoutputofthecommanddisplaysdetailedinformationaboutconfigured
prefixlists.Thesequencenumbers,theprefixlistentries,andthenumberoftimes
thatthecorrespondingprefixmatchedeachentry.
MonitoringPrefixLists
ISP1(config)# ip prefix-list MyFilter permit 10.0.1.0/24 ge 28
ISP1# show ip bgp prefix-list MyFilter
BGP table version is 21, local router ID is 10.0.40.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - inte
rnal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-
Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.16/28 0.0.0.0 0 32768 i
*> 10.0.1.32/28 0.0.0.0 0 32768 i
*> 10.0.1.48/28 0.0.0.0 0 32768 i
*> 10.0.1.64/28 0.0.0.0 0 32768 i
*> 10.0.1.80/28 0.0.0.0 0 32768 i
Displays all entries in BGP table with the first three octets equal to 10.0.1 and
with length of a subnet greater or equal to 28
Thisexampleshowsasimpleprefixlist-basedfilteringoftheBGPtable.Theprefix
listfilterpermitsallnetworkswiththefirstthreeoctetsequalto10.0.1andwithlength
ofasubnetgreaterorequalto28.Theshowipbgpprefix-listcommandoutput
displaysonlythenetworksthattheprefixlistfilterpermits.
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
Customers with multihomed networks are responsible for announcing their own
networks using BGP. Service providers with multihomed customers must take
precautions not to accept, use, or forward any erroneous routing information that
is received from their customers.
Prefix lists have a number of advantages over access lists, including faster
"permit" or "deny" determinations and easier CLI editing.
Prefix lists are configured using the ip prefix-list global configuration command.
Filter lists and prefix lists can be optionally applied on either incoming or
outgoing neighbors in any combination.
Prefix lists can filter incoming or outgoing BGP updates to neighbors and filter
routes that are being redistributed into the BGP process from other routing
protocols. Use the neighbor prefix-list router configuration command to
distribute BGP neighbor information as specified in a prefix list.
Certain Cisco IOS commands (such as the show ip prefix-list command) are
used to modify configured prefix list filters.
To display or monitor statistics about a prefix list or prefix list entries, you can
use the show ip prefix-list EXEC command.
Summary
Overview
YoucanuseORFasanextramechanismtominimizethenumberofupdatesthat
arerequestedfromaneighbor.Itwillreducethelinkbandwidthconsumptionand
CPUusewhenarouterrequestsarouterefresh.AnORFalsoallowsfilteringof
informationthatexternalnetworksshouldnotreceive(suchasRFC1918
information).Understandinghowtomonitoroutboundroutefilteringcapabilitiesis
alsoimportant.ABGPneighborthatsupportsspecificORFcapabilitieswillreport
thosecapabilitiestoamonitoringneighborandcanthensendafilterofthe
supportedtypetotheneighbor.
ThislessondiscussesthefunctionofoutboundroutefilteringinaBGPnetwork.The
formatandfunctionofORFmessagesarediscussed,aswellasthecommandsthat
enableORFnegotiationsandtheactivationofanORFprefixlist.Thecommands
thatareusedtotriggerarouterefresharealsodetailed.Finally,thereisa
discussiononhowtomonitortheoperationsofaconfiguredORFinaBGPnetwork.
Uponcompletingthislesson,youwillbeableto:
DescribethefunctionofoutboundroutefilteringinaBGPnetwork
Comparestandardinboundfilteringandoutboundroutefiltering
DescribethefunctionofBGPprefix-basedoutboundroutefiltering
DescribetheformatandfunctionofanORFmessage
IdentifytheCiscoIOScommandthatisrequiredtoenableORFnegotiationsand
activateanORFprefixlist
IdentifytheCiscoIOScommandthatisusedtotriggerarouterefreshmessage
IdentifytheCiscoIOScommandthatisrequiredtomonitortheoperationofa
configuredORF
UsingOutboundRouteFiltering
Outboundroutefilteringisaprefix-basedBGPfeaturethatisenabledthroughthe
advertisementofORFcapabilitiestopeerroutersandisintegratedinCiscoIOS
SoftwareRelease12.2(4)T.TheadvertisementoftheORFcapabilityindicatesthata
BGP-speakingrouterwillacceptaprefixlistfromaneighbor.Itwillapplytheprefix
listtolocallyconfiguredORFs(ifanyexist).Whenthiscapabilityisenabled,theBGP
speakercaninstallaninboundprefixlistfiltertotheremotepeerasanoutbound
filter,whichreducesunwantedroutingupdates.
The purpose of outbound route filtering is to reduce the amount of BGP traffic
and CPU use needed to process routing updates.
Routers exchange inbound filter configurations, which are used as outbound
filters on neighboring routers.
Filters are described in ORF entries.
ORF entries are part of the route refresh message.
ThestandardrouterefreshmessagecontainstheAFIforwhichtherefreshis
needed.Outboundroutefilteringisanextramechanismthatisusedtominimizethe
numberofupdatesthatarerequestedfromaneighbor.
ThismechanismreducesthelinkbandwidthconsumptionandCPUusewhena
routerrequestsarouterefresh.Filtersthattheroutersshouldusewiththeroute
refresharedescribedinORFentriesthatarepartoftherouterefreshmessage.
YoucanconfiguretheORFfeaturewithsend,receive,orsendandreceive
capabilities.ThelocalpeeradvertisestheORFcapabilityinthesendmode.The
remotepeerreceivestheORFcapabilityinreceivemodeandappliesthefilterasthe
outboundpolicy.ThelocalandremotepeersexchangeupdatestomaintaintheORF
foreachrouter.PeerroutersexchangeupdatesdependingontheORFprefixlist
capabilitythatisadvertised.Theremotepeerstartssendingupdatestothelocal
peerafteritreceivesarouterefreshrequestoranORFprefixlistwithanimmediate
status.
OutboundRouteFiltering
Thefigureillustratesthecomparisonbetweenstandardinboundfilteringand
outboundroutefiltering.
Outboundroutefilteringcanlimitthenumberofunwantedroutingupdates,whichwill
reducetheamountofresourcesthatarerequiredforroutingupdategenerationand
processing.Thisfeaturealsoreducestheamountofresourcesthatarerequiredto
receiveanddiscardroutes.Thereceivingrouterwouldotherwisefilteroutthese
routes,iftheORFfeaturewerenotavailable.
Theexampleshowstwoscenarios:
Thefirstexampleshowsthat600,000routesaresentfromAS1toAS2
neighbor,andtheinputfilteronAS2permitsonly100oftheseroutes.
ThesecondexampleshowshowarouterefreshwithafilterissentfromAS2to
theAS1neighbor.TheAS1neighborthenusesthefilterbeforesendingthe
updates.Thisway,only100updatesaresentfromAS1toAS2.
Inboundvs.OutboundFilteringExample
TheBGPprefix-basedoutboundroutefilteringfeatureusesBGPORFsendand
receivecapabilitiestominimizethenumberofBGPupdatesthataresentbetween
BGPpeers.Configuringthisfeaturecanhelpreducetheamountofsystem
resourcesthatarerequiredforgeneratingandprocessingroutingupdatesbyfiltering
outunwantedroutingupdatesatthesource.
Uses BGP ORF send and receive capabilities to minimize the number of BGP
updates that are sent between BGP peers
Helps to reduce the amount of system resources that are required for generating
and processing routing updates by filtering out unwanted routing updates at the
source
Limits the number of unwanted routing updates, which will reduce the amount of
resources that are required for routing update generation and processing
Reduces the amount of resources that are queried to receive and discard routes
that would otherwise be filtered out
TheBGPprefix-basedoutboundroutefilteringfeatureisenabledthroughthe
advertisementofORFcapabilitiestopeerrouters.TheadvertisementoftheORF
capabilityindicatesthataBGPspeakerwillacceptaprefixlistfromaneighbor.Itwill
applytheprefixlisttolocallyconfiguredORFs(ifanyexist).Whenthiscapabilityis
enabled,theBGPspeakercaninstalltheinboundprefixlistfiltertotheremotepeer
asanoutboundfilter,whichreducesunwantedroutingupdates.
TheBGPprefix-basedoutboundroutefilteringfeaturecanbeconfiguredwithsend,
receive,orsendandreceiveORFcapabilities.ThelocalpeeradvertisestheORF
capabilityinsendmode.TheremotepeerreceivestheORFcapabilityinreceive
modeandappliesthefilterasanoutboundpolicy.Thelocalandremotepeers
exchangeupdatestomaintaintheORFoneachrouter.Updatesareexchanged
betweenpeerroutersbyaddressfamilydependingontheORFprefixlistcapability
thatisadvertised.Theremotepeerstartssendingupdatestothelocalpeeraftera
routerefreshhasbeenconfiguredwiththeclearipbgpcommand.OrafteranORF
prefixlistwithimmediatestatusisprocessed.TheBGPspeakerwillcontinueto
applytheinboundprefixlisttoreceivedupdatesafterthespeakerpushesthe
inboundprefixlisttotheremotepeer.
BGPPrefix-BasedOutboundRouteFiltering
ItisimportanttounderstandthefieldsORFmessage.
ORF format
An ORF message consists of the following fields:
AFI/SAFI
ORF type
When to refresh
List of ORF entries
ORF entries depend on the ORF type.
The ORF capability needs to be negotiated for every supported ORF type.
AnORFmessagecontainsthefollowinginformation:
AFIandSAFI,forwhichthefiltershouldbeused.
ORFtype,whichidentifiesthetypeoffilter.
Whentorefresh(immediateordeferredrefresh).
ListofORFentrieswheretheactualfilterisdefined.
YoucanusetheAFI/SAFIcomponentoftheORFmessagetoprovideacoarselevel
ofgranularcontrolbylimitingtheORFtoonlytherouteswhoseNLRImatchesthe
configuredAFI/SAFIcomponent.
TherouterhastonegotiatetheORFcapabilityforeachORFtypethatissupported
intheORFmessage.
ORF types:
NLRI (ORF type = 1)
Filters that are based on the prefix
Communities (ORF type = 2)
Filters that are based on standard BGP community attributes
Extended communities (ORF type = 3)
Filters that are based on extended BGP community attributes
Prefix list (ORF type = 128)
Filters that are based on Cisco implementation of prefix filtering
ThevaluethatiscontainedintheORFtypedeterminesthecontentthatiscontained
intheORFmessage.
Currently,ORFtype0isreserved,ORFtypes1to127areassignedbytheIANA,
andORFtypes128to255arevendor-specific(andnotassignedbytheIANA).
CommonlyusedORFtypesareasfollows:
ORFtype1isusedtofilterbasedontheNLRI.
ORFtype2isusedtofilterbasedonstandardBGPcommunityattributes.
ORFtype3isusedtofilterbasedonextendedBGPcommunityattributes.
ORFtype128isusedtofilterbasedontheCiscoproprietaryimplementationof
prefixfiltering(prefixlists).
AFI/SAFI is IPv4 unicast.
ORF type is NLRI:
Action: ADD, DELETE, or DELETE ALL
Match: PERMIT or DENY
Scope: EXACT or REFINE
NLRI: Prefix
When: IMMEDIATE or DEFER
TheORF-typesettingdeterminesthecontentoftheORFvalue.AnORFtypeof
NLRI-basedfiltering(type1)usesthefollowingactions:
ADD:Addsalinetoaprefixlistfilterontheremotepeer
DELETE:Removesalinefromafilterthatwaspreviouslyinstalledonaremote
peer
DELETEALL:Removesallpreviouslyinstalledfiltersontheremotepeer
Foreachfilterentry,thereisamatchcomponentthatspecifieseitherPERMITor
OutboundRouteFilterMessage
DENY.APERMITasksthepeertosendupdateswithroutesthatmatchthesetof
entriesasspecifiedintheORF.DENYspecifiesthattheremotepeershouldnot
sendupdatesfortheentriesmatchingtheentriesthatarespecifiedintheORF.
ForprefixesspecifiedwithamatchcomponentofPERMIT,theremotepeerisasked
topassaprefixwithascopeofEXACT(anexactmatch)orREFINE(itssubnets).
AlsocontainedwithintheORFmessageisthewhen-to-refreshfield.Aroutercanset
thisfieldtoIMMEDIATE—askingtheremotepeertorefreshassoonasithas
finishedprocessingtheORFmessage.OritcansetittoDEFER—askingtheremote
peertowaituntilitreceivesasubsequentrouterefreshmessagewiththesame
AFI/SAFI.
YouneedtoenableORFnegotiationsandactivateanORFprefixlistwhen
configuringoutboundroutefiltering.
router(config-router)# neighbor ip-address capability orf prefix-
list [receive|send|both]
This command enables negotiation of prefix list ORF capability during session
setup.
The ORF-capable BGP speaker will install ORFs per neighbor.
Option:
"Both" allows sending and receiving of prefix lists.
"Send" allows only sending of prefix lists.
"Receive" allows only receiving of prefix lists.
Ciscorouterssupporttheuploadingoftheirprefixliststoaneighbor.Youneedto
usetheneighborip-addresscapabilityorfprefix-listreceivecommandto
advertisethiscapability.Youneedtouseneighborip-addresscapabilityorf
prefix-listsendcommandtouploadtheinboundprefixfiltertotheneighbor.The
uploadedfilteristhenusedontheneighboringrouterafterastaticallyconfigured
outboundprefixlist(ifitexists)isapplied.
Theneighborip-addresscapabilityorfprefix-listcommandenablesthe
negotiationoftheprefix-listORFcapabilityduringBGPsessionsetup.Theprefix-
list-basedORF(ORFtype=128)istheonlyORFtypethatCiscoIOSsoftware
supports.
neighbororfprefix-listCommand
ToadvertiseORFcapabilitiestoapeerrouter,usetheneighbororfprefix-list
commandinaddressfamilyorrouterconfigurationmode:
neighbor {ip-address} [capability] orf prefix-list [receive | send | both]
TodisableORFcapabilities,usethenoformofthiscommand.
no neighbor {ip-address} [capability] orf prefix-
list [receive | send | both]
SyntaxDescription
Parameter Description
ip-address TheIPaddressoftheneighborrouter
capability (Optional)InformsthespecifiedneighborthatthisrouterhasORF
capabilities
receive (Optional)EnablestheORFprefixlistcapabilityinreceivemode
send (Optional)EnablestheORFprefixlistcapabilityinsendmode
both (Optional)EnablestheORFprefixlistcapabilityinbothreceive
andsendmodes
AS1:
router bgp 100
neighbor 172.16.12.2 capability orf prefix-list receive
AS2:
ip prefix-list FILTER seq 5 permit 10.0.1.16/28
!
router bgp 1
neighbor 172.16.12.11 capability orf prefix-list send
ConfiguringOutboundRouteFiltering
1.
2.
3.
neighbor 172.16.12.11 prefix-list FILTER in
!
The command capability orf prefix-list send on one router requires capability
orf prefix-list receive on a neighboring router.
Theexampleshowstheconfigurationoftworouterswhereonerouterhasuploaded
aninputprefixlisttotheneighbortobeusedasanoutputfilter.
Thefollowingconfigurationstepsarenecessarytoenableoutboundroutefiltering:
Enablenegotiationofoutboundfilteringbasedonprefixlists.
Attachaninputprefixlisttoaneighbor.
Enablesendingofinputprefixlisttotheneighbor.
Intheexample,anORFhasbeenconfiguredonAS2toadvertisethefiltertoAS1.
AnIPprefixlistwiththenameFILTERiscreatedtospecifythe10.0.1.16/28subnet
foroutboundroutefiltering.TheORFsendcapabilityisconfiguredonAS2sothat
AS2canadvertisetheORFtoAS1.
AS1isconfiguredtoadvertisetheORFreceivecapabilitytoAS2.AS1willinstallthe
ORF,definedintheFILTERprefixlist,aftertheORFcapabilitieshavebeen
exchanged.AninboundsoftresetneedstobeinitiatedonAS1attheendofthis
configurationtoactivatetheORF.
IdentifytheCiscoIOScommandthatisusedtotriggerarouterefreshmessage.
router# clear ip bgp neighbor in [prefix-filter]
This command triggers a route refresh message.
This command includes a prefix list in the route refresh message if configured
and supported on both ends.
The prefix list is sent at session setup.
Use the prefix-filter option to refresh the remote filter.
Usetheclearipbgpneighborcommandwiththeprefix-filterkeywordtopushout
theexistingORFprefixlistsothatanewrouterefreshwillbereceivedfroma
neighbor.TheneighborwillusetheORFprefixlistthatwaspreviouslynegotiated.
Youneedtousetheclearipbgpneighborcommandonlywhenthefilterhasbeen
modifiedbecausetheneighborwillstorethefilterforsubsequentrouterefresh
requests.Theneighborwillthenusethefilteronallupdatestowardtherouterthat
originatedthefilter.
Youshouldentertheinkeywordwhenyouareusingthe
clearipbgpneighborcommandbecauseinboundroute
refreshisdesired;onlytheinboundprefixlistfilteris
pushedtotheneighborandusedbytheneighborinthe
outbounddirection.
Therouterwillignoretheprefix-filterkeywordifORFcapabilityhasnotbeen
received,orthesendcapabilityhasnotbeenenabled.
Whentheclearipbgpneighborcommandisusedwithouttheprefix-filterkeyword,
anormalrouterefreshisperformed.Youshouldalwaysusetheprefix-filter
keywordwhenORFinboundroutingpolicychangesoccur.
UsingOutboundRouteFiltering
Overview
Throughthisdiscovery,youwilllearnhowtoconfigureprefix-basedoutboundroute
filtering.YouwillcreateanoutboundroutefilterandconfiguretheR2routerto
advertisethefiltertotheISP1router.ThefilterwillallowtheISP1routertoadvertise
10.0.1.16/28networkonlytotheR2router.
Discovery9:Prefix-BasedOutboundRouteFiltering
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
R2 Ethernet0/0 172.16.22.2/24 ConnectiontoISP2
R2 Ethernet0/1 172.16.12.2/24 ConnectiontoISP1
R2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
10.0.0.1/28
10.0.0.17/28
10.0.0.33/28
10.0.0.49/28
10.0.0.65/28
Loopbackssimulate
LANnetworks
ISP1 Ethernet0/1 172.16.12.11/24 ConnectiontoR2
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
ISP1 Loopback21
Loopback37
Loopback40
10.0.21.1
10.0.37.1
10.0.40.1
Loopbackssimulate
extranetworksin
differentautonomous
systems.
ISP2 Ethernet0/0 172.16.22.22/24 ConnectiontoR2
ISP2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
Loopback7
Loopback8
Loopback9
10.0.2.1/28
10.0.2.17/28
10.0.2.33/28
10.0.2.49/28
10.0.2.65/28
10.0.2.81/28
10.0.2.97/28
10.0.2.113/28
10.0.2.129/28
Loopbackssimulate
LANnetworks
ISP2 Loopback21
Loopback37
Loopback40
10.0.21.1
10.0.37.1
10.0.40.1
Loopbackssimulate
extranetworksin
differentautonomous
systems.
IPaddressesandadvertisednetworksinBGParepreconfiguredasshownbelow:
JobAids
BGPisalsopreconfiguredasEBGP(R2toISP1andR2toISP2).TheISP1router
announcesthesenetworks:
10.0.1.0/28(assignedaddressspacetotheCustomer)
10.0.1.16/28
10.0.1.32/28
10.0.1.48/28
10.0.1.64/28
10.0.1.80/28
Step1
OntheISP1router,verifywhichnetworksareadvertisedviaBGPtotheR2router.
ISP1# show ip bgp neighbor 172.16.12.2 advertised-routes
BGP table version is 15, local router ID is 10.0.40.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.16/28 0.0.0.0 0 32768 i
*> 10.0.1.32/28 0.0.0.0 0 32768 i
*> 10.0.1.48/28 0.0.0.0 0 32768 i
*> 10.0.1.64/28 0.0.0.0 0 32768 i
*> 10.0.1.80/28 0.0.0.0 0 32768 i
*> 10.0.21.0/24 0.0.0.0 0 32768 i
*> 10.0.37.0/24 0.0.0.0 0 32768 i
*> 10.0.40.0/24 0.0.0.0 0 32768 i
YoushouldseetheseBGProutesbeingadvertisedtotheR2router:
Anysubnetsin10.0.1.0/24range.
Networks10.0.21.0/24,10.0.37.0/24and10.0.40.0/24.
Step2
OntheR2router,verifywhichroutesarereceivedviaBGPfromtheISP1router.
R2# show ip bgp
BGP table version is 162, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/24 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.0.64/28 0.0.0.0 0 32768 i
*> 10.0.1.0/28 172.16.12.11 0 0 100 i
*> 10.0.1.16/28 172.16.12.11 0 0 100 i
*> 10.0.1.32/28 172.16.12.11 0 0 100 i
*> 10.0.1.48/28 172.16.12.11 0 0 100 i
*> 10.0.1.64/28 172.16.12.11 0 0 100 i
*> 10.0.1.80/28 172.16.12.11 0 0 100 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
*> 10.0.2.16/28 172.16.22.22 0 0 200 i
*> 10.0.2.32/28 172.16.22.22 0 0 200 i
Network Next Hop Metric LocPrf Weight Path
*> 10.0.2.48/28 172.16.22.22 0 0 200 i
*> 10.0.2.64/28 172.16.22.22 0 0 200 i
*> 10.0.2.80/28 172.16.22.22 0 0 200 i
*> 10.0.2.96/28 172.16.22.22 0 0 200 i
*> 10.0.2.112/28 172.16.22.22 0 0 200 i
*> 10.0.2.128/28 172.16.22.22 0 0 200 i
* 10.0.21.0/24 172.16.12.11 0 0 100 37 40 21 i
*> 172.16.22.22 0 0 200 21 i
*> 10.0.37.0/24 172.16.12.11 0 0 100 37 i
* 172.16.22.22 0 0 200 21 40 37 i
*> 10.0.40.0/24 172.16.12.11 0 0 100 37 40 i
* 172.16.22.22 0 0 200 21 40
YoushouldseetheseBGProutesbeingreceivedontheR2routerfromtheISP1router:
Anysubnetsin10.0.1.0/24range.
Networks10.0.21.0/24,10.0.37.0/24and10.0.40.0/24.
Prefix-BasedOutboundRouteFiltering
DiscoverySteps
Step3
ConfiguretheISP1routertoadvertiseORFreceivecapabilityfromtheR2
neighbor.
ISP1(config)# router bgp 100
ISP1(config-router)# neighbor 172.16.12.2 capability orf prefix-
list receive
Step4
ConfiguretheR2routertoadvertiseORFsendcapabilitytotheISP1router.
R2(config)# router bgp 1
R2(config-router)# neighbor 172.16.12.11 capability orf prefix-
list send
Step5
OntheR2router,configuretheprefixlistwiththenameFILTER,whichwillonly
permit10.0.1.16/28network.Thisprefixlistwillbeusedforprefix-based
outboundroutefiltering.
R2(config)# ip prefix-list FILTER seq 5 permit 10.0.1.16/28
Step6
OntheR2router,applythe"FILTER"prefixlisttoincomingadvertisementsfrom
theISP1neighbor.
R2(config)# router bgp 1
R2(config-router)# neighbor 172.16.12.11 prefix-list FILTER in
Step7
ClearallBGPsessionsontherouterISP1.
ISP1# clear ip bgp *
Step8
OntheISP1router,verifythattheORFcapabilityexchangeisenabled.
ISP1# show ip bgp neighbors 172.16.12.2
BGP neighbor is 172.16.12.2, remote AS 1, external link
BGP version 4, remote router ID 10.0.0.65
BGP state = Established, up for 00:15:44
Last read 00:00:27, last write 00:00:09, hold time is 180, keepalive interval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable (disabled)
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN Capability: advertised and received
Address family IPv4 Unicast: advertised and received
Enhanced Refresh Capability: advertised and received
Multisession Capability:
Stateful switchover support enabled: NO for session 1
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 1 1
Notifications: 0 0
Updates: 2 2
Keepalives: 18 19
Route Refresh: 0 0
Total: 21 25
Default minimum time between advertisement runs is 30 seconds
For address family: IPv4 Unicast
Session: 172.16.12.2
BGP table version 15, neighbor version 15/0
Output queue size : 0
Index 28, Advertise bit 0
28 update-group member
AF-dependant capabilities:
Outbound Route Filter (ORF) type (128) Prefix-list:
Send-mode: received
Receive-mode: advertised
<...output omitted...>
YoushouldseethattheORFsendmodecapabilitywasreceivedandtheORFreceivemodecapability
wasadvertised.
Step9
OntheISP1router,verifythatithasreceivedanIPprefixlistthatdefinesan
outboundroutefilterforthe10.0.1.16/28subnet.
ISP1# show ip bgp neighbors 172.16.12.2 received prefix-filter
Address family: IPv4 Unicast
ip prefix-list 172.16.12.2: 1 entries
seq 5 permit 10.0.1.16/28
Step10
OntheISP1router,verifythatBGPisadvertising10.0.1.16/28subnetonlytotheR2
router.
ISP1# show ip bgp neighbor 172.16.12.2 advertised-routes
BGP table version is 16, local router ID is 10.0.40.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.1.16/28 0.0.0.0 0 32768 i
Step11
OntheR2router,verifythatonly10.0.1.16/28wasreceivedfromtheISP1router.
R2# show ip bgp
BGP table version is 137, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/24 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.0.64/28 0.0.0.0 0 32768 i
*> 10.0.1.16/28 172.16.12.11 0 0 100 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
*> 10.0.2.16/28 172.16.22.22 0 0 200 i
*> 10.0.2.32/28 172.16.22.22 0 0 200 i
*> 10.0.2.48/28 172.16.22.22 0 0 200 i
*> 10.0.2.64/28 172.16.22.22 0 0 200 i
*> 10.0.2.80/28 172.16.22.22 0 0 200 i
*> 10.0.2.96/28 172.16.22.22 0 0 200 i
*> 10.0.2.112/28 172.16.22.22 0 0 200 i
Network Next Hop Metric LocPrf Weight Path
*> 10.0.2.128/28 172.16.22.22 0 0 200 i
*> 10.0.21.0/24 172.16.22.22 0 0 200 21 i
*> 10.0.37.0/24 172.16.22.22 0 0 200 21 40 37 i
*> 10.0.40.0/24 172.16.22.22 0 0 200 21 40 i
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
The BGP prefix-based outbound route filtering feature uses BGP ORF send and
receive capabilities to minimize the number of BGP updates that are sent
between BGP peers.
Configuring ORF helps you to reduce the amount of system resources that are
required for generating and processing routing updates by filtering out unwanted
routing updates at the source.
The ORF message contains the information that is used to determine which
updates will be passed.
The neighbor ip-address capability orf prefix-list command with the send and
receive keywords enables ORF negotiations and activates an ORF prefix-list.
Use the clear ip bgp neighbor command to trigger a BGP route refresh.
With the show ip bgp neighbors command, neighbor-supported ORF
capabilities are displayed as "advertised, received," and a filter of the supported
type can be sent to the neighbor.
You can verify that prefix list was received on ORF receiver using the show ip
bgp neighbors ip-address received prefix command.
Summary
Overview
BGPisapowerfulroutingprotocolthatsupportsawidevarietyofadministrative
policycontrolsandrouteselectionfeatures.Youcannotachievemanycomplex
filteringgoalsandadministrativepoliciesbyusingonlysingle-purposefiltering
methodsorbycompoundingmultiplefilteringmethodstogether.Routemapsprovide
amethodtoperformvariouscompound,complexfilteringoperationswithinasingle
tool.Understandingtheoperationanduseofroutemapsisacriticalcomponentin
thesuccessfulimplementationofanylarge-scaleBGPdeployment.
Inthislesson,youwilllearnaboutroutemapsandhowyoucanusethemforBGP
filtering.Youwilllearnthecommandsthatarerequiredtouseroutemapswithprefix
lists.YouwillalsolearnhowtouseprefixlistsasBGPfiltersandhowtomonitor
previouslyconfiguredroutemaps.
Uponcompletingthislesson,youwillbeableto:
Describethehigh-levelfunctionofaroute-map
DescribethefunctionoftheBGPRoute-MapPolicyListSupportfeature
DescribethefunctionoftheBGPRoute-MapContinuefeature
IdentifytheCiscoIOScommandsthatarerequiredtoconfigurearoutemapto
matchagainstaprefixlist
IdentifywhereyoucanapplyroutemapsasroutefiltersinaBGPnetwork
IdentifytheCiscoIOScommandthatisrequiredtoenablearoutemapasa
BGProutefilter
IdentifytheCiscoIOScommandsthatarerequiredtomonitortheoperationofa
configuredroutemapthatisusedasaBGPfilter
ApplyingRouteMapsasBGPFilters
Aroutemapisafilter,andwhattheroutemapdeniesisdropped.Additionally,you
canusetheroutemaptomodifyattributesofthepermittedroutes.
Route maps are very complex access lists:
Access lists have lines.
Route maps contain statements.
Access lists use addresses and masks.
Route maps use match conditions.
With access lists, there is an access list number.
With route maps, there is a route map name.
Statements in route maps are numbered.
You can insert and delete statements in a route map.
You can edit match conditions in a statement.
Route map statements can modify matched routes with "set" options.
Routemapsaresimilartoaccesslists.Bothhaveasetofteststobeperformed,and
severaltestscanbedoneinsequence.Thefirstmatchproducestheresultofeither
"permit"or"deny."
Anaccesslisthasanumberoflines,eachindicatingatestingcondition.Theroute
mapismorecomplexthantheaccesslist.Theroutemapconsistsofseveralgroups
ofconfigurationlines;eachgroupiscalledastatement.Thestatementhasa
sequencenumberthatprovidestheopportunitytoremoveormodifyanexplicit
statementwithoutremovingtheentireroutemap.Thereisalsoanopportunitytoadd
anewstatementbetweentwoexistingstatements.
Eachroutemapstatementstartswithaconfigurationlineindicatingthenameofthe
routemap,thesequencenumber,andwhethertheresultshouldbepermittedor
deniedifthetestingmatches.Thestatementthencontinues,followingconfiguration
lineswiththematchclauses.Matchingcanbedoneinseveralways:testingonthe
prefix,theASpath,orsomeotherattribute.Thestatementconcludeswithoptional
"set"statements,whereattributesmaybemodifiedorset.
route-map name [permit|deny sequence]
match condition
match condition
set parameter
The default statement action is "permit."
A route that is not matched by any statement is dropped.
"Permit all" is achieved by specifying "permit" without a "match" clause.
Match conditions in one statement are ANDed together.
The first matching statement permits or denies the route.
Aroutemapconsistsofseveralstatements.Eachstatementstartswiththeroute
mapconfigurationline,onwhichthenameoftheroutemapmustbeindicated.A
goodpracticeistoalwaysindicatethepermitordenykeywordfollowedbya
sequencenumber.
Thematchingclausesforthestatementsarelistedonthematchlinesfollowingthe
routemapline.Theremaybeseveralmatchlines,eachreferringtoadifferenttestto
beperformed.Alltestsmustbepassedforthestatementtobematched.Ifanyofthe
matchlinetestsfails,thenextroutemapstatementistried.Statementsaretriedin
sequencenumberorder.Iftherearenomorestatementsintheroutemap,theresult
is,implicitly,"deny."
Ifallofthematchclausessucceed,thereisamatchforthestatementandthe
indicatedresultisused.Iftheresultistodeny,therouteisthensilentlyignored.If
theresultistopermit,therouteisacceptedandthesetclausesareapplied.Theset
clausesallowoneormoreattributestobechangedorsettospecificvaluesbefore
therouteisaccepted.
Route map conditions are specified in the match statement.
Route maps can match on:
Network number and subnet mask that are matched with an IP prefix list
Route originator
RouteMapOverview
BGP next-hop address
BGP origin
Tag attached to IGP route
AS path
BGP community that is attached to BGP route
IGP route type (internal/external ...)
Eachroutemapstatementcanhaveseveralmatchclauses,andeachmatchclause
isgivenitsownconfigurationline.Thematchclausereferstotheteststhatwillbe
madeonthecandidateroute.Testsofthecandidateroutecanbebasedonthe
followingcriteria:
IPnetworknumbersandsubnetmasks,byreferringtoaprefixlistoraccesslist
thatwillbeappliedontheroute.
Routeoriginator,byreferringtoaprefixlistoraccesslistthatwillbeappliedon
thevalueoftheoriginatorBGPattribute.
Nexthop,byreferringtoaprefixlistoraccesslistthatwillbeappliedonthe
valueofthenext-hopBGPattribute.
Origincode,bytestingthevalueoftheoriginBGPattribute.
TagvaluethatisattachedtoanIGProute—usedonlywhenredistributionfrom
anIGPintoBGPoccurs.
ASpath,byreferringtoanASpathaccesslistthatwillbeappliedonthevalue
oftheASpathBGPattribute.
Community,byreferringtoacommunitylistthatwillbeappliedonthevalueof
theCommunityBGPattribute.
IGProutetype,bytestingiftheIGProuteisinternalorexternal—usedonly
whenredistributionfromanIGPtoBGPoccurs.
Route maps can also change the attributes of BGP routes.
Route maps can set:
Origin
BGP next-hop
Weight
BGP community
Local preference
MED
Eachroutemapstatementmayhaveseveralsetclauses.Eachsetclauseisapplied
totheroutewhentheroutemapstatementpermitstheroute.Witharoutemap,the
followingcanbeset:
OriginBGPattribute
Next-hopBGPattribute
Weight
CommunityBGPattribute
LocalpreferenceBGPattribute
MEDBGPattribute,bysettingthemetric
TheBGPRoute-MapPolicyListSupportfeatureintroducesnewfunctionalitytoBGP
routemaps,allowingyoutogrouproutemapmatchclausesintonamedlists—policy
lists.Apolicylistfunctionslikeamacro.
Adds the capability for a network operator to group route map match clauses
into named lists that are called policy lists.
Simplifies the configuration of BGP routing policy in medium-size and large
networks. You can preconfigure policy lists with groups of match clauses and
then reference these policy lists within different route maps.
Eliminates need to manually reconfigure each recurring group of match clauses
that occur in multiple route map entries.
Whenapolicylistisreferencedinaroutemap,allofthematchclausesare
evaluatedandprocessedasiftheyhadbeenconfigureddirectlyintheroutemap.
TheBGPRouteMapPolicyListSupportfeaturesimplifiestheconfigurationofBGP
routingpolicyinmedium-sizeandlargenetworks.Anetworkoperatorcan
preconfigurepolicylistswithgroupsofmatchclausesandthenreferencethese
policylistswithindifferentroutemaps.Younolongerneedtomanuallyreconfigure
eachrecurringgroupofmatchclausesinmultipleroutemapentries.
Apolicylistislikearoutemapthatcontainsonlymatchclauses.Thepolicylistis
createdandthenreferencedwithinaroutemap.Therearenochangestomatch
clausesemanticsandroutemapfunctions.Matchclausesareconfiguredinpolicy
listswithpermitanddenystatements.Theroutemapevaluatesandprocesseseach
matchclauseandpermitsordeniesroutesbasedontheconfiguration.ANDandOR
semanticsintheroutemapfunctionthesamewayforpolicylistsastheydofor
matchandsetclauses.TherearesomecommandsthatarerelatedtotheBGP
Route-MapPolicyListSupportfeature:theippolicy-listcommand,thematch
policy-listcommand,andtheshowippolicy-listcommand.
router# ip policy-list policy-list-name {permit | deny}
Creates a BGP policy list
router# match policy-list policy-list-name
Configures a route map to evaluate and process a BGP policy list in a route map
router# show ip policy-list policy-list-name
Displays one or all filter lists
TocreateaBGPpolicylist,usetheippolicy-listcommandinthepolicymap
configurationmode.
ip policy-list policy-list-name {permit | deny}
Toremoveapolicy-list,usethenoformofthiscommand
no ip policy-list policy-list-name
SyntaxDescription
Parameter Description
policy-list-name Nameoftheconfiguredpolicylist
permit Permitsaccessformatchingconditions
deny Deniesaccesstomatchingconditions
ToconfigurearoutemaptoevaluateandprocessaBGPpolicylistinaroutemap,
usethematchpolicy-listcommandintheroutemapconfigurationmode.
match policy-list policy-list-name
Toremoveapathlistentry,usethenoformofthiscommand.
BGPRoute-MapPolicyListSupport
no match policy-list policy-list-name
SyntaxDescription
Parameter Description
policy-list-name Nameoftheconfiguredpolicylist
Todisplayinformationaboutaconfiguredpolicylistandpolicylistentries,usethe
showippolicy-listcommandintheuserEXECmode.
show ip policy-list policy-list-name
SyntaxDescription
Parameter Description
policy-list-name Nameoftheconfiguredpolicylist
ConfiguringPolicyListExamples
ThefollowingconfigurationexamplecreatesaBGPpolicy-listthatpermitsmatches
ontheASpathandMEDofarouter:
Router(config)# ip policy-list POLICY-LIST-NAME-1 permit
Router(config-policy-list)# match as-path 1
Router(config-policy-list)# match metric 10
Router(config-policy-list)# end
ThefollowingconfigurationexamplecreatesaBGPpolicylistthatpermitsmatches
onthespecifiedBGPcommunityandthenexthopofarouter:
Router(config)# ip policy-list POLICY-LIST-NAME-2 permit
Router(config-policy-list)# match community 20
Router(config-policy-list)# match metric 10
Router(config-policy-list)# ip community-list 20 permit 20:1
Router(config-policy-list)# end
ThefollowingconfigurationexamplecreatesaBGPpolicylistthatdeniesmatches
onthespecifiedBGPcommunityandthenexthopofarouter:
Router(config)# ip policy-list POLICY-LIST-NAME-3 deny
Router(config-policy-list)# match community 20
Router(config-policy-list)# match metric 10
Router(config-policy-list)# end
ConfiguringRouteMapstoReferencePolicyListExamples
TheconfigurationexamplesinthissectioncreateBGProutemapsthatreference
BGPpolicylistswiththeroute-maproutemapconfigurationcommand.
Thefollowingconfigurationexamplecreatesaroutemapthatreferencespolicylists
andseparatematchandsetclausesinthesameconfiguration.Thisexampleuses
ANDsemanticsbetweenPOLICY-LIST-NAME-1andPOLICY-LIST-NAME-2.
Router(config)# route-map MAP-NAME-1 10
Router(config-route-map)# match ip-address 1
Router(config-route-map)# match policy-list POLICY-LIST-NAME-1
Router(config-route-map)# match policy-list POLICY-LIST-NAME-2
Router(config-route-map)# set community 10:1
Router(config-route-map)# set local-preference 140
Router(config-route-map)# end
Thefollowingconfigurationexamplecreatesaroutemapthatreferencespolicylists
andseparatematchandsetclausesinthesameconfiguration.Thisexampleuses
ORsemanticsbetweenPOLICY-LIST-NAME-3andPOLICY-LIST-NAME-4.
Router(config)# route-map MAP-NAME-2 10
Router(config-route-map)# match policy-list POLICY-LIST-NAME-3 POLICY-
LIST-NAME-4
Router(config-route-map)# set community 10:1
Router(config-route-map)# set local-preference 140
Router(config-route-map)# end
VerifyingBGPRoute-MapPolicyListSupport
Toverifythatapolicylisthasbeencreated,usetheshowippolicy-listcommand.
Theoutputofthiscommanddisplaysthepolicylistnameandconfiguredmatch
clauses.Thefollowingsampleoutputissimilartotheoutputthatwillbedisplayed:
Router# show ip policy-list
policy-list POLICY-LIST-NAME-1 permit
Match clauses:
metric 20
policy-list POLICY-LIST-NAME-2 permit
Match clauses:
as-path (as-path filter): 1
Apolicylistnamecanbespecifiedwhentheshowip
policy-listcommandisentered.Thisoptioncanbeuseful
forfilteringtheoutputofthiscommandandverifyinga
singlepolicylist
Toverifythataroutemaphasbeencreatedandapolicylistisreferenced,usethe
showroute-mapcommand.Theoutputofthiscommanddisplaystheroutemap
nameandpolicy-liststhattheconfiguredroutemapsreference.Thefollowing
sampleoutputissimilartotheoutputthatwillbedisplayed:
Router# show route-map
route-map ROUTE-MAP-NAME-1, deny, sequence 10
Match clauses:
Set clauses:
Policy routing matches: 0 packets, 0 bytes
route-map ROUTE-MAP-NAME-1, permit, sequence 10
Match clauses:
IP Policy lists:
POLICY-LIST-NAME-1
Set clauses:
Policy routing matches: 0 packets, 0 bytes
TheBGPRoute-MapContinuefeatureintroducesthecontinueclausetotheBGP
routemapconfiguration.Thecontinueclauseprovidesmoreprogrammablepolicy
configurationandroutefiltering.Itintroducestheabilitytoexecuteextraentriesina
routemapafteranentryisexecutedwithsuccessfulmatchandsetclauses.
Continueclausesallowyoutoconfigureandorganizemoremodularpolicy
definitionstoreducethenumberofpolicyconfigurationsthatarerepeatedwithinthe
sameroutemap.
Introduces the continue clause to the BGP route map configuration, providing
more programmable policy configuration and route filtering
Enables you to execute extra entries in a route map after an entry is executed
with successful match and set clauses
Allows configuration and organization of more modular policy definitions to
reduce the number of policy configurations that are repeated within the same
route map
Allows modularization of network policy configuration so that repeated policy
definitions can be reduced within the same route map
Continueclausesprovideaprogrammablemethodtoorganizeandcontroltheflow
ofaroutemap.Routemapconfigurationwaslinearbeforethisfeaturewas
introduced.Continueclausesalsoallowyoutomodularizenetworkpolicy
configurationsothatrepeatedpolicydefinitionscanbereducedwithinthesame
routemap.
RouteMapOperationWithoutContinueClauses
Aroutemapevaluatesmatchclausesuntilasuccessfulmatchoccurs.Afterthe
matchoccurs,theroutemapstopsevaluatingmatchclausesandstartsexecuting
setclauses,intheorderinwhichtheywereconfigured.Ifasuccessfulmatchdoes
notoccur,theroutemap"fallsthrough"andevaluatesthenextsequencenumberof
theroutemap.Itrepeatsuntilallconfiguredroutemapentrieshavebeenevaluated
orasuccessfulmatchoccurs.Eachroutemapsequenceistaggedwithasequence
numbertoidentifytheentry.Routemapentriesareevaluatedinorder,startingwith
thelowestsequencenumberandendingwiththehighestsequencenumber.Ifthe
routemapcontainsonlysetclauses,thesetclausesareexecutedautomatically,and
theroutemapdoesnotevaluateanyotherroutemapentries.
RouteMapOperationwithContinueClauses
Whenacontinueclauseisconfigured,theroutemapcontinuestoevaluateand
executematchclausesinthespecifiedroutemapentryafterasuccessfulmatch
occurs.Thecontinueclausecanbeconfiguredtogoto(orjumpto)aspecificroute
mapentrybyspecifyingthesequencenumber.Or,ifasequencenumberisnot
specified,togotothenextsequencenumber.Thisbehavioriscalledan"implied
continue."Ifamatchclauseexists,thecontinueclauseisexecutedonlyifamatch
occurs.Ifnosuccessfulmatchesoccur,thecontinueclauseisignored.
Ifamatchclausedoesnotexistintheroutemapentrybutacontinueclausedoes,
thecontinueclauseisautomaticallyexecutedandgoestothespecifiedroutemap
entry.Ifamatchclauseexistsinaroutemapentry,thecontinueclauseisexecuted
onlywhenasuccessfulmatchoccurs.Whenasuccessfulmatchoccursanda
continueclauseexists,theroutemapexecutesthesetclausesandthengoestothe
specifiedroutemapentry.Ifthenextroutemapcontainsacontinueclause,theroute
mapexecutesthecontinueclauseifasuccessfulmatchoccurs.Ifacontinueclause
doesnotexistinthenextroutemap,theroutemapisevaluatednormally.Ifa
continueclauseexistsinthenextroutemapbutamatchdoesnotoccur,theroute
mapdoesnotcontinue.Itfallsthroughtothenextsequencenumber,ifoneexists.
Acontinueclausecanbeexecutedwithoutasuccessful
matchifaroutemapentrydoesnotcontainamatch
clause.
router# continue sequence-number
Configures a route map to go to a route map entry with a higher sequence
number
BGPRouteMapContinue
router# show route-map [map-name]
Displays configured route maps
YouwillusetwocommandswiththeBGPRoute-MapContinuefeature,the
continuecommandandtheshowroute-mapcommand.
Toconfigurearoutemaptogotoaroutemapentrywithahighersequencenumber,
usethecontinuecommandinroutemapconfigurationmode.
continue sequence-number
Toremoveacontinueclausefromaroutemap,usethenoformofthiscommand.
no continue
SyntaxDescription
Parameter Description
sequence-number (Optional)Routemapsequencenumber.
Ifaroutemapsequencenumberisnotspecifiedwhenconfiguringacontinue
clause,thecontinueclausecontinuestotheroutemapentrywiththenext
sequencenumber.Thisbehaviorisreferredtoasan"impliedcontinue"
Todisplaytheconfiguredroutemaps,usetheshowroute-mapcommandinthe
EXECmode.
show route-map [map-name]
SyntaxDescription
Parameter Description
map-name (Optional)Nameofaspecificroutemap
BGPRoute-MapContinueClauseConfigurationExample
Thefollowingexampleshowsthecontinueclauseconfigurationinaroutemap
sequence.
Thefirstcontinueclauseinroutemapentry10indicatesthattheroutemapwillgoto
routemapentry30ifasuccessfulmatchoccurs.Ifamatchdoesnotoccur,theroute
mapwillfallthroughtoroutemapentry20.Ifasuccessfulmatchoccursinroute
mapentry20,thesetactionwillbeexecutedandtheroutemapwillnotevaluateany
additionalroutemapentries.
Ifasuccessfulmatchdoesnotoccurinroutemapentry20,theroutemapwillfall
throughtoroutemapentry30.Thissequencedoesnotcontainamatchclause,so
thesetclausewillbeautomaticallyexecuted.Thecontinueclausewillgotothenext
routemapentrybecauseasequencenumberisnotspecified.
Iftherearenosuccessfulmatches,theroutemapwillfallthroughtoroutemapentry
30andexecutethesetclause,androutemapentry40willnotbeevaluated.
route-map ROUTE-MAP-NAME permit 10
match ip address 1
match metric 10
set as-path prepend 10
continue 30
!
route-map ROUTE-MAP-NAME permit 20
match ip address 2
match metric 20
set as-path prepend 10 10
!
route-map ROUTE-MAP-NAME permit 30
set as-path prepend 10 10 10
continue
!
route-map ROUTE-MAP-NAME permit 40
match community 10:1
set local-preference 104
BGPRoute-MapContinueClauseVerificationExample
Toverifytheconfigurationofcontinueclauses,usetheshowroute-mapcommand.
Theoutputofthiscommanddisplaysconfiguredroutemaps,match,set,and
continueclauses.Thefollowingsampleoutputissimilartotheoutputthatwillbe
displayed:
Router# show route-map
route-map ROUTE-MAP-NAME, permit, sequence 10
Match clauses:
ip address (access-lists): 1
metric 10
Continue: sequence 40
Set clauses:
as-path prepend 10
Policy routing matches: 0 packets, 0 bytes
route-map ROUTE-MAP-NAME, permit, sequence 20
Match clauses:
ip address (access-lists): 2
metric 20
Set clauses:
as-path prepend 10 10
Policy routing matches: 0 packets, 0 bytes
route-map ROUTE-MAP-NAME, permit, sequence 30
Match clauses:
Continue: to next entry 40
Set clauses:
as-path prepend 10 10 10
Policy routing matches: 0 packets, 0 bytes
route-map ROUTE-MAP-NAME, permit, sequence 40
Match clauses:
community (community-list filter): 10:1
Set clauses:
local-preference 104
Policy routing matches: 0 packets, 0 bytes
route-map LOCAL-POLICY-MAP, permit, sequence 10
Match clauses:
Set clauses:
community 655370
Policy routing matches: 0 packets, 0 bytes
Youcanuseprefixlistsasmatchcriteriainseveralmatchingstatementsintheroute
map.
router(config-route-map)# match ip address prefix-list list-name
Uses prefix list to match routes in route-map match condition
router(config-route-map)#match ip next-hop prefix-list list-name
Matches routes where the next hop matches the conditions in the prefix list
router(config-route-map)# match ip route-source prefix-list list-name
Matches routes that are received from BGP peer that matches the prefix list
Youcanusethematchipaddresscommandintheroutemapconfigurationmode
toperformpolicyroutingonpackets.Youcanusethesamecommandtodistribute
anyroutesthathaveadestinationnetworknumberaddressthatastandardaccess
list,anextendedaccesslist,oraprefixlistpermits.
match ip address {access-list-number [access-list-number... | access-list-
name...] | access-list-name [access-list-number...| access-list-
name] | prefix-list prefix-list-name [prefix-list-name...]}
Toremovethematchipaddressentry,usethenoformofthiscommand.
no match ip address {access-list-number [access-list-number... | access-
list-name...] | access-list-name [access-list-number...| access-list-
name] | prefix-list prefix-list-name [prefix-list-name...]}
SyntaxDescription
Parameter Description
access-list-
number...
Numberofastandardorextendedaccesslist.
Itcanbeanintegerfrom1to199.Theellipsisindicatesthatmultiplevaluescan
beentered.
access-list-name... Nameofastandardorextendedaccesslist.
Itcanbeanintegerfrom1to199.Theellipsisindicatesthatmultiplevaluescan
beentered.
prefix-list Distributesroutesbasedonaprefixlist.
prefix-list-name... Nameofaspecificprefixlist.
Theellipsisindicatesthatmultiplevaluescanbeentered.
Toredistributeanyroutesthathaveanext-hoprouteraddressthatispassedbyone
ofthespecifiedaccesslists,usethematchipnext-hopcommandintheroutemap
configurationmode.
match ip next-hop {access-list-number | access-list-name}[...access-list-
number | ...access-list-name]
Toremovethenexthopentry,usethenoformofthiscommand.
no match ip next-hop {access-list-number | access-list-name}[...access-
list-number | ...access-list-name]
SyntaxDescription
Parameter Description
access-list-number Numberofastandardorextendedaccess-list.
Itcanbeanintegerfrom1to199.
access-list-name Nameofastandardorextendedaccess-list.
Itcanbeanintegerfrom1to199.
Toredistributeroutesthathavebeenadvertisedbyroutersandaccessserversat
PrefixListUseinRouteMaps
theaddressthattheaccesslistsspecify,usethematchiproute-sourcecommand
intheroutemapconfigurationmode.
match ip route-source {access-list-number | access-list-name}[...access-
list-number | ...access-list-name]
Toremovetheroute-sourceentry,usethenoformofthiscommand.
no match ip route-source {access-list-number | access-list-name}
[...access-list-number | ...access-list-name]
SyntaxDescription
Parameter Description
access-list-number Numberofastandardorextendedaccesslist.
Itcanbeanintegerfrom1to199.
access-list-name Nameofastandardorextendedaccesslist.
Itcanbeanintegerfrom1to199.
Youcanoptionallyapplyfilterlists,prefixlists,androutemapstoeitherincomingor
outgoinginformationoranycombinationofthetwo.
Theincomingprefixlist,theincomingfilterlist,andtheincomingroutemapmustall
permittheroutesthatarereceivedfromaneighborbeforebeingacceptedintothe
BGPtable.Outgoingroutesmustpasstheoutgoingfilterlist,theoutgoingprefixlist,
andtheoutgoingroutemapbeforebeingtransmittedtotheneighbor.Whenarouter
isconfiguredtoredistributeroutinginformationfromanIGPintoBGP,theroutes
mustsuccessfullypassanyprefixlistorroutemapthatisappliedtothe
redistribution.ThishappensbeforearouteisinjectedintotheBGPtable.
BGPFilters
Youcanapplyaroutemaponincomingoroutgoingroutinginformationfora
neighbor.Theroutemapmustpermittheroutinginformationinordertobeaccepted.
Ifthereisnostatementintheroutemapexplicitlypermittingaroute,thentheroute
willbeimplicitlydeniedanddropped.
router(config-router)# neighbor ip-address route-map name [in | out]
This command applies a route map to incoming or outgoing BGP updates.
Prefixes not permitted by the route map are discarded.
Route maps can also change BGP attributes in incoming or outgoing updates.
Route maps, filter lists, and prefix lists are evaluated in sequence (effectively
ANDed together).
Thepermittedroutesmayhavetheirattributessetorchangedbythesetclausesin
theroutemap.Settingattributesonroutesisusefulwhenyouareinfluencingthe
routeselection.Oneofthestatementsintheroutemapcanpermitsomeroutesand
changetheirattributes.Anotherstatementintheroutemapcouldpermitotherroutes
andnotaltertheirattributes.Whenrouteselectionisperformed,theattributevalues
indicatethatonerouteispreferredovertheother.
Requirement: The customer will accept only a default route from both service
providers.
router bgp 1
neighbor Primary-ISP remote-as 100
neighbor Primary-ISP route-map filter in
neighbor Secondary-ISP remote-as 100
neighbor Secondary-ISP route-map filter in
!
route-map filter permit 10
match ip address prefix-list DefaultOnly
!
ip prefix-list DefaultOnly seq 10 permit 0.0.0.0/0
Inthisexample,thecustomerwillacceptonlyadefaultroutefrombothservice
providers.ThecustomerdoesnotwanttoputanyothernetworkintotheBGPtable
thanthedefaultroute.ThedefaultrouteisannouncedviaBGPtothecustomerfrom
bothserviceproviders.
Intheexample,theroutemapwithanamefilterisappliedininbounddirectionon
thecustomerrouterforbothserviceproviderneighbors.Theroutemapismatching
theprefixlist,whichpermitsdefaultrouteonly.
UsingRouteMapsasBGPFilters
Overview
Throughthisdiscovery,youwilllearnhowtoconfigureroutemapstofilterBGP
updates.YouasacustomerconfiguringrouterR2willacceptonlythedefaultroute
thatisannouncedviaBGPfromtheISP1router.Youwillnotacceptanyother
networksthatareannouncedviaBGPfromtheserviceprovider.
Discovery10:ConfigureRouteMapsasBGPFilters
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
R2 Ethernet0/0 172.16.22.2/24 ConnectiontoISP2
R2 Ethernet0/1 172.16.12.2/24 ConnectiontoISP1
R2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
10.0.0.1/28
10.0.0.17/28
10.0.0.33/28
10.0.0.49/28
10.0.0.65/28
Loopbackssimulate
LANnetworks
ISP1 Ethernet0/1 172.16.12.11/24 ConnectiontoR2
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
ISP1 Loopback21
Loopback37
Loopback40
10.0.21.1
10.0.37.1
10.0.40.1
Loopbackssimulate
extranetworksin
differentautonomous
systems.
ISP2 Ethernet0/0 172.16.22.22/24 ConnectiontoR2
ISP2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
Loopback7
Loopback8
Loopback9
10.0.2.1/28
10.0.2.17/28
10.0.2.33/28
10.0.2.49/28
10.0.2.65/28
10.0.2.81/28
10.0.2.97/28
10.0.2.113/28
10.0.2.129/28
Loopbackssimulate
LANnetworks
ISP2 Loopback21
Loopback37
Loopback40
10.0.21.1
10.0.37.1
10.0.40.1
Loopbackssimulate
extranetworksin
differentautonomous
systems.
IPaddressesandadvertisednetworksinBGParepreconfiguredasshownbelow:
JobAids
BGPisalsopreconfiguredasEBGP(R2toISP1andR2toISP2).TheISP1router
announcesthesenetworks:
Defaultroute0.0.0.0/0
10.0.1.16/28
10.0.1.32/28
10.0.1.48/28
10.0.1.64/28
10.0.1.80/28
Step1
OntheISP1router,verifythatthedefaultrouteisbeingannouncedinanEBGPsession
tothecustomerrouterR2.
ISP1# show ip bgp neighbor 172.16.12.2 advertised-routes
BGP table version is 13, local router ID is 10.0.40.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Originating default network 0.0.0.0
Network Next Hop Metric LocPrf Weight Path
*> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.16/28 0.0.0.0 0 32768 i
*> 10.0.1.32/28 0.0.0.0 0 32768 i
*> 10.0.1.48/28 0.0.0.0 0 32768 i
*> 10.0.1.64/28 0.0.0.0 0 32768 i
*> 10.0.1.80/28 0.0.0.0 0 32768 i
*> 10.0.21.0/24 0.0.0.0 0 32768 i
*> 10.0.37.0/24 0.0.0.0 0 32768 i
*> 10.0.40.0/24 0.0.0.0 0 32768 i
YoucanseethattheISP1routeroriginatesthedefaultroute.
Step2
OntheISP2router,verifythatthedefaultrouteisbeingannouncedinanEBGPsession
tothecustomerrouterR2.
ISP2# show ip bgp neighbors 172.16.22.2 advertised-routes
BGP table version is 20, local router ID is 10.0.40.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Originating default network 0.0.0.0
Network Next Hop Metric LocPrf Weight Path
*> 10.0.2.0/28 0.0.0.0 0 32768 i
*> 10.0.2.16/28 0.0.0.0 0 32768 i
*> 10.0.2.32/28 0.0.0.0 0 32768 i
*> 10.0.2.48/28 0.0.0.0 0 32768 i
*> 10.0.2.64/28 0.0.0.0 0 32768 i
*> 10.0.2.80/28 0.0.0.0 0 32768 i
*> 10.0.2.96/28 0.0.0.0 0 32768 i
*> 10.0.2.112/28 0.0.0.0 0 32768 i
*> 10.0.2.128/28 0.0.0.0 0 32768 i
*> 10.0.21.0/24 0.0.0.0 0 32768 i
*> 10.0.37.0/24 0.0.0.0 0 32768 i
*> 10.0.40.0/24 0.0.0.0 0 32768 i
Network Next Hop Metric LocPrf Weight Path
Total number of prefixes 12
YoucanseethattheISP2routeroriginatesthedefaultroute.
Step3
OntheR2router,verifythatthecustomerrouterR2receivesdefaultroutefromboth
neighbors,theISP1andISP2routers.YoushouldalsomakesurethattheR2router
receivesseveralotherroutesfrombothISProuters.
R2# show ip bgp
BGP table version is 26, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
* 0.0.0.0 172.16.22.22 0 200 i
*> 172.16.12.11 0 100 i
*> 10.0.0.0/24 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.0.64/28 0.0.0.0 0 32768 i
*> 10.0.1.0/28 172.16.12.11 0 0 100 i
*> 10.0.1.16/28 172.16.12.11 0 0 100 i
*> 10.0.1.32/28 172.16.12.11 0 0 100 i
*> 10.0.1.48/28 172.16.12.11 0 0 100 i
*> 10.0.1.64/28 172.16.12.11 0 0 100 i
*> 10.0.1.80/28 172.16.12.11 0 0 100 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
Network Next Hop Metric LocPrf Weight Path
*> 10.0.2.16/28 172.16.22.22 0 0 200 i
*> 10.0.2.32/28 172.16.22.22 0 0 200 i
*> 10.0.2.48/28 172.16.22.22 0 0 200 i
*> 10.0.2.64/28 172.16.22.22 0 0 200 i
*> 10.0.2.80/28 172.16.22.22 0 0 200 i
*> 10.0.2.96/28 172.16.22.22 0 0 200 i
*> 10.0.2.112/28 172.16.22.22 0 0 200 i
ConfigureRouteMapsasBGPFilters
DiscoverySteps
*> 10.0.2.112/28 172.16.22.22 0 0 200 i
*> 10.0.2.128/28 172.16.22.22 0 0 200 i
*> 10.0.21.0/24 172.16.22.22 0 0 200 21 i
* 172.16.12.11 0 0 100 37 40 21 i
* 10.0.37.0/24 172.16.22.22 0 0 200 21 40 37 i
*> 172.16.12.11 0 0 100 37 i
* 10.0.40.0/24 172.16.22.22 0 0 200 21 40 i
*> 172.16.12.11 0 0 100 37 40 i
YoucanseethatthedefaultrouteontheR2routerwasreceivedfromtwodifferentEBGP
neighbors:
OnedefaultroutewasreceivedfromtheISP1routerwiththeASpathattributesetto
100andBGPnexthopsetto172.16.12.11.ThisaddressistheIPaddressofthe
ISP1router.
AnotherdefaultroutewasreceivedfromtheISP2routerwiththeASpathattributeset
to200andBGPnexthopsetto172.16.22.22.ThisaddressistheIPaddressofISP2
router.
YoushouldreceivetheseBGProutesontheR2router:
Anysubnetsin10.0.1.0/24rangefromtheISP1router
Anysubnetsin10.0.2.0/24rangefromtheISP2router
Networks10.0.21.0/24,10.0.37.0/24,10.0.40.0/24fromboththeISP1andISP2
routers
Step4
ConfiguretheroutemapontheR2router.Theroutemapshouldpermitonlythe
defaultroute,thatisdefinedintheprefixlist,tobeacceptedfromtheISP1EBGP
neighbor.
R2(config)# ip prefix-list DefaultOnly permit 0.0.0.0/0
R2(config)# route-map ISP1 permit 10
R2(config-route-map)# match ip address prefix-list DefaultOnly
R2(config-route-map)# exit
R2(config)# router bgp 1
R2(config-router)# neighbor 172.16.12.11 route-map ISP1 in
Step5
EnabledebuggingofBGPupdatesontheR2router.
R2# debug ip bgp update
Step6
ClearallBGPsessionsontheR2router.
R2# clear ip bgp *
Step7
OntheR2router,observethatallnondefaultrouteBGPupdatesreceivedfromtheISP1routerweredeniedbecauseoftheconfiguredroutemap.Onlythe
defaultroutethatisreceivedviatheEBGPsessionfromtheISP1routershouldbeinstalledtotheroutingtable.
%BGP-5-ADJCHANGE: neighbor 172.16.12.11 Down User reset
*Mar 6 00:36:02.681: %BGP_SESSION-5-ADJCHANGE: neighbor 172.16.12.11 IPv4 Unicast topology base removed from session User reset
*Mar 6 00:36:02.681: %BGP-5-ADJCHANGE: neighbor 172.16.22.22 Down User reset
*Mar 6 00:36:02.681: %BGP_SESSION-5-ADJCHANGE: neighbor 172.16.22.22 IPv4 Unicast topology base removed from session User reset
*Mar 6 00:36:03.107: %BGP-5-ADJCHANGE: neighbor 172.16.12.11 Up
*Mar 6 00:36:03.109: %BGP-5-ADJCHANGE: neighbor 172.16.22.22 Up
*Mar 6 00:36:03.113: BGP(0): 172.16.12.11 rcvd UPDATE w/ attr: nexthop 172.16.12.11, origin i, metric 0, merged path 100 37 40, AS_PATH
*Mar 6 00:36:03.113: BGP(0): 172.16.12.11 rcvd 10.0.40.0/24 -- DENIED due to: route-map;
*Mar 6 00:36:03.113: BGP(0): 172.16.12.11 rcvd UPDATE w/ attr: nexthop 172.16.12.11, origin i, metric 0, merged path 100 37, AS_PATH
*Mar 6 00:36:03.113: BGP(0): 172.16.12.11 rcvd 10.0.37.0/24 -- DENIED due to: route-map;
*Mar 6 00:36:03.113: BGP(0): 172.16.12.11 rcvd UPDATE w/ attr: nexthop 172.16.12.11, origin i, metric 0, merged path 100 37 40 21, AS_PATH
*Mar 6 00:36:03.113: BGP(0): 172.16.12.11 rcvd 10.0.21.0/24 -- DENIED due to: route-map;
*Mar 6 00:36:03.113: BGP(0): 172.16.12.11 rcvd UPDATE w/ attr: nexthop 172.16.12.11, origin i, metric 0, merged path 100, AS_PATH
*Mar 6 00:36:03.113: BGP(0): 172.16.12.11 rcvd 10.0.1.0/28 -- DENIED due to: route-map;
*Mar 6 00:36:03.113: BGP(0): 172.16.12.11 rcvd 10.0.1.16/28 -- DENIED due to: route-map;
*Mar 6 00:36:03.113: BGP(0): 172.16.12.11 rcvd 10.0.1.32/28 -- DENIED due to: route-map;
*Mar 6 00:36:03.113: BGP(0): 172.16.12.11 rcvd 10.0.1.48/28 -- DENIED due to: route-map;
*Mar 6 00:36:03.113: BGP(0): 172.16.12.11 rcvd 10.0.1.64/28 -- DENIED due to: route-map;
*Mar 6 00:36:03.113: BGP(0): 172.16.12.11 rcvd 10.0.1.80/28 -- DENIED due to: route-map;
*Mar 6 00:36:03.114: BGP(0): 172.16.12.11 rcvd UPDATE w/ attr: nexthop 172.16.12.11, origin i, merged path 100, AS_PATH
*Mar 6 00:36:03.114: BGP(0): 172.16.12.11 rcvd 0.0.0.0/0
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd UPDATE w/ attr: nexthop 172.16.22.22, origin i, metric 0, merged path 200 21 40, AS_PATH
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd 10.0.40.0/24
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd UPDATE w/ attr: nexthop 172.16.22.22, origin i, metric 0, merged path 200 21 40 37, AS_PATH
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd 10.0.37.0/24
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd UPDATE w/ attr: nexthop 172.16.22.22, origin i, metric 0, merged path 200 21, AS_PATH
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd 10.0.21.0/24
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd UPDATE w/ attr: nexthop 172.16.22.22, origin i, metric 0, merged path 200, AS_PATH
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd 10.0.2.0/28
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd 10.0.2.16/28
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd 10.0.2.32/28
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd 10.0.2.48/28
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd 10.0.2.64/28
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd 10.0.2.80/28
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd 10.0.2.96/28
R2#
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd 10.0.2.112/28
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd 10.0.2.128/28
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd UPDATE w/ attr: nexthop 172.16.22.22, origin i, merged path 200, AS_PATH
*Mar 6 00:36:03.114: BGP(0): 172.16.22.22 rcvd 0.0.0.0/0
*Mar 6 00:36:04.130: BGP(0): Revise route installing 1 of 1 routes for 0.0.0.0/0 -> 172.16.12.11(global) to main IP table
*Mar 6 00:36:04.130: BGP(0): Revise route installing 1 of 1 routes for 10.0.2.0/28 -> 172.16.22.22(global) to main IP table
*Mar 6 00:36:04.130: BGP(0): Revise route installing 1 of 1 routes for 10.0.2.16/28 -> 172.16.22.22(global) to main IP table
*Mar 6 00:36:04.130: BGP(0): Revise route installing 1 of 1 routes for 10.0.2.32/28 -> 172.16.22.22(global) to main IP table
*Mar 6 00:36:04.130: BGP(0): Revise route installing 1 of 1 routes for 10.0.2.48/28 -> 172.16.22.22(global) to main IP table
*Mar 6 00:36:04.130: BGP(0): Revise route installing 1 of 1 routes for 10.0.2.48/28 -> 172.16.22.22(global) to main IP table
*Mar 6 00:36:04.130: BGP(0): Revise route installing 1 of 1 routes for 10.0.2.64/28 -> 172.16.22.22(global) to main IP table
*Mar 6 00:36:04.130: BGP(0): Revise route installing 1 of 1 routes for 10.0.2.80/28 -> 172.16.22.22(global) to main IP table
*Mar 6 00:36:04.130: BGP(0): Revise route installing 1 of 1 routes for 10.0.2.96/28 -> 172.16.22.22(global) to main IP table
*Mar 6 00:36:04.130: BGP(0): Revise route installing 1 of 1 routes for 10.0.2.112/28 -> 172.16.22.22(global) to main IP table
*Mar 6 00:36:04.130: BGP(0): Revise route installing 1 of 1 routes for 10.0.2.128/28 -> 172.16.22.22(global) to main IP table
Step8
OntheR2router,verifythecontentofBGPtable.
R2# show ip bgp
BGP table version is 19, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
* 0.0.0.0 172.16.22.22 0 200 i
*> 172.16.12.11 0 100 i
*> 10.0.0.0/24 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.0.64/28 0.0.0.0 0 32768 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
*> 10.0.2.16/28 172.16.22.22 0 0 200 i
*> 10.0.2.32/28 172.16.22.22 0 0 200 i
*> 10.0.2.48/28 172.16.22.22 0 0 200 i
*> 10.0.2.64/28 172.16.22.22 0 0 200 i
*> 10.0.2.80/28 172.16.22.22 0 0 200 i
*> 10.0.2.96/28 172.16.22.22 0 0 200 i
Network Next Hop Metric LocPrf Weight Path
*> 10.0.2.112/28 172.16.22.22 0 0 200 i
*> 10.0.2.128/28 172.16.22.22 0 0 200 i
*> 10.0.21.0/24 172.16.22.22 0 0 200 21 i
*> 10.0.37.0/24 172.16.22.22 0 0 200 21 40 37 i
*> 10.0.40.0/24 172.16.22.22 0 0 200 21 40 i
YoushouldonlyseethedefaultroutethatisinstalledforBGPupdatesfromtheISP1
routerwithnexthopIPaddress172.16.12.11.Allothernetworkswerereceivedfromthe
ISP2router.
DisplayingRoutesMatchingtheRouteMap
DisplayingRoutesMatchingtheRouteMap
router# show ip bgp route-map route-map-name
Displays all routes in BGP table matching the route map
Used for filtering the show ip bgp output on basis of BGP path attributes:
Community
Local preference
Weight
Origin
Next-hop
Can also filter based on prefixes
Allows powerful combined filtering
YoucanalsouseroutemapsforselectiveandpowerfulfilteringoftheBGPtable.
Theshowipbgproute-mapcommanddisplaystheselectedroutesfromaBGP
routingtablebasedonthecontentsofaroutemap.
AroutemapcanmatchtheroutesbasedonBGPpathattributes(localpreference,
community,weight,origin,next-hop)orprefixlistsandaccesslists(matchingIP
prefixes).Thepowerofroutemapfilteringliesinthepossibilityofcombiningdifferent
filters(forexample,filteringoncommunity,prefix,andnext-hopvalues).
Step9
ConfiguretheroutemapontheR2router.Theroutemapshouldpermitonly
networkspassingthroughtheautonomoussystem40.
R2(config)# ip as-path access-list 2 permit _40_
R2(config)# route-map PassAS40 permit 10
R2(config-route-map)# match as-path 2
Step10
OntheR2router,displaythecontentofBGPtablematchingtheconfiguredroutemap.
R2# show ip bgp route-map PassAS40
BGP table version is 19, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.37.0/24 172.16.22.22 0 0 200 21 40 37 i
*> 10.0.40.0/24 172.16.22.22 0 0 200 21 40 i
Youshouldonlyseethenetworkscontaining"40"intheASpath.
Youshouldonlyseethenetworkscontaining"40"intheASpath.
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
A route map is a filter that has the ability to drop denied routes as well as modify
attributes of the permitted routes.
The BGP Route-Map Policy List Support feature introduces new functionality to
BGP route maps. It adds the ability for a network operator to group route map
match clauses into named lists that are called policy lists.
The BGP Route-Map Continue feature introduces the continue clause to BGP
route map configuration. Continue clauses provide a programmable method to
organize and control the flow of a route map.
You can configure a route map to match against a prefix list by using the match
ip address, match ip next-hop, and match ip route-source commands.
Filter lists, prefix lists, and route maps can optionally all be applied on either
incoming or outgoing information in any combination.
A route map can be applied on incoming or outgoing routing information to or
from a neighbor. However, the route map must permit the routing information in
order to be accepted.
Monitoring route maps is possible using the show ip bgp and debug ip bgp
update commands.
Summary
Overview
BecauseofthehugevolumesofroutinginformationthatBGPiscapableofhandling,
traditionalroutingupdatemethodsarenotfeasible.RoutingpoliciesforaBGP
neighbormayincludefilteringmechanismssuchasroutemaps,distributelists,prefix
lists,andASpathfilterlists.Eachofthesefiltersmayaffectinboundoroutbound
routingtableupdates.
Wheneverthereisanadministrativechangeinaroutingpolicy,theBGPsession
mustberesetbeforethenewpolicycantakeeffect.Toaccomplishthistask,you
canusetwotypesofreset:hardresetandsoftreset.ClearingaBGPsessionusing
ahardresetinvalidatesthecacheandresultsinanegativeimpactontheoperation
ofnetworks,becausetheinformationinthecachebecomesunavailable.Asoftreset
isrecommendedbecauseitallowsroutingtablestobereconfiguredandactivated
withoutclearingtheBGPsession.
ThislessondiscussesroutingupdatesinaBGPenvironmentandthetraditional
methodsofforcingBGProuteupdatesafterchangesinafilterpolicy.Thefunction
andbenefitsofsoftreconfigurationandrouterefresharealsodiscussed.Thelesson
alsopresentsthecommandsthatarerequiredtoperformasoftreconfigurationand
routerefreshandexplainshowtomonitorandtroubleshootthesefeatures.
Uponcompletingthislesson,youwillbeableto:
IdentifythelimitationsofthetraditionalmethodsofforcingBGProuteupdates
afterchangingafilterpolicy
DescribethefunctionoftheBGPSoftResetEnhancementfeature
Describethefunctionandbenefitsoftherouterefreshfunction
IdentifytheCiscoIOScommandthatisrequiredtotriggerarouterefresh
IdentifytheCiscoIOScommandsthatarerequiredtomonitorrouterefresh
operation
ImplementingChangesinBGPPolicy
BGPcanpotentiallyhandlehugevolumesofroutinginformation.Butitcannot
handleitwhenyouchangeconfigurationlinesinfiltersorroutemaps.Inthiscase,
theroutercannotgothroughthehugetableofBGPinformationandcalculatewhich
entryisnolongervalidinthelocaltable.Norcantherouterdeterminewhichrouteor
routes,alreadyadvertised,shouldbewithdrawnfromaneighbor.Thereisan
obviousriskthatthefirstconfigurationchangewillbeimmediatelyfollowedbya
second,whichwouldcausethewholeprocesstostartalloveragain.
All filters apply only to new incoming and outgoing updates.
To change the outbound routing policy, you have to resend BGP updates to your
neighbors.
To change the inbound routing policy, you have to force your neighbor to resend
the updates to you.
The traditional mechanism is to clear BGP sessions.
Toavoidsuchaproblem,CiscoIOSsoftwareapplieschangesonlyontheupdates
thatarereceivedortransmittedaftertheconfigurationchangehasbeenperformed.
Thisapproachmeansthatthenewroutingpolicy,whichthenewfiltersenforce,is
appliedonlyonroutesthatarereceivedorsentafterthechange.Ifnetwork
administratorswouldliketoapplythepolicychangeonallroutes,theyhavetoforce
theroutertoletallroutespassthroughthenewfilter.
Ifthefilterisappliedtooutgoinginformation,therouterhastoresendtheentireBGP
tablethroughthenewfilter.Ifthefilterisappliedtoincominginformation,therouter
needsitsneighbortoresenditsentireBGPtablesothatitpassesthroughthenew
filters.
Traditionally,toaccomplishthesegoals,youhavetorndowntheaffectedBGP
sessionsaftercompletingaconfigurationchange.Afterthesessionsaredown,all
informationthatisreceivedonthosesessionsisinvalidatedandremovedfromthe
BGPtable.Also,theremoteneighborwilldetectasessiondownstate,andit
likewisewillinvalidatetheroutesthatarereceivedonthesession.Afteraperiodof
30to60seconds,thesessionsarere-establishedautomaticallyandtheentireBGP
tableisexchangedagain,butthroughthenewfilters.Thisprocess,however,
disruptspacketforwarding.
TraditionalLimitationsofClearingtheBGPSession
router# clear ip bgp {* | ip-address | peer-group-name}
This command tears down the BGP session with all neighbors, a specific
neighbor, or all neighbors in a peer group.
All BGP routes are lost after the session is torn down; connectivity through the
BGP neighbor is lost.
A new session is re-established within 30 to 60 seconds.
A full routing update is exchanged once the session is re-established, resulting
in enforcement of new routing policy.
Processing the full Internet routing table can take a long time.
Clearing the BGP session is a very disruptive way to implement routing
policies.
TheEXECcommandclearipbgptearsdownoneorseveralBGPsessions.The
BGPsessionsareterminated,andtheTCPconnectionsclosed.Theneighborsgo
intotheIdlestateandstaythereforapproximately30seconds.Next,theneighbor
sessiongoesintotheActivestate,andthesessionsarere-established.
Youcanimplementtheclearipbgpcommandwiththe*(asterisk)argument,which
appliestoallsessions.Youcanalsomakeareferencetoaspecificsessionorgroup
ofsessionstoteardown.
Whenthesessionisdown,allroutesthatarereceivedoverthesessionbyboth
routersareinvalidated.WhenthesessionisonceagainintheEstablishedstate,all
BGProuteshavetoberesentbybothpeers.Allroutesalsohavetopassthrough
thenewfilters,whichenforcesthenewpolicy.
ExchangingthecompleteInternetroutingtabletakestime,bandwidth,andCPU
resources.IPpacketforwardingtoandfromtheneighborisdownforseveral
TraditionalFilteringLimitations
minutes.Also,revokingandreannouncingtherouteswillberegisteredbytherestof
theInternetasaflapforeachroute.
Previously,toperformasoftresetforinboundroutingtableupdates,theneighbor
soft-reconfigurationcommanddirectedtheCiscoIOSsoftwareinthelocalBGP
routertostoreallreceived(inbound)routingpolicyupdateswithoutmodification.
Thismethodismemory-intensiveandnotrecommendedunlessabsolutely
necessary.(Outboundupdateshaveneverrequiredtheextramemoryandthis
featuredoesnotaffectthem.)
BGP Soft Reset Enhancement provides automatic support for dynamic soft reset
of inbound BGP routing table updates that is not dependent upon stored routing
table update information.
Requires no preconfiguration (as with the neighbor soft-reconfiguration
command).
Requires much less memory than the previous soft reset method for inbound
routing table updates.
Wheneverthereisachangeintheroutingpolicy,theBGPsessionmustbecleared,
orreset,forthenewpolicytotakeeffect.Therearetwotypesofreset,hardreset,
andsoftreset.ClearingaBGPsessionusingahardresetinvalidatesthecacheand
resultsinanegativeimpactontheoperationofnetworksastheinformationinthe
cachebecomesunavailable.Softresetisrecommendedbecauseitallowsrouting
tablestobereconfiguredandactivatedwithoutclearingtheBGPsession.Softreset
isdoneonper-neighborbasis.Therearetwotypesofsoftresets:
Dynamicinboundsoftreset:Whensoftresetisusedtogenerateinbound
updatesfromaneighbor
Outboundsoftreset:Whensoftresetisusedtosendanewsetofupdatesto
aneighbor.
TheBGPSoftResetEnhancementfeature,however,providesautomaticsupportfor
dynamicsoftresetofinboundBGProutingtableupdatesthatisnotdependenton
storedroutingtableupdateinformation.Thenewmethodrequiresno
preconfiguration(aswiththeneighborsoft-reconfigurationcommand)and
requiresmuchlessmemorythantheprevioussoftresetmethodforinboundrouting
tableupdates.
ThereareanumberofbenefitstotheBGPSoftResetEnhancementfeature:
Allowsdynamicrouterefreshrequests:Thisfeatureprovidesawaytoinitiate
nondisruptiveroutingpolicychanges.Itachievesthisbyallowingthedynamic
exchangeofrouterefreshrequestsbetweenBGProuters,andthesubsequent
readvertisementoftherespectiveoutboundroutingtables.
Requiresnopreconfiguration:Becausesupportforthesoftresetusingthe
routerefreshcapabilityisincludedinthisreleaseoftheCiscoIOSsoftware,no
furtherrouterconfigurationisrequired.Youcaninitiateasoftinboundreset
usingonlytheclearipbgpincommand.
Requiresnoadditionalmemoryresources:Unlikeasoftresetusingthe
storedinboundroutingtableupdatesprovidedbytheneighborsoft-
reconfigurationcommand,whenbothBGPpeerssupporttherouterefresh
capability,inboundroutingtableupdatesarenotstoredinthelocalBGProuter.
Thesoftresetrequestsareexchangeddynamically,andnoadditionalmemoryis
required.
Providesflexibility:Therearenowtwoavailablemethodsforinboundsoft
reset.Theoldermethod,usingstoredinboundroutingtableupdates,andthe
methodthatthisfeatureprovides,usingdynamicexchangeofupdate
information.
WhentheroutingpolicyofaBGPneighborchanges,thesessionmustbereset
(cleared)forthechangestotakeeffect.BecauseresettingaBGPsessioncanbe
disruptivetonetworks,asoftresetmethodisrecommendedforreconfiguringthe
routingtable.Previously,inordertoreconfiguretheinboundroutingtable,boththe
localBGProuterandtheBGPpeerfirstneededtobeconfiguredtostoreincoming
routingpolicyupdates.BGPsoftreconfigurationusedtobeconfiguredusingthe
neighborsoft-reconfigurationcommand.Moreresources,particularlymemory,
wererequiredtostoretheinboundroutingtableupdates.Theclearipbgp
commandcouldtheninitiatethesoftreset,whichgeneratedanewsetofinbound
routingtableupdatesusingthestoredinformation.
TheBGPSoftResetEnhancementfeatureprovidesanadditionalmethodforsoft
BGPSoftResetEnhancement
reset.Itallowsthedynamicexchangeofrouterefreshrequestsandrouting
informationbetweenBGProutersandthesubsequentreadvertisementofthe
respectiveoutboundroutingtable.Softresetusingtherouterefreshcapabilitydoes
notrequirepreconfigurationandconsumesnoadditionalmemoryresources.
Tousethisnewmethod,bothBGPpeersmustsupportthesoftrouterefresh
capability.ThiscapabilityisadvertisedintheOpenmessagethatissentwhena
peersendsitsroutingtableupdate.AnyrouterrunningBGPwiththissoftware
releaseautomaticallysupportstherouterefreshcapability.Routersrunningearlier
CiscoIOSsoftwarereleasesdonotsupporttherouterefreshcapabilityandmust
usetheoldersoftresetmethod.Ifthesoftresetfails,youcanstillcleartheBGP
session.However,itwillhaveanegativeimpactonnetworkoperationsandshould
beusedonlyasalastresort.
Outboundresetshaveneverrequiredpreconfigurationor
storingofroutingtableupdatesandremainunchangedby
theBGPSoftResetEnhancementfeature.
RouterefreshisoneofthecapabilitiesofBGP.Routersusetherouterefreshfeature
torequestaneighbortoresendalltheroutinginformationwhenitisneeded.
Route refresh is a BGP capability.
It is used to request a neighbor to resend routing information.
It is typically used after configuration changes to update the BGP table (route
map, distribute list, prefix list, filter list, weight, local preference, MED, etc.).
The traditional way of accomplishing this function is to clear the BGP session.
Inbound soft reconfiguration consumes memory on the receiving router.
It is needed only because there is no mechanism in standard BGP to
request retransmission of BGP routes.
BGP route refresh is an optional BGP capability that allows a BGP router to
request retransmission of BGP routes from a neighbor.
Thereareseveralwaysofrefreshingtheroutinginformationfromaneighbor:
Clearingtheneighborrelationship
Soft-clearingtheneighborrelationship(ifsoftreconfigurationisenabledforthis
specificneighbor)
Usingrouterefresh(iftheneighborsupportsthiscapability)
Thesoft-reconfigurationinboundfeatureconsumeslargevolumesofmemoryin
theInternetenvironment.Thenumberofroutesthatcanbereceivedfromapeer
routerontheInternetissolargethatitisnotfeasibletostoreanextracopy.
Theonlyreasonformakingtheextracopyistobeabletoreplaythedatathrough
thenewroutingpolicywithouttearingdownthesessionandre-establishingit.
Whatisneededisamechanismtoasktheneighborroutertodoa"clearsoft
outbound."Ifthiswerepossible,theextracopywouldnotbeneeded.The
neighboringrouter,ofcourse,hasitsowncopyinitsBGPtable,whichitcould
resendtothelocalrouterwheneveritissignaledtodoso.
ThereisnosuchmechanisminstandardBGP,butthereisanoptionalBGP
capabilitythatallowsoneroutertorequestarefreshfromitsneighbor:routerefresh.
ThetablecomparesthevariousmethodsofBGPsessionreset,statingthe
advantagesanddisadvantagesofeach.
Tousesoftresetwithoutpreconfiguration,bothBGPpeers
mustsupportthesoftrouterefreshcapability,whichis
advertisedintheOpenmessagethatissentwhenthe
peersestablishaTCPsession.RoutersthatrunCisco
IOSsoftwarereleasesearlierthanRelease12.1donot
supporttherouterefreshcapabilityandmustclearthe
BGPsessionusingtheneighborsoft-reconfiguration
command.
TypeofReset Advantages Disadvantages
Hardreset Nomemoryoverhead. TheprefixesintheBGP,IP,andFIB
tablesthattheneighborprovides,are
lost.
Notrecommended.
Outboundsoftreset Noconfiguration,nostoringof
routingtableupdates.
Doesnotresetinboundroutingtable
updates.
Dynamicinboundsoft
reset
DoesnotcleartheBGPsessionor
cache.
Doesnotrequirestoringofrouting
tableupdates,andhasnomemory
overhead.
BothBGProutersmustsupportthe
routerefreshcapability(CiscoIOS
SoftwareRelease12.1andlater
releases).
Configuredinboundsoft
reset(usestheneighbor
soft-reconfiguration
command)
CanbeusedwhenbothBGP
routersdonotsupportthe
automaticrouterefreshcapability.
Requirespreconfiguration.
Storesallreceived(inbound)routing
policyupdateswithoutmodification,
andisthusmemory-intensive.
Recommendedonlywhenabsolutely
necessary.
RouteRefresh
Step 1—Route refresh is negotiated when the BGP session is established.
Step 2—Inbound routing policy is changed on R2.
Step 3—Operator requests inbound route refresh.
Step 4—R2 sends route refresh message to R1.
Step 5—R1 resends all BGP routes to R2.
TheroutermustnegotiatetheabilitytousetherouterefreshfeaturewhentheBGP
sessionisfirstestablished.Thelocalrouterkeepsarecordthatthecapabilityis
availablewiththeneighbor.Thereisnoneedtokeepacopyoftherouting
informationthatisreceivedfromtheneighborifithastheabilitytorefresh.
Afterreconfiguringthefiltersandroutemapsthatwillimplementanewrouting
policy,youcanissuetheclearipbgpip-addresssoftincommandinthelocal
router.Theroutercheckswhethertherouterefreshcapabilityisavailable,andifitis,
requestsresendingoftheBGPtableoftheneighborinsteadofreplayingitsown
copy.
Usetheclearipbgp*incommand,tosendarouterefreshmessagetoall
neighborsorclearipbgpip-addressintosendarouterefreshmessagetoa
specificneighbor.
router# clear ip bgp {* | ip-address | peer-group-name } in
Sends a route refresh message to the neighbor or neighbors.
Only works if the neighbor has previously advertised the route refresh capability.
Youneednotusethesoftkeyword,becausesoftresetisautomaticallyassumed
whentherouterefreshcapabilityissupported.
ToresetaBGPconnectionwithBGPsoftreconfiguration,usetheclearipbgp
privilegedEXECcommandatthesystemprompt.
clear ip bgp {* | ip-address | peer-group-name} [soft [in | out]]
SyntaxDescription
Parameter Description
* ResetsallcurrentBGPsessions.
ip-address ResetsonlytheidentifiedBGPneighbor.
peer-group-name ResetsthespecifiedBGPpeergroup.
soft (Optional)Softreset.Doesnotresetthesession
in|out (Optional)Triggersinboundoroutboundsoftreconfiguration.
Iftheinoroutoptionisnotspecified,bothinboundandoutboundsoft
reset
aretriggered.
Whendoingclearipbgp*in,ifsoft-reconfiguration
inboundisconfiguredforaneighbor,itwillcauseasoft
action.Ifsoft-reconfigurationinboundisnotconfigured,
itwillcausearouterefresh.
ConfiguringRouteRefresh
YouneedtoverifywhethertheBGPneighborsupportstherouterefreshcapability.
R2# show ip bgp neighbors 172.16.22.22
BGP neighbor is 172.16.22.22, remote AS 200, external link
BGP version 4, remote router ID 10.0.40.1
BGP state = Established, up for 02:31:21
Last read 00:00:23, last write 00:00:49, hold time is 180, keepalive int
erval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable (disabled)
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN Capability: advertised and received
Address family IPv4 Unicast: advertised and received
Enhanced Refresh Capability: advertised and received
Verifies the support for route refresh capability.
Usetheshowipbgpneighborcommandtoseewhethertheneighborsupportsthe
routerefreshmessage.Intheexample,neighbor172.16.22.22supportsroute
refreshcapability.
R2# debug ip bgp
*Mar 10 05:06:53.243: BGP: ses global 172.16.22.22 (0xECFD0708:0) act Send
OPEN
*Mar 10 05:06:53.243: BGP: 172.16.22.22 active sending OPEN, version 4, my
as: 1, holdtime 180 seconds, ID A000041
*Mar 10 05:06:53.247: BGP: 172.16.22.22 active rcv message type 1, length
(excl. header) 38
*Mar 10 05:06:53.247: BGP: ses global 172.16.22.22 (0xECFD0708:0) act Rece
ive OPEN
*Mar 10 05:06:53.247: BGP: 172.16.22.22 active rcv OPEN, version 4, holdti
me 180 seconds
*Mar 10 05:06:53.247: BGP: 172.16.22.22 active OPEN has CAPABILITY code: 1
28, length 0
*Mar 10 05:06:53.247: BGP: 172.16.22.22 active OPEN has ROUTE-
REFRESH capability(old) for all address-families
*Mar 10 05:06:53.247: BGP: 172.16.22.22 active rcvd OPEN w/ optional param
eter type 2 (Capability) len 2
*Mar 10 05:06:53.247: BGP: 172.16.22.22 active OPEN has CAPABILITY code: 2
, length 0
*Mar 10 05:06:53.247: BGP: 172.16.22.22 active OPEN has ROUTE-
REFRESH capability(new) for all address-families
<... output omitted ...>
*Mar 10 05:06:53.248: BGP: 172.16.22.22 sending REFRESH_REQ(5) for afi/saf
i: 1/1, refresh code is 1
Debug output after BGP session reset
Usedebugipbgptodisplaythenegotiationofcapabilities.Debuggingdisplaysthe
receivedcapabilities.
Theexampleshowsadebuggingoutput,aftertheBGPsessionwasreset,usinga
hardreconfiguration.Youcansee,thataneighborisadvertisingbothold-styleand
standard(new-style)routerefresh.The"old"referstotheoldcapabilitycodeof128
thattheoldCiscoimplementationsofRouteRefreshused,beforetheRFC2918
wasinplace.The"new"referstotheRFC2918compliantcapabilitycode2thatall
recentBGPimplementationsthatsupporttheRouteRefreshuse,accordingtoRFC
2918.Afterthesessionhasbeenestablished,theroutersendsaninitialstandard
routerefreshmessagefortheaddressfamily1/1(IPv4unicast).
R2# debug ip bgp
R2# debug ip bgp updates
R2# clear ip bgp 172.16.22.22 in
*Mar 10 05:34:28.458: BGP: 172.16.22.22 sending REFRESH_REQ(5) for afi/saf
i: 1/1, refresh code is 0
*Mar 10 05:34:28.459: BGP: 172.16.22.22 rcv message type 5, length (excl.
header) 4
*Mar 10 05:34:28.459: BGP: 172.16.22.22 rcvd REFRESH_REQ for afi/safi: 1/1
, refresh code is 1
*Mar 10 05:34:28.459: BGP: nbr_topo global 172.16.22.22 IPv4 Unicast:base
(0xECFD0708:1) rcvd Refresh Start-of-RIB
*Mar 10 05:34:28.459: BGP: nbr_topo global 172.16.22.22 IPv4 Unicast:base
(0xECFD0708:1) refresh_epoch is 3
*Mar 10 05:34:28.460: BGP(0): 172.16.22.22 rcvd UPDATE w/ attr: nexthop 17
MonitoringRouteRefresh
2.16.22.22, origin i, metric 0, merged path 200, AS_PATH
*Mar 10 05:34:28.461: BGP(0): 172.16.22.22 rcvd 10.0.2.0/28
*Mar 10 05:34:28.461: BGP(0): 172.16.22.22 rcvd 10.0.2.16/28
*Mar 10 05:34:28.461: BGP(0): 172.16.22.22 rcvd 10.0.2.32/28
Debug output after route refresh
Debuggingalsoshowsarouterefreshmessagebeingsenttoaneighborafterthe
networkadministratorissuestheclearipbgpip-addressincommandfromthe
privilegedEXECmode.Whentheneighbor172.16.22.22receivestherouterefresh
message,itresendstheBGPupdatestotherouterR2.Updatesarethenprocessed
againsttheconfiguredBGPinboundfiltersontheR2router.
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
Because of the huge volumes of routing information, BGP cannot use traditional
routing update methods
The BGP Soft Reset Enhancement feature provides automatic support for
dynamic soft reset of inbound BGP routing table updates that is not dependent
upon stored routing table update information. This method requires no
preconfiguration and needs much less memory than the previous soft reset
method for inbound routing table updates.
Route refresh is a BGP capability that is used to request a neighbor to resend
routing information after configuration changes.
The clear ip bgp ip-address in command sends a route refresh message to the
neighboring router and executes if the neighbor has previously advertised the
route refresh capability.
To verify whether a neighbor supports route refresh, you can use the show ip
bgp neighbor command. To display the negotiation process, you can use the
debug ip bgp command.
Summary
Overview
Thistopicsummarizesthekeypointsthatwerediscussedinthismodule.
The multihomed customer network must exchange BGP information with both
ISP networks. Dynamic routing is required for full redundancy, and BGP is the
only protocol available that can be used in this scenario.
An AS-path filter is created by an AS-path access-list. The access-list is applied
to a set of routes from which to subset can be selected.
Use prefix-lists to filter incoming or outgoing BGP updates to neighbors and to
filter routes that are being redistributed into the BGP process from other routing
protocols.
Outbound route filtering is a mechanism that is used to minimize the number of
updates that are requested from a neighbor.
Route-maps provide a method to perform a variety of compound, complex
filtering operations (such as dropping denied routes and modifying attributes of
the permitted routes) within a single tool.
Soft reconfiguration provides the ability to run all routes through filters without
tearing down the sessions.
ThismodulediscussedBGProutefilteringandBGProuteselectionpolicies.The
moduledescribedmultihomedBGPnetworksandidentifiedtheneedforBGProute
selection.ThismodulealsoaddressedconfiguringBGPtoinfluencerouteselection
byusingAS-pathfilters,prefix-listfilters,androute-maps.Outboundroutefiltering
wasalsoexplained.Inaddition,detailsaboutsoftreconfigurationandrouterefresh
wereprovided.
ModuleSummary
Foradditionalinformation,refertotheseresources:
CiscoSystems,Inc.SampleConfigurationforBGPwithTwoDifferentService
Providers(Multihoming).
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/23675-
27.html
CiscoSystems,Inc.UsingRegularExpressionsinBGP.
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13754-
26.html
CiscoSystems,Inc.BGPCaseStudies"BGPCaseStudies1."
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/26634-
bgp-toc.html#BGPsec1
CiscoSystems,Inc.BGPCaseStudies"BGPCaseStudies3."
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/26634-
bgp-toc.html#sec3
CiscoSystems,Inc.ConfiguringBGP.
http://www.cisco.com/c/en/us/td/docs/ios/12_2/ip/configuration/guide/fipr_c/1cfbgp.html
CiscoSystems,Inc.BGPPrefix-BasedOutboundRouteFiltering.
http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-3e/irg-
iproute-bgp-xe-3e-book/irg-oubound-route-filtering.pdf
CiscoSystems,Inc.CompatibleSystemsSetupGuides:BGPConfigurationGuide.
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/17612-
bgp.html
CiscoSystems,Inc.BGPSoftResetEnhancement.
http://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/s_sftrst.html?
mdfid=275960490
References
Usethequestionsheretoreviewwhatyoulearnedinthismodule.Thecorrect
answersandsolutionsarefoundintheModuleSelf-CheckAnswerKey.
ModuleSelf-Check
1. Whataretworeasonswhyacustomerwouldwanttoconnecttotwo
ISPs?(Choosetwo.)(Source:"UsingMultihomedBGPNetworks")
ToexpandcapacityforInternettraffic.
TobetterprotectconfidentialinformationasittravelsthroughtheInternet.
Toprovideredundancytomission-criticalservicesthatareofferedover
theInternet.
ToefficientlyrouteInternettraffictotwodifferentdivisionswithinthe
company.
1. Whatarethetwotechnicalrequirementsformultihomedcustomers?
(Choosetwo.)(Source:"UsingMultihomedBGPNetworks")
TheISPsmustassignarangeofIPnetworknumberstothecustomer.
ThecustomernetworkmustexchangeBGPinformationwitheachISP
network.
Inmostcases,thecustomermusthaveitsownpublicASnumber.
ThecustomernetworkmustnotuseASpathfilters.
1. Whichofthefollowingstatementsbestillustratestheimportanceof
BGPpoliciesthatinfluencerouteselectioninamultihomedBGP
network?(Source:"UsingMultihomedBGPNetworks")
ThedefaultBGProuteselectiondoesnotalwaysresultinoptimum
routing.
ThedefaultBGProuteselectionalwaysresultsinoptimumrouting.
Aftertherouteselectionbehaviorhasbeenset,itcannotbechanged.
Thecustomerreceivesallroutesfrombothserviceproviders,giving
redundancy;therefore,BGPpoliciesarenotnecessary.
1. ThereisnoneedforISPsandcustomerstoinfluencetheBGPpath
selectionasBGPhasbuilt-inmechanismstofindtheoptimalwayto
reachthedestination.Trueorfalse?(Source:"UsingMultihomed
BGPNetworks")
true
false
1. Whichthreeofthefollowingarepotentialcustomerroutingpolicies?
(Choosethree.)(Source:"UsingMultihomedBGPNetworks")
Oneserviceproviderisdesignatedasprimary,andtheotherisabackup.
Trafficisload-balancedacrossbothISPnetworks.
TraffictowardaspecificdestinationgoesthroughonlyoneoftheISPs.
TraffictodirectcustomersoftheISPsgoesdirect;allothertrafficgoes
throughtheprimaryISP.
ThetwoISPsmayhavesimilarpeeringagreementswithotherISPs.
TheISPsusedefaultroutingtothecustomer,andthecustomerusesstatic
routingtotheISP(orISPs).
1. WhichstatementabouttheneedtoinfluenceBGProuteselectionin
aserviceproviderenvironmentisaccurate?(Source:"Using
MultihomedBGPNetworks")
IfbothISPconnectionsterminateinonesinglecustomerrouter,onlysome
routesthatarereceivedfromtheprimaryISPcanbeassignedaBGP
weight.
Inmostcases,itismoreoptimaltoreachothercustomersconnectedto
thebackupISPviathebackuplink,comparedwithreachingthemviathe
primarylink.
AllroutesthatarereceivedfromtheprimaryISPovertheprimarylinkare
assignedalocalpreferencevalue,whichislowerthanthedefaultvalueof
100.
WhenoneISPconnectionterminatesinonesinglecustomerrouter,all
routesthatarereceivedfromthatISPareassignedaBGPweight.
1. CustomercouldbecometransitASifnofilteringtomultihomedISPs
isperformed.Trueorfalse?(Source:"UsingMultihomedBGP
Networks")
true
false
1. Whichroutesshouldcustomerannouncetoserviceprovidersin
multihomedscenario?(Source:"UsingMultihomedBGPNetworks")
Customershouldannounceonlynetworksoriginatingincustomer’sAS.
CustomershouldannounceallnetworksreceivedfromprimaryISPto
secondaryISP.
CustomershouldannounceallnetworksreceivedfromsecondaryISPto
primaryISP.
Customershouldnotannounceanynetworks.
1. ISPshouldallowcustomertoannounceanypossiblenetworksvia
BGP.Trueorfalse?(Source:"UsingMultihomedBGPNetworks")
true
false
1. WhichthreegoalsrepresentappropriatereasonstoapplyASpath
filters?(Choosethree.)(Source:"EmployingASPathFilters")
Toensurethatonlylocallyoriginatedroutesareannounced.
TolimitroutesthatareadvertisedfromIBGPneighbors.
ToselectasubsetofallroutesbasedontheiroriginatingAS.
TolimitneighborrouteupdatestospecificAS-originatedroutes.
Toensurethatalldestinationautonomoussystemsshouldbereceived
fromaspecifiedneighbor.
Tochangetheweightorlocalpreferenceattributesforalldestination
autonomoussystems.
1. WhichASpathismatchedbytheregularexpression"72$"?
(Source:"EmployingASPathFilters")
2137221831727
273172715072
315277231991
725913682087
1. WhichregularexpressionwouldyouusetomatchallBGPnetworks
originatedinAS89?(Source:"EmployingASPathFilters")
^89$
^89_
_89_
_89$
1. WhichregularexpressionwouldyouusetomatchallBGPnetworks
goingthroughAS76?(Source:"EmployingASPathFilters")
^76$
^76_
_76_
_76$
1. WhichBGPnetworksarematchedbyusingregularexpression
^54$?(Source:"EmployingASPathFilters")
AllroutesthataresourcedinyourdirectlyconnectedneighboringAS54.
AllroutesthathaveAS54intheirASpaths.
AllroutesthataresourcedinAS54.
AlltheroutesthatarereachablebehindAS54.
1. Whatisthedifferencebetweentheregularexpressions"_100_"and
"_100$"?(Source:"EmployingASPathFilters")
Thefirstexpressionreferstoroutesthathavethesubstring"100"intheir
ASpaths;thesecondexpressionrefersonlytoroutesthataredirectly
connectedtoAS100.
Thefirstexpressionreferstoroutesthathavethesubstring"100"intheir
ASpaths;thesecondexpressionrefersonlytoroutesthatoriginatedinAS
100.
ThefirstexpressionreferstoroutesthatgothroughAS100;thesecond
expressionreferstoroutesthatoriginatedinAS100.
ThefirstexpressionreferstoroutesthataredirectlyconnectedtoAS100;
thesecondexpressionreferstoroutesthatoriginatedinAS100.
1. HowdoyoumatchASpathsthatcontainexactlytwosingle-digitAS
numbers?(Source:"EmployingASPathFilters")
usetheexpression"**"
usetheexpression".."
usetheexpression"[0-9]_[0-9]"
usetheexpression"^[0-9]_[0-9]$"
1. Whichthreestepsarerequiredtoapplyanewinboundroutingpolicy
toaneighbor?(Choosethree.)(Source:"EmployingASPath
Filters")
DefineanASpathaccesslist.
AttachtheASpathfiltertoinboundoroutboundupdatesforaspecific
BGPneighbor.
SendincomingandoutgoingASpathfilterstotheBGPneighbor.
Forcetheupdatestogothroughthenewfilter.
1. Howcanyoutestyourregularexpression?(Source:"EmployingAS
PathFilters")
showipbgpaccess-listcommand
showipbgpfiltercommand
showipbgpregexpcommand
showipbgpsummarycommand
1. Whataretworeasonsthatamultihomedcustomerneedsprefix-
lists?(Choosetwo.)(Source:"FilteringwithPrefix-Lists")
ToensurethatonlyvalidIPprefixesareannouncedtotheISPs.
Tosetalimitonthenumberofprefixesthatcanbeacceptedfromthe
ISPs.
TopreventthecustomerfromreceivingitsownIPprefixesfromtheISP.
ToverifythatthecustomerhasreceivedfullInternetroutetables.
1. Whichthreeofthefollowingchoicesareadvantagesofprefixlists
overaccesslists?(Choosethree.)(Source:"FilteringwithPrefix-
Lists")
significantperformanceimprovementonlongfilters
supportforincrementalupdates
abilitytoconsistofanynumberoflines,eachofwhichindicatesatestand
aresult
flexibility
morecomplexcommand-lineinterface
sequentialscanningofprefix-listswithinCiscoIOSsoftware
1. Whenyoudefineprefixlists,whataretworeasonstousesequence
numbers?(Choosetwo.)(Source:"FilteringwithPrefix-Lists")
Toreferencetheassociatedaccesslistfortheprefixlistentry.
ToprovideameansoflinkinganASpathfilterlisttotheprefixlist.
Toprovideanexecutionorderforprefix-listentries.
Toprovideameansofinsertingordeletinglistentries.
1. Whichofthefollowingstatementsisaccurateaboutthegeandle
parametersintheipprefix-listglobalconfigurationcommand
requiredtoconfigureprefix-listfilters?(Source:"FilteringwithPrefix-
Lists")
Nomatchisassumedwhenneithergenorleisspecified.
Therangeisassumedtobefromge-valueto142onlyifthegeattributeis
specified.
Therangeisassumedtobefromlento32onlyiftheleattributeis
specified.
Anexactmatchisassumedwhenneithergenorleisspecified.
1. Whichofthefollowingrouteswillbematchedbythetheprefixlist
10.0.1.0/24ge28?(Source:"FilteringwithPrefixLists")
10.0.1.0/26
10.1.1.0/28
10.0.1.0/29
10.0.0.0/28
1. Whichofthefollowingstatementsaboutimplementingprefixlistsin
aBGPnetworkisaccurate?(Source:"FilteringwithPrefixLists")
Youcanoptionallyapplyfilterlistsandprefixlistsoneitherincomingor
outgoingneighborsinanycombination.
Youcanoptionallyapplyfilterlistsandprefixlistsonlyonoutgoing
neighborsinanycombination
Eithertheincomingprefixlistortheincomingfilterlistmustpermitthe
routesthatarereceivedfromaneighborbeforetheyareacceptedintothe
BGPtable.
Outgoingroutesmustpasstheoutgoingprefixlistbeforebeingtransmitted
totheneighbor.
1. Whichthreestepsarerequiredtoapplyanewinboundroutingpolicy
(basedonprefixlist)toaneighbor?(Choosethree.)(Source:
"FilteringwithPrefixLists")
DefineanIPprefixlist.
AttachtheprefixlisttoinboundoroutboundupdatesforaspecificBGP
neighbor.
SendincomingandoutgoingASpathfilterstotheBGPneighbor.
Forcetheupdatestogothroughthenewfilter.
1. HowcanyouapplythesameprefixlisttomultipleBGPneighborson
arouter?(Source:"FilteringwithPrefixLists")
Byconfiguringaneighborprefix-liststatementforeachBGPpeer.
Byconfiguringaneighbordistribute-liststatementforeachneighbor.
ByusingtheBGPpeer-groupoptionwiththeneighborstatement.
ByconfiguringtheprefixlistasaglobalfilterundertheBGProuting
process.
1. Howcanyouusetheshowipprefix-listcommandtodisplaythe
prefixlistentrythatmatchesaspecificprefixandlength?(Source:
"FilteringwithPrefixLists")
notafeatureoftheshowipprefix-listcommand
byspecifyingthedetailkeyword
withthelongerkeywordtodisplayallmatchesexceptthosewithmore
specificentries
byspecifyingthefirst-matchkeyword
1. Whichofthefollowingbestdescribesthecapabilitiesofthe
proprietaryORFtypethatissupportedonCiscorouters?(Source:
"UsingOutboundRouteFiltering")
standardBGPcommunitiesfiltering
extendedBGPcommunitiesfiltering
ASpathfiltering
prefixlistfiltering
1. ORFfeatureismoreeffectivethanstandardinboundfilteringwhenit
comestotheamountofresourcesthatarerequiredforrouting
updategenerationandprocessing.Trueorfalse?(Source:"Using
OutboundRouteFiltering")
true
false
1. Whataretwokeybenefitstousingoutboundroutefiltering?(Choose
two.)(Source:"UsingOutboundRouteFiltering")
conservesCPUcycles
improvessecurity
reducesbandwidththatisusedbyunnecessaryroutingupdates
increasesneighboravailability
1. WhichtwoofthefollowingstatementsaboutBGPprefix-based
outboundroutefilteringareaccurate?(Choosetwo.)(Source:"Using
OutboundRouteFiltering")
UsesBGPORFsendandreceivecapabilitiestominimizethenumberof
BGPupdatesthataresentbetweenBGPpeers.
Canlimitthenumberofunwantedroutingupdates.
Increasestheamountofresourcesrequiredtoreceiveanddiscardroutes
thatwouldotherwisebefilteredout.
Canbeusedtoincreasetheamountofprocessingonarouterthatisnot
acceptingfullroutesfromaserviceprovidernetwork.
1. Howshouldyouconfiguretheneighborcapabilityorfprefix-list
commandonarouterthatisapplyingaprefixlistfilterasan
outboundroutepolicy?(Source:"UsingOutboundRouteFiltering")
send
receive
both
prefixfilter
1. WhichofthefollowingistrueregardingORFcapabilitiesoptions?
(Choosetwo.)(Source:"UsingOutboundRouteFiltering")
“both”optionallowssendingandreceivingofprefixlists.
“send”optionallowsreceivingofprefixlists.
“receive”optionallowsreceivingofprefixlists.
“both”optionallowsonlysendingofprefixlists.
1. WhataretwomethodsofdeterminingthatarouterhasORF
capabilitiesexchangeconfigured?(Choosetwo.)(Source:"Using
OutboundRouteFiltering")
showrunning-config|beginbgpcommand
showipbgpnegotiatecommand
showipbgpneighborscommand
showipprefix-listcommand
1. WhichcommandshouldbeusedtoverifythatIPprefix-listwas
receivedontheORFreceiver?(Source:"UsingOutboundRoute
Filtering")
showipbgpneighborsIP_addressreceivedprefix-filter
showipprefix-list
showipbgpneighborsIP_addresssendprefix-filter
showorfprefix-listreceived
1. WhataretwoprerequisitesbeforeyoucanconfigureORFprefixlist
functionality?(Choosetwo.)(Source:"UsingOutboundRoute
Filtering")
Arouterefreshmustbesentusingtheclearipbgpcommand.
ABGPpeeringsessionbetweentheORFroutersmustbeupandrunning.
ORFcapabilitiesmustbeenabledonbothrouters.
Youmustconfigureaprefixlistfilteronthereceivingrouter.
1. Whichofthefollowingstatementsaboutthefunctionofaroutemap
isaccurate?(Source:"ApplyingRouteMapsasBGPFilters")
Aroutemapcannotbeusedtomodifyattributesofthepermittedroutes.
Aroutemapisafilterthatusesaseriesofmatchconditions,andthat
whichisdeniedbytheroutemapisdropped.
Aroutemapislesscomplexthantheaccesslist.
Eachroutemapstatementstartswithaseriesofmatchclauses.
1. WhichthreeofthefollowingstatementsareaccurateabouttheBGP
RouteMapPolicyListSupportfeature?(Choosethree.)(Source:
"ApplyingRouteMapsasBGPFilters")
TheBGPRouteMapPolicyListSupportfeatureallowsanetworkoperator
togrouproute-mapmatchclausesintonamedlistscalledpolicy-lists.
Thenetworkoperatormanuallyreconfigureseachrecurringgroupof
matchclausesthatoccurinmultipleroute-mapentries.
TheANDandORsemanticsintheroutemapfunctiondifferentlyfor
policy-liststhanformatchandsetclauses.
TocreateaBGPpolicy-list,usetheippolicy-listcommandinpolicy-map
configurationmode.
Toconfigurearoute-aptoevaluateandprocessaBGPpolicy-listina
routemap,usethematchpolicy-listcommandinroute-mapconfiguration
mode.
Todisplayinformationaboutaconfiguredpolicy-listandpolicy-listentries,
usetheshowippolicy-listcommandinroute-mapconfigurationmode.
1. WhichtwoofthefollowingarefunctionsoftheBGPRouteMap
Continuefeature?(Choosetwo.)(Source:"ApplyingRouteMapsas
BGPFilters")
Providestheabilitytopauseifasequencenumberisnotspecified.
Providesthecapabilitytoexecuteadditionalentriesinaroute-mapafter
anentryisexecutedwithsuccessfulmatchandsetclauses.
Allowsmodularizationofnetworkpolicyconfigurationsothatrepeated
policydefinitionscanbeexpandedwithinthesameroutemap.
Allowsconfigurationandorganizationofmoremodularpolicydefinitionsto
reducethenumberofpolicyconfigurationsthatarerepeatedwithinthe
sameroutemap.
1. Whichofthefollowingcommandsisusedtodistributeanyroutes
thathaveadestinationnetworknumberaddressthatispermittedby
astandardaccesslist,anextendedaccesslist,oraprefixlist,orto
performpolicyroutingonpackets?(Source:"ApplyingRouteMaps
asBGPFilters")
matchipnext-hop
matchiproute-source
matchipaddress
showipbgproute-map
1. Howdoyouimplementa"permitall"statementwhenyouareusing
routemaps?(Source:"ApplyingRouteMapsasBGPFilters")
Bydefault,aroute-aphasan"implicitpermitany"statementifnomatchis
found.
Youmustconfigurearoutemapwitha"permit"parameterandnomatch
clause.
Youmustconfigurearoutemapwitha"deny"parameteranda"deny
none"clause.
Youmustconfigurearoutemapwitha"permitany"matchclause.
1. WhathappenstoincomingBGPupdatesthatdonotmatchany
route-mapmatchclauses?(Source:"ApplyingRouteMapsasBGP
Filters")
TheyareenteredintotheBGPtable.
TheyareenteredintotheBGPtableandmarkedwithaweightof32768.
TheyarenotacceptedbytherouterorenteredintotheBGPtable.
TheyareenteredintotheBGPtableifamatchingrouteexistsintheIP
routingtable.
1. WhichthreeBGPattributescanyousetusingroutemaps?(Choose
three.)(Source:"ApplyingRouteMapsasBGPFilters")
MED
pathorigin
administrativedistance
weightmetric
next-hop
atomicaggregate
1. Whataretworeasonsforusingroutemapsequencenumbers?
(Choosetwo.)(Source:"ApplyingRouteMapsasBGPFilters")
Toallowinsertionordeletionofroute-mapentries.
Toordertheexecutionsequenceofroute-mapmatchclauses.
Toprovideanorderedexecutionsequencefortheroutemap.
Tomapbetweenprefix-liststatementsandroute-mapmatchclauses.
1. WhichcommandshouldbeusedonR1(IPaddress1.1.1.1)toapply
aroutemapnamedFiltertoincomingBGPupdatescomingfromR2
(IPaddress2.2.2.2)?(Source:"ApplyingRouteMapsasBGP
Filters")
neighbor2.2.2.2route-mapFilterin
neighbor1.1.1.1.route-mapFilterin
neighbor2.2.2.2route-mapFilterout
neighbor1.1.1.1.route-mapFilterout
1. WhyisclearingaBGPsessionadisruptivechangeinroutingpolicy?
(Source:"ImplementingChangesinBGPPolicy")
ClearingaBGPsessiontakesalongtimeandcandisruptpacket
forwarding.
YoucannotrecoverinformationthatissentwhiletheBGPsessionisbeing
cleared.
Youcannotautomaticallyre-establishsessionsthataretorndownduring
theclearingoperation.
YoucannotselectivelyteardownBGPsessions;youmustclearsessions
withallneighbors.
1. Usingtheclearipbgp*command,allBGPsessionswillstayup
andnoBGProutesarelost.Trueorfalse?(Source:"Implementing
ChangesinBGPPolicy")
true
false
1. WhichtwoofthefollowingarefunctionsoftheBGPSoftReset
Enhancementfeature?(Choosetwo.)(Source:"Implementing
ChangesinBGPPolicy")
allowsdynamicrouterefreshrequests
requiresnopreconfiguration
providesnewermethodforinboundsoftresetthatusesstoredinbound
routingtableupdates
usesexpandedmemory
1. BGProuterefreshisBGPcapabilitythatallowsaBGProuterto
requestretransmissionofBGProutesfromaneighbor.Trueor
false?(Source:"ImplementingChangesinBGPPolicy")
true
false
1. Whichofthefollowingstatementsaboutthecommandthatis
requiredtoperformarouterefreshisaccurate?(Source:
"ImplementingChangesinBGPPolicy")
Youwillusetheclearipbgp*incommandtosendarouterefresh
messagetoallneighbors.
Youwillusetheclearipbgpip-addressincommandtosendaroute
refreshmessagetoallneighbors.
Youmustusethesoftkeywordwiththeclearipbgpcommandbecause
softresetisnotautomaticallyassumedwhentherouterefreshcapabilityis
supported.
Theclearipbgpcommandworkseveniftheneighborhasnotpreviously
advertisedtherouterefreshcapability.
1. WhichcommandwouldyouuseonR1(IPaddress1.1.1.1)to
requestfromneighboringR2(IPaddress2.2.2.2)toresendrouting
information?(Source:"ImplementingChangesinBGPPolicy")
clearipbgp1.1.1.1out
clearipbgp1.1.1.1in
clearipbgp2.2.2.2out
clearipbgp2.2.2.2in
1. WhichRFC2918compliantcapabilitycode2usedbyallrecentBGP
implementationswilltellyouthatroutersupportstheRouteRefresh
feature?(Source:"ImplementingChangesinBGPPolicy")
1
218
4
2
1. WhichcommandshouldbeusedtodisplaythenegotiationofBGP
capabilitiesaftersessionreset?(Source:"ImplementingChangesin
BGPPolicy")
debugip
debugbgppackets
debugipbgpupdates
debugipbgp
1. HowdoyoudeterminewhetheraBGPneighborsupportsroute
refresh?(Source:"ImplementingChangesinBGPPolicy")
AflagintheBGPtableindicatesthepresenceofrouterefreshcapability
Theshowipbgpneighborcommandindicateswhethertheoptionis
supported.
Initiatethedebugipbgpnegotiationcommandtoseewhethertherouter
hascompletedarouterefreshcapabilitiesexchange.
Executetheclearipbgp*command.Command-lineBGPstatus
messageswillindicaterouterefreshsupportcapabilities.
1. Whataretworeasonswhyacustomerwouldwanttoconnecttotwo
ISPs?(Choosetwo.)(Source:"UsingMultihomedBGPNetworks")
ToexpandcapacityforInternettraffic.
TobetterprotectconfidentialinformationasittravelsthroughtheInternet.
Toprovideredundancytomission-criticalservicesthatareofferedover
theInternet.
ToefficientlyrouteInternettraffictotwodifferentdivisionswithinthe
company.
AnswerKey
1. Whatarethetwotechnicalrequirementsformultihomedcustomers?
(Choosetwo.)(Source:"UsingMultihomedBGPNetworks")
TheISPsmustassignarangeofIPnetworknumberstothecustomer.
ThecustomernetworkmustexchangeBGPinformationwitheachISP
network.
Inmostcases,thecustomermusthaveitsownpublicASnumber.
ThecustomernetworkmustnotuseASpathfilters.
1. Whichofthefollowingstatementsbestillustratestheimportanceof
BGPpoliciesthatinfluencerouteselectioninamultihomedBGP
network?(Source:"UsingMultihomedBGPNetworks")
ThedefaultBGProuteselectiondoesnotalwaysresultinoptimum
routing.
ThedefaultBGProuteselectionalwaysresultsinoptimumrouting.
Aftertherouteselectionbehaviorhasbeenset,itcannotbechanged.
Thecustomerreceivesallroutesfrombothserviceproviders,giving
redundancy;therefore,BGPpoliciesarenotnecessary.
1. ThereisnoneedforISPsandcustomerstoinfluencetheBGPpath
selectionasBGPhasbuilt-inmechanismstofindtheoptimalwayto
reachthedestination.Trueorfalse?(Source:"UsingMultihomed
BGPNetworks")
true
false
1. Whichthreeofthefollowingarepotentialcustomerroutingpolicies?
(Choosethree.)(Source:"UsingMultihomedBGPNetworks")
Oneserviceproviderisdesignatedasprimary,andtheotherisabackup.
Trafficisload-balancedacrossbothISPnetworks.
TraffictowardaspecificdestinationgoesthroughonlyoneoftheISPs.
TraffictodirectcustomersoftheISPsgoesdirect;allothertrafficgoes
throughtheprimaryISP.
ThetwoISPsmayhavesimilarpeeringagreementswithotherISPs.
TheISPsusedefaultroutingtothecustomer,andthecustomerusesstatic
routingtotheISP(orISPs).
1. WhichstatementabouttheneedtoinfluenceBGProuteselectionin
aserviceproviderenvironmentisaccurate?(Source:"Using
MultihomedBGPNetworks")
IfbothISPconnectionsterminateinonesinglecustomerrouter,onlysome
routesthatarereceivedfromtheprimaryISPcanbeassignedaBGP
weight.
Inmostcases,itismoreoptimaltoreachothercustomersconnectedto
thebackupISPviathebackuplink,comparedwithreachingthemviathe
primarylink.
AllroutesthatarereceivedfromtheprimaryISPovertheprimarylinkare
assignedalocalpreferencevalue,whichislowerthanthedefaultvalueof
100.
WhenoneISPconnectionterminatesinonesinglecustomerrouter,all
routesthatarereceivedfromthatISPareassignedaBGPweight.
1. CustomercouldbecometransitASifnofilteringtomultihomedISPs
isperformed.Trueorfalse?(Source:"UsingMultihomedBGP
Networks")
true
false
1. Whichroutesshouldcustomerannouncetoserviceprovidersin
multihomedscenario?(Source:"UsingMultihomedBGPNetworks")
Customershouldannounceonlynetworksoriginatingincustomer’sAS.
CustomershouldannounceallnetworksreceivedfromprimaryISPto
secondaryISP.
CustomershouldannounceallnetworksreceivedfromsecondaryISPto
primaryISP.
Customershouldnotannounceanynetworks.
1. ISPshouldallowcustomertoannounceanypossiblenetworksvia
BGP.Trueorfalse?(Source:"UsingMultihomedBGPNetworks")
true
false
1. WhichthreegoalsrepresentappropriatereasonstoapplyASpath
filters?(Choosethree.)(Source:"EmployingASPathFilters")
Toensurethatonlylocallyoriginatedroutesareannounced.
TolimitroutesthatareadvertisedfromIBGPneighbors.
ToselectasubsetofallroutesbasedontheiroriginatingAS.
TolimitneighborrouteupdatestospecificAS-originatedroutes.
Toensurethatalldestinationautonomoussystemsshouldbereceived
fromaspecifiedneighbor.
Tochangetheweightorlocalpreferenceattributesforalldestination
autonomoussystems.
1. WhichASpathismatchedbytheregularexpression"72$"?
(Source:"EmployingASPathFilters")
2137221831727
273172715072
315277231991
725913682087
1. WhichregularexpressionwouldyouusetomatchallBGPnetworks
originatedinAS89?(Source:"EmployingASPathFilters")
^89$
^89_
_89_
_89$
1. WhichregularexpressionwouldyouusetomatchallBGPnetworks
goingthroughAS76?(Source:"EmployingASPathFilters")
^76$
^76_
_76_
_76$
1. WhichBGPnetworksarematchedbyusingregularexpression
^54$?(Source:"EmployingASPathFilters")
AllroutesthataresourcedinyourdirectlyconnectedneighboringAS54.
AllroutesthathaveAS54intheirASpaths.
AllroutesthataresourcedinAS54.
AlltheroutesthatarereachablebehindAS54.
1. Whatisthedifferencebetweentheregularexpressions"_100_"and
"_100$"?(Source:"EmployingASPathFilters")
Thefirstexpressionreferstoroutesthathavethesubstring"100"intheir
ASpaths;thesecondexpressionrefersonlytoroutesthataredirectly
connectedtoAS100.
Thefirstexpressionreferstoroutesthathavethesubstring"100"intheir
ASpaths;thesecondexpressionrefersonlytoroutesthatoriginatedinAS
100.
ThefirstexpressionreferstoroutesthatgothroughAS100;thesecond
expressionreferstoroutesthatoriginatedinAS100.
ThefirstexpressionreferstoroutesthataredirectlyconnectedtoAS100;
thesecondexpressionreferstoroutesthatoriginatedinAS100.
1. HowdoyoumatchASpathsthatcontainexactlytwosingle-digitAS
numbers?(Source:"EmployingASPathFilters")
usetheexpression"**"
usetheexpression".."
usetheexpression"[0-9]_[0-9]"
usetheexpression"^[0-9]_[0-9]$"
1. Whichthreestepsarerequiredtoapplyanewinboundroutingpolicy
toaneighbor?(Choosethree.)(Source:"EmployingASPath
Filters")
DefineanASpathaccesslist.
AttachtheASpathfiltertoinboundoroutboundupdatesforaspecific
BGPneighbor.
SendincomingandoutgoingASpathfilterstotheBGPneighbor.
Forcetheupdatestogothroughthenewfilter.
1. Howcanyoutestyourregularexpression?(Source:"EmployingAS
PathFilters")
showipbgpaccess-listcommand
showipbgpfiltercommand
showipbgpregexpcommand
showipbgpsummarycommand
1. Whataretworeasonsthatamultihomedcustomerneedsprefix-
lists?(Choosetwo.)(Source:"FilteringwithPrefix-Lists")
ToensurethatonlyvalidIPprefixesareannouncedtotheISPs.
Tosetalimitonthenumberofprefixesthatcanbeacceptedfromthe
ISPs.
TopreventthecustomerfromreceivingitsownIPprefixesfromtheISP.
ToverifythatthecustomerhasreceivedfullInternetroutetables.
1. Whichthreeofthefollowingchoicesareadvantagesofprefixlists
overaccesslists?(Choosethree.)(Source:"FilteringwithPrefix-
Lists")
significantperformanceimprovementonlongfilters
supportforincrementalupdates
abilitytoconsistofanynumberoflines,eachofwhichindicatesatestand
aresult
flexibility
morecomplexcommand-lineinterface
sequentialscanningofprefix-listswithinCiscoIOSsoftware
1. Whenyoudefineprefixlists,whataretworeasonstousesequence
numbers?(Choosetwo.)(Source:"FilteringwithPrefix-Lists")
Toreferencetheassociatedaccesslistfortheprefixlistentry.
ToprovideameansoflinkinganASpathfilterlisttotheprefixlist.
Toprovideanexecutionorderforprefix-listentries.
Toprovideameansofinsertingordeletinglistentries.
1. Whichofthefollowingstatementsisaccurateaboutthegeandle
parametersintheipprefix-listglobalconfigurationcommand
requiredtoconfigureprefix-listfilters?(Source:"FilteringwithPrefix-
Lists")
Nomatchisassumedwhenneithergenorleisspecified.
Therangeisassumedtobefromge-valueto142onlyifthegeattributeis
specified.
Therangeisassumedtobefromlento32onlyiftheleattributeis
specified.
Anexactmatchisassumedwhenneithergenorleisspecified.
1. Whichofthefollowingrouteswillbematchedbythetheprefixlist
10.0.1.0/24ge28?(Source:"FilteringwithPrefixLists")
10.0.1.0/26
10.1.1.0/28
10.0.1.0/29
10.0.0.0/28
1. Whichofthefollowingstatementsaboutimplementingprefixlistsin
aBGPnetworkisaccurate?(Source:"FilteringwithPrefixLists")
Youcanoptionallyapplyfilterlistsandprefixlistsoneitherincomingor
outgoingneighborsinanycombination.
Youcanoptionallyapplyfilterlistsandprefixlistsonlyonoutgoing
neighborsinanycombination
Eithertheincomingprefixlistortheincomingfilterlistmustpermitthe
routesthatarereceivedfromaneighborbeforetheyareacceptedintothe
BGPtable.
Outgoingroutesmustpasstheoutgoingprefixlistbeforebeingtransmitted
totheneighbor.
1. Whichthreestepsarerequiredtoapplyanewinboundroutingpolicy
(basedonprefixlist)toaneighbor?(Choosethree.)(Source:
"FilteringwithPrefixLists")
DefineanIPprefixlist.
AttachtheprefixlisttoinboundoroutboundupdatesforaspecificBGP
neighbor.
SendincomingandoutgoingASpathfilterstotheBGPneighbor.
Forcetheupdatestogothroughthenewfilter.
1. HowcanyouapplythesameprefixlisttomultipleBGPneighborson
arouter?(Source:"FilteringwithPrefixLists")
Byconfiguringaneighborprefix-liststatementforeachBGPpeer.
Byconfiguringaneighbordistribute-liststatementforeachneighbor.
ByusingtheBGPpeer-groupoptionwiththeneighborstatement.
ByconfiguringtheprefixlistasaglobalfilterundertheBGProuting
process.
1. Howcanyouusetheshowipprefix-listcommandtodisplaythe
prefixlistentrythatmatchesaspecificprefixandlength?(Source:
"FilteringwithPrefixLists")
notafeatureoftheshowipprefix-listcommand
byspecifyingthedetailkeyword
withthelongerkeywordtodisplayallmatchesexceptthosewithmore
specificentries
byspecifyingthefirst-matchkeyword
1. Whichofthefollowingbestdescribesthecapabilitiesofthe
proprietaryORFtypethatissupportedonCiscorouters?(Source:
"UsingOutboundRouteFiltering")
standardBGPcommunitiesfiltering
extendedBGPcommunitiesfiltering
ASpathfiltering
prefixlistfiltering
1. ORFfeatureismoreeffectivethanstandardinboundfilteringwhenit
comestotheamountofresourcesthatarerequiredforrouting
updategenerationandprocessing.Trueorfalse?(Source:"Using
OutboundRouteFiltering")
true
false
1. Whataretwokeybenefitstousingoutboundroutefiltering?(Choose
two.)(Source:"UsingOutboundRouteFiltering")
conservesCPUcycles
improvessecurity
reducesbandwidththatisusedbyunnecessaryroutingupdates
increasesneighboravailability
1. WhichtwoofthefollowingstatementsaboutBGPprefix-based
outboundroutefilteringareaccurate?(Choosetwo.)(Source:"Using
OutboundRouteFiltering")
UsesBGPORFsendandreceivecapabilitiestominimizethenumberof
BGPupdatesthataresentbetweenBGPpeers.
Canlimitthenumberofunwantedroutingupdates.
Increasestheamountofresourcesrequiredtoreceiveanddiscardroutes
thatwouldotherwisebefilteredout.
Canbeusedtoincreasetheamountofprocessingonarouterthatisnot
acceptingfullroutesfromaserviceprovidernetwork.
1. Howshouldyouconfiguretheneighborcapabilityorfprefix-list
commandonarouterthatisapplyingaprefixlistfilterasan
outboundroutepolicy?(Source:"UsingOutboundRouteFiltering")
send
receive
both
prefixfilter
1. WhichofthefollowingistrueregardingORFcapabilitiesoptions?
(Choosetwo.)(Source:"UsingOutboundRouteFiltering")
“both”optionallowssendingandreceivingofprefixlists.
“send”optionallowsreceivingofprefixlists.
“receive”optionallowsreceivingofprefixlists.
“both”optionallowsonlysendingofprefixlists.
1. WhataretwomethodsofdeterminingthatarouterhasORF
capabilitiesexchangeconfigured?(Choosetwo.)(Source:"Using
OutboundRouteFiltering")
showrunning-config|beginbgpcommand
showipbgpnegotiatecommand
showipbgpneighborscommand
showipprefix-listcommand
1. WhichcommandshouldbeusedtoverifythatIPprefix-listwas
receivedontheORFreceiver?(Source:"UsingOutboundRoute
Filtering")
showipbgpneighborsIP_addressreceivedprefix-filter
showipprefix-list
showipbgpneighborsIP_addresssendprefix-filter
showorfprefix-listreceived
1. WhataretwoprerequisitesbeforeyoucanconfigureORFprefixlist
functionality?(Choosetwo.)(Source:"UsingOutboundRoute
Filtering")
Arouterefreshmustbesentusingtheclearipbgpcommand.
ABGPpeeringsessionbetweentheORFroutersmustbeupandrunning.
ORFcapabilitiesmustbeenabledonbothrouters.
Youmustconfigureaprefixlistfilteronthereceivingrouter.
1. Whichofthefollowingstatementsaboutthefunctionofaroutemap
isaccurate?(Source:"ApplyingRouteMapsasBGPFilters")
Aroutemapcannotbeusedtomodifyattributesofthepermittedroutes.
Aroutemapisafilterthatusesaseriesofmatchconditions,andthat
whichisdeniedbytheroutemapisdropped.
Aroutemapislesscomplexthantheaccesslist.
Eachroutemapstatementstartswithaseriesofmatchclauses.
1. WhichthreeofthefollowingstatementsareaccurateabouttheBGP
RouteMapPolicyListSupportfeature?(Choosethree.)(Source:
"ApplyingRouteMapsasBGPFilters")
TheBGPRouteMapPolicyListSupportfeatureallowsanetworkoperator
togrouproute-mapmatchclausesintonamedlistscalledpolicy-lists.
Thenetworkoperatormanuallyreconfigureseachrecurringgroupof
matchclausesthatoccurinmultipleroute-mapentries.
TheANDandORsemanticsintheroutemapfunctiondifferentlyfor
policy-liststhanformatchandsetclauses.
TocreateaBGPpolicy-list,usetheippolicy-listcommandinpolicy-map
configurationmode.
Toconfigurearoute-aptoevaluateandprocessaBGPpolicy-listina
routemap,usethematchpolicy-listcommandinroute-mapconfiguration
mode.
Todisplayinformationaboutaconfiguredpolicy-listandpolicy-listentries,
usetheshowippolicy-listcommandinroute-mapconfigurationmode.
1. WhichtwoofthefollowingarefunctionsoftheBGPRouteMap
Continuefeature?(Choosetwo.)(Source:"ApplyingRouteMapsas
BGPFilters")
Providestheabilitytopauseifasequencenumberisnotspecified.
Providesthecapabilitytoexecuteadditionalentriesinaroute-mapafter
anentryisexecutedwithsuccessfulmatchandsetclauses.
Allowsmodularizationofnetworkpolicyconfigurationsothatrepeated
policydefinitionscanbeexpandedwithinthesameroutemap.
Allowsconfigurationandorganizationofmoremodularpolicydefinitionsto
reducethenumberofpolicyconfigurationsthatarerepeatedwithinthe
sameroutemap.
1. Whichofthefollowingcommandsisusedtodistributeanyroutes
thathaveadestinationnetworknumberaddressthatispermittedby
astandardaccesslist,anextendedaccesslist,oraprefixlist,orto
performpolicyroutingonpackets?(Source:"ApplyingRouteMaps
asBGPFilters")
matchipnext-hop
matchiproute-source
matchipaddress
showipbgproute-map
1. Howdoyouimplementa"permitall"statementwhenyouareusing
routemaps?(Source:"ApplyingRouteMapsasBGPFilters")
Bydefault,aroute-aphasan"implicitpermitany"statementifnomatchis
found.
Youmustconfigurearoutemapwitha"permit"parameterandnomatch
clause.
Youmustconfigurearoutemapwitha"deny"parameteranda"deny
none"clause.
Youmustconfigurearoutemapwitha"permitany"matchclause.
1. WhathappenstoincomingBGPupdatesthatdonotmatchany
route-mapmatchclauses?(Source:"ApplyingRouteMapsasBGP
Filters")
TheyareenteredintotheBGPtable.
TheyareenteredintotheBGPtableandmarkedwithaweightof32768.
TheyarenotacceptedbytherouterorenteredintotheBGPtable.
TheyareenteredintotheBGPtableifamatchingrouteexistsintheIP
routingtable.
1. WhichthreeBGPattributescanyousetusingroutemaps?(Choose
three.)(Source:"ApplyingRouteMapsasBGPFilters")
MED
pathorigin
administrativedistance
weightmetric
next-hop
atomicaggregate
1. Whataretworeasonsforusingroutemapsequencenumbers?
(Choosetwo.)(Source:"ApplyingRouteMapsasBGPFilters")
Toallowinsertionordeletionofroute-mapentries.
Toordertheexecutionsequenceofroute-mapmatchclauses.
Toprovideanorderedexecutionsequencefortheroutemap.
Tomapbetweenprefix-liststatementsandroute-mapmatchclauses.
1. WhichcommandshouldbeusedonR1(IPaddress1.1.1.1)toapply
aroutemapnamedFiltertoincomingBGPupdatescomingfromR2
(IPaddress2.2.2.2)?(Source:"ApplyingRouteMapsasBGP
Filters")
neighbor2.2.2.2route-mapFilterin
neighbor1.1.1.1.route-mapFilterin
neighbor2.2.2.2route-mapFilterout
neighbor1.1.1.1.route-mapFilterout
1. WhyisclearingaBGPsessionadisruptivechangeinroutingpolicy?
(Source:"ImplementingChangesinBGPPolicy")
ClearingaBGPsessiontakesalongtimeandcandisruptpacket
forwarding.
YoucannotrecoverinformationthatissentwhiletheBGPsessionisbeing
cleared.
Youcannotautomaticallyre-establishsessionsthataretorndownduring
theclearingoperation.
YoucannotselectivelyteardownBGPsessions;youmustclearsessions
withallneighbors.
1. Usingtheclearipbgp*command,allBGPsessionswillstayup
andnoBGProutesarelost.Trueorfalse?(Source:"Implementing
ChangesinBGPPolicy")
true
false
1. WhichtwoofthefollowingarefunctionsoftheBGPSoftReset
Enhancementfeature?(Choosetwo.)(Source:"Implementing
ChangesinBGPPolicy")
allowsdynamicrouterefreshrequests
requiresnopreconfiguration
providesnewermethodforinboundsoftresetthatusesstoredinbound
routingtableupdates
usesexpandedmemory
1. BGProuterefreshisBGPcapabilitythatallowsaBGProuterto
requestretransmissionofBGProutesfromaneighbor.Trueor
false?(Source:"ImplementingChangesinBGPPolicy")
true
false
1. Whichofthefollowingstatementsaboutthecommandthatis
requiredtoperformarouterefreshisaccurate?(Source:
"ImplementingChangesinBGPPolicy")
Youwillusetheclearipbgp*incommandtosendarouterefresh
messagetoallneighbors.
Youwillusetheclearipbgpip-addressincommandtosendaroute
refreshmessagetoallneighbors.
Youmustusethesoftkeywordwiththeclearipbgpcommandbecause
softresetisnotautomaticallyassumedwhentherouterefreshcapabilityis
supported.
Theclearipbgpcommandworkseveniftheneighborhasnotpreviously
advertisedtherouterefreshcapability.
1. WhichcommandwouldyouuseonR1(IPaddress1.1.1.1)to
requestfromneighboringR2(IPaddress2.2.2.2)toresendrouting
information?(Source:"ImplementingChangesinBGPPolicy")
clearipbgp1.1.1.1out
clearipbgp1.1.1.1in
clearipbgp2.2.2.2out
clearipbgp2.2.2.2in
1. WhichRFC2918compliantcapabilitycode2usedbyallrecentBGP
implementationswilltellyouthatroutersupportstheRouteRefresh
feature?(Source:"ImplementingChangesinBGPPolicy")
1
218
4
2
1. WhichcommandshouldbeusedtodisplaythenegotiationofBGP
capabilitiesaftersessionreset?(Source:"ImplementingChangesin
BGPPolicy")
debugip
debugbgppackets
debugipbgpupdates
debugipbgp
1. HowdoyoudeterminewhetheraBGPneighborsupportsroute
refresh?(Source:"ImplementingChangesinBGPPolicy")
AflagintheBGPtableindicatesthepresenceofrouterefreshcapability
Theshowipbgpneighborcommandindicateswhethertheoptionis
supported.
Initiatethedebugipbgpnegotiationcommandtoseewhethertherouter
hascompletedarouterefreshcapabilitiesexchange.
Executetheclearipbgp*command.Command-lineBGPstatus
messageswillindicaterouterefreshsupportcapabilities.
Introduction
RoutesthatarelearnedviaBGPhavepropertiesthatareassociatedwiththem.
Theyaidarouterindeterminingthebestroutetoadestinationwhenmultiplepaths
tothatparticulardestinationexist.ThesepropertiesarereferredtoasBGP
attributes.Inthismodule,youwilllearntheroleofBGPattributes,andhowtheir
presenceinfluencesrouteselectioninBGP.
UnderstandinghowBGPattributes,includingweight,localpreference,ASpath
prepending,MED,andBGPcommunities.influencerouteselectionisrequiredfor
thedesignofrobustnetworks.
Uponcompletingthismodule,youwillbeableto:
DefinehowtouseBGPweightattributetoinfluencetherouteselection
DefinehowtouseBGPlocalpreferenceattributetoinfluencetherouteselection
DefinehowtouseBGPASpathprependingtoinfluencethereturnpaththatis
selectedbytheneighboringautonomoussystems
DefinehowtouseMEDattributetoinfluencetherouteselection
DefinehowtouseBGOcommunityattributestoinfluencetherouteselection
RouteSelectionUsingAttributes
Overview
Whenconnectionstomultipleprovidersarerequired,itisimportantthatBGPselects
theoptimumroutefortraffictouse.Theoptimum,orbest,routemaynotbewhatyou
intendedduringthedesign,basedonthedesigncriteria,administrativepolicies,or
corporatemandate.Fortunately,BGPprovidesmanytoolsforyoutoinfluenceroute
selection.Oneofthesetoolsistheweightattribute.
Inthislesson,youwilllearnhowtoinfluenceBGProuteselectionbysettingthe
weightattributeofincomingBGProutes.Twomethodsthatareusedtosetthe
weightattribute,defaultweightandroutemaps,arediscussedinthislesson.Youwill
alsolearnhowtomonitortheBGPtabletoverifycorrectweightconfigurationand
properlyinfluencepathselection.
Uponcompletingthislesson,youwillbeableto:
ListBGProuteselectioncriteriaforbest-pathrouteselection
DescribetheuseofBGPweightstoinfluencetheBGProuteselectionprocess
IdentifytheCiscoIOScommandsthatarerequiredtoconfigureper-neighbor
weights
InfluencetheBGProuteselectionprocessbyconfiguringper-neighborweights
DescribehowtoinfluencetheBGProuteselectionprocessbyconfiguringBGP
weightswithroutemaps
SummarizeBGProuteselectionandfilteringtools
InfluencingBGPRouteSelectionwith
Weights
BGProuteselectioncriteriataketheweightparameterintoconsiderationfirst.Ifa
routerhastwoalternativepathstothesamedestination,andtheirweightvaluesare
different,BGPselectstheroutewiththehighestweightvalueasthebest.Onlywhen
thetwoalternativeshaveequalweight,thenextcriterion,localpreference,is
checked.
Prefer highest weight (local to router)
Prefer highest local preference (global within AS)
Prefer routes that the router originated
Prefer shorter AS paths (only length is compared)
Prefer lowest origin code (IGP < EGP < Incomplete)
Prefer lowest MED
Prefer external (EBGP) paths over internal (IBGP)
For IBGP paths, prefer path through closest IGP neighbor
For EBGP paths, prefer oldest (most stable) path
Prefer paths from router with the lower BGP router-ID
Ahighlocalpreferencevalueispreferredoveralowvalue.Onlywhenthetwo
alternativeshaveanequallocalpreference,thenextcriterionischecked.
BGPRouteSelectionCriteria
Theweightattributeislocaltoasinglerouteronly.BGPprotocolneverpropagates
theweightvalue,andthisvalueconstitutesaroutingpolicylocaltotherouter.You
havetwodifferentwayshowtoassigntheweightattributetoaroute.
BGP routing policy can be specified by using:
Weight: provides local routing policy (within a router)
Local preference: provides AS-wide routing policy
BGP weights are specified per neighbor.
Default weight
Complex criteria with route maps
Theweightattributecanbeassignedtoarouteusingoneofthefollowing:
Assignadefaultweightvaluetoalloftheroutesthatarereceivedfromaspecific
neighbor.Thisweightvalueindicatesthattheneighborispreferredoverthe
otherneighbors.
Applyaroutemaponincomingroutesfromaneighbortoselectsomeroutes
andassignweightvaluestothem.Rememberthataroutemapalsoactsasa
filterandwillsilentlydroptheroutesthattheroutemapdoesnotpermitinany
statement.
Ifconfigured,thedefaultweightassignmentonroutesthatarereceivedfroma
neighborisappliedfirst.Allroutesthatarereceivedfromtheneighborareassigned
aweightvalueasdefinedbythedefaultweight.
Whenaroutemapisapplied,itisconfiguredontherouter.Theroutemapcanbe
arbitrarilycomplexandcanusevariousselectioncriteria,suchasanetworknumber
orASpathtoselectroutes.Youcanaltersomeattributesoftheselectedroutes.The
routemapcansettheweightvaluesofpermittedroutes.Selectioncanbedonein
severalroutemapstatements,givingtheopportunitytoassignacertainweightvalue
tosomeroutesandanotherweightvaluetoothers.Aroutemapcanalsocompletely
filteroutroutes.
Ifyouwanttheroutingpolicytobeexchangedwithinall
theroutersintheAS,assignlocalpreferenceattributetoa
route.Thisattributeiscarriedwiththerouteonallinternal
BGPsessions.InthissituationallotherBGP-speaking
routerswithintheASreceivethesameinformation.
Normally,arouterassignsalocalpreferencetoaroute
thatisreceivedonanexternalBGPsessionbeforeitis
acceptedandenteredintheBGPtableoftheborder
router.Routerspropagatethelocalpreferenceattributeon
internalBGPsessionsonly.Thispolicyconstitutesa
routingpolicyfortheentireAS.
InfluencingBGPRouteSelection
IfyouwantallroutesfromtheBGPneighbortogetthespecifiedweight,youhaveto
configureper-neighborweights.
router(config-router)# neighbor ip-address weight weight
All routes from the BGP neighbor get the specified weight.
BGP routes with higher weight are preferred.
Weight is applied only to new incoming updates.
To enforce new weights, reestablish BGP sessions with your neighbors by using
the clear ip bgp command.
Toassignaweighttoaneighborconnection,usetheneighborweightrouter
configurationcommand.Toremoveaweightassignment,usethenoformofthis
command.
Allroutesthatarouterreceivesfromtheneighboraftertheconfigurationlineisin
placegettheweightvalueassigned.Tomakesurethatallroutesfromtheneighbor
receivethenewweightvalue,youcanrestarttheBGPsessionusingclearipbgp
command,forcingtheneighbortoresendallroutes.
Ifnoweightvalueisspecified,thedefaultvalueforroutes
learnedthroughanotherBGPpeeris0andthedefault
valueforroutessourcedbythelocalrouteris32768.
ConfiguringPer-NeighborWeights
Overview
Throughthisdiscovery,youwilllearnhowtoconfigureper-neighborweightsand
howtomonitorBGProuteselection.Youwillconfigureamultihomedcustomer's
router,R2,tousethelinktotheprimaryserviceprovider,ISP1,foralldestinations.
Discovery11:ConfigurePer-NeighborWeights
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
R2 Ethernet0/0 172.16.22.2/24 ConnectiontoISP2
R2 Ethernet0/1 172.16.12.2/24 ConnectiontoISP1
R2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
10.0.0.1/28
10.0.0.17/28
10.0.0.33/28
10.0.0.49/28
10.0.0.65/28
Loopbackssimulate
LANnetworks
ISP1 Ethernet0/1 172.16.12.11/24 ConnectiontoR2
ISP1 Ethernet0/2 172.16.100.11/24 ConnectiontoISP2
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
ISP2 Ethernet0/0 172.16.22.22/24 ConnectiontoR2
ISP2 Ethernet0/2 172.16.100.22/24 ConnectiontoISP1
ISP2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
Loopback7
Loopback8
Loopback9
10.0.2.1/28
10.0.2.17/28
10.0.2.33/28
10.0.2.49/28
10.0.2.65/28
10.0.2.81/28
10.0.2.97/28
10.0.2.113/28
10.0.2.129/28
Loopbackssimulate
LANnetworks
IPaddressesandadvertisednetworksinBGParepreconfiguredasshownbelow:
BGPisalsopreconfiguredasEBGP:
R2toISP1
R2toISP2
ISP1toISP2
JobAids
Step1
OntheR2router,verifytheinitialstateofdefaultweightsforallreceiveddestinations.
Toverifyweights,usetheshowipbgpcommand.
R2# show ip bgp
BGP table version is 65, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.0.64/28 0.0.0.0 0 32768 i
* 10.0.1.0/28 172.16.22.22 0 200 100 i
*> 172.16.12.11 0 0 100 i
* 10.0.1.16/28 172.16.22.22 0 200 100 i
*> 172.16.12.11 0 0 100 i
* 10.0.1.32/28 172.16.22.22 0 200 100 i
*> 172.16.12.11 0 0 100 i
* 10.0.1.48/28 172.16.22.22 0 200 100 i
*> 172.16.12.11 0 0 100 i
* 10.0.1.64/28 172.16.22.22 0 200 100 i
*> 172.16.12.11 0 0 100 i
Network Next Hop Metric LocPrf Weight Path
* 10.0.1.80/28 172.16.22.22 0 200 100 i
*> 172.16.12.11 0 0 100 i
* 10.0.2.0/28 172.16.12.11 0 100 200 i
*> 172.16.22.22 0 0 200 i
* 10.0.2.16/28 172.16.12.11 0 100 200 i
*> 172.16.22.22 0 0 200 i
* 10.0.2.32/28 172.16.12.11 0 100 200 i
*> 172.16.22.22 0 0 200 i
* 10.0.2.48/28 172.16.12.11 0 100 200 i
*> 172.16.22.22 0 0 200 i
* 10.0.2.64/28 172.16.12.11 0 100 200 i
*> 172.16.22.22 0 0 200 i
* 10.0.2.80/28 172.16.12.11 0 100 200 i
*> 172.16.22.22 0 0 200 i
* 10.0.2.96/28 172.16.12.11 0 100 200 i
*> 172.16.22.22 0 0 200 i
* 10.0.2.112/28 172.16.12.11 0 100 200 i
*> 172.16.22.22 0 0 200 i
* 10.0.2.128/28 172.16.12.11 0 100 200 i
*> 172.16.22.22 0 0 200 i
R2hasreceivedroutestothefollowingnetworks:
10.0.0.0/24networksareinjectedintotheBGPtablebythisrouter.Bydefault,locally
sourcedroutesareassignedaweightof32768.
10.0.1.0/24networksarereceivedfromISP1andISP2.Allrouteshavedefaultweight
valueof0assigned.
10.0.2.0/24networksarereceivedfromISP1andISP2.Allrouteshavedefaultweight
valueof0assigned.
Step2
OntheR2router,makesurethatroutereceivedfromtheISP1routerwillbe
preferredoverroutesreceivedfromtheISP2router.
ForroutesthatarereceivedfromISP1,configureaweightof150andforroutes
thatarereceivedfromISP2configureaweightof100.
NotethatBGProuteswithhigherweightarepreferred.
R2(config)# router bgp 1
R2(config-router)# neighbor 172.16.12.11 weight 150
R2(config-router)# neighbor 172.16.22.22 weight 100
ConfigurePer-NeighborWeights
DiscoverySteps
TheweightisconfiguredonR2onbothBGPsessions.Ahigherweightvalueis
giventotheroutesthatarereceivedfromtheprimaryISP,ISP1,comparedto
theroutesthatarereceivedfromthebackupISP,ISP2.
AnytimethattheR2receivesroutinginformationaboutthesameIPnetwork
numberfrombothISPs,theroutercomparestheweightsthatareassignedto
theroutes.TheroutesthatarereceivedfromtheISP1willalwayswinthis
comparison.
Step3
Re-establishBGPsessionsontheR2routertoenforcenewweights.
Tore-establishBGPsession,usetheclearipbgpcommand.
R2# clear ip bgp *
The*parameterofthecommandspecifiestoclearallpeers.
MonitoringBGPRouteSelectionandWeights
router> show ip bgp
Displays all BGP routes. Best routes marked with ">"
With every displayed route, weight is associated.
router> show ip bgp ip-prefix [mask subnet-mask]
Displays detailed information about all paths for a single prefix
Withoutanyargument,theshowipbgpcommanddisplaystheentireBGPtable.
Eachrouteisdisplayedinoneline.Thenetworknumberisdisplayed,andifthe
subnetmaskdiffersfromthenaturalmask,theprefixlengthisindicated.TheBGP
next-hopattribute,MED,localpreference,weight,ASpath,andorigincodeare
displayed.Localpreferenceisdisplayedonlyifitisnotthedefaultvalue.
Theprintoutissortedinnetworknumberorder.Ifthereismorethanoneroutetothe
samenetwork,thenetworknumberisprintedonthefirstlineonly.Theotherroutes
tothesamenetworkhavetheirnetworkfieldleftblankontheoutput.Thegreater-
than(">")characterindicatestheroutesthatareselectedasthebest.
Togetmoredetailedinformationaboutroutestoaspecificdestinationnetwork,you
canusethenetworknumber,andoptionallythesubnetmask,asanargumentonthe
commandline.Theseadditionsdisplaymoredetailedinformationaboutthatspecific
network.
First,ashortsummarythatindicatesthenetworknumberandprefixlengthis
displayed,alongwiththetableversionnumberforthisroute.Thenextlineindicates
howmanyalternativerouteshavebeenreceivedandwhichoneofthemtherouter
hasselectedasthebest.Therearethenacoupleoflinesforeachofthereceived
routestoreachthenetwork.Foreachoftheroutes,allattributesaredisplayed.The
oneselectedasthebestalsohastheword“best”displayed
Step4
OntheR2router,verifyreceivedroutestodestinationnetworksandtheirweights.
Toverifyreceivedroutes,usetheshowipbgpcommand.
R2# show ip bgp
BGP table version is 20, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.0.64/28 0.0.0.0 0 32768 i
* 10.0.1.0/28 172.16.22.22 100 200 100 i
*> 172.16.12.11 0 150 100 i
* 10.0.1.16/28 172.16.22.22 100 200 100 i
*> 172.16.12.11 0 150 100 i
* 10.0.1.32/28 172.16.22.22 100 200 100 i
*> 172.16.12.11 0 150 100 i
* 10.0.1.48/28 172.16.22.22 100 200 100 i
*> 172.16.12.11 0 150 100 i
* 10.0.1.64/28 172.16.22.22 100 200 100 i
*> 172.16.12.11 0 150 100 i
Network Next Hop Metric LocPrf Weight Path
* 10.0.1.80/28 172.16.22.22 100 200 100 i
*> 172.16.12.11 0 150 100 i
* 10.0.2.0/28 172.16.22.22 0 100 200 i
*> 172.16.12.11 150 100 200 i
* 10.0.2.16/28 172.16.22.22 0 100 200 i
*> 172.16.12.11 150 100 200 i
* 10.0.2.32/28 172.16.22.22 0 100 200 i
*> 172.16.12.11 150 100 200 i
* 10.0.2.48/28 172.16.22.22 0 100 200 i
*> 172.16.12.11 150 100 200 i
* 10.0.2.64/28 172.16.22.22 0 100 200 i
*> 172.16.12.11 150 100 200 i
* 10.0.2.80/28 172.16.22.22 0 100 200 i
*> 172.16.12.11 150 100 200 i
* 10.0.2.96/28 172.16.22.22 0 100 200 i
*> 172.16.12.11 150 100 200 i
* 10.0.2.112/28 172.16.22.22 0 100 200 i
*> 172.16.12.11 150 100 200 i
* 10.0.2.128/28 172.16.22.22 0 100 200 i
*> 172.16.12.11 150 100 200 i
R2hasreceivedroutesto15differentnetworksoutsideofitsownAS.
10.0.1.0/24networksarereceivedfromISP1andISP2.
10.0.2.0/24networksarereceivedfromISP1andISP2.
WhentherouteswerereceivedfromtheISP1,theweightvalue150wasassignedto
eachoftheroutes.WhentherouteswerereceivedfromtheISP2,theweightvalue100
wasassignedtoeachoftheroutes.TheR2thenmakestherouteselection—ithastwo
alternativepathsforeachdestinationanditselectsthepathviatheISP1asthebest.It
makesthisselectionbasedontheweightparameter,regardlessofotherBGPattributes,
suchasASpathlength.
Consequently,the10.0.2.0/24networksthataredirectlyconnectedtotheISP2willalso
bereachedviatheISP1.AlthoughtheyareactuallynetworksintheASoftheISP2,AS
200.
Step5
OntheR2router,verifydetailedinformationaboutthe10.0.2.0/28network.
Toverifydetailedinformation,usetheshowipbgpcommandwithanetwork
numberasanargument.
R2# show ip bgp 10.0.2.0/28
BGP routing table entry for 10.0.2.0/28, version 8
Paths: (2 available, best #2, table default)
Advertised to update-groups:
2
Refresh Epoch 2
200
172.16.22.22 from 172.16.22.22 (10.0.2.129)
Origin IGP, metric 0, localpref 100, weight 100, valid, external
Refresh Epoch 2
100 200
172.16.12.11 from 172.16.12.11 (10.0.1.81)
Origin IGP, localpref 100, weight 150, valid, external, best
Therearetwoalternativestoreachnetwork10.0.2.0/28.Eachofthemis
receivedfromdifferentneighbors,oneinAS100andanotherinAS200.The
network10.0.2.0/28iscreatedinAS200.
Therouteselectionmechanismhasselectedthesecondroutethatislistedas
thebest.Itwaschosenbecausetheweightvalueishigher.
Theroutemapisapowerfultooltoselectandalterroutinginformation.Whenyou
applyaroutemaptoincominginformationfromaBGPneighbor,eachreceived
updateisexaminedasitpassesthroughtheroutemap.Thesequencenumberof
thestatementsspecifytheorderofexecution.
Weights can be set with route maps in complex scenarios.
Routes can be matched on any combination of prefix lists, AS path filters, or
other BGP attributes.
Routes not matched by the route-map are discarded.
router bgp as-number
neighbor ip-address route-map route-map-name in
!
route-map route-map-name permit sequence
match condition
set weight weight
Thefirststatementintheroutemapforwhichallthematchclausesindicateamatch
istheonethatisused.Iftheroutemapsays"permit,"thesetclausesareappliedto
theroute,therouteisaccepted,andtheweightischanged.
Matchclausescanbearbitrarilycomplex.OneofthemcanrefertoanASpath
accesslistthatdoesmatchingonASpaths.Anothercanrefertoaprefixlistthat
doesmatchingontheannouncednetworknumber.Onlywhenallconfiguredmatch
clausesareevaluated,theroutemapstatementisusedanditsresult,permit,or
deny,applied.
Ifareceivedrouteisnotmatchedbyanyoftheroutemapstatements,theendofthe
routemapisreached.Theroutemaplogichasan"implicitdeny"rule,whichmeans
thatifnostatementselectsaroute,therouteisdiscarded.
Ifyoudonotdesirethe"implicitdeny"rule,addan"explicitpermitall"attheendof
theroutemaptooverruleit.Toensurethatsucharoutemapstatementisthelast
statement,youshouldassignitaveryhighsequencenumber.Itshouldnothaveany
matchclauseatall.Thelackofamatchclausemeans"matchall."Bynot
configuringanysetclause,youcanensurethatthisstatementdoesnotalterany
attributes.
ChangingWeightswithRouteMapsExample
router bgp 1
neighbor 172.16.12.11 route-map SetWeight200 in
!
route-map SetWeight200 permit 10
match as-path 47
set weight 200
!
route-map SetWeight200 permit 20
set weight 100
!
ip as-path access-list 47 permit _200$
AllreceivedroutesfromISP1neighborhavetheirASpathscheckedagainsttheAS
pathaccesslist47.ASpathaccesslist47,asreferencedbyroutemapstatement
number10,permitsthoserouteswithanASpaththatindicatesthattheyoriginated
inAS200.Routesthattheroutemapstatementnumber10intheSetWeight200
routemappermitsandselects,willhavetheirweightsetto200.Thesetclausein
theroutemapindicatesthenewweightvalue..
Routemapstatementnumber20thenteststheroutesthatarenotoriginatedinAS
ChangingWeightswithRouteMaps
200(routesthattheASpathaccesslist47doesnotpermit).Thisstatementdoesnot
includeamatchclause,indicatingthatallroutesarematched.Therefore,theroute
mapstatement20matchallroutesthattheroutemapstatementdoesnot.Theroute
maphasbeenconfiguredwithan“explicitpermitall”statementattheendofthe
routemap.
Routesthattheroutemapstatement20matches,havetheirweightsetto100.The
resultisthattherouteracceptstheroutesthatoriginatedinAS200(all10.0.2.0/24
networks)andassignstheweight200.Allothersareacceptedandassignedthe
weight100.Thisroutemapdoesnotdiscardanyroute.
Specifyingweightswithfilterlistsisnolongersupportedin
CiscoIOSSoftwareRelease12.1andlater.The
commandhasalreadybeenremovedfromCiscoIOS
SoftwareRelease12.1T.Thesereleasesuseanincoming
routemap,whereyoumatchanASpathwiththematch
as-pathcommandandsettheweightwiththesetweight
command.Whenyouareusingaroutemapasa
replacementforthefilterlistwiththeweightoption,make
surethatspecifyinga“permit”entryintheroutemap
withoutanassociatedmatchconditiondoesnotfilterall
otherroutes.
Prefixlistsandfilterlists,bothinandout,filteroutroutesanddiscardtheonesthat
arenotpermitted.Weightsettingisapplicableonlyonincomingroutesbecausea
routerneverpropagatestheweightattributetoitsneighbors.Routemapscanbe
filtersthatdiscardroutesbutcanalsobeusedtomodifyandsetvariousattributes
onbothincomingandoutgoingroutes.
Thefigureshowsallthepossibleapplicationsofprefixlists,filterlists,weights,and
routemaps.Theyareappliedintheorderindicated.
BGPRouteSelectionandFilteringToolsSummary
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
BGP uses a number of criteria for best-path route selection—the weight is taken
into consideration first.
The weight attribute can be assigned to a route using one of the following:
Default weight value can be assigned to all routes that are received from a
specific neighbor.
Route maps can be applied to neighbors to set the weight attribute of
received routes.
You can use the neighbor weight command to assign a weight value to all
routes that are received from a neighbor.
You can use the show ip bgp command to display all BGP routes, the routes
that BGP selected as best, and the weight attribute setting for each route.
Summary
Overview
SometimesyouneedtoinfluenceBGProuteselection.BGPprovidesmanytoolsto
achievethis.Oneofthesetoolsisalsothelocalpreferenceattribute.
Inthislesson,youwilllearnhowtoinfluenceBGProuteselectionbysettingtheBGP
localpreferenceattributeofincomingBGProutes.Localpreferenceissimilartothe
weightattributebutdiffersfromtheBGPweightattributeinthatweightislocaltoa
specificrouteronwhichitisconfigured.Twomethodsthatareusedtosetthelocal
preferenceattribute,defaultlocalpreferenceandroutemaps,arediscussedinthis
lesson.YouwillalsolearnhowtomonitortheBGPtabletoverifycorrectlocal
preferenceconfigurationandtoproperlyinfluencepathselection.
Uponcompletingthislesson,youwillbeableto:
ExplainwhyusingBGPweightsmightnotprovideconsistentBGProute
selectioninanAS
DescribehowtheBGPlocalpreferenceattributeinfluencesBGProuteselection
IdentifytheCiscoIOScommandthatisrequiredtoconfiguredefaultBGPlocal
preferenceonarouter
IdentifytheCiscoIOScommandsthatarerequiredtomonitorBGPlocal
preference
DescribehowtoconfiguredefaultBGPlocalpreferenceonarouter
IdentifytheCiscoIOScommandsthatarerequiredtoconfigureBGPlocal
preferenceusingroutemaps
DescribehowtoconfigureBGPlocalpreferenceusingroute-maps
SettingBGPLocalPreference
UsingBGPinautonomoussystemswithasingleneighborrelationshipusuallydoes
notrequireanyadvancedfeatures.Insituationsliketheoneshowninthefigure,
however,itisimportanttoensurethatthecustomerrouterschoosethecorrectlink.
Obviously,theroutershouldchoosethe2-Mbpslinkandusethe64-kbpslinkonly
forbackuppurposes.
Q: Which routing protocol must be run in AS 1?
A: You must run IBGP in AS 1.
Youhavetomakesurethattherouterselectstheupperlink(2-Mbpslink)asits
primarylinkandhastheabilitytoswitchovertothebackupifafailureoccurs.To
assurethis,youmustconfigureanIBGPsessionbetweenthetwoborderroutersin
AS1.
Q: How will you influence the route selection on routers in AS 1 so that they select
the fastest route?
A: Use weights on EBGP and IBGP sessions.
Onewayofchangingthedefaultrouteselectionistouseweights.Weightisan
attributethatislocallysignificanttoarouter.Weightisaproperty,orparameter,and
istherefore,notseenonanyneighboringrouters.WhendesigningBGPnetworks
usingweights,youasanetworkadministratorshouldsettheweightsonevery
router.Ifthereismorethanonepathforthesamenetwork,arouterwillchoosethe
onewiththehighestweight.Thedefaultvalueforweightis0.
Inthisexample,theR1routerinAS1setsaweightof100forroutesthatare
receivedoverthe2-MbpslinkfromAS100(primarylink).R1preferstheseroutesto
possibleinternalupdatesfromthebottomrouter,wherethedefaultweightis0.The
bottomroutersetsaweightof100forinternalroutesthatarereceivedfromthe
upperrouterandprefersthemtoroutesthatarereceivedfromAS200.Asaresult,
allpacketswillleavetheASthroughtheprimary2-Mbpslink.
ConsistentRouteSelectionWithintheAS
Run the traffic over the fastest line available.
Theconfigurationsthatareshowninthefiguredemonstratehowtochangethe
defaultweightonaper-neighborbasis.Ifyouusetheneighborweightcommand,
allnewlyarrivedupdateswillhaveaweightof100.Updatescomingfromtheother
neighborwillstillhavethedefaultweightof0.
Afteryouhaveappliedtheneighborweightcommand,arefreshisneededfromthe
neighbor.Therearethreewaysofdoingthis,dependingontheCiscoIOSversion:
Usetheclearipbgpneighboraddresscommandtocleartheneighbor
relationshipandre-establishittorefreshtheBGPentriesandapplytheweight.
Configuresoftreconfigurationfortheneighborandusetheclearcommand.You
canperformallsubsequentclearingbyusingtheclearipbgpneighbor
addresssoftincommand,whichdoesnotresettheneighborrelationship.The
CiscoIOSSoftwareRelease11.2andabovesupportsthesoftreconfiguration
feature..
Usetheclearipbgpneighboraddressincommandifbothneighboring
routerssupporttherouterefresh.TheCiscoIOSSoftwareRelease12.1and
abovesupportstherouterefreshfeature.
Whenyouaretryingtoimplementthisexamplewithweights,itrequirestworoute
mapsoneachrouterwithinAS1.Luckily,BGPhasasimilarmechanismthatyou
canuseforconsistentAS-widerouteselection:localpreference.
1.
2.
3.
Localpreferenceissimilartoweight;becauseitisanattribute,youcansetitonce
andthenviewitonneighboringrouterswithouthavingtoresetit.Thisattributehasa
defaultvalueof100,whichtherouterwillapplytolocallyoriginatedroutesand
updatesthatcomeinfromexternalneighbors.Updatesthatcomefrominternal
neighborsalreadyhavethelocalpreferenceattribute.
You can use local preference to ensure AS-wide route selection policy.
Any BGP router can set local preference when processing incoming route
updates, when doing redistribution, or when sending outgoing route updates.
Local preference is used to select routes with equal weight.
Local preference is stripped in outgoing EBGP updates except in EBGP updates
with confederation peers.
Localpreferenceisthesecond-strongestcriterionintherouteselectionprocess.If
therearetwoormorepathsavailableforthesamenetwork,arouterwillfirst
compareweight.Iftheweightsareequalforallpaths,therouterwillthencompare
thelocalpreferenceattributes.Thepathwiththehighestlocalpreferencevaluewill
bepreferred.
Thelocalpreferenceattributeisautomaticallystrippedoutofoutgoingupdatesto
EBGPsessions.Thispracticemeansthatyoucanusethisattributeonlywithina
singleAStoinfluencetherouteselectionprocess.
Local preference is the second strongest BGP route selection parameter.
Remember the BGP route selection rules:
Highest weight preferred (local to router).
Highest local preference preferred (global within AS).
Other BGP route selection rules.
Weights that are configured on a router override local preference settings.
To ensure consistent AS-wide route selection:
Do not change local preference within the AS.
Do not use BGP weights.
Localpreferenceisthesecond-strongestBGProuteselectionparameter.
Remembertherouteselectionrules:
Preferthehighestweight(localtorouter).
Preferthehighestlocalpreference(globalwithinAS).
ProcessallremainingBGProuteselectionrules.
Becauseyouasanadministratorcanusebothweightandlocalpreferenceto
manipulatetherouteselectionprocess,youmustdecidewhichonetouse.Iflocal
preferenceisused,theweightshouldbethesameforallpaths.
Youcanuseweightonanindividualroutertooverridelocalpreferencesettingsthat
areusedintherestoftheAS.
Inmostcases,itisenoughtochangethedefaultlocalpreferenceonupdates
comingfromexternalneighbors.Youshouldavoidchangingthelocalpreference
attributeoninternalsessionstopreventunnecessarycomplexityandunpredictable
behavior.
BGPLocalPreference
Youcanapplylocalpreferenceinthefollowingways:
Usearoutemapwiththesetlocal-preferencecommand.Youcanusethe
routemaponincomingupdatesfromallneighborsoronoutgoingupdatesto
internalneighbors(notrecommended).
Usethebgpdefaultlocal-preferencecommandtochangethedefaultlocal
preferencevaluethatisappliedtoallupdatesthatcomefromexternalneighbors
orthatoriginatelocally.
Youcanusethebgpdefaultlocal-preferencecommandinBGPconfiguration
modetochangethedefaultvalueoflocalpreference.Thenewdefaultvalueapplies
onlytolocallyoriginatedroutesandtheroutesthatarereceivedfromexternal
neighbors.
router(config-router)# bgp default local-preference preference
This command changes the default local preference value.
The specified value is applied to all routes that do not have local preference set
(EBGP routes).
The default value of this parameter is 100, allowing you to specify more
desirable or less desirable routers.
Ifyousetavaluelowerthanthedefaultof100,therouterwillpreferinternalpathsto
external(normallyarouterwouldpreferexternalroutes).
Ifyousetavaluehigherthan100,thenexternalpathswillbepreferredtoallinternal
paths(alsotheoneswithashorterASpath).
ConfiguringDefaultLocalPreference
Display nondefault local preference with the show ip bgp command.
Local preference is displayed in the show ip bgp prefix command.
Local preference is also displayed in BPG update debugging (only for inbound
updates).
Althoughlocalpreferenceisnotamandatoryattribute,itisappliedtoeveryroute.
Whenyouareusingtheshowipbgpcommand,alocallyapplieddefaultvalueis
notshown.Allothervaluesaredisplayed.Youshouldusetheshowipbgpprefix
commandtoalsodisplaythelocallyappliedvalue.
Theoutputoftheshowipbgpcommandwillnotdisplay
thelocalpreferencevalueifthevalueisthesameasthe
bgpdefaultlocal-preferencevalueinthelocalrouter.
Theoutputthatisdisplayedfromshowanddebugcommandswillvarydepending
ontheCiscoIOSversion.Newerversionstypicallydisplaymoreinformation.In
CiscoIOSSoftwareRelease12.0andinlaterversions,enablingdebuggingof
incomingroutingupdateswillalsodisplaythelocalpreferenceattribute.
MonitoringLocalPreference
Overview
Throughthisdiscovery,youwilllearnhowtoconfigureBGPdefaultlocalpreference.
YouwillinfluenceR2routertopreferthepathtoISP1networksviaR1,insteadof
usingthedirectlyconnectedlink.
Discovery12:ConfigureandMonitorLocalPreference
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
R1 Ethernet0/0 172.16.11.1/24 ConnectiontoISP1
R1 Ethernet0/2 192.168.12.1/24 ConnectiontoR2
R2 Ethernet0/0 172.16.22.2/24 ConnectiontoISP2
R2 Ethernet0/1 172.16.12.2/24 ConnectiontoISP1
R2 Ethernet0/2 192.168.12.2/24 ConnectiontoR1
R2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
10.0.0.1/28
10.0.0.17/28
10.0.0.33/28
10.0.0.49/28
10.0.0.65/28
Loopbackssimulate
LANnetworks
ISP1 Ethernet0/0 172.16.11.11/24 ConnectiontoR1
ISP1 Ethernet0/1 172.16.12.11/24 ConnectiontoR2
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
ISP2 Ethernet0/0 172.16.22.22/24 ConnectiontoR2
ISP2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
Loopback7
Loopback8
Loopback9
10.0.2.1/28
10.0.2.17/28
10.0.2.33/28
10.0.2.49/28
10.0.2.65/28
10.0.2.81/28
10.0.2.97/28
10.0.2.113/28
10.0.2.129/28
Loopbackssimulate
LANnetworks
IPaddressesandadvertisednetworksinBGParepreconfiguredasshownbelow:
BGPisalsopreconfigured:
EBGP
R1toISP1
R2toISP1
R2toISP2
JobAids
IBGP
R1toR2
Step1
OntheR2router,verifytheinitialstateofdefaultlocalpreferencesforallreceived
destinations.
R2# show ip bgp
BGP table version is 21, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.0.64/28 0.0.0.0 0 32768 i
* i 10.0.1.0/28 192.168.12.1 0 100 0 100 i
*> 172.16.12.11 0 0 100 i
* i 10.0.1.16/28 192.168.12.1 0 100 0 100 i
*> 172.16.12.11 0 0 100 i
* i 10.0.1.32/28 192.168.12.1 0 100 0 100 i
*> 172.16.12.11 0 0 100 i
* i 10.0.1.48/28 192.168.12.1 0 100 0 100 i
*> 172.16.12.11 0 0 100 i
* i 10.0.1.64/28 192.168.12.1 0 100 0 100 i
Network Next Hop Metric LocPrf Weight Path
*> 172.16.12.11 0 0 100 i
* i 10.0.1.80/28 192.168.12.1 0 100 0 100 i
*> 172.16.12.11 0 0 100 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
*> 10.0.2.16/28 172.16.22.22 0 0 200 i
*> 10.0.2.32/28 172.16.22.22 0 0 200 i
*> 10.0.2.48/28 172.16.22.22 0 0 200 i
*> 10.0.2.64/28 172.16.22.22 0 0 200 i
*> 10.0.2.80/28 172.16.22.22 0 0 200 i
*> 10.0.2.96/28 172.16.22.22 0 0 200 i
*> 10.0.2.112/28 172.16.22.22 0 0 200 i
*> 10.0.2.128/28 172.16.22.22 0 0 200 i
R2hasreceivedroutestothefollowingnetworks:
10.0.0.0/24networks(originatinglocallyonR2),forwhichtheapplieddefaultlocal
preferenceof100isnotdisplayed.
10.0.1.0/24networks(originatingontheISP1router)arereceivedfromR1andISP1:
ThefirstpathforthesenetworkswasreceivedfromR1(withnexthop
192.168.12.1),withlocalpreferencevalueof100.Localpreferenceofinternal
routeisdisplayedintheoutput.
ThesecondpathforthesenetworkswasreceivedfromISP1(withnexthop
172.16.12.11).Theapplieddefaultlocalpreferenceof100isnotdisplayed.
10.0.2.0/24networks(originatingontheISP2router)arereceivedfromISP2.The
applieddefaultlocalpreferenceof100isnotdisplayed.
For10.0.1.0/24networks,multiplepathsexistandthedecisionmechanismonR2prefers
theexternal(EBGP)pathsoverinternal(IBGP).
Step2
InfluencetheR2routertopreferthepathtoISP1networksviaR1router.
OnR1,applylocalpreferenceof120forallroutesthatarereceivedfrom
externalneighbor.
R1(config)# router bgp 1
R1(config-router)# bgp default local-preference 120
Step3
OntheR2router,verifyconfigureddefaultlocalpreferenceforallreceiveddestinations.
ConfigureandMonitorLocalPreference
DiscoverySteps
R2# show ip bgp
BGP table version is 20, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.0.64/28 0.0.0.0 0 32768 i
*>i 10.0.1.0/28 192.168.12.1 0 120 0 100 i
* 172.16.12.11 0 0 100 i
*>i 10.0.1.16/28 192.168.12.1 0 120 0 100 i
* 172.16.12.11 0 0 100 i
*>i 10.0.1.32/28 192.168.12.1 0 120 0 100 i
* 172.16.12.11 0 0 100 i
*>i 10.0.1.48/28 192.168.12.1 0 120 0 100 i
* 172.16.12.11 0 0 100 i
*>i 10.0.1.64/28 192.168.12.1 0 120 0 100 i
* 172.16.12.11 0 0 100 i
Network Next Hop Metric LocPrf Weight Path
*>i 10.0.1.80/28 192.168.12.1 0 120 0 100 i
* 172.16.12.11 0 0 100 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
*> 10.0.2.16/28 172.16.22.22 0 0 200 i
*> 10.0.2.32/28 172.16.22.22 0 0 200 i
*> 10.0.2.48/28 172.16.22.22 0 0 200 i
*> 10.0.2.64/28 172.16.22.22 0 0 200 i
*> 10.0.2.80/28 172.16.22.22 0 0 200 i
*> 10.0.2.96/28 172.16.22.22 0 0 200 i
*> 10.0.2.112/28 172.16.22.22 0 0 200 i
*> 10.0.2.128/28 172.16.22.22 0 0 200 i
R2hasreceivedroutestothefollowingnetworks:
10.0.0.0/24networksthatoriginatelocallyonR2,andtheapplieddefaultlocal
preferenceof100isnotdisplayed.
10.0.1.0/24networksarereceivedfromR1andISP1:
ThefirstpathforthesenetworkswasreceivedfromR1(nexthop192.168.12.1),
withachangedlocalpreferencevalueof120.Localpreferenceofinternalrouteis
displayedintheoutput.Notethatthisisthebestpathfor10.0.1.0/24networks,
markedwith>sign.
ThesecondpathforthesenetworkswasreceivedfromISP1(nexthop
172.16.12.11).Theapplieddefaultlocalpreferenceof100isnotdisplayed.
10.0.2.0/24networksarereceivedfromISP2.Theapplieddefaultlocalpreferenceof
100isnotdisplayed.
For10.0.1.0/24networks,multiplepathsexistandthedecisionmechanismonR2selects
aroutewithhigherlocalpreferenceattribute(120),sinceweightvalueisleftondefault,0.
Tohavemorecontroloversettinglocalpreference,youmaybeforcedtousearoute
map.
router(config)#
route-map name permit sequence
match condition
set local-preference value
Changes BGP local preference only for routes that the route map entry matches
router(config-router)# neighbor address route-map name in | out
Applies route map to incoming updates from specified neighbor or outgoing
updates to specified neighbor
Per-neighbor local preference that is configured by using a route map with no
match condition
Aroutemapcanhavemoresequencedstatements,eachwithadifferentsetlocal-
preferencecommand,andadifferentmatchcondition.Ifthereisnomatch
command,theroutemapstatementwillapplylocalpreferencetoallroutes.Youcan
thenapplytheroutemaptoBGProuteupdatesineithertheincomingoroutgoing
direction.
Applyingaroutemaptooutgoingupdatesonexternal
sessionswillhavenoeffectonlocalpreferenceinthe
neighboringAS.
Whenyouusearoutemaptosetlocalpreference,theroutemapistypicallyapplied
toincomingBGProutesthattheEBGPneighboradvertises.Thelocalrouteruses
thelocalpreferenceattributeinBGProuteselection.Inaddition,therouteralso
propagatestheattributetoallIBGPsessionsinthelocalAS.Normally,no
modificationsoflocalpreferencearemadeonIBGPsessions.Thisrestriction
ensuresthatallroutersinthelocalASusethesamelocalpreferencevalueand
makethesamedecisionintherouteselectionprocess.
Ifanetworkisnotmatchedinanyoftheroutemap
statements,thenetworkwillbefiltered.Topermit
unmatchednetworkswithoutsettingthelocalpreference
attribute,youshouldaddanotherroutemapstatement
withoutmatchorsetcommandstotheendoftheroute
map.Thisstatementshouldsimplypermittheremaining
networks.
ConfiguringLocalPreferencewithRouteMaps
Overview
Throughthisdiscovery,youwilllearnhowtoconfigurelocalpreferenceusingroute
maps.YouwillagainconfigureR2routertopreferthepathtoISP1networksviaR1,
insteadofusingthedirectlyconnectedlink.
Discovery13:ConfigureLocalPreferenceUsingRoute
Maps
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
R1 Ethernet0/0 172.16.11.1/24 ConnectiontoISP1
R1 Ethernet0/2 192.168.12.1/24 ConnectiontoR2
R2 Ethernet0/0 172.16.22.2/24 ConnectiontoISP2
R2 Ethernet0/1 172.16.12.2/24 ConnectiontoISP1
R2 Ethernet0/2 192.168.12.2/24 ConnectiontoR1
R2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
10.0.0.1/28
10.0.0.17/28
10.0.0.33/28
10.0.0.49/28
10.0.0.65/28
Loopbackssimulate
LANnetworks
ISP1 Ethernet0/0 172.16.11.11/24 ConnectiontoR1
ISP1 Ethernet0/1 172.16.12.11/24 ConnectiontoR2
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
ISP2 Ethernet0/0 172.16.22.22/24 ConnectiontoR2
ISP2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
Loopback7
Loopback8
Loopback9
10.0.2.1/28
10.0.2.17/28
10.0.2.33/28
10.0.2.49/28
10.0.2.65/28
10.0.2.81/28
10.0.2.97/28
10.0.2.113/28
10.0.2.129/28
Loopbackssimulate
LANnetworks
IPaddressesandadvertisednetworksinBGParepreconfiguredasshownbelow:
BGPisalsopreconfigured:
EBGP
R1toISP1
R2toISP1
R2toISP2
JobAids
IBGP
R1toR2
Step1
OntheR2router,verifytheinitialstateofdefaultlocalpreferencesforallreceived
destinations.
Toverifylocalpreferences,usetheshowipbgpcommand.
R2# show ip bgp
BGP table version is 21, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.0.64/28 0.0.0.0 0 32768 i
* i 10.0.1.0/28 192.168.12.1 0 100 0 100 i
*> 172.16.12.11 0 0 100 i
* i 10.0.1.16/28 192.168.12.1 0 100 0 100 i
*> 172.16.12.11 0 0 100 i
* i 10.0.1.32/28 192.168.12.1 0 100 0 100 i
*> 172.16.12.11 0 0 100 i
* i 10.0.1.48/28 192.168.12.1 0 100 0 100 i
*> 172.16.12.11 0 0 100 i
* i 10.0.1.64/28 192.168.12.1 0 100 0 100 i
Network Next Hop Metric LocPrf Weight Path
*> 172.16.12.11 0 0 100 i
* i 10.0.1.80/28 192.168.12.1 0 100 0 100 i
*> 172.16.12.11 0 0 100 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
*> 10.0.2.16/28 172.16.22.22 0 0 200 i
*> 10.0.2.32/28 172.16.22.22 0 0 200 i
*> 10.0.2.48/28 172.16.22.22 0 0 200 i
*> 10.0.2.64/28 172.16.22.22 0 0 200 i
*> 10.0.2.80/28 172.16.22.22 0 0 200 i
*> 10.0.2.96/28 172.16.22.22 0 0 200 i
*> 10.0.2.112/28 172.16.22.22 0 0 200 i
*> 10.0.2.128/28 172.16.22.22 0 0 200 i
R2hasreceivedroutestothefollowingnetworks:
10.0.0.0/24networksthatoriginatelocallyonR2,andtheapplieddefaultlocal
preferenceof100isnotdisplayed.
10.0.1.0/24networksarereceivedfromR1andISP1:
ThefirstpathforthesenetworkswasreceivedfromR1(nexthop192.168.12.1),
withlocalpreferencevalueof100.Localpreferenceofinternalrouteisdisplayed
intheoutput.
ThesecondpathforthesenetworkswasreceivedfromISP1(nexthop
172.16.12.11).Theapplieddefaultlocalpreferenceof100isnotdisplayed.
10.0.2.0/24networksarereceivedfromISP2.Theapplieddefaultlocalpreferenceof
100isnotdisplayed.
For10.0.1.0/24networks,multiplepathsexistandthedecisionmechanismonR2prefers
theexternal(EBGP)pathsoverinternal(IBGP).
Step2
OntheR2router,configureroutemaptosetlocalpreferenceon10.
Nametheroutemap"LP10."
R2(config)# route-map LP10 10
R2(config-route-map)# set local-preference 10
Step3
InfluencetheR2routertopreferthepathtoISP1networksviaR1router.Onthe
R2router,useconfiguredroutemaptosetlowerlocalpreferenceforroutesthat
ConfigureLocalPreferenceUsingRouteMaps
DiscoverySteps
arereceivedfromAS100.
ApplyroutemapLP10forroutesthatarereceivedfromAS100.
R2(config)# router bgp 1
R2(config-router)# neighbor 172.16.12.11 route-map LP10 in
Localpreferenceof10willnowbeappliedtoexternalupdatesfromISP1.
Consequently,R2willpreferthepathtonetworksinAS100viaR1,sinceithas
higherlocalpreferencevalue.
Step4
ClearallBGPsessionsontheR2router.
R2# clear ip bgp *
Step5
OntheR2router,verifylocalpreferenceforallreceiveddestinations.
R2# show ip bgp
BGP table version is 20, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.0.64/28 0.0.0.0 0 32768 i
*>i 10.0.1.0/28 192.168.12.1 0 100 0 100 i
* 172.16.12.11 0 10 0 100 i
*>i 10.0.1.16/28 192.168.12.1 0 100 0 100 i
* 172.16.12.11 0 10 0 100 i
*>i 10.0.1.32/28 192.168.12.1 0 100 0 100 i
* 172.16.12.11 0 10 0 100 i
*>i 10.0.1.48/28 192.168.12.1 0 100 0 100 i
* 172.16.12.11 0 10 0 100 i
*>i 10.0.1.64/28 192.168.12.1 0 100 0 100 i
* 172.16.12.11 0 10 0 100 i
Network Next Hop Metric LocPrf Weight Path
*>i 10.0.1.80/28 192.168.12.1 0 100 0 100 i
* 172.16.12.11 0 10 0 100 i
*> 10.0.2.0/28 172.16.22.22 0 0 200 i
*> 10.0.2.16/28 172.16.22.22 0 0 200 i
*> 10.0.2.32/28 172.16.22.22 0 0 200 i
*> 10.0.2.48/28 172.16.22.22 0 0 200 i
*> 10.0.2.64/28 172.16.22.22 0 0 200 i
*> 10.0.2.80/28 172.16.22.22 0 0 200 i
*> 10.0.2.96/28 172.16.22.22 0 0 200 i
*> 10.0.2.112/28 172.16.22.22 0 0 200 i
*> 10.0.2.128/28 172.16.22.22 0 0 200 i
R2hasreceivedroutestothefollowingnetworks:
10.0.0.0/24networksthatoriginatelocallyonR2,andtheapplieddefaultlocal
preferenceof100isnotdisplayed.
10.0.1.0/24networksarereceivedfromR1andISP1:
ThefirstpathforthesenetworkswasreceivedfromR1(nexthop192.168.12.1)
withadefaultlocalpreferencevalueof100.Localpreferenceofinternalrouteis
displayedintheoutput.
ThesecondpathforthesenetworkswasreceivedfromISP1(nexthop
172.16.12.11)withalocalpreferencevalueof10,setbytheroutemap.The
appliedlocalpreferenceisnotleftondefaultandisdisplayedintheoutput.
10.0.2.0/24networksarereceivedfromISP2.Theapplieddefaultlocalpreferenceof
100isnotdisplayed.
For10.0.1.0/24networks,multiplepathsexistandthedecisionmechanismonR2selects
theroutewithahigherlocalpreferenceattribute(100),sinceweightvalueislefton
default,0.ConsequentlythepathviaR1willbechosentoreach10.0.1.0/24networks.
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
Local preference is similar to the weight attribute. You can use both to influence
BGP path selection, but the weight is local to the specific router on which it is
configured.
You can use local preference to ensure AS-wide route selection policy because
it can be seen on neighboring routers without the need to reset it.
You should avoid mixing weight and local preference because weight has
priority when you are selecting the best path.
Configure local preference:
Using the bgp default local-preference preference command.
Using route map statements.
Summary
Overview
BGPprovidesmanytoolsforadministratorstoinfluencerouteselection.Youhave
alreadylearnedhowtoinfluenceBGProuteselectionforoutgoingtrafficbysetting
weightorlocalpreference.
Problemscanarisewhenadministrativepoliciesmandateaspecificreturnpathtobe
usedfortrafficreturningtotheAS.ASpathprependingpotentiallyallowsthe
customertoinfluencetherouteselectionofitsserviceproviders,sothepathfor
returningtraffic.Inthislesson,youwilllearnaboutASpathprependingandthe
CiscoIOScommandsthatarerequiredtoproperlyconfigureandmonitorASpath
configurations.Youwillalsolearnfilteringrequirementsforinfluencingroute
selectionusingASpathprepending.
Uponcompletingthislesson,youwillbeableto:
ExplaintheneedtoinfluenceBGPreturnpathselectioninaserviceprovider
environment
DescribethefunctionofASpathprependingandhowyoucanuseittofacilitate
properreturnpathselection
IdentifydesignconsiderationswhenyouareimplementingASpathprepending
toinfluencereturnpathselection
DescribehowtoconfigureASpathprependingonthecustomersideandAS
pathfilteringontheserviceproviderside
DescribeshowtheBGPHideLocal-AutonomousSystemfeaturesimplifiesthe
taskofchangingtheASnumberinaBGPnetwork
UsingASPathPrepending
ItisfairlyeasyforanAStoselecttheappropriatepathforoutgoingtraffic.Itismuch
morecomplicatedtoinfluenceotherautonomoussystemstoselecttheappropriate
pathfortrafficthatisreturningtoaspecificAS.
Requirement: The return traffic to the customer must arrive over the highest-
speed access link.
Ifyouconfigurethepreferredpathonlyforoutgoingtrafficandnotforincoming
(return)traffic,youwilllikelygetanasymmetricaltrafficflow.Itcanaswellresultin
suboptimalperformanceofthereturntraffic.Inthefigure,outgoingtrafficisdirected
tothehigh-speedline(2Mbps)asaresultofconfiguringlocalpreferenceorweight.
However,thereturntrafficfromAS200wouldtakethedefaultpathoverthelow-
speedline(64kbps).Thelow-speedlinewouldbealimitingfactorintheoverall
performancethatthenetworkcouldachieve.
Inthisexample,AS1requestsAS200tosendpacketstowardnetwork10.0.0.0/24
viaAS100.Thereasonforthisrequestistoimprovethenetworkperformanceand
minimizedelay.Assuming,ofcourse,thattheconnectivitybetweenAS100andAS
200isbetterthanthedirect64-kbpslinkbetweenAS200andAS1.
DefaultReturnPath
Result: The return traffic flows over the path with the shortest AS path length.
IfyoudonotconfigureanyBGPpathselectiontoolsontheroutetoinfluencethe
trafficflow,AS200willusetheshortestASpath.Thisactionwillresultinunwanted
behaviorbecausethereturntraffictoAS1willbesentoverthelow-speedWANlink.
AS1announcesnetwork10.0.0.0/24overEBGPsessionstobothAS100andAS
200.WhenAS1sendsEBGPupdates,itchangestheASpathattributeaccordingto
BGPspecifications.BothAS100andAS200receiveaBGPupdatefornetwork
10.0.0.0/24withtheASpathsetto1.
AS100selectstheroutefornetwork10.0.0.0/24thatitreceivedfromAS1asits
bestroute.AS100usesthatrouteandforwardsittoAS200.AccordingtoBGP
specifications,AS100alsochangestheASpathattribute.AS200receivestheroute
tonetwork10.0.0.0/24fromAS100withanASpathsetto1001.
AS200hasnowreceivedtwoalternativeroutestonetwork10.0.0.0/24(thedirect
ReturnPathSelectioninaMultihomedAS
routefromAS1andtheroutethroughAS100).Rememberthatyoudidnoconfigure
anythinginAS200toinfluencetheflowoftraffic.So,therouterwillusetheBGP
routeselectionruleofshortestASpathtoselectthebestreturnpathtonetwork
10.0.0.0/24.
ProperReturnPathSelection
Q: How do you select the proper return path from AS 200?
A: Use local preference in AS 200.
Q: Will the administrator of AS 200 configure it?
A: Unlikely.
Rememberthattheincomingtrafficflow(fromtheperspectiveofAS1)willbea
resultoftherouteselectionforoutgoingtrafficinAS200.Thetrafficthatisgoingout
fromAS200willendupasincomingtrafficinAS1.
AS200canconfiguresomechangesthatcausetherouteselectionprocessfor
outgoingtraffictoprefertoreachnetwork10.0.0.0/24viaAS100.Thesechanges
wouldresultinbehaviormatchingthedesiredadministrativepolicyforAS1,which
specifiesthatincomingtraffictotheASshouldbereceivedoverthehigh-speedlink.
OnewaytoaccomplishthedesiredadministrativepolicyinAS1istoreconfigurethe
routerinAS200,whichisreceivingEBGPupdatesdirectlyfromAS1.Youcan
configuretherouterinAS200toassignalocalpreferencevaluelessthanthe
defaultvalue(100)toallroutesthatarereceivedfromAS1.TherouterinAS200is
alsoconfiguredspecificallynottosetlocalpreferenceonEBGProutesthatare
receivedfromAS100.Thisconfigurationresultsinassignmentofthedefaultvalue
of100toallroutesreceivedfromAS100.Thisdifferenceinlocalpreferencevalues
causesAS200routerstoselectthepathviaAS100asthebestonetoreach
10.0.0.0/24networks.
However,alltheconfigurationworktocompletethisprocessmustbeperformedin
AS200.ThenetworkadministratorsofAS200wouldberequiredtomodifythe
routerconfigurationsinAS200tosatisfytheadministrativepolicyrequirementsof
AS1.Allchangesmustbedocumentedandmaintainedaccordingtotherulesand
proceduresthathavebeenadoptedbyAS200.
IfAS200isamajorISP,thenetworkadministratorsmostlikelyaretoobusydoing
otherthingstotailorrouterconfigurationsthatarebasedonthedemandofasingle
leaf(nontransit)ASthatlacksbandwidthonaredundantconnection.
BGPRouteSelectionRules
BGP route selection uses the following criteria:
Prefer largest weight.
Prefer largest local preference.
Prefer routes that the router originated.
Prefer shorter AS paths.
Use other route selection rules.
Manipulating the outgoing AS path length could result in proper return path
selection.
RecallthatBGProuteselectionusesthefollowingcriteria:
Preferthelargestweight.
Preferthelargestlocalpreference.
Preferroutesthattherouteroriginated.
PrefershorterASpaths.
Then,preferallotherrouteselectioncriteria.
Itisunlikelythatyou,astheoperatorofanAS,requestchangesinrouter
configurationsinanotherAS.Thislimitationmakesitvirtuallyimpossibletoinfluence
anotherAStoselectthedesiredpathbasedontheweightandlocalpreference
attributes.BothoptionswouldrequireconfigurationchangesintheneighboringAS.
Butifboththeweightandthelocalpreferenceparametersareleftattheirdefault
settings,theywillnotindicateadifference.Thisconfigurationcausestheroute
selectionprocesstocontinuedownthelistofselectioncriteria.Thethirdcriterionfor
selectionwillnotinfluencerouteselectioninthisscenario,becausenoneofthe
routesoriginateattherouterthatisperformingtherouteselection.Thefourth
criterionwillapply,however,becausetheASpathshavedifferentlengths.
IfyoudonotmanuallymanipulatetheASpathbysomeadministrativemeans,the
routerselectsthepathgoingoverthefewestnumberofASs,regardlessofthe
availablebandwidth.However,iftheASthatisattemptingtoinfluencetheincoming
trafficflowissendingoutEBGPupdateswithamanipulatedASpathattributeover
thatundesiredpath,thereceiverofthisupdateislesslikelytoselectitasthebest
becausetheASpathnowappearstobelonger.
ThebenefitofmanipulatingASpathstoinfluencetherouteselectionisthatthe
configurationthatisneededisdoneintheASthatisrequestingadesiredreturn
path.
YoucanmanipulateASpathsbyprependingASnumberstoexistingASpaths.
Normally,youperformASpathprependingonoutgoingEBGPupdatesoverthe
nondesiredreturnpath.TheASpathsentoutoverthenondesiredlinkbecomes
longerthantheASpathsentoutoverthepreferredpath.So,thenondesiredlinkis
nowlesslikelytobeusedasthereturnpath.
AS path prepending is known as manual manipulation of AS path length.
The AS path should be extended with multiple copies of the AS number of the
sender.
AS path prepending is used to:
Ensure proper return path selection.
Distribute the return traffic load for multihomed customers.
ThelengthoftheASpathisextendedbecauseextracopiesoftheASnumberofthe
senderareprependedto(addedtothebeginningof)theASpathattribute.Toavoid
clasheswithBGPlooppreventionmechanisms,youshouldprependnootherAS
numbertotheASpathattribute,exceptthatofthesendingAS.
IfyouprependanotherASnumberintheASpath,theroutersintheASthathas
beenprependedwillrejecttheupdatebecauseofBGPlooppreventionmechanisms.
Youcanconfigureprependingonarouterforallroutingupdatesthatyousendtoa
neighbororonlyonasubsetofthem.
ASPathPrependingExample
AdministrativepolicyinAS1prefersthatthelow-speedlinkisusedforbackup
purposesonly.Aslongasthehigh-speedlinkbetweenAS1andAS100is
available,alltrafficshouldflowtowardAS1usingthehigh-speedlink.
Result: The return traffic flows over the desired return path.
Toaccomplishthisgoal,youcanconfiguretherouterinAS1,thatsendsEBGP
updatestoAS200,toprependtwocopiesofASnumber"1"toASpath.
AS200receivestwoalternativeroutestoreachnetwork10.0.0.0/24:
TheupdatethatithasreceiveddirectlyfromAS1andhasamanipulatedAS
pathwithalengthofthree.
TheupdatethatithasreceivedviaAS100andwasnotmanuallymanipulated,
andthereforecontainsanASpathlengthoftwo.
WhenAS200startstheselectionprocessforbestroutetoreachnetwork
10.0.0.0/24,itcheckstheASpathlengthaftertheweightandlocalpreference
parameters.Inthiscase,neitherweightnorlocalpreferencehasbeenconfigured,
thelengthoftheASpathwillbethedecidingfactorintherouteselectionprocess.
Consequently,AS200preferstheshortestASpathandthusforwardspackets
towardnetwork10.0.0.0/24viaAS100.Thedesiredadministrativepolicyhasbeen
met,andAS1willreceiveincomingtrafficoverthehigh-speedlink.
IftheforwardingpathfromAS200viaAS100toAS1andnetwork10.0.0.0/24is
laterbroken,theBGPupdatetoreachnetwork10.0.0.0/24isrevoked.Incaseof
suchanetworkfailure,AS200willhaveonlyoneremainingpathtoreachnetwork
10.0.0.0/24.Therouteselectionprocessnowhasonlyonechoice,theroutedirectly
toAS1overthelow-speedWANlink.Thelow-speedlinkwillthereforeserveas
backuptothehigh-speedWANlink.
ASPathPrepending
Prepend the AS path with the AS number of the sender, not the AS number of
the receiver.
WhenyouaremanuallymanipulatingASpaths,theonlyvalidASnumberthatyou
canprependistheASnumberofthesender.PrependinganyotherASnumberwill
causeproblems.
Intheexample,AS1isprependingASnumber200.TheegressrouterperformsAS
pathprependingwhentherouteisonitswaytobetransmittedtoAS200.Afterthe
manualmanipulation,BGPautomaticallychangestheASpathaccordingtotheBGP
specifications.ThelocalASnumbershouldalwaysbeaddedfirstwhenupdatesare
sentoveranEBGPsession.Therefore,whenAS200receivestheBGPupdate,the
ASpathcontainsthevalue1200.TheASnumber200wassetbythemanual
manipulation,andtheASnumber1wasprependedautomaticallybyBGPbecause
theupdatewassentoveranEBGPsession.
WhentheedgerouterinAS200receivestheBGPupdate,itcheckstheASpathto
verifythattheBGPupdateswerenotpropagatedaccidentallybyaroutingloop.
BecausetheedgerouterfindsitsownASnumberintheASpath,itassumesthatthe
BGPupdatehasalreadybeeninAS200.AccordingtotheBGPspecification,the
updatewillbesilentlyignored.
NowassumethatAS1had,forthemanualmanipulation,usedadifferentAS
number,notitsownandnotASnumber200.WouldAS200nowhaveacceptedthe
update?Theanswerisyes.However,inthisscenario,aproblemwouldhave
appearedatalaterstagewhentheroutefinallyreachedtheactualASbelongingto
themanuallyprependedASnumber.ThisASwouldhaverejectedtheroutebecause
itwouldhavefounditsownASnumbersomewhereintheASpath.
HowmanycopiesoftheASnumberofthesendershouldyouprependtotheAS
path?Theanswerdependsonthegoalsoftheadministrativepolicy.Inthegeneral
case,itisnoteasytodeterminetheexactnumberofrequiredASnumbersto
prepend.ThesendingASdoesnotknowwhatalternativepathsareavailableto
otherautonomoussystems.
There is no exact mechanism to calculate the required prepended AS path length.
If a primary and backup scenario is desired:
Use a long prepended AS path over the backup link to ensure that the
primary AS path will always be shorter.
A long backup AS path consumes memory on every Internet router.
Experiment with various AS path lengths until the backup link is idle.
Add a few more AS numbers for improved security (unexpected changes in
the Internet).
If traffic load distribution is desired:
Start with a short prepended AS path, monitor link use, and extend the
prepended path length as needed.
Continuously monitor the link use and change the prepended AS path length
if required.
ThefollowingaretwotypicalcasesinwhichyoucanuseASpathprependingfor
returnpathselection:
Establishingaprimary/backuplink:Asanannouncedbackup(prepended)
routepropagatesthroughtheInternet,alltheroutersalongthewaythatreceive
therouteneedtostoreittogetherwithitsASpathattribute.Ifthisinformationis
long,itwillconsumeextramemoryintheserouters.However,routersforward
onlyroutesthatareselectedasbest.SoanASthatreceivesmultiple
alternativestoadestinationwillselecttheroutewiththeshortestASpathand
forwardonlythatroute.
Whenboththeprimaryandthesecondarylinkareup,theneighboringASwill
receivetworoutestothesamedestinationthatdifferonlyintheASpathlength.
TheroutewiththeshorterASpathwillbethenadvertisedthroughtheInternet.
Inthecasewheretheprimarylinkfails,theroutewiththelongerASpathisthe
onlyremainingroute.Asaresult,theprimaryrouteiswithdrawn,andthe
prependedrouteisadvertisedthroughtheInternet.Inthiscase,extramemory
willbeconsumedineachInternetrouterbecauseofthestorageofthe
prepended(longer)ASpath.
ThelongertheannouncedASpathtotheEBGPneighborconnectedviathe
backuplink,thelesslikelyitisthatincomingtrafficwillbereceivedfromthat
neighbor.You,asanetworkadministrator,canmakeacleverguessabouthow
manycopiesoftheASnumbertoprepend.Aftertheprependingisimplemented,
youhavetoexaminetheresult.Iftheexpectedresultisnotachieved,youcan
changetheconfigurationandprependafewmorecopiesoftheASnumber.
AfterASpathprependinghasgeneratedthedesiredresults,youmaytakethe
precautionofprependingafewmorecopiesoftheASnumbertotheASpath.
Thisactionprotectsthecustomerfrompacketsbeingroutedoverthebackup
linkatapossiblelaterstagewhenthetopologybetweenremoteautonomous
systemshasunexpectedlychanged,yieldingalongerASpathtoreachthe
primarylink.
Distributingtheloadofreturntraffic:Inamultihomedscenario,thereisno
waytoexactlypredeterminethevolumeoftrafficthatwillbereceivedovera
particularlink.Thetrafficloadondifferentlinkswillchangedependingonwhere
thesendersarelocated(whichautonomoussystemstheybelongto).The
networktopologyandthewaythatdifferentremoteautonomoussystemsare
interconnectedmayalsochangewithtime,changingtheloaddistribution.Only
constantmonitoringandfine-tuningwillensurethatthedesiredresultsare
achieved.
Inafirstattempt,youcanconfigurearouterthatisconnectedtoanoverused
linktoprependonlyafewextracopiesofthelocalASnumber.Afterthenetwork
hasbeengiventimetoconverge,youmustcheckthechangeinload
distribution.Monitoringoftheloadmustbedoneforaperiodlongenoughtobe
statisticallysignificant(severaldaysormore).Ifenoughvolumeoftraffichasnot
movedfromtheoverusedlinktotheunderusedlink,youmustprependmore
ASPathPrependingDesignConsiderations
copiesofthelocalASnumber.Thentheprocessofresendinglocalroutesand
monitoringtheresultsstartsalloveragain.
Overview
Inthisdiscovery,youwillfirstconfigureASpathprependingonthecustomerside.
Thenyouwillforamomentimaginethatyouareonaserviceprovidersideandyou
willconfigureASpathfilteringontheserviceproviderside.
Discovery14:ConfigureASPathPrepending
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
R1 Ethernet0/0 172.16.11.1/24 ConnectiontoISP1
R1 Ethernet0/2 192.168.12.1/24 ConnectiontoR2
R2 Ethernet0/0 172.16.22.2/24 ConnectiontoISP2
R2 Ethernet0/2 192.168.12.2/24 ConnectiontoR1
R2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
10.0.0.1/28
10.0.0.17/28
10.0.0.33/28
10.0.0.49/28
10.0.0.65/28
Loopbackssimulate
LANnetworks
ISP1 Ethernet0/0 172.16.11.11/24 ConnectiontoR1
ISP1 Ethernet0/2 172.16.100.11/24 ConnectiontoISP2
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
ISP2 Ethernet0/0 172.16.22.22/24 ConnectiontoR2
ISP2 Ethernet0/2 172.16.100.22/24 ConnectiontoISP1
ISP2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
Loopback7
Loopback8
Loopback9
10.0.2.1/28
10.0.2.17/28
10.0.2.33/28
10.0.2.49/28
10.0.2.65/28
10.0.2.81/28
10.0.2.97/28
10.0.2.113/28
10.0.2.129/28
Loopbackssimulate
LANnetworks
IPaddressesandadvertisednetworksinBGParepreconfiguredasshownbelow:
BGPisalsopreconfigured:
EBGP
R1toISP1
R2toISP2
ISP1toISP2
JobAids
IBGP
R1toR2
Step1
OntheISP1router,verifyinitialbestpathtoreachthe10.0.0.0/24networksinAS1.
ISP1# show ip bgp
BGP table version is 20, local router ID is 10.0.1.81
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
* 10.0.0.0/28 172.16.100.22 0 200 1 i
*> 172.16.11.1 0 1 i
* 10.0.0.16/28 172.16.100.22 0 200 1 i
*> 172.16.11.1 0 1 i
* 10.0.0.32/28 172.16.100.22 0 200 1 i
*> 172.16.11.1 0 1 i
* 10.0.0.48/28 172.16.100.22 0 200 1 i
*> 172.16.11.1 0 1 i
* 10.0.0.64/28 172.16.100.22 0 200 1 i
*> 172.16.11.1 0 1 i
*> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.16/28 0.0.0.0 0 32768 i
*> 10.0.1.32/28 0.0.0.0 0 32768 i
*> 10.0.1.48/28 0.0.0.0 0 32768 i
*> 10.0.1.64/28 0.0.0.0 0 32768 i
*> 10.0.1.80/28 0.0.0.0 0 32768 i
Network Next Hop Metric LocPrf Weight Path
* 10.0.2.0/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.16/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.32/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.48/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.64/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.80/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.96/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.112/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.128/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
ISP1receivedtworoutesto10.0.0.0/24networks:
ThefirstrouteisreceivedfromISP2(nexthop172.16.100.22),withtheASpath200
1.
ThesecondrouteisreceivedfromR1(nexthop172.16.11.1),withtheASpath1.
Sinceweightandlocalpreferenceattributesareleftondefaultvalues,ISP1choosesthe
pathwithshortestASpathlengthasthebest(theroutethatisreceivedfromR1).
IfyouwanttoinfluencethepathfromAS100to10.0.0.0/24networksinAS1,youhave
toconfigureASpathprepending.
ConfiguringAS-PathPrepending
YoucanconfiguremanualmanipulationoftheASpathattribute(prepending)usinga
routemapwiththesetas-pathprependcommand.Theroutemapisusedto
prependthespecifiedASnumberstooutgoingEBGProuteupdatesthatare
matchedwiththematchcondition,asspecifiedintheroutemap.ASpath
prependingiscompletedfirst,andthentherouteissubjecttothenormalASpath
modificationprocedureswhenitissentoveranEBGPsession.
router(config)#
route-map name permit sequence
ConfigureASPathPrepending
DiscoverySteps
match condition
set as-path prepend as-number [ as-number ... ]
Prepends the specified AS number sequence to the routes matched by the route
map entry.
AS numbers prepended to the AS path from the BGP table; the AS number of
the sender always prepended to the end.
router(config-router)# neighbor address route-map name out
Applies the route map to outgoing updates sent to the specified BGP neighbor.
Youcanalsousetheroute-maptoselectonlyasubsetofroutesthatshouldhave
theirASpathmanuallymanipulated.Usethesetas-pathprependcommandwith
theappropriateroute-mappermitstatement.
ChanginganoutgoingroutemapaffectsonlytheBGP
updatesthataresentafterthechange.Topropagatethe
newandlongerASpathwithallannouncedroutes,the
routesmustallberesentbytherouter.Todothis,usethe
privilegedEXECcommandclearipbgpwiththesoftout
qualifier.
Step2
YouwillnowconfiguretheASpathprependingontheR1router.Firstconfigure
route-mapontheR1router.
Nametheroutemap"PREPEND."SelectallBGProutesandprependfive
copiesof"1"totheexistingASpathattributethatisattachedtoeachroute.This
willmakesurethatalltrafficfromAS100to10.0.0.0/24networksinAS1willflow
throughISP2routerinAS200.
R1(config)# route-map PREPEND permit 10
R1(config-route-map)# set as-path prepend 1 1 1 1 1
Thelackofmatchconditionsintheroute-mapindicatesthatallroutesare
matched.TheASpath-string"11111"willbeprependedtotheexistingAS
path.
Step3
OntheR1router,applythe"PREPEND"routemaptoalloutgoingupdatestothe
ISP1EBGPneighbor.
R1(config)# router bgp 1
R1(config-router)# neighbor 172.16.11.11 route-map PREPEND out
AS100willnowreceivearoutetonetwork10.0.0.0/24fromR1withanASpath
containingtheASnumber1sixtimes(111111).
Step4
OntheR1router,reestablishBGPsessionstoenforcetheapplicationofASpath
prepending.
ToreestablishBGPsession,usetheclearipbgpcommand.
R1# clear ip bgp *
Step5
OntheISP1router,verifynewbestpathtoreachthe10.0.0.0/24networksinAS1.
ISP1# show ip bgp
BGP table version is 24, local router ID is 10.0.1.81
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.100.22 0 200 1 i
* 172.16.11.1 0 1 1 1 1 1 1 i
*> 10.0.0.16/28 172.16.100.22 0 200 1 i
* 172.16.11.1 0 1 1 1 1 1 1 i
*> 10.0.0.32/28 172.16.100.22 0 200 1 i
* 172.16.11.1 0 1 1 1 1 1 1 i
*> 10.0.0.48/28 172.16.100.22 0 200 1 i
* 172.16.11.1 0 1 1 1 1 1 1 i
*> 10.0.0.64/28 172.16.100.22 0 200 1 i
* 172.16.11.1 0 1 1 1 1 1 1 i
*> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.16/28 0.0.0.0 0 32768 i
*> 10.0.1.32/28 0.0.0.0 0 32768 i
*> 10.0.1.48/28 0.0.0.0 0 32768 i
*> 10.0.1.64/28 0.0.0.0 0 32768 i
*> 10.0.1.80/28 0.0.0.0 0 32768 i
Network Next Hop Metric LocPrf Weight Path
* 10.0.2.0/28 172.16.11.1 0 1 1 1 1 1 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.16/28 172.16.11.1 0 1 1 1 1 1 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.32/28 172.16.11.1 0 1 1 1 1 1 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.48/28 172.16.11.1 0 1 1 1 1 1 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.64/28 172.16.11.1 0 1 1 1 1 1 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.80/28 172.16.11.1 0 1 1 1 1 1 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.96/28 172.16.11.1 0 1 1 1 1 1 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.112/28 172.16.11.1 0 1 1 1 1 1 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.128/28 172.16.11.1 0 1 1 1 1 1 1 200 i
*> 172.16.100.22 0 0 200 i
ISP1receivedtworoutesto10.0.0.0/24networks:
ThefirstrouteisreceivedfromISP2(nexthop172.16.100.22),withtheASpath2001.
ThesecondrouteisreceivedfromR1(nexthop172.16.11.1),withtheASpath1111
11.
Youcanseenow,thatthereceivednetworkshavethe"11111"ASpaththatis
prependedtotheoriginalASpath.Consequently,thereceivedroutefromR1hasalonger
ASpaththantheroutereceivedfromISP2.Consequently,asweightandlocalpreference
valueareleftondefault,ISP1choosesthepaththatisreceivedfromISP2asthebest
(shortestASpathlength).
Note:WhenyouaremonitoringASpathprepending,therouterdoingtheprependingisnot
theproperpointtoobservetheresultsoftheASpathprependoperation:
Outputofthedebugipbgpupdatescommanddoesnotdisplaytheprependedpaths,
becausetheroutemapdoingtheprependingisappliedafterward.
Outputofshowroute-mapcommanddisplaystheconfigurationdetailsofaroute
map.However,thereisnoindicationofhowmanyrouteshavebeenmatchedbya
route-mapstatement.
AbetterplaceforobservingASpathprependingisontherouterreceivingtheBGPupdate
thatcontainstheprependedASpath.YoucanalsousethepatternofASnumber
sequencesinthereceivedASpathattributeofreceivedroutestofindtheroutesthathave
aprependedASpath.
ASPathFiltering
ServiceprovidersexpecttheircustomerstosendroutesthatoriginateonlyintheAS
ofthecustomer.However,becausecustomersmightnotdoso,proactivethinking
andcareoftherestoftheInternetcausetheserviceprovidertoimplementASpath
filtersonincomingupdates.
Thenetworkadministratoroftheserviceprovidercanconfigureindividualfiltersfor
eachneighbor.HoweverasingleASpathaccesslistpermittingonlyASpathswitha
lengthofexactlyoneASnumberisabettersolution.Thisaccesslistcanthenbe
uniformlyappliedtoallincomingroutesfromallcustomers.
WhenyouconfigureASpathprepending,youhaveto
agreewiththeserviceprovidersotheirASpathfilterswill
bemodifiedtosupportASpathprepending.
Step6
ImaginenowforamomentthatyouareaserviceproviderandconfigureASpath
filterstopreventASpathprepending.NOTE:Acustomernormallydoesnot
configuretheISPside.TheconfigurationonthesideofISPisdonebytheir
networkadministrator.
OntheISP1router,configureASpathfilter,afilterlist,thatwillnotsupportAS
pathprependingbutwillallowonlyASpathswithalengthofoneASnumber.
ApplythisfilerlisttoallincomingupdatesfromR1EBGPneighbor.
ISP1(config)# ip as-path access-list 10 permit ^[0-9]+$
ISP1(config)# router bgp 100
ISP1(config-router)# neighbor 172.16.11.1 filter-list 10 in
AfilterlistallowsonlyASpathswithalengthofoneASnumber—thisnumber
canbeanynumber.Consequently,allrouteswithaprependedASpathare
filteredout.
Step7
OntheISP1router,reestablishBGPsessionstoenforcetheapplicationoffilter
list.
ToreestablishBGPsession,usetheclearipbgpcommand.
ISP1# clear ip bgp *
Step8
OntheISP1router,verifyreceivedroutestoreachthe10.0.0.0/24networksinAS1.
ISP1# show ip bgp
BGP table version is 20, local router ID is 10.0.1.81
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.100.22 0 200 1 i
*> 10.0.0.16/28 172.16.100.22 0 200 1 i
*> 10.0.0.32/28 172.16.100.22 0 200 1 i
*> 10.0.0.48/28 172.16.100.22 0 200 1 i
*> 10.0.0.64/28 172.16.100.22 0 200 1 i
*> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.16/28 0.0.0.0 0 32768 i
*> 10.0.1.32/28 0.0.0.0 0 32768 i
*> 10.0.1.48/28 0.0.0.0 0 32768 i
*> 10.0.1.64/28 0.0.0.0 0 32768 i
*> 10.0.1.80/28 0.0.0.0 0 32768 i
*> 10.0.2.0/28 172.16.100.22 0 0 200 i
*> 10.0.2.16/28 172.16.100.22 0 0 200 i
*> 10.0.2.32/28 172.16.100.22 0 0 200 i
*> 10.0.2.48/28 172.16.100.22 0 0 200 i
Network Next Hop Metric LocPrf Weight Path
*> 10.0.2.64/28 172.16.100.22 0 0 200 i
*> 10.0.2.80/28 172.16.100.22 0 0 200 i
*> 10.0.2.96/28 172.16.100.22 0 0 200 i
*> 10.0.2.112/28 172.16.100.22 0 0 200 i
*> 10.0.2.128/28 172.16.100.22 0 0 200 i
ISP1nowreceivedonlyonerouteto10.0.0.0/24networks,theonefromISP2(nexthop
172.16.100.22).SinceR1isannouncingthenetworks10.0.0.0/24withaprependedAS
path,thefiler-listforincomingroutestoAS100filteredthoseroutesout.Thisfiltering
resultsinasituationwherethenetworks10.0.0.0/24arenotreachableoverthelink
betweenR1andISP1(AS1andAS100).Therefore,thebackupfunctionisnotavailable.
Networks10.0.0.0/24are,however,stillreachableviathepathgoingthroughAS200.
Step9
Ifyouwanttohaveabackuppathavailableto10.0.0.0/24networks,youhaveto
meetanagreementwithacustomerandallowASpathprependinginyourfilters.
OntheISP1router,modifyincomingfiltertoallowoneormultiplecopiesofthe
sameASnumber.Configureafilterforallroutesthatarereceiveddirectlyfrom
AS1.
ISP1(config)# ip as-path access-list 10 permit ^1(_1)*$
ISP1(config)# router bgp 100
ISP1(config-router)# neighbor 172.16.11.1 filter-list 10 in
YouconfiguredanindividualfilterforallroutesthatarereceiveddirectlyfromAS
1.TheASpathisrequiredtostartwith1,thenmultiplecopiesof1mayfollowit.
Ifyouhadmorecustomers,theywouldallrequireanindividualfilterlist,because
thecustomerisexplicitlyindicatedintheregularexpression.Analternative
wouldbetoimplementtheASpathfilterthatallowsanyASpathcontainingone
ormultiplecopiesofthesameASnumber.Forexampleusing^([0-9]+)(_\1)*$
filter,thatmatchesanyASpathbeginningwithanyASnumberandcontinues
withnoormultiplerepetitionsofthesameASnumber.
Step10
OntheISP1router,reestablishBGPsessionstoenforcetheapplicationofnew
filterlist.
Tore-establishBGPsession,usetheclearipbgpcommand.
ISP1# clear ip bgp *
Step11
OntheISP1router,verifynewreceivedroutestoreachthe10.0.0.0/24networksinAS1.
ISP1# show ip bgp
BGP table version is 20, local router ID is 10.0.1.81
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.100.22 0 200 1 i
* 172.16.11.1 0 1 1 1 1 1 1 i
*> 10.0.0.16/28 172.16.100.22 0 200 1 i
* 172.16.11.1 0 1 1 1 1 1 1 i
*> 10.0.0.32/28 172.16.100.22 0 200 1 i
* 172.16.11.1 0 1 1 1 1 1 1 i
*> 10.0.0.48/28 172.16.100.22 0 200 1 i
* 172.16.11.1 0 1 1 1 1 1 1 i
*> 10.0.0.64/28 172.16.100.22 0 200 1 i
* 172.16.11.1 0 1 1 1 1 1 1 i
*> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.16/28 0.0.0.0 0 32768 i
*> 10.0.1.32/28 0.0.0.0 0 32768 i
*> 10.0.1.48/28 0.0.0.0 0 32768 i
*> 10.0.1.64/28 0.0.0.0 0 32768 i
*> 10.0.1.80/28 0.0.0.0 0 32768 i
Network Next Hop Metric LocPrf Weight Path
*> 10.0.2.0/28 172.16.100.22 0 0 200 i
*> 10.0.2.16/28 172.16.100.22 0 0 200 i
*> 10.0.2.32/28 172.16.100.22 0 0 200 i
*> 10.0.2.48/28 172.16.100.22 0 0 200 i
*> 10.0.2.64/28 172.16.100.22 0 0 200 i
*> 10.0.2.80/28 172.16.100.22 0 0 200 i
*> 10.0.2.96/28 172.16.100.22 0 0 200 i
*> 10.0.2.112/28 172.16.100.22 0 0 200 i
*> 10.0.2.128/28 172.16.100.22 0 0 200 i
ISP1againreceivedtworoutesto10.0.0.0/24networks:
ThefirstrouteisreceivedfromISP2(nexthop172.16.100.22),withtheASpath200
1.
ThesecondrouteisreceivedfromR1(nexthop172.16.11.1),withtheASpath111
111.
Asweightandlocalpreferencevalueareleftondefault,ISP1choosesthepaththatis
receivedfromISP2asthebestbasedontheshortestASpathlength.
ChangingtheASnumbermaybenecessarywhentwoseparateBGPnetworksare
combinedunderasingleAS,asituationthattypicallyoccurswhenoneISP
purchasesanotherISP.HoweverchangingoftheASnumberinaBGPnetworkcan
bedifficult.Duringthetransition,IBGPpeerswillrejectexternalroutesfrompeers
withalocalASnumberintheASnumberpathtopreventroutingloops.TheBGP
HideLocal-AutonomousSystemfeatureallowsyoutotransparentlychangetheAS
numberfortheentireBGP.Italsoensuresthatroutescanbepropagatedthroughout
theAS,whiletheASnumbertransitionisincomplete.
Allows you to transparently change the AS number for the entire BGP network.
Ensures that routes can be propagated throughout the AS.
neighbor local-as command in address family or router configuration mode
allows customization of the AS number for EBGP peer groupings.
ToallowcustomizationoftheASnumberforEBGPpeergroupings,usethe
neighborlocal-ascommandinaddressfamilyorrouterconfigurationmode.To
disablethisfunction,usethenoformofthiscommand.
neighbor {ip-address | peer-group-name} local-as as-number [no-prepend]
SyntaxDescription
Parameter Description
ip-address IPaddressofthelocalBGP-speakingneighbor
peer-group-name NameofaBGPpeergroup.
as-number ValidASnumberfrom1to65535.
DonotspecifytheASnumbertowhichtheneighborbelongs.
no-prepend ConfigurestheroutertonotprependthelocalASnumbertoanyroutes
receivedfromanexternalpeer.
Theneighborlocal-ascommandisusedinitiallytoconfigureBGPpeerstosupport
twolocalASnumberstomaintainpeeringbetweentwoseparateBGPnetworks.
ThisconfigurationallowstheISPtoimmediatelymakethetransitionwithoutany
impactonexistingcustomerconfigurations.Whenthecustomerconfigurationshave
beenupdated,thenextstepistocompletethetransitionfromtheoldASnumberto
thenewASnumber.
Whentheneighborlocal-ascommandisconfiguredonaBGPpeer,thelocalAS
numberisautomaticallyprependedtoallroutesthatarelearnedfromEBGPpeers
bydefault.Thisbehavior,however,makeschangingtheASnumberforaservice
providerorlargeBGPnetworkdifficult.RouteswiththeprependedASnumberwill
berejectedbyIBGPpeersthatareconfiguredwiththesameASnumber.
Forexample,ifyouconfigureanIBGPpeerwiththeneighbor10.0.0.2local-as20
statement,allroutesthatarelearnedfromthe10.0.0.2externalpeerwill
automaticallyhavetheASnumber20prepended.Internalroutersthatare
configuredwiththeASnumber20willdetecttheseroutesasroutingloopsandreject
them.ThisbehaviorrequiresyoutochangetheASnumberforallIBGPpeersatthe
sametime.
BGPHideLocal-AutonomousSystem
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
If you do not configure the preferred path for incoming (return) traffic, the likely
result is an asymmetrical traffic flow as well as suboptimal performance of the
return traffic.
AS path prepending is performed on outgoing EBGP updates over the
nondesired return path. Configure it using a route-map with the set as-path
prepend command.
Monitor AS path prepending on the router that is receiving the prepended routes
because the prepended path will not be visible on the prepending router.
If you are a service provider with customers that use AS path prepending, you
must create new AS path filters to accommodate AS path lengths greater than
one AS number.
The BGP Hide Local-Autonomous System feature allows you to transparently
change the AS number for the entire BGP network. It also ensures that routes
can be propagated throughout the AS, while the AS number transition is
incomplete.
Summary
Overview
Whenconnectionstomultipleprovidersarerequired,itisimportantthatBGPselects
theoptimumroutefortraffictouse.Itisequallyimportantthattheselectedreturn
pathistheoptimumreturnpathintotheAS.Theoptimum,orbest,routemaynotbe
whatthenetworkdesignerintendedbasedondesigncriteria,administrativepolicies,
orcorporatemandate.Fortunately,BGPprovidesatoolforadministratorsto
influencerouteselection,theMEDattribute.
Inthislesson,youwilllearnhowtoinfluenceBGProuteselectionbysettingtheBGP
MEDattributeofoutgoingBGProutes.TwomethodsthatareusedtosettheMED
attribute,thedefaultMEDandroutemaps,arediscussed.Inaddition,youwillalso
learnthebasicMEDattributeconfiguration,advancedcommandstomanipulate
MEDproperties,aswellashowtomonitorandtroubleshoottheBGPtabletoverify
correctMEDconfigurationandtoproperlyinfluencepathselection.
Uponcompletingthislesson,youwillbeableto:
DescribehowtheMEDcanbeusedtofacilitateproperreturnpathselection
ExplainhowthevalueoftheMEDattributechangesinsideaBGPASand
betweendifferentBGPautonomoussystems
IdentifytheCiscoIOScommandthatisrequiredtoconfigureBGPMED
DescribehowtoconfigureandmonitorBGPMED
IdentifytheCiscoIOScommandsthatarerequiredtotroubleshoottheBGP
MEDconfigurations
IdentifytheCiscoIOScommandsthatarerequiredtoconfigureadvancedMED
featuresonCiscorouters
UnderstandingBGPMulti-Exit
Discriminators
Whenmultipleconnectionsbetweenprovidersarerequired,BGPattributessuchas
weightandlocalpreferencesolveonlyhalftheproblem:howtochoosetherightpath
outoftheAS.
Q: How can you make sure that the return traffic takes the right path?
Morecomplexhalfoftheproblemishowtoinfluenceneighboringautonomous
systemstochoosethecorrectreturnpathbackintotheAS.
Multi-ExitDiscriminator
TheMEDattributeisahinttoexternalneighborsaboutthepreferredpathintoanAS
whenmultipleentrypointsexist.
You can use the MED to influence path selection in neighbor autonomous systems.
An AS can specify its preferred entry point using the MED in outgoing EBGP
updates.
The MED is not propagated outside of a receiving AS.
The default value of the MED attribute is 0.
The MED is called the "metric" in Cisco IOS software.
The MED is a "weak" metric.
A lower MED value means more preferred.
YoucanapplytheMEDattributeonoutgoingupdatestoaneighboringASto
influencetherouteselectionprocessinthatAS.TheMEDattributeisusefulonly
whentherearemultipleentrypointsintoanAS.
TheMEDattribute,whichissenttoanexternalneighbor,willbeseenonlywithin
thatAS.AnASthatreceivesaroutethatcontainstheMEDattributewillnot
advertisethatMEDbeyonditslocalAS.
ThedefaultvalueoftheMEDattributeis0.AlowervalueoftheMEDattribute
indicatesamorepreferredpath.
TheMEDattributeisconsidereda"weak"metric.Incontrastwithweightandlocal
preference,arouterwillpreferapathwiththesmallestMEDvalue.Butonlyifthe
weight,localpreference,ASpath,andorigincodeattributesareequal.Usingthe
MEDmaynotyieldtheexpectedresultiftheneighboringASmodifiesanyofthe
strongerBGProuteselectionmechanisms.
ThetermthatisusedinCiscoIOSsoftwareforMEDis
"metric."Theterm"metric"alsoappliestotheset
commandthatisusedinroutemapsaswellasallshow
anddebugcommands.
SelectingtheProperReturnPath
ThefigureshowshowthevalueoftheMEDattributeisassigned,dependingupon
theroutinginformationsource.
YoumustconfigurearoutemaponaroutertomanuallyassignavaluetotheMED
attribute.ForthenetworksthatarealsopresentintheBGPtable,therouterassigns
adefaultvaluefromthemetricintheroutingtableandcopiesitintotheMED
attribute.TheMEDattributeisautomaticallyremovedonexternalsessionsifthe
attributedidnotoriginateinthelocalAS.
MEDPropagationinaBGPNetwork
TheMEDisnotamandatoryattribute,andtherenoMEDattributeisattachedtoa
routebydefault.Theonlyexceptionisiftherouterisoriginatingnetworksthathave
anexactmatchintheroutingtable(throughthenetworkcommandorthrough
redistribution).Inthatcase,therouterusesthemetricintheroutingtableasthe
MEDattributevalue.
You have two options, either you change the default MED or you change the MED
with route map.
Option 1:
router(config-router)# default-metric number
The MED is copied from the IGP cost in the router that sources the route
(through the network command or through route redistribution).
You can change the MED value for redistributed routes with the default-metric
command.
Option 2:
router(config)#
route-map name permit sequence
match condition
set metric value
Changes MED for routes that are matched by the route map entry
router(config-router)# neighbor address route-map name in | out
Applies a route map to incoming updates from a specified neighbor or to
outgoing updates to a specified neighbor.
Per-neighbor MED configured by using a route map with no match condition.
Usingthedefault-metriccommandinBGPconfigurationmodecausesall
redistributednetworkstohavethespecifiedMEDvalue.
Tosetthedefaultmetric(MED)valueforBGProutes,usethedefault-metric
numbercommandinrouterconfigurationmode.numberisadefaultmetricvaluethat
isappropriateforthespecifiedroutingprotocol.Toreturntothedefaultstate,usethe
noformofthiscommand.
YoucanalsousearoutemaptosettheMEDonincomingoroutgoingupdates.Use
thesetmetriccommandwithinroutemapconfigurationmodetosettheMED
attribute.
ChangingMED
Overview
Throughthisdiscovery,youwilllearnhowtoconfigureMEDusingroutemapsto
influencethereturnpathfromAS100to10.0.0.0/24networkinAS1.
Discovery15:ConfigureMED
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
R1 Ethernet0/0 172.16.11.1/24 ConnectiontoISP1
R1 Ethernet0/2 192.168.12.1/24 ConnectiontoR2
R2 Ethernet0/0 172.16.22.2/24 ConnectiontoISP2
R2 Ethernet0/1 172.16.12.2/24 ConnectiontoISP1
R2 Ethernet0/2 192.168.12.2/24 ConnectiontoR1
R2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
10.0.0.1/28
10.0.0.17/28
10.0.0.33/28
10.0.0.49/28
10.0.0.65/28
Loopbackssimulate
LANnetworks
ISP1 Ethernet0/0 172.16.11.11/24 ConnectiontoR1
ISP1 Ethernet0/1 172.16.12.11/24 ConnectiontoR2
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
ISP2 Ethernet0/0 172.16.22.22/24 ConnectiontoR2
ISP2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
Loopback7
Loopback8
Loopback9
10.0.2.1/28
10.0.2.17/28
10.0.2.33/28
10.0.2.49/28
10.0.2.65/28
10.0.2.81/28
10.0.2.97/28
10.0.2.113/28
10.0.2.129/28
Loopbackssimulate
LANnetworks
IPaddressesandadvertisednetworksinBGParepreconfiguredasshownbelow:
BGPisalsopreconfigured:
EBGP
R1toISP1
R2toISP1
R2toISP2
JobAids
IBGP
R1toR2
Step1
OntheISP1router,re-establishtheBGPsession.
Tore-establishBGPsession,usetheclearipbgpcommand.
ISP1# clear ip bgp *
Note:BecauseofthelimitationofIOL,youhavetoreestablishtheBGPsession
onISP1togetthesameresultsasdescribedinthenextstepofthelab.
Step2
OntheISP1router,verifytheinitialbestpathtoreach10.0.0.0/24network.
ISP1# show ip bgp
BGP table version is 20, local router ID is 10.0.1.81
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.11.1 0 1 i
* 172.16.12.2 0 0 1 i
*> 10.0.0.16/28 172.16.11.1 0 1 i
* 172.16.12.2 0 0 1 i
*> 10.0.0.32/28 172.16.11.1 0 1 i
* 172.16.12.2 0 0 1 i
*> 10.0.0.48/28 172.16.11.1 0 1 i
* 172.16.12.2 0 0 1 i
*> 10.0.0.64/28 172.16.11.1 0 1 i
* 172.16.12.2 0 0 1 i
*> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.16/28 0.0.0.0 0 32768 i
*> 10.0.1.32/28 0.0.0.0 0 32768 i
*> 10.0.1.48/28 0.0.0.0 0 32768 i
*> 10.0.1.64/28 0.0.0.0 0 32768 i
*> 10.0.1.80/28 0.0.0.0 0 32768 i
Network Next Hop Metric LocPrf Weight Path
*> 10.0.2.0/28 172.16.11.1 0 1 200 i
* 172.16.12.2 0 1 200 i
*> 10.0.2.16/28 172.16.11.1 0 1 200 i
* 172.16.12.2 0 1 200 i
*> 10.0.2.32/28 172.16.11.1 0 1 200 i
* 172.16.12.2 0 1 200 i
*> 10.0.2.48/28 172.16.11.1 0 1 200 i
* 172.16.12.2 0 1 200 i
*> 10.0.2.64/28 172.16.11.1 0 1 200 i
* 172.16.12.2 0 1 200 i
*> 10.0.2.80/28 172.16.11.1 0 1 200 i
* 172.16.12.2 0 1 200 i
*> 10.0.2.96/28 172.16.11.1 0 1 200 i
* 172.16.12.2 0 1 200 i
*> 10.0.2.112/28 172.16.11.1 0 1 200 i
* 172.16.12.2 0 1 200 i
*> 10.0.2.128/28 172.16.11.1 0 1 200 i
* 172.16.12.2 0 1 200 i
ISP1hasreceived10.0.0.0/24networksfromtwodifferentneighbors:
ThefirstpathwasreceivedfromR1(nexthop172.16.11.1).
ThesecondpathwasreceivedfromR2(nexthop172.16.12.2).
ISP1choosesthefirstpath(theonereceivedfromR1)asthebestpath.
Step3
OntheISP1router,checkwhywastheroutethatwasreceivedfromR1chosenasthebestpathto
10.0.0.0/24networks.
ISP1# show ip bgp
ConfigureMED
DiscoverySteps
BGP table version is 20, local router ID is 10.0.1.81
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.11.1 0 1 i
* 172.16.12.2 0 0 1 i
*> 10.0.0.16/28 172.16.11.1 0 1 i
* 172.16.12.2 0 0 1 i
*> 10.0.0.32/28 172.16.11.1 0 1 i
* 172.16.12.2 0 0 1 i
*> 10.0.0.48/28 172.16.11.1 0 1 i
* 172.16.12.2 0 0 1 i
*> 10.0.0.64/28 172.16.11.1 0 1 i
* 172.16.12.2 0 0 1 i
<... output omitted ...>
Ifyoufollowtherouteselectioncriteria,youcandeterminethat:
Theweightisnotconfiguredforanyneighbor.
Thelocalpreferenceisnotconfiguredforanyneighbor.
Routesdonotoriginatelocally.
BothrouteshavethesamelengthofASpath.
BothroutesareEGProutes.
SincebothpathsareEBGPpaths,theoldest(moststable)pathispreferred.Nextthingthatyou
havetoverifyisthedurationofsessionbetweenISP1andR1,andISP1andR2.
ISP1# show bgp summary
BGP router identifier 10.0.1.81, local AS number 100
BGP table version is 20, main routing table version 20
19 network entries using 2812 bytes of memory
32 path entries using 2048 bytes of memory
4/3 BGP path/bestpath attribute entries using 544 bytes of memory
2 BGP AS-PATH entries using 48 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 5452 total bytes of memory
BGP activity 95/76 prefixes, 160/128 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
172.16.11.1 4 1 25 25 20 0 0 00:17:40 13
172.16.12.2 4 1 28 25 20 0 0 00:17:40 13
Thedurationofbothsessionisthesame.Soobviously,therouter-IDbreaksthetie—thepathfrom
routerwiththelowerBGProuter-IDispreferred.
ISP1# show ip bgp neighbor 172.16.11.1
BGP neighbor is 172.16.11.1, remote AS 1, external link
BGP version 4, remote router ID 10.0.7.1
BGP state = Established, up for 00:21:49
Last read 00:00:42, last write 00:00:54, hold time is 180, keepalive
<... output omitted ...>
ISP1# show ip bgp neighbor 172.16.12.2
BGP neighbor is 172.16.12.2, remote AS 1, external link
BGP version 4, remote router ID 10.0.7.17
BGP state = Established, up for 00:23:40
Last read 00:00:44, last write 00:00:50, hold time is 180, keepalive
<... output omitted ...>
Therouter-IDofR1islowerthantherouter-IDofR2,sotheroutereceivedfromR1ispreferred.
Step4
Onthecustomerside,influencethereturningtrafficflowto10.0.0.0/24networks,
sotheISP1routerwilltakethepathviaR2routerinsteadthedefaultone.
ReconfigureMEDusingroutemaps.
Configureroutemap"MED"onbothrouters,R1andR2,topreferthelowerlink
toAS1.
R1(config)# route-map MED
R1(config-route-map)# set metric 500
R2(config)# route-map MED
R2(config-route-map)# set metric 100
LowerMEDvalueispreferredwhenchoosingthebestpath.IfyouwantR2tobe
chosenforthereturningtraffic,ithastobeconfiguredwithlowerMED.
Step5
OntheR1andR2routers,applyroutemapsforISP1neighborintheoutbound
direction.
R1(config)# router bgp 1
R1(config-router)# neighbor 172.16.11.11 route-map MED out
R2(config)# router bgp 1
R2(config-router)# neighbor 172.16.12.11 route-map MED out
Donotforgetthatthesolutionofinfluencingthereturningtrafficflow,ofcourse,
reliesontheneighboringASnotchangingtheweight,localpreference,ASpath,
ororigincodeattributesinupdatesthatitreceivesfromAS1.
Step6
OntheISP1router,reestablishBGPsessionstoenforcetheapplicationofMED.
ToreestablishBGPsession,usetheclearipbgpcommand.
ISP1# clear ip bgp *
Step7
OntheISP1router,verifynewbestpathtoreach10.0.0.0/24network.
ISP1# show ip bgp
BGP table version is 46, local router ID is 10.0.1.81
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
* 10.0.0.0/28 172.16.11.1 500 0 1 i
*> 172.16.12.2 100 0 1 i
* 10.0.0.16/28 172.16.11.1 500 0 1 i
*> 172.16.12.2 100 0 1 i
* 10.0.0.32/28 172.16.11.1 500 0 1 i
*> 172.16.12.2 100 0 1 i
* 10.0.0.48/28 172.16.11.1 500 0 1 i
*> 172.16.12.2 100 0 1 i
* 10.0.0.64/28 172.16.11.1 500 0 1 i
*> 172.16.12.2 100 0 1 i
*> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.16/28 0.0.0.0 0 32768 i
*> 10.0.1.32/28 0.0.0.0 0 32768 i
*> 10.0.1.48/28 0.0.0.0 0 32768 i
*> 10.0.1.64/28 0.0.0.0 0 32768 i
*> 10.0.1.80/28 0.0.0.0 0 32768 i
Network Next Hop Metric LocPrf Weight Path
* 10.0.2.0/28 172.16.11.1 500 0 1 200 i
*> 172.16.12.2 100 0 1 200 i
* 10.0.2.16/28 172.16.11.1 500 0 1 200 i
*> 172.16.12.2 100 0 1 200 i
* 10.0.2.32/28 172.16.11.1 500 0 1 200 i
*> 172.16.12.2 100 0 1 200 i
* 10.0.2.48/28 172.16.11.1 500 0 1 200 i
*> 172.16.12.2 100 0 1 200 i
* 10.0.2.64/28 172.16.11.1 500 0 1 200 i
*> 172.16.12.2 100 0 1 200 i
* 10.0.2.80/28 172.16.11.1 500 0 1 200 i
*> 172.16.12.2 100 0 1 200 i
* 10.0.2.96/28 172.16.11.1 500 0 1 200 i
*> 172.16.12.2 100 0 1 200 i
* 10.0.2.112/28 172.16.11.1 500 0 1 200 i
*> 172.16.12.2 100 0 1 200 i
* 10.0.2.128/28 172.16.11.1 500 0 1 200 i
*> 172.16.12.2 100 0 1 200 i
ISP1hasreceived10.0.0.0/24networksfromtwodifferentneighbors:
ThefirstpathwasreceivedfromR1(nexthop172.16.11.1),withtheMEDsetto500.
ThesecondpathwasreceivedfromR2(nexthop172.16.12.2),withtheMEDsetto
100.
ISP1choosesthepaththatwasreceivedfromR2asthebestpath,sinceithaslower
MEDconfigured.
NotethatMEDisdisplayedintheshowipbgpprintoutasthemetricfield.AllBGP-
relatedshowanddebugcommandsdisplaythevalueoftheMEDattribute.IftheMEDis
notconfigured,themetricfieldisleftblank.Whenlookingatdetailedinformationfora
specificnetwork(viatheshowipbgpprefixcommand),youwillseetheMEDonlyifthe
attributeexists.
Ifdebuggingisnecessarytotroubleshootaproblem,theMED,amongother
attributes,isdisplayedintheoutputs.
MED sent to a neighbor (after the outgoing route map) is displayed in debugging
outputs.
R1# debug ip bgp updates
BGP updates debugging is on for address family: IPv4 Unicast
R1# clear ip bgp 172.16.11.11 soft out
R1#
*Mar 9 01:58:12.497: BGP(0): (base) 172.16.11.11 send UPDATE (format) 10.
0.0.0/28, next 172.16.11.1, metric 500, path Local
*Mar 9 01:58:12.497: BGP(0): (base) 172.16.11.11 send UPDATE (format) 10.
0.0.16/28, next 172.16.11.1, metric 500, path Local
*Mar 9 01:58:12.497: BGP(0): (base) 172.16.11.11 send UPDATE (format) 10.
0.0.32/28, next 172.16.11.1, metric 500, path Local
TheMEDattributewassetwithanoutgoingroutemap.ThisMEDattribute("metric
500")isthensenttotheneighborintheupdate.
MED received from a neighbor is displayed in debugging outputs.
ISP1# debug ip bgp updates
BGP updates debugging is on for address family: IPv4 Unicast
ISP1# clear ip bgp 172.16.11.1
ISP1#
*Mar 9 02:31:35.697: BGP(0): 172.16.11.1 rcvd UPDATE w/ attr: nexthop 172
.16.11.1, origin i, metric 500, merged path 1, AS_PATH
*Mar 9 02:31:35.697: BGP(0): 172.16.11.1 rcvd 10.0.0.0/28
*Mar 9 02:31:35.697: BGP(0): 172.16.11.1 rcvd 10.0.0.16/28
*Mar 9 02:31:35.698: BGP(0): 172.16.11.1 rcvd 10.0.0.32/28
Thisdebuggingexampleshowsthereceivedupdatesfor10.0.0.0/28,10.0.0.0/16,
and10.0.0.32/28networks.TheMEDattribute("metric500")forallthreenetworksis
showninthereceivedupdate.TheMEDwassetontheneighboringrouter.
Original MED received from a neighbor (before the incoming route map
processing) is displayed in show ip bgp neighbor address received-routes.
ISP1# show ip bgp neighbor 172.16.11.1 received-routes
BGP table version is 59, local router ID is 10.0.1.81
Status codes: s suppressed, d damped, h history, * valid, > best, i - inte
rnal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-
Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
* 10.0.0.0/28 172.16.11.1 500 0 1 i
* 10.0.0.16/28 172.16.11.1 500 0 1 i
* 10.0.0.32/28 172.16.11.1 500 0 1 i
Total number of prefixes 3
YoucanchangetheMEDofthereceivedrouteonthereceivingrouter(onISP1for
thisexample).ToseetheoriginalMED,youneedtoenablesoftreconfigurationon
therouter.Theshowipbgpneighboraddressreceived-routescommanddisplays
theoriginalupdatesbeforeanyfiltersorroutemapshavefilteredorchangedthem.
Intheexample,theISP1routerreceivedthreenetworksfrom172.16.11.1neighbor
—10.0.0.0/28,10.0.0.16/28,and10.0.0.32/28.Allthreenetworkshavethemetricset
to500.
Both, original route and modified route, are displayed with a route map when
inbound soft reconfiguration is configured.
ISP1# show ip bgp 10.0.0.0/28
BGP routing table entry for 10.0.0.0/28, version 56
Paths: (3 available, best #1, table default)
Advertised to update-groups:
5
Refresh Epoch 2
1
TroubleshootingtheMED
172.16.11.1 from 172.16.11.1 (10.0.7.1)
Origin IGP, metric 50, localpref 100, valid, external, best
Refresh Epoch 2
1, (received-only)
172.16.11.1 from 172.16.11.1 (10.0.7.1)
Origin IGP, metric 500, localpref 100, valid, external
Only the modified route is displayed in the BGP table.
ISP1# show ip bgp
BGP table version is 163, local router ID is 10.0.1.81
Status codes: s suppressed, d damped, h history, * valid, > best, i - inte
rnal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-
Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.11.1 50 0 1 i
*> 10.0.0.16/28 172.16.11.1 50 0 1 i
*> 10.0.0.32/28 172.16.11.1 50 0 1 i
IfyouconfiguredtheroutemaptoinfluencetheMEDofreceivedroute,youcansee
theoriginalupdatestotheMEDattributebyusingtheshowipbgpprefixcommand.
Softreconfigurationhastobeenabled.Theoriginalversionsaremarkedwiththe
received-onlykeywordandfollowtheversionthatisintheglobalBGPtable.
Inthisexample,thereceivedupdatehadMEDattributesetto500,butavalueof50
waslaterappliedthroughaninboundroutemap.
1.
2.
3.
4.
SeveralrulesexistonwhenandhowyoushouldusetheMEDattribute.
By default, the MED is considered only during selection of routes from the same AS.
router(config-router)# bgp always-compare-med
With always-compare-med, the MED is also considered for routes coming from
a different AS.
If the MED is not attached to a BGP route, it is interpreted as value 0, and thus as
the best metric.
router(config-router)# bgp bestpath med missing-med-worst
With this command, a missing MED is interpreted as infinity (worst).
YoushouldusetheMEDintherouteselectionprocessonlyifboth(all)paths
comefromthesameAS.Usethebgpalways-compare-medcommandtoforce
theroutertocomparetheMEDevenifthepathscomefromdifferent
autonomoussystems.YouneedtoenablethisoptionintheentireAS;
otherwise,routingloopscanoccur.
AccordingtoaBGPstandarddescribingMED,youshouldregardamissing
MEDattributeasaninfinitevalue.CiscoIOSsoftware,ontheotherhand,
regardsamissingMEDattributeashavingavalueof0.Usethebgpbestpath
medmissing-med-worstcommandwhencombiningequipmentfromdifferent
vendors.Anevenbettersolutionistomakesurethateveryupdatecarriesa
MEDattribute.
By default, the MED is considered only during selection of routes from the same AS,
which does not include intra-confederation autonomous systems.
router(config-router)# bgp bestpath med confed
Use this command to allow routers to compare paths that are learned from
confederation peers.
router(config-router)# bgp deterministic-med
This command changes the BGP route selection procedure to a deterministic
but slower one.
YoumustusethebgpbestpathmedconfedcommandwhenyouusetheMED
withinaconfederationtoinfluencetherouteselectionprocess.Arouterwill
compareMEDvaluesfortheroutesthatoriginateintheconfederation.
WhenyouenableadeterministicMEDcomparison,youallowarouterto
compareMEDvaluesbeforeitconsidersBGProutetype(externalorinternal)
andIGPmetrictothenext-hopaddress.TherouterwillcompareMEDvalues
immediatelyaftertheASpathlength.
Ciscorecommendsenablingthebgpdeterministic-med
commandinallnewnetworkrollouts.Forexisting
networks,youmustdeploythecommandeitheronall
routersatthesametimeorincrementally,withcareto
avoidpossibleIBGProutingloops.
AdvancedMEDConfigurationExample
Thefollowingexampledemonstrateshowthebgpdeterministic-medandbgp
always-compare-medcommandscaninfluenceMED-basedpathselection.
ConsiderthefollowingBGProutesfornetwork172.16.0.0/16intheorderthatthey
arereceived:
entry 1: AS(PATH) 65500, med 150, external, rid 192.168.13.1
entry 2: AS(PATH) 65100, med 200, external, rid 1.1.1.1
entry 3: AS(PATH) 65500, med 100, internal, rid 192.168.8.4
BGPcomparesmultipleroutestoasingledestinationin
AdvancedMEDConfiguration
pairs,startingwiththenewestentryandmovingtoward
theoldestentry(startingatthetopofthelistandmoving
down).Forexample,entry1andentry2arecompared.
Thebetterofthesetwoisthencomparedtoentry3,and
soon.
Inthecasewherebothcommandsaredisabled,BGPcomparesentry1andentry2.
Entry2ischosenasthebetteronebecauseithasalowerrouter-ID.TheMEDisnot
checkedbecausethepathsarefromadifferentneighborAS.Next,entry2is
comparedtoentry3.BGPchoosesentry2asthebestpathbecauseitisexternal.
Inthecasewherethebgpdeterministic-medcommandhasbeendisabledandthe
bgpalways-compare-medcommandhasbeenenabled,BGPcomparesentry1to
entry2.Theseentriesarefromdifferentautonomoussystems,butbecausethebgp
always-compare-medcommandisenabled,theMEDisusedinthecomparison.
Entry1isthebetterofthesetwoentriesbecauseithasalowerMEDvalue.Next,
BGPcomparesentry1toentry3.TheMEDischeckedagainbecausetheentries
arenowfromthesameAS.BGPchoosesentry3asthebestpath.
Inthecasewherethebgpdeterministic-medcommandhasbeenenabledandthe
bgpalways-compare-medcommandhasbeendisabled,BGPgroupsroutesfrom
thesameAS.Thenitcomparesthebestentriesofeachgroup.TheBGPtablelooks
likethefollowing:
entry 1: AS(PATH) 65100, med 200, external, rid 1.1.1.1
entry 2: AS(PATH) 65500, med 100, internal, rid 192.168.8.4
entry 3: AS(PATH) 65500, med 150, external, rid 192.168.13.1
ThereisagroupforAS65100andagroupforAS65500.BGPcomparesthebest
entriesforeachgroup.Entry1isthebestofitsgroupbecauseitistheonlyroute
fromAS100.BGPcomparesentry1tothebestofgroupAS65500,entry2
(becauseithasthelowestMED).Becausethetwoentriesarenotfromthesame
neighborAS,theMEDisnotconsideredinthecomparison.TheEBGProutewins
overtheIBGProute,makingentry1thebestroute.
Ifthebgpalways-compare-medcommandwerealsoenabled,BGPwouldhave
takentheMEDintoaccountforthelastcomparisonandhaveselectedentry2asthe
bestpath.
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
The MED is a "weak" parameter in the route selection process—it is used only if
weight, local preference, AS path, and origin code are equal. By default, the
MED is compared only for paths that were received from the same AS.
There is no MED attribute that is attached to a route by default.
You can set the default metric value (MED) for BGP routes, using the
default-metric command in router configuration mode.
You can use a route map to set an arbitrary MED value to sent or received
routes.
You can configure advanced MED parameters to modify the default MED
behaviors.
Use the bgp always-compare-med to force the router to compare the MED
even if paths came from different autonomous systems.
Use the bgp bestpath med confed command when you use the MED
within a confederation to influence the route selection process
Summary
Overview
WhenconnectionstomultipleISPsarerequired,itisimportantthatBGPselectthe
optimumroutefortraffictouse.Itisequallyimportantthattheselectedreturnpathis
theoptimumreturnpathintotheAS.BGPprovidestoolsforadministratorsto
influencerouteselectionandBGPcommunityattributeisonesuchtool.
YouwilllearnhowtoinfluenceBGProuteselectionbysettingtheBGPcommunity
attributeonoutgoingBGProutestofacilitateproperreturnpathselection.Youwill
alsolearnabouttheconfigurationdetailsofBGPcommunities.
Uponcompletingthislesson,youwillbeableto:
Describetheissuesofreturnpathselectionformultihomedcustomersandwhy
youcannotusetheBGPattributesofweight,localpreference,andMEDtosolve
theseissues
DescribethebasicqualitiesofBGPcommunities
DescribehowBGPcommunitiesfacilitateproperreturnpathselection
ListthestepsthatarerequiredtosuccessfullydeploycommunitiesinaBGP-
basednetwork
IdentifytheCiscoIOScommandsthatarerequiredtodeployBGPcommunities
DescribethefunctionoftheBGPNamedCommunityListsfeature
DescribethefunctionoftheBGPCostCommunityfeature
DescribethefunctionoftheBGPLinkBandwidthfeature
DescribehowBGPsupportssequencedentriesinextendedcommunity-lists
AddressingBGPCommunities
Inthisexample,thecustomerandthebackupserviceproviderwouldliketoavoid
ASpathprependingandrelyonotherBGPtoolstoproperlyroutethereturntraffic
overthehighest-speedWANlink.
Q: How do you select the proper return path from AS 100 without using AS path
prepending in AS 1?
A: Use local preference in AS 100.
Q: Will the administrator of AS 100 configure it?
A: Unlikely.
UsingtheMEDtoinfluencethepreferredreturnpathisnotpossiblebecausethe
MEDcannotbepropagatedacrossseveralautonomoussystems.AS100would,
therefore,receivenetworksfromAS1directlywiththeMEDattributebutwould
receivenetworkswithoutaMEDattributefromAS200.Inanycase,BGProute
selectionwouldbebasedonthelengthoftheASpath.EveniftheMEDwaspresent
andusedtheshortestpath,thepathwouldstillbethroughtheslow64-kbpslink.
TheonlyoptionforresolvingthisissueistouselocalpreferenceinAS100.The
problemwiththissolutionisthatserviceprovidersnormallydonotrushtoimplement
everywishthattheircustomersmighthave.
Inthislesson,youwilllearnasolutiontothiscasestudythatusesthetransitive
optionalattributecalled"BGPcommunity"inconjunctionwithlocalpreference.
SelectingtheProperReturnPath
BGPcommunitiesareattributesthatareusedtogroupandfilterroutes.
Communitiesaredesignedtogiveyoutheabilitytoapplypoliciestolargenumbers
ofroutesbyusingmatchandsetclausesintheconfigurationofroutemaps.
Communitylistsareusedinthisprocesstoidentifyandfilterroutesbytheircommon
attributes.
BGP communities are a mean of tagging routes to ensure consistent filtering or
route selection policy.
Any BGP router can tag routes in incoming and outgoing routing updates or
when doing redistribution.
Any BGP router can filter routes in incoming or outgoing updates or select
preferred routes based on communities.
By default, communities are stripped in outgoing BGP updates.
AcommunityisanattributethatisusedtotagBGProutes.Aroutercanapplyitto
anyBGProutebyusingaroutemap.Otherrouterscanthenperformanyaction
basedonthetag(community)thatisattachedtotheroute.
TherecanbemorethanoneBGPcommunitythatisattachedtoasingleroute,but
therouters,bydefault,removecommunitiesinoutgoingBGPupdates.
The community attribute is a transitive optional attribute. Its value is a 32-bit
number (range 0 to 4,294,967,200).
Each network in a BGP routing table can be tagged with a set of communities.
The standards define several filtering-oriented communities:
no-advertise: Do not advertise routes to any peer.
no-export: Do not advertise routes to real EBGP peers.
local-as: Do not advertise routes to any EBGP peers.
internet: Advertise this route to the Internet community.
Routers that do not support communities pass them along unchanged.
Thecommunityattributeisa32-bittransitiveoptionalBGPattributethatwas
designedtogroupdestinationsandapplyroutingdecision(accept,prefer,
redistribute,andsoon)accordingtocommunities.Thisway,thecommunityattribute
allowseasyapplicationofadministrativepolicies.BGPcommunitiesprovidea
mechanismtoreduceBGPconfigurationcomplexityonaroutercontrollingthe
distributionofroutinginformation.
Asetofcommunityvalueshasbeenpredefined.Whenarouterreceivesaroutethat
hasbeenmarkedwithapredefinedcommunity,therouterwillperformaspecific,
predefinedactionthatisbasedonthatcommunitysettingasfollows:
no-advertise:Ifarouterreceivesanupdatecarryingthiscommunity,itwillnot
forwardittoanyneighbor.
no-export:Ifarouterreceivesanupdatecarryingthiscommunity,itwillnot
propagateittoanyexternalneighborsexcepttointra-confederationexternal
neighbors.Theno-exportattributeisthemostwidelyusedpredefined
communityattribute.
local-as:Thiscommunityhasasimilarmeaningtono-export,butitkeepsa
routewithinthelocalAS(ormember-ASwithintheconfederation).Therouteis
notsenttoexternalBGPneighborsortointra-confederationexternalneighbors.
internet:AdvertisethisroutetotheInternetcommunity.Allroutersbelongtoit.
Routersthatdonotsupportthecommunityattributewillpasstheattributetoother
neighborsbecauseitisatransitiveattribute.
Defining your own communities
A 32-bit community value is split into two parts:
High-order 16 bits contain the AS number of the AS that defines the
community meaning.
Low-order 16 bits have local significance.
Values of all zeroes and all ones in high-order 16 bits are reserved.
Cisco IOS parser allows you to specify a 32-bit community value as:
[AS-number]:[low-order-16-bits]
Communityattributesareusuallyusedbetweenneighboringautonomoussystems.
BGPCommunitiesOverview
FortheBGPcommunitiestobegloballyunique,apublicASnumbershouldbepart
ofthecommunityvalue.Forthisreason,youcanenterthecommunityvalueastwo
16-bitnumbersthatareseparatedbyacolon.Thefirstnumber(high-order16bits)
shouldbetheASnumberoftheASthatdefinesthecommunityvalue.Thesecond
numbershouldbeavaluethatisassignedacertainmeaning(thatis,translationofa
communityvalueintolocalpreferenceintheneighboringAS).
YoucanalsousecommunitiesinternallywithinanAS(toensureAS-widerouting
policy),inwhichcasethefirst16bitsshouldcontaintheASnumberofthelocalAS.
1.
2.
3.
4.
5.
6.
Define administrative policy goals.
Design filters and route selection policy to achieve administrative goals.
Define communities that signal individual goals.
Configure route tagging on entry points or let BGP neighbors tag the routes.
Configure community distribution.
Configure route filters and route selection parameters based on communities.
DesigningaBGPsolutionaroundBGPcommunitiesusuallyrequiresthefollowing
steps:
Defineadministrativepolicygoalsthatyouneedtoimplement.
Definethefiltersandrouteselectionpolicythatwillachievetherequiredgoals.
Assignacommunityvaluetoeachgoal.
Applycommunitiesonincomingupdatesfromneighboringautonomoussystems
ortelltheneighborstosetthecommunitiesthemselves.
EnablecommunitydistributionthroughoutyourAStoallowcommunity
propagation.
Matchcommunitieswithroutemapsandroutefilters,changeBGPattributes,or
influencetherouteselectionprocessbasedonthecommunitiesthatare
attachedtotheBGProutes.
UsingCommunitiesExample
Thisexampleshowshowyoucandefinegoalsandassigncommunitiestothem.
Define administrative policy goal:
Solve asymmetrical customer routing problems.
Design filters and path selection policy to achieve administrative goals:
Set local preference of customer routes to 50 for customers using the
backup ISP.
Define communities that signal individual goals:
Community 100:17 is used to indicate that the local preference of the route
should be lowered to 50.
Thistableliststhegoalsandthecommunityvalues.
Goal CommunityValue
Setlocalpreferenceof50. 100:17
Setlocalpreferenceof150. 100:18
PrependtheASpathoncewhensendingthenetworktoexternal
neighbors.
100:21
PrependtheASpathtwicewhensendingthenetworktoexternal
neighbors.
100:22
PrependtheASpaththreetimeswhensendingthenetworktoexternal
neighbors.
100:23
Allcustomersoftheserviceprovidershouldknowthislistsothattheycanusethe
BGPcommunitieswithouthavingtodiscusstheirusewiththeserviceprovider.
UsingCommunities
SeveralactivitiesarerequiredtosuccessfullydeployBGPcommunitiesinaBGP-
basednetwork.
Configure BGP communities as follows:
Configure route tagging with BGP communities.
Configure BGP community propagation.
Define BGP community access lists (community lists) to match BGP
communities.
Configure route maps that match on community lists and filter routes or set other
BGP attributes.
Apply route maps to incoming or outgoing updates.
ToconfigureBGPcommunitiesoncustomersside,youfirsthavetoconfigurea
routemapinwhichyouwillsetthecommunity.Alsothecommunitypropagationto
BGPneighborhastobeconfigured,otherwisethecommunityvaluesattachedto
outgoingBGPupdateswillbestripedout.
Ontheotherside,serviceproviderhastodefineBGPcommunityliststomatch
routesbasedonattachedBGPcommunities.Routemapshavetobeusedtofilteror
modifyBGProutingupdatesbasedonmatchingoncommunitylistsinincomingor
outgoingupdates.
1.ConfiguringRouteTaggingwithBGPCommunities
Firststepistosetcommunities,whichrequiresaroutemap.
router(config)#
route-map name
match condition
set community value [ value ... ] [additive]
Route tagging with communities is always done with a route map.
Communities that are specified in the set keyword overwrite existing
communities unless you specify the additive option.
router(config-router)# neighbor ip-address route-map map in | out
You have to apply a route map to inbound or outbound BGP updates.
router(config-router)# redistribute protocol route-map map
You can apply a route map to redistributed routes
Inaroute-mapconfigurationmode,youshouldusethesetcommunitycommandto
attachacommunityattribute(orasetofcommunities)toaroute.Youcanattachup
to32communitiestoasingleroutewithoneroutemapsetstatement.Ifyouusethe
keywordadditive,theoriginalcommunitiesarepreservedandtheroutersimply
appendsthenewcommunitiestotheroute.Omittingtheadditivekeywordresultsin
theoverwritingofanyoriginalcommunityattributes.
Youcanapplyaroutemaptoincomingoroutgoingupdates.Youcanalsouseit
withredistributionfromanotherroutingprotocol.
Aroutemapisafilteringmechanismthathasan"implicit
deny"forallnetworksthatarenotmatchedinanyroute
mapstatement.Ifaroutemapisnotintendedtofilter
routes,thenyoushouldaddanotherroutemapstatement
attheendtopermitallremainingnetworkswithout
changingit(nomatchandnosetcommandsareused
withinthatroutemapstatement).
Originally,CiscoIOSsoftwareacceptedanddisplayedBGPcommunityvaluesasa
single32-bitvalueinadigitalformat.NewerCiscoIOSversionssupportthenew
format,whereyoucansetorviewacommunityastwocolon-separated16-bit
numbers.Theglobalcommandipbgp-communitynew-formatisrecommendedon
allrouterswhenevercommunitiescontaintheASnumber.
ConfiguringBGPCommunities
1.
2.
3.
4.
2.ConfiguringCommunityPropagation
Secondstepistoenablingcommunitypropagationperneighborforallinternal
neighbors.Ifcommunitiesaresenttoexternalneighbors,youmustenable
communitypropagationforexternalneighbors.
router(config-router)# neighbor ip-address send-community
By default, communities are stripped in outgoing BGP updates.
You must manually configure community propagation to BGP neighbors.
BGP peer groups are ideal for configuring BGP community propagation toward
many neighbors.
AcommandthatnetworkadministratorscommonlyforgetwhenconfiguringBGP
communitiesistheneighborip-addresssend-communitycommand.This
commandisneededtopropagatecommunityattributestoBGPneighbors.Evenif
youuseanoutgoingroutemaptosetcommunities,bydefault,therouterwillstrip
outanycommunityvaluesthatareattachedtooutgoingBGPupdatesifyouhave
notconfiguredthiscommandforthespecificBGPneighbor.Youcanalsoapplythis
commandtoapeergroup.
3.DefiningBGPCommunityLists
Onaserviceproviderside,communitylistshavetobecreatedtobeusedwithin
routemapstomatchoncommunityvalues.
router(config)# ip community-list 1-99 permit|deny value [ value ... ]
This command defines a simple community list.
You can use the keyword internet to match any community.
router(config)# ip community-list 100-199 permit|deny regexp
This command defines an expanded community list.
Communities that are attached to a route are ordered, converted to string, and
matched with regular expression.
Use ".*" to match any community value.
Youcanuseastandardcommunitylisttofindcommunityattributesinrouting
updates.Astandardcommunitylistisdefinedbyitsassignedlistnumber,whichcan
rangefrom1to99.
ComparisonofCommunityListstoStandardIPAccessLists
Similarities Differences
Therouterevaluatesthelinesinthecommunity-
listsequentially.
Thekeywordinternetshouldbeusedtopermit
anycommunityvalue.
Ifnolinematchescommunitiesthatareattached
toaBGProute,therouteisimplicitlydenied.
Ifmorevaluesarelistedinasingleline,theyall
havetobeinanupdatetoproduceamatch.
Anexpandedcommunitylistisdefinedbyitsassignedlistnumber,whichcan
rangefrom100to199.Regularexpressionsareusedtomatchcommunity
attributes.Whenarouterprocessesalistofcommunitiesthatareattachedtoa
networkupdate,theyareconvertedintoanorderedstringofcharacters.This
exampleshowshowtheprocessisaccomplished:
Theoriginallistofcommunitiesinanupdate:
"10.0.0.0/24,NH=1.1.1.1,origin=I,AS-path=203040,community=10:101,
community=10:201,community=10:105,community=10:205"
Astringofcharacterscontaininganorderedlistofcommunityvalues:
"_10:101_10:105_10:201_10_205_"("_"representsaspace)
Aregularexpression:
"permit_10:.0[1-5]_"("_"representsanunderscorethatmatchesspaces)
Theresult:
Thisregularexpressionpermitstheroutebecauseitpermitsallrouteswith
communitieswherethefirst16bitscarrytheASnumber10andthesecond
5.
16bitscontain0astheseconddigitandanumberbetween1and5asthe
thirddigit;thefirstdigitcanbeanything(asindicatedbythe".").
Useregularexpression".*"topermitanycommunity.
4.MatchingBGPCommunitieswithRouteMaps
Onaserviceproviderside,routemapswherecommunitylistsareusedtomatchon
communityvalueshavetobecreated.Youcanthenuseroutemapstofilterbased
oncommunityvaluesortosetotherparametersorattributes(forexample,local
preference,MED,orASpathprepending).
Theseroutemapsarethenappliedtoincomingoroutgoingupdates.
router(config)#
route-map name permit | deny
match community clist-number [exact]
set attributes
Community lists are used in match conditions in route maps to match on
communities that are attached to BGP routes.
Route selection:
You can use route maps to set weights, local preference, or metric based on
BGP communities that are attached to the BGP route.
Normal route selection rules apply afterward. Routes not accepted by route
map are dropped.
Default filters:
Routes that are tagged with community no-export are sent to IBGP peers
and intra-confederation EBGP peers.
Routes that are tagged with local-as are sent to IBGP peers.
Routes that are tagged with no-advertise are not sent in any outgoing BGP
updates.
Useroutemapstomatchthenetworksthatcarryasubsetofcommunitiesthatthe
communitylistpermits.Otherparametersorattributescanthenbesetbasedon
communityvalues.Aroutemapwithacommunitylistmatchesarouteifatleast
somecommunitiesthatareattachedtotheroutematchthecommunitylist.Ifyou
usethekeywordexact,allcommunitiesthatareattachedtoaBGProutehavetobe
matchedbythecommunitylist.
YoucanusearoutemaptofilterormodifyBGProutingupdates.AnyBGP-related
setcommandscanbeusedtosetBGPparametersandattributes(thatis,weight,
localpreference,andMED).
Asmentionedbefore,therearesomepredefinedcommunityvaluesthatcause
routerstoautomaticallyfilterroutingupdates:
no-advertise:Ifarouterreceivesanupdatecarryingthiscommunity,therouter
willnotforwardittoanyneighbor.
local-as:Thiscommunityhasasimilarmeaningtono-export,butitkeepsa
routewithinthelocalsubautonomoussystem.Therouteisnotpropagatedto
intra-confederationexternalneighborsortoanyotherexternalneighbors.
no-export:Ifarouterreceivesanupdatecarryingthiscommunity,therouterwill
notpropagateittoanyexternalneighborsexcepttointra-confederationexternal
neighbors.
internet:ThisvalueadvertisesthisroutetotheInternetcommunity,towhichall
routersbelong.
Overview
Inthisdiscovery,youwillinfluencethereturningpath.Youwillconfigurelocal
preferencethroughthecommunities.Youwilldotheconfigurationonthecustomer
andontheserviceproviderside.
Discovery16:ConfigureLocalPreferenceUsingthe
Communities
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
R1 Ethernet0/0 172.16.11.1/24 ConnectiontoISP1
R1 Ethernet0/2 192.168.12.1/24 ConnectiontoR2
R2 Ethernet0/0 172.16.22.2/24 ConnectiontoISP2
R2 Ethernet0/2 192.168.12.2/24 ConnectiontoR1
R2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
10.0.0.1/28
10.0.0.17/28
10.0.0.33/28
10.0.0.49/28
10.0.0.65/28
Loopbackssimulate
LANnetworks
ISP1 Ethernet0/0 172.16.11.11/24 ConnectiontoR1
ISP1 Ethernet0/2 172.16.100.11/24 ConnectiontoISP2
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
ISP2 Ethernet0/0 172.16.22.22/24 ConnectiontoR2
ISP2 Ethernet0/2 172.16.100.22/24 ConnectiontoISP1
ISP2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
Loopback7
Loopback8
Loopback9
10.0.2.1/28
10.0.2.17/28
10.0.2.33/28
10.0.2.49/28
10.0.2.65/28
10.0.2.81/28
10.0.2.97/28
10.0.2.113/28
10.0.2.129/28
Loopbackssimulate
LANnetworks
IPaddressesandadvertisednetworksinBGParepreconfiguredasshownbelow:
BGPisalsopreconfigured:
EBGP
R1toISP1
R2toISP2
ISP1toISP2
JobAids
IBGP
R1toR2
Step1
OntheISP1router,verifyinitialbestpathtoreachthe10.0.0.0/24networksinAS1.
ISP1# show ip bgp
BGP table version is 31, local router ID is 10.0.1.81
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
* 10.0.0.0/28 172.16.100.22 0 200 1 i
*> 172.16.11.1 0 1 i
* 10.0.0.16/28 172.16.100.22 0 200 1 i
*> 172.16.11.1 0 1 i
* 10.0.0.32/28 172.16.100.22 0 200 1 i
*> 172.16.11.1 0 1 i
* 10.0.0.48/28 172.16.100.22 0 200 1 i
*> 172.16.11.1 0 1 i
* 10.0.0.64/28 172.16.100.22 0 200 1 i
*> 172.16.11.1 0 1 i
*> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.16/28 0.0.0.0 0 32768 i
*> 10.0.1.32/28 0.0.0.0 0 32768 i
*> 10.0.1.48/28 0.0.0.0 0 32768 i
Network Next Hop Metric LocPrf Weight Path
*> 10.0.1.64/28 0.0.0.0 0 32768 i
*> 10.0.1.80/28 0.0.0.0 0 32768 i
* 10.0.2.0/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.16/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.32/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.48/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.64/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.80/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.96/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.112/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
* 10.0.2.128/28 172.16.11.1 0 1 200 i
*> 172.16.100.22 0 0 200 i
ISP1receivedtworoutesto10.0.0.0/24networks:
FirstrouteisreceivedfromISP2(nexthop172.16.100.22)
SecondrouteisreceivedfromR1(nexthop172.16.11.1),
Sinceweightandlocalpreferenceattributesareleftondefaultvalues,ISP1choosesthe
pathwithshortestASpathlengthasthebest(theroutethatisreceivedfromR1).
IfyouwanttoinfluencethereturningtraffictoAS1fromAS100togoviaAS200(and
notdirectly),youcandoitbyusingcommunitylistsinAS1.IntheupdatefromthatAS,
youcansendacommunitylist.Thiscommunitylistcantheninfluencethevalueof,for
example,localpreferenceinAS100forallroutesthatarereceivedfromAS1.
Step2
ConfigurecommunitylisttoinfluencethereturningtrafficpathtoyourAS(AS1).
UsecommunityforchangingthelocalpreferencevalueonISP'sside.Firstdo
thecustomer'ssideoftheconfigurationandconfigureroutemapontheR1
routertotagrouteswithBGPcommunities.
Nametheroutemap"SetCom."SelectallBGProutesandsetcommunityvalue
of"100:17."Youhavealreadyagreedwithaserviceprovider,that"100:17"
communitywillsetlocalpreferenceof50onaserviceprovider'sside.Sincethe
defaultlocalpreferenceis100,lowerlocalpreference(50)meansthattheroute
willnotbechosenasthebestone.
ConfigureLocalPreferenceUsingtheCommunities
DiscoverySteps
R1(config)# route-map SetCom permit 10
R1(config-route-map)# set community 100:17
Becauseyoudidnotenteranymatchstatement,allnetworksarepermitted.If
youwantedtosetcommunitiesonspecificroutes,youcoulduseastandard
accesslisttomatchagainst,withthematchipaddresscommandintheroute
map.
Step3
OntheR1router,applythe"SetCom"routemaptoalloutgoingupdatestothe
ISP1EBGPneighbor.
R1(config)# router bgp 1
R1(config-router)# neighbor 172.16.11.11 route-map SetCom out
Step4
Remember,thatyouhavetoenablepropagationofcommunityattributestoBGP
neighbors.Otherwisetherouterwillstripoutanycommunityvaluesattachedto
outgoingupdates.
EnablecommunitypropagationontheR1router.
R1(config)# router bgp 1
R1(config-router)# neighbor 172.16.11.11 send-community
YouenabledcommunitypropagationfromAS1toAS100.
Step5
Nowmoveontheserviceprovider'ssideanddefineBGPcommunitylistthatwill
matchcommunitiessetinAS1.
OntheISP1router,configurecommunitylisttomatchcommunitiesthatwere
previouslysetontheR1router.
ISP1(config)# ip community-list 7 permit 100:17
Step6
ThelastthingthatyouhavetodoistotranslatereceivedcommunitiesfromAS1
intolocalpreference.OntheISP1router,configureroutemaptomatchBGP
communitiesandtochangethelocalpreferencevalueofmatchedcommunities.
OntheISP1router,configureroutemap"SetLocalPref,"thatwillusethe
configuredcommunitylisttofindacommunity100:17andwillsetlocal-
preferenceof50.Donotforgettopermitallotherroutes,thatdonotcontainthe
rightcommunity,withoutchanginganythingintheupdate.
ISP1(config)# route-map SetLocalPref permit 10
ISP1(config-route-map)# match community 7
ISP1(config-route-map)# set local-preference 50
ISP1(config-route-map)# exit
ISP1(config)# route-map SetLocalPref permit 1000
Routemapusesacommunitylist,thatwasconfiguredinpreviousstep,tofind
community100:17.Ifthecommunitylistmatchesoneofthecommunity
attributes,thesetcommandisexecuted,thelocal-preferenceischanged,and
therouteispermitted.
Iftheroutedoesnotcontaintherightcommunity,therouteissimplypermitted
byroutemapstatement1000withoutchanginganythingintheupdate.
Step7
OntheISP1router,applytheconfiguredroutemaptoallincomingupdatesfrom
R1EBGPneighbor.
ISP1(config)# router bgp 100
ISP1(config-router)# neighbor 172.16.11.1 route-map SetLocalPref in
TheresultwillbethatAS100willsetlocalpreferenceof50toallroutesreceived
fromAS1.So,itwillprefertheotherpath(thepathviaAS200)toreachAS1.
Step8
OntheISP1router,re-establishBGPsessiontoenforcetheapplicationofnew
routemap.
ToreestablishBGPsession,usetheclearipbgpcommand.
ISP1# clear ip bgp *
Step9
OntheISP1router,verifynewreceivedroutestoreachthe10.0.0.0/24networksinAS1.
ISP1# show ip bgp
BGP table version is 21, local router ID is 10.0.1.81
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.100.22 0 200 1 i
* 172.16.11.1 50 0 1 i
*> 10.0.0.16/28 172.16.100.22 0 200 1 i
* 172.16.11.1 50 0 1 i
*> 10.0.0.32/28 172.16.100.22 0 200 1 i
* 172.16.11.1 50 0 1 i
*> 10.0.0.48/28 172.16.100.22 0 200 1 i
* 172.16.11.1 50 0 1 i
*> 10.0.0.64/28 172.16.100.22 0 200 1 i
* 172.16.11.1 50 0 1 i
*> 10.0.1.0/28 0.0.0.0 0 32768 i
*> 10.0.1.16/28 0.0.0.0 0 32768 i
*> 10.0.1.32/28 0.0.0.0 0 32768 i
*> 10.0.1.48/28 0.0.0.0 0 32768 i
Network Next Hop Metric LocPrf Weight Path
*> 10.0.1.64/28 0.0.0.0 0 32768 i
*> 10.0.1.80/28 0.0.0.0 0 32768 i
*> 10.0.2.0/28 172.16.100.22 0 0 200 i
* 172.16.11.1 50 0 1 200 i
*> 10.0.2.16/28 172.16.100.22 0 0 200 i
* 172.16.11.1 50 0 1 200 i
*> 10.0.2.32/28 172.16.100.22 0 0 200 i
* 172.16.11.1 50 0 1 200 i
*> 10.0.2.48/28 172.16.100.22 0 0 200 i
* 172.16.11.1 50 0 1 200 i
*> 10.0.2.64/28 172.16.100.22 0 0 200 i
* 172.16.11.1 50 0 1 200 i
*> 10.0.2.80/28 172.16.100.22 0 0 200 i
* 172.16.11.1 50 0 1 200 i
*> 10.0.2.96/28 172.16.100.22 0 0 200 i
* 172.16.11.1 50 0 1 200 i
*> 10.0.2.112/28 172.16.100.22 0 0 200 i
* 172.16.11.1 50 0 1 200 i
*> 10.0.2.128/28 172.16.100.22 0 0 200 i
* 172.16.11.1 50 0 1 200 i
ISP1againreceivedtworoutesto10.0.0.0/24networks:
ThefirstrouteisreceivedfromISP2(nexthop172.16.100.22),withdefaultlocal
preferenceof100.(Rememberthattheapplieddefaultlocalpreferenceisnot
displayed.)
ThesecondrouteisreceivedfromR1(nexthop172.16.11.1),withchangedlocal
preferenceof50.
ISP1choosesthepaththatisreceivedfromISP2asthebestbasedonthehighestlocal-
preferencevalue.
MonitoringCommunities
Becauseacommunityisanattributethatcanappearmorethanonceinasingle
update,theshowipbgpcommanddoesnotshowit.
Communities are displayed in show ip bgp prefix printout.
Communities are not displayed in debugging outputs.
Routes in the BGP table that are tagged with a set of communities or routes that
match a community list can be displayed.
All routes in a BGP table that have at least one attached community are
displayed in show ip bgp community printout.
Ifthekeywordcommunityisincludedinshowipbgpcommand,allnetworksthat
haveatleastonecommunityattributearedisplayed.However,youcanview
communitiesonlyifyouusetheshowipbgpprefixcommand.
Ifyouusetheshowipbgpcommunity-listclistcommand,allnetworksthatthe
communitylistpermitsarelisted.Usingtheshowipbgpcommunityas:nn[as:nn
...]command,allroutesinaBGPtablehavingallthespecifiedcommunitiesthatare
attachedaredisplayed.Ifthekeywordexactisaddedattheend,onlythenetworks
thatmatchexactlyaredisplayed.
Step10
OntheISP1router,verifyallthenetworks,thathaveatleastonecommunityattribute.
ISP1# show ip bgp community
BGP table version is 21, local router ID is 10.0.1.81
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
* 10.0.0.0/28 172.16.11.1 50 0 1 i
* 10.0.0.16/28 172.16.11.1 50 0 1 i
* 10.0.0.32/28 172.16.11.1 50 0 1 i
* 10.0.0.48/28 172.16.11.1 50 0 1 i
* 10.0.0.64/28 172.16.11.1 50 0 1 i
* 10.0.2.0/28 172.16.11.1 50 0 1 200 i
* 10.0.2.16/28 172.16.11.1 50 0 1 200 i
* 10.0.2.32/28 172.16.11.1 50 0 1 200 i
* 10.0.2.48/28 172.16.11.1 50 0 1 200 i
* 10.0.2.64/28 172.16.11.1 50 0 1 200 i
* 10.0.2.80/28 172.16.11.1 50 0 1 200 i
* 10.0.2.96/28 172.16.11.1 50 0 1 200 i
* 10.0.2.112/28 172.16.11.1 50 0 1 200 i
* 10.0.2.128/28 172.16.11.1 50 0 1 200 i
Youcanseethatallnetworks,10.0.0.0/24and10.0.2.0/24,receivedfromAS1arelisted
intheoutput.TheyarelistedbecauseyouconfiguredroutetaggingwithBGP
communitiesonR1inAS1,forallupdatesthataresenttoAS100.
Step11
OntheISP1router,verifythecommunityattributeforoneofthe10.0.0.0/24
networks,the10.0.0.0/28network.
ISP1# show ip bgp 10.0.0.0/28
BGP routing table entry for 10.0.0.0/28, version 2
Paths: (2 available, best #1, table default)
Advertised to update-groups:
3
Refresh Epoch 2
200 1
172.16.100.22 from 172.16.100.22 (10.0.2.129)
Origin IGP, localpref 100, valid, external, best
Refresh Epoch 1
1
172.16.11.1 from 172.16.11.1 (172.16.11.1)
Origin IGP, localpref 50, valid, external
Community: 6553617
YoucanseethatISP1receivedtworoutesto10.0.0.0/28network:
FirstrouteisreceivedfromISP2,withdefaultlocalpreferenceof100.
SecondrouteisreceivedfromR1,withthecommunityattribute6553617
andconsequentlyithaschangedlocalpreferenceof50.
Notethatthecommunityisdisplayedasasingle32-bitvalue(6553617).Ifyou
wantittobeinaformofas:nn,youhavetoconfigurethesupportfornew-format
ontherouter,usingtheipbgp-communitynew-formatcommand.
Step12
OntheISP1router,enablenewBGPcommunityformat.
ISP1(config)# ip bgp-community new-format
Step13
OntheISP1router,againverifythecommunityattributefor10.0.0.0/28network.
ISP1# show ip bgp 10.0.0.0/28
BGP routing table entry for 10.0.0.0/28, version 2
Paths: (2 available, best #1, table default)
Advertised to update-groups:
3
Refresh Epoch 2
200 1
172.16.100.22 from 172.16.100.22 (10.0.2.129)
Origin IGP, localpref 100, valid, external, best
Refresh Epoch 1
1
172.16.11.1 from 172.16.11.1 (172.16.11.1)
Origin IGP, localpref 50, valid, external
Community: 100:17
Nowthecommunityisdisplayedastwocolon-separated16-bitnumbers,
100:17.
TheBGPNamedCommunityListsfeatureintroducesanewtypeofcommunitylist
thatiscalledthenamedcommunitylist.Anamedcommunitylistcanbeconfigured
withregularexpressionsorwithnumberedcommunitylists.
Allows you to assign meaningful names to community lists and increases the
number of community lists that can be configured.
Can be configured with regular expressions and with numbered community lists.
Increases the number of community lists that a network operator can configure
—there is no limitation on the number of named community lists that can be
configured.
Commands:
Use the ip community-list command to create a numbered or named
community list.
Use the match community command in route-map configuration mode, to
match a BGP community.
Use the set com-list delete command in route-map configuration mode to
remove communities from the community attribute of an update.
TheBGPNamedCommunityListsfeatureallowsyoutoassignmeaningfulnamesto
communitylists.Allrulesofnumberedcommunitiesapplytonamedcommunitylists,
withoneexception.Theexceptionisthatthereisnolimitationonthenumberof
communityattributesthatcanbeconfiguredforanamedcommunitylist.Both,
standardandexpanded,communitylistshavealimitationof100communitygroups
thatcanbeconfiguredwithineachtypeoflist.Anamedcommunitylistdoesnot
havethislimitation.
ipcommunity-listCommand
TocreateanumberedornamedcommunitylistforBGPandtocontrolaccesstoit,
usetheipcommunity-listcommandinglobalconfigurationmode.Todeletethe
communitylist,usethenoformofthiscommand.
ip community-list {standard-list-number | expanded-list-number [regular-
expression] | {standard | expanded} community-list-
name} {permit| deny} community-number | regular-expression
SyntaxDescription
Parameter Description
standard-list-number Specifiesastandardcommunitylistnumberfrom1to99that
identifiesoneormorepermitordenygroupsofcommunities.
expanded-list-number Specifiesanexpandedcommunitylistnumberfrom100to199
thatidentifiesoneormorepermitordenygroupsofcommunities.
standard Configuresastandardnamedcommunitylist.
expanded Configuresanexpandednamedcommunitylist.
community-list-name Thecommunitylistname.
permit Permitsaccessforamatchingcondition.
deny Deniesaccessforamatchingcondition.
community-number Communitynumberthatyouconfigurewiththesetcommunity
command.
Validvalueofcommunitynumberisoneofthefollowing:
Anumberfrom1to4294967200.Youcanspecifyasinglenumberormultiple
numbersandyouseparatethembyaspace.
internet—TheInternetcommunity.
no-export—Routeswiththiscommunityaresenttopeersinother
subautonomoussystemswithinaconfederation.Donotadvertisethisrouteto
anEBGPpeer.Externalsystemsarethesystemsoutsidetheconfederation.If
thereisnoconfederation,anexternalsystemisanyEBGPpeer.
local-as—Sendthisroutetopeersinothersubautonomoussystemswithinthe
localconfederation.Donotadvertisethisroutetoanexternalsystem.
BGPNamedCommunityLists
no-advertise—Donotadvertisethisroutetoanypeer(internalorexternal).
matchcommunityCommand
TomatchaBGPcommunity,usethematchcommunitycommandinroute-map
configurationmode.
Toremovethematchcommunitycommandfromtheconfigurationfileandrestore
thesystemtoitsdefaultconditionwherethesoftwareremovestheBGPcommunity
listentry,usethenoformofthiscommand.
match community standard-list-number | expanded-list-number | community-
list-name [exact]
SyntaxDescription
Parameter Description
standard-list-number Specifiesastandardcommunitylistnumberfrom1to99thatidentifies
oneormorepermitordenygroupsofcommunities.
expanded-list-number Specifiesanexpandedcommunitylistnumberfrom100to199that
identifiesoneormorepermitordenygroupsofcommunities.
community-list-name Thecommunitylistname.
exact (Optional)Indicatesthatanexactmatchisrequired.
Allofthecommunitiesandonlythosecommunitiesthatarespecified
mustbe
present.
setcomm-listdeleteCommand
Toremovecommunitiesfromthecommunityattributeofaninboundoroutbound
update,usethesetcomm-listdeletecommandinroute-mapconfigurationmode.
Tonegateaprevioussetcomm-listdeletecommand,usethenoformofthis
command.
set comm-list community-list-number | community-list-name delete
SyntaxDescription
Parameter Description
community-list-number Astandardorexpandedcommunitylistnumber.
community-list-name Astandardorexpandedcommunitylistname.
BGPNamedCommunityListExamples
Thefollowingexamplecreatesastandardcommunitylistthatpermitsallroutes
excepttherouteswiththecommunities5and10or10and15:
Router(config)# ip community-list 1 deny 5 10
Router(config)# ip community-list 1 deny 10 15
Router(config)# ip community-list 1 permit internet
Thefollowingexamplecreatesastandardcommunitylistthatpermitsallroutes
withinthelocalAS:
Router(config)# ip community-list 1 permit local-as
Thefollowingexamplecreatesastandardnamedcommunitylistwiththename
COMMUNITY_AthatpermitsallrouteswithinthelocalASanddeniesallrouteswith
theinternetcommunityattribute:
Router(config)# ip community-list standard COMMUNITY_A permit local-AS
Router(config)# ip community-list standard COMMUNITY_A deny internet
Thefollowingexamplecreatesanexpandednamedcommunitylistwiththename
COMMUNITY_BthatwillnotadvertiseroutestoEBGPpeers:
Router(config)# ip community-list expanded COMMUNITY_B permit no-export
Thefollowingexamplecreatesanamedcommunitylistwiththename
COMMUNITY_CthatwillnotadvertisethisroutetoanyEBGPorEBGPpeers:
Router(config)# ip community-list expanded COMMUNITY_C permit no-advertise
Thefollowingexampleusesaregularexpression.Theexamplecreatesafilterthat
willdenyallcommunitiesthatcontainanumber:
Router(config)# ip community-list 100 deny [0-9]*
Thecostcommunityisanontransitiveextendedcommunityattributethatispassed
toIBGPandconfederationpeersbutnottoEBGPpeers.
Allows you to customize the BGP best-path selection process for a local AS or
confederation.
Applied to internal routes by configuring the set extcommunity cost command
in a route map.
Influences the BGP best-path selection process at the POI.
Can be used as a "tie breaker" during the best-path selection process.
TheconfigurationoftheBGPCostCommunityfeatureallowsyoutocustomizethe
BGPbest-pathselectionprocessforalocalASorconfederationbyassigningcost
valuestospecificroutes.
Tocreateasetclausetoapplythecostcommunityattributetoroutesthatpass
througharoutemap,usethesetextcommunitycostcommandinroute-map
configurationmode.Todeletethecostcommunitysetclause,usethenoformofthis
command.
set extcommunity cost[igp] community-id cost-value
SyntaxDescription
Parameter Description
igp TheIGPPOI.
Theconfigurationofthiskeywordforcesthecostcommunitytobe
evaluatedaftertheIGPdistancetothenexthophasbeen
compared.
community-id TheIDfortheconfiguredextendedcommunity.
Therangeisfrom0to255.
cost-value Theconfiguredcostthatissetformatchingpathsintheroute
map.
Therangeisfrom0to4294967295.
ThecostcommunitysetclauseisconfiguredwithacostcommunityIDnumber(0to
255)andcostnumber(0to4294967295).Thecostnumbervaluedeterminesthe
preferenceforthepath.Thepathwiththelowestcostcommunitynumberis
preferred.Pathsthatarenotspecificallyconfiguredwiththecostcommunityattribute
areassignedadefaultcostnumbervalueof2147483647(themidpointbetween0
and4294967295)andevaluatedbythebest-pathselectionprocessaccordingly.
Whentwopathshavebeenconfiguredwiththesamecostnumbervalue,thepath
selectionprocessprefersthepathwiththelowestcostcommunityID.Thecost
extendedcommunityattributeispropagatedtoIBGPpeerswhenextended
communityexchangeisenabledwiththeneighborsend-communitycommand.
ThecostcommunityattributeinfluencestheBGPbest-pathselectionprocessatthe
POI.Bydefault,thePOIfollowstheIGPmetriccomparison.WhenBGPreceives
multiplepathstothesamedestination,itusesthebest-pathselectionprocessto
determinewhichpathisthebestpath.BGPautomaticallymakesthedecisionand
installsthebestpathintotheroutingtable.ThePOIallowsyoutoassigna
preferencetoaspecificpathwhenmultipleequal-costpathsareavailable.IfthePOI
isnotvalidforlocalbest-pathselection,thecostcommunityattributeissilently
ignored.
MultiplepathscanbeconfiguredwiththecostcommunityattributeforthesamePOI.
ThepathwiththelowestcostcommunityIDisconsideredfirst.Inotherwords,allof
thecostcommunitypathsforaspecificPOIareconsidered,startingwiththeone
withthelowestcostcommunityID.Pathsthatdonotcontainthecostcommunity(for
thePOIandcommunityIDbeingevaluated)areassignedthedefaultcommunity
costvalue(2147483647).Ifthecostcommunityvaluesareequal,thencost
communitycomparisonproceedstothenext-lowestcommunityIDforthisPOI.
ApplyingthecostcommunityattributeatthePOIallowsyoutoassignavaluetoa
pathoriginatedorlearnedbyapeerinanypartofthelocalASorconfederation.The
costcommunitycanbeusedasa"tiebreaker"duringthebestpathselection
process.Multipleinstancesofthecostcommunitycanbeconfiguredforseparate
equal-costpathswithinthesameASorconfederation.Forexample,alower-cost
communityvaluecanbeappliedtoaspecificexitpathinanetworkwithmultiple
BGPCostCommunity
equal-costexitpoints.Then,theBGPbest-pathselectionprocesswillpreferthe
specificexitpath.
BGPCostCommunityExample
Thefollowingexampleconfigurationshowstheconfigurationoftheset
extcommunitycostcommand.ThisexampleappliesthecostcommunityIDof1
andcostcommunityvalueof100totheroutesthattheroutemappermits.This
configurationwillcausethebest-pathselectionprocesstopreferthisrouteover
otherequal-costpathsthatthisroutemapsequencedoesnotpermit.
Router(config)# router bgp 50000
Router(config-router)# neighbor 10.0.0.1 remote-as 50000
Router(config-router)# neighbor 10.0.0.1 update-source Loopback 0
Router(config-router)# address-family ipv4
Router(config-router-af)# neighbor 10.0.0.1 activate
Router(config-router-af)# neighbor 10.0.0.1 route-map COST1 in
Router(config-router-af)# neighbor 10.0.0.1 send-community both
Router(config-router-af)# exit
Router(config)# route-map COST1 permit 10
Router(config-route-map)# match ip-address 1
Router(config-route-map)# set extcommunity cost 1 100
TheBGPLinkBandwidthfeatureisusedtoenablemultipathloadbalancingfor
externallinkswithunequalbandwidthcapacity.ThisfeaturesupportsIBGP,EBGP
multipathloadbalancing,andEIGRPmultipathloadbalancinginMPLSVPNs.When
thisfeatureisenabled,routeslearnedfromdirectlyconnectedexternalneighbors
arepropagatedthroughtheIBGPnetworkwiththebandwidthofthesourceexternal
link.
Use it to enable multipath load balancing for external links with unequal
bandwidth capacity.
You enable it under an IPv4 or VPNv4 address family sessions by entering the
bgp dmzlink-bw command.
Routes that are learned from directly connected external neighbor are
propagated through the IBGP network with the bandwidth of the source external
link.
YouenabletheBGPLinkBandwidthfeatureunderanIPv4orVPNv4address-family
sessionbyenteringthebgpdmzlink-bwcommand.
TheBGPLinkBandwidthfeatureallowsBGPtobeconfiguredtosendtrafficover
multipleIBGPorEBGPlearnedpaths.Thetrafficthatissentonthosepathsis
proportionaltothebandwidthofthelinksthatareusedtoexittheAS.The
configurationofthisfeaturecanbeusedwithEBGPandIBGPmultipathfeaturesto
enableunequal-costloadbalancingovermultiplelinks.Unequal-costloadbalancing
overlinkswithunequalbandwidthwasnotpossibleinBGPbeforetheBGPLink
Bandwidthfeaturewasintroduced.
ThelinkbandwidthextendedcommunityattributeindicatesthepreferenceofanAS
exitlinkintermsofbandwidth.Thisextendedcommunityisappliedtoexternallinks
betweendirectlyconnectedEBGPpeersbyenteringtheneighbordmzlink-bw
command.ThelinkbandwidthextendedcommunityattributeispropagatedtoIBGP
peerswhenextendedcommunityexchangeisenabledwiththeneighborsend-
communitycommand.
bgpdmzlink-bwCommand
ToconfigureBGPtodistributetrafficproportionallyoverexternallinkswithunequal
bandwidthwhenmultipathloadbalancingisenabled,usethebgpdmzlink-bw
commandinaddressfamilyconfigurationmode.Todisabletrafficdistribution
proportionaltothelinkbandwidth,usethenoformofthiscommand.
bgp dmzlink-bw
Thiscommandhasnokeywordsorarguments.
neighbordmzlink-bwCommand
ToconfigureBGPtoadvertisethebandwidthoflinksthatareusedtoexitanAS,use
theneighbordmzlink-bwcommandinaddressfamilyconfigurationmode.To
disablelinkbandwidthadvertisement,usethenoformofthiscommand.
neighbor ip-address dmzlink-bw
SyntaxDescription
Parameter Description
ip-address TheIPaddressthatidentifiestheexternalinterface
Thelinkbandwidthextendedcommunityattributeisa4-bytevaluethatisconfigured
foralinkontheDMZinterfacethatconnectstwosingle-hopEBGPpeers.Thelink
bandwidthextendedcommunityattributeisusedasatraffic-sharingvaluerelativeto
otherpathswhileforwardingtraffic.Twopathsaredesignatedasequalforload
balancingiftheweight,localpreference,ASpathlength,MED,andIGPcostsare
thesame.
BGPLinkBandwidthExample
Inthisexample,theBGPLinkBandwidthfeatureisconfiguredsothatBGPwill
BGPLinkBandwidthFeature
distributetrafficproportionallytothebandwidthofeachexternallink.
Thefigureshowstwoexternalautonomoussystems.Threelinksconnectthetwo
ASs,andeachAScarriesadifferentamountofbandwidth(unequal-costlinks).
Multipathloadbalancingisenabled,andtrafficisbalancedproportionally.
FirstR1isconfiguredtosupportIBGPmultipathloadbalancingandtoexchangethe
BGPextendedcommunityattributewithIBGPneighbors,R2on10.0.1.2IPaddress
andR3on10.0.1.3IPaddress.
R1(config)# router bgp 100
R1(config-router)# neighbor 10.0.1.2 remote-as 100
R1(config-router)# neighbor 10.0.1.2 update-source Loopback 0
R1(config-router)# neighbor 10.0.1.3 remote-as 100
R1(config-router)# neighbor 10.0.1.3 update-source Loopback 0
R1(config-router)# address-family ipv4
R1(config-router)# bgp dmzlink-bw
R1(config-router-af)# neighbor 10.0.1.2 activate
R1(config-router-af)# neighbor 10.0.1.2 send-community both
R1(config-router-af)# neighbor 10.0.1.3 activate
R1(config-router-af)# neighbor 10.0.1.3 send-community both
R1(config-router-af)# maximum-paths ibgp 6
R2isconfiguredtosupportmultipathloadbalancing,todistributeR4(172.16.100.1
IPaddress)andR5(172.16.200.2IPaddress)linktrafficproportionallytothe
bandwidthofeachlink,andtoadvertisethebandwidthoftheselinkstoIBGP
neighborsasanextendedcommunity:
R2(config)# router bgp 100
R2(config-router)# neighbor 10.0.1.1 remote-as 100
R2(config-router)# neighbor 10.0.1.1 update-source Loopback 0
R2(config-router)# neighbor 10.0.1.3 remote-as 100
R2(config-router)# neighbor 10.0.1.3 update-source Loopback 0
R2(config-router)# neighbor 172.16.100.1 remote-as 200
R2(config-router)# neighbor 172.16.100.1 ebgp-multihop 1
R2(config-router)# neighbor 172.16.200.2 remote-as 200
R2(config-router)# neighbor 172.16.200.2 ebgp-multihop 1
R2(config-router)# address-family ipv4
R2(config-router-af)# bgp dmzlink-bw
R2(config-router-af)# neighbor 10.0.1.1 activate
R2(config-router-af)# neighbor 10.0.1.1 next-hop-self
R2(config-router-af)# neighbor 10.0.1.1 send-community both
R2(config-router-af)# neighbor 10.0.1.3 activate
R2(config-router-af)# neighbor 10.0.1.3 next-hop-self
R2(config-router-af)# neighbor 10.0.1.3 send-community both
R2(config-router-af)# neighbor 172.16.100.1 activate
R2(config-router-af)# neighbor 172.16.100.1 dmzlink-bw
R2(config-router-af)# neighbor 172.16.200.2 activate
R2(config-router-af)# neighbor 172.16.200.2 dmzlink-bw
R2(config-router-af)# maximum-paths ibgp 6
R2(config-router-af)# maximum-paths 6
AlsoR3isconfiguredtosupportmultipathloadbalancingandtoadvertisethe
bandwidthofthelinkwithR5(172.16.300.3IPaddress)toIBGPneighborsasan
extendedcommunity:
R3(config)# router bgp 100
R3(config-router)# neighbor 10.0.1.1 remote-as 100
R3(config-router)# neighbor 10.0.1.1 update-source Loopback 0
R3(config-router)# neighbor 10.0.1.2 remote-as 100
R3(config-router)# neighbor 10.0.1.2 update-source Loopback 0
R3(config-router)# neighbor 172.16.300.3 remote-as 200
R3(config-router)# neighbor 172.16.300.3 ebgp-multihop 1
R3(config-router)# address-family ipv4
R3(config-router-af)# bgp dmzlink-bw
R3(config-router-af)# neighbor 10.0.1.1 activate
R3(config-router-af)# neighbor 10.0.1.1 send-community both
R3(config-router-af)# neighbor 10.0.1.1 next-hop-self
R3(config-router-af)# neighbor 10.0.1.2 activate
R3(config-router-af)# neighbor 10.0.1.2 send-community both
R3(config-router-af)# neighbor 10.0.1.2 next-hop-self
R3(config-router-af)# neighbor 172.16.300.3 activate
R3(config-router-af)# neighbor 172.16.300.3 dmzlink-bw
R3(config-router-af)# maximum-paths ibgp 6
R3(config-router-af)# maximum-paths 6
TheBGPSequencedEntriesinExtendedCommunityListsfeatureallowsyou
automaticsequencingofindividualentriesinBGPextendedcommunitylists.Italso
providesyoutheabilitytoremoveorresequenceextendedcommunitylistentries
withoutdeletingtheentireexistingextendedcommunitylist.
Allows you automatic sequencing of individual entries in BGP extended
community lists.
Provides you the ability to remove or resequence extended community list
entries without deleting the entire existing extended community list.
Configures sequence numbers for extended community list entries.
Resequences existing sequence numbers for extended community list entries.
Configures an extended community list to use default values.
BothnamedandnumberedextendedcommunitylistscanbeconfiguredinIP
extendedcommunitylistconfigurationmode.ToenterIPextendedcommunitylist
configurationmode,issuetheipextcommunity-listcommandwitheitherthe
expandedorstandardkeywordfollowedbytheextendedcommunitylistname.This
configurationmodesupportsallofthefunctionsthatareavailableinglobal
configurationmode.Inaddition,youcanperformthefollowingoperations:
Configuresequencenumbersforextendedcommunitylistentries.
Resequenceexistingsequencenumbersforextendedcommunitylistentries.
Configureanextendedcommunitylisttousedefaultvalues.
ipextcommunity-listCommand
Tocreateanextendedcommunitylistandcontrolaccesstoit,usetheip
extcommunity-listcommandinglobalconfigurationmode.Todeletetheentire
communitylist,usethenoformofthiscommand.
ip extcommunity-list expanded-list-number | expanded list-
name {permit| deny} [regular-expression] | standard-list-
number | standard list-name {permit| deny} [rt extcom-value] [soo extcom-
value]
SyntaxDescription
Parameter Description
expanded-list-number Anexpandedlistnumberfrom100to500thatidentifiesoneor
morepermitordenygroupsofextendedcommunities.
standard-list-number Astandardlistnumberfrom1to99thatidentifiesoneormore
permitordenygroupsofextendedcommunities.
expandedlist-name Createsanexpandednamedextendedcommunitylistandenters
IPextendedcommunitylistconfigurationmode.
standardlist-name CreatesastandardnamedextendedcommunitylistandentersIP
extendedcommunitylistconfigurationmode.
permit Permitsaccessforamatchingcondition.
deny Deniesaccessforamatchingcondition.
regular-expression (Optional)Aninputstringpatterntomatchagainst.
rt (Optional)SpecifiestheRTextendedcommunityattribute.
Thertkeywordcanbeconfiguredonlywithstandardextended
communitylistsandnotexpandedcommunitylists.
soo (Optional)SpecifiestheSOOextendedcommunityattribute.
Thesookeywordcanbeconfiguredonlywithstandardextended
communitylistsandnotexpandedcommunitylists.
extcom-value SpecifiestheRTorSOOextendedcommunityvalue.
Thevaluecanbeoneofthefollowingcombinations:autonomous-
system-number:network-numberorip-address:network-number
sequence-number (Optional)Thesequencenumberofanamedornumbered
extendedcommunitylist.
Thisvaluecanbeanumberfrom1to2147483647.
BGPSupportforSequencedEntriesinExtended
CommunityLists
default (Optional)Setsakeywordorargumenttodefaultbehavioror
value.
exit (Optional)ExitsIPextendedcommunitylistconfigurationmode.
resequence (Optional)Changesthesequencesofextendedcommunitylist
entriestothedefaultsequencenumberingortothespecified
sequencenumbering.
Extendedcommunityentriesaresequencedby10-number
incrementsbydefault.
starting-sequence (Optional)Specifiesthenumberforthefirstentryinanextended
communitylist.
sequence-increment (Optional)Specifiestheincrementrangeforeachsubsequent
extendedcommunitylistentry.
Parameter Description
SequencedandResequencedExtendedCommunityListEntry
Example
Thefollowingexamplecreatesandconfiguresanamedextendedcommunitylistthat
willpermitroutesonlyfromRT64512:10,65000:20,64535:30andSOO65535:40.
Allotherroutesareimplicitlydenied.
Router(config)# ip extcommunity-list standard NAMED_LIST
Router(config-extcom-list)# 1 permit rt 64512:10
Router(config-extcom-list)# 2 permit rt 65000:20
Router(config-extcom-list)# 3 permit rt 64535:30
Router(config-extcom-list)# 4 permit soo 65535:40
Router(config-extcom-list)# end
Resequencetheextendedcommunitylistentriesinthenamedcommunitylistthatis
configuredabove.Thefirstentryisresequencedtothenumber50andtherangefor
eachsubsequententrytofollowby100(forexample,150,250,350,andsoon).
Router(config)# ip extcommunity-list standard NAMED_LIST
Router(config-extcom-list)# resequence 50 100
Router(config-extcom-list)# end
Todisplayroutesthatthenamedextendedcommunitylistpermits,usetheshowip
extcommunity-listEXECcommand.Theoutputshowstheconfigurationfromthe
firstexampleafterithasbeenresequencedwithuser-definedvalues.
Router> show ip extcommunity-list
Standard extended community-list NAMED_LIST
50 permit RT:64512:10
150 permit RT:65000:20
250 permit RT:64535:30
350 permit SoO:65535:40
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
You can use the BGP community attribute to create an AS-wide routing policy or
to provide services to neighboring autonomous systems.
A community is an attribute that is used to tag BGP routes that you can use to
manipulate path selection and enforce administrative policies.
To configure community attribute, you should:
Configure a route map with a set community command.
Configure propagation of BGP communities on the routers on a per-neighbor
basis
Configure community lists to match against the community attribute as a
method of route selection
Configure a route map to match networks that carry communities that the
community list permits and apply policies to them.
The BGP Named Community Lists feature allows you to assign meaningful
names to community lists and increases the number of community lists that can
be configured.
The configuration of the BGP Cost Community feature allows you to assign
cost values to specific routes. This way, you customize the BGP best path
selection process for a local AS or confederation.
The BGP Link Bandwidth feature allows you multipath load balancing for
external links with unequal bandwidth capacity.
BGP Support for Sequenced Entries in Extended Community Lists allows
automatic sequencing of individual entries in BGP extended community lists. It
also enables you to remove or resequence extended community list entries
without deleting the entire existing extended community list.
Summary
Overview
Thistopicsummarizesthekeypointsthatwerediscussedinthismodule.
The following BGP attributes influence the route selection:
Weight (The first criterion in BGP route selection.)
You can configure a default weight or use route-maps to configure it.
Local preference (The second-strongest criterion in the route selection
process.)
You can configure a default local preference or use route-maps to configure
it.
AS-path
You can perform AS-path prepending on outgoing EBGP updates over the
nondesired return path.
MED (A "weak" parameter in the route selection process.)
Community
Used to tag BGP routes that you can use to manipulate path selection and
enforce administrative policies.
ModuleSummary
Usethequestionsheretoreviewwhatyoulearnedinthismodule.Thecorrect
answersandsolutionsarefoundintheModuleSelf-CheckAnswerKey.
ModuleSelf-Check
1. Numberthe10BGPselectioncriteriaintheorderinwhichtheyare
used,fromthefirsttothelast,whenselectingtheBGPpaththatis
submittedtotheIProutingtable.(Source:"InfluencingBGPRoute
SelectionwithWeights")
PrefertheshortestASpath. 5
PreferthepaththroughtheclosestIGP
neighbor. 4
Prefertheroutethatwasoriginatedbythe
localrouter. 6
PreferanEBGPpathoveranIBGPpath. 3
PreferthepathwiththelowestneighborBGP
routerID. 2
Preferthehighestweight. 8
PrefertheoldestrouteforEBGPpaths. 1
Preferthelowestorigincode(IGP<EGP<
incomplete). 7
PreferthelowestMED. 10
Preferthehighestlocalpreference. 9
1. WhatisthedefaultweightforroutesthatarereceivedfromaBGP
neighbor?(Source:"InfluencingBGPRouteSelectionwithWeights")
0
100
32768
ItdependsontheCiscoIOSrelease.
1. Whenaretheweightsthatareconfiguredonaneighborenforced?
(Source:"InfluencingBGPRouteSelectionwithWeights")
Beforethenewweightscantakeeffect,theBGPprocessontherouter
mustberemovedandreconfigured.
Theroutermustfirstberebootedforthenewweightstotakeeffect.
ThenewweightswillbeappliedaftertheBGPupdateintervalof30
minutesexpires.
Thenewweightconfigurationisappliedtoallroutesthatarereceived
followingtheconfigurationchange.
1. Ifyouwantallroutesreceivedfromtheneighbortohavethenew
weightvalueapplied,youhavetoreestablishBGPsession.Trueor
false?(Source:"InfluencingBGPRouteSelectionwithWeights")
true
false
1. Howcouldyouimplementaprimary/backupISProutingpolicyby
usingweights?(Source:"InfluencingBGPRouteSelectionwith
Weights")
AssignhigherweightstoallroutesthatarereceivedfromthebackupISP.
AssignlowerweightstoallroutesthatarereceivedfromthebackupISP.
AssignhigherweightstoallroutesthatarereceivedfromtheprimaryISP.
AssignlowerweightstoallroutesthatarereceivedfromtheprimaryISP.
1. Whenyouareusingroute-mapstomodifyweights,whathappensby
defaulttoaroutethatdoesnotmatchanyoftheroute-map
statements?(Source:"InfluencingBGPRouteSelectionwith
Weights")
Therouteisacceptedwiththeweightattributeunmodified.
Therouteisdiscarded.
TherouteisinsertedintotheBGPtablebutnotintotheIProutingtable.
Anerrorisdisplayedontherouterconsoleandinrouterdebugs.
1. Whichmethodofinfluencingrouteselectionwithweightsisthelast
tobeappliedonanincominginterface?(Source:"InfluencingBGP
RouteSelectionwithWeights")
prefix-list
route-map
filter-listweight
defaultweight
1. Youcansettheweightusingaroutemapappliedtoaneighborin
theoutgoingdirection.Trueorfalse?(Source:"InfluencingBGP
RouteSelectionwithWeights")
true
false
1. WhichtwoofthefollowingstatementsarecorrectregardingBGP
routeselection?(Choosetwo.)(Source:"InfluencingBGPRoute
SelectionwithWeights")
Iftworouteshavethesameweightattribute,theroutewiththelowestlocal
preferenceischosen.
Theroutewiththehighestweightisalwayschosenfirst.
TheweightattributeisglobalwithinanAS.
Theweightattributeisonlylocaltothelocalrouter.
Theweightvalueispropagatedbyallrouters.
1. Whatisakeydifferencebetweenthelocalpreferenceandweight
attributes?(Source:"SettingBGPLocalPreference")
Localpreferenceislocaltotherouteonwhichitisconfigured.
LocalpreferenceislocaltotheASwithinwhichithasbeenconfigured.
LocalpreferenceislocaltotheBGPadministrativedomain.
LocalpreferenceisglobaltoaBGPdomain.
1. Whatisthedefaultlocalpreferenceforroutesthatarereceivedfrom
aBGPneighbor?(Source:"SettingBGPLocalPreference")
0
100
Thereisnodefaultlocalpreferencevalue.
ItdependsontheCiscoIOSrelease.
1. Whichtwostatementsregardinglocalpreferencearetrue?(Choose
two.)(Source:"SettingBGPLocalPreference")
Thehighervalueforlocalpreferenceispreferred.
LocalpreferenceisusedonlybetweenEBGPneighbors
Thelowervalueforlocalpreferenceispreferred.
LocalpreferenceisusedonlybetweenIBGPneighbors
1. Whichtwoofthefollowingstatementsabouttheinfluenceoflocal
preferenceonBGProuteselectionisaccurate?(Choosetwo.)
(Source:"SettingBGPLocalPreference")
Whenyousetlocalpreference,youcanviewitonneighboringrouters,but
youmustresetit.
YoucanuselocalpreferencetoensureAS-widerouteselectionpolicy.
Localpreferenceisusedtoselectrouteswithunequalweights.
Localpreferenceisthesecond-strongestcriterionintherouteselection
process.
1. Letsaywehavetworoutestothesamedestination,fistonehasa
weightof50andalocalpreferenceof150,whiletheotheronehasa
weightof150andalocalpreferenceof50.Thefistroutewillbe
selectedasthebestone,sinceithashigherlocalpreferencevalue.
Trueorfalse?(Source:"SettingBGPLocalPreference")
true
false
1. WhichCiscoIOScommandisusedtochangethedefaultvalueof
localpreference?(Source:"SettingBGPLocalPreference")
setlocal-preference
bgpdefaultlocal-preference
showipbgp
showipbgpprefix
1. WhichCiscoIOScommandisusedtoconfigureBGPlocal
preferencewithroute-mapstatements?(Source:"SettingBGPLocal
Preference")
setlocal-preference
bgpdefaultlocal-preference
showipbgp
showipbgpprefix
1. Locallyapplieddefaultvaluecanbedisplayedwithshowipbgp
command.Trueorfalse?(Source:"SettingBGPLocalPreference")
true
false
1. WhichCiscoIOScommandisnecessarytodisplaythelocally
appliedBGPvalue?(Source:"SettingBGPLocalPreference")
showbgppreferencedetail
showipbgp
showipbgpdetail
showipbgpprefix
1. WhichtwostatementsregardingtheASpatharetrue?(Choose
two.)(Source:"UsingASPathPrepending")
TheshorterASpathispreferred.
ThelongerASpathispreferred.
TheASpathisprependedandexchangedbetweenautonomoussystems.
TheASpathislocaltoanAS.
1. WhatisASpathprepending?(Source:"UsingASPathPrepending")
Whenarouter,sendingaBGPupdate,addstheASnumberoftherouter
fromwhichitreceivedtheroutetotheASpathattribute.
Whenarouter,sendingaBGPupdate,addstheASnumberoftherouter
towhichitissendingtheroutetotheASpathattribute.
Whenarouter,sendingaBGPupdate,addsitsASnumbertotheASpath
attributemultipletimes.
WhenarouterusestheASpathattributeinrouteselection.
1. TheASpathwillbetherouteselectioncriterionthatisusedwhen
whichofthefollowingistrue?(Source:"UsingASPathPrepending")
ItisthefirstcriterionthatisusedinBGProuteselection.
Itisusedwhenthereisnodifferenceinweight,localpreference,orroute
origination.
ItisusedwhentheMEDisidenticalonthecandidateroutes.
Theweight,localpreference,MED,andoriginattributesmustbeidentical
beforetheASpathattributeisusedforrouteselection.
1. WhichcommanddoyouusetomanipulatetheASpathattribute?
(Source:"UsingASPathPrepending")
Theglobalconfigurationcommandsetas-pathprependas-number.
Therouterconfigurationcommandsetas-pathprependas-number.
Thesetas-pathprependas-numbercommandinaroute-map.
Theinterfaceglobalcommandsetas-pathprependas-number.
1. TheconfigurationbelowisfromarouterinAS347,whichis
advertisingnetwork11.0.0.0/8toanEBGPneighbor2.0.0.2inAS
529.WhatarethecontentsoftheASpathattributeforroute
11.0.0.0/8onarouterthatisresidinginAS529?(Source:"UsingAS
PathPrepending")route-mapaddASpermit10
setas-pathprepend347347347
routerbgp347
neighbor2.0.0.2remote-as529
neighbor2.0.0.2route-mapaddASout
347347347
347347347347
529347347347
529347347347347
1. WhydoyouneedtouseASpathprepending?(Source:"UsingAS
PathPrepending")
ASpathprependingallowsacustomertopotentiallyinfluencereturnpath
routeselection.
ASpathprependingisusedonacustomerroutertocontroloutgoingroute
updates.
ServiceprovidersuseASpathprependingtocontrolincomingupdates
fromacustomerAS.
ASpathprependingisusedbetweenserviceprovidersthatareconnected
tothesamecustomerAStodeterminewhichwillbetheprimarylinktothe
customer.
1. WhichregularexpressionwillallowanyASpathnumber,butwitha
lengthofone?(Source:"UsingASPathPrepending")
^[0-9]+$
^[0-9](_[0-9])*$
^_[0-9]$
^[1-8]+$
1. HowdoesASpathprependingaffectarouter?(Source:"UsingAS
PathPrepending")
ASpathprependingissimplyatermthatisusedtodescribewhenarouter
usestheASpathattributeinrouteselectionandhencedoesnotaffect
routerresources.
ThelongertheASpathattributeattachedtoBGPupdates,themorerouter
memoryrequirementsincrease.
ASpathprependingdoesnotimpacttherouterbecauseCiscoIOS
softwarerecognizesthatASpathprependingisinuseandstoresasingle
ASnumberwithapointertothenumberofASpathprepends.
ASpathprependingcausestheroutertooperateinprocess-switching
modebecausetheBGPupdatemustbestored,manipulated,andthen
rewrittentoaccommodatethenewASpathattribute.
1. Whichtwoofthefollowingarecharacteristicsofthefunctionofthe
BGPHideLocal-AutonomousSystemfeature?(Choosetwo.)
(Source:"UsingASPathPrepending")
AllowsyoutotransparentlychangetheASnumberfortheentireBGP
network.
EnsuresthatroutescanbepropagatedthroughouttheAS.
AllowscustomizationoftheASnumberforEBGPpeergroupingsthrough
thesetas-pathcommand.
ChangestheASnumberforallIBGPpeersatthesametime.
1. WhatisthetypicalapplicationoftheMEDattribute?(Source:
"UnderstandingBGPMulti-ExitDiscriminators")
ToinfluencepathselectionoutofanoriginatingAS.
Toprovideastrongmetrictoselectthebestpathwhenmultipleroutes
exist.
TohaveaBGPattributetraversingmanyautonomoussystemswhile
influencingpathselection.
ToinfluencethereturnpathoftrafficbackintoanAS.
1. WhatisthedefaultMEDvalue?(Source:"UnderstandingBGPMulti-
ExitDiscriminators")
0
100
ThereisnodefaultMEDvalue.
ItdependsontheCiscoIOSrelease.
1. WhatarethreeBGPattributesthatarecomparedbeforetheMED?
(Choosethree.)(Source:"UnderstandingBGPMulti-Exit
Discriminators")
largestweight
originatedroutes
ASpathlength
lowestIPaddress
1. WhichtwostatementsabouttheCiscoIOScommandthatis
requiredtoconfigurechangestothedefaultBGPMEDonaCisco
IOSrouterareaccurate?(Choosetwo.)(Source:"Understanding
BGPMulti-ExitDiscriminators")
TheMEDisamandatoryattribute.
Usingthedefault-metriccommandinBGPconfigurationmodewillcause
oneredistributednetworktohavethespecifiedMEDvalue.
ThereisnoMEDattributethatisattachedtoaroutebydefault.
Tosetthedefaultmetricvalue(MED)forBGProutes,usethedefault-
metriccommand.
1. WhichtwostatementsabouttheCiscoIOScommandsthatare
requiredtoconfigurechangestotheBGPMEDattributewithroute-
mapstatementsareaccurate?(Choosetwo.)(Source:
"UnderstandingBGPMulti-ExitDiscriminators")
Thesetmetriccommandisusedwithinroute-mapconfigurationmodeto
settheMEDattribute.
Theneighboraddressroute-mapnamein|outcommandappliesa
route-maptoincomingupdatesfromallneighbors.
Per-neighborMEDisconfiguredbyusingaroute-mapwithamatch
condition.
Youcanusearoute-maptosettheMEDonincomingoroutgoing
updates.
1. WhichtwoofthefollowingstatementsabouttheCiscoIOS
commandsthatarerequiredtoconfigureadvancedMEDfeatureson
Ciscoroutersareaccurate?(Choosetwo.)(Source:"Understanding
BGPMulti-ExitDiscriminators")
Thebgpbestpathmedconfedcommandallowsrouterstocompare
pathslearnedfromconfederationpeers.
WhenyouenableadeterministicMEDcomparison,youallowarouterto
compareMEDvaluesafteritconsidersBGProutetype(externalor
internal)andIGPmetrictothenext-hopaddress.
Usethebgpalways-compare-medcommandtoforcetherouterto
comparetheMEDevenifthepathscomefromdifferentautonomous
systems.
CiscoIOSsoftware,ontheotherhand,regardsamissingMEDattribute
ashavingavalueof1.
1. Ifyouconfigureinboundsoftreconfigurationwitharoute-mapand
issuetheshowipbgpprefixcommand,whichvalueoftheMED
attributeisdisplayed?(Source:"UnderstandingBGPMulti-Exit
Discriminators")
Onlytheoriginalroute(noMED)isdisplayed.
Boththeoriginalrouteandthemodifiedroutearedisplayed.
Onlythemodifiedrouteisdisplayed.
TheMEDattributeisnotdisplayedwiththeshowipbgpprefixcommand.
1. WhichofthefollowingstatementsabouttheCiscoIOScommands
thatarerequiredtotroubleshootBGPMEDconfigurationsona
Ciscorouterisaccurate?(Source:"UnderstandingBGPMulti-Exit
Discriminators")
ToseetheoriginalMED,youneedtoenablehardreconfigurationonthe
router.
Thecommandshowipbgpneighboraddressreceived-routesdisplays
theoriginalupdatesbeforeanyfiltersorroute-mapshavefilteredor
changedthem.
Ifhardreconfigurationisenabled,theoriginalupdatestotheMEDattribute
areavailablebyusingtheshowipbgpprefixcommand.
IssuingtheshowiproutecommandwilldisplaytheMEDvalue.
1. WhichtwostatementsregardingtheMEDaretrue?(Choosetwo.)
(Source:"UnderstandingBGPMulti-ExitDiscriminators")
ThehighervaluefortheMEDispreferred.
ThelowervaluefortheMEDispreferred.
TheMEDisexchangedbetweenautonomoussystems.
TheMEDislocaltoanAS.
1. WhataretworeasonswhyitisnotfeasibletousetheMEDto
influencereturnpathselectionwhenmultipleautonomoussystems
areinvolved?(Choosetwo.)(Source:"AddressingBGP
Communities")
TheMEDattributeisdesignedtoinfluenceoutboundpathselectiononly.
TheASpathattributewouldbeusedforpathselectionregardlessofany
configuredMEDvalue.
Theweightattributewillalwaysbeused,giventhatitisfirstintheBGP
routeselectionprocess.
TheMEDcannotbepropagatedacrossseveralautonomoussystems.
1. DoesthecommunityattributehaveanyinfluenceonBGPpath
selection?(Source:"AddressingBGPCommunities")
No,communitiesaresimplytagsthatareappliedtoBGProutes.
No,communitiesarenontransitiveattributes.
Yes,BGPpathsareselectedbasedonthevalueinthecommunitytag.
Yes,thecommunityattributeispartoftheBGProuteselectionprocess.
1. MatchthestepswiththeactionsdescribinghowBGPcommunities
canfacilitateproperreturnpathselection.(Source:"AddressingBGP
Communities")
Defineadministrativepolicygoalsthatyou
needtoimplement. Step3
Enablecommunitydistributionthroughout
yourAStoallowcommunitypropagation. Step5
Definethefiltersandrouteselectionpolicy
thatwillachievetherequiredgoals. Step2
Applycommunitiesonincomingupdates
fromneighboringautonomoussystemsortell
theneighborstosetthecommunities
themselves.
Step1
Matchcommunitieswithroute-mapsand
routefilters,changeBGPattributes,or
influencetherouteselectionprocessbased
onthecommunitiesthatareattachedtothe
BGProutes.
Step4
Assignacommunityvaluetoeachgoal. Step6
1. HowmanycommunitytagscanbeattachedtoasingleBGProute?
(Source:"AddressingBGPCommunities")
1
32
256
Itdependsonthenumberthatisconfiguredwiththeipbgpcommunity
command.
1. Whichthreeofthefollowingareactivitiesthatarerequiredto
successfullydeployBGPcommunitiesinaBGP-basednetwork?
(Choosethree.)(Source:"AddressingBGPCommunities")
Settingcommunities,whichrequiresaroute-map.
Creatingcommunity-liststobeusedwithinroute-mapstomatchon
communityvalues.
Enablingcommunitypropagationperneighborfordesignatedinternal
neighbors.
Creatingroute-mapswherecommunity-listsareusedtomatchon
communityvalues.
1. WhichtwoofthefollowingstatementsabouttheCiscoIOS
commandsthatarerequiredtoconfigureroutetaggingwithBGP
communitiesareaccurate?(Choosetwo.)(Source:"Addressing
BGPCommunities")
Youcanattachupto35communitiestoasingleroutewithoneroute-map
setstatement.
Omittingtheadditivekeywordfromthesetcommunitycommandresults
inoverwritinganyoriginalcommunityattributes.
Theglobalcommandipbgp-communitynew-formatisrecommendedon
allrouterswhenevercommunitiescontaintheASnumber.
Youcannotusearoute-mapwithredistributionfromanotherrouting
protocol.
1. WhichofthefollowingstatementsabouttheCiscoIOScommand
thatisrequiredtoenableBGPcommunitypropagationtoBGP
neighborsisaccurate?(Source:"AddressingBGPCommunities")
CommunitypropagationtoBGPneighborsisautomaticallyconfigured.
ItisnecessarytomanuallystripcommunitiesinoutgoingBGPupdates.
Theneighborip-addresssend-communitycommandcannotbeapplied
toapeergroup.
Theneighborip-addresssend-communitycommandisneededto
propagatecommunityattributestoBGPneighbors.
1. WhichmatchcriteriaarespecifiedinastandardBGPcommunity-
list?(Source:"AddressingBGPCommunities")
destinationIPaddresses
regularexpressions
communityattributevalues
ASnumbers
1. Whatistheresultoftaggingaroutewiththeno-exportcommunity?
(Source:"AddressingBGPCommunities")
TheroutewillnotbeadvertisedwithinthelocalAS.
TheupstreamASwillnotbeallowedtoexporttheroute.
Theroutecannotbeexportedtoanotherroutingprotocol.
Therouterwillnotpropagatetheroutetoanyexternalneighborsexceptto
intra-confederationexternalneighbors.
1. MatchthefunctionstotheCiscoIOScommandsthatmonitorBGP
communities.(Source:"AddressingBGPCommunities")
DisplaysallroutesinaBGPtablethathave
allthespecifiedcommunitiesattached. showipbgpcommunity
DisplaysallroutesinBGPtablethathave
exactlythespecifiedcommunitiesattached. showipbgpcommunity-listclist
DisplaysallroutesinaBGPtablethathave
atleastonecommunityattached. showipbgpcommunityas:nn[as:nn...]
DisplaysallroutesinBGPtablethatmatch
community-listclist.
showipbgpcommunityas:nn[as:nn...]
exact
1. WhichtwoofthefollowingstatementsaboutthefunctionoftheBGP
LinkBandwidthfeatureareaccurate?(Choosetwo.)(Source:
"AddressingBGPCommunities")
TheBGPLinkBandwidthfeatureisusedtoenablemultipathload
balancingforexternallinkswithunequalbandwidthcapacity.
WhentheBGPLinkBandwidthfeatureisenabled,routeslearnedfrom
directlyconnectedexternalneighborarepropagatedthroughtheIBGP
networkwiththebandwidthofthesourceexternallink.
TheconfigurationoftheBGPLinkBandwidthfeaturecanbeusedonly
withIBGPmultipathfeaturestoenableunequal-costloadbalancingover
multiplelinks.
ToconfigureBGPtoadvertisethebandwidthoflinksthatareusedtoexit
anASusethebgpdmzlink-bwcommand.
1. WhichthreeofthefollowingfunctionsofBGPnamedcommunity-
listsareaccurate?(Choosethree.)(Source:"AddressingBGP
Communities")
Allowsthenetworkoperatortoassignmeaningfulnamestocommunity-
lists
Setslimitsonthenumberofnamedcommunity-listthatcanbeconfigured
Cannotbeconfiguredwithregularexpressionsandwithnumbered
community-lists.
Increasesthenumberofcommunity-liststhatcanbeconfiguredbya
networkoperator.
Appliespoliciestolargenumbersofroutesbyusingmatchandset
clausesintheconfigurationofroutemaps.
Setslimitof200communitygroupsthatcanbeconfiguredwithineach
typeoflist.
1. WhichtwoofthefollowingstatementsabouttheBGPCost
Communityfeatureareaccurate?(Choosetwo.)(Source:
"AddressingBGPCommunities")
Thepathwiththehighestcostcommunitynumberispreferred.
ThecostcommunityattributeinfluencestheBGPbestpathselection
processatthePOI.
Thecostcommunitycanbeusedasa"tiebreaker"duringthebest-path
selectionprocess.
Ifthecostcommunityvaluesareequal,thencostcommunitycomparison
proceedstothenexthighestcommunityIDforthisPOI.
1. WhichthreeofthefollowingarefunctionsoftheBGPSupportfor
SequencedEntriesinExtendedCommunityLists?(Choosethree.)
(Source:"AddressingBGPCommunities")
AllowsautomaticsequencingofindividualentriesinBGPextended
community-lists.
Providestheabilitytoremoveorresequenceextendedcommunity-list
entrieswithoutdeletingtheentireexistingextendedcommunity-list.
Configuresanextendedcommunity-listtousecustomvalues.
Removesorresequencesextendedcommunity-listentrieswhiledeleting
theentireexistingextendedcommunity-list.
Isactivatedbytheipextcommunity-listcommandinglobalconfiguration
mode.
Configuressequencenumbersforstandardcommunity-listentries.
1. Numberthe10BGPselectioncriteriaintheorderinwhichtheyare
used,fromthefirsttothelast,whenselectingtheBGPpaththatis
submittedtotheIProutingtable.(Source:"InfluencingBGPRoute
SelectionwithWeights")
AnswerKey
Preferthehighestweight. 1
PrefertheshortestASpath. 4
PreferanEBGPpathoveranIBGPpath. 7
Preferthehighestlocalpreference. 2
Preferthelowestorigincode(IGP<EGP<
incomplete). 5
PreferthepaththroughtheclosestIGP
neighbor. 8
Prefertheroutethatwasoriginatedbythe
localrouter. 3
PreferthepathwiththelowestneighborBGP
routerID. 10
PrefertheoldestrouteforEBGPpaths. 9
PreferthelowestMED. 6
1. WhatisthedefaultweightforroutesthatarereceivedfromaBGP
neighbor?(Source:"InfluencingBGPRouteSelectionwithWeights")
0
100
32768
ItdependsontheCiscoIOSrelease.
1. Whenaretheweightsthatareconfiguredonaneighborenforced?
(Source:"InfluencingBGPRouteSelectionwithWeights")
Beforethenewweightscantakeeffect,theBGPprocessontherouter
mustberemovedandreconfigured.
Theroutermustfirstberebootedforthenewweightstotakeeffect.
ThenewweightswillbeappliedaftertheBGPupdateintervalof30
minutesexpires.
Thenewweightconfigurationisappliedtoallroutesthatarereceived
followingtheconfigurationchange.
1. Ifyouwantallroutesreceivedfromtheneighbortohavethenew
weightvalueapplied,youhavetoreestablishBGPsession.Trueor
false?(Source:"InfluencingBGPRouteSelectionwithWeights")
true
false
1. Howcouldyouimplementaprimary/backupISProutingpolicyby
usingweights?(Source:"InfluencingBGPRouteSelectionwith
Weights")
AssignhigherweightstoallroutesthatarereceivedfromthebackupISP.
AssignlowerweightstoallroutesthatarereceivedfromthebackupISP.
AssignhigherweightstoallroutesthatarereceivedfromtheprimaryISP.
AssignlowerweightstoallroutesthatarereceivedfromtheprimaryISP.
1. Whenyouareusingroute-mapstomodifyweights,whathappensby
defaulttoaroutethatdoesnotmatchanyoftheroute-map
statements?(Source:"InfluencingBGPRouteSelectionwith
Weights")
Therouteisacceptedwiththeweightattributeunmodified.
Therouteisdiscarded.
TherouteisinsertedintotheBGPtablebutnotintotheIProutingtable.
Anerrorisdisplayedontherouterconsoleandinrouterdebugs.
1. Whichmethodofinfluencingrouteselectionwithweightsisthelast
tobeappliedonanincominginterface?(Source:"InfluencingBGP
RouteSelectionwithWeights")
prefix-list
route-map
filter-listweight
defaultweight
1. Youcansettheweightusingaroutemapappliedtoaneighborin
theoutgoingdirection.Trueorfalse?(Source:"InfluencingBGP
RouteSelectionwithWeights")
true
false
1. WhichtwoofthefollowingstatementsarecorrectregardingBGP
routeselection?(Choosetwo.)(Source:"InfluencingBGPRoute
SelectionwithWeights")
Iftworouteshavethesameweightattribute,theroutewiththelowestlocal
preferenceischosen.
Theroutewiththehighestweightisalwayschosenfirst.
TheweightattributeisglobalwithinanAS.
Theweightattributeisonlylocaltothelocalrouter.
Theweightvalueispropagatedbyallrouters.
1. Whatisakeydifferencebetweenthelocalpreferenceandweight
attributes?(Source:"SettingBGPLocalPreference")
Localpreferenceislocaltotherouteonwhichitisconfigured.
LocalpreferenceislocaltotheASwithinwhichithasbeenconfigured.
LocalpreferenceislocaltotheBGPadministrativedomain.
LocalpreferenceisglobaltoaBGPdomain.
1. Whatisthedefaultlocalpreferenceforroutesthatarereceivedfrom
aBGPneighbor?(Source:"SettingBGPLocalPreference")
0
100
Thereisnodefaultlocalpreferencevalue.
ItdependsontheCiscoIOSrelease.
1. Whichtwostatementsregardinglocalpreferencearetrue?(Choose
two.)(Source:"SettingBGPLocalPreference")
Thehighervalueforlocalpreferenceispreferred.
LocalpreferenceisusedonlybetweenEBGPneighbors
Thelowervalueforlocalpreferenceispreferred.
LocalpreferenceisusedonlybetweenIBGPneighbors
1. Whichtwoofthefollowingstatementsabouttheinfluenceoflocal
preferenceonBGProuteselectionisaccurate?(Choosetwo.)
(Source:"SettingBGPLocalPreference")
Whenyousetlocalpreference,youcanviewitonneighboringrouters,but
youmustresetit.
YoucanuselocalpreferencetoensureAS-widerouteselectionpolicy.
Localpreferenceisusedtoselectrouteswithunequalweights.
Localpreferenceisthesecond-strongestcriterionintherouteselection
process.
1. Letsaywehavetworoutestothesamedestination,fistonehasa
weightof50andalocalpreferenceof150,whiletheotheronehasa
weightof150andalocalpreferenceof50.Thefistroutewillbe
selectedasthebestone,sinceithashigherlocalpreferencevalue.
Trueorfalse?(Source:"SettingBGPLocalPreference")
true
false
1. WhichCiscoIOScommandisusedtochangethedefaultvalueof
localpreference?(Source:"SettingBGPLocalPreference")
setlocal-preference
bgpdefaultlocal-preference
showipbgp
showipbgpprefix
1. WhichCiscoIOScommandisusedtoconfigureBGPlocal
preferencewithroute-mapstatements?(Source:"SettingBGPLocal
Preference")
setlocal-preference
bgpdefaultlocal-preference
showipbgp
showipbgpprefix
1. Locallyapplieddefaultvaluecanbedisplayedwithshowipbgp
command.Trueorfalse?(Source:"SettingBGPLocalPreference")
true
false
1. WhichCiscoIOScommandisnecessarytodisplaythelocally
appliedBGPvalue?(Source:"SettingBGPLocalPreference")
showbgppreferencedetail
showipbgp
showipbgpdetail
showipbgpprefix
1. WhichtwostatementsregardingtheASpatharetrue?(Choose
two.)(Source:"UsingASPathPrepending")
TheshorterASpathispreferred.
ThelongerASpathispreferred.
TheASpathisprependedandexchangedbetweenautonomoussystems.
TheASpathislocaltoanAS.
1. WhatisASpathprepending?(Source:"UsingASPathPrepending")
Whenarouter,sendingaBGPupdate,addstheASnumberoftherouter
fromwhichitreceivedtheroutetotheASpathattribute.
Whenarouter,sendingaBGPupdate,addstheASnumberoftherouter
towhichitissendingtheroutetotheASpathattribute.
Whenarouter,sendingaBGPupdate,addsitsASnumbertotheASpath
attributemultipletimes.
WhenarouterusestheASpathattributeinrouteselection.
1. TheASpathwillbetherouteselectioncriterionthatisusedwhen
whichofthefollowingistrue?(Source:"UsingASPathPrepending")
ItisthefirstcriterionthatisusedinBGProuteselection.
Itisusedwhenthereisnodifferenceinweight,localpreference,orroute
origination.
ItisusedwhentheMEDisidenticalonthecandidateroutes.
Theweight,localpreference,MED,andoriginattributesmustbeidentical
beforetheASpathattributeisusedforrouteselection.
1. WhichcommanddoyouusetomanipulatetheASpathattribute?
(Source:"UsingASPathPrepending")
Theglobalconfigurationcommandsetas-pathprependas-number.
Therouterconfigurationcommandsetas-pathprependas-number.
Thesetas-pathprependas-numbercommandinaroute-map.
Theinterfaceglobalcommandsetas-pathprependas-number.
1. TheconfigurationbelowisfromarouterinAS347,whichis
advertisingnetwork11.0.0.0/8toanEBGPneighbor2.0.0.2inAS
529.WhatarethecontentsoftheASpathattributeforroute
11.0.0.0/8onarouterthatisresidinginAS529?(Source:"UsingAS
PathPrepending")route-mapaddASpermit10
setas-pathprepend347347347
routerbgp347
neighbor2.0.0.2remote-as529
neighbor2.0.0.2route-mapaddASout
347347347
347347347347
529347347347
529347347347347
1. WhydoyouneedtouseASpathprepending?(Source:"UsingAS
PathPrepending")
ASpathprependingallowsacustomertopotentiallyinfluencereturnpath
routeselection.
ASpathprependingisusedonacustomerroutertocontroloutgoingroute
updates.
ServiceprovidersuseASpathprependingtocontrolincomingupdates
fromacustomerAS.
ASpathprependingisusedbetweenserviceprovidersthatareconnected
tothesamecustomerAStodeterminewhichwillbetheprimarylinktothe
customer.
1. WhichregularexpressionwillallowanyASpathnumber,butwitha
lengthofone?(Source:"UsingASPathPrepending")
^[0-9]+$
^[0-9](_[0-9])*$
^_[0-9]$
^[1-8]+$
1. HowdoesASpathprependingaffectarouter?(Source:"UsingAS
PathPrepending")
ASpathprependingissimplyatermthatisusedtodescribewhenarouter
usestheASpathattributeinrouteselectionandhencedoesnotaffect
routerresources.
ThelongertheASpathattributeattachedtoBGPupdates,themorerouter
memoryrequirementsincrease.
ASpathprependingdoesnotimpacttherouterbecauseCiscoIOS
softwarerecognizesthatASpathprependingisinuseandstoresasingle
ASnumberwithapointertothenumberofASpathprepends.
ASpathprependingcausestheroutertooperateinprocess-switching
modebecausetheBGPupdatemustbestored,manipulated,andthen
rewrittentoaccommodatethenewASpathattribute.
1. Whichtwoofthefollowingarecharacteristicsofthefunctionofthe
BGPHideLocal-AutonomousSystemfeature?(Choosetwo.)
(Source:"UsingASPathPrepending")
AllowsyoutotransparentlychangetheASnumberfortheentireBGP
network.
EnsuresthatroutescanbepropagatedthroughouttheAS.
AllowscustomizationoftheASnumberforEBGPpeergroupingsthrough
thesetas-pathcommand.
ChangestheASnumberforallIBGPpeersatthesametime.
1. WhatisthetypicalapplicationoftheMEDattribute?(Source:
"UnderstandingBGPMulti-ExitDiscriminators")
ToinfluencepathselectionoutofanoriginatingAS.
Toprovideastrongmetrictoselectthebestpathwhenmultipleroutes
exist.
TohaveaBGPattributetraversingmanyautonomoussystemswhile
influencingpathselection.
ToinfluencethereturnpathoftrafficbackintoanAS.
1. WhatisthedefaultMEDvalue?(Source:"UnderstandingBGPMulti-
ExitDiscriminators")
0
100
ThereisnodefaultMEDvalue.
ItdependsontheCiscoIOSrelease.
1. WhatarethreeBGPattributesthatarecomparedbeforetheMED?
(Choosethree.)(Source:"UnderstandingBGPMulti-Exit
Discriminators")
largestweight
originatedroutes
ASpathlength
lowestIPaddress
1. WhichtwostatementsabouttheCiscoIOScommandthatis
requiredtoconfigurechangestothedefaultBGPMEDonaCisco
IOSrouterareaccurate?(Choosetwo.)(Source:"Understanding
BGPMulti-ExitDiscriminators")
TheMEDisamandatoryattribute.
Usingthedefault-metriccommandinBGPconfigurationmodewillcause
oneredistributednetworktohavethespecifiedMEDvalue.
ThereisnoMEDattributethatisattachedtoaroutebydefault.
Tosetthedefaultmetricvalue(MED)forBGProutes,usethedefault-
metriccommand.
1. WhichtwostatementsabouttheCiscoIOScommandsthatare
requiredtoconfigurechangestotheBGPMEDattributewithroute-
mapstatementsareaccurate?(Choosetwo.)(Source:
"UnderstandingBGPMulti-ExitDiscriminators")
Thesetmetriccommandisusedwithinroute-mapconfigurationmodeto
settheMEDattribute.
Theneighboraddressroute-mapnamein|outcommandappliesa
route-maptoincomingupdatesfromallneighbors.
Per-neighborMEDisconfiguredbyusingaroute-mapwithamatch
condition.
Youcanusearoute-maptosettheMEDonincomingoroutgoing
updates.
1. WhichtwoofthefollowingstatementsabouttheCiscoIOS
commandsthatarerequiredtoconfigureadvancedMEDfeatureson
Ciscoroutersareaccurate?(Choosetwo.)(Source:"Understanding
BGPMulti-ExitDiscriminators")
Thebgpbestpathmedconfedcommandallowsrouterstocompare
pathslearnedfromconfederationpeers.
WhenyouenableadeterministicMEDcomparison,youallowarouterto
compareMEDvaluesafteritconsidersBGProutetype(externalor
internal)andIGPmetrictothenext-hopaddress.
Usethebgpalways-compare-medcommandtoforcetherouterto
comparetheMEDevenifthepathscomefromdifferentautonomous
systems.
CiscoIOSsoftware,ontheotherhand,regardsamissingMEDattribute
ashavingavalueof1.
1. Ifyouconfigureinboundsoftreconfigurationwitharoute-mapand
issuetheshowipbgpprefixcommand,whichvalueoftheMED
attributeisdisplayed?(Source:"UnderstandingBGPMulti-Exit
Discriminators")
Onlytheoriginalroute(noMED)isdisplayed.
Boththeoriginalrouteandthemodifiedroutearedisplayed.
Onlythemodifiedrouteisdisplayed.
TheMEDattributeisnotdisplayedwiththeshowipbgpprefixcommand.
1. WhichofthefollowingstatementsabouttheCiscoIOScommands
thatarerequiredtotroubleshootBGPMEDconfigurationsona
Ciscorouterisaccurate?(Source:"UnderstandingBGPMulti-Exit
Discriminators")
ToseetheoriginalMED,youneedtoenablehardreconfigurationonthe
router.
Thecommandshowipbgpneighboraddressreceived-routesdisplays
theoriginalupdatesbeforeanyfiltersorroute-mapshavefilteredor
changedthem.
Ifhardreconfigurationisenabled,theoriginalupdatestotheMEDattribute
areavailablebyusingtheshowipbgpprefixcommand.
IssuingtheshowiproutecommandwilldisplaytheMEDvalue.
1. WhichtwostatementsregardingtheMEDaretrue?(Choosetwo.)
(Source:"UnderstandingBGPMulti-ExitDiscriminators")
ThehighervaluefortheMEDispreferred.
ThelowervaluefortheMEDispreferred.
TheMEDisexchangedbetweenautonomoussystems.
TheMEDislocaltoanAS.
1. WhataretworeasonswhyitisnotfeasibletousetheMEDto
influencereturnpathselectionwhenmultipleautonomoussystems
areinvolved?(Choosetwo.)(Source:"AddressingBGP
Communities")
TheMEDattributeisdesignedtoinfluenceoutboundpathselectiononly.
TheASpathattributewouldbeusedforpathselectionregardlessofany
configuredMEDvalue.
Theweightattributewillalwaysbeused,giventhatitisfirstintheBGP
routeselectionprocess.
TheMEDcannotbepropagatedacrossseveralautonomoussystems.
1. DoesthecommunityattributehaveanyinfluenceonBGPpath
selection?(Source:"AddressingBGPCommunities")
No,communitiesaresimplytagsthatareappliedtoBGProutes.
No,communitiesarenontransitiveattributes.
Yes,BGPpathsareselectedbasedonthevalueinthecommunitytag.
Yes,thecommunityattributeispartoftheBGProuteselectionprocess.
1. MatchthestepswiththeactionsdescribinghowBGPcommunities
canfacilitateproperreturnpathselection.(Source:"AddressingBGP
Communities")
Defineadministrativepolicygoalsthatyou
needtoimplement. Step1
Definethefiltersandrouteselectionpolicy
thatwillachievetherequiredgoals. Step2
Assignacommunityvaluetoeachgoal. Step3
Applycommunitiesonincomingupdates
fromneighboringautonomoussystemsortell
theneighborstosetthecommunities
themselves.
Step4
Matchcommunitieswithroute-mapsand
routefilters,changeBGPattributes,or
influencetherouteselectionprocessbased
onthecommunitiesthatareattachedtothe
BGProutes.
Step5
Enablecommunitydistributionthroughout
yourAStoallowcommunitypropagation. Step6
1. HowmanycommunitytagscanbeattachedtoasingleBGProute?
(Source:"AddressingBGPCommunities")
1
32
256
Itdependsonthenumberthatisconfiguredwiththeipbgpcommunity
command.
1. Whichthreeofthefollowingareactivitiesthatarerequiredto
successfullydeployBGPcommunitiesinaBGP-basednetwork?
(Choosethree.)(Source:"AddressingBGPCommunities")
Settingcommunities,whichrequiresaroute-map.
Creatingcommunity-liststobeusedwithinroute-mapstomatchon
communityvalues.
Enablingcommunitypropagationperneighborfordesignatedinternal
neighbors.
Creatingroute-mapswherecommunity-listsareusedtomatchon
communityvalues.
1. WhichtwoofthefollowingstatementsabouttheCiscoIOS
commandsthatarerequiredtoconfigureroutetaggingwithBGP
communitiesareaccurate?(Choosetwo.)(Source:"Addressing
BGPCommunities")
Youcanattachupto35communitiestoasingleroutewithoneroute-map
setstatement.
Omittingtheadditivekeywordfromthesetcommunitycommandresults
inoverwritinganyoriginalcommunityattributes.
Theglobalcommandipbgp-communitynew-formatisrecommendedon
allrouterswhenevercommunitiescontaintheASnumber.
Youcannotusearoute-mapwithredistributionfromanotherrouting
protocol.
1. WhichofthefollowingstatementsabouttheCiscoIOScommand
thatisrequiredtoenableBGPcommunitypropagationtoBGP
neighborsisaccurate?(Source:"AddressingBGPCommunities")
CommunitypropagationtoBGPneighborsisautomaticallyconfigured.
ItisnecessarytomanuallystripcommunitiesinoutgoingBGPupdates.
Theneighborip-addresssend-communitycommandcannotbeapplied
toapeergroup.
Theneighborip-addresssend-communitycommandisneededto
propagatecommunityattributestoBGPneighbors.
1. WhichmatchcriteriaarespecifiedinastandardBGPcommunity-
list?(Source:"AddressingBGPCommunities")
destinationIPaddresses
regularexpressions
communityattributevalues
ASnumbers
1. Whatistheresultoftaggingaroutewiththeno-exportcommunity?
(Source:"AddressingBGPCommunities")
TheroutewillnotbeadvertisedwithinthelocalAS.
TheupstreamASwillnotbeallowedtoexporttheroute.
Theroutecannotbeexportedtoanotherroutingprotocol.
Therouterwillnotpropagatetheroutetoanyexternalneighborsexceptto
intra-confederationexternalneighbors.
1. MatchthefunctionstotheCiscoIOScommandsthatmonitorBGP
communities.(Source:"AddressingBGPCommunities")
DisplaysallroutesinaBGPtablethathave
atleastonecommunityattached. showipbgpcommunity
DisplaysallroutesinaBGPtablethathave
allthespecifiedcommunitiesattached. showipbgpcommunityas:nn[as:nn...]
DisplaysallroutesinBGPtablethathave
exactlythespecifiedcommunitiesattached.
showipbgpcommunityas:nn[as:nn...]
exact
DisplaysallroutesinBGPtablethatmatch
community-listclist.showipbgpcommunity-listclist
1. WhichtwoofthefollowingstatementsaboutthefunctionoftheBGP
LinkBandwidthfeatureareaccurate?(Choosetwo.)(Source:
"AddressingBGPCommunities")
TheBGPLinkBandwidthfeatureisusedtoenablemultipathload
balancingforexternallinkswithunequalbandwidthcapacity.
WhentheBGPLinkBandwidthfeatureisenabled,routeslearnedfrom
directlyconnectedexternalneighborarepropagatedthroughtheIBGP
networkwiththebandwidthofthesourceexternallink.
TheconfigurationoftheBGPLinkBandwidthfeaturecanbeusedonly
withIBGPmultipathfeaturestoenableunequal-costloadbalancingover
multiplelinks.
ToconfigureBGPtoadvertisethebandwidthoflinksthatareusedtoexit
anASusethebgpdmzlink-bwcommand.
1. WhichthreeofthefollowingfunctionsofBGPnamedcommunity-
listsareaccurate?(Choosethree.)(Source:"AddressingBGP
Communities")
Allowsthenetworkoperatortoassignmeaningfulnamestocommunity-
lists
Setslimitsonthenumberofnamedcommunity-listthatcanbeconfigured
Cannotbeconfiguredwithregularexpressionsandwithnumbered
community-lists.
Increasesthenumberofcommunity-liststhatcanbeconfiguredbya
networkoperator.
Appliespoliciestolargenumbersofroutesbyusingmatchandset
clausesintheconfigurationofroutemaps.
Setslimitof200communitygroupsthatcanbeconfiguredwithineach
typeoflist.
1. WhichtwoofthefollowingstatementsabouttheBGPCost
Communityfeatureareaccurate?(Choosetwo.)(Source:
"AddressingBGPCommunities")
Thepathwiththehighestcostcommunitynumberispreferred.
ThecostcommunityattributeinfluencestheBGPbestpathselection
processatthePOI.
Thecostcommunitycanbeusedasa"tiebreaker"duringthebest-path
selectionprocess.
Ifthecostcommunityvaluesareequal,thencostcommunitycomparison
proceedstothenexthighestcommunityIDforthisPOI.
1. WhichthreeofthefollowingarefunctionsoftheBGPSupportfor
SequencedEntriesinExtendedCommunityLists?(Choosethree.)
(Source:"AddressingBGPCommunities")
AllowsautomaticsequencingofindividualentriesinBGPextended
community-lists.
Providestheabilitytoremoveorresequenceextendedcommunity-list
entrieswithoutdeletingtheentireexistingextendedcommunity-list.
Configuresanextendedcommunity-listtousecustomvalues.
Removesorresequencesextendedcommunity-listentrieswhiledeleting
theentireexistingextendedcommunity-list.
Isactivatedbytheipextcommunity-listcommandinglobalconfiguration
mode.
Configuressequencenumbersforstandardcommunity-listentries.
Introduction
Customerswhoareconnectingtoaserviceprovidermustconsidervarious
requirementsandchoosetheproperconnectivitytype.Ingeneral,threetypesof
connectivityareavailable:connectingtoasingleserviceprovider,connectingtoa
singleserviceproviderusingtwolinks,andconnectingtotwoindependentservice
providers.Basedontheconnectivitytypetheproperroutingmethods,IPaddressing,
andASnumberhavetobechosen.Afteralloftheseinputparametersareknownthe
customerandserviceprovidercanimplementthedesiredconnectivitytype.
Uponcompletingthismodule,youwillbeableto:
DescribetherequirementstoconnectcustomernetworkstotheInternetina
serviceproviderenvironment
Toimplementcustomerconnectivitybyusingstaticrouting
DefineshowtoimplementMultihomedcustomerconnectivityusingBGPtoa
singleISP
DefinehowtoimplementMultihomedcustomerconnectivityusingBGPto
multipleISPs
Customer-to-ProviderConnectivitywith
BGP
Overview
CustomersconnecttotheInternetbyusingserviceproviderstoenableapplications
suchasintranetconnectivitywithVPNs,extranetconnectivitywithsuppliers,and
otherInternetapplications.WhenplanningnetworkconnectivitytoanISP,youasa
networkdesignermustgivecarefulconsiderationtothevariousaspectsofthe
connectivity.Theseaspectsincludephysicalconnectiontypes,theredundancythat
thechosenconnectionmethodprovides,IPaddressingrequirements,andAS
numberingconsiderations,ifthenetworkdesignisgoingtomeetbothbusinessand
technicalrequirementsoftheapplicationsthatareplannedforthenetwork.
Inthislesson,youwilldiscusssolutionsforconnectingcustomernetworkstoservice
providers.Youwillalsodiscusscustomernetworkredundancyrequirements,routing
requirements,IPaddressingrequirements,andASnumberingrequirements.
Uponcompletingthislesson,youwillbeableto:
Identifyvariousphysicalconnectionsthatthecustomersusetoconnecttoa
serviceprovider
Describethelevelsofredundancythatisprovidedbyeachphysicalconnection
typethatthecustomersusetoconnecttoaserviceprovider
Identifyvariousroutingschemesthatthecustomersusetoconnecttoaservice
provider
Describeroutingschemesthatareappropriateforeachphysicalconnectiontype
thatthecustomersusetoconnecttoaserviceprovider
Describetheaddressingschemesthatthecustomersusetoconnecttoa
serviceprovider
DescribetheASnumberingschemesthatthecustomersusetoconnecttoa
serviceprovider
UnderstandingCustomer-to-Provider
ConnectivityRequirements
Internet customers have a wide range of connectivity and redundancy requirements:
Single-homed customers:
Single permanent connection to a single service provider.
Multiple permanent connections to a single provider in primary and backup
configuration.
Multiple permanent connections to a single provider used for load sharing of
traffic.
Multihomed customers:
Multiple permanent connections to multiple service providers for maximum
redundancy.
Serviceprovidercustomershavedifferentrequirements,suchasredundancy,
stability,security,flexibility,fortheirInternetconnectivity.Thesedifferent
requirementsresultindifferentsolutions:
AsinglepermanentconnectiontooneISP.Thissolutionmeetstherequirements
formostofthecustomers.
MultiplepermanentconnectionstooneISP,inwhichoneofthelinesisprimary
andtheotherlineisusedforbackuponly.Thissetupalsoprovidesredundancy
onthelinks.Comparedtoadial-upbackup,apermanentbackuplinkis
preferredforvariousreasons.Forexample,theseverebandwidthlimitationson
dial-uplinesandthetimethatisrequiredtoestablishadial-upconnection.
MultiplepermanentconnectionstooneISP,whichisusedforloadsharingof
traffic.Thissolutiongivesredundancyonthelinksbutalsoprovidesextra
bandwidth.
MultiplepermanentconnectionstomorethanoneISP.Thissolutionprovides
thehighestlevelofredundancy,becausenotonlydoesitcopewithlink-level
failuresbutalsowithfailureswithinthenetworkofaserviceprovider.
Customer-to-ProviderConnectivityTypes
Thelevelofredundancydependsonaphysicalconnectiontypethatthecustomers
usetoconnecttoaserviceprovider.
SinglePermanentConnection
The simplest setup is a single link between the customer network and the
service provider.
There is no redundancy for link, equipment, or service provider failure.
AsinglepermanentconnectiontooneISPisthemostcommonsetup.Thissetupis
alsothesimplesttoimplement.
Thecustomerconnectstoasingleserviceproviderusingasinglelink.Thecustomer
hasanedgerouterthatisconnectedtotheedgeroutersoftheISP.Thisconnection
ispermanentandcouldbealeasedline,aFrameRelay,orATMPVC,DSL,FFTx,
cable,orsomethingequivalent.
Youdonothaveredundancyinthissolution.Anyfailureonthepermanentlinkor
eitherofthetwoedgerouterscausesacompleteoutageoftheservice.Serious
failureswithintheISPnetworkthataffectallcustomersofthisISPalsoaffectthe
customerinthisexample.
MultiplePermanentConnectionsProvidingRedundancy
To increase redundancy, customers install several physical links to the service
provider.
Redundant links are used as follows:
Primary and backup.
For load sharing.
Redundancy is for link or equipment failure.
There is no redundancy for service provider failure.
Inthissetup,youhaveoneCErouterthatisconnectedtoonePErouter,and
anotherCEthatisconnectedtoanotherPErouter.Ifoneoftheseroutersfails,only
oneoftheconnectionsbreaksdown;theotherconnectionisstillavailable.
Thesetwolinksmaybeimplementedintwoscenarios:
Primaryandbackup:Onelinkisalwaysactive,whileanotheroneisthebackup
andisusedonlywhentheprimarylinkfails.BackupPVCsinFrameRelayor
ATMnetworkscanbeverycost-efficientincasethatthesePVCscarryonlya
verysmallvolumeoftrafficandthattheprimarypathisavailable.
Loadsharing:Bothlinksareusedatthesametimetoprovideloadbalancing.
Thedistributionoftheloadoverthelinksismorecomplicatedthanwhenboth
linksterminateinthesamerouter.
Again,becausethecustomerisconnectedtoasingleISP,redundancyisprovided
forlink,andcustomer,orISPdevicefailure.ThefailuresintheISPnetworkwillstill
CustomerRedundantConnectivity
affectthecustomer.Thissolutioncanmitigaterouterandlinkfailures,butitcannot
mitigateISPfailures.
MultiplePermanentConnectionsProvidingLoadSharing
Customers wanting to increase their access speed can install several physical links
between a pair of routers.
There is redundancy for link failure.
There is no redundancy for equipment failure.
Load sharing in this setup is optimal.
Inthissetup,youhaveasinglerouterinthecustomernetworkthatisconnectedtoa
singlerouterintheISPnetwork.Theredundancyislimitedtothelinklevelbecause
routerfailuresarenotcovered.Usingtwoparallellinksbetweentworoutersallows
foranoptimaldistributionofloadoverthelinks.
DependingontheswitchingpaththatisusedinthecustomerandtheISProuters,
loadsharingcanbeperformedbasedonvariousoptions.Onthedestinationaddress
only(fastswitching),onsource-destinationaddresspairs(defaultbehaviorforCEF),
oronapacket-by-packetbasis(processswitchingorCEF).
Asinthepreviousexamples,becausethecustomerisconnectedtoasingleISP,
seriousISPnetworkfailurewillaffectthecustomerregardlessofthelinkbackup.
ConnectionstoMultipleServiceProviders
Customers with maximum redundancy requirements install physical links to multiple
service providers.
Redundant links are used as follows:
Primary and backup.
Primary and backup with direct traffic.
For load sharing.
There is redundancy for link, equipment, or service provider failure.
Intheexample,twoCEroutershaveonepermanentconnectioneachtoadifferent
ISP.Linkanddevicefailuresarecoveredinthesamewayasintheprevious
examples.However,becausethetwoconnectionsinthisexamplegototwodifferent
ISPs,theredundancyalsocoversproblemswithinoneISPnetwork.Ifoneservice
providerfails,trafficwillstillflowthroughanotherserviceprovider,andthehighest
levelofredundancyisachieved.
Inthissetup,threescenariosarepossible:
Primaryandbackup:Onelinkisalwaysactive,whileanotheroneisthebackup
andisusedonlywhentheprimarylinkfails.
Primaryandbackupwithdirecttraffictothebackupserviceprovider:One
linkisactive,andanotheroneisthebackup.Exceptforthetrafficthatshouldgo
directlytoserviceprovider2.
Loadsharing:Bothlinksareusedatthesametimetoprovideloadbalancing.
Inallscenarios,thecustomerhastocooperatewithaserviceprovidertoachievethe
desiredsetup.Loadsharingbetweenthelinkscanneverbeoptimal.Equal
distributionofthereturntrafficloadfromtheInternetoverthetwoseparatelinks
cannotbeachieved.Distributionoftheloadofoutgoingtrafficisdonebasedon
destinationaddresses.Slowlyadjustingtherouterconfigurationparametersand
observingthelinktrafficloadchangescanhelpyouachieveanacceptable
distributionoftrafficbetweentwolinks.
AvoidinganyloadonthebackuplinkmayrequireassistancefromtheISPtowhich
thebackuplinkisconnected.
DifferentsolutionsforconnectingacustomernetworktothenetworkofanISP
requiredifferentmethodsofroutinginformationexchange.
Static or dynamic routing can be used between an Internet customer and a
service provider.
BGP is the only acceptable dynamic routing protocol.
Because of its lower complexity, static routing is preferred where possible.
Routingoptions:
Staticrouting:Staticroutingispreferredbecauseofitslowercomplexity.Ina
normalcase,thecustomernetworkmusthaveadefaultroutetotheISPnetwork
andtheISPnetworkmusthavearoutetotheIPprefixesthatthecustomerhas
initsnetwork.Asalways,staticroutingprovidesverylowredundancy,ifany.
Dynamicrouting:Dynamicroutingprovidesredundancy.Thecustomerandthe
ISPnetworksmustbeconfiguredtoexchangeacommonroutingprotocol.BGP
istheonlychoicebecauseofthelargevolumesofroutinginformation,the
inherentsecuritymechanismsofBGP,andtheabilityofBGPtohandlerouting
policies.
Customer-to-ProviderRoutingSchemes
Differentroutingschemesareappropriateforeachphysicalconnectiontypethatthe
customersusetoconnecttoaserviceprovider.
RoutingScheme:CustomerwithaSingleConnectiontoa
ServiceProvider
Whenthecustomerhasasingleconnectiontotheserviceprovider,staticroutingis
usuallyadequate.Thephysicaltopologydoesnotprovideanyredundancy,anditis
thereforeunnecessarytoaddthecomplexityofdynamicrouting.
Customers with a single connection to a service provider typically do not require
BGP:
Static routing is usually adequate.
The static route for the provider-assigned address space of the customer is
on the PE router.
The static default route is on the CE router.
BGP should not be used in this setup.
Inthiscase,keepthenetworksimplebyavoidingtheuseofBGP.Ifthereisstatic
routing,adefaultrouteisneededontheCErouterthatpointstothePErouter.A
staticrouteforthecustomernetworkisneededonthePErouter,whichpointstothe
CErouter.
IfyouuseBGP,theserviceprovidersendsadefaultroutetothecustomer,andthe
customeradvertisesitsownaddressspacetotheserviceprovider.
RoutingScheme:CustomerwithMultipleConnectionstoa
ServiceProvider
Static routing is preferred if physical link failure can be detected.
Traffic will enter a "black hole" if the physical link failure is not detected.
Multiplepermanentconnectionsbetweenasinglerouteronthecustomernetwork
andasinglerouterontheserviceprovidernetworkshouldbeconfiguredwithstatic
routing.Thisconfigurationenablesthelink-levelprocedurestodetectalinkfailure.
Withthistypeofconnection,twodefaultroutesareconfiguredontheCErouterthat
pointtothePErouter.TwostaticroutesareconfiguredonPErouter,pointingtoboth
linksbetweenthecustomerandtheISP.Ifeitherofthelinksfails,thelink-level
proceduresshoulddetectthisfailureandplacetheinterfaceinadownstate.Inthis
case,thestaticrouteisinvalidandisnotusedforforwardingpackets.Therouterwill
thenforwardallpacketsovertheremaininglink.
Ifthelink-levelprocedurescannotdetectalinkfailure,thestaticroutepointingout
overthefailedlinkisstillvalid.Theroutercontinuestousethisstaticroutetosend
someofthetrafficoutonthefailedinterface.Thissituationeffectivelycreatesa
"blackhole"forsomeofthetraffic.
CustomerRoutingSchemes
You can still use static routing if link and service provider equipment failure can
be detected.
BGP between the customer and the service provider is usually used in this
setup.
The service provider originates the default route (in the primary or secondary
scenario) or specific routes (in the load-balancing scenario).
The customer originates its address space.
Youcanalsousestaticroutingformultiplepermanentconnectionsbetweentwo
differentroutersonthecustomernetworktotwodifferentroutersontheservice
providernetwork.However,youcanusestaticroutingonlyifthelink-level
procedurescandetectlinkfailures.Whenoneoftheconnectionsislost,thelink-
levelproceduredetectsthislossandplacestheinterfaceinadownstate.Because
theinterfaceisinthedownstate,thestaticroutethatpointsoutofthedown
interfacebecomesinvalid.Asaresult,thecustomer'srouterstopstheredistribution
ofthedefaultrouteintothecustomerIGP.Theprovider'srouterstopsthe
redistributionofthestaticroute,thatpointsthecustomernetwork,intotheservice
providerBGP.
However,customersthatrequiretheuseofmultipleconnectionsandmultiplerouters
veryoftendonotrelyonthelink-levelprocedures.Abetteroptiontodetectlink
failuresisusingBGPbetweenCEandPErouters.BecauseBGPuseshandshaking
andreliabletransfer,italwaysdetectsafailedlinkorfailedremoterouter.BGPcan
alsobeusedtoachieveaprimaryorsecondaryscenarioorloadbalancingoftraffic
overbothlinks.
RoutingScheme:CustomerwithConnectionstoMultiple
ServiceProviders
Static routing is not possible.
BGP must be used:
The service provider originates the default route (in the primary or secondary
scenario), the default route, and service provider-owned routes (in the
primary or secondary scenario with different traffic scenario), or all routers
(in the load-balancing scenario).
The customer originates its address space.
MultiplepermanentconnectionstomorethanoneISPalwaysrequiretheuseof
dynamicroutingwithBGP.Thecustomersthatrequirethistypeofconnectiondonot
wantonlytoprotectthenetworkconnectivityfromlinkfailuresorremoterouter
failures.Theyalsowanttoprotecttheirnetworkconnectivityfromseriousproblems
intheISPnetwork.
MonitoringthelinkstatuscannotdetectaprobleminsideoneoftheISPnetworks.If
thelinkisstillupandtheISPedgerouterisstillup,thelink-levelproceduresdonot
indicateanyproblems.However,theISPnetworkmaysufferfromsevereproblems.
AnISPnetworkcanbepartitionedordisconnectedfromtherestoftheInternet
withouthavinganyproblemswiththeedgerouterandtheaccesslinetothe
customernetwork.
TheonlywaytodetectthissituationistouseBGPwithbothISPsandreceivefull
Internetroutingfrombothofthem.WhenoneoftheISPshasproblems,theedge
router,beingtheBGPneighborofthecustomer,withdrawstheroutesthatitcanno
longerreach.ThisactionmeansthatthecustomerroutersknowwhichInternet
routeseachISPcanreachatthemoment.
CEroutersadvertisethecustomernetworktotheserviceprovider.PErouters
advertiseonlythedefaultrouteorthedefaultrouteplustheserviceprovider-owned
networksorallroutestotheCErouters.Thelevelofroutesthattheserviceprovider
advertisesisdeterminedbasedontherequiredcustomerscenario.
Customerswhoareconnectedtoasingleserviceproviderusuallyhavetheir
addressspaceassignedbytheserviceprovider,whichisknownasPAaddresses.
CustomersthatareconnectedtomorethanoneISPshould,ifpossible,assigntheir
ownaddressspaceandnothaveaddressesthataredelegatedfromanyoftheir
ISPs.SuchassignedaddressesarecalledPIaddresses.
Single-homed customers:
Customer gets the address space from the provider—PA address space.
Single (link) IP address or multiple (subnet) IP address.
If service provider changes, readdressing is needed.
Inside the customer network, private addresses are used, with a combination of
NAT.
Multihomed customers:
Customer assigns its own address space—PI address space
Customer has to advertise address space to all service providers.
If service provider changes, readdressing is not needed.
Single-HomedCustomers:CustomersthatareconnectedtoasingleISPusually
gettheiraddressspaceassignedbytheISP.Customerscaneitherrequestasingle
IPaddress,whichisusedontheaccesslinkaswell,ortheycanrequestanentireIP
subnet.Aserviceproviderisusuallyassignedalargeaddressspacetodelegateto
itscustomers.AllcustomersofoneISPgettheiraddressesfromoneaddressspace
orafewaddressspaces.ItisverylikelythattheISPisabletoaggregatethe
customeraddressesbeforesendingtheroutestotherestoftheInternet.
Ifthecustomershoulddecidetochangeitsserviceprovider,thecustomermust
returnitsPAaddressestotheoldISP.Thecustomerthenreceivesanew
assignmentofPAaddressesfromthenewISP.Otherwise,theISPsarenolonger
abletoperformefficientaddressaggregation.Theconsequenceforthecustomeris
thatthecustomerhastorenumberitsnetworkwhenitchangesitsserviceprovider.
MultihomedCustomers:CustomersthatareconnectedtomultipleISPsusually
assigntheirownaddressspace.Noneoftheserviceprovidersadvertisesthe
address,andthecustomerhastoadvertiseittobothserviceproviders.This
arrangementmeansthatnoISPcanaggregatethecustomerroutesbeforesending
themtotherestoftheInternet.TheroutespropagatethroughtheInternetwiththe
prefixlengthsgiven.
SomelargeISPsfilteroutrouteswithlongprefixes.ISPsdonotwanttopopulate
theirroutingtableswithmanyexplicitroutesthatshouldhavebeenaggregatedinto
aroutesummarybeforetheyweresenttothem.Asaresult,thecustomer
announcingsmallblocksofPIaddresses,whichcannotbeaggregated,maynotbe
reachablefromallpartsoftheInternet.AlargerblockofPIaddressessolvesthe
problem.
Ifthecustomershoulddecidetochangeitsserviceprovider,renumberingofthe
networkisnotneeded.
AmultihomedcustomercaninsomecasesusePA
addresses.Theaddressspacemustbeassignedfrom
oneoftheISPs.Whenthecustomerannouncestheblock
ofPAaddressestobothISPs,bothshouldpropagatethe
addressestotherestoftheInternet.Theproviderthat
assignedtheaddressspaceshouldalsoannouncethe
largerblockofaddresses,ofwhichthecustomeris
announcingasubset.OtherISPsnowreceivetwo
alternateexplicitroutesandanoverlappingroute
summary.Filteringoutexplicitroutesismorelikelyatthis
timebecausetheotherISPsrecognizetheseasroutes
thatcanbeaggregated.IftheotherISPsfilteroutthe
moreexplicitroutes,thecustomerisstillreachableaslong
asbothprovidersareannouncingtheoverlappingroute
summary.
CustomerAddressingRequirements
SomecustomersdecidetouseprivateaddresseswithintheirnetworkanddoNATat
theconnectionpointtotheISP.Thissetupmeansthatcustomersrequireonlyavery
smallportionofpublicaddressesfromtheISP.Inthefigure,onlycustomerDMZhas
beenassignedpublicaddresses.Thecustomernetworkisconnectedtothe
customerDMZusingtwoalternatefirewallswithbothfirewallsdoingNAT.
Inthiscase,thecustomerrequiresonlyaverysmallblockofpublicaddresses.
TheseaddressescanbePAaddresses.Ifthecustomerdecidestochangeits
serviceprovider,addressesarerenumberedonlyattheNATpoint.Therestofthe
customernetworkdoesnotneedtoberenumbered.
Dependingonacustomerconnectivitytosingleserviceproviderortomultiple
serviceproviders,theassignedASnumbershouldbeprivateorpublic.
Single-homed customers:
If static routing is used, the AS number is not needed.
If BGP is desired, private AS number (64512 - 65535) can be used.
Multihomed customers:
Public AS number has to be used.
CustomerswhoareconnectedtoasingleserviceproviderusuallydonotuseBGP,
soanASnumberisnotneeded.IfacustomerneedsaBGPtodetectlinkfailures,
theserviceprovidercanassignaprivateASnumbertothecustomer.
CustomerswhoareconnectedtomultipleserviceprovidersuseBGPandthey
shouldbeassignedapublicASnumber.
ASNumberAllocation:Single-HomedCustomers
Customers running BGP with the service provider need their own BGP AS
number.
Private AS numbers (64512-65535) can be used for customers who are
connected to a single service provider.
BGPrequirestheuseofASnumbers.WhenBGPisconfigured,theASnumberis
mandatoryinformation.PublicASnumbersareascarceresource,however.
CustomersshouldusepublicASnumbersonlywhentheyarerequired.Acustomer
thatusesBGPtoexchangeroutinginformationwithonlyoneISPdoesnotrequirea
publicASnumber.ThiscustomercanuseaprivateASnumber.
AnISPnetworkthatisrunningBGPwithsomeofitscustomersmustdetermine
whetherapublicoraprivateASnumberisrequiredforeachcustomer.Whenthe
customercanuseaprivateASnumber,theISPmustallocateonefromtherangeof
privateASnumbers(64512to65535).TheISPmustmakesurenottoassignanyof
theprivateASnumberstomorethanonecustomer.
WhentheISPreceivesBGProutesfromthecustomer,theISProutersseethe
privateASnumberintheASpath.TheISPtreatsthisprivatenumberasanyother
ASnumber.However,beforetheISPpropagatesanyoftheseroutestotherestof
theInternet,itmustremovetheprivateASnumbersfromtheASpath.ThesameAS
numbermaybeinusebysomeoneelse.AftertheprivateASnumberisremoved,
therouteappearsasbelongingtothepublicASoftheISP.
ASNumberAllocation:MultihomedCustomers
Multihomed customers must run BGP with their service providers.
CustomerASNumberAllocation
Multihomed customers must use public AS numbers for their autonomous
systems.
AmultihomedcustomerrequiresapublicASnumberandmustrunBGPwithbothof
itsISPs.ThecustomershouldnotuseaprivateASnumberbecausebothISPsmust
propagatethecustomerroutestotherestoftheInternet.Ifthecustomerdoesusea
privateASnumber,andbothISPsremovethenumberbeforesendingittotherest
oftheInternet,thenthecustomerrouteswillappeartobelocalinthepublicASof
bothISPs.TomakeBGPworkcorrectly,multihomedcustomersneedtoavoidthis
situation.
WiththehelpoftheASnumbertranslationfeature,private
ASnumberscanalsobeusedformultihomedcustomers,
butthistypeofconfigurationisnotencouraged.
MultihomedcustomersarecorrectlyconnectedtotheInternetbyassigningapublic
ASnumbertothecustomernetwork.ThispublicASnumberappearsintheASpath
andserviceprovidershouldpropagateittotherestoftheInternet.Thecustomer
networkisnowreachablebytherestoftheInternetthroughbothproviders.Internet
endpointsusetheroutewiththeshortestASpathasthebestroutetothecustomer
network.
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
Different customers have different requirements, especially for redundancy for
their Internet connections. These connectivity options include:
Single-homed: One or multiple connections to a single ISP.
Multihomed: Multiple connections to different ISPs.
Routing, static or dynamic, that is used between a customer and service
provider depends on the selected connectivity type.
If dynamic routing is required, BGP is the only option.
IP addressing assignment depends on the selected connectivity type:
Single-homed: PA address space.
Multihomed: PI address space.
AS numbering depends on the selected connectivity type:
Single-homed: No AS number, or private AS number
Multihomed: Public AS number.
Summary
Overview
WhenacustomercanconnecttotheInternetthrougheitherasingleconnectiontoa
serviceproviderormultipleconnectionstothesameISP,staticroutingisthe
simplestroutingapproachtoimplementbetweenthecustomerandprovider.When
networkadministratorsareimplementingcustomer-to-providerconnectivitywith
staticroutes,knowledgeofstaticroutingimplementationguidelineswillaidin
successfullydeployingstaticroutingnetworkconfigurations.
Uponcompletingthislesson,youwillbeableto:
Identifywhentousestaticroutingbetweenacustomerandaserviceproviderin
aBGPenvironment
Describethecharacteristicsofstaticroutingbetweenacustomerandaservice
providerinaBGPenvironment
Identifydesignconsiderationsforpropagatingstaticroutesinaserviceprovider
network
ConfigurestaticroutepropagationinaBGPenvironmentwithdifferentservice
levels
Configureatypicalbackupsetupthatusesstaticroutingbetweenacustomer
andaserviceproviderinaBGPenvironment
Describethelimitationsoffloatingstaticrouteswhentheyareusedintypical
backupstaticroutingscenariosandthecorrectiveactionstoovercomethese
limitations
Describethecharacteristicsofloadsharingwhenyouareconfiguringstatic
routingbetweenacustomerandaserviceprovider
ImplementingCustomerConnectivityUsing
StaticRouting
Staticroutingisthebestsolutiontoimplementwhenyoudonothaveredundancyin
thenetworktopology.
Static routing should be used only with single-homed customers in the following
examples:
Customers with a single connection to the Internet.
Customers with multiple connections to the same service provider in
environments where link and equipment failure can be detected.
Dynamic routing with BGP must be used with multihomed customers.
Asingleconnectionbetweenthecustomernetworkandtheserviceprovidernetwork
doesnotprovideanyredundancy.Ifthelinkgoesdown,theconnectionislost
regardlessofwhichroutingprotocolisconfiguredinthecustomerorprovider
network.Whenthereareredundantconnectionsbetweenthecustomernetworkand
thenetworkofasingleserviceprovider,staticroutingcanbeusedunderspecific
circumstances.
TheCErouterthatisusinganIGPmustconditionallyannounceastaticdefault
route.IfthelinktooneoftheCEroutersgoesdown,thentheroutermustbeableto
detectthefailureandinvalidatethestaticdefaultroute.Announcementofthisrouter
asadefaultgatewaythatisusinganIGPmustnowcease.Likewise,onthePE
routers,thestaticroutesthatarepointingtothecustomernetworksmustbe
invalidatedifthelinkbetweenthemgoesdown,andredistributiontoBGPis
thereforestopped.
Iflink-levelprocedurescannotdetectalinkfailure,theinterfaceremainsintheup
state.Thestaticroutesarenotinvalidated,andpacketsareforwardedintoa"black
hole."Sincetheroutercannotdetectafailureatthelinklevelinthesecases,BGP
mustbeusedbetweenthecustomerandtheprovider.
BGPmustalsobeusedbetweenthecustomerandtheserviceprovidernetworks
whenthecustomerismultihomed.Thisistrueregardlessofwhichlinkfailure
detectionmechanismsareinuse.
WhentoUseStaticRouting?
WhenstaticroutingisimplementedbetweenthecustomernetworkandtheISP
network,theCEmustannounceitselfasadefaultgatewayoragatewayoflast
resort.ThisproceduremustbedoneusingtheIGPwithinthecustomernetwork.
Differentrouterswithinthecustomernetworkmustbeabletoselectthebestrouteto
theexitpointofthenetwork.
The customer network must announce a default route:
Redistribute default route into customer IGP if the customer is running
EIGRP.
Use default-information originate if the customer is running OSPF or RIP.
Customer-connected routes should be redistributed into service provider BGP.
Redistribute static routes into BGP, not core IGP.
Specific customer routes should not be advertised to other autonomous
systems:
Mark customer routes with the no-export community when redistributed into
BGP.
Use static route tags for consistent tagging.
DifferentIGPsusedifferentmethodsofannouncingarouterasagatewayoflast
resort.EIGRPusestheconceptofadefaultnetwork,whileOSPFandRIPsend
reachabilityinformationaboutnetwork0.0.0.0/0.Ineithercase,thenetwork
operatorsofthecustomernetworkareresponsibleforconfiguringtheirnetworkto
usetheCErouterasagatewayoflastresort.
Whenstaticroutingisusedbetweenthecustomerandtheprovider,thePEmust
propagateastaticroutethatpointstothecustomernetwork.Itmustpropagateitto
allotherrouterswithintheISPnetwork,andalsototherestoftheInternet.Theroute
isredistributedintoBGP.
CustomerroutesshouldnotberedistributedintotheIGPoftheISPnetwork.Care
shouldbetakenthattheIGPoftheISPnetworkdoesnotcarrytoomanyroutes.
RedistributingcustomerroutesintotheIGPcouldpotentiallycausepoor
performanceandmighteventuallycauseacompleteshutdownofIGProutingatthe
serviceprovider.
ImaginethatacustomerusesPAaddressesandtheISPannouncesalargeblockof
addressesforwhichthenetworkofthiscustomerisonlyasmallportionoftheblock.
Theserviceprovidershouldnotpropagatetheroutesofthiscustomertotherestof
theInternet.TheISPshouldcarefullyredistributeonlythoseconnectedroutesthata
singleIPaddresscustomeruses.TherestoftheInternetshouldreceiveonlyan
announcementcontainingthelargerblockofaddresses,asummaryroute.
AneasywayofachievingthissetupistousecommunitieswithintheISPnetwork.
YoucanmakeindividualroutesineligibleforadvertisingoutsideofthelocalASby
settingthepredefinedBGPno-exportcommunity.ToensurethattheBGP
communitiesarepropagated,atleastoverallIBGPsessions,thenetworkoperators
oftheISPnetworkmustconfigureasend-communityoptionforallIBGPneighbors.
Ifarouterinaprovidernetworkreceivesanupdatecarryingtheno-export
community,therouterwillnotpropagatetheupdatetoanyexternalBGPneighbors.
Meaningtheupdateisnotpropagatedtootherserviceproviders.
Communitiesaresetbyusingroutemaps.Aroutemap
canselectroutesbasedonvariousattributes.Oneof
theseattributesistheroutetag.Throughconfiguration,the
routercanassignaroutetagtospecificstaticroutes.This
optionmeansthatthenetworkoperatorsoftheISP
networkcaninventaschemeoftagging.Forexampleall
staticroutesthatshouldnotbepropagatedtoother
autonomoussystemsareassignedaspecifictag.Thena
routemapcanselectallrouteswiththattagandassign
themtheno-exportcommunity.
CharacteristicsofStaticRoutingExample
Inthefigure,thecustomernetworkisconnectedtotheInternetbyusingasingle
permanentconnectiontoasingleserviceprovider.
CharacteristicsofStaticRouting
Inthiscase,aroutingprotocoldoesnotaddanyredundancyandwouldonlyadd
complexity.
TheconfigurationofCEandPErouterisshownintheexample.Accesslinkisused,
anddirectlyconnectedroutesareredistributedintoBGPonthePErouter.TheCE
routerisconfiguredwithstaticdefaultroute,whichpointstotheserial0interface.If
theserialinterfacegoesdown,theroutebecomesinvalid.Thedefaultrouteisalso
redistributedintoOSPF,whichisusedinthecustomernetwork.Thereforetherouter
announcesadefaultrouteintoOSPFaslongasithasavaliddefaultrouteitself.
ThePErouteralsohasastaticroute,declaringthecustomerIPnetworknumber
209.165.201.0/24asreachableovertheserial0interface.Italsobecomesinvalidif
theinterfacegoesintothedownstate.ThePEroutermustforwardthisinformation
toallotherISProutersandtotherestoftheInternet.Thisactionisaccomplishedby
redistributingthestaticrouteintoBGP.Aslongasthestaticrouteisvalid,theBGP
announcesit.TotherestoftheInternet,thecustomernetworkappearsasreachable
withintheASoftheISP.AsfarastherestoftheInternetisconcerned,thecustomer
isapartoftheserviceproviderAS.
1.
2.
3.
4.
Youcaneasilyextendtheprincipleofusingtagswhenyouareconfiguringstatic
routes.Basedonthosetags,youcanassigndifferentcommunitiestoimplementa
morecomplexroutingpolicy.
Identify all possible combination of services that are offered to a customer,
including QoS services.
Assign a tag to each combination of services.
Configure a route map that matches the defined tags and sets BGP
communities or other BGP attributes.
Redistribute static routes into BGP through a route map.
For each customer, configure a static route toward the customer with the proper
tag.
Topropagatestaticroutesinaserviceprovidernetwork,completethesesteps:
Identifyalldifferentservicelevelsthatareofferedtocustomersandthenallthe
differentcombinationsoftheseservicelevels.
Assigneachcombinationitsowntagvalueanditsowncommunity.
Configurearoutemap,whichselectsrouteswitheachoftheassignedtagsand
setsthecorrespondingcommunityvalue.Becausetheprocessingofaroute
mapstopswhenthematchclauseofastatementismet,eachrouteshouldbe
assignedasinglecombinationofcommunitiesonly.Therefore,youmusttake
greatcaretoassignatagandacommunitycombinationtoeachcombinationof
servicesthatareprovided.
WhenthePEroutersredistributestaticroutesintoBGP,
theseroutesmustpassthroughtheroutemap.BGP
assignsthecorrectcommunitydependingonthetag
valuesthataregivenontheconfigurationlineforeachof
thestaticroutes.
Finally,configurestaticroutes.Beforeyouconfigureastaticrouteforaspecific
customer,youmustidentifythecombinationoftheservicesthatareprovidedto
thiscustomer.Thenyoumustlookupthecorrespondingtagvalue.Afteryou
haveconfiguredtheroute,youmustassignthetag.
Withthisroutingpolicy,everystaticroutetoacustomernetworkisassignedatag
andtheredistributedBGProuteisassignedacorrespondingcommunity.TheBGP
communitiesthatareattachedtotheroutessignaltootherroutersintheISP
networkwhichparticularservicecombinationyoushoulduse.
StaticRoutePropagationExample
Ascenariowithvariedservicelevelsisshown.Staticroutepropagationisconfigured
inaBGPenvironment.
Sample service offering:
Addressing:
PA address blocks are not propagated to upstream ISPs.
PI address blocks are propagated to upstream ISP.
Quality of service:
Normal customers.
Gold customers.
Define static route tags.
Advertise
CustomerRoute
QoSType Route
Tag
Community
Values
No Normal 1000 no-export300:31000
Yes Normal 1001 300:31000
No Gold 2000 no-export300:32000
Yes Gold 2001 300:32000
Inthisscenario,theserviceproviderofferstwodifferentservicelevelstoits
customers:NormalandGold.CustomersarealsoassignedIPaddressblocks.
DesigningStaticRoutePropagationinaService
ProviderNetwork
SomecustomershavePAaddresses,whichtheISPdoesnotannounceasexplicit
routes.ThelargeroutesummaryblockthattheISPannouncescoversthese
customers.OthercustomersusePIaddressesthatmustbeexplicitlyannouncedto
theInternetbytheserviceprovider.
TherearetwodifferentQoSservices,NormalandGold,andtherearebothPAand
PIaddresses.Sothetotalnumberofcombinationstocoverthenetworkpolicyis
four:
NormalQoSroutesthattheISPassignsandshouldnotbeexplicitlyannounced.
NormalQoSroutesthatarePIroutesandshouldbeexplicitlyannounced.
GoldQoSroutesthattheISPassignsandshouldnotbeexplicitlyannounced.
GoldQoSroutesthatarePIroutesandshouldbeexplicitlyannounced.
Eachofthesefourcombinationsreceivesitsowntagvalueanditsowncommunity
combination.
NetworkoperatorsconfigurearoutemapinthePE1routerthathasthestaticroutes
tothecustomernetwork.Redistributionoftheconfiguredcustomerstaticroutesinto
BGPisalsoperformedatthePE1router.
Aroutemapcanmatchanindividualrouteinasingleroute-mapstatementonly.A
singletagvalue,representingeachcombinationofservicesmustbeassignedtothe
staticroutesbytherouter.Whenarouteismatched,theinterpretationoftheroute
mapforthatindividualroutestops.Theroutemaphasonestatementforeach
combination,andeachstatementmatchesatagvalueandassignsthe
correspondingcommunitycombinationforthattag.
TheroutemapisappliedduringtheredistributionofcustomerstaticroutesintoBGP
atthePErouter.Becausetheroutemaphasno"permitany"statementattheend,
thestaticroutesthatarenotassignedanyofthetagsbeingusedarenot
redistributed.Theroutemapfilterstheseroutesout,forcingthenetworkoperatorsto
makeatagassignmenttoallcustomerroutes.Furthermore,theroutemapfiltering
canhelptheadministratorcatchconfigurationentryerrors,thusgivingallcustomers
theservicecombinationthattheyareentitledto.
ThefigureshowshowthePE1routerusestheroutemapnamed"IntoBGP"when
redistributingstaticroutesintoBGP.Becausetheroutemapassignscommunity
valuesthatotherrouterswithintheISPnetworkwilluse,networkoperatorsmust
configureallIBGPneighborswiththesend-communityqualifier.
Whenconnectingcustomers,thenetworkoperatorsidentifywhichservice
combinationtouseforthisparticularcustomer.Thethreeservicesthatare
associatedwiththisparticularcustomerareasfollows:
ApplynormalQoS.
UseaPAnetworknumber.
Donotenabletheprovidertoexplicitlyannouncecustomerroutes.
Astaticroutetothecustomerisconfiguredandassignedtheappropriatetagvalue
of1000,whichrepresentsthespecifiedservicesthatareassignedtothecustomer.
Theshowiproutecommanddisplaysinformationfromtheroutingtableabout
subnet209.165.201.0/24.Therouteislearnedbystaticconfigurationandis
redistributedviaBGP.Therouterhasassignedatagvalueof1000tothecustomer
routebyusingastaticallyassignedtag.Theroutemustpassthroughtheroutemap
intoBGPbeforebeinginsertedintotheBGPtable.
TheshowipbgpcommanddisplaysinformationfromtheBGPtableaboutsubnet
209.165.201.0/24.TherouteislocalwithinthisASandthisroutersourcedit.The
BGPcommunities300:31000andno-exporthavebeenassignedbytherouterto
theredistributedcustomerroute.Theywereassignedbyusingtheprovider-defined
routemapbeforeinsertingthecustomerrouteintotheBGPtable.
Imagineacasewherethecustomernetworkhastwoconnectionstoasingleservice
provider,usingtworoutersoneachside.Oneconnectionbetweenthecustomer
networkandtheISPistheprimaryconnection,andtheotherconnectionisusedfor
backuppurposesonly.Iflink-levelprocedurescandetectlinkfailuresandafailurein
theremoterouter,thenstaticroutingcanbeusedbetweenthecustomerand
providernetworks.
Default routes on the CE router and redistribution into customer IGP:
Floating static routes are configured on the backup router.
Static route on the PE router and redistribution into service provider BGP:
Floating static routes are configured on the backup router.
Separately tag primary and backup static routes and match them for
redistribution.
Asinthepreviousexample,wherenobackuplinkisavailable,theCE1routerhasa
staticdefaultroutetowardtheISP.ThePE1routerhasstaticroutestowardthe
customer.ThecustomerrouterredistributesthestaticdefaultrouteintoitsIGP.The
ISProuterredistributesthestaticroutesintoBGP.
Iftheprimarylinkgoesdown,thelink-levelproceduressettheinterfacetothedown
state.Thiscausesthestaticroutespointingoutthroughtheinterfacetobeinvalid
andremovingtheroutesfromtheroutingtable.Whentheinterfacechangesbackto
theupstate,thestaticroutewillreappearintheroutingtable.
Theappearanceoftherouteintheroutingtableconditionstheredistributionof
routesintoanyroutingprotocol.Thus,iftheinterfacegoesdown,therouterremoves
thestaticroutefromitsroutingtable,andtherouteiswithdrawnfromtherouting
protocol.Whenthestaticroutereappears,theredistributionprocessinsertsitinto
theroutingprotocolagain.
TheCE2routeralsousesadefaultstaticroutetowardtheISPviathebackuplink.
TheCE2routerisalsoredistributingthedefaultrouteintotheIGP.However,the
staticroutethatisusedisafloatingstaticroute,whichisassignedahighAD—
higherthantheadministrativedistanceofthecustomerIGP.Aslongastheprimary
linkworks,theIGPprovidestheCE2routerwiththeprimarydefaultroute.Because
ofthehigherAD,thebackupstaticdefaultrouteisnotinstalledintotheCE2routing
table.Becausethestaticrouteisnotintheroutingtable,itisnotredistributed.Ifthe
primarylinkfails,theIGPnolongerfeedstheCE2routerwithadefaultroute.The
backupstaticdefaultrouteistheonlyremainingdefaultroute.Therefore,therouter
willinstallthefloatingdefaultrouteintoitsroutingtableandthenredistributeitinto
theIGP.
ThePE2routercanalsousefloatingstaticroutes,whichareredistributedintothe
ISPBGPprocess.
BGPBackupwithStaticRoutes
Inthefigure,thecustomernetworkandtheISPnetworkareconnectedusingleased
lines.Both,CE1andCE2,haveastaticdefaultroutetowardtheserialinterface
leadingtotheISP.Bothroutersalsodoredistributionofthedefaultrouteintothe
OSPFprotocol,whichisbeingusedasanIGPwithinthecustomernetwork.
However,thestaticdefaultrouteintheCE2routerisconfiguredwithanADvalueset
to250.ThisADvalueishigherthantheADvaluesofanyroutingprotocol.This
configurationmeansthataslongastheCE2receivesthedefaultroutebyOSPF,the
staticdefaultrouteisnotused.
Whentheprimarylinkgoesdown,thestaticdefaultrouteintheprimaryrouterisnot
valid.TheOSPFprotocolstopsannouncingthedefaultroute,becausethedefault-
informationoriginatecommandmakesOSPFcontingentontheavailabilityofthat
staticdefaultrouteintheroutingtablebeforeannouncement.
TheCE2nowinstallsitsstaticdefaultrouteintheroutingtable.Theconditionsfor
announcingthedefaultroutebyOSPFaremetandtherestofthecustomerrouters
seetheCE2asthegatewayoflastresort.
WhenfloatingstaticroutesareconfiguredonthePErouters,theyarealso
redistributedintoBGP.Thisconfigurationmakesthingsalittlebitmorecomplicated.
Thenetworkoperatorconfiguresafloatingstaticroutetothecustomersubnet
209.165.201.0/24.InthePE2router,thefloatingstaticrouteisassignedthesame
tagvalueasthetagvaluebeingusedinthePE1router.Theroutemap"IntoBGP"is
thesameasinthePE1routerandprovidestheroutestothecustomernetworkwith
thesamecommunities.ThesecommunitiesarebasedonthesameQoSleveland
theindicationwhethertoexplicitlyannouncerouteprefixtotherestoftheInternet.
Thefloatingstaticroutehastheadministrativedistancesetto250.Thisvalueis
higherthananyroutingprotocol.WhenthePE2routernolongerreceivesany
routingprotocolinformationaboutthecustomernetworks,therouterwill
automaticallyinstallthefloatingstaticrouteandthenredistributeitintoBGP.
BasedonBGProuteselectionrules,theredistributedfloatingstaticroutewillalways
remainthepreferredpathifanextraBGPconfigurationisnotperformedonthePE
router.Thispreferencemeansthatregardlessofwhethertheprimarylinkcomes
back,thePE2selectsthelocallysourcedrouteasthebestroute.Therefore,thePE2
continuestoannounceapathtowardthecustomernetwork.Thebackuplinkdoes
notgobacktotheIdlestate.
PE2# show ip bgp 209.165.201.0
BGP routing table entry for 209.165.201.0/24, version 7
Paths: (2 available, best #1, table default, not advertised to EBGP peer)
Advertised to non peer-group peers:
10.3.0.5
Local
0.0.0.0 from 0.0.0.0 (10.0.3.6)
Origin incomplete, metric 0, localpref 100, weight 32768, valid, sou
rced, best
Community: 300:31000 no-export
Local
10.0.3.2 (metric 128) from 10.0.3.5 (1.0.0.2)
Origin incomplete, metric 0, localpref 100, valid, internal
Orginator: 1.0.0.2, Cluster list: 10.0.3.5
Community: 300:31000 no-export
The BGP table on the service provider backup router contains the floating static
route.
Inthisexample,theshowipbgpcommandisusedinthePE2routeroftheprovider
todisplaytheinformationaboutthecustomernetwork209.165.201.0/24.The
primarylinkhascomeback,sothePE2nowseestwoalternateroutes.Thefirst
routeistheroutethattherouteritselfhasredistributedintoBGPusingthefloating
staticroute.ThisrouteislocallysourcedbythisASandhasbeenassignedaweight
valueof32768.ThesecondroutePE2receivedbyIBGPfromthePE1router.This
ASalsosourcesthisroute,butnoweightvalueisassigned.
TheBGProuteselectionalgorithmselectstheroutewithweightvalue32768asthe
best.Asaresult,theroutethatwasreceivedfromthePE1routerisnotacandidate
tobeinstalledintheroutingtable.Itnevercompeteswiththefloatingstaticroute.
Thefloatingstaticroutestaysintheroutingtable,andredistributionoftheroute
continuesuntilthebackuplinkgoesdownandtheroutebecomesinvalid.
Unfortunately,floatingstaticroutesdonotworkcorrectlywithBGP.Aftertheyare
inserted,thefloatingstaticrouteisneverremovedfromtheroutingtableevenifthe
primarylinkcomesback.
Limitations and corrections:
Floating static routes do not work correctly with BGP.
Weight has to be lowered to default value for other BGP routes to be
considered.
BGP local preference has to be changed for floating static routes that are
redistributed into BGP, to make sure that other routes take precedence.
Administrative distance cannot be matched with a route map; extra tags need to
be defined for static routes.
WheneveryouusefloatingstaticroutesincombinationwithredistributionintoBGP,
youwillneedtotakeextraconfigurationsteps.YouhavetoensurethattheBGP
routeselectionalgorithmselectstheprimaryrouteasthebestBGProutewhenit
reappears:
WhenarouterredistributesafloatingstaticrouteintoBGP,theweightvaluethat
isassignedtothefloatingstaticroutemustbereduced(forexamplesetto0).
Otherwise,thefloatingstaticroutewillalwaysbeselectedasthebestBGProute
afterthefirstfailureoftheprimarylinkoccurs.
Theroutermustalsoassignlocalpreferencevaluestothefloatingstaticroute,
sothatthefloatingstaticroutehasalowerlocalpreferencethantheprimary
route(forexample50).Thisassignmentensuresthattheprimaryrouteis
selectedasthebestBGProuteonotherrouterswithinProvidernetwork.
ThesetworequirementsmustbespecifiedonthePErouterintheroutemap
"IntoBGP"thatisusedfortheredistribution.Theroutemapmustselectthefloating
staticroutesandsetweightandlocalpreference.However,aroutemapcannotdo
matchingbasedontheADvaluethathasbeenassignedtoastaticroute.Some
othermeansarerequiredfortheroutemaptodistinguishbetweennormalstatic
routeswithnormalweightandlocalpreferenceandthefloatingstaticoneswith
modifiedvalues.
Thesolutionistocreateextratagvaluesforthissetofstaticroutes.Thetagvalue
mustnotonlyreflecttheQoSlevelandwhethertoannouncetheroute.Thetag
valuemustalsoindicateifitisaprimaryrouteorabackuproute.
Advertise
Customer
Route
Backup QoSType Tag Community
Values
LocalPreference
Normal 1000 no-export
300:31000
100
Yes Normal 1010 no-export
300:31000
50
Yes Normal 1001 300:31000 100
Yes Yes Normal 1011 300:31000 50
Gold 2000 no-export
300:32000
100
Yes Gold 2010 no-export
300:32000
50
Yes Gold 2001 300:32000 100
Yes Yes Gold 2011 300:32000 50
Eighttagvalueshavecurrentlybeenidentified.Eachtagvalueindicatesaspecific
combinationofexplicitroutepropagation(backuporprimary)andQoSlevel.
WhennetworkoperatorsconfigurestaticroutesinthePErouter,theymustconsider
whichofthecombinationsthattheyshouldusefortheroute.Theroutemapthat
theyusewhenredistributingthestaticroutesintoBGPmustbeconfiguredto
recognizealleightcombinations.Itmustsettheappropriateweightandcommunity
andlocalpreferencevaluestothesecombinations.
FloatingStaticRouteswithBGP
Theconfigurationoutputinthefiguredisplaysthefirsthalfofthe"IntoBGP"route
map.Theoutputshowshowmatchclausesidentifyfouroftheeightdifferenttags.
Foreachofthetagvalues,theroutemapsetsthecommunity,thelocalpreference,
and,insomecases,theweight.
ThedisplayedhalfoftheroutemapdealsonlywiththefourtagsthatindicateQoS
Normal.SoallstatementsintheconfigurationdisplayhavesettheBGPcommunity
attributeto300:31000.
Thesecondpartoftheroutemapthatisshowndealswiththefourtagsthatindicate
QoSGold,whichareconfiguredtosettheBGPcommunityattributeto300:32000.
Tagvaluesof1000,1010,2000,and2010indicatethattherouteshouldnotbe
explicitlypropagated.Theroutesthataprovidershouldnotexplicitlyadvertisetothe
restoftheInternetareassignedtheno-exportcommunitybytheroutemap.
Tagvalues1010,1011,2010,and2011allindicatethattherouteisabackuproute.
Thosetagshavetheirweightvaluesetto0andtheirlocalpreferencevaluesetto
50.Thesesettingsensurethatonthereturnofafailedprimaryroute,thePErouter
willselecttheprimaryrouteasitsbestpathandremovethebackupfloatingstatic
routefromitsroutetable.
Acustomerthathastwoconnectionstoaserviceprovidercanalsodecideto
balancetrafficoverbothlinks.Ifbothlinksareactive,trafficshouldbebalanced
acrossbothlinks.Ifthereislinkfailure,onelinkshouldcareforalltraffic.
OutgoingTrafficLoadBalancing
Outgoing traffic load balancing:
On each CE router, the default route is redistributed into IGP.
Each CE router uses the closest CE router as the exit point.
Balanced load sharing is achieved if the CE routers are collocated.
Loadsharingofoutgoingcustomertrafficisaccomplishedbyconfiguringastandard
defaultstaticrouteinbothCErouters.Eachstaticrouteisvalidaslongastheserial
linkfromtheCEroutertoPErouterisup.Whenbothstaticroutesarevalid,bothCE
routersannouncethedefaultrouteintothecustomernetwork.
Theremainingroutersinthecustomernetworkseetwocandidategatewaysoflast
resort.Theseremainingrouterschoosetheclosestone,withrespecttotheIGP
metric.Thepartofthenetworkthatisclosertotheuppermostexitpointusesthat
exitpointforalloutgoingtraffic.Theotherpartofthenetworkusestheother(lower)
exitpoint.
Ifbothexitpointsarecollocated,theyareequallydistantfromeachoftheother
routersinthecustomernetwork.Eachrouterwithinthecustomernetworktherefore
usesloadsharingoftrafficsentoutofbothexitpoints.
ReturnTrafficLoadBalancing
Load sharing of return traffic is impossible to achieve with multiple edge routers.
All provider routers select the same BGP route to the destination.
All return traffic arrives at the same PE router.
Theserviceproviderroutersknowabouttworoutestothecustomernetwork.Those
routesarelearnedthroughBGP.BGPwasdesignedtoselectonlyoneroute,which
isthebestroutefromacollectionofroutestodestination,allowingnoloadsharing.
ThePEroutersthatreceivethesameBGProutefromtwoCErouterswillalways
selectthecloserCErouter.IfallotherBGPattributesareequal,theIBGProutewith
theclosernexthopisselected.ThepartoftheISPnetworkthatisclosertothe
uppermostconnectionusesthatconnection.TheotherpartoftheISPnetworkuses
theother(lower)connection.
Ifbothconnectionpointsarecollocated,allPEroutersselectthesameIBGProute
basedonrouter-ID(becausetheIGPmetricsarealwaysequal).Soallthereturn
trafficissentoverasinglelinktowardthecustomernetwork,resultinginnoload
sharing.
LoadSharingwithStaticRoutes
SinceCiscoIOSSoftwareRelease12.2,theIBGP
multipathload-sharingfeatureenablestheBGP-speaking
routertoselectmultipleIBGPpathsasthebestpathstoa
destination.Thebestpathsormultipathsaretheninstalled
intheIProutingtableoftherouter.
You can optimize return traffic load sharing:
Each PE router advertises only part of the customer address space into the
provider backbone.
Every PE router also advertises the whole customer address space for backup
purposes.
Load sharing is not optimal—every link will carry return traffic for part of the
customer address space.
Toobtainbettercontrolofthereturntrafficload,thecustomeraddressspacemust
beadvertisedtothePEroutersusingmultiple,moreexplicitroutes.Theupperedge
routercouldadvertisehalftheaddressspace,andtheloweredgeroutercould
advertisetheotherhalf.Forbackupreasons,theyalsoshouldbothadvertisethe
entireaddressspaceasalargerroutesummary.
Aslongasbothpathsareavailable,thetrafficfromtheISPtothecustomerusesthe
mostexplicitroute.Inthiscase,twoexplicitroutesareusedtosendthetraffic
representingonehalfoftheaddressspaceoveronelinkandtrafficrepresentingthe
otherhalfoftheaddressspaceovertheotherlink.
Loadsharinginthiswaydoesnotresultinanequalloadonthelinksbutrathera
statisticallybaseddistributionofthetrafficloadoverthelinks.
LoadSharingwithStaticRoutesExample
Youcanachieveloadbalancingoftheincomingtrafficbyadvertisingdifferentparts
ofthecustomernetworkbydifferentPErouters.TheupperPEroutercouldadvertise
one-halfoftheaddressspace,andthelowerPEroutercouldadvertisetheother
half.Forbackupreasons,thePEroutersalsoshouldadvertisetheentireaddress
spaceasalargerroutesummary.
Intheexample,thecustomeraddressspace209.165.201.0/24ispartitionedintotwo
smallerblocks:209.165.201.0/25and209.165.201.128/25.
ThePE1routeradvertisestherouteto209.165.201.0/25,andthePE2router
advertisestherouteto209.165.201.128/25.BothPEroutersalsoadvertisethe
entireaddressspace209.165.201.0/24.
Aslongasbothpathsareavailable,thetrafficfromserviceprovidertothecustomer
usesthemostexplicitroute.Twoexplicitroutesareusedtosendtraffic.Trafficto
destinationsinthe209.165.201.0/25rangeissenttotheupperconnectionspoint.
Traffictodestinationinthe209.165.201.128/25rangeisdirectedtothelower
connectionpoint.
Loadsharinginthiswaydoesnotresultinanequalload
onthelinksbutratherastatisticallybaseddistributionof
thetrafficloadoverthelinks.
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
Static routing can be used for:
Customers with single connection to service provider.
Customers with multiple connections to service provider—primary and
backup path, or load balancing.
In static routing, the customer network must announce a default route.
Customer routes should be carried in BGP, not core IGP; and routes to subnets
of the provider address block should not be propagated to other ASs.
When you are using static routes in a backup scenario, floating static routes are
used on the backup routers.
Depending on the origin of the customer address space, the provider may elect
not to advertise the customer space, choosing to advertise a larger aggregate
route instead.
Summary
Overview
Whenmultipleconnectionstothesameserviceprovideraretheonlymeansthata
customerhasofconnectingtotheInternet,itisimportantthattheconnectionsare
correctlyconfigured.Theymustensureproperinteractionbetweenthecustomer
andtheserviceprovidernetwork.Itisalsoimportanttounderstandhowtoconfigure
routingprotocolssothatcustomerbackuporload-balancingrequirementsaremet.
Inthislesson,youwilllearnabouttheuseofmultipleconnectionsbetweena
customerandasingleISPforbackupandload-sharingpurposes.
Uponcompletingthislesson,youwillbeableto:
ConfigureBGPonacustomernetworktoestablishroutingbetweena
multihomedcustomerandasingleserviceprovider
Configureconditionaladvertisingofacustomeraddressspacewhenyouare
usingBGPtoestablishroutingbetweenamultihomedcustomerandasingle
serviceprovider
ConfigureBGPonaserviceprovidernetworktoestablishroutingbetweena
multihomedcustomerandasingleserviceprovider
DisablethepropagationofprivateASnumberstoEBGPpeersinaservice
providernetworkwhereamultihomedcustomerisadvertisingprivatenumbers
intheASpath
DescribetheBGPSupportforDual-ASConfigurationforNetworkASMigrations
feature
Configureatypicalbackupsetupbetweenamultihomedcustomerandasingle
serviceproviderinaBGPenvironment
Describehowyoucanimplementloadsharingbetweenamultihomedcustomer
andasingleserviceproviderinaBGPenvironment
IdentifytheCiscoIOScommandthatisrequiredtoconfigureloadsharing
betweenamultihomedcustomerandasingleserviceproviderusingBGP
multipath
Configureloadsharingbetweenamultihomedcustomerandasingleservice
providerusingEBGPmultihop
ConnectingaCustomertoaSingleService
Provider
Intheexample,thecustomernetworkisconnectedtoaserviceprovidernetworkby
usingmultiplepermanentlinks.BGPisusedtoexchangeroutinginformation
betweenthecustomerandtheprovider.
BGP is run between the customer and the service provider.
The customer advertises allocated address space into BGP. The customer can
use a private AS number.
The service provider advertises only the default route to the customer. CE
routers advertise the default route to the customer network by using IGP.
The service provider has to deploy inbound BGP filters.
SelectingBGPastheroutingprotocolbetweenthecustomerandtheprovider
networkensuresthatalinkfailureorthefailureofaremoterouterisdetected.Inthis
scenario,thecustomerdoesnotrequiretheuseofapublicASnumberorfull
Internetrouting.Instead,aprivateASnumberisassignedtothecustomernetwork,
andtheISPsendsadefaultroutetothecustomerthroughBGP.
Thereisasignificantdifferenceinthiscase,ascomparedtoanetworkscenario
wherestaticroutesandredistributionareused.RouterswithintheprivateASofthe
customernowadvertisethecustomerroutesviaBGP.Thus,thecustomeris
responsibleforannouncingitsownaddressspace.TheISPreceivesroutesfromthe
customerandconditionallypropagatesthem(similartostaticrouting).Ifthe
customerusesPAaddressspaceandtheISPcansummarizetheaddressspace,it
willnotpropagatetheexplicitroutesfromthecustomertotheInternet.Theprivate
ASnumberintheASpathattributemustberemovedbeforetheISPcanpropagate
anyofthecustomerroutes.
ThecustomerisnowcreatingBGProutesthattheISPreceives.Soanyerrorthat
thecustomermakescaninfluenceroutingoperationwithintheprovidernetworkand
ifpropagated,withintheInternetasawhole.Announcingaroutetoanetworkto
whichthecustomerhasnotbeenassignedmaycauseroutingproblems.Thereis
alwaysariskthatsuchroutingproblemscanoccurinaserviceprovidernetwork.
However,theriskismuchgreaterwhenthecustomer,whosenetworkadministrators
usuallyhavelessexperiencewithBGP,enterstheconfiguration.
Toreducetheriskoferroneousrouteadvertising,theISPshouldalwaysfilterany
BGPinformationthatcomesfromthecustomernetwork.TheISPshouldreject
routestonetworksthatarenotexpectedtobeinthecustomerAS.Routesthat
containanASpathwithunexpectedASnumbersshouldalsoberejected.
BGPConfigurationonMultihomedCustomerRoutersExample
Inthefigure,thecustomerhasbeenassignedtheprivateASnumber65001.
CE1(config)# ip route 209.165.201.0 255.255.255.0 null 0
CE1(config)# router bgp 65001
CE1(config-router)# neighbor 209.165.201.2 remote-as 65001
CE1(config-router)# neighbor 172.16.33.33 remote-as 300
CE1(config-router)# network 209.165.201.0 mask 255.255.255.0
CE1(config-router)# exit
CE1(config)# router ospf 1
BGPConfigurationonCustomerRouters
CE1(config-router)# default-information originate
The customer address space is advertised on every CE router.
CE routers run IBGP between themselves and advertise the default route to the
rest of the customer network.
BothCEroutersareconfiguredtorunBGPandshouldadvertiseallofthecustomer
networkswiththenetworkcommand.Ifonlyonerouteradvertisesthenetwork,a
singlepointoffailurehasbeenintroduced.Thetwocustomeredgeroutersmustalso
runIBGPbetweenthemtomakecommondecisionsregardingBGProuting
information.
EachCErouterhasanEBGPsessionwiththePErouterontheothersideofthe
link.OverthatEBGPsession,theISPannouncesonlyadefaultroutetothe
customerAS.WhenEBGPreceivesthedefaultroute,itinstallsitintheroutingtable
andredistributesitintotheIGP,inthiscaseOSPF,ofthecustomer.
Thecustomershouldannounceaddressesaslargeaspossible(thelargerthe
addressspacethatcanbeaggregated,thebetter).TheBGPadvertisementis
configuredontheCEroutersusingthenetworkcommand.Theappearanceofa
correspondingnetworkorsubnetintheroutingtableoftheedgerouterconditions
therouteadvertisement.Becausethereisachancethatthecustomersubnettedthe
addressspaceintosmallersubnets,thecompletecustomeraddressspacewillnot
bepresentedintheCEroutingtable.Thesolutionistomanuallyentertherouteinto
theroutingtablebyconfiguringastaticroutetonull0.However,thestaticrouteis
alwayspresentintheroutingtable.AndtheBGPadvertisementisalwaysperformed
fromtheCErouter,eveniftheCEroutercannotaccesstherestofthecustomer
network.
The CE router should announce the customer address space into BGP.
The CE router should advertise the customer space only if the customer network
is reachable from the CE router (conditional advertising).
The CE router should stop advertising the customer address space if the CE
router loses connectivity with the customer core.
The reachability of the customer core network is tested using a static route. The
static route should point to the core network next hop that is learned via IGP, not
to null 0.
IftheCErouterlosesconnectivitytotherestofthecustomernetworkbutisstill
connectedtotheISPnetwork,theBGPadvertisementmustcease.Inthiscase,
BGPadvertisementcanbestoppedifBGPadvertisementsareboundtothe
reachabilitystatusofaspecificsubnetinthecoreofthecustomernetwork,
accordingtothecustomerIGP.
Theproblemwithusingastaticroutetonull0isthatitconditionsthenetwork
statementintheBGPconfigurationsothatBGPalwaysadvertisestheroute.Ifthe
CErouterlosesconnectivitywiththerestofthecustomernetwork,therouter
continuestoadvertisetheentirecustomeraddressspace.TheISPnetworkreceives
avalidroutefromtheCErouter.Trafficissenttothisrouter,butbecausetherouter
haslostconnectivitywiththerestofthenetwork,thetrafficisdropped.Orthetraffic
isroutedtothenull0interfaceusingthestaticroute.
Thesolutionistocreateastaticrouteforthecustomer-assignedaddresses,which
pointstoanexthopthatwaslearnedthroughanIGP.Thestaticroutewillbeinthe
routingtableonlyifthenexthopisreachable.Thestaticroutewouldbevalidandthe
routewouldbeadvertisedthroughBGP.IftheCErouterlosesconnectivityfromthe
restofthecustomernetwork,thenexthopbecomesunavailableandthestaticroute
isnotvalidanymore.Becausethecustomerrouteisnotpresentintheroutingtable
anymore,BGPwillceasetoadvertisethecustomerroutetotheISP.Thecustomer
networkwillthenbeaccessibleoverthebackupconnection,ifavailable.
ConditionalBGPAdvertisinginMultihomedCustomer
NetworksExample
Inthisexample,thecustomernetworkalsousestheaddressspace13.5.0.0/16.
ConditionalBGPAdvertisinginCustomerNetworks
CE2(config)# ip route 13.5.0.0 255.255.0.0 13.5.1.1
CE2(config)# router bgp 65001
CE2(config-router)# neighbor 209.165.201.1 remote-as 65001
CE2(config-router)# neighbor 172.16.33.33 remote-as 300
CE2(config-router)# network 13.5.0.0 mask 255.255.0.0
CE2(config-router)# exit
CE2(config)# router ospf 1
CE2(config-router)# default-information originate
Theaddressspaceisfurthersubnettedatthecustomersite.Oneofthesubnets
(subnet13.5.1.0/24)isidentifiedasbeingthecentralpartofthecustomercore
network.
TheCEroutersparticipateintheIGProutingofthecustomer.Thisparticipation
meansthattheseroutershaveinformationaboutwhichsubnetswithintheaddress
space13.5.0.0/16arecurrentlyreachable.Ifthesesubnetsareavailable,thereisan
explicitroutetoeachofthem.Ifanyofthesubnetsgodown,orifthepathtoward
themgoesdown,theroutetothatsubnetisremovedfromtheroutingtable.
TheBGPadvertisementineachoftheCEroutersisconfiguredtoadvertisethefull
addressspacethatthecustomeruses.WhentheCEroutersadvertisethisroute,the
ISPnetwork,andtherestoftheInternet,seecustomer'scompleteaddressspaceas
onesingleroute,13.5.0.0/16.
Theappearanceofthestaticroute,IProute13.5.0.0255.255.0.013.5.1.1conditions
theadvertisementofthecustomeraddressspacebyBGP.Ifthestaticrouteisvalid,
thentheBGProute13.5.0.0/16isadvertised.Thestaticrouteisarecursiveroute—
theroutertakesanotherlookintheroutingtablefortheaddress13.5.1.1before
determiningwhattodowiththestaticroute.Theideaisthat13.5.1.1isreachablevia
theIGP.TheIGPannouncesthesubnet13.5.1.0/24.Ifthissubnetisreachableby
theedgerouter,thenthestaticrouteto13.5.0.0/16isvalid.Ifthereisnorouteto
13.5.1.1,thenthestaticrouteisinvalid.
Thecondition,whetherornottoadvertisetheentire
customeraddressspace13.5.0.0/16,iscontrolledbythe
IGPreachabilityofasinglesubnet,13.5.1.0/24.
TheIGPconfigurationalsoincludesoriginationofthedefaultroutebybothCE
routers.
IntheISPnetwork,thetwoPEroutersmusthaveBGPsessionsconfiguredwiththe
customer.ThereisnopointinfeedingthefullInternetroutingtabletothecustomer.
Thetablecontainsthesamesetofroutesforbothlinksandthecustomeralways
usestheISPforalltraffictowardtheInternet.Injectionofadefaultrouteinthe
customernetworkwouldaccomplishthesametask.
The service provider must:
(Conditionally) Advertise a default route to the customer through BGP.
Filter incoming BGP updates with a prefix list to verify that the customer
announces only the assigned address space
Filter incoming BGP updates with an AS path filter list to verify that the customer
uses only its own AS number
Optionally, the no-export community should be set on customer routes.
Thecustomerisresponsibleforitsownadvertisements.Becausecustomersare
muchlesslikelytobeexperiencedinBGPconfigurationthantheISP,theyaremore
likelytomakeerrors.Therefore,theISPmustprotectitselfandtherestofthe
Internetfromthoseerrors.
Theserviceprovidershoulduseafilterthatallowsonlycustomer-assignedroutes
anddeniesanyotherroute.Thisfilterensuresthatprivateaddressspaceorany
otherillegalnetworksthatthecustomererroneouslyannounces,neverreachtheISP
BGPtable.FilteringbasedontheASpathalsoprovidessomeprotectionfrom
customerconfigurationerrors.OnlyroutesthatoriginatedwithinthecustomerAS
shouldbeallowed.Afilterlistperformsthischeck.
IfthecustomeraddressspaceisPAaddressspaceanditrepresentsonlyasmall
partofalargerblockthattheISPannounces,theexplicitBGProutesthatare
receivedfromthecustomerdonotneedtobeadvertisedtotherestoftheInternet.
TheISPcanannouncethelargeblock,attractinganytraffictowardanysubnetwithin
theblock.AfterthetrafficenterstheISPnetwork,themoreexplicitroutestothe
customernetworkareavailableandused.Inthiscase,thePEroutercantagthe
BGProutesthatarereceivedfromthecustomerwiththeno-exportwell-known
community.ThiscommunitypreventstheISPfromsendingtheroutestoanyother
AS.
router(config-router)# neighbor ip-address default-originate
By default, the default route (0.0.0.0/0) is not advertised in outgoing BGP
updates.
The neighbor default-originate command advertises the default route to a
BGP neighbor even if the default route is not present in the BGP table.
ThedefaultrouteisnotsentthroughtheoutboundBGP
filters(prefixlist,filterlist,orroutemap).
Thedefaultroute,0.0.0.0/0,isnotadvertisedinoutgoingBGPupdatesunlessitis
explicitlyconfigured.Theneighbordefault-originaterouterconfigurationcommand
isusedtoinitiatetheadvertisementofthedefaultroutetoaneighbor.
BGPdoesnocheckingbeforethedefaultrouteisadvertised.Thedefaultroutedoes
notneedtobepresentintheBGPtablebeforeitisadvertisedusingthiscommand.
Thedefaultrouteisalsosentwithoutbeingfilteredbyanyoutgoingprefixlists,filter-
lists,orroutemaps.
BGPConfigurationonServiceProviderRoutersExample
BGPConfigurationonServiceProviderRouters
ThisexampleshowstheconfigurationofthePErouter.
PE1(config)# router bgp 300
PE1(config-router)# neighbor 172.16.33.3 remote-as 65001
PE1(config-router)# neighbor 172.16.33.3 default-originate
PE1(config-router)# neighbor 172.16.33.3 prefix-list DefaultOnly out
PE1(config-router)# neighbor 172.16.33.3 prefix-list Customer1 in
PE1(config-router)# neighbor 172.16.33.3 filer-list 15 in
PE1(config-router)# neighbor 172.16.33.3 route-map AllCustomerIn in
PE1(config-router)# exit
PE1(config)# ip as-path access-list 15 permit ^65001(_65001)*$
PE1(config)# ip prefix-list DefaultOnly permit 0.0.0.0/0
PE1(config)# ip prefix-list Customer1 permit 209.165.201.0/24 le 32
PE1(config)# ip prefix-list Provider permit 209.165.0.0/16 le 32
PE1(config)# route-map AllCustomersIn permit 10
PE1(config-route-map)# match ip prefix-list Provider
PE1(config-route-map)# set community no-export additive
PE1(config-route-map)# exit
PE1(config)# route-map AllCustomersIn permit 1000
ThecustomerisassignedtheprivateASnumber65001.TheBGPsessionis
openedwiththecustomerIPaddress,172.16.33.3.
TheISPsendsthedefaultrouteonlytothecustomer.Thisrouteisconfiguredusing
firstthedefault-originatecommandandthentheprefixlistDefaultOnly.
ReceivedroutesfromthecustomermustfirstpasstheprefixlistCustomer1.Thereis
onededicatedprefixlistforeachindividualcustomer;theprefixlistpermitsonlythe
routesthatthecustomerisallowedtoannounce.Iftheprefixlistallowstheroutes,
theroutesmustalsopassthefilterlistnamed"15."Inthiscase,thefilterallowsthe
privateASofthecustomerinanynumberofrepetitions,aslongasitistheonlyAS
numberinthepath.ThisfilterlistallowsASpathprependingconfigurationsonthe
customerside.Ifbothprefixlistsandthefilterlistallowthereceivedroute,thenthe
routemapAllCustomersInisapplied.
Theroutemapisageneralroutemapthatisusedforallcustomers.Itchecksevery
routethatisreceived,viatheprefixlistProvider.Iftherouteiswithinthebigblockof
PAaddressspacethattheISPannouncestotherestoftheInternet,thecustomer
routeismarkedwiththeno-exportcommunity.Thismarkmeansthattherouteis
usedwithintheISPASonlyandisnotsenttotherestoftheInternet.
Routesthatarereceivedfromthecustomer,andareallowedbytheprefixlistand
filterlist,butdonotfallwithinthePAaddressspace,areallowedbytheroutemap
andarenotchangedinanyway.TheISPpropagatestheseroutestotherestofthe
Internet.
RoutesthattheISPreceivesfromthecustomerarepropagatedtotherestofthe
InternetonlyiftheyarepartofthePIaddressspace.
A customer can use a private AS.
Private AS numbers should not be advertised into the Internet.
Private AS numbers must be removed from the AS path before the customer
BGP routes are advertised to other service providers.
A private AS number can be removed from the tail of the AS path using the
remove-private-as keyword.
WhentheISPreceivesBGProutesfromthecustomer,theASpathattributeofthe
receivedroutescontainsonlytheASnumberofthecustomer.Ifthecustomeruses
ASpathprepending,theremaybeseveralrepetitionsofthecustomerASnumberin
theASpath.IftheserviceproviderpropagatesthecustomerroutestotheInternet,
theASnumberofthecustomerwillbepresentintheASpathunlessitisexplicitly
removed.
IfthecustomerhasbeenassignedaprivateASnumber,
thisASnumbermustneverbeadvertisedbyanyrouterto
therestoftheInternet.Theserviceproviderhastomake
surethattheprivateASnumberisremovedfromtheBGP
routeandthattheserviceproviderASnumberispresent
inthetailoftheASpath.
RemovalofaprivateASnumberfromtheASpathisaccomplishedbyusingthe
remove-private-ascommandontheISPEBGPsessionswiththerestofthe
Internet.Inthefigure,removaloftheprivateASnumberoccursontheEBGP
sessionbetweenAS300andtheInternet.
router(config-router)# neighbor ip-address remove-private-as
The command modifies AS path processing on outgoing updates that are sent to
the specified neighbor.
Private AS numbers are removed from the tail of the AS path before the update
is sent.
Private AS numbers followed by a public AS number are not removed.
The AS number of the sender is prepended to the AS path after this operation.
ToremoveprivateASnumbersfromtheASpathinoutboundroutingupdates,use
theneighborremove-private-asrouterconfigurationcommand.Todisablethis
function,usethenoformofthiscommand.
neighbor {ip-address| peer-group-name} remove-private-as
SyntaxDescription
Parameter Description
ip-address IPaddressoftheBGP-speakingneighbor
peer-group-name NameofaBGPpeergroup
Usethiscommandontheserviceprovideregressrouters.Beforetheservice
provideradvertisesanyofthecustomerroutesoftheISPtotherestoftheInternet,
theprivateASnumbersmustberemoved.ThecommandremovesthoseAS
numbersiftheyareatthetailendoftheASpath.
RemovingaPrivateASNumbers
PrivateASnumbersfollowedbypublicASnumbersare
notremovedbecausethecommand'svisibilityisonlyon
thelast(tailend)ASnumber.
TheASnumberoftheISPisautomaticallyprependedtotheASpathattributeafter
theremove-private-asoperationhascompleted.ThissituationmeansthattheAS
numberoftheISPhasnotalreadybeenprependedtotheASpathattributewhen
thetailoftheASpathischeckedforprivateASnumbers.
RemovingaPrivateASNumbersExample
Inthisexample,theserviceproviderAS(300)receivesroutesfromthecustomer.
PE3(config)# router bgp 300
PE3(config-router)# neighbor 10.2.3.3 remote-as 200
PE3(config-router)# neighbor 10.2.3.3 remove-private-as
TheISPassignstheprivateASnumber65001tothecustomer;therefore,theroutes
thattheproviderreceiveshaveanASpathcontainingonlyAS65001.This
informationshouldbekeptandusedwithintheISPnetworkandshouldneverbe
propagatedtotherestoftheInternet(AS200inthisexample).
ThePE3routerinAS300hasbeenconfiguredtoremoveprivateASnumberson
EBGProutestowardAS200.IfprivateASnumbersappearinthetailendoftheAS
path(beforeAS300isadded),theyareremoved.
ThisconfigurationmustbeappliedtoallegressroutersinAS300thatserveEBGP
neighborsleadingtootherISPs.NoprivateASnumbermaybepresentinanAS
pathofaroutethatispropagatedtoanetworkusingapublicASnumber.
ASmigrationcanbenecessarywhenatelecommunicationsproviderorISP
purchasesanothernetwork.Itisdesirablefortheprovidertobeabletointegratethe
secondASwithoutdisruptingexistingcustomerpeeringarrangements.However,the
amountofconfigurationthatisrequiredinthecustomernetworkscanmakethis
migrationacumbersometaskthatisdifficulttocompletewithoutdisruptingservice.
Allows you to merge a secondary AS under the primary AS without disrupting
customer peering sessions.
Allows a router to appear to external peers as a member of secondary AS
during the AS migration.
Allows a network operator to merge the ASs and then later migrate customers to
new configurations during normal service windows without disrupting existing
peering arrangements
If misconfigured, increases the possibility of creating routing loops
TheBGPSupportforDualASConfigurationforNetworkASMigrationsfeature
allowsyoutomergeasecondaryASunderaprimaryASwithoutdisrupting
customerpeeringsessions.Theconfigurationofthisfeatureistransparentto
customernetworks.Thisfeatureallowsaroutertoappeartoexternalpeersasa
memberofsecondaryASduringtheASmigration.Italsoallowsthenetwork
operatortomergetheASandthenlatermigratecustomerstonewconfigurations
duringnormalservicewindowswithoutdisruptingexistingpeeringarrangements.
Theneighborlocal-ascommandisusedtocustomizetheASpathattributeby
addingandremovingASnumbersforroutesthatarereceivedfromEBGP
neighbors.Usethiscommandinaddress-familyorrouterconfigurationmode.To
disableASpathattributecustomization,usethenoformofthiscommand.
neighbor ip-address local-as [as-number[no-prepend [replace-as [dual-
as]]]]
SyntaxDescription
Parameter Description
ip-address SpecifiestheIPaddressoftheEBGPneighbor.
as-number SpecifiesanASnumbertoprependtotheASpathattribute.
TherangeofvaluesforthisargumentisanyvalidASnumberfrom1to
65535.
no-prepend (Optional)DoesnotprependthelocalASnumbertoanyroutes
receivedfromtheEBGPneighbor.
replace-as (Optional)PrependsonlythelocalASnumbertotheASpathattribute.
TheASnumberfromthelocalBGProutingprocessisnotprepended.
dual-as (Optional)ConfigurestheEBGPneighbortoestablishapeering
sessionusingtherealASnumberfromthelocalBGProutingprocess.
OrbyusingtheASnumberthatisconfiguredwiththeip-address
argument(local-as)
ASpathcustomizationincreasesthepossibilitythat
routingloopscanbecreatedifitismisconfigured.The
largerthenumberofcustomerpeerings,thegreaterthe
risk.Youcanminimizethispossibilitybyapplyingpolicies
ontheingressinterfacestoblocktheASnumberthatisin
transitionorroutesthathavenolocal-asconfiguration.
BGPprependstheASnumberfromeachBGPnetwork
thataroutetraversestomaintainnetworkreachability
informationandtopreventroutingloops.Thisfeature
shouldbeconfiguredonlyfortheASmigrationandshould
bedeconfiguredafterthetransitionhasbeencompleted.
Thisprocedureshouldbeattemptedonlybyan
experiencednetworkoperator,becauseroutingloopscan
becreatedwithimproperconfiguration.
BGPSupportforDualASConfigurationforNetworkAS
Migrations
Dual-ASConfigurationExample
ThefollowingexamplesshowhowDualASConfigurationforNetworkASMigrations
featureisusedtomergetwoautonomoussystemswithoutinterruptingpeering
arrangementswiththecustomernetwork.
If a service provider migrates to a new AS number, customers have to change
the BGP configuration.
A merged service provider can present itself to the customer using the old AS
number, so the customer is not required to change the BGP configuration.
Theneighborlocal-ascommandisconfiguredtoallowPE1tomaintainpeering
sessionsthroughAS100andAS200.CE1isacustomerrouterthatrunsaBGP
routingprocessinAS1andisconfiguredtopeerwithAS200.
PE1(config)# router bgp 100
PE1(config-router)# no synchronization
PE1(config-router)# neighbor 172.16.33.3 remote-as 1
PE1(config-router)# neighbor 172.16.33.3 local-as 200 no-prepend replace-
as dual-as
PE1(config)# router bgp 200
PE1(config-router)# neigbor 172.16.33.3 remote-as 1
CE1(config)# router bgp 1
CE1(config-router)# neighbor 172.16.33.34 remote-as 200
Afterthetransitioniscomplete,theconfigurationonCE1canbeupdatedtopeer
withAS100duringanormalmaintenancewindoworduringotherscheduled
downtime.
CE1(config-router)# neighbor 172.16.33.34 remote-as 100
Dual-ASConfederationConfigurationExample
ThefollowingexamplecanbeusedinplaceofthePE1configurationintheprevious
example.Theonlydifferencebetweentheseconfigurationsisthatinthisexample
PE1isconfiguredtobepartofaconfederation.
PE1(config)# router bgp 65100
PE1(config-router)# no synchronization
PE1(config-router)# bgp confederation identifier 100
PE1(config-router)# neighbor 172.16.33.3 remote-as 1
PE1(config-router)# neighbor 172.16.33.3 local-as 200 no-prepend replace-
as dual-as
Replace-ASConfigurationExample
ThefollowingexamplestripsprivateAS65100fromoutboundroutingupdatesfor
the172.16.33.3neighborandreplacesitwithpublicAS1.
PE1(config)# router bgp 65100
PE1(config-router)# neighbor 172.16.33.3 local-as 1 no-prepend replace-as
WhenacustomerusesBGPonmultiplelinksbetweenitsnetworkandtheISP
network,thecustomerissolelyresponsibleforcontrollinghowitusesthelinks.The
customercanchoosetouseitslinksinaprimaryorbackupscenarioorinaload-
sharingscenario.
The CE routers control the route selection entirely.
Outgoing traffic:
Local preference is used to select primary or backup link.
Incoming traffic:
MED is used to advertise the primary or backup link to the ISP. This is no
reliable, because the ISP can change the local preference.
No service provider configuration is required.
Ifonelinkisprimary,thentheothershouldbeusedforbackuponly.Thecustomer
canusethelocalpreferenceconfigurationtodirectalloutgoingtrafficoverthe
primarylink.
IncomingtraffictothecustomeriscontrolledbyusingeitherASpathprependingor
theMED.BecausethecustomerhasmultipleconnectionstothesameAS,theMED
istheidealattributetouse.WhenthecustomerannouncesitsroutestotheISP,a
worse(orhigh)MEDvalueonthebackuplinkisset.Alsoagood(orlow)valueon
theprimarylinkisset.
ThereceivingEBGPpeerchecksMEDandASpathlengthonlyiftheweightand
localpreferenceattributeshavenotbeenconfigured.Inthiscase,theISPshouldnot
useanyoftheseconfigurationoptions.TheISPshouldrelysolelyontheattributes
thatithasreceivedfromthecustomer.However,iftheISPsets,forexample,the
localpreferenceforcustomerroutes,theISPwillcontrolthepathselectionfortraffic
tothecustomer.Thispathselectioniswhyyoucannotrelyonthefactthatyour
trafficdistributionscenariowillworkasintended.
Primary/BackupLinkSelectionExample
ThecustomerisconnectedtotheISPovertwopermanentconnections.The
customerusestheupperconnectionastheprimaryconnectionandthelower
connectionasthebackup.
TheBGPconfigurationontheISPsideistransparent.Thistransparencymeansthat
noparticularpreferenceisconfiguredtousetheupperorlowerconnection.TheISP
reliesontheattributevaluesthatarereceivedfromthecustomer.
TheCE1routerisconfiguredtosetlocalpreferencetothevalue100onallEBGP
routesthatarereceived.TheCE2routersetsthelocalpreferenceattributetoa
valueof50.Thisconfigurationmeansthattheoutgoingtraffictowardanydestination
thattheISPannouncesisprimarilysentovertheupperlink.Itmakestheprimary
routerthepreferredone.
IncomingtraffictothecustomerisdirectedtotheprimarylinkbyusingtheMED.On
theCE1router,allroutesthataresenttotheISPhavetheirMEDattributesettothe
value100bytheroutemapnamed"LowMEDout."OntheCE2router,allroutesthat
BackupSolutionswithBGP
aresenttotheISPhavetheirMEDattributesettothevalue200bytheroutemap
named"HiMEDout."BecausetheISPreceivestherouteswithallotherattributesset
tothesamevalues,theMEDvaluesdirecttrafficforthecustomertotheprimarylink.
Thismakestheprimaryrouterthepreferredonefromtheserviceproviderpointof
view.
Loadsharingofoutgoingtrafficfromthecustomernetworkisidenticaltothestatic
routingscenario.ThecustomerIGPisconfiguredtosendinformationabouta
gatewayoflastresort.Thereisnodifferencewhethertheedgeroutergetsitsdefault
bystaticroutingorbyincomingEBGPupdates.
You can implement load sharing of return traffic in a number of ways:
Outgoing traffic:
Load sharing is identical to the static routing scenario. The IGP determines
it.
Incoming traffic:
Announce portions of the customer address space to each upstream router.
Configure BGP multipath support in the service provider network.
Use EBGP multihop in environments where parallel links run between a pair
of routers.
LoadsharingofthereturntrafficcomingbacktothecustomernetworkfromtheISP
canbeimplementedinanumberofways:
Thecustomercandivideitsaddressspaceintoseveralannouncements.TheCE
routercansendeachannouncementoveroneofitsEBGPsessionswiththe
ISP.Forbackuppurposes,thecustomershouldadvertisetheentireaddress
spaceoverallofitsEBGPsessions.TheISPnowusesthemostexplicitroute
rule.Aslongasbothlinksareup,trafficwithdestinationswithinonepartofthe
customeraddressspaceisroutedoveroneofthelinks.Traffictotheotherpart
isroutedovertheotherlink.
Ifthecustomerannouncesequivalentroutesoverbothlinks,theISProutersuse
theclosestconnectionregardingtheIGPoftheISP.IfanISProuterhasan
equivalentdistancetobothconnectionpoints,theuseofthemaximum-paths
(orBGPmultipath)optioncausesloadsharing.
IfthemultiplelinksbetweenthecustomerandtheISPnetworkterminateinone
singlerouteronthecustomersideandonesinglerouterontheISPside,thetwo
routersmustestablishtheirEBGPsessionfromloopbackinterfacetoloopback
interface.Staticordynamicroutingisrequiredforoneroutertogetinformation
onhowtoreachtheloopbackinterfaceoftheotherrouter.Theuseoftheebgp-
multihopoptionisalsorequiredbecausetheaddressoftheneighborisnot
directlyconnected.
LoadSharing
Bydefault,BGProuteselectionrulesselectone,andonlyone,routeasthebest.If
therearetwoidenticalroutes,thetiebreakeriseitherthemoststablerouteorthe
router-IDofthepeerrouterthatisadvertisingtheroute.However,whenthe
maximum-pathsrouterconfigurationcommandisused,theBGProuteselection
processwillselectmorethanonerouteasbestiftheyareidentical.Theroutesare
allinstalledintheroutingtable,andloadsharingtakesplace.
BGP Multipath:
By default, BGP selects a single path as the best path and installs it in the IP
routing table.
If configured, a BGP router can select up to six identical BGP routes as the best
routes.
router(config-router)# maximum-paths number
With maximum-paths configured, a BGP router can select several identical
EBGP routes as the best routes and install them in the IP routing table for load-
sharing purposes.
AroutercanuseeitheronlyIBGProutesoronlyEBGProutesforloadbalancingbut
notbothatthesametimebecausethiscouldleadtoroutingloops.BGPmultipath
canbeenabledseparatelyforIBGPandEBGProutes.Aroutercaninstalluptosix
BGProutesintheIProutingtable.Theactualtypeofloadsharing(per-sessionor
per-packet)thatoccursbetweentheroutesdependsontheswitchingmodethatis
used.
TocontrolthemaximumnumberofparallelroutesthatanIProutingprotocolcan
support,usethemaximum-pathscommandinaddressfamilyorrouter
configurationmode.Torestorethedefaultvalue,usethenoformofthiscommand.
maximum-paths number
SyntaxDescription
Parameter Description
number MaximumnumberofparallelroutesthatanIProutingprotocolinstalls
inaroutingtable,intherangeof1to6.
LoadsharingbetweenalternativeBGProutesisachieved
onlyiftheEBGProutesareidenticalaccordingtoallBGP
routeselectionrulesandmaximum-pathsisconfigured
withavaluelargerthan1.
LoadSharingwithBGPMultipath
Whentwoadjacentroutershavemultiplelinksbetweenthem,youcanconfigurethe
EBGPsessionfromloopbackinterfacetoloopbackinterface.Inthiscase,youmust
usetheebgp-multihopoptiontomaketheBGPsessiongointotheactivestate.
Theremustbestaticordynamicroutinginusetoprovidebothrouterswith
informationonhowtoreachtheloopbackinterfacesofeachother.Otherwise,their
EBGPsessiondoesnotcompleteestablishment.
EBGP Multihop:
Used when parallel links between routers exist. Loopback interfaces have to be
used for BGP peering.
Because of recursive lookup, load sharing toward a BGP destination always
occurs if there are several equal-cost paths to the BGP next hop.
By default, EBGP neighbors must be directly connected.
The ebgp-multihop [TTL] command declares an EBGP neighbor to be distant
(several hops away).
Routingtotheloopbackinterfaceoftheneighboringrouterisrequiredtoestablish
theEBGPsession.Itisalsousedintherecursivelookupwhentherouterinstallsthe
routesinitsroutingtable.Thetworoutestotheloopbackinterfaceofthe
neighboringroutershouldbeequivalentforloadsharingtooccur.
Afterconfiguration,onesingleEBGPsessionisestablishedbetweenthetworouters.
Thissessionisusedtoexchangetheroutinginformation.ThereisonlyoneBGP
routetoeachdestination,andithasanexthopthatreferstotheloopbackinterface
oftheotherrouter.
TheloadbalancingisautomaticasaresultoftherecursiveroutelookupofBGP
nexthops.TheIProutingtablewillcontainasingleroutingentrytoreachthe
customerprefixbuttheCiscoExpressForwarding(CEF)table(whichcontainsthe
resultsoftherecursivelookup)willhavetwoequal-costpathstoreachthecustomer
prefix.
Bydefault,EBGPneighborsmustbedirectlyconnected.Arouterverifiesthatan
EBGPneighborisreachableasdirectlyconnectedoveroneoftherouterinterfaces
beforethesessiongoesintotheactivestate.ForanEBGPsession,IPpacketsthat
carrytheTCPsegmentswithBGPinformationarealsosentusingaTTLvaluesetto
thevalue1.Thisvaluemeansthattheycannotberouted.
Theebgp-multihopneighborconfigurationcommandchangesthisbehavior.
Althoughtheneighborisseveralhopsaway,thesessiongoesintotheActivestate,
andpacketsstarttobeexchanged.TheTTLvalueoftheIPpacketsissettoavalue
largerthan1.Ifnovalueisspecifiedonthecommandline,255isused.
Usetheebgp-multihopcommandwhenyouareestablishingEBGPsessions
betweenloopbackinterfacesforload-sharingpurposes.Youmusttakegreatcare
whenusingebgp-multihop.Properpacketforwardingreliesonalltheintermediate
routersalongthepathtotheEBGPpeertomakethecorrectforwardingdecision.If
theintermediateroutershaveacorrectpathtotheEBGPpeerbutawrongpathto
thefinaldestination,thepacketmaygetintoaroutingloop.
LoadSharingwithEBGPMultihopExample
Inthefigure,thecustomernetworkandtheISPnetworkareconnectedviasingle
routeroneachside.Twoparallellinksareused.
LoadSharingwithEBGPMultihop
Inthiscase,onlyoneEBGPsessionisconfiguredbetweenthecustomerand
providerrouters.Thesessionshouldbeestablishedfromtheloopbackinterfaceon
oneroutertotheloopbackinterfaceontheother.
Eachofthetwoedgeroutershastwostatichostroutesthatpointtotheloopback
interfaceontheotherrouter.TheEBGPsessionisestablishedfromloopbackto
loopbackusingebgp-multihop.
ThecustomerreceivesanEBGProutefromtheISPwiththenexthopsetto
10.0.3.1.TheCErouterperformsarecursivelookupandfindsthatitcanreach
10.0.3.1via172.16.33.33andvia172.16.34.34.Thesetworoutesareequivalent.
Therefore,theroutetothefinaldestinationisinstalledintheroutingtableofthe
customerrouterusingbothpaths.
Dependingontheswitchingmodeinuse,loadsharingisdoneperpacket,per
destination,orpersourceanddestinationpair.
Inthisexample,link-levelproceduresensurethatifoneofthelinksgoesdown,the
correspondingstaticlinkgoesdown.AllBGProutesintheroutingtablethatrelyon
thestaticroutetothelinkthatwentdownareinvalidated.However,theBGProutes
intheroutingtablethatrelyontheremaininglinkarestillvalidandused.
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
When a customer has multiple connections to a single ISP and the link-level
procedures cannot detect a link failure, a routing protocol is required. For
security reasons, this routing protocol must be BGP.
Customer does not necessarily use a public AS number, it can also use private
AS numbers that range from 64512 to 65535. BGP must remove private AS
numbers.
The ISP should advertise a default route to the customer through BGP. The
provider should also use incoming filters to ensure that the customer advertises
only the correct address space and AS number.
The BGP Support for Dual AS Configuration for Network AS Migrations feature
allows you to merge a secondary AS under a primary AS without disrupting
customer peering sessions.
By announcing portions of its address space, a customer can use maximum
paths and EBGP multihop to provide load sharing over multiple links.
Summary
Overview
Whenacustomerrequiresthemaximumredundancyinitsnetworkdesign,itshould
implementamultihomedstrategythatusesmultipleserviceproviders.This
configurationrequiresspecificconsiderationstobeimplementedproperly.
AddressingandASnumberselectionareimportantconsiderationsthataffectthe
networkimplementation.Itisalsoimportanttounderstandhowtoconfigurerouting
protocolssothatcustomerbackuporload-sharingrequirementsaremet.
Inthislesson,youwilldiscussmultipleconnectionsbetweenacustomerand
multipleserviceprovidersforbackupandload-sharingpurposes.IncludingtheBGP
characteristicsthatareusedtoconfiguresuchconnectivity.Youwillalsodiscuss
addressselection,privateASnumbertranslation,andconfigurationofthenetworkto
supporteitherbackuplinksorloadsharing.
Uponcompletingthislesson,youwillbeableto:
DescribeBGPconfigurationcharacteristicsthatareusedtoestablishrouting
betweenamultihomedcustomerandmultipleserviceproviders
Describeaddressingstrategiesthatareavailabletoamultihomedcustomerthat
isconnectedtomultipleserviceproviders
DescribeASnumberingstrategiesthatareavailabletoamultihomedcustomer
thatisconnectedtomultipleserviceproviders
DescribetheoperationofASnumbertranslation
Describehowyoucanimplementatypicalbackupsetupbetweenamultihomed
customerandmultipleserviceprovidersinaBGPenvironment
DescribetheuseofBGPattributestoinfluenceinboundlinkselectionin
customernetworksthataremultihomedtomultipleserviceproviders
Describehowyoucanimplementloadsharingbetweenamultihomedcustomer
andmultipleserviceprovidersinaBGPenvironment
ConnectingaMultihomedCustomerto
MultipleServiceProviders
Thehighestlevelofredundancytonetworkfailuresisachievedinnetworkdesigns
thatconnectthecustomernetworktotwodifferentserviceprovidernetworks.
CustomersusethisoptionwhentherequirementforresilientInternetconnectivityis
veryhigh.Thisrequirementalsoinvolvesduplicationofequipmenttomakethe
customernetworkfullyredundant.
BGP is run between the customer and the service provider.
The customer advertises the allocated address space into BGP. It is
recommended to use a public AS number.
The service provider has to deploy inbound BGP filters.
The service provider advertises the following to the customer:
Default route, default route + local networks, or full Internet routing table
BecausethecustomerhastousePIaddressspace,itistheresponsibilityofthe
customertoadvertisethecustomeraddressspace.Theonlyroutingoptionthatcan
beusedinthisscenarioisBGPbecauseserviceprovidersrunonlyBGPwith
customers.Itisnotenoughtodetectlinkfailuresorafailureintheremoterouterby
link-levelprocedures.Failuresthatoccurbeyondthedirectlyconnectedroutermust
alsobedetected,andtheonlymeansofdetectingthesefailuresisbyusingarouting
protocol.TheonlyroutingprotocolthatissuitedfortheInternetenvironmentisBGP.
Correctlyconfigured,BGPtakescareofreroutinginthefollowingsituations:
LinkfailurebetweenthecustomernetworkandthenetworkofoneoftheInternet
serviceproviders.
Edgerouterfailureoneitherthecustomerortheserviceproviderside.
LinkfailureorrouterfailurewithinthecustomernetworkthatcausestheCE
routertoloseconnectivitywiththecustomernetworkcore.Thissituation
requirescorrectconfigurationofrouteadvertisementasdescribedinanearlier
lesson.
LinkfailureorrouterfailurewithintheISPnetworkthatcausesthePErouterto
loseconnectivitywiththerestoftheInternet
MultihomedcustomershavemultiplepermanentlinkstodifferentISPs.Thelinks
shouldterminateindifferentedgeroutersinthecustomernetwork.Otherwise,oneof
themajoradvantages,resiliencetorouterfailure,islost.
MultihomedcustomersshoulduseBGPwithbothISPs.Thecustomershould
advertiseitsaddressspacetobothproviders.Therouteadvertisementshouldbe
configuredinbothCErouters.Theadvertisementshouldbeconditionedattheedge
routersbytheappropriateroutepoliciesleadingtowardthecoreofthecustomer
network.Thissetupisanalogoustotheoneconfiguredwhenyouareconnectinga
multihomedcustomernetworktoasingleserviceprovider.
Thecustomershouldtakecarenottomoveanyroutinginformationbetweenthetwo
ISPs.Itmustuseoutgoingfilterstopreventanyroutethatisreceivedfromoneofthe
ISPsfrombeingpropagatedtotheother.Otherwise,thecustomernetworkappears
asatransitnetworkbetweenthetwoISPs.
BothISPsmustapplyfiltersontheincomingBGPinformationfromthecustomer.
ThesefilterswillprotectthemselvesandtherestoftheInternetfromerrorsinthe
BGPconfigurationofthecustomer.Eachoftheserviceprovidersmustacceptroutes
fromthecustomerthatindicatenetworkswithinthecustomeraddressspaceonly.
ASpathfilterlistsshouldbeimplementedonthePErouterstoallowincomingroutes
onlyiftheyhavethecorrectASpathattributevalue.IftheincomingfiltersontheISP
edgerouteracceptcustomerroutes,thentheserviceprovidershouldpropagate
thoseroutestotherestoftheInternet.
BothISPsmustprovidethecustomerwithatleastsomeBGProutes.Dependingon
thecustomerrequirements,thevolumeofBGProutestheISPprovidescanvary.It
BGPConfigurationforMultihomedCustomers
canbethedefaultrouteonly,thedefaultroutewithlocalroutesaswell,orthefull
Internetroutingtable.
Beforeconfiguringthemultihomednetwork,youneedtoconsiderthefollowing
questions:
Shouldanyofthelinksbeusedasprimaryandtheothersasbackup?
Shouldbothlinkssharetheload?
Whataddressspaceisthecustomerusing?IsthecustomeraddressspacePA
orPI?
WhatASnumberisthecustomerusing?(Isthecustomerusingapublicora
privateASnumber?)
Ifthecustomerhasitsownaddressspace,itshouldannounceittobothservice
providers.Bothprovidersareresponsibleforpropagatingthecustomerroutestothe
restoftheInternetwithoutdoinganysummarization.
Provider-independent address space:
If the customer owns the address space, there should be no limitations
regarding announcing it to both service providers.
Provider-assigned address space:
If the customer uses ISP-assigned small address blocks, then there is no
purpose in using BGP to provide redundant connectivity. NAT is easier to
implement and solves the problem of the reverse path.
However,ifthecustomerusesasmallblockofaddressesthatoneoftheISPs
assigned,analternativedesignthatdoesnotinvolveBGPexist.Thecustomercan
usetwodifferentPAaddressspacesanddoNAT.WithNAT,theroutertranslates
trafficgoingoutoveroneofitsconnectionstooneofthePAaddresses.Ifthetraffic
goesouttheotherway,theaddressesaretranslatedtoanaddressfromtheaddress
spaceoftheotherprovider.
MultihomedCustomerAddressSpaceSelection
TheuseofBGPrequiresanASnumber.Thepreferredoptionistousearegistered,
publicASnumber.However,registeredASnumbersareassignedonlytothosewho
reallyneedthembecausepublicASnumbersareascarceresource.Acustomer
withBGPsessionstomultipleISPsmustusearegistered,publicASnumber.A
customerthatisconnectedtoonlyoneISPdoesnotrequireapublicASnumber.In
thatcase,aprivateASnumberintherangefrom64512to65535issufficient.
Registered, public AS number (recommended):
Preferred option, but difficult to get.
Does not require ISPs to assign a private AS number.
Consistent routing information in the Internet.
Private AS number (discouraged):
Easier to get (even easier with AS translation).
One private AS number: The customer has to be able to use the same
private AS number with multiple providers.
Multiple private AS numbers: Each service provider assigns a private AS
number to the customer and the customer then uses one of them internally;
the others have to be translated.
Causes inconsistent routing information.
WheneverthecustomerisassignedapublicASnumber,therearenoconflictsinthe
BGPsetupbecausethenumberisguaranteedtobeuniquewithintheInternet.Both,
thecustomerandtheserviceprovidermakerouteannouncementswithout
tamperingwiththeASpath.Asaresult,theserviceproviderpropagatesaconsistent
ASpathinformationtotherestoftheInternet.
IncaseswherethecustomerdoesnothaveapublicASnumber,itmustusea
privateASnumber.BecauseprivateASnumbersarenotpropagatedtotheInternet,
severalnetworkadministratorscanmakethisassignmentindependentlyofeach
other.Inthiscase,ASnumbersarereused,whichconservesASnumberspace.A
serviceprovidernormallyassignsprivateASnumberstoitscustomers.This
arrangementensuresthatuniqueprivateASnumbersareusedamongthe
customersofasingleISP.
WhenacustomerisgoingtobemultihomedandtheprivateASnumberthathas
alreadybeenassignedbyoneoftheISPscomesinconflictwithanASnumberthat
hasbeenassignedbytheotherISP,thecustomerneedstoconsiderrenumbering
thecustomerAS.Ifthetwoserviceproviderscanreachacommonagreementon
whichprivateASnumberthemultihomedcustomershoulduse,renumberingisa
solution.Ifnocommonagreementcanbemadeorifrenumbering,forsomereason,
isnotanoption,AStranslationmustbeconfiguredonthecustomernetwork.
NoroutershouldeverpropagateprivateASnumberstotherestoftheInternet.An
ISPcankeeptrackoftheprivateASnumbersthatithasassignedtoitscustomers
andavoidreuseorconflictswithinthatscope.However,assoonasthescopeis
widenedtoincludeotherISPs,conflictswillhappen.EachISP,therefore,removes
privateASnumbersfromtheASpathbeforesendingroutesoutsideitsownAS.
WhentherouteswiththeprivateASnumbersremovedarepropagatedtotherestof
theInternet,theASpathlooksasthoughtherouteswereoriginatedwithinthepublic
ASoftheISP.AllinformationabouttheprivateASlyingbehindthepublicASislost.
Inthecaseofamultihomedcustomer,thecustomerroutesarefirstpropagatedinto
eachoftheautonomoussystemsofitsISPs.Inthenextstep,theprivateASnumber
isremovedfromtheroutes,astheroutesarepropagatedtotherestoftheInternet.
NowthecustomerroutesappeartobeoriginatingintheASsofbothISPs.Toan
outsideobserver,thereisnowanASpathinconsistencybecauseitlooksasthough
thesameroutebelongstodifferentASs.
MultihomedCustomerASNumberSelection
Thefigureshowsacasewhereacustomerismultihomedbutforcedtousetwo
privateASnumbers(forexample,becauseofthescarcityofpublicASnumbers).
On one EBGP adjacency, the real AS number is used.
On the other EBGP adjacency, the AS number is translated to the one assigned
by the second ISP.
Inthefigure,ServiceProvider1hasassignedtheprivateASnumber65053tothe
customer.Serviceprovider2didnotagreetousethisprivateASnumberwhen
connectingtothecustomer.Instead,serviceprovider2hasassignedtheprivateAS
number65286.
ThecustomernowhastwodifferentprivateASnumbers:65053and65286.The
customerdecidestouse65053internally.AllrouterBGPconfigurationlineshave
65053astheASnumber.ThecustomerusesASnumber65286onlywhen
establishingtheEBGPsessiontoAS200.
Intheexample,ServiceProvider1(AS100)hasanEBGPsessiontothecustomer
wheretheASnumber65053isusedatthecustomerend.Serviceprovider2(AS
200)hasanEBGPsessiontothecustomerwheretheASnumber65286isusedat
thecustomerend.TranslationbetweenthesetwoprivateASnumberstakesplacein
theCErouteraspartoftheEBGPsessiontoAS200.
router(config-router)# neighbor ip-address local-as private-as [no-
prepend [replace-as]]
Optionally, the service providers can assign two different private AS numbers to
the customer.
Internally, the customer can use an ISP-assigned AS number or even any other
private AS number.
Externally, the customer appears as one private AS to ISP 1 and as a different
private AS to ISP 2.
Note: When you are using this option, the AS path of the customer network
contains two AS numbers. The ISP has to adapt the incoming AS path filters.
Theneighborip-addresslocal-asprivate-asrouterconfigurationcommandisused
toindicatetheASnumberthatthelocalrouterusesasitslocalASnumberinthe
BGPOpenmessage.TheremoterouterisassumedtohaveanEBGPsessiontothe
indicatedlocalAS.
Internally,thecustomernetworkusesanotherprivateASnumber.Whenroutesare
senttotheneighbor,theinternalASnumberisautomaticallyprependedintheAS
pathfirst,andthenthespecifiedlocalASnumberisprependedaswell.Asa
consequence,theISPreceivestherouteswithanASpathwithbothASnumbersin
it.TheISPhastoadaptitsincomingfilter-listsasaresultofthissituation.
Whenakeywordno-prependisused,routerdoesnotprependthelocal
autonomoussystemnumbertoanyroutesreceivedfromtheEBGPneighbor.
Whenakeywordreplace-asisused,routerreplacestherealautonomoussystem
numberwiththelocalautonomoussystemnumberintheeBGPupdates.The
autonomoussystemnumberfromthelocalBGProutingprocessisnotprepended.
Someserviceprovidersmightbeunwillingtochangetheir
ASpathinputfilters,leavingthecustomernootheroption
thantouseapublicASnumberortoconnecttoasingle
ISPwithaprivateASnumber.
ASNumberTranslation
WhenusingBGPonmultiplelinksbetweenacustomerandseveralserviceprovider
networks,thecustomerissolelyresponsibleforcontrollingtheuseofthelinks
betweenthemforoutgoingtraffic.Thecustomerchooseswhethertousetheselinks
inaprimaryandbackup,oraload-sharingconfiguration.
Intheprimaryandbackupscenario,onlythedefaultrouteisrequiredfromeach
serviceprovider.Thecustomerimplementsroutingpolicies.
Outgoing link selection:
You can use the same solution as with multihomed customers connected to
one service provider.
A higher local preference for the default route comes from the primary
service provider.
Incoming link selection:
You cannot use the MED because it can be sent only to the neighboring AS
and no farther.
You must use other means such as BGP communities or AS path
prepending to achieve incoming link selection.
Only the default route is required from both service providers.
Ifonelinkisprimaryandtheotherisbackup,thecustomercanusethelocal
preferenceattributeintheconfigurationtodirectalloutgoingtrafficovertheprimary
link.Thecustomerselectsaprimaryserviceproviderforoutgoingtrafficbysettinga
higherlocalpreferenceforroutesthatarereceivedfromthepreferredISP.This
configurationisnodifferentthantheconfigurationthatisusedforcustomerswith
multipleconnectionsrunningBGPtoasingleserviceprovider.
Controllingtheloaddistributionofincomingtrafficovermultiplelinksismoredifficult
inthemultihomedscenariowhenlinkstomultipleserviceprovidersareused.You
cannotusetheMEDwhenthecustomerconnectstomultipleprovidersbecausethe
updatesaresenttotwodifferentASs.RecallthattheMEDisusedonlywhenyou
compareroutesthatarereceivedfromasingledirectlyconnectedASovertwo
parallellinks.
Therefore,routeselectiondecisionswillmostlikelyusetheASpathattributeand
prefertheroutewiththeshortestASpathlength.ThecustomercanconfigureAS
pathprependingofroutesthataresenttothebackupserviceprovidertolengthen
theASpath.ThereturningtrafficoriginatingfromtheInternetwillthereforechoosea
shorterpaththroughtheprimaryserviceprovider.
PrimaryandBackupLinkSelection
Toremoveincomingtrafficfromthebackuplink,thecustomermustinfluenceroute
selectioninthebackupAS.ThebackupISPmustbeforcedtoprefertheprimary
pathtoreachthecustomernetwork,althoughthischoicemeansselectingaroute
withalongerASpath.
BGP communities:
Customer sets the appropriate BGP community attribute on updates that are
sent to the backup ISP.
Requires the ISP to translate the BGP community attribute to a local preference
attribute that is lower than the default value of 100.
May not work in all situations.
AS path prepending:
Multiple copies of customer AS number prepended to the AS path to lengthen
the AS path sent over the backup link.
Customer not dependent on the provider configuration.
Always works.
Onewaytoinfluencerouteselectionistouselocalpreferenceinthenetworkofthe
backupISP.Usinglocalpreferencecreatesanadministrativescalabilityissueifeach
customerrequiresitsuse,becausetheISPmustmaintaintheconfiguration.
OnescalablewayofsettinglocalpreferenceinanISPnetworkistouse
communities.Thecustomersetsawell-knowncommunityvalueontheroutesthat
aresenttothebackupISP.TheISPrecognizesthecommunityandsetsthelocal
preferencefortheseroutes.ThissolutionisavailableonlyiftheISPhas
implementedandannouncedtheuseofcommunities.Ifcommunitiesandalocal
preferencesettingareused,routeselectionoccursonlyiftherearealternative
routestocompare.
AnotherwayofinfluencingrouteselectioninthebackupISPistodoASpath
prependingbeforesendingtheadvertisementtothebackupISP.Whenthecustomer
sendsroutesoverthebackuplink,multiplecopiesofitsownASnumberare
prependedtotheASpathofeachroute.ThebackupISPreceivestheroutesand
makesnormalroute-selectiondecisions.Nospecialweightorlocalpreference
settingsareused;theBGProuteselectionisbasedexclusivelyontheASpath
length.Nospecialconfigurationisrequiredintheserviceprovidernetwork.
BGPIncomingLinkSelectionUsingBGPCommunities
Intheexamplehere,thebackupserviceprovider2(AS200)hasdefinedthe
meaningofcommunity200:50.WhenAS200receivesrouteswiththiscommunity,
thelocalpreferenceissetto50.
ThecustomerAS1isadvertisingtherouteovertheprimarylinkwithoutany
communities.ItisreceivedbyAS100andpropagatedtoAS200.WhenAS200
receivestherouteviaAS100,thereisnocommunityset.AS200thereforeassigns
thedefaultlocalpreferencevalueof100.
Thecustomerisalsoadvertisingtherouteoverthebackuplink.However,inthis
case,theroutehasthecommunity200:50set.WhenAS200receivesthisroute,it
recognizesthecommunity,andthelocalpreferencevalueissetto50.
RouteselectionisnowperformedinAS200.Theroutethathasbeenreceivedvia
AS100ispreferredbasedonthelocalpreferencevalues.
BGPIncomingLinkSelection
Evenwhentheuseofcommunitiesiscorrectlyconfigured,thedesiredload
distributionmaynotalwaysbeachieved.Asthisexampleshows,AS200doesnot
alwaysreceivetheprimaryroute,althoughnothingiswronginthenetwork.
ThecustomerAS1sendsrouteswithcommunity200:50overthebackuplinktoAS
200.AS200receivestheroutesandsetsthelocalpreferenceto50.IfAS200over
sometimeselectsthedirectlyconnectedpathtoAS1asthebest,itpropagatesthe
routetoAS300.AstherouteispropagatedovertheEBGPsessionbetweenAS200
andAS300,thelocalpreferencevaluethatisusedwithinAS200islost.
AS300doesnothaveanyuseforthecommunity200:50becausethiscommunityis
definedandimplementedonlywithinAS200.Potentially,thecommunityvaluecan
alsobestrippedoffduringBGProutepropagation.
CustomerAS1alsosendstheroutesovertheprimarylinktoAS100.Theroutes
arepropagatedtoAS300,whichnowseestwoalternativeroutestothedestination
networkswithinAS1.TherouterinAS300doesnotuseweightnorlocalpreference
ascriteriaforreachingAS1.BothalternativeshaveequalASpathlengths.
TherouteselectiondecisionthatwillbemadeinAS300ishardtopredict,butthe
outcomedefinitelyinfluencestherouteselectiondecisionthatwasmadeinAS200.
IfAS300preferstheroutetothecustomernetworkviaAS200foranyreason,then
thesecond-bestalternativeviaAS100andtheprimarylinkisneverpropagatedto
AS200.
Inthiscase,AS200neverseestheprimarypathandhastosticktothebackuplink
andannouncetheroutetoAS300.Thenetworkhasreachedasteadystatewhen
thetrafficusesthebackuplinkalthoughtheprimarylinkisavailable.
BGPIncomingLinkSelectionUsingASPathPrepending
Inthisexample,thecustomerAS1isperformingASpathprependingonthebackup
link.ThreecopiesofthecustomerASnumber(1)areprependedtotheASpath.As
theroutegoesoutovertheEBGPsession,BGPprependsthelocalASnumberto
theASpathattribute.AS200receivesroutesfromAS1overthebackuplinkwithan
ASpathlengthoffour.ThisistheoriginalAS1plusthreeprependedcopiesthatthe
CErouterappliedtotheASpathattribute.
ThecustomeradvertisesnetworkswithoutASpathprependingovertheprimarylink.
AS100receivesrouteswithanASpathlengthofoneandpropagatestheseroutes
toAS300,whichthenreceivesthemwithanASpathlengthoftwo.
If,forashorttime,AS300receivedthecustomerroutesviaAS200,theASpath
lengthofthoserouteswouldhavebeenfive.Inthatcase,AS300selectstheroute
fromAS100asthebestandpropagatesittoAS200.
AS200nowseesbothalternatives.Thecustomerroutesthathavebeenreceived
directlyfromthecustomerhaveanASpathlengthoffour.Theroutesthathavebeen
receivedviaAS300haveanASpathlengthofthree.Becausenoweightorlocal
preferenceisconfiguredinthisexample,AS200selectstherouteviaAS300asthe
best.
Thedesiredresult,tohavealltrafficenterthecustomernetworkviatheprimarylink,
isnowachieved.
IfthebackupISPisimplementingincomingASpathfilters
forthiscustomerwiththelengthoftheASpathequalto
one,theISPhastochangetheconfigurationoftheAS
pathfilterforthecustomer.TheISPcaneithercreatea
newfilter,allowingmultiplecopiesofthecustomerAS
numberonlyforthiscustomer,oruseregularexpression
variablestocreateacommonfilterforallcustomersthat
belongtoonepeergroup.
LoadsharingoverlinkstotwodifferentISPscanbecomparedtodoingloadsharing
overtwoparallellinkstoasingleISP.Theonlydifferenceisthatthereisonlyone
optionthatisavailabletocontrolincomingtraffic.Controllingloaddistributionofthe
outgoingtrafficisconfiguredinthesamewayaswhenamultihomedcustomer
connectstoasingleserviceprovider.
Load sharing for outgoing traffic:
You can use the same solution as with multihomed customers that are
connected to one service provider.
A higher local preference for half of the routes comes from one service
provider. A higher local preference for the other half of the routes comes
from the other service provider.
Load sharing for incoming traffic:
The only load-sharing option that you can use in this setup is to separate the
address space into two or more smaller address blocks.
Some traffic analysis is needed to fine-tune the address space separation
according to link bandwidths.
You should use AS path prepending to ensure symmetric routing as well as
backup for noncontiguous address blocks.
The customer divides the address space into two announcements.
Thecustomercancontroltheloaddistributionofincomingtrafficbasedonthetraffic
destination.Thecustomerdividesitsaddressspaceintoseveralannouncements.
OneannouncementissenttooneoftheISPs.Anotherannouncementissenttothe
otherISP.Forbackuppurposes,thecustomerannouncestheentireaddressspace
tobothISPs.TheISPsnowusethemost-explicitrouterule.Aslongasbothlinks
areup,trafficwithdestinationswithinonepartofthecustomeraddressspaceis
routedoveroneofthelinks.Traffictotheotherpartisroutedovertheotherlink.
Itisverydifficulttopredictthevolumeoftrafficthatwillbedirectedtoeachpartof
thecustomeraddressspace.Youshouldmonitortheresultsofchangingroute
updatesbywatchingtheloadonthelinksbeforeandafterimplementingthechange.
Iftheloaddistributionisnotsatisfactory,youcanfurthermodifythedivisionofthe
addressspace.Youmustthenchecktheloadonthelinksagainandfurtherfine-tune
theconfiguration.
Acustomermaydecidetouseboththedivisionofaddressspaceintoseveral
advertisementsandASpathprependingtogether.Somepartofthecustomer
addressspacemaybeadvertisedbythecustomernetworkwithalongerASpath
overoneofthelinkstofine-tunetheload.Also,theremaybecaseswherethereare
noncontiguoussubnetsthatcannotbedividedbecausetheprefixeswouldbetoo
long.Thesesubnetsareevenlydistributedbetweenthelinksinaprimary/backup
configuration.
LoadSharingwithMultipleProviders
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
A customer that is multihomed to multiple BGP service providers must advertise
its address space to both ISPs, but has to make sure that it does not become a
transit network.
Customers who are connected to multiple ISPs must use an AS number that all
ISPs agree to.
You can use AS number translation to prepend a different AS number to the
AS path, which allows the customer to use a single private AS number in the
network.
Primary and backup connectivity:
Outgoing route selection is achieved by using local preference.
Incoming route selection is achieved by using either BGP communities or
AS path prepending.
The key for load-sharing is to divide the customer address space.
Summary
Overview
Thistopicsummarizesthekeypointsthatwerediscussedinthismodule.
Customer requirements influence the planning of connectivity between a
customer and an ISP, including:
Physical connection type (the redundancy provided)
IP addressing
AS numbering
Static or dynamic routing can be used between a customer and a service
provider.
Static routing: Customers with a single or multiple connections to the
service provider (link detection and device failure can be detected)
Dynamic routing (BGP): Multihomed customers.
When using dual links and BGP, BGP attribute manipulation is required to
achieve the required traffic distribution pattern:
Primary and backup link
Load sharing
ModuleSummary
Foradditionalinformation,refertotheseresources:
CiscoSystems,Inc.SampleConfigurationforBGPwithTwoDifferentServiceProviders
(Multihoming).
http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a008009456d.shtml
CiscoSystems,Inc.RemovingPrivateAutonomousSystemNumbersinBGP.
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13756-32.html
CiscoSystems,Inc.HowBGPRoutersUsetheMulti-ExitDiscriminatorforBestPathSelection.
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13759-37.html
CiscoSystems,Inc.ConfiguringtheBGPLocal-ASFeature.
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13761-39.html
CiscoSystems,Inc.LoadSharingwithBGPinSingleandMultihomedEnvironments:Sample
Configurations.
http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00800945bf.shtml
References
Usethequestionsheretoreviewwhatyoulearnedinthismodule.Thecorrect
answersandsolutionsarefoundintheModuleSelf-CheckAnswerKey.
ModuleSelf-Check
1. Ifacustomerrequiresextrabandwidthandredundancy,which
approachispreferred?(Source:"UnderstandingCustomer-to-
ProviderConnectivityRequirements")
AsinglepermanentconnectiontooneISP.
PermanentconnectionstomorethanoneISP.
Dial-upconnectionstomorethanoneISP.
MultiplepermanentconnectionstooneISP.
1. Whichtworoutingoptionscouldbeusedwhenconnectinga
customertoaserviceprovider?(Choosetwo.)(Source:
"UnderstandingCustomer-to-ProviderConnectivityRequirements")
staticrouting
OSPF
IS-IS
BGP
1. Acustomerwouldliketoconnecttoaserviceprovider.Whichtwo
requirementsshouldbeconsideredbeforedecidingonatypeof
connectivity?(Choosetwo.)(Source:"UnderstandingCustomer-to-
ProviderConnectivityRequirements")
applicationavailability
redundancy
costownership
flexibility
1. Whichtypeofredundancydomultiplepermanentconnectionsthat
provideload-sharingconfigurationdisplay?(Source:"Understanding
Customer-to-ProviderConnectivityRequirements")
link
equipment
serviceprovider
routingprotocol
1. Inacustomer-to-providerroutingscheme,whichmethodofrouting
ispreferredbecauseofitslowercomplexity?(Source:
"UnderstandingCustomer-to-ProviderConnectivityRequirements")
policy-basedrouting
dynamicrouting
contentrouting
staticrouting
1. Whyisitthatwithmultiplepermanentconnectionstomorethanone
ISP,theuseofdynamicroutingwithBGPisrequired?(Source:
"UnderstandingCustomer-to-ProviderConnectivityRequirements")
Whenoneoftheconnectionsislost,thelinkleveldetectsthislossand
placestheinterfaceinadownstate.
MonitoringofthelinkstatuscannotdetectaprobleminsideoneoftheISP
networks.
StaticroutesdetectproblemsinsideoneoftheISPnetworks.
Itisnotrequired,andstaticroutingmaybeused.
1. Whatcanbedonewhenacustomerisassignedonlyaverysmall
subnetofpublicaddresses?(Source:"UnderstandingCustomer-to-
ProviderConnectivityRequirements")
Purchasemoreaddressesasrequired.
UseNAT.
Addaserviceprovider.
Addlinkstothesameserviceprovider.
1. Whataretwodifferentaddressingschemesthatcustomersuseto
connecttoaserviceprovider?(Choosetwo.)(Source:
"UnderstandingCustomer-to-ProviderConnectivityRequirements")
provider-independent
customer-independent
provider-assigned
customer-assigned
1. Whichtwoofthefollowingcriteriaarerequiredforacustomertobe
multihomedtomultipleISPs?(Choosetwo.)(Source:
"UnderstandingCustomer-to-ProviderConnectivityRequirements")
ThecustomermusthaveapublicASnumber.
ThecustomermusthaveaprivateASnumber.
ThecustomermustrunBGPwithbothofitsISPs.
ThecustomermustrunBGPwithoneISPandmayusestaticroutingwith
theother.
1. Floatingstaticroutesdonotdemandanyadditionalconfigurationto
workwithBGP.Trueorfalse?(Source:"ImplementingCustomer
ConnectivityUsingStaticRouting")
True
False
1. Whenstaticroutingisimplementedbetweenthecustomernetwork
andtheISPnetwork,thecustomermustannounceadefaultroute.
Trueorfalse?(Source:"ImplementingCustomerConnectivityUsing
StaticRouting")
True
False
1. Whataretworequirementsforbeingabletousestaticroutingas
partofinstallingredundantconnectionsbetweenthecustomer
networkandasingleserviceprovidernetwork?(Choosetwo.)
(Source:"ImplementingCustomerConnectivityUsingStatic
Routing")
Theroutermustbeabletodetectalinkfailure.
ThedefaultroutemustbeannouncedusingthecustomerIGP.
Ifonelinkgoesdown,theinterfacemustremaininanupstate.
ThecustomerIGPmustcontinuetoadvertisethestaticdefaultroute.
1. Acustomerroutethatshouldnotbeannouncedtotherestofthe
Internetismarkedusingwhat?(Source:"ImplementingCustomer
ConnectivityUsingStaticRouting")
aroutetag
theexportcommunity
theno-exportcommunity
thepublicaddressfilter
1. Whenyouaredesigningastaticroutepropagationinaservice
providernetwork,whichthreestepsmustyoutake?(Choosethree.)
(Source:"ImplementingCustomerConnectivityUsingStatic
Routing")
Assignatagtoeachcombinationofservices.
Configureacommunitythatmatchesdefinedtags.
RedistributestaticroutesintoBGPthrougharoutemap.
Identifyallpossiblecombinationsofservicesthatareofferedtoa
customer.
1. Whatdoesaroutemapassignthatwillbeusedbyotherrouters
withinanetwork?(Source:"ImplementingCustomerConnectivity
UsingStaticRouting")
atag
communityvalues
publicaddressing
QoS
1. Whichthreekeypiecesofinformationcanyouderivefromthe
followingroutercommandoutput?(Choosethree.)(Source:
"ImplementingCustomerConnectivityUsingStaticRouting")CE2#
showipbgp209.165.201.0
BGProutingtableentryfor209.165.201.0/24,version7
Paths:(2available,best#1,notadvertisedtoEBGPpeer)
Advertisedtononpeer-grouppeers:
10.3.0.5
Local
0.0.0.0from0.0.0.0(10.3.0.6)
Originincomplete,metric0,localpref100,weight32768,valid,
sourced,best
Community:1:31000no-export
Local
10.3.0.2(metric128)from10.3.0.5(1.0.0.2)
Originincomplete,metric0,localpref100,valid,internal
Originator:1.0.0.2,Clusterlist:10.3.0.5
Community:1:31000no-export
Theprimarylinkhascomebackup,sothebackuprouternowseestwo
alternateroutes.
Theprimarylinkhasnotcomebackup,butthebackuprouterstillsees
twoalternateroutes.
ThefirstrouteistheroutethattherouteritselfhasredistributedintoBGP
usingthefloatingstaticroute.ThisrouteislocallysourcedbytheASand
hasbeenassignedaweightvalueof32768.
ThesecondrouteistheonethathasbeenreceivedbyIBGPfromthe
primaryedgerouter.TheASalsosourcesthisroute,butnoweightvalue
isassigned.
1. Whichtwothingscanyoudotoovercometheproblemsthatoccur
whenafloatingstaticrouteisredistributedintoBGP?(Choosetwo.)
(Source:"ImplementingCustomerConnectivityUsingStatic
Routing")
Youmustraisetheweightvalue.
Youmustlowertheweightvalue.
YoumustsettheADatahighervaluethanallotherroutes.
Youmustassignlocalpreferencevalues,givingthefloatingstaticroutea
lowerlocalpreferencevaluethantheprimaryroute.
1. Whatarethethreecharacteristicsofusingstaticroutesduringload
sharingofoutgoingtraffic?(Choosethree.)(Source:"Implementing
CustomerConnectivityUsingStaticRouting")
Outgoingtrafficloadsharingiseasytoachieve.
Eachcustomerrouterusestheclosestcustomeredgerouterastheexit
point.
Balancedloadsharingisachievedifthecustomeredgeroutersare
collocated.
Localpreferencevaluesmustbeassigned,givingthefloatingstaticroutea
lowerlocalpreferencevaluethantheprimaryroute.
1. Whatarethethreeresponsibilitiesofthecustomerwhenthe
customerhasmultipleconnectionstoasingleserviceprovider?
(Choosethree.)(Source:"ConnectingaCustomertoaSingle
ServiceProvider")
CustomeredgeroutersmustrunIBGPbetweenthem.
Thecustomermustadvertiseadefaultroute.
Thecustomermustconditionallyadvertiseitsassignedaddressspaceinto
BGP.
ThecustomeredgeroutersmustrunEBGPwiththeprovider.
1. Giventhefollowingroutercommandoutput,whichmethodhasbeen
usedtoinfluencereturntrafficinaprimary/backuplink
implementationforthecustomerwithmultipleconnectionstoa
serviceprovider?(Source:"ConnectingaCustomertoaSingle
ServiceProvider")ISP#showipbgp
BGPtableversionis5,localrouterIDis10.0.33.34
Statuscodes:ssuppressed,ddamped,hhistory,*valid,>best,i-
internal
Origincodes:i-IGP,e-EGP,?-incomplete
NetworkNextHopMetricLocPrfWeightPath
*10.10.20.0/24192.168.63.310000100100i
*>192.168.64.420000400100i
*>30.30.30.0/24192.168.63.300100I
*>40.40.40.0/24192.168.64.400400I
MED
localpreference
weight
ASpathprepending
1. Whatarethreeresponsibilitiesoftheproviderrouterwhen
supportingacustomerwithmultipleconnections?(Choosethree.)
(Source:"ConnectingaCustomertoaSingleServiceProvider")
TheprovidermustadvertiseadefaultroutetothecustomerthroughBGP.
Theprovidermustfiltercustomerroutestoverifythatproperaddressingis
used.
TheprovidermustremovetheprivateASnumber,ifitisinusebythe
customer.
TheprovidermustconfigurenewASpathfilterstoallowASpath
prepending;otherwise,aprimary/backuplinkcannotbeestablished.
1. WhatwilloccurifprivateASnumbersareadvertisedtotheInternet?
(Source:"ConnectingaCustomertoaSingleServiceProvider")
TheInternetwillnotbeabletoroutepackets.
InternetrouterscoulddroproutesbasedonBGPloop-prevention
mechanisms.
Customerloadbalancingwillnotfunction.
Customerconfigurationsfortheprimary/backuplinkusingASpath
prependingwillnotfunction.
1. WhichtwoBGPconfigurationsarerequiredtoproperlyimplementa
backupsolutionforacustomerthatisconnectedtoasingleprovider
viamultipleconnections?(Choosetwo.)(Source:"Connectinga
CustomertoaSingleServiceProvider")
Thecustomershouldsetlocalpreferencetoinfluenceoutgoingroute
selection.
Thecustomershouldsettheweightattributetoinfluenceoutgoingpath
selection.
ThecustomershouldsettheMEDoneachroutetoinfluencereturnpath
selection.
ThecustomershouldconfigureASpathprependingtoensureproper
outgoingpathselection.
1. Acustomerrouterhasbeenconfiguredwithmaximumpathssettoa
valueof4.Giventhefollowingroutercommandoutput,overhow
manylinkswilltherouterneedtoperformloadbalancing?(Source:
"ConnectingaCustomertoaSingleServiceProvider")CE1#show
ipbgp
BGPtableversionis5,localrouterIDis10.0.33.34
Statuscodes:ssuppressed,ddamped,hhistory,*valid,>best,i-
internal
Origincodes:i-IGP,e-EGP,?-incomplete
NetworkNextHopMetricLocPrfWeightPath
*10.10.20.0/24192.168.63.30300100100i
*>192.168.64.40400100i
*192.168.65.50500100i
*>30.30.30.0/24192.168.63.300300i
*>40.40.40.0/24192.168.64.400400i
Therouterwilluseonlythepathmarkedas"best"byBGP.
Therouterwillperformloadbalancingovertwopathstoreachnetwork
10.10.20.0/24.
Therouterwillperformloadbalancingoverthreepathstoreachnetwork
10.10.20.0/24.
Thereisnotenoughinformationtodeterminethecorrectanswer.
1. Whichthreemethodscanyouusetoprovideloadsharingover
networklinksbetweenacustomerandasingleprovider?(Choose
three.)(Source:"ConnectingaCustomertoaSingleService
Provider")
Advertisingofsplitaddressingspacetotheprovider.
Configuringebgp-multihopbetweenthecustomerandtheprovider.
UsingtheBGPmaximum-pathscommandtoperformloadbalancingover
parallellinks.
Configuringmultiplestaticroutesthatpointtotheprovider.
1. WhyisitnotrequiredtoconfiguremaximumpathsundertheBGP
routingprocesswhenloadbalancingisbeingperformedbecause
theebgp-multihopcommandhasbeenconfigured?(Source:
"ConnectingaCustomertoaSingleServiceProvider")
Bydefault,BGPwillperformloadbalancingoveruptofourpaths,
configurableuptosix.
ThestaticrouteorIGPprocessisresponsibleforloadbalancinginthis
configuration.
ConfiguringmultihopenablesmaximumpathsequaltotheTTLsettingof
theneighborebgp-multihopcommand.
Configuringebgp-multipathisarequiredcomponentofebgp-multihop
loadbalancing.
1. Whichtwoofthefollowingcharacteristicsaccuratelydescribethe
BGPSupportforDualASConfigurationforNetworkASMigrations
feature?(Choosetwo.)(Source:"ConnectingaCustomertoaSingle
ServiceProvider")
AllowsyoutomergeasecondaryASunderaprimaryASwithout
disruptingcustomerpeeringsessions.
Allowsaroutertoappear,toexternalpeers,asamemberofprimaryAS
duringtheASmigration.
Allowsaroutertoappear,toexternalpeers,asamemberofsecondary
ASduringtheASmigration.
Eliminatesthepossibilitythatroutingloopscanbecreated.
1. AmultihomedcustomerisusingASnumber65550internally.The
customerisconnectedtotwodifferentproviders.ISP1(inAS200)
hasassignedthecustomeranASnumberofAS65101.ISP2(inAS
300)hasassignedthecustomeranASnumberofAS65201.Given
thatthecustomerwilluseASnumbertranslationforitsinternalAS,
whatistheASpathattribute(attachedtoroutesthatoriginatedinthe
customernetwork)thatwillbedisplayedonarouterinthenetwork
ofISP2?(Source:"ConnectingaMultihomedCustomertoMultiple
ServiceProviders")
65550i
65201i
6520165550i
30065201i
1. Whichthreemethodscanyouusetoprovideloadsharingover
networklinksbetweenamultihomedcustomerandmultiple
providers?(Choosethree.)(Source:"ConnectingaMultihomed
CustomertoMultipleServiceProviders")
Advertisingofsplitaddressingspacetotheprovider.
Configuringofmultiplestaticroutesthatpointtotheprovider.
UsingtheBGPmaximum-pathscommandtoperformloadsharingover
parallellinks.
ASpathprependingtofine-tunetheload-sharingconfiguration.
1. WhatarethreeBGPconfigurationcharacteristicsofamultihomed
customerthatisconnectedtomultipleproviders?(Choosethree.)
(Source:"ConnectingaMultihomedCustomertoMultipleService
Providers")
Thecustomerannouncesassignedaddressingtoitsprovidersthrough
BGP.
ThecustomerannouncesadefaultroutetoitsnetworkthroughBGP.
Theproviderannouncesadefaultroute,localroutes,orfullInternet
routingtothecustomerviaBGP.
Thecustomerconfiguresoutboundfilterstopreventitsnetworkfrom
becomingatransitarea.
1. AmultihomedcustomerisusingASnumber1024andisconnected
totwodifferentproviders(ISP1:AS200andISP2:AS300).The
customerhasconfiguredtheMEDtoensureaproperreturnpathso
thatISP1istheprimaryproviderandISP2isthebackupprovider.
Unfortunately,returntrafficcontinuestousethebackuplink.Whatis
apossiblecauseofthisproblem?(Source:"Connectinga
MultihomedCustomertoMultipleServiceProviders")
ThebackupproviderisignoringtheMEDattributeonreceivedroutes.
TheMEDattributecannotbesenttothebackupproviderbecauseitis
localtoAS1024only.
ThecustomerhasnotsettheproperBGPcommunitiestoallowthe
primaryandbackupproviderstocorrectlysettheMEDattribute.
TheMEDcannotbeusedinthisscenario,becauseitwillnotbeadvertised
toprovidersupstreamofISP2.
1. Whatarethreeimportantconsiderationsforcustomersthatwishto
connecttomultipleproviders?(Choosethree.)(Source:"Connecting
aMultihomedCustomertoMultipleServiceProviders")
ThecustomerhastoconsiderwhethertousePAorPIaddressspace.
ThecustomerhastodecidewhethertousestaticroutesorBGPto
connecttoupstreamproviders.
ThecustomerhastodecidewhethertouseapublicASnumberora
privateASnumberscheme.
Thecustomerhastodecidewhethertoperformloadsharingorusea
primary/backupimplementationoverredundantlinks.
1. WhichASnumberselectionisthebestpossiblechoicefora
customerthatismultihomedtomultipleproviders?(Source:
"ConnectingaMultihomedCustomertoMultipleServiceProviders")
AsinglepublicASnumber.
AsingleprivateASnumber.
TwoprivateASnumbersthatareusedinconjunctionwithASnumber
translation.
MultipleprivateASnumbers,oneusedinternallybythecustomerandthe
othersusedinconjunctionwithASnumbertranslationforeachprovider.
1. Giventhefollowingroutercommandoutput,whichtwomethods
havebeenconfiguredtoinfluencereturntrafficinaprimary/backup
linkforthismultihomedcustomer?(Choosetwo.)(Source:
"ConnectingaMultihomedCustomertoMultipleService
Providers")ISP#showipbgp
BGPtableversionis5,localrouterIDis10.0.33.34
Statuscodes:ssuppressed,ddamped,hhistory,*valid,>best,i-
internal
Origincodes:i-IGP,e-EGP,?-incomplete
NetworkNextHopMetricLocPrfWeightPath
*10.10.20.0/24192.168.63.320000100100100i
*>192.168.64.40300100i
*>30.30.30.0/24192.168.63.300300100i
*>40.40.40.0/24192.168.64.400100i
MED
localpreference
splitaddressadvertisement
ASpathprepending
1. Youareacustomerconnectedtotwoserviceproviderswithonelink
toeachprovider.Youwouldliketoachieveprimaryandbackup
trafficdistribution.WhichtwoBGPattributesdoyouhaveto
manipulate?(Choosetwo.)(Source:"ConnectingaMultihomed
CustomertoMultipleServiceProviders")
localpreferenceforincomingtraffic
localpreferenceforoutgoingtraffic
ASpathforincomingtraffic
MEDforincomingtraffic
1. AmultihomedcustomercangetonlyoneprivateASnumber,
regardlessthenumberofserviceproviders.Trueorfalse?(Source:
"ConnectingaMultihomedCustomertoMultipleServiceProviders")
True
False
1. Ifacustomerrequiresextrabandwidthandredundancy,which
approachispreferred?(Source:"UnderstandingCustomer-to-
ProviderConnectivityRequirements")
AsinglepermanentconnectiontooneISP.
PermanentconnectionstomorethanoneISP.
Dial-upconnectionstomorethanoneISP.
MultiplepermanentconnectionstooneISP.
AnswerKey
1. Whichtworoutingoptionscouldbeusedwhenconnectinga
customertoaserviceprovider?(Choosetwo.)(Source:
"UnderstandingCustomer-to-ProviderConnectivityRequirements")
staticrouting
OSPF
IS-IS
BGP
1. Acustomerwouldliketoconnecttoaserviceprovider.Whichtwo
requirementsshouldbeconsideredbeforedecidingonatypeof
connectivity?(Choosetwo.)(Source:"UnderstandingCustomer-to-
ProviderConnectivityRequirements")
applicationavailability
redundancy
costownership
flexibility
1. Whichtypeofredundancydomultiplepermanentconnectionsthat
provideload-sharingconfigurationdisplay?(Source:"Understanding
Customer-to-ProviderConnectivityRequirements")
link
equipment
serviceprovider
routingprotocol
1. Inacustomer-to-providerroutingscheme,whichmethodofrouting
ispreferredbecauseofitslowercomplexity?(Source:
"UnderstandingCustomer-to-ProviderConnectivityRequirements")
policy-basedrouting
dynamicrouting
contentrouting
staticrouting
1. Whyisitthatwithmultiplepermanentconnectionstomorethanone
ISP,theuseofdynamicroutingwithBGPisrequired?(Source:
"UnderstandingCustomer-to-ProviderConnectivityRequirements")
Whenoneoftheconnectionsislost,thelinkleveldetectsthislossand
placestheinterfaceinadownstate.
MonitoringofthelinkstatuscannotdetectaprobleminsideoneoftheISP
networks.
StaticroutesdetectproblemsinsideoneoftheISPnetworks.
Itisnotrequired,andstaticroutingmaybeused.
1. Whatcanbedonewhenacustomerisassignedonlyaverysmall
subnetofpublicaddresses?(Source:"UnderstandingCustomer-to-
ProviderConnectivityRequirements")
Purchasemoreaddressesasrequired.
UseNAT.
Addaserviceprovider.
Addlinkstothesameserviceprovider.
1. Whataretwodifferentaddressingschemesthatcustomersuseto
connecttoaserviceprovider?(Choosetwo.)(Source:
"UnderstandingCustomer-to-ProviderConnectivityRequirements")
provider-independent
customer-independent
provider-assigned
customer-assigned
1. Whichtwoofthefollowingcriteriaarerequiredforacustomertobe
multihomedtomultipleISPs?(Choosetwo.)(Source:
"UnderstandingCustomer-to-ProviderConnectivityRequirements")
ThecustomermusthaveapublicASnumber.
ThecustomermusthaveaprivateASnumber.
ThecustomermustrunBGPwithbothofitsISPs.
ThecustomermustrunBGPwithoneISPandmayusestaticroutingwith
theother.
1. Floatingstaticroutesdonotdemandanyadditionalconfigurationto
workwithBGP.Trueorfalse?(Source:"ImplementingCustomer
ConnectivityUsingStaticRouting")
True
False
1. Whenstaticroutingisimplementedbetweenthecustomernetwork
andtheISPnetwork,thecustomermustannounceadefaultroute.
Trueorfalse?(Source:"ImplementingCustomerConnectivityUsing
StaticRouting")
True
False
1. Whataretworequirementsforbeingabletousestaticroutingas
partofinstallingredundantconnectionsbetweenthecustomer
networkandasingleserviceprovidernetwork?(Choosetwo.)
(Source:"ImplementingCustomerConnectivityUsingStatic
Routing")
Theroutermustbeabletodetectalinkfailure.
ThedefaultroutemustbeannouncedusingthecustomerIGP.
Ifonelinkgoesdown,theinterfacemustremaininanupstate.
ThecustomerIGPmustcontinuetoadvertisethestaticdefaultroute.
1. Acustomerroutethatshouldnotbeannouncedtotherestofthe
Internetismarkedusingwhat?(Source:"ImplementingCustomer
ConnectivityUsingStaticRouting")
aroutetag
theexportcommunity
theno-exportcommunity
thepublicaddressfilter
1. Whenyouaredesigningastaticroutepropagationinaservice
providernetwork,whichthreestepsmustyoutake?(Choosethree.)
(Source:"ImplementingCustomerConnectivityUsingStatic
Routing")
Assignatagtoeachcombinationofservices.
Configureacommunitythatmatchesdefinedtags.
RedistributestaticroutesintoBGPthrougharoutemap.
Identifyallpossiblecombinationsofservicesthatareofferedtoa
customer.
1. Whatdoesaroutemapassignthatwillbeusedbyotherrouters
withinanetwork?(Source:"ImplementingCustomerConnectivity
UsingStaticRouting")
atag
communityvalues
publicaddressing
QoS
1. Whichthreekeypiecesofinformationcanyouderivefromthe
followingroutercommandoutput?(Choosethree.)(Source:
"ImplementingCustomerConnectivityUsingStaticRouting")CE2#
showipbgp209.165.201.0
BGProutingtableentryfor209.165.201.0/24,version7
Paths:(2available,best#1,notadvertisedtoEBGPpeer)
Advertisedtononpeer-grouppeers:
10.3.0.5
Local
0.0.0.0from0.0.0.0(10.3.0.6)
Originincomplete,metric0,localpref100,weight32768,valid,
sourced,best
Community:1:31000no-export
Local
10.3.0.2(metric128)from10.3.0.5(1.0.0.2)
Originincomplete,metric0,localpref100,valid,internal
Originator:1.0.0.2,Clusterlist:10.3.0.5
Community:1:31000no-export
Theprimarylinkhascomebackup,sothebackuprouternowseestwo
alternateroutes.
Theprimarylinkhasnotcomebackup,butthebackuprouterstillsees
twoalternateroutes.
ThefirstrouteistheroutethattherouteritselfhasredistributedintoBGP
usingthefloatingstaticroute.ThisrouteislocallysourcedbytheASand
hasbeenassignedaweightvalueof32768.
ThesecondrouteistheonethathasbeenreceivedbyIBGPfromthe
primaryedgerouter.TheASalsosourcesthisroute,butnoweightvalue
isassigned.
1. Whichtwothingscanyoudotoovercometheproblemsthatoccur
whenafloatingstaticrouteisredistributedintoBGP?(Choosetwo.)
(Source:"ImplementingCustomerConnectivityUsingStatic
Routing")
Youmustraisetheweightvalue.
Youmustlowertheweightvalue.
YoumustsettheADatahighervaluethanallotherroutes.
Youmustassignlocalpreferencevalues,givingthefloatingstaticroutea
lowerlocalpreferencevaluethantheprimaryroute.
1. Whatarethethreecharacteristicsofusingstaticroutesduringload
sharingofoutgoingtraffic?(Choosethree.)(Source:"Implementing
CustomerConnectivityUsingStaticRouting")
Outgoingtrafficloadsharingiseasytoachieve.
Eachcustomerrouterusestheclosestcustomeredgerouterastheexit
point.
Balancedloadsharingisachievedifthecustomeredgeroutersare
collocated.
Localpreferencevaluesmustbeassigned,givingthefloatingstaticroutea
lowerlocalpreferencevaluethantheprimaryroute.
1. Whatarethethreeresponsibilitiesofthecustomerwhenthe
customerhasmultipleconnectionstoasingleserviceprovider?
(Choosethree.)(Source:"ConnectingaCustomertoaSingle
ServiceProvider")
CustomeredgeroutersmustrunIBGPbetweenthem.
Thecustomermustadvertiseadefaultroute.
Thecustomermustconditionallyadvertiseitsassignedaddressspaceinto
BGP.
ThecustomeredgeroutersmustrunEBGPwiththeprovider.
1. Giventhefollowingroutercommandoutput,whichmethodhasbeen
usedtoinfluencereturntrafficinaprimary/backuplink
implementationforthecustomerwithmultipleconnectionstoa
serviceprovider?(Source:"ConnectingaCustomertoaSingle
ServiceProvider")ISP#showipbgp
BGPtableversionis5,localrouterIDis10.0.33.34
Statuscodes:ssuppressed,ddamped,hhistory,*valid,>best,i-
internal
Origincodes:i-IGP,e-EGP,?-incomplete
NetworkNextHopMetricLocPrfWeightPath
*10.10.20.0/24192.168.63.310000100100i
*>192.168.64.420000400100i
*>30.30.30.0/24192.168.63.300100I
*>40.40.40.0/24192.168.64.400400I
MED
localpreference
weight
ASpathprepending
1. Whatarethreeresponsibilitiesoftheproviderrouterwhen
supportingacustomerwithmultipleconnections?(Choosethree.)
(Source:"ConnectingaCustomertoaSingleServiceProvider")
TheprovidermustadvertiseadefaultroutetothecustomerthroughBGP.
Theprovidermustfiltercustomerroutestoverifythatproperaddressingis
used.
TheprovidermustremovetheprivateASnumber,ifitisinusebythe
customer.
TheprovidermustconfigurenewASpathfilterstoallowASpath
prepending;otherwise,aprimary/backuplinkcannotbeestablished.
1. WhatwilloccurifprivateASnumbersareadvertisedtotheInternet?
(Source:"ConnectingaCustomertoaSingleServiceProvider")
TheInternetwillnotbeabletoroutepackets.
InternetrouterscoulddroproutesbasedonBGPloop-prevention
mechanisms.
Customerloadbalancingwillnotfunction.
Customerconfigurationsfortheprimary/backuplinkusingASpath
prependingwillnotfunction.
1. WhichtwoBGPconfigurationsarerequiredtoproperlyimplementa
backupsolutionforacustomerthatisconnectedtoasingleprovider
viamultipleconnections?(Choosetwo.)(Source:"Connectinga
CustomertoaSingleServiceProvider")
Thecustomershouldsetlocalpreferencetoinfluenceoutgoingroute
selection.
Thecustomershouldsettheweightattributetoinfluenceoutgoingpath
selection.
ThecustomershouldsettheMEDoneachroutetoinfluencereturnpath
selection.
ThecustomershouldconfigureASpathprependingtoensureproper
outgoingpathselection.
1. Acustomerrouterhasbeenconfiguredwithmaximumpathssettoa
valueof4.Giventhefollowingroutercommandoutput,overhow
manylinkswilltherouterneedtoperformloadbalancing?(Source:
"ConnectingaCustomertoaSingleServiceProvider")CE1#show
ipbgp
BGPtableversionis5,localrouterIDis10.0.33.34
Statuscodes:ssuppressed,ddamped,hhistory,*valid,>best,i-
internal
Origincodes:i-IGP,e-EGP,?-incomplete
NetworkNextHopMetricLocPrfWeightPath
*10.10.20.0/24192.168.63.30300100100i
*>192.168.64.40400100i
*192.168.65.50500100i
*>30.30.30.0/24192.168.63.300300i
*>40.40.40.0/24192.168.64.400400i
Therouterwilluseonlythepathmarkedas"best"byBGP.
Therouterwillperformloadbalancingovertwopathstoreachnetwork
10.10.20.0/24.
Therouterwillperformloadbalancingoverthreepathstoreachnetwork
10.10.20.0/24.
Thereisnotenoughinformationtodeterminethecorrectanswer.
1. Whichthreemethodscanyouusetoprovideloadsharingover
networklinksbetweenacustomerandasingleprovider?(Choose
three.)(Source:"ConnectingaCustomertoaSingleService
Provider")
Advertisingofsplitaddressingspacetotheprovider.
Configuringebgp-multihopbetweenthecustomerandtheprovider.
UsingtheBGPmaximum-pathscommandtoperformloadbalancingover
parallellinks.
Configuringmultiplestaticroutesthatpointtotheprovider.
1. WhyisitnotrequiredtoconfiguremaximumpathsundertheBGP
routingprocesswhenloadbalancingisbeingperformedbecause
theebgp-multihopcommandhasbeenconfigured?(Source:
"ConnectingaCustomertoaSingleServiceProvider")
Bydefault,BGPwillperformloadbalancingoveruptofourpaths,
configurableuptosix.
ThestaticrouteorIGPprocessisresponsibleforloadbalancinginthis
configuration.
ConfiguringmultihopenablesmaximumpathsequaltotheTTLsettingof
theneighborebgp-multihopcommand.
Configuringebgp-multipathisarequiredcomponentofebgp-multihop
loadbalancing.
1. Whichtwoofthefollowingcharacteristicsaccuratelydescribethe
BGPSupportforDualASConfigurationforNetworkASMigrations
feature?(Choosetwo.)(Source:"ConnectingaCustomertoaSingle
ServiceProvider")
AllowsyoutomergeasecondaryASunderaprimaryASwithout
disruptingcustomerpeeringsessions.
Allowsaroutertoappear,toexternalpeers,asamemberofprimaryAS
duringtheASmigration.
Allowsaroutertoappear,toexternalpeers,asamemberofsecondary
ASduringtheASmigration.
Eliminatesthepossibilitythatroutingloopscanbecreated.
1. AmultihomedcustomerisusingASnumber65550internally.The
customerisconnectedtotwodifferentproviders.ISP1(inAS200)
hasassignedthecustomeranASnumberofAS65101.ISP2(inAS
300)hasassignedthecustomeranASnumberofAS65201.Given
thatthecustomerwilluseASnumbertranslationforitsinternalAS,
whatistheASpathattribute(attachedtoroutesthatoriginatedinthe
customernetwork)thatwillbedisplayedonarouterinthenetwork
ofISP2?(Source:"ConnectingaMultihomedCustomertoMultiple
ServiceProviders")
65550i
65201i
6520165550i
30065201i
1. Whichthreemethodscanyouusetoprovideloadsharingover
networklinksbetweenamultihomedcustomerandmultiple
providers?(Choosethree.)(Source:"ConnectingaMultihomed
CustomertoMultipleServiceProviders")
Advertisingofsplitaddressingspacetotheprovider.
Configuringofmultiplestaticroutesthatpointtotheprovider.
UsingtheBGPmaximum-pathscommandtoperformloadsharingover
parallellinks.
ASpathprependingtofine-tunetheload-sharingconfiguration.
1. WhatarethreeBGPconfigurationcharacteristicsofamultihomed
customerthatisconnectedtomultipleproviders?(Choosethree.)
(Source:"ConnectingaMultihomedCustomertoMultipleService
Providers")
Thecustomerannouncesassignedaddressingtoitsprovidersthrough
BGP.
ThecustomerannouncesadefaultroutetoitsnetworkthroughBGP.
Theproviderannouncesadefaultroute,localroutes,orfullInternet
routingtothecustomerviaBGP.
Thecustomerconfiguresoutboundfilterstopreventitsnetworkfrom
becomingatransitarea.
1. AmultihomedcustomerisusingASnumber1024andisconnected
totwodifferentproviders(ISP1:AS200andISP2:AS300).The
customerhasconfiguredtheMEDtoensureaproperreturnpathso
thatISP1istheprimaryproviderandISP2isthebackupprovider.
Unfortunately,returntrafficcontinuestousethebackuplink.Whatis
apossiblecauseofthisproblem?(Source:"Connectinga
MultihomedCustomertoMultipleServiceProviders")
ThebackupproviderisignoringtheMEDattributeonreceivedroutes.
TheMEDattributecannotbesenttothebackupproviderbecauseitis
localtoAS1024only.
ThecustomerhasnotsettheproperBGPcommunitiestoallowthe
primaryandbackupproviderstocorrectlysettheMEDattribute.
TheMEDcannotbeusedinthisscenario,becauseitwillnotbeadvertised
toprovidersupstreamofISP2.
1. Whatarethreeimportantconsiderationsforcustomersthatwishto
connecttomultipleproviders?(Choosethree.)(Source:"Connecting
aMultihomedCustomertoMultipleServiceProviders")
ThecustomerhastoconsiderwhethertousePAorPIaddressspace.
ThecustomerhastodecidewhethertousestaticroutesorBGPto
connecttoupstreamproviders.
ThecustomerhastodecidewhethertouseapublicASnumberora
privateASnumberscheme.
Thecustomerhastodecidewhethertoperformloadsharingorusea
primary/backupimplementationoverredundantlinks.
1. WhichASnumberselectionisthebestpossiblechoicefora
customerthatismultihomedtomultipleproviders?(Source:
"ConnectingaMultihomedCustomertoMultipleServiceProviders")
AsinglepublicASnumber.
AsingleprivateASnumber.
TwoprivateASnumbersthatareusedinconjunctionwithASnumber
translation.
MultipleprivateASnumbers,oneusedinternallybythecustomerandthe
othersusedinconjunctionwithASnumbertranslationforeachprovider.
1. Giventhefollowingroutercommandoutput,whichtwomethods
havebeenconfiguredtoinfluencereturntrafficinaprimary/backup
linkforthismultihomedcustomer?(Choosetwo.)(Source:
"ConnectingaMultihomedCustomertoMultipleService
Providers")ISP#showipbgp
BGPtableversionis5,localrouterIDis10.0.33.34
Statuscodes:ssuppressed,ddamped,hhistory,*valid,>best,i-
internal
Origincodes:i-IGP,e-EGP,?-incomplete
NetworkNextHopMetricLocPrfWeightPath
*10.10.20.0/24192.168.63.320000100100100i
*>192.168.64.40300100i
*>30.30.30.0/24192.168.63.300300100i
*>40.40.40.0/24192.168.64.400100i
MED
localpreference
splitaddressadvertisement
ASpathprepending
1. Youareacustomerconnectedtotwoserviceproviderswithonelink
toeachprovider.Youwouldliketoachieveprimaryandbackup
trafficdistribution.WhichtwoBGPattributesdoyouhaveto
manipulate?(Choosetwo.)(Source:"ConnectingaMultihomed
CustomertoMultipleServiceProviders")
localpreferenceforincomingtraffic
localpreferenceforoutgoingtraffic
ASpathforincomingtraffic
MEDforincomingtraffic
1. AmultihomedcustomercangetonlyoneprivateASnumber,
regardlessthenumberofserviceproviders.Trueorfalse?(Source:
"ConnectingaMultihomedCustomertoMultipleServiceProviders")
True
False
Introduction
InstandardBGPimplementations,allBGProuterswithinanASmustbefully
meshedsothatallexternalroutinginformationcanbedistributedamongtheother
routersthatresidewithintheAS.Therefore,withinanAS,allroutersmustestablish
TCPsessionswithallotherBGProuters.AstheASgrows,scalabilitychallenges
arisebecauseofanever-increasingnumberofTCPsessionsanddemandsfor
routerCPUandmemoryresources.
Thismodulediscussesnetworkscalabilityconcernsthatarecommontolarge,
complexserviceprovidernetworks.ThemodulealsodiscussesBGProutereflectors
asscalabilitymechanismthatallowsyoutosteerawayfromBGPfull-mesh
requirementsandimprovenetworkscalabilitybyreducingthenumberofTCP
sessionsthatarerequiredwithinanAS.AlsodiscussedinthismodulearetheCisco
IOScommandsthatareneededtoconfigureandmonitorBGProutereflectors.
Uponcompletingthismodule,youwillbeableto:
Describecommonroutingscalabilityissuesinserviceprovidernetworks
DescribethefunctionofroutereflectorsinaBGPenvironment
ConfigureproperoperationofroutereflectorstomodifyIBGPsplit-horizonrules
inanexistingIBGPnetwork
ScalingServiceProviderNetworks
Overview
ProperlyscalingIPaddressing,IGPs,andBGPisacommonareaofconcerntoall
serviceprovidersandcanbethedifferencebetweenasuccessfulandaproblematic
BGPimplementation.Serviceprovidernetworksarecomplexandmustmeetthe
administrativepolicyandroutingdemandsoftheinternalnetwork,different
customers,andotherproviders.Properscalingiscrucialtothesuccessofthe
network.InteractionsbetweenIGPsandtheBGP,specificallywhenyousupport
internalrouting,customerconnectivity,andtransittraffic(andtheadministrative
policiesthatmatch),canbequitecomplex.Furthermore,thelargenumberof
prefixesthatarerequiredtosupportfullInternetroutingrequiresadministratorsto
fullycharacterizeIGPandBGPinteractionsforinternalnetworksandcustomers
alike.
Thislessondiscussesnetworkscalabilityconcernscommontolarge,complex
serviceprovidernetworks.IncludedinthislessonisadescriptionofatypicalISP
networkanddiscussionofthepropagationofinternalandcustomerrouting
information.AlsoscalingconsiderationsforIGPsandBGP,andscalingofIP
addressinginserviceprovidernetworksaredescribed.
Uponcompletingthislesson,youwillbeableto:
Describethebasicstructureofserviceprovidernetworks
Describethepropagationofinternalandcustomerroutesinserviceprovider
networks
DescribeproperscalingofIGPsandBGPinserviceprovidernetworks
DescribescalingissuesthatarerelevanttoIPaddressinginISPnetworks
ScalingIGPandBGPinServiceProvider
Networks
ThecommonserviceprovidernetworkrunsEBGPorstaticroutingwithcustomers.
EBGPisalwaysusedastheroutingprotocolbetweendifferentserviceproviders.
Runs BGP or static routing with customer.
Exchanges routes with other service providers via BGP.
Runs IBGP between its own BGP speakers.
Runs one instance of IGP (OSPF or IS-IS).
IGP used for internal routes only.
IBGPisrequiredintheprovidernetworkbecauseallEBGP-speakingroutersinan
ASmustexchangeexternalroutesviaIBGP.Also,non-EBGPspeakersarerequired
totakepartintheIBGPexchangeiftheyareinatransitpathandforwardpackets
basedondestinationIPaddresses.
TheserviceprovidernetworkalsorunsanIGP.TheprotocolsofchoiceareOSPF
andIS-IS.TheIGPisusedfortwopurposes:
ProvidesIPconnectivitybetweenallIBGPspeakerssothatTCPsessionsfor
IBGPcanbeestablishedbetweenBGP-speakingrouters
ProvidesoptimalroutingtotheBGPnext-hopaddress
AsingleIGPshouldbeusedwithintheentireAS.Thissetupfacilitateseffective
packetforwardingfromtheingressroutertoegressrouters.TheIGPisconfiguredto
carryinternalroutesonly,includinginternallinksandloopbackaddressesofthe
routers.Forperformanceandscalabilityreasons,nocustomerroutesorexternal
routesshouldbeinjectedintotheIGP.
Thetypicalserviceprovidernetworkconsistsofanetworkcorethatconnectsvarious
edgedevices.Someoftheedgedevicesconnectcustomers;othersconnecttoother
serviceproviders.
TheedgedevicesthatconnecttootherserviceprovidersuseEBGPtoexchange
routinginformation.Theedgedevicesthatconnectcustomersuseeitherstatic
routingorEBGP.
UnlessMPLSisconfiguredontheserviceproviderbackbone,routersinatransit
patharealsorequiredtohavefullroutinginformation.Therefore,theserouterstake
partintheIBGProutingexchange.
AnIGPisalsorequiredwithintheserviceprovidernetwork.TheIGPisusedtocarry
internalroutes,includingtheloopbackinterfaceaddressesofIBGP-speaking
routers.TheIGPprovidesreachabilityinformationtoestablishIBGPsessionsandto
performtherecursiveroutinglookupfortheBGPnexthop.
CommonServiceProviderNetwork
PE routers use EBGP or static routing with CE routers.
PE and P routers use full-mesh IBGP routing.
The provider core IGP is a single instance of IS-IS or OSPF and is used only
within the service provider core network.
Optimal routing between PEs is desired.
CEroutersconnectviaIPtoPErouters.Inmanycases,thePEroutersusestatic
routingtocustomernetworks.ThePEroutersadvertisestaticroutestotherestof
theserviceprovidernetworkandtootherautonomoussystemsusingBGP.PEand
Proutersusefull-meshIBGProutingtoexchangeBGProutinginformation.
ServiceprovidersuseBGProutingwiththecustomerwhenredundancyrequiresthe
useofaroutingprotocol.
TheserviceproviderbackbonetypicallyusesasingleinstanceofeitherIS-ISor
OSPFasitsIGP.TheIGPisusedwithintheproviderbackboneonly.Theprovider
backboneexchangesnoIGProutinginformationwithcustomerroutersorwith
routersinotherautonomoussystems.
ItisimportanttoavoidsendinganyunnecessaryroutinginformationintheIGP.The
IGPperformsbestifitcarriesasfewroutesaspossible.Optimally,theIGPshould
containonlyinformationaboutBGPnexthopsandroutesthatareinternaltothe
serviceprovidernetwork,enablingtheestablishmentofIBGPsessions.
BGP route propagation
BGP carries customer routes.
BGP carries other provider routes.
IGP route propagation
IGP is responsible only for the resolution of BGP next hop and internal
routes
Do not redistribute BGP into IGP.
IGP performance and convergence time suffer if many are carried.
No IGP is capable of carrying full Internet routes.
A full Internet routing table has exceeded 600,000 routes.
AllotherroutinginformationshouldbecarriedinBGP,whichisdesignedtoscalefor
largevolumesofroutinginformation.Customerroutesandtheroutesfromother
serviceprovidersshouldbecarriedinBGP.Theseroutesshouldnotbepropagated
fromBGPintotheproviderIGP.
IGPperformanceandconvergencetimesufferiftheIGPcarriesalargernumberof
routes.Thedesigngoalshouldbetominimizethevolumeofroutinginformationthat
theIGPcarries.Naturally,thenumberofrouteflapsisalsoreducedasthenumber
ofroutesisreduced.
BGPscalestoamuchlargervolumeofroutinginformationbecauseoftheinherent
qualitiesofthedesignofBGP.Potentially,theBGProutersoftheserviceprovider
canreceivethefullInternetroutingtable,whichhasexceeded600,000routes.You
shouldthereforeneverredistributetheroutinginformationthattheBGPreceivedinto
theIGP,becausenoIGPiscapableofcarryingseveraltensofthousandsofroutes.
RoutingInformationExchangewithOtherServiceProviders
BGP is used to exchange routing information with upstream service providers:
Service provider sends summary of service provider-owned address space to
upstream service provider.
Service provider sends prefixes owned by customers using independent
address space.
Upstream service provider sends full Internet routing table to the service
provider.
Thefigureshowshowroutesareexchangedbetweenserviceproviders.
ProvideredgeroutersuseBGPtoexchangeroutinginformationwithotherservice
providernetworksforredundancyandscalabilityreasons.
RoutinginformationfromcustomersisreceivedatPEroutersusingEBGPandthen
propagatedusingIBGPtotherestoftheserviceP-network.AtanotherPErouter,
theroutinginformationisfurtherpropagatedtoadifferentserviceproviderusing
EBGPwithotherautonomoussystems.Theserviceprovidersendsasummaryof
serviceprovider-ownedspacetoupstreamserviceproviders,alongwithprefixes
ownedbythecustomerusingprovider-independentaddressspace.Theupstream
serviceprovidersendsthefullInternetroutingtabletotheserviceprovider,which
allowstheserviceprovidertoimplementaprimary/backupscenariooraload-
balancingscenariowhenpeeringwithmultipleupstreamserviceproviders.
RoutePropagationinServiceProviderNetworks
RoutingInformationExchangewithCustomers
BGP with customer:
Customer advertises its address space.
Service provider advertises default route, service provider-owned routes and
default route, or full Internet routing table.
Static routing with customer:
Customer uses default route.
Service provider uses static route on the PE router for customer address
space. Static route is redistributed into BGP on the PE router.
Thefigureshowshowcustomersexchangerouteswiththeserviceprovider.Inthis
case,thecustomeradvertisesitsaddressspacetotheserviceprovider.Theservice
provideradvertisestothecustomereitherthedefaultroute,serviceprovider-owned
routesandthedefaultroute,orthefullInternetroutingtable,basedonthecustomer
requirements.
Whenthecustomerdoesnotrequireredundancy,staticroutingisused.Inthiscase,
thecustomerconfiguresastaticdefaultroutethatpointstothePErouterofthe
serviceprovider.Theserviceproviderconfiguresastaticrouteforthecustomer
networkthatpointstotheCErouter.ThePErouterredistributesstaticroutesinto
BGP.TheserviceP-networkthenusesBGPtopropagatetheinformationtotherest
oftheserviceP-networkusingIBGP.Theserviceprovideralsoadvertisescustomer
routinginformationtootherautonomoussystemsusingBGP.
Next-HopResolution
The core IGP of the service provider should carry information only about
backbone links and loopback addresses.
next-hop-self feature on the PE routers removes the need to include access
links in IGP, and thus prevents route flapping if access link flaps.
ThefigureshowswhyIGPisneededintheserviceprovidercorenetwork.TheIGP
usedintheserviceprovidercoreshouldcarryinformationonlyaboutbackbonelinks
andloopbackaddresses.TheserviceprovidershoulduseBGPtocarryallother
information.
UsetheBGPnext-hop-selfcommandwhenBGProutingisexchangedwiththe
customerorotherserviceproviders.Usingthenext-hop-selfcommandresultsin
theBGPnexthopbeingsettotheloopbackaddressoftheserviceprovideredge
routerandnottotheaccesslinkaddressofthecustomer.TheIGPcanthenbe
relievedoftheburdenofcarryinginformationabouttheaccesslink.Thebenefitof
notcarryingcustomerlinkinformationisthataflappingaccesslinkwillnotdisturb
theserviceproviderIGP.
AsshownintheroutingtableonthePE2router,thenexthopforthecustomer
networkpointstotheaddressofthePE1route.ThisisusuallytheIPaddressofthe
loopbackinterface.BecausethePE1andPE2routersarenotdirectlyconnected,an
IGProutingprotocolisneededthatcarriesinformationaboutcorelinksand
loopbacksacrosstheserviceprovidernetwork.
BGPandIGPhavedifferentresponsibilitiesinserviceprovidernetworks.
IGP responsibilities:
Carrying route to BGP next hop.
Providing optimal path to next hop.
Converging to alternate path so that BGP peering is maintained.
BGP responsibilities:
Generating BGP update.
Scaling BGP policies.
Scaling IBGP mesh.
Reducing impact of flapping routes.
TheIGPisresponsibleforthefollowing:
CarryingroutestotheBGPnexthopstofacilitaterecursiverouting.
Providinganoptimalpathtothenexthop,therebyoptimizingpacketflowtoward
allBGPdestinations.
Convergingtoanalternatepathinthecaseoflostlinksorroutersinaredundant
network(whichshouldbequicksothatBGPsessionsarenotlost).
TheBGPisresponsibleforthefollowing:
GeneratingBGPupdatesaboutreachableandunreachablenetworks.
ImplementingandscalingtheBGProutingpolicy,whichcanbequite
cumbersomeinlargeserviceprovidernetworkswithmanyEBGP-speaking
routers.
ImplementingandscalingIBGPsessionsbetweenallBGP-speakingroutersin
theAS.
Reducingtheimpactofindividualflappingroutesthroughroutesummarization.
ScalingIGP
InscalinganIGP,itisimportanttolimitthenumberofroutesthattheIGPcarries.
Optimally,theIGPcarriesonlyloopbackinterfacesandinternallinks.
Loopbacks and internal links carried only.
Good addressing structure within the POP required.
Loopback addresses taken out of a different address space and not
summarized.
Summarization of internal link addresses on POP level.
Optimal routes to loopbacks needed only (with proper summarization).
ThenumberofroutesthattheIGPcarriedcanbeevenfurtherreducedwithroute
summarization.However,caremustbetakenbecauseloopbackaddressesshould
neverbesummarized.Routesummarizationalwaysintroducestheriskof
suboptimalroutingandshouldbecarefullyplanned,becauseitisimportantthat
recursiveroutinglookupalwaysusesoptimalroutingtothenexthop.Also,inan
MPLSenvironment,anLSPmustbeunbrokenbetweenedgerouters,and
summarizingloopbackinterfaceswillbreaktheLSP.
InternallinkscanalwaysbesummarizedbecausetheyarenotusedasBGPnext-
hopaddresses.Tofacilitateproperroutesummarization,internallinksandloopback
interfacesonaroutershouldbeassignedaddressesfromtwodifferentaddress
spaces.Also,theinternallinksofaroutershouldbeassignedaddressesdepending
uponwhichPOPtheroutersbelongto.
Ifimplementedcorrectly,allinternalrouterlinksinonePOPcanbesummarizedat
thePOPlevelandinjectedintothecoreasasingleroute.But,allrouterloopback
addresseswithinthePOParestillpropagatedintothecoreasindividualhostroutes,
givingoptimalroutingtoallloopbackinterfaces.
ScalingBGP
ScalingBGPincludesscalingbothIBGPandEBGPaswellasscalingBGPupdates
androutingtablesize.
BGP policy scaling:
ScalingServiceProviderRoutingProtocols
The AS routing policy should be unitary and easy to maintain.
This goal is achieved by reusing the same configuration in all EBGP-
speaking routers.
IBGP mesh scaling:
Avoid unnecessary duplicate updates over a physical link.
Full-mesh IBGP is not needed since there are other technologies and
features available
Updates and table size scaling:
Route summarization is the key to scalability.
ThetaskofscalingBGPactuallyinvolvesthreedifferentandindependentscaling
tasks:
BGPpolicyscaling:TheASroutingpolicyshouldbeunitaryandeasyto
maintain.DifferentedgeroutersofthesameASshouldnotusedifferentpolicies
andtherebyadvertisedifferentroutestoneighboringautonomoussystems.
Regardlessofwhichrouteriscurrentlyactive,thesameroutingpolicyshouldbe
inplace.Administratively,replicationofthesameroutingpoliciesrequiresthe
sameconfigurationlinesinseveraledgerouters.
IBGPmeshscaling:AllBGP-speakingroutersmustbeupdatedwithconsistent
IBGPinformation.InthetraditionalBGPapproach,ensuringconsistentrouting
informationwasachievedbyestablishingafullmeshofIBGPsessionsbetween
allrouterswithintheAS.AnIBGPfullmeshiscertainlynotscalable,andseveral
toolsarenowavailabletoachievethesameresultswithoutthefullmesh.
Updatesandtablesizescaling:Thenumberofroutesintheroutingtableand
thenumberofupdatesthataresentandreceivedrepresentthethirdscaling
task.Routesummarizationisthekeytothisscalability.
InternalIPaddressingintheserviceprovidercorenetworkcanbesimplifiedto
reducetheuseofpublicaddressesandtosimplifyconfiguration.
This list shows how to scale addressing in Service Provider Core Network:
Private or public IP addresses can be used.
Private addresses on core links and loopbacks display private IP addresses in a
traceroute when run from customers.
MPLS with TTL propagation disabled solves the traceroute issue.
Private addresses on loopbacks and core links call for careful external routing to
prevent advertisement of private addresses to customers or upstream service
providers.
Otherwise, use public addresses in service provider core networks.
ForIPv4,publicorprivateIPaddressescanbeusedoncorelinks.However,using
privateaddresseshassomedrawbacks.Privateaddressesontheproviderinternal
linkswillcausetroubleforthetracerouteapplication.Whenthetraceroute
commandisexecutedfromthecustomer,thecustomerwillseeprivateIPaddresses
inthetracerouteprintout.UsingMPLSwithoutTTLpropagationintheservice
P-networkcaneasilyovercomethetracerouteproblemwithprivateaddresses.If
thesefunctionsareused,theprovidernetworkwillappearasasinglehoptothe
tracerouteapplication.Theintermediaterouterswillbeinvisibleandthuscanuse
privateaddresses.
Usingprivateaddressesontheserviceproviderrouterloopbackinterfacesis
possible.However,youmusttakecarenottoadvertiseanyprivateaddressestoany
otherAS.
Aruleofsafetyistopreventtheannouncementofanyprivateaddressesbyusing
prefixliststhatareappliedonoutgoingupdatestoexternalneighbors.Thesame
prefixlistmechanismcanalsobeusedontheprovideredgerouters.Itcanbeused
topreventacceptingprivateaddressesfromanyotherASiftheotherAS,by
mistake,announcesprivateaddresses.
ScalingServiceProviderAddressing
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
Service providers most commonly use integrated IS-IS and OSPF as interior
gateway protocols and BGP as the exterior gateway protocol.
BGP is used to carry customer routes while IGPs are used to carry service
provider internal prefix reachability information.
BGP allows ISP clients to acquire information about all or some networks
reachable through the ISP.
ISP can use static routing or BGP to direct traffic going to the customers to the
correct links.
Next-hop-self can be used to avoid redistributing transit segments into IGP on
iBGP neighbors.
When BGP networks grow, various actions must be taken to make them
scalable, for iBGP scalability use route reflectors.
When IP networks grow, several aspects of addressing need to be considered to
reduce sizes of routing tables and to avoid consuming too many addresses.
Summary
Overview
LargeBGPnetworkscannotproperlyscalewithoutrelyingonperformance-
enhancingtoolssuchasroutereflectorsandconfederations.Routereflectorsenable
BGProutinginformationtobedistributedinafashionthatdoesnotrequirea
physicalfullymeshednetwork.Networkoverheadisreducedbydecreasingthe
numberofTCPconnectionsthatarerequiredtodistributeroutinginformationandby
lesseningrouterCPUandmemoryrequirements.
ThislessonintroducesBGProutereflectorsbyexplainingwhytheyimproveBGP
scalability.Modifiedsplit-horizonrules,appliedwhenyouareusingroutereflectors,
arealsodiscussed.Thelessonalsodescribesthevariousredundancymechanisms
thatareusedwithroutereflectors,includingroutereflectorclusters.Thelesson
introducesnetworkdesignrulesthatyoushouldfollowwhenimplementinganetwork
withBGProutereflectors.Italsoliststhepotentialissuesthatcanariseifthe
networkdesignrulesarenotadheredto.Thelessonconcludesbydescribingthe
conceptofhierarchicalroutereflectors.
Uponcompletingthislesson,youwillbeableto:
ExplaintheneedforBGProutereflectorsinBGPtransitbackbones
ExplainhowroutereflectorsmodifytraditionalIBGPsplit-horizonrules
Explainthebenefitsofdeployingredundantroutereflectors
Explainhowroutereflectorclusterspreventloopsinthedeploymentofroute
reflectorsinredundantconfigurations
Describeadditionalroutereflectormechanismsthathavebeendesignedto
preventroutingloops
ListthenetworkdesignrulesforimplementingBGProutereflectors
Listthepotentialissuesthatcanariseifyoudonotfollowtheroutereflector
networkdesignrules
Explainthefunctionofhierarchicalroutereflectors
IntroducingandDesigningRouteReflectors
ClassicIBGPsplit-horizonrulesspecifythatupdatesthatarereceivedonanEBGP
sessionshouldbeforwardedonallIBGPandEBGPsessions.However,updates
thatarereceivedonanIBGPsessionshouldbeforwardedonlytoallEBGP
sessions.ThisrulerequiresaBGPboundaryroutertobeabletosendrouting
updatestoallotherBGP-speakingroutersinitsownASdirectlythroughaseparate
IBGPsessiontoeachofthem.
IBGP requires a full mesh between all BGP-speaking routers.
Large number of TCP sessions.
Unnecessary duplicate routing traffic.
Solutions:
Route reflectors modify IBGP split-horizon rules.
BGP confederations modify IBGP AS-path processing.
TheprimaryreasonfortheIBGPsplit-horizonruleistoavoidroutinginformation
loopswithintheAS.IftheinformationthatisreceivedthroughanIBGPsessionis
forwardedonotherIBGPsessions,theinformationmightcomebacktothe
originator.Thisinformationisthenforwardedagaininanever-endingloop.The
originatorwouldnotdetecttheloopbecausenoBGPattributesarechangedon
IBGPsessions.
ThegeneraldesignruleinclassicIBGPistohaveafullmeshofIBGPsessions.But
afullmeshofIBGPsessionsbetweennnumberofrouterswouldrequire(n*(n-1))
/2IBGPsessions.Forexample,arouterwithanASthatcontains10routerswould
require(10*(10-1))/2=45IBGPsessions.Imaginethenumberofsessions(and
theassociatedrouterconfiguration)thatwouldberequiredforasingleAScontaining
500routers.
EveryIBGPsessionusesasingleTCPsessiontoanotherIBGPpeer.Anupdate
thatmustbesenttoallIBGPpeersmustbesentoneachoftheindividualTCP
sessions.Ifarouterisattachedtotherestofthenetworkoverjustasinglelink,this
singlelinkhastocarryallTCP/IPpacketsforallIBGPsessions.Thisrequirement
resultsinmultiplicationoftheupdateoverthesinglelink.
RoutereflectorsareBGPscalabilitymechanismsthatenableroutinginformationto
beredistributedtoallrouterswithinanASwhileeliminatingtheneedforafully
meshedtopologywithintheAS.ThisfeaturereducesthenumberofTCPsessions
thatmustbemaintained,loweringnetworkoverheadandCPUandmemoryresource
requirements.
Twodifferentsolutionsareavailabletoachievegreaterscalabilitywhenyouare
facedwiththefull-meshrulesofIBGPautonomoussystems:
RoutereflectorsmodifytheclassicIBGPsplit-horizonruleandallowaparticular
routertoforwardincomingIBGPupdatestoanoutgoingIBGPsessionunder
certainconditions.Thisrouterbecomesaconcentrationrouter,oraroute
reflector.
BGPconfederations(notcoveredinthiscourse)introducetheconceptofa
numberofsmallerautonomoussystemswithintheoriginalAS.Thesmall
autonomoussystemsexchangeBGPupdatesbetweenthemusingintra-
confederationEBGPsessions.
IBGPScalabilityIssuesinaTransitAS
InclassicIBGP,theBGPboundaryrouterneedstoforwardtheroutethatisreceived
fromanEBGPpeertoeveryotherrouterwithinitsownAS.ItusesadedicatedIBGP
sessionforeachrouter.Also,theBGPboundaryrouterforwardsroutesthatarouter
sourcesinthesameway.Toalloweveryroutertoupdateeveryotherrouter,afull
meshofIBGPsessionsisrequired.
Classic IBGP: IBGP routes are not propagated to other IBGP peers.
Full mesh of IBGP peers is therefore required.
ThefigureshowshowtherouterreceivesroutingupdateonEBGPsessionand
forwardsthisupdatetoallIBGPneighbors.WhenIBGPneighborsreceivethis
updateonIBGPsessions,theseupdatesarenotforwardedtoanyotherIBGP
sessionsbydefault.
Route reflector can propagate IBGP routes to other IBGP peers.
Full mesh of IBGP peers is no longer required.
TheIBGProutereflectordesignrelaxestheneedforafullmesh.Therouterthatis
configuredasaroutereflector,undercertainconditions,willrelayupdatesthatare
receivedthroughanIBGPsessiontoanotherIBGPsession.Thiscapabilityrequires
modificationsoftheclassicIBGPsplit-horizonrules.
Theroutereflectorconceptintroducesprocessingoverheadontheconcentration
routerand,ifitisconfiguredincorrectly,cancauseroutingloopsandinstability.
RouteReflectorSplit-HorizonRules
Whenyouimplementaroute-reflector-basedIBGPnetwork,theBGProutersare
dividedintoroutereflectors(whichimplementmodifiedsplit-horizonrules)and
clients(whicharebehavingliketraditionalIBGProuters).
Routereflectorclientsareexcludedfromthefullmesh.Theycanhaveanynumber
ofEBGPsessionsbutmayhaveonlyoneIBGPsession,thesessionwiththeirroute
reflector.ClientsconformtotheclassicIBGPsplit-horizonrulesandforwarda
receivedroutefromEBGPontheirIBGPneighborsessions.Buttheroutereflector
conformstotheroutereflectorsplit-horizonrulesandrecognizesthatithasanIBGP
sessiontoaclient.WhentheIBGPupdateisreceivedfromtheclient,theroute
reflectorforwardstheupdatetootherIBGPneighbors,thereforealleviatingtheIBGP
full-meshrequirementforitsclients.
Similarly,whentheroutereflectorreceivesanIBGPupdatefromaneighborthatis
notitsclient,itforwardstheupdatetoallofitsclients.
ForwardingofanIBGPupdateinaroutereflectordoesnotchangethenext-hop
attributeoranyothercommonBGPattribute.Thisfeaturemeansthattheclientwill
usetheoptimumroutewiththerecursiverouting,regardlessofthewaythatithas
receivedtheBGProute.
Anotherexampleofroutepropagationinaroutereflector-enablednetwork:
NonclientsconformtotheclassicIBGPsplit-horizonrulesandforwarda
receivedroutefromEBGPontheirIBGPneighborsessions.
Theupperroutereflectorthatreceivesaroutefromanonclientsendstheroute
toEBGPpeersandclientsonly.ItisthereasonwhytheIBGPupdateisnotsent
tothebottomroutereflectoraswell.
TheclientreceivestheIBGPupdateandsendsittotheEBGPpeers.
ThetablepresentsdetailedIBGPsplit-horizonrulesasmodifiedbytheintroduction
ofBGProutereflectors.Forpurposesofdefinition,a"routereflector"isaBGP
speakerthatcanadvertiseIBGPlearnedrouterstoanotherIBGPpeerand,hence,
canreflectroutes.IBGPpeersoftheroutereflectorfallundertwocategories:
"clients"and"nonclients."Theroutereflectoranditsclientsforma"cluster."AllIBGP
peersoftheroutereflectorthatarenotpartoftheclusterarenonclients.A"classic"
IBGProuterisarouterthatdoesnotsupportroutereflectorfunctionality.
TypeofRouter IncomingUpdateFrom IsForwardedTo
Classic EBGPpeer Allpeers(IBGPandEBGP)
IBGPpeer EBGPpeers
Routereflector EBGPpeer Allpeers(IBGPandEBGP)
NonclientIBGPpeer EBGPpeersandclients
ClientIBGPpeer Allpeersbutthesender
Client EBGPpeer Allpeers(IBGPandEBGP)
IBGPpeer EBGPpeers
ClientsmayhaveanynumberofEBGPpeersbutmayhaveIBGPsessionsonlywith
theirroutereflectororreflectors.Ifthereflectorfails,itsclientcannolongersend
BGPupdatesto,orreceivethemfrom,therestoftheAS.Theroutereflectoris,
therefore,asinglepointoffailure.
Clients that have an IBGP session with only one route reflector will not be able
to send any BGP updates if the route reflector fails.
Clients should establish an IBGP session with at least two route reflectors using
different physical connections.
Toavoidintroducingasinglepointoffailureintothenetwork,theroutereflector
functionalitymustbeasredundantasthephysicalnetwork.Ifaclientwillstillbe
physicallyattachedtothenetworkafteritsroutereflectorhasfailed,theclientshould
havearedundantroutereflector.Thus,inallhighlyavailablenetworks,route
reflectorsmustberedundant.
Redundant reflectors solve the high-availability requirement.
The concept of clusters is introduced to prevent IBGP routing loops between
route reflectors.
AclientmayhaveIBGPsessionstomorethanoneroutereflectortoavoidasingle
pointoffailure.Eachclientwillreceivethesameroutefrombothofitsreflectors.
BothroutereflectorswillreceivethesameIBGPupdatefromtheirclient,andthey
willbothreflecttheupdatetotherestoftheclients.Additionally,bothroutereflectors
willgetupdatedfromthefullmeshandreflectthoseupdatestotheirclients.Asa
result,eachclientwillgettwocopiesofallroutes.Undercertaincircumstances
(particularlywhenyouuseweightsonIBGPsessionstoinfluenceBGProute
selection),improperroutereflectioncanresultinanIBGProutingloop.Andthis
routingloopisimpossibletodetect.ExtraBGPattributesarethusnecessaryto
preventtheseroutingloops.
RedundantRouteReflectors
Arouterthatisactingasaroutereflectorclientdoesnotrequireanyspecific
configuration.ItsimplyhasfewerIBGPsessionsthanitwouldhaveifitwerepartof
thefullmesh.Butimproperlyconfiguringtheclienttoalsobeareflectorcouldeasily
causealoop.AnIBGProutecominginfromoneoftherealreflectorstotheclient
couldbeforwardedbytheclient,erroneouslyactingasreflector,totheother
reflector.
A group of redundant route reflectors and their clients form a cluster.
Each cluster must have a unique cluster ID.
Each time a route is reflected, the cluster ID is added to the cluster-list BGP
attribute.
The route that already contains the local cluster ID in the cluster list is not
reflected.
RoutereflectorclusterspreventIBGProutingloopsinredundantroutereflector
designs.
Theroleofthenetworkdesigneristoproperlyidentifywhichroutereflectorsand
theirclientswillformacluster.Thedesignerassignstotheclusteracluster-ID
numberthatisuniquewithintheAS.
Thecluster-IDnumbermustbeconfiguredintheroute
reflectors.Theclientsshouldnotbeconfiguredwiththis
information.
Aroutereflectorroutercanreflectroutesonlywithinasinglecluster.Aroutereflector
can,however,participateinanotherclusterbutonlyasaclient.Aclientcanfunction
asaclientonlytoaroutereflectorbelongingtothesamecluster.
Whenarouteisreflected,thereflectorcreatesthecluster-listattributeandattaches
ittotherouteifitdoesnotexist.Itthensetsitscluster-IDnumberinthecluster-listor
addsitscluster-IDnumbertoanalreadyexistingcluster-listattribute.Iftheroute,for
anyreason,iseverreflectedbacktothesamereflector,itwillrecognizeitscluster-
IDnumberinthecluster-listandnotforwarditagain.Thefirstroutereflectorthat
reflectstheroutealsosetsaBGPattribute,called"originator-ID,"andaddsittothe
BGProuter-IDofitsclient.
Thecluster-listandoriginator-IDattributesare
nontransitiveoptionalBGPattributes,allowingroutersthat
donotsupportroutereflectorfunctionalitytocoexistwith
routereflectorsandtheirclientsinthesameAS.
Basedoncluster-listandoriginator-IDattributes,routerscanimplementtwoloop-
preventionmechanisms:
AnyrouterthatreceivesanIBGPupdatewiththeoriginator-IDattributesettoits
ownBGProuter-IDwillignorethatupdate.
AnyroutereflectorthatreceivesanIBGPupdatewithitscluster-IDalreadyin
thecluster-listwillignorethatupdate.
Intheexample,theclientintheclusterforwardsthereceivedEBGPupdatetoboth
reflectors.TheroutereflectorsforwardtheupdateintotheIBGPfullmesh.This
behaviormeansthattheysendtheupdatetoeachotheraswell.Butwhenaroute
reflectorreceivesaBGPupdatefromanotherroutereflector,itrecognizestheir
commonclusterIDnumberinthecluster-listattribute.Therefore,thenewlyreceived
routeupdateisignored.
RouteReflectorClusters
Whenarouteisreflected,theroutereflectorsetstheoriginator-IDBGPattribute
(nontransitiveoptionalBGPattribute)totherouter-IDofthepeerfromwhichit
receivedtheroute.Anyrouterthatreceivesaroutewithitsownrouter-IDinthe
originator-IDattributesilentlyignoresthatroute.
Every time a route is reflected, the router-ID of the originating IBGP router is
stored in the originator-ID BGP attribute.
A router receiving an IBGP route with originator-ID set to its own router-ID
ignores that route.
The BGP path selection procedure is modified to take into account cluster-list
and originator-ID.
BGPpathselectionruleshavebeenmodifiedtoselectthebestrouteinscenarios
wherearoutermightreceivereflectedandnonreflectedroutesorseveralreflected
routes:
ThetraditionalBGPpathselectionparameterssuchasweight,localpreference,
origin,andMEDarecomparedfirst.
Iftheseparametersareequal,theroutesthatarereceivedfromEBGP
neighborsarepreferredoverroutesthatarereceivedfromIBGPneighbors.
WhenarouterreceivestwoIBGProutes,thenonreflectedroutes(routeswithno
originator-IDattribute)arepreferredoverreflectedroutes.
Thereflectedrouteswithshortercluster-listsarepreferredoverrouteswith
longercluster-lists.
Iftheadditionalroute-reflector-orientedselectioncriteriadonotyieldadecision,
therestofthetraditionalBGPpathselectionrulesarefollowed.
AdditionalRouteReflectorLoop-PreventionMechanisms
Thephysicaltopologyofthenetworkcouldserveasaguidetoroutereflector
design.
Route reflector rules:
Route reflector rules divide a transit AS into smaller areas (called clusters).
Each cluster contains route reflectors and route reflector clients.
Routers that do not support route reflector functionality act as a one-router
cluster or as a route reflector client.
ImplementingroutereflectorswithinthetransitASwillcreatesmallerareas(or
groups)ofrouters.Thesesmallergroupingsofroutersarecalledclusters.Acluster
consistsofroutereflectorrouters,eitherredundantornonredundant,andtheclient
routersthatareconnectedtothem.
IndesigningtheimplementationofroutereflectorswithinatransitAS,identifya
groupofperipheralroutersthatarephysicallyconnectedtothesamebackbone
routerorrouters.Considertheperipheralroutersasclientsandthebackbone
routersasroutereflectors.Then,considerthisgroupofrouterstoformacluster.
IBGP session rules:
All clients in a cluster must establish IBGP sessions with and only with all route
reflectors in the cluster.
An IBGP full mesh between all route reflectors within the AS is required.
Routers that are not route reflectors can participate in the IBGP full mesh or be
route reflector clients.
TheprincipalgoalfordesigningnetworkswithBGProutereflectorsistoreducethe
sizeofthefullmeshofIBGPsessionsbyexcludingsomeroutersfromthemesh.
Theroutersthatareexcludedfromthefullmesh,theclients,havetosendtheirIBGP
informationto,andreceiveitfrom,atleastonerouterthatbelongstothefullmesh,
theroutereflector.Thus,thefullmeshisstillthere,butitissmaller,andallroute
reflectorshavetobepartofit.
AllclientsinaclustershouldhaveIBGPsessionswithalltheirroutereflectorsand
theirroutereflectorsonly.Ifaclientdoesnothavesessionswithallthereflectorsin
thecluster,theredundancyisviolated.IfaclienthasIBGPsessionstoroutersother
thantheroutereflectors,unnecessaryroutingtrafficisgenerated.
Bothclientsandotherrouters,thatarenotroutereflectors,obeytheclassicIBGP
split-horizonrules.Thus,non-route-reflectorroutersareeitherclientstoareflectoror
areparticipatingdirectlyinthefullmesh.
Intheexample,therearetwoclustersandonestandaloneBGProuters.Inthearea
thatislabeled“redundantcluster,”thethreeclientroutersandthetworoutereflector
routersmakeupthecluster.EachofthethreeclientroutershasanIBGPsession
withthetworoutereflectorsandonlywiththosetworoutereflectors.
Inthenonredundantarea,theclientrouterhasasinglephysicalconnectiontoa
routereflectorrouter.Thesetworoutersformanonredundantcluster.Therouterthat
isdesignatedastheroutereflectorintheclusterisalreadyasinglepointoffailurein
thisphysicaldesign.Afailureofthisrouterwillpreventtheclientsintheclusterfrom
reachingtherestofthenetwork.Therefore,thereisnonewsinglepointoffailure
thatisintroducedwhentherouterisconfiguredastheonlyroutereflectorinthis
cluster.TheclienthasasingleIBGPsessiontotheroutereflector.
Thestandalonerouterthatisshownisnotconfiguredasaroutereflector,norisita
NetworkDesignwithRouteReflectors
clienttoanyotherroutereflector.Thisotherrouterservesasanexampleofwherea
non-route-reflectorrouterparticipatesinthefullmesh.Thisrouterhasafullmeshof
IBGPsessionswithallroutereflectors,eachreflectorrepresentingitscluster.
Potential problems that can occur when you deviate form the route reflector network
design rules:
Issue: Result:
Clientsdonothavesessionswithallreflectorsina
cluster.
ClientswillnotreceiveallIBGProutes.
Clientshavesessionswithreflectorsinseveral
clusters.
Clientswillreceiveduplicatecopiesofthesame
route.
ClientshaveIBGPsessionswithotherclients. Clientswillreceiveduplicatecopiesofthesame
route.
TwonontransitiveoptionalBGPattributes,originator-IDandcluster-list,areboth
usedtopreventfatalloopsofinformation.Theuseofthesetwoattributesmakesa
networkfairlyinsensitivetopoorconfiguration.However,foroptimalperformance,
youmusthaveanoptimalconfiguration.Herearesomeoftheproblemsthatcould
occurifyoudeviatefromroutereflectornetworkdesignrules:
IfroutereflectorsarenotconnectedwithIBGPsessionsinafullmesh,some
clusterswillnothavealltheroutes.
IfaclienthasIBGPsessionswithsomeroutereflectorsinacluster,butnotwith
allofthem,theclientmightmisssomeBGProutes.
IfaclienthasIBGPsessionstoroutereflectorsthatbelongtodifferentclusters,
theclientwillforwardtheBGPupdatefromtheclientintothefullmeshwith
differentcluster-IDsinthecluster-listattribute.WhentheBGPupdateentersthe
mesh,itwillreachtheotherroutereflector,whichwill,unnecessarily,acceptthe
routeasvalidandforwarditintoitscluster.Thissituation,inturn,causes
unnecessaryduplicationofupdatestotheclients.
IfaclienthasIBGPsessionstootherclientsinthesamecluster,thoseclients
willreceiveunnecessaryduplicationsofupdates.
PotentialNetworkIssues
Networkdesignerscanbuildroutereflectorclustersinhierarchies.Withhierarchies,
arouterservingasaroutereflectorinoneclustercanactasaclientinanother
cluster.
Problem:
In very large networks, a single layer of route reflectors might not be enough.
Solution:
A hierarchy of route reflectors can be established.
A route reflector can be a client of another route reflector.
The hierarchy can be as deep as needed.
Clientsarenotconfiguredtoberoutereflectorclients;theysimplyhavefewerIBGP
sessions.However,anetworkdesignermustconfigurearoutereflector.In
configuringanIBGPsessiononaroutereflector,thedesignermustconfigurethe
sessiontoreachaclient,sotheroutereflectorIBGPsplit-horizonrulestostart
working.AllotherIBGPsessionsthatareconfiguredontheroutereflectorareapart
ofthefullmesh.Also,thedesignermustconfigurethecluster-IDontheroute
reflector.
ArouterthatisconfiguredtobearoutereflectorwillstillhaveordinaryIBGP
sessionsthatarepartofthefullmesh.Ifthesesessionsarereducedinnumberand
onlyafewremain,andtheremainingonesreachasecondlevelofroutereflectors,a
hierarchyofroutereflectorsiscreated.
Whenadesignerbuildsafirstlevelofclusters,theremainingfullmeshissmaller
thanwhenallroutersbelongedtoit.Butifitislargeenough,thedesignercanbuild
anextralevelofroutereflectors.
Thefigureshowsanexampleofhierarchicalroutereflectors.
Thefirstlevelofhierarchyreducedtheoriginalfullmeshof12routers(allroutersin
theserviceprovidernetwork)toafullmeshofsevenrouters(thelowerthreeroute
reflectorsandtheuppertworoutereflectorsandtwoclients).Thesecondlevelof
routereflectorclusterswasbuiltbycreatingcluster27.Thissecondstepfurther
reducedthefullmeshofsevenrouterstoafullmeshconsistingofonlytworouters
(upperroutereflectors).Onlythetworoutereflectorsincluster27shouldbe
connectedinafullmesh.
WhenaclientinthelowestlevelreceivesanEBGPupdate,itwillforwardtheupdate
onallconfiguredIBGPsessionstoaroutereflector.Theroutereflectorrecognizes
BGPupdatesthatarereceivedfromconfiguredclientsandwillforwardthese
updatestoallotherclientsthatusenormalIBGPsessions.Theupdate,sentona
normalIBGPsession,willbeasecond-levelclientupdatetothesecond-levelroute
reflector.Thesecond-levelroutereflectorwillrecognizethattheupdatewas
receivedfromaclient,andwillforwardittoallotherclientsandintothefullmesh.
HierarchicalRouteReflectors
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
BGP route reflectors were introduced to free the network designers from IBGP
full-mesh requirements that prevent large networks from scaling.
BGP route reflectors modify IBGP split-horizon rules. All routes that are received
from a route reflector client are sent to all other IBGP neighbors. All routes that
are received from a nonclient IBGP neighbor are sent to all route reflector
clients.
A route reflector is a single point of failure, and therefore redundancy should be
implemented in a network containing route reflectors.
Route reflector clusters were introduced in the BGP route reflector architecture
to support redundancy, preventing IBGP routing loops in redundant route
reflector designs.
The originator-ID and cluster-list BGP attributes were introduced to prevent
routing loops in route reflector environments.
All route reflectors in a cluster should have IBGP sessions to all clients in the
cluster. The route reflectors also participate in the IBGP full mesh, and they
should have no other IBGP sessions.
When the route reflector clients do not have IBGP sessions with all route
reflectors in the cluster, they might not receive all IBGP routes.
Route reflector clusters can be built in hierarchies. A router that is a route
reflector in one cluster can act as client in another cluster.
Summary
Overview
LargeBGPnetworkscannotproperlyscalewithoutrelyingonperformance-
enhancingtoolssuchasroutereflectors.RoutereflectorsenableBGProuting
informationtobedistributedinafashionthatdoesnotrequireaphysicalfull-mesh
network.Implementingsuchanetworkrequiresknowledgeofthestepstoproperly
migrateandconfigureroutereflectorsandthecommandsthatareusedtoverifythe
operationofaconfigurednetwork.
Thislessonintroducesthestepsthatarerequiredtosuccessfullymigrateanexisting
AStoBGProutereflectors.ItalsoliststheCiscoIOScommandsthatarerequiredto
configureandmonitorroutereflectors.
Uponcompletingthislesson,youwillbeableto:
ListthestepstomigrateanexistingIBGPbackbonetoabackbonewithroute
reflectors
IdentifytheconfigurationchangesandrelatedCiscoIOScommandsthatare
requiredtoconfigureroutereflectorsonaBGPbackbone
IdentifytheCiscoIOScommandsthatarerequiredtomonitoraBGPbackbone
thatcontainsroutereflectors
ConfiguringandMonitoringRoute
Reflectors
1.
2.
ThephysicaltopologyoftheASservesasaguidetodesigningclusters.Youshould
introducenoadditionalsinglepointsoffailurewhenyouaredeployingroute
reflectors.Ifthephysicaltopologyisredundant,agoodpracticeistohaveredundant
routereflectors.Ifthephysicaltopologyisnotredundant,introducinga
nonredundantclusterdoesnotaddasinglepointoffailurebecausethenetworkwas
alreadynonredundant.
Divide the AS into areas (clusters).
Assign a cluster-ID to each area.
On route reflector clients, retain only IBGP sessions with route reflectors in their
cluster.
On route reflectors, retain only IBGP sessions with other route reflectors and
clients in their cluster.
Configure cluster-ID on every route reflector.
Configure clients on every route reflector.
Thefollowingplanningandpreparationstepsarerequiredbeforeyoumigratefroma
fullmeshofIBGPsessionstoaroutereflectordesign:
Identifyagroupofperipheralroutersthatarephysicallyconnectedtothesame
setofbackbonerouters.Considertheperipheralroutersasclientsandthe
backboneroutersasroutereflectors.Lettheroutersformacluster.Makesure
thatnorouterbelongstotwodifferentclusters,becausethissetupwould
representanillegalconfiguration.
Createanumberingplanthatindicateshownumbersareassignedtothe
clustersinthenetwork.Theplanmustmakesuretouniquelyidentifyeachofthe
clusterswithintheAS.ClustersarenotseenfromoutsidetheAS,sotheplan
doesnotneedtobecoordinatedwithanyotherAS.Toeasetroubleshooting,it
isrecommendedthatnumberslowerthan256areused,becausecluster-IDsare
displayedinIPaddressformat.
Thedefaultvalueofacluster-IDistheBGProuter-IDof
theroutereflector.Ifyoudecidetoimplement
nonredundantclusters,youdonothavetoplanthe
cluster-IDnumbers,becausetheBGProuter-IDsshould
beunique.
RouteReflectorBackboneMigration
Configuration changes that are required to configure BGP route reflectors:
Configure cluster-ID on route reflectors.
Configure BGP neighbors as route reflector clients on the route reflectors.
No configuration is needed on the route reflector clients.
Make sure that IBGP neighbor is removed on both ends of the IBGP session.
Aspartoftheplanningandpreparationthatisnecessarytomigratefromafullmesh
ofIBGPsessionstoaroutereflectordesign,youneedtomakethefollowing
configurationchanges:
Configurethepropercluster-IDvalueontheroutereflectors.
ConfiguretheroutereflectorwithinformationaboutwhichIBGPneighbor
sessionsarereachingtheirclients.
Intheclients,removeallIBGPsessionstoneighborsthatarenotroutereflectors
intheclientcluster.
MakesurethattheIBGPneighborisremovedonbothendsoftheIBGP
session.
Cisco IOS commands that are required to configure BGP route reflectors:
router(config-router)# bgp cluster-id cluster-id
Optionally assigns a cluster-ID to the route reflector (default value is router-ID)
Required only for clusters with redundant reflectors
router(config-router)# neighbor ip-address route-reflector-client
On route reflector configures an IBGP neighbor to be a client of this reflector.
Usethebgpcluster-idcommandtoconfigurethecluster-IDiftheBGPclusterhas
redundantroutereflectors.
bgp cluster-id cluster-id
Toremovethecluster-ID,usethenoformofthiscommand.
no bgp cluster-id cluster-id
SyntaxDescription
Parameter Description
cluster-id Cluster-IDoftherouteractingasaroutereflector.
Thecluster-IDisamaximumof4bytes.
ThiscommandisusedtoconfiguretherouterasaBGProutereflectorandconfigure
thespecifiedneighborasitsclient.Whenalltheclientsaredisabled,thelocalrouter
isnolongeraroutereflector.
neighbor ip-address route-reflector-client
Toindicatethattheneighborisnotaclient,usethenoformofthiscommand.
no neighbor ip-address route-reflector-client
SyntaxDescription
Parameter Description
ip-address NeighborIPaddress
Bydefault,thereisnoroutereflectorintheAS.
ConfiguringRouteReflectorsExample
Inthisexample,therouter1.0.0.1hasbeenconfiguredasaroutereflector.
ConfiguringRouteReflectors
Therouterswithrouter-ID1.0.0.1and1.0.0.2areroutereflectorsinaclusterthat
hasbeenassignedcluster-ID175.Therouterswithrouter-ID1.0.0.3and1.0.0.4are
clientstothesetworoutereflectors.
Thefigureshowsaportionoftheconfigurationinrouter1.0.0.1.Thecluster-IDis
assignedtotherouterundertherouterbgpprocessdefinitionoftherouter
configuration.Aftertherouterhasbeenassigned,theroutereflectorclient
configurationisaddedundertherouterbgpprocessforthetwoneighborsthat
identifythetwosessionsreachingclients.
BothclientsneedtohaveestablishedIBGPsessiontobothroutereflectors.IBGP
sessionbetweenroutereflectorsisalsoneeded.Routereflector1.0.0.1alsohas
establishedIBGPsessiontononclientrouter1.0.0.6andEBGPsessiontorouter
2.7.1.1onautonomoussystem222.
Overview
Throughthisdiscovery,youwilllearnhowtoeliminatetheneedforfullmeshIBGP
sessionsbyintroducingtheroutereflectorfeatureintotheautonomoussystem.
Discovery17:ConfigureRouteReflector
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
ISP1 Ethernet0/0 172.16.11.11/24 ConnectiontoR1
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
R1 Ethernet0/0 172.16.11.1/24 ConnectiontoISP1
R1 Ethernet0/3 192.168.13.1/24 ConnectiontoR3
R1 Loopback1 10.0.0.1/28 Loopbacksimulates
LANnetwork
R3 Ethernet0/2 192.168.34.3/24 ConnectiontoR4
R3 Ethernet0/3 192.168.13.3/24 ConnectiontoR1
R3 Loopback1 10.0.0.17/28 Loopbacksimulates
LANnetwork
R4 Ethernet0/0 172.16.34.4/24 ConnectiontoISP3B
R4 Ethernet0/3 192.168.24.4/24 ConnectiontoR2
R4 Loopback1 10.0.0.49/28 Loopbacksimulates
LANnetwork
ISP3B Ethernet0/0 172.16.34.34/24 ConnectiontoR4
ISP3B Loopback1
Loopback2
Loopback3
Loopback4
10.0.3.1/28
10.0.3.17/28
10.0.3.33/28
10.0.3.49/28
Loopbackssimulate
LANnetworks
IPaddressesandIGParepreconfiguredasshowninthetopologybelow:
TheseBGPsessionshavebeenpreconfigured:
EBGPsessionbetweenISP1andR1routers
EBGPsessionbetweenISP3BandR4routers
IBGPsessionbetweenR1andR3routers
IBGPsessionbetweenR3andR4routers
NotethatthereisnoIBGPsessionbetweenR1andR4routers.OSPFprotocolhas
alsobeenpreconfiguredasIGProutingprotocol,makingsurethatallinternallinks,
JobAids
loopbacks,andnexthopsareaccessibleinautonomoussystemAS1.
Step1
OntheR1router,verifywhichBGPsessionsareestablished.
R1# show bgp summary
BGP router identifier 10.0.0.1, local AS number 1
BGP table version is 101, main routing table version 101
8 network entries using 1184 bytes of memory
8 path entries using 512 bytes of memory
4/3 BGP path/bestpath attribute entries using 544 bytes of memory
1 BGP rrinfo entries using 24 bytes of memory
2 BGP AS-PATH entries using 48 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 2312 total bytes of memory
BGP activity 55/42 prefixes, 86/78 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.0.0.17 4 1 8 6 101 0 0 00:00:22 1
172.16.11.11 4 100 109 118 101 0 0 01:33:46 6
YoushouldonlyseetheBGPsessionsestablished:
EBGPsessiontotheISP1router(neighbor172.16.11.11)
IBGPsessiontotheR3router(neighbor10.0.0.17)
Step2
OntheR3router,verifywhichBGPsessionsareestablished.
R3# show bgp summary
BGP router identifier 10.0.0.17, local AS number 1
BGP table version is 198, main routing table version 198
13 network entries using 1924 bytes of memory
13 path entries using 832 bytes of memory
4/4 BGP path/bestpath attribute entries using 544 bytes of memory
2 BGP AS-PATH entries using 48 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 3348 total bytes of memory
BGP activity 37/24 prefixes, 109/96 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.0.0.1 4 1 7 8 198 0 0 00:00:51 7
10.0.0.49 4 1 6 7 198 0 0 00:00:38 5
YoushouldonlyseetheBGPsessionsestablished:
IBGPsessiontotheR1router(neighbor10.0.0.1)
IBGPsessiontotheR4router(neighbor10.0.0.49)
Step3
OntheR4router,verifywhichBGPsessionsareestablished.
R4# show bgp summary
BGP router identifier 10.0.0.49, local AS number 1
BGP table version is 145, main routing table version 145
6 network entries using 888 bytes of memory
6 path entries using 384 bytes of memory
3/3 BGP path/bestpath attribute entries using 408 bytes of memory
1 BGP AS-PATH entries using 24 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 1704 total bytes of memory
BGP activity 61/55 prefixes, 109/103 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.0.0.17 4 1 8 7 145 0 0 00:01:03 1
172.16.34.34 4 300 109 122 145 0 0 01:34:45 4
YoushouldonlyseetheBGPsessionsestablished:
ConfiguringRouteReflector
DiscoverySteps
EBGPsessiontotheISP3Brouter(neighbor172.16.34.34)
IBGPsessiontotheR3router(neighbor10.0.0.17)
NotethatthereisnoIBGPsessionbetweenroutersR1andR4.
Step4
OntheR1router,verifywhichroutesareadvertisedtotherouterR3.
R1# show ip bgp neighbors 10.0.0.17 advertised-routes
BGP table version is 53, local router ID is 10.0.0.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 0.0.0.0 0 32768 i
*> 10.0.1.0/28 172.16.11.11 0 0 100 i
*> 10.0.1.16/28 172.16.11.11 0 0 100 i
*> 10.0.1.32/28 172.16.11.11 0 0 100 i
*> 10.0.1.48/28 172.16.11.11 0 0 100 i
*> 10.0.1.64/28 172.16.11.11 0 0 100 i
*> 10.0.1.80/28 172.16.11.11 0 0 100 i
YoushouldseethatR1routeradvertisestheseprefixes:
Locallyoriginatedprefix10.0.0.0/28
AllprefixesoriginatingontheISP1router(10.0.1.0/24subnets)inAS100.
Step5
OntheR3router,verifythecontentoftheBGPtable.
R3# show ip bgp
BGP table version is 102, local router ID is 10.0.0.17
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*>i 10.0.0.0/28 10.0.0.1 0 100 0 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*>i 10.0.0.48/28 10.0.0.49 0 100 0 i
*>i 10.0.1.0/28 10.0.0.1 0 100 0 100 i
*>i 10.0.1.16/28 10.0.0.1 0 100 0 100 i
*>i 10.0.1.32/28 10.0.0.1 0 100 0 100 i
*>i 10.0.1.48/28 10.0.0.1 0 100 0 100 i
*>i 10.0.1.64/28 10.0.0.1 0 100 0 100 i
*>i 10.0.1.80/28 10.0.0.1 0 100 0 100 i
*>i 10.0.3.0/28 10.0.0.49 0 100 0 300 i
*>i 10.0.3.16/28 10.0.0.49 0 100 0 300 i
*>i 10.0.3.32/28 10.0.0.49 0 100 0 300 i
*>i 10.0.3.48/28 10.0.0.49 0 100 0 300 i
YoushouldseethatprefixesoriginatingontheISP1router(10.0.1.0/24prefixes)were
receivedontheR3router.
Step6
OntheR3router,verifywhichroutesareadvertisedtotherouterR4.
R3# show ip bgp neighbors 10.0.0.49 advertised-routes
BGP table version is 102, local router ID is 10.0.0.17
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.16/28 0.0.0.0 0 32768 i
Total number of prefixes 1
Youshouldseethatonlylocallyoriginatedprefix(10.0.0.16/28)isadvertisedtotheR4
router.SplithorizonrulepreventsrouterR3topassreceivedupdatesfromrouterR1to
besenttotheR4router.
Step7
OntheR4router,verifythecontentoftheBGPtable.
R4# show ip bgp
BGP table version is 81, local router ID is 10.0.0.49
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*>i 10.0.0.16/28 10.0.0.17 0 100 0 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.3.0/28 172.16.34.34 0 0 300 i
*> 10.0.3.16/28 172.16.34.34 0 0 300 i
*> 10.0.3.32/28 172.16.34.34 0 0 300 i
*> 10.0.3.48/28 172.16.34.34 0 0 300 i
YoushouldseethatprefixesoriginatingontheISP1router(10.0.1.0/24prefixes)werenot
receivedontheR4router.
NOTE:FortheprefixesoriginatingontheISP1routertobereceivedontheR4router,
eitherIBGPsessionbetweenR1andR4needstobeconfiguredortherouterR3should
beconfiguredastheroutereflectortobreakthesplithorizonruleforIBGPsessions.
Step8
OntheISP3Brouter,verifythecontentoftheBGPtable.
ISP3B# show ip bgp
BGP table version is 75, local router ID is 10.0.3.49
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.16/28 172.16.34.4 0 1 i
*> 10.0.0.48/28 172.16.34.4 0 0 1 i
*> 10.0.3.0/28 0.0.0.0 0 32768 i
*> 10.0.3.16/28 0.0.0.0 0 32768 i
*> 10.0.3.32/28 0.0.0.0 0 32768 i
*> 10.0.3.48/28 0.0.0.0 0 32768 i
YoushouldseeasaconsequenceoftheIBGPsplithorizonrulethatprefixesoriginating
ontheISP1router(10.0.1.0/24prefixes)werenotreceivedontheISP3Brouter.
Step9
ConfiguretheBGPclusterIDof1ontheR3router.ConfiguretheR3routertobe
routereflectorforclientrouterR1andR4.
R3(config)# router bgp 1
R3(config-router)# bgp cluster-id 1
R3(config-router)# neighbor 10.0.0.1 route-reflector-client
R3(config-router)# neighbor 10.0.0.49 route-reflector-client
NOTE:IBGPsessionisrestoredautomaticallyafterroutereflectorclientis
configuredontheroutereflectorrouter.
MonitoringRouteReflectors
MonitoringRouteReflectors
Commands that are used to monitor route reflector operation:
router# show ip bgp neighbors
Displays whether a neighbor is a route reflector client.
router# show ip bgp network/lenght
Displays the routes that are received from the client as seen on the reflector.
Displays reflected routes as seen on the client (with originator ID and cluster ID
information)
TodisplayinformationabouttheTCPandBGPconnectionstoneighbors,usethe
showipbgpneighborsEXECcommand.
show ip bgp neighbors [address]
Inthiscase,theshowipbgpneighborscommandisusedontherouternottosee
routesorpathsthathavebeenreceivedbuttoseethestatusoftheneighbor
session,sonootherqualifiersthantheoptionalIPaddressaregiven.Thecommand
thatisissuedontheroutereflectorrouter,indicatesthattheneighborisaroute
reflectorclient.
TodisplayentriesintheBGProutingtable,usetheshowipbgpEXECcommand.
show ip bgp[network/lenght [longer-prefixes]
WhendetailsaredisplayedforaspecificrouteentryintheBGPtable,theclusterlist
andoriginatorIDattributesarealsoshown.Ifyouissuethiscommandontheroute
reflector,youcouldseethataparticularentrywasreceivedfromaroutereflector
client.Ifyouissuethiscommandontheroutereflectorclient,youcouldseeentries
intheBGPtable,thatwerereflectedbytheroutereflector.
Step10
OntheR3router(whichactsasaroutereflector),verifythattheR1routerisconfiguredasroutereflector
client.
R3# show ip bgp neighbors 10.0.0.1
BGP neighbor is 10.0.0.1, remote AS 1, internal link
BGP version 4, remote router ID 10.0.0.1
BGP state = Established, up for 00:07:39
Last read 00:00:41, last write 00:00:22, hold time is 180, keepalive interval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable (disabled)
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN Capability: advertised and received
Address family IPv4 Unicast: advertised and received
Enhanced Refresh Capability: advertised and received
Multisession Capability:
Stateful switchover support enabled: NO for session 1
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 1 1
Notifications: 0 0
Updates: 7 3
Keepalives: 10 10
Route Refresh: 0 0
Total: 18 14
Default minimum time between advertisement runs is 0 seconds
For address family: IPv4 Unicast
Session: 10.0.0.1
BGP table version 150, neighbor version 150/0
Output queue size : 0
Index 5, Advertise bit 1
Route-Reflector Client
5 update-group member
Slow-peer detection is disabled
Slow-peer split-update-group dynamic is disabled
Step11
OntheR3router(whichactsasaroutereflector),verifyadditionalinformationin
theBGPtableforprefix10.0.1.0/28,whichisoriginatedontheISP1router.Make
surethattheroutewasreceivedfromtheroutereflectorclient.
R3# show ip bgp 10.0.1.0/28
BGP routing table entry for 10.0.1.0/28, version 134
Paths: (1 available, best #1, table default)
Advertised to update-groups:
5
Refresh Epoch 1
100, (Received from a RR-client)
10.0.0.1 (metric 11) from 10.0.0.1 (10.0.0.1)
Origin IGP, metric 0, localpref 100, valid, internal, best
Step12
OntheR4router(whichactsasaroutereflectorclient),verifyadditional
informationintheBGPtableforprefix10.0.1.0/28,whichisoriginatedonthe
ISP1router.Makesurethattheroutewasreflectedfromtheroutereflector
routerR3.
R4# show ip bgp 10.0.1.0/28
BGP routing table entry for 10.0.1.0/28, version 99
Paths: (1 available, best #1, table default)
Advertised to update-groups:
6
Refresh Epoch 2
100
10.0.0.1 (metric 21) from 10.0.0.17 (10.0.0.17)
Origin IGP, metric 0, localpref 100, valid, internal, best
Originator: 10.0.0.1, Cluster list: 0.0.0.1
YoushouldseethatoriginatorofthisreflectedroutewasroutereflectorclientR1
(10.0.0.1)andtheBGPclusterIDis1.
Step13
OntheR3router,verifywhichroutesareadvertisedtotheR4router.
R3# show ip bgp neighbors 10.0.0.49 advertised-routes
BGP table version is 150, local router ID is 10.0.0.17
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*>i 10.0.0.0/28 10.0.0.1 0 100 0 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*>i 10.0.0.48/28 10.0.0.49 0 100 0 i
*>i 10.0.1.0/28 10.0.0.1 0 100 0 100 i
*>i 10.0.1.16/28 10.0.0.1 0 100 0 100 i
*>i 10.0.1.32/28 10.0.0.1 0 100 0 100 i
*>i 10.0.1.48/28 10.0.0.1 0 100 0 100 i
*>i 10.0.1.64/28 10.0.0.1 0 100 0 100 i
*>i 10.0.1.80/28 10.0.0.1 0 100 0 100 i
*>i 10.0.3.0/28 10.0.0.49 0 100 0 300 i
*>i 10.0.3.16/28 10.0.0.49 0 100 0 300 i
*>i 10.0.3.32/28 10.0.0.49 0 100 0 300 i
*>i 10.0.3.48/28 10.0.0.49 0 100 0 300 i
Network Next Hop Metric LocPrf Weight Path
Total number of prefixes 13
YoushouldseethatprefixesoriginatedontheISP1(10.0.1.0/24subnets)routerare
beingadvertisedfromtheR3routertotheIBGPneighborR4becauseoftherouterR3
actingasaroutereflector.
Step14
OntheR4router,verifythecontentoftheBGPtable.
R4# show ip bgp
BGP table version is 106, local router ID is 10.0.0.49
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*>i 10.0.0.0/28 10.0.0.1 0 100 0 i
*>i 10.0.0.16/28 10.0.0.17 0 100 0 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*>i 10.0.1.0/28 10.0.0.1 0 100 0 100 i
*>i 10.0.1.16/28 10.0.0.1 0 100 0 100 i
*>i 10.0.1.32/28 10.0.0.1 0 100 0 100 i
*>i 10.0.1.48/28 10.0.0.1 0 100 0 100 i
*>i 10.0.1.64/28 10.0.0.1 0 100 0 100 i
*>i 10.0.1.80/28 10.0.0.1 0 100 0 100 i
*> 10.0.3.0/28 172.16.34.34 0 0 300 i
*> 10.0.3.16/28 172.16.34.34 0 0 300 i
*> 10.0.3.32/28 172.16.34.34 0 0 300 i
*> 10.0.3.48/28 172.16.34.34 0 0 300 i
YoushouldseethatprefixesoriginatingontheISP1router(10.0.1.0/24prefixes)
werereceivedontheR4router,becausetherouterR3isconfiguredasaroutereflector.
Step15
OntheISP3Brouter,verifythecontentoftheBGPtable.
ISP3B# show ip bgp
BGP table version is 84, local router ID is 10.0.3.49
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.34.4 0 1 i
*> 10.0.0.16/28 172.16.34.4 0 1 i
*> 10.0.0.48/28 172.16.34.4 0 0 1 i
*> 10.0.1.0/28 172.16.34.4 0 1 100 i
*> 10.0.1.16/28 172.16.34.4 0 1 100 i
*> 10.0.1.32/28 172.16.34.4 0 1 100 i
*> 10.0.1.48/28 172.16.34.4 0 1 100 i
*> 10.0.1.64/28 172.16.34.4 0 1 100 i
*> 10.0.1.80/28 172.16.34.4 0 1 100 i
*> 10.0.3.0/28 0.0.0.0 0 32768 i
*> 10.0.3.16/28 0.0.0.0 0 32768 i
*> 10.0.3.32/28 0.0.0.0 0 32768 i
*> 10.0.3.48/28 0.0.0.0 0 32768 i
YoushouldseethatprefixesoriginatingontheISP1router(10.0.1.0/24prefixes)were
receivedontheISP3Brouter.
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
A proper migration plan is important because the change to BGP confederation
involves a major reconfiguration of BGP routing.
The neighbor ip-address route-reflector-client command specifies an IBGP
neighbor to be a client of this reflector
The show ip bgp neighbors command has been modified to display whether a
BGP neighbor is part of a BGP confederation.
Summary
Overview
Thistopicsummarizesthekeypointsthatwerediscussedinthismodule.
Service providers use an IGP to carry internal routes and to provide optimal
routing between POPs, the information that is needed for IBGP sessions to be
established, and the addresses that are required for BGP next-hop resolution.
Route reflectors enable BGP routing information to be distributed in a fashion
that does not require a physical fully meshed network.
Route reflector clusters can be built in hierarchies. A router that is a route
reflector in one cluster can act as a client in another cluster.
There are only two Cisco IOS commands that are used to configure route
reflectors: bgp cluster-id and neighbor ip-address route-reflector-client.
ModuleSummary
Usethequestionsheretoreviewwhatyoulearnedinthismodule.Thecorrect
answersandsolutionsarefoundintheModuleSelf-CheckAnswerKey.
ModuleSelf-Check
1. Whichthreecharacteristicsarecommontotypicalserviceprovider
networks?(Choosethree.)(Source:"ScalingIGPandBGPin
ServiceProviderNetworks")
TheprovidernetworkusestwoIGPs,oneforcustomerroutesandonefor
internalproviderroutes.
ServiceprovidersexchangerouteswithotherprovidersusingBGP.
ServiceprovidersrunIBGPwithintheirnetworkinadditiontotheirIGP
requirements.
ServiceproviderstypicallyuseeitherstaticroutesorEBGPwiththeir
customers.
1. WhatisthetypicalroleofanIGPwithinaserviceprovidernetwork?
(Source:"ScalingIGPandBGPinServiceProviderNetworks")
TheIGPcarriescustomerroutesforredistributionintoBGPattheprovider
edge.
TheIGPadvertisesadefaultroutetocustomersoftheserviceprovider.
TheIGPresolvesnext-hopIPaddresses.
TheIGPcarriesBGProutesacrosstheprovidernetwork.
1. TheIGPprotocolisnotneededintheserviceprovidernetwork,as
informationaboutnexthopattributegetsadvertisedviaBGP
updates.Trueorfalse?(Source:"ScalingIGPandBGPinService
ProviderNetworks")
true
false
1. WhyshouldyouavoidtheuseofprivateIPaddressinginservice
providernetworks?(Source:"ScalingIGPandBGPinService
ProviderNetworks")
Privateaddressingcanpreventcustomernetworktroubleshootingutilities
suchastraceroutefromfunctioningcorrectly.
PrivateIPaddressingisnotallowedontheInternetandwillnotfunctionin
aserviceprovidernetwork.
PrivateIPaddressingpreventstheserviceproviderfromproperly
summarizingcustomerroutesifitisalsousingprivateaddressspace.
PrivateIPaddressingpreventsserviceproviderapplicationssuchas
MPLSfromoperatingproperlyinanInternet-supportingenvironment.
1. InserviceprovidernetworkItisrecommendedtotoredistributeBGP
routesintoIGPprotocol.Trueorfalse?(Source:"ScalingIGPand
BGPinServiceProviderNetworks")
true
false
1. WhenusingnexthopselffeatureonthePErouterintheservice
providernetwork,IGPcanberelievedoftheburdenofcarrying
informationabouttheaccesslink.Trueorfalse?(Source:"Scaling
IGPandBGPinServiceProviderNetworks")
true
false
1. WhichthreerequirementsarekeytoproperlyscalingBGPina
serviceproviderenvironment?(Choosethree.)(Source:"Scaling
IGPandBGPinServiceProviderNetworks")
IBGPfull-meshscalingtoolstoreduceduplicatetrafficwithintheAS
summarizationofcustomerroutestoreducethenumberofprefixesthat
arecarried
improvementinBGPconvergencetimebyusingtheIGPforroute
propagationwithintheproviderAS
properscalingoftheAS-wideroutingpolicytoeaseadministrationand
maintenancerequirements
1. WhichthreerequirementsarekeytoproperlyscalingBGPina
serviceproviderenvironment?(Choosethree.)(Source:"Scaling
IGPandBGPinServiceProviderNetworks")
IBGPfull-meshscalingtoolstoreduceduplicatetrafficwithintheAS
summarizationofcustomerroutestoreducethenumberofprefixesthat
arecarried
improvementinBGPconvergencetimebyusingtheIGPforroute
propagationwithintheproviderAS
properscalingoftheAS-wideroutingpolicytoeaseadministrationand
maintenancerequirements
1. InthetraditionalBGPapproach,ensuringconsistentrouting
informationwasachievedbyestablishingafullmeshofIBGP
sessionsbetweenallrouterswithintheAS.Trueorfalse?(Source:
"ScalingIGPandBGPinServiceProviderNetworks")
true
false
1. WhatisthemainproblemthatissolvedbyimplementingBGProute
reflectors?(Source:"IntroducingandDesigningRouteReflectors")
thelargenumberofroutesthatarecarriedintheIGPwhenBGPis
deployed
theabilityofBGPtoscaleasingleASinalargenetwork
theneedforahomogeneousmethodofapplyingpoliciestoroutesthatare
carriedthroughanAS
theabilitytosupportservice-levelparameterswithgreaterease
1. HowdoesaroutereflectormodifytheIBGPsplit-horizonrule?
(Source:"IntroducingandDesigningRouteReflectors")
forwardsEBGPupdatestoallpeers(IBGPandEBGP)
treatsallneighborsasEBGPpeers,whicheliminatestheIBGPmesh
requirements
forwardsIBGPupdatesfromclientstootherIBGPneighbors
appendsthecluster-IDtotheASpath,whichallowspeerstobetreatedas
EBGPneighbors
1. WhenincomingBGPupdateisreceivedfromEBGPpeeronroute
reflector,theupdateisforwardedtobothIBGPandEBGPpeers.
Trueorfalse?(Source:"IntroducingandDesigningRoute
Reflectors")
true
false
1. Whyareredundantroutereflectorsmandatoryinanyhigh-
availabilitynetworkdesign?(Source:"IntroducingandDesigning
RouteReflectors")
Allneighborspeerwiththeroutereflector,andalargenumberof
neighborscanmaketheroutereflectorrouterunstable.
EBGPpeerscaninjectBGPupdatesintotheASonlythroughtheroute
reflector.
Routereflectorsmaintainmoreroutinginformation,whichmakesthem
morepronetocongestionandfailure.
ClientscanformIBGPrelationshipsonlywiththeroutereflector.
1. Whatisthemainreasonforimplementingredundantroutereflectors
withclusters?(Source:"IntroducingandDesigningRoute
Reflectors")
toeliminateroutingloopsinredundantconfigurations
tolimitthenumberofneighborsessionswitheachroutereflector
toprovideanotherscalabilitymechanismtargetedatremovingtheIBGP
full-meshrequirement
toenhancesecuritywithintheAS
1. Howdoestheoriginator-IDattributeassistintheeliminationof
routingloopsthatarecausedbyredundantroutereflectordesigns?
(Source:"IntroducingandDesigningRouteReflectors")
Iftheoriginator-IDmatchestherouter-IDofthereflector,localpreference
issetontheroutetomakeitabackup.
Theoriginator-IDattributeissettothecluster-IDtoensurethataroute
traversestheASonlyonetime.
Arouterthatreceivesarouteinwhichtheoriginator-IDmatchesitsrouter-
IDwillignorethatroute.
Theoriginator-IDallowstheroutertoknowiftherouteoriginatedlocallyor
fromanexternalsourcesothatadministrativedistancerulesfortheroute
canbeverified.
1. WhatcanoccurifaclienthasIBGPneighborrelationshipswithother
routersthatarenotconfiguredasroutereflectors?(Source:
"IntroducingandDesigningRouteReflectors")
Thisisaninvalidconfiguration.
Theclientwillnotifytheroutereflectorandbepromotedtoaroutereflector
aswell.
RoutingblackholescanoccurandcauselosttrafficinsidetheAS.
Unnecessaryroutingtrafficwillbegenerated.
1. WhichpotentialproblemcanoccurifaclientdoesnothaveanIBGP
sessionwithallroutereflectorsinacluster?(Source:"Introducing
andDesigningRouteReflectors")
Thisisaninvalidconfiguration.
TheclientmightnotreceiveallBGProutes.
EBGProutesthatarereceivedbytheclientwillnotbedistributedproperly
throughouttheAS.
Duplicateroutingtrafficwillbesenttotheclient.
1. Whichproblemarehierarchicalroutereflectorsdesignedtosolve?
(Source:"IntroducingandDesigningRouteReflectors")
lackofaconsistentapplicationofsecurityandroutingpoliciesthroughout
theAS
scalabilityofautonomoussystemsinverylargeroutingdomains
routingloopscausedbyredundantclusterconfigurations
administrativeoverheadwhenyouareimplementingrouterreflector
networkdesigns
1. WhichtwoBGPparametersdoyouhavetoconfigureonaroute
reflector?(Choosetwo.)(Source:"ConfiguringandMonitoringRoute
Reflectors")
cluster-ID
originator-ID
cluster-list
routereflectorclients
1. Whatarethreemigrationstepsthatarerequiredtoconvertfroma
fullymeshedIBGPAStoanASthatisbasedonroutereflectors?
(Choosethree.)(Source:"ConfiguringandMonitoringRoute
Reflectors")
removeunnecessaryIBGPsessions
configuretheclientsontheroutereflectors
configureIBGPsessionsbetweenroutereflectorclients
configurethecluster-IDontheroutereflectors
1. Whichcommandshouldyouusetoidentifyroutereflectorclients
withoutinspectingtherouterconfiguration?(Source:"Configuring
andMonitoringRouteReflectors")
showipbgpprefix
showipbgpneighbors
showipbgpclients
showipbgpsummary
1. Thecommandneighborip-addressroute-reflector-
clientcommandshouldbeusedonclient.Trueorfalse?(Source:
"ConfiguringandMonitoringRouteReflectors")
true
false
1. Whichthreecharacteristicsarecommontotypicalserviceprovider
networks?(Choosethree.)(Source:"ScalingIGPandBGPin
ServiceProviderNetworks")
TheprovidernetworkusestwoIGPs,oneforcustomerroutesandonefor
internalproviderroutes.
ServiceprovidersexchangerouteswithotherprovidersusingBGP.
ServiceprovidersrunIBGPwithintheirnetworkinadditiontotheirIGP
requirements.
ServiceproviderstypicallyuseeitherstaticroutesorEBGPwiththeir
customers.
AnswerKey
1. WhatisthetypicalroleofanIGPwithinaserviceprovidernetwork?
(Source:"ScalingIGPandBGPinServiceProviderNetworks")
TheIGPcarriescustomerroutesforredistributionintoBGPattheprovider
edge.
TheIGPadvertisesadefaultroutetocustomersoftheserviceprovider.
TheIGPresolvesnext-hopIPaddresses.
TheIGPcarriesBGProutesacrosstheprovidernetwork.
1. TheIGPprotocolisnotneededintheserviceprovidernetwork,as
informationaboutnexthopattributegetsadvertisedviaBGP
updates.Trueorfalse?(Source:"ScalingIGPandBGPinService
ProviderNetworks")
true
false
1. WhyshouldyouavoidtheuseofprivateIPaddressinginservice
providernetworks?(Source:"ScalingIGPandBGPinService
ProviderNetworks")
Privateaddressingcanpreventcustomernetworktroubleshootingutilities
suchastraceroutefromfunctioningcorrectly.
PrivateIPaddressingisnotallowedontheInternetandwillnotfunctionin
aserviceprovidernetwork.
PrivateIPaddressingpreventstheserviceproviderfromproperly
summarizingcustomerroutesifitisalsousingprivateaddressspace.
PrivateIPaddressingpreventsserviceproviderapplicationssuchas
MPLSfromoperatingproperlyinanInternet-supportingenvironment.
1. InserviceprovidernetworkItisrecommendedtotoredistributeBGP
routesintoIGPprotocol.Trueorfalse?(Source:"ScalingIGPand
BGPinServiceProviderNetworks")
true
false
1. WhenusingnexthopselffeatureonthePErouterintheservice
providernetwork,IGPcanberelievedoftheburdenofcarrying
informationabouttheaccesslink.Trueorfalse?(Source:"Scaling
IGPandBGPinServiceProviderNetworks")
true
false
1. WhichthreerequirementsarekeytoproperlyscalingBGPina
serviceproviderenvironment?(Choosethree.)(Source:"Scaling
IGPandBGPinServiceProviderNetworks")
IBGPfull-meshscalingtoolstoreduceduplicatetrafficwithintheAS
summarizationofcustomerroutestoreducethenumberofprefixesthat
arecarried
improvementinBGPconvergencetimebyusingtheIGPforroute
propagationwithintheproviderAS
properscalingoftheAS-wideroutingpolicytoeaseadministrationand
maintenancerequirements
1. WhichthreerequirementsarekeytoproperlyscalingBGPina
serviceproviderenvironment?(Choosethree.)(Source:"Scaling
IGPandBGPinServiceProviderNetworks")
IBGPfull-meshscalingtoolstoreduceduplicatetrafficwithintheAS
summarizationofcustomerroutestoreducethenumberofprefixesthat
arecarried
improvementinBGPconvergencetimebyusingtheIGPforroute
propagationwithintheproviderAS
properscalingoftheAS-wideroutingpolicytoeaseadministrationand
maintenancerequirements
1. InthetraditionalBGPapproach,ensuringconsistentrouting
informationwasachievedbyestablishingafullmeshofIBGP
sessionsbetweenallrouterswithintheAS.Trueorfalse?(Source:
"ScalingIGPandBGPinServiceProviderNetworks")
true
false
1. WhatisthemainproblemthatissolvedbyimplementingBGProute
reflectors?(Source:"IntroducingandDesigningRouteReflectors")
thelargenumberofroutesthatarecarriedintheIGPwhenBGPis
deployed
theabilityofBGPtoscaleasingleASinalargenetwork
theneedforahomogeneousmethodofapplyingpoliciestoroutesthatare
carriedthroughanAS
theabilitytosupportservice-levelparameterswithgreaterease
1. HowdoesaroutereflectormodifytheIBGPsplit-horizonrule?
(Source:"IntroducingandDesigningRouteReflectors")
forwardsEBGPupdatestoallpeers(IBGPandEBGP)
treatsallneighborsasEBGPpeers,whicheliminatestheIBGPmesh
requirements
forwardsIBGPupdatesfromclientstootherIBGPneighbors
appendsthecluster-IDtotheASpath,whichallowspeerstobetreatedas
EBGPneighbors
1. WhenincomingBGPupdateisreceivedfromEBGPpeeronroute
reflector,theupdateisforwardedtobothIBGPandEBGPpeers.
Trueorfalse?(Source:"IntroducingandDesigningRoute
Reflectors")
true
false
1. Whyareredundantroutereflectorsmandatoryinanyhigh-
availabilitynetworkdesign?(Source:"IntroducingandDesigning
RouteReflectors")
Allneighborspeerwiththeroutereflector,andalargenumberof
neighborscanmaketheroutereflectorrouterunstable.
EBGPpeerscaninjectBGPupdatesintotheASonlythroughtheroute
reflector.
Routereflectorsmaintainmoreroutinginformation,whichmakesthem
morepronetocongestionandfailure.
ClientscanformIBGPrelationshipsonlywiththeroutereflector.
1. Whatisthemainreasonforimplementingredundantroutereflectors
withclusters?(Source:"IntroducingandDesigningRoute
Reflectors")
toeliminateroutingloopsinredundantconfigurations
tolimitthenumberofneighborsessionswitheachroutereflector
toprovideanotherscalabilitymechanismtargetedatremovingtheIBGP
full-meshrequirement
toenhancesecuritywithintheAS
1. Howdoestheoriginator-IDattributeassistintheeliminationof
routingloopsthatarecausedbyredundantroutereflectordesigns?
(Source:"IntroducingandDesigningRouteReflectors")
Iftheoriginator-IDmatchestherouter-IDofthereflector,localpreference
issetontheroutetomakeitabackup.
Theoriginator-IDattributeissettothecluster-IDtoensurethataroute
traversestheASonlyonetime.
Arouterthatreceivesarouteinwhichtheoriginator-IDmatchesitsrouter-
IDwillignorethatroute.
Theoriginator-IDallowstheroutertoknowiftherouteoriginatedlocallyor
fromanexternalsourcesothatadministrativedistancerulesfortheroute
canbeverified.
1. WhatcanoccurifaclienthasIBGPneighborrelationshipswithother
routersthatarenotconfiguredasroutereflectors?(Source:
"IntroducingandDesigningRouteReflectors")
Thisisaninvalidconfiguration.
Theclientwillnotifytheroutereflectorandbepromotedtoaroutereflector
aswell.
RoutingblackholescanoccurandcauselosttrafficinsidetheAS.
Unnecessaryroutingtrafficwillbegenerated.
1. WhichpotentialproblemcanoccurifaclientdoesnothaveanIBGP
sessionwithallroutereflectorsinacluster?(Source:"Introducing
andDesigningRouteReflectors")
Thisisaninvalidconfiguration.
TheclientmightnotreceiveallBGProutes.
EBGProutesthatarereceivedbytheclientwillnotbedistributedproperly
throughouttheAS.
Duplicateroutingtrafficwillbesenttotheclient.
1. Whichproblemarehierarchicalroutereflectorsdesignedtosolve?
(Source:"IntroducingandDesigningRouteReflectors")
lackofaconsistentapplicationofsecurityandroutingpoliciesthroughout
theAS
scalabilityofautonomoussystemsinverylargeroutingdomains
routingloopscausedbyredundantclusterconfigurations
administrativeoverheadwhenyouareimplementingrouterreflector
networkdesigns
1. WhichtwoBGPparametersdoyouhavetoconfigureonaroute
reflector?(Choosetwo.)(Source:"ConfiguringandMonitoringRoute
Reflectors")
cluster-ID
originator-ID
cluster-list
routereflectorclients
1. Whatarethreemigrationstepsthatarerequiredtoconvertfroma
fullymeshedIBGPAStoanASthatisbasedonroutereflectors?
(Choosethree.)(Source:"ConfiguringandMonitoringRoute
Reflectors")
removeunnecessaryIBGPsessions
configuretheclientsontheroutereflectors
configureIBGPsessionsbetweenroutereflectorclients
configurethecluster-IDontheroutereflectors
1. Whichcommandshouldyouusetoidentifyroutereflectorclients
withoutinspectingtherouterconfiguration?(Source:"Configuring
andMonitoringRouteReflectors")
showipbgpprefix
showipbgpneighbors
showipbgpclients
showipbgpsummary
1. Thecommandneighborip-addressroute-reflector-
clientcommandshouldbeusedonclient.Trueorfalse?(Source:
"ConfiguringandMonitoringRouteReflectors")
true
false
Introduction
BGPisdesignedforreliabilityandscalability.Assuch,ithasbecomethestandard
protocolthatisusedtocarrythemorethan600,000prefixesintheInternettoday.
BGPalsohasatremendousamountofflexibilitywithregardtoadministrativepolicy
controls,routeselection,performancetuning,andscalabilityfeatures.
YouwilllearnaboutadvancedBGPconfigurationtoolsthataredesignedtoimprove
BGPscalabilityandperformance.Toolsthatarediscussedinthismoduleinclude
convergencetimereductionfeatures,limitingthenumberofprefixes,peergroups,
androutedampening.
Uponcompletingthismodule,youwillbeableto:
ConfigureCiscoIOSperformanceimprovementstoreduceBGPconvergence
time
ConfigureBGPtolimitthenumberofprefixesthatarereceivedfromaneighbor
UseBGPpeergroupstosharecommonconfigurationparametersbetween
multipleBGPpeers
Useroutedampeningtominimizetheimpactofunstableroutes
OptimizingBGPScalability
Overview
AsthenumberofroutesintheInternetincreases,demandsonrouterCPUand
memoryresourcesontherouterinaserviceproviderwillincrease.BGPprocessing
affectsbothrouterresourcesandnetworkconvergencetime.Itisimportantthatthe
networkconvergenceisasfastaspossibletoensureaccurateroutinginformation
betweendomains.Itisalsoimportantthatrouterresourcesareoptimizedwhenever
possible.CiscoIOSperformanceimprovementsforBGParedesignedtoaid
networkadministratorsinachievingthesegoals.
Inthislesson,youwilllearnaboutvariousCiscoIOSperformanceimprovements
thathavebeendesignedtoreduceBGPconvergencetime.Includedinthislesson
arediscussionsofconvergence,BGProutingprocesses,andtheeffectsofBGP
routingprocessesonrouterCPUresources.Thelessonalsodiscussesthe
commandsthatarerequiredtoconfigureandmonitorBGPforvariousCiscoIOS
performanceimprovements.ImprovementsthatarementionedarePMTUdiscovery,
inputholdqueue,BGPPIC,BFD,BGPNSFawareness,BGPscantime,
advertisementinterval,keepalive,andhold-downtimers.
Uponcompletingthislesson,youwillbeableto:
DescribeconvergenceinBGPnetworks
DescribetheBGProuterprocessesandtheirfunctions
DescribetheeffectsofBGPprocessesonrouterCPUresources
DescribethefeaturesthatcanbeusedtoimproveBGPconvergence
DescribeusingPMTUdiscovery
Describeincreasingtheinputqueuedepth
DescribetheBGPprefixindependentconvergencefeatureandconfiguration
DescribetheBGPBFDfeatureandconfiguration
DescribethefunctionoftheBGPNonstopForwardingAwarenessfeature
ExplainBGPscantimerconfiguration
ExplainBGPadvertisementintervalconfiguration
ExplainBGPkeepaliveandhold-downtimerconfiguration
ImprovingBGPConvergence
AsthenumberofroutesintheInternetroutingtablegrows,serviceprovidersand
largeenterprisecustomersareexperiencingadramaticincreaseinthetimethat
BGPtakestoconverge.Networksthatonceconvergedin10or15minutesmaynow
takeuptoanhourinsomecases,andevenlongerinextremesituations.Ingeneral,
convergenceisdefinedastheprocessofbringingallroutingtablestoastateof
consistency.
As the number of routes in the Internet routing table grows, the time it takes for
BGP to converge increases.
The Internet currently contains more than 300,000 prefixes.
Network convergence times can range from 10 minutes to more than 1 hour.
BGP is considered converged when:
All routes have been accepted.
All routes have been installed in the routing table.
The input queue and output queue for all peers is 0.
The table version for all peers equals the table version of the BGP table.
TheBGProutingprotocolisconsideredconvergedwhenthefollowingconditionsare
true:
Allrouteshavebeenaccepted.
Allrouteshavebeeninstalledintheroutingtable.
Theinputqueueandoutputqueueforallpeersis0.
ThetableversionforallpeersequalsthetableversionoftheBGPtable.
Convergencetimeisanimportantconsiderationinanetwork,because
nonconvergednetworkscancauseroutingloops,packetdelays,andevenpacket
lossasaresultofblackholes.
BGPConvergence
Ingeneral,aCiscoIOSprocessconsistsoftheindividualthreadsandassociated
datathatperformroutertasks,suchassystemmaintenance,packetswitching,and
implementingroutingprotocols.
BGPconsistsofseveralprocesses,eachofthemrunningatdifferenttimes,
dependingonthetaskthatishandles.
Process Description Interval
BGPopen PerformsBGPpeerestablishment. Atinitialization,whenestablishinga
TCPconnectionwithaBGPpeer.
BGPI/O HandlesqueuingandprocessingofBGP
packets(updatesandkeepalives).
AsBGPcontrolpacketsare
received.
BGPscanner WalkstheBGPtableandconfirms
reachabilityofthenexthops.BGP
scanneralsochecksconditional
advertisementtodeterminewhether
BGPshouldadvertiseconditionprefixes.
Performsroutedampening.
Every60seconds.
BGProuter CalculatesthebestBGPpathand
processesanyroutechanges.Italso
sendsandreceivesroutes,establishes
peers,andinteractswiththerouting
informationbase.
Oncepersecondandwhenadding,
removing,orsoft-reconfiguringa
BGPpeer.
BGP scanner and BGP router are responsible for many calculations and can
lead to high CPU utilization.
SeveralprocessesthatareexecutedontherouterenableBGPtorun.Youcanuse
theshowprocesscpu|includeBGPcommandtoseethevolumeofCPU
resourcesthatareconsumed(utilization)becauseofrunningBGPprocesses.
Athreadisaninformationplaceholderthatallowsasingle
processtobehalted(interrupted)ontheroutersothatthe
CPUcanserviceanotherprocess.Theinformationthatis
containedwithinthethreadallowstheinterruptedprocess
torestartexactlywhereitleftoffwhentheCPUisreadyto
continuetoservicethatprocessthread.
ThefigureliststhefunctionofeachoftheBGProuterprocessesandhowofteneach
processisexecutedontherouter.Itshowsthateachprocessrunsatdifferenttimes,
dependingonthetasksthatthisspecificprocesshandles.BecauseBGPscanner
andBGProuterareresponsibleformanycalculations,youmaynoticehighCPU
utilizationduringtherunningofeitheroneoftheseprocesses.
BGPProcesses
RunningBGProuterprocessesaffectsrouterCPUresources.
BGP scanner process:
High CPU utilization stemming from the BGP scanner process can be expected
for short durations on a router carrying a large Internet routing table.
While the BGP scanner runs, low-priority processes need to wait a longer time
to access the CPU.
BGP router process:
The BGP router process runs about once per second to check for work.
The BGP router consumes all free CPU cycles.
OnroutersthatcarryalargeInternetroutingtable,youcanexpecthighCPU
utilizationstemmingfromtheBGPscannerprocessforshortperiodsoftime.Once
perminute,theBGPscanner"walks"(scans)theBGProutingtableandperforms
importantmaintenancetasks.Thesetasksincludecheckingthenexthopthatis
referencedintheBGPtableoftherouterandverifyingthatthenext-hopdevicescan
bereached.Thus,alargeBGPtabletakesanequallylargeamountoftimetobe
walkedandvalidated.
TheBGPscannerwalkstheBGProutingtabletoupdateanydatastructuresand
walksthetableforrouteredistributionpurposes.Inthiscontext,theroutingtableis
alsoknownastheRIB,whichtherouteroutputswhentheshowiproutecommand
isexecuted.Bothtablesarestoredseparatelyintheroutermemoryandcanbevery
large,thusconsumingCPUandmemoryresources.
TheBGPscannerrunsthroughtheentireBGPtable.Sothedurationofthehigh
CPUutilizationconditionthattheBGPscannerprocesscausesvarieswiththe
numberofneighborsandthenumberofroutesthatarelearnedperneighbor.
WhiletheBGPscannerruns,low-priorityprocessesneedtowaitalongertimeto
accesstheCPU.Onelow-priorityprocesscontrolsICMPpacketssuchaspings.
Packetsthataredestinedtoorhaveoriginatedfromtheroutermayexperience
higherthanexpectedlatencybecausetheICMPprocessmustwaitbehindtheBGP
scanner.TheBGPscannerprocessrunsforsometime,andissuspended,then
ICMPrunsandissuspended,thentheBGPscannerruns,andsoon.Incontrast,
pingssentthrougharoutershouldbeswitchedviaCiscoExpressForwarding(CEF)
andshouldnotexperienceanyadditionallatency.Whenyouaretroubleshooting
periodicspikesinlatency,compareforwardingtimesforpacketsthatareforwarded
througharouterversuspacketsthatareprocesseddirectlybytheCPUonthe
router.
TheBGProuterprocessrunsaboutoncepersecondtocheckforwork.BGP
convergencedefinesthedurationbetweenthetimewhenthefirstBGPpeeris
establishedandthepointatwhichBGPisconverged.Toensuretheshortest
possibleconvergencetimes,theBGProuterconsumesallfreeCPUcycles.
However,afteritstarts,itrelinquishes(orsuspends)theCPUintermittently.
CPUEffectsofBGPProcessesExample
ConvergencetimeisadirectmeasurementofhowlongtheBGProuterprocessruns
ontheCPU,notthetotaltimethattheprocessisactuallyrunning.Thisexample
investigatesthehighCPUutilizationconditiononarouterduringBGPconvergence
asBGPexchangesprefixeswithtwoEBGPpeers.
CaptureabaselinefornormalCPUutilizationbeforestartingthetest.
R1# show process cpu
CPU utilization for five seconds: 0%/0%; one minute: 4%; five minutes: 5%
Aftertheteststarts,theCPUreaches100percentutilization.Theshow
processcpucommandshowsthattheBGProutercauseshighCPUcondition,
denotedby139(theCiscoIOSprocessIDfortheBGProuter)inthefollowing
output:
R1# show process cpu
CPU utilization for five seconds: 100%/0%; one minute: 99%; five minutes:
81%
[output omitted] 139 6795740 1020252 6660 88.34% 91.63% 74.01% 0
BGP Router
CPUEffectsofBGPProcesses
Monitortherouterbycapturingmultipleoutputsoftheshowipbgpsummary
andshowprocesscpucommandsduringtheevent.Theshowipbgp
summarycommandcapturesthestateoftheBGPneighbors.
R1# show ip bgp summary
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxR
cd
192.168.12.2 4 64512 309453 157389 19981 0 253 22:06:44 11163
3
172.16.11.11 4 65101 188934 1047 40081 41 0 00:07:51 58
430
WhentheroutercompletesprefixexchangewithitsBGPpeers,theCPU
utilizationratesshouldreturntonormallevels.Thecomputed1-minuteand5-
minuteaverageswillsettlebackdownaswellbutmayshowhigherthannormal
levelsforalongerperiodthanthe5-secondrate.
R1# show process cpu
CPU utilization for five seconds: 3%/0%; one minute: 82%; five minutes: 91
%
UsingtheoutputfromtheshowcommandswillallowyoutocomputetheBGP
convergencetime.Inparticular,theUp/Downcolumnoftheshowipbgp
summarycommandiscomparedtothestartandstoptimesofthehighCPU
utilizationcondition.Typically,BGPconvergencecantakeseveralminuteswhen
routersexchangealargeInternetroutingtable.
BGPconvergencecanoftenbeanissueinnetworksrequiringquickpropagationof
routinginformation.
Queuing to TCP peer connections
BGP automatically queues data aggressively from the BGP output queue
Deploy BGP peer groups
Simplifies BGP configuration and enhances BGP scalability.
Enable the path MTU feature
Improves efficiency by dynamically determining the largest MTU that you
can use without creating packets that need to be fragmented.
Implement BFD:
Reduces BGP convergence by fast detection of neighbor failure.
Implement BGP PIC:
Reduces convergence by storing BGP backup/alternate path in RIB and FIB.
Increase interface input queues
Improves convergence by reducing dropped TCP ACKs.
Enable BGP NSF Awareness feature
Forwards packets even during failure and thus, saves resources.
ToreduceBGPconvergencetimeandthehighCPUutilizationthatarunningBGP
processcauses,thefollowingperformanceimprovementfeaturesareavailable:
QueuingtoTCPpeerconnections:Insteadofqueuingdataoncepersecond,
BGPnowqueuesdataaggressivelyfromtheBGPoutputqueuetotheTCP
socketforeachpeeruntiltheoutputqueueshavedrainedcompletely.Because
BGPnowsendsatafasterrate,itconvergesmorequickly.
BGPpeergroups:ThemajorbenefitofspecifyingaBGPpeergroupisthatit
reducesthevolumeofsystemresources(CPUandmemory)thatareusedin
BGPupdategeneration.PeergroupsalsosimplifyBGPconfiguration.Many
repetitiveconfigurationelements(suchasfilters)areappliedbytherouteronly
once(tothepeergroup)insteadofapplyingthemtoeachneighbor.Peergroups
allowtheroutingtabletobecheckedonlyonceandallowupdatestobe
replicatedtoallotherin-syncpeergroupmembers.Itdependsonthenumberof
peergroupmembers,thenumberofprefixesinthetable,andthenumberof
prefixesthatareadvertised.Consequentlytheycansignificantlyreducerouter
resourcerequirements.
BidirectionalForwardingDetection:BFDisadetectionprotocoldesignedto
providefastforwardingpathfailuredetectiontimesforallmediatypes,
encapsulations,topologies,androutingprotocols.
BGPPrefixIndependentConvergence:TheBGPPICfeatureimprovesBGP
convergenceafteranetworkfailure.Thisconvergenceisapplicabletobothcore
andedgefailuresandcanbeusedinbothIPandMPLSnetworks.TheBGPPIC
featurecreatesandstoresabackup/alternatepathintheRIB,FIB,andCisco
ExpressForwarding(CEF).Whenafailureisdetected,thebackup/alternate
pathcanimmediatelytakeover,thusenablingfastfailover.
PathMTUfeature:AllTCPsessionsareboundedbyalimitonthenumberof
bytesthatasinglepacketcantransport.Thislimit,whichisknownastheMSS,
is536bytesbydefault.Inotherwords,TCPbreaksuppacketsinatransmit
queueinto536-bytechunksbeforepassingpacketsdowntotheIPlayer.The
advantageofa536-byteMSSisthatpacketsarenotlikelytobefragmentedat
anIPdevicealongthepathtothedestination,becausemostlinksuseanMTU
ofatleast1500bytes.Thedisadvantageisthatsmallerpacketsincreasethe
amountofbandwidththatisusedfortransportoverhead.
BecauseBGPbuildsaTCPconnectiontoallpeers,a536-byteMSSaffects
BGPconvergencetimes.ThesolutionistoenablethePMTUfeature.Youcan
usethisfeaturetodynamicallydeterminehowlargetheMSSvaluecanbe
withoutcreatingpacketsthatneedtobefragmented.PMTUallowsTCPto
determinethesmallestMTUsizeamongalllinksinaTCPsession.TCPthen
usesthisMTUvalue,minusroomfortheIPandTCPheaders,astheMSSfor
thesession.
Increaseinterfaceinputqueues:IfBGPisadvertisingthousandsofroutesto
manyneighbors,TCPmusttransmitthousandsofpackets.BGPpeersreceive
thesepacketsandsendTCPACKstotheadvertisingBGPspeaker,causingthe
BGPspeakertoreceiveafloodofTCPACKsinashorttime.IftheACKsarrive
ImprovingBGPConvergence
ataratethatistoohighfortherouterCPU,packetsbackupininboundinterface
queues.Bydefault,routerinterfacesuseaninputqueuesizeof75packets.In
addition,specialcontrolpacketssuchasBGPupdatesuseaspecialqueuewith
SPD.Thisspecialqueueholds100packets.DuringBGPconvergence,TCP
ACKscanquicklyfillthe175spotsofinputbuffering,causingnewlyarriving
packetstobedropped.Onrouterswith15ormoreBGPpeersthatalso
exchangethefullInternetroutingtable,morethan10,000dropsperinterface
perminutemaybeseen.Increasingtheinterfaceinputqueuedepthhelps
reducethenumberofdroppedTCPACKs,reducingtheamountofworkthat
BGPmustdotoconverge.
BGPNonstopForwardingAwarenessfeature:NSFawarenessallowsaNSF-
awareroutertoassistNSF-capableandNSF-awareneighborstocontinue
forwardingpacketsduringaswitchoveroperationorduringawell-knownfailure
condition.Therouter-holdtimersetsthemaximumtimethattheNSF-aware
routerwillholdknownroutesforanNSF-capableneighbor.Thedeploymentof
BGPNSFawarenesscanimprovetheoverallnetworkstabilitybyreducingthe
amountofresourcesthatarenormallyrequiredforreestablishingpeeringwitha
failedrouter.
BGP convergence can also be improved to some extent by:
Lowering scan time interval for the BGP scanner process.
Lowering advertisement interval between BGP neighbors.
Lowering keepalive and hold-down timers.
Limitation:
Not recommended in routers dealing with large BGP tables.
Could lead to CPU or memory exhaustion.
Lower hold-down timers could lead to undesired session terminations.
YoualsoneedtoimproveBGPconvergenceincertainscenarios;forexample,in
networksusingtheconditionaladvertisementfeature.TherearefourextraBGP
parametersthatyoucanusetoinfluenceBGPconvergencespeed:
Scantime:ControllingtheBGPscannerprocess,responsibleforverifying
informationintheBGPtable
Advertisementinterval:Controllingtherateatwhichsuccessive
advertisementsaresenttoaBGPneighbor
Keepalivetimers:ControllingtheBGPsessionbysendingkeepalivemessages
atspecifictimeinterval.
Hold-downtimers:ControllingtheBGPsessionbywaitingforsuccessive
keepalivemessagefromaBGPneighbor.
Networkadministratorsmusttakecarewhenconfiguringtheseparameters.Setting
thevaluestoolowforaspecificnetworkenvironmentcouldleadtoasignificant
consumptionofrouterresources.ThelargertheBGPtablesandthemoreunstable
theBGPnetwork,thegreaterthedangerofexhaustingtheresourcesofarouter.
Loweringthekeepaliveandhold-downtimerscouldleadtoundesiredsession
terminationswhenkeepalivesarenotreceivedduetonetworkcongestion.
PMTUdiscoveryfeaturecanbeusedtoreduceBGPconvergence.PMTUdiscovery
isamethodformaximizingtheuseofavailablebandwidthinthenetworkbetween
theendpointsofaTCPconnection.
PMTU discovery is used to automatically determine TCP MSS used for TCP
connections from a router.
Prior to Cisco IOS Release 15.0, the default TCP MSS value for BGP was 536
bytes.
From Cisco IOS Release 15.0, PMTU is enabled by default.
Small TCP MSS affects BGP convergence:
Higher TCP MSS can improve BGP convergence.
router(config)# ip tcp path-mtu-discovery [age-timer {minutes | infinite}]
The command enables the PMTU discovery feature for all new TCP connections
from the router.
The age timer is a time interval for how often TCP re-estimates the path
MTU with a larger MSS (default is 10 minutes).
PMTUdiscoveryworksbysettingtheDFoptionbitintheIPheadersofoutgoing
packets.Then,anydevicealongthepathwhoseMTUissmallerthanthepacketwill
dropit.ThenitwillsendbackanICMPFragmentationNeeded(Type3,Code4)
messagecontainingitsMTU,allowingthesourcehosttoreduceitsPMTU
appropriately.TheprocessrepeatsuntiltheMTUissmallenoughtotraversethe
entirepathwithoutfragmentation.
ThedefaultTCPMSSforBGPtrafficis536bytes.EnablingPMTUdiscovery,and
thususingahigherMSSforBGPtraffic,cansignificantlyimproveBGP
convergence,sinceittakesfewerpacketstosendBGPupdates.
ToenablethePMTUdiscoveryfeatureforallnewTCPconnectionsfromtherouter,
usetheiptcppath-mtu-discoveryglobalconfigurationcommand.Todisablethe
function,usethenoformofthiscommand:
ip tcp path-mtu discovery [age-timer {minutes | infinite}]
SyntaxDescription
Parameter Description
agetimerminutes (Optional)Timeinterval(inminutes)afterwhichTCPre-estimates
thePMTUwithalargerMSS.Themaximumintervalis30
minutes;thedefaultis10minutes.
age-timerinfinite (Optional)Turnsofftheagetimer.
TheagetimerisatimeintervalforhowoftenTCPre-estimatesthePMTUwitha
largerMSS.Thedefaultvalueoftheagetimeris10minutes,butitcanbemanually
configuredupto30minutesordisabled(settoinfinite).IftheMSSthatisusedfor
theconnectionissmallerthanthepeerconnectioncanhandle,therouterwill
attempttousealargerMSSeachtimethattheagetimerexpires.Thediscovery
processisstoppedwheneitherthesentMSSisaslargeasthepeernegotiatedor
theuserhasdisabledthetimerontherouter.Youcanturnofftheagetimerby
settingitto"infinite."
MonitoringPMTUDiscovery
ToverifytheTCPMSSthatisusedbetweenBGPneighbors,usetheshowipbgp
neighbors.Youcanuseincludemaxdatafiltertodisplayrelevantoutputonly,as
shownintheexample.
router# show ip bgp neighbor | include max data
Datagrams (max data segment is 536 bytes):
Datagrams (max data segment is 536 bytes):
Datagrams (max data segment is 536 bytes):
Datagrams (max data segment is 536 bytes):
The default MSS is 536 bytes (before Cisco IOS Release 15.0).
PMTUDiscovery
router# show ip bgp neighbor | include max data
Datagrams (max data segment is 1460 bytes):
Datagrams (max data segment is 1460 bytes):
Datagrams (max data segment is 1460 bytes):
Datagrams (max data segment is 1460 bytes):
After enabling of the PMTU discovery feature, the MSS has been increased.
ThefirstexampleshowsthedefaultsizeoftheMSS,536bytes,beforethePMTU
discoveryfeatureisenabledontherouter.Afterusingtheiptcppath-mtu-
discoverycommandtoenablePMTUdiscovery,therouterdynamicallydetermines
howlargetheMSScanbewithoutcreatingIPpacketsthatrequirefragmentation.In
theexampleatthebottom,theoutputshowsthatthePMTUfeaturehasbeen
enabled.Theshowipbgpneighbors|includemaxdatacommandhasbeenused
todeterminethatthePMTUdiscoveryfeaturehassettheMSSto1460bytes.
AnotherfeaturecanbeusedtoimproveBGPconvergenceistoincreaseinput
queuedepth.BGProutersmightexperiencepacketdropsonaninterfacedueto
largenumberofTCPACKsegments,whichareusedtoacknowledgereceiptof
BGPupdates.
Input queue on an interface specifies how many packets can be queued before
dropping the packets.
BGP routers with several peers might experience packet drops on an interface
due to many TCP ACK segments.
router(config-if)# hold-queue length in
This command limits the size of the IP queue on an interface.
The default input hold-queue limit is 75 packets, configurable from 0 to
65,535 packets.
A length of 1000 will normally resolve problems that are caused by input
queue drops of TCP ACKs.
Eachinterfaceownsaninputqueueintowhichincomingpacketsareplacedtoawait
processingbytherouter.Frequently,therateatwhichincomingpacketsareplaced
intheinputqueueexceedstherateatwhichtheroutercanprocessthepackets.
Eachinputqueuehasasizethatindicatesthemaximumnumberofpacketsthatcan
beplacedinthequeue.Aftertheinputqueuebecomesfull,theinterfacedropsany
newincomingpackets.
TospecifythesizeoftheIPinputoroutputqueueonaninterface,usethehold-
queuecommandininterfaceconfigurationmode.Torestorethedefaultvaluesfor
aninterface,usethenoformofthiscommandwiththeappropriatekeyword.
hold-queue length {in | out}
SyntaxDescription
Parameter Description
length Integerthatspecifiesthemaximumnumberofpacketsinthe
queue.Therangeofallowedvaluesis0to65535.
in Specifiestheinputqueue.Thedefaultis75packets.For
asynchronousinterfaces,thedefaultis10packets.Theselimits
preventamalfunctioninginterfacefromconsuminganexcessive
amountofmemory.
out Specifiestheoutputqueue.Thedefaultis40packets.For
asynchronousinterfaces,thedefaultis10packets.Theselimits
preventamalfunctioninginterfacefromconsuminganexcessive
amountofmemory.
Severalconsiderationsshouldbetakenintoaccountwhen
increasinginterfaceinputqueues.Increasingtheinterface
inputqueueswillincreasememoryutilization.
Increasingtheholdqueuecanhavedetrimentaleffectson
networkroutingandresponsetimes.Forprotocolsthat
useSEQorACKpacketstodetermineround-triptimes,do
notincreasetheoutputqueue.Droppingpacketsinstead
informshoststoslowdowntransmissionstomatch
availablebandwidth.Thisapproachisgenerallybetter
thanhavingduplicatecopiesofthesamepacketwithinthe
network(whichcanhappenwithlargeholdqueues).
TheCisco12000SeriesnowusesadefaultSPD
headroomvalueof1000.Itretainsthedefaultinputqueue
sizeof75.Usetheshowspdcommandtoviewthese
specialinputqueues.
MonitoringPMTUInputQueueDepth
Usetheshowinterface{interface-identifier}commandtodisplaysthecurrentinput
IncreasingInputQueueDepth
queuelevelsandthenumberofincomingpacketsdropped.
router# show interface GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up
Hardware is AmdP2, address is aabb.cc00.4900 (bia aabb.cc00.4900)
MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
ARP type: ARPA, ARP Timeout 04:00:00
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Displays interface information, including input queue depth.
Theinputqueuew/x/y/zcounterdisplaysthecurrentinputqueue.wisthecurrent
numberofpacketsintheinputqueueandxisthequeuedepth.Thedropscounter,
y,indicatesthenumberofincomingpacketsthathavebeendropped.Inthe
example,thereisnopacketintheinputqueue,thedepthofqueueisleftondefault,
75,andnopacketweredropped.
Ifthecurrentnumberofpacketsintheinputqueueisconsistentlyat,orgreaterthan,
80percentofthecurrentsizeoftheinputqueue,thesizeoftheinputqueuemay
requiretuningtoaccommodatetherateofincomingpackets.Evenifthecurrent
numberofpacketsintheinputqueueneverseemstoapproachthesizeoftheinput
queue,burstsofpacketsmaystillbeoverflowingthequeue.Ifthedropscounteris
increasingatahighrate,thesizeoftheinputqueuemayrequiretuningto
accommodatethebursts.
1.
2.
3.
4.
5.
BGPPrefixIndependentConvergencefeatureimprovesBGPconvergenceaftera
networkfailure.Itcalculatesasecondbetspath,alongwiththeprimarybestpath
andstoresbothpathsintheCiscoExpressForwarding(CEF).
BGP prefix independent convergence characteristics:
PIC enhances BGP convergence, regardless of the number of BGP prefixes.
PIC stores the BGP backup/alternate path for each prefix in BGP, RIB, and FIB
tables.
When the primary goes down, Cisco Express Forwarding quickly selects a
different egress port for the affected destination.
Undernormalcircumstances,BGPcantakeseveralsecondstoafewminutesto
convergeafteranetworkchange.Atahighlevel,BGPgoesthroughthefollowing
process:
BGPlearnsoffailuresthrougheitherIGPorBFDeventsorinterfaceevents.
BGPwithdrawstheroutesfromtheRIB,andtheRIBwithdrawstheroutesfrom
theFIBanddistributedFIB.Thisprocessclearsthedatapathfortheaffected
prefixes.
BGPsendswithdrawmessagestoitsneighbors.
BGPcalculatesthenextbestpathtotheaffectedprefixes.
BGPinsertsthenextbestpathforaffectedprefixesintotheRIB,andtheRIB
installsthemintheFIBanddistributedFIB.
Thisprocesstakesafewsecondsorafewminutestocomplete.Thetimedepends
onthelatencyofthenetwork,theconvergencetimeacrossthenetwork,andthe
localloadonthedevices.Thedataplaneconvergesonlyafterthecontrolplane
converges.
TheBGPPICfunctionalityisachievedbyadditionalfunctionalityintheBGP,RIB,
andCEF.
BGPPICaffectsprefixesunderIPv4andVPNv4addressfamilies.Forthose
prefixes,BGPcalculatesasecondbestpath,alongwiththeprimarybestpath.(The
secondbestpathiscalledthebackup/alternatepath.)BGPinstallsthebestpathand
thebackup/alternatepathsfortheaffectedprefixesintotheBGPRIB.The
backup/alternatepathprovidesafastreroutemechanismtocounterasinglenetwork
failure.
ForBGPPIC,RIBinstallsanalternatepathperrouteifoneisavailable.Withthe
BGPPICfunctionality,aRIBthatselectsaBGProutecontainingabackup/alternate
pathinstallsthebackup/alternatepathwiththebestpath.
WithBGPPIC,CEFstoresanalternatepathperprefix.Whentheprimarypathgoes
down,CEFsearchesforthebackup/alternatepathinaprefix-independentmanner.
CEFalsolistenstoBFDeventstorapidlydetectlocalfailures.Upondetectionofa
failure,CEFdetectsthealternatenexthopforallprefixesthatareaffectedbythe
failure.
BGPPICConfiguration
ToenableBGPPIConCiscoIOSSoftwarerouters,firstenterBGPconfiguration
modeusingtherouterbgpcommand.Then,entertheappropriateaddressfamily
configurationmodeandusethebgpadditional-pathsinstallcommandtoenable
BGPPIC.
BGPPrefixIndependentConvergence
Intheexample,CE1andCE2areconfiguredwiththeBGPPICfeature.BGP
computesPE1asthebestpathandPE2asthebackup/alternatepath,andinstalls
boththeroutesintotheRIBandCEFplane.
NopoliciesshouldbesetonCE1andCE2fortheEBGPpeersPE1andPE2.Both
CEroutersmustpointtotheEBGProuteasthenexthop.OnCE1,thenexthopto
reachtheInternetisthroughPE1.OnCE2,thebestpathtoreachtheInternetis
PE2.CE2advertisesitselfasthenexthoptoCE1,andCE1doesthesametoCE2.
Asaresult,CE1hastwopathsforthespecificprefix.Itusuallyselectsthedirectly
connectedEBGPpathovertheIBGPpath,accordingtothebestpathselection
rules.Similarly,CE2hastwopaths,anEBGPpaththroughPE2andanIBGPpath
throughCE1-PE1.
IftheCE1-PE1linkorPE1goesdown,BGPrecomputesthebestpath,removingthe
nexthopPE1fromRIBandreinstallingCE2asthenexthopintotheRIBandCEF.
CE1automaticallygetsabackup/alternaterepairpathintoCEF,andthetrafficloss
duringforwardingisnowinsubseconds,achievingfastconvergence.
PE1# show ip bgp
BGP table version is 65, local router ID is 10.0.3.81
Status codes: s suppressed, d damped, h history, * valid, > best, i - inte
rnal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-
Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 209.165.201.0/24 1.0.0.101 0 1 i
*bi 10.0.100.2 0 100 0 1 i
Youcanverifythebackuproutebyissuingshowipbgpcommand.Intheexample,
youcanseethatPE1routerhastwopathstoreachthe209.165.201.0/24networkat
thecustomerside.Theinternalroute,viaPE2router,ismarkedasthebackuproute.
Additionally,youcanverifythatthebackuppathisreallystoredintheCEF,by
issuingshowipcefprefixdetailcommand.Thebackuprouteislabeledas"repair."
WithoutBGPPICfeatureenabled,onlyonepathisstoredintheCEF.
PE1# show ip cef 209.165.201.0/24 detail
209.165.201.0/24, epoch 0, flags rib only nolabel, rib defined all labels
recursive via 1.0.0.101
attached to Serial0
recursive via 10.0.100.2, repair
attached to Ethernet0/0
BFDprovidesalow-overhead,short-durationmethodofdetectingfailuresinthe
forwardingpathbetweentwoadjacentrouters,includingtheinterfaces,datalinks,
andforwardingplanes.BFDisadetectionprotocolthatyouenableattheinterface
androutingprotocollevels.CiscosupportstheBFDasynchronousmode,which
dependsonthesendingofBFDcontrolpacketsbetweentwosystemstoactivate
andmaintainBFDneighborsessionsbetweenrouters.Therefore,inorderforaBFD
sessiontobecreated,youmustconfigureBFDonbothBFDpeers.OnceBFDhas
beenenabledontheinterfacesandattherouterlevelfortheappropriaterouting
protocols,aBFDsessioniscreated,BFDtimersarenegotiated,andtheBFDpeers
willbegintosendBFDcontrolpacketstoeachotheratthenegotiatedinterval.
Extremely lightweight hello protocol that uses UDP to test bidirectional
communication.
Used to detect failures in the forwarding path between two adjacent routers.
Millisecond resolution of forwarding plane failure.
Relies on routing protocols to detect neighbors.
BFDprovidesfastBFDpeerfailuredetectiontimesindependentlyofallmediatypes,
encapsulations,topologies,androutingprotocolsBGP,EIGRP,IS-IS,andOSPF.By
sendingrapidfailuredetectionnoticestotheroutingprotocolsinthelocalrouterto
initiatetheroutingtablerecalculationprocess,BFDcontributestogreatlyreduced
overallnetworkconvergencetime.
TheBFDprotocolhasnodiscoverymechanismstodetectneighbors;itisdesigned
solelyasanagentforotherapplicationsrequiringfastfailuredetection.Whenevera
routingprotocolthatisconfiguredtouseBFDdetectsanewneighbor,itrequests
availabilitytrackingfromBFD.
BFDcanrelyoncontrolpacketsoronechopackets.EchopacketsareIPpackets
addressedtotherouteritselfbutsenttotheLayer2addressofthenext-hopnode.
Theechopacketsthoroughlytestthecompletebidirectionalforwardingpath
betweenadjacentroutersbecausetheyhavetobetransmittedbytheoriginating
router,propagatedtotheadjacentrouter,receivedbyitsinterfacelogic,switchedby
itsforwardingengine,andsentbacktotheoriginator(becausetheIPpacketis
addressedtotherouteritself).
Forexample,whenR1sendsaBFDechopacket,itsetsthedestinationIPaddress
inthepackettoitsowninterfaceIPaddressandtheMACaddressintheLayer2
frameheadertotheMACaddressoftheneighbor(R2).WhenR2receivesthe
packet,itperformsaLayer3lookupandsendsthepackettowarditsfinaldestination
(backtoR1).
BFD operation:
Routing protocol (BFD client) bootstraps BFD to create a BFD session to a
neighbor:
BFD client receives link status change notification.
Receive and transmit intervals are negotiated and configurable.
The two systems agree on a method to detect failure.
In case of failure, BFD notifies the BFD client:
BidirectionalForwardingDetectionforBGP
The BFD client independently decides on the action.
Whenaroutingprotocol(BGP,forexample)discoversaneighbor,itsendsarequest
tothelocalBFDprocesstoinitiateaBFDneighborsessionwiththeBGPneighbor
router.ThentheBFDneighborsessionwiththeBGPneighborrouterisestablished.
Ifthereisafailureonthelinkbetweenneighbors,theBFDneighborsessionwiththe
BGPneighborrouteristorndown.BFDnotifiesthelocalBGPprocessthattheBFD
neighborisnolongerreachable.ThelocalBGPprocesstearsdowntheBGP
neighborrelationship.Ifanalternativepathisavailable,therouterswillimmediately
startconvergingonit.
IfmultipleroutingprotocolswanttoestablishBFD
sessionswiththesameremotesystemforthesame
routedprotocol(IPv4orIPv6),allmustshareasingleBFD
session.
BFDConfiguration
ToconfigureBFD,youfirsthavetoenabletheBFD,usingthebfdinterval
commandintheinterfaceconfigurationmode.
bfd interval send-timer min_rx receive-timer multiplier multiplier
SyntaxDescription
Parameter Description
intervalsend-timer Therate,inmilliseconds,atwhichBFDcontrolpacketswillbe
senttoBFDpeers.Thevalidrangeforthesend-timerisfrom50
to999.
min_rxreceiver-timer Therate,inmilliseconds,atwhichBFDcontrolpacketswillbe
expectedtobereceivedfromBFDpeers.Thevalidrangeforthe
receive-timerisfrom50to999.
multiplermultipler ThenumberofBFDpacketsthatcanbelostbeforetheBFDpeer
isdeclared"down."
ThenyouhavetoenableBFDsupportforBGP,usingtheneighborfall-overbfd
command.
neighbor ip-address fall-over bfd
SyntaxDescription
Parameter Description
fall-overbfd EnablesBFDforindividualneighbor.
Usually,whenanetworkingdevicerestarts,allroutingpeersofthatdevicedetect
thatthedevicewentdownandthencamebackup.Thistransitionresultsinwhatis
calledaroutingflap,whichcouldspreadacrossmultipleroutingdomains.Routing
flapsthatarecausedbyroutingrestartscreateroutinginstabilities,whichare
detrimentaltotheoverallnetworkperformance.CiscoNonstopForwarding(NSF),
alsoknownasgracefulrestart,workswiththeSSOtominimizetheamountoftimea
networkisunavailabletoitsusersfollowingaswitchover.Themainobjectiveof
CiscoNSFistocontinueforwardingIPpacketsfollowingarouteprocessor
switchoverinplatformswithdualRPs,thusreducingnetworkinstability.
CiscoNonstopForwarding
Cisco NSF is applicable in platforms with dual RPs and works together with
SSO.
Cisco NSF allows:
Routing neighbor relationships remain established during SSO.
Routes on neighboring routers remain valid.
Forwarding of data packets continues while the routing process on the new
RP converges.
Cisco NSF is supported by:
Routing protocols (OSPF, IS-IS, EIGRP, BGP)
Forwarding operation (Cisco Express Forwarding)
The device must be Cisco NSF-capable.
The neighboring device must be Cisco NSF-aware.
CiscoNSFallowsfortheforwardingofdatapacketstocontinuealongknownroutes
whiletheroutingprotocolinformationisbeingrestoredfollowingaswitchover.With
NSF,peernetworkingdevicesdonotexperienceroutingflaps.Datatrafficis
forwardedthroughintelligentlinecardsordualforwardingprocessors,whilethe
standbyRPassumescontrolfromthefailedactiveRPduringaswitchover.
CiscoNSFissupportedbyfourprotocolsforrouting(BGP,EIGRP,OSPF,andIS-IS
),andbyCiscoExpressForwarding(CEF)forforwarding.Theroutingprotocols
havebeenenhancedwithCiscoNSFcapabilityandawareness,meaningthat
routersrunningtheseprotocolscandetectaswitchoverandtakethenecessary
actionstocontinueforwardingnetworktrafficandtorecoverrouteinformationfrom
thepeerdevices.
AnetworkingdeviceissaidtobeCiscoNSF-awareifitisrunningNSF-compatible
software.AdeviceissaidtobeCiscoNSF-capableifithasbeenconfiguredto
supportNSFandcanrebuildroutinginformationfromNSF-awareorNSF-capable
neighbors.
CiscoNSFdependsonCEFtocontinueforwardingpacketsduringswitchoverwhile
theroutingprotocolsrebuildtheRIBtables.Oncetheroutingprotocolshave
converged,CEFupdatestheFIBtableandremovesoldrouteentries.CEF,inturn,
updatesthelinecardswiththenewFIBinformation.
Cisco NSF overview:
One RP is active, one is standby.
Cisco Express Forwarding on the active RP synchronizes the FIB and adjacency
table to the standby RP.
Upon switchover, the new active RP uses the old FIB and adjacency table to
forward packets while the routing protocol reconverges.
Routing protocol must:
Establish neighbor relationship without causing a reset of neighbor
relationship
Learn routing information
As the routing protocol starts to repopulate the RIB, it updates Cisco Express
Forwarding.
CiscoNSFalwaysoperatestogetherwithSSO.InspecificCisconetworkingdevices
thatsupportdualRPs,SSOestablishesoneoftheRPsastheactiveprocessor
whiletheotherRPisdesignatedasthestandbyprocessor,andthensynchronizes
theFIBandadjacencytablebetweenthem.Aswitchoverfromtheactivetothe
BGPNonstopForwardingAwareness
standbyprocessoroccurswhentheactiveRPfails,isremovedfromthenetworking
device,orismanuallytakendownformaintenance.Duringtheswitchover,thenew
activeRPusestheoldFIBandadjacencytabletoforwardpacketswhiletherouting
protocolreconverges.OncetheroutingprotocolshaveupdatedtheRIB,CEF
updatestheFIBtableandremovesoldrouteentries.CEF,inturn,updatestheline
cardswiththenewFIBinformation.
Theroutingprotocols(forexample,BGP)runonlyontheactiveRP,andthey
receiveroutingupdatesfromtheirneighborrouters.Routingprotocolsdonotrunon
thestandbyRP.IfCiscoNSFisnotconfigured,aneighboringdevicewillteardown
theadjacencyduringaswitchover.Theresultwouldbearoutingflapthatwould
spreadacrossthenetwork.
UsingCiscoNSF,theroutingprotocolsonanNSF-capabledevicerequeststhatthe
NSF-awareneighbordevicessendroutinginformationtohelprebuildtherouting
tables.TheCiscoNSF-awareneighborwillnotteardowntheneighborrelationship.
Itwillre-establishtheneighborrelationshipwiththeCiscoNSF-capabledevice,and
willsendroutinginformationtosynchronizetheroutingtableonanNSF-capable
device.
ACiscoNSF-awareroutercanpeerwithCiscoNSF-capableroutersandfacilitate
theresynchronizationofroutinginformationwithsuchrouters.
BGPNonstopForwardingAwareness
TheBGPNSFAwarenessfeatureallowstheBGPpeersofthefailingroutertoretain
theroutinginformationthatisadvertisedbythefailingrouter.Italsocontinuetouse
thisinformationuntilthefailedrouterhasreturnedtonormaloperatingbehaviorand
isabletoexchangeroutinginformation.Thepeeringsessionismaintained
throughouttheentireNSFoperation.
Minimizes the effects of the following:
Well-known failure conditions (for example, a stuck-in-active event)
Unexpected events (for example, an RP switchover operation)
Scheduled events (for example, a hitless software upgrade)
router(config-router)# bgp graceful-restart
Globally enables BGP NSF awareness.
TheBGPNSFfeatureprovidesanNSF-awarerouterwiththecapabilitytodetecta
neighborthatisundergoinganSSOoperation,maintainthepeeringsessionwiththis
neighbor,retainknownroutes,andcontinuetoforwardpacketsfortheseroutes.
BGPsupportforCiscoNSFrequiresthatneighborroutersareNSF-awareorNSF-
capable.ThegracefulrestartmechanismalsoenablesCiscoNSFawarenessin
BGP.ArouterthatisNSF-awarefunctionslikearouterthatisNSF-capable,with
oneexception:AnNSF-awareroutercannotperformanSSOoperation.However,a
routerthatisNSF-awarecanmaintainapeeringrelationshipwithaNSF-capable
neighborduringanNSFSSOoperation.Itcanholdroutesforthisneighborduring
theSSOoperation.
AnNSF-awareroutermustbeupandcompletely
convergedwiththenetworkbeforeitcanassistanNSF-
capablerouterinanNSFrestartoperation.
ThedeploymentofBGPNSFawarenesscanminimizetheeffectsofthefollowing:
Well-knownfailureconditions(forexample,astuck-in-activeevent)
Unexpectedevents(forexample,anRPswitchoveroperation)
Scheduledevents(forexample,ahitlesssoftwareupgrade)
BGPNSFimprovestheoverallnetworkstabilitybyreducingtheamountof
resourcesthatarenormallyrequiredforreestablishingpeeringwithafailedrouter.
WhenaCiscoNSF-capablerouterbeginsaBGPsessionwithaBGPpeer,itsends
anOPENmessagetothepeer.Includedinthemessageisadeclarationthatthe
CiscoNSF-capableorNSF-awarerouterhas"gracefulrestartcapability."IftheBGP
peerhasreceivedthiscapability,itisawarethatthedevicesendingthemessageis
CiscoNSF-capable.BoththeCiscoNSF-capablerouteranditsBGPpeers(NSF-
awarepeers)needtoexchangethegracefulrestartcapabilityintheirOPEN
messagesatthetimeofsessionestablishment.Ifbothpeersdonotexchangethe
gracefulrestartcapability,thesessionwillnotbecapableofgracefulrestart.
IftheBGPsessionislostduringtheRPswitchover,theCiscoNSF-awareBGPpeer
marksalltheroutesthatareassociatedwiththeNSF-capablerouterasstale.
However,itcontinuestousetheseroutestomakeforwardingdecisionsforatime.
ThisfunctionalitymeansthatnopacketsarelostwhilethenewlyactiveRPiswaiting
forconvergenceoftheroutinginformationwiththeBGPpeers.
AfteranRPswitchoveroccurs,theCiscoNSF-capablerouterre-establishesthe
sessionwiththeBGPpeer.Inestablishingthenewsession,itsendsanewgraceful
restartmessagethatidentifiestheCiscoNSF-capablerouterashavingrestarted.
Atthispoint,theroutinginformationisexchangedbetweenthetwoBGPpeers.
Oncethisexchangeiscomplete,theCiscoNSF-capabledeviceusestherouting
informationtoupdatetheRIBandtheFIBwiththenewforwardinginformation.The
CiscoNSF-awaredeviceusesthenetworkinformationtoremovestaleroutesfrom
itsBGPtable.Oncethestaleroutesareremoved,theBGPprotocolisfully
converged.
IfaBGPpeerdoesnotsupportthegracefulrestartcapability,itwillignorethe
gracefulrestartcapabilityinanOPENmessagebutwillestablishaBGPsessionwith
theCiscoNSF-capabledevice.Thisfunctionalitywillallowinteroperabilitywithnon-
NSF-awareBGPpeers(andwithoutNSFfunctionality),buttheBGPsessionwith
non-NSF-awareBGPpeerswillnotbecapableofgracefulrestart.
BGPNSFawarenessisnotenabledbydefault.Togloballyenableit,usebgp
graceful-restartrouterconfigurationcommand.TodisabletheBGPgracefulrestart
capabilitygloballyforallBGPneighbors,usethenoformofthiscommand.
bgp grafecul-restart [restart-time seconds] [stalepath-time seconds]
SyntaxDescription
Parameter Description
restart-timeseconds (Optional)Themaximumtimeperiodthatthelocalrouterwillwait
foragraceful-restart-capableneighbortoreturntonormal
operationafterarestarteventoccurs.Thedefaultvalueforthis
argumentis120seconds.Thevalidrangeofvaluesisfrom1to
3600seconds.
stalepath-timeseconds (Optional)Themaximumtimeperiodthatthelocalrouterwillhold
stalepathsforarestartingpeer.Allstalepathsaredeletedafter
thistimerexpires.Thedefaultvalueforthisargumentis360
seconds.Thevalidrangeofvaluesisfrom1to3600seconds
NSFawarenessisenabledautomaticallyinsupported
softwareimagesforInteriorGatewayProtocols,suchas
EIGRP,IS-IS,andOSPF.
BGPconvergencecanalsobereducedbyusingcertainBGPtimers.BGPscantime
isoneofthetimersthatcanbeusedtoinfluenceBGPconvergencespeed.
Defines how often BGP scanner process scans the BGP table.
If lowered, can improve convergence
Needed to confirm that next hops are still available.
The BGP scanner process is also responsible for advanced features such as
conditional advertisement check and performing route dampening.
Set to 60 seconds by default.
router(config-router)# bgp scan-time scanner-interval
Changes the default value of BGP scanner process runs.
TheBGPscannerprocesswalks(scans)theBGPtableandconfirmsthe
reachabilityofnexthops.AchangeofthisstatustriggersanewBGProuteselection
forthenetwork.TherouterthenpropagatethechangestoestablishedBGP
neighbors.IncreasingtheBGPscannerprocessfrequencywillmaketherouterfind
achangedstatusmorequickly,butitwillalsoconsumemoreCPUresources.
TheBGPscannerprocessisalsoresponsibleforsomeadvancedBGPfeatures.It
checkstheconditionaladvertisementtodeterminewhetherBGPshouldadvertise
conditionalprefixesorperformroutedampening.
ToconfigurenondefaulttimeintervalforrepetitionsoftheBGPscannerprocess,use
bgpscan-timecommand.
bgp scan-time [import] scanner-interval
SyntaxDescription
Parameter Description
import (Optional)ConfiguresimportprocessingofVPNv4unicastrouting
informationfromBGProutersintoroutingtables.
scanner-interval SpecifiesthescanningintervalofBGProutinginformation.Valid
valuesthatareusedforselectingthedesiredscanninginterval
arefrom5to60seconds.Bydefault,thescanningintervalis60
seconds.
MonitoringBGPScanTime
YoucanchecktheconfiguredBGPscanintervalusingtheshowipbgpsummary
command.
R1# show ip bgp summary
BGP router identifier 192.168.12.1, local AS number 1
BGP table version is 87, main routing table version 87
11 network entries using 1628 bytes of memory
16 path entries using 1024 bytes of memory
3/2 BGP path/bestpath attribute entries using 408 bytes of memory
1 BGP AS-PATH entries using 24 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 3084 total bytes of memory
BGP activity 44/33 prefixes, 84/68 paths, scan interval 60 secs
<... output omitted ...>
Scan interval is defined per BGP router process and address family.
TheconfiguredBGPscanintervalwillapplytotheentireBGProutingprotocol
process.
BGPNext-HopTracking
BGPmonitorsthenexthopofinstalledroutestoverifynext-hopreachabilityandto
select,install,andvalidatetheBGPbestpath.Bydefault,theBGPscannerisused
topolltheRIBforthisinformationevery60seconds.Duringthe60secondtime
periodbetweenscancycles,IGPinstabilityorothernetworkfailurescancauseblack
holesandroutingloopstotemporarilyform.
BGPScanTime
TheBGPnext-hoptrackingfeaturepreventsblackholeroutingduringthe60second
periodofscan-timer.Scan-timerisalreadymostlyreplacedinbasicfunctionsbythe
BGPNHT.
The replacement for scan-timer.
Prevents black-hole routing during the scan-timer period.
It is event driven.
Automatically tracks BGP prefixes, and rapidly report next-hop changes to the
BGP routing process.
Enabled by default.
router(config-router)# bgp nexthop trigger [delay seconds | enable]
Controls the BGP NGT feature:
Enables it, if it was previously disabled.
Changes the interval between checks.
BGPnext-hoptrackingisanevent-drivensystemthatprovidesfasterconvergencein
on-demandfashionratherthanperiodicbasedonscan-timer.ItmonitorsRIBfor
next-hoprelatedchangesforbothEBGPandIBGPprefixes.Itreportschangesto
theBGProutingprocess.
ThisoptimizationimprovesoverallBGPconvergencebyreducingtheresponsetime
tonext-hopchangesforroutesinstalledintheRIB.Whenabestpathcalculationis
runinbetweenBGPscannercycles,onlynext-hopchangesaretrackedand
processed.
BGPNHTfeatureisenabledbydefault.Disablingnexthopaddresstrackingmaybe
usefulifyournetworkhasunstableIGPpeersandroutedampeningisnotresolving
thestabilityissues.Tocontrolthisfeature,usethebgpnexthoptriggercommand
intheaddressfamilyconfigurationmodeoftheroutingprocess.
bgp nexthop trigger { delay seconds | enable}
SyntaxDescription
Parameter Description
delayseconds Changesthedelayintervalbetweenchecksonupdatednext-hop
routesthatareinstalledintheroutingtable.Validvaluesforthe
delayarefrom0to100seconds.Thedefaultis5seconds.
enable EnablesBGPnext-hoptracking.
Thebgpscan-timecommandisignoredifyourrouterhas
BGPnext-hoptrackingenabledfortheaddress-family.
BGPadvertisementintervalisanotheroptiontoinfluenceBGPconvergencespeed.
Defines a time which has to elapse between two successive updates about the
same destination that are sent to a neighbor.
If lowered, can improve convergence
Default values are different for IBGP and EBGP neighbors:
30 seconds for EBGP neighbors
0 seconds for IBGP neighbors
router(config-router)# neighbor [ip-address | peer-group-
name] advertisement-interval seconds
Changes the default time interval in the sending of BGP routing updates for a
specific neighbor:
Advertisementintervaltimercontrolstherateatwhichsuccessiveadvertisements
aresenttoaBGPneighbor.WhenaBGP-speakingroutersendsarouteupdatetoa
neighborforaspecificdestination,itisnotallowedtosendanotherupdatetothe
neighboraboutthesamedestinationuntilatimeequaltotheadvertisementinterval
haselapsed.So,theadvertisementintervaltimeractsasaformofratelimitingona
per-destinationbasis,eventhoughthevalueoftheadvertisementintervalis
configuredforeachneighbor.
ThedefaultvaluesaredifferentforIBGPandEBGPpeers.ForIBGPpeers,the
intervalissetto0seconds.ForEBGPpeers,theintervalissetto30seconds.For
EBGPpeersthatareconfiguredinaVRFinstance,theintervalisalsosetto0
seconds.
Itisimportanttonotethatwhiletherouteriswaitingfortheadvertisementinterval
timertoexpire,theroutercanstillreceiveandprocessrouteupdatesfromBGP
neighbors.Changingtheadvertisementintervaldoesnotrate-limitBGProute
selection(inboundupdatesandsubsequentprocessing)butonlytherateofoutgoing
routeadvertisements.
WhenfasterpropagationofsuccessiveBGPupdates(whicharebatchedandrate-
limited)isrequired,thenetworkadministratorcanlowerthedefaultvalueofthe
advertisementinterval,thusimprovingconvergence.
YoucanmodifythedefaultadvertisementintervalforaspecificBGPpeerusingthe
neighboradvertisement-intervalrouterconfigurationcommand.
neighbor [ip-address | peer-group-name] advertisement-interval seconds
SyntaxDescription
Parameter Description
ip-address NeighborIPaddress.
peer-group-name NameofaBGPpeergroup.IfaBGPpeergroupisspecifiedby
usingthepeer-group-nameargument,allmembersofthepeer
groupwillinheritthecharacteristicthatisconfiguredwiththis
command.
seconds Timeinseconds.Thevalidvalueisfrom0to600seconds.The
defaultis30secondsforexternalpeersand5secondsforinternal
peers.
InroutersthathandlelargeBGPtablesandlessstable
networks,loweringtheadvertisementintervalcould
potentiallyleadtoconsuminglargeportionsofrouter
resources.
MonitoringtheBGPAdvertisementInterval
BGPAdvertisementInterval
R1# show ip bgp neighbors 192.168.12.2
BGP neighbor is 192.168.12.2, remote AS 1, internal link
<... output omitted ...>
Default minimum time between advertisement runs is 0 seconds
<... output omitted ...>
R1# show ip bgp neighbors 172.16.11.11
BGP neighbor is 172.16.11.11, remote AS 100, external link
<... output omitted ...>
Default minimum time between advertisement runs is 30 seconds
For address family: IPv4 Unicast
Session: 172.16.11.11
BGP table version 102, neighbor version 102/0
<... output omitted ...>
Minimum time between advertisement runs is 15 seconds
<... output omitted ...>
YoucanexaminethecurrentlyconfiguredBGPadvertisementintervalwiththeshow
ipbgpneighborscommand.Theadvertisementintervalisdefinedforaspecific
neighborinaspecificBGPaddressfamily.Actualvaluesoftheadvertisement
intervalarethereforestatedunderthespecificaddress-familyportionoftheneighbor
output.
Theexampleshowstheoutputforinternalneighbor,wherethedefaulttimeris0
seconds.ThesecondexampleshowsthechangeddefaulttimerforEBGP,30
seconds.Aftertheparameterwaschanged,thetimerswitchedto15seconds.
BGPkeepaliveandhold-downtimersaretwoBGPtimersthatinfluenceBGP
convergenceaswell.
BGP keepalive timer:
Defines a time between successive keepalive messages.
Set to 60 seconds by default.
BGP hold-down timer:
Defines how long a router will wait from the last received keepalive or update
message before declaring the session dead.
Set to 3x keepalive = 180 seconds by default.
router(config-router)# neighbor [ip-address | peer-group-
name] timers keepalive holdtime [min-holdtime]
ThekeepalivetimerdefineshowoftenaBGProuterwillprobeaBGPneighborby
sendingkeepalivemessages.Theholddowntimerdefineshowlongarouterwill
waitfromthelastreceivedkeepaliveorupdatemessagebeforedeclaringthe
sessiondead.
Loweringthekeepaliveandhold-downtimerswillimproveBGPconvergence.
Bydefault,thekeepalivetimerissetto60seconds,andthehold-downtimerissetto
180seconds.EveniftheIProutingtableindicatesthattheneighborisnolonger
reachable,BGPwillnottriggertheconvergenceprocessuntiltheBGPsessionhold-
downtimerexpires.
Toconfigureanondefaultvalueofthekeepaliveand/orhold-downtimers,usethe
neighbortimersrouterconfigurationcommand.
neighbor [ip-address | peer-group-name] timers keepalive holdtime [min-
holdtime]
SyntaxDescription
Parameter Description
ip-address NeighborIPaddress.
peer-group-name NameofaBGPpeergroup.IfaBGPpeergroupisspecifiedby
usingthepeer-group-nameargument,allmembersofthepeer
groupwillinheritthecharacteristicthatisconfiguredwiththis
command.
keepalive Timeinsecondswithwhichthekeepalivemessagesaresentto
thepeer.Thevalidvalueisfrom0to65535seconds.Thedefault
is60seconds.
holdtime Timeinsecondsafternotreceivingakeepalivemessageand
declaringapeerdead.Thevalidvalueisfrom0to65535
seconds.Thedefaultis180seconds.
min-holdtime (Optional)Interval(inseconds)specifyingtheminimum
acceptablehold-timefromaBGPneighbor.Theminimum
acceptablehold-timemustbelessthan,orequalto,theinterval
thatisspecifiedintheholdtimeargument.Thevalidvalueisfrom
0to65535.
NewkeepaliveswilltakeeffectonlyafternewBGP
sessionestablishmentastheyaretransferredand
negotiatedtothelowervaluesfromthetwopeersinthe
OPENmessages.ThereforeyouneedtoresettheBGP
session.
MonitoringBGPKeepaliveandHold-DownTimers
R1# show ip bgp neighbors
BGP neighbor is 172.16.11.11, remote AS 100, external link
BGP version 4, remote router ID 10.0.1.81
BGP state = Established, up for 06:54:31
Last read 00:00:02, last write 00:00:22, hold time is 180, keepalive int
BGPKeepaliveandHold-DownTimers
erval is 60 seconds
<... output omitted ...>
Verifies keepalive and hold-down timers.
YoucanexaminethecurrentlyconfiguredBGPkeepaliveandhold-downtimersfor
eachneighborwiththeshowipbgpneighborscommand.
Intheexample,bothtimersareleftondefault:keepalivetimeris60secondsand
hold-downtimeris180seconds.
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
BGP is considered converged when there are no outstanding updates to be sent
to neighbors.
Several features are provided in BGP to improve convergence time:
PMTU discovery benefits BGP as it benefits any transmission over IP.
Input queue depth can be increased to improve performance.
PIC enhances BGP convergence regardless of number of BGP prefixes.
BFD reduces conversion time by providing rapid detection of failed peers.
NSF awareness reduces the amount of resources by continue forwarding
packets during a failure condition.
You can also lower the BGP timers to influence BGP convergence:
Scan timer
Advertisement interval
Keepalive and hold-down timers
Summary
Overview
Therearecurrentlymorethan600,000prefixesontheInternet.Therearemany
circumstancesinwhichnetworkadministratorsdonotneedordesiretheirroutersto
carryfullInternetrouting.Furthermore,thereisaneedtoprovideprotectivecontrols
oncustomer-facingrouterstoensurethataconfigurationerrordoesnotcausethe
accidentaladvertisementofprefixesfromASsthatdidnotoriginatethem.
BGPisdesignedforreliabilityandscalability.Assuch,ithasatremendousamount
offlexibilityregardingadministrativepolicycontrols,routeselection,and
performancetuningandscalabilityfeatures.AnadvancedBGPconfigurationtool
hasbeendesignedtoimproveBGPscalabilityandperformancebyreducingthe
numberofprefixesthatarouterreceivesfromaBGPneighbor.
Uponcompletingthislesson,youwillbeableto:
Describetheneedforlimitingthenumberofroutesthatarereceivedfroma
BGPneighbor
IdentifytheCiscoIOScommandthatisrequiredtoconfiguretheBGProute
limiting
DescribehowtoconfigureandmonitorBGPmaximum-prefixfunction
LimitingtheNumberofPrefixesReceived
fromaBGPNeighbor
Incomingroutepolicies,whichareappliedonBGProuters,indicatetheBGP
attributevaluesthatarouteshouldhaveinordertobeaccepted.Routepoliciescan
beappliedthatmatchroutesbasedonthenetworknumberortheBGPattributes
thatareattachedtoaroute.Themostcommonlyappliedfilteringusedinroute
policiesistheonethatmatchesthecontentsoftheASpathattribute.
All filtering mechanisms specify only what you are willing to accept but not how
much.
A misconfigured BGP neighbor can send a huge number of prefixes that can
exhaust the memory of a router or overload the CPU (several Internet-wide
incidents have already occurred).
BGP maximum prefixes limiting is used to establish a hard limit on the number
of prefixes that are received from a neighbor.
AnISPwithamultihomedcustomermayuseroutepoliciestoensurethattheroutes
thatarereceivedfromthecustomeroriginatewithintheASofthecustomer.Using
anASpathaccesslistisonemethodofachievingthisgoal.
Ifyouconfigureroutepoliciesinacustomerrouterimproperly,itmayaccidentally
causethecustomertoreceivemanyInternetroutes.Evenworse,afaulty
configurationmaycausecustomertoadvertiseprefixesasthoughtheroutes
originatedinsidethecustomerAS.ThissituationwouldresultinaBGPtableinthe
ISProuterthatlistsmanyofthepossibledestinationnetworksontheInternetas
reachableinthecustomerAS.TheASwouldcontainonlyasingleentry,the
customerAS.TheBGProuteselectionintheISPnetworkwouldselectthoseroutes
asthebest,basedontheASpathlength.Asaresult,itwoulddirectmuchofthe
providertrafficthatisintendedfortheInternettothecustomernetwork.
AroutepolicythatisbasedonanASpathfilterintheISProuterwillnotpreventthis
accident.TheroutesthatthecustomersendshavetheanticipatedASpathvalue.A
routepolicythatisbasedoncustomerprefixesandthatdistinctlyidentifiesand
permitseachofthenetworknumbersthatthecustomermayadvertisewillprevent
theaccident.However,sucharoutingpolicyisdifficulttomaintain.
BGP router terminates peering when a number of maximum prefixes exceeded.
You can configure a router to:
Generate a logging message when a specified percentage of the maximum
prefixes is reached.
Reestablish BGP peering after a specified time (from 1 to 65535 minutes).
Generate a logging message when the maximum prefix limit is exceeded,
instead of terminating BGP peering.
Ascalablesolutiontotheneedforlimitingthenumberofroutes(prefixes)thatare
receivedfromaBGPneighboristouseBGPmaximumprefixeslimiting.Thisfeature
enablesyoutospecifyhowmanyroutesaroutercanreceivefromaneighbor.
Whenthenumberofprefixesthatisreceivedfromthepeerforagivenaddress
familyexceedsthemaximumlimit,aceasenotificationmessageissenttothe
neighbor.Alsothepeeringwiththeneighboristerminated
Youcanalsoconfiguretheroutertogeneratealoggingmessagewhenaconfigured
percentageofthemaximumprefixesisreached.Youcanalsoconfiguretherouterto
automaticallyreestablishapeeringsessionthathasbeendisabledbecausethe
maximumprefixlimithasbeenexceeded.Insteadofceasingthepeeringwitha
neighbor,youcanconfiguretheroutertogenerateanddisplayawarningwhena
configuredpercentageofthemaximumprefixeshasbeenreached.
BGPRouteLimiting
TheBGProutelimitingcanbeconfiguredwiththeuseofmaximum-prefixfunction.
router(config-router)# neighbor ip-address maximum-
prefix maximum [threshold] [warning-only] [restart restart-interval]
This command controls how many prefixes can be received from a neighbor.
The optional threshold parameter specifies the percentage where a warning
message is logged (default is 75%).
The optional warning-only keyword specifies the action on exceeding the
maximum number (default is to drop the neighbor relationship).
The optional restart keyword instructs the router to try to re-establish the
session after the specified interval in minutes.
TocontrolhowmanyprefixesaBGProutercanreceivefromaneighbor,usethe
neighbormaximum-prefixrouterconfigurationcommand.
neighbor {ip-address| peer-group-name} maximum-
prefix maximum [threshold] [warning-only] [restart restart-interval]
SyntaxDescription
Parameter Description
ip-address IPaddressoftheneighbor.
peer-group-name NameofaBGPpeergroup.
maximum Maximumnumberofprefixesthatareallowedfromthisneighbor.
threshold (Optional)Integerspecifyingatwhatpercentageofmaximumthe
routerstartstogenerateawarningmessage.Therangeis1to
100percent.Thedefaultis75percent.
warning-only (Optional)Allowstheroutertogeneratealogmessagewhenthe
maximumisexceeded,insteadofterminatingthepeering.
restart (Optional)Configurestheroutertoautomaticallyre-establisha
peeringsessionthathasbeendisabledbecausethemaximum-
prefixlimithasbeenexceeded.Theconfigurablerangeofthe
restartintervalisfrom1to65535minutes.
ThiscommandallowsyoutoconfigureamaximumnumberofprefixesthataBGP
routerisallowedtoreceivefromapeer.Itaddsanothermechanism(inadditionto
distributelists,filterlists,androutemaps)tocontrolprefixesthatarereceivedfroma
peer.
Whenthenumberofreceivedprefixesexceedsthemaximumnumberthatis
configured,therouterterminatesthepeering(bydefault).However,ifthewarning-
onlykeywordisconfigured,theroutersendsalogmessagebutcontinuespeering
withthesender.Ifthepeeristerminated,thepeersessionremainsdownuntilthe
clearipbgpcommandisissuedontherouter,unlessyouhaveincludedtherestart
keywordintheconfiguration.
TheBGPRestartSessionAfterMax-PrefixLimitfeatureenhancesthecapabilitiesof
theneighbormaximum-prefixcommandwiththeintroductionoftherestart
keyword.Thisenhancementallowsyoutoconfigurethetimeintervalafterwhicha
routerre-establishesapeeringsessionwhenthenumberofprefixesthathavebeen
receivedfromapeerhasexceededthemaximumprefixlimit.Therestartkeyword
hasaconfigurabletimerargumentthatisspecifiedinminutes.Thetimerangeofthe
timerargumentisfrom1to65535.
Thisfeatureattemptstore-establishadisabledpeeringsessionatthetimeinterval
thatyouconfigure.However,theconfigurationoftherestarttimeralonecannot
changeorcorrectapeerthatissendinganexcessivenumberofprefixes.Youwill
needtoreconfigurethemaximum-prefixlimitorreducethenumberofprefixesthat
aresentfromthepeer.Apeerthatisconfiguredtosendtoomanyprefixescan
causeinstabilityinthenetwork,whereanexcessivenumberofprefixesarerapidly
advertisedandwithdrawn.Inthiscase,thewarning-onlykeywordcanbe
configuredtodisabletherestartcapabilitywhileyoucorrecttheunderlyingproblem.
Youcanusethebgpdampeningcommandtoconfigure
ConfiguringtheBGPRouteLimiting
thedampeningofaflappingrouteorinterfacewhenapeer
issendingtoomanyprefixesandcausingnetwork
instability.Youshouldneedtherestartcommandonly
whenyouaretroubleshootingortuningarouterthatis
sendinganexcessivenumberofprefixes.
Overview
Throughthisdiscovery,youwilllearnhowtoconfigureBGProutelimitingusingthe
maximum-prefixfunction.Youwillconfigureaserviceprovider'srouter,ISP1,tolimit
themaximumnumberofprefixesthatarereceivedfromamultihomedcustomer's
router,R2.
Discovery18:ConfigureBGPRouteLimiting
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
R2 Ethernet0/0 172.16.22.2/24 ConnectiontoISP2
R2 Ethernet0/1 172.16.12.2/24 ConnectiontoISP1
R2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
Loopback7
Loopback8
Loopback9
10.0.0.1/28
10.0.0.17/28
10.0.0.33/28
10.0.0.49/28
10.0.0.65/28
10.0.0.81/28
10.0.0.97/28
10.0.0.113/28
10.0.0.129/28
Loopbackssimulate
LANnetworks
ISP1 Ethernet0/1 172.16.12.11/24 ConnectiontoR2
ISP1 Ethernet0/2 172.16.100.11/24 ConnectiontoISP2
ISP2 Ethernet0/0 172.16.22.22/24 ConnectiontoR2
ISP2 Ethernet0/2 172.16.100.22/24 ConnectiontoISP1
IPaddressesandadvertisednetworksinBGParepreconfiguredasshownbelow:
BGPisalsopreconfiguredasEBGP:
R2toISP1
R2toISP2
JobAids
Step1
OntheISP1andISP2routers,verifyinitialstateofBGPtable.Verifythatyoureceived
routestofourdestinationnetworksfromR2.
Usetheshowipbgpcommand.
ISP1# show ip bgp
BGP table version is 5, local router ID is 172.16.12.11
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.12.2 0 0 1 i
*> 10.0.0.16/28 172.16.12.2 0 0 1 i
*> 10.0.0.32/28 172.16.12.2 0 0 1 i
*> 10.0.0.48/28 172.16.12.2 0 0 1 i
*> 10.0.0.64/28 172.16.12.2 0 0 1 i
ISP2# show ip bgp
BGP table version is 5, local router ID is 172.16.22.22
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.22.2 0 0 1 i
*> 10.0.0.16/28 172.16.22.2 0 0 1 i
*> 10.0.0.32/28 172.16.22.2 0 0 1 i
*> 10.0.0.48/28 172.16.22.2 0 0 1 i
*> 10.0.0.64/28 172.16.22.2 0 0 1 i
Both,ISP1andISP2,havereceivedroutestofivedifferentnetworks—10.0.0.0/28,
10.0.0.16/28,10.0.0.32/28,10.0.0.48/28,and10.0.0.64/24.
Step2
OntheISP1router,limitthenumberofreceivedprefixesfromR2.
ISP1shouldreceivenomorethan8prefixesfromR2,andshouldstart
generatingwarningmessageswhenthenumberofreceivedprefixesexceeds
75%ofthemaximumspecified.
ISP1(config)# router bgp 100
ISP1(config-router)# neighbor 172.16.12.2 maximum-prefix 8 75
Step3
OntheR2router,advertisethe10.0.0.80/28and10.0.0.96/28networksinBGP.
Toadvertisenetworks,configurethefollowingcommandsonR2.
NOTE:BeforeconfiguringR2,makesurethatyoualsohaveaconsolewindow
openforISP1router.Soyouwillbeabletoseethegeneratedwarning
messages.
R2(config)# router bgp 1
R2(config-router)# network 10.0.0.80 mask 255.255.255.240
R2(config-router)# network 10.0.0.96 mask 255.255.255.240
Step4
OntheISP1router,observethewarningmessagegenerated.
ISP1#
*Apr 2 12:07:55.964: %BGP-4-
MAXPFX: Number of prefixes received from 172.16.12.2 (afi 0) reaches 7, max 8
Thewarningmessagewasgenerated,becauseISP1received7prefixesfromR2.The
ConfigureBGPRouteLimiting
DiscoverySteps
Thewarningmessagewasgenerated,becauseISP1received7prefixesfromR2.The
maximumissetto8,sothethresholdof75%wasexceeded.
Step5
OntheISP1router,verifythatyoustillhavealldestination,thatR2advertises,intheBGP
table.
ISP1# show ip bgp
BGP table version is 8, local router ID is 172.16.12.11
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.12.2 0 0 1 i
*> 10.0.0.16/28 172.16.12.2 0 0 1 i
*> 10.0.0.32/28 172.16.12.2 0 0 1 i
*> 10.0.0.48/28 172.16.12.2 0 0 1 i
*> 10.0.0.64/28 172.16.12.2 0 0 1 i
*> 10.0.0.80/28 172.16.12.2 0 0 1 i
*> 10.0.0.96/28 172.16.12.2 0 0 1 i
Althoughthewarningwasgenerated,ISP1stillhasall7networksthatarereceivedfrom
R2initsBGPtable.
Step6
OntheR2router,alsoadvertisethe10.0.0.112/28and10.0.0.128/28networks
inBGP.
Toadvertisenetworks,configurethefollowingcommandsonR2.
NOTE:BeforeconfiguringR2,makesurethatyoualsohaveaconsolewindow
openforISP1router.Soyouwillbeabletoseethegeneratedmessages.
R2(config)# router bgp 1
R2(config-router)# network 10.0.0.112 mask 255.255.255.240
R2(config-router)# network 10.0.0.128 mask 255.255.255.240
Step7
OntheISP1router,observethegeneratedmessages.
ISP1#
*Apr 2 12:22:01.859: %BGP-4-MAXPFX: Number of prefixes received from 172.16.12.2 (afi 0) reaches 8, max 8
ISP1#
*Apr 2 12:22:32.575: %BGP-3-
MAXPFXEXCEED: Number of prefixes received from 172.16.12.2 (afi 0): 9 exceeds limit 8
*Apr 2 12:22:32.575: %BGP-5-ADJCHANGE: neighbor 172.16.12.2 Down BGP Notification sent
*Apr 2 12:22:32.575: %BGP-3-NOTIFICATION: sent to neighbor 172.16.12.2 3/1 (update malformed) 0 bytes
*Apr 2 12:22:32.575: %BGP-4-MSGDUMP: unsupported or mal-formatted message received from 172.16.12.2:
FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF 0037 0200 0000 1B40 0101 0040 0206 0201
0000 0001 4003 04AC 100C 0280 0404 0000 0000 1C0A 0000 80
ISP1#
*Apr 2 12:22:32.575: %BGP_SESSION-5-
ADJCHANGE: neighbor 172.16.12.2 IPv4 Unicast topology base removed from session BGP Notification sent
ISP1#
*Apr 2 12:23:41.141: %BGP-3-
NOTIFICATION: sent to neighbor 172.16.12.2 passive 2/8 (no supported AFI/SAFI) 3 bytes 000101 (timer expired)
Thebeginningoftheoutputfirstshowsthatthetotalnumberofreceivedprefixeshasreached8,whichisoverthe
thresholdtogenerateawarningmessage.WhenyoustartedtoadvertisethesecondnetworkinBGP,thesecondmessage
wasgenerated.Thetotalnumberofreceivedprefixesisnow9,whichisabovetheconfiguredlimit(8).Asaresult,the
sessionwiththeneighbor,R2,wasterminated.
Step8
OntheISP1andISP2routers,verifythestateofBGPtable.
ISP1# show ip bgp
ISP1#
ISP2# show ip bgp
BGP table version is 10, local router ID is 172.16.22.22
BGP table version is 10, local router ID is 172.16.22.22
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/28 172.16.22.2 0 0 1 i
*> 10.0.0.16/28 172.16.22.2 0 0 1 i
*> 10.0.0.32/28 172.16.22.2 0 0 1 i
*> 10.0.0.48/28 172.16.22.2 0 0 1 i
*> 10.0.0.64/28 172.16.22.2 0 0 1 i
*> 10.0.0.80/28 172.16.22.2 0 0 1 i
*> 10.0.0.96/28 172.16.22.2 0 0 1 i
*> 10.0.0.112/28 172.16.22.2 0 0 1 i
*> 10.0.0.128/28 172.16.22.2 0 0 1 i
ISP1hasnoroutesintheBGPtable,sinceR2wastheonlyneighborandthesession
wasterminatedduetoexceededmaximumnumberofreceivedprefixes.Youdidnot
configureroutelimitingonISP2,soyoucanseeroutesforallninedestinationsinitsBGP
table.
MonitoringtheBGPRouteLimiting
router# show ip bgp neighbors [address]
For neighbors with the maximum-prefix function configured, displays the
maximum number of prefixes and the warning threshold.
For neighbors exceeding the maximum number of prefixes, displays the reason
that the BGP session is idle.
YoucanusetheshowipbgpneighborscommandtomonitorthestatusofBGP
neighbors.Amongotherthings,thecommanddisplaysinformationabouthowmany
prefixesaBGProuterhasreceivedfromaneighborandifanylimitshavebeen
configured.
Ifthepeerexceededtheconfiguredmaximumnumberofprefixes,theoutputofthe
showipbgpneighborscommandshowsthereasonforresettingthesession.Asa
resultofthesessionbeingreset,theBGPsessionwillremainintheIdlestate.
ToforcetheneighborfromtheIdlestateintotheActivestateandtore-establishthe
BGPsession,youmustissuetheclearipbgpip-addresscommandforthe
neighbor.Except,ifyouhavespecifiedtherestartkeywordintheconfiguration.In
thiscase,theroutertriestore-establishtheBGPsessionautomaticallyafterthe
expirationoftheconfiguredrestarttimeoutinterval.
Step9
OntheISP1router,verifyinformationabouttheR2neighbor.
Toverifyinformationabouttheneighbor,usetheshowipbgpneighbors
commandwithaneighborIPaddressasanargument.
ISP1# show ip bgp neighbor 172.16.12.2
BGP neighbor is 172.16.12.2, remote AS 1, external link
BGP version 4, remote router ID 10.0.0.129
BGP state = Idle
Neighbor sessions:
0 active, is not multisession capable (disabled)
Stateful switchover support enabled: NO
Default minimum time between advertisement runs is 30 seconds
For address family: IPv4 Unicast
BGP table version 17, neighbor version 1/17
Output queue size : 0
Index 0, Advertise bit 0
Address family not supported notification sent
Slow-peer detection is disabled
Slow-peer split-update-group dynamic is disabled
Peer had exceeded the max. no. of prefixes configured.
Maximum prefixes allowed 8
Threshold for warning message 75%
Reduce the no. of prefix and clear ip bgp 172.16.12.2 to restore peering
<... output omitted ...>
ThestateofBGPsessionwithR2routerisIdle.Thereasonisthatthepeerhad
exceededthemaximumnumberofprefixesconfigured.Youcansethatthe
maximumallowedprefixesforthisneighborare8andthethresholdforwarning
maximumallowedprefixesforthisneighborare8andthethresholdforwarning
messagesissetto75%.Thesearethevaluesthatyouconfiguredearlier.
ToforcetheneighborfromtheIdlestateintotheActivestateandtore-establishthe
BGPsession,youhavetoreducethenumberofprefixesandcleartheBGPsession
withtheneighbor.
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
An improperly configured filter in a customer router may accidentally cause the
customer router to receive many Internet routes.
Maximum prefix limitation will shut down peering session if too many prefixes
are received.
Use neighbor maximum-prefix command to configure route limiting—a
maximum number of prefixes that a BGP router is allowed to receive from a
peer.
Summary
Overview
ScalingrouterstomeetthedemandsoffullInternetroutingandassociated
administrativepoliciesrequiresprotocolslikeBGPwithembeddedscalability
mechanisms.Inenvironmentswherenetworkadministratorsmustconfiguremany
BGPpeers,peergroupsareascalabilitytoolthatreducesbothadministrative
overheadandrouterresourcerequirements.
TypicalserviceprovidernetworksusuallycontainBGP-speakingroutersthatconsist
ofmanyneighborsthatareconfiguredwiththesameadministrativepolicies.These
policiescanbeoutboundroutemaps,distributelists,filterlists,updatesource,and
soon.Youcangroupneighborswiththesameupdatepoliciesintopeergroupsto
simplifyconfigurationand,moreimportantly,tomakeBGPupdatesmoreefficient.In
thislesson,youwilllearnaboutpeergroupsasaBGPscalabilitymechanism.
Uponcompletingthislesson,youwillbeableto:
DescribetheneedforBGPpeergroups
DescribetheperformancebenefitsofusingBGPpeergroups
DescribethelimitationsofBGPpeergroups
IdentifytheCiscoIOScommandsthatarerequiredtoconfigureBGPpeer
groups
DescribetheconfigurationofBGPpeergroups
ExplaintheBGPpeergroupconfigurationexamples
DescribeBGPdynamicupdategroups
DescribeBGPpeertemplates
DescribetheBGPpeertemplateinheritance
IdentifytheCiscoIOScommandsthatarerequiredtoconfigureBGPpeer
templates
ImplementingBGPPeerGroups
Inmanycases,youasanetworkadministratormustconfigureasinglerouterwith
manyneighbors,eachneighborhavingparameterssimilartotheothers.This
situationmaycausetime-consumingconfigurationwork,becausethenyouhaveto
configurealmostthesameparametersforalloftheneighbors.Imagineyouhavea
serviceprovidernetworkthathasanedgerouterwithmanycustomersattachedtoit,
whereeachcustomerrequiresitsownBGPsession.Youmayfindthatallofthe
BGPsessionstoitscustomerroutershavealmostidenticalconfigurations.
BGP routers could have many neighbors with similar requirements:
Provider edge router with many customer connections.
BGP route reflector with many IBGP peers.
Provider edge router at an exchange point.
Most of the parameters that are specified for the BGP neighbors are identical,
with a few exceptions.
Solution is to group common parameters in a BGP peer group.
Likewise,IBGPsessionsarealmostalwaysidenticallyconfigured.Ifafullmeshis
deployedwithinanAS,manypeerconfigurationsmightexist.RecallthatanAS
containingonly15routerswillrequire([15*14]/2)=105neighborsessionstomeet
thefull-meshrequirementofBGP.Configuring105neighborswithduplicate
parametersleadtoatremendousamountofredundantconfiguration.
Toeasetheburdenofconfiguringmanyneighborswithidenticalorsimilar
parameters(forexample,routemaps,filterlists,orprefixlists),theconceptofpeer
groupswasintroduced.Youasanetworkadministratorcanconfigureatemplate,or
peergroupwithalltheBGPparametersthataretobeappliedtomanyBGPpeers.
ActualBGPneighborsareboundtothepeergroup,andyouapplythepeergroup
configurationoneachoftheBGPsessions.BGPneighborsofasingleroutercanbe
dividedintoseveralgroups,eachgrouphavingitsownBGPparameters.Actual
neighborsarethenboundtotheappropriategroup,resultinginanoptimumBGP
configuration.
BGP peer group creates a neighbor parameter template.
Configurable parameters include the following:
Community propagation
Update-source and next-hop-self
EBGP multihop
Authentication password
Neighbor weight
Prefix lists, filer lists, and route maps
Individual parameters that are specified in a peer group can be overridden on a
neighbor-by-neighbor basis.
Onarouter,thepeergroupiscreatedasatemplate.Thetemplatecanbe,among
others,configuredtodothefollowing:
Propagate,ornotpropagate,thecommunityattribute.
UsetheIPaddressofaspecificinterfaceasthesourceaddresswhenopening
theTCPsessionorusethenext-hop-selffeature.
Use,ornotuse,theEBGPmultihopfunction.
Use,ornotuse,MD5authenticationontheBGPsessions.
Filteroutanyincomingoroutgoingroutesusingaprefixlist,afilterlist,anda
routemap.
Assignaparticularweightvaluetotheroutesthatarereceived.
Whenactualneighboringroutersareassignedtothepeergrouponarouter,allof
theattributesthatareconfiguredforthepeergroupareappliedtoallpeergroup
members.CiscoIOSsoftwareoptimizestheoutgoingroutesbyrunningthroughthe
outgoingfiltersandroutingpoliciesonlyonceandthenreplicatingtheresultstoeach
ofthepeergroupmembers.Inreality,CiscoIOSsoftwareassignsapeergroup
leader,forwhichthesoftwaregeneratesanupdate,andthisupdateisreplicatedby
theleadertoallothermembersofthepeergroup.
Someparametersthatareconfiguredonthepeergroupcanbeoverriddenby
neighborconfigurations.Butonlyiftheindividualconfigurationsapplyonincoming
updates.Outgoingupdatesarealwayspreparedforthepeergroupleaderandthen
BGPPeerGroupsOverview
replicatedtotheothermembersofthepeergroup.
PeerGroupsExample—CustomerConnections
Thisexampleillustratesaserviceprovidernetworkwithagroupofcustomer
autonomoussystemsthatcanbetreatedinthesame(oraverysimilar)way.
Thefigureshowsanexamplewherepeergroupsareuseful.Thecustomer
autonomoussystemsareallassumedtoannouncelocalnetworksonly.Allcustomer
ASsshouldreceiveBGPupdateswiththesamesetofInternetroutes,andthe
customerASsareallassumedtogenerateonlyafewprefixes.
Thissituationmakestheneighborconfigurationalmostidenticalforeachofthe
customers,withonlyafewchangesthatarespecifictoeachneighbor.
Inthisscenario,theuseofthepeergroupfunctionishighlydesirable.Youasa
networkadministratorcanconfigureBGPneighborsinthecustomerASsusinga
singlepeergroup.Youconfigurethepeergrouptemplatewithreferencestoroute
policies,authenticationsettings,andthemaximumnumberofreceivedprefixes.
ThentheIPaddressesofthecustomerroutersareboundtothepeergroup,andthe
peergroupconfigurationisappliedtoallneighbors.
PeerGroupExample—BGPRouteReflector
ThisexampleillustratestheBGProutereflector.
AnotherexampleofusingpeergroupsiswithintheentirelocalAS,ontheroute
reflectors,whereeveryIBGPsessionisconfiguredidentically.IfarouterintheASis
suppliedwithsomeinformation,thenalltheroutersshouldbesuppliedwiththe
sameinformation.Otherwise,aninconsistentroutingpolicywithintheASmight
causeinconsistentroutingorapplicationofBGPpolicies.
ThepeergroupfunctionisagoodtooltomakesurethatallIBGPpeersreceivethe
sameconfigurationinformation.Youcanconfigureapeergrouptemplatewiththe
requiredparameters.YoucanconfiguretheneighborASnumber,enableofthe
send-communityoption,setoftheupdatesourcetoaloopbackinterface,and
routerauthenticationmechanisms.Then,alltheinternalneighborIPaddressesare
boundtothepeergroup,andthepeergroupconfigurationisappliedtoallofthem.
ThisapproachensuresaconsistentroutingpolicywithintheAS.
Inaserviceprovidernetwork,theroutersthatareassignedasroutereflectorsare
therouterswiththelargestnumberofIBGPsessions.Thesearetherouterswhere
thepeergroupfunctionismostuseful.
PeerGroupExample—EdgeRouterataPeeringPoint
Thisexampleillustratesanedgerouteratapeeringpoint.
Anotherexampleofusingpeergroupsisontheedgerouterthatislocatedinthe
networkwheretheserviceproviderexchangesrouteswithotherserviceproviders.
Fromtheedgerouter,theserviceproviderAScanpeerwithmanyotherservice
providers.
AllpeeringASsshouldreceivethesamesetofroutes,namelytherouteslocaltothe
serviceproviderASandtheroutesthatarereceivedfromcustomerASs.Also,all
routesthatarereceivedbytheserviceproviderpeeringrouterfromallpeeringASs
areprocessedalmostidentically.Thecharacteristicoftheexchangenetworkisthe
sameregardlessofwhichneighbortheroutesarereceivedfrom.Ifthepeeringpoint
isanFDDI,ATM,GigabitEthernet,orDPTnetwork,thepreferenceofusingthe
networkforpacketexchangemaybedifferent.However,foreachsinglepeering
point,allneighborsarereachableoverthesamenetwork,andthepreferenceis
quitelikelytobethesame.
Additionally,anumberofotherparameterscouldbethesame,suchasremoving
privateASnumbersandlimitingthenumberofroutesreceived.Inthesecases,you
canapplytheseparametersonthepeergrouptemplatebeforetheactualIP
addressesoftheneighborsareboundtothepeergroup.
Asdescribedhere,peergroupscanbeusedtocombinecommonBGPconfiguration
intoapeergroup.Neighborsarethenconfiguredbyassigningthemtothepeer
group.Bydefault,routerbuildsBGPupdatesforeachneighborindividually.Building
BGPupdatesinvolvesanumberofrouter-CPU-consumingtasks,includingscanning
theBGPtableandapplyingvariousoutgoingfilteringmechanisms(filterlists,route
maps,andprefixlists).Thesetasksmeanthatwhenarouterisconfiguredwitha
largenumberofneighbors,theCPUloadgrowsproportionally.
By default, router builds individual BGP updates for each BGP neighbor.
Common configuration can be combined into a peer group.
Neighbors are configured by assigning them to the peer group.
A single BGP update is then built for all members of a BGP peer group.
The CPU load does not increase linearly with the increased number of
neighbors.
Use peer groups wherever possible to reduce the CPU load of the BGP
process.
However,withtheuseofpeergroups,someoftherouterCPUutilizationthatis
imposedbyBGPupdategenerationissignificantlyreduced.Theuseofpeergroups
allowstheroutertoruntheBGPupdate(includingalloutgoingfilterprocessing)only
oncefortheentirepeergroup.Therouter,afterithasfinishedbuildingtheBGP
update,sendsittoeachmemberofthepeergroup.TheactualTCPtransmission
stillhastobedoneonaper-neighborbasisbecauseoftheconnection-oriented
characteristicsofBGPsessions.
RouterCPUloaddoesincreasewhentherearemoreneighborsofarouter,dueof
increasedTCPworkload,buttheuseofpeergroupscansignificantlyreducethe
increase.Therefore,youshouldusepeergroupswheneverpossibletoreducethe
CPUload.
BGPpeergroupsarethefundamentalBGPscalabilitytool
andshouldbeusedinallenvironmentswherearouterhas
alargenumberofBGPneighbors.
RoutersbuiltBGPupdatesforeachneighborindividually
beforetheintroductionofdynamicupdategroups,which
wereintroducedinCiscoIOSRelease12.0(24)S.
Dynamicupdategroups,whichareenabledbydefault,
allowrouterstodynamicallycalculateandoptimize
updatesthataresenttoneighborthatsharethesame
outboundpolicies.Althoughthisnewfeatureeffectively
mademanuallydefinedpeergroupsobsolete,many
administratorsstillusepeergroupsastheyoffer
convenienceduringconfiguration.
BGPPeerGroupsasaPerformanceTool
BGPpeergroupswereintroducedprimarilyforCPUusageoptimization.Because
therouterbuildsonlyoneupdateforallmembersofthesamepeergroup,some
restrictionsapplytomembersofthepeergroup.
Peer groups have a number of limitations because of the way that they are used to
build BGP updates.
Peer groups were intended to be used only for CPU optimization.
Awkward with similar but not identical configuration policies.
IBGP and EBGP neighbors cannot be mixed in a peer group.
Per-neighbor BGP parameters that affect outbound updates cannot be changed
for peer group members.
BGPparametersandoutboundroutingpoliciescannotdifferbetweengroup
membersbecausedoingsocouldcausedifferentupdatestobesenttotwo
membersofthesamepeergroup.Routercreatesonlyoneupdate,whichisthen
replicatedtoallmembers.Therefore,peergroupsareawkwardwhenusedona
routerwithseveralneighborsthathavesimilarbutnotcompletelyidenticalpolicies.
AlsoEBGPandIBGPupdatesareverydifferent.Thesamepeergroupcannotbe
usedforEBGPandIBGPneighbors,eventhoughapartoftheconfigurationisthe
sameforbothtypesofneighbors.Anexamplewouldbeifyouwantedtochangethe
BGPkeepaliveandhold-downtimersforbothIBGPandEBGPsessions.Usingpeer
groups,youmustspecifythetimersinbothpeergroups(IBGPandEBGP),because
theyusedifferentconfigurationsduetothedifferentnatureofIBGPandEBGP
sessions.AlsoEBGPupdateshavetheASpathattributechanged.IBGPsessions
passonlythelocalpreferenceattribute.TheMEDattributethatisreceivedfroma
remoteASispassedontoIBGPsessionsbutisremovedbeforeitissentonan
EBGPsession.ThereforecommonBGPparametersmustbereplicatedacrossall
peergroups,andthereisnowaytouseonlyonepeergroupforcommon
configuration.
BGPPeerGroupLimitations
1.
2.
3.
4.
TofinishBGPpeergroupconfiguration,youfirsthavetocreateapeergroup,
specifyparameterforthepeergroup,andthenassignneighborsintothepeergroup.
router(config-router)# neighbor group-name peer-group
Creates a BGP peer group (peer groups names are case-sensitive) .
router(config-router)# neighbor group-name any-BGP-parameter
Specifies any BGP parameter for the peer group.
router(config-router)# neighbor ip-address peer-group peer-group-name
Assigns a BGP neighbor to a peer group. The neighbor inherits all the BGP
parameters that are specified for the peer group.
router(config-router)# neighbor ip-address any-BGP-parameter
Overrides a BGP parameter that is specified for the peer group with a neighbor-
specific parameter.
ToconfigureBGPpeergroupsonCiscoIOSrouters,performthefollowingsteps:
CreateaBGPpeergroup.
TocreateaBGPpeergroup,usetheneighborpeer-grouprouterconfiguration
command.
neighbor peer-group-name peer-group
SpecifyparametersfortheBGPpeergroup.
Afteryouhavecreatedapeergroup,youcanconfigureitwiththeneighbor
commands.Bydefault,membersofthepeergroupinheritalltheconfiguration
optionsofthepeergroup.Youcanalsoconfigurememberstooverridetheoptions
thatdonotaffectoutboundupdates.Peergroupmemberswillalwaysinheritthe
followingconfigurationoptions:remote-as(ifconfigured),version,update-source,
out-route-map,out-filter-list,out-dist-list,minimum-advertisement-interval,and
next-hop-self.Allpeergroupmemberswillinheritchangesthataremadetothe
peergroup.
CreateaBGPneighbor.
Assignthisneighborintothepeergroup.
ToconfigureaBGPneighbortobeamemberofapeergroup,usetheneighbor
peer-grouprouterconfigurationcommand.
neighbor ip-address peer-group peer-group-name
SyntaxDescription
Parameter Description
ip-address IPaddressoftheBGPneighborthatbelongstothepeergroup
thatisspecifiedbythetag.
peer-group-name NameoftheBGPpeergrouptowhichthisneighborbelongs.
Afteryouhaveassignedanactualneighbortobeamemberofthepeergroup,all
configurationsthataremadetothepeergrouptemplatearethenappliedtoallthe
neighborsthatareassignedtothatpeergroup.Throughconfiguration,peergroup
configurationsmaybeoverriddenforanindividualneighbor,aslongasthechanges
applyonlytoincomingupdates.Rememberthatoutgoingupdatesarepreparedonly
onceandthenreplicated.
Thepeergroupisaverypowerfultoolwhennetwork
administratorsaredealingwithalargenumberof
neighborswithalmostidenticalconfigurations.However,if
anyofthecustomersrequireroutinginformationthat
ConfiguringBGPPeerGroups
differsfromthatofothermembersofthepeergroup,then
thatneighbormustberemovedfromthepeergroupand
configuredasanindividualneighbor.
Overview
Throughthisdiscovery,youwilllearnhowtoconfigureBGPpeergroup.Apeer
groupwillbeusedforallcustomeroftheserviceproviderbecausetheyshare
identicalroutingpolicy.Youwillputallneighborconfigurationinthepeergroup
configurationandthepeerswilltheninheritthepeergroupconfiguration.
Discovery19:ConfigureBGPPeerGroups
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
ISP1 Ethernet0/1 172.16.11.11/24 ConnectiontoR1
ISP1 Ethernet0/2 172.16.12.11/24 ConnectiontoR2
ISP1 Ethernet0/3 172.16.13.11/24 ConnectiontoR3
R1 Ethernet0/1 172.16.11.1/24 ConnectiontoISP1
R1 Loopback1 10.1.0.1/24 Loopbacksimulates
LANnetwork
R2 Ethernet0/2 172.16.12.2/24 ConnectiontoISP1
R2 Loopback1 10.2.0.1/24 Loopbacksimulates
LANnetwork
R3 Ethernet0/3 172.16.13.3/24 ConnectiontoISP1
R3 Loopback1 10.3.0.1/24 Loopbacksimulates
LANnetwork
IPaddressesandadvertisednetworksinBGParepreconfiguredasshownbelow:
BGPisalsopreconfiguredasEBGP:
ISP1toR1
ISP1toR2
ISP1toR3
JobAids
Step1
OntheISP1router,verifythatyoureceivenetworks,thatallthreecustomersadvertise.
ISP1# show ip bgp
BGP table version is 4, local router ID is 172.16.13.11
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.1.0.0/24 172.16.11.1 0 150 0 1 i
*> 10.2.0.0/24 172.16.12.2 0 150 0 2 i
*> 10.3.0.0/24 172.16.13.3 0 150 0 3 i
YoucanseethatISP1routerinAS100receivedthreenetworks,eachfromdifferent
customer:
10.1.0.0/24networkoriginatesinthefirstcustomer'sAS1.
10.2.0.0/24networkoriginatesinthesecondcustomer'sAS2.
10.3.0.0/24networkoriginatesinthethirdcustomer'sAS3.
Notethatallthreenetworkshavelocalpreferencesetto150.Routemapisusedtoset
thelocalpreferenceforallthreeneighbors.
ISP1# show route-map
route-map Filter, permit, sequence 10
Match clauses:
Set clauses:
local-preference 150
Policy routing matches: 0 packets, 0 bytes
Step2
OntheISP1router,verifytheBGPconfiguration.
ISP1# show running-config | section bgp
router bgp 100
bgp log-neighbor-changes
neighbor 172.16.11.1 remote-as 1
neighbor 172.16.11.1 prefix-list Customer in
neighbor 172.16.11.1 route-map Filter in
neighbor 172.16.11.1 filter-list 10 in
neighbor 172.16.12.2 remote-as 2
neighbor 172.16.12.2 prefix-list Customer in
neighbor 172.16.12.2 route-map Filter in
neighbor 172.16.12.2 filter-list 10 in
neighbor 172.16.13.3 remote-as 3
neighbor 172.16.13.3 prefix-list Customer in
neighbor 172.16.13.3 route-map Filter in
neighbor 172.16.13.3 filter-list 10 in
Allthreeneighborsareconfiguredwiththesameparameters.Insteadof
configuringthesameconfigurationthreetimes,youcanputalltheconfiguration
intothepeergroupconfigurationandassignalltheneighbortothispeergroup.
Step3
OntheISP1router,createapeergroupCUSTOMERS.
ISP1(config)# router bgp 100
ISP1(config-router)# neighbor CUSTOMERS peer-group
Step4
OntheISP1router,moveallper-neighborBGPconfigurationintopeergroup
configuration.
ConfigureBGPPeerGroups
DiscoverySteps
Removethefilterlist10,prefixlistCustomer,androutemapFilterfromallthree
neighbors.ApplythisthreeroutingpoliciestotheCUSTOMERSpeergroup
ISP1(config)# router bgp 100
ISP1(config-router)# neighbor CUSTOMERS prefix-list Customer in
ISP1(config-router)# neighbor CUSTOMERS route-map Filter in
ISP1(config-router)# neighbor CUSTOMERS filter-list 10 in
ISP1(config-router)# no neighbor 172.16.11.1 prefix-list Customer in
ISP1(config-router)# no neighbor 172.16.11.1 route-map Filter in
ISP1(config-router)# no neighbor 172.16.11.1 filter-list 10 in
ISP1(config-router)# no neighbor 172.16.12.2 prefix-list Customer in
ISP1(config-router)# no neighbor 172.16.12.2 route-map Filter in
ISP1(config-router)# no neighbor 172.16.12.2 filter-list 10 in
ISP1(config-router)# no neighbor 172.16.13.3 prefix-list Customer in
ISP1(config-router)# no neighbor 172.16.13.3 route-map Filter in
ISP1(config-router)# no neighbor 172.16.13.3 filter-list 10 in
ApplyingtheroutingpoliciesonlytothepeergroupsimplifiestheBGP
configurationandimprovesBGPscalability.Insteadofconfiguringthesame
parametersforeachneighbor,younowconfigurethemonlyonceandthen
assignasmanyneighbortothepeergroupasyouwant.
Step5
OntheISP1router,assignallthreeneighborstotheCUSTOMERSpeergroup.
ISP1(config)# router bgp 100
ISP1(config-router)# neighbor 172.16.11.1 peer-group CUSTOMERS
ISP1(config-router)# neighbor 172.16.12.2 peer-group CUSTOMERS
ISP1(config-router)# neighbor 172.16.13.3 peer-group CUSTOMERS
Thisconfigurationenablestheneighborsthatareassignedtothepeergroup,to
inheritcommonconfigurationfromthepeergroup.Youdonotneedtoconfigure
eachindividualneighbor.
MonitoringPeerGroups
router# show ip bgp peer-group [peer-group-name]
Displays the definition of the specified peer group or all peer groups
router# show ip bgp peer-group [peer-group-name] summary
Displays summary status of all neighbors in the peer group
router# clear ip bgp [peer-group-name] [soft [in | out]]
Clears BGP session with all peer group members
TodisplayinformationaboutBGPpeergroups,usetheshowipbgppeer-group
command.Ifyouusesummarykeyword,onlyasummaryofthestatusofallthe
membersofapeergroupwillbeshown.
ToresettheBGPsessionswithallthemembersofapeergroup,usetheclearip
bgpcommand.
clear ip bgp {* | neighbor-address | peer-group-name} [soft [in | out]]
SyntaxDescription
Parameter Description
*ResetsallcurrentBGPsessions.
neighbor-address ResetsonlythespecifiedBGPneighbor.
peer-group-name ResetsonlythespecifiedBGPpeergroup.
soft (Optional)Initiatessoftreconfiguration.
in|out (Optional)Triggersinboundoroutboundsoftreconfiguration.If
youdonotspecifytheinoroutoption,bothinboundand
outboundsoftreconfigurationaretriggered.
Parameter Description
Step6
OntheISP1router,verifythatallprefixlist,filterlist,androutemapconfiguration
isnowappliedtothepeergroupCUSTOMER.
ISP1# show ip bgp peer-group
BGP peer-group is CUSTOMERS
BGP version 4
Neighbor sessions:
0 active, is not multisession capable (disabled)
Default minimum time between advertisement runs is 30 seconds
For address family: IPv4 Unicast
BGP neighbor is CUSTOMERS, peer-group external, members:
172.16.11.1 172.16.12.2 172.16.13.3
Index 0, Advertise bit 0
Incoming update prefix filter list is Customer
Incoming update AS path filter list is 10
Route map for incoming advertisements is Filter
Update messages formatted 0, replicated 0
Number of NLRIs in the update sent: max 0, min 0
Youcanseethatallparametersconfiguredforthepeergrouparelistedinthe
output:
Prefixlist"Customer"forincomingupdates.
Filterlist"10"forincomingupdates.
Routemap"Filter"forincomingupdates.
NotethatthepeergroupshouldhaveonlyIBGPmembersorEBGPmembers.
ThispeergroupcontainsEBGPpeers—172.16.11.1,172.16.12.2,and
172.16.13.3.Thesearethepeersthatinherittheconfigurationfromthepeer
group.
Anotheroptiontoverifytheconfigurationthatisappliedtothepeergroupisto
simplyviewtheBGPconfigurationontheISP1.
ISP1# show running-config | section bgp
router bgp 100
bgp log-neighbor-changes
neighbor CUSTOMERS peer-group
neighbor CUSTOMERS prefix-list Customer in
neighbor CUSTOMERS route-map Filter in
neighbor CUSTOMERS filter-list 10 in
neighbor 172.16.11.1 remote-as 1
neighbor 172.16.11.1 peer-group CUSTOMERS
neighbor 172.16.12.2 remote-as 2
neighbor 172.16.12.2 peer-group CUSTOMERS
neighbor 172.16.13.3 remote-as 3
neighbor 172.16.13.3 peer-group CUSTOMERS
Step7
OntheISP1router,verifythatR1,R2,andR3peersarepartoftheCUSTOMERSpeergroup.
ISP1# show ip bgp peer-group CUSTOMERS summary
BGP router identifier 172.16.13.11, local AS number 100
BGP table version is 10, main routing table version 10
3 network entries using 444 bytes of memory
3 path entries using 192 bytes of memory
6/3 BGP path/bestpath attribute entries using 816 bytes of memory
3 BGP AS-PATH entries using 72 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 1524 total bytes of memory
BGP activity 24/21 prefixes, 27/24 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
172.16.11.1 4 1 18 23 10 0 0 00:12:10 1
172.16.12.2 4 2 18 23 10 0 0 00:11:58 1
172.16.13.3 4 3 17 21 10 0 0 00:11:46 1
TheoutputlistsyouallthemembersofCUSTOMERSpeergroup
R1(172.16.11.1)
R2(172.16.12.2)
R3(172.16.13.3)
Anotheroptiontoverifythattheneighborisamemberofthepeergroupisusingtheshowipbgp
neighborcommand.
ISP1# show ip bgp neighbor 172.16.11.1
BGP neighbor is 172.16.11.1, remote AS 1, external link
Member of peer-group CUSTOMERS for session parameters
BGP version 4, remote router ID 10.1.0.1
BGP state = Established, up for 00:12:39
Last read 00:00:44, last write 00:00:28, hold time is 180, keepalive interval is 60 seconds
<... output omitted ...>
ISP1# show ip bgp neighbor 172.16.12.2
BGP neighbor is 172.16.12.2, remote AS 2, external link
Member of peer-group CUSTOMERS for session parameters
BGP version 4, remote router ID 10.2.0.1
BGP state = Established, up for 00:13:00
Last read 00:00:13, last write 00:00:12, hold time is 180, keepalive interval is 60 seconds
<... output omitted ...>
ISP1# show ip bgp neighbor 172.16.13.3
BGP neighbor is 172.16.13.3, remote AS 3, external link
Member of peer-group CUSTOMERS for session parameters
BGP version 4, remote router ID 10.3.0.1
BGP state = Established, up for 00:13:08
Last read 00:00:12, last write 00:00:28, hold time is 180, keepalive interval is 60 seconds
<... output omitted ...>
Step8
OntheISP1router,verifythatyoustillreceivenetworks,thatallthreecustomers
advertise.Verifythatthelocalpreferenceisstillsetto150forallthenetworks.
ISP1# show ip bgp
BGP table version is 10, local router ID is 172.16.13.11
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.1.0.0/24 172.16.11.1 0 150 0 1 i
*> 10.2.0.0/24 172.16.12.2 0 150 0 2 i
*> 10.3.0.0/24 172.16.13.3 0 150 0 3 i
YoucanseethatISP1routerinAS100receivedthreenetworks,eachfromdifferent
customer:
10.1.0.0/24networkoriginatesinthefirstcustomer'sAS1.
10.2.0.0/24networkoriginatesinthesecondcustomer'sAS2.
10.3.0.0/24networkoriginatesinthethirdcustomer'sAS3.
Allthreenetworksstillhavelocalpreferencesetto150.Eventhoughtheroutemapthat
setsthelocalpreferenceisnowincludedinthepeergroupconfiguration,theneighbors
inheritedtheconfigurationfromthepeergroup.
PeerGroupConfigurationExample—BGPRouteReflector
Inthisexample,arouteractingasaBGProutereflectorhasfourIBGPneighbors.
router(config)# router bgp 100
router(config-router)# neighbor IBGP_peers peer-group
router(config-router)# neighbor IBGP_peers remote-as 100
router(config-router)# neighbor IBGP_peers update-source loopback 0
router(config-router)# neighbor IBGP_peers password c73Dx8K
router(config-router)# neighbor IBGP_peers send-community
router(config-router)# neighbor 10.0.1.3 peer-group IBGP_peers
router(config-router)# neighbor 10.0.1.4 peer-group IBGP_peers
router(config-router)# neighbor 10.0.1.6 peer-group IBGP_peers
router(config-router)# neighbor 10.0.1.8 peer-group IBGP_peers
InalargeAS,someroutersmayhavemanyIBGPsessions.Apeergroupthatis
named"IBGP_peers"iscreatedtohandlealloftheIBGPsessions.Thepeergroup
iscreatedandconfiguredwiththeremoteAS,update-source,authentication,and
community-passingparameters.Whentheactualneighborsareconfiguredas
membersofthepeergroup,alltheseconfigurationparameterswillapplytoallofthe
neighbors.
InthecaseofIBGP,theremoteAScanalsobepartofthepeergroupconfiguration
sincetheASnumberisthesameforallpeergroupmembers.
ThepeergroupisaverypowerfultoolwhenyouaredealingwithmanyIBGP
neighbors.Youcangivealloftheneighborsthesameconfigurationtoensurea
consistentAS-wideroutingpolicy.
PeerGroupConfigurationExample—EdgeRouterataPeering
Point
Inthisexample,therouterinAS100isbeingconfiguredwithapeergroupnamed
"Peering"
PE(config)# router bgp 100
PE(config-router)# neighbor Peering peer-group
PE(config-router)# neighbor Peering filter-list 10 in
PE(config-router)# neighbor Peering route-map PeerMap out
PE(config-router)# neighbor Peering maximum-prefix 50
PE(config-router)# neighbor Peering remove-private-as
PE(config-router)# neighbor 209.165.207.7 remote-as 700
PE(config-router)# neighbor 209.165.207.7 peer-group Peering
PE(config-router)# neighbor 209.165.208.8 remote-as 800
PE(config-router)# neighbor 209.165.208.8 peer-group Peering
PE(config-router)# no neighbor 209.165.208.8 maximum-prefix 50
PE(config-router)# no neighbor 209.165.208.8 filter-list 10 in
PE(config-router)# neighbor 209.165.209.9 remote-as 900
PE(config-router)# neighbor 209.165.209.9 peer-group Peering
BGPPeerGroupConfigurationExamples
Thispeergroupisusedforallpeerprovidersbecausetheyshareanalmostidentical
routingpolicy.Thepeergroupisfirstcreatedasatemplate,whichisconfiguredwith
anincomingASpathfilterlist(list10)andanoutgoingroutemapnamed"PeerMap."
Themaximumnumberofreceivedprefixesisalsosetinthepeergroupto50.The
peergrouphasalsobeenconfiguredtoremoveprivateASnumbersfromallAS
pathsbeforetheroutesaresenttothepeerAS.TheseASnumbersareintherange
64512to65535inclusive.
TheneighborsinAS700,AS800,andAS900arethenassignedtothepeergroup,
meaningthattherouterinAS100willattempttoopenBGPsessionswiththose
routers.IftheBGPsessionsaresuccessfullyestablished,filterlist10andtheroute
mapPeerMap,asconfiguredinthepeergroup,willbeappliedtoincomingand
outgoingroutesfrombothneighbors,respectively.
Asdefinedintherouterconfiguration,filterlist10filtersoutanyincomingroutesfrom
peergroupmembersunlessotherwisespecified.However,inthecaseofthe
neighborinAS800,theindividualconfigurationofnofilter-list10willoverridethe
peergroupconfiguration.Thus,thefilterlistwillnotbeusedforthisneighbor.The
limitationonthenumberofreceivedroutesfromAS800isalsoremovedfromthe
neighborinAS800.
Thepeergroupisaverypowerfultoolwhenyouare
dealingwithalargenumberofneighborswithalmost
identicalconfigurations.However,ifanyofthecustomers
thatareassignedtothepeergrouprequirerouting
informationthatisdifferentfromothermembersofthe
peergroup,thenthatneighbormustberemovedfromthe
peergroupandconfiguredindividually.
IntheCiscoIOS12.0(24)SreleaseCiscointroduceddynamicupdatepeergroups,a
featurethatenablestherouterstoautomaticallygroupBGPneighborintogroups
andgenerateupdatemessagesaccordingly.
Separates BGP update generation from neighbor configuration
Introduces a new algorithm that dynamically calculates and optimizes update
groups of neighbors that share the same outbound policies.
Requires no additional configuration.
Optimal BGP update message generation occurs automatically and
independently
InpreviousversionsofCiscoIOSsoftware,BGPupdatemessagesweregrouped
basedonpeergroupconfigurations.ThismethodofgroupingneighborsforBGP
updatemessagegenerationreducedtheamountofsystemprocessingresources
thatareneededtoprocesstheroutingtable.Thismethod,however,hadthe
followinglimitations:
Allneighborsthatsharedthesamepeergroupconfigurationalsohadtoshare
thesameoutboundroutingpolicies.
Allneighborshadtobelongtothesamepeergroupandaddressfamily.
Neighborsthatareconfiguredindifferentpeergroupscannotbelongtodifferent
addressfamilies.
Theselimitationsexistedtobalanceoptimalupdategenerationandreplication
againstpeergroupconfiguration.Theselimitationsalsocausedthenetworkoperator
toconfiguresmallerpeergroups,whichreducedtheefficiencyofupdatemessage
generation.
TheBGPdynamicupdategroupsfeaturecontainsanalgorithmthatdynamically
calculatesandoptimizesupdategroupsofneighborsthatshareoutboundpolices
andcansharetheupdatemessages.TheBGPdynamicupdategroupsfeature
separatesupdategroupreplicationfrompeergroupconfiguration,improving
convergencetimeandflexibilityofneighborconfiguration.TheBGPupdategroups
requirenoconfiguration,andarouteroptimizesBGPupdatemessagegeneration
automatically.
Whenachangetotheconfigurationoccurs,therouterautomaticallyrecalculates
updategroupmembershipsandappliesthechangesbytriggeringanoutboundsoft
resetaftera3-minutetimerexpires.Thisbehaviorisdesignedtoprovidethe
networkoperatorwithtimetochangetheconfigurationifamistakeismade.
Asoftreset,whichisperformedonaper-neighborbasis,
doesnotcleartheBGPsessionandfacilitatesthe
applicationofnewpolicies.Therearetwomethodsof
performingasoftreset.Dynamicinboundsoftresetis
usedtogenerateinboundupdatesfromaneighbor.An
outboundsoftresetisusedtosendanewsetofupdates
toaneighbor.
clear ip bgp update-group [index-group | ip-address]
Clears BGP update-group member sessions.
debug ip bgp groups [index-group | ip-address]
Displays information that is related to the processing of BGP update-groups.
show ip bgp replication [index-group | ip-address]
Displays update replication statistics for BGP update-groups.
show ip bgp update-group [index-group | ip-address] [summary]
Displays information about BGP update-groups.
Ifnoargumentisspecified,theclearipbgpupdate-groupcommandrecalculates
allupdate-groups.Specificindexnumbersforupdategroupsandinformationabout
BGPDynamicUpdatePeerGroupsFeature
update-groupmembershipisdisplayedintheoutputoftheshowipbgpupdate-
groupanddebugipbgpgroupscommands.
clearipbgpupdate-group
ToclearBGPupdate-groupmembersessions,usetheclearipbgpupdate-group
commandinprivilegedEXECmode.
clear ip bgp update-group [index-group | ip-address]
SyntaxDescription
Parameter Description
index-group (Optional)Specifiesthattheupdate-groupwithcorresponding
indexnumberwillbereset.Therangeofupdate-groupindex
numbersisfrom1to4294967295.
ip-address (Optional)SpecifiestheIPaddressofasinglepeerthatwillbe
reset.
debugipbgpgroups
Theoutputofthedebugipbgpgroupscommanddisplaysyouinformationabout
update-groupcalculationsandtheadditionandremovalofupdate-groupmembers.
Informationaboutpeergroupsandpeer-policyandpeer-sessiontemplatesisalso
displayedintheoutputofthiscommandasneighborconfigurationschange.
debug ip bgp groups [index-group | ip-address]
SyntaxDescription
Parameter Description
index-group (Optional)Specifiesthatupdate-groupdebugginginformationfor
thecorrespondingindexnumberwillbedisplayed.Therangeof
update-groupindexnumbersisfrom1to4294967295.
ip-address (Optional)Specifiesthatupdate-groupdebugginginformationfor
asinglepeerwillbedisplayed.
Thefollowingexampleoutputfromthedebugipbgpgroupscommandshowsthe
recalculationofupdate-groupsaftertheclearipbgpgroupscommandwasissued:
ISP1# debug ip bgp groups
5w4d: %BGP-5-ADJCHANGE: neighbor 172.16.11.1 Down User reset
5w4d: BGP-
DYN(0): Comparing neighbor 172.16.11.1 flags 0x0 cap 0x0 and updgrp 2 fl0
5w4d: BGP-DYN(0): Update-
group 2 flags 0x0 cap 0x0 policies same as 172.16.11.1 fl0
5w4d: %BGP-5-ADJCHANGE: neighbor 172.16.12.2 Down User reset
5w4d: BGP-
DYN(0): Comparing neighbor 172.16.12.2 flags 0x0 cap 0x0 and updgrp 2 fl0
5w4d: BGP-DYN(0): Update-
group 2 flags 0x0 cap 0x0 policies same as 172.16.12.2 fl0
5w4d: %BGP-5-ADJCHANGE: neighbor 172.16.100.22 Down User reset
5w4d: BGP-
DYN(0): Comparing neighbor 172.16.100.22 flags 0x0 cap 0x0 and updgrp 2 fl
0
5w4d: BGP-DYN(0): Update-
group 2 flags 0x0 cap 0x0 policies same as 172.16.100.22 fl0
5w4d: %BGP-5-ADJCHANGE: neighbor 172.16.11.1 Up
5w4d: %BGP-5-ADJCHANGE: neighbor 172.16.100.22 Up
5w4d: %BGP-5-ADJCHANGE: neighbor 172.16.12.2 Up
Theoutputofthiscommandcanbeveryverbose,soyou
shouldnotdeploythiscommandinaproductionnetwork
unlessyouaretroubleshootingaproblem.
showipbgpreplication
TodisplayupdatereplicationstatisticsforBGPupdate-groups,usetheshowipbgp
replicationcommand.
show ip bgp replication [index-group | ip-address]
SyntaxDescription
Parameter Description
index-group (Optional)Specifiesthatupdatereplicationstatisticsforthe
update-groupwithcorrespondingindexnumberwillbedisplayed.
Therangeofupdate-groupindexnumbersisfrom1to
4294967295.
ip-address (Optional)SpecifiestheIPaddressofasingleneighborforwhich
update-groupstatisticswillbedisplayed.
Thefollowingsampleoutputfromtheshowipbgpreplicationcommandshows
update-groupreplicationinformationforallforneighbors:
ISP1# show ip bgp replication
Curren
t Next
Index Members Leader MsgFmt MsgRepl Csize Versio
n Version
1 2 172.16.11.1 0 0 0/1000 1
/0
2 1 172.16.100.22 0 0 0/1000
1/0
showipbgpupdate-group
TodisplayinformationaboutBGPupdate-groups,usetheshowipbgpupdate-
group.
show ip bgp update-group [index-group | ip-address] [summary]
SyntaxDescription
Parameter Description
index-group (Optional)Specifiesthatupdatereplicationstatisticsforthe
update-groupwithcorrespondingindexnumberwillbedisplayed.
Therangeofupdate-groupindexnumbersisfrom1to
4294967295.
ip-address (Optional)SpecifiestheIPaddressofasingleneighborforwhich
update-groupstatisticswillbedisplayed.
summary (Optional)Displaysasummaryofupdate-groupmember
information.
Theoutputcanbefilteredtoshowinformationforasingleindex-
grouporpeerwiththeindex-grouporip-addressargument.
Thefollowingsampleoutputfromtheshowipbgpupdate-groupcommandshows
update-groupinformationforallneighbors:
ISP1# show ip bgp update-group
BGP version 4 update-group 1, external, Address Family: IPv4 Unicast
BGP Update version : 6/0, messages 0
Topology: global, highest version: 6, tail marker: 6
Format state: Current working (OK, last minimum advertisement interval)
Refresh blocked (not in list, last not in list)
Update messages formatted 1, replicated 2, current 0, refresh 0, limit 1
000
Number of NLRIs in the update sent: max 5, min 0
Minimum time between advertisement runs is 30 seconds
Has 1 member:
172.16.100.22
BGP version 4 update-group 2, external, Address Family: IPv4 Unicast
BGP Update version : 6/0, messages 0
Topology: global, highest version: 6, tail marker: 6
Format state: Current working (OK, last minimum advertisement interval)
Refresh blocked (not in list, last not in list)
Route map for outgoing advertisements is CUST1
Update messages formatted 1, replicated 2, current 0, refresh 0, limit 1
000
Number of NLRIs in the update sent: max 5, min 0
Minimum time between advertisement runs is 30 seconds
Has 2 members:
172.16.12.2 172.16.11.1
TheBGPdynamicupdategroupsfeatureseparatespeergroupconfigurationfrom
updategroupgeneration.BGPneighborconfigurationisnolongerrestrictedby
outboundroutingpolicies,andupdategroupscanbelongtodifferentaddress
families.
Peer templates contain configuration pattern that can be applied to neighbors
that share common policies.
They are reusable and support inheritance, which allows you to group and apply
distinct neighbor configurations for BGP neighbors.
You can define very complex configuration patterns through the use of
inheritance.
EventhoughBGPupdatemessagegenerationhasbeenseparatedfrompeergroup
configuration,peergroupconfigurationstillhasthefollowinglimitations:
Aneighborcanbelongonlytoonepeergroup.
Neighborsthatbelongtodifferentaddressfamiliescannotbelongtothesame
peergroup.
Differentsetsofpoliciescannotbegroupedandappliedtoaneighbor.
Toaddressthelimitationsofpeergroups,theBGPpeertemplatesfeaturewas
introducedalongwiththeBGPdynamicupdategroupsfeature.
Apeertemplateisaconfigurationpatternthatcanbeappliedtoneighborsthatshare
commonpolicies.Peertemplatesarereusableandsupportinheritance,whichallows
youtogroupandapplydistinctneighborconfigurationsforBGPneighborsthatshare
commonpolicies.Peertemplatesalsoallowyoutodefineverycomplex
configurationpatterns,sinceapeertemplatecaninheritaconfigurationfromanother
peertemplate.
Theexampleinthefigurepresentspeertemplateinheritance.Onepeertemplate,
calledBGP,isconfiguredandcontainsparametersthatarecommontoallBGP
neighbors.Intheexample,suchparametersaretheBGPkeepaliveandhold-down
timers.ThenonepeertemplateisconfiguredforIBGPandoneforEBGPneighbors.
BothtemplatesinheritsettingsfromthecommonpeertemplatethatiscalledBGP
andaddotherparametersthatarecommononlytoIBGPorEBGPsettings,
respectively.Thereforetemplatescanbecreatedmoreeffectivelywithfewer
configurationswhencomparedtoBGPpeergroups.
Two types of peer templates are available:
Peer session template: Used to group and apply the configuration of general
session commands that are common to all address-family configuration modes.
Peer policy template: Used to group and apply the configuration of commands
that are applied within specific address-family configuration modes.
Therearetwotypesofpeertemplates:
Peersessiontemplatesareusedtogroupandapplytheconfigurationofgeneral
sessioncommandsthatarecommontoalladdress-familiesconfiguration
modes.
Peerpolicytemplatesareusedtogroupandapplytheconfigurationof
commandsthatareappliedwithinspecificaddress-familiesconfigurationmodes.
Peertemplatesimprovetheflexibilityandenhancethecapabilityofneighbor
configuration.Peertemplatesalsoprovideanalternativetopeergroupconfiguration
andovercomesomelimitationsofpeergroups.BGPpeerroutersusingpeer
templatesalsobenefitfromautomaticupdategroupconfiguration.Withthe
configurationoftheBGPpeertemplatesthesupportoftheBGPdynamicupdate
BGPPeerTemplatesOverview
peergroupsfeature,younolongerneedtoconfigurepeergroupsinBGP.You
benefitfromimprovedconfigurationflexibilityandfasterconvergence.
TheconfigurationofBGPpeertemplatesdoesnotconflict
withorrestrictpeergroupconfiguration,andpeergroups
arestillsupportedinCiscoIOSReleasesthatsupport
BGPpeertemplates.However,aBGPneighborcannotbe
configuredtoworkwithbothpeergroupsandpeer
templates.ABGPneighborcanbeconfiguredtobelong
onlytoapeergrouportoinheritpoliciesfrompeer
templates.
Theinheritancecapabilityisakeycomponentofpeer-templateoperation.
Inheritanceinapeertemplateissimilartothenodeandtreestructuresthatare
commonlyfoundingeneralcomputing—forexample,fileanddirectorytrees.Apeer
templatecandirectlyorindirectlyinheritaconfigurationfromanotherpeertemplate.
Thedirectlyinheritedpeertemplaterepresentsthetreeinthestructure,andthe
indirectlyinheritedpeertemplaterepresentsanodeinthetree.Becauseeachnode
alsosupportsinheritance,branchescanbecreatedthatapplytheconfigurationsof
allindirectlyinheritedpeertemplateswithinachainthattracesbacktothedirectly
inheritedpeertemplateorthesourceofthetree.Thisstructureeliminatestheneed
torepeatconfigurationstatementsthatarecommonlyreappliedtogroupsof
neighbors.Commonconfigurationstatementscannowbeappliedonceandthen
indirectlyinheritedbypeertemplatesthatareappliedtoneighborgroupswith
commonconfigurations.
The BGP peer templates inheritance characteristics are as follows:
A session template can inherit configuration from another session template.
A policy template can inherit configuration from another policy template.
Neighbors can inherit from a session and a policy template.
Inheritanceexpandsthescalabilityandflexibilityofneighborconfiguration.Itallows
youtochaintogetherpeer-templateconfigurationstocreatesimpleconfigurations
thatinheritcommonconfigurationstatementsorcomplexconfigurationsthatapply
specificconfigurationstatementsalongwithcommoninheritedconfigurations.
Generalsessioncommandscanbeconfiguredonceinapeersessiontemplateand
thenappliedtomanyneighborsthroughthedirectapplicationofapeersession
template.Orthroughindirectinheritancefromapeersessiontemplate.The
configurationofpeersessiontemplatessimplifiestheconfigurationofgeneral
sessioncommandsthatarecommonlyappliedtoallneighborswithinanAS.
Peersessiontemplatessupportdirectandindirectinheritance.Apeercanbe
configuredwithonlyonepeersessiontemplateatatime,andthatpeersession
templatecancontainonlyoneindirectlyinheritedpeersessiontemplate.
Ifyouattempttoconfiguremorethanoneinheritstatement
withasinglepeersessiontemplate,anerrormessagewill
bedisplayed.
ThisbehaviorallowsaBGPneighbortodirectlyinheritonlyonesessiontemplate
andindirectlyinherituptosevenadditionalpeersessiontemplates.Thispractice
allowsyoutoapplyamaximumofeightpeersessionconfigurationstoaneighbor.
Theconfigurationfromthedirectlyinheritedpeersessiontemplate,andthe
configurationsfromuptosevenindirectlyinheritedpeersessiontemplates.Inherited
peersessionconfigurationsareevaluatedfirstandappliedstartingwiththelastnode
inthebranchandendingwiththedirectlyappliedpeersessiontemplate
configurationatthesourceofthetree.Thedirectlyappliedpeersessiontemplatewill
havepriorityoverinheritedpeersessiontemplateconfigurations.Anyconfiguration
statementsthatareduplicatedininheritedpeersessiontemplateswillbeoverwritten
bythedirectlyappliedpeersessiontemplate.Meaningthatifageneralsession
commandisreappliedwithadifferentvalue,thesubsequentvaluewillhavepriority
andoverwritethepreviousvaluethatwasconfiguredintheindirectlyinherited
template.
PeerpolicytemplatesareusedtoconfigureBGPpolicycommandsthatare
configuredforneighborsthatbelongtospecificaddressfamilies.Likepeersession
templates,peerpolicytemplatesareconfiguredonceandthenappliedtomany
neighborsthroughthedirectapplicationofapeerpolicytemplateorthrough
BGPPeerTemplatesInheritance
inheritancefrompeerpolicytemplates.Theconfigurationofpeerpolicytemplates
simplifiestheconfigurationofBGPpolicycommandsthatareappliedtoallneighbors
withinanAS
Likepeersessiontemplates,apeerpolicytemplatesupportsinheritance.However,
thereareminordifferences.Adirectlyappliedpeerpolicytemplatecandirectlyor
indirectlyinheritconfigurationsfromuptosevenpeerpolicytemplates.Thatis,a
totalofeightpeerpolicytemplatescanbeappliedtoaneighbororneighborgroup.
Inheritedpeerpolicytemplatesareconfiguredwithsequencenumberslikeroute
maps.Aninheritedpeerpolicytemplate,likearoutemap,isevaluatedstartingwith
theinheritstatementwiththelowestsequencenumberandendingwiththehighest
sequencenumber.However,thedifferenceisthatapeerpolicytemplatewillnot
collapselikearoutemap.Everysequenceisevaluated,andifaBGPpolicy
commandisreappliedwithadifferentvalue,anypreviousvaluefromalower
sequencenumberwillbeoverwritten.
Thedirectlyappliedpeerpolicytemplateandtheinheritstatementwiththehighest
sequencenumberwillalwayshavepriorityandbeappliedlast.Commandsthatare
reappliedinsubsequentpeertemplateswillalwaysoverwritethepreviousvalues.
Thisbehaviorisdesignedtoallowyoutoapplycommonpolicyconfigurationsto
largeneighborgroupsandspecificpolicyconfigurationsonlytocertainneighbors
andneighborgroups,withoutduplicatingindividualpolicyconfigurationcommands.
Theconfigurationofpeerpolicytemplatessimplifiesandimprovestheflexibilityof
BGPconfiguration.Aspecificpolicycanbeconfiguredonceandreferencedmany
times.Becauseapeerpolicysupportsuptoeightlevelsofinheritance,specificand
complexBGPpoliciescanalsobecreated.
Generalsessioncommandsthatarecommonforneighborsthatareconfiguredin
differentaddressfamiliescanbeconfiguredwithinthesamepeersessiontemplate.
Peersessiontemplatesarecreatedandconfiguredinpeersessionconfiguration
mode.Onlygeneralsessioncommandscanbeconfiguredinapeersession
template.BGPpolicyconfigurationcommandsthatareconfiguredonlyforspecific
addressfamiliesconfigurationmodesareconfiguredwithpeerpolicytemplates.
PeerSessionCommands
template peer-session session-template-name
Creates a peer session.
inherit peer-session session-template-name
Configures a peer session template to inherit the configuration of another peer
session template.
neighbor ip-address inherit peer-session session-template-name
Sends a peer session template to a neighbor so that the neighbor can inherit the
configuration.
Tocreateapeersessiontemplateandentersessiontemplateconfigurationmode,
usethetemplatepeer-sessioncommandinrouterconfigurationmode.
template peer-session session-template-name
SyntaxDescription
Parameter Description
session-template-name Nameortagforthepeersessiontemplate
Toconfigureapeersessiontemplatetoinherittheconfigurationofanotherpeer
sessiontemplate,usetheinheritpeer-sessioncommandinsession-template
configurationmode.
inherit peer-session session-template-name
SyntaxDescription
Parameter Description
session-template-name Nameortagforthepeersessiontemplate
Theneighborinheritpeer-sessioncommandisusedtosendlocallyconfigured
sessiontemplatestothespecifiedneighbor.Ifthesessiontemplateisconfiguredto
inheritconfigurationsfromothersessiontemplates,thespecifiedneighborwillalso
indirectlyinherittheseconfigurationsfromtheothersessiontemplates.Aneighbor
candirectlyinheritonlyonepeersessiontemplateandindirectlyinherituptoseven
peersessiontemplates.
Usetheneighborinheritpeer-sessioncommandinaddressfamilyorrouter
configurationmode.
neighbor ip-address inherit peer-session session-template-name
SyntaxDescription
Parameter Description
ip-address IPaddressoftheneighbor
session-template-name Nameortagforthepeersessiontemplate
ABGPneighborcannotbeconfiguredtoworkwithboth
peergroupsandpeertemplates.ABGPneighborcanbe
configuredtobelongonlytoapeergrouportoinherit
BGPPeerTemplatesConfiguration
policiesonlyfrompeertemplates.
PeerPolicyCommands
Peerpolicytemplatessupportonlygeneralpolicycommands.BGPpolicy
configurationcommandsthatareconfiguredonlyforspecificaddressfamilies
configurationmodesareconfiguredwithpeerpolicytemplates.
inherit peer-policy policy-template-name sequence-number
Configures a peer policy template to inherit the configuration from another peer
policy template.
neighbor ip-address inherit peer-policy policy-template-name
Sends a peer policy template to a neighbor so that the neighbor can inherit the
configuration.
Toconfigureapeerpolicytemplatetoinherittheconfigurationfromanotherpeer
policytemplate,usetheinheritpeer-policycommandinpolicy-template
configurationmode.
inherit peer-policy policy-template-name sequence-number
SyntaxDescription
Parameter Description
peer-policy-name Nameofthepeerpolicytemplatetobeinherited.
sequence-number Sequencenumberthatsetstheorderinwhichthepeerpolicy
templateisevaluated.Likearoutemapsequencenumber,the
lowestsequencenumberisevaluatedfirst.
Theneighborinheritpeer-policycommandisusedtosendlocallyconfigured
policytemplatestothespecifiedneighbor.Ifthepolicytemplateisconfiguredto
inheritconfigurationsfromotherpeerpolicytemplates,thespecifiedneighborwill
alsoindirectlyinherittheseconfigurationsfromtheotherpeerpolicytemplates.A
directlyappliedpeerpolicytemplatecandirectlyorindirectlyinheritconfigurations
fromuptosevenpeerpolicytemplates.So,atotalofeightpeerpolicytemplatescan
beappliedtoaneighbororneighborgroup.
neighbor ip-address inherit peer-policy policy-template-name
SyntaxDescription
Parameter Description
ip-address IPaddressoftheneighbor
policy-template-name Nameortagforthepeerpolicytemplate
BGPPeerTemplatesVerification
show ip bgp template peer-session [session-template-name]
Displays peer policy template configurations.
show ip bgp template peer-policy [policy-template-name]
Displays locally configured peer policy templates.
Theshowipbgptemplatepeer-sessioncommandisusedtodisplaylocally
configuredpeersessiontemplates.Theoutputcanbefilteredtodisplayasingle
peersessiontemplatewiththepeer-session-nameargument.Thiscommandalso
supportsallstandardoutputmodifiers.
show ip bgp template peer-session [session-template-name]
SyntaxDescription
Parameter Description
session-template-name (Optional)Nameofalocallyconfiguredpeersessiontemplate
Todisplaylocallyconfiguredpeerpolicytemplates,usetheshowipbgptemplate
peer-policycommandinEXECmode.
show ip bgp template peer-policy [policy-template-name]
SyntaxDescription
Parameter Description
policy-template-name (Optional)Nameofalocallyconfiguredpeerpolicytemplate
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
BGP peer groups were designed primarily for CPU optimization. They can also
be used for configuration optimization.
BGP peer groups are used on a router with several neighbors that have
similar, but not completely identical polices.
Peer groups have limitations because of the way that they are used to build
BGP updates:
Per-neighbor BGP parameters that affect outbound updates cannot be
changed for peer group members.
IBGP and EBGP neighbors cannot be mixed in a peer group.
To configure BGP peer groups, create a BGP peer group, specify parameters
for the BGP peer group, create a BGP neighbor, and then assign a neighbor to
the peer group.
BGP dynamic update groups separate BGP update generation from neighbor
configuration.
BGP peer templates are reusable configuration patterns that support
inheritance.
Peer session templates—contain configuration that is common to all
address families.
Peer policy templates—contain configuration that is applied within a specific
address family.
Summary
Overview
EvenwhenaBGPimplementationiscorrectlyconfiguredandhighlyrobust,the
performanceoftheroutingprocessonanygivenrouterislimited.Limitingthe
propagationofunstableroutes,specificallywhentheyarenotbeneficialtothe
network,becomesanimportantissuebecauseitreducestheprocessing
requirementsoftherouterthatisforcedtoprocessroutingtablestatechanges.
RoutedampeningisaBGPfeaturethathasbeendesignedtoreduceBGP
processingrequirementsbyminimizingthepropagationofunstableroutestoBGP
peers.ASborderrouters,inanyBGPimplementation,cannotrelyuponexternal
peerstosufficientlyshieldtheASfromroutingtableinstability.Routedampening
allowsrouteinstabilitytobecontainedatanASborderrouterthatbordersthe
instability.
Thislessondescribesthepurposeandoperationoftheroute-dampeningfeatureon
CiscoIOSrouters.AlsodiscussedinthislessonaretheCiscoIOScommandsthat
arerequiredtoenableroutedampening,modifydefaultdampeningparameters,and
releasearoutethathasbeensuppressedbecauseofdampening.TheCiscoIOS
commandsthatareusedtomonitorroutedampeningarealsodiscussed.
Uponcompletingthislesson,youwillbeableto:
DescribethepurposeofBGProutedampening
DescribetheoperationofBGProutedampening
IdentifytheCiscoIOScommandsthatarerequiredtoconfigureBGProute
dampening
IdentifytheCiscoIOScommandsthatarerequiredtoreleasedampenedroutes
andmonitorBGProute-dampening
UsingBGPRouteDampening
BGPistheonlyroutingprotocolthatisdesignedforlargeinternetworkswiththe
specificintentionofcarryingalargenumberofprefixes.Thereareseveral
mechanismsthatarebuiltintoBGPthatensuremaximumrouterstability.
Designed to reduce router processing load that is caused by unstable routes
Prevents sustained routing oscillations without affecting other well-behaved
routes
Defined in RFC 2439: BGP Route Flap Dampening
A tool that is designed to help minimize the number of BGP updates
Other update reduction tools:
Batching of BGP updates
Per-neighbor update timers
Forexample,aBGProuterdoesnotforwardBGProutingupdatesimmediatelyafter
receivingthem.EverytimeaBGProutersendsanupdate,itstartsa5-secondtimer
forinternalneighborsanda30-secondtimerforexternalneighbors.Nonewupdates
canbesentuntilthattimerexpires.Theresultisthatifaroutercontainsalinkthatis
flappingrepeatedly(available,thenunavailable,thenavailable,thenunavailable,
andsoon)atarateofoncepersecond,externalroutersseetheflapatamuch
slowerrate.Routersthatareexternaltothesourceoftheflaparenotforcedto
recalculatethebestpatheverysecondbut,atmost,every30seconds.
Reducingtherateatwhichneighboringroutersprocessflappingrouteshelpswith
reducingtherequirementstoprocesstheBGPupdate.However,routersthat
processroutingupdatesforunstableroutesarestillwastingresourcesin
determiningthebestroutetothedestination.Becausetheunstablerouteis
oscillatingbetweenupanddown,eachrouteupdatethatarouterreceivescausesit
toprocesstheunstableroutealloveragain.Abetterapproachistoremovethe
updateabouttherouteuntilitcanbeguaranteedthatthedestinationismorestable.
Withthisgoalinmind,anextraBGPscalabilitymechanism,routeflapdampening,
wascreatedtoreducerouteupdateprocessingrequirementsbysuppressing
unstableroutes.
Minimizes the amount of BGP update processing in the Internet by suppressing
unstable (flapping) routes
Does not suppress routes that occasionally flap
Suppresses routes that are likely to flap in the future based on the history of
their behavior
Flap = Remove route
Suppress = Do not use a route after it reappears
MostserviceprovidersholdroutinginformationfortheentireInternet.Therefore,a
flappinglinksomewhereintheInternetcancauseallroutersintheInternettokeep
processingchangesbecauseofonesinglelink.If,however,oneoftheautonomous
systemsintheInternetimplementsroutedampening,theflappingnetworkis
suppressed.Therouteisnotpropagatedfurthertootherautonomoussystemsuntil
theconfiguredrulesofroutedampeningallowit.
A"flap"referstoaroutethatisrepeatedlyavailable,thenunavailable,then
available,thenunavailable,andsoon.Ifarouteflapsonceortwice,itistypicallynot
consideredaflapfromanadministrativeperspective.Iftheflappinghappensmore
often,however,thereisprobablysomethingwrongwiththedestinationandtheroute
shouldbesuppressed.TheBGProuterstoresasuppressedrouteintheBGPtable
butdoesnotconsideritintheBGPpath-selectionprocessanddoesnottherefore
propagateittootherBGPneighborsoruseitfordataforwarding.
BGPRouteDampening
ABGProuterwithroutedampeningenabledkeepstrackofallroutes(eventhe
routesthatareunreachable)soitcanrecallthepenaltiesthatareassignedtoeach
route.
Each time an EBGP route flaps, it gets 1000 penalty points (IGBP routes are not
dampened).
The penalty that is placed on a route decays according to the exponential decay
algorithm.
When the penalty exceeds the suppress limit, the route is dampened (no longer
used or propagated to other neighbors).
A dampened route is propagated when the penalty drops below the reuse limit.
Everytimearouteflapoccurs,theflappingroutereceives1000penaltypoints.The
penaltyisgraduallydecreasedbyusingadecayingalgorithm.Ifarouteflapsseveral
times,itwillbepenalized(gainenoughpenaltypoints)andsubsequentlyreachand
exceedthesuppresslimit.
Anyroutethatreachesthesuppresslimitisnolongerforwardedtootherneighbors
untiltheassignedpenaltyisonceagainbelowthereuselimit.Anexponentialdecay
algorithmreducespenaltypointsthatareappliedtoaflappingroute.Afterthe
numberofpenaltypointsthatareassignedtoaroutefallsbelowthereuselimit,the
BGProuteronceagainadvertisestheroute.
The flap history is forgotten, when the penalty drops below half of the reuse limit.
A route is never dampened for more time than the maximum suppress limit.
An unreachable route with a flap history is put in the history state—it stays in the
BGP table but only to maintain the flap history.
A penalty is applied on the individual path in the BGP table, not on the IP prefix.
Arouterstopstrackingpenaltypointswhentheyarebelowhalfofthereuselimit.
Themaximumsuppresslimitdefinesthemaximumdurationthataroutecanbe
suppressedafterithasbeensuppressed.
Afterroutedampeningisenabled,therouterneverremovesaroutefromtheBGP
table.AroutethataBGPneighborhaswithdrawncanstillbeseenintheBGPtable
andismarkedwithan"h"(historystate).
Apenaltyisalwaysappliedtoapathandnotaprefix.Ifoneofthepathsisflapping,
itdoesnotmeanthatthedestinationisflapping.
BGPRouteDampeningOperation
ThistopicidentifiestheCiscoIOScommandsthatarerequiredtoconfigureBGP
routedampening.
router(config-router)# bgp dampening [half-life reuse suppress max-
suppress-time] [route-map map-name]
Configures BGP route dampening
BGP dampening parameters:
half-life Decay time in which the penalty is halved
suppress Value when the route starts dampening
reuse Value when the dampened route is reused
max-suppress-time Maximum time to suppress the route
route-map Name of route-map controlling dampening
Toenableroutedampening,usethebgpdampeningcommand.Optionally,youcan
changethedefaultsettingsoftheroute-dampeningparameters.
Routeflapdampeningrequiresthefollowingparameters:
half-life:Thehalf-lifeisthetimethatisneededforthepenaltytohalve(defaultis
15minutes).
suppress:Whenaroutehasmorepenaltypointsthanthesuppresslimit,the
routeissuppressed(defaultis2000).
reuse:Aftertheflappinghasstoppedandthepenaltyforaroutehasfallen
belowthereuselimit,therouteisunsuppressed(defaultis750).
max-suppress-time:Noroutecanbesuppressedlongerthanthemax-suppress-
timeminutes(defaultis1hour;maximumis255minutes).
Youcanspecifythefourrouteflap-dampeningparametersdirectlywiththebgp
dampeningcommand.Alternatively,youcancreatearoute-mapthatspecifies
differentdampeningparametersfordifferentsetsofroutes,andthenyoucanapply
theroute-mapwiththebgpdampeningroute-mapcommand.
Most Internet service providers use default values:
A flapping route is dampened after three successive flaps.
A route stays suppressed for approximately 30 minutes.
Net result: The route is lost for 30 minutes if a BGP session with a neighbor is
cleared three times in succession.
Default dampening parameter values are:
half-life 15 minutes
suppress 2000
reuse 750
max-suppress-time 60 minutes (4x half-life)
per-flap penalty 1000 (nonconfigurable)
Thissamplecalculationshowshowlongaroutethatflapsthreetimesissuppressed
withthedefaultvaluesoftheCiscoIOSroute-dampeningparameters.Eachtime
thatarouteflaps,itaccumulatesanother1000points.Afterthethirdflap,theroute
hasalmost3000points.Rememberthatthepenaltyisgraduallydecreasedbyusing
adecayingalgorithm,causingareductioninthenumberofpointsthattheroute
accumulates.Ittakes15minutesforthepenaltytodropbelow1500(providedthere
arenofurtherflaps)andanother15minutestodropbelowthereuselimitof750.
Manyserviceproviderschangethedefaultparameterstoallowamaximum
suppresstimeofseveralhours.
Usingtheclearipbgp*commandisregardedasaflapto
neighboringautonomoussystems.Usingthiscommand
severaltimesmaycauseneighboringautonomous
systemstosuppressprefixesforsometimeevenifthere
isnothingwrongwiththeroute.
Usingtheclearipbgp*[soft][in|out]commandisnot
regardedasaflap,andthiscommandshouldbeused
insteadofclearipbgp*forclearingtheneighbor
ConfiguringBGPRouteDampening
relationships.
Youcanchangealldefaultvaluesifyouspecifythemintheoptionalparametersof
thebgpdampeningcommand.Theper-flappenaltyistheonlyvaluethatisnot
configurable.
router(config-route-map)# set dampening half-life reuse suppress max-
suppress-time
This command sets the BGP dampening parameters for individual routes that a
route map entry matches.
Apply this route-map to the bgp dampening command instead of specifying
individual parameters.
Applications:
Less aggressive dampening of routes toward root DNS servers (or other
servers)
Dampening of smaller prefixes more aggressively
Selective dampening based on BGP neighbor and route-map match criteria
Manyserviceprovidersprefertoimplementselectivedampening.Largerprefixesare
usuallylesslikelytoflapandshouldnotbepenalizedasaggressivelyasthesmaller
prefixesthatpopulatemostoftheBGPtable.
Youcanusearoute-mapincombinationwithaprefix-listtomatchonprefixlength
andtosetdifferentroute-dampeningparametersforlargerprefixesthanforsmaller
ones.Apracticalserviceproviderpolicyistousearoute-maptoexcluderootDNS
serversfromdampeningaltogether.
Youcanthenattachtheroute-maptotheBGProute-dampeningprocesswiththe
bgpdampeningroute-mapcommand.
Overview
Throughthisdiscovery,youwilllearnhowtoconfigureBGProutedampening.You
willenableroutedampeningontheR2router.Youwillsimulateflappingofoneof
theadvertisednetworksontheISP1router.Consequently,thenetworkflappingwill
getsuppressedontheR2router,whereroutedampeningisenabled.
Discovery20:ConfigureBGPRouteDampening
Topology
Ifyoushutdownaninterfaceonarealrouterorswitch,the
connecteddevicewillseeitas"down/down."Dueto
virtualizationspecifics,IOLbehaviorisslightlydifferent.If
youshutdownaninterfaceonarouterorswitch,the
connecteddevicewillseeitas"up/up."InIOL,thestatus
ofaninterfacecanonlybe"up/up"or"administratively
down/down."
DeviceInformation
Device Interface IPaddress Description
R2 Ethernet0/0 172.16.22.2/24 ConnectiontoISP2
R2 Ethernet0/1 172.16.12.2/24 ConnectiontoISP1
R2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
10.0.0.1/28
10.0.0.17/28
10.0.0.33/28
10.0.0.49/28
10.0.0.65/28
Loopbackssimulate
LANnetworks
ISP1 Ethernet0/1 172.16.12.11/24 ConnectiontoR2
ISP1 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.1.1/28
10.0.1.17/28
10.0.1.33/28
10.0.1.49/28
10.0.1.65/28
10.0.1.81/28
Loopbackssimulate
LANnetworks
ISP1 Loopback21
Loopback37
Loopback40
10.0.21.1
10.0.37.1
10.0.40.1
Loopbackssimulate
extranetworksin
differentautonomous
systems.
ISP2 Ethernet0/0 172.16.22.22/24 ConnectiontoR2
ISP2 Loopback1
Loopback2
Loopback3
Loopback4
Loopback5
Loopback6
10.0.2.1/28
10.0.2.17/28
10.0.2.33/28
10.0.2.49/28
10.0.2.65/28
10.0.2.81/28
Loopbackssimulate
LANnetworks
ISP2 Loopback21
Loopback37
Loopback40
10.0.21.1
10.0.37.1
10.0.40.1
Loopbackssimulate
extranetworksin
differentASs.
IPaddressesandadvertisednetworksinBGParepreconfiguredasshownbelow:
JobAids
BGPisalsopreconfiguredasEBGP(R2toISP1andR2toISP2).TheISP1router
announcesthesenetworks:
10.0.1.0/28
10.0.1.16/28
10.0.1.32/28
10.0.1.48/28
10.0.1.64/28
10.0.1.80/28
Step1
OntheR2router,verifythatyoureceive10.0.1.48/28routetotheBGPtablefromthe
ISP1neighbor.
R2# show ip bgp
BGP table version is 24, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/24 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.0.64/28 0.0.0.0 0 32768 i
*> 10.0.1.0/28 172.16.12.11 0 0 100 i
*> 10.0.1.16/28 172.16.12.11 0 0 100 i
*> 10.0.1.32/28 172.16.12.11 0 0 100 i
*> 10.0.1.48/28 172.16.12.11 0 0 100 i
*> 10.0.1.64/28 172.16.12.11 0 0 100 i
*> 10.0.1.80/28 172.16.12.11 0 0 100 i
<... output omitted...>
Step2
EnableBGProutedampeningontheR2routerwithdefaultvaluesfor
dampeningparameters.
R2(config)# router bgp1
R2(config-router)# bgp dampening
Step3
EnabledebuggingforBGPdampeningontheR2router.
R2# debug ip bgp dampening
Step4
OntheISP1router,disableandenableinterfaceLoopback4(10.0.1.49/28)for
atleastthreetimes.Allowyourselfatleastaminutebeforenextcommandis
applied.
ISP1(config)# interface Loopback 4
ISP1(config-if)# shutdown
!wait for at least 30 seconds!
ISP1(config-if)# no shutdown
!wait for 30 seconds!
!repeat these steps for at least three times!
NOTE:Youwillsimulatetheflappingofthe10.0.1.48/28networkbyenabling
anddisablingloopbackinterfacefewtimesinshortamountoftime.
Step5
Observethedebuggingoutput.EachtimethatyoudisabletheinterfaceLoopback4,the10.0.1/48
networkwithexistingpathgetschargedwith1000points.Oncethesuppresspenaltypassesthe
thresholdof2000andyouenabletheinterfaceLoopback4,network10.0.1.0/48getssuppressed.
R2#
*Apr 1 08:36:29.423: EvD: charge penalty 1000, new accum. penalty 1000, flap count 1
*Apr 1 08:36:29.423: EvD: unsuppress item left in reuse timer array with penalty 1000
*Apr 1 08:36:29.423: BGP(0): charge penalty for 10.0.1.48/28 path 100 with halflife-
time 15 reuse/suppress 750/2000
*Apr 1 08:36:29.423: BGP(0): flapped 1 times since 00:00:00. New penalty is 1000
ConfigureBGPRouteDampening
DiscoverySteps
R2#
*Apr 1 08:37:12.319: EvD: accum. penalty decayed to 969 after 42 second(s)
R2#
*Apr 1 08:37:30.196: EvD: accum. penalty 957, not suppressed
R2#
*Apr 1 08:38:01.030: EvD: accum. penalty decayed to 935 after 31 second(s)
*Apr 1 08:38:01.030: EvD: charge penalty 1000, new accum. penalty 1935, flap count 2
*Apr 1 08:38:01.030: EvD: unsuppress item left in reuse timer array with penalty 1935
*Apr 1 08:38:01.030: BGP(0): charge penalty for 10.0.1.48/28 path 100 with halflife-
time 15 reuse/suppress 750/2000
*Apr 1 08:38:01.030: BGP(0): flapped 2 times since 00:01:31. New penalty is 1935
R2#
*Apr 1 08:38:12.380: EvD: accum. penalty decayed to 1920 after 11 second(s)
R2#
*Apr 1 08:38:31.858: EvD: accum. penalty 1890, not suppressed
R2#
*Apr 1 08:39:02.687: EvD: accum. penalty decayed to 1846 after 31 second(s)
*Apr 1 08:39:02.687: EvD: charge penalty 1000, new accum. penalty 2846, flap count 3
*Apr 1 08:39:02.687: EvD: unsuppress item left in reuse timer array with penalty 2846
*Apr 1 08:39:02.687: BGP(0): charge penalty for 10.0.1.48/28 path 100 with halflife-
time 15 reuse/suppress 750/2000
*Apr 1 08:39:02.687: BGP(0): flapped 3 times since 00:02:33. New penalty is 2846
R2#
*Apr 1 08:39:12.402: EvD: accum. penalty decayed to 2835 after 9 second(s)
R2#
*Apr 1 08:39:32.484: BGP(0): suppress 10.0.1.48/28 path 100 for 00:28:30 (penalty 2791)
*Apr 1 08:39:32.484: halflife-time 15, reuse/suppress 750/2000
*Apr 1 08:39:32.484: EvD: accum. penalty 2791, now suppressed with a reuse intervals of 171
MonitoringRouteDampening
router# show ip bgp dampened-paths
Displays the dampened routes
router# show ip bgp flap-statistics [{regexp regexp} | {filter-
list access-list} | {ip-address mask [longer-prefix]}]
Displays flap statistics for all routes with dampening history
Can match routes against regular expressions, AS-path access-lists, a specific
route, or more specific routes
router# debug ip bgp dampening
Displays the BGP dampening events
Thepenaltythatisplacedonanetworkisdecayeduntilthereuselimitisreached,
uponwhichtherouteisonceagainadvertised.Everytimethatarouteflapoccurs,
thepenaltyisrecalculated.Intherouter,thepenaltyisencodedasthetimethatit
takesforthepenaltytodecaybelowthereuselimit.Athalfofthereuselimit,the
dampeninginformationfortheroutetothenetworkisremoved.
Usetheshowipbgpdampened-pathscommandtolistallroutesthatarecurrently
suppressedbecauseofdampening.
Usetheshowipbgpflap-statisticscommandtolistallroutesthathaveapenalty
thatisstillabovethetime-to-forgetlimit.Youcanalsousethiscommandin
combinationwithregularexpressionsandfilter-lists.
Theshowipbgpflap-statisticsprefixcommanddisplaysdetaileddampening
informationaboutaspecificnetwork.
Theshowipbgpflap-statisticsprefixmasklonger-prefixcommanddisplays
dampeninginformationaboutaspecificnetworkanditssubnets.
UsethedebugipbgpdampeningcommandtodisplayBGPdampeningeventsas
theyoccurinrealtime.
Theexampleshowshow,afterthefirstflap(whenaroutebecomesunreachable),
therouterwithdrawstheroute.Howevertherouterkeepstherouteinitsown
databasetokeeptrackofthepenalty.Therouteentersthehistorystate.
Ifaprefixisinthehistorystate,itmeansthatitiscurrentlyunreachablebutthatthe
informationhasbeenkeptintheBGPtabletokeeptrackofthepenalty.
Theexamplehereshowshow,afterthethirdflap,thepenaltyoftherouteexceeds
thesuppresslimit,andtheroutecouldbesuppressed.Whentherouteexceedsthe
suppresslimit,oneoftwothingscouldhappen:
Therouterwillputtherouteinthehistorystateiftherouteiscurrently
unreachable.
Therouterwillsuppresstherouteiftherouteiscurrentlyreachable.
Thepenaltyoftherouteisdecreasedfollowinganexponentialcurve.Afterawhile,
thepenaltydropsbelowthesuppresslimit,buttherouteisnotyetreleased.The
routeisreleasedonlyafterthepenaltydropsfurtherbelowthereuselimit.Inthe
examplehere,therouteflapsagain,furtherincreasingthepenalty.
Intheexamplehere,theroutehasstabilized,andthepenaltygraduallydropsbelow
thereuselimit.Atthatpoint,theBGProuterreleasestheroute,anditcannowbe
selectedasthebestBGPpath.Iftherouterselectsthereleasedrouteasthebest
BGPpath,itisalsopropagatedtoBGPneighborsandusedfordataforwarding.
Whenthepenaltythatisassociatedwitharoutedropsbelowhalfofthereuselimit,
therouterclearsthepenaltyandtheflaphistorythatareassociatedwiththeroute.
Step6
OntheR2router,verifythe10.0.1.0/48entryintheBGPtable.
R2# show ip bgp 10.0.1.48/28
BGP routing table entry for 10.0.1.48/28, version 47
Paths: (1 available, no best path)
Not advertised to any peer
Refresh Epoch 1
100, (suppressed due to dampening)
172.16.12.11 from 172.16.12.11 (10.0.40.1)
Origin IGP, metric 0, localpref 100, valid, external
Dampinfo: penalty 2674, flapped 3 times in 00:04:00, reuse in 00:05:39
Youshouldseethatnetwork10.0.1.0/48receivedvia172.16.12.11neighborgets
suppressedduetodampening.
Step7
OntheR2router,verifythecontentofthewholeBGPtable.
R2# show ip bgp
BGP table version is 47, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/24 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.0.64/28 0.0.0.0 0 32768 i
*> 10.0.1.0/28 172.16.12.11 0 0 100 i
*> 10.0.1.16/28 172.16.12.11 0 0 100 i
*> 10.0.1.32/28 172.16.12.11 0 0 100 i
*d 10.0.1.48/28 172.16.12.11 0 0 100 i
*> 10.0.1.64/28 172.16.12.11 0 0 100 i
*> 10.0.1.80/28 172.16.12.11 0 0 100 i
<... output omitted...>
Youshouldseenetwork10.0.1.0/48markedasdampened.
Step8
OntheR2router,displayalldampenedpaths.
R2# show ip bgp dampening dampened-paths
BGP table version is 47, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network From Reuse Path
*d 10.0.1.48/28 172.16.12.11 00:04:49 100 i
Youshouldseethatnetwork10.0.1.0/48markedasdampenedforapathvia
172.16.12.11neighborinautonomoussystem100.
Step9
OntheR2router,displayflappingstatisticsforBGProutedampening.
R2# show ip bgp dampening flap-statistics
BGP table version is 47, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network From Flaps Duration Reuse Path
*d 10.0.1.48/28 172.16.12.11 3 00:05:20 00:04:19 100
Youshouldseethatnetwork10.0.1.0/48flappedthreetimes.
Step10
OntheR2router,displayBGPdampeningparameters.
R2# show ip bgp dampening parameters
dampening 15 750 2000 60 (DEFAULT)
Half-life time : 15 mins Decay Time : 2320 secs
Max suppress penalty: 12000 Max suppress time: 60 mins
Suppress penalty : 2000 Reuse penalty : 750
YoushouldseethedefaultBGPdampeningparameters,forinstancefor
suppresspenalty.Thedefaultvalueis2000.
ReleasingDampenedRoutes
MonitoringRouteReflectors
router# clear ip bgp ip-address flap-
statistics [{regexp regexp} | {filter-list list-name} | {ip-
address network-mask}]
Clears the flap statistics but does not release dampened routes.
router# clear ip bgp dampening [ip-address network-mask]
Releases all the dampened routes or just the specified network.
Flap statistics or dampened routes are also cleared when the BGP session with
the neighbor is lost.
Therearetwotimersthatarecalculatedforeveryroutewhenitflaps:
Timetoforget:Thetimethatittakesbeforeallflaphistoryisdeleted.Usingthe
clearipbgpflap-statisticscommanddeletesallpenaltyinformation,butitdoes
notreleasethedampenedpaths.
Timetoreuse:Thetimethatittakesbeforearoutecanbeconsideredforbest-
pathprocessing.Usingtheclearipbgpdampeningcommandresetsthistimer
forallnetworksorforspecifiednetworkssothattheyarenolongersuppressed.
Theflapstatistics,however,arestillkept,andthenextflapwillcausethe
previouslydampenedpathstobesuppressedagain.
clearipbgpflap-statistics
ToclearBGPflapstatistics,usetheclearipbgpflap-statisticsprivilegedEXEC
command.
clearipbgpip-addressflap-statistics[{regexpregexp}|{filter-listlist-name}|
{ip-addressnetwork-mask}]
SyntaxDescription
Parameter Description
ip-address (Optional)ClearsflapstatisticsforasingleentryatthisIP
address.Ifthisargumentisplacedbeforeflap-statistics,the
routerclearsflapstatisticsforallpathsfromtheneighboratthis
address.
regexpregexp (Optional)Clearsflapstatisticsforallthepathsthatmatchthe
regularexpression.
filter-listlist-name (Optional)Clearsflapstatisticsforallthepathsthatpassthe
access-list.
network-mask (Optional)Networkmaskthatisappliedtotheip-address
argument.
clearipbgpdampening
ToclearBGProute-dampeninginformationandunsuppressthesuppressedroutes,
usetheclearipbgpdampeningprivilegedEXECcommand.
clearipbgpdampening[ip-addressnetwork-mask]
SyntaxDescription
Parameter Description
ip-address (Optional)ClearsflapstatisticsforasingleentryatthisIP
address.Ifthisargumentisplacedbeforeflap-statistics,the
routerclearsflapstatisticsforallpathsfromtheneighboratthis
address.
network-mask (Optional)Networkmaskthatisappliedtotheip-address
argument.
Step11
CleartheBGPdampeninginformationontheR2router.
R2# clear ip bgp dampening
Step12
OntheR2router,verifythe10.0.1.0/48entryintheBGPtable.
R2# show ip bgp 10.0.1.48/28
BGP routing table entry for 10.0.1.48/28, version 48
Paths: (1 available, best #1, table default)
Advertised to update-groups:
2
Refresh Epoch 1
100
172.16.12.11 from 172.16.12.11 (10.0.40.1)
Origin IGP, metric 0, localpref 100, valid, external, best
Youshouldseethatnetwork10.0.1.0/48receivedvia172.16.12.11neighboris
notsuppressedanymore.
Step13
OntheR2router,verifythecontentofthewholeBGPtable.
R2# show ip bgp
BGP table version is 48, local router ID is 10.0.0.65
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.0/24 0.0.0.0 0 32768 i
*> 10.0.0.16/28 0.0.0.0 0 32768 i
*> 10.0.0.32/28 0.0.0.0 0 32768 i
*> 10.0.0.48/28 0.0.0.0 0 32768 i
*> 10.0.0.64/28 0.0.0.0 0 32768 i
*> 10.0.1.0/28 172.16.12.11 0 0 100 i
*> 10.0.1.16/28 172.16.12.11 0 0 100 i
*> 10.0.1.32/28 172.16.12.11 0 0 100 i
*> 10.0.1.48/28 172.16.12.11 0 0 100 i
*> 10.0.1.64/28 172.16.12.11 0 0 100 i
*> 10.0.1.80/28 172.16.12.11 0 0 100 i
<...output omitted...>
Youshouldseethatnetwork10.0.1.0/48isnotmarkedasdampenedanymore.
Thistopicsummarizesthekeypointsthatwerediscussedinthislesson.
Route dampening is a BGP feature that is designed to reduce BGP processing
requirements by minimizing the propagation of unstable routes to BGP peers.
A router with route dampening enabled keeps track of all routes and the
penalties that are assigned to them.
Each time a flap occurs, the flapping route receives 1000 penalty points.
If the route penalties reach the suppress limit, the route is no longer forwarded
to other neighbors until the assigned penalty has decayed below the reuse limit.
You can implement route dampening with the bgp dampening command either
globally in the BGP process or selectively using a route-map.
Use the clear ip bgp flap-statistics command to delete all penalty information
without releasing the dampened paths.
The clear ip bgp dampening command clears dampening information and
releases suppressed routes.
The show ip bgp dampened-paths command lists all routes that are currently
suppressed because of dampening, the
Summary
Overview
Thistopicsummarizesthekeypointsthatwerediscussedinthismodule.
Increased BGP convergence time and high CPU utilization caused by the BGP
scanner and BGP router processes can be reduced with Cisco IOS tools such
as PMTU discovery, BGP PIC, BFD, as well as by adjusting BGP timers and
intervals..
The maximum-prefix function is a scalable solution that limits the number of
routes that a BGP router can receive from a specific neighbor.
Peer groups are a fundamental BGP scalability tool and should be used in all
environments where a router has a large number of BGP neighbors.
Route dampening is a BGP feature that is designed to reduce BGP processing
requirements by minimizing the propagation of unstable routes to BGP peers.
ModuleSummary
Foradditionalinformation,refertotheseresources:
CiscoSystems,Inc.BGPRestartNeighborSessionAfterMaximum-PrefixLimit
Reached.
http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/15-
mt/irg-15-mt-book/BGP-Sub-Codes-for-BGP-Cease-Notification.pdf
CiscoSystems,Inc.BGPPeerGroups.
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-
bgp/13755-29.html
CiscoSystems,Inc.BGPCaseStudies"BGPCaseStudies4."
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-
bgp/26634-bgp-toc.html#case4
References
Usethequestionsheretoreviewwhatyoulearnedinthismodule.Thecorrect
answersandsolutionsarefoundintheModuleSelf-CheckAnswerKey.
ModuleSelf-Check
1. WhatarethreecharacteristicsofaconvergedBGPnetwork?
(Choosethree.)
Theinputqueueandoutputqueueforallpeersis0.
AllroutesintheBGPtablehavebeeninstalledintheroutingtable.
ThetableversionforallpeersequalsthetableversionoftheBGPtable.
Allrouteshavebeenaccepted.
1. WhichtwoofthefollowingmodificationsresultinimprovedBGP
convergence?(Choosetwo.)(Source:"ImprovingBGP
Convergence")
IncreasingthedefaultvalueofBGPholdtime.
LoweringthedefaultvalueofBGPscantime.
Increasingthedefaultvalueoftheneighboradvertisementintervals.
Loweringthedefaultvalueoftheneighboradvertisementintervals.
1. WhatisthemaintaskoftheBGPscannerprocess?(Source:
"ImprovingBGPConvergence")
SendsroutingupdatestoBGPneighbors.
WalkstheBGPtableforroutestoenterintotheIProutingtable.
ConfirmsthereachabilityofBGPnexthops.
ScanstherouterconfigurationtoestablishandmaintainBGPneighbors.
1. OneofyourBGPcoreroutersisexperiencingperiodicslow
responsestopingpacketsthatarebeingdirectedtoitfromthe
networkmanagementconsole.Therouterhasjustbeenconfigured
toreceivefullInternetroutes,andyoususpectthattheBGProuter
processiscausingCPUutilizationissuesinthecorerouter.Which
tworoutercommandsshouldyouusetoconfirmyoursuspicion?
(Choosetwo.)(Source:"ImprovingBGPConvergence")
showiproute
showipbgpsummary
showprocesscpu
showmemory
1. LookattheoutputoftheshowinterfacesEthernet0/0command.
HowhastheinterfacebeenmodifiedtoimproveBGPconvergence?
(Source:"ImprovingBGPConvergence")router#showinterfaces
Ethernet0/0
Ethernet0/0isup,lineprotocolisup
HardwareisDEC21140,addressis0000.0c0c.1111(bia
0002.eaa3.5a60)
Internetaddressis112.64.101.17255.255.255.240
MTU1460bytes,BW100000Kbit,DLY100usec,rely255/255,load
200/255
EncapsulationARPA,loopbacknotset,keepalivenotset,hdx,
100BaseTX
ARPtype:ARPA,ARPTimeout4:00:00
Lastinputnever,output0:00:16,outputhang0:28:01
Lastclearingof"showinterface"counters0:20:05
Outputqueue25/40,0drops;inputqueue50/500,1470drops
5minuteinputrate21666400bits/sec,1855packets/sec
5minuteoutputrate72221bits/sec,618packets/sec
Theoutputqueuehasbeendecreasedtoexpeditepacketforwardingout
theFastEthernetinterface.
Thedropthresholdoftheinputqueuehasbeensettobeginrandomly
discardingpacketsafterthequeuereaches50packetsdeep.
PMTUdiscoveryhasbeenenabledbysettingtheinterfaceMSSto1460
bytes.
PMTUdiscoveryhasbeenenabledbysettingtheinterfaceMSSto1460
bytes.
1. Refertotheoutput.Whichtwoparameterswouldindicatethatthe
BGPnetworkhasconverged?(Choosetwo.)(Source:"Improving
BGPConvergence")router#showipbgpsummary
BGProuteridentifier172.16.0.4,localASnumber1
BGPtableversionis16,mainroutingtableversion16
20networkentriesand20pathsusing2826bytesofmemory
8BGPpathattributeentriesusing480bytesofmemory
7BGPAS-PATHentriesusing168bytesofmemory
3BGPcommunityentriesusing72bytesofmemory
0BGProute-mapcacheentriesusing0bytesofmemory
3BGPfilter-listcacheentriesusing36bytesofmemory
BGPactivity20/0prefixes,24/4paths,scaninterval120secs
NeighborVASMsgRcvdMsgSentTblVerInQOutQ
Up/DownState/PfxRcd
172.16.0.1413030160000:23:135
172.16.0.2413330160000:23:155
172.16.0.3412730160000:23:145
192.168.21.994993135160000:23:045
TheTblVerforallneighborsis16.
Vissetto4forallneighbors.
TheInQandOutQforallneighborsis0.
AllneighborsareintheEstablishedstateandhavethesamePfxRcd
value.
1. Refertotheoutput.HowfrequentlytheBGPscannerprocesswill
runontherouter?(Source:"ImprovingBGPConvergence")router#
showipbgpsummary
BGProuteridentifier172.16.0.4,localASnumber1
BGPtableversionis16,mainroutingtableversion16
20networkentriesand20pathsusing2826bytesofmemory
8BGPpathattributeentriesusing480bytesofmemory
7BGPAS-PATHentriesusing168bytesofmemory
3BGPcommunityentriesusing72bytesofmemory
0BGProute-mapcacheentriesusing0bytesofmemory
3BGPfilter-listcacheentriesusing36bytesofmemory
BGPactivity20/0prefixes,24/4paths,scaninterval120secs
NeighborVASMsgRcvdMsgSentTblVerInQOutQ
Up/DownState/PfxRcd
172.16.0.1413030160000:23:135
172.16.0.2413330160000:23:155
172.16.0.3412730160000:23:145
192.168.21.994993135160000:23:045
Bydefault,theprocesswillrunevery60seconds.
Theprocesshasrun16timesandwillrunagainwhenthenextBGP
updatearrives.
Theprocesswillrunonthisrouterevery120seconds.
Itcannotbedeterminedfromthisoutput.
1. Whataretwopotentialissuesthatarecausedbymodifyingthe
defaultscantimeandadvertisementintervalonaBGProuter?
(Choosetwo.)(Source:"ImprovingBGPConvergence")
RouterCPUresourcescanbeexhausted.
Routermemoryresourcescanbedepleted.
Routingloopsaremorelikely.
BGPcouldconvergefasterthantheIGPandcausenetworkblackholes.
1. WhichoptioncanbeusedtosupporttrafficforwardingduringRP
switchover?(Source:"ImprovingBPGConvergence")
NonstopForwarding
CiscoExpressForwarding
PerfectForwardingRouting
1. Loweringthekeepalivetimerbetweensentkeepalivemessagestoa
BGPneighborwillimproveBGPconvergence.Trueorfalse?
(Source:"ImprovingBGPConvergence")
True
False
1. WhatarethreereasonstolimitthenumberofBGPprefixesthatare
receivedfromaneighbor?(Choosethree.)(Source:"Limitingthe
NumberofPrefixesReceivedfromaBGPNeighbor")
Topreventdenial-of-serviceattacks.
Toprotectagainstincorrectrouterconfigurationontheneighborside.
TopreventredundantroutinginformationfrombeingloadedintotheBGP
table.
ToavoidoverloadingroutermemoryandCPUresources.
1. InwhichtwosituationswouldadirectlyconnectedBGPneighbor
stayintheIdlestate?(Choosetwo.)(Source:"LimitingtheNumber
ofPrefixesReceivedfromaBGPNeighbor")
Theneighborhasexceededthemaximumnumberofallowedprefixes.
Themaximum-prefixthresholdhasbeenreached.
Therestartoptionhasnotbeenspecifiedwiththemaximum-prefix
command.
Theneighborismorethanonehopaway.
1. Whichtwoofthefollowingcharacteristicsaccuratelydescribethe
showipbgpneighborscommand?(Choosetwo.)(Source:
"LimitingtheNumberofPrefixesReceivedfromaBGPNeighbor")
Forneighborswiththemaximum-prefixfunctionconfigured,displaysthe
maximumnumberofprefixesandthewarningthreshold.
Forneighborsexceedingthemaximumnumberofprefixes,displaysthe
reasonthattheBGPsessionisidle.
Forneighborswithunstableroutes,displaysthefeasiblesuccessorfor
thoseroutes.
Forneighborsinconfederations,displaystheroutereflectorstatusof
thoseneighbors.
1. WhatistheneedforBGPpeergroups?(Source:"Implementing
BGPPeerGroups")
Canbeusedtoconfigurethesamesetofparametersforanumberof
BGPneighborsinacommontemplate.
CanbeusedtoallowanonymousBGPneighbors.
AllowEBGPpeerstobeconfiguredwiththesameASnumberand
parameters.
CanbeusedtohidetheidentityofBGPpeersfromexternalneighbors.
1. WhichofthefollowingstatementsaboutthebenefitofBGPpeer
groupsisaccurate?(Source:"ImplementingBGPPeerGroups")
WithBGPpeergroups,alloftherouterCPUutilizationthatisimposedby
BGPupdategenerationissignificantlyreduced.
WithBGPpeergroups,someoftherouterCPUutilizationthatisimposed
byBGPupdategenerationissignificantlyreduced.
Networkadministratorsshouldusepeergroupstomakesmallernetworks
moreproductive.
WithBGPpeergroups,neighborrelationshipsareautomaticallycreated.
1. WhataretwolimitationsofBGPpeergroupsonCiscorouters?
(Choosetwo.)(Source:"ImplementingBGPPeerGroups")
EBGPandIBGPneighborscannotbemembersofthesamepeergroup.
AllroutersinthepeergroupmustbelongtothesameAS.
Peergroupmemberscannotcontaindifferentoutboundfiltering
mechanisms.
Peergroupmembersmusthavethesameinboundfilteringmechanisms.
1. WhichtwoofthefollowingcharacteristicsaccuratelydescribeBGP
peergroups?(Choosetwo.)(Source:"ImplementingBGPPeer
Groups")
ABGPpeergroupcreatesaneighborparametertemplate.
Whenactualneighboringroutersareassignedtothepeergroupona
router,alloftheattributesthatareconfiguredforthepeergroupare
appliedtoselectedpeergroupmembers.
Oneoftheconfigurableparametersincludescommunitypropagation.
Individualparametersspecifiedinapeergroupcannotbeoverriddenona
neighbor-by-neighborbasis.
1. Whichtwoofthefollowingcharacteristicsaccuratelydescribethe
functionoftheBGPDynamicUpdatePeer-Groupsfeature?(Choose
two.)(Source:"ImplementingBGPPeerGroups")
Doesnotprovidetheoperatorwithtimetochangetheconfigurationifa
mistakeismade.
SeparatesBGPupdategenerationfrompeergroupconfiguration.
Doesnotrequireanyconfigurationbythenetworkoperator.
Requiresminimalconfigurationbythenetworkoperator.
1. Whichtwoofthefollowingstatementsaccuratelydescribethe
functionoftheBGPpeertemplatesfeature?(Choosetwo.)(Source:
"ImplementingBGPPeerGroups")
NetworkoperatorsmuststillconfiguresomepeergroupsinBGP,evenif
usingtheBGPConfigurationUsingPeerTemplatesfeature.
Peertemplatesovercomealllimitationsofpeergroups.
Peertemplatesimprovetheflexibilityandenhancethecapabilityof
neighborconfiguration.
Youcanchaintogetherpeertemplateconfigurationstocreatesimpleor
complexconfigurations.
1. WhatarethreestepsthatarerequiredtoproperlyconfigureBGP
peergroupsonCiscorouters?(Choosethree.)(Source:
"ImplementingBGPPeerGroups")
SpecifyparametersfortheBGPpeergroup
CreateaBGPpeergroup
EnablethepeergroupbyclearingtheBGPsession
Assignaneighborintothepeergroup
1. Whichcommanddoyouusetodisplaythesummarystatusofall
neighborsinapeergroup?(Source:"ImplementingBGPPeer
Groups")
showipbgp
showpeer-groupsummary
showipbgpneighbor
showipbgppeer-groupsummary
1. WhichtwodescriptionsofthepurposeofBGProutedampeningare
accurate?(Choosetwo.)(Source:"UsingBGPRouteDampening")
AtooldesignedtohelpminimizethenumberofBGPupdates
Suppressesroutesthatoccasionallyflap
Designedtoreducerouterprocessingloadcausedbyunstableroutes
Preventssustainedroutingoscillation,withsomeeffectonotherwell-
behavedroutes
1. WhichtwomechanismsarebuiltintoBGPtomakeitmorescalable
byreducingtheroute-processingrequirementsofBGProuters?
(Choosetwo.)(Source:"UsingBGPRouteDampening")
Splithorizon
Routedampening
Synchronization
Per-neighborupdatetimers
1. WhataretwothingsthathappentoanEBGProutethathasbecome
unreachablewhenBGProutedampeningisused?(Choosetwo.)
(Source:"UsingBGPRouteDampening")
ItisremovedfromtheIProutingtable.
ItisremovedfromtheBGPtable.
ItremainsintheIProutingtableaslongasitspenaltyremainsgreater
than50percentofthereuselimit.
ItiskeptintheBGPtableandmarkedasahistoryentry.
1. WhataretwomethodsofenablingroutedampeningonaCisco
router?(Choosetwo.)(Source:"UsingBGPRouteDampening",
Topic:"ConfiguringBGPRouteDampening")
Globally,byenablingroutedampeninginglobalrouterconfigurationmode
Globally,byenablingroutedampeningundertheBGProutingprocess
Onspecificroutesbyenablingroutedampeningonaspecificinterface
Byusingaroute-mapintheBGPprocesstoapplyroutedampeningto
specificroutes
1. WhichtwothingscouldhappentoaBGProutethatispenalized
abovethereuselimitbuthasanassignedpenaltythatisunderthe
suppresslimit?(Choosetwo.)(Source:"UsingBGPRoute
Dampening")
TherouteissuppressedfromBGPupdatesifitisreachable.
TherouteismarkedasahistoryentryintheBGPtable.
TherouteiswithdrawnfromtheIProutingtable.
Theroutecontinuestobeadvertised.
1. WhatarethreecharacteristicsofaconvergedBGPnetwork?
(Choosethree.)
Theinputqueueandoutputqueueforallpeersis0.
AllroutesintheBGPtablehavebeeninstalledintheroutingtable.
ThetableversionforallpeersequalsthetableversionoftheBGPtable.
Allrouteshavebeenaccepted.
AnswerKey
1. WhichtwoofthefollowingmodificationsresultinimprovedBGP
convergence?(Choosetwo.)(Source:"ImprovingBGP
Convergence")
IncreasingthedefaultvalueofBGPholdtime.
LoweringthedefaultvalueofBGPscantime.
Increasingthedefaultvalueoftheneighboradvertisementintervals.
Loweringthedefaultvalueoftheneighboradvertisementintervals.
1. WhatisthemaintaskoftheBGPscannerprocess?(Source:
"ImprovingBGPConvergence")
SendsroutingupdatestoBGPneighbors.
WalkstheBGPtableforroutestoenterintotheIProutingtable.
ConfirmsthereachabilityofBGPnexthops.
ScanstherouterconfigurationtoestablishandmaintainBGPneighbors.
1. OneofyourBGPcoreroutersisexperiencingperiodicslow
responsestopingpacketsthatarebeingdirectedtoitfromthe
networkmanagementconsole.Therouterhasjustbeenconfigured
toreceivefullInternetroutes,andyoususpectthattheBGProuter
processiscausingCPUutilizationissuesinthecorerouter.Which
tworoutercommandsshouldyouusetoconfirmyoursuspicion?
(Choosetwo.)(Source:"ImprovingBGPConvergence")
showiproute
showipbgpsummary
showprocesscpu
showmemory
1. LookattheoutputoftheshowinterfacesEthernet0/0command.
HowhastheinterfacebeenmodifiedtoimproveBGPconvergence?
(Source:"ImprovingBGPConvergence")router#showinterfaces
Ethernet0/0
Ethernet0/0isup,lineprotocolisup
HardwareisDEC21140,addressis0000.0c0c.1111(bia
0002.eaa3.5a60)
Internetaddressis112.64.101.17255.255.255.240
MTU1460bytes,BW100000Kbit,DLY100usec,rely255/255,load
200/255
EncapsulationARPA,loopbacknotset,keepalivenotset,hdx,
100BaseTX
ARPtype:ARPA,ARPTimeout4:00:00
Lastinputnever,output0:00:16,outputhang0:28:01
Lastclearingof"showinterface"counters0:20:05
Outputqueue25/40,0drops;inputqueue50/500,1470drops
5minuteinputrate21666400bits/sec,1855packets/sec
5minuteoutputrate72221bits/sec,618packets/sec
Theoutputqueuehasbeendecreasedtoexpeditepacketforwardingout
theFastEthernetinterface.
Thedropthresholdoftheinputqueuehasbeensettobeginrandomly
discardingpacketsafterthequeuereaches50packetsdeep.
PMTUdiscoveryhasbeenenabledbysettingtheinterfaceMSSto1460
bytes.
PMTUdiscoveryhasbeenenabledbysettingtheinterfaceMSSto1460
bytes.
1. Refertotheoutput.Whichtwoparameterswouldindicatethatthe
BGPnetworkhasconverged?(Choosetwo.)(Source:"Improving
BGPConvergence")router#showipbgpsummary
BGProuteridentifier172.16.0.4,localASnumber1
BGPtableversionis16,mainroutingtableversion16
20networkentriesand20pathsusing2826bytesofmemory
8BGPpathattributeentriesusing480bytesofmemory
7BGPAS-PATHentriesusing168bytesofmemory
3BGPcommunityentriesusing72bytesofmemory
0BGProute-mapcacheentriesusing0bytesofmemory
3BGPfilter-listcacheentriesusing36bytesofmemory
BGPactivity20/0prefixes,24/4paths,scaninterval120secs
NeighborVASMsgRcvdMsgSentTblVerInQOutQ
Up/DownState/PfxRcd
172.16.0.1413030160000:23:135
172.16.0.2413330160000:23:155
172.16.0.3412730160000:23:145
192.168.21.994993135160000:23:045
TheTblVerforallneighborsis16.
Vissetto4forallneighbors.
TheInQandOutQforallneighborsis0.
AllneighborsareintheEstablishedstateandhavethesamePfxRcd
value.
1. Refertotheoutput.HowfrequentlytheBGPscannerprocesswill
runontherouter?(Source:"ImprovingBGPConvergence")router#
showipbgpsummary
BGProuteridentifier172.16.0.4,localASnumber1
BGPtableversionis16,mainroutingtableversion16
20networkentriesand20pathsusing2826bytesofmemory
8BGPpathattributeentriesusing480bytesofmemory
7BGPAS-PATHentriesusing168bytesofmemory
3BGPcommunityentriesusing72bytesofmemory
0BGProute-mapcacheentriesusing0bytesofmemory
3BGPfilter-listcacheentriesusing36bytesofmemory
BGPactivity20/0prefixes,24/4paths,scaninterval120secs
NeighborVASMsgRcvdMsgSentTblVerInQOutQ
Up/DownState/PfxRcd
172.16.0.1413030160000:23:135
172.16.0.2413330160000:23:155
172.16.0.3412730160000:23:145
192.168.21.994993135160000:23:045
Bydefault,theprocesswillrunevery60seconds.
Theprocesshasrun16timesandwillrunagainwhenthenextBGP
updatearrives.
Theprocesswillrunonthisrouterevery120seconds.
Itcannotbedeterminedfromthisoutput.
1. Whataretwopotentialissuesthatarecausedbymodifyingthe
defaultscantimeandadvertisementintervalonaBGProuter?
(Choosetwo.)(Source:"ImprovingBGPConvergence")
RouterCPUresourcescanbeexhausted.
Routermemoryresourcescanbedepleted.
Routingloopsaremorelikely.
BGPcouldconvergefasterthantheIGPandcausenetworkblackholes.
1. WhichoptioncanbeusedtosupporttrafficforwardingduringRP
switchover?(Source:"ImprovingBPGConvergence")
NonstopForwarding
CiscoExpressForwarding
PerfectForwardingRouting
1. Loweringthekeepalivetimerbetweensentkeepalivemessagestoa
BGPneighborwillimproveBGPconvergence.Trueorfalse?
(Source:"ImprovingBGPConvergence")
True
False
1. WhatarethreereasonstolimitthenumberofBGPprefixesthatare
receivedfromaneighbor?(Choosethree.)(Source:"Limitingthe
NumberofPrefixesReceivedfromaBGPNeighbor")
Topreventdenial-of-serviceattacks.
Toprotectagainstincorrectrouterconfigurationontheneighborside.
TopreventredundantroutinginformationfrombeingloadedintotheBGP
table.
ToavoidoverloadingroutermemoryandCPUresources.
1. InwhichtwosituationswouldadirectlyconnectedBGPneighbor
stayintheIdlestate?(Choosetwo.)(Source:"LimitingtheNumber
ofPrefixesReceivedfromaBGPNeighbor")
Theneighborhasexceededthemaximumnumberofallowedprefixes.
Themaximum-prefixthresholdhasbeenreached.
Therestartoptionhasnotbeenspecifiedwiththemaximum-prefix
command.
Theneighborismorethanonehopaway.
1. Whichtwoofthefollowingcharacteristicsaccuratelydescribethe
showipbgpneighborscommand?(Choosetwo.)(Source:
"LimitingtheNumberofPrefixesReceivedfromaBGPNeighbor")
Forneighborswiththemaximum-prefixfunctionconfigured,displaysthe
maximumnumberofprefixesandthewarningthreshold.
Forneighborsexceedingthemaximumnumberofprefixes,displaysthe
reasonthattheBGPsessionisidle.
Forneighborswithunstableroutes,displaysthefeasiblesuccessorfor
thoseroutes.
Forneighborsinconfederations,displaystheroutereflectorstatusof
thoseneighbors.
1. WhatistheneedforBGPpeergroups?(Source:"Implementing
BGPPeerGroups")
Canbeusedtoconfigurethesamesetofparametersforanumberof
BGPneighborsinacommontemplate.
CanbeusedtoallowanonymousBGPneighbors.
AllowEBGPpeerstobeconfiguredwiththesameASnumberand
parameters.
CanbeusedtohidetheidentityofBGPpeersfromexternalneighbors.
1. WhichofthefollowingstatementsaboutthebenefitofBGPpeer
groupsisaccurate?(Source:"ImplementingBGPPeerGroups")
WithBGPpeergroups,alloftherouterCPUutilizationthatisimposedby
BGPupdategenerationissignificantlyreduced.
WithBGPpeergroups,someoftherouterCPUutilizationthatisimposed
byBGPupdategenerationissignificantlyreduced.
Networkadministratorsshouldusepeergroupstomakesmallernetworks
moreproductive.
WithBGPpeergroups,neighborrelationshipsareautomaticallycreated.
1. WhataretwolimitationsofBGPpeergroupsonCiscorouters?
(Choosetwo.)(Source:"ImplementingBGPPeerGroups")
EBGPandIBGPneighborscannotbemembersofthesamepeergroup.
AllroutersinthepeergroupmustbelongtothesameAS.
Peergroupmemberscannotcontaindifferentoutboundfiltering
mechanisms.
Peergroupmembersmusthavethesameinboundfilteringmechanisms.
1. WhichtwoofthefollowingcharacteristicsaccuratelydescribeBGP
peergroups?(Choosetwo.)(Source:"ImplementingBGPPeer
Groups")
ABGPpeergroupcreatesaneighborparametertemplate.
Whenactualneighboringroutersareassignedtothepeergroupona
router,alloftheattributesthatareconfiguredforthepeergroupare
appliedtoselectedpeergroupmembers.
Oneoftheconfigurableparametersincludescommunitypropagation.
Individualparametersspecifiedinapeergroupcannotbeoverriddenona
neighbor-by-neighborbasis.
1. Whichtwoofthefollowingcharacteristicsaccuratelydescribethe
functionoftheBGPDynamicUpdatePeer-Groupsfeature?(Choose
two.)(Source:"ImplementingBGPPeerGroups")
Doesnotprovidetheoperatorwithtimetochangetheconfigurationifa
mistakeismade.
SeparatesBGPupdategenerationfrompeergroupconfiguration.
Doesnotrequireanyconfigurationbythenetworkoperator.
Requiresminimalconfigurationbythenetworkoperator.
1. Whichtwoofthefollowingstatementsaccuratelydescribethe
functionoftheBGPpeertemplatesfeature?(Choosetwo.)(Source:
"ImplementingBGPPeerGroups")
NetworkoperatorsmuststillconfiguresomepeergroupsinBGP,evenif
usingtheBGPConfigurationUsingPeerTemplatesfeature.
Peertemplatesovercomealllimitationsofpeergroups.
Peertemplatesimprovetheflexibilityandenhancethecapabilityof
neighborconfiguration.
Youcanchaintogetherpeertemplateconfigurationstocreatesimpleor
complexconfigurations.
1. WhatarethreestepsthatarerequiredtoproperlyconfigureBGP
peergroupsonCiscorouters?(Choosethree.)(Source:
"ImplementingBGPPeerGroups")
SpecifyparametersfortheBGPpeergroup
CreateaBGPpeergroup
EnablethepeergroupbyclearingtheBGPsession
Assignaneighborintothepeergroup
1. Whichcommanddoyouusetodisplaythesummarystatusofall
neighborsinapeergroup?(Source:"ImplementingBGPPeer
Groups")
showipbgp
showpeer-groupsummary
showipbgpneighbor
showipbgppeer-groupsummary
1. WhichtwodescriptionsofthepurposeofBGProutedampeningare
accurate?(Choosetwo.)(Source:"UsingBGPRouteDampening")
AtooldesignedtohelpminimizethenumberofBGPupdates
Suppressesroutesthatoccasionallyflap
Designedtoreducerouterprocessingloadcausedbyunstableroutes
Preventssustainedroutingoscillation,withsomeeffectonotherwell-
behavedroutes
1. WhichtwomechanismsarebuiltintoBGPtomakeitmorescalable
byreducingtheroute-processingrequirementsofBGProuters?
(Choosetwo.)(Source:"UsingBGPRouteDampening")
Splithorizon
Routedampening
Synchronization
Per-neighborupdatetimers
1. WhataretwothingsthathappentoanEBGProutethathasbecome
unreachablewhenBGProutedampeningisused?(Choosetwo.)
(Source:"UsingBGPRouteDampening")
ItisremovedfromtheIProutingtable.
ItisremovedfromtheBGPtable.
ItremainsintheIProutingtableaslongasitspenaltyremainsgreater
than50percentofthereuselimit.
ItiskeptintheBGPtableandmarkedasahistoryentry.
1. WhataretwomethodsofenablingroutedampeningonaCisco
router?(Choosetwo.)(Source:"UsingBGPRouteDampening",
Topic:"ConfiguringBGPRouteDampening")
Globally,byenablingroutedampeninginglobalrouterconfigurationmode
Globally,byenablingroutedampeningundertheBGProutingprocess
Onspecificroutesbyenablingroutedampeningonaspecificinterface
Byusingaroute-mapintheBGPprocesstoapplyroutedampeningto
specificroutes
1. WhichtwothingscouldhappentoaBGProutethatispenalized
abovethereuselimitbuthasanassignedpenaltythatisunderthe
suppresslimit?(Choosetwo.)(Source:"UsingBGPRoute
Dampening")
TherouteissuppressedfromBGPupdatesifitisreachable.
TherouteismarkedasahistoryentryintheBGPtable.
TherouteiswithdrawnfromtheIProutingtable.
Theroutecontinuestobeadvertised.
Glossary
ACK
acknowledgment. Notification sent from one network device to another to acknowledge that some event occurred (for
example,thereceiptofamessage).
AD
administrative distance. Rating of the trustworthiness of a routing information source. Administrative distance often is
expressedasanumericalvaluebetween0and255.Thehigherthevalue,thelowerthetrustworthinessrating.
AFI
AddressFamilyIdentifier.
ARIN
American Registry for Internet Numbers. A nonprofit organization that administers and registers IP numbers for the
geographicalareasthatarecurrentlymanagedbyNetworkSolutions(InterNIC).Thoseareasinclude,butarenotlimitedto,
NorthAmerica,SouthAmerica,SouthAfrica,andtheCaribbean.
ARP
AddressResolutionProtocol.InternetprotocolthatisusedtomapanIPaddresstoaMACaddress.DefinedinRFC826.
AS
autonomous system. A collection of networks under a common administration sharing a common routing strategy.
Autonomoussystems aresubdividedby areas.An autonomous systemmustbe assigned a unique 16-bit number by the
IANA.
ATM
AsynchronousTransferMode.Theinternationalstandardforcellrelayinwhichmultipleservicetypes(suchasvoice,video,
ordata)areconveyedinfixed-length(53-byte)cells.Fixed-lengthcellsallowcellprocessingtooccurinhardware,thereby
reducingtransitdelays.ATMisdesignedtotakeadvantageofhigh-speedtransmissionmedia,suchasE3,SONET,andT3.
BFD
Bidirectional Forwarding Detection. A detection protocol that is designed to provide fast forwarding-path failure detection
timesformediatypes,encapsulations,topologies,androutingprotocols.
BGP
Border Gateway Protocol. Interdomain routing protocol that replaces EGP. BGP exchanges reachability information with
otherBGPsystems.ItisdefinedbyRFC1163.
BGP4
BorderGatewayProtocolversion4.
CE
customeredge.Identifiesthenetworkdevices,connectedtoaprovidernetwork,thatareundertheadministrativecontrolof
thecustomer.
CLI
command-lineinterface.Aninterfacethatallowstheusertointeractwiththeoperatingsystembyenteringcommandsand
optionalarguments.TheUNIXoperatingsystemandDOSprovideCLIs.
CPU
central processing unit. The hardware within a computer system or smartphone that carries out the instructions of a
computerprogrambyperformingthebasicarithmetical,logical,andinput-outputoperationsofthesystem.
DF
DonotFragment
DMVPN
DynamicMultipointVPN.
DMZ
demilitarizedzone.
DNS
DomainNameSystem.SystemusedontheInternetfortranslatingnamesofnetworknodesintoaddresses.
DoS
denialofservice.Anintentionalorunintentionalattackonadevicethatmakestheresourceunavailabletoperformitsnormal
function.
DPT
DynamicPacketTransport
DSL
digital subscriber line. Public network technology that delivers high bandwidth over conventional copper wiring at limited
distances. There are four types of DSL:ADSL, HDSL, SDSL, and VDSL. All are provisioned via modem pairs, with one
modemlocatedatacentralofficeandtheotheratthecustomersite.BecausemostDSLtechnologiesdonotusethewhole
bandwidthofthetwistedpair,thereisroomremainingforavoicechannel.
EBGP
ExteriorBorderGatewayProtocol
EGP
Exterior Gateway Protocol. It's the Internet protocol for exchanging routing information between autonomous systems.
Documentedin RFC 904.This isnot to beconfusedwiththe general termexteriorgatewayprotocol.EGPisan obsolete
protocolthatwasreplacedbyBGP.
EIGRP
Enhanced InteriorGateway Routing Protocol.It's the advanced version of IGRPdeveloped by Cisco. Itprovides superior
convergence properties and operating efficiency, and it combines the advantages of link-state protocols with those of
distancevectorprotocols.
Ethernet
Baseband LAN specification invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment
Corporation.EthernetnetworksuseCSMA/CDandrunoveravarietyofcabletypesat10Mbps.Ethernetissimilartothe
IEEE802.3seriesofstandards.ItisthemostcommonlyusedLANtechnologybecauseitsprotocoliseasytounderstand,
implement, manage, andmaintain.Itallowslow-costnetwork implementations,providesextensivetopologicalflexibilityfor
networkinstallation,andguaranteessuccessful interconnectionandoperationofstandards-compliantproducts,regardless
ofmanufacturer.
FastEthernet
Anyofanumberof100-MbpsEthernetspecifications.FastEthernetoffersaspeedincrease10timesthatofthe10BaseT
Ethernetspecificationwhilepreservingsuchqualitiesasframeformat,MACmechanisms,andMTU.Suchsimilaritiesallow
theuseofexisting10BaseTapplicationsandnetworkmanagementtoolsonFastEthernetnetworks.Basedonanextension
totheIEEE802.3specification.
FDDI
Fiber Distributed Data Interface. LAN standard, defined by ANSI X3T9.5, specifying a 100-Mbps token-passing network
usingfiber-opticcable,withtransmissiondistancesofupto2km.FDDIusesadual-ringarchitecturetoprovideredundancy.
FFTx
FiberFromThe"X",whereXstandsfordifferenttypeofdeployment(Building,Home,Premisses,etc.)
FIB
forwardinginformationbase.
FrameRelay
Industry-standard,packet-switcheddatalinklayerprotocolthathandlesmultiplevirtualcircuitsbetweenconnecteddevices.
GigabitEthernet
Standard for a high-speed Ethernet, approved by the IEEE (Institute of Electrical and Electronics Engineers) 802.3z
standardscommitteein1996.
IANA
Internet Assigned Numbers Authority. Organization operated under the auspices of the ISOC as a part of the IAB. IANA
delegatesauthorityforIPaddress-spaceallocationanddomain-nameassignmenttotheInterNICandotherorganizations.
IANAalso maintains a database of assigned protocol identifiers that is used in the TCP/IP stack, including autonomous
systemnumbers.
IBGP
InternalBorderGatewayProtocol.
ICMP
InternetControlMessageProtocol.NetworklayerInternetprotocolthatreportserrorsandprovidesotherinformationthatis
relevanttoIPpacketprocessing.DocumentedinRFC792.
IGP
interiorgatewayprotocol.Internetprotocolusedtoexchangeroutinginformationwithinanautonomoussystem.Examplesof
commonInternetIGPsincludeIGRP,OSPF,andRIP.
IOL
SpecialbuildofCiscoIOSSoftwareforLinux,createdspeciallyforvirtualizedenvironments.
IP
Internet Protocol. Network layer protocol in the TCP/IP stack offering a connectionless internetwork service. IP provides
featuresforaddressing,type-of-servicespecification,fragmentationandreassembly,andsecurity.DefinedinRFC791.
IPaddress
A 32-bit addressassigned tohostsusingTCP/IP.An IPaddress belongsto one offiveclasses(A, B, C, D,or E) and is
writtenas4octetsseparatedbyperiods(dotteddecimalformat).Each addressconsistsofanetworknumber,anoptional
subnetworknumber,andahostnumber.Thenetworkandsubnetworknumberstogetherareusedforrouting,andthehost
numberisusedtoaddressanindividualhostwithinthenetworkorsubnetwork.Asubnetmaskisused toextractnetwork
andsubnetworkinformationfromtheIPaddress.CIDRprovidesanewwayofrepresentingIPaddressesandsubnetmasks.
AlsocalledanInternetaddress.
IPv4
IPversion4.InternetProtocolversion4isthefourthversioninthedevelopmentoftheIPandthefirstversionoftheprotocol
tobewidelydeployed.AlongwithIPv6,IPv4isatthecoreofstandards-basedinternetworkingmethodsoftheInternet.IPv4
isstillusedtoroutemosttrafficacrosstheInternet.IPv4isaconnectionlessprotocolforuseonpacket-switchedlinklayer
networks(forexample,Ethernet).Itoperatesonabest-effortdeliverymodelinthatitdoesnotguaranteedeliveryanddoes
notassurepropersequencingoravoidanceofduplicatedelivery.
IPv6
IPversion6.ReplacementforthecurrentversionofIP(version4).IPv6includessupportforflowIDinthepacketheader,
whichcanbeusedtoidentifyflows.FormerlycalledIPng(nextgeneration).
IS-IS
IntermediateSystem-to-IntermediateSystem.OSIlink-statehierarchicalroutingprotocolbasedonDECnetPhaseVrouting,
wherebyISs(routers)exchangeroutinginformationbasedonasinglemetrictodeterminenetworktopology.
ISP
Internetserviceprovider.CompanythatprovidesInternetaccesstoothercompaniesandindividuals.
LAN
local-areanetwork.Ahigh-speed,low-errordatanetworkcoveringarelativelysmallgeographicarea(uptoafewthousand
meters). LANs connect workstations,peripherals, terminals, and other devices ina single building or othergeographically
limitedarea.LAN standards specifycablingand signalingat thephysicaland datalink layersof theOSImodel.Ethernet,
FDDI,andTokenRingarewidelyusedLANtechnologies.
LSA
link-stateadvertisement.Abroadcastpacketusedbylink-stateprotocolsthatcontainsinformationaboutneighborsandpath
costs.LSAsareusedbythereceivingrouterstomaintaintheirroutingtables.SometimescalledanLSP.
LSP
label-switchedpath.
MAC
MediaAccessControl.ThelowerofthetwosublayersofthedatalinklayerthatisdefinedbytheIEEE.TheMACsublayer
handlesaccesstosharedmedia,suchaswhethertokenpassingorcontentionwillbeused.
MACaddress
astandardizeddatalinklayeraddressthatisrequiredforeveryportordevicethatconnectstoaLAN.Otherdevicesinthe
network use these addresses to locate specific ports in the network and to create and update routing tables and data
structures.AMACaddressis6byteslongandiscontrolledbytheIEEE.Itisalsoknownasahardwareaddress,MAClayer
address,andphysicaladdress.
MD5
MessageDigest5.Aone-wayhashingalgorithmthatproducesa128-bithash.BothMD5andSecureHashAlgorithm(SHA)
are variations on MD4 andaredesigned tostrengthen thesecurity ofthe MD4 hashing algorithm. Cisco uses hashes for
authentication within the IPsec framework. MD5 is also used for message authentication in SNMP v.2. MD5 verifies the
integrityofthecommunication,authenticatestheorigin,andchecksfortimeliness.
MED
multi-exitdiscriminator.
MPLS
MultiprotocolLabelSwitching.aswitchingmethodthatforwardsIPtrafficusingalabel.Thislabelinstructstheroutersand
theswitchesinthenetworkwheretoforwardthepacketsbasedonpre-establishedIProutinginformation.
MSS
maximumsegmentsize.ATCPparameterthatspecifiesthemaximumamountofdatathatadevicecanreceiveinasingle
TCPsegment.
MTU
maximumtransmissionunit.Themaximumpacketsize,inbytes,thataparticularinterfacecanhandle.
NAT
Network Address Translation. A mechanism for reducing the need for globally unique IP addresses. NAT allows an
organization with addresses that are not globally unique to connect to the Internet by translating these addresses into
globallyroutableaddressspace.AlsoknownasNetworkAddressTranslator.
NHT
Nexthoptracking
NLRI
NetworkLayerReachabilityInformation.
NSF
NonstopForwarding.
ORF
outboundroutefilter.
OSPF
Open Shortest Path First. Link-state, hierarchical IGP routing algorithm proposed as a successor to RIP in the Internet
community. OSPF features include least-cost routing, multipath routing, and load balancing. OSPF was derived from an
earlyversionoftheIS-ISprotocol.
PA
provider-assigned.
PAaddress
provider-assignedaddress.
PE
provideredge.Identifiesthenetworkdevices,undertheadministrativecontroloftheprovider,thatconnecttoCEdevices.
PI
provider-independent.
PIaddress
provider-independentaddress.
PIC
Prefix-IndependentConvergence.
PMTU
pathmaximumtransmissionunit.
P-network
providernetwork.
POI
pointofinsertion.
POP
PostOfficeProtocol.Protocolthatclientemailapplicationsusetoretrievemailfromamailserver.
Prouter
providerrouter.
PVC
permanent virtual circuit (or connection). Virtual circuit that is permanently established. PVCs save bandwidth associated
with circuit establishment and tear down in situations where certain virtual circuits must exist all the time. In ATM
terminology,calledapermanentvirtualconnection.
QoS
quality of service. Measure of performance for a transmission system that reflects its transmission quality and service
availability.
RFC
Request for Comments. Document series that is used as the primary means for communicating information about the
Internet.SomeRFCsaredesignatedbytheIABasInternetstandards.MostRFCsdocumentprotocolspecifications,suchas
TelnetandFTP,butsomeRFCsarehumorousorhistorical.RFCsareavailableonlinefromnumeroussources.
RIB
routinginformationbase.
RIP
RoutingInformationProtocol.Adistance-vectorroutingprotocolthatuseshopcountasaroutingmetric.
RIPE
ReseauxIPEuropeens.GroupformedtocoordinateandpromoteTCP/IP-basednetworksinEurope.
RP
routeprocessor.
RST
reset.Atypeofmessage.
RT
routetarget.
SAFI
SubsequentAddressFamilyIdentifier.
SOO
SiteofOrigin.
SPD
SelectivePacketDiscard.
SSO
StatefulSwitchover.
SYN
synchronization.
SYN-ACK
synchronization-acknowledgment.
TCP
Transmission Control Protocol. Connection-oriented transport layer protocol that provides reliable full-duplex data
transmission.TCPispartoftheTCP/IPprotocolstack.
TCP/IP
TransmissionControlProtocol/InternetProtocol.CommonnameforthesuiteofprotocolsdevelopedbytheU.S.DoDinthe
1970stosupporttheconstructionofworldwideinternetworks.TCPandIParethetwobest-knownprotocolsinthesuite.
TLV
type,length,value.
TTL
TimetoLive.Amechanismthatlimitsthelifespanorlifetimeofdatainacomputerornetwork.
UNIX
operating system developed in 1969 at Bell Laboratories. UNIX has gone through several iterations since its inception.
These include UNIX 4.3 BSD (Berkeley Standard Distribution), developed at the University of California at Berkeley, and
UNIXSystemV,Release4.0,developedbyAT&T.
VPN
virtualprivatenetwork.Enables IPtraffictotravel securelyoverapublicTCP/IPnetworkby encrypting alltrafficfrom one
networktoanother.AVPNusestunnelingtoencryptallinformationattheIPlevel.
VPNv4
virtualprivatenetworkversion4.EnablesIPtraffictotravelsecurelyoverapublicTCP/IPnetworkbyencryptingall traffic
fromonenetworktoanother.AVPNusestunnelingtoencryptallinformationattheIPlevel.
VRF
aVPNrouting/forwardinginstance.AVRFconsistsofanIProutingtable,aderivedforwardingtable,asetofinterfacesthat
use the forwarding table, and a set of rules and routing protocols that determine what goes into the forwarding table. In
general,aVRFincludestheroutinginformationthatdefinesacustomerVPNsitethatisattachedtoaPErouter.
WAN
wide-area network. Data communications network that serves users across a broad geographic area and often uses
transmissiondevicesprovidedbycommoncarriers.FrameRelay,SMDS,andX.25areexamplesofWANs.

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