Configuring BGP Routing Policy.docx

Configuring BGP Routing Policy.docx

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ConfiguringBGPRoutingPolicy

ConfiguringBGPRoutingPolicy

RoutingpolicydetermineshowthesystemhandlestheroutesitreceivesfromandsendstoBGPpeersorotherroutingprotocols.Inmanycases,routingpolicyconsistsoffilteringroutes,acceptingcertainroutes,acceptingandmodifyingotherroutes,andrejectingsomeroutes.Youcanthinkofroutingpolicyasawaytocontroltheflowofroutesintoandoutofthesystem.

Youcanuseoneormoreofthefollowingmechanismstoconfigureroutingpolicy:

Accesslists

Prefixtrees

Communitylists

Routemaps

Prefixlists

TheremainderofthissectionprovidesdetailedinformationonusingthesefeatureswithBGP.Beforeproceeding,pleaseseeERXRoutingProtocolsConfigurationGuide,Vol. 1,Chapter 1,ConfiguringRoutingPolicy,forathoroughbackgroundonhowthesefeaturesworkingeneral.

TypesofBGPRouteMaps

Aroutemapconsistsofmatchclausesandsetclauses.Matchclauses,whichconsistofamatchcommand,specifytheattributevaluesthatdeterminewhetheraroutematchestheroutemap.Setclauses,whichconsistofasetcommand,modifythespecifiedattributesofroutesthatpassallmatchclausesintheroutemap.

BGProutemapscanbeappliedtoinboundroutes,outboundroutes,andredistributedroutes.BGProutemapsareoftwotypes,thosethatsupportbothmatchandsetclauses,andthosethatsupportonlymatchclauses.

Thematch-and-setroutemapsconsistoftheroutemapsconfiguredwithanyofthecommandslistedinTable 1-8.

Table 1-8Commandsthatcreatematch-and-setroutemaps 

aggregate-addressattribute-map

redistributeroute-map

bgpdampeningroute-map

table-map

neighborroute-mapin

vrfimport-map

neighborroute-mapout

vrfexport-map

BGPsupportstheclauseslistedinTable 1-9formatch-and-setroutemaps.

Table 1-9ClausessupportedinBGPmatch-and-setroutemaps 

matchas-path

setas-pathprepend

matchcommunity

setcomm-listdelete

matchdistance

setcommunity

matchextcommunity

setdampening

matchipaddress

setextcommunity

matchipnext-hop

setipnext-hop

matchlevel

setlocal-preference

matchmetric

setmetric

matchmetric-type

setmetric-type

matchroute-type

setorigin

matchtag

settag

setweight

Thematch-onlyroutemapsconsistoftheroutemapsconfiguredwithanyofthecommandslistedinTable 1-10.YoucanuseanyofthematchclauseslistedinTable 1-9intheseroutemaps.Setclauseshavenoeffectintheseroutemaps.

Table 1-10Commandsthatcreatematch-onlyroutemaps

aggregateadvertise-map

aggregatesupport-map

BGPdoesnotsupporttheclauseslistedinTable 1-11.However,seethesectionlaterinthischapter,ApplyingTableMaps,forexceptionsforroutemapsappliedviathetable-mapcommand.

Table 1-11ClausesnotsupportedinBGProutemaps 

setautomatic-tag

setlevel

setdistance

setroute-type

    matchas-path

∙UsetomatchanAS-pathaccesslist.

∙TheimplementedweightisbasedonthefirstmatchedASpath.

∙Example

host1（config）#route-mapnyc1permit10

host1（config-route-map）#matchas-pathpathlist5

∙Usethenoversiontodeletethematchclausefromaroutemaporaspecifiedvaluefromthematchclause.

    matchcommunity

∙Usetomatchacommunitylist.

∙Supportedforinboundandoutboundroutemaps.

∙Example

host1（config）#route-mapnyc1permit10

host1（config-route-map）#matchcommunitycomm5

∙Usethenoversiontodeletethematchclausefromaroutemaporaspecifiedvaluefromthematchclause.

    matchdistance

∙Usetomatchanyroutesthathavethespecifiedadministrativedistance.

∙Example

host1（config）#route-mapnyc1permit10

host1（config-route-map）#matchdistance25

∙Usethenoversiontodeletethematchclausefromaroutemaporaspecifiedvaluefromthematchclause.

    matchextcommunity

∙Usetomatchanextendedcommunitylistinaroutemap.

∙Youcanspecifyoneormoreextendedcommunitylistnamesinamatchclause.Ifyouspecifymorethanoneextendedcommunitylist,thelistsarelogicalORed.

∙Example

host1（config）#route-mapnyc1permit10

host1（config-route-map）#matchextcommunitytopeka10

∙Usethenoversiontoremovethematchclausefromaroutemaporaspecifiedvaluefromthematchclause.

    matchipaddress

∙Usetomatchanyroutethathasadestinationnetworknumberthatispermittedbyanaccesslist,aprefixlist,oraprefixtree,orperformspolicyroutingonpackets.

∙Example

host1（config）#route-mapnyc1permit10

host1（config-route-map）#matchipaddressprefix-treeboston

∙Usethenoversiontodeletethematchclausefromaroutemaporaspecifiedvaluefromthematchclause.

    matchipnext-hop

∙Usetomatchanyroutesthathaveanext-hoprouteraddresspassedbythespecifiedaccesslist,prefixlist,orprefixtree.

∙Example

host1（config）#route-mapnyc1permit10

host1（config-route-map）#matchipnext-hop5192.54.24.1

∙Usethenoversiontodeletethematchclausefromaroutemaporaspecifiedvaluefromthematchclause.

    matchlevel

∙Usetomatchroutesforthespecifiedtype.

∙Example

host1（config）#route-mapnyc1permit10

host1（config-route-map）#matchlevel-1

∙Usethenoversiontodeletethematchclausefromaroutemaporaspecifiedvaluefromthematchclause.

    matchmetric

∙Usetomatcharouteforthespecifiedmetricvalue.

∙Example

host1（config）#route-mapnyc1permit10

host1（config-route-map）#matchmetric10

∙Usethenoversiontodeletethematchclausefromaroutemaporaspecifiedvaluefromthematchclause.

    matchmetric-type

∙Usetomatcharouteforthespecifiedmetrictype.

∙Example

host1（config）#route-mapnyc1permit10

host1（config-route-map）#matchmetric-typeexternal

∙Usethenoversiontodeletethematchclausefromaroutemap.

    matchroute-type

∙Usetomatcharouteforthespecifiedroutetype.

∙Example

host1（config）#route-mapnyc1permit10

host1（config-route-map）#matchroute-typelevel-1

∙Usethenoversiontodeletethematchclausefromaroutemaporaspecifiedvaluefromthematchclause.

    matchtag

∙Usetomatchthetagvalueofthedestinationroutingprotocol.

∙Example

host1（config）#route-map1

host1（config-route-map）#matchtag25

∙Usethenoversiontodeletethematchclausefromaroutemaporaspecifiedvaluefromthematchclause.

    neighborroute-map

∙Usetoapplyaroutemaptoincomingoroutgoingroutes.

∙Ifyouspecifyanoutboundroutemap,BGPadvertisesonlyroutesthatmatchatleastonesectionoftheroutemap.

∙Aclausewithmultiplevaluesmatchesaroutehavinganyofthevalues;thatis,themultiplevaluesarelogicalORed.

∙IfyouspecifyaBGPpeergroupbyusingthepeerGroupNameargument,allthemembersofthepeergroupinheritthecharacteristicconfiguredwiththiscommandunlessitisoverriddenforaspecificpeer.However,youcannotconfigureamemberofapeergrouptooverridetheinheritedpeergroupcharacteristicforoutboundpolicy

∙Newpolicyvaluesareappliedtoallroutesthataresent（outboundpolicy）orreceived（inboundpolicy）afteryouissuethecommand.

ToapplythenewpolicytoroutesthatarealreadypresentintheBGPRIB,youmustusetheclearipbgpcommandtoperformasoftclearorhardclearofthecurrentBGPsession.

BehaviorisdifferentforoutboundpoliciesconfiguredforpeergroupsforwhichyouhaveenabledAdj-RIBs-Out.IfyouchangetheoutboundpolicyforsuchapeergroupandwanttofilltheAdj-RIBs-Outtableforthatpeergroupwiththeresultsofthenewpolicy,youmustusetheclearipbgppeer-groupcommandtoperformahardclearoroutboundsoftclearofthepeergroup.YoucannotmerelyperformahardclearoroutboundsoftclearforindividualpeergroupmembersbecausethatcausesBGPtoresendonlythecontentsoftheAdj-RIBs-Outtable.

∙Usethenoversiontoremovetheroutemap.

    route-map

∙Usetodefinetheconditionsforredistributingroutesfromoneroutingprotocolintoanother,andforfilteringormodifyingupdatessenttoorreceivedfrompeers.

∙Eachroute-mapcommandhasalistofmatchandsetcommandsassociatedwithit.

∙Thematchcommandsspecifythematchcriteria—theconditionsunderwhichredistributionisallowedforthecurrentroutemap.

∙Thesetcommandsspecifythesetactions—theredistributionactionstoperformifthecriteriaenforcedbythematchcommandsareset.

∙Useroutemapswhenyouwishtohavedetailedcontroloverhowroutesareredistributedbetweenroutingprocesses.

∙Thedestinationroutingprotocolistheoneyouspecifywiththeroutercommand.

∙Thesourceroutingprotocolistheoneyouspecifywiththeredistributecommand.

∙Aclausewithmultiplevaluesmatchesaroutehavinganyofthevalues;thatis,themultiplevaluesarelogicalORed.

∙IfyouspecifyaBGPpeergroupbyusingthepeerGroupNameargument,allthemembersofthepeergroupinheritthecharacteristicconfiguredwiththiscommandunlessitisoverriddenforaspecificpeer.

∙Usethenoversiontodeletetheroutemap.

    setas-pathprepend

∙UsetomodifyanASpathforBGProutesbyprependingoneormoreASnumbersoralistofASnumberstothepathlist.

∙TheonlyglobalBGPmetricavailabletoinfluencethebest-pathselectionistheAS-pathlength.ByvaryingthelengthoftheASpath,aBGPspeakercaninfluencethebest-pathselectionbyapeerfartheraway.

∙Example

host1（config）#route-mapnyc1permit10

host1（config-route-map）#setas-pathprependlistlist10

∙Usethenoversiontodeletethesetclausefromaroutemap.

    setcomm-listdelete

∙Usetoremovecommunitiesspecifiedbythecommunitylistfromthecommunityattributeofroutesmatchingt