思科ccna杂cisco中文ccnp003.bgp基本配置

BGP路由互连本节内容:BGPCommandsIBGPPeeringIssueBGPNeighborUpdateSourceAddressNext-HopBehavior(AS的下一跳)EBGP多跳特性(MULTIHOP)IBGPandRedistributionIBGPSplitHorizonRuleBGPSynchronizationBGPCommandsrouterbgpautonomous-systemRouter(config)#Thiscommand,withnomands,doesnotactivateBGP.OnlyoneinstanceofBGPcanbeconfiguredontherouteratasingletime.Theautonomoussystemnumberidentifiestheautonomoussystemtowhichtherouterbelongs.Theautonomoussystemnumberinthiscommandiscomparedtotheautonomoussystemnumberslistedinneighborstatementstodetermineiftheneighborisaninternalorexternalneighbor.BGPneighborCommandneighbor{ip-address|peer-group-name}

remote-asautonomous-systemRouter(config-router)#TheneighborcommandactivatesaBGPsessionwith

thisneighbor.Thetermremote-asshowswhatASthisneighborisin.ThisASnumberisusedtodetermineiftheneighborisinternalorexternal.Thiscommandisusedforbothexternalandinternalneighbors.TheIPaddressspecifiedisthedestinationaddressofBGPpacketsgoingtothisneighbor.ThisroutermusthaveanIPpathwaytoreachthisneighbor

beforeitcansetupaBGPrelationship.BGPneighborCommandneighbor{ip-address|peer-group-name}shutdown

Router(config-router)#noneighbor{ip-address|peer-group-name}shutdown

Router(config-router)#AdministrativelybringsdownaBGPneighborUsedformaintenanceandpolicychangestoprevent

routeflappingRe-enablesaBGPneighborthathasbeenadministrativelyshutdownExample:BGPneighborCommandIBGPPeeringIssueBGPNeighborUpdateSourceAddressneighbor{ip-address|peer-group-name}update-sourceinterface-typeinterface-numberRouter(config-router)#ThiscommandallowstheBGPprocesstousetheIPaddressofaspecifiedinterfaceasthesourceIPaddressofallBGP

updatestothatneighbor.Aloopbackinterfaceisusuallyused,asitwillbeavailable

aslongastherouterisoperational.(当有多条物理连接时,可用来容错.)TheIPaddressusedinthiscommandwillbethedestination

IPaddressofallBGPupdatesandshouldbetheloopback

interfaceoftheotherrouter.Theupdate-sourcecommandisnormallyusedonlywith

IBGPneighbors.(只能用于IBGP间,EBGP间必须直接连接.)TheaddressofanEBGPneighbormustbedirectlyconnectedby

default.TheloopbackofanEBGPneighborisnotdirectlyconnected.Example:BGPUsingLoopbackAddressesThenext-hop-selfCommandneighbor{ip-address|peer-group-name}next-hop-selfRouter(config-router)#Forcesallupdatesforthisneighbortobe

advertisedwiththisrouterasthenexthop.TheIPaddressusedforthenext-hop-selfwill

bethesameasthesourceIPaddressofthe

BGPpacket.用来手动强迫BGP使用它自己的IP地址作为下一跳属性参数,而不让BGP协议来选择要使用的下一跳地址.一般在NBMA的网络中很常用.EBGP邻居向IBGP邻居传递时,不改变下一跳；由于BGP是一种AS－BY－AS的路由选择协议；使用缺省设置时，跳数是按AS传递的，所以就存在下一跳的问题。Next-HopBehavior(AS的下一跳)BGPisanAS-by-ASroutingprotocol,notarouter-by-routerroutingprotocol.InBGP,thenexthopdoesnotmeanthenextrouter,itmeans

theIPaddresstoreachthe

nextAS.RouterAadvertises

networkto

routerBinEBGP,with

anexthopof.RouterBadvertises

inIBGPto

routerC,keeping

asthenext-hopaddress.B原封不动的告诉了C,到的下一跳为.会有什么问题出现呢?C为什么下一跳不可达.NextHoponaMultiaccessNetworkThefollowingtakesplaceinamultiaccessnetwork:RouterBadvertises

networkto

routerAinEBGPwith

anexthopof,

not.Thisavoids

anunnecessaryhop.BGPisbeingefficientby

informingAS64520ofthe

bestentrypointintoAS65000

fornetwork.RouterBinAS65000alsoadvertisestoAS64520that

thebestentrypointforeachnetworkinAS64600isthe

nexthopofrouterCbecausethatisthebestpathwayto

transitAS65000toAS64600fromAS64520.Thenext-hop-selfCommandneighbor{ip-address|peer-group-name}next-hop-selfRouter(config-router)#Forcesallupdatesforthisneighbortobe

advertisedwiththisrouterasthenexthop.TheIPaddressusedforthenext-hop-selfwill

bethesameasthesourceIPaddressofthe

BGPpacket.Example:next-hop-selfConfigurationEBGP多跳特性(MULTIHOP)1,中间隔着一个非BGP路由器,也可以维持EBGP会话.2,多跳EBGP是CISCOIOS提供的一种选项,它允许EBGP对等路由器被逻辑连接起来,即使处于它们之间的路由器不支持BGP也无妨.3,这种情况在实际中尽量不要采用.Example:ebgp-multihopCommandIBGPandRedistributionAtransit(转接)ASshouldrunIBGPonallroutersbecausethefullInternetroutingtableistoolargetoredistributeintoanIGP.B和E可以通过IGP联结在一起.我们可以将BGP再发布到IGP中,将路由发布出去.(redistribute,network);但BGP路由表太大了,IGP路由器会……由互联网络上的路由表太大了,如果发布到IGP中,会使网络崩溃,所以在转接AS中所有路由器都要运行BGP.IBGPSplitHorizonRule复习:IGP的水平分割:端口的水平分割.而BGP水平分割是基于邻居关系的.Bydefault,routeslearnedviaIBGPareneverpropagatedtootherIBGPpeers.(缺省情况下,路由器自己学习到的IBPG路由绝不会传给其他IBGP邻居;目的是为了确保路由环路不在AS内产生.)因为这个原因,IBGP邻居关系必须是全互联的;可以物理互连或逻辑全互联.PartialMeshIGPRoutingIssueswithoutFullyMeshedIBGPRouterCwilldropthepackettonetwork.RouterCisnotrunningIBGP;therefore,ithasnotlearnedabouttheroutetonetworkfromrouterB.Inthisexample,routerBandrouterEarenotredistributingBGPintoOSPF.(这个例子,是因为没有做BGP到OSPF的重发布;所以我们必须为每个连接指定BGP邻居;或者将BGP发布到IGP中.)思考:如果没有全互联,则会出现什么问题?要将BGP发布到IGP中,要求BGP和IGP同时互相学习路由,才能确保路由可达.

当然,运行的是全互联,运行的是BGP,则可以自由可达.(参考同步案例)BGPSynchronizationSynchronizationrule:Donotuseoradvertise

toanexternalneighboraroutelearnedbyIBGPuntilamatchingroutehasbeenlearnedfromanIGP.(当IBGP和IGP路由均出现了,才会向EBGP宣告.)EnsuresconsistencyofinformationthroughouttheASAvoidsblackholeswithintheASSafetoturnoffifallroutersintheASarerunningfull-meshIBGP(如果所有路由器都运行全网IBGP,关闭同步.)nosynchronization

Router(config-router)#DisablesBGPsynchronizationsothatarouterwilladvertiseroutesinBGPwithoutlearningtheminIGPExample:BGPSynchronizationIfsynchronizationison(thedefault),then:RoutersA,C,andDwouldnotuseoradvertisetherouteto

untiltheyreceivethematchingrouteviaanIGP.RouterEwouldnothearabout.Ifsynchronizationisoff,then:RoutersA,C,andDwoulduseandadvertisetheroutethey

receiveviaIBGP;routerEwouldhearabout.IfrouterEsendstrafficfor,routersA,C,andD

wouldroutethepacketscorrectlytorouterB.Example:BGPSynchronizationIfsynchronizationison(thedefault),then:路由器B将通过IBGP向AS65500中的其他路由器通告到的路由.路由器B将使用到的路由,并且将它放置到它的路由表中.RoutersA,C,andDwouldnotuseoradvertisetherouteto

untiltheyreceivethematchingrouteviaanIGP(也即,因为没有运行IGP,所以这些路由器将永远不使用或通告此条路由).RouterEwouldnothearabout.（它也没有到的转发路径）

Ifsynchronizationisoff,then:RoutersA,C,andDwoulduseandadvertisetheroutethey

receiveviaIBGP;routerEwouldhearabout.(直接使用IBGP接收到的路由,并将它放到自己的路由表中;前提是ACD能够到达的下一跳地址)IfrouterEsendstrafficfor,routersA,C,andD

wouldroutethepacketscorrectlytorouterB.(路由器E将听到有关的信息,并会有一条到的路由.)RunBGPonCoreRoutersandTurnOffSynchronizationOSPFprocessesalllocalpacketsforthenetworks

ownedbyAS65001.BGPprocessesallpacketstransitingacrossAS65001

tootherAutonomousSystems.BGP总结：1，在EBGP之间配置BGP即可。2，对于EBGP，下一跳是发送更新的邻居路由器的IP地址。3，对于IBGP，BGP协议要求由EBGP所通告的下一跳应该原封不动的被传输到IBGP；这时应该在收到EBGP路由的边界路由器上手动改变下一跳.这样路由器C才能到达EBGP路由器的网络。（改变下一跳，就是手动改变BGP路由器的缺省行为，并让它将自己作为发送给邻居的路由的下一跳地址进行通告，有时是必要的，例如在一个特殊的环境中。）4，由于互联网络上的路由表太大了,如果发布到IGP中,会使网络崩溃,