思科ccna雜cisco中文ccnp003.bgp基本配置

BGP路由互連本節(jié)內(nèi)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.(當(dāng)有多條物理連接時(shí),可用來容錯(cuò).)TheIPaddressusedinthiscommandwillbethedestination

IPaddressofallBGPupdatesandshouldbetheloopback

interfaceoftheotherrouter.Theupdate-sourcecommandisnormallyusedonlywith

IBGPneighbors.(只能用于IBGP間,EBGP間必須直接連接.)TheaddressofanEBGPneighbormustbedirectlyconnectedby

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

advertisedwiththisrouterasthenexthop.TheIPaddressusedforthenext-hop-selfwill

bethesameasthesourceIPaddressofthe

BGPpacket.用來手動(dòng)強(qiáng)迫BGP使用它自己的IP地址作為下一跳屬性參數(shù),而不讓BGP協(xié)議來選擇要使用的下一跳地址.一般在NBMA的網(wǎng)絡(luò)中很常用.EBGP鄰居向IBGP鄰居傳遞時(shí),不改變下一跳；由于BGP是一種AS－BY－AS的路由選擇協(xié)議；使用缺省設(shè)置時(shí)，跳數(shù)是按AS傳遞的，所以就存在下一跳的問題。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原封不動(dòng)的告訴了C,到的下一跳為.會(huì)有什么問題出現(xiàn)呢?C為什么下一跳不可達(dá).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,中間隔著一個(gè)非BGP路由器,也可以維持EBGP會(huì)話.2,多跳EBGP是CISCOIOS提供的一種選項(xiàng),它允許EBGP對等路由器被邏輯連接起來,即使處于它們之間的路由器不支持BGP也無妨.3,這種情況在實(shí)際中盡量不要采用.Example:ebgp-multihopCommandIBGPandRedistributionAtransit(轉(zhuǎn)接)ASshouldrunIBGPonallroutersbecausethefullInternetroutingtableistoolargetoredistributeintoanIGP.B和E可以通過IGP聯(lián)結(jié)在一起.我們可以將BGP再發(fā)布到IGP中,將路由發(fā)布出去.(redistribute,network);但BGP路由表太大了,IGP路由器會(huì)……由互聯(lián)網(wǎng)絡(luò)上的路由表太大了,如果發(fā)布到IGP中,會(huì)使網(wǎng)絡(luò)崩潰,所以在轉(zhuǎn)接AS中所有路由器都要運(yùn)行BGP.IBGPSplitHorizonRule復(fù)習(xí):IGP的水平分割:端口的水平分割.而BGP水平分割是基于鄰居關(guān)系的.Bydefault,routeslearnedviaIBGPareneverpropagatedtootherIBGPpeers.(缺省情況下,路由器自己學(xué)習(xí)到的IBPG路由絕不會(huì)傳給其他IBGP鄰居;目的是為了確保路由環(huán)路不在AS內(nèi)產(chǎn)生.)因?yàn)檫@個(gè)原因,IBGP鄰居關(guān)系必須是全互聯(lián)的;可以物理互連或邏輯全互聯(lián).PartialMeshIGPRoutingIssueswithoutFullyMeshedIBGPRouterCwilldropthepackettonetwork.RouterCisnotrunningIBGP;therefore,ithasnotlearnedabouttheroutetonetworkfromrouterB.Inthisexample,routerBandrouterEarenotredistributingBGPintoOSPF.(這個(gè)例子,是因?yàn)闆]有做BGP到OSPF的重發(fā)布;所以我們必須為每個(gè)連接指定BGP鄰居;或者將BGP發(fā)布到IGP中.)思考:如果沒有全互聯(lián),則會(huì)出現(xiàn)什么問題?要將BGP發(fā)布到IGP中,要求BGP和IGP同時(shí)互相學(xué)習(xí)路由,才能確保路由可達(dá).

當(dāng)然,運(yùn)行的是全互聯(lián),運(yùn)行的是BGP,則可以自由可達(dá).(參考同步案例)BGPSynchronizationSynchronizationrule:Donotuseoradvertise

toanexternalneighboraroutelearnedbyIBGPuntilamatchingroutehasbeenlearnedfromanIGP.(當(dāng)IBGP和IGP路由均出現(xiàn)了,才會(huì)向EBGP宣告.)EnsuresconsistencyofinformationthroughouttheASAvoidsblackholeswithintheASSafetoturnoffifallroutersintheASarerunningfull-meshIBGP(如果所有路由器都運(yùn)行全網(wǎng)IBGP,關(guān)閉同步.)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將通過IBGP向AS65500中的其他路由器通告到的路由.路由器B將使用到的路由,并且將它放置到它的路由表中.RoutersA,C,andDwouldnotuseoradvertisetherouteto

untiltheyreceivethematchingrouteviaanIGP(也即,因?yàn)闆]有運(yùn)行IGP,所以這些路由器將永遠(yuǎn)不使用或通告此條路由).RouterEwouldnothearabout.（它也沒有到的轉(zhuǎn)發(fā)路徑）

Ifsynchronizationisoff,then:RoutersA,C,andDwoulduseandadvertisetheroutethey

receiveviaIBGP;routerEwouldhearabout.(直接使用IBGP接收到的路由,并將它放到自己的路由表中;前提是ACD能夠到達(dá)的下一跳地址)IfrouterEsendstrafficfor,routersA,C,andD

wouldroutethepacketscorrectlytorouterB.(路由器E將聽到有關(guān)的信息,并會(huì)有一條到的路由.)RunBGPonCoreRoutersandTurnOffSynchronizationOSPFprocessesalllocalpacketsforthenetworks

ownedbyAS65001.BGPprocessesallpacketstransitingacrossAS65001

tootherAutonomousSystems.BGP總結(jié)：1，在EBGP之間配置BGP即可。2，對于EBGP，下一跳是發(fā)送更新的鄰居路由器的IP地址。3，對于IBGP，BGP協(xié)議要求由EBGP所通告的下一跳應(yīng)該原封不動(dòng)的被傳輸?shù)絀BGP；這時(shí)應(yīng)該在收到EBGP路由的邊界路由器上手動(dòng)改變下一跳.這樣路由器C才能到達(dá)EBGP路由器的網(wǎng)絡(luò)。（改變下一跳，就是手動(dòng)改變BGP路由器的缺省行為，并讓它將自己作為發(fā)送給鄰居的路由的下一跳地址進(jìn)行通告，有時(shí)是必要的，例如在一個(gè)特殊的環(huán)境中。）4，由于互聯(lián)網(wǎng)絡(luò)上的路由表太大了,如果發(fā)布到IGP中,會(huì)使網(wǎng)絡(luò)崩潰,