115-Distance-Vector-Rou教學(xué)講解課件

?2002,CiscoSystems,Inc.Allrightsreserved.?2002,CiscoSystems,Inc.Allrightsreserved.2DistanceVectorRoutingObjectivesUponcompletingthislesson,youwillbeableto:Describe（描述）thefeaturesoffered（提供的功能）bydistancevectorroutingprotocolsandgiveexamplesofeachDescribetheissuesassociated（引發(fā)的問題）withdistancevectorroutingandidentifysolutionstothoseissues（這些問題的解決方案）Routerspassperiodiccopiesofroutingtabletoneighbor

routersandaccumulatedistancevectors.DistanceVectorRoutingProtocols距離：多遠矢量：哪個方向周期性地（RIP：30S）累計Routersdiscoverthebestpathto

destinationsfromeachneighbor.SourcesofInformationandDiscoveringRoutes信息來源和路由獲取Selectingthe

BestRoutewithMetrics度量值Updatesproceedstep-by-step

fromroutertorouter.Maintaining維護RoutingInformation拓撲變化引起路由表更新路由器的更新進程更新路由表路由器A在下一個更新周期發(fā)出路由表更新信息路由表更新Eachnodemaintainsthedistancefromitselftoeachpossibledestinationnetwork.每一個節(jié)點自行維護它到每一個可能的目標網(wǎng)絡(luò)的距離InconsistentRoutingEntries路由條目不一致距離矢量路由協(xié)議會出現(xiàn)路由條目不一致的情況Slowconvergenceproducesinconsistentrouting.慢的收斂會引起路由條目的不一致InconsistentRoutingEntries(Cont.)RouterCconcludes（推斷）thatthebestpathtonetwork10.4.0.0isthroughrouterB.InconsistentRoutingEntries(Cont.)RouterAupdatesitstabletoreflectthenewbuterroneous（不正確的）hopcount.InconsistentRoutingEntries(Cont.)Hopcountfornetwork10.4.0.0countstoinfinity.CounttoInfinity距離運算到無窮大Definealimitonthenumberofhopstopreventinfiniteloops.定義一個跳數(shù)的上限防止無窮大的環(huán)路DefiningaMaximum規(guī)定一個最大的數(shù)Packetsfornetwork10.4.0.0bounce(loop)betweenroutersBandC.RoutingLoops路由環(huán)路去往10.4.0.0的數(shù)據(jù)包Itisneverusefultosendinformationaboutaroutebackinthedirectionfromwhichtheoriginalinformationcame.不向原始路由更新的方向再次發(fā)送路由更新信息SplitHorizon水平分割Routersadvertisethedistanceofroutesthathavegonedowntoinfinity.路由器向鄰居路由發(fā)送故障路由的更新信息，將其度量值標為無窮大

RoutePoisoning路由中毒Poisonreverseoverridessplithorizon.毒性反轉(zhuǎn)是唯一覆蓋水平分割PoisonReverse毒性反轉(zhuǎn)鄰居路由收到路由中毒信息后，向所有鄰居發(fā)布路由中毒信息，目的是為了快速收斂RoutePoisoning路由中毒VSPoisonReverse毒性反轉(zhuǎn)路由中毒是指在路由信息在路由表中失效時，先將度量值變?yōu)闊o窮大，而不是馬上從路由表中刪掉這條路由信息。(這句話要理解，如RIP協(xié)議中，其度量值變?yōu)?6，意味著路由不可達)再將其信息發(fā)布出去，這樣相鄰的路由器就得知這條路由己無效了……毒性反轉(zhuǎn)與路由中毒,概念是不一樣的，它是指收到路由中毒消息的路由器，不遵守水平分割原則將中毒消息轉(zhuǎn)發(fā)給所有的相鄰路由器，也包括發(fā)送中毒信息的源路由器，也就是通告相鄰路由器這條路由信息己失效了……主要目的是達到快速收斂的目的。Therouterkeepsanentryforthenetwork’spossibledown

state,allowingtimeforotherrouterstorecomputeforthistopologychange.路由保持一條網(wǎng)絡(luò)可以已經(jīng)down的狀態(tài)記錄，并打開一個計時器，在計時器內(nèi)關(guān)于此路由的更新信息被抑制。HolddownTimers抑制計時器Theroutersendsupdateswhenachangeinitsroutingtableoccurs.當(dāng)路由表發(fā)生改變時，路由器馬上傳遞路由更新而不用等待下一個更新周期TriggeredUpdates觸化更新DistanceVectorOperationDistanceVectorOperation(Cont.)DistanceVectorOperation(Cont.)DistanceVectorOperation(Cont.)DistanceVectorOperation(Cont.)DistanceVectorOperation(Cont.)SummaryDistancevector-basedroutingalgorithms

(alsoknownasBellman-Fordalgorithms)passperiodiccopiesofaroutingtablefromroutertorouter.Whenthetopologyinadistancevectorprotocolinternetworkchanges,routingtableupdatesmustoccur.Aswiththenetworkdiscoveryprocess,topologychangeupdatesproceedstep-by-stepfromroutertorouter.Whenmaintainingtheroutinginformation,inconsistenciescanoccuriftheinternetwork’sslowconvergenceonanewconfigurationcausesincorrectroutingentries.Summary(Cont.)Theconditioncalledcounttoinfinityariseswhenroutingtableupdatescontinuetoincreasethemetrictoadestinationthatcannotbereached,ratherthanmarkingthedestinationasunreachable.Aroutingloopoccurswhentwoormoreroutershaveincorrectroutinginformationindicatingthatavalidpathtoanunreachabledestinatio