11解决路由环路的机制

介绍距离矢量路由协议,Medium-SizedRoutedNetworkConstruction,教学内容：,什么是距离矢量路由协议如果来维持动态路由信息动态路由协议会出现的问题如何来解决动态路由协议会出的的环路问题重点理解路由环路的概念,DistanceVectorRoutingProtocols,Routerspassperiodiccopiesoftheirroutingtabletoneighboringroutersandaccumulatedistancevectors.,SourcesofInformationandDiscoveringRoutes,Routersdiscoverthebestpathtodestinationsfromeachneighbor.,MaintainingRoutingInformation,RIP：30IGRP：90,InconsistentRoutingEntries:CountingtoInfinityandRoutingLoops,Eachnodemaintainsthedistancefromitselftoeachpossibledestinationnetwork.,CountingtoInfinity,Slowconvergenceproducesinconsistentrouting.,CountingtoInfinity(Cont.),RouterCconcludesthatthebestpathtonetwork10.4.0.0isthroughrouterB.,CountingtoInfinity(Cont.),RouterAupdatesitstabletoreflectthenewbuterroneoushopcount.,CountingtoInfinity(Cont.),Thehopcountfornetwork10.4.0.0countstoinfinity.,RoutingLoops,Packetsfornetwork10.4.0.0bounce(loop)betweenroutersBandC.,1、DefiningaMaximum（定义最大度量值）,Alimitissetonthenumberofhopstopreventinfiniteloops.,2、SplitHorizon（水平分割）,Itisneverusefultosendinformationaboutaroutebackinthedirectionfromwhichtheoriginalinformationcame.,3、RoutePoisoning（路由抑制）,Routersadvertisethedistanceofroutesthathavegonedowntoinfinity（无穷大，设置为不可达-down）,4、PoisonReverse（反向抑制）,Poisonreverseoverridessplithorizon(毒性逆转).,5、Hold-DownTimers（抑制定时器）,Therouterkeepsanentryforthe“possiblydown”stateinthenetwork,allowingtimeforotherrouterstorecomputeforthistopologychange.(在抑制时间内等待网络恢复正常，C会将自己的路由更新正常，然后再去更新其他路由器的路由信息。),6、TriggeredUpdates（触发更新）,Theroutersendsupdateswhenachangeinitsroutingtableoccurs.（当路由表发生变化时，更新报文会立即广播给相邻的路由器，而不是等30