Ender204426777乾颐堂CCNAv3.0课程ImplementingScalableMedium-SizedNetworksBuildingRedundantSwitchedTopologies生成树乾颐堂CCNAv3.0Ender204426777IssuesinRedundantTopologies•Aredundanttopologyeliminatessinglepointsoffailure.•Aredundantswitchtopologycausesbroadcaststorms,multipleframecopies,andMACaddresstableinstabilityproblems冗余带来的问题是广播风暴、多点帧的拷贝以及MAC地址表不稳定等问题•Aloop-avoidancemechanismisrequired.EnderICND2—1-18乾颐堂CCNAv3.0Ender204426777IssuesinRedundantTopologies(Cont.)LoopresolutionwithSpanningTreeProtocol:•Providesaloop-freeredundantnetworktopologybyplacingcertainportsintoablockingstate解决环路的途径是在逻辑上把某些点解置于block状态•PublishedintheIEEE802.1Dspecification.EnderICND2—1-19乾颐堂CCNAv3.0Ender204426777Spanning-TreeOperationThespanning-treealgorithmfollowsthesesteps:1.Electsarootbridge选取一个根交换机2.Electsarootportforeachnon-rootswitch每个非根交换机选取一个根端口，用来收取最优的BPDU3.Electsadesignatedportforeachsegment，在每个segment选取一个指定端口，传输最优的BPDU4.PortstransitiontoforwardingorblockingstateEnderICND2—1-20BridgeID乾颐堂CCNAv3.0Ender204426777Spanning-TreeOperation(Cont.)Step1:Electarootbridge.•DecisionbasedonlowestBID（交换机B）EnderICND2—1-21乾颐堂CCNAv3.0Ender204426777Spanning-TreeOperation(Cont.)选择根端口Step2:Electarootportforeachnon-rootswitch.•Decisionbasedonlowestrootpathcost.到达根的最小的cost指•Ifnecessary,tiesarebrokenbyupstreamBIDandportIDvalues.BridgeID和端口ID作为候选EnderICND2—1-22乾颐堂CCNAv3.0Ender204426777Spanning-TreeOperation(Cont.)选择指定端口Step3:Electadesignatedportforeachsegment.•Decisionisbasedonthelowestrootpathcost-到达根的最小的cost值Ifnecessary,tiesarebrokenbyupstreamBIDandportID.BridgeID和端口ID作为候选EnderICND2—1-23乾颐堂CCNAv3.0Ender204426777Spanning-TreeOperation(Cont.)其他端口处于Bolck状态Step4:Theportstransitiontotheforwardingorblockingstate.•Rootportsanddesignatedportstransitiontotheforwardingstate.•Otherportsstayintheblockingstate.EnderICND2—1-24乾颐...
