STP基本概念的理解.docx

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1、STP基本概念STPDefinitionsBridgeID:bridgepriority+MACAddressBridgePriority:0-65,535(defaultis32,768,incrementsof4096)PortID:portpriority+portnumberPortPriority:0-240(defaultis128,incrementsof16)PathCost:Thecumulativecostofalllinksbetweentheswitchandtheroot

2、bridgeRootBridgeSelectionSTPexchangeinformationbymulticastingBPDUsframeevery2seconds(bydefault).TheBridge-ID(priority+MACaddress)isusedbySTPtodeterminetherootbridge.Iftwoswitcheshavethesamepriority,thentheonehaslowerMACaddresswillbetherootbridge.STPPa

3、thSelectionSpanningtreebuildsthetreestructureattemptingtousethefastestlinksithasavailablefortheactivepaths.STPusesthefollowingstepstoselectitspaths:1.LowestrootbridgeID(BIP)2.Lowestpathcosttotheroot(pathcostisconfigurableonsometypeofswitch/bridge)3.Lo

4、westsenderbridgeID4.LowestsenderportID(PID)STPConvergence1.RootbridgeelectionEachVLANelectsonerootbridge.Allportsontherootbridgeactasdesignatedports,whichsendandreceivetrafficaswellasBPDUs.Thebridgewiththelowestprioritybecomesroot.2.Rootportsaredeterm

5、inedonallnon-rootbridgesEachnon-rootbridgeisassignedasinglerootportthatsendsandreceivestraffic.Therootportischosenbasedontheportwiththelowest-costpathbetweenthenon-rootbridgeandtherootbridge.Iftwopathsareequalcost,theportwiththelowestportID(priority+p

6、ortnumber)willwin.3.DesignatedportselectionEachsegmenthasasingledesignatedport.Designatedportsarechosenfromonnon-rootportsthathavethelowestpathcosttotherootbridge.Intheeventofatie,thebridgeIDactsasatiebreaker(lowestwins).Allportsonarootbridgearedesign

7、atedports.STPPortRolesRootportOnnon-rootbridgesonlyForwardstraffictowardstherootbridgeOnlyoneperswitchCanpopulatetheMACtableDesignatedportOnrootandnon-rootbridgesAllportsonrootbridgearedesignatedportsReceivesandforwardsframestowardstherootbridgeasneed

8、edOnlyonepersegmentCanpopulatetheMACtableNondesignatedportDoesnotforwardpackets(blocking)DoesnotpopulatetheMACtableDisabledportAportthatisshutdownSTPPortStatesBlockingInnondesignatedstatusanddoesnotforwardframesReceivesBPDUstodeterminerootswit