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Configuring Spanning Tree Bridge Parameters

SecureStack B2 Configuration Guide 9-3

learningandthepriorityvectorisworsethanthatalreadyheldbytheport.IfadisputedBPDUis

received,theportisforcedtothelisteningstate.WhenaninferiordesignatedBPDUwiththe

learningbitsetisreceivedonadesignatedport,itsstateissettodiscardingto

preventloop

formation.NotethattheDisputemechanismisalwaysactiveregardlessoftheconfiguration

settingofLoopProtection.

LoopProtectoperatesasaperport,perMSTinstancefeature.Itshouldbesetoninter‐switch

links.Itiscomprisedofseveralrelatedfunctions:

• Controlofportforwardingstatebased

onreceptionofagreementBPDUs

• ControlofportforwardingstatebasedonreceptionofdisputedBPDUs

• Communicatingportnon‐forwardingstatusthrough trapsandsyslogmessages

•Disablingaportbasedonfrequencyoffailureevents

PortforwardingstateinthedesignatedportisgatedbyatimerthatissetuponBPDUreception.

It

isanalogoustothercvdInfoWhiletimertheportuseswhenreceivingrootinformationintheroot/

alternate/backuprole.

TherearetwooperationalmodesforLoopProtectonaport.Iftheportisconnectedtoadevice

knowntoimplementLoopProtect,itusesfullfunctionalmode.Otherwisetheport

operatesin

limitedfunctionalmode.

ConnectiontoaLoopProtectswitchguaranteesthatthealternateagreementmechanismis

implemented.Thismeansthedesignatedportcanrelyonreceivingaresponsetoitsproposal

regardlessoftheroleoftheconnectedport,whichhastwoimportantimplications.First,the

designatedport

connectedtoanon‐rootportmaytransitiontoforwarding.Second,thereisno

ambiguitywhenatimeouthappens;aLoopProtecteventhasoccurred.

Infullfunctionalmode,whenatype2BPDUisreceivedandtheportisdesignatedandpoint‐to‐

point,thetimerissetto3

timeshelloTime.Inlimitedfunctionalmodethereistheadditional

requirementthattheflagsfieldindicatearootrole.IftheportisaboundaryporttheMSTIsfor

thatportfollowtheCIST,thatis,theMSTIporttimersaresetaccordingtotheCISTporttimer.If

the

portisinternaltotheregionthentheMSTIportti mersaresetindependentlyusingthe

particularMSTImessage.

MessageageexpirationandtheexpirationoftheLoopProtecttimerarebothLoopProtectevents.

Anoticelevelsyslogmessageisproducedforeachsuchevent.Trapsmaybeconfiguredto

report

theseeventsaswell.AsyslogmessageandtrapmaybeconfiguredfordisputedBPDUs.

Itisalsoconfigura bletoforcethelockingofaSID/portfortheoccurrenceofoneormoreevents.

Whentheconfigurednumberofeventshappenwithinagivenwindowoftime,theportis

forced

intoblockingandheldthereuntilitismanuallyunlockedviamanagement.

Configuring Spanning Tree Bridge Parameters

Purpose

TodisplayandsetSpanningTreebridgeparameters,includingdevicepriorities,hellotime,

maximumwaittime,forwarddelay,pathcost,andtopologychangetrapsuppression.

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