Enterasys SecureStack C2 - Configuring Spanning Tree Bridge Parameters

Configuring Spanning Tree Bridge Parameters
SecureStack C2 Configuration Guide 7-3
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timers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configurable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.