Specifications

Understanding and Configuring Loop Protect
March 14, 2011 Page 23 of 29
Port Modes and Event Triggers
PortsworkintwoLoopProtectoperationalmodes.Iftheportisconfiguredsothatitisconnected
toaswitchingdeviceknowntoimplementLoopProtect,itusesfullfunctional(enhanced)mode.
Otherwise,itoperatesinlimitedfunctional(standard )mode.
ConnectiontoaLoopProtectswitchingdevi ceguaranteesthatthe
alternateagreement
mechanismisimplementedand,therefore,thedesignatedportcanrelyonreceivingaresponseto
itsproposalregardlessoftheroleoftheconnectedport.This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mode,whenatype2BPDUisreceivedandtheportisdesignatedandpoint‐to‐point,the
timerissetto3ti meshelloTime.Limitedmodeaddsafurtherrequirementthattheflagsfieldin
theBPDU
indicatesarootrole.Iftheportisaboundaryport,theMSTIsforthatportfollowthe
CIST(forexampleif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.
LoopProtectinitializestheMSTItimertozeroanddoesnotallowthedesignatedporttotransition
fromlisteningtolearninguntilthetimerbecomesnon‐zero.If theportisnotdesignated,the timer
doesnotapply.Itsstateiscontrolledthroughnormalprotocolbehavior.
A
disputedBPDUisoneinwhichtheflagsfieldindicatesadesignatedrole,alearningstate,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.
Messageageexpirationandtheexpirationofthe
LoopProtecttimerarebotheventsforwhich
LoopProtectgeneratesanoticelevelsyslogmessage.Youcan alsoconfiguretrapstoreportthese
events,aswellasasyslogmessageandtrapfordisputedBPDUs.
Inaddition,youcanconfigureLoopProtecttoforcethelockingofaSID/portwhen
oneormore
eventsoccurs.Whentheconfigurednumberofeventshappenwithinagivenwindowoftime,the
portwillbeforcedintoblockingandheldthereuntilyoumanuallyunlockit.
Example: Basic Loop Protect Configuration
ThefollowingsampleconfigurationshowshowLoopProtectfunctionsinabasicSpanningTree
topology.
IntheexampleinFigure 8,Switch1istherootbridgewithBPDUsbeingsenttobothSwitch2and
3.(DesignatedportsarelabeledDandrootportsarelabeledR.)Switch3hasplaced
theportthat
connectstoSwitch2inablockingstate.