Specifications

Understanding and Configuring Loop Protect
March 14, 2011 Page 22 of 29
Monitoring SpanGuard Status and Settings
UsethecommandsinTable 7toreviewSpanGuardstatusandsettings.
Understanding and Configuring Loop Protect
ThissectionprovidesinformationaboutthefollowingLoopProtecttopicsandtasks:
• WhatIsLoopProtect?(page22)
• HowDoesItOperate?(page22)
• ConfiguringLoopProtect(page25)
What Is Loop Protect?
AsdescribedpreviouslyintheoverviewofLoopProtectonpage5,thisfeaturepreventsorshort
circuitsloopformationinyournetwork.Itdoesthisbyrequiringportstoreceivetype2BPDUs
(RSTP/MSTP)onpoint‐to‐pointinter‐switchlinks(ISLs)beforetheirstatesareallowedtobecome
forwarding.
Further,ifaBPDUtimeoutoccursonaport,itsstatebecomesnon‐forwardinguntila
BPDUisreceived.
Inthisway,bothupstreamanddownstreamfacingportsareprotected.Whenarootoralternate
portlosesitspathtotherootbridgeduetoamessageageexpiration,it
takesontheroleof
designatedportandwillnotforwardtrafficuntilaBPDUisreceived.
WhenaportisintendedtobethedesignatedportinanISL,itconstantlyproposesandwillnot
forwarduntilaBPDUisreceived.This protectsagainstmisconfigurationandprotocolfailureby
the
connectedbridge.
How Does It Operate?
LoopProtectoperatesasaperport,perMSTinstancefeatureandshould besetonISLs.Itis
comprisedofseveralrelatedfunctions,including:
• ControllingportforwardingstatebasedonreceptionofagreementBPDUs
• Controlling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
Table 7 Commands for Monitoring SpanGuard
Task Command
Display the status of SpanGuard on the device. show spantree spanguard
Display the status of the SpanGuard lock function on
one or more ports.
show spantree spanguardlock [port
port-string]
Display the SpanGuard timeout setting. show spantree spanguardtimeout
Display the status of the SpanGuard trap function. show spantree spanguardtrapenable