Enterasys Networks DFE-Gold Series Spanning Tree Features, Loop Protect

Overview: Single, Rapid and Multiple Spanning Tree Protocols
6-2 Spanning Tree Configuration
FordetailsoncreatingSpanningTreeinstances,referto“setspantreemsti”onpage 6‐14.
FordetailsonmappingSpanningTreeinstancestoVLANs,referto“setspantreemstmap”on
page 6‐15.
Spanning Tree Features
TheMatrixSeriesdevicemeetstherequirementsoftheSpanningTreeProtocolsbyperformingthe
followingfunctions:
• CreatingasingleSpanningTreefromanyarrangementofswitchingorbridgingelements.
• Compensatingautomaticallyforthefailure,removal,oradditionofanydeviceinanactive
datapath.
•Achievingportchangesinshorttimeintervals,whichestablishesastableactivetopology
quicklywithminimalnetworkdisturbance.
•Usingaminimumamountofcommunicationsbandwidthtoaccomplishtheoperationofthe
SpanningTreeProtocol.
• Reconfiguringtheactivetopologyinamannerthatistransparenttostationstransmittingand
receivingdatapackets.
• ManagingthetopologyinaconsistentandreproduciblemannerthroughtheuseofSpanning
TreeProtocolparameters.
Loop Protect
TheLoopProtectfeaturepreventsorshortcircuitsloopformationinanetworkwithredundant
paths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listeninguntilaBPDUisreceived.
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.It
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,andwillrevertto
listeningifitfailstogetaresponse.Thisprotectsagainstmisconfigurationandprotocolfailureby
theconnectedbridge.
TheDisputedBPDUmechanismprotectsagainstloopinginsituationswherethereisoneway
communication.AdisputedBPDUisoneinwhichtheflagsfieldindicatesadesignatedroleand
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
Note: MSTP and RSTP are fully compatible and interoperable with each other and with legacy STP
802.1D.