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Spanning Tree Configuration Summary
9-2 Spanning Tree Configuration
MSTP
TheIEEE802.1sMultipleSpanningTreeProtocol(MSTP)buildsupon802.1DandRSTPby
optimizingutilizationofredundantlinksbetweenswitchesinanetwork.Whenredundantlinks
existbetweenapairofswitchesrunningsingleSTP,onelinkisforwardingwhiletheothersare
blockingforalltrafficflowingbetweenthe
twoswitches.Theblockinglinksareeffectivelyused
onlyiftheforwardinglinkgoesdown.MSTPassignseachVLANpresentonthenetworktoa
particularSpanningTreeinstance,allowing eachswitchporttobeinadistinctstateforeachsuch
instance:blockingforoneSpanningTreewhileforwarding
foranother. Thus,traffic associated
withonesetofVLANscantraverseaparticularinter‐switchlink,whiletrafficassociatedwith
anothersetofVLANscanbeblockedonthatlink.IfVLANsareassignedtoSpanningTrees
wisely,nointer‐switchlinkwillbecompletelyidle,maximizingnetworkutilization.
For
detailsoncreatingSpanningTreeinstances,referto“setspantreemsti” onpage 9 ‐12 .
FordetailsonmappingSpanningTreeinstancestoVLANs,referto“setspantreemstmap”on
page 9 ‐14.
Spanning Tree Features
TheSecureStackC3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
Note: MSTP and RSTP are fully compatible and interoperable with each other and with legacy
STP 802.1D.
Note: The term “bridge” is used as an equivalent to the term “switch” or “device” in this document.