Enterasys D2 D2G124-12P

Spanning Tree Configuration Summary
6-2 Spanning Tree Configuration
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 6‐12.
FordetailsonmappingSpanningTreeinstancestoVLANs,referto“setspantreemstmap”on
page 6 ‐14.
Spanning Tree Features
TheD‐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operat ion 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/M STP)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
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.