Enterasys Networks D-Series Spanning Tree Features, Loop Protect

Spanning Tree Configuration Summary

8-2 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 8‐12.

FordetailsonmappingSpanningTreeinstancestoVLANs,referto“setspantreemstmap”on

page8‐14 .

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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

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.