Enterasys SecureStack B3 User Manual

Spanning Tree Configuration Summary
7-2 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spa ntreemsti”onpage 7‐12.
FordetailsonmappingSpanningTreeinstancestoVLANs,referto“setspantreemstmap”on
page 7‐14.
Spanning Tree Features
TheSecureStackB3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/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.

Enterasys SecureStack B3 Specifications

General

Brand	Enterasys
Model	SecureStack B3
Category	Switch
Language	English