Enterasys b5 User Manual

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EnterasysB5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forwa rdi ng.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alt ernat e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reve rt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.

Enterasys b5 Specifications

General

Brand	Enterasys
Model	b5
Category	Other
Language	English