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Overview of Spanning Tree Protocols

Matrix DFE Series and N-SA User’s Guide 5-3

Spanning Tree (IEEE 802.1D)

TheSpanningTreeProtocol(STP)definedinIEEE802.1Dallowsbridgestodynamicallydiscover

asubsetofthetopology thatisloop‐free.Theloop‐freetreethatisdiscoveredcontainspathsto

everyLANsegment.

TheSpanningTreeProtocolisusedtoeliminatedataloopsinanEthernetnetworkby

creatinga

treewherethereisonlyonedataroutebetweenanytwoendstations.STPblocksredundant data

paths.Shouldapathbecomeunreachable,STPautomaticallyactivatesablockedpath.Shoulda

bridgebeaddedcreatingaredundantpath,STPblocksoneofthepaths.STPcanalsochange

data

pathsbasedonacostchange.

AllbridgesthatsupportthespanningtreeexchangeinformationusingBridgeProtocolDataUnit

(BPDU)messages.UsingtheinformationexchangedbytheBPDUs,STPdesignatesabridgefor

eachswitchedLANsegment,andonerootbridgeforthespanningtree.Therootbridgeis

the

logicalcenterofthespanningtreeandisusedtodeterminewhichpathstoblock andwhichto

open.

Anetworkadministratorcandeterminethetopologyofthespanningtreebyadjustingthebridge

priority,portpriority,andpathcost.Thebridgepriorityassignsthebridge’srelativepriority

compared

tootherbridges.Theportpriorityassignstheport’spriorityinrelationtotheother

portsonthesamebridge.Bydefault,theportcostisavalueassignedtotheportbasedonthe

speedoftheport.Thefasterthespeed,thelowerthecost.Thishelpstodetermine

thequickest

pathbetweentherootbridgeandaspecifieddestination.Thesegmentattachedtotherootbridge

normallyhasapathcostofzero.

EachbridgehasaBridgeIdentification(BID),whichisderivedfromthebridge’sMACaddress

andbridgepriority.ThebridgewiththelowestBIDbecomes

therootbridge.

Rapid Spanning Tree (IEEE 802.1w)

TheRapidSpanningTreeProtocol(RSTP)definedinIEEE802.1wenhancestheSTPbyallowing

thenetworktopologytoreconvergeinasignificantlysmalleramountoftime.Thisispartially

accomplishedbyitsabilitytobe independentoftheprotocoltimervalueswhenconfiguring the

activetopologyofaLAN.

Multiple Spanning Trees (IEEE 802.1s)

TheMultipleSpanningTreeProtocol(MSTP),definedinIEEE802.1s,allowsforincreased

bandwidthutilizationandoptimalloadbalancingacrossredundantlinks.Itfurtherexpandsupon

STPandRSTPwiththefollowingfeatures:

• BackwardscompatibilitywithSTPandRSTP.

•AbilitytocreateasingleCommonandInternalSpanningTree(CIST)thatrepresents

the

connectivityoftheentirenetwork.

•Userscangroupanynumberofdevicesintoindividualregions,witheachregionbehaving

andpresentingitselfasasingledevicetotherestofthenetwork.

•Aregioncancontainmultipleinstancesofthespanningtree,whereeachinstancecansupport

multipleVLANs.

Note: The terms path cost and port cost are sometimes used interchangeably. Normally, the port

cost is the value assigned to a specific port, while path cost, especially in a BPDU, is the sum of the

port costs in a path.

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