User`s guide

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.