Specifications

STP Overview
March 14, 2011 Page 3 of 29
• IEEE802.1s(MultipleSpanningTreeProtocol)
• IEEE802.1t(Updateto802.1D)
Asdescribedpreviously,STPresolvestheproblemsofphysicalloopsinanetworkbyestablishing
oneprimarypathbetweenanytwodevices.Itdoesthisbyenablingswitchingdevicestoexchange
informationusingBridgeProtocolDataUnit(BPDU)messages.STPuses
BPDUstodesignate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.
IfyouarefamiliarwithSTPoperationand
wishtoadjustthedefaultsinyournetwork,you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
RapidSpanningTree(RSTP)optimizesconvergenceinaproperlyconfigurednetworkby
significantlyreducingthetimetoreconfigurethenetwork’sactivetopologywhenphysical
topologyorconfigurationparameterchangesoccur.RSTPprovidesrapidconnectivityfollowing
thefailureofaswitchingdevice,switchport,oraLAN.
Multiple Spanning Trees
MultipleSpanningTree(MSTP)optimizestheutilizationofredundantlinksbetweenswitching
devicesinanetwork.It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.
MSTP
enhances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switching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: MSTP and RSTP are fully compatible and interoperable with each other and with legacy STP.