Specifications

Understanding How Spanning Tree Operates
March 14, 2011 Page 6 of 29
Understanding How Spanning Tree Operates
ThissectionprovidesyouwithamoredetailedunderstandingofhowtheSpanningTreeoperates
inatypicalnetworkenvironment.Thefollowingconceptsarecovered.
• ElectingtheRootBridge(page6)
• AssigningPathCosts(page6)
• DeterminingtheDesignatedBridge(page6)
• IdentifyingPortRolesandAssigningPortStates(page6)
• MSTPOperation(page7)
Electing the Root Bridge
Arootbridgeisthelogicalcenterof theSpanningTreetopology.Rootisdeterminedwhenbridges
periodicallyadvertisetheirSTPinformationtotheirneighborsbytransmittingBPDUscontaining
theirrootIDandbridgeID.Eachreceivingbridgeanalyzesthisinformation,electingthebrid ge
withthelowestbridgeIDasroot.
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Assigning Path Costs
SpanningTreeassignseachLANsegmentapathcost,whichisaportspeed‐basedvalue
associatedwitheachlinkandtherelative“cost”totraversethatlink.
Determining the Designated Bridge
SpanningTreecalculatesadesignatedbridge,whichisthebridgeofferingthelowestpathcostto
therootbridge.Ifpathcostsareequal,thedesignatedbridge istheonewiththelowerbridgeID.
Eachbridgeisservicedbyonlyonedesignatedbridge.Therootbridgeservesasthedesignated

bridgeforallbridgestowhichitisdirectlyattached.Foreachbridge,SpanningTreecalculatesall
possiblepathsbacktotherootbridge.Ifthepathcostisequalfrommultiplepaths,thedesignated
bridgewillbedeterminedbythelowestbridgeID.
Identifying Port Roles and Assigning Port States
Oneachbridge,SpanningTreeidentifiesarootport,whichistheportthatprovidesthebestpath
totherootbridge,andadesignatedport,whichistheportthatforwar dsconfigurationBPDUs
fromtherootbridge.Ifabridgeiselectedasthedesignatedbridgeforotherdownstreamdevices,
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thentheportsthatconnecttothesedownstreamdevicesaredenotedasdesignatedports.
SpanningTreeusesthefollowingparameterstodeterminerootportsanddesignatedports:
• Pathcosttoroot
•DesignatedbridgeID
•DesignatedbridgeportID
STP’smaingoalistoensurethateverylinkconnectingarootportanda
designatedport
transitionstotheforwardingstateasquicklyaspossible.TheseandotherSTPportroles
(describedinTable 1)willdictateforwardingandassignmentoftheportstates(describedin
Table 2).