Specifications

How Do I Implement Spanning Trees?
March 14, 2011 Page 2 of 29
wouldcontinuetocirculateendlesslybetweenbothswitchingdevices.WithoutSpanningTree
blockingoneofthelinks,therewouldbenothingatlayer2tostopthisloopfromhappeningand
unnecessarilyconsumingnetworkmemory.Asadministrator,youwouldbeforcedtomanually
disableoneofthelinksbetweenSwitch1
and2fortheFigure 1networktooperate.
Figure 1 Redundant Link Causes a Loop in a Non-STP Network
WithSpanning Treerunningonyournetworkdevices,therewouldbenoneedforyouto
manuallydisablelinks.STPwouldautomaticallyblockoneoftheredundantpaths,asshownin
Figure 2,restoringasmoothdatatransferbetweenSwitch1and2
andendusers.Intheeventthat
theprimary(unblocked)pathfailed,STPwouldplacetheblockedpathbackintoserviceand
blockthefailedlink.Whenenabled,itwoulddothisautomatically,withoutadministrative
intervention.
Figure 2 Loop Avoided When STP Blocks a Duplicate Path
How Do I Implement Spanning Trees?
Bydefault,SpanningTreeisenabledgloballyonN‐Series,S‐Series,K‐Series,stackable,and
standaloneswitchdevicesandenabledonallports.ThedefaultversionissettoMSTPmode,an
enhancementofthestandard802.1D(see“MultipleSpanningTrees”onpage3).Inmostnetworks,
thesedefaults
shouldnotbechanged.However,ifyouareknowledgeableaboutSpanningTrees
andconfiguringSTPalgorithms,youwillbeabletoadjustparameterstofinetuneSTP
performanceinyournetworkasdescribedinthisdocument.ByusingtheSpanningTree
monitoringcommandsdescribedhere,youwillalsobeable
tobetterunderstandthedefaultSTP
configurationonyourEnterasysdeviceandhowitoperatesinyournetwork.
STP Overview
EnterasysswitchdevicessupporttheSpanningTreeProtocol(STP),RapidSpanningTreeProtocol
(RSTP),andMultipleSpanningTreeProtocol(MSTP)asdefinedinthefollowingstandardsand
describedinthisdocument:
• IEEE802.1D(SpanningTreeProtocol)
• IEEE802.1w(RapidSpanningTreeProtocol)