Specifications

Understanding How Spanning Tree Operates
March 14, 2011 Page 9 of 29
•FormatSelector–Oneoctetinlengthandisalways0.Itcannotbeadministrativelychanged.
• ConfigurationName–Auser‐assigned,casesensitivenamegiventotheregion.The
maximumlengthofthenameis32octets.
• RevisionLevel–Twooctetsinlength.Thedefaultvalueof0maybe
administrativelychanged.
• ConfigurationDigest–16‐octetHMAC‐MD5signaturecreatedfromtheconfiguredVLAN
Identification(VID)/FilteringIdentification(FID)toMultipleSpanningTreeInstances(MSTI)
mappings.Alldevicesmusthaveidenticalmappingstohaveidenticalconfigurationdigests.
TheMSTregiondesignatesoneCISTregionalrootbridgefortheregion,regardless
ofthenumber
ofMSTIs.TheregionalrootprovidestheconnectivityfromtheregiontotheCISTrootwhenthe
CISTrootliesoutsidetheregion.
Multiple Spanning Tree Instances (MSTI)
InsidetheMSTregion,aseparatetopologyismaintainedfromtheoutsideworld.EachMST
regionmaycontainupto64differentMSTIs.TheEnterasysswitchdevicemapsVLANIDs(VIDs)
andFilteringIDs(FIDs )toeachotherinaone‐to‐onecorrelation;forexample,FID3=VID3.
VID/
FIDsaremappedtodifferentMSTIstocreateatypeofloadbalancing.
Determining FID-to-SID Mappings
VLANsaremappedtoMSTIsthroughaFID‐to‐SIDmappingcorrelationwhichisthekeyelement
inMSTPconfiguration.EachVLANisassociatedtoaFIDand,duringMSTIcreation,VLANsare
mappedtoSpanningTreeIDsusingtheirFIDassociation.Thismappingiscontainedwithinthe
MSTconfigurationdigest
describedintheprevioussection anddisplayedinthefollowing
example.Bydefault,everybridgewillhaveaFID‐to‐SIDmappingthatequalsVLANFID1/SID0.
UsethiscommandtodetermineMSTIconfigurationidentifierinformation,andwhetherornot
thereisamisconfigurationduetonon‐matchingconfigurationidentifier
components:
show spantree mstcfgid
Example
ThisexampleshowshowtodisplayMSTIconfigurationidentifierinformation.Inthiscase,this
bridgebelongsto“Region1”:
Enterasys->show spantree mstcfgid
MST Configuration Identifier:
Format Selector: 0
Configuration Name: Region1
Revision Level: 88
Configuration Digest: 6d:d7:93:10:91:c9:69:ff:48:f2:ef:bf:cd:8b:cc:de
InorderforotherbridgestobelongtoRegion1,allfourelementsofthosebridges’configuration
IDoutputmustmatch.Theonlydefaultvaluethatmustbechangedforthistohappenisthe
configurationnamesetting.
UsethiscommandtochangetheconfigurationnamefromthedefaultbridgeMACaddress
value
to“Region1”:
set spantree mstcfgid cfgname Region1
SinceanMSTIisaseparateSpanningTree,eachMSTIhasitsownrootinsidetheMSTregion.
Figure 4andFigure 5showtwoMSTIsinasingleregion.Switchingdevice3istherootforMSTI1,