Specifications

Understanding How Spanning Tree Operates
March 14, 2011 Page 8 of 29
MSTPisthedefaultSpanningTreemodeonallEnterasysswitchdevices.
ThefollowingconceptsinvolvedinMSTPoperationaredescribedinthissection:
• CommonandInternalSpanningTree(CIST)(page8)
• MSTRegion(page8)
• MultipleSpanningTreeInstances(MSTI)(page9)
Common and Internal Spanning Tree (CIST)
MSTPusesallSpanningTreeregioninformat iontocreateasingleCommonandInternal
SpanningTree(CIST)thatrepresentstheconnectivityoftheentirenetwork.Thisisequivalentto
thesingleSpanningTreeusedforSTPandRSTP.
TheMSTPenablednetworkcontainsoneCISTandaminimumofatleast
oneMSTregion.A
typicalnetworkmaycont ainnumerousMSTregionsaswellasseparateLANsegmentsrunning
legacySTPandRSTPSpanningTreeprotocols.
TheCISTcontainsarootbridge,whichistherootoftheSpanningTreeforthenetwork.TheCIST
rootisnotnecessarilylocatedinsidean
MSTregion.EachregioncontainsaCISTregionalroot,
unlesstheCISTrootispartoftheregion.BridgesinanMSTPtopologycomparetheirreceived
BPDUstocalculatetheirshortestpathtotheCISTroot,CISTregionalrootandMSTIregionalroot.
MST Region
AnMSTregionisagroupofdevicesthatareconfiguredtogethertoformalogicalregion.The
MSTregionpresentsitselftotherestofthenetworkasasingleswitchingdevice,whichsimplifies
administration.Pathcostisonlyincrementedwhentrafficentersorleavestheregion,regardless
ofthe
numberofdeviceswithintheregion.EachLANcanonlybeamemberofoneregion.
Figure 3showsthattheMSTregionappearsasasingleswitchingdevicetoDevices1and2,but
reallyconsistsofthreedevices.
Figure 3 Example of an MST Region
Foraswitchingdevicetobeconsideredaspartofan
MSTregion,itmustbeadministratively
configuredwiththesameconfigurationidentifierinformationasallotherdevicesintheMST
region.Theconfigurationidentifierconsistsofthefollowing fourseparateparts:
MST Region
Device 1 Device 2