User`s guide

Overview of Spanning Tree Protocols

5-4 Configuring Spanning Trees

•Inadditiontousinghellotime,forwarddelay,andmaxageinformation,MSTPalsoutilizes

thehopcountforimprovedperformance.

MSTPcanautomaticallydete cttheversionofspanningtreebeingusedonaLANandsendoutthe

equivalenttypeofBPDU.Inaddition,MSTPincorporatesaforceversionfeature

wheretheuser

mayforceMSTPtobehaveasSTPorRSTP.

Common and Internal Spanning Tree (CIST)

MSTPusesallSTP,RSTPandMultipleSpanningTree(MST)regioninformation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contain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.

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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 5‐1showsthattheMSTregionappearsasasingledevicetoDevices1and2,butreally

consistsofthreedevices.

Figure 5-1 Example of an MST Region

ForadevicetobeconsideredaspartofanMSTregion,

itmustbeadministrativelyconfigured

withthesameconfigurationidentifierinforma tionasallotherdevicesintheMSTregion.The

configurationidentifierconsistsoffourseparateparts:

MST Region

Device 1 Device 2