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Question on MSTP regions.

  • 1.  Question on MSTP regions.

    Posted 10-09-2020 10:06



    Is the MSTI information of one region present in the MST BPDUs of another region?


    Here I have two Regions, each with several MSTIs.



    Region A                         Region B


    I understand that the BPDU between EX1 and EX2 has information on all of Region A's MSTIs. 

    Does EX2's BPDU over the CST to EX3 contain information on all of Region A's MSTIs?

            If not, how does EX2 know to remove it.

            If yes, does EX3's BPDU to EX4 contain info on Regions A's MSTIs along with Region B's? If not, how does EX3 know to remove it?





  • 2.  Re: Question on MSTP regions.

    Posted 10-13-2020 06:50

    Hello Deepak,


    All common spanning tree environments (CST) contain CST, which is used to interconnect individual MST regions or MST regions with independent RSTP devices. A single root bridge is elected and tasked with path calculation for the CST.  If the MSTI is not matching, the root election will be based on the priority of the CST and not the MSTI.  If the priority of the MSTI is 4k and for the CST is 32k.  The priority to elect the root bridge when the MSTI does not match will be 32k.


    Hope this helps,




  • 3.  Re: Question on MSTP regions.

    Posted 10-15-2020 04:08

    MSTI ID's are local to each region, so you can reuse the same MSTI ID in different regions.


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