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Problems electing (correct) CST root

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  • 1.  Problems electing (correct) CST root

    Posted Oct 19, 2020 04:36 AM

    Hi.

     

    We have 25 switches, connected in a tree structure from two core switches.

    We have MSTP enabled with the core switch assigned switch priority 4096 and the subsequent switches with rising numbers based on their path of the tree.

    Currently, the last switch in the largest branch of the tree is elected as CST root, and we are having some problems figuring out why - so any input is highly appreciated.

     

    Here is the output from 3 switches:

     

    Core switch: 

    Multiple Spanning Tree (MST) Information

    STP Enabled : Yes
    Force Version : MSTP-operation
    IST Mapped VLANs : 1-4094
    Switch MAC Address : 9020c2-6a9860
    Switch Priority : 4096
    Max Age : 20
    Max Hops : 20
    Forward Delay : 15

    Topology Change Count : 459
    Time Since Last Change : 43 mins

    CST Root MAC Address : 883a30-390480
    CST Root Priority : 0
    CST Root Path Cost : 16000
    CST Root Port : 1/16

    IST Regional Root MAC Address : 9020c2-6a9860
    IST Regional Root Priority : 4096
    IST Regional Root Path Cost : 0
    IST Remaining Hops : 20

     

    2nd Switch on the branch where the root is elected:

    Multiple Spanning Tree (MST) Information

    STP Enabled : Yes
    Force Version : MSTP-operation
    IST Mapped VLANs : 1-4094
    Switch MAC Address : 883a30-390480
    Switch Priority : 32768
    Max Age : 20
    Max Hops : 20
    Forward Delay : 15

    Topology Change Count : 5
    Time Since Last Change : 33 mins

    CST Root MAC Address : 883a30-390480
    CST Root Priority : 0
    CST Root Path Cost : 0
    CST Root Port : This switch is root

    IST Regional Root MAC Address : 883a30-390480
    IST Regional Root Priority : 0
    IST Regional Root Path Cost : 0
    IST Remaining Hops : 20

     

    Switch elected as root bridge (4th switch on the branch)

    Multiple Spanning Tree (MST) Information

    STP Enabled : Yes
    Force Version : MSTP-operation
    IST Mapped VLANs : 1-4094
    Switch MAC Address : 9020c2-44ce40
    Switch Priority : 49152
    Max Age : 20
    Max Hops : 20
    Forward Delay : 15

    Topology Change Count : 124
    Time Since Last Change : 52 mins

    CST Root MAC Address : 883a30-390480
    CST Root Priority : 0
    CST Root Path Cost : 20000
    CST Root Port : 28

    IST Regional Root MAC Address : 9020c2-44ce40
    IST Regional Root Priority : 49152
    IST Regional Root Path Cost : 0
    IST Remaining Hops : 20

     

     

    When viewing the mst-config, I do see that there is a mismatch between "MST Configuration Name" on all switches, but the MST Configuration Revision : 0
    MST Configuration Digest : 0xAC36177F50283CD4B83821D8AB26DE62 

    are all identical.

     

    Full output of one mst-config:

    MST Configuration Identifier Information

    MST Configuration Name : 9020c244ce40
    MST Configuration Revision : 0
    MST Configuration Digest : 0xAC36177F50283CD4B83821D8AB26DE62

    IST Mapped VLANs : 1-4094

    Instance ID Mapped VLANs
    ----------- ---------------------------------------------------------

     

     

    Regards

    Thomas



  • 2.  RE: Problems electing (correct) CST root

    Posted Oct 19, 2020 05:58 AM

    All your involved switches MUST use the same MST Configuration Name.



  • 3.  RE: Problems electing (correct) CST root

    Posted Oct 19, 2020 06:08 AM

    Thank you for the reply - this leads me to the next, logical, question: How do I enforce the same configuration name on all switches?

     

    To clarify a little: I do know of the spanning-tree config-name setting, but do I really have to perform this action on all swicthes? Or is it possible to set this on the core switch and let it propagate to all?

    /Thomas



  • 4.  RE: Problems electing (correct) CST root
    Best Answer

    Posted Oct 19, 2020 07:29 AM

    AFAIK you need to setup the same MST region configuration name on all involved switches being part of the very same MST region (clearly!).

     

    It's important that these MST:

     

    Region Name: via the spanning-tree config-name command

    Region Revision Number: via the spanning-tree config-revision command

     

    are the same across all various involved switches part of the same MST region you decided to setup.



  • 5.  RE: Problems electing (correct) CST root

    Posted Oct 19, 2020 07:42 AM

    Then that will be done. Appreciate the feedback, thank you!

     

    /Thomas



  • 6.  RE: Problems electing (correct) CST root

    Posted Oct 19, 2020 09:13 AM

    Glad you fixed.

     

    I should have added this (it was a little bit too implicit and so I forgot to clearly wrote it, a little bit redundant but it's just to clarify):

     

    All involved switches will be in the same MST Region IF each switch is configured with the same:

     

    1. Configuration Name
    2. Configuration Revision Number
    3. Mapping of "VLANs-to-MST-Instance"

    thus if you need them into the same MST Region then you need to configure the above parameters in all involved switches.

     



  • 7.  RE: Problems electing (correct) CST root

    Posted 18 days ago
    hope this post is still active..
    is this the case also for mst 0 (default instance) do I need to set the same name, revision number for mst 0?

    ------------------------------
    Amir Katz
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  • 8.  RE: Problems electing (correct) CST root

    Posted 18 days ago
    If all involved switches are planned to be part of the same - default, in your case - MST Region the same rules apply (in other terms: a common MST region's configuration name and revision number should be used on all involved switches being part of - partecipating into - the very same MST region).

    ------------------------------
    Davide Poletto
    ------------------------------