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
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
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.
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:
-
Configuration Name
-
Configuration Revision Number
-
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.
Original Message
Original Message:
Sent: Oct 19, 2020 09:12 AM
From: Davide Poletto
Subject: Problems electing (correct) CST root
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:
-
Configuration Name
-
Configuration Revision Number
-
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.
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
------------------------------
Original Message
Original Message:
Sent: Nov 10, 2020 02:49 AM
From: Amir Katz
Subject: Problems electing (correct) CST root
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
Original Message:
Sent: Oct 19, 2020 09:12 AM
From: Davide Poletto
Subject: Problems electing (correct) CST root
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:
-
Configuration Name
-
Configuration Revision Number
-
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.
