Question about MSTP bridge priority

[kay.kang]

I've set one switch as root primary for MST instance 0 and 1. (I see the switch has locally priority 0 for two instances) 

And set as root secondary for MST instance 2. (I see the switch has locally priority 4096 for the instance 2).

When I checked the bridge ID for each instance on nexus switches(Non Root Bridge),

it shows bridge ID for each instance increased by instance number.

(For example, the root bridge priority for Instance 0 is 0 and it shows 1 for instance 1 and shows 4098 for instance 2).   

I am not sure if this is a default action of MST or STP.

[M02@rt37 · Accepted solution]

Hello @kay.kang 

It is the expected behavior of the MSTP, Yes ! MSTP allows multiple spanning trees to be configured within a single physical network, where each instance can have a unique spanning tree topology.

MSTP Bridge Priority Calculation ?
The priority for a Bridge id in MSTP is calculated by adding the bridge priority (a configurable value) to the MST instance number.

When you set a bridge priority for a specific MST instance, MSTP will adjust the priority for each instance by adding the instance number to the priority value you configured. This allows for unique bridge priorities across different MST instances.

For instance 0, if the bridge prioriti is set to 0 for instance 0, the priority remains 0 for that instance.

For instance 1, the bridge priority would be 0 (base priority) + 1 (instance number) = 1.
For instance 2, with a base priority of 4096, the priority would be 4096 (base priority) + 2 (instance number) = 4098.

The priorities are calculated in your case ?
- Instance 0: Base priority is 0 + Instance number 0 = 0.
- Instance 1: Base priority is 0 + Instance number 1 = 1.
- Instance 2: Base priority is 4096 + Instance number 2 = 4098.

This behavior is by design to ensure each instance has a unique priority value, facilitating correct MSTP operation.