MST load balancing

[Nader ibrahimi]

Hello to everyone 

I was practicing MST with multi-region, and I am quite concerned about how to perform load balancing in MST.

SW-2's 0/1 is the master port for instance 1 and SW-2 is also the regional root  if I want to make SW-3's 0/0 the master port, is it possible, and if it is, how do I perform load balancing? 

[M02@rt37]

Hello @Nader ibrahimi 

You can make SW-3’s 0/0 the master port for Instance 1 by adjusting the path cost and bridge priority. In MST, load balancing is achieved by assigning different VLANs to different MSTi and controlling the spanning tree topology through cost manipulation and priority settings.

Since SW-2's 0/1 is currently the master por and SW-2 is the regional root for Instance 1, you need to adjust the path cost so that SW-3's 0/0 becomes the preferred master port. Lowering the path cost of SW-3’s 0/0 towards the root will make it the better choice for the master port, while increasing the cost on SW-2’s 0/1 will make it less preferable.

Additionally, if SW-2 is the regional root, adjusting SW-3’s bridge priority will help shift the rgional root role to SW-3. To ensure load balancing, VLANs should be distributed across multiple MST instances, such as assigning VLAN 10-50 to Instance 1 and VLAN 51-100 to Instance 2. 

[Nader ibrahimi]

If I change the path cost, then the regional root will change. If I want to change one master port for IMSTI-2 in SW-3, then this won't effect the master port. So the result is we can change the master ports.

Rule

1. Regional root is the master port for all MSTI except MSTi0. 

[soniabraga8795 · Accepted solution]

Yes, it’s definitely possible to influence which port becomes the master port in MST (Multiple Spanning Tree) and achieve load balancing across instances.

To make SW 3’s 0/0 port the master port for instance 1, you need to adjust the port priorities and path costs. Here’s a general approach:

1. Lower Port Priority on SW 3’s 0/0 port:
   On SW-3, specifically for instance 1, you can set a lower port priority for the 0/0 interface. MST selects the master port based on the port priority and path cost.

2.  interface gigabitEthernet 0/0
   spanning-tree mst 1 port priority 0

3. Adjust Path Costs (if needed):
   You can manually tweak the path cost to influence which link is preferred. Lower path cost = more preferred.