MST Root Bridge Selection and Port Blocking Behavior

In a Multiple Spanning Tree (MST) environment with three instances (the Internal Spanning Tree (IST), instance 1, and instance 2), each instance can have a different root bridge based on bridge priority settings. When configuring a priority of 4096 for different switches across instances:

• The switch with priority 4096 will become root bridge for that instance.
• Default bridge priority is 32768.
• Bridge ID uses priority first, then MAC address as tiebreaker.

A switch can have no blocked ports across all instances even if it's root bridge for only one instance. Take a look at this topology for example, and assume the three instances have been configured here:

Imagine the priorities are set up such that:

1. For IST: If Switch1 is root, it will have no blocked ports
2. For Instance 1: If Switch2 is root, Switch3's port could be blocked based on STP parameters
3. For Instance 2: If Switch3 is root, Switch2's port could be blocked based on STP parameters

This configuration allows Switch1 to maintain all ports in forwarding state across all instances, while still only being root bridge for one instance. The actual port blocking depends on:

• Path cost
• Bridge priority
• Port priority
• Port ID

Links

https://networklessons.com/spanning-tree/multiple-spanning-tree-mst