Multiple Spanning Tree (MST)

Lessons Discussion
77

Hello NetworkGuy

Take a look at this network topology:

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There are only so many possible STP logical topologies that you can have. For example, the possibilities are:

  • SW1 is the root with Gi0/1 of SW2 as the blocked port
  • SW1 is the root with Gi0/1 of SW3 as the blocked port
  • SW2 is the root with Gi0/1 of SW1 as the blocked port
  • SW2 is the root with Gi0/0 of SW3 as the blocked port
  • SW3 is the root with Gi0/0 of SW2 as the blocked port
  • SW3 is the root with Gi0/0 of SW1 as the blocked port

Now imagine that you have 100 VLANs configured on these switches. With PVST+ you will need 100 instances of STP. Those instances will necessarily match one of the above 6 logical STP topologies. That means that you will have multiple instances of the same logical STP topology.

MST groups VLANs that share the exact same logical STP topology, thus needing only 6 instances of STP rather than 100. Does that make sense?

I hope this has been helpful!

Laz

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