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Multiple Spanning Tree Protocol (MSTP)

How a region maps several VLANs to a shared tree while allowing different groups to take different paths.

Map VLANs to instances

MSTP lets an operator assign multiple VLANs to one spanning tree instance. All VLANs mapped to that instance follow the same loop-free topology. Another instance can select a different root and forwarding path. This can use redundant links without running a separate tree for every VLAN.

Keep the region configuration consistent

Switches in the same MST region must agree on the region name, revision and VLAN-to-instance mapping. If any of these differ, the link is treated as a region boundary. The Common and Internal Spanning Tree connects regions and bridges outside them. Additional MST instances operate within their own region.

Try it in a lab

  1. Create one MST region and map VLANs 10 and 20 to instance 1 and VLAN 30 to instance 2. Verify that all switches report the same region configuration.
  2. Set a different root for instance 2. Compare port roles for the two instances. VLANs 10 and 20 should use the same spanning tree, while VLAN 30 can use a different path.

Sources