Multiple Spanning Tree (MST) Related Basic Configuration

MST is nothing but grouping per VLAN RSTP instances. Back end calculations are same as in Rapid PVST. Grouping multiple instances of Rapid PVST minimizes the processor overhead in managing the topology.

Let's see how it will be configured..

VLAN 11-15 are created in all switches.

By default, Cisco switches run PVST. For all 6 VLANs (including the VLAN 1), there will be 6 PVST instances on all 3 switches.






Let's see what are the roots by default calculations here.

SW-1#show spanning-tree root

Now let's change the mode of STP to MST on all switches..

SW-1(config)#spanning-tree mode mst
SW-2(config)#spanning-tree mode mst
SW-3(config)#spanning-tree mode mst

Now let's see the show spanning-tree root & show spanning-tree outputs..

Now we can see that there is only one instance for all 6 VLANs. There is only one root. Because I entered only the global mode command just to change the mode of STP, all default MST settings have been applied. We can see them by show spanning-tree mst configuration command..

As we can see the revision is 0 and all VLANs are mapped to instance 0. Also there is no name for the region. MST region is just a group of switches which shares same revision number and VLAN to STP instance mappings.

We can configure these parameters in mst configuration mode as well for more controllability. But all the switches in same region must have same configuration.
Note:- these parameters can be pushed through VTP version 3..

SW-1(config)#spanning-tree mst configuration 
SW-1(config-mst)#name REGION-A
SW-1(config-mst)#revision 1
SW-1(config-mst)#instance 1 vlan 11-13
SW-1(config-mst)#instance 2 vlan 14-15

SW-2(config)#spanning-tree mst configuration 
SW-2(config-mst)#name REGION-A
SW-2(config-mst)#revision 1
SW-2(config-mst)#instance 1 vlan 11-13
SW-2(config-mst)#instance 2 vlan 14-15

SW-3(config)#spanning-tree mst configuration 
SW-3(config-mst)#name REGION-A
SW-3(config-mst)#revision 1
SW-3(config-mst)#instance 1 vlan 11-13
SW-3(config-mst)#instance 2 vlan 14-15

Related show command outputs are as following..

Now let's make SW-2 the root for instance 2 by the spanning-tree mst 2 root primary macro command on SW-2..

Note:- 

If it is a migration from other STP to MST, it is best practice to start configuration from mst configuration sub mode and finally enable mode globally starting from root switch..