Multiple Spanning Tree – MST


The next section of the CCIE R&S v5 Blueprint that I was running through was “1.1.f (i) PVST+/RPVST+/MST”, specifically MST or Multiple Spanning Tree. Pretty straight forward process around this one and here is the summary that I will give for it as there is not too much to it.

Multiple Spanning Tree, or MST, allow you as the admin to configure multiple instances of spanning tree on the same switch. Sometimes, especially in smaller environments, there may be a single switch acting as the “core”. A lot of the time that switch will be the root switch for every vlan when it comes to spanning tree. In a small environment that is just fine and usually works as it should. In larger environments or other special situations, there may be a reason to use MST. My example would be a large company with a location made up of two or more buildings. All vlans are shared between all buildings. My example has a very basic diagram that looks like this:


Each switch represents a “core or distribution” switch in each of three buildings. Vlans 2 and 3 are primarily used in building 1, vlans 4 and 5 in building 2, and 6 and 7 for building 3. All vlans are used by users moving between the three buildings though. For your setup though you might want SW1 to be the root switch for the vlans primarily used in building 1, SW2 for building 2, and SW3 for building 3. MST can allow you to have multiple instances to accomplish each of these tasks. I am focusing on SW1’s config, but it is very easy to see how this config changes for the other two switches.

spanning-tree mst configuration
name MST1
revision 1
instance 1 vlan 2,3
instance 2 vlan 4,5
instance 3 vlan 6,7

That is the start of the MST configuration where the instances are created and vlans are added to each. The region of MST can also be assigned a common name here that is shared among the switches in this group. The next step is to assign your priorities to each of these instances. That way, you can force this switch to be root in one instance and non-root in another instance.

spanning-tree mst 1 priority 0
spanning-tree mst 2 priority 4096
spanning-tree mst 3 priority 4096

That assigns SW1 as the root switch for instance 1 and then it will be in place to be the backup for instances 2 and 3 because for each of these instances, SW2 and SW3 will have a priority of 0 respectively. The last thing to do is to set the global spanning-tree mode to MST for the whole switch. One more command to enter and that is:

spanning-tree mode mst

That’s all there is to it! You now have three instances running under MST with specific vlans and priorities assigned to each. You can then continue to assign your global spanning-tree setting as well such as the different portfast options, etc.

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