I have a scenario of six switches (C2960S, Version 12.2) running out of PVST instances so a transition to MST is inevitable. Seeing how small scale the setup is, I assume I could shut a few links to ensure there are no loops, turn off spanning-tree and then configure MST.

Once done I should be able 'no shut' the shutted links. Am I right in it being this straight forward, or are there any caveats with my approach?

switch topology

UPDATE 1: I did convert the root bridge, after shutting enough links to not have a ring/loop, to mst using the following configuration:

#spanning-tree mst configuration
#name data
#revision 10
#instance 1 vlan 1-1024
#spanning-tree mode mst

After this I lost connection between the root bridge and a neighbor switch. The state of the port was BKN. I proceeded to check all trunks native vlans, but couldn't find a mismatch. I quickly reverted and here I am, with not much more information.

UPDATED QUESTION 1: Given the simple topology, if I disable enough links to prevent the ring and thus loops, could I not disable stp altogether and then set up mst on a switch at a time?

  • do you care how long the outages are during the pvst to mst transition? Commented Oct 26, 2013 at 20:11
  • 2
    How are you running out of 'instances' how many vlans are we talking about here?
    – fredpbaker
    Commented Oct 27, 2013 at 21:56
  • @fredpbaker, keep in mind that with PVST you have one spanning-tree instance per VLAN per port carrying the VLAN. So a 48 port switch with one VLAN is already using 48 instances. Throw in a couple trunk/tagged ports and just over 40 VLANs (few with more trunk/tagged ports) and you can easily exceed the 128 instance limit of a 2960S.
    – YLearn
    Commented Nov 14, 2013 at 1:04
  • BKN indicates a configuration error, and MSTP is preventing things from going haywire by going into this state. We need to see your configurations to assist further... also see the NE question checklist Commented Jan 4, 2014 at 12:14
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Commented Aug 8, 2017 at 15:53

1 Answer 1


I recently went through a similar migration that eradicated proprietary protocols (i.e., PVST+, HSRP, etc.); our transition was focused on posturing us for a 'vendor neutral' network. Since you're running Cisco devices, you're likely running PVST+.

Cisco covers this in more detail, but the crux of it is fairly simple: establish a hierarchy and, contrary to what common sense would tell you, start from the core and move outward.

Set your priorities in a manner that cascades down. Base this hierarchy on what is closest to a central node. All nodes participating in this STP setup should be on a common structure, so for simplicities sake, manually set one root bridge with the spanning-tree vlan xx,xx root primary.

Once you're ready for the migration to begin, start with the core. The reason for this is because MST is backwards compatible with PVST, not the other way around. A PVST core won't be able to communicate with BPDUs from MST nodes; those trunk links will drop. If everything is going right, you'll see a small drop while the new STP protocol comes online and figures out the current lay of the land.

Then just trickle these protocol changes down all the way to your furthest nodes.

End-users might not even notice the small blip while MST elects a root bridge.

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