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It was decided recently to migrate a large campus network from the short mode STP costs (16-bit) to the long mode STP costs (32-bit).

I have clearly found it documented that this needs to be a "universal" configuration in that all switches need to support it and it needs to configured on all switches, however I have found very few discussions on how to migrate from short mode to long mode.

Most spanning tree costs in the campus network are dynamic, but there are some places where the costs were set manually.

  1. When configuring long mode, would it be better to move from the root of spanning tree outwards to the edge or from the edge in towards the root?
  2. How will the current manual costs be affected and should these be adjusted before hand in some fashion?
  3. What issues should we be aware of before making this change?
  4. If there were a few switches that did not support long mode and were left in short mode, would this still work? If so, should we be aware of?
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  1. I would run the change from root outwards but I don't think there are any use cases for doing it each way (someone correct me if I'm wrong).

  2. Manual costs will stay exactly the same so you should at some point convert these to fall in line with your new costs once you have changed over.

  3. Be aware of the fact that during topology changes or before you change over the manual metrics you may be running a sub-optimal L2 path between nodes.

  4. It is recommended that all your switches run the same cost type and reference bandwidth, although I can't see any fatal flaws in not having them all the same it again could give you an inconsistent / sub-optimal L2 path.

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