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Spanning tree instances on a old switches and newer ones

I used to use rapid pvst on network , with a mix of rapid switches and some older switches running the old  (802.1D?) pvst

anyway , i encountered  a very big problem in that some switches could only do a maximum number of 128 spanning tree instances before they complain

so i migrated to MST to fix this issue ,   i have a question..

1) how do i know the max number of stp instances a switch can handle before it has a heart attack?

Also , if you run MST and you have some switches that run the older 801.2D pvst -- do they just have 1 spanning tree instance with the MST switches for all vlans?

1 REPLY
Cisco Employee

Re: Spanning tree instances on a old switches and newer ones

Hi Rob,

Regarding the maximum number of MST instances, I don't know any particular command for that. Usually, this limit is described in the Configuration Guide for the particular switch platform and IOS version. Have you had a look there? Let me know the switch type and IOS version and I'll gladly point you towards the proper document.

The MSTP and 802.1D or PVST/PVST+ cooperation is a complicated topic. I suggest reading these documents:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml

http://blog.ine.com/2008/07/27/mstp-tutorial-part-i-inside-a-region/

http://blog.ine.com/2008/09/24/mstp-tutorial-part-ii-outside-a-region/

In a very short summary: if the outside world is pure 802.1D then only the MST instance 0 interacts with the outside world, and the state of the boundary port (Blocking/Discarding, Forwarding) influences all instances (and therefore, all VLANs).

If the outside world is PVST/RPVST then the boundary port replicates MST instance 0 BPDUs onto outgoing PVST/RPVST BPDUs on all known VLANs, and maps the incoming PVST/RPVST BPDUs onto the MST instance 0. There are some things that must be met in order for this "PVST Simulation" to work properly - see the referenced documents for more information.

Best regards,

Peter

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