MSTP and PVST cohabitation

Unanswered Question
Feb 15th, 2010

Hi,

We are implementing this topology in our customer network.

Core routers are 7609-S connected via 10G links to each other  and 1G links to the customer platform which contains 3650 cisco switches and Sun

servers.

.

                                                 architecture.GIF

    The problem is that core switches in each site run MSTP, however 3560 switches run PVST. The solution works fine since both protocol are compatible but we have facing some problem especially when troublshooting or when adding new vlans (loops,...).We couldn't find any valuable document describing the installation and troubleshooting a network with PVST and MSTP.

So what we need to know if there's some technical references about this subject and what kind of commun problems we risk to face with this type of topology.

Thank you

Zied

I have this problem too.
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Peter Paluch Mon, 02/15/2010 - 08:45

Hello Zied,

There were a couple of documents I have found useful when deploying the MSTP and optionally interacting with PVST.

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

This document is a very fine while not-so-deep explanation about the MSTP and some of the caveats of deploying it.

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

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

These two articles by Petr Lapukhov are a wonderful explanation of how indeed does the MSTP work. I have to admit I have never seen a more detailed explanation of what is going on inside MSTP than this guy's articles.

Perhaps you will find some of these documents helpful.

As far as I am concerned, I have never deployed the MSTP in large networks but I am aware of these limitations:

  1. Making changes to the region configuration (name, revision number, mapping VLANs to instances) after MSTP is running may lead to disruptions in the connectivity, as they lead to creation of several regions. These disruptions should not, to my knowledge, be permanent nor long-termed but I hope that someone else will confirm this statement of mine. Several people have suggested that the assignment of VLANs to instances shall be done in forward, including even those VLANs which are not created yet. This way, you can create the VLAN without actually changing the MSTP region configuration, and it will be immediately mapped to a particular instance that you have defined beforehand. This works for Cisco but it may not be applicable for different vendors - for example the HP ProCurve switches do not allow you to map a VLAN to an instance if that VLAN is not created yet (sadly).
  2. When configuring the root bridge, it is the best option to make sure that a root bridge is present inside the MSTP region for all VLANs. If a bridge outside the region (i.e. a PVST switch) is the root bridge for more than one VLAN, it may cause permanent connectivity issues because of inability of the MSTP/PVST translation to cope with external root bridges having different BIDs. The different BIDs are caused by the VLAN number embedded in the BID of a switch.

Best regards,

Peter

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