Clarification on understanding how MST and PVST interoperate

Unanswered Question
Feb 24th, 2009

I'm playing around with MST and PVST in a lab and I'm seeing some behavior that seems confusing to me, so I'm looking for a bit of clarification.

For PVST, every VLAN does it's own STP, thus each STP is it's own CST for other PVST switches.

In an MST region, the IST instance 0 is the CIST for that region and that IST instance 0/CIST acts as the CST when speaking to non MST devices.

In a Cisco MST/PVST scenario, the MST switch detects a PVST neighbor so the MST IST instance 0 CIST (CST) sends BPDUs for each of the vlans that it knows about in it's CIST to those PVST switches.

The part that I'm not understanding is when you add an instance inside an MST region (where that region is connected to a PVST device), that VLAN is then removed from instance 0. If instance 0 is what sends the BPDUs to a PVST device, how can my PVST switch possibly be seeing a BPDU for the VLAN that is no longer in instance 0?

Along the same lines, what I'm seeing on my PVST switch doesn't help with that confusion :) It sees the MST switch as type P2p Peer(STP) where the MST switch sees the PVST switch as type P2p Peer(PVST). If the PVST switch sees the MST switch as a STP device, how can it receive BPDUs for multiple VLANs? I suppose if the MST switch is sending BPDUs on VLAN1, that sorta makes sense, but again, if that's the case, I don't understand how the PVST switch can see a BPDU for VLAN900 if the MST switch is sending STP BPDUs on VLAN1. It's my understanding that the old-style STP doesn't send any VLAN information.

I have this problem too.
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pkurdziel Tue, 02/24/2009 - 19:49

MST can interact with legacy bridges that run PVST+ on a per-port basis, so it is not a problem to mix both types of bridges if interactions are clearly understood. Always try to keep the root of the CST and IST inside the region. If you interact with a PVST+ bridge through a trunk, ensure that the MST bridge is the root for all VLANs that are allowed on that trunk. Do not use PVST bridges as the root of CST.

Ensure that all PVST spanning tree root bridges have lower (numerically higher) priority than the CST root bridge.

http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a00807b075f.shtml#mst1

jlixfeld Tue, 02/24/2009 - 21:17

The root of the CST and IST are inside the region; the MST bridge is the root bridge, so it is also the root bridge for the PVST bridge.

I have read the posted link a couple of times prior, but am still confused about how the MST CST can send PVST+ BPDUs for a VLAN that has been removed from instance 0 because it was added to instance 1.

The port(s) between the MST switch and PVST switch are trunking all VLANs and the VLANs are the same on the MST switch as on the PVST switch.

In my lab, I have MST and PVST interoperating correctly. I'm just trying to understand why it's operating correctly because my understanding of the protocol is clearly wrong since I expect the VLAN in instance 1 to be unknown to the PVST switch, yet it is not.

mbacklund Fri, 03/20/2009 - 05:48

When you connect MST with PVST+,you leaving the MST region which meen that pvst+ only gets MST0 BPDU:s on every VLAN and you follow MST0 topology. In PVST+ on every vlan. Internal instances, your instance 1 follow the MST0 topology at the boundary ports, pvst+.

vaughancahill Fri, 03/20/2009 - 08:33

mbacklund is right,

MSTI0 is the IST/CST region inside MST.

So think of MSTI0 as the CST region outside of MST.

The MST to PVST+ interaction replicates the BPDU from IST for each vlan. This simulates a PVST+ neighbour.

The following link describes what you are after, but the entire page is also worth a read "Understanding Multiple Spanning Tree Protocol (802.1s)"

Interaction Between the MST Region and the Outside World

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

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