MST and PVRST+ integration

Answered Question
Nov 9th, 2007
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I'm planning to integrate a new network running MST with a legacy network running PVRST+.


There will be a trunk supporting 2 vlan's between the 2 networks, with an additional trunk to be added later supporting the same vlans.


Reading up on this, PVRST will see the MST domain as one switch.


The bit I'm not clear on, will PVRST see a seperate STA instance for each vlan on the trunk, or is the link considered as one network?


If its per vlan then I should be able to make each trunk the root for one of the vlans so I can distribute traffic across both paths, that would be ideal.


Correct Answer by Francois Tallet about 9 years 4 months ago

Our MST implementation will detect the PVST neighbor and start sending one PVST BPDU per vlan. Those per-vlan BPDUs will be a copy of the CIST information (instance 0).

So each vlan instance will receive BPDU and think it has a PVST peer, but in fact, all of them are talking to a single MST instance. Doing this kind of trick has a constraint. The MST bridge must see consistent BPDUs coming from the PVST neighbor. By consistent I mean that the CIST information must be better or worse than the information sent by any of your two vlan. If for example the information of MST 0 is better than vlan 1 but worse than vlan 2, you will enter an inconsistency mode.

The PVST/MST interaction is relatively complex. I really regret that we did not go for something less powerful but simpler. Anyway, what you are trying to achieve should work, as long as you pay attention when the root for the different vlans are. I recommend you put the root on the side of the MST region. This way, all the MST ports are designated, and you can still do load balancing on a per vlan basis with PVST.

See also: http://www.cisco.com/warp/public/473/147.html#mst_region_world

Regards,

Francois

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Correct Answer
Francois Tallet Fri, 11/09/2007 - 07:15
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Our MST implementation will detect the PVST neighbor and start sending one PVST BPDU per vlan. Those per-vlan BPDUs will be a copy of the CIST information (instance 0).

So each vlan instance will receive BPDU and think it has a PVST peer, but in fact, all of them are talking to a single MST instance. Doing this kind of trick has a constraint. The MST bridge must see consistent BPDUs coming from the PVST neighbor. By consistent I mean that the CIST information must be better or worse than the information sent by any of your two vlan. If for example the information of MST 0 is better than vlan 1 but worse than vlan 2, you will enter an inconsistency mode.

The PVST/MST interaction is relatively complex. I really regret that we did not go for something less powerful but simpler. Anyway, what you are trying to achieve should work, as long as you pay attention when the root for the different vlans are. I recommend you put the root on the side of the MST region. This way, all the MST ports are designated, and you can still do load balancing on a per vlan basis with PVST.

See also: http://www.cisco.com/warp/public/473/147.html#mst_region_world

Regards,

Francois

aacole Mon, 11/12/2007 - 08:38
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  • Bronze, 100 points or more

Thanks Francois for the information. I've put the root for the VLAN's in the MST region, but not yet implemented the second trunk link, so cannot really determine if this is working the way I wanted, I'll have another look at how I've set this up.


Andy

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