MSTP instance 0 multiple regions

[jacek.jagielka]

Case:

network containing a mix of Catalyst switches (which don't support classic RSTP) and Meraki switches (which support only RSTP) chained together in many ways, so there is no real benefit of using real MSTP.

A stack of Merakis is the root of the network, there are only a few vlans.

Question:

If I would setup all Catalyst switches to MSTP, leave the default mapping (all vlans to instance 0) and create a different region for each switch in that network (so in practice only changing the mst region on each switch) wouldn't this be some kind of RSTP "simulation" using MSTP? Then you don't need to deal with chosing regional roots or so, everyone is trying to get to the network root "as fast as possible".

What negative impact could this cause?

I went trough a lot of documents, some of them saying that we should avoid mapping vlans to instance 0 or have as less regions as possible, but not explaining why, any idea?

When I tested this solution the only weird result I got was that one link connecting to Catalyst switches (no redundand links) was seen from one side as "p2p internal" and from the other as "p2p bound(RSTP)" - is this bad?

And everything was working like a charm to be honest.

[paul driver]

Hello

Have a looks at a recently post regards Meraki and MSTP - here

[jacek.jagielka]

Hi Paul,

I know about the compatibility between RPVST and RSTP, and thanks for pointing this out, but this was not the question :-)

[paul driver]

Hello

@jacek.jagielka wrote:

would setup all Catalyst switches to MSTP, leave the default mapping (all vlans to instance 0) and create a different region for each switch in that network (so in practice only changing the mst region on each switch) wouldn't this be some kind of RSTP "simulation" using MSTP? Then you don't need to deal with chosing regional roots or so, everyone is trying to get to the network root "as fast as possible".

You would have multiple mstp cist regional roots anyway given that each individual switch would be in its own region and as all the vlans would be in the one instance I see no point in using MSTP  plus the fact the meraki's dont support it your CST root would see the merakis and all the other switches as an stp boundary’s which personally wouldn’t make any sense. 

[jacek.jagielka]

The point in using MSTP is to not use RPVST, which for example requires VLAN 1 to be active everywhere to avoid loops and speak to other RSTP devices.

It's better to use RSTP in a very mixed environment, instead of Cisco Proprietary RPVST protocol in my opinion.