MST Root behaviours between two regions

[D@1984]

Hi,

I have two switches that are configured for region A, and now I need to connect them to some switches that are in region B. there is concern around spanning tree loops and I want to make sure my configuration is right.

so my understanding is that as regions are different, they don't exchange mst instances, but vlans coordinated only via CIST. so they should elect a CIST bridge and then that decides how traffic flow and which redundant ports to get blocked. The trunk link between regions going to be configured for vlan 100 & 200. they are in MST4 in region B and MST1 in region A (I assume this doesn't matter as regions are not the same)

there is a link between my A and B switches and now I'm connecting a1-b1 and a2-b2. a1 has the priority of 0, so am I right that a1 will be collected as CIST root? so B switches see A switches as one virtual switch that is elected as root so one of the links in B region will be forwarding and the other is blocked for vlan 100 that is extended between two regions? 

Is the priority of 0 enough config to make sure A is selected as bridge root? or do I need other considerations? the region A and B switches are from different vendors, do I need any specific config to make this work?  on the port config itself (ports that connect both switches) do I need any bpdu protection like BPDU filter? what are the recommendation config for scenarios like this?

Thanks

[paul driver]

Hello
If you have region A already active with all the switches that need to be attached -And your intention is to just attach region B – Then as long as this regions current core switch(s) have a MST priority less preferred then region A – (so higher than priority 0) you will be okay so therefore there would be no need to change region B mst switches to region A

Additionally, if you wish at any time for Region A not to have root paths towards Region B then you could apply guard-root on region A boundary links.

Lastly - I would advise though NOT to run bpdufilter if you have multiple inter - region boundary links.