Hello
TBH there is no real requirement to have multiple mst or mst and non mst regions although they can be incurred in various ways though.
Such as when you need to join different switching domains together (merger of companies) or when you have migration of a non mst spanning-tree mode to mst, these will create separate spanning-tree domains
The links between these stp domains are called boundaries and you can have multiple redundant boundary interconnected links between different regions but LB wouldnt really viable, for these boundary interconnections need to run over a special spanning tree called CST (common spanning tree)
This CST which isn’t configurable but it is used by spanning-tree to allow communication for the differing stp domains to interact between each other in a consistent manner so they can elect the port state/roles of each boundary switch and more importantly which stp domain will become the root for the entire stp estate as role referred to as a CIST root.
So having multiple stp domains id say would be a disadvantage as most probably you would not have a full administrative control over the entire switching estate, plus you would obviously be running separate stp regions/ modes/mst instances etc...so as ive stated LB wouldn’t really be a viable option.
Furthermore if your just in a migration process say hardware and stp mode refresh then really you wouldn’t want to have any redundant physical links between the old/new stp domains (excluding aggregate ports)
you would just filter stp as a whole on its single boundary link whilst the migration is in process, this will isolate either stp domain but would not cause any stp changes within either domain but still allow communication between regions.
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Kind Regards
Paul
