The MDS uses VSANs, which are vritual fabrics, so it is possible to have a VSAN completely contained in a single MDS, or have that VSAN span multiple MDS. You are correct, if the VSAN spans multiple MDS, then each MDS in that VSAN will require a unique domain ID.
Commands to help you on the CLI
show vsan membership
what ports are in what VSAN number
show fcdomain domain-list vsan 100
shows all domain IDs that this switch knows about in VSAN 100 (for example). The domain with the [Local] is this switch. The one with [Principle] next to it, is the principle switch for this VSAN.
show topology vsan 100
shows what interfaces this switch is using and what interfaces the remote switch is using to cary VSAN 100 between the switches.
To view how devices register, and what the MDS has learned from the attached devices.
show fcns database vsan 100
show fcns database detail vsan 100
I use VSAN 100 as an example, yours may be different.
Hope this helps,
Mike
