Interesting functionality what is the use case for that scenario vice simply using one vlan for both subnets??
Obviously there seems to be a reason to have two VLANS vice one and normally if there is some degree of sharing (common printer etc) then firewall can be made so that the connectivity needed exists at layer 3.
- Company acquisition with overlapping VLAN's
- Intentionally merging 2 tagged VLAN's as if it were one, like a router-on-a-stick config at layer2 during the above (requires MSTP to not cause a loop)
- Connections to other providers/customers i.e. they specify they need connectivity on VLAN 10 but you already use that VLAN for something else. You can bring it in on 500 and rewrite as 10 to them
- I've had to do this when I needed to split 2 customers for PPPoE policy routing (Customer A goes 1 way, Customer Y goes the other) but it was cleaner and easier to not modify things on the customer side as I wanted all radio configs to be exactly the same so if/when that customer leaves the radio is re-used elsewhere it keeps working without needing anything changed. Was a mikrotik in front of it that simply rewrote VLAN X as VLAN Y, easier on the installers
- I suppose you could use it for tagging of services in a way. i.e. VLAN10 everywhere, as above makes it easier with equipment moves, but you rewrite as '1110' for location 1, '1210' for location 2 etc. Q-in-Q would be better but maybe its not supported on some equipment