Hi Ulrich,
This is purely for simplifying the implementation at that stage.
There are two STP roles that can be forwarding in STP: root role and designated role.
By requiring that the FabricPath region is the root, we're in fact requiring that the forwarding edge ports are designated only, i.e. we don't support the root port role (by extension, we don't support the alternate role either BTW).
When a bridge has a root port, it receives better STP information on this root port and needs to distribute it on all its designated port. So if we supported the root role, we would need to support the propagation of STP information across the FabricPath network. Potentially, we would also have to sync some STP state at the edge of the fabric before allowing a FabricPath link core port to start forwarding traffic. All those problems are just avoided by enforcing that the FabricPath region is the root. Everything is simple.
Note that we have the code to do this STP propagation/sync across the fabric. We'll probably release it at some point in the future. For now, this restriction has not had too much impact on FabricPath deployment.
Regards,
Francois
