Fabric Path STP Interaction
Fabric Path and STP interaction is based on core ports and edge ports. Let’s first discuss edge ports, as edge ports are connected to classical ethernet domain so, edge ports process the BPDU and participates in STP topology calculation.
However Core ports are those ports which are part of only fabric path domain so these ports don’t participate in STP and neither process any BPDU and nor transmits any BPDU. Even TCN are also not propagated to other domain connected to same fabric path fabric.
Due to this above behaviour, entire Cisco fabric Path domain appears as a single STP bridge to any Classical Ethernet STP domains and work like the all Cisco fabric Path bridges share a common bridge ID C84C.75FA.6000 and this BID is statically defines and cannot be configured by any administrator.
You must configure all Cisco FabricPath Edge Switch as STP root by following commands spanning-tree vlan X root primary to make the edge switch root for all vlan participating in STP. Even though multiple edge switch are connected to fabric Path domain, then all those switch must have same bridge priority value.
You should also configure STP root-guard on all edge ports which are going to connect to classical STP ethernet switch to make sure that all edge switch should be root.
TCN Forwarding in fabric Path:
By default STP BPDU and TCNs are not forwarded through Cisco fabric Path core port. But there are some configuration option available by which we can enable fabric Path edge switch to forward TCN through Fabric path domain to support certain topologies.
Below Figure states the case why TCNs must be carried over fabric path domain to optimize reconvergence on switch or failed interface.