A loop prevention mechanism is required at Layer 2 to stop wayward traffic from crippling the network. Reverse Path Forwarding Check (RPFC) is the chosen method of suppressing loop traffic with SPBM. RPFC was originally designed for IP traffic at Layer 3 where it checks the source address of the packet against the routing entry in the routing table. The source address must match the route for the port it came in on otherwise the packet is illegitimate and therefore dropped.
With SPBM, the node matches the source B-MAC address against the ingress port to establish validity. If the frame is not supposed to come in that port, it is immediately suppressed imposing a guaranteed loop control. If there is no VLAN FDB entry to the source MAC address with the outgoing port as the ingress port, the frame will be dropped.