Multicast implements a reverse path forwarding check (RPF). An RPF checks the path that multicast packets take between their sources and the destinations to prevent loops. Multicast requires that an incoming interface be the outgoing interface used by unicast routing to reach the source of the multicast packet. RPF forwards a multicast packet only if it is received on an interface that is used by the router to route to the source.
If the forwarding paths are modified due to routing topology changes, any dynamic filters that may have been applied must be reevaluated. If filters are removed, the associated alarms are also cleared.