After RPF succeeds and the virtual router has a route entry in
its multicast routing information base (mRIB), the virtual router
maintains source-based tree entries (S,G) and shared tree entries
(*,G) in its multicast forwarding information base (multicast forwarding
table or mFIB). Each entry includes the source IP address, multicast
group, incoming interface (RPF interface) and outgoing interface
list. There can be multiple outgoing interfaces for an entry because
the shortest path tree can branch at the router, and the router
must forward the packet out multiple interfaces to reach receivers
of the group that are located down different paths. When the virtual
router uses the mFIB to forward a multicast packet, it matches an
(S,G) entry before it attempts to match a (*,G) entry.