3Com Switch 7750 Configuration Guide Guide

Forwarding Mechanism of Multicast Packets 383
receiver sites can receive the packets. Therefore the forwarding process of
multicast is more complicated than unicast.
In order to guarantee the transmission of multicast packets in the network,
multicast packets must be forwarded based on unicast routing tables or those
specially provided to multicast (such as an MBGP multicast routing table). In
addition, to prevent the interfaces from receiving the same information from
different peers, routers must check the receiving interfaces. This check mechanism
is reverse path forwarding (RPF) check, which is the basis of performing multicast
forwarding for most multicast routing protocols.
Based on source addresses, multicast routers judge whether multicast packets
come from specified interfaces, that is, RPF check determines whether inbound
interfaces are correct by comparing the interfaces that the packets reach with the
interfaces that the packets should reach. If the router resides on a shortest path
tree (SPT), the interface that multicast packets should reach points to the multicast
source. If the router resides on a rendezvous point tree (RPT), the interface that
multicast packets should reach points to the rendezvous point (RP). When
multicast data packets reach the router, if RPF check passes, the router forwards
the data packets based on multicast forwarding entries; otherwise, the data
packets are dropped.