Multicast Packet Forwarding Mechanism
In a multicast model, a multicast source sends information to the host group identified by the multicast
group address in the destination address field of IP multicast packets. Therefore, to deliver multicast
packets to receivers located in different parts of the network, multicast routers on the forwarding path
usually need to forward multicast packets received on one incoming interface to multiple outgoing
interfaces. Compared with a unicast model, a multicast model is more complex in the following aspects.
To ensure multicast packet transmission in the network, unicast routing tables or multicast routing
tables (for example, the MBGP routing table) specially provided for multicast must be used as
guidance for multicast forwarding.
To process the same multicast information from different peers received on different interfaces of
the same device, every multicast packet is subject to a reverse path forwarding (RPF) check on the
incoming interface. The result of the RPF check determines whether the packet will be forwarded
or discarded. The RPF check mechanism is the basis for most multicast routing protocols to
implement multicast forwarding.