11
Multicast packet forwarding mechanism
In a multicast model, multicast receivers of a multicast group are usually located at different areas on
the network. They are identified by the same multicast group address. To deliver multicast packets to
these receivers, a multicast source encapsulates the multicast data in an IP packet with the multicast
group address as the destination address. Multicast routers on the forwarding paths forward
multicast packets that an incoming interface receives through multiple outgoing interfaces.
Compared to a unicast model, a multicast model is more complex in the following aspects:
•
To ensure multicast packet transmission in the network, different routing tables are used for
multicast forwarding. These routing tables include unicast routing tables, routing tables for
multicast (for example, the MBGP routing table), and static multicast routing tables.
•
To process the same multicast information from different peers received on different interfaces,
the multicast device performs an RPF check on each multicast packet. The RPF check result
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.
