IP Multicast

Unicast IP traffic is aways destined for a single interface. Special considerations must be taken for multicast traffic, which can have many destinations.

Reverse Path Forwarding

Routers perform an RPF test on every multicast packet they receive. The source IP address and interface are checked the routing table to ensure the packet arrived on the same interface that would be used to send traffic to the source address. This is the reverse of normal packet forwarding, in which the destination address is looked up.

Multicast Trees

Multicast traffic must be routed in a loop-free tree, away from the source. These are distribution trees.

Multicast routes are display in the form (S,G), displaying the source and group IP addresses.

Two types of distribution trees:

• Shared - A common set of links over which all multicast traffic flows; administratively pre-defined
• Source-rooted - A separate route exists for each source

Shared trees can be extended with source-rooted trees at rendezvous points (RPs).

Multicast Routing Protocols

Modern multicast routing protocols include:

• Protocol Independent Multicast (PIM)
• Multicast OSPF (MOSPF)
• Multicast BGP (MBGP)
• Distance Vector Multicast Routing Protocol (DVMRP)

Dense-mode multicast routing protocols assume all hosts are interested in receiving multicast traffic, and prune out only hosts which explicitly ask not to receive it.

Sparse-mode protocols work the opposite way, only forwarding multicast traffic to hosts who explicitly request it.