A source sends its data to an IP address within a special block of addresses reserved for multicast streams (224.0.0.0 to 239.255.255.255). Each such IP address identifies a multicast group.
Systems that wish to receive data being sent to a multicast group need to register with a designated router (DR) using the Internet Group Management Protocol (IGMP). The data for the group is sent to a rendezvous point (RP) which acts as the initial distribution point. Routers between the RP and each DR (with registered receivers) work out how to route the data for each group to each DR using the minimum number of data replications. This set of paths forms the multicast distribution tree. Once data for a multicast group reaches the DR at the end of tree, it is sent into the network that has registered receivers in it.
The path information for group trees is established using Protocol-Independent Multicast (PIM). Once traffic is flowing from sources to receiving networks, PIM can determine that the best path may be one that does not include the RP. RPs use an additional protocol, Multicast Source Discovery Protocol (MSDP), to set up peer-to-peer relationships in order to create multicast trees that span autonomous systems, and to create redundancy groups.
| ABBREVIATION | PROTOCOL | DESCRIPTION |
|---|---|---|
| PIM | Protocol-Independent Multicast | Sets up and maintains the multicast distribution tree within and between networks. Can be configured in sparse mode, dense mode, or sparse-dense mode. |
| IGMP | Internet Group Management Protocol | Allows hosts to register in a multicast group in order to receive traffic sent into that group by sources. |
| MSDP | Multicast Source Discovery Protocol | Allows multicast rendezvous points to communicate with each other between Border Gateway Patrol (BGP) autonomous systems and is also used to support rendezvous point redundancy. |
