Understanding Multicast

Multicast allows a source to send a single copy of data using a single IP address from a well-defined range for an entire group of recipients (a multicast group). A source sends data to a multicast group by simply setting the destination IP address of the datagram to be the multicast group address. Sources do not need to register in any way before they can begin sending data to a group, and do not need to be members of the group themselves. Routers between the source and recipients use the group address to route the data, forwarding duplicate data packets only when the path to recipients diverges.

Hosts that wish to receive data from the multicast group join the group by sending a message to a multicast router on a local interface, using a multicast group membership discovery protocol, such as IGMP (IPv4) or MLD (IPv6). For more IGMP information, see Overview. For more MLD information, see Multicast Listener Discovery (MLD) Configuration.

Multicast routers communicate among themselves using a multicast routing protocol, such as DVMRP, PIM-SM, or PIM-DM. These protocols calculate a multicast distribution tree (PIM-SM) or source-based tree (DVMRP and PIM-DM) of recipients to ensure that:

• Multicast traffic reaches all recipients that have joined the multicast group
• Multicast traffic does not reach networks that do not have any such recipients (unless the network is a transit network on the way to other recipients)
• The number of identical copies of the same data flowing over the same link is minimized

For more information, see Understanding Distance Vector Multicast Routing Protocol (DVMRP) and Understanding PIM.