Multicast Concept

Multicast transmits data from one source to multiple receivers. Figure 1-3 shows the multicast transmission model. HostA and HostC are interested in information sent from Source and request the information. The data sent from Source is received only by HostA and HostC.

Figure 1-3 Multicast transmission

Entities in an IP multicast model are:

• Multicast group: a group of receivers identified by a multicast IP address. User hosts (or other receiver devices) that have joined a multicast group become members of the group and can identify and receive the IP packets destined for the multicast group address.
• Multicast source: a sender of multicast data. "Source" in Figure 1-3 is a multicast source. A multicast source can simultaneously send data to multiple multicast groups. Multiple multicast sources can send data to a multicast group simultaneously. Generally, a multicast source does not need to join a multicast group. The source designated router (DR) manages multicast source registration and setup of shortest path trees.
• Multicast group member: a host that has joined a multicast group. HostA and HostC in Figure 1-3 are multicast group members. Memberships of a multicast group change dynamically. Hosts can join or leave a multicast group at any time. Members of a multicast group can be located anywhere on a network.
• Multicast router: a router or Layer 3 switch that supports IP multicast. The routers in Figure 1-3 are multicast routers. They provide multicast routing functions, and those connected to user network segments also provide multicast member management functions.

Table 1-1 provides an analogy to explain the concept of IP multicast.