1-4z Each receiver is a multicast group member (“Receiver” in Figure 1-3).z All receivers interested in the same information form a multicast group. Multicast groups are notsubject to geographic restrictions.z A router that supports Layer 3 multicast is called multicast router or Layer 3 multicast device. Inaddition to providing multicast routing, a multicast router can also manage multicast groupmembers.For a better understanding of the multicast concept, you can use the analogy of a transmission of TVprograms, as shown in Table 1-1.Table 1-1 An analogy between TV transmission and multicast transmissionStep TV transmission Multicast transmission1 A TV station transmits a TV program througha television channel.A multicast source sends multicast data to amulticast group.2 A user tunes the TV set to the channel. A receiver joins the multicast group.3 The user starts to watch the TV programtransmitted by the TV station via the channel.The receiver starts to receive the multicast datathat the source sends to the multicast group.4 The user turns off the TV set. The receiver leaves the multicast group.z A multicast source does not necessarily belong to a multicast group. Namely, a multicast source isnot necessarily a multicast data receiver.z A multicast source can send data to multiple multicast groups at the same time, and multiplemulticast sources can send data to the same multicast group at the same time.Common Notations in MulticastTwo notations are commonly used in multicast:z (*, G): Indicates a rendezvous point tree (RPT), or a multicast packet that any multicastsource sends to multicast group G. Here “*” represents any multicast source, while “G”represents a specific multicast group.z (S, G): Indicates a shortest path tree (SPT), or a multicast packet that multicast source Ssends to multicast group G. Here “S” represents a specific multicast source, while “G”represents a specific multicast group.For details about the concepts RPT and SPT, see “PIM Configuration”.
