1-4z 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 to thetransmission of TV programs, as shown in Table 1-1.Table 1-1 An analogy between TV transmission and multicast transmissionStep TV transmission Multicast transmission1A TV station transmits a TVprogram through a televisionchannel.A multicast source sendsmulticast data to a multicastgroup.2 A user tunes the TV set to thechannel.A receiver joins the multicastgroup.3The user starts to watch the TVprogram transmitted by the TVstation via the channel.The receiver starts to receivethe multicast data that thesource sends to the multicastgroup.4 The user turns off the TV set. The receiver leaves themulticast 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 multicast packet that any multicast source sends to multicast group G. Here “*”represents any multicast source, while “G” represents a specific multicast group.z (S, G): Indicates a multicast packet that multicast source S sends to multicast group G. Here “S”represents a specific multicast source, while “G” represents a specific multicast group.Advantages and Applications of MulticastAdvantages of multicastAdvantages of multicast include:z Enhanced efficiency: Multicast decreases network traffic and reduces server load and CPU load.z Optimal performance: Multicast reduces redundant traffic.z Distributive application: Multicast makes multiple-point application possible.
