2-4Figure 1-3 Multicast transmissionThe multicast source (Source in the figure) sends only one copy of the information to a multicast group.Host B, Host D and Host E, which are receivers of the information, need to join the multicast group.The routers on the network duplicate and forward the information based on the distribution of the groupmembers. Finally, the information is correctly delivered to Host B, Host D, and Host E.To sum up, the advantages of multicast are summarized as follows:z Over unicast: As multicast traffic flows to the node the farthest possible from the source before it isreplicated and distributed, an increase of the number of hosts will not increase the load of thesource and will not remarkably add to network resource usage.z Over broadcast: As multicast data is sent only to the receivers that need it, multicast uses thenetwork bandwidth reasonably and enhances network security. In addition, data broadcast isconfined to the same subnet, while multicast is not.Features of MulticastMulticast has the following features:z A multicast group is a multicast receiver set identified by an IP multicast address. Hosts join amulticast group to become members of the multicast group, before they can receive the multicastdata addressed to that multicast group. Typically, a multicast source does not need to join amulticast group.z An information sender is referred to as a multicast source (Source in Figure 1-3). A multicastsource can send data to multiple multicast groups at the same time, and multiple multicastsources can send data to the same multicast group at the same time.z All hosts that have joined a multicast group become members of the multicast group (Receiver inFigure 1-3). The group memberships are dynamic. Hosts can join or leave multicast groups at anytime. Multicast groups are not subject to geographic restrictions.z Routers or Layer 3 switches that support Layer 3 multicast are called multicast routers or Layer 3multicast devices. In addition to providing the multicast routing function, a multicast router can alsomanage multicast group memberships on stub subnets with attached group members. A multicastrouter itself can be a multicast group member.
