H3C Technologies H3C S7500E Series Switches - Manual (Page 21)

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Figure 1-3 Multicast transmission
The 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 group members. 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 is replicated and distributed, an increase of the number of hosts will not increase the load of
the source 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 the
network bandwidth reasonably and enhances network security. In addition, data broadcast is
confined to the same subnet, while multicast is not.
Features of Multicast
Multicast has the following features:
z A multicast group is a multicast receiver set identified by an IP multicast address. Hosts join a
multicast group to become members of the multicast group, before they can receive the
multicast data addressed to that multicast group. Typically, a multicast source does not need to
join a multicast group.
z An information sender is referred to as a multicast source (Source in Figure 1-3 ). A multicast
source can send data to multiple multicast groups at the same time, and multiple multicast
sources 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
in Figure 1-3 ). The group memberships are dynamic. Hosts can join or leave multicast groups
at any time. 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
3 multicast devices. In addition to providing the multicast routing function, a multicast router can