3Com 7757 Configuration Manual page 415

Multicast solves this problem. When some users on a network require specified
information, the multicast information sender (namely, the multicast source) sends
the information only once. With tree-type routes established for multicast data
packets through a multicast routing protocol, the packets are duplicated and
distributed at the nearest nodes as shown in Figure 102:
Figure 102 Information transmission in the multicast mode
Source
Server
Packets for the multicast group
Assume that users B, D and E need the information. To transmit the information to
the right users, it is necessary to group users B, D and E into a receiver set. The
routers on the network duplicate and distribute the information based on the
distribution of the receivers in this set. Finally, the information is correctly delivered
to users B, D, and E.
The advantages of multicast over unicast are as follows:
No matter how many receivers exist, there is only one copy of the same
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multicast data flow on each link.
With the multicast mode used to transmit information, an increase of the
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number of users does not add to the network burden remarkably.
The advantages of multicast over broadcast are as follows:
A multicast data flow can be sent only to the receiver that requires the data.
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Multicast brings no waste of network resources and makes proper use of
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bandwidth.
In the multicast mode, network components can be divided in to the following
roles:
An information sender is referred to as a multicast source.
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Multiple receivers receiving the same information form a multicast group.
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Multicast group is not limited by physical area.
Multicast Overview
Host A
Receiver
Host B
Host C
Receiver
Host D
Receiver
Host E
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