3Com Switch 7750 Configuration Guide Guide

Multicast Overview 377
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 87: 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
multicast data flow on each link.
■ With the multicast mode used to transmit information, an increase of the
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.
■ Multicast brings no waste of network resources and makes proper use of
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.
■ Multiple receivers receiving the same information form a multicast group.
Multicast group is not limited by physical area.
■ Each receiver receiving multicast information is a multicast group member.
■ A router providing multicast routing is a multicast router. The multicast router
can be a member of one or multiple multicast groups, and it can also manage
members of the multicast groups.
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CAUTION: A multicast source does not necessarily belong to a multicast group. A
multicast source sends packets to a multicast group, and it is not necessarily a
receiver. Multiple multicast sources can send packets to the same multicast group
at the same time.
There may be routers that do not support multicast on the network. A multicast
router encapsulates multicast packets in unicast IP packets in the tunnel mode,
and then sends them to the neighboring multicast routers through the router that
do no support multicast. The neighboring multicast routers remove the header of
the unicast IP packets, and then continue to multicast the packets, thus avoiding
changing the network structure greatly.
Advantages and
Applications of
Multicast
Advantages of multicast
Advantages of multicast include:
■ Enhanced efficiency: Multicast decreases network traffic and reduces server
load and CPU load.