3Com Switch 8800 Advanced Software V5 Configuration Guide

Introduction to Multicast 493
for multicast packets through multicast routing protocols, the packets are
replicated only where the tree branches, as shown in
Figure 157:
Figure 157 Multicast transmission
Assume that Hosts B, D and E need the information. To receive the information
correctly, these hosts need to join a receiver set, which is known as a multicast
group. The multicast routers on the network duplicate and forward the
information based on the distribution of the receivers in this set. Finally, the
information is correctly delivered to Hosts B, D, and E.
To sum up, multicast has the following advantages:
■ 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 remarkably add to the network load.
■ Over broadcast: As multicast data is sent only to the receivers that need it,
multicast uses the network bandwidth reasonably and brings no waste of
network resources, and enhances network security.
Roles in Multicast The following roles are involved in multicast transmission:
■ An information sender is referred to as a Multicast Source ("Source" in
Figure 157).
■ Each receiver is a Multicast Group Member ("Receiver" in Figure 157).
■ All receivers interested in the same information form a Multicast Group.
Multicast groups are not subject to geographic restrictions.
■ A router that supports Layer 3 multicast is called multicast router or Layer 3
multicast device. In addition to providing the multicast routing function, a
multicast router can also manage multicast group members. In practice,
switches that support Layer 3 multicast also act as Layer 3 multicast devices.
Source
Server
Receiver
Receiver
Receiver
Host A
Host B
Host C
Host D
Host E
Packets for the multicast group