3Com Switch 4800G 24-Port Configuration Manual page 543

Figure 164 Multicast transmission
Source
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 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:
■
■
Roles in Multicast The following roles are involved in multicast transmission:
■
■
■
■
For a better understanding of the multicast concept, you can assimilate multicast
transmission to the transmission of TV programs, as shown in Table 51.
Server
Packets for the multicast group
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.
An information sender is referred to as a Multicast Source ("Source" in
Figure 164).
Each receiver is a Multicast Group Member ("Receiver" in Figure 164).
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.
Introduction to Multicast
Host A
Receiver
Host B
Host C
Receiver
Host D
Receiver
Host E
543