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Multimedia Traffic Control with IP Multicast (IGMP)
More on IGMP Operation
General IGMP Operation
With IGMP enabled, the switch examines the IGMP packets it receives to learn
which of its ports are linked to IGMP hosts and multicast routers/queriers
belonging to any multicast group. The switch becomes a querier if it does not
discover a multicast router/querier on the network.
Once the switch learns the port location of the hosts belonging to any partic-
ular multicast group, it can direct group traffic to only those ports, resulting
in bandwidth savings on ports where group members do not reside.
The following example illustrates this operation.
Figure 12-3 on page 12-12 shows a network running IGMP.
PCs 1 and 4, switch 2, and all of the routers are members of an IP multicast
group. (The routers operate as queriers.)
Switch 1 ignores IGMP traffic and does not distinguish between IP multi-
cast group members and non-members. Thus, it is sending large amounts
of unwanted multicast traffic out the ports to PCs 2 and 3.
Switch 2 is recognizing IGMP traffic and learns that PC 4 is in the IP
multicast group receiving multicast data from the video server (PC X).
Switch 2 then sends the multicast data only to the port for PC 4, thus
avoiding unwanted multicast traffic on the ports for PCs 5 and 6.
12-11