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 12-3on page 12-12shows 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.

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