Configuring the Switch

IP Multicast (IGMP) Service Features—Multimedia Traffic Control

Role of the Switch

When IGMP is enabled on the switch, it 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

To become a querier if a multicast router/querier is not discovered on the network

Once the switch learns the port location of the hosts belonging to any particular 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 6-19on page page 6-40shows 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 multicast 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.

Switch the Configuring

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