Configuring Advanced

Features

 

 

Configuring Advanced Features

Multimedia Traffic Control with IP Multicast (IGMP)

a switch is configured to support IGMP with the querier feature enabled.) A set of hosts, routers, and/or switches that send or receive multicast data streams to or from the same source(s) is termed a multicast group, and all devices in the group use the same multicast group address. The multicast group running version 2 of IGMP uses three fundamental types of messages to communicate:

■Query: A message sent from the querier (multicast router or switch) asking for a response from each host belonging to the multicast group. If a multicast router supporting IGMP is not present, then the switch must assume this function in order to elicit group membership information from the hosts on the network. (If you need to disable the querier feature, you can do so through the CLI, using the IGMP configuration MIB. See “Changing the Querier Configuration Setting” on page “Configuring the Querier Function” on page 9-97.)

■Report: A message sent by a host to the querier to indicate that the host wants to be or is a member of a given group indicated in the report message.

■Leave Group: A message sent by a host to the querier to indicate that the host has ceased to be a member of a specific multicast group.

Thus, IGMP identifies members of a multicast group (within a subnet) and allows IGMP-configured hosts (and routers) to join or leave multicast groups.

IGMP Data. To display data showing active group addresses, reports, que- ries, querier access port, and active group address data (port, type, and access), see “IP Multicast (IGMP) Status” on page 7-19.

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 9-67on page 9-99shows 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.)

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