MNS-BB Software User Guide
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12.5 How IGMP Operates
The Internet Group Management Protocol (IGMP) is an internal protocol of the Internet Protocol (IP)
suite. IP manages multicast traffic by using switches, multicast routers, and hosts that support IGMP.
(In the LE2425A and LEV2525A Switches implementation of IGMP, a multicast router is not
necessary as long as 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”)
• 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.
12.6 IGMP Data.
To display data showing active group addresses, reports, queries, querier access port, and active
group address data (port, type, and access), see “CLI Section”.
12.7 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 (below) 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 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.
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