KB.15.15

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, the switch must assume this function to elicit

group membership information from the hosts on the network. If you need

to disable the querier feature, do so through the CLI using the IGMP

configuration MIB.

Report (Join) 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.

Note on IGMP version 3 support:

When an IGMPv3 Join is received by the switch, it accepts the host request and begins to forward

the IGMP traffic. This means that ports that have not joined the group and are not connected to

routers or the IGMP Querier will not receive the group's multicast traffic.

The switch does not support the IGMPv3 "Exclude Source" or "Include Source" options in the Join

Reports. Rather, the group is simply joined from all sources.

The switch does not support becoming a version 3 Querier. It becomes a version 2 Querier in the

absence of any other Querier on the network.

An IP multicast packet includes the multicast group (address) to which the packet belongs. When

an IGMP client connected to a switch port needs to receive multicast traffic from a specific group,

it joins the group by sending an IGMP report (join request) to the network. (The multicast group

specified in the join request is determined by the requesting application running on the IGMP

client.) When a networking device with IGMP enabled receives the join request for a specific

group, it forwards any IP multicast traffic it receives for that group through the port on which the

join request was received. When the client is ready to leave the multicast group, it sends a Leave

Group message to the network and ceases to be a group member. When the leave request is

detected, the appropriate IGMP device ceases transmitting traffic for the designated multicast group

through the port on which the leave request was received (as long as there are no other current

members of that group on the affected port.)

Thus, IGMP identifies members of a multicast group (within a subnet) and allows IGMP-configured

hosts (and routers) to join or leave multicast groups.

To display IGMP data showing active group addresses, reports, queries, querier access port, and

active group address data (port, type, and access), see the Management and Configuration Guide

for your switch.

Operation with or without IP addressing

You can configure IGMP on VLANs that do not have IP addressing. The benefit of IGMP without

IP addressing is a reduction in the number of IP addresses you have to use and configure. This can

be significant in a network with a large number of VLANs. The limitation on IGMP without IP

addressing is that the switch cannot become Querier on any VLANs for which it has no IP

address—so the network administrator must ensure that another IGMP device will act as Querier.

It is also advisable to have an additional IGMP device available as a backup Querier.

26 Multimedia Traffic Control with IP Multicast (IGMP)