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 CLI commands.
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:
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.
Chapter 2 Multimedia traffic control with IP multicast (IGMP)
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