Multimedia Traffic Control with IP Multicast (IGMP)
How IGMP Operates
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 Hewlett-Pack-
ard’s 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
“Configuring the Querier Function” on page 2-11.)
■ 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 When an IGMPv3 Join is received by the switch, it accepts the host request
version 3
and begins to forward the IGMP traffic. This means that ports which have not
support
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 will become 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
2-12
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