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 ProCurve’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.
Message Types
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 4-12
Multimedia Traffic Control with IP Multicast (IGMP)
■ 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.
IGMP Querying/Snooping
A Layer 2 switch supporting IGMP Snooping can passively snoop on IGMP Query, Report, and Leave (IGMP version 2) packets transferred between IP Multicast routers/switches and IP Multicast hosts to determine the IP Multicast group membership. IGMP snooping checks IGMP packets passing through the network, picks out the group registration, and configures Multicasting accordingly.
Without IGMP Querying/Snooping, Multicast traffic is treated in the same manner as a Broadcast transmission, which forwards packets to all ports on the network. With IGMP Querying/Snooping, Multicast traffic is only forwarded to ports that are members of that Multicast group. IGMP Snooping generates no additional network traffic, which significantly reduces the Multicast traffic passing through your switch.
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