HP 6120G/XG HP ProCurve Series 6120 Blade Switches Multicast and Routing Guide - Page 31

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-Packard'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 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 which 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 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