Administrator Guide

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Internet Group Management Protocol (IGMP)
Multicast is premised on identifying many hosts by a single destination IP address; hosts represented by the same IP address are a
multicast group.
IGMP is a Layer 3 multicast protocol that hosts use to join or leave a multicast group. Multicast routing protocols (such as protocol-
independent multicast [PIM]) use the information in IGMP messages to discover which groups are active and to populate the
multicast routing table.
IGMP Protocol Overview
IGMP has three versions. Version 3 obsoletes and is backwards-compatible with version 2; version 2 obsoletes version 1.
IGMP Version 2
IGMP version 2 improves on version 1 by specifying IGMP Leave messages, which allows hosts to notify routers that they no longer
care about trac for a particular group.
Leave messages reduce the amount of time that the router takes to stop forwarding trac for a group to a subnet (leave latency)
after the last host leaves the group. In version 1 hosts quietly leave groups, and the router waits for a query response timer several
times the value of the query interval to expire before it stops forwarding trac.
To receive multicast trac from a particular source, a host must join the multicast group to which the source is sending trac. A
host that is a member of a group is called a receiver. A host may join many groups, and may join or leave any group at any time. A
host joins and leaves a multicast group by sending an IGMP message to its IGMP Querier. The querier is the router that surveys a
subnet for multicast receivers and processes survey responses to populate the multicast routing table.
IGMP messages are encapsulated in IP packets, as shown in the following illustration.
Figure 44. IGMP Messages in IP Packets
Internet Group Management Protocol (IGMP)
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