Administrative Distance, Access Lists, Route Manipulation, and IP Multicast - CCIE Routing and Switching Exam Certification Guide - page 491

IGMPv1

The first RFC describing IGMP (RFC 1112) was written in 1989. It describes the host extension
of multicasting. IGMPv1 protocol provides simple message types for communication between
hosts and routers. These messages are as follows:
•

Membership query— Sent by the host to request to join a multicast group

•

Membership report— Sent by the router to check if hosts are still in multicast groups in
the segment

The problem with IGMPv1 is the latency involved with a host leaving a group. With IGMPv1,
the router periodically sends out membership reports, and a host waits for the membership
report message to leave a group.

IGMPv2

IGMPv2 provides improvement over IGMPv1 by allowing faster termination or leaving of
multicast groups.

IGMPv2 includes the following three message types, plus one for backward compatibility:
•

Membership query —Sent to indicate that a host wants to join a group.

•

Version 2 membership report —Message sent to the group address with the multicast
group members (IP addresses). The message is sent to verify that hosts are still in
multicast groups on the segment.

•

Leave group —Sent by the hosts to indicate that a host is leaving a group, sent to
destination 224.0.0.2. The message is sent without having to wait for the IGMPv2
membership report message.

•

Version 1 membership report —For backward compatibility with IGMPv1 hosts.

IGMP is enabled on an interface when the multicast routing protocol is enabled, such as PIM.
The version of IGMP can be configured for the interface with the ip igmp version [ 1 | 2 ]
command. The known multicast groups on a router can be queried with the show ip igmp
groups command.

IGMPv2 Designated Querier
Multicast querier routers send host membership query messages to discover which multicast
groups have members on the router's attached networks. Hosts respond with IGMP report
messages that indicate that the host wants to become a member of the group. Host-query
messages are addressed to the all-hosts multicast group, which has the address 224.0.0.1,
and have an IP TTL value of 1.