Internet Group Management Protocol version 2 (IGMPv2) introduced support for group specific queries and querier elections. IGMPv1 queries are general queries sent to the 224.0.0.1 all hosts multicast address. IGMPv2 and IGMPv3 queries are either general queries, which are sent to 224.0.0.1, or group specific queries, which are sent only to members of a particular multicast group.

Although IGMPv3 supports querier elections, this feature was introduced in IGMPv2. The router with the lowest IP address on the subnet is elected as the querier. The querier is responsible for periodically sending out membership query messages to determine whether any hosts want to receive multicast packets for the multicast group. If at least one host responds with a membership report message, the querier will continue to send those multicast packets on that network segment.

Although the Request for Comments (RFC) standard for IGMP query messages is 125 seconds, Cisco uses a default query interval of 60 seconds for all IGMP versions. The query interval determines how often the querier sends out membership query messages. If no member has responded to the query message within three times the query interval, the interface is pruned.

Cisco uses a default querier timeout of two times the query interval, or 120 seconds, for IGMPv2 and IGMPv3? IGMPv1 does not support querier elections. The querier timeout isused to trigger querier elections. If an IGMP device has not received a query message from the querier within the querier timeout period, a querier election is triggered and a new querier is elected.

IGMPv3 introduced support for Source Specific Multicast (SSM). SSM enables IGMPv3 hosts to specify the source addresses from which they will accept multicast traffic. To enable SSM, you should issue the ip pim ssm command from global configuration mode, the ip pim {sparse mode | sparse-dense mode} command from interface configuration mode, and the ip igmp version 3 command from interface configuration mode.

Although IGMPv3 supports leave group messages, this feature was introduced in IGMPv2.

In IGMPv1, a host leaves a multicast group silently. In IGMPv2 and IGMPv3, a host sendsan IGMP leave message when it wants to leave a multicast group. IGMP routers maintain the IP address of the last reporter, which is the last host that sent a membership report message for that multicast group. If the last reporter leaves a multicast group, the IGMP router immediately sends a membership query message to determine whether any interested hosts remain.

IGMPv3 does not support host membership report suppression; in fact, IGMPv3 removed support for host membership report suppression. This feature, which is supported in IGMPv1 and IGMPv2, prevents the sending of a membership report if a similar report is detected from another host on the network. IGMPv3 removes this restriction.