IGMPv2 - leave group message mechanism
When implementing a multicast topology, IGMP is used to allow hosts to announce to a multicast router their desire to receive multicast traffic. IGMPv2 improves the way in which hosts stop receiving multicast traffic in GMPv1. Specifically, when a host no longer wants to listen to a multicast group address then it will report to the router that it has stopped listening.
Once a host leaves a multicast group, it will send a leave group message to the multicast router. The router will then lower the expiration timer to 2 seconds (or 2000 ms) for the interface on which the message was received, which is the router interface serving that host. The router will then send out a membership query with the multicast group address as the destination to determine if there is still anyone else interested in receiving this traffic.
If there is no response, the group is deleted from the multicast multicast routing table.