How Hosts Join and Leave Multicast Groups

Hosts can join multicast groups in either of two ways:

• By sending an unsolicited IGMP join message to a multicast router that specifies the IP multicast that the host is attempting to join.
• By sending an IGMP join message in response to a general query from a multicast router.

A multicast router continues to forward multicast traffic to a VLAN provided that at least one host on that VLAN responds to the periodic general IGMP queries. For a host to remain a member of a multicast group, therefore, it must continue to respond to the periodic general IGMP queries.

To leave a multicast group, a host can either not respond to the periodic general IGMP queries, which results in a “silent leave” (the only leave option for hosts connected to switches running IGMPv1), or send a group-specific IGMPv2 leave message.