When a host leaves a multicast group (group G, for example), the host sends an IGMP Leave message of group G to the multicast device. Because of the Report message suppression mechanism in IGMPv2, the multicast device cannot determine whether another host exists in group G. Therefore, the multicast device triggers a query on group G. If another host exists in group G, the host sends the IGMP Report message of group G to the multicast device. If the multicast device sends the query on group G a specified number of times but does not receive IGMP Report messages from any host, the multicast device deletes information about group G and stops forwarding multicast data of group G.
If a multicast device is directly connected to an access device on which IGMP proxy is enabled, when the access device leaves group G and sends the IGMP Leave message of group G to the multicast device, the multicast device can identify that group G contains no receivers and will not trigger the IGMP Query message. Then, the multicast device deletes all records of group G and stops forwarding data of group G. This is called IGMP Prompt-Leave.
After IGMP Prompt-Leave is enabled on a multicast device, the multicast device does not trigger IGMP Query messages destined for the multicast group when the multicast device receives IGMP Leave messages from the multicast group. In this case, the multicast device deletes all records about the multicast group and stops forwarding the data of the multicast group. In this manner, the multicast device responds faster to IGMP Leave messages.