3Com 2928 User Guide - Page 286

forwarding entry for the member port corresponding to the host from the forwarding table when its aging timer expires. When an IGMPv2 or IGMPv3 host leaves a multicast group, the host sends an IGMP leave message to the multicast router to announce that it has left the multicast group. When the switch receives a group-specific IGMP leave group message on a member port, it first checks whether a forwarding table entry for that group exists, and, if one exists, whether its outgoing port list contains that port. z If the forwarding table entry does not exist or if its outgoing port list does not contain the port, the switch discards the IGMP leave group message instead of forwarding it to any port. z If the forwarding table entry exists and its outgoing port list contains the port, the switch forwards the IGMP leave group message to the router ports in the VLAN. Because the switch does not know whether any other member hosts for that group still exist under the port to which the leave message arrived, the switch does not immediately remove the port from the outgoing port list; instead, the switch resets the member port aging timer for that port. Upon receiving the IGMP leave group message from a host, the IGMP querier resolves from the message the address of the multicast group that the host just left and sends an IGMP group-specific query to that multicast group through the port that received the leave group message. Upon hearing the IGMP group-specific query, the switch forwards it through all its router ports in the VLAN and all member ports for that multicast group, and performs the following to the port (in case it is a dynamic member port) before the member port aging timer of the port expires: z If any IGMP report in response to the group-specific query is heard on a member port before its aging timer expires, this means that some host attached to the port is receiving or expecting to receive multicast data for that multicast group. The switch resets the aging timer of the member port. z If no IGMP report in response to the group-specific query is heard on a member port before its aging timer expires, this means that no hosts attached to the port are still listening to that group address. The switch removes the port from the outgoing port list of the forwarding table entry for that multicast group when the aging timer expires. Protocols and Standards IGMP snooping is documented in: z RFC 4541: Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches Configuring IGMP Snooping Configuration Task List Perform the tasks in Table 1-1 to configure IGMP snooping. Table 1-1 IGMP snooping configuration task list Task Enabling IGMP snooping Globally Required Disabled by default. Remarks 1-4