Protocols And Standards; Configuring Igmp Snooping; Configuration Task List - 3Com Baseline 2928 PWR Plus User Manual

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.
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.
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:
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.
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:
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 IGMP snooping configuration task list
Task
Enabling IGMP snooping
Globally
Table 1-1 to configure IGMP snooping.
Required
Disabled by default.
Remarks
1-4