HP V1910, User Manual

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After 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. After 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 before the member port aging timer of the port expires (in case it is
a dynamic member port):
 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.
IGMP snooping querier
In an IP multicast network running IGMP, a multicast router or Layer 3 multicast switch is responsible for
sending IGMP general queries, so that all Layer 3 multicast devices can establish and maintain multicast
forwarding entries, thus to forward multicast traffic correctly at the network layer. This router or Layer 3
switch is called an IGMP querier.
However, a Layer 2 multicast switch does not support IGMP, and therefore cannot send general queries by
default. By enabling IGMP snooping on a Layer 2 switch in a VLAN where multicast traffic needs to be
Layer-2 switched only and no multicast routers are present, the Layer 2 switch will act as the IGMP snooping
querier to send IGMP queries, thus allowing multicast forwarding entries to be established and maintained
at the data link layer.
Protocols and standards

RFC 4541, Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener
Discovery (MLD) Snooping Switches
Configuring IGMP snooping
Configuration task list
Task Remarks
Enabling IGMP snooping
globally
Required
Disabled by default.