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IGMP Messages
The switch, running IGMP Snooping, processes the IGMP messages of different types as
follows.
1. IGMP Query Message
IGMP query message, sent by the router, falls into two types, IGMP general query message and
IGMP group-specific-query message. The router regularly sends IGMP general message to
query if the multicast groups contain any member. When receiving IGMP leave message, the
receiving port of the router will send IGMP group-specific-query message to the multicast
group and the switch will forward IGMP group-specific-query message to check if other
members in the multicast group of the port need this multicast.
When receiving IGMP general query message, the switch will forward them to all other ports in
the VLAN owning the receiving port. The receiving port will be processed: if the receiving port
is not a router port yet, it will be added to the router port list with its router port time specified; if
the receiving port is already a router port, its router port time will be directly reset.
When receiving IGMP group-specific-query message, the switch will send the group-specific
query message to the members of the multicast group being queried.
2. IGMP Report Message
IGMP report message is sent by the host when it applies for joining a multicast group or
responses to the IGMP query message from the router.
When receiving IGMP report message, the switch will send the report message via the router
port in the VLAN as well as analyze the message to get the address of the multicast group the
host applies for joining. The receiving port will be processed: if the receiving port is a new
member port, it will be added to the multicast address table with its member port time specified;
if the receiving port is already a member port, its member port time will be directly reset.
3. Member Leave Message
The host will send IGMP leave message when leaving a multicast group to inform the router of
its leaving.
When Immediate Leave is not enabled in a VLAN and a leave message is received on a port of
this VLAN, the switch will generate Multicast-Address-Specific Queries (MASQs) on this port to
check if there are other members in this multicast group. The user can control when a port
membership is removed for an existing address in terms of the number and interval of MASQs.
If there is no Report message received from this port during the switch maximum response
time, the port on which the MASQ was sent is deleted from the multicast group. If the deleted
port is the last member of the multicast group, the multicast group is also deleted. The switch
will send leave message to the router ports of the VLAN.
In IPv4，Layer 2 switches can use IGMP Snooping to limit the flooding of multicast traffic by
dynamically configuring Layer 2 interfaces so that IPv4 multicast data is selectively forwarded
to a list of ports that want to receive the data. This list is constructed by snooping IPv4
multicast control packets.
IGMP Snooping Fundamentals
1. Ports
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