TP-Link T1700X-16TS User Manual page 120

IGMP Messages
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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
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1. Ports
Router Port: Indicates the switch port directly connected to the multicast router.
Member Port: Indicates a switch port connected to a multicast group member.
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