Tp-Link T2600G-28MPS (TL-SG3424P) [159/379] Igmp snooping

selectively forwarded to a list of ports that want to receive the data, instead of being flooded to

all ports in a VLAN. The list is constructed and maintained by snooping IPv6 multicast control

packets. MLD snooping performs a similar function in IPv6 as IGMP snooping in IPv4.

The Multicast module is mainly for multicast management configuration of the switch, including

three submenus: IGMP Snooping, MLD Snooping and Multicast Table.

9.1 IGMP Snooping



IGMP Snooping Process

The switch, running IGMP Snooping, listens to the IGMP messages transmitted between the

host and the router, and tracks the IGMP messages and the registered port. When receiving

IGMP report message, the switch adds the port to the multicast address table; when the switch

listens to IGMP leave message from the host, the router sends the Group-Specific Query

message of the port to check if other hosts need this multicast, if yes, the router will receive

IGMP report message; if no, the router will receive no response from the hosts and the switch

will remove the port from the multicast address table. The router regularly sends IGMP query

messages. After receiving the IGMP query messages, the switch will remove the port from the

multicast address table if the switch receives no IGMP report message from the host within a

period of time.



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.

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