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Figure 8-3 Multicast Address Table
IGMP Snooping
In the network, the hosts apply to the near Router for joining (leaving) a multicast group by sending
IGMP (Internet Group Management Protocol) messages. When the up-stream device forwards
down the multicast data, the switch is responsible for sending them to the hosts. IGMP Snooping is
a multicast control mechanism, which can be used on the switch for dynamic registration of the
multicast group. The switch, running IGMP Snooping, manages and controls the multicast group via
listening to and processing the IGMP messages transmitted between the hosts and the multicast
router, thereby effectively prevents multicast groups being broadcasted in the network.
The Multicast module is mainly for multicast management configuration of the switch, including
four submenus: IGMP Snooping, Multicast IP, Multicast Filter and Packet Statistics.
8.1 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,
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