Netis ST3208, Руководство пользователя

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Figure 4-3-15
4.3.10 IGMP Snooping
Internet Group Management Protocol Snooping (IGMP Snooping) is a multicast constraining
mechanism that runs on Layer 2 devices to manage and control multicast groups.
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.