3com 4210 PWR 9-Port 3CR17341-91-ME ユーザーズマニュアル

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

By analyzing received IGMP messages, a Layer 2 device running IGMP Snooping 
establishes mappings between ports and multicast MAC addresses and forwards 
multicast data based on these mappings. 

As shown in Figure 62, when IGMP Snooping is not running on the switch, 
multicast packets are broadcast to all devices at Layer 2. When IGMP Snooping is 
running on the switch, multicast packets for known multicast groups are multicast 
to the receivers, rather than broadcast to all hosts, at Layer 2. However, multicast 
packets for unknown multicast groups are still broadcast at Layer 2.

Figure 62   Before and after IGMP Snooping is enabled on Layer 2 device

As shown in Figure 63, Router A connects to the multicast source, IGMP Snooping 
runs on Switch A and Switch B, Host A and Host C are receiver hosts (namely, 
multicast group members).