3Com 2928 User Guide - Page 283

1 IGMP snooping Overview Internet Group Management Protocol Snooping (IGMP snooping) is a multicast constraining mechanism that runs on Layer 2 devices to manage and control multicast groups. Principle of IGMP Snooping 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 1-1, when IGMP snooping is not running on the switch, multicast packets are flooded to all devices at Layer 2. However, 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. Figure 1-1 Multicast forwarding before and after IGMP snooping runs Multicast packet transmission without IGMP Snooping Multicast packet transmission when IGMP Snooping runs Source Multicast router Source Multicast router Layer 2 switch Layer 2 switch Host A Receiver Host B Multicast packets Host C Receiver Host A Receiver Host B Host C Receiver IGMP Snooping Related Ports As shown in Figure 1-2, 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). 1-1