1-13 IGMP Snooping ConfigurationWhen configuring IGMP snooping, go to these sections for information you are interested in:z IGMP Snooping Overviewz Configuring IGMP Snoopingz Displaying and Maintaining IGMP Snoopingz IGMP Snooping Configuration Examplesz Troubleshooting IGMP SnoopingIn this manual, the term “router” refers to a router in the generic sense or a Layer 3 Ethernet switchrunning an IP multicast protocol.IGMP Snooping OverviewInternet Group Management Protocol Snooping (IGMP Snooping) is a multicast constrainingmechanism that runs on Layer 2 devices to manage and control multicast groups.Principle of IGMP SnoopingBy analyzing received IGMP messages, a Layer 2 device running IGMP Snooping establishesmappings between ports and multicast MAC addresses and forwards multicast data based on thesemappings.As shown in Figure 3-1, when IGMP Snooping is not running on the switch, multicast packets arebroadcast to all devices at Layer 2. When IGMP Snooping is running on the switch, multicast packets forknown 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.
