2-12 IGMP Snooping ConfigurationWhen configuring IGMP snooping, go to the following sections for information you are interested in:z IGMP Snooping Overviewz IGMP Snooping Configuration Task Listz Displaying and Maintaining IGMP Snoopingz IGMP Snooping Configuration Examplesz Troubleshooting IGMP Snooping ConfigurationIGMP 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 2-1, when IGMP snooping is not running on the switch, multicast packets areflooded 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.Figure 2-1 Before and after IGMP snooping is enabled on the Layer 2 deviceMulticast packet transmissionwithout IGMP SnoopingSourceMulticast routerHost AReceiverHost BHost CReceiverMulticast packetsLayer 2 switchMulticast packet transmissionwhen IGMP Snooping runsSourceMulticast routerHost AReceiverHost BHost CReceiverLayer 2 switchIGMP snooping forwards multicast data to only the receivers requiring it at Layer 2. It brings thefollowing advantages:
