1-11 IGMP snoopingOverviewInternet 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 1-1, when IGMP snooping is not running on the switch, multicast packets areflooded to all devices at Layer 2. However, when IGMP snooping is running on the switch, multicastpackets for known multicast groups are multicast to the receivers, rather than broadcast to all hosts, atLayer 2.Figure 1-1 Multicast forwarding before and after IGMP snooping runsMulticast packet transmissionwithout IGMP SnoopingSourceMulticast routerHost AReceiverHost BHost CReceiverMulticast packetsLayer 2 switchMulticast packet transmissionwhen IGMP Snooping runsSourceMulticast routerHost AReceiverHost BHost CReceiverLayer 2 switchIGMP Snooping Related PortsAs shown in Figure 1-2, Router A connects to the multicast source, IGMP snooping runs on Switch Aand Switch B, Host A and Host C are receiver hosts (namely, multicast group members).
