Operation Manual – MulticastH3C S3100 Series Ethernet Switches Chapter 2 IGMP Snooping Configuration2-1Chapter 2 IGMP Snooping Configuration2.1 IGMP Snooping OverviewInternet Group Management Protocol Snooping (IGMP Snooping) is a multicastconstraining mechanism that runs on Layer 2 devices to manage and control multicastgroups.2.1.1 Principle of IGMP SnoopingBy analyzing received IGMP messages, a Layer 2 device running IGMP Snoopingestablishes mappings between ports and multicast MAC addresses and forwardsmulticast data based on these mappings.As shown in Figure 2-1, when IGMP Snooping is not running on the switch, multicastpackets are broadcast to all devices at Layer 2. When IGMP Snooping is running on theswitch, multicast packets for known multicast groups are multicast to the receivers,rather than broadcast to all hosts, at Layer 2. However, multicast packets for unknownmulticast groups are still broadcast at Layer 2.Multicast packet transmissionwithout IGMP SnoopingSourceMulticast routerHost AReceiverHost BHost CReceiverMulticast packetsLayer 2 switchMulticast packet transmissionwhen IGMP Snooping runsSourceMulticast routerHost AReceiverHost BHost CReceiverLayer 2 switchFigure 2-1 Before and after IGMP Snooping is enabled on Layer 2 device
