1-11Figure 1-6 Positions of Layer 2 multicast protocolsSourceReceiver Receivermulticast packetsIGMP Snooping2) IGMP SnoopingRunning on Layer 2 devices, Internet Group Management Protocol Snooping (IGMP Snooping) aremulticast constraining mechanisms that manage and control multicast groups by listening to andanalyzing IGMP messages exchanged between the hosts and Layer 3 multicast devices, thuseffectively controlling the flooding of multicast data in a Layer 2 network.Multicast Packet Forwarding MechanismIn a multicast model, a multicast source sends information to the host group identified by the multicastgroup address in the destination address field of the IP packets. Therefore, to deliver multicast packetsto receivers located in different parts of the network, multicast routers on the forwarding path usuallyneed to forward multicast packets received on one incoming interface to multiple outgoing interfaces.Compared with a unicast model, a multicast model is more complex in the following aspects.z In the network, multicast packet transmission is based on the guidance of the multicast forwardingtable derived from the unicast routing table or the multicast routing table specially provided formulticast.z To process the same multicast information from different peers received on different interfaces ofthe same device, every multicast packet is subject to a reverse path forwarding (RPF) check on theincoming interface. The result of the RPF check determines whether the packet will be forwardedor discarded. The RPF check mechanism is the basis for most multicast routing protocols toimplement multicast forwarding.The RPF mechanism enables multicast devices to forward multicast packets correctly based on themulticast route configuration. In addition, the RPF mechanism also helps avoid data loops caused byvarious reasons.Implementation of the RPF MechanismUpon receiving a multicast packet that a multicast source S sends to a multicast group G, the multicastdevice first searches its multicast forwarding table:1) If the corresponding (S, G) entry exists, and the interface on which the packet actually arrived is theincoming interface in the multicast forwarding table, the router forwards the packet to all theoutgoing interfaces.
