Operation Manual – Multicast OverviewH3C S9500 Series Routing Switches Chapter 1 Multicast Overview1-9For the SSM model, multicast routes are not divided into inter-domain routes andintra-domain routes. Because receivers are aware of the position of the multicastsource, dedicated multicast forwarding paths are established through PIM-SM formulticast transmission.II. Layer 2 Multicast ProtocolsLayer 2 multicast protocols include IGMP Snooping and multicast VLAN. Figure 1-6shows where they work in the network.SourceReceiver ReceiverIPv4 Multicast packetsIGMP SnoopingMulticast VLANFigure 1-6 Positions of Layer 2 multicast protocols1) IGMP SnoopingIGMP Snooping is a multicast constraining mechanism running on Layer 2 devices. Bylistening to and analyzing IGMP messages exchanged between hosts and Layer 3multicast devices, IGMP Snooping manages and controls multicast groups, thuseffectively suppressing flooding of multicast data in Layer 2 networks.2) Multicast VLANIn the traditional multicast-on-demand mode, when users in different VLANs on a Layer2 device need multicast information, the upstream Layer 3 device needs to deliver aseparate copy of the multicast data to each VLAN on the Layer 2 device. With themulticast VLAN feature enabled on the Layer 2 device, the Layer 3 device needs tosend only one copy of multicast data to the multicast VLAN on the Layer 2 device. Thisavoids waste of network bandwidth and extra burden on the Layer 3 device.1.3 RPF Mechanism for IP Multicast PacketsTo ensure that multicast packets reach a router along the shortest path, the multicastrouter must check the receiving interface of multicast packets depending on the unicastrouting table or a unicast routing table independently provided for multicast. This check
