Operation Manual – MulticastH3C S5500-EI Series Ethernet Switches Chapter 1 Multicast Overview1-131) IGMP Snooping/MLD SnoopingRunning on Layer 2 devices, Internet Group Management Protocol Snooping (IGMPSnooping) and Multicast Listener Discovery Snooping (MLD Snooping) are multicastconstraining mechanisms that manage and control multicast groups by listening to andanalyzing IGMP or MLD messages exchanged between the hosts and Layer 3multicast devices, thus effectively controlling the flooding of multicast data in a Layer 2network.2) Multicast VLAN/IPv6 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 forward aseparate copy of the multicast data to each VLAN of the Layer 2 device. With themulticast VLAN or IPv6 multicast VLAN feature enabled on the Layer 2 device, theLayer 3 multicast device needs to send only one copy of multicast to the multicastVLAN or IPv6 multicast VLAN on the Layer 2 device. This avoids waste of networkbandwidth and extra burden on the Layer 3 device.1.4 Multicast Packet Forwarding MechanismIn a multicast model, a multicast source sends information to the host group identifiedby the multicast group address in the destination address field of IP multicast packets.Therefore, to deliver multicast packets to receivers located in different parts of thenetwork, multicast routers on the forwarding path usually need to forward multicastpackets received on one incoming interface to multiple outgoing interfaces. Comparedwith a unicast model, a multicast model is more complex in the following aspects.z To ensure multicast packet transmission in the network, unicast routing tables ormulticast routing tables specially provided for multicast must be used as guidancefor multicast forwarding.z To process the same multicast information from different peers received ondifferent interfaces of the same device, every multicast packet is subject to areverse path forwarding (RPF) check on the incoming interface. The result of theRPF check determines whether the packet will be forwarded or discarded. TheRPF check mechanism is the basis for most multicast routing protocols toimplement multicast forwarding.Note:For details about the RPF mechanism, refer to RPF Mechanism.
