1-10one. Based on the forwarding mechanism, PIM comes in two modes – dense mode (often referredto as PIM-DM) and sparse mode (often referred to as PIM-SM).z An inter-domain multicast routing protocol is used for delivery of multicast information between twoASs. So far, mature solutions include multicast source discovery protocol (MSDP).For the SSM model, multicast routes are not divided into inter-domain routes and intra-domain routes.Since receivers know the position of the multicast source, channels established through PIM-SM aresufficient for multicast information transport.Layer 2 multicast protocolsLayer 2 multicast protocols include IGMP Snooping and multicast VLAN Figure 1-6 shows where theseprotocols are in the network.Figure 1-6 Position of Layer 2 multicast protocols1) IGMP SnoopingRunning on Layer 2 devices, Internet Group Management Protocol Snooping (IGMP Snooping) ismulticast 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.2) Multicast VLANIn the traditional multicast-on-demand mode, when users in different VLANs on a Layer 2 device needmulticast information, the upstream Layer 3 device needs to forward a separate copy of the multicastdata to each VLAN of the Layer 2 device. With the multicast VLAN feature enabled on the Layer 2device, the Layer 3 multicast device needs to send only one copy of multicast to the multicast VLANmulticast VLAN on the Layer 2 device. This avoids waste of network bandwidth and extra burden on theLayer 3 device.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 IP multicast packets. Therefore, to deliver multicastpackets to receivers located in different parts of the network, multicast routers on the forwarding pathusually need to forward multicast packets received on one incoming interface to multiple outgoinginterfaces. Compared with a unicast model, a multicast model is more complex in the following aspects.
