11Multicast protocolsGenerally, Layer 3 multicast refers to IP multicast working at the network layer. The correspondingmulticast protocols are Layer 3 multicast protocols, which include IGMP, MLD, PIM, IPv6 PIM, MSDP,MBGP, and IPv6 MBGP. Layer 2 multicast refers to IP multicast working at the data link layer. Thecorresponding multicast protocols are Layer 2 multicast protocols, which include IGMP snooping, MLDsnooping, PIM snooping, IPv6 PIM snooping, multicast VLAN, and IPv6 multicast VLAN.IGMP snooping, PIM snooping, multicast VLAN, IGMP, PIM, MSDP, and MBGP are for IPv4, and MLDsnooping, IPv6 PIM snooping, IPv6 multicast VLAN, MLD, IPv6 PIM, and IPv6 MBGP are for IPv6.This section provides only general descriptions about applications and functions of the Layer 2 and Layer3 multicast protocols in a network. For more information about these protocols, see the related chapters.Layer 3 multicast protocolsLayer 3 multicast protocols include multicast group management protocols and multicast routingprotocols.Figure 8 Positions of Layer 3 multicast protocols• Multicast group management protocolsTypically, the Internet Group Management Protocol (IGMP) or Multicast Listener Discovery Protocol(MLD) is used between hosts and Layer 3 multicast devices that directly connect to the hosts. Theseprotocols define the mechanism of establishing and maintaining group memberships betweenhosts and Layer 3 multicast devices.• Multicast routing protocolsA multicast routing protocol runs on Layer 3 multicast devices to establish and maintain multicastroutes and forward multicast packets correctly and efficiently. Multicast routes constitute loop-freedata transmission paths from a data source to multiple receivers, namely, a multicast distributiontree.
