Multicast protocols

	NOTE: Generally, Layer 3 multicast refers to IP multicast working at the network layer. The related multicast protocols are Layer 3 multicast protocols, which include IGMP/MLD, PIM/IPv6 PIM, MSDP, and MBGP/IPv6 MBGP. Layer 2 multicast refers to IP multicast working at the data link layer. The related multicast protocols are Layer 2 multicast protocols, which include IGMP snooping/MLD snooping, and multicast VLAN/IPv6 multicast VLAN. IGMP snooping, IGMP, multicast VLAN, PIM, MSDP, and MBGP are for IPv4, MLD snooping, MLD, IPv6 multicast VLAN, IPv6 PIM, and IPv6 MBGP are for IPv6. This section provides only general descriptions about applications and functions of the Layer 2 and Layer 3 multicast protocols in a network. For more information about these protocols, see related chapters.

Layer 3 multicast protocols include multicast group management protocols and multicast routing protocols.

1. Multicast group management protocols

Typically, the internet group management protocol (IGMP) or multicast listener discovery protocol (MLD) is used between hosts and Layer 3 multicast devices that directly connected to the hosts. These protocols define the mechanism of establishing and maintaining group memberships between hosts and Layer 3 multicast devices.

1. Multicast routing protocols

A multicast routing protocol runs on Layer 3 multicast devices to establish and maintain multicast routes and forward multicast packets correctly and efficiently. Multicast routes constitute loop-free data transmission paths from a data source to multiple receivers, namely, a multicast distribution tree.

In the ASM model, multicast routes include intra-domain routes and inter-domain routes.

• An intra-domain multicast routing protocol discovers multicast sources and builds multicast distribution trees within an AS to deliver multicast data to receivers. Among a variety of mature intra-domain multicast routing protocols, protocol independent multicast (PIM) is most widely used. Based on the forwarding mechanism, PIM has the dense mode (often called "PIM-DM"), and sparse mode (often called "PIM-SM").

• An inter-domain multicast routing protocol delivers multicast information between two ASs. Mature solutions include Multicast Source Discovery Protocol (MSDP) and Multicast Border Gateway Protocol (MBGP). MSDP propagates multicast source information among different ASs, and MBGP is an extension of the Multi-protocol Border Gateway Protocol (MP-BGP) for exchanging multicast routing information among different ASs.

For the SSM model, multicast routes are not divided into intra-domain routes and inter-domain routes. Because receivers know the positions of the multicast sources, channels established through PIM-SM are sufficient for the transport of multicast information.

Layer 2 multicast protocols include IGMP snooping/MLD snooping and multicast VLAN/IPv6 multicast VLAN.

1. IGMP snooping/MLD snooping

IGMP snooping and MLD snooping are multicast constraining mechanisms that run on Layer 2 devices. They manage and control multicast groups by listening to and analyzing IGMP or MLD messages exchanged between the hosts and Layer 3 multicast devices, effectively controlling the flooding of multicast data in a Layer 2 network.

1. Multicast VLAN/IPv6 multicast VLAN

In the traditional multicast-on-demand mode, when users in different VLANs on a Layer 2 device need multicast information, the upstream Layer 3 device needs to forward a separate copy of the multicast data to each VLAN of the Layer 2 device. When the multicast VLAN or IPv6 multicast VLAN feature is enabled on the Layer 2 device, the Layer 3 multicast device sends only one copy of the multicast data to the multicast VLAN or IPv6 multicast VLAN on the Layer 2 device. This approach avoids waste of network bandwidth and extra burden on the Layer 3 device.