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Layer 3 multicast protocols
Layer 3 multicast protocols include multicast group management protocols and multicast routing
protocols.
Figure 8 Positions of Layer 3 multicast 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 connect to the hosts. These protocols
define the mechanism of establishing and maintaining group memberships between hosts and Layer 3
multicast devices.
2. 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 includes 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 Multiprotocol 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
Layer 2 multicast protocols include IGMP snooping, MLD snooping, multicast VLAN, and IPv6 multicast
VLAN.
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