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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.
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.
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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
Layer 2 multicast protocols include IGMP snooping/MLD snooping and multicast VLAN/IPv6
multicast VLAN.
Figure 9 Positions of Layer 2 multicast protocols
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.
2.
Multicast VLAN/IPv6 multicast VLAN