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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 dense mode (often referred to as
"PIM-DM"), and sparse mode (often referred to as "PIM-SM").
{ An inter-domain multicast routing protocol is used for delivery of multicast information between
two ASs. So far, mature solutions include Multicast Source Discovery Protocol (MSDP) and
Multicast Border Gateway Protocol (MBGP). MSDP propagates multicast source information
among different ASs. 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 position of the multicast source, 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, PIM snooping, IPv6 PIM snooping,
multicast VLAN, and IPv6 multicast VLAN.
Figure 9 Positions of Layer 2 multicast protocols
• IGMP snooping and MLD snooping
IGMP snooping and MLD snooping are multicast constraining mechanisms that run on Layer 2
devices. They manage and control multicast groups by monitoring 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.
• PIM snooping and IPv6 PIM snooping
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