1-11
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.
z An intra-domain multicast routing protocol is used to discover multicast sources and build multicast
distribution trees within an AS so 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 comes in two modes – dense mode (often
referred to as PIM-DM) and sparse mode (often referred to as PIM-SM).
z 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 is used to propagate multicast source information
among different ASs, while MBGP, an extension of the Multi-protocol Border Gateway Protocol
(MP-BGP), is used 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.
Since receivers know the positions of the multicast sources, channels established through PIM-SM are
sufficient for multicast information transport.
Layer 2 multicast protocols
Layer 2 multicast protocols include IGMP Snooping and multicast VLAN. Figure 1-9 shows where these
protocols are in the network.
Figure 1-9 Positions of Layer 2 multicast protocols
Source
Receiver Receiver
IPv4/IPv6 multicast packets
IGMP Snooping
/MLD Snooping
Multicast VLAN
/IPv6 Multicast VLAN
1) IGMP snooping/MLD snooping
Running on Layer 2 devices, IGMP snooping and MLD snooping are multicast constraining
mechanisms that manage and control multicast groups by listening to and analyzing IGMP or MLD
messages exchanged between the hosts and Layer 3 multicast devices, thus effectively controlling the
flooding of multicast data in a Layer 2 network.
2) Multicast VLAN/IPv6 multicast VLAN
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