10
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. This effectively
controls the flooding of multicast data in a Layer 2 network.
•
PIM snooping and IPv6 PIM snooping:
PIM snooping and IPv6 PIM snooping run on Layer 2 devices. They determine which ports are
interested in multicast data by analyzing the received IPv6 PIM messages. Then, they add the
ports to a multicast forwarding entry. In this way, multicast data can be forwarded to only the
ports that are interested in the data.
•
Multicast VLAN and 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 must 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
