3Com Switch 8800 Advanced Software V5 Configuration Guide

500 CHAPTER 37: MULTICAST OVERVIEW
establishing and maintaining group memberships between hosts and Layer
multicast devices.
2 Multicast routing protocols
A multicast routing protocol runs between Layer 3 multicast devices to establish
and maintain multicast routes and forward multicast packets correctly and
efficiently. A multicast route is a loop-free data transmission path from a data
source to multiple receivers. Namely, it is a multicast distribution tree.
In the ASM model, multicast routes come in intra-domain routes and inter-domain
routes.
■ An intra-domain multicast routing protocol is used to discover multicast
sources and build multicast distribution trees with an autonomous system (AS)
so as to deliver multicast data to receivers. Among a variety of mature
intra-domain multicast routing protocols, protocol independent multicast (PIM)
is a most popular one. It delivers information to receivers by discovering the
multicast source and establishing multicast distribution trees. Based on the
forwarding mechanism, PIM comes in two modes - dense mode (PIM-DM) and
sparse mode (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).
For the SSM model, multicast routes are not divided into inter-domain routes and
intra-domain routes. Since receivers know the position of the multicast source,
channels established through PIM-SD are sufficient for multicast information
transport.
Layer 2 multicast protocols
Layer 2 multicast protocols include IGMP Snooping/MLD Snooping and multicast
VLAN.
Figure 161 shows where these protocols are in the network.