3Com Switch 7750 Configuration Guide Guide

382 CHAPTER 39: MULTICAST OVERVIEW
IP Multicast Protocols IP multicast protocols include the multicast group management protocol and the
multicast routing protocol.
Figure 90describes the positions of the protocols
related to multicast in the network.
Figure 90 Positions of protocols related to multicast
Multicast group management protocol
Internet group management protocol (IGMP) is adopted between a host and its
directly-connected multicast routers. This protocol defines the mechanism of
establishing and maintaining multicast group membership between hosts and
multicast routers.
There are three versions for IGMP currently, including IGMPv1, IGMPv2 and
IGMPv3. A new version is compatible with the old ones.
Multicast routing protocols
A multicast routing protocol operates between multicast routers to establish and
maintain multicast routes and forward multicast packets accurately and effectively.
A multicast route establishes a loop-free data transport path (also known as
multicast distribution tree) from a data source to multiple receivers.
Multicast routes include intra-domain routes and inter-domain routes:
■ Intra-domain multicast routes have been quite mature. Protocol independent
multicast (PIM) is the most commonly used protocol currently. PIM transmits
information to receivers by means of multicast source discovery and multicast
distribution tree establishment. According to forwarding mechanisms, PIM
includes PIM dense mode (PIM-DM) and PIM sparse mode (PIM-SM).
■ The key problem for inter-domain routes is how to transmit information
between autonomous systems (AS). Currently, multicast source discovery
protocol is a relatively mature solution.
Forwarding
Mechanism of
Multicast Packets
In a multicast model, a multicast source host transports information to the
multicast group, which is identified by the multicast group address in the
destination address field of an IP data packet. Unlike a unicast model, a multicast
model must forward data packets to multiple external interfaces so that all
Server
Multicast
User D
User E
User C
IGMP
IGMP
User A
User B
IGMP
MBGP/MSDP
PIM
PIM
AS1
AS2