3Com Switch 8800 Advanced Software V5 Configuration Guide

Multicast Architecture 495
ASM model
In the ASM model, any sender can become a multicast source and send
information to a multicast group; numbers of receivers can join a multicast group
identified by a group address and obtain multicast information addressed to that
multicast group. In this model, receivers are not ware of the position of a multicast
source in advance. However, they can join or leave the multicast group at any
time.
SFM model
The SFM model is derived from the ASM model. From the view of a sender, the
two models have the same multicast group membership architecture.
Functionally, the SFM model is an extension of the ASM model. In the SFM model,
the upper layer software checks the source address of received multicast packets
so as to permit or deny multicast traffic from specific sources. Therefore, receivers
can receive the multicast data from only part of the multicast sources. From the
view of a receiver, multicast sources are not all valid: they are filtered.
SSM model
In practice, users may be interested in the multicast data from only certain
multicast sources. The SSM model provides a transmission service that allows users
to specify the multicast sources they are interested in at the client side.
The radical difference between the SSM model and the ASM model is that in the
SSM model, receivers already know the locations of the multicast sources by some
other means. In addition, the SSM model uses a multicast address range that is
different from that of the ASM module, and dedicated multicast forwarding paths
are established between receivers and the specified multicast sources.
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For details about the concepts of SPT and RPT, refer to “PIM Configuration” on
page 563.
Multicast Architecture
Multicast Mechanism The purpose of an IPv6 multicast technology is to carry information, by multicast,
from a multicast source to the receivers.
IP multicast involves the following questions:
■ Where should the multicast source transmit information to? (multicast
addressing)
■ What receivers exist on the network? (host registration)
■ How should information be transmitted to the receivers? (multicast routing)