3Com 4007, Implementation Manual, Page: 208 / 684

208 C
HAPTER
11: IP M
ULTICAST
F
ILTERING WITH
IGMP
This chapter describes IGMP functions on Layer 2 Switching Modules.
IGMP is also supported on Multilayer Switching Modules, but is described
in the context of IP multicast routing. See Chapter 18
for more
information.
The management interfaces display “
cb9000
” and refer to the
Management Module as the Enterprise Management Engine (EME)
because the heritage of the Switch 4007 is the CoreBuilder
®
9000 switch.
Overview
To transport their content to a community of network users,
bandwidth-hungry applications often generate packets in the IP multicast
format. Many network standards and protocols have been designed to
create an efficient delivery system for IP multicast traffic from the source
(usually a server) to the destinations (users). IGMP is one of these
protocols.
If IGMP functions are disabled or not present in a Layer 2 switch, the
switch floods all IP multicast packets to all ports — that is, it operates in
compliance with the
IEEE 802.1D MAC Bridges base standard. If IGMP
functions are present and enabled, a switch can forward IP multicast
traffic only to ports that require it and filter it on other ports.
Defined in Internet RFC 1112 and RFC 2236, IGMP performs two main
functions in a Layer 2 switch:
snooping
and
querying
. Descriptions of
these functions and how they work together are explained later in this
chapter.
Benefits Support for IGMP in Layer 2 devices benefits your network in many ways:
■
IGMP reduces the amount of bandwidth that an IP multicast stream
would otherwise occupy at the edge of an IP multicast delivery tree; it
is especially useful in flat network designs (large broadcast domains
with cascading switches).
■
IGMP reduces the amount of unwanted traffic that a host encounters
on its Ethernet segment, which may be critical for users with
low-speed connections to the network.