3Com 4007 Implementation Manual page 209

IGMP requires minimal configuration in network devices and hosts.
For example:
Snooping and querying functions can be easily enabled in
switching devices.
IP-capable end stations do not usually require any special
configuration because IGMP is already part of the IP protocol stack.
Because more IP multicast applications are available each year, support
for IGMP in switches helps prolong the life span of existing network
topologies and available bandwidth.
To understand the fundamental benefit that IGMP provides for users
attached to a switch, see Figure 8.
Figure 8 IP Multicast Traffic Flow Before and After IGMP Snooping
Switch floods IP multicast
traffic to all ports
BEFORE
IP multicast
application
sources
Switch forwards IP
multicast traffic only to
Overview 209
ports that lead to
group members
AFTER