HP J3100B Installation And Configuration Manual page 123

The next figure (7-14) shows a network running IP multicasting using IGMP
without a multicast router. In this case, the IGMP-configured switch runs as
a querier.
PCs 2, 5, and 6 are members of the same IP multicast group.
IGMP is configured on switches 3 and 4. Either of these switches can operate
as querier because a multicast router is not present on the network. (If an
IGMP switch does not detect a querier, it automatically assumes this role,
assuming the querier feature is enabled—the default—within IGMP.)
IGMP is NOT
Running Here
Switch # 2 Switch # 1
PC #1
Figure 7-14. Isolating IP Multicast Traffic in a Network
In the above figure, the multicast group traffic does not go to switch 1 and
beyond because either the port on switch 3 that connects to switch 1 has
been configured as blocked or there are no hosts off of switch 1 or switch
2 that belong to the multicast group.
For PC #1 to become a member of the same multicast group without
flooding IP multicast traffic on all ports of switches 1 and 2, IGMP must
be configured on both switches 1 and 2.
Switch # 1
Switch # 3
Switch #4
PC #2
PC # 5
Advanced Concepts
IP Multicast (IGMP)
IGMP IS
Running Here
Multicast
Data Stream
PC #6
7-25