HP J3100B Installation And Configuration Manual page 122

Advanced Concepts
IP Multicast (IGMP)
Once the switch learns the port location of the hosts belonging to any partic-
ular multicast group, it can direct group traffic to only those ports, resulting
in bandwidth savings on ports where group members do not reside. The
following example illustrates this operation.
Figure 7-13 shows a network running IGMP.
Figure 7-13. The Advantage of Using IGMP
7-24
PCs 1 and 4, Switch #2, and all of the routers are members of an IP
multicast group. (The routers operate as queriers.)
Switch #1 ignores IGMP traffic and does not distinguish between IP
multicast group members and non-members. Thus, it is sending large
amounts of unwanted multicast traffic out the ports to PCs 2 and 3.
Switch #2 is recognizing IGMP traffic and learns that PC #4 is in the IP
multicast group receiving multicast data from the video server (PC X).
Switch #2 then sends the multicast data only to the port for PC #4, thus
avoiding unwanted multicast traffic on the ports for PCs #5 and #6.
Router
Router
IGMP is NOT
Running Here
Switch # 1
PC #1
Video
PC #3
Client
PC #2
Router
Router
IGMP IS
Switch # 2
Running Here
PC # 4
Video
Client
PC # 5
Multicast
Data Stream
PC X
Video
Server
PC #6