Multimedia Traffic Control with IP Multicast (IGMP)
More on IGMP Operation
Switch 4
PC 1
PC 2
PC 5
PC 6
Multicast
Data Stream
Switch 3 is
running IGMP
and operating
as the querier.
Switch 1
Switch 2
IGMP is NOT
running on
switches 1
and 2.
Switch 3
IGMP IS
running on
switch 4.
Server
Figure 12-4. 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 (through IGMP or another means) or there
are no hosts connected to 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, and the port on Switch 3 that
connects to Switch 1 must be unblocked. (Blocking can be due to the
current IGMP configuration or to another reason unrelated to IGMP.)
Data-Driven IGMP
An IP multicast packet includes the multicast group (address) to which the
packet belongs. When an IGMP client connected to a switch port needs to
receive multicast traffic from a specific group, it joins the group by sending
an IGMP report (join request) through the switch to the network. (The
multicast group specified in the join request is determined by the requesting
application running on the IGMP client.) When a networking device with IGMP
enabled receives the join request for a specific group, it forwards any IP
12-13
