M
ULTICAST
C
ONFIGURATION
2-87
This switch uses IGMP (Internet Group Management Protocol) to
query for any attached hosts that want to receive a specific
multicast service. It identifies the ports containing hosts requesting
to join the service and sends data out to those ports only. It then
propagates the service request up to any neighboring multicast
switch/router to ensure that it will continue to receive the multicast
service. This procedure is called multica st filtering.
The purpose of IP multicast filtering is to optimize a switched
network’s performance, so multicast packets will only be
forwarded to those ports containing multicast group hosts or
multicast routers/switches, instead of flooding traffic to all ports in
the subnet (VLAN).
Configuring IGMP Parameters
You can configure the switch to forward multicast traffic
intelligently. Based on the IGMP query and report messages, the
switch forwards traffic only to the ports that request multicast
traffic. This prevents the switch from broadcasting the traffic to all
ports and possibly disrupting network performance.
Command Usage
•IGMP Snooping – This switch can passively snoop on IGMP
Query and Report packets transferred between IP multicast
routers/switches and IP multicast host groups to identify the IP
multicast group members. It simply monitors the IGMP packets
passing through it, picks out the group registration inform ation,
and configures multicast filters accordingly.
•IGMP Query – A router, or multicast-enabled switch, can
periodically ask their hosts if they want to receive multicast
traffic. If there is more than one router/switch on the LAN
performing IP multicasting, one of these devices is elected
“querier” and assumes the role of querying the LAN for group
members. It then propagates the service requests on to an y