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| Multicast Filtering
Layer 2 IGMP (Snooping and Query for IPv4)
– 608 –
Figure 357: Multicast Filtering Concept
This switch can use Internet Group Management Protocol (IGMP) to filter
multicast traffic. IGMP Snooping can be used to passively monitor or
“snoop” on exchanges between attached hosts and an IGMP-enabled
device, most commonly a multicast router. In this way, the switch can
discover the ports that want to join a multicast group, and set its filters
accordingly.
If there is no multicast router attached to the local subnet, multicast traffic
and query messages may not be received by the switch. In this case (Layer
2) IGMP Query can be used to actively ask the attached hosts if they want
to receive a specific multicast service. IGMP Query thereby 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.
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).
You can also configure a single network-wide multicast VLAN shared by
hosts residing in other standard or private VLAN groups, preserving
security and data isolation "Multicast VLAN Registration for IPv4" on
page 642.
LAYER 2 IGMP (SNOOPING AND QUERY FOR IPV4)
IGMP Snooping and Query – If multicast routing is not supported on other
switches in your network, you can use IGMP Snooping and IGMP Query
(page 610) to monitor IGMP service requests passing between multicast
clients and servers, and dynamically configure the switch ports which need
to forward multicast traffic. IGMP Snooping conserves bandwidth on
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