Chapter 14
| Multicast Filtering
Layer 2 IGMP (Snooping and Query for IPv4)
– 540 –
Figure 336: 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 584.
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 542) 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 network segments where no node has
expressed interest in receiving a specific multicast service. For switches that do not
support multicast routing, or where multicast routing is already enabled on other
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