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Multicast Filtering
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Multicast Filtering
Multicasting is used to support real-time applications such
as videoconferencing or streaming audio. A multicast
server does not have to establish a separate connection
with each client. It merely broadcasts its service to the
network, and any hosts that want to receive the multicast
register with their local multicast switch/router. Although
this approach reduces the network overhead required by
a multicast server, the broadcast traffic must be carefully
pruned at every multicast switch/router it passes through
to ensure that traffic is only passed on to the hosts which
subscribed to this service.
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 multicast 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).
Layer 2 IGMP (Snooping and Query)
IGMP Snooping and Query – If multicast routing is not supported on other switches
in your network, you can use IGMP Snooping and Query (page 3-110) to monitor
IGMP service requests passing between multicast clients and servers, and
dynamically configure the switch ports which need to forward multicast traffic.
Static IGMP Router Interface – If IGMP snooping cannot locate the IGMP querier,
you can manually designate a known IGMP querier (i.e., a multicast router/switch)
connected over the network to an interface on your switch (page 3-112). This
interface will then join all the current multicast groups supported by the attached
router/switch to ensure that multicast traffic is passed to all appropriate interfaces
within the switch.
Static IGMP Host Interface – For multicast applications that you need to control
more carefully, you can manually assign a multicast service to specific interfaces on
the switch (page 3-114).
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