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28-1
Chapter 28: 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 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).
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 IGMP Query (page 28-2) 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 28-5). This
interface will then join all the current multicast groups supported by the attached
Unicast
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Multicast
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