14-1
C
HAPTER
14
M
ULTICAST
F
ILTERING
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
Unicast
Flow
Multicast
Flow
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