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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).
This switch not only supports IP multicast filtering by passively monitoring IGMP
query and report messages and multicast routing probe messages to register
end-stations as multicast group members, but also supports the DVMRP and
PIM-DM multicast routing protocols required to forward multicast traffic to other
subnets (page 3-264 and 3-271).
IGMP Protocol
The Internet Group Management Protocol (IGMP) runs between hosts and their
immediately adjacent multicast router/switch. IGMP is a multicast host registration
protocol that allows any host to inform its local router that it wants to receive
transmissions addressed to a specific multicast group.
A router, or multicast-enabled switch, can periodically ask their hosts if they want to
receive multicast traffic. If there is more than one router/switch on the LAN
performing IP multicasting, one of these devices is elected “querier” and assumes
the role of querying the LAN for group members. It then propagates the service
requests on to any adjacent multicast switch/router to ensure that it will continue to
receive the multicast service.
Unicast
Flow
Multicast
Flow