Filter
Top Products
Configuring the Switch
3-212
3
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 3-213) to monitor
IGMP service requests passing between multicast clients and servers, and
dynamically configure the switch ports which need to forward multicast traffic.
When using IGMPv3 snooping, service requests from IGMP Version 1, 2 or 3 hosts
are all forwarded to the upstream router as IGMPv3 reports. The primary
enhancement provided by IGMPv3 snooping is in keeping track of information about
the specific multicast sources which downstream IGMPv3 hosts have requested or
refused. The switch maintains information about both multicast groups and
channels, where a group indicates a multicast flow for which the hosts have not
requested a specific source (the only option for IGMPv1 and v2 hosts unless
statically configured on the switch), and a channel indicates a flow for which the
hosts have requested service from a specific source.
Only IGMPv3 hosts can request service from a specific multicast source. When
downstream hosts request service from a specific source for a multicast service,
Unicast
Flow
Multicast
Flow