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Multicast Filtering
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Multicast Filtering
Overview
Multicasting is used to support real-time applications such as video conferencing 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 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 IGMP Query 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 (see the Specifying Interfaces Attached to a
Multicast Router section on page 152). 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 (see the Assigning
Ports to Multicast Services section on page 153).
FIG. 175 Unicast Flow vs. Multicast Flow
Unicast
Flow
Multicast
Flow