Configuring the Switch
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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).
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.
Based on the group membership information learned from IGMP, a router/switch can
determine which (if any) multicast traffic needs to be forwarded to each of its ports.
At Layer 3, multicast routers use this information, along with a multicast routing
protocol such as DVMRP or PIM, to support IP multicasting across the Internet.
Note that IGMP neither alters nor routes IP multicast packets. A multicast routing
protocol must be used to deliver IP multicast packets across different subnetworks.
Therefore, when DVMRP or PIM routing is enabled for a subnet on this switch, you
also need to enable IGMP.
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 3-161) 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 3-164). 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 (page 3-166).
