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Configuring the Switch
3-170
3
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 3-171) 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 3-174). 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 3-176).
IGMP Query (Layer 2 or 3) – IGMP Query can only be enabled globally at Layer 2,
but can be enabled for individual VLAN interfaces at Layer 3 (page 3-176). However,
note that Layer 2 query is disabled if Layer 3 query is enabled.
L3 - network core
(multicast routing)
L2 - edge switches
(snooping and query)
L2 switch to end nodes
(snooping on IGMP clients)