20.
IGMP Snooping
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traffic to the VLAN. The switch forwards multicast group traffic only to those hosts listed in
the forwarding table for that multicast group.
When hosts want to leave a multicast group, they can leave silently or send a
leave
message.
When the switch receives a
leave
message from a host, it sends out a MAC-based general
query to determine if any other devices connected to that interface are interested in traffic for
the specific multicast group. The switch then updates the forwarding table for that MAC
group so that only those hosts interested in receiving multicast traffic for the group are listed
in the forwarding table. If the router receives no reports from a VLAN, it removes the group
for the VLAN from its IGMP cache.
Immediate-Leave Processing
IGMP snooping
Immediate-Leave
processing allows the switch to remove an interface that
sends a
leave
message from the forwarding table, without first sending out MAC-based
general queries to the interface. The VLAN interface is pruned from the multicast tree for the
multicast group specified in the original
leave
message. Immediate-Leave processing ensures
optimal bandwidth management for all hosts on a switched network, even when multiple
multicast groups are in use simultaneously.
IGMP Snooping Commands
The following IGMP Snooping commands are available.
Table 20-1 IGMP Snooping Commands
C o m m a n d D e s c r i p t i o n
ip igmp snooping
Enables IGMP snooping in all existing VLAN interfaces.
ip igmp snooping vlan
Enables the VLAN id for the multicast group.
ip igmp snooping vlan mrouter
Adds a multicast router port (adds a static connection to
a multicast router).
ip igmp snooping vlan static
Configures a Host or physical interface statically to join a
multicast group.
ip igmp snooping vlan immediate-
leave
Enables IGMP Immediate-Leave Processing on the VLAN
interface.
ip igmp snooping for-all
Forwards all multicast via the port list.
ip igmp snooping forbidden
Forbids IGMP snooping via the port list.
ip igmp snooping send-query
Queries sending.
show ip igmp snooping
Displays snooping configuration information for the
switch or for a specified VLAN.
show ip igmp snooping mrouter
Displays information on dynamically learned and
manually configured multicast router interfaces.