CLI Reference Guide-R04

Chapter 25 | Multicast Filtering Commands
IGMP Snooping
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◆ If a topology change notification (TCN) is received, and all the uplink ports are
subsequently deleted, a timeout mechanism is used to delete all of the
currently learned multicast channels.
◆ When a new uplink port starts up, the switch sends unsolicited reports for all
current learned channels out through the new uplink port.
◆ By default, the switch immediately enters into “multicast flooding mode” when
a spanning tree topology change occurs. In this mode, multicast traffic will be
flooded to all VLAN ports. If many ports have subscribed to different multicast
groups, flooding may cause excessive loading on the link between the switch
and the end host. Flooding may be disabled to avoid this, causing multicast
traffic to be delivered only to those ports on which multicast group members
have been learned.
◆ When the spanning tree topology changes, the root bridge sends a proxy
query to quickly re-learn the host membership/port relations for multicast
channels. The root bridge also sends an unsolicited Multicast Router Discover
(MRD) request to quickly locate the multicast routers in this VLAN.
The proxy query and unsolicited MRD request are flooded to all VLAN ports
except for the receiving port when the switch receives such packets.
Example
The following example enables TCN flooding.
Console(config)#ip igmp snooping tcn-flood
Console(config)#
ip igmp snooping
tcn-query-solicit
This command instructs the switch to send out an IGMP general query solicitation
when a spanning tree topology change notification (TCN) occurs. Use the no form
to disable this feature.
Syntax
[no] ip igmp snooping tcn-query-solicit
Default Setting
Disabled
Command Mode
Global Configuration
Command Usage
◆ When the root bridge in a spanning tree receives a topology change
notification for a VLAN where IGMP snooping is enabled, it issues a global IGMP
leave message (query solicitation). When a switch receives this solicitation, it
floods it to all ports in the VLAN where the spanning tree change occurred.