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22-13
Catalyst 2960 and 2960-S Switch Software Configuration Guide
OL-8603-09
Chapter 22 Configuring IGMP Snooping and MVR
Configuring IGMP Snooping
Recovering from Flood Mode
When a topology change occurs, the spanning-tree root sends a special IGMP leave message (also known
as global leave) with the group multicast address 0.0.0.0. However, when you enable the ip igmp
snooping tcn query solicit global configuration command, the switch sends the global leave message
whether or not it is the spanning-tree root. When the router receives this special leave, it immediately
sends general queries, which expedite the process of recovering from the flood mode during the TCN
event. Leaves are always sent if the switch is the spanning-tree root regardless of this configuration
command. By default, query solicitation is disabled.
Beginning in privileged EXEC mode, follow these steps to enable the switch to send the global leave
message whether or not it is the spanning-tree root:
To return to the default query solicitation, use the no ip igmp snooping tcn query solicit global
configuration command.
Disabling Multicast Flooding During a TCN Event
When the switch receives a TCN, multicast traffic is flooded to all the ports until 2 general queries are
received. If the switch has many ports with attached hosts that are subscribed to different multicast
groups, this flooding might exceed the capacity of the link and cause packet loss. You can use the ip
igmp snooping tcn flood interface configuration command to control this behavior.
Beginning in privileged EXEC mode, follow these steps to disable multicast flooding on an interface:
To re-enable multicast flooding on an interface, use the ip igmp snooping tcn flood interface
configuration command.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
ip igmp snooping tcn query solicit Send an IGMP leave message (global leave) to speed the process of
recovering from the flood mode caused during a TCN event. By
default, query solicitation is disabled.
Step 3
end Return to privileged EXEC mode.
Step 4
show ip igmp snooping Verify the TCN settings.
Step 5
copy running-config startup-config (Optional) Save your entries in the configuration file.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
interface interface-id Specify the interface to be configured, and enter interface
configuration mode.
Step 3
no ip igmp snooping tcn flood Disable the flooding of multicast traffic during a spanning-tree TCN
event.
By default, multicast flooding is enabled on an interface.
Step 4
exit Return to privileged EXEC mode.
Step 5
show ip igmp snooping Verify the TCN settings.
Step 6
copy running-config startup-config (Optional) Save your entries in the configuration file.