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24-13
Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02
Chapter 24 Configuring IGMP Snooping and MVR
Configuring IGMP Snooping
Configuring TCN-Related Commands
These sections describe how to control flooded multicast traffic during a TCN event:
• Controlling the Multicast Flooding Time After a TCN Event, page 24-13
• Recovering from Flood Mode, page 24-13
• Disabling Multicast Flooding During a TCN Event, page 24-14
Controlling the Multicast Flooding Time After a TCN Event
You can control the time that multicast traffic is flooded after a TCN event by using the ip igmp
snooping tcn flood query count global configuration command. This command configures the number
of general queries for which multicast data traffic is flooded after a TCN event. Some examples of TCN
events are when the client changed its location and the receiver is on same port that was blocked but is
now forwarding, and when a port went down without sending a leave message.
If you set the TCN flood query count to 1 by using the ip igmp snooping tcn flood query count
command, the flooding stops after receiving 1 general query. If you set the count to 7, the flooding until
7 general queries are received. Groups are relearned based on the general queries received during the
TCN event.
Beginning in privileged EXEC mode, follow these steps to configure the TCN flood query count:
To return to the default flooding query count, use the no ip igmp snooping tcn flood query count global
configuration command.
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.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
ip igmp snooping tcn flood query count
count
Specify the number of IGMP general queries for which the multicast
traffic is flooded. The range is 1 to 10. By default, the flooding query
count is 2.
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.