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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:
Command
Step 1
configure terminal
Step 2
ip igmp snooping tcn query solicit
Step 3
end
Step 4
show ip igmp snooping
Step 5
copy running-config startup-config
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:
Command
Step 1
configure terminal
Step 2
interface interface-id
Step 3
no ip igmp snooping tcn flood
Step 4
exit
Step 5
show ip igmp snooping
Step 6
copy running-config startup-config
To re-enable multicast flooding on an interface, use the ip igmp snooping tcn flood interface
configuration command.
Catalyst 2928 Switch Software Configuration Guide
21-12
Purpose
Enter global configuration mode.
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.
Return to privileged EXEC mode.
Verify the TCN settings.
(Optional) Save your entries in the configuration file.
Purpose
Enter global configuration mode.
Specify the interface to be configured, and enter interface
configuration mode.
Disable the flooding of multicast traffic during a spanning-tree TCN
event.
By default, multicast flooding is enabled on an interface.
Return to privileged EXEC mode.
Verify the TCN settings.
(Optional) Save your entries in the configuration file.
Chapter 21
Configuring IGMP Snooping
OL-23389-01
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Troubleshooting
