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z If you have not enabled the loopback port auto-shutdown function on the access port, the port will
automatically resume the normal forwarding state after the loop is removed.
2) If a loop is found on a trunk or hybrid port, the system sends log and trap messages to the terminal.
If you have additionally enabled the loopback port control function or the loopback port
auto-shutdown function, the system will deal with the port accordingly:
z If the loopback port control function is enabled on the port (with the loopback-detection control
enable
command), the system will set the port to the block state (ports in this state cannot forward
data packets), send log and trap messages to the terminal, and remove the corresponding MAC
forwarding entry. After the loop is removed, the port will automatically resume the normal
forwarding state.
z If the loopback port auto-shutdown function is enabled on the port (with the loopback-detection
shutdown enable
command), the system will shut down the port and send log and trap messages
to the terminal. After the loop is removed, you need to use the undo shutdown command to bring
up the port.
The loopback detection function on a specific port can take effect only after you enable the loopback
detection function globally (in system view) and on the port (in the specified port view).
By default, the loopback detection function is enabled globally and on ports if the device boots with the
default configuration file (config.def); if the device boots with null configuration, this function is disabled.
Related command: loopback-detection control enable, loopback-detection shutdown enable
Example
# Enable the loopback detection function on Ethernet 1/0/1.
<Sysname> system-view
System View: return to User View with Ctrl+Z.
[Sysname] loopback-detection enable
[Sysname] interface ethernet 1/0/1
[Sysname-Ethernet1/0/1] loopback-detection enable
loopback-detection interface-list enable
Syntax
loopback-detection interface-list enable
undo loopback-detection interface-list enable
View
System view
Parameter
interface-list: Ethernet port list, in the form of interface-list = { interface-type interface-number [ to
interface-type interface-number ] } &<1-10>, where
z interface-type is the port type, and interface-number is the port number.
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