H3C s3100 series Command Manual page 166

Use the undo loopback-detection enable command to disable the loopback detection function on the
port.
1) If a loop is found on an access port, 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.
If you have enabled the loopback port auto-shutdown function (with the
shutdown enable
and trap messages to the terminal. After the loop is removed, you need to use the undo shutdown
command to bring up the port.
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:
If the loopback port control function is enabled on the port (with the
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.
If the loopback port auto-shutdown function is enabled on the port (with the
shutdown enable
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
command) on the access port, the system will shut down the port, and send log
command), the system will shut down the port and send log and trap messages
1-21
loopback-detection
loopback-detection control
loopback-detection