3Com 4500 PWR 26-Port Configuration Manual page 157

If you have not enabled the loopback port auto-shutdown function on the 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, 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, 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.
You cannot enable both the loopback port control function and the loopback port auto-shutdown
function on a port. If you do so, the function configured later will take effect.
Table 12-1 Configure loopback detection for Ethernet port(s)
Operation
Enter system view
Enable loopback
detection globally
Enable loopback
detection on the
specified ports in bulk
Set the interval for
performing port loopback
detection
Enter Ethernet port view
Enable loopback
detection on a specified
port
Command
system-view
loopback-detection enable
loopback-detection
interface-list enable
loopback-detection
interval-time time
interface interface-type
interface-number
loopback-detection enable
12-6
Remarks
—
Optional
By default, the global loopback
detection function is enabled if the
device boots with the default
configuration file (config.def); if the
device boots with null configuration,
this function is disabled.
Optional
By default, the loopback detection
function is enabled on ports if the
device boots with the default
configuration file (config.def); if the
device boots with null configuration,
this function is disabled.
Optional
The default is 30 seconds.
—
Optional
By default, the loopback detection
function is enabled on ports if the
device boots with the default
configuration file (config.def); if the
device boots with null configuration,
this function is disabled.