3Com E4500-24 Cli Configuration Manual page 128

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 merely sends log messages to the terminal
but does not set the port to the block state or remove the corresponding MAC forwarding entry.
You can also further control the loopback port by enabling one of the following function on it (note
that, the following two function are mutually exclusive and the latest function configured takes
effect):
Enable the loopback port control function on the port: the system sets the port to the block state
(where the port cannot forward data packets), sends log messages to the terminal, and removes
the corresponding MAC forwarding entry. After the loop is removed, the port automatically resumes
the normal forwarding state.
Enable the loopback port auto-shutdown function on the port: the system shuts down the port and
sends log messages to the terminal. After the loop is removed, the port does not automatically
resume the normal forwarding state. Instead, you need to use the undo shutdown command to
bring up the port.
Follow these steps to configure loopback detection for an Ethernet port:
To do...
Enter system view
Enable loopback
detection globally
Set the interval for
performing port loopback
detection
Enter Ethernet port view
Enable the loopback port
auto-shutdown function
Enable loopback port
control on the trunk or
hybrid port
Configure the system to
run loopback detection
on all VLANs of the
current trunk or hybrid
port
Exit to system view
Use the command...
system-view
loopback-detection enable
loopback-detection
interval-time time
interface interface-type
interface-number
loopback-detection
shutdown enable
loopback-detection control
enable
loopback-detection
per-vlan enable
quit
1-6
Remarks
—
Optional
By default, the global loopback
detection function is enabled if the
device boots with the default
configuration file (config.def);
By default, this function is disabled. if
the device boots with null configuration,
Optional
The default is 30 seconds.
—
Optional
By default, the loopback port
auto-shutdown function is enabled on
ports if the device boots with the default
configuration file (config.def);
By default, this function is disabled if
the device boots with null configuration
Optional
By default, the loopback port control
function is enabled on ports if the
device boots with the default
configuration file (config.def);
By default, this function is disabled if
the device boots with null configuration
Optional
By default, the system runs loopback
detection only on the default VLAN of
the current trunk or hybrid port.
—