H3C Technologies H3C S12500 Series Switches - Manual (Page 198)

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• Shutdown—When detecting a looped port, besides printing log information and sending trap
messages, the system also shuts down the port to disable it from receiving and sending frames
(including loopback detection frames). After the port status detection timer (set by the
shutdown-interval command) expires, the system will bring up the port. For more information about
the port status detection timer, see Fundamentals Configuration Guide.
Loop status auto recovery
After the switch detects a loop on a port, the switch continues to monitor the loopback detection frames.
If no loopback detection frame is received within three times the loopback detection interval, the switch
concludes that the loop is removed and notifies the users of this event. This process is known as loop
status auto recovery.
Loop status auto recovery applies only when the loopback detection action is none. When the loopback
detection action is shutdown, the switch automatically shuts down looped ports and thus removes the
loop. After the port status detection timer (set by the shutdown-interval command) expires, the system will
bring up the port. If the loop still exists, the system shuts down the port again.
When a network loop occurs, the switch discards some of the frames to reduce the load. If the loopback
detection frames are among the discarded frames, the loop status auto recovery function on the switch
will erroneously conclude that the loop has already been removed. To avoid this, set the loopback
detection action to shutdown, or manually remove the loop when the switch reports the occurrence of the
loop if you set the loopback detection action to none.
Loopback detection configuration task list
Task Remarks
Enabling loopback detection Required.
Configuring the loopback detection action Optional.
Configuring the loopback detection interval Optional.
Enabling loopback detection
You can enable the loopback detection function in system view or VLAN view. After you enable loopback
detection for a VLAN, the system performs loopback detection on all the ports in the VLAN.
Enabling loopback detection in system view
In system view, you can bulk enable loopback detection for multiple or all VLANs.
To enable loopback detection in system view:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enable loopback detection.
loopback-detection enable vlan
{ vlan-list | all }
By default, this function is disabled.