3com 5500-ei pwr Reference Guide

 

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undo loopback-detection enable 

System view or Ethernet port view 

None 

Use the loopback-detection enable command to enable the loopback detection feature on ports to 

detect whether external loopback occurs on a port.  

Use the undo loopback-detection enable command to disable the loopback detection feature on port.  

1)  If a loop is found on an access 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.  

z 

If you have also enabled 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, you need to use 

the undo shutdown command to bring up the port. 

z 

If you have not enabled the loopback port auto-shutdown function on the port, the port 

automatically resumes 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 functions are mutually exclusive, and the latest function configured takes 

effect): 

z 

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. 

z 

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. 

 

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).