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nradia_FTNT
Staff
nradia_FTNT
Staff
May 22, 2025

Technical Tip: How to do a loopback test for SFP ports

  • May 22, 2025
  • 0 replies
  • 2664 views
Description This article describes steps to perform a hardware test of SFP ports using loopback cables.
Scope FortiGate 6.x, FortiGate 7.x.
Solution

To perform a loopback test on SFP ports in a FortiGate firewall, the goal is to verify that the port is functioning correctly (both transmitting and receiving data).

 

An optical transceiver loopback test checks if the transceiver's hardware is working by connecting its transmitter (Tx) and receiver (Rx). To do this, a loopback cable or a single fiber strand is needed. Here is how to perform the test:

  1. Prepare the fiber: Separate one strand of fiber from a duplex fiber cable and remove the collars from both ends.

  2. Insert the transceiver: Plug the optical transceiver into the firewall. 

  3. Create the loopback connection: Connect the isolated fiber or loopback cable to the Tx and Rx ports on the transceiver. Use of a fiber loopback module is optional and possible. 

  4. Check the indicator light: If the light stays on, the hardware works. If it does not, further troubleshooting is required.

 

Check port link status:
Once the cable is connected:
Log into the FortiGate via the CLI or the GUI.
In the CLI, run the following:

 

get system interface transceiver

get system interface transceiver port25  (Port25 is an example.)

 

Alternatively, check basic link status:

 

Example Output:

 

FortiGate-1801F (global) # get sys interface transceiver port25
Interface port25 - SFP/SFP+/SFP28, 10GBASE-SR
  Diagnostics  :            Implemented
  Vendor Name  :            AVAGO           
  Part No.     :            AFBR-709ASMZ    
  Serial No.   :            AA181530UFX     
  Measurement  Unit         Value        High Alarm   High Warning Low Warning  Low Alarm
  ------------ ------------ ------------ ------------ ------------ ------------ ------------              
  Temperature  (Celsius)     22.8         90.0         85.0          0.0         -5.0       
  Voltage      (Volts)       3.30         3.60         3.46         3.13         3.00       
  Tx Bias      (mA)          6.64        10.50        10.50         2.50         2.50       
  Rx Power     (dBm)         -3.3          3.0         -1.0        -15.0        -15.0       
  Tx Power     (dBm)         -2.1          3.0         -1.0         -5.0         -9.0       
  ++ : high alarm, + : high warning, - : low warning, -- : low alarm, ? : suspect.

 

To test connectivity between two ports, use a regular SFP cable by connecting the two ports.


Other useful commands:

 

diagnose hardware deviceinfo nic port25
diagnose hardware deviceinfo nic <interface-name>

 

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