How to test the optical ports of two switches

Testing an optical port involves checking the transceiver status, optical power levels, and link connectivity using CLI commands or external fiber testing tools.

Step 1: Physical Inspection

Start by visually inspecting the fiber port and connectors. Ensure there is no physical damage, dirt, or dust on the connectors, and that the fiber cables are properly seated in the SFP or optical module .

Step 2: Access the Switch CLI

1. Connect to the switch via console cable, SSH, or Telnet.
2. Log in with the appropriate credentials.
3. Enter privileged EXEC mode by typing enable and providing the enable password if prompted .

Step 3: Check Port and Transceiver Status

Use the following commands to verify the optical port:

• show interface status – Displays all interfaces and their link status (UP/DOWN) and type, helping identify fiber ports .
• show interface – Provides detailed information about a specific port, including speed, duplex, errors, and link state .
• show interface transceiver or show interfaces transceiver details – Displays TX (transmit) and RX (receive) optical power levels, temperature, voltage, and other diagnostics . Check that the link state is UP and that the optical power levels are within the acceptable range specified by the transceiver manufacturer. Significant discrepancies between TX and RX levels on opposite ends may indicate a fiber issue .

Step 4: Compare Optical Levels

If both ends of the fiber link are accessible:

• Compare the TX of one side with the RX of the other.
• A large difference or RX reading near -40 dBm or lower indicates a signal loss or broken fiber .

Step 5: Use External Fiber Testing Tools (Optional)

For more precise testing:

• Optical Power Meter – Measures the actual optical power received.
• OTDR (Optical Time-Domain Reflectometer) – Detects breaks, bends, or faults along the fiber length .

Step 6: Troubleshooting

• If the port shows LOS (Loss of Signal) or RX levels are too low, check the fiber cable, connectors, and SFP module compatibility .
• Clean connectors and reseat the SFP module if necessary.
• Replace the fiber or transceiver if the problem persists. By following these steps, you can effectively test and verify the optical port on a switch, ensuring proper fiber connectivity and signal integrity.