Check fiber optic loss status on the switch

You can check fiber optic loss on a Cisco switch by using SFP Digital Optical Monitoring (DOM) commands to view TX/RX power and calculate link loss.Step 1: Verify Physical Connections

Before using software diagnostics, inspect the fiber optic cables and connectors:

• Check for physical damage such as cuts, kinks, or abrasions along the cable. Even minor damage can cause significant signal loss .
• Inspect connectors at both ends for dust or debris and ensure they are clean and securely plugged into the correct ports .
• Check bend radius to ensure the fiber is not bent beyond its minimum radius, which can cause attenuation .

Step 2: Confirm SFP Recognition

Ensure the SFP module is recognized by the switch:

• Use show inventory to list installed SFPs and verify part numbers, serial numbers, and vendor information .
• If the SFP is not recognized, the port may be err-disabled, or the module may be unsupported.

Step 3: Check Optical Power Levels

Cisco switches support Digital Optical Monitoring (DOM), which provides real-time TX and RX power readings in dBm:

• Use the command show interfaces transceiver detail (Catalyst switches) or show fiber-ports optical-transceiver (SMB switches) to view optical power .
• The output shows Transmit (TX) power and Receive (RX) power for each SFP. Compare these values to the SFP's specified thresholds (Low Warn / High Warn) to detect potential signal issues .

Step 4: Calculate Fiber Link Loss

To determine the real-time loss of a fiber link:

• Measure the TX power from the sending switch and the RX power at the receiving switch.
• Calculate the loss as: Loss (dB) = TX Power (dBm) - RX Power (dBm)
• Repeat for both directions if needed to ensure symmetrical performance .

Step 5: Monitor Interface and Error Counters

• Use show interfaces to check operational status, speed, and duplex settings .
• Use show interfaces counters errors to view input/output errors, CRC errors, or frame errors, which may indicate fiber degradation .
• Check system logs with show logging | include for err-disable messages or unsupported transceiver alerts .

Step 6: Troubleshooting Tips

• Ensure the SFP supports DOM; otherwise, optical power readings will not be available .
• Use Cisco-compatible SFPs to avoid unpredictable behavior and inaccurate readings .
• If TX/RX values are outside acceptable ranges, inspect connectors, clean fiber ends, or replace damaged cables. By following these steps, you can effectively monitor fiber optic loss, detect degradation early, and maintain reliable switch-to-switch connectivity.