Fiber Optic Module Diagnostic & Troubleshooting Cheat-Sheet

Purpose

Quick reference for interpreting Digital Optical Monitoring (DOM) values on fiber optic modules (SFP, SFP+, QSFP, etc), identifying acceptable, caution, and unacceptable levels, and general issue troubleshooting examples. The suggested ranges is meant to cover a general ground across different make/model fiber optic modules.

Parameter Ranges

Distance Cheat Sheet 

Troubleshooting Examples

If RX Power Is Extremely Low (Alarm Condition)

• Indicates no usable optical signal.

Possible causes:

• Fiber unplugged or broken
• Far-end transmitter down
• Dirty or damaged connectors
• Wrong fiber type or polarity
• Faulty optic

If RX remains in alarm after reseat and cleaning → likely far-end TX is off/faulty or the path is open (physical break or discontinuity in the fiber link).

• This can happen due to:

• Unplugged connector at one end or both ends.
• Fiber break (damaged cable or cut).
• Patch panel mispatch (fiber not connected through the correct ports).
• Disconnected jumper in the middle of the run.

If Laser Bias Current Is High (Warning or Alarm)

• Indicates the transmitter’s laser requires more current to maintain output power.

Possible causes:

• Aging or degraded laser diode
• Elevated operating temperature
• Contaminated or damaged fiber optical path increasing loss

If bias remains high after cleaning and reseating → the fiber optic module or the fiber run itself is nearing end-of-life and should be scheduled for replacement.

If TX Power Is Below Preferred Range

• Indicates the transmitter fiber optic module is outputting less optical power than expected.

Possible causes:

• Dirty or damaged TX connector
• Laser degradation inside the fiber optic line
• Incorrect or incompatible SFP type

If TX Power remains low after cleaning and reseating → fiber optic module or fiber optic line may be failing and should be replaced.

If RX Power Is Too High (Warning or Alarm)

• Indicates the receiver is being overpowered, which can cause bit errors.

Possible causes:

• Very short fiber run without attenuation
• Incorrect fiber optic module type for the link distance
• Missing optical attenuator on short-reach connections

If RX remains high → add an attenuator or use optical modules that are rated for short distances.

If Voltage Is Out of Range

• Indicates the SFP is receiving unstable or incorrect supply voltage.

Possible causes:

• Host device (switch or endpoint) power rail instability
• Failing voltage regulator inside the fiber optic module 

If voltage remains out of range after reseating → check switch power health or replace the fiber optic module.

If Temperature Is Near or Above Limit

• Indicates the optic is operating in a high-temperature environment.

Possible causes:

• Poor airflow or blocked vents
• High ambient temperature in the rack
• Overloaded switch chassis

If temperature remains high after improving airflow → relocate optic or replace if thermal issues persist.

Fast Troubleshooting Flow

1. Clear and re-read alarms/DOM; verify link state on both ends.
2. Reseat optics; clean LC connectors on both ends; reseat again.
3. Confirm fiber grade and length:

• OM1 ≈ 33 m, OM2 ≈ 82 m
• OM3 ≈ 300 m, OM4 ≈ 400 m, OM5 ≈ 400 m

4. Check polarity/crosses at patch panels (A↔B).
5. Swap components in order: patch → local SFP → remote SFP → port.
6. If RX remains in alarm → far-end TX is off/faulty or path is open.

Useful Commands

show transceiver detail

show transceiver threshold-violation interface <port>

configure terminal

interface <port>

 transceiver-monitoring enable

end

Notes

• Warn = caution message, device may still operate but concerning thresholds have been recorded; Alarm = failure message. Entirely likely the device is non-functional. Stay comfortably inside the suggested ranges above for reliability.
• Track trends: rising Bias >20 mA or RX near Warn often precede link issues.