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
Parameter
|
Preferred Range
|
Caution Range
|
Unacceptable Range
|
Notes
|
Temperature (°C)
|
0–70
|
70–100 or −20–0
|
≥100 or ≤−45
|
39 °C typical; airflow matters.
|
Voltage (V)
|
3.0–3.5
|
3.5–3.6 or 2.9–3.0
|
≥3.6 or ≤2.9
|
Stable 3.2–3.35 V is common.
|
Laser Bias (mA)
|
2–15
|
15–20
|
≥25 or ≤1
|
Rising bias indicates aging.
|
RX Power (dBm)
|
−13 to 0
|
−13 to −15
|
≤−15
|
Strong RX = −3 dBm typical short link.
|
TX Power (dBm)
|
−7.5 to 0
|
−7.5 to −9.5
|
≤−9.5
|
−4 dBm typical for SR optics.
|
Distance Cheat Sheet
|
Type
|
Core/Cladding (um)
|
Fast Ethernet 100Mb
|
Gigabit GbE
|
10Gigabit 10GbE
|
40Gigabit 40GbE
|
100Gigabit 100GbE
|
40G SWDM4
|
100G SWDM4
| |
Multimode
|
OM1
|
62.5 / 125
|
2km
|
275m
|
33m
|
-
|
-
|
-
|
-
| |
Multimode
|
|
OM2
|
50 / 125
|
2km
|
550m
|
82m
|
-
|
-
|
-
|
-
|
Multimode
|
|
OM3
|
50 / 125
|
2km
|
800m
|
300m
|
100m
|
100m
|
240m
|
75m
|
Multimode
|
|
OM4
|
50 / 125
|
2km
|
1100m
|
400m
|
150m
|
150m
|
350m
|
100m
|
Multimode
|
|
OM5
|
50 / 125
|
2km
|
1100m
|
400m
|
150m
|
150m
|
440m
|
150m
|
Single Mode
|
OS1/OS2
|
9 / 125
|
40km
|
100km
|
40km
|
40km
|
40km
|
-
|
-
|
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
Clear and re-read alarms/DOM; verify link state on both ends.
Reseat optics; clean LC connectors on both ends; reseat again.
Confirm fiber grade and length:
OM1 ≈ 33 m, OM2 ≈ 82 m
OM3 ≈ 300 m, OM4 ≈ 400 m, OM5 ≈ 400 m
Check polarity/crosses at patch panels (A↔B).
Swap components in order: patch → local SFP → remote SFP → port.
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.