Fiberchecks

If there is fiber between switches there is a good way to see what is going on:

Check power between fiber cables

INI-LON-EDGE-8A#show interface te4/1/1 transceiver detail ITU Channel not available (Wavelength not available), Transceiver is internally calibrated. mA: milliamperes, dBm: decibels (milliwatts), NA or N/A: not applicable. ++ : high alarm, + : high warning, - : low warning, — : low alarm. A2D readouts (if they differ), are reported in parentheses. The threshold values are calibrated.

                            High Alarm  High Warn  Low Warn   Low Alarm
         Temperature        Threshold   Threshold  Threshold  Threshold

Port (Celsius) (Celsius) (Celsius) (Celsius) (Celsius)

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Te4/1/1 17.5 75.0 70.0 0.0 -5.0

                            High Alarm  High Warn  Low Warn   Low Alarm
         Voltage            Threshold   Threshold  Threshold  Threshold

Port (Volts) (Volts) (Volts) (Volts) (Volts)

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Te4/1/1 3.28 3.63 3.46 3.13 2.97

                              High Alarm  High Warn  Low Warn   Low Alarm
             Current          Threshold   Threshold  Threshold  Threshold

Port Lane (milliamperes) (mA) (mA) (mA) (mA)

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Te4/1/1 N/A 5.8 12.0 10.5 2.5 1.0

             Optical          High Alarm  High Warn  Low Warn   Low Alarm
             Transmit Power   Threshold   Threshold  Threshold  Threshold

Port Lane (dBm) (dBm) (dBm) (dBm) (dBm)

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Te4/1/1 N/A -2.7 2.00 -1.00 -7.30 -11.31

             Optical          High Alarm  High Warn  Low Warn   Low Alarm
             Receive Power    Threshold   Threshold  Threshold  Threshold

Port Lane (dBm) (dBm) (dBm) (dBm) (dBm)

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Te4/1/1 N/A -40.0 2.00 -1.00 -9.91 -13.91

This is a good example where there is an issue as we see -40 dBm, means most likely there is a faulty cable.