Zero transmit output with normal reception usually indicates a faulty optical module, misconfiguration, or a transmitter-specific issue, while the receiver remains functional.

Possible Causes

1. Optical Module Failure If the module's transmit optical power is zero, it may indicate a failure of the laser or transmitter circuitry, even if the receiver is still operational . This is a common failure mode in SFP, SFP+, or XFP modules.
2. Dirty or Damaged Transmitter End Contamination on the transmitter lens or damage to the fiber can prevent light emission, while the receiver can still detect incoming signals from the opposite end .
3. Incorrect Port Configuration Mismatched speed, duplex, or interface type between the module and the switch/router port can prevent transmission, even if reception works .
4. DDM or Threshold Alarms Digital Diagnostic Monitoring (DDM) may report low or zero transmit power, signaling that the module is outside its operational range . This can be verified via CLI commands like show interfaces transceiver detail.

Troubleshooting Steps

1. Check DDM Readings Verify the transmit power, receive power, and module alarms. If transmit power is zero or below the minimum threshold, the module is likely faulty .
2. Loopback Test Perform a self-loop test by connecting the module's transmit port to its own receive port using a short fiber. If the port does not go UP, the module or port is defective .
3. Inspect Fiber and Connectors Clean the fiber connectors and check for physical damage. Replace the fiber if necessary .
4. Swap Modules or Ports Replace the optical module with a known working one. If the new module transmits normally, the original module is defective. Alternatively, test the module in another compatible port .
5. Verify Configuration Ensure the port speed, duplex, and interface type match the module specifications. Adjust settings if needed .

Recommended Solution

• Replace the optical module if DDM shows zero transmit power and all other checks (fiber, port, configuration) are normal .
• Use vendor-approved modules to avoid compatibility issues .
• Maintain proper rack cooling and avoid overheating, as thermal stress can affect module transmit performance . By following these steps, you can isolate whether the zero output is due to a faulty module, fiber issue, or configuration mismatch, ensuring reliable network operation.