Technical Tip: Optical Power value displaying as 0 for QSFP+ Transceiver

[Ted]

Description	This article describes why the Optical Tx/Rx Power fields may show 0 dBm in the CLI output of get system interface transceiver, even though the 40G QSFP+ interface is operational, traffic flows normally, and no hardware issues are present.
Scope	FortiGate.
Solution	This behavior is not a bug with the transceiver or the FortiGate. Instead, it reflects differences in Digital Diagnostics Monitoring capabilities across QSFP+ module part numbers. The root cause is that the 40GBASE‑SR4 QSFP+ Optical Transceiver module with part number FTL410QE2C does not support Digital Diagnostic Monitoring, including optical power monitoring capabilities. Therefore, the FortiGate cannot display the optical power readings for this module through the get system interface transceiver command. On the other hand, the same module with a different part number, FTL410QE4C, does include built‑in Digital Diagnostics Monitoring features, and optical power readings are correctly reported when this transceiver is used. Example: Port 23 - FTL410QE4C: Supports built‑in digital diagnostic functions, including Tx/Rx optical power monitoring. Port 24 - FTL410QE2C: Provides only partial digital diagnostics. Does not support optical power monitoring. Note: No action is required if the port is functional and the link is stable. If optical power monitoring is required for operational or diagnostic purposes, use a module that supports Digital Diagnostics Monitoring features, such as the FTL410QE4C. Related documents: 40GBASE‑SR4 QSFP+ Optical Transceiver module with part number FTL410QE4C 40GBASE‑SR4 QSFP+ Optical Transceiver module with part number FTL410QE2C