A 25G Ethernet connection between an EX4400 and QFX5120 may establish physical link but fail to pass traffic when different optical technologies are used on each side of the connection. In this scenario, one end of the link was using a Single-Mode (LR) SFP28 transceiver while the other end was using a Multi-Mode (SR) SFP28 transceiver.

Although the interfaces intermittently showed link-up status, the mismatch generated Forward Error Correction (FEC) errors and prevented reliable data transmission. The issue can be mistaken for an optics compatibility, firmware, or hardware problem because certain optics combinations may partially function or display link activity.

• 25G link fails to pass traffic despite interfaces showing operational link state.
• Link may intermittently come up and go down.
• Local fault messages observed on the interface.
• High counts of FEC corrected and uncorrected errors.
• Different behavior observed when swapping optics.
• LR optics may establish link while SR optics appear non-functional.
• No configuration issues found on interfaces or aggregated Ethernet bundles.

Example log:

Jul 22 14:44:00 QFX5120 fpc0 Local fault detected on port 74 (et-0/0/46)

Jul 22 14:44:00 QFX5120 fpc0 DCBCM: dcbcm_drv_link_chg: et-0/0/46 is not an SFP-T, not reading link partner info

Jul 22 14:44:00 QFX5120 fpc0 DCBCM: dcbcm_drv_link_chg: et-0/0/46 is not an SFP-T, not reading link partner info

Jul 22 14:44:00 QFX5120 fpc0 Setting Port Status LED for pic_slot 0, port 46, ifd_channel 0, led_state GREEN

• High numbers of FEC errors observed on both devices:

From the QFX5120, interface et-1/0/46:

 Ethernet FEC Mode :         FEC74

  FEC Codeword size            0

  FEC Codeword rate          0.000

 Ethernet FEC statistics       Errors

  FEC Corrected Errors       1194721

  FEC Uncorrected Errors       764777

  FEC Corrected Errors Rate        0

  FEC Uncorrected Errors Rate       0

From the EX4400, interface et-1/2/3:

 Ethernet FEC Mode :         FEC74

  FEC Codeword size            0

  FEC Codeword rate          0.000

 Ethernet FEC statistics       Errors

  FEC Corrected Errors        245937

  FEC Uncorrected Errors        584

  FEC Corrected Errors Rate        0

• Replacing fibers, ports, trunk configuration, and optics does not consistently resolve the issue.
• Link behavior changes depending on the optic type used.

Verify that both ends of the fiber link use the same optical technology and matching fiber type:

SR optic ↔ SR optic using multimode fiber.

LR optic ↔ LR optic using single-mode fiber.

Do not mix SR and LR transceivers on opposite ends of the same link.

After replacing the mismatched optic and ensuring both sides used Single-Mode (LR) optics, the FEC errors ceased, the link stabilized, and traffic passed normally.

A mismatch between Single-Mode and Multi-Mode SFP28 optics caused optical compatibility issues that resulted in FEC errors, link instability, and traffic forwarding failures. The problem was not related to software defects, interface configuration, or hardware failure.