How to force 25G FEC on MCX512A-ACAT

I have a 25G MCX512A-ACAT and windows 10 I use mlxlink to configure.

When using an optical 25GBASE-LR module, the 25G only works with FC fec. I can configure the remote side to RS fec or disable the fec but this doesn’t work, even when I set the fec using the command mlxlink --fec RS --fec_speed 25G. This command appears to have no effect.

When I use a DAC cable, only RS fec seems to work. also here using the command “mlxlink --fec FC --fec_speed 25G” seems to have no effect at all.

The remote side is a FPGA device and completely agnostic to the type of module with no auto-negotiation and connections from this device to ethernet switches work without problems.

It appears the use of FEC on the Mellanox card is restricted to the type of module and the FEC settings are ignored. How can I force the FEC to a specific setting ?

Attached are some logs of the link status with the DAC cable and optical module for both RS and FC fec.


mlxlink_logs.txt (7.07 KB)