What command is needed to measure BER (Bit Error Rate)?

In NVIDIA networking environments, specifically those utilizing InfiniBand or high-speed Ethernet via ConnectX adapters, monitoring the physical link quality is critical for preventing packet loss and RDMA retransmissions. The mlxlink tool is part of the NVIDIA Firmware Tools (MFT) package and is the primary utility for checking the status and health of the physical link. Using the -d flag specifies the device (e.g., /dev/mst/mt4123_pciconf0), while the -c (counters) and -e (error counters/BER) flags provide a detailed readout of the link's performance. Bit Error Rate (BER) is a fundamental metric for signal integrity. NVIDIA systems typically distinguish between "Raw BER" (errors before Forward Error Correction) and "Effective BER" (errors remaining after FEC). A high BER often points to a failing transceiver, a dirty fiber connector, or a marginal DAC cable. While ethtool can show general statistics in Ethernet mode, mlxlink is the verified method for granular BER measurement across InfiniBand and high-speed fabrics, allowing engineers to determine if a link meets the "Error-Free" operation standards required for large-scale AI collective communications like NCCL.