An engineer needs to validate 400G DAC cable signal integrity in a DGX cluster. Which CVT metric best identifies marginal cables needing replacement?

The most direct and accurate metric for identifying physical signal integrity issues in a DAC cable is the Symbol Bit Error Rate (BER). The NVIDIA Cluster Validation Tool (CVT) monitors the physical layer (PHY) error counters on network adapters and switches. A non-zero or increasing symbol BER indicates that data bits are being corrupted during transmission over the physical medium. This is a definitive sign of a hardware problem, such as a marginal cable, a poor physical connection, or a faulty port, making it the best metric for proactively identifying cables that require replacement before they cause significant performance degradation or link failure.

Bandwidth: Low bandwidth is a symptom of a high BER (which causes retransmissions), not the root-cause metric for physical signal integrity.

Latency: Latency is primarily influenced by distance, congestion, and device processing times, not the signal quality of a short DAC cable.

Packet Drops: Packet drops occur at a higher network layer, often as a result of CRC failures caused by a high BER at the physical layer.

1. NVIDIA DGX SuperPOD and DGX POD Validation Checklist, Section "Physical Network Validation". This document specifies checking for InfiniBand/Ethernet symbol errors and CRC errors as a critical validation step.

2. NVIDIA Networking Documentation for Cumulus Linux, "Monitoring Interface Health". The documentation details using tools like ethtool to check for PHY-level errors, which CVT automates.

3. J. Kurose, K. Ross, "Computer Networking: A Top-Down Approach", Chapter on The Link Layer. University courseware and standard textbooks establish BER as a fundamental metric for physical layer link quality.