Symptom: Ethernet link up at 100G. High CRC error rate. TCP retransmits. Throughput 40% of line rate.
Measure MAC FCS/CRC errors over a defined interval and correlate them with PHY/PCS counters and captured traffic.
PHY status: per-lane BER, eye margin, temperature.
For a lane-specific error pattern, compare channel margin, lane mapping, clocks and receiver configuration before deciding the cause.
Try lower speed (50G/25G) — does CRC improve?
Treat narrow margin as a hypothesis to test with controlled settings and traffic; clean counters may reflect the observation point or test duration.
Assigning FCS errors to the PHY without correlating PCS, interface, configuration and physical evidence.
Link up doesn't mean clean link — check error counters.
Ignoring per-lane margin in multi-lane Ethernet.