The strongest indicator of a cable signal problem is repeated CRC errors combined with intermittent port flapping. CRC errors indicate corrupted frames at the link layer, often caused by poor signal integrity, damaged cables, bad transceivers, dirty optical connectors, excessive bend radius, marginal DAC quality, or unsupported media. Intermittent port flapping means the link repeatedly transitions down and back up, which strongly suggests a physical-layer instability rather than a higher-level application issue. Seeing the expected link speed in ifconfig is not enough to prove signal quality; a link can negotiate at the correct speed while still accumulating errors under load. Successful pings under 2 ms also do not validate high-speed fabric quality because ICMP traffic is lightweight and does not stress the link like RDMA, NCCL, or storage traffic. In NVIDIA AI clusters, physical-layer faults can severely affect distributed training because retransmissions, packet drops, and link resets create stragglers in collective communication. Cable validation should include switch counters, BER or FEC data where available, port stability, transceiver telemetry, and sustained workload or fabric testing.