cabling

Links that never come up and links that come up and flap are different faults with different causes. The reach change between 400G and 800G explains most first-time failures, and the counters separate the rest. The order to check, and the one physical measurement that settles it.

A rail-optimized fabric makes this fault findable because the eight rails are identical by construction, so any asymmetry is a defect. Bisecting by rail, then by switch, then by node, then by port narrows 512 links to one in four steps, and the counters say which of three physical causes it is.

The rule is one sentence and the execution is thousands of connections where a single transposition creates a fault that is invisible until a collective runs. The mapping, the labelling scheme that makes errors findable, and the verification that has to happen before anyone accepts the cluster.

Wire GPU number three of every node to the same switch and the collective that matters most never leaves it. The rank mapping that makes the layout pay, the switch and cable count for a thousand GPUs worked out, and what happens to a job whose ranks are assigned in the wrong order.

Three networks, not one, and the mistake that hurts most is putting two of them on the same wires. The port and switch counts for each, the cable plan, the split of the bill between switches and optics, and the two decisions that are hard to change once the cluster is racked.