Fat-tree
A multi-tier leaf/spine network topology providing full, non-blocking bandwidth between any pair of nodes.
Current numbers
144 × 800GQuantum-X800 radix (72 OSFP); 2-tier fat-tree to 10,368 NICs, tens of thousands of GPUs in 3 tiers
8× 400 Gb/sback-end NICs per training node = 3.2 Tb/s/node; 1:1 non-blocking 8-rail fat-tree
38-77%rail-only network-cost cut vs full-bisection fat-tree (spine eliminated; cross-rail rides the scale-up domain via PXN-style forwarding)