Fat-tree
A multi-tier leaf/spine-derived network topology with increasing aggregate bandwidth toward the root. Oversubscription, path diversity, routing, link speeds, and failure state determine whether a particular deployment is nonblocking; the topology name alone does not.
Current numbers
144 × 800GQuantum-X800 radix (72 OSFP): two-tier fat-tree to 10,368 NICs; three tiers reach 746,496 on paper (k³/4)
38-77%paper-modeled rail-only network-cost reduction vs full-bisection fat-tree