Leaf–spine network
Beginner
Two layers of switches: the lower ones connect to servers, and every lower switch has a cable to every upper switch.
Novice
A two-tier network. Leaf switches (also called top-of-rack switches) connect to servers; spine switches connect only to leaves; every leaf links to every spine. Any server reaches any other in at most three switch hops.
Expert
A two-stage folded Clos. With radix and no oversubscription: leaves, spines, endpoints, and equal-cost paths between any two leaves.