Spine-leaf (folded Clos)
Two layers of switches. Servers plug into a "leaf"; every leaf connects to every "spine." Any server can reach any other server by going up to a spine and back down — there is no single box in the middle that everything has to squeeze through.
Need more room between servers? Add a spine, and every leaf gets another path up at once — you lift the whole roof instead of widening one doorway. Need more servers? Add a leaf. Growth stays smooth and predictable, which is exactly why modern datacenters are built this way.
Detailed explanation
Folded Clos: N leaves x M spines with full bisection — every leaf carries at least one uplink to every spine. Host-to-host (east-west) traffic is ECMP-load-balanced across all spines, so there is no hairpin through a congested core.
Scale bandwidth by adding spines (one extra uplink per leaf per spine). Scale host count by adding leaves (linear). Oversubscription is set by the leaf uplink:downlink ratio and is uniform across the fabric, unlike 3-tier where it compounds per layer.