Fat tree: definition · Chip Field Guide

Beginner

A network shaped like a tree whose branches get thicker toward the top, so the top doesn’t become a traffic jam.

Novice

A tree-shaped network, proposed by Charles Leiserson in 1985, in which the links get more numerous (fatter) closer to the root. In datacenters it is built from many identical switches in layers so every layer has as much total bandwidth as the one below.

Expert

Leiserson’s universal network; in practice the $k$ -ary fat-tree of Al-Fares et al.: $k$ pods of $k/2$ edge and $k/2$ aggregation switches, $(k/2)^2$ cores, $k^3/4$ hosts, all from identical $k$ -port switches. A folded Clos by another name.