Clos network: definition · Chip Field Guide

Beginner

A way to build one huge switch out of many small ones arranged in stages.

Novice

A multi-stage switching network, described by Charles Clos in 1953 for telephone exchanges, in which every switch of one stage connects to every switch of the next. With enough switches in the middle it can connect any free input to any free output.

Expert

Three-stage (or more) network of small crossbars. With $n$ inputs per ingress switch and $m$ middle switches it is strictly non-blocking for $m \ge 2n - 1$ and rearrangeably non-blocking for $m \ge n$ . Data-center fabrics “fold” it so ingress and egress stages are the same leaf switches.