A while back, I was sitting in a meeting where the presenter described switching from a “traditional, hierarchical data center fabric” to a spine-and-leaf (while drawing CLOS, in all capital letters, on the whiteboard). He pointed out that the spine-and-leaf design is simpler because it only has two tiers rather than three.

There is so much wrong with this I almost winced in physical pain. Traditional hierarchical designs are not fabrics. Spine-and-leaf fabrics are not CLOS, but Clos, fabrics. Clos fabrics have three stages, not two—even if we draw them “folded” so you only see two apparent levels to the fabric. In fact, all spine-and-leaf fabrics always have an odd number of stages, and they are stages, not tiers.