Any-to-any connectivity is an O(x^2) (quadratic) problem.
When you build a fabric, you can add new pizza-boxes in a linear fashion as long as the existing boxes have spare ports to plug in the new boxes.
As soon as the spare ports run out, the quadratic hits.
Then the choices are either:
- Reduce any-to-any bandwidth to recover spare ports.
- Replace all the boxes by larger ones.
- Add a tier. Adds hops and increases convergence.