This paper gives a generalized description of fat tree topology, and invented the notation:

This notation means the fat tree has height $h$, and on level $i$ ($0\le i\le h-1$) each node has $w_{i+1}$ parent nodes and $m_i$ children nodes. Level 0 is the bottom-most layer, so assumed $m_0=0$. So for the degree-3 fat-tree that I always use, it can be expressed by $XGFT(3;3,3,6;1,3,3)$.

Bibliographic data

   title = "On Generalized Fat Trees",
   author = "Sabine R. Öhring and Maximilian Ibel and Sajal K. Das and Mohan J. Kumar",
   booktitle = "Proc. 9th International Parallel Processing Symposium",
   Mon = "Apr",
   pages = "37--44",
   address = "Santa Barbara, CA",
   year = "1995",