[Beowulf] torus versus (fat) tree topologies

[Chris Sideroff]

I looking for some general comments about the pros/cons of torus (2D/3D) 
and (fat) tree network topologies, for HPC of course.

My investigation thus far has led me to believe that one reason a torus 
topology might be better is because it eliminates the need for a switch. 
  On the other hand fat tree interconnects seem to dominate the 
larger(est) clusters out there, why?

I'm looking more for comments about implementation and maintenance 
experiences with either topology (or both).  It seems that most of the 
available high-performance network products give _roughly_ the same 
overall performance so I don't want to start a long debate about the 
competing products - just looking for topology differences, not company 
A vs company B.

Thanks for any comments

Chris Sideroff