[dgaudet]

more clos advantages:

- simplified operations: can service/remove/add ToRs without affecting global routing or the forwarding capacity of the fabric.

- tor uplink flexibility: if a rack has higher bandwidth needs then double up the links (4 vs 8 in your example)

- tor location mobility: it's a lot easier to manage 4 fiber runs per tor than it is to manage 43 different fiber runs... backhaul to 4 spine blocks vs. a complex web of interconnect spreading all over the datacenter floor. with the fly network it's unlikely you can move a rack once it has been placed, at least not until you're ready to tear down the whole fabric and build something new. so you better get your rack density and layout just right. with tor you're stuck with your spine locations, but everything else can be moved around.

the fly advantages for homogeneous supercomputers built and decom'd N years later are clear... but for datacenters which grow and evolve with heterogeneous devices, fly doesn't seem to really hold up well compared to clos.