NF-A4x10 FLX, but I would look at the Delta fan AFB0412LB-F00.
NF-A4x10 FLX - 4500RPM, 17.9 dBA, 4.83 CFM, 1.78 mmH2O, 0.6W, 40 x 40 x 10
AFB0412LB-F00 - 5000RPM, 18.5 dBA, 6.50 CFM, 3.67 mmH2O, 0.6W, 40 x 40 x 15
If you have a switching loop and STP is disabled, it will take down the network with broadcast storms. STP eliminates the switching loop by disabling multiple connections to switches. STP allows you to have multiple links between switches for redundancy without the risk of creating switching loops. Just make sure you do not have a switching loop and you can run with STP disabled.