In this illustration, we once again see sixteen server racks, each one topped with a TOR switch. TOR switches 1-4 connect to a tier-2 switch, as do 5-8, 9-12, and 12-16. Each tier-2 switch connects to a tier-1 switch above it, but also connects to each other tier-1 switch so that each tier-2 switch connects to four tier-1 switches.