Oversubscription (downlink:uplink)
You can wire up more server bandwidth than the switch can actually carry upstream, betting that not everyone talks at once. A 5:1 oversubscribed switch means if every server tried to push full speed simultaneously, only one in five would get it.
Most of the time the bet pays off — real traffic is bursty and uneven. The pain comes when a lot of tenants get busy together (a backup window, a viral spike): the upstream link saturates, latency climbs, and everyone behind that switch feels it at once.
Detailed explanation
Aggregate downlink (host-facing) capacity divided by usable uplink capacity. Non-blocking = 1:1; anything above 1.0 is oversubscribed. The switch ASIC's bisection bandwidth is the hard ceiling regardless of how many ports you light up — see switch fabric capacity.