Oversubscription (Network)

Oversubscription rates:

1. Access Layer to Distribution Layer => 20:1
2. Distribution Layer to Core Layer => 4:1
3. Data Center links => 1:1

How to count oversubscription rates:

for example you have 48 1Gb/s ports Access Layer switch with one uplink to the Distribution Layer, so your rate is 47*1Gb/s:1*1Gb/s => 47:1 it can be ok, but recommendation is to have 20:1, so you can achieve this by lowering speed of Access Layer ports to 100Mb/s:

47*100Mb/s:1*1Gb/s => 4700Mb/s:1000Mb/s => 4,7:1 this rate is much better.
Also, you can utilize extra port for uplink (LAG=Etherchannel):
46*1Gb/s:2*1Gb/s => 4600:2000 => 23:1
Oversubscription with incorrect rates (given rates are just recommendations and you can calculate your own rates using ports load and dropped packets counters). Using proper oversubscription rates don't supplace QoS it just ensures (theoretically) that devices buffer will not be overloaded and that all packets will be delivered.