Compare access bandwidth with uplink bandwidth to get the oversubscription ratio and the uplinks a target needs.
The ratio assumes every access port could transmit at line rate simultaneously, which is the worst case the fabric must survive. A 1:1 result is non-blocking; 3:1 is the common enterprise compromise; storage and AI fabrics are usually built at 1:1. Oversubscription is the single number that predicts incast drops, and it is far cheaper to add uplinks at build time than to re-cable a live rack.
Oversubscription Ratio
oversubscription = (access ports × access speed) ÷ (uplink ports × uplink speed); uplinks for a target = access capacity ÷ (target × uplink speed), rounded up.
oversubscription = (access ports × access speed) ÷ (uplink ports × uplink speed); uplinks for a target = access capacity ÷ (target × uplink speed), rounded up. The ratio assumes every access port could transmit at line rate simultaneously, which is the worst case the fabric must survive. A 1:1 result is non-blocking; 3:1 is the common enterprise compromise; storage and AI fabrics are usually built at 1:1.
Oversubscription is the single number that predicts incast drops, and it is far cheaper to add uplinks at build time than to re-cable a live rack.
This calculator takes 5 inputs: Access ports, Access port speed, Uplink ports, Uplink port speed, Target ratio (x:1). The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.