Network Design - Switch oversubscription ratios

When designing a network using the three-tier model, an important aspect of the design should take into account what is known as the oversubscription ratio.

This is the ratio between the total downlink bandwidth and the total uplink bandwidth for a switch. For example, a distribution switch may have 48 1Gbps connected to access switches (downlinks), and one 10 Gbps port connected to the core layer (uplinks). The total aggregate bandwidth of the downlinks is 48 Gbps while the total aggregate uplink is 10 Gbps. The oversubscription ratio is 48:10 or 4.8:1.

The rule-of-thumb recommendation for data oversubscription is 20:1 for access ports on the access-to-distribution uplink. The recommendation is 4:1 for the distribution-to-core links. When you use these oversubscription ratios, congestion may occur infrequently on the uplinks. QoS is needed for these occasions. If congestion is frequently occurring, the design does not have sufficient uplink bandwidth.

For a spine and leaf architecture the oversubscription is essentially determined by the number of spine switches and leaf switches.