Campus lan and Wireless lan solution Design Guide

 
Example oversubscription ratio – access-layer switch stack 
Figure 5. 
For example, let’s say you deploy four 48-port switches each of which supports mGig up to 10 Gbps on 12 
access ports, 1 Gbps on the other 36 ports, with fixed 4x10 Gbps uplink ports. However, you decide to use 
only four 10 Gbps uplinks spread across two switches in the stack. This configuration would provide up to 40 
Gbps uplink bandwidth in a MEC configuration, when both switches are operational. However, you now have 
192 access ports.
The maximum potential bandwidth usage of the switch ports would be 48 x 10 Gbps = 480 Gbps plus 144 x 1 
Gbps = 144 Gbps, for a total 624 Gbps. The maximum uplink oversubscription ratio would be 624 : 40 or 
15.6:1 assuming all mGig ports were operating at 10 Gbps and all the 1 Gbps ports were being used.
More realistically, you may have the following actually connected to the switch: 
● 
32 Catalyst 9100 Series APs operating at 5 Gbps connected across the switch stack
● 
128 Cisco IP Phones and/or end-user devices (PCs, Mac’s etc.) operating at 1 Gbps connected across 
the switch stack 
● 
32 unused ports across the switch stack for future capacity 
The actual potential bandwidth usage of the switch ports would be 32 x 5 Gbps = 160 Gbps plus 128 x 1 Gbps 
= 128 Gbps, for a total 288 Gbps. Therefore a more realistic view of the oversubscription ratio is 288 Gbps : 40 
Gbps, or 7.2:1.
Again, this configuration provides for additional capacity, in that you have an additional 12 x 10 Gbps unused 
uplinks across the switch stack, as additional devices require mGig port speeds, as devices transition to 10 
Gbps speeds, and as you add additional switches in the switch stack. 
If one of the switches with uplinks were to fail, the number of uplinks decreases to 2 x 10 Gbps. However, the 
number of ports also decreases by 48 switch ports. In this scenario, it is important to balance the devices 
(operating at mGig speeds and at 1 Gbps speeds) across all switches to maintain a similar oversubscription 
ratio. For example, APs operating at 5 Gbps should be spread equally across the switches in the stack. This is 
already a best practice for AP high availability in a wireless deployment. Assuming all devices were equally 
distributed across the four switches in the example above, if one of the switches with uplinks were to fail, the 
actual oversubscription ratio would increase from approximately 7:2:1 to 10.8:1.
Another best practice is to select switches without uplinks as the active and standby of the switch stack, as 
shown in the figure above. Uplinks should be provisioned on the member switches. This way, if the active 

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switch of the stack fails, you don’t have a double failure – meaning that you lose both the active switch and half 
of your uplinks.