Hello Alan

Sorry about the confusion. Let me try to clarify. When looking at a three-tier network design model, we have something like this:

Now, imagine that those computers at the bottom left each have a gigabit connection to that access switch. Now also imagine that there are 48 of those PCs connected to that access switch. Finally, imagine that the uplinks from that access switch are also 1Gbps speeds as shown.

- What is the total downlink aggregate speed to the PCs? 48 Gbps
- What is the total uplink aggregate speed to the distribution layer? 2 Gbps.
- What is the oversubscription ratio on that access switch? 48:2 or 24:1.

Ideally, this should be less than 20:1.

Now in a different scenario, take a look at the following diagram:

If you have a distribution switch such as the one labeled D1 in the diagram above, and you have four downlinks to the access layer, each of 10 Gbps, and two uplinks to the core layer, each at 10 Gbps, what is the oversubscription ratio?

- What is the total downlink aggregate speed to the access layer? 40 Gbps
- What is the total uplink aggregate speed to the core layer? 20 Gbps.
- What is the oversubscription ratio on that access switch? 40:20 or 2:1.

This value is acceptable because the rule of thumb states it should be no larger than 4:1.

The oversubscription ratios are always calculated on a per switch basis, so you have to calculate them based on each individual switch. It’s simply the ratio of the aggregate downlink speed to the aggregate uplink speed.

This will tell you how many access switches you can connect to each distribution switch.

So each access switch has an aggregate downlink of 150 Gbps, and an aggregate uplink of 20 Gbps for an oversubscription ratio of 150:20 or 15:2 which is well below the 20:1 rule of thumb for the access layer.

For each distribution switch, you have 8 downlinks at 10 Gbps, and two uplinks at 10 Gbps. So the aggregate downlink speed is 80 Gbps, the aggregate uplink speed is at 20 Gbps, so your oversubscription ratio is 80:20 or 4:1 which is at the upper limit of the rule of thumb, so you’re OK.

I hope this has been helpful!

Laz