As I have given various people already individually the formulas needed to calculate how much bandwidth is required I figured I would share this as well. If you are doing a stretched VSAN design you will want to read this excellent paper by Jase McCarty. This paper describes the bandwidth requirements between the “data sites” and from the data sites to the “witness site”. It provides the formula needed, and it will show you that the “general guidelines” provided during launch were relatively conservative. In many cases especially the the connection to the witness location can be low bandwidth. Just have a read when you are designing a stretched VSAN and do the math.