While this stage is referred to as requirements in the waterfall model, I prefer to refer to it as assess, because in this phase, you are gathering all the required inputs that will form your infrastructure design.
In this phase, it's really important to identify and interact with the key stakeholders, and to collect all the requirements in order to deliver a design that meets all customer and business needs. Sometimes, the importance of this phase is underestimated, or it is simply not performed at all, perhaps because of time or budget constraints. This can be a huge risk for the project itself.
Also, remember that if you are providing a professional service (as a consultant or as a solution partner), you should prepare for this phase with pre-engagement planning and a kick-off in order to be prepared for the engagement. Furthermore, if the project is handled internally...