We study the reversal mechanism between opposite closed flux states of ferromagnetic nanorings driven by an azimuthal magnetic field. The reversal proceeds via the formation of domain walls, and we show that the role of interacting nucleation sites is essential for the process to take place. Such nucleation is seen to create domain walls with the right topological charge conditions for domain-wall formation. Given the symmetry of the system, we utilize an energetic description as a function of the azimuthal field magnitude, which clearly reveals the different stages of this reversal process. The annihilation of the domain walls that is necessary for the reversal process to complete is controlling the field value at the final stage of the process. Such a fundamental mechanism for ring reversal has several implications and will guide the design of the various data-storage-device proposals based on nanorings.