Internal solitary waves (hereafter ISWs) are stable nonlinear waves propagating in regions of strong density gradients common in geophysical flows. The purpose of the present work is to describe the generation of internal solitary waves at the interface of a two layer fluid, by the periodic oscillation of a topography. This academic configuration is inspired by oceanic observations. Direct numerical simulations, using the numerical model Symphonie-NH, are used to give insights into the physical parameters controlling the generation of these high amplitude interfacial waves in the primary generation case. The dynamics of the propagating ISWs is successfully compared with a simple Korteweg-de Vries scheme, showing that primarily generated ISWs propagate in an unimodal manner, and confirming that their stability relies on the balance between nonlinear and dispersive effects. Finally, the role of the topography in the primary generation process is quantitatively described by varying its shape. We show the existence of a topographic control of the primary generation of ISWs. A nondimensional parameter based on the ratio of the interfacial wavelength and the typical topography width is introduced to describe this spatial selection criterion.