This paper aims at developing a finite-element model, which simulates a planar impact experiment on a ductile metallic porous material. When the shockwave passes through a porous medium, the voids face a rapid collapse, and the material particles in the vicinity of the voids are subjected to very high acceleration. Acceleration forces generated at the microscale level may affect the overall response of the porous material. Analyzing the effect of the initial porosity, size, shape, and spatial distribution of voids on the plastic shock velocity and the shock structure was the main objective of this work. It is found that, for a given impact velocity, the plastic shock velocity depends solely on the material parameters of the dense material and the initial porosity. However, the size, shape and spatial distribution of voids, together with the initial porosity, play a significant role on the structure of the plastic shock.