ZRP (Zone Routing Protocol) is a hybrid routing protocol that combines proactive and reactive techniques. It distinguishes between two areas: proactive (or intra-zone) corresponding to the N-neighborhood in which it applies a proactive technique and reactive (or inter-zone) corresponding to the rest of the network (excluding the N-neighborhood), in which the reactive approach is applied. This hybrid protocol relates on of the fact that distant nodes communicate with each other less often than the neighboring nodes. Therefore it is more obvious to keep in the local topology only nodes at a reduced number of hops. A global knowledge of the entire network is generally expensive and less useful. We study in this paper the impact of the network size, the traffic load and the zone radius on ZRP performances. The obtained results show that a radius zone of 3 is a preferred and optimal value compared to radius 2 when the traffic load is important.