Security of transferred data is a big concern for data networks users. One of the best security solutions is the IPSec Virtual Private Networking (IPSec VPN). In this paper, The Impact of additional traffic generated by IPSec protocol on the network's resources performance is discussed. Network simulation is a major part of this paper. OPNET simulation software is used. Analyzing the impact of applying IPSec VPN to secure a flow of traffic from a remote sites connected through IPSec tunnels to one server is the methodology of the study, using OPNET.