Traditional IP based networks, Frame Relay and ATM networks have many disadvantages in the management operation of large networks such as cost, security, scalability and flexibility. To solve this, an MPLS-based VPN networking is introduced that can work with existing deployed backbones and allow organizations to interconnect the dispersed sites and remote workers through secure links by using public internet. In this paper we tried to provide a better understanding of services provider large networks management operation; through focusing on the MPLS VPN technology. To get get familiar with the MPLS VPN networks behavior, we chose the interaction between routing protocols as a case study to evaluate the MPLS VPN networks performance. This work mainly focus on studying the effect of the interior gateway routing protocols in the MPLS VPN networks. We chose two famous protocols for investigation (OSPF and IS-IS) and we proposed two different scenarios and simulated them by using OPNET Modeler; then a deep analyses of the network performance was provided.