This paper proposes a dynamic routing optimal model in IP/MPLS over WDM networks. A corresponding on-line integrated routing algorithm minimizing the cost of total lightpaths (MCTLP) is designed accordingly. MCTLP optimizes the network resource utilization by integrally assigning bandwidth allocations on both IP layer and wavelength links usage on WDM layer. The performance comparison of MCTLP with two representative routing algorithms used in IP/MPLS over WDM networks via simulation shows that MCTLP can use less links on IP layer and WDM layer to carry IP traffic flows, resulting in accepting more connection requests and reducing the traffic blocking probability efficiently.