In this paper, we discuss the "free-riding" traffic problem in routing overlay networks, which is mainly caused by a policy mismatch between the overlay routing and the underlay IP routing. We first define the free-riding problem in a routing overlay network, and construct mathematical models to calculate the amount of the free-riding traffic in routing overlay networks with various path-selection metrics. We also present the numerical examples to estimate the effects of the overlay routing. We conclude that there can be a significant amount of free-riding traffic in an actual Internet environment.