Engineering a large IP backbone network without a view of internal link state is challenging. Previous algorithms assume that probes experience fixed routes in networks. As there are load-balancing equipments in networks, probes are delivered across random routes. This results in invalidation of the prevous algorithms. New algorithm proposed in this paper uses CGF (cumulate generating function) to infer delay characteristics of the internal link under stochastic routes. Simulation results prove that the algorithm could resolve the delay inference in the networks with load-balancing route. Based on the delay characteristics of the internal link, the bottleneck link can be located.