The well-known Clos network has been extensively used for telephone switching, multiprocessor interconnection and data communications. Much work has been done to develop analytical models for understanding the routing blocking probability of the Clos network. However, none of the analytical models for estimating the blocking probability of this type of network have taken into account the very real possibility of the interstage links in the network failing. In this paper, we consider the routing between arbitrary network inputs and outputs in the Clos network in the presence of interstage link faults. In particular, we present an analytical model for the routing blocking probability of the Clos network which incorporates the probability of interstage link failure to allow for a more realistic and useful determination of the approximation of blocking probability. We also conduct extensive simulations to validate the model. Our analytical and simulation results demonstrate that for a relatively small interstage link failure probability, the blocking behavior of the Clos network is similar to that of a fault-free network, and indicate that the Clos network has a good fault-tolerant capability. The new integrated analytical model can guide network designers in the determination of the effects of network failure on the overall connecting capability of the network and allows for the examination of the relationship between network utilization and network failure.