In this paper, we identify the topological and routing technique constraints on the performance of optical backbone WDM networks with wavelength conversion at the nodes and propose techniques to overcome this constraints so that the network performance is enhanced. For any session request, a lightpath has to be established on the shortest path between the node pairs. As each lightpath is a substantial revenue and long-lived, the network operator would like no lightpath request rejection (call blocking) or would like to accomodate more calls in the network before he has to upgrade his network. Hence given a network, we are proposing an analytical method to identify the critical links of the network which will cause early blocking, for the fixed shortest path routing. Once the critical links are identified, we would like to analyse the physical location of these critical links on the network topology and propose a new fixed routing technique namely, restricted routing by which the performance is enhanced as it balances the load uniformly among the links. In this routing technique, not all the calls have the shortest path. We compare the performance of the restricted routing with the optimal performance where all the links have equal blocking probability. The analysis is carried out on some of the standard backbone networks and the results show that by the restricted routing there is a significant performance improvement. ©2008 IEEE.