The ever increasing global Internet traffic is resulting in a serious upgrade of the current optical networks' capacity. The legacy infrastructure can be enhanced not only by increasing the capacity, but also by adopting advance modulation formats, having increased spectral efficiency at higher data rate. In a mixed-line-rate (MLR) optical network, different line rates, on different wavelengths, can coexist on the same fiber. Further, studies have shown that migration to data rates higher than 10Gbps requires implementation of phase modulation schemes. However, the co-existing On-Off Keying (OOK) channels cause critical physical layer impairments (PLIs) to the phase modulated channels, mainly due to cross-phase modulation (XPM), which in turn limits the network's performance. In order to mitigate this effect, a more sophisticated PLI-Routing and Wavelength Assignment (PLI-RWA) scheme needs to be adopted. In this work, we investigate the critical impairment for each data rate and the way it affects the quality of transmission (QoT). We propose a novel PLI-RWA algorithm for MLR optical networks. The proposed algorithm is compared through simulations with the existing shortest path and minimum hop routing schemes.