With the ever-increasing traffic demands, the infrastructure of current 10 Gbit/s optical network needs to be enhanced. The legacy infrastructure can be enhanced not only by increasing the capacity, but also by adapting advance modulation format, having increased spectral efficiency at higher data rate. In all-optical mixed line rate (MLR) network, feasibility of a lightpath is determined by physical layer impairment (PLI) accumulation. Contrary to physical layer impairment-aware routing and wavelength assignment (PLIA-RWA) algorithm applicable for a 10 Gbit/s WDM network, that selects a feasible route-keeping the signal to noise ratio value at the receiver above the threshold limit, a new Routing, Wavelength, Modulation format assignment (RWMFA) algorithm is required for a MLR optical network. This article reviews the major PLIs present in an optical fiber, emphasizing those that result in performance degradation of an MLR system. The article also surveys the advance modulation formats that are spectrally efficient and noise resistant. The article further presents a survey of different RWMFA (PLIA-RWA) algorithms for MLR networks and finally identifies several open problems for future research.