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We consider the problem of planning a mixed line rate (MLR) wavelength division multiplexing (WDM) transport optical network. In such networks, different modulation formats are usually employed to support transmission at different line rates. Previously proposed planning algorithms have used a transmission reach bound for each modulation format/line rate, mainly driven by single line rate systems. However, transmission experiments in MLR networks have shown that physical layer interference phenomena are more severe among transmissions that utilize different modulation formats. Thus, the transmission reach of a connection with a specific modulation format/line rate depends also on the other connections that copropagate with it in the network. To plan an MLR WDM network, we present routing and wavelength assignment algorithms that adapt the transmission reach of each connection according to the use of the modulation formats/line rates in the network. The proposed algorithms are able to plan the network so as to alleviate cross-rate interference effects, enabling the establishment of connections of acceptable quality over paths that would otherwise be prohibited.