A method of configuring transmission wavelengths in a passive optical network comprising a wavelength selective routing element between first and second locations. The method comprises: at a first location: a. iteratively generating and transmitting a first optical signal at different ones of a plurality of wavelengths until a second optical signal is received at the first location; and then b. ceasing transmission of the first optical signal and then recommencing transmission of the first optical signal at the wavelength being transmitted when the second optical signal was received; and at a second location, remote from the first location: c. waiting until the first optical signal is received at the second location; d. iteratively generating and transmitting the second optical signal at different ones of a plurality of wavelengths until the first optical signal is no longer received at the second location; and e. maintaining generation and transmission of the second optical signal at the wavelength being transmitted when the first optical signal was no longer received.