A method (10, 30, 50, 70, 80) of configuring transmission wavelengths in a passive optical network comprising a wavelength selective routing element between first and second locations, the method characterised by: at the 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 (12, 32, 34, 36, 38, 40, 72); and then b. ceasing transmission of the first optical signal for a first time period (14, 74, 76) and then recommencing transmission of the first optical signal at the said wavelength being transmitted when the second optical signal was received (16, 78); and at the second location, remote from the first location: c. waiting until the first optical signal is received at the second location (18, 82, 84); d. iteratively generating and transmitting the second optical signal at different ones of a plurality, N, of wavelengths until the first optical signal is no longer received (20, 52, 54, 56, 58, 60, 82) at the second location; and e. maintaining generation and transmission of the second optical signal at the said wavelength being transmitted when the first optical signal was no longer received. at the 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 (12, 32, 34, 36, 38, 40, 72); and then b. ceasing transmission of the first optical signal for a first time period (14, 74, 76) and then recommencing transmission of the first optical signal at the said wavelength being transmitted when the second optical signal was received (16, 78); and 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 (12, 32, 34, 36, 38, 40, 72); and then b. ceasing transmission of the first optical signal for a first time period (14, 74, 76) and then recommencing transmission of the first optical signal at the said wavelength being transmitted when the second optical signal was received (16, 78); and at the second location, remote from the first location: c. waiting until the first optical signal is received at the second location (18, 82, 84); d. iteratively generating and transmitting the second optical signal at different ones of a plurality, N, of wavelengths until the first optical signal is no longer received (20, 52, 54, 56, 58, 60, 82) at the second location; and e. maintaining generation and transmission of the second optical signal at the said wavelength being transmitted when the first optical signal was no longer received. c. waiting until the first optical signal is received at the second location (18, 82, 84); d. iteratively generating and transmitting the second optical signal at different ones of a plurality, N, of wavelengths until the first optical signal is no longer received (20, 52, 54, 56, 58, 60, 82) at the second location; and e. maintaining generation and transmission of the second optical signal at the said wavelength being transmitted when the first optical signal was no longer received.