A method implemented in a network apparatus used in a wavelength division multiplexing (WDM) optical network is disclosed. The method includes (a) selecting unconsidered virtual link (VL) (i,j) with a maximum cost, a cost being requested line rate rij on VL(i,j)×shortest distance between nodes i and j, (b) selecting unconsidered route k out of K-shortest routes between nodes i and j of VL(i,j), (c) determining a bit map of unconsidered route k, (d) finding a modulation format that supports requested line rate rij with minimum spectrum ⌈ r ij s z ⌉ , where Sz is spectral efficiency with which the modulation format transmits a channel, (e) finding ⌈ r ij s z ⌉ consecutive spectrum slots at M lowest wavelengths in the bit map of selected route k, and (f) determining fragmentation factor Fkm after provisioning a channel at each wavelength m on selected route k, where 1≦m≦M. Other apparatuses, systems, and methods also are disclosed.