In this paper we present three wavelength routing algorithms, "end-to-end", concatenated shortest path" and "hierarchical routing" for large-scale optical networks where optical paths traverse a number of networking segments interconnected by gateways. We analyze them with respect to the type of traffic, i.e. global vs. local and consider different requirements on wavelength routing based on gateway adaptation capabilities (wavelength merging, conversion or waveband interchange). Finally, for examples of regional all-optical networks interconnected over an all-optical WDM backbone under a variety of traffic conditions, we present and compare numerical results of blocking performances of different routing algorithms and gateway selection strategies.