With the increasing deployment of wavelength division multiplexing (WDM) optical networks, the need for advanced lightpath provisioning algorithms and protocols in a multi-domain setting is becoming evident. In order to increase efficiency by relaxing the wavelength continuity constraint in WDM optical networks, wavelength converters are often placed at certain nodes in the network. In this paper, we study the efficiency of using converters in a multi-domain setting. We have made simple but important modifications to existing optical inter-domain routing protocols in order to utilize the power of wavelength converters and have tested their performance. These modifications can be seamlessly integrated into these protocols (i.e., without changing their algorithmic aspects) to significantly reduce their blocking ratio. We also show that there is a clear performance difference among the considered protocols.