To reduce network infrastructure cost, network operators want to integrate interoperable and open transponders, since these transponders allow application of the latest technologies over legacy networks at a competitive price. This process of using third-party transponders in a given network is commonly called "alien wavelength" support. Yet, moving toward interoperability raises several challenges: incompatible physical parameters, vendor lock-in, and proprietary software. Manual set up of alien wavelengths has been demonstrated, but automating this process is essential to enable alien wavelength operation in the field. This paper sheds light on the prevailing literature on the concept of alien wavelengths, taking into account many challenges that accompany the concept on its various levels. We particularly focus on several approaches proposed in the literature: protocol message translation, controller cooperation, and open line systems, including our own proposals, an OpenROADM-based approach and a RSVP-TE-based approach. We analyze these approaches with respect to several criteria: applicability to legacy equipment, added operational cost, and the offered level of interoperability and openness. According to these characteristics, we note that, even though some non-interoperable approaches might be applicable to legacy equipment and easy to maintain, they remain infeasible with advanced alien scenarios (high level of interoperability). Finally, a combination has to be made between at least two of the given approaches in order to get the optimal solution.