Software-defined networking (SDN) is an innovative technology that aims to improve the network control, while providing an improved usage of virtual resources deployed across modern networks. Recently, telco operators started to adapt the SDN technology in their transport networks, thus boosting the research activities around the Transport-SDN (T-SDN) paradigm. The integration of SDN in transportation networks, however, poses numerous challenges related to the design of efficient routing approaches, working in large-scale and distributed networks. In this context, this paper investigates the design issues addressed by the current scientific literature and provides a cross-comparison of routing techniques published during recent years. The analysis of key features and drawbacks characterizing the reviewed methodologies reveals the lack of routing strategies that could ensure both energy effectiveness and better bandwidth requirements also known as quality of service in a real-time scenario, which is the primary requirement of the modern large-scale telecommunication networks.