Current solutions for network virtualization in the transport network relies on Virtual Local Area Network (VLAN) and Virtual Private Network (VPN) technologies, which facilitate logically isolated connectivity, providing data traffic seperation. However, co-existing VLAN/VPNs on top of a common infrastructure may compete for the same physical resources and introduce performance degradation, especially in case of congestion. This paper introduces Flex-Ethernet (Flex-E) and investigates its use as candidate interface solution for facilitating “hard” slicing in transport networks. This study sheds light on how Flex-E together with newly developed techniques like Segment Routing (SR), can enable a dynamic path allocation with strict performance guarantees. The standardization gaps and open challenges are also elaborated.