China Telecom China Telecom Huawei Technologies Huawei Technologies Enhanced VPN (VPN+) aims to provide enhanced VPN service to support some application's needs of enhanced isolation and stringent performance requirements. VPN+ requires integration between the overlay VPN and the underlay network. A Virtual Transport Network (VTN) is a virtual underlay network which consists of a subset of the network topology and network resources allocated from the physical network. A VTN could be used as the underlay for one or a group of VPN+ services. In some network scenarios, each VTN can be associated with a unique logicial network topology. This document describes a mechanism to build the SR based VTNs using IS-IS Multi-Topology together with other well-defined IS-IS extensions.