Building a departmental heterogeneous virtual private network

Virtual private networks' building by using both own (private) and rented (open) channels for heterogeneous computer complexes' cooperation is typical while creating large departmental systems. The problem exacerbated by the necessity of retaining former investments in equipment and software, which do not support up-to-date network protocols and architectures. This article attempts to systematize the problems that arise while building such networks and analyze the methods of their solution.