Routing alternatives for virtual private networks

Virtual private networks (VPNs) are a popular method for connecting enterprises over a public network infrastructure. Using an IP-based backbone has the potential to significantly reduce the capital expenditure of the service providers and allow them to offer a variety of legacy and new services using a common infrastructure. Recently, a variety of methods have been proposed that enable implementation of VPN services over IP-based backbone networks. Scalability is one of the basic requirements in all the IP-based VPN architectures. In this paper, we focus on cases where the provider must support explicit routing of customer´s traffic, and thus have to implement some form of constrained path computation in its backbone network. Routing protocols that support constrained path computation have been known to cause high volumes of routing protocol control traffic. This overhead can seriously affect the operation of the backbone routers and limit the scalability of the VPN architecture. We propose a mechanism that significantly reduces the overhead from routing protocol messages in the core of the network by exchanging resource availability information only between the edge routers. We show that with relatively straightforward changes to the protocols used in the backbone, it is possible to achieve considerable savings in the amount of the routing protocol control overhead without compromising routing performance.