13.1 Overview of VPNs

VPNs were initially introduced through technologies such as Frame Relay and ATM. These technologies make use of virtual circuits for communicating between devices. These virtual connections are private and identified by a DLCI or VPI/VCI. These identifiers are used to keep different users' traffic in different circuits, while passing the traffic on the same physical infrastructure. Thus the term " virtual private network has evolved from the private circuits of the past to the very secure and separate networks seen today."

The key concept that has remained the same with VPNs is that the data of many different customers' traffic can be sent across the same physical medium, but separated by using the virtual connection. As VPNs continue to evolve , new and better ways are being found to implement the technology. Modern-day VPN technology service providers are using VPNs to help reduce costs for themselves and their customers by replacing multiple networks, whether they are Frame Relay leased line or otherwise , while still providing similar service offerings to their customers.

To provide a more thorough understanding of VPN technology and its implementations , it is necessary first to introduce some of the terms used in the world of VPNs. This section identifies the devices used in BGP/MPLS VPNs, and the network displayed in Figure 13-1 gives a general topological view of a VPN-enabled network. Notice the customer edge, or CE, device. This is typically customer- premise equipment that connects to the provider's network and is usually a router or a switch. The provider edge, or PE, device is a router located in the provider network that connects to the CE devices. These routers support VPN and label capabilities. PE routers are connected across the provider network through tunnels. The provider router (P) devices are routers that are used in the core of the service provider network and do not connect to any CE devices. The P routers must support MPLS, but do not have to support VPN functionality. These routers are used as a part of the physical path that makes up the logical tunnel between the PE routers.

Figure 13-1. VPN Routers