8.2 Overlay Model

At the present moment the most common techniques for providing VPN services are based on the overlay model. With the overlay model, each site has a router that is connected via point-to-point links to routers in other sites. A site may have one or more such routers, which may be connected to all other sites or to only a subset of all other sites. The technology used to offer point-to-point links could be leased lines, Frame Relay circuits, or ATM circuits. We call the network that consists of these point-to-point links and the routers attached to these links a virtual backbone. It is this virtual backbone that provides connectivity among sites.

To illustrate the overlay model, consider an example shown in Figure 8.1. In this example we have two VPNs, A and B. The cloud in the middle represents a VPN service provider. VPN A consists of three sites, Site 1, Site 2, and Site 3. Router R _A1 at Site 1 is connected via either Frame Relay or ATM circuits to router R _A2 at Site 2 and router R _A3 at Site 3. Likewise, router R _A2 at Site 2 is connected to router R _A1 at Site 1 and router R _A3 at Site 3. Thus, in VPN A we have a complete mesh of intersite connectivity. The Frame Relay or ATM virtual circuits are provided by the VPN service provider. VPN B also consists of three sites,...