8.3 The Peer Model

In the previous section we described VPN solutions based on the overlay model. In the rest of this chapter we describe a VPN solution that is based on a different model the peer model. The major goal of this solution is to overcome the fundamental limitations associated with the overlay model. Specifically, this model aims at enabling VPN service providers to support very large-scale VPN service offerings (thousands to millions of VPNs per service provider), where most of the customers have little or no IP routing expertise, and to support a diverse population of customers, where some VPNs would consist of just a few sites, while others would have hundreds or even thousands of sites per VPN. At the same time, it aims at keeping the cost of offering the VPN service low.

The solution we describe is built around several key technologies:

• Constrained distribution of routing information

• Multiple forwarding tables

• Use of a new type of addresses, VPN-IP addresses

• MPLS

In the rest of this chapter we look at how these technologies, used together, provide a solution that meets the goals outlined in the previous paragraph.

To illustrate the peer model, consider the example shown in Figure 8.2. The middle cloud is a collection of one or more service providers that offer VPN services. Around the cloud we have sites that form VPNs. In this picture we show two Virtual Private Networks, A and B. A site is connected to a VPN service provider by a...