TABLE OF CONTENTS As MPLS developed, it became apparent that MPLS VPNs could provide a flexible VPN solution to service providers and ISPs alike. In order to meet these needs, the Layer-3 MPLS VPN was developed.
MPLS VPN topology and design has developed rapidly. Over the past few years, MPLS VPNs have grown from Layer-3 VPNs into variety of options, including any-to-any protocols, such as Cisco s AToM (Any Transport over MPLS). The proliferation of VPNs at both Layer-2 and Layer-3 has led to much confusion. Figure 7.16 represents a top-layer drawing of the VPN tree showing the relationship of Layer-2 and Layer-3 VPNs. The drawing shows current relationships at the time of printing; however, it should continue to serve as a top-level view of MPLS VPNs.
As you can see from the drawing, there are many VPN solutions. We will work our way through this chart to discover how MPLS developed historically, define its concepts, and identify the benefits and challenges of each of the various MPLS VPN solutions.
Before we talk about the details of MPLS Layer-3 VPNs, we need to introduce a model of VPN called the Peer Model. In the Peer Model, the PE and CE are peers, i.e., they exchange IP routing information in a peer-to-peer relationship. The VPN tunnels are established in the core of the MPLS network.
In Figure 7.15, we saw that the CE communicates to the PE using standard...
