MPLS-Enabled Applications: Emerging Developments and New Technologies

7.3. THE OVERLAY VPN MODEL

The overlay model is the most intuitive VPN model. If it is connectivity that the customer wants, what can be simpler than connecting the customer sites via point-to-point links between routers at the various sites? The point-to-point links could be Frame Relay or ATM circuits, leased lines or IP-over-IP tunnels such as Generic Route Encapsulation (GRE) or IP Security (IPSec). What is provided is a virtual backbone for the customer's network, overlaid on top of the provider's infrastructure. Designated routers at the different customer sites (the customer edge routers, or CE routers) peer with each other and exchange routing information, thus allowing traffic to flow over the links between the different sites.

In this model, the provider is oblivious of the internal structure and addressing in the customer's network and provides only a transport service. Provisioning the circuits between the customer sites assumes knowledge of the traffic matrix between sites. However, in most cases it is not the traffic matrix that is known but the average traffic sourced and received, thus making it difficult to estimate the bandwidth required. After the circuits are set up, the bandwidth that is not used is wasted, making the solution expensive. One more interesting note on provisioning involves the case where Frame Relay or ATM is used. In this case, increasing the bandwidth between sites may require provisioning of new circuits, which can take a long time to set ...