Optimal Traffic Flow

Because the ATM or Frame Relay switches are purely Layer 2 devices, the routers interconnect through them by means of virtual circuits created between them. For any router to send traffic directly to any other router at the edge, a virtual circuit must be created between them directly. Creating the virtual circuits manually is tedious. In any case, if the requirement is the any-to-any connection between sites, it is necessary to have a full mesh of virtual circuits between the sites, which is cumbersome and costly. If the sites are only interconnected as in Figure 1-8, the traffic from CE1 to CE3 must first go through CE2.

Figure 1-8 Non-Fully Meshed Overlay ATM Network

Service Providers' ATM Network


Switch Switch

Virtual \ Virtual

Circuit ATM / Circuit



The result is that the traffic crosses the ATM backbone twice and takes a detour through the router CE2. When using MPLS VPN as discussed in the previous section, the traffic flows directly—thus optimally—between all customer sites. For the traffic to flow optimally between the sites in the case of an overlay VPN model, all sites must be interconnected, thus demanding a fully meshed design of links or virtual circuits.

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