Available Technologies and Options

Layer 2 technologies, such as Frame Relay and ATM, have long been deployed to provide a VPN-like service. The attributes of both technologies are quite similar, as follows:

• A virtual circuit has bi-directionality.

• A virtual circuit is established via signaling.

• A fixed hierarchy exists of a virtual path or virtual circuit.

• The virtual circuit is connection oriented and not tied to an IP control plane.

• A single route exists between the source and destination.

• A full-mesh of VCs is required to have any-to-any connectivity.

A typical topology for Layer 2 implementations has been hub and spoke, in which all VCs terminate at a central locationfor example, at the enterprise headquarters. Hub and spoke topologies are depicted in Figure 3-1.

Figure 3-1. Typical Frame Relay Topologies

The attributes of a Layer 2 technology, such as Frame Relay, include the following:

• Secure, closed user group connectivity exists amongst corporate sites.

• Statistical performance guarantees throughput via permanent virtual circuit (PVC) constructs with a committed information rate (CIR) and excess information rate (EIR).

• Approximately 80% of the traffic over a Frame Relay network is IP.

As an unbundled service, Frame Relay is Layer 2centric where the target market consists of enterprise customers who implement their own corporate virtual private networks (VPNs). The enterprise purchases a PVC from a service provider; consequently the enterprise is responsible for designing the VPN topology and managing the customer edge router (CE) IP routing, quality of service (QoS) policies, and application prioritization. For a service provider, Layer 2 virtual circuits are easy to sell, manage, and bill.

Another type of service using Frame Relay technology (there is a similar service in ATM service) is a bundled Frame Relay managed router service, which has a look and feel similar to that of an IP VPN. The target market is customers who want to outsource a VPN (Layer 2-based) to a service provider. The enterprise customer subscribes to Layer 2-based VPN services and is not involved in the PVC complexity discussions. The service provider must manage the PVC complexity, the corresponding topology, and the CE and address customer routing, application prioritization, and service level agreement management issues.

What are the possible limitations of a Layer 2 technology, such as Frame Relay, as customers request value-added services, such as a service providerhosted IP telephony? The service provider must provision a full mesh of PVCs among all sitesfor example, a VPN with 50 sites would require 1225 PVCs. Due to the requirement to prioritize Voice over IP (VoIP), the service provider must deploy separate voice and data PVCs. With shared service providerhosted PBXs and offnet gateways, the service provider must provision PVCs from each customer site to the service provider data center. As a result, enterprise customers often do not accept a bill for the cost-prohibitive PVC mesh and the service provider consequently bears the cost itself. So, scalable value-added service architecture is needed, and MPLS technology possesses attributes that contribute to a scalable architecture for managed VPNs with value-added service elements.