MPLS-Enabled Applications: Emerging Developments and New Technologies

11.3. COMPARISON OF LAYER 2 VPNs AND LAYER 3 VPNs

The introduction to the Layer 3 VPN chapter (Chapter 7) discussed the two main models that exist for VPN connectivity: the overlay model and the peer model. BGP/MPLS-based Layer 3 VPNs fall within the peer model. In contrast, when an enterprise builds a Layer 2 VPN, by buying Layer 2 transport services from the service provider they are building an overlay network. Hence the differences between Layer 2 and Layer 3 VPNs are as follows:

1. In the Layer 2 case, no routing interaction occurs between the customer and service provider. In the L3VPN case, the CE and PE router can exchange routes.

2. In the Layer 2 case, the customer can run any type of Layer 3 protocol between sites. The SP network is simply transporting Layer 2 frames and hence is unaware of the Layer 3 protocol that is in use. Although IP is prevalent in many enterprise networks, non-IP protocols such as IPX or SNA are often in use. This would preclude the use of a Layer 3 VPN to transport that type of traffic.

3. Multiple (logical) interfaces between each CE and the corresponding PE are required in the Layer 2 case, one per remote CE that each CE needs to connect to. For example, if the CE routers are fully meshed and there are 10 CE routers in total, each CE needs nine interfaces (e.g. DLCIs, VCs or VLANs, depending on the media type) to the PE, each leading to one of the remote CE routers. In the Layer 3 VPN case, one connection between each CE and the local PE is sufficient ...