Most popular were Frame Relay or ATM technologies, providing VPN service at Layer 2. The provider had a Frame Relay or ATM backbone and supplied Layer 2 connectivity to the customer routers. This was commonly referred to as the overlay model.
The service provider might have actually owned or managed the edge routers that were connected to the customer network. The point is that the routers were physically at the customer premises.
Peer-to-peer VPN networks existed, but they were not popular. The main reason is that they were not easy to deploy and maintain because they needed distribute lists, IP packet filters, or GRE tunnels. As explained in Chapter 1, MPLS VPN is an example of a highly scalable peer-to-peer VPN model.
The CE router does not peer with any of the CE routers from the other sites across the service provider network, as with the overlay model. The name peer-to-peer model is derived from the fact that the CE and PE form a peer at Layer 3.
Virtual routing/forwarding (VRF): is a VPN routing and forwarding instance. It is the name for the combination of the VPN routing table, the VRF Cisco Express Forwarding (CEF) table, and the associated IP routing protocols on the PE router.
A PE router holds the global IP routing table, but also a VRF routing table per VPN connected to the PE. Continue reading “MPLS Fundamentals: 5 – MPLS-VPN”