Pre-MPLS Protocols
Before MPLS, the most popular WAN protocols were ATM and Frame Relay. Cost-effective
WAN networks were built to carry various protocols. With the popularity of the Internet, IP
became the most popular protocol. IP was everywhere. VPNs were created over these WAN
protocols. Customers leased ATM links and Frame Relay links or used leased lines and built
their own private network over it. Because the routers of the provider supplied a Layer 2 service
toward the Layer 3 customer routers, the separation and isolation between different customer
networks were guaranteed. These kinds of networks are referred to as overlay networks.