What is GRE (Generic Routing Encapsulation)?

Generic Routing Encapsulation (GRE) makes it possible to encapsulate, over an Internet Protocol network, a large assortment of network layer protocols inside virtual point-to-point connection. This permits the source and objective switches to work as if they have a virtual point-to-point association with one another. GRE tunnels permit VPNs across wide area networks (WANs).