(Generic Routing Encapsulation) A tunneling protocol developed by Cisco that allows network layer packets to contain packets from a different protocol. It is widely used to tunnel protocols inside IP packets for virtual private networks (VPNs). For example, the point-to-point tunneling protocol (PPTP) is based on GRE, and GRE is used with IPsec to transmit routing protocol data from one router to another, which IPsec does not natively support. See tunneling protocol, PPTP and IPsec.