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Developed by Cisco Systems, Generic Routing Encapsulation (GRE) is a tunneling protocol that makes it possible to encapsulate, over an Internet Protocol network, a large variety of network layer protocols inside virtual point-to-point links. GRE is defined by RFC 2784 and as a tunneling protocol, carries OSI layer 3 protocols in the network. GRE creates a private point-to-point connection, just like that of a virtual private network. Therefore, it finds widespread usage in the creation of VPNs (with PPTP and IPsec). Unlike IP-to-IP tunneling, GRE can transport IPv6 and multicast traffic between networks.
Generic Routing Encapsulation encapsulates a payload, which is an inner packet, that is to be delivered to a destination, which is an outer IP packet. Endpoints that support GRE can then send such routed encapsulated packages through IP networks. In the process, the payload naturally comes across several routers, which do not parse the payload but rather only the outer IP packet. Thus, in this manner the payload is forwarded to the endpoint, which is the destination. When the payload reaches the GRE tunneling endpoint, the encapsulation is removed (de-encapsulation) and the inner packet is available.
GRE offers a connection that is both stateless and private. However, it is not considered a secure protocol because it lacks encryption. An alternative in this regard would be a protocol like IPsec Encapsulation Security Payload.