generic routing encapsulation (GRE)
Generic Routing Encapsulation (GRE) is a standard for tunneling. GRE allows IP protocols and other Layer 3 protocols to be transmitted over an IP network. In the GRE protocol, a data packet with a payload is encapsulated in another packet and tunneled through IP networks.
The outer encapsulating data packet is routed by the routers through the GRE tunnel to the endpoint. The encapsulated data packet is not analyzed. When the encapsulating data packet reaches the destination station, the inner data packet containing the payload data is decapsulated.
The GRE tunnel packet consists of the tunnel header, the GRE header, and the payload. The tunnel header contains the address information, the GRE header contains information about the encapsulated tunnel protocol and the encryption algorithms, and the payload includes the protocol header of the encapsulated tunnel protocol and the plain payload data.
GRE tunneling can be used to transport multicast and IPv6 data packets between networks. Examples of such tunneling include Ethernet over MPLS( EoMPLS) and Virtual Private LAN Service over GRE (VPLSoGRE). Generic Routing Encapsulation is described in RFC 2784.