Generic routing encapsulation (GRE) is a tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links or point-to-multipoint links over an Internet Protocol network.
It may be viewed as a separator between two different protocol stacks, one acting as a carrier for another.
[4] For performance reasons, GRE can also be encapsulated in UDP packets.
The extended version of the GRE packet header[6] is represented below: A standard GRE packet header structure[7] is represented in the diagram below.
The newer structure superseded the original structure:[1] The original GRE RFC defined further fields in the packet header which became obsolete in the current standard: The Point-to-Point Tunneling Protocol (PPTP) [8] uses a variant GRE packet header structure, represented below.