Generic Routing Encapsulation

Generic Routing Encapsulation (GRE) is a developed by Cisco Systems that can encapsulate a wide variety of network layer inside virtual point-to-point links over an network.

Based on the principles of protocol layering in OSI, protocol encapsulation, not specifically GRE, breaks the layering order. It may be viewed as a separator between two different protocol stacks, one acting as a carrier for another.

GRE packets that are encapsulated within use IP protocol .

A standard GRE packet header structure, as defined by RFC 2784 and RFC 2890, is represented in the diagram below.

The (PPTP), defined in RFC 2637, uses a variant GRE packet header structure, represented below. PPTP creates a GRE tunnel through which the PPTP GRE packets are sent.

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