Configuring Generic Routing Encapsulation (GRE) tunnels on Cisco IOS

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Overview

Generic routing encapsulation (GRE) is a tunneling protocol which was initially developed by Cisco, and later it has been adopted as an industry standard in RFC 2784. GRE allows the encapsulation of a wide variety of network layer protocols inside virtual point-to-point links. This means that the original packet is encapsulated inside a GRE header and a new IP header containing the source and the destination of the tunnel endpoints. The GRE protocol does not provide any security for the data being transported so if encryption is needed GRE must be used in conjunction with IPsec protocol. Some of the reasons for using GRE are the need to transport multicast traffic, or to provide workarounds for networks with limited hops. In this article we will demonstrate how two networks which do not have reachability can be connected through an GRE tunnel.

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