4.1.c Implement and troubleshoot encapsulation

4.1.c [i] GRE

Tunneling provides a mechanism to transport packets of one protocol within another protocol, the indirection. The protocol that is carried is known as the passenger protocol, and the protocol that is used for carrying the passenger protocol is known as the transport protocol. Generic Routing Encapsulation (GRE) is one of the available tunneling mechanisms which uses IP as the transport protocol and can be used for carrying many different passenger protocols. The tunnels behave as virtual point-to-point links that have two endpoints identified by the tunnel source and tunnel destination addresses at each endpoint.

Configuring a GRE tunnel involves creating a tunnel interface, which is a logical interface. Then you must configure the tunnel endpoints for the tunnel interface.

To configure the tunnel source and destination, issue the

tunnel source {ip-address | interface-type} and tunnel destination {host-name | ip-address} commands under the interface configuration mode for the tunnel.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4443-4452).  . Kindle Edition.

http://www.cisco.com/en/US/tech/tk827/tk369/tech_configuration_examples_list.html