GRE protocol functionality adds one additional protocol on Cisco's multimedia core platforms (ASR 5500 or higher) to support mobile users to connect to their enterprise networks through Generic Routing Encapsulation (GRE).
GRE tunnels can be used by the enterprise customers of a carrier 1) To transport AAA packets corresponding to an APN over a GRE tunnel to the corporate AAA servers and, 2) To transport the enterprise subscriber packets over the GRE tunnel to the corporation gateway.
The corporate servers may have private IP addresses and hence the addresses belonging to different enterprises may be overlapping. Each enterprise needs to be in a unique virtual routing domain, known as VRF. To differentiate the tunnels between same set of local and remote ends, GRE Key will be used as a differentiator.
It is a common technique to enable multi-protocol local networks over a single-protocol backbone, to connect non-contiguous networks and allow virtual private networks across WANs. This mechanism encapsulates data packets from one protocol inside a different protocol and transports the data packets unchanged across a foreign network. It is important to note that GRE tunneling does not provide security to the encapsulated protocol, as there is no encryption involved (like IPSEC offers, for example).
GRE Tunneling consists of three main components:
Passenger protocol-protocol being encapsulated. For example: CLNS, IPv4 and IPv6.
Carrier protocol-protocol that does the encapsulating. For example: GRE, IP-in-IP, L2TP, MPLS and IPSec.
Transport protocol-protocol used to carry the encapsulated protocol. The main transport protocol is IP.