Virtual Private Networks with Level 2 Protocols have been developed mainly as Remote Access Networks: in other words, allow a remote user to connect through an Internet line (e.g. dial-up connection) on the internal network of a company.
The tunnels can be created either between a pair of router-to-routers or between two (host-to-host) terminal nodes. The establishment of a passageway can be implemented in a point-to-point or point-to-point topology: point-to-point has less administrative in terms of establishment and maintenance.
Tunnelling is the encapsulation technique an entire data packet/frame in a package/frame different protocol. The headline of tunnelling protocol is attached to the original package while the transfer/transmission is carried out using the new protocol.
Thus, when such a packet is routed to the node destination, runs through the network through a logical path, which is referred to as a tunnel. When the destination node receives the package, converts it to its original format. It is noted that the tunnelling technology can be developed in the second or third level of the OSI model.
One of the advantages of tunnelling is that interconnected VPN subnets do not require unique network addresses. That’s what I’m important when the majority of organisations today use private addresses. Also a VPN using tunnelling may be created with or without the knowledge of the network provider; could be “passed” through successive network providers.
The Cisco GRE (Generic Routing Encapsulation) mechanism used for tunnelling between source routers and router-to-router. GRE tunnels provide a special path along a shared WAN infrastructure that is not belongs to only one client user (e.g. Internet) and encapsulates the move with new package headings to ensure the distribution to a specific destination. A GRE tunnel between the source router and the router Destination.
Packages to be forwarded along the encapsulated with a GRE heading, transported in accordance with the length of the passage and at the end of it the GRE heading is removed.