Abstract: Open Shortest Path First (OSPF) routing protocol is a dynamic routing protocol which employs link-state routing and Dijkstra’s shortest path algorithm. Using LSA’s (Link State Advertisements) the router’s local routing topology is advertised to all other routers in the same OSPF area. OSPF employs a hierarchical network design using Areas. Area 0 is called the backbone and other areas are connected to the backbone area or area 0 using virtual-Links. Hierarchical network architecture has two major advantages. Firstly, the router does not have detailed information about network topology outside of its area, which Thereby reduces the size of its database and helps in fast convergence. Secondly, there is systematic grouping and communication when the network is subdivided into areas, which is a logical collection of OSPF network, routers and links that have the same area identification. Troubleshooting is made easier. In this paper we explain in detail, the connection of other OSPF areas to the backbone area or area 0 using virtual-Links. We connect area 10 to area 0 via virtual-link. Also, a virtual-Link can be used to connect to the backbone through non-backbone area. The area through which a virtual-link is configured is known as the transit area (which will never be a stub area) and it consists of full routing information (should and must).

Keywords: Open Shortest Path First (OSPF) routing protocol, link-state routing, Dijkstra’s shortest path algorithm, LSA’s, OSPF area, virtual-Link and transit area.