2.2 Single Area OSPF concepts
2.2.1 OSPF overview
Open Shortest Path First (OSPF) is a link-state routing protocol based on open standards. It is described in several standards of the Internet Engineering Task Force (IETF). The most recent description is RFC 2328. The Open in OSPF means that it is open to the public and is non-proprietary.
OSPF is becoming the preferred IGP protocol when compared with RIP v1 and RIP v2 because it is scalable. RIP is limited to 15 hops, it converges slowly, and it sometimes chooses slow routes because it ignores critical factors such as bandwidth in route determination.   OSPF overcomes these limitations and proves to be a robust and scalable routing protocol suitable for the networks of today. OSPF can be used and configured as a single area for small networks.  It can also be used for large networks. OSPF routing scales to large networks if hierarchical network design principles are used. 
Large OSPF networks use a hierarchical design. Multiple areas connect to a distribution area, area 0, also called the backbone. This design approach allows for extensive control of routing updates. Defining areas reduces routing overhead, speeds up convergence, confines network instability to an area and improves performance
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