"Open Shortest Path First is a robust link-state interior
gateway protocol (IGP). People use OSPF when they discover that RIP
just isn’t going to work for their larger network, or when
they need very fast convergence. This installment of Networking 101
will provide a conceptual overview of OSPF, and the second part of
our OSPF coverage will delve a bit deeper into the protocol itself,
as well as OSPF area configurations.
"OSPF is the most widely used IGP. When we discuss IGPs,
we’re talking about one routing domain, or Autonomous System
(AS). Imagine a medium-sized company with multiple buildings and
departments, all connected together and sharing two redundant
Internet links. All of the buildings on-site are part of the same
AS. But with OSPF we also have the concept of an Area, which allows
further segmentation, perhaps by department in each building."