Chapter 2 Advanced Routing for FortiOS 5.0 : Open Shortest Path First (OSPF) : OSPF Background and concepts
  
OSPF Background and concepts
OSPF (Open Shortest Path First) is a link-state interior routing protocol, that is widely used in large enterprise organizations. It only routes packets within a single autonomous system (AS). This is different from BGP as BGP can communicate between ASes.
This section includes:
Background
The parts and terminology of OSPF
How OSPF works