Open Shortest Path First (OSPF) is a link-state routing protocol as per RFC 2328. It belongs to the category Interior Gateway Protocols (IGP). This involves routers regularly exchanging link-status information via link-state advertisements (LSA). The routers use multicast to automatically discover one another on the local network. OSPF is generally used for the exchange of internal routing information in large networks (LANs).
Each router has an identical copy of the database (link state database, LSDB), which routers use to determine the best routes (Shortest Path First, SPF) using the Dijkstra algorithm.
In contrast, BGP is one of the Exterior Gateway Protocols (EGP) and is typically used to route between autonomous systems or within VPNs.
