What is OSPF and how it works?

Open Shortest Path First (OSPF) is a dynamic routing protocol that handles routing for IP traffic.

Originally designed in the 1980s, OSPF is defined for IPv4 in protocol version 2 by RFC 2328 (1998). The updates for IPv6 are specified as OSPF Version 3 in RFC 5340 (2008).

OSPF is a link-state routing protocol, it runs over IPv4 and IPv6, but does not use a transport protocol. It encapsulates its data directly in IP packets with protocol number 89. This is in contrast to other routing protocols, such as the Routing Information Protocol (RIP) and the Border Gateway Protocol (BGP). OSPF implements its own transport error detection and correction functions.

OSPF establishes and maintains neighbor relationships in order to exchange routing updates with available routers and constructs a topology map of the network. It supports complex networks with multiple routers, including backup routers, to balance traffic load on multiple links to other subnets. Neighboring routers in the same broadcast domain or at each end of a point-to-point link communicate with each other via the OSPF protocol. Routers form adjacencies when they have detected each other. This detection is initiated when a router identifies itself in a hello protocol packet. Upon acknowledgment, this establishes a two-way state and the most basic relationship.