Open Shortest Path First (OSPF)

Open Shortest Path First (OSPF) is a routing protocol developed for Internet Protocol (IP) networks
by the interior gateway protocol (IGP) working group of the Internet Engineering Task Force
(IETF). The working group was formed in 1988 to design an IGP based on the shortest path first
(SPF) algorithm for use in the Internet. Similar to the Interior Gateway Routing Protocol (IGRP),
OSPF was created because in the mid-1980s, the Routing Information Protocol (RIP) was
increasingly unable to serve large, heterogeneous internetworks. This chapter examines the OSPF
routing environment, underlying routing algorithm and general protocol components.
OSPF was derived from several research efforts, including Bolt, Beranek, Newman’s (BBN’s) SPF
algorithm developed in 1978 for the ARPANET (a landmark packet-switching network developed in
the early 1970s by BBN), Dr. Radia Perlman’s research on fault-tolerant broadcasting of routing
information (1988), BBN’s work on area routing (1986), and an early version of OSI’s Intermediate
System-to-Intermediate System (IS-IS) routing protocol.
OSPF has two primary characteristics. The first is that the protocol is open, which means that its
specification is in the public domain. The OSPF specification is published as Request For Comments
(RFC) 1247. The second principal characteristic is that OSPF is based on the SPF algorithm, which
sometimes is referred to as the Dijkstra algorithm, named for the person credited with its creation.
OSPF is a link-state routing protocol that calls for the sending of link-state advertisements (LSAs)
to all other routers within the same hierarchical area. Information on attached interfaces, metrics
used, and other variables is included in OSPF LSAs. As OSPF routers accumulate link-state
information, they use the SPF algorithm to calculate the shortest path to each node.
As a link-state routing protocol, OSPF contrasts with RIP and IGRP, which are distance-vector
routing protocols. Routers running the distance-vector algorithm send all or a portion of their routing
tables in routing-update messages to their neighbors.