C H A P T E R46
• Discuss the use of autonomous systems.
• Describe the use of the Sorts Path First algorithm.
• Discuss the additional features of OSPF.
Open Shortest Path First
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 incapable of serving
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, and 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