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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)                 9  TCP/IP Interior Routing Protocols (RIP, OSPF, GGP, HELLO, IGRP, EIGRP)                      9  Open Shortest Path First (OSPF)

Open Shortest Path First (OSPF) 1 2 OSPF Basic Topology and the Link State Database (LSDB)

In the early days of TCP/IP, the Routing Information Protocol (RIP) became the standard protocol for routing within an autonomous system (AS), almost by default. RIP had two big things going for it: it was simple and easy to use, and it was included in the popular Berkeley Standard Distribution (BSD) of UNIX starting in 1982. Most organizations using TCP/IP started out with relatively small networks, and were able to use RIP with some degree of success.

However, as we discussed in our look at RIP, that protocol has some serious technical issues, and these are exacerbated when it is used on a larger AS. Many of its problems are due to it being a distance-vector protocol—the algorithm itself simply limits the ability of RIP to choose the best route and adapt to changing network conditions. Other problems with RIP were based on its implementation, such as the selection of a value of 16 for “infinity” that made it impossible to use RIP in a situation where more than 15 hops might occur between devices. Problems such as the lack of classless addressing support were addressed in Version 2 of RIP, but the basic difficulties with the protocol as a whole persist.

The Internet Engineering Task Force (IETF) recognized that RIP by itself simply would not meet the needs of all autonomous systems on the Internet. They formed a working group in 1988 to develop a new routing protocol based on the more capable link-state algorithm, also called shortest path first (SPF). Research into this type of protocol had already begun as early as the 1970s, with some of it conducted on the ARPAnet, the predecessor of the Internet upon which much of TCP/IP was developed.

This new protocol was called Open Shortest Path First, or OSPF, and its name conveys two of its most important characteristics. The first word refers to the fact that the protocol, like all TCP/IP standards, was developed using the open and public RFC process, so it is not proprietary and no license is required to use it. The SPF portion of the name refers to the type of algorithm it uses, which is designed to allow routers to dynamically determine the shortest path between any two networks.

The first version of OSPF was described in RFC 1131, published in October 1989. This was quickly replaced by OSPF Version 2 in July 1991, described in RFC 1247. Since then there have been several revisions to the OSPF Version 2 standard, in RFCs 1583, 2178, and 2328, with the last of these the current standard. OSPF Version 2 is the only version in use today, so it is usually what is meant when people (including myself) refer to “OSPF”.

Open Shortest Path First (OSPF) 1 2 OSPF Basic Topology and the Link State Database (LSDB)