1. Introduction

This document is a specification of the Open Shortest Path First (OSPF) TCP/IP internet routing protocol. OSPF is classified as an Interior Gateway Protocol (IGP). This means that it distributes routing information between routers belonging to a single Autonomous System. The OSPF protocol is based on link-state or SPF technology. This is a departure from the Bellman-Ford base used by traditional TCP/IP internet routing protocols.

The OSPF protocol was developed by the OSPF working group of the Internet Engineering Task Force. It has been designed expressly for the TCP/IP internet environment, including explicit support for IP subnetting, TOS-based routing and the tagging of externally-derived routing information. OSPF also provides for the authentication of routing updates, and utilizes IP multicast when sending/receiving the updates. In addition, much work has been done to produce a protocol that responds quickly to topology changes, yet involves small amounts of routing protocol traffic.

The author would like to thank Fred Baker, Jeffrey Burgan, Rob Coltun, Dino Farinacci, Vince Fuller, Phanindra Jujjavarapu, Milo Medin, Kannan Varadhan and the rest of the OSPF working group for the ideas and support they have given to this project.

• 1.1. Protocol overview
• 1.2. Definitions of commonly used terms
• 1.3. Brief history of link-state routing technology
• 1.4. Organization of this document