ScreenOS Cookbook

Chapter 16. OSPF

16.0. Introduction

Open Shortest Path First (OSPF) is arguably the most common and widespread interior gateway routing protocol running today. OSPF is scalable, relatively simple to configure, and robust. OSPF has a twin brother, which is Integrated Intermediate System to Intermediate System (IS-IS). Both protocols are link-state routing protocols. OSPF was first documented in RFC 1247 by John T. Moy in 1991. The most current RFC for OSPF is RFC 2328.

Routing protocols are typically considered to be one of two types: distance-vector or link-state. In distance-vector routing protocols such as the Routing Information Protocol (RIP), routers make local routing decisions and report the results to their neighbors, which in turn do the same and report their results to their neighbors, and so on. This process has been termed routing by rumor because each router has to trust the accuracy of the information received from its neighbors. In the second type of protocol, link-state protocols, routers report their link information, including interface types and IP addresses, to all other routers in the network. Each router then uses this information about links to independently build a topology view and thus a routing table. In a properly functioning network, all routers running a link-state routing protocol will have an identical view of the network. Unlike distance-vector routing protocols, link-state routing protocols do not share local routing decisions with other routers. ...