JUNOS Cookbook

Chapter 12. OSPF

Introduction

The Open Shortest Path First ( OSPF) protocol is an IGP that routes packets within a single AS, or domain. The IETF began work on OSPF in the late 1980s to develop a replacement for RIP, which was the only routing protocol at the time, because people felt that a stronger routing protocol was needed and the link-state algorithm looked promising. OSPF was implemented by router vendors in the early 1990s and was eventually standardized by the IETF in 1997 as OSPF Version 1. The current standard is Version 2, defined in RFC 2328. Much of the OSPF design was lifted from IS-IS, which is an ISO routing-protocol standard developed at the same time. OSPF was designed specifically for TCP/IP and explicitly supports IP subnetting and the tagging of externally derived routing information. OSPF also provides for the authentication of routing updates. RFC 2740 defines OSPF for IPv6.

As an IGP, OSPF works within a domain, which usually corresponds to an administrative boundary and focuses on determining the most efficient routes to destinations within a domain. EGPs, on the other hand, primarily focus on policy rather than on the most efficient routing.

OSPF is a link-state protocol and uses link-state advertisements (LSAs) to describe the network topology. Each OSPF router generates LSAs that describe the topology it sees and floods the LSAs throughout the domain. As a result, each router ends up with a link-state database that describes the same network topology. ...