OSPF (Open Shortest Path First) Protocol

OSPF (Open Shortest Path First) Protocol is an open standard routing protocol. It allows routers to exchange routing information. This tutorial provides a brief introduction to OSPF (Open Shortest Path First) and explains the differences between different OSPF versions.

What is OSPF?

Routers connect different IP subnets in the network. To connect subnets, they learn all network paths, select the best route for every subnet from all available paths, and save the chosen path in the routing table.

A routing protocol automates this process. There are many routing protocols. Based on how they calculate the best path, you can categorize them into three types: distance vector, link state, and hybrid.

The metric used by routing protocols to calculate the best and shortest path is known as the metric. Distance vector routing protocols use the number of routers in the path as the metric. RIPv1 and RIPv2 are distance vector routing protocols. A hybrid routing protocol uses bandwidth and delay as the metric. EIGRP is a hybrid routing protocol. Link State routing protocols use path cost as the metric. OSPF is a link-state routing protocol.

OSPF Overview

OSPF stands for Open Shortest Path First. RFC 2328 describes it. It is an open standard routing protocol. It dynamically learns all network paths, selects the best route for every subnet, and adds that to the routing table. It uses path cost to choose the shortest path from all available paths.

OSPF Standards

Open standards allow you to run OSPF on all routers. If you use OSPF as the routing protocol, you can use routers from different vendors in the same network. For example, you can use routers from Cisco and Juniper in the same network.

OSPF Versions

OSPF has three versions: OSPFv1, OSPFv2, and OSPFv3. OSPFv1 was developed in the mid-1980s to overcome limitations, deficiencies, and scalability problems of the routing protocols of that time. In 1998, OSPFv1 was updated by OSPFv2 to support modern infrastructure and networks.

OSPFv2 includes all the features and characteristics that modern networks need. It is one of the most popular and widely used routing protocols. It was developed when IPv6 was not in use. Because of this, support for IPv6 was not added to OSPFv2.

To support IPv6, instead of updating OSPFv2, a new version of OSPF was developed. It was OSPFv3. It was released in 2008. The following table compares the features of OSPFv2 and OSPFv3.

Since OSPFv1 has been updated and replaced by OSPFv2, network administrators commonly use the term OSPF to refer to OSPFv2. Because of this, unless the version of OSPF is explicitly mentioned, you can consider all references to OSPF to be OSPFv2.

Conclusion

This tutorial is the first part of the tutorial series on OSPF. It introduced OSPF and explained the differences between different OSPF versions. Learning these differences helps you choose the correct OSPF version for your network and understand the configuration steps explained in the following parts.