Content

| Overview

Open Shortest Path First (OSPF) is a link-state routing protocol based on open standards. Described in several RFCs, most recently RFC 2328, the Open in Open Shortest Path First means that OSPF is open to the public and nonproprietary. Among nonproprietary routing protocols, such as RIP v1 and RIP v2, OSPF is preferred because of its remarkable scalability. Recall that both versions of RIP are very limited. RIP cannot scale beyond 15 hops. It converges slowly, and it chooses suboptimal routes that ignore critical factors such as bandwidth. OSPF addresses all of these limitations and proves to be a powerful, scalable routing protocol.

The considerable capability that OSPF has to scale is achieved through hierarchical design and the use of areas. By defining areas in a properly designed network, an administrator can reduce routing overhead and improve performance. To achieve scalability OSPF relies on complex communications and relationships to maintain a comprehensive link-state database. However, as an OSPF network scales to 100, 500, or even 1000 routers, link-state databases can balloon to include thousands of links. To help OSPF routers route more efficiently and to preserve their CPU and memory resources for the business of switching packets, network engineers divide OSPF networks into multiple areas.

This module describes how to create and configure OSPF. Specifically, this module examines the different OSPF area types, which include stubby, totally stubby, and not-so-stubby areas (NSSAs). Each of these different area types uses a special advertisement to exchange routing information with the rest of the OSPF network. Therefore, link-state advertisements (LSAs) will be studied in detail. The Area 0 backbone rule and how virtual links can work around backbone connectivity problems will also be reviewed. Finally, this module surveys important show commands that can be used to verify multiarea OSPF operation.