Link-state routing algorithms, also known as shortest path first (SPF) algorithms, maintain a complex database of topology information. A link-state routing algorithm maintains full knowledge of distant routers and how they interconnect. In contrast, distance vector algorithms provide nonspecific information about distant networks and no knowledge of distant routers.

Understanding the operation of link-state routing protocols is critical in understanding how to enable, verify, and troubleshoot their operation. This module explains how link-state routing protocols work, outlines their features, describes the algorithm they use, and points out the advantages and disadvantages of link-state routing.

Early routing protocols like RIP were all distance vector protocols. Many of the important protocols in use today are also distance vector protocols, including RIP v2, IGRP, and EIGRP. However, as networks grew in size and complexity, some of the limitations of distance vector routing protocols became apparent. Routers in a network using a distance vector scheme could only guess at the network topology based on the full routing tables received from neighboring routers. Bandwidth usage is high because of periodic exchange of routing updates, and network convergence is slow resulting in poor routing decisions.

Link-state routing protocols differ from distance vector protocols. Link-state protocols flood routing information allowing every router to have a complete view of the network topology. Triggered updates allow efficient use of bandwidth and faster convergence. Changes in the state of a link are sent to all routers in the network as soon as the change occurs.

One of the most important link-state protocols is Open Shortest Path First (OSPF). OSPF is based on open standards, which means it can be developed and improved by multiple vendors. It is a complex protocol that is a challenge to implement in a large network. The basics of OSPF are covered in this module.

OSPF configuration on a Cisco router is similar to the configuration of other routing protocols. As with other routing protocols, the OSPF routing process must be enabled and networks must be identified that will be announced by OSPF. However, OSPF has a number of features and configuration procedures that are unique. These features make OSPF a powerful choice for a routing protocol and make OSPF configuration a very challenging process.

In complex large networks, OSPF can be configured to span many areas and several different area types. The ability to design and implement large OSPF networks begins with the ability to configure OSPF in a single area. This module also discusses the configuration of single area OSPF.

Students completing this module should be able to:

• Identify the key features of link-state routing
• Explain how link-state routing information is maintained
• Discuss the link-state routing algorithm
• Examine the advantages and disadvantages of link-state routing
• Compare and contrast link-state routing with distance vector routing
• Enable OSPF on a router
• Configure a loopback address to set router priority
• Change OSPF route preference by modifying the cost metric
• Configure OSPF authentication
• Change OSPF timers
• Describe the steps to create and propagate a default route
• Use show commands to verify OSPF operation
• Configure the OSPF routing process
• Define key OSPF terms
• Describe the OSPF network types
• Describe the OSPF Hello protocol
• Identify the basics steps in the operation of OSPF