Content Overview The two main classes of interior gateway routing protocols (IGP) are distance vector and link-state. Both types of routing protocols are concerned with finding routes through autonomous systems. Distance vector and link-state routing protocols use different methods to accomplish the same tasks. 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

Content 2.1 Link-State Routing Protocol 2.1.1 Overview of link-state routing Link-state routing protocols perform in a very different way from distance vector protocols. Understanding the difference between distance vector and link-state protocols is vital for network administrators. One essential difference is that distance vector protocols use a simpler method of exchanging routing information. Figure outlines the characteristics of both distance vector and link-state routing protocols.Link-state routing algorithms maintain a complex database of topology information. While the distance vector algorithm has nonspecific information about distant networks and no knowledge of distant routers, a link-state routing algorithm maintains full knowledge of distant routers and how they interconnect. Interactive Media Activity Drag and Drop: Link-State Routing Overview When the student has completed this activity, the student will be able to identify the differences between distance vector and link-state routing protocols.

Content 2.1 Link-State Routing Protocol 2.1.2 Link-state routing protocol features Link-state routing protocols collect routing information from all other routers in the network or within a defined area of the network. Once all of the information is collected, each router, independently of the other routers, calculates its best paths to all destinations in the network. Because each router maintains its own view of the network, it is less likely to propagate incorrect information provided by any of its neighboring routers. Link-state routing protocols perform the following functions:

Respond quickly to network changes
Send triggered updates only when a network change has occurred
Send periodic updates known as link-state refreshes
Use a hello mechanism to determine the reachability of neighbors

Each router keeps track of the state or condition of its directly connected neighbors by multicasting hello packets. Each router also keeps track of all the routers in its network or area of the network by using link-state advertisements (LSAs). The hello packets contain information about the networks that are attached to the router. In Figure , P4 knows about its neighbors, P1 and P3, on Perth3 network. The LSAs provide updates on the state of links that are interfaces on other routers in the network. A router running a link-state protocol has the following features:

Uses the hello information and LSAs it receives from other routers to build a database about the network
Uses the shortest path first (SPF) algorithm to calculate the shortest route to each network
Stores this route information in its routing table

Content 2.1 Link-State Routing Protocol 2.1.3 How routing information is maintained Link-state routing uses the following features:

Link-state advertisements (LSAs)
A topological database
The shortest path first (SPF) algorithm
The resulting SPF tree
A routing table of paths and ports to each network to determine the best paths for packets

Link-state routing protocols were designed to overcome the limitations of distance vector routing protocols. For example, distance vector protocols only exchange routing updates with immediate neighbors while link-state routing protocols exchange routing information across a much larger area. When a failure occurs in the network, such as a neighbor becomes unreachable, link-state protocols flood LSAs using a special multicast address throughout an area. Each link-state router takes a copy of the LSA and updates its link-state, or topological database. The link-state router will then forward the LSA to all neighboring devices. LSAs cause every router within the area to recalculate routes. Because LSAs need to be flooded throughout an area, and all routers within that area need to recalculate their routing tables, the number of link-state routers that can be in an area should be limited. A link is the same as an interface on a router. The state of the link is a description