of an interface and the relationship to its neighboring routers. For example, a description of the interface would include the IP address of the interface, the subnet mask, the type of network to which it is connected, the routers connected to that network, and so on. The collection of link-states forms a link-state database, sometimes called a topological database. The link-state database is used to calculate the best paths through the network. Link-state routers find the best paths to destinations. Link-state routers do this by applying the Dijkstra shortest path first (SPF) algorithm against the link-state database to build the shortest path first tree, with the local router as the root. The best paths are then selected from the SPF tree and placed in the routing table.

Content 2.1 Link-State Routing Protocol 2.1.4 Link-state routing algorithms Link-state routing algorithms maintain a complex database of the network topology by exchanging link-state advertisements (LSAs) with other routers in a network. This section describes the link-state routing algorithm.Link-state routing algorithms have the following characteristics:

They are known collectively as shortest path first (SPF) protocols.
They maintain a complex database of the network topology.
They are based on the Dijkstra algorithm.

Unlike distance vector protocols, link-state protocols develop and maintain full knowledge of the network routers as well as how they interconnect. This is achieved through the exchange of link-state advertisements (LSAs) with other routers in a network. Each router that exchanges LSAs constructs a topological database using all received LSAs. An SPF algorithm is then used to compute reachability to networked destinations. This information is used to update the routing table. This process can discover changes in the network topology caused by component failure or network growth. LSA exchange is triggered by an event in the network instead of periodic updates. This can greatly speed up the convergence process because there is no need to wait for a series of timers to expire before the networked routers can begin to converge. If the network shown in Figure uses a link-state routing protocol, there would be no concern about connectivity between routers A and B. Depending on the actual protocol employed and the metrics selected, it is highly likely that the routing protocol could discriminate between the two paths to the same destination and try to use the best one.

Shown in Figure are the routing entries in the table for Router A, to Router D. In this example, a link-state protocol would remember both routes. Some link-state protocols provide a way to assess the performance capabilities of the two routes and choose the best one. If the route through Router C was the more preferred path and experienced operational difficulties, such as congestion or component failure, the link-state routing protocol would detect this change and and begin forwarding packets through Router B.

Content 2.1 Link-State Routing Protocol 2.1.5 Advantages and disadvantages of link-state routing The following list contains many of the advantages that link-state routing protocols have over the traditional distance vector algorithms, such as Routing Information Protocol (RIP v1) or Interior Gateway Routing Protocol (IGRP):

Link-state protocols use cost metrics to choose paths through the network. The cost metric reflects the capacity of the links on those paths.
Link-state protocols use triggered, flooded updates and can immediately report changes in the network topology to all routers in the network. This immediate reporting generally leads to fast convergence times.
Each router has a complete and synchronized picture of the network. Therefore, it is very difficult for routing loops to occur.
Routers always use the latest set of information on which to base their routing decisions because LSAs are sequenced and aged.
The link-state database sizes can be minimized with careful network design. This leads to smaller Dijkstra calculations and faster convergence.
Every router is capable of mapping a copy of the entire network architecture, at least of its own area of the network. This attribute can greatly assist troubleshooting.
Classless interdomain routing (CIDR) and variable-length subnet masking (VLSM) are supported.

The following are some disadvantages of link-state routing protocols:

They require more memory and processing power than distance vector routers, which can make link-state routing cost-prohibitive for organizations with small budgets and legacy hardware.
They require strict hierarchical network design, so that a network can be broken into smaller areas to reduce the size of the topology tables.
They require an administrator with a good understanding of link-state routing.
They flood the network with LSAs during the initial discovery process, which can significantly decrease the capability of the network to transport data. This flooding process can noticeably degrade the network performance depending on the available bandwidth and the number of routers exchanging information.

Content 2.1 Link-State Routing Protocol 2.1.6 Compare and contrast distance vector and link-state routing All distance vector protocols learn routes and then send these routes to directly connected neighbors. However, link-state routers advertise the states of their links to all other routers in the area so that each router can build a complete link-state database. These advertisements are called link-state advertisements (LSAs). Unlike distance vector routers, link-state routers can form special relationships with their neighbors and other link-state routers. This is to ensure that the LSA information is properly and efficiently exchanged.The initial flood of LSAs provides routers with the information that they need to build a link-state database. Routing updates occur only when the network changes. If there is no changes, the routing updates occur after a specific interval. If the network changes, a partial update is sent immediately. The partial update only contains contains information about links that have changed, not a complete routing table. An administrator concerned about WAN link utilization will find these partial and infrequent updates an efficient alternative to distance vector routing, which sends out a complete routing table every 30 seconds. When a change occurs, link-state routers are all notified simultaneously by the partial update. Distance vector routers wait for neighbors to note the change, implement the change, and then pass it to the neighboring routers. The benefits of link-state routing over distance vector protocols include faster convergence and improved bandwidth utilization. Link-state protocols support classless interdomain routing (CIDR) and variable-length subnet mask (VLSM). This makes them a good choice for complex, scalable networks. In fact, link-state protocols generally outperform distance vector protocols on any size network. Link-state protocols are not implemented on every network because they require more memory and processing power than distance vector protocols and can overwhelm slower equipment. Another reason they are not more widely implemented is the fact that link-state protocols are quite complex. This would require well-trained administrators to correctly configure and maintain them.

Content 2.2 Single Area OSPF Concepts 2.2.1 OSPF overview Open Shortest Path First (OSPF) is a link-state routing protocol based on open standards. It is described in several standards of the Internet Engineering Task Force (IETF). The most recent description is RFC 2328. The Open in OSPF means that it is open to the public and is non-proprietary. OSPF is becoming the preferred IGP protocol when compared with RIP v1 and RIP v2 because it is scalable. RIP is limited to 15 hops, it converges slowly, and it