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.
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