31 Days Before Your CCNA Exam

Day 18 157Figure 18-3 shows the rather basic flooding process, with R8 sending the original LSA for itself,and the other routers flooding the LSA by forwarding it until every router has a copy.After the LSA has been flooded, even if the LSAs do not change, link-state protocols do requireperiodic reflooding of the LSAs by default every 30 minutes. However, if an LSA changes, therouter immediately floods the changed LSA. For example, if Router R8’s LAN interface failed, R8would need to reflood the R8 LSA, stating that the interface is now down.Calculating the Dijkstra AlgorithmThe flooding process alone does not cause a router to learn what routes to add to the IP routingtable. Link-state protocols must then find and add routes to the IP routing table using the DijkstraShortest Path First (SPF) algorithm.The SPF algorithm is run on the LSDB to create the SPF tree. The LSDB holds all the informationabout all the possible routers and links. Each router must view itself as the starting point, and eachsubnet as the destination, and use the SPF algorithm to build its own SPF tree to pick the bestroute to each subnet.Figure 18-4 shows a graphical view of the results of the SPF algorithm run by router R1 when tryingto find the best route to reach subnet 172.16.3.0/24 (based on Figure 18-3).Figure 18-4 SPF Tree to Find R1’s Route to 172.16.3.0/24Cost 10Cost 20R1Cost 30R2Cost 60R5Cost 30R7Cost 180R3Cost 20R6Cost 40R4Cost 5R8Cost 10Possible RouteSubnet X(172.16.3.0/24)To pick the best route, a router’s SPF algorithm adds the cost associated with each link betweenitself and the destination subnet, over each possible route. Figure 18-4 shows the costs associatedwith each route beside the links, with the dashed lines showing the three routes R1 finds betweenitself and subnet X (172.16.3.0/24).