31 Days Before Your CCNA Exam

Day 12 231The following criteria is used to elect the DR and BDR:1. DR: Router with the highest OSPF interface priority.2. BDR: Router with the second highest OSPF interface priority.3. If OSPF interface priorities are equal, the highest router ID is used to break the tie.When the DR is elected, it remains the DR until one of the following conditions occurs:■■■The DR fails.The OSPF process on the DR fails.The multiaccess interface on the DR fails.If the DR fails, the BDR assumes the role of DR, and an election is held to choose a new BDR. Ifa new router enters the network after the DR and BDR have been elected, it will not become theDR or the BDR even if it has a higher OSPF interface priority or router ID than the current DR orBDR. The new router can be elected the BDR if the current DR or BDR fails. If the current DRfails, the BDR will become the DR, and the new router can be elected the new BDR.Without additional configuration, you can control the routers that win the DR and BDR electionsby doing either of the following:■■Boot the DR first, followed by the BDR, and then boot all other routers.Shut down the interface on all routers, followed by a no shutdown on the DR, then the BDR,and then all other routers.However, the recommended way to control DR/BDR elections is to change the interface priority,which we review in the “OSPF Configuration” section.OSPF AlgorithmEach OSPF router maintains a link-state database containing the LSAs received from all otherrouters. When a router has received all the LSAs and built its local link-state database, OSPF usesDijkstra’s shortest path first (SPF) algorithm to create an SPF tree. This algorithm accumulatescosts along each path, from source to destination. The SPF tree is then used to populate the IProuting table with the best paths to each network.For example, in Figure 12-4 each path is labeled with an arbitrary value for cost. The cost of theshortest path for R2 to send packets to the LAN attached to R3 is 27 (20 + 5 + 2 = 27). Notice thatthis cost is not 27 for all routers to reach the LAN attached to R3. Each router determines its owncost to each destination in the topology. In other words, each router uses the SPF algorithm to calculatethe cost of each path to a network and determines the best path to that network from its ownperspective.