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9tut.com ccna dumps.R1-R2-R3-R4-R8 Right 30 + 60 + 20 + 5 + 10 = 125Based on the results of the SPF algorithm for the LSDB, R1 adds a route arriving at subnet 172.16.3.0/24 to its routing table, and the next hop of that route is R5.In a real OSPF network, the engineer can do the same calculation if he knows the OSPF overhead for each interface. Using the network diagram, the engineer can examine all routes, add up the costs, and predict the metric value for each route. Tip: Depending on the area design, OSPF uses a different process to calculate costs. When using a single-area design, the example around Figure 8-15 best matches the OSPF logic.The logic.Managing DistanceOSPF can select the best route for each subnet based on the OSPF metric, which was discussed earlier in this chapter. However, the I0S must be able to select the best route to reach a subnet even if the router is aware of routes from different sources. To make such a selection, the router uses the concept known as administrative distance (AD).First, imagine a common situation where all routers use OSPF, and only use OSPF as the only IPv4 routing protocol. A router learns routes through OSPF, but it also learns connected routes. It may also have some ip route commands that define static IPv4 routes. It is also possible that a router knows the connected routes, static routes, and OSPF routes for a subnet. How does this router choose which route to use?Whenever a router must choose between competing connected, static, and OSPF routes, by default the router uses the connected route. Why? While this may seem like just good common sense, the reason a router chooses a connected route is actually that it174 Chapter 8 Implementing IPv4 OSPF has a lower (better) AD and a connected route has an AD of 0. The default AD for static routes is 1,and the default AD for OSPF routes is no. To explain this concept even further, in some cases, companies need to use multiple routing protocols. For example, if two companies are connecting their networks in order to exchange information, they need to exchange certain routing information. If one company uses OSPF and the other uses EIGRP on at least one router, then both OSPF and EIGRP (Enhanced Interior Gateway RoutingProtocol) must be used. Then, through a process called route redistribution, that router can accept routes known through OSPF and advertise them to EIGRP, and vice versa. Depending on the network topology, a router may learn routes to a subnet through both OSPF and EIGRP. In this case, because the metrics for each routing protocol are based on different information, IOS cannot compare metrics. In this case, IOS also selects the best route based on administrative distance. the AD value is configured on one router and is not exchanged with other routers. Table 8-6 lists the various sources of routing information. and the default AD value.8.2 Basic Contents 175