9tut.com ccna

9tut.com ccna.As you can see from these LSAs, they contain useful information, but they do not contain the specific information that a router needs to add to its IPv4 routing table. In other words, the router cannot just copy the information from the LSDB to a route in the IPv4 routing table. The router must perform an SPF calculation, select the best route, and add a route: a route with a subnet number and mask, an outgoing interface, and a next-hop router IP address.Although engineers do not need to know the details of the SPF calculation, they do need to know how to predict which routes the SPF will select as the best routes.The SPF algorithm calculates all routes for the subnet, that is, all possible routes from the router to the destination subnet. If multiple routes exist, the router compares the metric values and selects the route with the best (lowest) metric value, adding it to the routing table. Although the SPF calculation can be complex, the engineer can calculate the metric value for each route using the network diagram, the router's state information, and simple addition to predict which route the SPF will select.Once the SPF identifies a route, it calculates the route's metric value according to the following rules./^-Sum the OSPF interface cost for all outgoing interfaces in the route. [Cranky Figure 8-15 shows an example of three routes that reach subnet X (172.16.3.0/24) at the bottom of this graph from R1.8.2 Basics 173Figure 8-15 Finding the SPF tree for the route from R1 to 172.16.3.0/24 Tip: The OSPF (only) considers the outgoing interface cost in each route. It does not add the cost of the incoming interfaces in the route. Table 8-5 lists the three routes and their cumulative costs shown in Figure 8-15. which shows that the best route for R1 to reach 172.16.3.0/24 first goes through R5.Table 8-5 compares the three alternative routes for R1 to reach 172.16.3.0/24Routes Location in Figure 8-15 Cumulative CostR1-R7-R8 Left 10 + 180+ 10 = 200R1-R5-R6-R8 Middle 20 + 30 + 40+ 10= 100