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best cisco dumps provider.At the same time, EIGRP on Router E has to determine whether the other two routes reaching Subnet 1 can be used immediately if the route through Router D fails for whatever reason. Only feasible successor routes can be used. To satisfy the feasibility condition, the RD of the alternative route must be smaller than the FD of the successor route. Figure 9-14 shows an updated version of Figure 9-13. router E uses the following logic to determine that the route through router B is not a feasible successor route but that the route through router C is a feasible successor route.Router E compares the value of FD of 14000 to the value of RD through B of 19000. the RD is worse than the FD. so this route is not a feasible successor route.Router E compares the value of FD 14000 with the RD value of the route through C (13000). the RD is better than FD, so this route is a feasible successor route., router E topology metric for subnet 1 RD router B- 1900015000 router C- 17500 13000 router D -14 000 10 000Subnet 1 metric 14 000, via router DFigure 9-14 Route through Router C is a feasible successor route8.3 Exam Prep 201 If the route through Router D to Subnet 1 fails, then Router E can immediately add the route through Router C to the routing table and does not have to worry about creating a loopback. In this example, convergence occurs almost immediately. Query and Answer Processing When a route fails and there are no feasible successor routes, EIGRP uses the Diffusing UpdateAlgorithm (DUAL).DUAL finds a loopback-free free route to the involved subnet by sending a query. When a new route is found, DUAL adds it to the routing table.The DUAL method of EIGRP simply acknowledges the existence of a route by sending a message before deciding to replace the failed route with an optional route, and that route does not generate a loopback. For example, as shown in Figure 9-14, assume that routers C and D have both failed and that router E has no feasible successor to subnet 1, but there is clearly an available physical path through router B. To use this route, Router E sends an EIGRPquery message to its functioning neighbor (in this case, Router B). The route from Router B to Subnet 1 is still working well, so Router B replies to Router E with an EIGRPreply message that simply states the details of this working route to Subnet 1, and confirms that this route is still available. Router E then adds this new route to subnet 1 to the routing table, and does not have to worry about loopbacks. Replacing a failed route with a viable successor takes very little time, typically less than 1 second or 2 seconds. When queries and answers are required, convergence takes slightly longer, but in most networks, convergence can still be completed in less than 10 seconds.9.3 Pre-test Preparation9.3.1 Reviewing the Exam EssentialsNote the "Exam Points" signs in the margins of this chapter. Table 9-4 lists the content of these exam points.