You administer the partial network shown above. All of the routers run EIGRP. RouterA has the following FD and AD values for the 10.1.2.0/24 network, which is connected to RouterE:

The variance is set to the default value on RouterA.

Which of the following statements is correct? (Select the best answer.)

A. The route through RouterB is the only feasible successor.
B. Theroute through RouterC is the only feasible successor.

C. The route through RouterD is the only feasible successor.

D. The routes through RouterB and RouterC are feasible successors.
E. The routes through RouterB and RouterD are feasible successors.
F. Theroutes through RouterC and RouterD are feasible successors.
G. None of the routes qualifies as a feasible successor.

Explanation:

The route through RouterD is the only feasible successor. A feasible successor is a backup route to a destination. To qualify as a feasible successor, the advertising router must be closer to the destination than the router to which the route is advertised. To ensure that the advertising router meets this feasibility condition, the router must have an advertised distance (AD) that is less than the feasible distance (FD) of the successor? the successor is the router with the lowest FD. The AD, sometimes called the reported distance (RD), is the cost that the nexthop router has calculated for the route, and the FD is the total distance to the destination network through that nexthop router. If the AD of a route is lower than the FD of the successor, it is a feasible successor.
The route through RouterC is not a feasible successor? it is the successor because it has the lowest FD, 5362821. The AD of the route through RouterD is 3907449. Because the AD of RouterD is less than the FD of RouterC, the route through RouterD is a feasible successor.

The route through RouterB is not a feasible successor, because the AD of RouterB, 5470303, is higher than the FD of RouterC, 5362821. If the AD of RouterB were lower than the FD of RouterC, the routes through RouterB and RouterD would both be feasible successors.

In some situations, a destination route might have no feasible successors. For example, there would be no feasible successors if there were no alternate routes to a destination or if there were no routes with an AD lower than the FD of the successor. Finally, there would be no feasible successors if all of the routes qualified as successors. Enhanced Interior Gateway Routing Protocol (EIGRP) can load balance over equalcost paths, and EIGRP can proportionally load balance over unequalcost paths if the variance value is changed from its default value of 1.

Reference:
Cisco: Introduction to EIGRP