EIGRP saves details like next-hop router for alternate routes about the topology in its database.This will allow EIGRP for quick convergence.
R1 has two paths to reach the network 172.16.1.0. One is via R2 and other is via R6.
The route via R2 has a cost of 20307200 and a reported distance of 307200.
The route via R6 has a cost of 20307200 and a reported distance of 307200.
Reported distance is the distance from the router advertising the route to the network.
This means , the reported distance from router R2 is the metric to get to network 172.16.1.0 from R2 and the reported distance from R6 is the metric to get to network 172.16.1.0 from R6.
EIGRP select s the route via R2 as the best path and uses the metric via R2 as the feasible distance.
As you can see the reported distance to the network 172.16.1.0 via R6 is less than the feasible distance, EIGRP uses the path R1-R6-R5-R4 as the feasible successor.
If the link between R1 and R2 goes down then the metric of the path via R6 will be used as the feasible distance.
Feasible Distance
The best metric to a destination network along a path which includes the metric to the neighbor advertising that path.Reported Distance
Total metric to the destination network along a path as advertised by the next-hop router.Successor
The route with the best metric is called a successor.Feasible Successor
A path whose advertised distance is less than the feasible distance of the current successor is called a feasible successor.
R1 has two paths to reach the network 172.16.1.0. One is via R2 and other is via R6.
The route via R2 has a cost of 20307200 and a reported distance of 307200.
The route via R6 has a cost of 20307200 and a reported distance of 307200.
Reported distance is the distance from the router advertising the route to the network.
This means , the reported distance from router R2 is the metric to get to network 172.16.1.0 from R2 and the reported distance from R6 is the metric to get to network 172.16.1.0 from R6.
EIGRP select s the route via R2 as the best path and uses the metric via R2 as the feasible distance.
As you can see the reported distance to the network 172.16.1.0 via R6 is less than the feasible distance, EIGRP uses the path R1-R6-R5-R4 as the feasible successor.
If the link between R1 and R2 goes down then the metric of the path via R6 will be used as the feasible distance.