And this is where most people start assuming that it's always going to be the case. Which of course is not true :)
Let's test this quickly based on following topology:
From R1's perspective it has three different paths to reach the destination - 126.96.36.199
Now let's review R1's routing table to first find which path is preferred.
As we can see, the middle path has been chosen as best path based on Dual Algorithm. Now let's next review R1's EIGRP topology table to figure out if we have feasible successor chosen at all and if So than which path.
Let's verify Feasible Distance (FD) and Reported Distance (RD) for path through R2 by shutting down the path through R3 and R4.
Now here is an interesting scenario:
> Path through R3 is the best path (Lowest Metric)
> Path through R4 is meeting feasibility condition (making it feasible successor). But overall cost to destination is worst if we compare all three paths metric.
> Path through R2 is actually second best path based on total metric but got out of the equation as it failed feasibility condition
Though we can see that topology doesn't include any potential link which can lead traffic back to original source while forwarding traffic towards destination 188.8.131.52 , but EIGRP fails to recognize this fact.
This is where we find this true that eventually EIGRP is an Advance Distance Vector protocol as it tries to avoid any possible looping with help of Dual Algorithm but is not always successful to find it's goal. But not as good as a Link State Routing protocol which would have the complete picture of the topology.
Now in this scenario what you think would happen if Successor Route fails ?
If we go by theory discussed earlier in the post, EIGRP feasible successor should take over. Right ?
But that would mean sub-optimal routing.
But don't worry. EIGRP is intelligent enough still and it finds Optimal Path here based on overall cost to destination and avoiding sub-optimal path.
To my surprise during this test two commands didn't work the way I expected. Which I must figure out sometime :)