3.5.c Implement and troubleshoot loop free path selection

3.5.c [i] RD, FD, FC, successor, feasible successor

Feasible distance is the best metric along a path to a destination network, including the metric to the neighbor advertising that path.

Reported distance is the total metric along a path to a destination network as advertised by an upstream neighbor. A feasible successor is a path whose reported distance is less than the feasible distance (current best path).

A feasible successor for a particular destination is a next hop router that is guaranteed not to be a part of a routing loop. This condition is verified by testing the feasibility condition.

Thus, every successor is also a feasible successor. However, in most references about EIGRP the term feasible successor is used to denote only those routes which provide a loop-free path but which are not successors (i.e. they do not provide the least distance). From this point of view, for a reachable destination there is always at least one successor, however, there might not be any feasible successors.

The feasibility condition (FC) is a sufficient condition for routing loop prevention in EIGRP-routed network. It is used to select the successors and feasible successors that are guaranteed to be on a loop-free route to a destination. Its formulation is strikingly simple:

● If, for a destination, a neighbor router advertises a distance that is strictly lower than our feasible distance, then this neighbor lies on a loop-free route to this destination. or in other words,

● If, for a destination , a neighbor router tells us that it is closer to the destination than we have ever been, then this neighbor lies on a loop-free route to this destination.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3095-3098).  . Kindle Edition.

http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/16406-eigrp-toc.html#basictheory