If the receiving router has a Feasible Distance to a particular network and it receives an update from a neighbour with a lower advertised distance (Reported Distance) to that network, then there is a Feasibility Condition. In this instance, the neighbour becomes a Feasible Successor for that route because it is one hop closer to the destination network.
What you should note here, is that the metric for a neighbour to reach a particular network (i.e. the Reported Distance) must always be less than the metric (Feasible Distance) for the local router to reach that same network. This way routing loops are avoided. This is why routes that have Reported Distances larger than the Feasible Distance are not entered into the Topology table, so that they can never be considered as successors, since the route is likely to loop back through that local router.
Regards,
