3.5.d [i] General operations
The lowest cost route is calculated by adding the cost between the next hop router and the destination (Reported Distance, RD), and the cost between the local router and the next hop. This sum is the Feasible Distance (FD).
The successor is the next hop router to the destination that the local router has chosen. Multiple successors may exist if they provide equal cost paths.
If a backup path to the destination exists it may become a feasible successor. To qualify, this next hop router must report an RD that is less than the FD of the successor.