What is the difference between an FD and an RD in EIGRP?

In EIGRP (Enhanced Interior Gateway Routing Protocol), the terms FD (Feasible Distance) and RD (Reported Distance) refer to critical components of the routing decision process. The correct answer states that the RD is reported by a neighbor, while the FD represents the calculated route distance on the local EIGRP router.

The Feasible Distance is the lowest calculated metric to reach a destination as determined by the local router. This metric takes into account all potential paths to the destination and is used to make routing decisions. It influences the selection of the best route among multiple available paths.

On the other hand, the Reported Distance is the distance to the destination reported by a neighboring router. This value indicates how far the neighbor perceives itself to be from the destination and is essential in EIGRP's feasibility condition to determine if a route can be used.

Thus, the distinction between these two metrics lies in their origin: the RD comes from a neighbor's perspective, whereas the FD is derived from the local router's calculations. Understanding these differences is crucial for grasping how EIGRP maintains loop-free operation and optimizes routing paths through its advanced metrics.