Best Route Calculation Feasible Distance and Reported Distance

In EIGRP, the best path to a destination network is calculated based on the metric of the feasible successor, also known as the Successor Route. The feasible distance (FD) is the metric of the best path to a destination network that is currently in the routing table. The reported distance (RD) is the metric advertised by a neighboring router for a particular path to a destination network.

When a router receives an EIGRP update message from a neighboring router, it adds the RD to its own FD to calculate the total metric to the destination network. If the new total metric is lower than the current FD, the router replaces the current path with the new path, updates its routing table, and sends updates to its neighbors.

EIGRP uses the Diffusing Update Algorithm (DUAL) to prevent routing loops and to ensure that the best path is always selected. When a link failure occurs, EIGRP quickly recalculates the best path by searching for a feasible successor, which is a backup route that has a metric lower than the current FD. If a feasible successor is found, it becomes the new Successor Route, and the router switches to this path without sending queries to its neighbors. If a feasible successor is not found, EIGRP sends queries to its neighbors to find a new path to the destination network.