EIGRP tracks primary and backup routes:
Successor: The neighbor with the lowest metric to a destination. This route is installed in the routing table.
Feasible Successor: A backup neighbor whose Reported Distance is less than the current Feasible Distance. Kept in the topology table only.
Example: Router A reaches network X
- Via B: RD = , total metric =
- Via C: RD = , total metric =
B is the successor. C is a feasible successor because . If B fails, A switches to C immediately.