CompTIA Network+ (N10-009)Network ImplementationMedium
An EIGRP router has a successor route to a destination network with a feasible distance of 20. A neighboring router advertises an alternate path with a reported distance of 15. According to the Feasibility Condition, when will this alternate path be installed as a feasible successor in the topology table?
- AOnly if the alternate path uses the same next-hop router as the successor
- BImmediately, because its reported distance is lower than the successor's feasible distance
- COnly after the successor route fails and a full recomputation occurs
- DNever, because only one path per destination can exist in the topology table
Show answer & explanationAnswer & explanation
Correct answer: B. Immediately, because its reported distance is lower than the successor's feasible distance
EIGRP's Feasibility Condition states that a neighbor's route qualifies as a feasible successor if its reported distance (RD) is less than the local router's feasible distance (FD) for that destination. Since 15 < 20, this condition is met immediately, and the route is added as a backup feasible successor without needing to converge via query/reply (no need to go into Active state).
Why the other options are wrong
- A. Feasible successors are evaluated by metric comparison, not next-hop matching.
- C. Feasible successors are pre-computed and stored in advance, not after failure.
- D. EIGRP topology table can hold multiple feasible successors for fast failover.
EIGRP Feasible Successor
A backup route in EIGRP's topology table whose reported distance is less than the successor's feasible distance, allowing instant failover without recomputation.
- Feasibility Condition: RD < FD
- Provides loop-free backup paths
- Enables fast convergence without going Active
Memory trick: If the neighbor's reported cost beats your best cost, keep it as backup.