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350-401 Practice Question: Is used by EIGRP to calculate the feasible…
Which of the following is used by EIGRP to calculate the feasible distance (FD) of a route?
⚠ Common exam trap
Cisco often tests the distinction between Feasible Distance (FD) and Advertised Distance (AD), and the trap here is that candidates confuse FD with just the link cost or one metric component (like delay or bandwidth), rather than recognizing it as the sum of the AD and the cost to the successor.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The sum of the advertised distance (AD) of the successor and the link cost to the successor.
EIGRP calculates the Feasible Distance (FD) as the sum of the Advertised Distance (AD) from the successor neighbor and the link cost (metric) to that neighbor. This represents the total metric from the local router to the destination network via that path. The FD is used to determine the best route (successor) and to compare against feasible successors.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The sum of the advertised distance (AD) of the successor and the link cost to the successor.
Why this is correct
The feasible distance (FD) is the best metric to a destination in EIGRP, computed by taking the metric reported by the next-hop router (the advertised distance, or AD) and adding the local link cost to that next hop. This sum yields the total end-to-end metric from the local router to the destination via the successor. The successor is chosen precisely because it minimizes this composite value, making the FD the definitive metric for route selection and for verifying loop-free paths in DUAL.
- ✗
The lowest hop count among all paths to the destination.
Why it's wrong here
EIGRP does not use hop count as a primary metric; its composite metric defaults to a blend of bandwidth and delay (K1=1, K3=1). Hop count is at most a secondary tie-breaker in certain older configurations and does not appear in the metric calculation that determines the feasible distance. Therefore, selecting the lowest hop count would ignore the actual link characteristics like bandwidth and delay, which are what EIGRP uses to compute FD.
- ✗
The highest bandwidth among all paths to the destination.
Why it's wrong here
While bandwidth is a factor in EIGRP's composite metric, the feasible distance is not simply the highest bandwidth path. EIGRP uses the *minimum* bandwidth along the path (the bottleneck) in its calculation, because that reflects the actual throughput limitation, and then combines it with cumulative delay and optionally other K values. Merely choosing the highest bandwidth would overlook the impact of path delay and the fact that a single slow link can degrade the entire path's metric.
- ✗
The sum of all delays along the path.
Why it's wrong here
Delay is indeed a component of EIGRP's composite metric (default K3=1), but it is not the only component. The feasible distance also incorporates the inverse of the minimum bandwidth (K1=1) and, if configured, reliability and load. Moreover, the advertised distance from the successor already includes that neighbor's own composite metric to the destination, so the FD is not merely a sum of local interface delays—it is a composite of multiple link attributes evaluated across the entire path.
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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