hardMultiple ChoiceObjective-mapped
300-410 Practice Question: Is troubleshooting an EIGRP issue where a route…
A network engineer is troubleshooting an EIGRP issue where a route is flapping in and out of the routing table. The engineer checks the logs and sees messages indicating that the route is being learned from two different neighbors, but the metric keeps changing. The route is a summary route. What is the most likely cause of the flapping?
⚠ Common exam trap
It's easy for candidates to assume flapping is caused by neighbor adjacency issues (like duplicate router IDs) or interface misconfigurations, rather than recognizing that metric instability between multiple sources of the same summary route is the root cause.
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 summary route is being advertised by multiple routers with different metrics.
When a summary route is learned from multiple EIGRP neighbors with different metrics, the router will alternate between the best paths as the metrics change, causing the route to flap in and out of the routing table. EIGRP uses the feasible successor logic, but if the metric from each neighbor varies (e.g., due to unequal link speeds or load), the route may be repeatedly inserted and withdrawn as the topology table updates.
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 summary route is being advertised by multiple routers with different metrics.
Why this is correct
Correct because if the summary route is originated by multiple routers, the metric may vary, causing the router to flap between the best paths.
- ✗
The EIGRP stub feature is configured on one of the neighbors.
Why it's wrong here
Incorrect because stub routing prevents the router from being a transit router, but it does not cause route flapping due to metric changes.
- ✗
The passive-interface command is applied to the interface receiving the summary route.
Why it's wrong here
Incorrect because passive-interface would prevent the router from sending hellos, not cause flapping of a learned route.
- ✗
The EIGRP router ID is the same on both neighbors.
Why it's wrong here
Incorrect because duplicate router IDs can cause route instability, but they would not cause a summary route to flap specifically due to metric changes.
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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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.