- A
The summary route is being advertised by multiple routers with different metrics.
Correct because if the summary route is originated by multiple routers, the metric may vary, causing the router to flap between the best paths.
- B
The EIGRP stub feature is configured on one of the neighbors.
Why wrong: Incorrect because stub routing prevents the router from being a transit router, but it does not cause route flapping due to metric changes.
- C
The passive-interface command is applied to the interface receiving the summary route.
Why wrong: Incorrect because passive-interface would prevent the router from sending hellos, not cause flapping of a learned route.
- D
The EIGRP router ID is the same on both neighbors.
Why wrong: 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 Answer
The answer is that the summary route is being advertised by multiple routers with different metrics, causing EIGRP route flapping. This occurs because when a summary route is learned from two different neighbors, the router compares the metrics and selects the best path; if the metrics are inconsistent, the router constantly switches between the two advertisements, removing and reinserting the route from the routing table. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of EIGRP summarization behavior and metric consistency, often appearing as a troubleshooting question where logs show fluctuating metrics for a summary route. A common trap is to blame a mismatched K-value or a redistribution issue, but the core problem is that multiple routers originate the same summary with differing composite metrics. Memory tip: “Multiple metrics, multiple neighbors—flapping follows.”
300-410 EIGRP Troubleshooting Practice Question
This 300-410 practice question tests your understanding of eigrp troubleshooting. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
If a summary route is being learned from two different neighbors, and the metric changes, it could be because one of the neighbors is advertising the summary route with a different metric, causing the router to constantly switch between the two paths. However, the most common cause is that the summary route is being originated by multiple routers, and the metric is not consistent, leading to instability.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
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.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 300-410 question test?
EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The summary route is being advertised by multiple routers with different metrics. — If a summary route is being learned from two different neighbors, and the metric changes, it could be because one of the neighbors is advertising the summary route with a different metric, causing the router to constantly switch between the two paths. However, the most common cause is that the summary route is being originated by multiple routers, and the metric is not consistent, leading to instability.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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