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300-410 Practice Question: Is troubleshooting an OSPFv2 route flapping issue
A network engineer is troubleshooting an OSPFv2 route flapping issue. The router R1 is learning a route to 192.168.1.0/24 via two different paths: one through R2 and one through R3. The route is flapping between the two paths every few seconds. 'show ip ospf interface' shows that both interfaces are stable. What is the most likely 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 routers R2 and R3 have the same OSPF router ID.
Route flapping can be caused by a number of issues, but if the interfaces are stable, the problem is likely due to a routing loop or a metric issue. One common cause is that the routers are using the same router ID, causing OSPF to see the same route from two different neighbors as equal-cost paths, but the route may be flapping due to a change in the metric. Another cause is that the link between R1 and R2 is flapping at Layer 1, but the stem says the interfaces are stable. The most likely cause is that the routers have duplicate router IDs, which can cause OSPF to treat the same route as two different routes and cause instability.
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 routers R2 and R3 have the same OSPF router ID.
Why this is correct
Duplicate router IDs can cause OSPF to see the same route from two different neighbors as different, leading to route flapping.
- ✗
The route is being redistributed by both R2 and R3 with different metrics.
Why it's wrong here
Different metrics would cause the router to prefer one path, not flap.
- ✗
The OSPF network type on the interfaces is set to broadcast, causing DR/BDR instability.
Why it's wrong here
If the interfaces are stable, the DR/BDR election is stable.
- ✗
The link between R1 and R2 has a high error rate causing intermittent packet loss.
Why it's wrong here
The stem says the interfaces are stable, indicating no Layer 1 issues.
Visual reference
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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JA
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.