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300-410 Practice Question: Is troubleshooting an OSPF adjacency that is…
A network engineer is troubleshooting an OSPF adjacency that is flapping between two routers. The adjacency forms and then drops repeatedly. Both routers are configured for BFD on the OSPF interface. The engineer checks the BFD session and sees it is up, but the OSPF neighbor state transitions from FULL to DOWN every few seconds. What is the most likely cause of this issue?
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 OSPF dead interval is mismatched between the two routers.
The BFD session is up, so BFD is not causing the flap. The issue is likely a mismatch in OSPF parameters that causes the adjacency to reset independently of BFD.
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 BFD timers are set too low on one router.
Why it's wrong here
BFD timers do not affect OSPF adjacency stability if the BFD session is up; the flap is due to OSPF itself.
- ✓
The OSPF dead interval is mismatched between the two routers.
Why this is correct
A mismatch in OSPF dead interval (or hello interval) causes the adjacency to reset even if BFD is healthy, because OSPF uses its own keepalive mechanism.
- ✗
The interface is flapping due to a physical issue.
Why it's wrong here
If the interface were flapping, the BFD session would also go down, but the BFD session remains up.
- ✗
BFD is configured with the 'strict-mode' command, causing OSPF to ignore BFD state.
Why it's wrong here
Strict-mode does not cause OSPF to ignore BFD; it makes BFD more aggressive in failing over.
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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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.