The answer is an OSPF dead interval mismatch between the two routers. While BFD provides rapid failure detection, a mismatched dead interval causes OSPF to independently reset the adjacency because each router expects Hello packets at different intervals—when one router’s Dead timer expires before receiving the other’s Hello, the neighbor state drops to DOWN, even though the BFD session remains stable and up. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to isolate Layer 3 protocol issues from underlying fast-detect mechanisms; a common trap is to blame BFD when it is actually functioning correctly. Remember that BFD only accelerates failure detection—it does not override OSPF’s own timer-based adjacency logic. A useful memory tip: “BFD is the lookout, but OSPF’s Dead timer is the executioner.”
300-410 Bidirectional Forwarding Detection (BFD) Practice Question
This 300-410 practice question tests your understanding of bidirectional forwarding detection (bfd). 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 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?
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.
Why wrong: BFD timers do not affect OSPF adjacency stability if the BFD session is up; the flap is due to OSPF itself.
B
The OSPF dead interval is mismatched between the two routers.
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.
C
The interface is flapping due to a physical issue.
Why wrong: If the interface were flapping, the BFD session would also go down, but the BFD session remains up.
D
BFD is configured with the 'strict-mode' command, causing OSPF to ignore BFD state.
Why wrong: Strict-mode does not cause OSPF to ignore BFD; it makes BFD more aggressive in failing over.
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.
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 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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
Bidirectional Forwarding Detection (BFD) — This question tests Bidirectional Forwarding Detection (BFD) — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: 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.
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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