- A
An MTU mismatch between the two routers causes the DBD packets to be dropped, preventing OSPF from progressing past EXSTART.
Correct. OSPF DBD packets are sized based on the interface MTU. A mismatch causes the larger DBD to be dropped, halting the adjacency while BFD remains up.
- B
BFD is not configured with the 'bfd all-interfaces' command under OSPF, causing OSPF to ignore BFD state changes.
Why wrong: Incorrect. The 'bfd all-interfaces' command enables BFD for all OSPF interfaces, but the issue here is not BFD integration; it is an MTU mismatch.
- C
The OSPF network type is set to point-to-multipoint on one side and broadcast on the other, preventing the adjacency from forming.
Why wrong: Incorrect. A network type mismatch would prevent the routers from reaching EXSTART at all; they would remain stuck in INIT or 2-WAY.
- D
The BFD minimum interval is set too low, causing BFD to flap and reset the OSPF adjacency repeatedly.
Why wrong: Incorrect. A flapping BFD session would cause OSPF to reset the adjacency, not remain stuck in EXSTART.
300-410 Bidirectional Forwarding Detection (BFD) Practice Question
This 300-410 practice question tests your understanding of bidirectional forwarding detection (bfd). This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
An engineer configures BFD on an OSPF link between two routers. The BFD session comes up, but OSPF remains stuck in EXSTART state. The engineer verifies that IP connectivity is fine and OSPF hello packets are exchanged. What is the most likely cause?
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
An MTU mismatch between the two routers causes the DBD packets to be dropped, preventing OSPF from progressing past EXSTART.
BFD can cause OSPF to remain stuck in EXSTART if there is an MTU mismatch on the link. OSPF uses the interface MTU to determine the size of Database Description (DBD) packets. If the MTU is mismatched, the larger DBD packet is silently dropped by the receiving router, preventing the exchange of LSAs. BFD does not detect this because BFD packets are small and unaffected by MTU issues.
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.
- ✓
An MTU mismatch between the two routers causes the DBD packets to be dropped, preventing OSPF from progressing past EXSTART.
Why this is correct
Correct. OSPF DBD packets are sized based on the interface MTU. A mismatch causes the larger DBD to be dropped, halting the adjacency while BFD remains up.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
BFD is not configured with the 'bfd all-interfaces' command under OSPF, causing OSPF to ignore BFD state changes.
- ✗
The OSPF network type is set to point-to-multipoint on one side and broadcast on the other, preventing the adjacency from forming.
Why it's wrong here
Incorrect. A network type mismatch would prevent the routers from reaching EXSTART at all; they would remain stuck in INIT or 2-WAY.
- ✗
The BFD minimum interval is set too low, causing BFD to flap and reset the OSPF adjacency repeatedly.
Why it's wrong here
Incorrect. A flapping BFD session would cause OSPF to reset the adjacency, not remain stuck in EXSTART.
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.
Trap categories for this question
Command / output trap
Incorrect. The 'bfd all-interfaces' command enables BFD for all OSPF interfaces, but the issue here is not BFD integration; it is an MTU mismatch.
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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Bidirectional Forwarding Detection (BFD) — study guide chapter
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FAQ
Questions learners often ask
What does this 300-410 question test?
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: An MTU mismatch between the two routers causes the DBD packets to be dropped, preventing OSPF from progressing past EXSTART. — BFD can cause OSPF to remain stuck in EXSTART if there is an MTU mismatch on the link. OSPF uses the interface MTU to determine the size of Database Description (DBD) packets. If the MTU is mismatched, the larger DBD packet is silently dropped by the receiving router, preventing the exchange of LSAs. BFD does not detect this because BFD packets are small and unaffected by MTU issues.
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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