- A
The 'no neighbor' command is configured under the af-interface, disabling EIGRP on that interface.
Correct. In named mode, the 'no neighbor' command under the af-interface prevents EIGRP from forming adjacencies on that interface, even if BFD is up.
- B
The 'bfd' command is configured under the af-interface, but the 'bfd all-interfaces' command is missing under the EIGRP process.
Why wrong: Incorrect. In named mode, BFD is enabled per af-interface, not globally. The 'bfd all-interfaces' command is not used in named mode.
- C
The EIGRP router ID is the same on both routers, causing a conflict.
Why wrong: Incorrect. EIGRP does not use router ID for adjacency formation; it uses K values and AS number.
- D
The interface is configured with 'ip bandwidth-percent eigrp' set to 0, preventing EIGRP from using the interface.
Why wrong: Incorrect. The bandwidth-percent command affects how much bandwidth EIGRP can use, not adjacency formation.
Quick Answer
The answer is that the 'no neighbor' command is configured under the af-interface, which disables EIGRP on that interface. This is the most likely explanation for an EIGRP named mode BFD adjacency failure because, even when the BFD session itself is up and the K values match, the af-interface configuration in named mode can independently suppress neighbor formation. The 'no neighbor' command effectively tells EIGRP to ignore that interface for adjacency building, so BFD, which is also configured under the same af-interface, has no EIGRP neighbor to monitor. On the Cisco CCNP ENARSI 300-410 exam, this is a classic trap: candidates often focus on BFD misconfiguration or mismatched parameters, but the real culprit is a subtle interface-level disable. The exam tests your understanding that named mode separates interface policy from the address-family, and a single 'no neighbor' can silently block adjacency. Memory tip: think of it as "BFD can't knock on a door that EIGRP has already locked with 'no neighbor'."
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 for EIGRP in named mode. The BFD session is up, but the EIGRP neighbor relationship is not forming. The engineer checks that the K values match and that the interfaces are up. What is the most likely explanation?
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 'no neighbor' command is configured under the af-interface, disabling EIGRP on that interface.
In EIGRP named mode, the 'af-interface' configuration can have different settings than classic mode. Specifically, the 'no neighbor' command under the af-interface can disable EIGRP on that interface, preventing adjacency formation. BFD is configured under the af-interface as well, but if the interface is disabled for EIGRP, BFD will not help.
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 'no neighbor' command is configured under the af-interface, disabling EIGRP on that interface.
Why this is correct
Correct. In named mode, the 'no neighbor' command under the af-interface prevents EIGRP from forming adjacencies on that interface, even if BFD is up.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The 'bfd' command is configured under the af-interface, but the 'bfd all-interfaces' command is missing under the EIGRP process.
Why it's wrong here
Incorrect. In named mode, BFD is enabled per af-interface, not globally. The 'bfd all-interfaces' command is not used in named mode.
- ✗
The EIGRP router ID is the same on both routers, causing a conflict.
- ✗
The interface is configured with 'ip bandwidth-percent eigrp' set to 0, preventing EIGRP from using the interface.
Why it's wrong here
Incorrect. The bandwidth-percent command affects how much bandwidth EIGRP can use, not adjacency formation.
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. In named mode, BFD is enabled per af-interface, not globally. The 'bfd all-interfaces' command is not used in named mode.
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: The 'no neighbor' command is configured under the af-interface, disabling EIGRP on that interface. — In EIGRP named mode, the 'af-interface' configuration can have different settings than classic mode. Specifically, the 'no neighbor' command under the af-interface can disable EIGRP on that interface, preventing adjacency formation. BFD is configured under the af-interface as well, but if the interface is disabled for EIGRP, BFD will not help.
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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