- A
BFD cannot be used with OSPF virtual links; BFD only works on physical interfaces.
Correct. OSPF virtual links are not supported with BFD. The BFD session on the underlying physical interface does not affect the virtual link adjacency.
- B
The virtual link is configured with a different hello interval than the BFD interval, causing a mismatch.
Why wrong: Incorrect. BFD and OSPF hello intervals are independent; they do not need to match.
- C
The 'bfd all-interfaces' command is not configured under the OSPF process, so OSPF ignores BFD for virtual links.
Why wrong: Incorrect. Even with 'bfd all-interfaces', BFD does not apply to virtual links.
- D
The virtual link is using a different authentication type than the BFD session.
Why wrong: Incorrect. BFD and OSPF authentication are separate; they do not need to match.
Quick Answer
The answer is that BFD cannot be used with OSPF virtual links, so the adjacency will not form despite a working BFD session. This is because BFD operates only on physical interfaces or subinterfaces, not on logical constructs like virtual links, which traverse multiple hops through a transit area. Even if the underlying physical link between the virtual link endpoints has a functional BFD session, OSPF ignores that BFD state for the virtual link adjacency, leaving it vulnerable to failure from other issues such as mismatched area IDs or authentication. On the Cisco CCNP ENARSI 300-410 exam, this question tests your understanding of BFD’s architectural limitation—it is a common trap where candidates assume BFD works on any OSPF neighbor relationship. Remember the memory tip: “BFD needs a wire, not a virtual wire.”
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 that is part of a virtual link. The BFD session between the two endpoints of the virtual link is up, but OSPF adjacency over the virtual link is not forming. 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
BFD cannot be used with OSPF virtual links; BFD only works on physical interfaces.
BFD does not support virtual links in OSPF. BFD can only be configured on physical interfaces or subinterfaces, not on virtual links. Therefore, even if BFD is up on the underlying physical link, it will not help detect failures of the virtual link. The virtual link adjacency may fail due to other issues, such as mismatched area IDs or authentication.
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.
- ✓
BFD cannot be used with OSPF virtual links; BFD only works on physical interfaces.
Why this is correct
Correct. OSPF virtual links are not supported with BFD. The BFD session on the underlying physical interface does not affect the virtual link adjacency.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The virtual link is configured with a different hello interval than the BFD interval, causing a mismatch.
Why it's wrong here
Incorrect. BFD and OSPF hello intervals are independent; they do not need to match.
- ✗
The 'bfd all-interfaces' command is not configured under the OSPF process, so OSPF ignores BFD for virtual links.
Why it's wrong here
Incorrect. Even with 'bfd all-interfaces', BFD does not apply to virtual links.
- ✗
The virtual link is using a different authentication type than the BFD session.
Why it's wrong here
Incorrect. BFD and OSPF authentication are separate; they do not need to match.
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.
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.
- →
Bidirectional Forwarding Detection (BFD) — study guide chapter
Learn the concepts, then practise the questions
- →
Bidirectional Forwarding Detection (BFD) practice questions
Targeted practice on this topic area only
- →
All 300-410 questions
2,152 questions across all exam domains
- →
Cisco CCNP ENARSI 300-410 study guide
Full concept coverage aligned to exam objectives
- →
300-410 practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related 300-410 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Layer 3 Technologies practice questions
Practise 300-410 questions linked to Layer 3 Technologies.
EIGRP Troubleshooting practice questions
Practise 300-410 questions linked to EIGRP Troubleshooting.
OSPF Troubleshooting (v2/v3) practice questions
Practise 300-410 questions linked to OSPF Troubleshooting (v2/v3).
BGP Troubleshooting practice questions
Practise 300-410 questions linked to BGP Troubleshooting.
Route Redistribution practice questions
Practise 300-410 questions linked to Route Redistribution.
Policy-Based Routing (PBR) practice questions
Practise 300-410 questions linked to Policy-Based Routing (PBR).
VRF-Lite practice questions
Practise 300-410 questions linked to VRF-Lite.
Route Maps and Route Filtering practice questions
Practise 300-410 questions linked to Route Maps and Route Filtering.
Administrative Distance practice questions
Practise 300-410 questions linked to Administrative Distance.
Route Summarization practice questions
Practise 300-410 questions linked to Route Summarization.
Bidirectional Forwarding Detection (BFD) practice questions
Practise 300-410 questions linked to Bidirectional Forwarding Detection (BFD).
VPN Technologies practice questions
Practise 300-410 questions linked to VPN Technologies.
Practice this exam
Start a free 300-410 practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
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: BFD cannot be used with OSPF virtual links; BFD only works on physical interfaces. — BFD does not support virtual links in OSPF. BFD can only be configured on physical interfaces or subinterfaces, not on virtual links. Therefore, even if BFD is up on the underlying physical link, it will not help detect failures of the virtual link. The virtual link adjacency may fail due to other issues, such as mismatched area IDs or authentication.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Keep practising
More 300-410 practice questions
- Drag and drop the steps to negotiate an IKEv2 IPsec site-to-site tunnel into the correct order, from first to last.
- Drag and drop the steps to troubleshoot an IPsec site-to-site VPN adjacency failure into the correct order, from first t…
- Drag and drop the steps to verify and validate the operational state of an IPsec site-to-site VPN into the correct order…
- Drag and drop the steps to configure a GRE tunnel for IPv6 over IPv4 into the correct order, from first to last.
- Drag and drop the steps to troubleshoot IPv6 over IPv4 tunnel adjacency or connectivity failures into the correct order,…
- Drag and drop the steps to verify and validate the operational state of an IPv6 tunneling technique into the correct ord…
Last reviewed: Jun 18, 2026
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.