- A
The eBGP neighbor is not on the same subnet, and the 'ebgp-multihop' command is missing, preventing TCP connection.
Why wrong: The ebgp-multihop command is not required for directly connected eBGP peers on the same subnet. This is a common misdiagnosis.
- B
The BFD session is using a different source IP than the BGP update-source, causing BGP to ignore BFD state.
Correct. The mismatch between BFD source and BGP update-source causes BGP to ignore the BFD session, potentially leading to Idle state.
- C
The BGP neighbor is configured with a password, but BFD does not support authentication, causing a mismatch.
Why wrong: BGP authentication (TCP MD5) and BFD authentication are independent. A password configured for BGP does not affect BFD, and both can coexist.
- D
The 'bgp bestpath med missing-as-worst' command is configured, causing BGP to reject the neighbor.
Why wrong: The 'bgp bestpath med missing-as-worst' command influences path selection but does not affect BGP session establishment.
300-410 BFD (Bidirectional Forwarding Detection) 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. A key principle to apply: bFD (Bidirectional Forwarding Detection). 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 BGP between two directly connected eBGP peers. The BFD session is up, but BGP remains in the Idle state. The engineer verifies that the BGP configuration is correct and that TCP port 179 is reachable. 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
The BFD session is using a different source IP than the BGP update-source, causing BGP to ignore BFD state.
In this scenario, BFD is up but BGP remains in Idle state. Since TCP port 179 is reachable and BGP configuration is correct, the issue is likely that the BFD session is using a different source IP than the BGP update-source. BFD for BGP must monitor the same path as the BGP peering; otherwise, BGP may not use the BFD session for fast detection, and in some implementations, BGP may remain in Idle until a properly matched BFD session is established. The ebgp-multihop command is not required for directly connected peers on the same subnet.
Key principle: BFD (Bidirectional Forwarding Detection)
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 eBGP neighbor is not on the same subnet, and the 'ebgp-multihop' command is missing, preventing TCP connection.
Why it's wrong here
The ebgp-multihop command is not required for directly connected eBGP peers on the same subnet. This is a common misdiagnosis.
- ✓
The BFD session is using a different source IP than the BGP update-source, causing BGP to ignore BFD state.
Why this is correct
Correct. The mismatch between BFD source and BGP update-source causes BGP to ignore the BFD session, potentially leading to Idle state.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
BFD (Bidirectional Forwarding Detection)
- ✗
The BGP neighbor is configured with a password, but BFD does not support authentication, causing a mismatch.
- ✗
The 'bgp bestpath med missing-as-worst' command is configured, causing BGP to reject the neighbor.
Why it's wrong here
The 'bgp bestpath med missing-as-worst' command influences path selection but does not affect BGP session establishment.
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common trap is to assume ebgp-multihop is required when BGP is stuck in Idle, but for directly connected peers on the same subnet, this is not needed. Instead, check that the BFD source interface matches the BGP update-source.
Trap categories for this question
Command / output trap
The ebgp-multihop command is not required for directly connected eBGP peers on the same subnet. This is a common misdiagnosis.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- BFD (Bidirectional Forwarding Detection)
- BGP update-source
- BFD source IP
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
BFD (Bidirectional Forwarding Detection)
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review bFD (Bidirectional Forwarding Detection), then practise related 300-410 questions on the same topic to reinforce the concept.
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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) — BFD (Bidirectional Forwarding Detection).
What is the correct answer to this question?
The correct answer is: The BFD session is using a different source IP than the BGP update-source, causing BGP to ignore BFD state. — In this scenario, BFD is up but BGP remains in Idle state. Since TCP port 179 is reachable and BGP configuration is correct, the issue is likely that the BFD session is using a different source IP than the BGP update-source. BFD for BGP must monitor the same path as the BGP peering; otherwise, BGP may not use the BFD session for fast detection, and in some implementations, BGP may remain in Idle until a properly matched BFD session is established. The ebgp-multihop command is not required for directly connected peers on the same subnet.
What should I do if I get this 300-410 question wrong?
Review bFD (Bidirectional Forwarding Detection), then practise related 300-410 questions on the same topic to reinforce the concept.
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?
BFD (Bidirectional Forwarding Detection)
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Last reviewed: Jun 18, 2026
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