- A
The BGP hold timer is set to 30 seconds on one router and 90 seconds on the other.
A mismatch in BGP hold timer can cause the session to reset when the hold timer expires; the BFD session being up does not affect BGP's own keepalive mechanism.
- B
The BFD session is using echo mode, which is not supported for BGP fall-over.
Why wrong: Echo mode is supported and does not cause BGP flapping.
- C
The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor is configured with 'bfd interval 100 min_rx 100 multiplier 3'.
Why wrong: BFD parameter negotiation allows different values; as long as the session is up, this is not the issue.
- D
The BGP session is using EBGP multihop, and the TTL is set to 2.
Why wrong: If the routers are directly connected, EBGP multihop is not needed; TTL mismatch would prevent BGP from forming, not cause flapping.
Quick Answer
The answer is a BGP hold timer mismatch, where one router is configured with a 30-second hold timer and the other with 90 seconds. This is the most likely cause because the BFD session is up, confirming the physical and BFD link are healthy, but the BGP session flaps every 30 seconds—exactly the interval of the shorter hold timer. When BGP fall-over bfd is configured, BGP relies on BFD for fast failure detection, but a hold timer mismatch still causes BGP to reset the session when the routers cannot agree on the timer during the OPEN negotiation. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to distinguish between BFD-related failures and BGP configuration issues; a common trap is assuming BFD is the culprit when it is actually a BGP timer mismatch. Remember the memory tip: BFD checks the pipe, but BGP timers set the pace—if the hold times don’t match, the session will flap at the shorter timer’s pace.
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 a BGP session that is flapping. The routers are connected via a direct Ethernet cable. BFD is configured for the BGP session. The engineer checks the BFD session and sees it is 'Up'. However, the BGP session goes down every 30 seconds. The BGP configuration includes 'neighbor 10.0.0.2 fall-over bfd'. 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 BGP hold timer is set to 30 seconds on one router and 90 seconds on the other.
The BGP fall-over bfd command causes BGP to monitor the BFD session. If the BFD session is up but BGP is flapping, the issue is likely a BGP configuration problem, such as a mismatch in hold timer or update-source.
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 BGP hold timer is set to 30 seconds on one router and 90 seconds on the other.
Why this is correct
A mismatch in BGP hold timer can cause the session to reset when the hold timer expires; the BFD session being up does not affect BGP's own keepalive mechanism.
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 session is using echo mode, which is not supported for BGP fall-over.
Why it's wrong here
Echo mode is supported and does not cause BGP flapping.
- ✗
The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor is configured with 'bfd interval 100 min_rx 100 multiplier 3'.
Why it's wrong here
BFD parameter negotiation allows different values; as long as the session is up, this is not the issue.
- ✗
The BGP session is using EBGP multihop, and the TTL is set to 2.
Why it's wrong here
If the routers are directly connected, EBGP multihop is not needed; TTL mismatch would prevent BGP from forming, not cause flapping.
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.
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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 BGP hold timer is set to 30 seconds on one router and 90 seconds on the other. — The BGP fall-over bfd command causes BGP to monitor the BFD session. If the BFD session is up but BGP is flapping, the issue is likely a BGP configuration problem, such as a mismatch in hold timer or update-source.
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
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.
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