- A
The hold timer values are mismatched between the two routers.
Correct because a hold timer mismatch causes the router with the smaller hold time to expire, leading to session flaps.
- B
The MTU on the link is mismatched, causing BGP packets to be fragmented.
Why wrong: Incorrect because MTU mismatch typically causes packet drops or failure to establish the session, not hold timer expiry.
- C
The TTL for eBGP is set to 1, and the routers are not directly connected.
Why wrong: Incorrect because the scenario states they are directly connected, so TTL=1 is fine; TTL issues would prevent session establishment, not cause hold timer expiry.
- D
The BGP update timer is set too high, causing delays in sending updates.
Why wrong: Incorrect because update timers do not affect keepalive messages; hold timer expiry is related to keepalive frequency, not update frequency.
Quick Answer
The answer is a mismatch in BGP hold timer values between the two routers. This is correct because the BGP notification code 4/0 (Hold Timer Expired) explicitly indicates that the router stopped receiving keepalive or update messages within its configured hold time. When two eBGP peers have different hold timers, the router with the shorter timer will time out first, tearing down the session and causing the flapping cycle you observe—even though IP connectivity and the rest of the configuration are fine. On the ENCOR 350-401 exam, this scenario tests your understanding of BGP session establishment and timer negotiation; a common trap is to blame IP reachability or ACLs when the logs clearly point to a timer expiration. Remember that BGP uses the smaller of the two proposed hold timers during negotiation, but if one router is manually configured with a value lower than the other’s minimum, the session will flap. Memory tip: “Hold timer mismatch = shorter timer wins the race to reset.”
350-401 BGP Practice Question
This 350-401 practice question tests your understanding of bgp. 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 BGP peering between two routers in different autonomous systems. The peering is established over a directly connected Ethernet link. The engineer notices that the BGP session is flapping every few minutes. The configuration on both routers appears correct, and the IP connectivity is stable. The engineer checks the BGP logs and sees messages like 'BGP-3-NOTIFICATION: sent 4/0 (Hold Timer Expired)'. What is the most likely cause of this issue?
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 hold timer values are mismatched between the two routers.
The BGP notification 'Hold Timer Expired' indicates that the router did not receive keepalive or update messages within the configured hold time. When hold timers are mismatched between two eBGP peers, the router with the shorter hold timer will expire first, causing the session to reset. Since the IP connectivity is stable and the configuration appears correct, a mismatch in hold timer values is the most direct cause of the flapping session.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 hold timer values are mismatched between the two routers.
Why this is correct
Correct because a hold timer mismatch causes the router with the smaller hold time to expire, leading to session flaps.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The MTU on the link is mismatched, causing BGP packets to be fragmented.
Why it's wrong here
Incorrect because MTU mismatch typically causes packet drops or failure to establish the session, not hold timer expiry.
- ✗
The TTL for eBGP is set to 1, and the routers are not directly connected.
Why it's wrong here
Incorrect because the scenario states they are directly connected, so TTL=1 is fine; TTL issues would prevent session establishment, not cause hold timer expiry.
- ✗
The BGP update timer is set too high, causing delays in sending updates.
Why it's wrong here
Incorrect because update timers do not affect keepalive messages; hold timer expiry is related to keepalive frequency, not update frequency.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between hold timer mismatch (which causes a notification and session reset) versus other common BGP issues like MTU or TTL, where candidates may incorrectly assume that any BGP flapping is due to connectivity or packet loss rather than timer negotiation.
Trap categories for this question
Scenario analysis trap
Incorrect because the scenario states they are directly connected, so TTL=1 is fine; TTL issues would prevent session establishment, not cause hold timer expiry.
Detailed technical explanation
How to think about this question
BGP uses a hold timer negotiated during the OPEN message exchange; each peer proposes a hold time, and the session uses the smaller of the two values. If one router is configured with a hold time of 30 seconds and the other with 180 seconds, the router with 30 seconds will expect keepalives every 10 seconds (one-third of hold time). If the other router sends keepalives every 60 seconds (based on its 180-second hold), the first router will time out after 30 seconds, causing the notification. In real-world scenarios, this often occurs after a configuration change or when using default values from different IOS versions.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
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
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 350-401 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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BGP — study guide chapter
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FAQ
Questions learners often ask
What does this 350-401 question test?
BGP — This question tests BGP — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The hold timer values are mismatched between the two routers. — The BGP notification 'Hold Timer Expired' indicates that the router did not receive keepalive or update messages within the configured hold time. When hold timers are mismatched between two eBGP peers, the router with the shorter hold timer will expire first, causing the session to reset. Since the IP connectivity is stable and the configuration appears correct, a mismatch in hold timer values is the most direct cause of the flapping session.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This 350-401 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 350-401 exam.
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