- A
The BGP community string is not set correctly
Why wrong: Communities affect routing policies, not session stability.
- B
MTU mismatch between the customer router and the AWS Direct Connect endpoint
Packet drops cause BGP session instability.
- C
Incorrect BGP ASN configuration on the virtual interface
Why wrong: Would prevent BGP from establishing.
- D
Bidirectional Forwarding Detection (BFD) is not enabled
Why wrong: BFD speeds up failure detection, does not cause flapping.
Question 1,309 of 1,705
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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 company has a Direct Connect connection with a private VIF to a VPC. They notice that the BGP session is flapping every few minutes. The network team confirms that the customer router and AWS router are configured correctly. What is the MOST likely cause of the BGP flapping?
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
✓
MTU mismatch between the customer router and the AWS Direct Connect endpoint
B is correct because an MTU mismatch between the customer router and the AWS Direct Connect endpoint can cause BGP session flapping. When the MTU is mismatched, larger BGP keepalive or update packets may be fragmented or dropped, leading to hold timer expirations and session resets. This occurs even if both routers are configured correctly for BGP, as the issue lies at the Layer 2/3 boundary rather than the BGP configuration itself.
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 BGP community string is not set correctly
Why it's wrong here
Communities affect routing policies, not session stability.
- ✓
MTU mismatch between the customer router and the AWS Direct Connect endpoint
Why this is correct
Packet drops cause BGP session instability.
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.
- ✗
Incorrect BGP ASN configuration on the virtual interface
Why it's wrong here
Would prevent BGP from establishing.
- ✗
Bidirectional Forwarding Detection (BFD) is not enabled
Why it's wrong here
BFD speeds up failure detection, does not cause flapping.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The ANS-C01 exam often tests the misconception that BGP flapping is always caused by BGP configuration errors (ASN, communities) or missing features like BFD, when in reality Layer 2/3 issues like MTU mismatch are a common root cause in Direct Connect scenarios.
Detailed technical explanation
How to think about this question
Under the hood, Direct Connect uses 802.1Q VLAN tagging and typically expects an MTU of 1500 bytes (or 1522 with VLAN tags) on the customer side. If the customer router is configured with a larger MTU (e.g., 9000 jumbo frames) without proper path MTU discovery or fragmentation handling, BGP packets exceeding the AWS endpoint's MTU are silently dropped. This triggers repeated hold timer expirations, causing the BGP session to flap every few minutes as the routers attempt to re-establish the session.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: MTU mismatch between the customer router and the AWS Direct Connect endpoint — B is correct because an MTU mismatch between the customer router and the AWS Direct Connect endpoint can cause BGP session flapping. When the MTU is mismatched, larger BGP keepalive or update packets may be fragmented or dropped, leading to hold timer expirations and session resets. This occurs even if both routers are configured correctly for BGP, as the issue lies at the Layer 2/3 boundary rather than the BGP configuration itself.
What should I do if I get this ANS-C01 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: Jul 4, 2026
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services 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 ANS-C01 exam.
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