The answer is an MTU mismatch between the customer router and the AWS Direct Connect endpoint. When the maximum transmission unit differs, large packets—such as BGP keepalives or update messages encapsulated in larger frames—are silently dropped, causing the BGP hold timer to expire and the session to flap every few minutes. This scenario tests your understanding of how Layer 2 and Layer 3 interact on a private VIF, a common trap on the AWS Certified Advanced Networking Specialty ANS-C01 exam where candidates often overlook MTU as a cause of intermittent flapping, instead blaming BFD or ASN misconfigurations. Remember that BGP sessions can establish with a mismatched MTU but fail under load, so the key symptom is periodic resets rather than a failure to peer. Memory tip: “MTU drop makes BGP stop” — if the session flaps but establishes, check the packet size, not the config.
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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.
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 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
Option A is correct because if the maximum transmission unit (MTU) is mismatched, large packets may be dropped, causing BGP to reset. Option B is wrong because a single mismatch in ASN would prevent the session from establishing. Option C is wrong because BFD does not cause flapping; it detects failures faster. Option D is wrong because community strings are not used in BGP peering.
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
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
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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.
Identify which ANS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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 — Option A is correct because if the maximum transmission unit (MTU) is mismatched, large packets may be dropped, causing BGP to reset. Option B is wrong because a single mismatch in ASN would prevent the session from establishing. Option C is wrong because BFD does not cause flapping; it detects failures faster. Option D is wrong because community strings are not used in BGP peering.
What should I do if I get this ANS-C01 question wrong?
Identify which ANS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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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Question Discussion
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