The answer is incorrect MTU size causing packet fragmentation and asymmetric routing causing stateful firewall drops. When the MTU is misconfigured, packets exceeding the path MTU are fragmented or dropped, leading to the intermittent drops you see even when the tunnel status shows UP. Asymmetric routing, where traffic takes different paths in each direction, causes stateful firewalls to drop return packets that don’t match an existing session, creating the same sporadic failures. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your ability to distinguish between causes that degrade performance versus those that kill the tunnel entirely—a common trap is confusing DPD or BGP timer issues with intermittent drops, which would instead reset the connection. Remember: if the tunnel is UP but traffic drops, think “MTU and asymmetry,” not timers.
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 network engineer is troubleshooting connectivity between an on-premises data center and a VPC over an AWS Site-to-Site VPN. The tunnel status shows UP, but traffic from on-premises to the VPC is intermittently dropped. Which TWO of the following are likely causes? (Choose 2.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Asymmetric routing across multiple tunnels
Incorrect MTU settings can cause fragmentation issues leading to drops, and asymmetric routing can cause stateful firewalls to drop return traffic. Dead Peer Detection (DPD) timeout would cause the tunnel to go down, not intermittent drops; BGP hold timer expiration would cause BGP session reset; IPsec SA lifetime mismatch would cause periodic rekey issues but not typical intermittent drops.
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.
✗
Dead Peer Detection (DPD) timeout configuration
Why it's wrong here
DPD timeout would bring the tunnel down, not cause intermittent drops.
✗
BGP hold timer expiration
Why it's wrong here
Hold timer expiration resets BGP session, not intermittent drops.
✓
Asymmetric routing across multiple tunnels
Why this is correct
Asymmetric routing can cause stateful devices to drop packets.
Related concept
Read the scenario before looking for a memorised answer.
✓
Incorrect MTU size causing packet fragmentation
Why this is correct
MTU mismatch leads to fragmentation and potential drops.
Related concept
Read the scenario before looking for a memorised answer.
✗
IPsec SA lifetime mismatch between peers
Why it's wrong here
SA mismatch causes rekey failures, not typical intermittent drops.
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: Asymmetric routing across multiple tunnels — Incorrect MTU settings can cause fragmentation issues leading to drops, and asymmetric routing can cause stateful firewalls to drop return traffic. Dead Peer Detection (DPD) timeout would cause the tunnel to go down, not intermittent drops; BGP hold timer expiration would cause BGP session reset; IPsec SA lifetime mismatch would cause periodic rekey issues but not typical intermittent drops.
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.
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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