The answer is an IPsec encryption or integrity algorithm mismatch between the AWS VPN endpoint and the on-premises device for Tunnel2. This is correct because AWS Site-to-Site VPN configures each tunnel independently with its own pre-shared key and IPsec security parameters; a mismatch in algorithms, such as AES-128 versus AES-256 or SHA-1 versus SHA-256, will cause the IKE negotiation to fail for that specific tunnel while leaving the other tunnel operational. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of per-tunnel configuration and the fact that a single tunnel down—not both—points to a misconfiguration on that tunnel rather than a broader connectivity or routing issue. A common trap is assuming both tunnels share identical settings, but AWS explicitly separates them for redundancy. Memory tip: "One tunnel down, one up? Check the crypto cup"—meaning inspect the IPsec algorithms and keys for the failed tunnel only.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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.
Exhibit
Refer to the exhibit.
```
Tunnel1:
State: UP
Last Status Change: 2024-03-15 10:23:45 UTC
Details: Tunnel is in UP state with BGP established.
Tunnel2:
State: DOWN
Last Status Change: 2024-03-15 10:25:12 UTC
Details: Tunnel is in DOWN state due to phase 2 negotiation failure.
```
A network engineer has configured an AWS Site-to-Site VPN connection between a VPC and an on-premises network. The engineer checks the VPN status and sees the output above. What is the MOST likely cause of Tunnel2 being down?
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.
Refer to the exhibit.
```
Tunnel1:
State: UP
Last Status Change: 2024-03-15 10:23:45 UTC
Details: Tunnel is in UP state with BGP established.
Tunnel2:
State: DOWN
Last Status Change: 2024-03-15 10:25:12 UTC
Details: Tunnel is in DOWN state due to phase 2 negotiation failure.
```
A
The BGP ASN on the on-premises device is misconfigured.
Why wrong: BGP issues would not cause Phase 2 failure.
B
The route tables in the VPC do not have a route to the on-premises network.
Why wrong: Routing does not affect tunnel status.
C
The IKE pre-shared key used for Tunnel2 is incorrect.
Why wrong: Incorrect PSK would cause Phase 1 failure, not Phase 2.
D
The IPsec encryption or integrity algorithms do not match between the AWS VPN endpoint and the on-premises device.
Mismatched IPsec parameters cause Phase 2 negotiation failure.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The IPsec encryption or integrity algorithms do not match between the AWS VPN endpoint and the on-premises device.
Tunnel2 is down while Tunnel1 is up, indicating a per-tunnel configuration mismatch. The most common cause is a mismatch in IPsec security parameters (encryption, integrity, or DH group) between the AWS VPN endpoint and the on-premises device for that specific tunnel. AWS uses separate pre-shared keys and IPsec settings per tunnel, so a mismatch in algorithms would affect only the misconfigured tunnel.
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 ASN on the on-premises device is misconfigured.
The route tables in the VPC do not have a route to the on-premises network.
Why it's wrong here
Routing does not affect tunnel status.
✗
The IKE pre-shared key used for Tunnel2 is incorrect.
Why it's wrong here
Incorrect PSK would cause Phase 1 failure, not Phase 2.
✓
The IPsec encryption or integrity algorithms do not match between the AWS VPN endpoint and the on-premises device.
Why this is correct
Mismatched IPsec parameters cause Phase 2 negotiation failure.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the distinction between IKE phase 1 (pre-shared key, authentication) and IPsec phase 2 (encryption/integrity algorithms); candidates mistakenly attribute a single tunnel failure to a pre-shared key mismatch, but AWS assigns unique keys per tunnel, so a key error would affect only that tunnel—making algorithm mismatch the more subtle and likely cause when only one tunnel is down.
Detailed technical explanation
How to think about this question
AWS Site-to-Site VPN creates two tunnels for high availability, each with independent IPsec security associations (SAs). The IKE negotiation for each tunnel uses its own pre-shared key and can be configured with different encryption/integrity algorithms (e.g., AES256, SHA256, DH group 14). A mismatch in IPsec transform sets—such as using AES128 on one side and AES256 on the other—will cause phase 2 to fail, leaving the tunnel in a 'DOWN' state while the other tunnel remains operational.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The IPsec encryption or integrity algorithms do not match between the AWS VPN endpoint and the on-premises device. — Tunnel2 is down while Tunnel1 is up, indicating a per-tunnel configuration mismatch. The most common cause is a mismatch in IPsec security parameters (encryption, integrity, or DH group) between the AWS VPN endpoint and the on-premises device for that specific tunnel. AWS uses separate pre-shared keys and IPsec settings per tunnel, so a mismatch in algorithms would affect only the misconfigured tunnel.
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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