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Question 1,087 of 1,705

Network Design →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the web tier instances are still using the private IP address of the Network Load Balancer instead of the VPC endpoint DNS name or its private IP addresses. This is the most likely cause because after migrating from VPC peering to AWS PrivateLink, the routing path changes entirely: the VPC endpoint provides interface endpoints with IP addresses from Account A’s subnet, while the NLB’s private IP remains in Account B’s subnet. Since the VPC peering connection is removed, traffic directed to the NLB’s original IP has no valid route and is dropped, even if security groups and NACLs permit it. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how PrivateLink decouples service consumers from the underlying resource IPs, and it’s a common trap where candidates assume the endpoint simply extends the old peering path. Remember the memory tip: “DNS or die” — after a PrivateLink migration, always update client configurations to use the endpoint’s DNS name, not the old resource IP.

ANS-C01 Network Design Practice Question

This ANS-C01 practice question tests your understanding of network design. 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 large e-commerce company operates a multi-tier application across multiple AWS accounts. The web tier is in a VPC (10.0.0.0/16) in Account A, and the application tier is in a separate VPC (10.1.0.0/16) in Account B. Both VPCs are connected via a VPC peering connection. The application tier uses an NLB to distribute traffic to EC2 instances in private subnets. The web tier sends traffic to the NLB's private IP address. Recently, the company migrated the application tier to use AWS PrivateLink instead of the VPC peering connection, creating a VPC endpoint service in Account B and an interface VPC endpoint in Account A. After the migration, the web tier cannot connect to the application tier. The security groups and NACLs allow the traffic. Which of the following is the MOST likely cause of the connectivity 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.

Question 1hardmultiple choice

Review the full subnetting walkthrough →

A
The web tier instances are using the private IP address of the Network Load Balancer instead of the VPC endpoint DNS name or private IP addresses.
Correct; after migrating to PrivateLink, the web tier should use the VPC endpoint's DNS name or its assigned private IPs, not the NLB's IP directly.
B
The route tables in Account A's VPC do not have a route to the VPC endpoint service in Account B.
Why wrong: Incorrect; routes to VPC endpoints are managed automatically by the endpoint, not manually added.
C
The VPC endpoint in Account A requires an Internet Gateway (IGW) to route traffic to the VPC endpoint service in Account B.
Why wrong: Incorrect; PrivateLink uses private IPs and does not require IGW.
D
The Network Load Balancer in Account B is not configured to support TCP traffic.
Why wrong: Incorrect; NLB supports TCP by default.

Full breakdown with real-world context →

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 web tier instances are using the private IP address of the Network Load Balancer instead of the VPC endpoint DNS name or private IP addresses.

After migrating from VPC peering to AWS PrivateLink, the web tier instances must use the VPC endpoint's DNS name or its private IP addresses to connect to the application tier. The VPC endpoint provides a different set of IP addresses (from the subnet in Account A) than the NLB's private IP, which remains in Account B. Since the web tier continues to send traffic to the NLB's private IP, the packets are not routed through the VPC endpoint, causing connectivity failure because the VPC peering connection is no longer available.

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 web tier instances are using the private IP address of the Network Load Balancer instead of the VPC endpoint DNS name or private IP addresses.
Why this is correct
Correct; after migrating to PrivateLink, the web tier should use the VPC endpoint's DNS name or its assigned private IPs, not the NLB's IP directly.
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 route tables in Account A's VPC do not have a route to the VPC endpoint service in Account B.
Why it's wrong here
Incorrect; routes to VPC endpoints are managed automatically by the endpoint, not manually added.
✗
The VPC endpoint in Account A requires an Internet Gateway (IGW) to route traffic to the VPC endpoint service in Account B.
Why it's wrong here
Incorrect; PrivateLink uses private IPs and does not require IGW.
✗
The Network Load Balancer in Account B is not configured to support TCP traffic.
Why it's wrong here
Incorrect; NLB supports TCP by default.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates assume PrivateLink transparently forwards traffic to the original NLB IP, but in reality the consumer must use the endpoint's DNS name or IP, not the service's original IP.

Detailed technical explanation

How to think about this question

When using AWS PrivateLink, the interface VPC endpoint creates an elastic network interface (ENI) in the consumer VPC (Account A) with a private IP from the subnet. The consumer must direct traffic to this ENI's IP or the endpoint-specific DNS name (which resolves to these IPs), not to the NLB's original private IP. The NLB's private IP is only reachable within the provider VPC (Account B) or via peering; after peering is removed, that IP is unreachable from Account A.

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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.

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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FAQ

Questions learners often ask

What does this ANS-C01 question test?

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 web tier instances are using the private IP address of the Network Load Balancer instead of the VPC endpoint DNS name or private IP addresses. — After migrating from VPC peering to AWS PrivateLink, the web tier instances must use the VPC endpoint's DNS name or its private IP addresses to connect to the application tier. The VPC endpoint provides a different set of IP addresses (from the subnet in Account A) than the NLB's private IP, which remains in Account B. Since the web tier continues to send traffic to the NLB's private IP, the packets are not routed through the VPC endpoint, causing connectivity failure because the VPC peering connection is no longer available.

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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