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Question 25 of 1,705

Network Implementation →hardMultiple ChoiceObjective-mapped

Why On-Premises Route Advertisement Is Critical for VPN

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 VPC with a CIDR of 10.0.0.0/16. They create a subnet 10.0.1.0/24 and launch an EC2 instance with a private IP 10.0.1.5. The instance needs to communicate with an on-premises server at 172.16.0.10 over a VPN connection. The VPN connection uses a Virtual Private Gateway. The VPC route table has a route 172.16.0.0/16 pointing to the VPG. The instance cannot reach the on-premises server. What is the most likely cause?

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.

A
The security group attached to the instance does not allow outbound traffic to 172.16.0.10.
Why wrong: Security groups are stateful; outbound traffic is allowed by default.
B
The instance's route table does not have a route to the internet gateway.
Why wrong: Internet gateway is not needed for VPN traffic; the route table already has a route to the VPG.
C
The network ACL for the subnet denies outbound traffic to the on-premises IP range.
Why wrong: Default NACL allows all outbound traffic; if custom, it would need to be verified.
D
The on-premises router is not advertising the 172.16.0.0/16 route to the Virtual Private Gateway.
Without the route advertisement, the VPG does not know how to reach the on-premises network.

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 on-premises router is not advertising the 172.16.0.0/16 route to the Virtual Private Gateway.

Option D is correct because the VPN connection requires the on-premises router to advertise the 172.16.0.0/16 route to the Virtual Private Gateway (VPG) via BGP (or static route configuration) for the VPG to forward traffic to the on-premises network. Without this route advertisement, the VPG will not have a path to 172.16.0.10, even though the VPC route table has a route pointing to the VPG. The instance's traffic reaches the VPG, but the VPG cannot forward it because it lacks the necessary routing information from the on-premises side.

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 security group attached to the instance does not allow outbound traffic to 172.16.0.10.
Why it's wrong here
Security groups are stateful; outbound traffic is allowed by default.
✗
The instance's route table does not have a route to the internet gateway.
Why it's wrong here
Internet gateway is not needed for VPN traffic; the route table already has a route to the VPG.
✗
The network ACL for the subnet denies outbound traffic to the on-premises IP range.
Why it's wrong here
Default NACL allows all outbound traffic; if custom, it would need to be verified.
✓
The on-premises router is not advertising the 172.16.0.0/16 route to the Virtual Private Gateway.
Why this is correct
Without the route advertisement, the VPG does not know how to reach the on-premises network.
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

The trap here is that candidates often assume that adding a route in the VPC route table pointing to the VPG is sufficient for connectivity, overlooking the requirement that the on-premises router must also advertise the destination network to the VPG via BGP or have a static route configured.

Detailed technical explanation

How to think about this question

In AWS VPN connections using a Virtual Private Gateway, BGP is typically used to exchange routes between the on-premises router and the VPG. The VPG will only forward traffic to the on-premises network for prefixes that are advertised via BGP or configured as static routes. If the on-premises router does not advertise the 172.16.0.0/16 prefix, the VPG will not install a corresponding route in its forwarding table, causing traffic to be dropped even though the VPC route table has a route to the VPG. This is a common misconfiguration where the VPC-side route is present but the BGP session or static route on the on-premises side is missing.

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.

Visual reference

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 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: The on-premises router is not advertising the 172.16.0.0/16 route to the Virtual Private Gateway. — Option D is correct because the VPN connection requires the on-premises router to advertise the 172.16.0.0/16 route to the Virtual Private Gateway (VPG) via BGP (or static route configuration) for the VPG to forward traffic to the on-premises network. Without this route advertisement, the VPG will not have a path to 172.16.0.10, even though the VPC route table has a route pointing to the VPG. The instance's traffic reaches the VPG, but the VPG cannot forward it because it lacks the necessary routing information from the on-premises side.

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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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.