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Network Implementation →easyMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the on-premises network is advertising a default route (0.0.0.0/0) via BGP, which overrides the VPC’s local route. This occurs because when a VPN BGP default route override is propagated into the VPC route tables, the more specific on-premises CIDR route is effectively superseded by the default route, causing traffic destined for the on-premises network to be sent back out the VPN gateway instead of being delivered locally to EC2 instances. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of BGP route propagation and the precedence of static versus dynamic routes in VPC route tables—a common trap is assuming a more specific static route will always win, but a propagated default can break connectivity if route propagation is enabled. Remember the memory tip: “Default drowns the local; check propagation before you panic.”

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 company is deploying a hybrid network architecture with an AWS Site-to-Site VPN connection between its on-premises network and a VPC. The on-premises network uses BGP to advertise routes to the VPN connection. After the VPN is established, the on-premises network cannot reach EC2 instances in the VPC. The VPC route table has a route for the on-premises CIDR block pointing to the VPN gateway. What is the most likely cause of this 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 1easymultiple choice

Open the full BGP breakdown →

A
The VPN connection is not using the correct pre-shared key.
Why wrong: Incorrect pre-shared key would prevent the VPN tunnel from establishing, but the VPN is already established.
B
The security group attached to the EC2 instances does not allow inbound traffic from the on-premises CIDR.
Why wrong: Security groups are stateful; if the EC2 instance initiates outbound traffic, the return traffic is allowed regardless of inbound rules.
C
The on-premises network is advertising a default route (0.0.0.0/0) via BGP, which is overriding the VPC's local route.
BGP route propagation can cause the VPC route table to learn a default route from on-premises, which may cause traffic destined for the VPC CIDR to be sent back to the VPN gateway instead of staying local.
D
The VPC's network ACLs are blocking inbound traffic from the on-premises network.
Why wrong: Network ACLs are stateless and would affect traffic in both directions; however, the issue is routing, not filtering.

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 network is advertising a default route (0.0.0.0/0) via BGP, which is overriding the VPC's local route.

When the on-premises network advertises a default route (0.0.0.0/0) via BGP over the VPN connection, the VPC propagates that route into its route tables (if route propagation is enabled). This default route can override the VPC's local route for the on-premises CIDR, causing traffic destined for the on-premises network to be sent back out the VPN gateway instead of being delivered locally, effectively breaking connectivity to EC2 instances.

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 VPN connection is not using the correct pre-shared key.
Why it's wrong here
Incorrect pre-shared key would prevent the VPN tunnel from establishing, but the VPN is already established.
✗
The security group attached to the EC2 instances does not allow inbound traffic from the on-premises CIDR.
Why it's wrong here
Security groups are stateful; if the EC2 instance initiates outbound traffic, the return traffic is allowed regardless of inbound rules.
✓
The on-premises network is advertising a default route (0.0.0.0/0) via BGP, which is overriding the VPC's local route.
Why this is correct
BGP route propagation can cause the VPC route table to learn a default route from on-premises, which may cause traffic destined for the VPC CIDR to be sent back to the VPN gateway instead of staying local.
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 VPC's network ACLs are blocking inbound traffic from the on-premises network.
Why it's wrong here
Network ACLs are stateless and would affect traffic in both directions; however, the issue is routing, not filtering.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates often focus on security group or NACL misconfigurations, but the real issue is a routing conflict caused by BGP advertising a default route that overrides the VPC's local route, a classic hybrid networking pitfall.

Detailed technical explanation

How to think about this question

In AWS, when a VPC receives a BGP route from a VPN connection with route propagation enabled, the propagated route (including 0.0.0.0/0) is added to the route table with a lower priority than a specific static route but can still override the local route if the local route is not more specific. The VPC's local route for the on-premises CIDR is a static route, but if the on-premises advertises a default route, it can cause asymmetric routing where traffic from the VPC to on-premises is sent via the VPN, but the return traffic from on-premises is sent directly to the VPC, leading to connectivity failure.

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.

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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 network is advertising a default route (0.0.0.0/0) via BGP, which is overriding the VPC's local route. — When the on-premises network advertises a default route (0.0.0.0/0) via BGP over the VPN connection, the VPC propagates that route into its route tables (if route propagation is enabled). This default route can override the VPC's local route for the on-premises CIDR, causing traffic destined for the on-premises network to be sent back out the VPN gateway instead of being delivered locally, effectively breaking connectivity to EC2 instances.

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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