The correct answer is to check the VPC route tables to ensure routes to the on-premises network point to the virtual private gateway. Even when a Site-to-Site VPN tunnel is up and BGP peering is established, traffic from on-premises to AWS will fail if the VPC lacks a route directing return traffic for the on-premises CIDR block to the virtual private gateway (VGW). Without this explicit route, the VPC has no path for return packets, causing them to be dropped or blackholed, which breaks connectivity despite the tunnel being active. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how route tables govern traffic flow in hybrid networks—a common trap is assuming a healthy VPN tunnel guarantees bidirectional traffic. Remember the memory tip: “Tunnel up, route down” means the link is alive, but without a VPC route to the VGW, your traffic is dead on arrival.
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. 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 uses AWS Site-to-Site VPN to connect its on-premises network to AWS. The VPN connection is established, but traffic from on-premises to AWS is not working. The on-premises network team confirms that the on-premises firewall is allowing traffic to the VPC CIDR. What should the network engineer check in AWS to resolve the issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check the VPC route tables to ensure routes to the on-premises network point to the virtual private gateway.
Option D is correct because even if the VPN tunnel is up and BGP is peering, traffic will not flow unless the VPC route tables have a route pointing to the virtual private gateway (VGW) for the on-premises CIDR. Without this route, the VPC has no path to forward return traffic back to the on-premises network, causing asymmetric routing or blackholing.
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.
Review the customer gateway configuration for incorrect BGP settings.
Why it's wrong here
The customer gateway is on-premises; the question asks what to check in AWS.
✗
Ensure the virtual private gateway is attached to the correct VPC.
Why it's wrong here
If the VPN is established, the virtual private gateway is attached.
✓
Check the VPC route tables to ensure routes to the on-premises network point to the virtual private gateway.
Why this is correct
Missing routes in the VPC route table would prevent inbound traffic from reaching instances.
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 assume a tunnel status of UP (Option A) guarantees traffic flow, but AWS explicitly separates tunnel health from routing configuration, and the exam tests this distinction by requiring you to check the VPC route tables for the correct target.
Detailed technical explanation
How to think about this question
In AWS Site-to-Site VPN, the VPC route tables must contain a static route (or propagated route from BGP) with the destination set to the on-premises CIDR and the target set to the VGW (vgw-xxx). Without this route, the VPC’s subnet routing table has no entry for the on-premises network, so any return traffic from instances in the VPC is dropped. This is a common misconfiguration when using static routing or when route propagation is not enabled on the route table.
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 Management and Operations — This question tests Network Management and Operations — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Check the VPC route tables to ensure routes to the on-premises network point to the virtual private gateway. — Option D is correct because even if the VPN tunnel is up and BGP is peering, traffic will not flow unless the VPC route tables have a route pointing to the virtual private gateway (VGW) for the on-premises CIDR. Without this route, the VPC has no path to forward return traffic back to the on-premises network, causing asymmetric routing or blackholing.
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.
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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