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 network engineer is troubleshooting connectivity between two VPCs (VPC-A and VPC-B) connected via a VPC peering connection. Both VPCs have CIDR blocks: VPC-A = 10.0.0.0/16, VPC-B = 10.1.0.0/16. An EC2 instance in VPC-A (10.0.1.10) cannot ping an EC2 instance in VPC-B (10.1.1.10). Security groups and NACLs allow all traffic. The route tables are configured as follows: In VPC-A, a route to 10.1.0.0/16 via the peering connection. In VPC-B, a route to 10.0.0.0/16 via the peering connection. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
There is an intermediate VPC or on-premises network that routes traffic incorrectly due to the lack of transitive routing.
VPC peering connections do not support transitive routing. Even though the route tables in VPC-A and VPC-B correctly point to each other via the peering connection, if there is an intermediate VPC or on-premises network involved in the path, traffic cannot be forwarded through that intermediate point. The ping fails because the peering connection is a direct, one-to-one link and does not allow routing through a third network.
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 route tables are missing routes to the peering connection.
Why it's wrong here
Routes are present.
✓
There is an intermediate VPC or on-premises network that routes traffic incorrectly due to the lack of transitive routing.
Why this is correct
VPC peering does not allow transitive routing; any intermediate hop would break connectivity.
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 assume VPC peering supports transitive routing like a traditional router, but AWS explicitly disallows it, so even with correct routes, traffic cannot traverse an intermediate VPC.
Detailed technical explanation
How to think about this question
In AWS, VPC peering connections are non-transitive, meaning that if VPC-A is peered with VPC-B and VPC-B is peered with VPC-C, VPC-A cannot communicate with VPC-C through VPC-B. This behavior is defined by the AWS VPC peering model, which requires explicit peering and route table entries for each pair. A common real-world scenario is when a central VPC (e.g., a shared services VPC) is used to connect multiple VPCs, but traffic must be routed through a transit gateway or VPN instead of relying on peering transitivity.
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 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: There is an intermediate VPC or on-premises network that routes traffic incorrectly due to the lack of transitive routing. — VPC peering connections do not support transitive routing. Even though the route tables in VPC-A and VPC-B correctly point to each other via the peering connection, if there is an intermediate VPC or on-premises network involved in the path, traffic cannot be forwarded through that intermediate point. The ping fails because the peering connection is a direct, one-to-one link and does not allow routing through a third network.
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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Question Discussion
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