The answer is the VPC peering transitive routing limitation. This is the most likely cause because VPC peering does not support transitive routing, meaning that if VPC A is peered with VPC B and also connected to an on-premises network via a Site-to-Site VPN, traffic from VPC B cannot hop through VPC A to reach the on-premises network. Each VPC must have its own direct VPN connection or use a transit gateway to enable transitive routing between multiple networks. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept frequently appears as a trick question where a peered VPC cannot reach an external network through another VPC, testing your understanding that VPC peering is a one-to-one, non-transitive relationship. A common trap is assuming that a VPN connection in one VPC automatically extends to all peered VPCs. Remember the mnemonic: “Peering is point-to-point, not a transit point.”
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 company has a VPC with CIDR 10.0.0.0/16. They have an on-premises network with CIDR 172.16.0.0/12 connected via AWS Site-to-Site VPN. The company also has a second VPC (VPC B) with CIDR 10.1.0.0/16 peered with the first VPC. They notice that instances in VPC B cannot reach the on-premises network. 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: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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
✓
VPC peering does not support transitive routing.
VPC peering does not support transitive routing. This means that if VPC A is peered with VPC B and also connected to an on-premises network via VPN, traffic from VPC B cannot use VPC A as a transit point to reach the on-premises network. Each VPC must have its own direct connection to the on-premises network, or a transit gateway must be used to enable transitive routing.
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.
✗
Route propagation is disabled in VPC B.
Why it's wrong here
Route propagation in VPC B would not help because routes from VPN are not propagated to peered VPCs.
✗
The VPN connection is not compatible with VPC peering.
Transitive routing is not supported; on-premises cannot reach VPC B through VPC peering.
Clue confirmation
The clue words "first", "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 VPC peering works like a router or a hub-and-spoke model, not realizing that AWS explicitly disables transitive routing across VPC peering connections to prevent unintended network loops and complexity.
Detailed technical explanation
How to think about this question
Under the hood, VPC peering is a one-to-one, non-transitive relationship defined by the AWS VPC peering specification. Traffic between peered VPCs is routed directly using the VPC route tables, but there is no mechanism to forward traffic from one peering connection to another or to a VPNgateway. In a real-world scenario, to enable connectivity between multiple VPCs and an on-premises network, you would use a transit gateway, which acts as a central hub and supports transitive routing across attachments.
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 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: VPC peering does not support transitive routing. — VPC peering does not support transitive routing. This means that if VPC A is peered with VPC B and also connected to an on-premises network via VPN, traffic from VPC B cannot use VPC A as a transit point to reach the on-premises network. Each VPC must have its own direct connection to the on-premises network, or a transit gateway must be used to enable transitive routing.
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: "first", "most likely". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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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