- A
The route tables in both VPCs do not have the route propagation enabled.
Why wrong: Route propagation is for VPN connections, not VPC peering.
- B
The VPC CIDR blocks overlap.
Why wrong: VPC peering requires non-overlapping CIDR blocks for successful communication.
- C
There is a VPN connection attached to both VPCs that creates a transitive routing issue.
VPC peering does not support transitive routing; if there is another connection that could create a transitive route, it may cause unexpected behavior.
- D
The security group in VPC A does not allow inbound ICMP from VPC B.
Why wrong: Security groups are stateful; if outbound ICMP is allowed, return traffic is automatically allowed.
- E
The network ACLs are not configured correctly for return traffic.
Why wrong: NACLs are stateless, but the engineer verified they are correct.
Quick Answer
The answer is a VPN connection attached to both VPCs creating a transitive routing issue. This is the correct choice because VPC peering explicitly does not support transitive routing—traffic must flow directly between the peered VPCs and cannot hop through an intermediate network. When both VPCs have a VPN connection to the same on-premises network, that VPN path can override the direct peering route, causing ICMP pings to fail even when route tables and security groups are correctly configured. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of VPC peering’s fundamental limitation: it is a one-to-one, non-transitive relationship. A common trap is assuming that because routes and security groups look correct, the issue must be elsewhere, but the hidden VPN route silently steals traffic. Memory tip: “Peering is direct, not a transit—VPNs can hijack the path in an instant.”
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 networking engineer is troubleshooting connectivity issues between two VPCs that are peered using a VPC peering connection. The VPCs are in different AWS accounts. The engineer has verified that the route tables are correct and the security groups allow traffic. However, ICMP ping fails from an instance in VPC A to an instance in VPC B. What is a likely cause?
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
There is a VPN connection attached to both VPCs that creates a transitive routing issue.
Option C is correct because a VPN connection attached to both VPCs can introduce transitive routing, which is not supported by VPC peering. VPC peering does not allow transitive routing; traffic must flow directly between the peered VPCs. If both VPCs have a VPN connection to the same on-premises network or another network, the VPN can create a path that interferes with the direct peering path, causing ICMP pings to fail even when route tables and security groups are correctly configured.
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 in both VPCs do not have the route propagation enabled.
Why it's wrong here
Route propagation is for VPN connections, not VPC peering.
- ✗
The VPC CIDR blocks overlap.
Why it's wrong here
VPC peering requires non-overlapping CIDR blocks for successful communication.
- ✓
There is a VPN connection attached to both VPCs that creates a transitive routing issue.
Why this is correct
VPC peering does not support transitive routing; if there is another connection that could create a transitive route, it may cause unexpected behavior.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The security group in VPC A does not allow inbound ICMP from VPC B.
Why it's wrong here
Security groups are stateful; if outbound ICMP is allowed, return traffic is automatically allowed.
- ✗
The network ACLs are not configured correctly for return traffic.
Why it's wrong here
NACLs are stateless, but the engineer verified they are correct.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the concept that VPC peering does not support transitive routing, and candidates may overlook how a VPN connection can inadvertently create a transitive path that breaks direct peering traffic.
Detailed technical explanation
How to think about this question
VPC peering is a non-transitive, one-to-one connection; traffic cannot hop from VPC A to VPC B to VPC C via peering. When a VPN connection is attached to both VPCs, the VPN can create a transitive path (e.g., VPC A -> VPN -> on-premises -> VPN -> VPC B), which may cause asymmetric routing or blackholing if the VPC peering route is also present. This is a common misconfiguration in hybrid cloud architectures where multiple VPCs connect to the same on-premises network via VPN.
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
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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: There is a VPN connection attached to both VPCs that creates a transitive routing issue. — Option C is correct because a VPN connection attached to both VPCs can introduce transitive routing, which is not supported by VPC peering. VPC peering does not allow transitive routing; traffic must flow directly between the peered VPCs. If both VPCs have a VPN connection to the same on-premises network or another network, the VPN can create a path that interferes with the direct peering path, causing ICMP pings to fail even when route tables and security groups are correctly configured.
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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Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more ways this is tested on ANS-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network engineer is troubleshooting connectivity between two VPCs that are peered. The route tables are correct, and security groups allow traffic. However, ICMP ping fails. What is the most likely cause?
hard- A.The route tables are not propagated
- ✓ B.VPC peering does not support ICMP
- C.The VPCs are in different regions
- D.The security groups are blocking ICMP
Why B: VPC peering does not support ICMP; it only supports TCP and UDP traffic.
Last reviewed: Jun 30, 2026
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.
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