- A
The VPCs have overlapping CIDR blocks.
Why wrong: Overlapping CIDR blocks would cause routing issues, but VPC-A and VPC-B have distinct CIDR ranges (10.0.0.0/16 and 10.1.0.0/16), so this is not the cause.
- B
Security groups are blocking ICMP traffic.
Why wrong: The scenario explicitly states that security groups and NACLs allow all traffic, so they are not blocking ICMP.
- C
The route tables are missing routes to the peering connection.
While the main route table contains the peering routes, the subnet's custom route table may lack these routes. This is a common misconfiguration that prevents traffic from being routed to the peered VPC.
- D
There is an intermediate VPC or on-premises network that routes traffic incorrectly due to the lack of transitive routing.
Why wrong: There is no intermediate VPC or on-premises network mentioned in the scenario, and VPC peering does not support transitive routing, but that is irrelevant here as only two VPCs are involved. The actual issue is more likely a subnet route table association problem.
VPC Peering No Transitive Routing: A Critical Limitation
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. A key principle to apply: vPC Peering. 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 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 route tables are missing routes to the peering connection.
The most likely cause is that the route tables containing the peering routes are not associated with the subnets where the EC2 instances are located. In AWS, each subnet must have an associated route table. If the main route table has the peering routes but the subnet's custom route table does not, the traffic will not be routed correctly. Since security groups and NACLs allow all traffic and the CIDRs are not overlapping, the only remaining possibility is a route table association issue.
Key principle: VPC Peering
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 VPCs have overlapping CIDR blocks.
Why it's wrong here
Overlapping CIDR blocks would cause routing issues, but VPC-A and VPC-B have distinct CIDR ranges (10.0.0.0/16 and 10.1.0.0/16), so this is not the cause.
- ✗
Security groups are blocking ICMP traffic.
Why it's wrong here
The scenario explicitly states that security groups and NACLs allow all traffic, so they are not blocking ICMP.
- ✓
The route tables are missing routes to the peering connection.
Why this is correct
While the main route table contains the peering routes, the subnet's custom route table may lack these routes. This is a common misconfiguration that prevents traffic from being routed to the peered VPC.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
VPC Peering
- ✗
There is an intermediate VPC or on-premises network that routes traffic incorrectly due to the lack of transitive routing.
Why it's wrong here
There is no intermediate VPC or on-premises network mentioned in the scenario, and VPC peering does not support transitive routing, but that is irrelevant here as only two VPCs are involved. The actual issue is more likely a subnet route table association problem.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap is that candidates often assume that simply adding a route to the VPC's main route table is sufficient. However, each subnet must have its own route table association. If the subnet's route table does not have the peering route, connectivity fails even if the main route table is correctly configured.
Trap categories for this question
Scenario analysis trap
The scenario explicitly states that security groups and NACLs allow all traffic, so they are not blocking ICMP.
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
- VPC Peering
- Route Table Association
- Subnet Routing
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
VPC Peering
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
Visual reference
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Implementation — This question tests Network Implementation — VPC Peering.
What is the correct answer to this question?
The correct answer is: The route tables are missing routes to the peering connection. — The most likely cause is that the route tables containing the peering routes are not associated with the subnets where the EC2 instances are located. In AWS, each subnet must have an associated route table. If the main route table has the peering routes but the subnet's custom route table does not, the traffic will not be routed correctly. Since security groups and NACLs allow all traffic and the CIDRs are not overlapping, the only remaining possibility is a route table association issue.
What should I do if I get this ANS-C01 question wrong?
Review vPC Peering, then practise related ANS-C01 questions on the same topic to reinforce the concept.
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?
VPC Peering
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Last reviewed: Jun 11, 2026
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