- A
The VPC Peering connection does not support transitive routing through an intermediate VPC or on-premises network.
VPC Peering is non-transitive; if there is a VPN or another VPC in the path, traffic will be dropped.
- B
The VPC Peering connection requires an IAM role to be assumed for cross-account communication.
Why wrong: VPC Peering can work cross-account without IAM roles; only the route tables and security groups need configuration.
- C
The security group in VPC A does not allow inbound traffic from VPC B's CIDR.
Why wrong: Security groups are stateful; if outbound is allowed, return traffic is automatically allowed.
- D
The network ACL in VPC A does not allow return traffic from VPC B.
Why wrong: NACLs are stateless and must allow both inbound and outbound traffic; but the issue is likely not NACLs if security groups are configured.
Quick Answer
The answer is the VPC Peering connection does not support transitive routing through an intermediate VPC or on-premises network. This is the most likely cause because VPC Peering is a non-transitive, one-to-one relationship; if traffic must pass through a third VPC, a VPN gateway, or a Direct Connect connection to reach the destination, the peering link will drop the packets even if route tables and security groups are correctly configured. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this limitation is a classic trap—candidates often overlook that VPC Peering cannot forward traffic beyond its direct pair, unlike Transit Gateway which supports transitive routing. The exam tests your ability to distinguish between stateful security groups (which automatically allow return traffic) and the inherent architectural constraint of peering. Remember the mnemonic: “Peering is a point-to-point pipe, not a hub-and-spoke highway.”
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 security engineer is troubleshooting connectivity issues between two VPCs connected via a VPC Peering connection. The VPCs are in different accounts. The security groups in both VPCs allow traffic between the CIDRs. The route tables have the appropriate entries. However, instances in VPC A cannot communicate with instances in VPC B. 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 VPC Peering connection does not support transitive routing through an intermediate VPC or on-premises network.
Option B is correct because VPC Peering connections do not support transitive routing, so if there is an intermediate gateway, traffic will not flow. Option A is incorrect because security groups are stateful and allow return traffic automatically. Option C is incorrect because VPC Peering does not require an IAM role. Option D is incorrect because NACLs are stateless and return traffic must be allowed, but the problem states security groups allow traffic, not NACLs.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 VPC Peering connection does not support transitive routing through an intermediate VPC or on-premises network.
- ✗
The VPC Peering connection requires an IAM role to be assumed for cross-account communication.
Why it's wrong here
VPC Peering can work cross-account without IAM roles; only the route tables and security groups need configuration.
- ✗
The security group in VPC A does not allow inbound traffic from VPC B's CIDR.
Why it's wrong here
Security groups are stateful; if outbound is allowed, return traffic is automatically allowed.
- ✗
The network ACL in VPC A does not allow return traffic from VPC B.
Why it's wrong here
NACLs are stateless and must allow both inbound and outbound traffic; but the issue is likely not NACLs if security groups are configured.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The VPC Peering connection does not support transitive routing through an intermediate VPC or on-premises network. — Option B is correct because VPC Peering connections do not support transitive routing, so if there is an intermediate gateway, traffic will not flow. Option A is incorrect because security groups are stateful and allow return traffic automatically. Option C is incorrect because VPC Peering does not require an IAM role. Option D is incorrect because NACLs are stateless and return traffic must be allowed, but the problem states security groups allow traffic, not NACLs.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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Same concept, more angles
2 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 connected via a VPC peering connection. Security groups and NACLs are configured correctly. The engineer verifies that the route tables have the necessary entries. However, traffic from an EC2 instance in VPC A to an RDS instance in VPC B fails. Which TWO additional checks should be performed? (Choose two.)
medium- ✓ A.Check that the security group attached to the RDS instance allows inbound traffic from the CIDR block of VPC A.
- B.Verify that the network ACLs in both VPCs have appropriate inbound and outbound rules for the traffic.
- C.Confirm that the VPCs have an Internet Gateway attached.
- D.Check that the EC2 instance has an IAM role that allows it to communicate with RDS.
- ✓ E.Ensure that the VPC peering connection is in the 'active' state.
Why A: Option B is correct because a VPC peering connection must accept the request; if it's in 'pending-acceptance' or 'rejected' state, traffic will not flow. Option D is correct because security group rules must allow inbound traffic from the peer VPC's CIDR; misconfigured rules can block traffic. Option A is wrong because NACLs are stateless and if correctly configured should allow traffic; but the issue is likely elsewhere. Option C is wrong because IAM roles are not required for VPC peering. Option E is wrong because the issue is not about Internet Gateway; it's about VPC peering.
Variation 2. A network engineer is troubleshooting connectivity issues between two VPCs that are peered. The VPCs are in the same region and the peering connection is in the 'active' state. Security groups in both VPCs allow all traffic. However, instances in VPC A cannot reach instances in VPC B. What is the most likely cause?
medium- A.Security groups are blocking traffic between the VPCs
- B.The VPC peering connection is in the 'pending-acceptance' state
- ✓ C.Route tables in one or both VPCs do not have routes pointing to the peering connection
- D.Network ACLs are blocking traffic between the VPCs
Why C: VPC peering requires route table entries in both VPCs to direct traffic to the peering connection. Option A is correct because missing routes are a common issue. Option B is wrong because the peering is active. Option C is wrong because security groups allow all. Option D is wrong because NACLs are not mentioned as blocking.
Last reviewed: Jun 20, 2026
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