The answer is that the route tables in VPC A point to a VPN gateway instead of the VPC peering connection. This is correct because VPC peering does not support transitive routing, meaning traffic cannot hop from one VPC through a VPN gateway or another VPC to reach a peered destination; the route must point directly to the peering ID. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the fundamental limitation that VPC peering is a one-to-one, non-transitive relationship, often appearing as a trap where engineers assume a VPN gateway can act as a hub for multiple peered VPCs. A common memory tip is to think of VPC peering as a direct point-to-point link—if the route table shows anything other than the peering connection (like a VPN gateway or internet gateway), the traffic will fail due to the transitive routing limitation. Remember: no hops, no hubs, just direct peering paths.
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. 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 issues between two VPCs that are peered. The VPCs are in the same region but different accounts. The engineer verifies that the route tables and security group rules are correctly configured. However, instances in VPC A cannot ping 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The route tables in VPC A point to a VPN gateway instead of the VPC peering connection
Option D is correct because VPC peering does not support transitive routing; if traffic goes through a VPN or another VPC, it will not work. Option A is wrong because security groups are stateful and return traffic is allowed. Option B is wrong because NACLs are stateless but can be checked; however, if correctly configured, they would not block. Option C is wrong because the VPC peering connection is established.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Network ACLs are not configured to allow inbound ICMP
Why it's wrong here
NACLs can be checked; if correct, they are not the issue.
✓
The route tables in VPC A point to a VPN gateway instead of the VPC peering connection
Why this is correct
Transitive routing is not supported; routes must point directly to the peering connection.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
Security groups are stateful and block return traffic
Why it's wrong here
Stateful security groups allow return traffic automatically.
✗
The VPC peering connection is in the 'failed' state
Why it's wrong here
If failed, no traffic would flow at all.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related ANS-C01 ACL questions on filtering logic and placement.
Network Management and Operations — This question tests Network Management and Operations — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The route tables in VPC A point to a VPN gateway instead of the VPC peering connection — Option D is correct because VPC peering does not support transitive routing; if traffic goes through a VPN or another VPC, it will not work. Option A is wrong because security groups are stateful and return traffic is allowed. Option B is wrong because NACLs are stateless but can be checked; however, if correctly configured, they would not block. Option C is wrong because the VPC peering connection is established.
What should I do if I get this ANS-C01 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related ANS-C01 ACL questions on filtering logic and placement.
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?
Standard ACLs match source addresses.
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