- A
The VPC network ACL is blocking inbound traffic from on-premises
Why wrong: Possible but less likely than security group; NACLs are stateless.
- B
The direct connect virtual interface is configured for a different VLAN
Why wrong: Would affect all traffic, not just RDS.
- C
The RDS database security group is not allowing inbound traffic from the on-premises CIDR
Security groups are stateful and often the cause of such issues.
- D
The VPC route table does not have a route to the on-premises network
Why wrong: EC2 is reachable, so routing is fine.
Question 28 of 1,705
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 network engineer is troubleshooting connectivity issues between an on-premises network and a VPC over a Direct Connect private VIF. The on-premises network can reach EC2 instances in the VPC, but cannot reach an RDS database in the same VPC. The VPC has a single subnet. 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 RDS database security group is not allowing inbound traffic from the on-premises CIDR
Since the on-premises network can reach EC2 instances but not the RDS database, the issue is likely at the security group level, not the network ACL or routing. Security groups are stateful and act as a virtual firewall for instances; by default, they deny all inbound traffic unless explicitly allowed. The RDS database's security group must have an inbound rule permitting traffic from the on-premises CIDR range, and its absence would block the connection while leaving EC2 access unaffected.
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 VPC network ACL is blocking inbound traffic from on-premises
Why it's wrong here
Possible but less likely than security group; NACLs are stateless.
- ✗
The direct connect virtual interface is configured for a different VLAN
Why it's wrong here
Would affect all traffic, not just RDS.
- ✓
The RDS database security group is not allowing inbound traffic from the on-premises CIDR
Why this is correct
Security groups are stateful and often the cause of such issues.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The VPC route table does not have a route to the on-premises network
Why it's wrong here
EC2 is reachable, so routing is fine.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The ANS-C01 exam often tests the distinction between security groups and network ACLs, and the trap here is that candidates assume a subnet-level ACL (Option A) is the cause when the symptom is specific to one resource type, overlooking that security groups operate at the instance level and can selectively block traffic.
Detailed technical explanation
How to think about this question
Security groups are stateful, meaning that if you allow inbound traffic, the outbound return traffic is automatically permitted regardless of outbound rules. In contrast, network ACLs are stateless and require explicit rules for both inbound and outbound traffic. For RDS, security groups are often the first line of defense, and misconfigurations here are common because RDS instances are not directly exposed to the internet and rely solely on security group rules for access control.
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 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
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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: The RDS database security group is not allowing inbound traffic from the on-premises CIDR — Since the on-premises network can reach EC2 instances but not the RDS database, the issue is likely at the security group level, not the network ACL or routing. Security groups are stateful and act as a virtual firewall for instances; by default, they deny all inbound traffic unless explicitly allowed. The RDS database's security group must have an inbound rule permitting traffic from the on-premises CIDR range, and its absence would block the connection while leaving EC2 access unaffected.
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: "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?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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