The answer is the RDS database security group not allowing inbound traffic from the on-premises CIDR. This is correct because security groups act as a virtual firewall at the instance level, and while the EC2 instance’s security group permits the on-premises traffic, the RDS security group is a separate, independent filter that must explicitly allow the same source. Since the VPC route table and Direct Connect private VIF are working for EC2, the routing path is functional, isolating the issue to the database’s stateful security group rather than a stateless NACL or routing misconfiguration. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your ability to distinguish between network-layer routing and host-level firewall rules, a common trap where engineers overlook that RDS security groups default to denying all inbound traffic. A quick memory tip: EC2 reachable, RDS blocked? Check the security group’s inbound rules first, not the route table.
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 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
Option C is correct because the RDS security group likely does not allow inbound traffic from the on-premises CIDR. Option A is wrong because VPC route table would affect all traffic to the subnet. Option B is wrong if the security group is not the issue; NACLs are stateless and could be the cause but security group is more likely for RDS. Option D is wrong because Direct Connect routing is working for EC2.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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
Access ports place end devices into a single VLAN.
✗
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: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
Access ports place end devices into a single VLAN.
Trunk ports carry multiple VLANs between switches.
Allowed VLAN lists decide which VLANs can cross a trunk.
Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
→Use show vlan brief to verify access VLANs.
→Use show interfaces trunk to verify trunk state and allowed VLANs.
→Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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 VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related ANS-C01 questions on switching, trunking, and access-port configuration.
Network Implementation — This question tests Network Implementation — Access ports place end devices into a single VLAN..
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 — Option C is correct because the RDS security group likely does not allow inbound traffic from the on-premises CIDR. Option A is wrong because VPC route table would affect all traffic to the subnet. Option B is wrong if the security group is not the issue; NACLs are stateless and could be the cause but security group is more likely for RDS. Option D is wrong because Direct Connect routing is working for EC2.
What should I do if I get this ANS-C01 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related ANS-C01 questions on switching, trunking, and access-port configuration.
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?
Access ports place end devices into a single VLAN.
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