The answer is a missing return route on the on-premises router pointing the VPC CIDR back to the Direct Connect virtual interface. This is the most likely cause because while the EC2 instance can send outbound packets to the on-premises server via the VPC route table entry directing traffic to the virtual private gateway, the on-premises firewall logs confirm those packets arrive. However, for the on-premises server to reply, its router must have a specific route directing the VPC CIDR block back through the Direct Connect interface; without it, the return traffic follows the default route—often the internet—where it is either dropped by the VPC security group or never reaches the EC2 instance. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding that Direct Connect connectivity troubleshooting requires verifying bidirectional routing, not just outbound paths. A common trap is assuming the VPC configuration alone ensures connectivity, but the on-premises side must explicitly route return traffic. Memory tip: “Packets need a round-trip ticket—both sides must have a route back to the Direct Connect interface.”
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. 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 an Amazon EC2 instance in a VPC and an on-premises server over a Direct Connect virtual interface. The EC2 instance has a security group that allows outbound traffic to the on-premises CIDR block (10.0.0.0/16). The VPC has a route table entry pointing the on-premises CIDR to the virtual private gateway. The on-premises firewall shows that packets are received from the EC2 instance but responses are not reaching the instance. 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 on-premises router does not have a route pointing the VPC CIDR back to the Direct Connect interface.
The on-premises firewall logs show packets are received from the EC2 instance, but responses are not reaching it. This indicates a routing issue on the on-premises side: the on-premises router must have a route pointing the VPC CIDR back to the Direct Connect interface (virtual interface) for return traffic to be forwarded correctly. Without this return route, the on-premises server sends responses via its default route (likely the internet), which are dropped by the VPC security group or never reach the EC2 instance.
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 on-premises router does not have a route pointing the VPC CIDR back to the Direct Connect interface.
Why this is correct
Without a return route, responses from on-premises are dropped.
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 network ACL for the subnet is blocking outbound traffic to the on-premises CIDR.
Why it's wrong here
Outbound traffic is reaching on-premises, so NACL is not blocking outbound.
✗
The virtual private gateway is not attached to the VPC.
Why it's wrong here
If the gateway were not attached, outbound traffic would not reach on-premises.
✗
The security group does not allow inbound traffic from the on-premises server.
Why it's wrong here
Security groups are stateful; if outbound is allowed, return traffic is automatically allowed.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume security groups or network ACLs are the cause of asymmetric connectivity issues, but the real problem is the missing return route on the on-premises side, which is a common misconfiguration in hybrid networking scenarios.
Detailed technical explanation
How to think about this question
Direct Connect uses BGP to exchange routes between the on-premises router and the AWS virtual private gateway. The on-premises router must advertise the VPC CIDR back to AWS via BGP, or have a static route pointing the VPC CIDR to the Direct Connect interface, to ensure symmetric routing. Asymmetric routing (where traffic goes out Direct Connect but returns via the internet) will be blocked by the VPC security group because the return packet's source IP is not the on-premises server's private IP, or by the VPC's route table if the return path is not via the virtual private gateway.
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 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Infrastructure Security — This question tests Infrastructure Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The on-premises router does not have a route pointing the VPC CIDR back to the Direct Connect interface. — The on-premises firewall logs show packets are received from the EC2 instance, but responses are not reaching it. This indicates a routing issue on the on-premises side: the on-premises router must have a route pointing the VPC CIDR back to the Direct Connect interface (virtual interface) for return traffic to be forwarded correctly. Without this return route, the on-premises server sends responses via its default route (likely the internet), which are dropped by the VPC security group or never reach the EC2 instance.
What should I do if I get this SCS-C02 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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This SCS-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SCS-C02 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.