- A
The Network ACL associated with the subnets is blocking the return traffic for TCP connections on ephemeral ports
NACLs are stateless; they require explicit inbound and outbound rules. ICMP may be allowed, but TCP return traffic is blocked if ephemeral ports are not allowed in the outbound NACL.
- B
The VPC peering connection is not properly configured for TCP traffic
Why wrong: VPC peering connections do not filter traffic by protocol; they connect the VPCs and rely on route tables and security groups for filtering.
- C
The route tables in the VPCs do not contain a route for the other VPC's CIDR
Why wrong: If route tables were missing, ICMP would also fail because ICMP requires routing just like TCP.
- D
The security group on the EC2 instance is blocking inbound TCP traffic on port 443
Why wrong: The question states that security groups allow all inbound and outbound traffic, so this cannot be the cause.
Quick Answer
The answer is the Network ACL blocking return traffic on ephemeral ports. This is because NACLs are stateless, so they require explicit rules for both inbound and outbound traffic; while ICMP succeeds as it uses a single protocol number, TCP port 443 (HTTPS) triggers the target instance to send return traffic from a random high ephemeral port (1024-65535). If the NACL’s outbound rules do not allow this ephemeral range, the TCP handshake fails even when security groups are fully open. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of the critical difference between stateful security groups and stateless NACLs, a common trap where candidates assume “allow all” in security groups covers everything. Remember the mnemonic: “Stateless NACLs need two-way streets; ephemeral ports are the return ticket.”
SOA-C02 Networking and Content Delivery Practice Question
This SOA-C02 practice question tests your understanding of networking and content delivery. 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 SysOps administrator is troubleshooting connectivity issues between Amazon EC2 instances in two different VPCs that are connected via a VPC peering connection. The instances can successfully send ICMP (ping) traffic, but TCP connections on port 443 (HTTPS) fail. The security groups of both instances allow all inbound and outbound traffic. What is the most likely cause of the issue?
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 Network ACL associated with the subnets is blocking the return traffic for TCP connections on ephemeral ports
The correct answer is A because the Network ACL (NACL) is stateless, meaning it must explicitly allow both inbound and outbound traffic. While ICMP (ping) works because it doesn't rely on ephemeral ports for return traffic, TCP connections on port 443 require the return traffic to come from the target instance on a high ephemeral port (typically 1024-65535). If the NACL's outbound rules block these ephemeral ports, the TCP handshake fails, even though the security groups allow all traffic.
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 Network ACL associated with the subnets is blocking the return traffic for TCP connections on ephemeral ports
Why this is correct
NACLs are stateless; they require explicit inbound and outbound rules. ICMP may be allowed, but TCP return traffic is blocked if ephemeral ports are not allowed in the outbound NACL.
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 peering connection is not properly configured for TCP traffic
Why it's wrong here
VPC peering connections do not filter traffic by protocol; they connect the VPCs and rely on route tables and security groups for filtering.
- ✗
The route tables in the VPCs do not contain a route for the other VPC's CIDR
- ✗
The security group on the EC2 instance is blocking inbound TCP traffic on port 443
Why it's wrong here
The question states that security groups allow all inbound and outbound traffic, so this cannot be the cause.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume security groups are the only firewall layer, overlooking that Network ACLs are stateless and require explicit rules for ephemeral port return traffic, which is why ICMP works but TCP fails.
Detailed technical explanation
How to think about this question
Network ACLs are stateless firewalls that evaluate traffic in both directions independently. For a TCP connection initiated from VPC A to VPC B on port 443, the outbound rule in VPC A's NACL must allow the destination port 443, and the inbound rule in VPC B's NACL must allow source port 443. However, the return traffic from VPC B to VPC A uses a random ephemeral source port (e.g., 49152-65535) and destination port 443. If VPC A's NACL outbound rules do not allow these ephemeral ports, the TCP SYN-ACK packet is dropped, causing a connection timeout. This is a common misconfiguration because security groups are stateful and automatically allow return traffic, but NACLs are not.
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.
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.
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FAQ
Questions learners often ask
What does this SOA-C02 question test?
Networking and Content Delivery — This question tests Networking and Content Delivery — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Network ACL associated with the subnets is blocking the return traffic for TCP connections on ephemeral ports — The correct answer is A because the Network ACL (NACL) is stateless, meaning it must explicitly allow both inbound and outbound traffic. While ICMP (ping) works because it doesn't rely on ephemeral ports for return traffic, TCP connections on port 443 require the return traffic to come from the target instance on a high ephemeral port (typically 1024-65535). If the NACL's outbound rules block these ephemeral ports, the TCP handshake fails, even though the security groups allow all traffic.
What should I do if I get this SOA-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
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Last reviewed: Jun 11, 2026
This SOA-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 SOA-C02 exam.
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