- A
The network ACL in the VPC is blocking outbound traffic.
Why wrong: NACLs are stateless; if ping works, both inbound and outbound rules allow ICMP. However, TCP could be blocked if NACL rules are different for TCP, but ping success suggests NACLs are not the issue.
- B
The security group for the EC2 instance does not allow inbound HTTPS traffic.
Security groups are stateful, so outbound return traffic is allowed automatically. The inbound rule for HTTPS must be present.
- C
The BGP session is in the 'Idle' state.
Why wrong: Ping works, so BGP is up and routes are exchanged.
- D
The route table associated with the virtual private gateway does not have a route to the on-premises network.
Why wrong: The virtual private gateway does not have its own route table; routes are added to the VPC route tables.
Quick Answer
The answer is that the EC2 instance’s security group lacks an inbound rule for HTTPS (TCP 443). Ping succeeds because ICMP traffic is often allowed by default or via a separate rule, confirming Layer 3 connectivity and ruling out routing, BGP, or the Direct Connect virtual interface as the issue. The failure of application traffic specifically on port 443 points directly to a stateful firewall at the instance level—the security group—which implicitly denies all inbound traffic unless explicitly permitted. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your ability to isolate Layer 4 filtering from Layer 3 reachability, a common trap where engineers blame the Direct Connect link or VPC routing when the real culprit is an overly restrictive security group. Remember the mnemonic: “Ping is ICMP, apps need TCP—check the SG for the port you need.”
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 from an on-premises network to an AWS VPC over a Direct Connect private VIF. The VPC has a virtual private gateway attached. The on-premises router can ping the private IP of an EC2 instance in the VPC, but application traffic (TCP port 443) fails. 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 security group for the EC2 instance does not allow inbound HTTPS traffic.
Since the on-premises router can ping the EC2 instance's private IP, Layer 3 connectivity is working, which rules out routing and BGP issues. The failure of TCP port 443 specifically points to a firewall or access control blocking the application traffic. A security group acts as a virtual firewall at the instance level, and by default it denies all inbound traffic unless explicitly allowed. Therefore, the most likely cause is that the security group for the EC2 instance does not have an inbound rule permitting HTTPS (TCP 443) 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 in the VPC is blocking outbound traffic.
- ✓
The security group for the EC2 instance does not allow inbound HTTPS traffic.
Why this is correct
Security groups are stateful, so outbound return traffic is allowed automatically. The inbound rule for HTTPS must be present.
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 BGP session is in the 'Idle' state.
Why it's wrong here
Ping works, so BGP is up and routes are exchanged.
- ✗
The route table associated with the virtual private gateway does not have a route to the on-premises network.
Why it's wrong here
The virtual private gateway does not have its own route table; routes are added to the VPC route tables.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume ping success implies full application-layer connectivity, but security groups can selectively block specific protocols or ports while allowing ICMP, leading to a false sense of network health.
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. However, they operate at the instance level (ENI), not the subnet level, and default to deny-all inbound. In contrast, network ACLs are stateless and require explicit rules for both inbound and outbound traffic. A common real-world scenario is when ICMP (ping) is allowed by a security group but TCP port 443 is not, causing application failure despite successful ping tests.
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
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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 security group for the EC2 instance does not allow inbound HTTPS traffic. — Since the on-premises router can ping the EC2 instance's private IP, Layer 3 connectivity is working, which rules out routing and BGP issues. The failure of TCP port 443 specifically points to a firewall or access control blocking the application traffic. A security group acts as a virtual firewall at the instance level, and by default it denies all inbound traffic unless explicitly allowed. Therefore, the most likely cause is that the security group for the EC2 instance does not have an inbound rule permitting HTTPS (TCP 443) traffic.
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: Jun 11, 2026
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