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Question 265 of 1,748

Infrastructure Security →mediumMultiple ChoiceObjective-mapped

Troubleshoot Connectivity: When Network ACLs Block Traffic Between Subnets

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 two EC2 instances in the same VPC but different subnets. Both instances have security groups that allow all traffic from each other's security group. However, traffic is still blocked. 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.

A
The instances are in different VPCs.
Why wrong: The scenario states same VPC.
B
The network ACL for one or both subnets is blocking the traffic.
Network ACLs are stateless and can block traffic even if security groups allow it.
C
The route tables do not have a route between the subnets.
Why wrong: Route tables enable routing, but the main VPC route table typically allows local traffic.
D
VPC Flow Logs are not enabled.
Why wrong: Flow logs do not affect traffic flow.

Full breakdown with real-world context →

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 for one or both subnets is blocking the traffic.

The most likely cause is that the network ACL (NACL) for one or both subnets is blocking the traffic. Security groups are stateful and allow traffic based on rules, but NACLs are stateless and require explicit inbound and outbound rules for traffic to flow. Even if security groups permit all traffic between the instances, a NACL denying the traffic (e.g., by having a default deny rule or missing ephemeral port ranges) will block it. Since the instances are in different subnets, the NACL associated with each subnet must allow the traffic in both directions.

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 instances are in different VPCs.
Why it's wrong here
The scenario states same VPC.
✓
The network ACL for one or both subnets is blocking the traffic.
Why this is correct
Network ACLs are stateless and can block traffic even if security groups allow it.
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 route tables do not have a route between the subnets.
Why it's wrong here
Route tables enable routing, but the main VPC route table typically allows local traffic.
✗
VPC Flow Logs are not enabled.
Why it's wrong here
Flow logs do not affect traffic flow.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates often assume security groups alone control all traffic and overlook the stateless nature of network ACLs, especially when instances are in different subnets where NACLs apply at the subnet boundary.

Trap categories for this question

Scenario analysis trap
The scenario states same VPC.

Detailed technical explanation

How to think about this question

Network ACLs are stateless, meaning they evaluate each packet independently without tracking connection state. For traffic to pass between subnets, the NACL must allow both the outbound traffic from the source subnet (including ephemeral ports for return traffic) and the inbound traffic to the destination subnet. In contrast, security groups are stateful and automatically allow return traffic, which is why the security group rule allowing all traffic from each other's group is insufficient if NACLs are restrictive. A common misconfiguration is forgetting to allow ephemeral ports (1024-65535) in the outbound NACL rule for the source subnet.

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

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 SCS-C02 question test?

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 network ACL for one or both subnets is blocking the traffic. — The most likely cause is that the network ACL (NACL) for one or both subnets is blocking the traffic. Security groups are stateful and allow traffic based on rules, but NACLs are stateless and require explicit inbound and outbound rules for traffic to flow. Even if security groups permit all traffic between the instances, a NACL denying the traffic (e.g., by having a default deny rule or missing ephemeral port ranges) will block it. Since the instances are in different subnets, the NACL associated with each subnet must allow the traffic in both directions.

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.

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