ANS-C01 Network ACL Statelessness Practice Question

Exhibit

Refer to the exhibit. The following is an excerpt from a VPC Flow Log:

2 123456789010 eni-12345678 10.0.1.5 10.0.2.10 443 12345 6 10 1000 1234567890 1234567890 ACCEPT OK
2 123456789010 eni-12345678 10.0.1.5 10.0.2.10 443 12346 6 10 1000 1234567890 1234567890 REJECT OK

A network engineer is reviewing VPC Flow Logs for a VPC. The logs show that traffic from 10.0.1.5 to 10.0.2.10 on port 443 is being accepted and rejected intermittently. Both instances are in the same VPC. What is the most likely cause?

⚠ Common exam trap

The ANS-C01 exam often tests the distinction between stateful security groups and stateless network ACLs. Candidates may incorrectly assume that intermittent traffic issues are due to security group rules, but the key indicator is the 'intermittent' pattern—a hallmark of stateless firewall behavior where response traffic can be blocked if outbound rules are missing.

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 is blocking traffic.

VPC Flow Logs show intermittent ACCEPT and REJECT for traffic from 10.0.1.5 to 10.0.2.10 on port 443. Since the traffic is within the same VPC, the route table is not involved (Option C is unlikely). Security groups are stateful, so even if the destination security group denies outbound traffic, return traffic for the established inbound connection is still allowed; thus Option A is incorrect. The intermittent behavior is characteristic of a stateless network ACL (NACL). A NACL is applied at the subnet level and evaluates inbound and outbound rules independently. If the NACL allows inbound traffic but blocks outbound response traffic (or vice versa), the flow will be partially accepted (initial packets) and then rejected when return packets hit the blocking rule. This matches the logs showing both ACCEPT and REJECT for the same flow. Therefore, the most likely cause is a misconfigured NACL (Option B).

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 security group on the destination instance is configured to allow inbound HTTPS but deny outbound traffic.
Why it's wrong here
Incorrect. Security groups are stateful, so outbound deny rules on the destination do not affect established inbound connections. Return traffic for allowed inbound connections is automatically permitted regardless of outbound rules.
✓
The network ACL is blocking traffic.
Why this is correct
Correct. A network ACL is stateless and requires both inbound and outbound rules to allow traffic. If outbound rules block response traffic, the flow will show intermittent ACCEPT and REJECT.
✗
The route table is misconfigured.
Why it's wrong here
Incorrect. Route tables direct traffic between subnets; if they were misconfigured, the traffic would not reach the destination at all, not intermittently.
✗
The source instance's security group is denying outbound traffic.
Why it's wrong here
Incorrect. The source instance's security group outbound rules do not affect the destination's response; security groups are stateful and allow return traffic for outbound connections initiated by the source.

Visual reference

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ANS-C01 Network ACL Statelessness Practice Question

Exhibit

Why each option matters

Option-by-option breakdown

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