Question 1,670 of 1,621
ANS-C01 Network Design Practice Question
A company is expanding its on-premises data center to AWS using a hybrid cloud architecture. They have established an AWS Direct Connect connection with a private virtual interface to a VPC. The on-premises network uses BGP to exchange routes with the VPC. The network engineer notices that the on-premises network can reach some EC2 instances in the VPC but not others. All EC2 instances are in the same subnet (10.0.1.0/24) and have private IP addresses. The Direct Connect virtual interface is configured with the VPC CIDR (10.0.0.0/16) advertised to on-premises. The on-premises firewall logs show that traffic to the unreachable instances is being dropped. What is the most likely cause?
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 attached to the unreachable instances is not allowing inbound traffic from the on-premises network.
Security groups are stateful and control inbound traffic; if the security group does not allow inbound traffic from on-premises, it will be dropped. Option A is wrong because the subnet route table has a route to the virtual private gateway, which is correct. Option B is wrong because the VPC CIDR is advertised. Option C is wrong because network ACLs are stateless and need both inbound and outbound rules; but if they were misconfigured, all instances would be affected.
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 subnet route table does not have a route to the on-premises network.
Why it's wrong here
Wrong: The route to the virtual private gateway covers all on-premises traffic.
- ✗
The BGP session is not advertising the on-premises routes to the VPC.
Why it's wrong here
Wrong: The issue is on-premises cannot reach instances, not the other way.
- ✗
The network ACL on the subnet is blocking inbound traffic from on-premises.
Why it's wrong here
Wrong: If network ACL blocked, all instances would be unreachable.
- ✓
The security group attached to the unreachable instances is not allowing inbound traffic from the on-premises network.
Why this is correct
Correct: Security groups can be different per instance, causing inconsistent reachability.
Visual reference
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Last reviewed: Jun 20, 2026
This ANS-C01 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 ANS-C01 exam.
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