ANS-C01 Network Management and Operations Practice Question
A network engineer is troubleshooting an issue where an on-premises server cannot reach an EC2 instance in a VPC over a Site-to-Site VPN. The VPN tunnel is up, and BGP is established. The engineer checks the route tables and sees the on-premises CIDR in the VPC route table pointing to the virtual private gateway. 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 on-premises router does not have a route back to the VPC CIDR pointing to the VPN tunnel.
Although the VPN tunnel is up and BGP is established, the on-premises router must have a route pointing back to the VPC CIDR through the VPN tunnel for return traffic. Without this route, traffic from the EC2 instance to the on-premises server cannot be routed back, causing a one-way connectivity issue. Option A is incorrect because a mismatch in pre-shared keys would prevent the tunnel from coming up. Option C is incorrect because network ACLs are stateless and would affect traffic in both directions if misconfigured, not just one-way. Option D is incorrect because security group rules affect inbound traffic only; the issue here is return traffic from the EC2 instance, which is outbound from the VPC perspective, and the security group does not control outbound traffic in this context.
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 VPN tunnel is not passing traffic due to a mismatch in pre-shared keys.
Why it's wrong here
If keys mismatched, the tunnel would not be up.
- ✓
The on-premises router does not have a route back to the VPC CIDR pointing to the VPN tunnel.
Why this is correct
Without a return route, the on-premises server cannot send traffic back to the EC2 instance.
- ✗
The network ACLs in the VPC are blocking the traffic.
Why it's wrong here
NACLs are stateless and would block traffic in both directions if misconfigured.
- ✗
The security group attached to the EC2 instance is blocking inbound traffic from the on-premises CIDR.
Why it's wrong here
Security groups are stateful; if inbound is blocked, outbound would also be affected but the issue is one-way reachability.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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