ANS-C01 Network Implementation Practice Question
A company has a VPC with a CIDR of 10.0.0.0/16. They create a subnet 10.0.1.0/24 and launch an EC2 instance with a private IP 10.0.1.5. The instance needs to communicate with an on-premises server at 172.16.0.10 over a VPN connection. The VPN connection uses a Virtual Private Gateway. The VPC route table has a route 172.16.0.0/16 pointing to the VPG. The instance cannot reach the on-premises server. What is the most likely cause?
⚠ Common exam trap
Many exam-takers assume that adding a route in the VPC route table pointing to the VPG is sufficient for connectivity, overlooking the requirement that the on-premises router must also advertise the destination network to the VPG via BGP or have a static route configured.
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 is not advertising the 172.16.0.0/16 route to the Virtual Private Gateway.
The VPN connection requires the on-premises router to advertise the 172.16.0.0/16 route to the Virtual Private Gateway (VPG) via BGP (or static route configuration) for the VPG to forward traffic to the on-premises network. Without this route advertisement, the VPG will not have a path to 172.16.0.10, even though the VPC route table has a route pointing to the VPG. The instance's traffic reaches the VPG, but the VPG cannot forward it because it lacks the necessary routing information from the on-premises side.
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 attached to the instance does not allow outbound traffic to 172.16.0.10.
Why it's wrong here
Security groups are stateful; outbound traffic is allowed by default.
- ✗
The instance's route table does not have a route to the internet gateway.
Why it's wrong here
Internet gateway is not needed for VPN traffic; the route table already has a route to the VPG.
- ✗
The network ACL for the subnet denies outbound traffic to the on-premises IP range.
Why it's wrong here
Default NACL allows all outbound traffic; if custom, it would need to be verified.
- ✓
The on-premises router is not advertising the 172.16.0.0/16 route to the Virtual Private Gateway.
Why this is correct
Without the route advertisement, the VPG does not know how to reach the on-premises network.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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