Site-to-Site VPN Route Propagation
A company is connecting their on-premises data center to AWS using a site-to-site VPN. The customer gateway device has a dynamic routing configuration with BGP. The VPN connection is established, but the VPC route table does not contain the on-premises network routes. What is the most likely cause?
Quick Answer
The answer is that route propagation is not enabled on the VPC route table. For a site-to-site VPN with dynamic routing using BGP, the VPC route table must have route propagation explicitly enabled to automatically inject the BGP-learned prefixes from the on-premises network. Without this setting, even though the VPN tunnel and BGP session are established, the routes are advertised but never installed into the VPC’s routing table. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the critical distinction between establishing a BGP session and actually propagating those routes into the VPC—a common trap is assuming a healthy BGP session automatically populates routes. Remember the key step: enable route propagation on the VPC route table for dynamic VPNs, or the routes stay in the BGP table but never reach the VPC. Memory tip: “Propagation is the permission slip for BGP routes to enter the VPC route table.”
⚠ Common exam trap
Many candidates assume that if the VPN tunnel and BGP session are up, the routes will automatically appear in the VPC route table, overlooking the explicit requirement to enable route propagation on the VPC route table.
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
✓
Route propagation is not enabled on the VPC route table.
When using a site-to-site VPN with dynamic routing (BGP), the VPC route table must have route propagation enabled to automatically import the BGP-advertised routes from the virtual private gateway (VGW). Without route propagation, the VPC route table will not contain the on-premises network routes, even though the VPN tunnel and BGP session are established.
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 customer gateway device is not configured with the correct BGP ASN.
Why it's wrong here
Incorrect ASN would prevent BGP session establishment, but the VPN is established.
- ✗
The VPC route table has static routes pointing to the virtual private gateway.
Why it's wrong here
Static routes would work but dynamic BGP requires propagation.
- ✓
Route propagation is not enabled on the VPC route table.
Why this is correct
Route propagation must be enabled for dynamic BGP routes to be added.
- ✗
The VPN connection is using static routing instead of dynamic routing.
Why it's wrong here
The question states dynamic routing with BGP.
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Same concept, more angles
1 more way this is tested on ANS-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company wants to connect its on-premises data center to a VPC using AWS Site-to-Site VPN. Which of the following is required to establish the VPN connection?
easy- A.Direct Connect gateway associated with the VPC
- B.Internet gateway attached to the VPC
- ✓ C.Virtual private gateway attached to the VPC and a customer gateway representing the on-premises router
- D.VPC endpoint for the VPN service
Why C: To establish a Site-to-Site VPN, you need a virtual private gateway (VGW) or transit gateway on the AWS side, and a customer gateway (CGW) representing the on-premises router. Option C is correct. Option A is incorrect because a Direct Connect gateway is used for Direct Connect connections, not VPN. Option B is incorrect because an internet gateway is used for public internet traffic, not VPN. Option D is incorrect because a VPC endpoint is for private access to AWS services, not for establishing a VPN connection.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.