ANS-C01 Network Management and Operations Practice Question
A network engineer is configuring a site-to-site VPN connection between an on-premises network and AWS. The VPN tunnel is established, but traffic is not flowing. Which THREE components should the engineer check?
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
✓
Security group rules on the VPC resources to allow inbound traffic from on-premises
Options A, B, and E are correct. Security group rules must allow inbound traffic from the on-premises network (A). On-premises firewall rules must permit IPsec traffic (B). The VPC route table must have a route to the on-premises CIDR pointing to the virtual private gateway (E). Option C is incorrect because Network ACLs are stateless and require explicit inbound rules for return traffic; however, they are not the primary components to check when troubleshooting a site-to-site VPN that is established but not passing traffic. Option D is incorrect because an Internet Gateway is not used for VPN traffic; VPN traffic goes through the virtual private gateway.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Security group rules on the VPC resources to allow inbound traffic from on-premises
Why this is correct
Security groups control traffic flow.
- ✓
On-premises firewall rules to allow IPsec traffic
Why this is correct
Firewall must allow IPsec (UDP 500, 4500, ESP).
- ✗
Network ACLs for the subnet to allow return traffic
Why it's wrong here
NACLs are stateless; if outbound is allowed, return is automatic.
- ✗
Internet Gateway attachment to the VPC
Why it's wrong here
Internet Gateway is for internet traffic, not VPN.
- ✓
VPC route table for a route to the on-premises CIDR pointing to the virtual private gateway
Why this is correct
Without this route, traffic cannot reach the VPN.
Visual reference
Quick reference
VPN Protocol Comparison
| Protocol | Port | Encryption | Authentication | Use Case |
|---|---|---|---|---|
| IKEv2 / IPsec | UDP 500 / 4500 | AES-256 | Certificates / PSK | Site-to-site & remote access |
| SSL / TLS VPN | TCP 443 | TLS 1.3 | Certificates / MFA | Clientless remote access |
| L2TP / IPsec | UDP 1701 | AES (IPsec) | PSK / Certificates | Legacy remote access |
| WireGuard | UDP 51820 | ChaCha20 | Public keys | Modern high-performance VPN |
| PPTP | TCP 1723 | MPPE (weak) | MS-CHAPv2 | Legacy — avoid in production |
PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.
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Same concept, more angles
2 more ways 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 has set up a Site-to-Site VPN connection between its on-premises network and AWS. The VPN tunnel shows as 'UP' but traffic is not flowing. What should the engineer check?
medium- A.Ensure the customer gateway is configured correctly
- B.Verify the security group rules for the VPN connection
- C.Check the internet gateway route table
- ✓ D.Verify that route propagation is enabled on the VPC route table
Why D: Route propagation from the virtual private gateway to the VPC route table must be enabled. Option A is wrong because the tunnel state is up, so security group is not likely the issue (security group affects instances, not VPN endpoint). Option B is wrong because the internet gateway is for public internet access, not VPN. Option C is wrong because the customer gateway is the on-premises endpoint; it is configured but if routes are not propagated, traffic won't flow.
Variation 2. A company has an AWS Site-to-Site VPN connection between an on-premises network and a VPC. The VPN uses virtual private gateways and static routes. The network team reports that the VPN tunnel is up, but traffic from the on-premises network cannot reach some EC2 instances in the VPC. The EC2 instances have security groups that allow inbound traffic from the on-premises network. The VPC route table has a route pointing to the virtual private gateway for the on-premises CIDR. The tunnel status shows 'UP' from both sides. What is the MOST likely cause of the connectivity issue?
medium- ✓ A.The VPC route table does not have a route for the on-premises subnet that the traffic originates from, but only for a larger CIDR.
- B.The VPC has a network ACL that denies inbound traffic from the on-premises CIDR.
- C.The customer gateway device is using a different pre-shared key than configured in AWS.
- D.The virtual private gateway is not attached to the correct VPC.
Why A: The security group may be allowing traffic from the on-premises CIDR but not from the tunnel endpoint IP. However, the more common issue is that the on-premises network's source IP is being translated or the VPC route table is missing a route for the specific subnet. Option A is a typical cause: if the on-premises CIDR is not exactly matched, the VPC may not route traffic back. Option C could cause issues if the VPN is not in the main route table. Option D would cause tunnel issues.
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.