The answer is the VPC route table, security groups, and the on-premises firewall. When a site-to-site VPN tunnel is up but no traffic flows, the most common cause is a missing or misdirected route in the VPC route table that fails to point the on-premises CIDR block to the virtual private gateway; without this route, AWS has no path to forward traffic into the tunnel. Additionally, security groups must explicitly permit inbound traffic from the on-premises network, and the on-premises firewall must allow IPsec traffic through, as both sides of the tunnel require proper access controls. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the three-layer verification model—routing, security group statefulness, and firewall rules—and often appears as a trap where candidates mistakenly check the internet gateway, which is irrelevant for VPN traffic, or NACLs, which are stateless and typically not the culprit if outbound rules are correct. Remember the mnemonic “Route, SG, Firewall” to quickly isolate why a tunnel is up but silent.
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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 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, C, and D are correct. The route table must have a route to the on-premises CIDR via the virtual private gateway. Security groups must allow traffic from on-premises. The on-premises firewall must allow IPsec traffic. Option B is incorrect because the internet gateway is not needed for VPN traffic. Option E is incorrect because NACLs are stateless and usually allow return traffic if outbound is permitted.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Security group rules on the VPC resources to allow inbound traffic from on-premises — Options A, C, and D are correct. The route table must have a route to the on-premises CIDR via the virtual private gateway. Security groups must allow traffic from on-premises. The on-premises firewall must allow IPsec traffic. Option B is incorrect because the internet gateway is not needed for VPN traffic. Option E is incorrect because NACLs are stateless and usually allow return traffic if outbound is permitted.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
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: Option D is correct because 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.
Last reviewed: Jun 20, 2026
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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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