- A
Review the IPsec phase 2 settings, including the traffic selectors.
Phase 2 parameters or mismatched encryption domains could cause the tunnel to be UP but not pass traffic.
- B
Check the IKE phase 1 parameters (e.g., encryption, hash).
Why wrong: Phase 1 mismatch would also prevent the tunnel from being UP.
- C
Verify that the pre-shared keys match.
Why wrong: Mismatched PSK would prevent phase 1, so the tunnel would not be UP.
- D
Confirm that the VPC route table has a route to the on-premises subnet.
Why wrong: The engineer already checked route tables and found static routes correct.
Question 1,441 of 1,705
VPN Tunnel UP But No Traffic: Troubleshoot IPsec Phase 2
This ANS-C01 practice question tests your understanding of network management and operations. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 troubleshooting a VPN connection between an AWS Virtual Private Gateway and an on-premises Cisco ASA. The tunnel status shows 'UP' but no traffic passes. The engineer checks the route tables and finds the correct static routes on both sides. What should the engineer check next?
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
✓
Review the IPsec phase 2 settings, including the traffic selectors.
If the tunnel is up but no traffic passes, the issue is often phase 2 IPsec security associations (SAs) or mismatch in encryption domains. Unlike phase 1, phase 2 can fail silently.
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.
- ✓
Review the IPsec phase 2 settings, including the traffic selectors.
Why this is correct
Phase 2 parameters or mismatched encryption domains could cause the tunnel to be UP but not pass traffic.
Related concept
CIDR notation defines the prefix length.
- ✗
Check the IKE phase 1 parameters (e.g., encryption, hash).
Why it's wrong here
Phase 1 mismatch would also prevent the tunnel from being UP.
- ✗
Verify that the pre-shared keys match.
Why it's wrong here
Mismatched PSK would prevent phase 1, so the tunnel would not be UP.
- ✗
Confirm that the VPC route table has a route to the on-premises subnet.
Why it's wrong here
The engineer already checked route tables and found static routes correct.
Common exam traps
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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.
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.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
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: Review the IPsec phase 2 settings, including the traffic selectors. — If the tunnel is up but no traffic passes, the issue is often phase 2 IPsec security associations (SAs) or mismatch in encryption domains. Unlike phase 1, phase 2 can fail silently.
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 →
Same concept, more angles
1 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 network engineer is configuring a Site-to-Site VPN connection between an on-premises network and AWS. The VPN tunnel status shows 'UP' but traffic is not passing. The engineer checks the route tables and finds that the VPC route table has a route pointing to the virtual private gateway for the on-premises CIDR. What is the most likely missing configuration?
medium- A.The VPC route table does not have a route for the on-premises CIDR pointing to the virtual private gateway
- ✓ B.The security group of the EC2 instances does not allow inbound traffic from on-premises
- C.The VPN tunnel is using the wrong pre-shared key
- D.The on-premises router is not advertising the VPC CIDR over BGP
Why B: Since the VPN tunnel status is 'UP', the pre-shared key and Phase 2 parameters are correctly configured. With the VPC route table already pointing to the virtual private gateway for the on-premises CIDR, the most likely missing configuration is that the security group associated with the EC2 instances in the VPC does not allow inbound traffic from the on-premises network. Security groups act as a virtual firewall for instances, and inbound traffic from on-premises must be explicitly allowed. Option A is incorrect because the stem states the route exists. Option C is incorrect because a tunnel UP status confirms the pre-shared keys match. Option D is less likely because even if the on-premises router is not advertising the VPC CIDR, that would affect return traffic but not necessarily all traffic; also the question does not specify BGP usage.
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Last reviewed: Jun 20, 2026
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