300-410 VPN Technologies Practice Question
A network administrator is troubleshooting an IPsec site-to-site VPN between two Cisco routers. The VPN tunnel is up, but traffic from the local LAN to the remote LAN is not passing. The administrator verifies that the crypto ACLs match on both peers and that routing is correct. Which of the following is the most likely cause?
⚠ Common exam trap
The trap here is assuming that a successful tunnel establishment guarantees data flow; NAT can silently break IPsec by altering packets before encryption.
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
✓
NAT is translating the traffic before it is encrypted, causing the IPsec peer to drop the packets.
When NAT and IPsec are configured on the same router, outbound traffic may be translated by NAT before it is encrypted. If the translated source address does not match the crypto ACL, the remote peer will drop the packets because they do not match the interesting traffic. The tunnel remains up because Phase 1 and Phase 2 SAs are established, but data traffic fails. The fix is to configure a NAT exemption (deny statement) for the VPN traffic in the NAT ACL.
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 transform set is misconfigured with mismatched encryption algorithms.
Why it's wrong here
If the transform sets were mismatched, the IPsec tunnel would not have established successfully. The administrator confirmed that the VPN tunnel is up, which means the transform sets, encryption, and hashing algorithms negotiated successfully. A mismatch would cause Phase 2 negotiation to fail, and the tunnel would not be up.
- ✗
The crypto map is applied to the wrong interface.
Why it's wrong here
If the crypto map were applied to the wrong interface, the tunnel would likely not establish at all, or traffic would not be encrypted/decrypted properly. However, the tunnel is up, indicating that the crypto map is correctly applied to the outgoing interface. Applying it to the wrong interface would typically prevent Phase 1 or Phase 2 from completing.
- ✗
The IPsec SA lifetime is set to a lower value than the ISAKMP SA lifetime.
Why it's wrong here
The IPsec SA lifetime and ISAKMP SA lifetime are independent. The IPsec SA lifetime being lower than the ISAKMP SA lifetime does not prevent traffic from passing; it simply means that the IPsec SA will rekey more frequently. The tunnel would still pass traffic between rekey intervals. This is not a cause for complete traffic failure.
- ✓
NAT is translating the traffic before it is encrypted, causing the IPsec peer to drop the packets.
Why this is correct
If NAT is applied to the outbound interface before the crypto map, the source IP of the packets may be translated to the router's public IP, which does not match the crypto ACL. The IPsec peer will then drop the packets because they do not match the expected source subnet. This is a common issue when NAT and IPsec are configured on the same interface. The solution is to configure NAT exemption for the VPN traffic.
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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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
This 300-410 practice question is part of Courseiva's free Cisco 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 300-410 exam.