mediumMultiple ChoiceObjective-mapped
300-410 Practice Question: Is troubleshooting an IPsec site-to-site VPN…
A network engineer is troubleshooting an IPsec site-to-site VPN between two routers. The tunnel interface is up/up, but traffic from the local LAN to the remote LAN is not passing. The engineer checks the crypto map and sees it is applied to the outside interface. What is the most likely cause of the traffic failure?
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
✓
The access list in the crypto map does not match the LAN-to-LAN traffic.
The tunnel interface being up/up indicates the GRE tunnel is operational, but traffic may still fail if the crypto map is not correctly triggering IPsec encryption for the actual data traffic. The most common cause is a missing or incorrect access list in the crypto map that defines interesting traffic.
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 crypto map is not applied to the tunnel interface.
Why it's wrong here
Incorrect because the crypto map should be applied to the physical outside interface, not the tunnel interface.
- ✓
The access list in the crypto map does not match the LAN-to-LAN traffic.
Why this is correct
Correct because the crypto map uses an access list to define which traffic is encrypted; if it does not match the actual LAN subnets, traffic will be sent unencrypted and may be dropped by the remote router.
- ✗
The IPsec transform set is missing the esp-aes encryption algorithm.
Why it's wrong here
Incorrect because the transform set configuration is for the encryption and authentication algorithms; a missing algorithm would cause phase 2 negotiation failure, but the tunnel is up.
- ✗
The IKE phase 1 proposal is mismatched between the two routers.
Why it's wrong here
Incorrect because if IKE phase 1 were mismatched, the tunnel would not come up at all.
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 by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.