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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?
⚠ Common exam trap
The trap here is assuming that 'tunnel up/up' means the IPsec configuration is fully correct — candidates overlook that the crypto ACL is a separate traffic-selection mechanism that can be misconfigured even when IKE and IPsec SAs are established.
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 crypto map's access list (the 'match address' ACL) defines which traffic is considered interesting and therefore encrypted/tunneled. If the tunnel is up/up but LAN-to-LAN traffic isn't passing, the most common cause is that the ACL doesn't match the actual source/destination subnet pairs, so the router either doesn't encrypt the traffic or drops it as non-interesting. The tunnel interface being up only confirms IKE/IPsec SA establishment, not that the correct traffic is being selected.
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
Crypto maps are applied to physical interfaces carrying the encrypted traffic, never to tunnel interfaces; the tunnel interface is the logical endpoint, not an encryption point. It tempts engineers because tunnel interfaces seem central to the VPN. The real fault is likely missing interesting traffic or routing.
- ✓
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
A missing esp-aes proposal would prevent Phase 2 SA establishment, leaving the tunnel down rather than up/up. Transform sets are negotiated during quick mode; with the tunnel established, the failure lies in interesting-traffic matching or routing, not encryption algorithm selection.
- ✗
The IKE phase 1 proposal is mismatched between the two routers.
Why it's wrong here
An IKE phase 1 proposal mismatch prevents the tunnel from reaching up/up, so it cannot explain traffic failure on an established tunnel. It is tempting because proposal mismatches are a common IPsec fault, but here the failure lies in phase 2 selectors or routing.
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.