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300-410 Practice Question: Is troubleshooting an IPsec site-to-site VPN that…
A network engineer is troubleshooting an IPsec site-to-site VPN that stopped working after a recent configuration change. The engineer runs 'show crypto isakmp sa' and sees an active IKE SA, but 'show crypto ipsec sa' shows no IPsec SAs. What is the most likely cause?
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 IPsec transform set on one router does not match the transform set on the other.
An active IKE SA indicates that IKE phase 1 completed successfully, but no IPsec SAs means phase 2 failed. The most common cause is a mismatch in the IPsec transform set or the crypto map access list between the two routers.
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 IPsec transform set on one router does not match the transform set on the other.
Why this is correct
Correct because during phase 2, the routers negotiate the transform set; if they do not match, the IPsec SA cannot be established.
- ✗
The pre-shared key is incorrect on one of the routers.
Why it's wrong here
Incorrect because a pre-shared key mismatch would cause IKE phase 1 to fail, not phase 2.
- ✗
The tunnel interface is down.
Why it's wrong here
Incorrect because the issue is with IPsec SAs, not the tunnel interface.
- ✗
The IKE proposal is mismatched.
Why it's wrong here
Incorrect because an IKE proposal mismatch would cause phase 1 to fail, but phase 1 is active.
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.