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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 where the tunnel is not coming up. The engineer runs 'show crypto isakmp sa' and sees no active IKE SAs. The peer IP address is correctly configured. What should the engineer check first?
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
✓
Check the IP connectivity between the two public IP addresses using ping.
The absence of IKE SAs indicates that IKE phase 1 negotiation has not started or failed. The first step is to verify that the routers can reach each other at the IP layer, as a connectivity issue will prevent any IKE exchange.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Verify that the crypto map is correctly applied to the outside interface.
Why it's wrong here
Incorrect because the crypto map application is important but if IKE SAs are not even appearing, the issue is more fundamental—connectivity.
- ✓
Check the IP connectivity between the two public IP addresses using ping.
Why this is correct
Correct because without basic IP reachability, IKE packets cannot be exchanged, and the VPN cannot establish.
- ✗
Check the IPsec transform set configuration on both routers.
Why it's wrong here
Incorrect because transform set is used in phase 2; phase 1 must complete first.
- ✗
Verify the pre-shared key is identical on both routers.
Why it's wrong here
Incorrect because a pre-shared key mismatch would cause IKE phase 1 to fail, but the engineer should first verify basic connectivity before checking authentication.
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.