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300-410 Practice Question: Which Diffie-Hellman group is considered the…
Which Diffie-Hellman group is considered the minimum recommended for secure IPsec site-to-site VPNs according to current best practices?
⚠ Common exam trap
Cisco often tests the misconception that Group 5 (1536-bit) is still acceptable because it was once the default in older IOS versions, but current best practices require at least Group 14 (2048-bit) for compliance with security standards like NIST SP 800-131A.
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
✓
Group 14
Diffie-Hellman Group 14 (2048-bit MODP) is the minimum recommended for secure IPsec site-to-site VPNs because it provides sufficient key strength against modern computational attacks, including those from quantum-capable adversaries in the near future. Groups 1 and 2 (768-bit and 1024-bit) are considered weak and deprecated due to the Logjam attack (CVE-2015-4000) and advances in factoring, while Group 5 (1536-bit) is also no longer recommended as it does not meet current NIST SP 800-131A guidelines for minimum 2048-bit Diffie-Hellman strength.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Group 1
Why it's wrong here
Group 1 uses 768-bit modulus, which is insecure.
- ✗
Group 2
Why it's wrong here
Group 2 uses 1024-bit modulus, now considered weak.
- ✓
Group 14
Why this is correct
Group 14 uses 2048-bit modulus, the current minimum recommendation.
- ✗
Group 5
Why it's wrong here
Group 5 uses 1536-bit modulus, which is better but not the minimum recommended; group 14 is preferred.
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.