Quick Answer
The minimum recommended Diffie-Hellman group for IPsec is Group 14. This 2048-bit MODP group is the baseline because it offers sufficient key strength to resist modern computational attacks, including the Logjam attack (CVE-2015-4000) that exploits weak 768-bit and 1024-bit groups, and it aligns with NIST SP 800-131A guidelines requiring at least 2048-bit Diffie-Hellman for secure key exchange. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your ability to select appropriate cryptographic parameters for site-to-site VPNs, often appearing in multiple-choice questions where Group 1, 2, or 5 are traps—these are deprecated or considered weak. A common memory tip is to remember that "14 is the floor for security"—anything lower is a security risk, and Group 14 is the safe starting point for modern IPsec deployments.
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which Diffie-Hellman group is considered the minimum recommended for secure IPsec site-to-site VPNs according to current best practices?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Clue: "minimum / minimize"
Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Group 1
Why wrong: Group 1 uses 768-bit modulus, which is insecure.
Group 2
Why wrong: Group 2 uses 1024-bit modulus, now considered weak.
Group 14
Group 14 uses 2048-bit modulus, the current minimum recommendation.
Group 5
Why wrong: Group 5 uses 1536-bit modulus, which is better but not the minimum recommended; group 14 is preferred.
Answer choices
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
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.
Clue confirmation
The clue words "best", "minimum / minimize" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
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.
Common exam traps
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.
Detailed technical explanation
Diffie-Hellman groups define the modulus size (p) and generator (g) for the key exchange; Group 14 uses a 2048-bit prime, which provides approximately 112 bits of symmetric-equivalent security. In real-world IPsec configurations, the DH group is set under the crypto isakmp policy (e.g., 'crypto isakmp policy 10 dh group 14'), and using a weaker group can allow an attacker to perform a man-in-the-middle attack by solving the discrete logarithm problem with sufficient resources. The Logjam attack specifically demonstrated that 1024-bit DH groups can be broken by nation-state adversaries, making Groups 1, 2, and 5 obsolete for secure site-to-site VPNs.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
IPsec Site-to-Site VPN — This question tests IPsec Site-to-Site VPN — Read the scenario before looking for a memorised answer..
The correct answer is: 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "best", "minimum / minimize". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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