Quick Answer
The correct answer is DH group 1 (768-bit), which is the default Diffie-Hellman group used in IKEv1 Phase 1 on Cisco IOS routers. This default is defined within the built-in ISAKMP policy 100, which is automatically present when no custom policy is configured; if you do not explicitly specify a DH group, the router defaults to group 1 to ensure backward compatibility with older peers. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of IPsec site-to-site VPN default behaviors, often appearing as a trap where candidates assume a stronger group like 2 or 5 is the default. A common memory tip is to remember that "1" is the loneliest number—and the weakest default—so always explicitly configure a higher group like 14 or 19 for production environments.
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
In IPsec site-to-site VPN, what is the default Diffie-Hellman (DH) group used in IKEv1 phase 1 on Cisco IOS?
DH group 1 (768-bit)
When no DH group is specified in IKE policy, Cisco IOS defaults to group 1.
DH group 2 (1024-bit)
Why wrong: Group 2 is not the default; it must be configured.
DH group 5 (1536-bit)
Why wrong: Group 5 requires explicit configuration.
No default DH group; negotiation fails if not set
Why wrong: IOS defaults to group 1 if not specified.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
DH group 1 (768-bit)
In Cisco IOS, the default Diffie-Hellman group for IKEv1 Phase 1 is DH group 1 (768-bit). This is defined in the default ISAKMP policy (policy 100) that is automatically present when no custom policy is configured. If no DH group is explicitly specified, the router uses group 1 by default, which provides the lowest security level but ensures backward compatibility.
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.
DH group 1 (768-bit)
Why this is correct
When no DH group is specified in IKE policy, Cisco IOS defaults to group 1.
Related concept
Read the scenario before looking for a memorised answer.
DH group 2 (1024-bit)
Why it's wrong here
Group 2 is not the default; it must be configured.
DH group 5 (1536-bit)
Why it's wrong here
Group 5 requires explicit configuration.
No default DH group; negotiation fails if not set
Why it's wrong here
IOS defaults to group 1 if not specified.
Common exam traps
Cisco often tests the fact that there is a default DH group (group 1) in IKEv1 Phase 1, leading candidates to incorrectly assume that no default exists or that a stronger group like 2 or 5 is the default.
Detailed technical explanation
The default ISAKMP policy on Cisco IOS (policy 100) uses DES encryption, MD5 hash, pre-shared key authentication, and DH group 1. This default is often overlooked in production because it is weak; modern deployments typically use DH group 14 or 19 for stronger security. The DH group determines the strength of the key exchange—group 1 uses a 768-bit prime modulus, which is vulnerable to modern attacks and should be avoided in favor of higher groups.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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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: DH group 1 (768-bit) — In Cisco IOS, the default Diffie-Hellman group for IKEv1 Phase 1 is DH group 1 (768-bit). This is defined in the default ISAKMP policy (policy 100) that is automatically present when no custom policy is configured. If no DH group is explicitly specified, the router uses group 1 by default, which provides the lowest security level but ensures backward compatibility.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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