hardMultiple ChoiceObjective-mapped
300-410 Practice Question: In IPsec site-to-site VPN, what is the default…
In IPsec site-to-site VPN, what is the default Diffie-Hellman (DH) group used in IKEv1 phase 1 on Cisco IOS?
⚠ Common exam trap
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.
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
✓
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.
Answer analysis
Option-by-option breakdown
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.
- ✗
DH group 2 (1024-bit)
Why it's wrong here
DH group 2 is incorrect because the default Diffie-Hellman group for IKEv1 phase 1 on Cisco IOS is DH group 1 (768-bit). Cisco IOS automatically selects DH group 1 when no specific group is explicitly configured in the ISAKMP policy. However, DH group 2 (1024-bit) is a very common and widely supported group, frequently chosen for explicit configuration to ensure interoperability with various VPN peers or when a stronger key exchange than group 1 is desired without moving to more computationally intensive groups.
- ✗
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.
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.