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350-401 Practice Question: Is configuring a site-to-site IPsec VPN between…
A network engineer is configuring a site-to-site IPsec VPN between two Cisco routers. The engineer wants to ensure that the VPN tunnel uses the strongest possible encryption and authentication algorithms. The engineer configures the following: crypto isakmp policy 10, authentication pre-share, encryption aes-256, group 14, lifetime 86400. On the remote router, the engineer configures: crypto isakmp policy 10, authentication pre-share, encryption aes-256, group 14, lifetime 86400. The tunnel fails to establish. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the fact that the hash algorithm is a mandatory parameter in an ISAKMP policy, and candidates mistakenly assume that omitting it will default to a consistent value across routers, leading to a mismatch.
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
✓
The hash algorithm is not specified and defaults may differ between routers.
The most likely cause is that the hash algorithm is not specified in the ISAKMP policy. By default, Cisco IOS uses SHA-1 (or MD5 on older versions) for the hash algorithm, but if one router defaults to SHA-1 and the other defaults to MD5, the IKE Phase 1 proposals will not match, causing the tunnel to fail. The configuration must explicitly include the `hash` command to ensure both peers agree on the same hash algorithm.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The lifetimes are set too high; they should be 3600 seconds.
Why it's wrong here
The lifetimes match on both routers, so this is not the cause.
- ✓
The hash algorithm is not specified and defaults may differ between routers.
Why this is correct
Correct because the default hash algorithm can vary, causing a mismatch.
- ✗
The Diffie-Hellman group 14 is not supported on these routers.
Why it's wrong here
Group 14 is widely supported on modern Cisco routers.
- ✗
Pre-shared keys cannot be used with AES-256 encryption.
Why it's wrong here
Pre-shared keys are compatible with AES-256.
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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Last reviewed: Jul 4, 2026
This 350-401 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 350-401 exam.
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