hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer configures IPsec between two routers…
An engineer configures IPsec between two routers using a site-to-site VPN with IKEv1. The configuration uses `crypto isakmp policy 10` with authentication pre-share and encryption aes. On the peer, the policy is configured with authentication pre-share and encryption 3des. Unexpectedly, the IKE phase 1 negotiation fails. Which is the most likely explanation?
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 encryption algorithms (AES vs 3DES) do not match, causing IKE phase 1 to fail.
IKEv1 requires that the encryption algorithm, hash, authentication method, and Diffie-Hellman group match exactly between peers. The encryption algorithm mismatch (AES vs 3DES) causes the IKE proposal to be rejected. Even though both use pre-shared keys, the encryption mismatch is a common edge case.
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 encryption algorithms (AES vs 3DES) do not match, causing IKE phase 1 to fail.
Why this is correct
IKEv1 requires exact match of all parameters in the proposal.
- ✗
The pre-shared key must be configured globally, not under the policy.
Why it's wrong here
Pre-shared key is configured with `crypto isakmp key` command, not under the policy.
- ✗
The IKE policy must have the same priority number on both ends.
Why it's wrong here
Priority numbers are local and do not need to match.
- ✗
The authentication method must be `rsa-sig` for site-to-site VPNs.
Why it's wrong here
Pre-shared key is valid for site-to-site VPNs.
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
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