hardMultiple Choice
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?
⚠ Common exam trap
The trap here is assuming that IKE policy priority numbers must match between peers, or that the pre-shared key lives inside the policy — both are common misconceptions that distract from the real requirement that the transform attributes (encryption, hash, auth, DH group) must match.
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.
IKE phase 1 (ISAKMP SA) negotiation requires both peers to agree on a matching transform set: encryption algorithm, hash algorithm, authentication method, and Diffie-Hellman group. Here the local policy uses AES while the peer uses 3DES, so no common encryption proposal exists and the ISAKMP SA cannot be built. The mismatch causes the negotiation to fail before phase 2 (IPsec SA) is even attempted.
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 phase 1 requires both peers to agree on the same encryption algorithm within a matching policy. AES on one peer and 3DES on the other produce no common proposal, so the ISAKMP security association negotiation fails before phase 2 begins.
- ✗
The pre-shared key must be configured globally, not under the policy.
Why it's wrong here
The pre-shared key is always configured globally via `crypto isakmp key`, never inside the policy, so this is not the fault. That global command is the correct place when defining the key for pre-shared-key authentication between peers, independent of any policy attributes.
- ✗
The IKE policy must have the same priority number on both ends.
Why it's wrong here
Priority numbers are locally significant and need not match between peers; each side selects its own policy order. Identical priorities are configured when administrators want mirrored policy sequences for manageability, not because IKE requires the numbers to correspond during negotiation.
- ✗
The authentication method must be `rsa-sig` for site-to-site VPNs.
Why it's wrong here
`rsa-sig` is optional, used when peers authenticate with certificates rather than a shared secret. Pre-shared keys remain valid for site-to-site VPNs, so the failure stems from the differing encryption transforms, not from the authentication method chosen on either router.
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 and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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