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. The tunnel does not come up, and the debug output shows 'received unexpected payload type'. Both routers are configured with pre-shared keys. 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 IKE phase 1 transform sets are mismatched; one router uses AES-256 while the other uses AES-128.
The 'received unexpected payload type' error typically indicates a mismatch in the IKE phase 1 parameters, such as encryption, hash, or DH group. Even if the pre-shared key matches, a mismatch in transform sets or IKE proposals causes the error.
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 pre-shared keys do not match on both sides.
Why it's wrong here
A pre-shared key mismatch usually results in 'invalid payload' or authentication failures, not 'unexpected payload type'.
- ✓
The IKE phase 1 transform sets are mismatched; one router uses AES-256 while the other uses AES-128.
Why this is correct
Mismatched encryption or hash algorithms cause the IKE negotiation to fail with 'unexpected payload type' because the proposals do not align.
- ✗
The IPsec phase 2 transform sets are mismatched.
Why it's wrong here
Phase 2 mismatches occur after phase 1 is established; the error occurs during phase 1.
- ✗
The access-list defining interesting traffic is missing on one router.
Why it's wrong here
Missing interesting traffic prevents the tunnel from being triggered, but the error occurs during negotiation, not before.
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.