- A
The pre-shared keys do not match on both sides.
Why wrong: A pre-shared key mismatch usually results in 'invalid payload' or authentication failures, not 'unexpected payload type'.
- B
The IKE phase 1 transform sets are mismatched; one router uses AES-256 while the other uses AES-128.
Mismatched encryption or hash algorithms cause the IKE negotiation to fail with 'unexpected payload type' because the proposals do not align.
- C
The IPsec phase 2 transform sets are mismatched.
Why wrong: Phase 2 mismatches occur after phase 1 is established; the error occurs during phase 1.
- D
The access-list defining interesting traffic is missing on one router.
Why wrong: Missing interesting traffic prevents the tunnel from being triggered, but the error occurs during negotiation, not before.
Quick Answer
The answer is a mismatch in the IKE phase 1 transform sets, such as one router using AES-256 while the other uses AES-128. This is the most likely explanation because the debug output ‘received unexpected payload type’ directly indicates that the two peers cannot agree on the security parameters for the ISAKMP SA; even when pre-shared keys match, a discrepancy in encryption, hash algorithm, or Diffie-Hellman group prevents phase 1 negotiation from completing. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of IPsec IKE phase 1 mismatch troubleshooting, often appearing as a trap where candidates assume the pre-shared key is the sole culprit. A common memory tip is to think of “transform sets as a handshake—if one side offers AES-128 and the other expects AES-256, the handshake fails before any keys are exchanged.”
300-410 Administrative Distance Practice Question
This 300-410 practice question tests your understanding of administrative distance. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
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.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Administrative Distance — This question tests Administrative Distance — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 300-410 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 18, 2026
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