- 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.
IPsec IKE Phase 1 Mismatch
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.
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.”
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: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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
Standard ACLs match source addresses.
- ✗
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: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
- Standard ACLs match source addresses.
- Extended ACLs can match source, destination, protocol and ports.
- The first matching ACL entry is used.
- There is usually an implicit deny at the end.
TExam Day Tips
- Check inbound versus outbound direction.
- Read the ACL from top to bottom.
- Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
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.
What to study next
Got this wrong? Here's your next step.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Administrative Distance — This question tests Administrative Distance — Standard ACLs match source addresses..
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 ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.
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?
Standard ACLs match source addresses.
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Last reviewed: Jun 18, 2026
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