- A
IKE phase 1 is successful; the transform set is accepted.
Why wrong: The output says 'atts are not acceptable'.
- B
IKE phase 1 fails due to transform set mismatch.
The attributes are not acceptable, indicating a mismatch.
- C
IKE phase 1 fails due to authentication failure.
Why wrong: No authentication failure is shown; the issue is with the transform set.
- D
IKE phase 1 fails due to lifetime mismatch.
Why wrong: The lifetime is shown but not indicated as the cause; the general 'atts are not acceptable' covers all attributes.
Question 1,512 of 2,152
How to Identify IKE Phase 1 Transform Set Mismatch Using Debug
This 300-410 practice question tests your understanding of device management. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A network engineer runs the following command to troubleshoot IPsec IKE phase 1:
R1# debug crypto isakmp
ISAKMP: (0:0:N/A:0) Starting aggressive mode exchange ISAKMP: (0:0:N/A:0) processing SA payload ISAKMP: (0:0:N/A:0) Checking ISAKMP transform 1 against priority 1 policy ISAKMP: (0:0:N/A:0) encryption 3DES ISAKMP: (0:0:N/A:0) hash SHA ISAKMP: (0:0:N/A:0) group 2 ISAKMP: (0:0:N/A:0) auth pre-share ISAKMP: (0:0:N/A:0) life type in seconds ISAKMP: (0:0:N/A:0) life duration (basic) of 86400 ISAKMP: (0:0:N/A:0) atts are not acceptable
What does this output indicate?
Quick Answer
The answer is that the debug output indicates an IKE phase 1 failure caused by a transform set mismatch. This is correct because the `debug crypto isakmp` output shows the router processing the SA payload and checking the received transform against its local policy, but it concludes with "atts are not acceptable," meaning the encryption (3DES), hash (SHA), Diffie-Hellman group (2), authentication (pre-share), or lifetime (86400 seconds) does not match what the peer is offering. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to interpret ISAKMP debug messages and troubleshoot IPsec VPN misconfigurations; a common trap is assuming the failure is due to a preshared key mismatch, but the key clue is the explicit "atts are not acceptable" line before any authentication attempt. Memory tip: think of the acronym "E-HAL" for the transform attributes—Encryption, Hash, Authentication, Lifetime—and if any one of these doesn't align between peers, you'll see that "not acceptable" message.
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
✓
IKE phase 1 fails due to transform set mismatch.
The debug output shows that the ISAKMP transform (3DES, SHA, group 2, pre-share) is being checked against the local policy, and the message 'atts are not acceptable' indicates a mismatch. This means the proposed transform set does not match any configured IKE policy on the responder, causing IKE phase 1 to fail. The failure is specifically due to a transform set mismatch, not authentication or lifetime issues.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
IKE phase 1 is successful; the transform set is accepted.
Why it's wrong here
The output says 'atts are not acceptable'.
- ✓
IKE phase 1 fails due to transform set mismatch.
Why this is correct
The attributes are not acceptable, indicating a mismatch.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
IKE phase 1 fails due to authentication failure.
Why it's wrong here
No authentication failure is shown; the issue is with the transform set.
- ✗
IKE phase 1 fails due to lifetime mismatch.
Why it's wrong here
The lifetime is shown but not indicated as the cause; the general 'atts are not acceptable' covers all attributes.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between transform set mismatch and authentication failure, where candidates mistakenly assume that any failure in IKE phase 1 is due to authentication, but the debug message 'atts are not acceptable' specifically points to a proposal mismatch.
Trap categories for this question
Command / output trap
The output says 'atts are not acceptable'.
Detailed technical explanation
How to think about this question
IKE phase 1 uses a proposal/response exchange where the initiator sends a list of transform sets (encryption, hash, DH group, auth method, lifetime) and the responder selects the first matching policy. The debug message 'atts are not acceptable' occurs when none of the proposed transforms match the responder's configured policy, even if individual attributes like encryption or hash are correct. In aggressive mode, the exchange is faster but less secure, and a transform mismatch will cause the responder to send a 'not acceptable' notification, terminating the negotiation.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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
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FAQ
Questions learners often ask
What does this 300-410 question test?
Device Management — This question tests Device Management — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: IKE phase 1 fails due to transform set mismatch. — The debug output shows that the ISAKMP transform (3DES, SHA, group 2, pre-share) is being checked against the local policy, and the message 'atts are not acceptable' indicates a mismatch. This means the proposed transform set does not match any configured IKE policy on the responder, causing IKE phase 1 to fail. The failure is specifically due to a transform set mismatch, not authentication or lifetime issues.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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