hardMultiple Choice
300-410 Practice Question: Runs the following command to troubleshoot IPsec…
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?
⚠ Common exam trap
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.
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.
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 line "atts are not acceptable" means the responder rejected every offered transform, so no matching policy exists and phase 1 fails. Successful negotiation would instead log "atts are acceptable" followed by a completed SA. The debug is used precisely to confirm such mismatches.
- ✓
IKE phase 1 fails due to transform set mismatch.
Why this is correct
The debug shows the responder comparing its priority 1 ISAKMP policy against the initiator's proposed transform (3DES, SHA, group 2, pre-share, 86400s). The message 'atts are not acceptable' means no attribute set matched, so phase 1 fails on transform mismatch.
- ✗
IKE phase 1 fails due to authentication failure.
Why it's wrong here
Authentication failure would appear after the transform set is accepted, with messages about pre-shared key mismatch. Here the transform is rejected before authentication, so the mismatch lies in the phase 1 policy attributes (encryption, hash, group or lifetime), not credentials.
- ✗
IKE phase 1 fails due to lifetime mismatch.
Why it's wrong here
The lifetime values are not compared before the transform set is rejected; the mismatch lies in the encryption, hash, group or authentication attributes. Lifetime mismatch is diagnosed when both peers agree on transforms but disagree on duration, which is configured separately under the ISAKMP policy.
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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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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