hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer configures an IPsec site-to-site VPN…
An engineer configures an IPsec site-to-site VPN using IKEv1 with aggressive mode. The VPN tunnel establishes, but after some time, the tunnel goes down and re-establishes repeatedly. The engineer notices that the ISAKMP SA lifetime is set to 86400 seconds on one router and 3600 seconds on the other. What is the most likely explanation for the instability?
⚠ Common exam trap
Cisco often tests the subtle difference that aggressive mode does not negotiate ISAKMP SA lifetimes, so candidates mistakenly assume both sides must match or that the mismatch only affects phase 2, when in fact the responder's lifetime is used unilaterally, causing re-key timing issues.
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
✓
Aggressive mode does not negotiate ISAKMP SA lifetimes; the responder's lifetime is used, so the SA expires at different times, causing re-key failures.
In IKEv1 aggressive mode, the ISAKMP SA lifetime is not negotiated; the responder's configured lifetime is used. With lifetimes of 86400 seconds (24 hours) on one router and 3600 seconds (1 hour) on the other, the responder will enforce its own 3600-second lifetime. When the SA expires after 3600 seconds, the re-key occurs, but the initiator still expects the longer lifetime, causing a mismatch in the re-key timing and leading to repeated tunnel drops and re-establishments.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Aggressive mode does not negotiate ISAKMP SA lifetimes; the responder's lifetime is used, so the SA expires at different times, causing re-key failures.
Why this is correct
In aggressive mode, the initiator sends its proposal, but the responder's lifetime is used for the SA. If the lifetimes differ, the SA will expire on one side first, causing the tunnel to drop and re-establish.
- ✗
The IPsec transform set uses ESP with null encryption, which is incompatible with aggressive mode.
Why it's wrong here
Null encryption is supported in aggressive mode; this would not cause tunnel instability.
- ✗
The ISAKMP SA lifetime mismatch causes the IPsec SA to be re-keyed with different parameters, leading to a transform set mismatch.
Why it's wrong here
The IPsec SA parameters are negotiated separately; lifetime mismatch affects only the ISAKMP SA.
- ✗
Aggressive mode requires pre-shared keys to be identical, and a mismatch causes the tunnel to drop after the first re-key.
Why it's wrong here
Pre-shared key mismatch would prevent the tunnel from establishing initially.
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.