- A
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 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.
- B
The IPsec transform set uses ESP with null encryption, which is incompatible with aggressive mode.
Why wrong: Null encryption is supported in aggressive mode; this would not cause tunnel instability.
- C
The ISAKMP SA lifetime mismatch causes the IPsec SA to be re-keyed with different parameters, leading to a transform set mismatch.
Why wrong: The IPsec SA parameters are negotiated separately; lifetime mismatch affects only the ISAKMP SA.
- D
Aggressive mode requires pre-shared keys to be identical, and a mismatch causes the tunnel to drop after the first re-key.
Why wrong: Pre-shared key mismatch would prevent the tunnel from establishing initially.
Quick Answer
The answer is that aggressive mode’s failure to negotiate ISAKMP SA lifetimes directly causes the tunnel instability. In IKEv1 aggressive mode, the responder’s configured lifetime is enforced without negotiation, so when one router has a lifetime of 86400 seconds and the other has 3600 seconds, the responder forces a re-key after one hour while the initiator still expects the longer duration. This mismatch in re-key timing leads to repeated tunnel drops and re-establishments. On the Cisco CCNP ENARSI 300-410 exam, this question tests your understanding of IKEv1 aggressive mode behavior versus main mode, where lifetimes are negotiated; a common trap is assuming both peers will agree on a lifetime. Remember the memory tip: “Aggressive mode is a dictator, not a diplomat—the responder’s lifetime always wins.”
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. 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 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?
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
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.
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.
- ✓
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
IKEv1 aggressive mode uses fewer exchanges (3 packets) than main mode (6 packets) and does not include the ISAKMP SA lifetime in the negotiation; the responder's lifetime is silently accepted. This behavior is documented in RFC 2409, where the lifetime attribute is optional in aggressive mode. In real-world deployments, mismatched lifetimes can cause the IKE SA to expire on one side while the other still considers it valid, leading to re-key failures and tunnel flapping until the administrator aligns the lifetimes.
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 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
- →
IPsec Site-to-Site VPN — study guide chapter
Learn the concepts, then practise the questions
- →
IPsec Site-to-Site VPN practice questions
Targeted practice on this topic area only
- →
All 300-410 questions
2,152 questions across all exam domains
- →
Cisco CCNP ENARSI 300-410 study guide
Full concept coverage aligned to exam objectives
- →
300-410 practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related 300-410 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Layer 3 Technologies practice questions
Practise 300-410 questions linked to Layer 3 Technologies.
EIGRP Troubleshooting practice questions
Practise 300-410 questions linked to EIGRP Troubleshooting.
OSPF Troubleshooting (v2/v3) practice questions
Practise 300-410 questions linked to OSPF Troubleshooting (v2/v3).
BGP Troubleshooting practice questions
Practise 300-410 questions linked to BGP Troubleshooting.
Route Redistribution practice questions
Practise 300-410 questions linked to Route Redistribution.
Policy-Based Routing (PBR) practice questions
Practise 300-410 questions linked to Policy-Based Routing (PBR).
VRF-Lite practice questions
Practise 300-410 questions linked to VRF-Lite.
Route Maps and Route Filtering practice questions
Practise 300-410 questions linked to Route Maps and Route Filtering.
Administrative Distance practice questions
Practise 300-410 questions linked to Administrative Distance.
Route Summarization practice questions
Practise 300-410 questions linked to Route Summarization.
Bidirectional Forwarding Detection (BFD) practice questions
Practise 300-410 questions linked to Bidirectional Forwarding Detection (BFD).
VPN Technologies practice questions
Practise 300-410 questions linked to VPN Technologies.
Practice this exam
Start a free 300-410 practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
FAQ
Questions learners often ask
What does this 300-410 question test?
IPsec Site-to-Site VPN — This question tests IPsec Site-to-Site VPN — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
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.
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?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Last reviewed: Jun 24, 2026
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.