The answer is that an ISAKMP lifetime mismatch will cause IKE phase 1 to fail. During IKE phase 1 negotiation, both peers exchange their configured ISAKMP lifetimes, and Cisco IOS requires these values to be identical for the session to succeed; if one peer uses 3600 seconds while the other uses the default of 86400 seconds, the negotiation fails because the devices cannot agree on a common lifetime. This concept is a frequent trap on the Cisco CCNP ENARSI 300-410 exam, where candidates often overlook that while many IKE parameters like encryption or Diffie-Hellman group can be negotiated, the ISAKMP lifetime must match exactly on both ends—a mismatch silently breaks the tunnel without an obvious error. To remember this, think of the phrase "lifetime locks the link": if the lifetimes don't lock together, phase 1 won't unlock the tunnel.
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. 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.
Consider the following configuration on router R2:
crypto isakmp policy 10
encryption aes 256
authentication pre-share
group 14
lifetime 3600
!
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 will fail due to ISAKMP lifetime mismatch.
The correct answer is B because the ISAKMP lifetime configured on R2 is 3600 seconds (1 hour), but the question implies the remote peer (192.168.1.1) has a different lifetime (likely the default of 86400 seconds). During IKE phase 1 negotiation, the peers propose their lifetimes, and if they do not match, the session fails because Cisco IOS requires identical lifetime values for successful phase 1 establishment. This mismatch causes the IKE phase 1 to fail, preventing the IPsec tunnel from forming.
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.
✗
The IPsec tunnel will establish successfully because the transform sets match.
Why it's wrong here
Even though transform sets match, the ISAKMP lifetime mismatch will prevent IKE phase 1 completion.
✓
The IKE phase 1 will fail due to ISAKMP lifetime mismatch.
Why this is correct
ISAKMP policies must match exactly, including lifetime. A mismatch causes negotiation failure.
Related concept
Read the scenario before looking for a memorised answer.
✗
The tunnel will work but will rekey every hour due to the shorter lifetime.
Why it's wrong here
The tunnel cannot establish at all because IKE phase 1 fails.
✗
The pre-shared key mismatch will cause the tunnel to fail.
Why it's wrong here
Both sides use the same key 'secretkey', so that is not an issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that only transform sets (phase 2) need to match, while IKE phase 1 parameters like lifetime are overlooked, causing candidates to incorrectly select option A.
Detailed technical explanation
How to think about this question
In Cisco IOS, IKE phase 1 negotiation uses a proposal exchange where each parameter (encryption, hash, DH group, authentication, lifetime) must match exactly between peers. The lifetime is a critical parameter; if one peer proposes 3600 seconds and the other proposes 86400 seconds (default), the negotiation fails with a 'no acceptable proposal' error. This behavior is defined in RFC 2409 (IKE) and Cisco's implementation enforces strict matching, unlike some vendors that accept the smaller lifetime. In real-world scenarios, this often catches administrators who forget to align lifetimes when using non-default values.
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 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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: The IKE phase 1 will fail due to ISAKMP lifetime mismatch. — The correct answer is B because the ISAKMP lifetime configured on R2 is 3600 seconds (1 hour), but the question implies the remote peer (192.168.1.1) has a different lifetime (likely the default of 86400 seconds). During IKE phase 1 negotiation, the peers propose their lifetimes, and if they do not match, the session fails because Cisco IOS requires identical lifetime values for successful phase 1 establishment. This mismatch causes the IKE phase 1 to fail, preventing the IPsec tunnel from forming.
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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Question Discussion
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