- A
Mismatched IKE version.
Why wrong: A mismatched IKE version would prevent the tunnel from establishing entirely.
- B
Dead Peer Detection (DPD) interval too long.
Why wrong: Long DPD interval would delay detection but not cause frequent rekeys.
- C
The rekey time settings are too short.
Short rekey intervals cause the tunnel to renegotiate frequently, leading to flapping.
- D
Incorrect local or peer ID.
Why wrong: Incorrect IDs cause authentication failure, not flapping.
Question 478 of 516
Why Is My VPN Tunnel Flapping? Fix Short Rekey Time Settings
This PCNSE practice question tests your understanding of troubleshoot. 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.
An administrator is troubleshooting VPN tunnel flapping. The logs show multiple Phase 2 rekeys. The tunnel uses IKEv2 with pre-shared key. What is the most likely cause?
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
✓
The rekey time settings are too short.
Frequent Phase 2 rekeys indicate that the IPsec security associations (SAs) are being renegotiated too often. With IKEv2, the rekey time settings (e.g., lifetime seconds or kilobytes) control how long a Phase 2 SA remains active before it must be refreshed. If these values are set too short, the tunnel will flap as SAs are constantly re-established, causing intermittent connectivity.
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.
- ✗
Mismatched IKE version.
Why it's wrong here
A mismatched IKE version would prevent the tunnel from establishing entirely.
- ✗
Dead Peer Detection (DPD) interval too long.
Why it's wrong here
Long DPD interval would delay detection but not cause frequent rekeys.
- ✓
The rekey time settings are too short.
Why this is correct
Short rekey intervals cause the tunnel to renegotiate frequently, leading to flapping.
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.
- ✗
Incorrect local or peer ID.
Why it's wrong here
Incorrect IDs cause authentication failure, not flapping.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse rekey flapping with DPD or misconfiguration issues, but the specific log entry of 'multiple Phase 2 rekeys' directly points to the SA lifetime being too short, not to peer reachability or identity problems.
Detailed technical explanation
How to think about this question
In IKEv2, Phase 2 rekeys are triggered by the IPsec SA lifetime, which is negotiated during the CREATE_CHILD_SA exchange. The default lifetime is often 3600 seconds (1 hour) or 4 GB of traffic, but if an administrator sets it to an extremely low value (e.g., 60 seconds), the tunnel will flap as the SA is torn down and rebuilt. This behavior is distinct from Phase 1 rekeys, which affect the IKE SA and are less disruptive to data traffic.
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 PCNSE 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
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this PCNSE question test?
Troubleshoot — This question tests Troubleshoot — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The rekey time settings are too short. — Frequent Phase 2 rekeys indicate that the IPsec security associations (SAs) are being renegotiated too often. With IKEv2, the rekey time settings (e.g., lifetime seconds or kilobytes) control how long a Phase 2 SA remains active before it must be refreshed. If these values are set too short, the tunnel will flap as SAs are constantly re-established, causing intermittent connectivity.
What should I do if I get this PCNSE 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.
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Last reviewed: Jul 4, 2026
This PCNSE practice question is part of Courseiva's free Palo Alto Networks 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 PCNSE exam.
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