- A
Increase the MTU on the WAN interfaces to 1500 bytes on all firewalls.
Why wrong: MTU mismatch can cause fragmentation, but the logs show rekey failures, not fragmentation errors; also, MTU increase may not help if the path already supports 1500.
- B
Change the encryption algorithm from AES-256 to 3DES on all peers.
Why wrong: The encryption algorithm is unlikely to cause rekey failures; mismatched lifetimes or DPD settings are the typical cause.
- C
Migrate all VPN connections from IPsec to SSL VPN using clientless access.
Why wrong: This is a major redesign and not targeted at the symptom; rekey failures are specific to IPsec Phase 2.
- D
Adjust the Dead Peer Detection (DPD) intervals and Phase 2 lifetime settings to be consistent across all sites.
Consistent DPD timers and lifetimes prevent premature rekey attempts or missed rekeys, stabilizing the tunnels.
CISSP Communication and Network Security Practice Question
This CISSP practice question tests your understanding of communication and network security. 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.
A multinational corporation maintains site-to-site IPsec VPN tunnels between its headquarters and three regional branch offices. Over the past week, the tunnels have been dropping intermittently, causing disruption to real-time applications. The network team checked logs and found frequent 'Phase 2 rekey failure' messages. The tunnels are configured with IKEv1 and preshared keys. The headquarters uses a Cisco ASA, and the branches use various vendors' firewalls. The team verified that firewall policies allow IPsec traffic, and there is no packet loss on the WAN links. Which action should the team take to resolve the issue most effectively?
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
Adjust the Dead Peer Detection (DPD) intervals and Phase 2 lifetime settings to be consistent across all sites.
The frequent 'Phase 2 rekey failure' messages indicate a mismatch in IPsec security association (SA) parameters between the Cisco ASA and the branch firewalls. IKEv1 Phase 2 lifetimes and Dead Peer Detection (DPD) intervals must be consistent across all peers; otherwise, one side may attempt to rekey or declare the peer dead while the other expects a different timing, causing intermittent tunnel drops. Adjusting these values to match across all sites resolves the rekey failures without compromising security or requiring a protocol migration.
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.
- ✗
Increase the MTU on the WAN interfaces to 1500 bytes on all firewalls.
Why it's wrong here
MTU mismatch can cause fragmentation, but the logs show rekey failures, not fragmentation errors; also, MTU increase may not help if the path already supports 1500.
- ✗
Change the encryption algorithm from AES-256 to 3DES on all peers.
Why it's wrong here
The encryption algorithm is unlikely to cause rekey failures; mismatched lifetimes or DPD settings are the typical cause.
- ✗
Migrate all VPN connections from IPsec to SSL VPN using clientless access.
Why it's wrong here
This is a major redesign and not targeted at the symptom; rekey failures are specific to IPsec Phase 2.
- ✓
Adjust the Dead Peer Detection (DPD) intervals and Phase 2 lifetime settings to be consistent across all sites.
Why this is correct
Consistent DPD timers and lifetimes prevent premature rekey attempts or missed rekeys, stabilizing the tunnels.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the misconception that rekey failures are caused by encryption algorithm mismatches or MTU issues, but the real cause is almost always inconsistent Phase 2 lifetimes or DPD intervals when using IKEv1 with multiple vendor firewalls.
Trap categories for this question
Command / output trap
MTU mismatch can cause fragmentation, but the logs show rekey failures, not fragmentation errors; also, MTU increase may not help if the path already supports 1500.
Detailed technical explanation
How to think about this question
In IKEv1, Phase 2 (IPsec SA) lifetimes are negotiated during Quick Mode; if peers have mismatched lifetimes (e.g., 3600 seconds on ASA vs. 28800 seconds on a branch firewall), the side with the shorter lifetime will initiate a rekey that the other side may reject or ignore, leading to 'Phase 2 rekey failure' errors. DPD (RFC 3706) uses R-U-THERE messages to detect dead peers; inconsistent DPD intervals can cause premature tunnel teardown if one side expects a response before the other sends keepalives. Real-world troubleshooting often involves using 'show crypto ipsec sa' and 'debug crypto isakmp' to compare lifetime values and DPD settings across all peers.
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 developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach 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.
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FAQ
Questions learners often ask
What does this CISSP question test?
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Adjust the Dead Peer Detection (DPD) intervals and Phase 2 lifetime settings to be consistent across all sites. — The frequent 'Phase 2 rekey failure' messages indicate a mismatch in IPsec security association (SA) parameters between the Cisco ASA and the branch firewalls. IKEv1 Phase 2 lifetimes and Dead Peer Detection (DPD) intervals must be consistent across all peers; otherwise, one side may attempt to rekey or declare the peer dead while the other expects a different timing, causing intermittent tunnel drops. Adjusting these values to match across all sites resolves the rekey failures without compromising security or requiring a protocol migration.
What should I do if I get this CISSP 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.
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 30, 2026
This CISSP practice question is part of Courseiva's free ISC2 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 CISSP exam.
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