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CISSP Practice Question: A multinational corporation maintains…
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?
⚠ Common exam trap
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.
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.
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
Increasing the Maximum Transmission Unit (MTU) on WAN interfaces primarily addresses packet fragmentation issues, not directly IPSec rekey failures. Rekey failures typically stem from mismatches in security association (SA) parameters, timers, or authentication, not packet size. While excessive fragmentation could theoretically impede rekey message delivery, the core problem of a rekey failure lies in the negotiation process itself, making MTU adjustment an unlikely solution for this specific symptom.
- ✗
Change the encryption algorithm from AES-256 to 3DES on all peers.
Why it's wrong here
Changing the encryption algorithm from AES-256 to 3DES is a downgrade in security and does not resolve rekey failures. If the initial IPSec Security Association (SA) was established successfully, the chosen encryption algorithm is not the root cause of subsequent rekeying problems. Rekey failures are almost always attributed to mismatched Phase 1 or Phase 2 lifetimes, Dead Peer Detection (DPD) settings, or authentication issues, not the cryptographic strength of the agreed-upon algorithm.
- ✗
Migrate all VPN connections from IPsec to SSL VPN using clientless access.
Why it's wrong here
Migrating all VPN connections from IPSec to SSL VPN using clientless access is a drastic architectural change that does not target the specific symptom of IPSec rekey failures. This action would involve a complete redesign and redeployment of the VPN infrastructure, incurring significant cost and downtime, rather than troubleshooting and resolving a configuration issue within the existing IPSec tunnels. The problem is with IPSec's SA management, not the choice of VPN technology itself.
- ✓
Adjust the Dead Peer Detection (DPD) intervals and Phase 2 lifetime settings to be consistent across all sites.
Why this is correct
Inconsistent Dead Peer Detection (DPD) intervals can cause one peer to prematurely declare the other dead and tear down the tunnel, disrupting rekey attempts. Similarly, mismatched Phase 2 Security Association (SA) lifetimes will cause peers to attempt rekeying at different times, leading to negotiation failures. Ensuring these critical parameters are synchronized across all sites allows for coordinated rekeying and stable tunnel operation, preventing premature disconnections and rekey failures.
Visual reference
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.
Go deeper
Related to this question
Learn chapter
Security Governance and Principles
Key term
Security
Security in IT is the practice of protecting systems, networks, and data from unauthorized access, damage, or theft.
Key term
Firewall
A firewall is a network security system that monitors and controls incoming and outgoing traffic based on predetermined security rules to protect trusted internal networks from untrusted external networks.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.