hardMultiple ChoiceObjective-mapped
CISSP Practice Question: Is troubleshooting a site-to-site IPsec VPN…
A security engineer is troubleshooting a site-to-site IPsec VPN between two firewalls. The tunnel status shows Phase 1 is up but Phase 2 is not. Which of the following is the most likely cause?
⚠ Common exam trap
ISC2 often tests the distinction between Phase 1 and Phase 2 failures, and the trap here is that candidates mistakenly attribute Phase 2 failures to authentication or encryption mismatches, which actually affect Phase 1, not the traffic selector negotiation in Phase 2.
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
✓
Mismatched proxy IDs (traffic selectors)
Phase 1 (IKE SA) establishes a secure channel for key exchange, while Phase 2 (IPsec SA) negotiates the specific traffic to be encrypted. If Phase 1 is up but Phase 2 fails, the most common cause is a mismatch in proxy IDs (traffic selectors), such as local/remote subnets or ports, which prevents the two peers from agreeing on which traffic to protect. This is distinct from authentication or encryption mismatches, which would typically cause Phase 1 to fail.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Incorrect pre-shared key
Why it's wrong here
An incorrect pre-shared key (PSK) would cause the Internet Key Exchange (IKE) Phase 1 to fail, not Phase 2. During Phase 1, the PSK is used as a shared secret to authenticate the IPSec peers and derive initial cryptographic keys for securing the IKE communication channel itself. If the PSKs do not match on both ends, the mutual authentication process cannot complete successfully, preventing the establishment of the IKE Security Association (SA) necessary for any subsequent Phase 2 negotiation.
- ✗
Mismatched authentication algorithm
Why it's wrong here
A mismatched authentication algorithm would also lead to a failure during IKE Phase 1. In Phase 1, peers negotiate and agree upon cryptographic parameters, including the authentication algorithm (e.g., MD5, SHA-1, SHA-256) used to ensure the integrity and authenticity of the IKE messages exchanged. If the configured algorithms do not align or if one peer proposes an unsupported algorithm, the IKE Phase 1 SA cannot be established, thereby halting the tunnel setup before Phase 2 can even commence.
- ✗
Firewall rule blocking IKE traffic
Why it's wrong here
A firewall rule blocking IKE traffic would prevent the establishment of IKE Phase 1. IKE uses specific UDP ports, typically 500 for IKEv1 and 4500 for NAT traversal, to initiate and conduct its negotiation. If a firewall on either peer or an intermediate network device blocks these essential ports, the initial IKE messages cannot reach their destination, effectively stopping the entire IPSec tunnel setup process at its earliest stage, long before Phase 2 parameters are considered.
- ✓
Mismatched proxy IDs (traffic selectors)
Why this is correct
Mismatched proxy IDs, also known as traffic selectors, are a common cause for IKE Phase 2 failures. Proxy IDs define the specific source and destination IP addresses, subnets, and protocols that are permitted to traverse the IPSec tunnel and will be protected by the IPSec Security Association (SA). If the local and remote proxy IDs do not precisely match, the Phase 2 SA cannot be successfully established, even if Phase 1 completed, thus preventing the actual data encryption tunnel from forming.
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
IPsec
IPsec is a suite of protocols used to secure Internet Protocol (IP) communications by encrypting and authenticating each IP packet in a data stream.
Key term
Security
Security in IT is the practice of protecting systems, networks, and data from unauthorized access, damage, or theft.
About these practice questions
Courseiva writes every CISSP question from scratch — 747 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
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.