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 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?
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 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.
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.
✗
Incorrect pre-shared key
Why it's wrong here
Incorrect pre-shared key would cause Phase 1 to fail, not Phase 2.
✗
Mismatched authentication algorithm
Why it's wrong here
Mismatched authentication algorithm would also cause Phase 1 to fail.
✗
Firewall rule blocking IKE traffic
Why it's wrong here
If IKE were blocked, Phase 1 would not establish.
✓
Mismatched proxy IDs (traffic selectors)
Why this is correct
Proxy IDs define which traffic should be encrypted; if they don't match, Phase 2 fails.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
Proxy IDs (traffic selectors) are defined in the IPsec transform set and specify the source and destination subnets, ports, and protocol to be encrypted. In Cisco IOS, these are configured with the 'crypto map' or 'ipsec profile' commands, and mismatches often occur when one side uses a broader subnet (e.g., 10.0.0.0/8) while the other uses a narrower one (e.g., 10.0.1.0/24). Debug commands like 'debug crypto ipsec' or 'debug crypto isakmp' can reveal the exact mismatch in the Phase 2 proposal.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
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: 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.
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.
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.
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 →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.