The answer is that the VPN server is not listening on UDP port 500. This is correct because the symptom—internal users can reach internet websites but cannot reach the external VPN server—isolates the problem to the host itself rather than the network path or firewall, which is already verified as correctly configured. In IPsec-based VPNs, UDP port 500 is the standard port for Internet Key Exchange (IKE) traffic; if the VPN server’s service is stopped, misconfigured, or bound to a different port, it will not respond to incoming IKE requests, even when firewall rules are perfect. On the CISSP exam, this scenario tests your ability to distinguish between network-layer issues (firewall, routing) and host-level failures, a common trap where candidates blame the firewall first. Remember the mnemonic “Host First, Firewall Last”—when general internet access works but a specific service fails, always check if the server is actually listening on the required port.
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 network where internal users can access internet websites but cannot reach the company's external VPN server (IP 203.0.113.50, UDP port 500). The firewall rule for VPN traffic is correctly configured. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The VPN server is not listening on UDP port 500.
Option D is correct because the symptom—internal users can reach internet websites but cannot reach the external VPN server—indicates a host-level issue rather than a network or firewall problem. Since the firewall rule for VPN traffic is correctly configured and other traffic flows normally, the most likely cause is that the VPN server itself is not listening on UDP port 500, which is the standard port for IPsec IKE (Internet Key Exchange) traffic. This could be due to a misconfiguration, service failure, or the server being configured to use a different port or protocol.
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.
✗
The VPN server is using TCP port 443 instead of UDP 500.
The firewall rule is applied to the wrong interface.
Why it's wrong here
The rule is correctly configured per the question.
✗
The firewall is stateful and blocking the return traffic.
Why it's wrong here
Stateful firewalls allow return traffic for established sessions.
✓
The VPN server is not listening on UDP port 500.
Why this is correct
If the server does not have the VPN service running, it won't respond, causing the client to time out.
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
The trap here is that candidates often assume a firewall misconfiguration (like stateful blocking or wrong interface) is the cause, but the question explicitly states the firewall rule is correctly configured, forcing you to look at the endpoint itself—a classic CISSP test of reading comprehension and layered troubleshooting.
Detailed technical explanation
How to think about this question
IPsec IKE uses UDP port 500 for initial key exchange and UDP port 4500 for NAT traversal (NAT-T). If the VPN server is not listening on UDP 500, it will not respond to IKE initiation packets, causing the connection to time out. This can be verified with a port scan (e.g., `nmap -sU -p 500 203.0.113.50`) or by checking the server's IPsec service status (e.g., `ipsec status` on Linux or the Windows IPsec Policy Agent). In real-world scenarios, misconfigured VPN servers often default to TCP 443 for SSL VPNs, leaving UDP 500 unbound.
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: The VPN server is not listening on UDP port 500. — Option D is correct because the symptom—internal users can reach internet websites but cannot reach the external VPN server—indicates a host-level issue rather than a network or firewall problem. Since the firewall rule for VPN traffic is correctly configured and other traffic flows normally, the most likely cause is that the VPN server itself is not listening on UDP port 500, which is the standard port for IPsec IKE (Internet Key Exchange) traffic. This could be due to a misconfiguration, service failure, or the server being configured to use a different port or protocol.
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.
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Question Discussion
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