The correct answer is to create a service object for SIP and set 'alg-mode' to 'disable'. This works because disabling the SIP ALG on the firewall policy stops the FortiGate from inspecting and modifying SIP traffic at the application layer, which is a common troubleshooting step when the SIP ALG interferes with VoIP traffic by altering SIP headers or signaling in ways that break compatibility with specific providers or endpoints. On the Fortinet NSE 4 Network Security Professional exam, this scenario tests your understanding of how application-layer gateways can disrupt voice traffic and how to selectively disable them using a custom service object rather than a blanket policy change. A common trap is assuming the SIP ALG must always be enabled for VoIP to work, but in practice, many modern SIP endpoints handle their own NAT traversal, making the ALG a source of problems. Memory tip: think "SIP ALG off, voice calls soft" — disabling the ALG often smooths out call setup and media flow.
NSE4 Firewall Policies and NAT Practice Question
This NSE4 practice question tests your understanding of firewall policies and nat. 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 FortiGate admin is troubleshooting a policy that should allow VoIP traffic. The admin suspects that the SIP ALG is interfering. Which TWO actions should the admin take to verify or resolve the issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Disable the SIP ALG on the firewall policy
Option A is correct because disabling the SIP ALG on the firewall policy stops the FortiGate from inspecting and modifying SIP traffic at the application layer. This is a common troubleshooting step when the SIP ALG interferes with VoIP traffic, as it can alter SIP headers or signaling in ways that break compatibility with certain VoIP providers or endpoints.
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.
✓
Disable the SIP ALG on the firewall policy
Why this is correct
Disabling the ALG for the policy prevents ALG from modifying packets.
Related concept
Read the scenario before looking for a memorised answer.
✗
Enable 'set sip-nat-trace' on the policy
Why it's wrong here
This is for SIP NAT tracing, not directly for ALG issues.
✗
Enable 'set ssl-ssh-profile' on the policy
Why it's wrong here
SSL/SSH profile is for encryption inspection, not SIP ALG.
✗
Increase the session TTL for SIP
Why it's wrong here
TTL adjustments do not address ALG interference.
✓
Create a service object for SIP and set 'alg-mode' to 'disable'
Why this is correct
This disables the ALG for the SIP service globally.
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 may confuse 'sip-nat-trace' (a diagnostic tool) with a fix for ALG interference, or think increasing session TTL addresses the problem, when the real solution is to disable the ALG's application-layer processing.
Detailed technical explanation
How to think about this question
The SIP ALG in FortiGate performs deep packet inspection (DPI) on SIP traffic, rewriting headers like Contact and Via to maintain NAT traversal. However, this can cause issues with encrypted SIP (TLS) or non-standard implementations. Disabling the ALG per-policy (Option A) or per-service (Option E) stops this rewriting, while 'set sip-nat-trace' (Option B) only logs the ALG's actions without changing them. In real-world scenarios, VoIP providers often recommend disabling the ALG to prevent one-way audio or registration failures.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation 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.
Firewall Policies and NAT — This question tests Firewall Policies and NAT — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Disable the SIP ALG on the firewall policy — Option A is correct because disabling the SIP ALG on the firewall policy stops the FortiGate from inspecting and modifying SIP traffic at the application layer. This is a common troubleshooting step when the SIP ALG interferes with VoIP traffic, as it can alter SIP headers or signaling in ways that break compatibility with certain VoIP providers or endpoints.
What should I do if I get this NSE4 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.
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Question Discussion
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