Central SNAT vs Policy-Based NAT Precedence
An administrator configures a policy-based NAT rule to translate traffic from 10.0.0.0/8 to 203.0.113.1 using an IP Pool with overload. Later, they also enable Central SNAT for the same traffic. The traffic is not being NAT'd as expected. What is the MOST likely reason?
Quick Answer
The answer is that Central SNAT overrides policy-based NAT when both are configured for the same traffic. This happens because Fortinet’s FortiOS processes Central SNAT rules at a higher priority in the NAT evaluation order, meaning the policy-based NAT rule is effectively ignored once a Central SNAT rule matches the same source and destination. On the Fortinet NSE 4 Network Security Professional exam, this concept tests your understanding of NAT precedence and the distinction between policy-based NAT (configured within a firewall policy) and Central SNAT (configured under Policy & Objects > NAT). A common trap is assuming policy-based NAT will still apply as a fallback, but FortiOS strictly enforces Central SNAT as the primary mechanism. To remember: think “Central rules centralize control—they take the front seat over policy-based NAT.”
⚠ Common exam trap
Many candidates assume policy-based NAT is always applied because it is configured directly on the firewall policy, but FortiGate's Central SNAT has a higher precedence, leading to unexpected NAT behavior when both are enabled.
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
✓
Central SNAT overrides policy-based NAT
Central SNAT (Source NAT) is a centralized NAT policy that takes precedence over policy-based NAT rules when both are configured for the same traffic. In FortiGate, Central SNAT is evaluated before policy-based NAT, and if a matching Central SNAT rule exists, it will override any policy-based NAT configuration. This is by design to provide a more predictable and manageable NAT architecture.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Both NAT methods are applied, causing double NAT
Why it's wrong here
Only one method is applied, not both.
- ✓
Central SNAT overrides policy-based NAT
Why this is correct
When Central NAT is enabled, policy-based NAT rules are ignored for the matching traffic.
- ✗
The IP Pool used in policy-based NAT is also used in Central SNAT, causing a conflict
Why it's wrong here
The conflict is due to precedence, not pool usage.
- ✗
Policy-based NAT always overrides Central SNAT
Why it's wrong here
Central SNAT has higher precedence.
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Same concept, more angles
2 more ways this is tested on NSE4
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A FortiGate administrator configures a Central SNAT policy to translate internal IPs to a single public IP for internet access. However, traffic from a specific internal server (10.0.1.100) must use a different public IP. The administrator also creates a policy-based NAT rule in the firewall policy for that server. Which NAT method takes precedence?
hard- ✓ A.Central SNAT takes precedence over policy-based NAT
- B.Policy-based NAT takes precedence because it is more specific
- C.Central SNAT takes precedence because it is evaluated after policy-based NAT
- D.The most recently created rule takes precedence
Why A: In FortiGate, when both Central SNAT and policy-based NAT (configured within a firewall policy) are present, Central SNAT takes precedence. This is because Central SNAT is evaluated before policy-based NAT in the NAT processing order, and once a match is found in Central SNAT, the system applies it and does not proceed to policy-based NAT. The specific server's traffic (10.0.1.100) would still be subject to the Central SNAT rule unless a more specific Central SNAT rule is created for that IP.
Variation 2. An administrator configures Central SNAT with a dynamic IP pool for internet-bound traffic. Some users report that certain applications fail when they should be translated to a specific public IP. The administrator checks the policy-based NAT rules and finds none. What is the most likely reason for the failure?
hard- ✓ A.A higher priority Central SNAT rule matches the traffic first
- B.The traffic is being dropped by a security profile
- C.The firewall policy has NAT disabled
- D.The IP pool is configured on the wrong interface
Why A: Central SNAT rules are evaluated in order of priority, and the first matching rule is applied. If a higher-priority Central SNAT rule matches the traffic before the intended rule with the specific public IP, the traffic will be translated to the IP defined in that higher-priority rule, causing the applications to fail. Since no policy-based NAT rules exist, the issue lies in the Central SNAT rule priority order.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This NSE4 practice question is part of Courseiva's free Fortinet 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 NSE4 exam.