- A
Enable 'arp-response' on the phase1 interface
When the hub receives traffic for a spoke, it needs to respond to ARP requests for the spoke's IP on the phase1 interface; otherwise, the hub won't forward.
- B
Configure static routes on the hub for each spoke's local subnet
The hub needs routes to each spoke's subnet so it can forward traffic between spokes.
- C
Set 'mode-cfg' to enable on the hub phase1
Why wrong: Mode-config is for assigning IPs to dial-up clients, not needed in hub-and-spoke.
- D
Enable 'auto-discovery-sender' on the hub
Why wrong: Auto-discovery is for dynamic VPN, not for hub-and-spoke with static tunnels.
- E
Enable 'add-route' on each phase1 interface
Why wrong: Enabling 'add-route' on the hub would automatically create routes for each spoke, which can conflict with the manual routes needed for spoke-to-spoke. It should be disabled.
Quick Answer
The answer is that the hub FortiGate must have ARP reply enabled on its phase1 interface, static routes configured for each spoke’s local subnet, and the add-route option disabled on its phase1 interfaces. These three settings are required because in a hub-and-spoke IPsec VPN, spoke-to-spoke traffic must be routed through the hub, meaning the hub must proxy-ARP for remote spoke IPs on its VPN interface, explicitly know how to reach each spoke’s subnet via static routes, and avoid automatically creating conflicting dynamic routes that would override those manual entries. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this scenario tests your understanding of how the hub acts as a transit router for inter-spoke communication, often appearing as a multi-select question where the common trap is forgetting to disable add-route, which would cause the hub to install its own routes and break the manual static path. A helpful memory tip is “ARP, Route, No Auto”—the hub must answer ARP for spokes, have their routes, and stop automatic route creation to let spoke-to-spoke traffic flow correctly.
NSE4 Authentication and VPN Practice Question
This NSE4 practice question tests your understanding of authentication and vpn. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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.
An administrator is configuring a hub-and-spoke IPsec VPN with a FortiGate as the hub. The spokes must be able to communicate with each other through the hub. Which THREE settings must be enabled on the hub FortiGate?
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
Enable 'arp-response' on the phase1 interface
For spoke-to-spoke traffic to pass through the hub, the hub must have ARP reply enabled (so it responds for the remote spokes' IPs on its phase1 interface), must add the spoke subnets to its routing table, and must have 'add-route' disabled on the hub's phase1 interfaces to prevent automatic route creation that conflicts with manual routes.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Enable 'arp-response' on the phase1 interface
- ✓
Configure static routes on the hub for each spoke's local subnet
Why this is correct
The hub needs routes to each spoke's subnet so it can forward traffic between spokes.
Related concept
CIDR notation defines the prefix length.
- ✗
Set 'mode-cfg' to enable on the hub phase1
Why it's wrong here
Mode-config is for assigning IPs to dial-up clients, not needed in hub-and-spoke.
- ✗
Enable 'auto-discovery-sender' on the hub
Why it's wrong here
Auto-discovery is for dynamic VPN, not for hub-and-spoke with static tunnels.
- ✗
Enable 'add-route' on each phase1 interface
Why it's wrong here
Enabling 'add-route' on the hub would automatically create routes for each spoke, which can conflict with the manual routes needed for spoke-to-spoke. It should be disabled.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related NSE4 subnetting questions on CIDR, address ranges, and subnet selection.
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Authentication and VPN — study guide chapter
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Authentication and VPN practice questions
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FAQ
Questions learners often ask
What does this NSE4 question test?
Authentication and VPN — This question tests Authentication and VPN — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Enable 'arp-response' on the phase1 interface — For spoke-to-spoke traffic to pass through the hub, the hub must have ARP reply enabled (so it responds for the remote spokes' IPs on its phase1 interface), must add the spoke subnets to its routing table, and must have 'add-route' disabled on the hub's phase1 interfaces to prevent automatic route creation that conflicts with manual routes.
What should I do if I get this NSE4 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related NSE4 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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 →
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 is configuring a hub-and-spoke IPsec VPN. The hub has multiple Phase 2 selectors for each spoke. What is the recommended way to simplify configuration on the hub when adding new spokes?
medium- ✓ A.Use a single Phase 2 selector with 0.0.0.0/0.0.0.0 for both local and remote
- B.Configure each spoke in a separate VDOM
- C.Use aggressive mode for Phase 1
- D.Use policy-based VPN instead of route-based
Why A: Using a single Phase 2 with 0.0.0.0/0 as local/remote subnet allows the hub to accept all traffic without needing per-spoke Phase 2 selectors. But security-conscious admins often use specific selectors. The question asks for simplification; dynamic routing (BGP) is even better.
Variation 2. A FortiGate administrator is configuring a hub-and-spoke IPsec VPN with three spokes. Each spoke has a dial-up connection to the hub. The hub uses a dynamic DNS name. Which THREE settings are necessary on each spoke to establish the VPN?
medium- A.A static route on the spoke for the hub's local networks
- ✓ B.The pre-shared key or certificate for authentication
- ✓ C.Hub's public IP address or FQDN as remote gateway
- ✓ D.The Phase 2 proposal (encryption, authentication, etc.)
- E.NAT enabled on the spoke tunnel interface
Why B: For a dial-up IPsec VPN, each spoke needs the hub's public IP or FQDN as the remote gateway. Authentication can be via pre-shared key or certificate. The spoke must also have a Phase 2 proposal that matches the hub's configuration.
Last reviewed: Jun 21, 2026
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.
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