Configuring Dual WAN Failover & Load Balancing on USG FLEX H with FWA515

Created December 11, 2025 10:10 - Updated January 12, 2026 14:55

Serhii Boiarynov

The purpose of this article is to provide technical guidance for implementing dual-WAN internet access with automatic switchover and optional traffic distribution. This document presents a practical configuration of two WAN interfaces on a Zyxel USG FLEX 200H in the Nebula environment. The primary connection is a wired ISP link, while the second WAN uses a Zyxel FWA5015 5G router. Based on the official documentation, the following step-by-step configuration process is provided.

Failover — using a second WAN (FWA515) as a backup channel.
Load Balancing — simultaneous operation of two WANs via a WAN trunk.

In Nebula, go to Site-wide > Configure > Firewall > Interface, set WAN1 according to the ISP’s parameters, and configure WAN2 as a DHCP client when the FWA505 operates in router mode.

Configure a Primary and Backup WAN

Note: Load balancing and trunk configuration can currently be done only from the local web interface, so you must log in to the device via its local GUI to set this up.

Note on Selecting a Load Balancing Algorithm

In configurations specifically designed for a WAN backup scenario, where the primary WAN is set to Active and the secondary interface is assigned as Passive, the choice of load balancing algorithm (Weighted Round Robin, Least Load First, or Spillover) does not affect how traffic is distributed. A Passive interface does not participate in balancing and is used exclusively as a standby link.

As a result, a Primary + Backup setup always functions as a pure failover: all traffic flows through WAN1, and WAN2 is activated only when the primary connection becomes unavailable. Spillover is commonly selected for such designs, as it logically matches the “primary + backup WAN” approach and allows adding load thresholds later if partial load distribution becomes desirable.

However, when working with three WAN interfaces, the behavior depends on the specific configuration and which interfaces are set to Active or Passive.

Practical Scenarios for WAN Back-up

This scenario describes how to configure a backup WAN connection where the primary WAN link is used under normal conditions, and the secondary WAN link is activated automatically only if the primary link fails. This is a common and recommended setup for offices that require stable and uninterrupted internet access.

Verify WAN Interfaces

Before configuring the WAN Trunk, make sure that both WAN interfaces are correctly created and active.

Menu path:
Network → Interface → Interface

Check the following:

Both interfaces (for example, ge1 and ge2) are assigned to the WAN zone
Each interface has a valid IP configuration:
- Static IP address, or
- DHCP, depending on the ISP
The physical ports are connected and the link status is Up

Example:

ge1 (WAN) — primary internet connection
ge2 (WAN) — backup internet connection

Create a User-Defined WAN Trunk

(Photo 2: Trunk creation and member settings)

Go to:
Network → Interface → Trunk

Click Add and configure:

Name: Wan_Backup
Algorithm: Weighted Round Robin (WRR)

Add WAN interfaces to the trunk:

ge1 (WAN) → Mode: Active
ge2 (WAN) → Mode: Passive

This ensures that the backup WAN is used only when the primary WAN fails.

Practical Scenarios for WAN Load Balancing

Spillover Scenario

Primary WAN (WAN1): Unlimited bandwidth, preferred for regular traffic.
Secondary WAN (WAN2): Metered connection, used only when primary exceeds 800 Mbps.
Outcome: Regular traffic utilizes WAN1, and only excess traffic spills over to WAN2, saving costs on metered usage.

Least Load First Scenario

Multiple WAN Connections: Different bandwidth capacities.
Outcome: Traffic is dynamically routed to the interface with the least load, optimizing the usage of available bandwidth and improving overall network performance.

Configuring WAN Load Balancing

Navigate to Network Interface:
- Go to the Network section in your firewall’s configuration interface.
- Select Interface and then Trunk.

Create User-Defined Trunk:
- Click on the Add button to create a new trunk.
- Choose the load-balancing algorithm.
Set Load Balancing Index:
- Select the appropriate load balancing index (Inbound, Outbound, or Sum).
Add Interfaces:
- Add the WAN interfaces you want to include in the trunk.
- Specify the bandwidth limits for each interface

Set Default Trunk Routing:

After creating a user-defined trunk, go to the Network section and select Interface.
Click on Default Trunk and choose the user-defined trunk profile to set it as the default WAN load balancing policy.