Why Does the USG FLEX H Series Not Allow Configure Multiple WAN Interfaces in the Same Subnet?

Zyxel_Stanley

Question:
Why does the USG FLEX H series show the error “Duplicate static IP or subnet detected” when assigning multiple WAN interfaces within the same subnet, and is there a way to make this configuration work?

Answer:
The USG FLEX H series is designed to prohibit assigning multiple WAN interfaces within the same subnet. This limitation is intentional and cannot be overridden through configuration.
Because multiple WAN interfaces sharing the same subnet can create routing conflicts, ARP instability, and operational issues, the system blocks this setup entirely.

This limitation cannot be resolved through firewall settings.