Why can't the same subnet be used on WAN and LAN on USG FLEX H?

Zyxel_Emily

Question:
Why can't the same subnet be used on WAN and LAN on USG FLEX H?

Answer:
In the case of USG FLEX H device, it does not allow configuring a LAN and WAN interface within the same subnet. Attempting this on other devices, such as ATP or USG FLEX, may lead to problems, known as ARP flux. ARP flux occurs when a device with multiple network interfaces on the same subnet responds to ARP requests from various interfaces instead of a designated one, leading to network instability.

Although ATP/USG FLEX device does not block such misconfigurations, but it's strongly recommended to avoid using the same subnet for different interfaces.