How automatic internet failover works

A failover (dual-WAN) router connects to your main line and to a backup. It regularly checks whether the main line can reach the internet. When the checks fail, it sends traffic over the backup, then switches back once the main line recovers.

The pieces

Primary WAN: your cable, fiber, or DSL modem.
Secondary WAN: a cellular modem, Starlink, or a second line.
Health checks: the router pings or tests reachable hosts every few seconds.
Failover: traffic moves to the backup. Connections already open may drop and reconnect, and payment terminals usually retry.
Failback: traffic returns to the primary once it's stable.

Glossary

Dual-WAN: a router with two internet inputs.
Load balancing: using both lines at once (different from failover).
Data cap: monthly allowance on cellular backup plans.
UPS: battery that keeps the modem and router running in a power cut.