Most appliance models include a "fail-to-wire" (Ethernet bypass) feature for inline mode. If power fails, a relay closes and the input and output ports become electrically connected, allowing the Ethernet signal to pass through from one port to the other as if the appliance were not there. In fail-to-wire mode, the appliance looks like a cross-over cable connecting the two ports.
Any failure of the appliance hardware or software also closes the relay. When the appliance is restarted, the bypass relay remains closed until the appliance is fully initialized, maintaining network continuity at all times. This feature is automatic and requires no user configuration.
When the bypass relay is closed, the appliance's bridge ports are inaccessible.
If carrier is lost on one of the bridge ports, the carrier is dropped on the other bridge port to ensure that the link-down condition is propagated to the device on the other side of the appliance. Units that monitor link state (such as routers) are thus notified of conditions on the other side of the bridge.