Product Documentation

Ethernet Bypass and Link-Down Propagation

Aug 09, 2017

Note: Link-Down propagation was added to the SD-WAN 2000, 3000, 4000, and 5000 appliances with the 7.2.1 release.

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.

Link-down propagation has two operating modes:

  • If the Primary port is not enabled, the link-down state on one bridge port is mirrored briefly on the other bridge port, and then the port is re-enabled. This allows the appliance to be reached through the still-connected port for management, HA heartbeat, and other tasks.
  • If the Primary port is enabled, the appliance assumes (without checking) that the Primary port is used for management, HA heartbeat, and other tasks. The link-down condition on one bridge port is mirrored persistently on the other port, until carrier is restored or the unit is rebooted. This is true even if the Primary port is enabled in the GUI but not connected to a network, so the Primary port should be disabled (the default) when not in use.