If a network adapter's hardware allows its Media Access Control (MAC) address to be reprogrammed, you can configure the adapter as a ``backup'' device for an existing ``primary'' device. If the primary device fails or it loses its physical connection, the system will automatically switch to using the backup device. This functionality is commonly referred to as ``device failover''. It is also possible to use the Network Configuration Manager to switch between primary and backup devices manually.
You may only configure a backup device after you have configured the primary device. When configuring a backup device, you are asked to select the primary device that it will support.
One backup device may be configured for each primary device. However, you can also configure a backup device for an adapter that is itself a backup device, so creating a ``chain'' of backup devices. Up to 16 devices can exist in a chain: one primary plus 15 backup devices. For example, in a system with four network adapters, you could configure two adapters as primary devices and two adapters as backup devices, one primary device with a chain of two backup devices plus one primary device with no backup device, or one primary device with a chain of three backup devices.
When a network adapter device driver indicates that an adapter has failed or that its network connection has been lost, or the administrator suspends its operation, the system searches down the chain of devices until it finds a backup device that is available to be opened for use. The end of a chain is also implicitly connected back to the head of the chain. This means that the primary device is effectively the backup device for the final device configured in the chain.
An adapter may also be configured as a backup device for more than one primary device although there is little utility in doing this. As soon as it selected for use when one of its primary devices fails, it is no longer available for use by the other primary device.
Network adapters on some high-availability systems can be replaced without shutting down the system and turning off the power. Using a ``hot plug'' adapter as the primary device allows a system to remain connected to the network via the backup device while the primary device is being replaced. The Hot Plug Manager is used to administer such devices. See ``Managing Hot Plugable controllers'' for more information.