If only one TCP/IP instance is using the OSA-Express device, or when multiple TCP/IP instances are using the same OSA-Express device but you want all instances to share the same physical MAC address of the device, you can optionally have the OSA route unicast packets to unregistered IP addresses by using OSA's primary (PRIROUTER) and secondary (SECROUTER) router support. This function enables a single TCP/IP stack, on a per-protocol (IPv4 and IPv6) basis, to register and act as a router stack on a per-OSA basis. Secondary routers can also be configured to provide for conditions in which the primary router becomes unavailable and the secondary router takes over for the primary router. The primary router stack is the only stack to which OSA forwards packets when the destination IP address has not been previously registered with OSA. If no active TCP/IP instance using this device is defined as the primary router (PRIROUTER) or secondary router (SECROUTER), the device discards the datagram. If the PRIROUTER or SECROUTER value is not specified, the default value is NONROUTER.