For QDIO devices, z/OS® Communications Server and OSA-Express provide functions that control how incoming unicast datagrams are routed, especially when the OSA-Express is shared by multiple TCP/IP instances.

When TCP/IP activates a QDIO device, each TCP/IP registers each of its home IP addresses with OSA-Express. (TCP/IP also dynamically registers any updates to its set of home IP addresses with OSA-Express.) This allows OSA-Express to route datagrams destined for a registered IP address to the correct TCP/IP instance.

However, when packets are received for IP addresses that are not registered by any TCP/IP stack, the device needs a method for determining which stack, if any, should receive the packet. Two functions are available to accomplish this; OSA-Express Virtual MAC (VMAC) routing, and primary and secondary routing.