OSA-Express virtual MAC routing

If multiple TCP/IP instances are sharing an OSA-Express feature, the preferred method of routing is to define or generate a virtual MAC (VMAC) for each stack and for each protocol being used (IPv4 or IPv6). For IPv4, this results in the OSA-Express feature using the VMAC address rather than the physical burned in MAC for all ARPs sent for that TCP/IP stack's registered IP addresses, and using the VMAC as the source MAC address for all packets sent from that stack. In this way, all routers on the same LAN as the OSA-Express feature use only the VMAC address as the destination for all packets destined for that specific TCP/IP stack. From a network routing perspective, the OSA-Express feature with this VMAC appears as a dedicated device to that TCP/IP stack.

This simplifies a shared OSA configuration significantly. The routers on the LAN always send any packets destined for a particular TCP/IP stack to the VMAC defined for that stack. The OSA-Express feature knows by VMAC address exactly which stack should receive a given packet. Even if the IP address is not registered with the OSA-Express feature, if the packet is destined for that VMAC, the router has determined which stack should be the intermediate router, and the OSA can forward the packet directly to that stack. If the stack is not an intermediate router, the capability is provided for a stack to indicate to the OSA that it wants to receive packets to registered IP addresses only.

This simplification is true for IPv6 as well. TCP/IP uses the VMAC address for all neighbor discovery address resolution flows for that stack's IP addresses, and likewise uses the VMAC as the source MAC address for all IPv6 packets sent from that stack. Again, from a network perspective, the OSA-Express feature with this VMAC appears as a dedicated device to that stack.

The VMAC address can be defined in the stack, or it can be generated by the OSA. If generated by the OSA, it is guaranteed to be unique from all other physical MAC addresses and from all other VMAC addresses generated by any OSA-Express feature.

Rule: If VMACs are defined in the stack, they should be defined as locally administered MAC addresses, and should be unique addresses for the local LAN on which they are.

Guidelines:

If the OSA is configured for both IPv4 (using DEVICE and LINK) and IPv6 for a stack, then you can define the same VMAC for both the INTERFACE statement and the LINK statement, or you can define one VMAC on the LINK statement for IPv4 usage and a different VMAC on the INTERFACE statement for IPv6 usage. If the OSA is configured for both IPv4 (using the INTERFACE statement) and IPv6 for a stack, then you must define one VMAC on the INTERFACE statement for IPv4 usage, and a different VMAC on the INTERFACE statement for IPv6 usage.
A VLAN ID can be associated with an OSA-Express link or interface that is defined with a VMAC. For more information about VMAC routing, see z/OS Communications Server: SNA Network Implementation Guide.

To enable virtual MAC support, you must be running at least an IBM® System z9® Enterprise Class (z9® EC) or z9 Business Class (z9 BC), and an OSA-Express feature with OSA Layer 3 Virtual MAC support. OSA Layer 3 Virtual MAC support is not available for Fast Ethernet. For more information, see the 2094DEVICE Preventive Service Planning (PSP) bucket and the 2096DEVICE Preventive Service Planning (PSP) bucket.