Layer 2 and layer 3

In layer 2 mode, OSA routing to the destination is based on MAC addresses. A local MAC address is assigned to each interface of a Linux® instance and registered in the OSA Address Table. These MAC addresses are unique and different from the MAC address of the OSA adapter.

In layer 3 mode, all interfaces of all Linux instances share the MAC address of the OSA adapter. OSA routing to the destination Linux instance is based on IP addresses.

The layer 2 discipline (qeth_l2)

The layer 2 discipline supports:

• OSA devices and z/VM® virtual NICs that couple to VSWITCHes or QDIO guest LANs running in layer 2 mode
• HiperSockets devices (as of System z10®)
• OSM (OSA-Express for Unified Resource Manager) devices
• OSX (OSA-Express for zBX) devices for IEDN

The layer 2 discipline is the default setup for OSA. On HiperSockets the default continues to be layer 3.

For z/VM NICs that are coupled to a guest LAN or VSWITCH, the qeth device driver detects the required layer and configures it automatically. If a qeth device is created before the NIC is coupled, the qeth device driver defaults to layer 2.

The layer 3 discipline (qeth_l3)

The layer 3 discipline supports:

• OSA devices and z/VM virtual NICs that couple to VSWITCHes or QDIO guest LANs running in layer 3 mode (with faked link layer headers)
• HiperSockets and HiperSockets guest LAN devices that are running in layer 3 mode (with faked link layer headers)
• OSX (OSA-Express for zBX) devices for IEDN

This discipline supports those devices that are not capable of running in layer 2 mode. Not all Linux networking features are supported and others need special setup or configuration. Some performance-critical applications might benefit from being layer 3.

Layer 2 and layer 3 interfaces cannot communicate within a HiperSockets LAN or within a VSWITCH or guest LAN. However, a shared OSA adapter can convert traffic between layer 2 and layer 3 networks.