MAC headers in layer 3 mode

Red Hat Enterprise Linux 8.6 LPAR mode z/VM guest

A qeth layer 3 mode device driver is an Ethernet offload engine for IPv4 and a partial Ethernet offload engine for IPv6. Hence, there are some special things to understand about the layer 3 mode.

To support IPv6 and protocols other than IPv4, the device driver registers a layer 3 card as an Ethernet device to the Linux® TCP/IP stack.

In layer 3 mode, the OSA-Express adapter in QDIO mode removes the MAC header with the MAC address from incoming IPv4 packets. It uses the registered IP addresses to forward a packet to the recipient TCP/IP stack. Thus the OSA-Express adapter is able to deliver IPv4 packets to the correct Linux images. Apart from broadcast packets, a Linux image can get only packets for IP addresses it configured in the stack and registered with the OSA-Express adapter.

Figure 1. MAC address handling in layer3 mode
This graphic is described in the text that surrounds it.

The OSA-Express QDIO microcode builds MAC headers for outgoing IPv4 packets and removes them from incoming IPv4 packets. Hence, the operating systems' network stacks send and receive only IPv4 packets without MAC headers.

This lack of MAC headers can be a problem for applications that expect MAC headers. For examples of how such problems can be resolved, see Setting up for DHCP with IPv4.