MAC headers in layer 2 mode

6.10 LPAR mode z/VM guest

In LAN environments, data packets find their destination through Media Access Control (MAC) addresses in their MAC header.

This graphic is described in the surrounding text. — Figure 1. Standard IPv4 processing

MAC address handling as shown in Figure 1 applies to non-mainframe environments and a mainframe environment with an OSA-Express adapter where the layer2 option is enabled.

The layer2 option keeps the MAC addresses on incoming packets. Incoming and outgoing packets are complete with a MAC header at all stages between the Linux® network stack and the LAN as shown in Figure 1. This layer2-based forwarding requires unique MAC addresses for all concerned Linux instances.

In layer 2 mode, the Linux TCP/IP stack has full control over the MAC headers and the neighbor lookup. The Linux TCP/IP stack does not configure IPv4 or IPv6 addresses into the hardware, but requires a unique MAC address for the card.

For Linux as a z/VM® guest, the qeth device driver obtains a MAC address for each L2 device from the z/VM host. No configuration is necessary.

For Linux in LPAR mode with a directly attached OSA adapter in QDIO mode, assign a unique MAC address. You can set a MAC address by issuing the command:

ip link set addr <MAC address> dev <interface>

Note: Be sure not to assign the MAC address of the OSA-Express adapter to your Linux instance.

For OSX and OSM CHPIDs, you cannot set your own MAC addresses. Linux uses the MAC addresses defined by the Unified Resource Manager.

For HiperSockets connections, a MAC address is generated.