A recent System z microcode update for the OSA cards in zEC12 / zBC12 may require an adaptation of the bonding setup in Linux. If you are using Linux bonding in active_backup mode and you are using the same MAC address for all the slaves (fail_over_mac=0 or not specified) and your are on the lastest machines you probably want to check if you need to change your setup.
You can find the complete details in the restriction list on Developerworks.