Administrator Guide

12 : BIOS vs. UEFI | Doc ID 20444677 | June 2018
Figure 4 UEFI Boot Order Configuration in RACADM.
4.2 UEFI Boot from Local Media
As a general rule, operating systems installed in a traditional BIOS environment will not be bootable in UEFI
boot mode. There are no reliable means for converting or upgrading traditional bootable media to a UEFI-
bootable form, other than re-installing the operating system when the BIOS is in UEFI boot mode.
The boot mode must be configured before installing operating systems or other bootable software. Operating
system installers detect the current boot mode and provide tools for formatting the media accordingly. If the
boot mode is UEFI, the installer will format the media using the GPT partitioning scheme. If the boot mode is
BIOS, the installer uses the traditional MBR scheme. Operating systems also specify different boot loaders for
the two boot modes.
4.3 UEFI PXE Boot Configuration
PXE is used to execute an operating system’s bootstrap program using a network connection. The PXE Client
sends a DHCP request with PXE specific options. The DHCP server response contains the Network
Bootstrap Program (NBP) filename and a list of TFTP boot servers. The PXE client downloads the NBP and
then executes it to complete the boot process.
These are the primary differences between UEFI PXE and Legacy PXE:
• In UEFI boot mode the Network Bootstrap Program (NBP) must be a UEFI bootable image (PE/COFF
format).
• If UEFI PXE Boot is being used to install an OS, that OS will be installed in UEFI boot mode. If the
boot mode is changed later, the OS must be re-installed in the new boot mode.
• If a chainloader like iPXE is used it can take advantage of the Universal Network Device Interface
(UNDI) embedded in the NIC to support network adapters that would not be supported in legacy
mode.
• Because of the additional structure and security of UEFI, it will take a little longer to load the NBP.
• Legacy PXE firmware (option ROMs) on NICs may support options beyond PXE like IPv4/IPv6 HTTP
boot and iSCSI boot. In UEFI boot mode, equivalent functionality is available through HTTP boot and