The Definitive Guide to Installing ESXi using iPXE...

[MultimediaMan]

Memdisk via iPXE vs. ISO Boot HTTP via iPXE:

Memdisk via iPXE does the following things:

1) Emulates a CD-ROM allowing a Network-Based Install.
2) Masks actual system RAM because the environment memdisk creates "hides" a certain amount of RAM to allow for the ISO - This amount is generally 2x ISO Size (Maximum 2GB - I think).
3) Preloads the ISO into memory before executing the ISO. This slows boot time a bit.

ISO Boot over HTTP via iPXE:

1) Does not emulate a CD-ROM. It is a Block Device.
2) Does not mask system RAM.
3) Executes as it reads: Faster than memdisk and no "preloading" of the ISO is required.
4) Does not hold the ISO as a readable device once the ISO is loaded unless loaded into the iBFT.

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Updated to reflect two more methods of installing ESXi using iPXE.