When attempting to PXE/network boot a Dell Optiplex 990, you may receive the following error:
Intel(R) Boot Agent PXE Base Code (PXE-2.1 build 089)
Copyright (C) 1997-2010, Intel Corporation
PXE-E61: Media test failure, check cable
PEX-M0F: Exiting Intel Boot Agent.
The issue appears to be caused with several different managed switches that include PoE (Power over Ethernet) that cause the media test portion of the network boot process to fail.
If you receive this error, check the BIOS revision on the computer. BIOS revision A06 resolves this issue, but there are newer versions available. Also be aware that you may have to install multiple BIOS updates to get the Optiplex 990 to the latest version.