Update: This patch has been accepted into gPXE. I just released a patch to gPXE that modifies the dhcp command so that it can iterate over multiple interfaces. The stock dhcp command only accepts a single interface as an argument, which can be a problem if you are trying to boot on a machine with multiple interfaces. The builtin autoboot commands attempts to resolve this, but is only useful if you expect to receive appropriate boot parameters from your dhcp server.