@fradlo ok well that rules out stp then.
I guess the next round of testing should be this
Lets get a pcap (packet capture) of the pxe booting process. Since the fog server and the target computer are still connected to the same switch I want you to follow this process (I know you are wireshark capable, but lets follow this route): https://forums.fogproject.org/topic/9673/when-dhcp-pxe-booting-process-goes-bad-and-you-have-no-clue Lets add to the filter sting port 80 too, so we can capture http requests to the FOG server. Run the pcap all the way through registration. Upload the pcap to a google drive or drop box. You can either post the link here or DM me the link so I can take a look at it.
We need to rule out anything in your environment that could be causing this issue. It may be worth setting up isc-dhcp server on your fog server for a test. Keep that unmanaged switch between your FOG server and your target computer and unplug the unmanaged switch from your business switch. and then with isc-dhcp server running see if pxe booting behaves any differently. I have 790s in our office and they pxe boot just fine. So unless the bios is the original release on the 990s pxe booting should just work.
