Recently at my new workplace, we’ve been having issues getting new Lenovo ThinkCentre M700’s to (Legacy) PXE boot. The lights on the NIC actually went out when it tried to PXE boot! (Booting from a boot USB thumb drive worked fine.) Some folks out on the SCCM Subreddit (https://www.reddit.com/r/SCCM/) were having similar issues with various systems based on the new Skylake chipset, like our small Lenovo M700’s.
After someone on the Subreddit stated that they successfully PXE booted using a 100mb unmanaged switch as a workaround, I decided to investigate. I worked with one of our (Cisco) Network Engineers to test changing settings on one of our switch ports. He suggested disabling POE (Power-Over-Ethernet) and his idea was spot on. The new M700 was able to PXE boot after he made the change!
I haven’t seen anything official from Lenovo on the issue, but testing disabling POE on a port is worth a shot if you’re in a similar situation.