iMAC with Broadcom 14e4-1686 BCM57766 : link status down

[speleo14]

Sorry to mix in... as I mentioned, I have the same problem, and maybe this information can help:

We have several iMacs of that model; some we bought more than a year ago, and I installed Windows on some of them using iPXE and WDS - without any problems! It's only the ones that we bought recently that will not PXE-boot.
It seems as if the ones that came with Sierra (10.12) pre-installed will not work with iPXE while the ones that had El Capitan (10.11) are OK.

I checked with System Profiler to find any differences between these two, hmm, versions of that model and found the following:

iMac 27'' Retina 5K Late 2015, bought in May 2016 (iMac17,1):
Boot ROM: IM171.0105.B05
SMC Version: 2.33f10

Ethernet:
Broadcom 57766-A1:
Manufacturer ID: 0x14e4
Device ID: 0x1686
Versions-ID: 0x0001
Kext-Name: AppleBCM5701Ethernet.kext
Firmware-Version: 57766a-v1.15, 0xad0d59c9
Version: 10.1.12

OS: 10.11.3


iMac 27'' Retina 5K Late 2015, bought in March 2017 (iMac17,1):
Boot ROM: IM171.0105.B15
SMC Version: 2.34f2

Ethernet:
Broadcom 57766-A1:
Manufacturer ID: 0x14e4
Device ID: 0x1686
Versions-ID: 0x0001
Kext-Name: AppleBCM5701Ethernet.kext
Firmware-Version: 57766a-v1.15, 0xad0d59c9
Version: 10.2.7

OS: 10.12.3

Differences are in bold. Both versions use the very same ethernet NIC, the same kext and the same firmware on the NIC, just the "version" (whatever this is) is different. However, the Boot ROM and SMC versions are different on the two iMacs.
Thus, I suspect that nothing is wrong with the Broadcom NIC and its iPXE driver, but with the newer BootROM / firmware on the newer iMacs.

I have no idea if this helps... Unfortunately, I don't have one of the older "versions" of the iMac at hand right now, they're all out with my users, so I can't compare iPXE boots, but I could try to get one back for a day or two - then we could try to hunt down differences?