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The actual production environment is more complicated and it may have issue during the process. Generally speaking, based on my understanding, the easier way is to put a PXE-enabled DP in this third-party suppliers site if the third-party suppliers could connect with the primary site by VPN. If we set the DHCP server and PXE-enabled DP in the same subnet of the suppliers site, the PXE process could be smooth.
The other way is that we could use stand-alone media to image devices in the third-party suppliers site.
For more details, please click this helpful guide to create offline image in SCCM.
Have a good day!
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