While there are a couple methods to boot a bare metal machine (or a reimage) I like the convenience of PXE. However, it doesn’t come without it’s pitfalls. The process for PXE booting works similar to this:
At step 3, if DHCP Options 66 / 67 are configured, the NIC will boot to the information returned by DHCP. If those options are not configured the NIC will send a broadcast message out asking for a PXE service point. This becomes a problem if the PXE server (SCCM) is not on the same subnet as the device requesting the PXE boot. This is where IP helpers come into play.
If your DP/PXE server does not reside on the same subnet as the device that is PXE booting you will have to setup an IP Helper (or configure DHCP Options 66 / 67 - but this is not recommended by Microsoft). An IP helper (setup on a Layer 3 device - typically the router for the subnet) will basically take this broadcast packet and make sure that the request gets to the appropriate device on the appropriate subnet.