If your enterprise network switches run standard Spanning Tree Protocol (STP) without optimization, the network port may undergo loop-checking routines for up to 30 to 45 seconds when the computer changes states. As the target hardware transitions from the motherboard BIOS over to WinPE, the switch port temporarily drops and resets. Because WinPE expects an immediate IP address response to look for its variable file, the STP listening delay causes the connection attempt to time out entirely. 3. Client and Server Clock Desynchronization
Open the SCCM console:
Then test the variable file download: