If you have a test sccm server or you just migrating to another server you need a usb stick in order for the client to pxe boot into the new server while the old server is still running the pxe. You can accomplish this by creating a discovery image for your sccm server.
- Create a normal tasksequence
- Select the task Sequences and press the creat task sequence media button in the ribbon
- Follow the wizard.
- make sure you select bootable image and set the media file location.
- Now that you have an bootable ISO set it on a USB. I normally do this with rufus2-2.exe
- Boot up the client from USB. it should now contact your New system center server