When using ConfigMgr as a update source for WSUS updates, I prefer downloading updates from the internet. In some cases however it's better to use an existing WSUS server as an update source. This because the internet line speed is to slow, or updates are already downloaded on another server. You can use that one for downloading the first batch off software updates. Very handy if you ask me! Just select to "Download software updates from a location on my network" and updates will be downloaded within a few minutes.
This can be done on the following location:
-ConfigMgr console > Software Library > Software Updates > All Software Updates > Download (selection) > Download software updates from a location on my network (Browse)
Just use it to your advantage! No need to download updates again, if they are available already on an existing WSUS server.