We are setting up Portal and are going to have to deal with external requirements keeping some of our ArcGIS servers at previous versions. So far I have identified servers at 10.2 and 10.3 and our SDE is currently at 10.1. I believe I understand that Portal can not be installed at 10.5 due to underlying gdb changes (not sure how this effects portal, it doesnt seem to do that). What is best Portal practice in this situation. The other servers are being held back by much larger projects which will move along albeit very slowly into new versions.