We had a network that had been built up over the past 20 years, so first thing I had to do was try and map what we had - mixture of unmanaged switches, CAT5, media converters, fibre, old fat ethernet (powered up, polling, but not operating...), and then I had to go through all the devices connected :
PLCs, Programming PCs, spare ethernet ports, SCADA PCs, remote racks with no apparent owner.
So I built up a spreadsheet of each potential node, allocated a main managed switch to each node (8 in total) then planned the main fibre routes.
From each main node then decided how many devices had to be connected - identified as many as I could, but none of the CAT5 cables were numbered, so still many unknowns.
Once I got the big picture, got 2 vendors in to give us a robust network design, then went for Rockwell Stratix. We designed the cabinets, and power supplies and UPS to the cabinets...spent about £170k in total so far, and we are almost up and running.