Being very familiar with AutoCAD, Visio and SmartDraw, the easiest and the cheapest way to handle this, in my opinion, would be to use SmartDraw. These guys give you 30-days free trial, and the amount of symbol libraries available will simply blow your mind. 3COM, Cisco, 100 other vendors, 7' rack, wire managers, you name it. I think you will complete your drafting project in couple of hours, let alone 30 days trial period.
Here is the link:
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It is a shameless plug as well as a shortcut to the Tech Edition download. Once you got the software installed, get the update from their site and in this updates you'll have all the libraries you are looking for. Then start SmartDraw, and in the new file Wizard dialog that comes up, select Rack Diagram, and just start populating it. The software downloads
3COM library automatically as you are trying to use 3COM symbol fro the first time.