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im designing a network for a small business. There are three departments in the business retail sales - 80 employees, sale support - 10 employees, accounts - 15 employees. I've decided to use star topology. It will have one router connecting to many switches. I was thinking of using multiple 48 port switches. Would it be good to have one switch for each department or otherwise and how many switch should i use. The business is expecting expansion in the next five years.

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closed as too localized by Sven, pauska, Iain, Bart De Vos, Ward Oct 5 '11 at 13:48

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Don't forget to account for the physical layout and cabling runs. In some cases it may be may be cheaper to run one bit of fibre across the building to the router (and have a more local switch) rather than 20 GigE lines. – Michael Kohne Oct 5 '11 at 10:37
@MichaelKohne, you should turn this into a proper answer, I think. – Simon Richter Oct 5 '11 at 11:05

I don't think there's one answer to this kind of question, not least because it will depend wildly on how the network is used: 48 salesmen shifting giant PDFs of technical information will need a much different network from 48 salesmen sending text invoices and making the odd VOIP call.

But two general principles I've never found to be false over the years are:

  1. Make sure that the uplinks between the switches are much faster than the switch ports themselves. At least 10 times faster, but if possible, 100 times is good. This means the central switch/router needs to have the fastest ports you can provision, and if that also means running fibre, well, suck it up. In some cases this means putting switches out on the floors with a lower port density, to avoid over-subscribing the uplink, which means more uplinks and more central switch ports. Again, suck it up.

  2. All the switches should be manageable. This used to be contentious, but now manageable switches are coming way down in price, you shouldn't need to think twice about it. This means you can VLAN when needed, interrogate the switch to identify problem talkers, mirror certain ports to examine problem traffic, potentially traffic-shape all the way from the desktop to prioritise eg VOIP, and just generally makes your life much easier in the long run.

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Fibre is cheaper than copper anyway. – Simon Richter Oct 5 '11 at 11:03
I hadn't counted, but if that's true it's useful to know; even though the switch ports are often more so, and there's the multiplicity of possible terminators to contend with, for long runs you may well have a very good argument. – MadHatter Oct 5 '11 at 11:23

You're over-thinking this a bit. All you need is however many switches are required to provide the number of ports you need to feed, plus those required for daisy-chaining. No more, no less. When the business expands simply add more switches.

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You should be looking a good quality managed switches for corporate use. Think in terms of having one vlan per department, rather than one switch per department. Provisioning one 48 port switch to cover sales support & accounts would be more than enough. Two more should cover retail sales. Using vlans gives you far more flexibility when you need to increase port density as the organisation grows.

In a star topology I would suggest you have two switches at the centre of the star to prevent a failure of that central switch severing the network completely. Using a stacked pair of Cisco 3750's as your router & core switch is probably a good option if you have the budget. That would allow you to use port channelling for your uplinks to the 'core' to increase the bandwidth of the uplinks and build in some resilience.

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