# How to calculate/estimate the capacity of a switch theoretically [closed]

I don't know whether this is the correct place to ask this question. If not, could any one tell me some sites or forums to ask such questions? Thanks in advance.

My question is the same meaning as the title roughly, but more specific. Given a shared memory architecture of a switch, how to calculate or estimate the total capacity of that switch theoretically? I've seen some articles saying 5Gb/s, so how to get 5Gb/s?

Any guidelines (e.g. books, papers etc.) will be greatly appreciated.

Jfhu

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## closed as not a real question by Iain, Chopper3Aug 9 '11 at 13:43

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

Check the manufacturers documentation ? – Iain Aug 9 '11 at 11:43
Maybe I did't make myself clear. What I want to know is the calculating of capacity(or throughput) of a switch with its design architecture, in my case is a shared memory, presenting. – machinarium Aug 9 '11 at 13:29
Note that switch capacity is measured in both packet throughput (pps) and medium capacity (Gbps). It is very common for switches to have insufficient capability to saturate both measures at the same time. – Chris S Aug 10 '11 at 2:30

The term you're looking for is "backplane speed" for most switches. This is the maximum data a switch can move in an instant. Most switches you'd use in the office or datacenter should list this speed somewhere on the spec sheet. Things get more complicated in blade-style and stacked switches as that concept is more fuzzy, since each blade/stacked-switch can have its own backplane speed and a separate speed for the blade-chassis/switch interconnect channel.

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First, thanks for your reply. So how to get the backplane speed for non blade-style switches, are there some guidelines? – machinarium Aug 9 '11 at 13:30
@machinarium Look on the spec sheets. Take this one. Under Tech Specs -> Performance is a "switching capacity" stat of 176 Gbps. – sysadmin1138 Aug 9 '11 at 15:36
I've heard `backplane speed` and `switched fabric`, seems to vary by manufacturer. – Chris S Aug 10 '11 at 2:27

Most if not all manufacturers very clearly state their switches capabilities on a model-by-model basis. Let us know what make/model you're looking at and I'll find it for you.

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OK. It's just about the calculation/estimation of switch capacity according to its shared memory architecture, not of some specific manufactured switch. – machinarium Aug 9 '11 at 13:35
There is no rule for that, it's vendor specific - shared memory could entirely dictate how fast a switch is, not at all or somewhere inbetween. This question isn't answerable in the state it's in. – Chopper3 Aug 9 '11 at 13:43
Sorry for my poor English and the confusion. I just wonder why the designer of the shared memory architecture switches say their switch could operate with up to 5 Gb/s capacity. How they get the 5 Gb/s capacity? If it's vendor specific actually, sorry for my question. – machinarium Aug 9 '11 at 14:23
They'll have looked at the entire end-to-end design of their switches looking for the slowest part, whatever that may be, that's their maximum throughput figure. Shared memory may or may not be the slowest point in the chain for one particular model. – Chopper3 Aug 9 '11 at 14:33

What do you mean by capacity?

• Ports: If you have a 24 port switch, the capacity of it is 24.
• Bandwidth: If you have a Gigabit switch, the bandwidth capacity is 2 Gigabit/s at Full-Duplex
• Backplane speed: Having a 24 port Gigabit switch, the backplane should be able to handle 48 Gigabit/s. If not, you can't use the theoretical "capacity".
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The backplane speed is what I mean. I expect to get some formulas or expressions whose results are 2 Gigabit/s or some other value(5 Gb/s in my case), and the parameters of formulas/expressions are related to the shared memory architecture. – machinarium Aug 9 '11 at 13:42