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switch throughput

Posted on 2011-03-13
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Last Modified: 2012-08-13
If we have a 100M switch, if that every port is providing 100M or sum of all the traffic passing through the switch is 100M? If every port, and is it that uplink on 24-port sw is carrying 23x100M or 2.3G worth of data? Mostly, the uplinks are 1G fiber, can they boost up to 2.3G???
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Question by:totaram
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by:AustinComputerLabs
ID: 35124316
A 100m switch has 100m per port, if full duplex (and almost everything is now) 100m up and 100m down.
A 100m hub has 100m total
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by:totaram
ID: 35124828
Austin;
If every port is operating @ 100m, will it not slow down the uplink link considerably? Is there way to around it??
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by:AustinComputerLabs
ID: 35124919
That is the max of each, they seldom need or use it.
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by:totaram
ID: 35124976
Austin;
We have applications downloading 1 gig on different ports via FTP, and am looking for a suitable solution, for sure 100 meg will be used.
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by:Don Johnston
ID: 35127582
If you have a 24-port, 100mbps switch with a single gig uplink port, then that port will be oversubscribed. (If all 24 ports are sending simultaneously, they will be generating 2.4gbps of traffic.)

On some switches, there will be two gig uplink ports that you can channel so that you end up with 2gbps of uplink throughput. It will still be oversubscribed, but not as much. When you consider the chances of all 24-ports receiving data at max throughput as the same time, the 400mbps of oversubscription isn't that big of a problem.
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by:totaram
ID: 35130074
donjohnston;
I see your point with oversubscription of 400m, which is not a big deal. But if the switch ports are 10/100/1000 and 2 uplinks are 10g each for 48port switch, it would be considerable oversubscription... is there any way around it?
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by:Don Johnston
ID: 35133132
Nope. You'd be dealing with over 100% over-subscription.  

So it comes down to the traffic patterns. In a situation like this, you would want to know that the devices connected to your switch would not be generating over 20gbps of traffic simultaneously. But even if they did, the traffic would hopefully not exceed the buffer capacity until the utilization dropped back below 20gbps.
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by:totaram
ID: 35138825
Is having two uplinks also called dual personality? Was wondering if vendors ever thought of providing 4 uplinks instead of 2???
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decoleur earned 25 total points
ID: 35138955
the other thing that you should look at is how the interfaces themselves are oversubscribed.

you can have 100M interfaces that are tied 4 to an ASIC with a 4 times over subscription, each ASIC can handle 100M full duplex but all 4 interfaces will not be able to overload the ASIC once the 100M threshold has been reached. Furthermore those ASICs are connected to each other on a bus that has a finite capacity often called a backplane or a switch fabric. in this example you could have 24 ports that are 4 times oversubscribed into 6 asics tied to a common backplane that is limited to 1G total throughput so at every step along the way there is over subscription and the distinct possibility of packet loss as a result of saturation.

this is entirely dependent on the switch hardware architecture. this limits are physical capacity limitations and cannot be overcome.

the hardware that is less over subscribed is more expensive but you can get an indication by the interfaces speeds as they relate to the back plane capacity.

let us know if there is more that you need to assist you in making your decisions.

-t
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by:Don Johnston
Don Johnston earned 25 total points
ID: 35139639
Yes, some vendors will have four 10 gig ports on a 48-port switch for 1.2:1 oversubscription.
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