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I am in charge of a network of which a portion of it looks like this: One DLink DGS-1210-24 (1) as the main switch. One DLink DGS-1210-16 (2) connected with cat5e 250 feet away connected on (1) One DLink DGS-1210-16 (3) connected with fiber 500 feet away connected on (2) One trendnet 8-port gig switch (4) connected with cat5e 50 feet away connected on (1)

(2) and (3) are connected with multimode fiber and I have 1 miniGBIC on each switch (model DEM-311GT).

I plug in a laptop on (1) and start iperf, then I connect another laptop on (2) and run iperf and get 50 Mbits

When I plug in the laptops on (2) and (3) and run iperf, I get 50Mbits.

I run iperf on 2 servers plugged in on the same switch (1) and I get 1 Gbit

If I run iperf on a computer that is plugged in on (4) and the server on (1) I get 97 Mbits

Can anyone suggest something to try and figure out why there is such a big drop in throughput?


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Most likely, the 50 foot of cat5e isn't wired as a Gigabit cable. Having wires that can carry Gigabit is just one of the requirements. (Having all four signal pairs properly wired through, each on a twisted pair, is another.) – David Schwartz Apr 19 '12 at 19:31
up vote 2 down vote accepted

There is a lot of 5E cable around that is sub-par.

Even if it says 5E on the box or on the cable itself that doesn't mean it can actually do it.

It is quite common for such cable to fall back to 100 Mb/s or even less.

Getting Gigabit over more than 30 feet is usually quite a feat in those cases.

That would explain the 97 Mb/s between (1) and (4).

As for the 50 Mb/s when switch 2 is involved (on both the copper and the fiber link if I read the question correctly). I'm guessing that switch 2 has a very weird config problem or is faulty.

Other possibility that comes to mind is that the long run of 5e cable between (1) and (2) is causing a lot of errors with which switch 2 can't cope with.

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But if the issue was just the copper, shouldn't I get better throughput than 50MBits when I connect (2) and (3) since they are connected with fiber? – Nic Apr 19 '12 at 23:10
Not if the laptop's link to the switch was the limiting factor. – David Schwartz Apr 20 '12 at 1:41
It appears that the laptops I was using were the limiting factors. I installed iperf on a macbook pro and I am getting 1 gig between (1), (2) and (3). I guess (4)'s limit would be the cheap cable I used. Thanks – Nic Apr 20 '12 at 18:17
Glad to be of some service :-) – Tonny Apr 20 '12 at 19:11

We use quite a bit of DLink here, sadly.

Have you verified that every single on of those links between the switches is actually operating at the speed that you want them to? No sense poking about with performance testing software if you don't know that those links are running at full rated speed.

There's lights on the front of those switches above each port. Make sure each one of those is reporting the desired speed.

share|improve this answer
All of them are at 1 gig – Nic Apr 19 '12 at 21:24
Then you really need to check your switch's interface to make sure you don't have problems. Try your switch's advanced web interface > Monitoring > Statistics. – Magellan Apr 20 '12 at 2:06

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