[mrtg] Re: Link speed

Alex van den Bogaerdt alex at slot.hollandcasino.nl
Mon Apr 30 23:01:01 MEST 2001

Alejandro Cabrera Obed wrote:

> A couple of days ago, I've installed the MRGT in order to mesure the link
> speed corresponding to th e32 Kbps line. I configured the MRTG with the
> router attached to that link and in the weekly report, I take 27 Kbps as the
> maximun link speed (32 - 27 = 5 Kbps under the limit).

You have set "MaxBytes[target]: 3375" so your graphs will show 27000 bps
as being 100%
I think you also should set "AbsMax[target]: 4000" so MRTG will know
that there can be 32kbps on this line. (maybe you want a slightly higher
number, such as 4500)

> I ask you: how can I do to test the link in order to prove that the maximun
> link speed is 32 Kbps like my ISP assures ???
> (My ISP says that the CIR is 32 Kbps).

MRTG monitors in 5-minute intervals.  If you want to see 100% usage
(or, in your case, 118%) you need the line fully utilized during at
least one complete interval.  You start the transfer of this file
in one interval (this will be *somewhere* during the interval), the
next interval will be the one where the line is fully utilized.

So, you need a file large enough to use up all available bandwidth
during two intervals (usually 10 minutes) at the maximum possible
rate of 32kbps this means:

10 minutes * 60 seconds_per_minute * 32kbps = 10*60*32000 = 19200000
bits which is 2,400,000 bytes.  Any file larger than this will do.

Transfer this file from or to a site nearby and make sure that other
site can deliver the bandwidth you have: in other words, your
connection should be the bottle neck.

The transfer time for this file is usually *not* accurate as there
will be other traffic on the line.  However, MRTG will "see" all of
the traffic so its numbers should be accurate.

 / alex at slot.hollandcasino.nl                  alex at ergens.op.het.net \
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