[mrtg] Re: How to read data accurately from Accton 3512A and 3524A??

Alex van den Bogaerdt alex at ergens.op.HET.NET
Sat Jul 20 00:36:58 MEST 2002

Ming Fong Lei wrote:

> You can see the our measurements at http://ipserver.ee.ntu.edu.tw/data.html

I can't.  Apart from getting quite an amount of packet loss, the
server (or a router along the path) denies the connection.

> From 1630 to 1700 hour, I was receiving several large files (3~4 Gb) from a
> FTP outside our campus, the reading from my FTP client software is
> ~2800kbyte/s, however the reading from mrtg during this period of time is
> only about 2300kbit/s.  Apparently there is something wrong which either the
> agent itself or the MRTG program.  I've checked the data flow with the
> software provided with the hub, and it is correct.  So it must be with the
> way my MRTG is set up.

2800 kBps and a 5-minute polling interval means the counter will increase
by 2800000 * 300 = 840000000 bytes.  This is less than 4294967296 so that
shouldn't impose a problem.  The data transfer must have lasted at least
20 minutes so at least three MRTG intervals have been used entirely.

Two things to do:

1) Find a way to get the interface counter values by other means,
   preferably both by using SNMP and by *not* using SNMP
2) do another test, this time comparing the several data sources:
   -a- take the following counters
      i) top of the log file, just after mrtg has run
     ii) counters using SNMP
    iii) counters from another source (i.e. device command line)
   -b- wait until mrtg runs again, then fetch the same counters again.

MRTG calculates the rate by doing the following:

   [current counter value]  -  [previous counter value]
  [current time in seconds] - [previous time in seconds]

You can do the same and compare to see what fails.

If there's other traffic on the link the data rate may have been
~ 15 MBps (assuming your link can handle this).  If so, the counters
increase with 300 * 15 MBps = 4.5 GB.  This is more than what a
32-bit counter can handle; the counter wraps within one polling
interval and MRTG "thinks" there has been an increase of 4.5 G - 2^32.
This increase when divided by 300 seconds to get Bps is in the kB range.

To solve this kind of problem: use 64-bit counters, see the archives.

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