[mrtg] MRTG accuracy issue

Gabor vasmegyei at yahoo.com
Mon May 14 14:21:38 CEST 2007

Dear MRTG guys,
  This is my first post to this mailing list. I use MRTG to monitor numerous things on our network. Recently an accuracy issue was brought to my attention: MRTG shows too little traffic for Cisco Catalyst 65xx switch gig interfaces. To test this theory, we conducted 2 tests.
  Test #1: connected a sniffer to the switch (Sniffer Pro software), monitored all traffic through gig ports and got Sniffer Pro to draw its own graph. FYI, the sniffer was set up on a 100 Mb port but we monitored a Gig links with no more than 60-70 Mb traffic. We found that the MRTG graph showed roughly 30% less traffic than the Sniffer Pro graph.
  Test #2: we cleared the counters on some gig interfaces. After a few hours, then days, we checked the counters, noted the amount of traffic going through, calculated the average and compared it to the MRTG graph. We came to the same conclusion, namely that there was a roughly 30% difference.
  For now we just consider the 30% discrepency when looking at MRTG graphs for gig ports. It seems that MRTG is accurate for slower ports.
  So, I wonder if any of you good folks have run into this problem. Is there a special config I have to do for Gig ports or 65xx switches? I would appreciate some thoughts, thanks in advance.
  This is how I create my config:
  # /usr/local/mrtg-2/bin/cfgmaker --global 'Htmldir: /data2/web/mrtg/gig/hostname' --global 'Imagedir: /data2/web/mrtg/gig/hostname' --global 'Logdir: /data2/web/mrtg/g
ig/hostname' --global 'RunAsDaemon: Yes' --global 'Interval: 5' --ifref=nr --ifdesc=ip --noreversedns --global 'Options[_]: bits,growright' --output /data2/data/mrtg/c
fgs/hostname.cfg communitystring at hostname
  A typical gig link:
  ### Interface 855 >> Descr: '' | Name: '1/1' | Ip: '' | Eth: '00-01-64-75-cf-6d' ###
  Target[hostname_855]: 855:communitystring at hostname:
SetEnv[hostname_855]: MRTG_INT_IP="" MRTG_INT_DESCR=""
MaxBytes[hostname_855]: 125000000
Title[hostname_855]: 855 -- hostname.domain.name
PageTop[hostname_855]: <H1>855 -- hostname.domain.name</H1>
   <TR><TD>System:</TD>     <TD>hostname.domain.name in </TD></TR>
   <TR><TD>Maintainer:</TD> <TD>Contact info</TD></TR>
   <TR><TD>Description:</TD><TD>  </TD></TR>
   <TR><TD>ifType:</TD>     <TD>ethernetCsmacd (6)</TD></TR>
   <TR><TD>ifName:</TD>     <TD>1/1</TD></TR>
   <TR><TD>Port Name:</TD>  <TD>Gig link to Hostname</TD></TR>
   <TR><TD>Max Speed:</TD>  <TD>1000.0 Mbits/s</TD></TR>

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