[mrtg] MRTG accuracy issue

Koelstra, J. (Jan) JKoelstra at MINSZW.NL
Mon May 14 14:47:05 CEST 2007


When monitoring links with speeds above 100Mbits/s you have to use the 64bit counters.
The 32bit counters do roll over too quickly to give reliable statistics for this link speeds.

MRTG can deal with roll overs, but is unable to see the difference between 1, 2 or more roll overs in a polling interval.

To get the 64bit counters just add :::::2 after the hostname. For details see http://oss.oetiker.ch/mrtg/doc/mrtg-reference.en.html#item_snmpv2c



	-----Original Message-----
	From: mrtg-bounces at lists.oetiker.ch [mailto:mrtg-bounces at lists.oetiker.ch] On Behalf Of Gabor
	Sent: Monday, May 14, 2007 2:22 PM
	To: mrtg at lists.oetiker.ch
	Subject: [mrtg] MRTG accuracy issue

	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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