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MRTG-FAQ(1)			     mrtg			   MRTG-FAQ(1)

       mrtg-faq	- How to get help if you have problems with MRTG

       MRTG seems to raise a lot of questions. There are a number of resources
       apart from the documentation where you can find help for	mrtg.

       In the following	sections you'll	find some additonal Frequently Asked
       Questions, with Answers.

   Why is there	no "@#$%" (my native language) version of MRTG?
       Nobody has contributed a	@#$%.pmd file yet. Go into the
       mrtg-2.17.4/translate directory and create your own translation file.
       When you	are happy with it send it to me	for inclusion with the next
       mrtg release.

   I need a script to make mrtg	work with my xyz device.
       Probably	this has already been done. Check the stuff in the
       mrtg-2.17.4/contrib directory. There is a file called 00INDEX in	that
       directory which tells what you can find in there.

   How does this SNMP thing work
       There are many resources	on the net that	explain	SNMP.  Take a look at
       this article from the Linux Journal by David Guerrero

       And at this rather long document	from CISCO.

   The images created by MRTG look very	strange.
       Remove the *-{week,day,month,year}.png files and	start MRTG again.
       Using MRTG for the first	time, you might	have to	do this	twice. This
       will also help when you introduce new routers into the cfg file.

   What	is my Community	Name?
       Ask the person in charge	of your	Router or try 'public',	as this	is the
       default Community Name.

   My graphs show a flat line during an	outage.	Why ?
       Well, the short answer is that when an SNMP query goes out and a
       response	doesn't	come back, MRTG	has to assume something	to put in the
       graph, and by default it	assumes	that the last answer we	got back is
       probably	closer to the truth than zero.	This assumption	is not perfect
       (as you have noticed).  It's a trade-off	that happens to	fail during a
       total outage.

       If this is an unacceptable trade-off, use the unknaszero	option.

       You may want to know what you're	trading	off, so	in the spirit of
       trade-offs, here's the long answer:

       The problem is that MRTG	doesn't	know *why* the data didn't come	back,
       all it knows is that it didn't come back.  It has to do something, and
       it assumes it's a stray lost packet rather than an outage.

       Why don't we always assume the circuit is down and use zero, which will
       (we think) be more nearly right?	 Well, it turns	out that you may be
       taking advantage	of MRTG's "assume last"	behaviour without being	aware
       of it.

       MRTG uses SNMP (Simple Network Management Protocol) to collect data,
       and SNMP	uses UDP (User Datagram	Protocol) to ship packets around.  UDP
       is connectionless (not guaranteed) unlike TCP where packets are tracked
       and acknowledged	and, if	needed,	retransmitted.	UDP just throws
       packets at the network and hopes	they arrive.  Sometimes	they don't.

       One likely cause	of lost	SNMP data is congestion; another is busy
       routers.	 Other possibilities include transient telecommunications
       problems, router	buffer overflows (which	may or may not be congestion-
       related), "dirty	lines" (links with high	error rates), and acts of God.
       These things happen all the time; we just don't notice because many
       interactive services are	TCP-based and the lost packets get
       retransmitted automatically.

       In the above cases where	some SNMP packets are lost but traffic is
       flowing,	assuming zero is the wrong thing to do - you end up with a
       graph that looks	like it's missing teeth	whenever the link fills	up.
       MRTG interpolates the lost data to produce a smoother graph which is
       more accurate in	cases of intermittent packet loss.  But	with V2.8.4
       and above, you can use the "unknaszero" option to produce whichever
       graph is	best under the conditions typical for your network.

       Tobias Oetiker <>

2.17.4				  2012-01-12			   MRTG-FAQ(1)


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