NAME

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

SYNOPSIS

       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.

FAQ

       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-2.17.7/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-2.17.7/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

        http://www.david-guerrero.com/papers/snmp/

       And at this rather long document from CISCO.

        http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/snmp.htm

   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.

AUTHOR

       Tobias Oetiker <tobi@oetiker.ch>