mrtg-faq.txt
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- MRTG-FAQ(1) mrtg MRTG-FAQ(1)
- NNAAMMEE
- mrtg-faq - How to get help if you have problems with MRTG
- SSYYNNOOPPSSIISS
- 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.
- FFAAQQ
- In the following sections you'll find some additonal Fre-
- quently Asked Questions, with Answers.
- WWhhyy iiss tthheerree nnoo ""@@##$$%%"" ((mmyy nnaattiivvee llaanngguuaaggee)) vveerrssiioonn ooff
- MMRRTTGG??
- Nobody has contributed a _@_#_$_%_._p_m_d file yet. Go into the
- _m_r_t_g_-_2_._1_3_._2_/_t_r_a_n_s_l_a_t_e directory and create your own trans-
- lation file. When you are happy with it send it to me for
- inclusion with the next mrtg release.
- II nneeeedd aa ssccrriipptt ttoo mmaakkee mmrrttgg wwoorrkk wwiitthh mmyy xxyyzz ddeevviiccee..
- Probably this has already been done. Check the stuff in
- the _m_r_t_g_-_2_._1_3_._2_/_c_o_n_t_r_i_b directory. There is a file called
- _0_0_I_N_D_E_X in that directory which tells what you can find in
- there.
- HHooww ddooeess tthhiiss SSNNMMPP tthhiinngg wwoorrkk
- 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
- TThhee iimmaaggeess ccrreeaatteedd bbyy MMRRTTGG llooookk vveerryy ssttrraannggee..
- 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.
- WWhhaatt iiss mmyy CCoommmmuunniittyy NNaammee??
- Ask the person in charge of your Router or try 'public',
- as this is the default Community Name.
- MMyy ggrraapphhss sshhooww aa ffllaatt lliinnee dduurriinngg aann oouuttaaggee.. WWhhyy ??
- Well, the short answer is that when an SNMP query goes out
- and a response doesn't come back, MRTG has to assume some-
- thing 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 dur-
- ing a total outage.
- If this is an unacceptable trade-off, use the uunnkknnaasszzeerroo
- 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 guar-
- anteed) unlike TCP where packets are tracked and acknowl-
- edged 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 accu-
- rate 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 typi-
- cal for your network.
- AAUUTTHHOORR
- Tobias Oetiker <oetiker@ee.ethz.ch>
- 2.13.2 2006-02-03 MRTG-FAQ(1)