faq(1)
NAME
help - 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
ArrayThe 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 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 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 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.
AUTHOR
- Tobias Oetiker <oetiker@ee.ethz.ch>