[mrtg] totals do not add up when graphing multiple targets

Jan Ferré Jan.Ferre at uni-c.dk
Thu Feb 12 16:23:14 CET 2009

Now we're getting close to a problem of mine - sorry to mix it up:
I do have a lot of rrd's with data which I want to add up. I do that
with rrdcgi to make a graph of the sum. This usually works fine - unless
there is an 'unknown' value in the originating rrd's. This will make the
sum unknown and I get no printout.

Do anybody know if it's possible to require: 'Treat unknown as zero'? Or
how should I circumvent the problem?

The task comes from wanting a total of all the connections to a switch -
but if a new interface is used, the 'old' total will be ruined as the
new interface has 'unknown' values ahead of doing mrtg.

Jan Ferré

McDonald, Dan wrote:
> On Wed, 2009-02-11 at 05:19 -0800, pvh wrote:
>> Hello,
>> I've recently had to install MRTG on Windows with the latest ActivePerl at
>> work to monitor a server.
>> What I'm doing is graphing all active interfaces (as per the default
>> cfgmaker configuration), and I added a totals graph which is simply
>> something like that:
>> 1:community at ip + 2:community at ip + 3:community at ip .. 
> So, when one counter rolls over, mrtg has no way of knowing (and
> correcting)
>> I'm working with fast ethernet interfaces (all 32 bit counters) here.
> Thus rollovers are frequent.
> The only way to accurately do this is to collect the three counters
> separately, and use routers2.cgi (or some other similarly enlightened
> front end, but there aren't any others published with this feature) and
> build a stacked graph to show the actual total.
> Note that that requires RRDtool as a backend
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Jan Ferré - UNI-C Netdrift - DTU Bygning 304, DK-2800 Lyngby
tlf. +45 3587 8935, cell. +45 2160 8012, fax. +45 3587 8990
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