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

pvh mrtg at dynmail.b34tz.net
Thu Feb 12 17:03:49 CET 2009

Thanks for your reply Dan.

I don't really understand how I could be experiencing counter roll-overs?
This would be an issue when a 32 bit counter wraps around in less than 5
minutes, which cannot be happening on my fast ethernet interfaces (which btw
don't even go above 10% utilization).

My MRTG config looks like this:
one target for each interface (as generated by cfgmaker)
and finally after all individual interfaces I added the total, by simply
copy/pasting the target of each of the above interfaces with + signs in

>From the documentation I learned that MRTG is smart about multiple
occurrences of the same target in a configuration file: it will not query a
second time but reuse the previously saved value. So all my individual
values are fine and it's up to MRTG to add them up, and at this stage I
cannot imagine that counter wrap is still an issue - surely MRTG (well, Perl
I guess) is capable of handling this properly and is not just adding up my
8x32 bit counters into a single 32bit counter?

Besides, wouldn't counter wrap result in _lower_ values than expected? (what
I see is about 3 times _higher_ values than I should get). 
Anyway if that's what happening then I don't understand the point of being
able to add/subtract/divide targets in MRTG if it doesn't work? (what I'm
doing is exactly the example given in the MRTG docs, aggregate a few
interfaces together, only I have 8 of them and not 2).

Thanks a lot for your time!



McDonald, Dan wrote:
> On Wed, 2009-02-11 at 05:19 -0800, pvh wrote:
>> 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
> -- 
> Daniel J McDonald, CCIE #2495, CISSP #78281, CNX
> Austin Energy
> http://www.austinenergy.com
> _______________________________________________
> mrtg mailing list
> mrtg at lists.oetiker.ch
> https://lists.oetiker.ch/cgi-bin/listinfo/mrtg

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