[mrtg] Re: Spike after rebooting routers

Makai József jozsef.makai at siemens.com
Thu Apr 24 14:50:13 MEST 2003

Hi Michael,

This phenomenon is absolutely normal.

In the router there are counters, and the mrtg checks the values in every 5

After reading out the values the counters continues counting without zeroing
their values.

If the next readout is higher, mrtg calculates the normal difference between
the two values, and stores it.

If the next readout is lower then the previous one, that means that the
counter reached the 32 or 64 bit limit and continues counting from zero.

In this case (the current value of a counter is lower then the previous one)
mrtg takes into consideration the overflow of the counters.

(mrtg can calculate only with one overflow, so if you wanted to increase the
default 5 minutes interval, you must take this into consideration!)

After a reboot the counters are reset, and the calculated values are
practically never real values. Either lower or higher due to the above
mentioned. (The spikes, what you find strange.)

I hope this helps,


> Hi there,
> I have a strange problem running MRTG against several cisco routers:
> when doing a IOS-update (or only reload) I sometimes get high 
> spikes in *some* interface graphs.
> The ifIndex doesn't change and everything works fine besides 
> from that, we're also talkking about upgrading minor 
> release-versions without any noticeable other change in SNMP 
> on the router. After the spike everything works fine again; 
> It's quite annoying because it messes up the graphs' scale 
> und my traffic totals, and fixing the spikes in the RRDs is 
> also a time consuming task..
> Is there any known workaround out there to prevent this 
> (besides from setting AbsMax lower), either manual in advance 
> to a (scheduled) reload/upgrade or preferably something 
> generic in mrtg-config (might using snmpv2c help) ?
> another question: is there any other tool like killspike.pl 
> which allows me to specify the time-period or somthing 
> similar) where I want to kill spikes ?
> some of my config: mrtg-2.9.29 / RRDtool 1.0.41 on Red Hat Linux 7.3 
> thanks
> Michael

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