[mrtg] Re: Broken Graphs

[Grendel]

Hi Joseph,
	It would be closer to 114 Mbps. Did the math out and it looked
something like this:

(2^32 * 8) / (5 * 60) = 114,532,461.22666_

	SNMPv1 uses 32 bit counters to count the bytes that pass through
interfaces. Mrtg measures speed by subtracting the difference between
counter values at 5 minute (by default) intervals. If this difference
from one to the next is negative, it assumes the counter looped. If the
difference is positive, then mrtg has no way telling if the counter
looped around and went a bit further, or increased only a little bit.
	In its original form, mrtg allows a mimimum poll interval of 5 minutes.
So if an interface counts more than 2^32 bytes passing through in a 5
minute span, the counter loops. Long story short, using SNMPv1 with an
unmolested mrtg you can measure a maximum of 114 Mbps. To get around
this you can either edit the mrtg perl executable and shorten the
minimum allowed interval which gets you up to around 5 * 114, or you can
enable SNMPv2c on your router which lets you access 64 bit counters. The
mrtg config reference page tells more about how to configure targets to
access those 64 bit counters. Each solutiuon has its own additional
burdens.

HTH

James Overbeck
interQ
Tokyo, Japan

"Brian E. Seppanen" wrote:
> 
> On Thu, 9 Nov 2000, Joseph Smith wrote:
> 
> >
> > Hi all,
> >
> >       I am pulling stats on a few OC-3 interfaces and as soon as my
> > bandwidth hits about 120 megabits my graph just dies off,  not completely
> > just to 100kb to 1000kb. I know that my traffic is not actually dropping
> > off. Any thoughts Thanks
> 
> In my experience it has been due to the MaxBytes setting being too low.
> It's obvious when you look at the graph and it's kind of striped, and you
> noticed its striped only at the peak points.  Find out the true MaxBytes
> and restart mrtg.
> 
> HTH
> 
> Brian Seppanen
> Charter Communications
> Regional Data Center                            906-228-4226 ext 23
> Marquette, MI                                   seppy at chartermi.net
> 
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