[mrtg] Re: Mrtg does not display high values of traffic correctly ??

[tony bourke]

Hi Richard, 

The answer, as people have already stated, is to use SNMPv2.  

Here is a blurb on the technical reasons for the errors with v1:

One limitation to keep in mind is that with SNMPv1 MRTG or any other SNMP
program) cannot accurately measure bandwidth above about 114 Mbps
(Megabits per second) based on 5-minute intervals. The reason is that the
default interface counter is a 32-bit number, and a 32-bit number will
rollover twice in a period of 5 minutes at more than 114 Mbps. (232 =
4,294,967,296 bytes, multiplied by 8 (bits in a byte), divided by 300
seconds in 5 minutes, is 114,532,641 bits per second, or roughly 114
Mbps.) MRTG or any other SNMP program cannot correctly account for a
double rollover. The graph will appear to peak at about 114 Mbps and never
go beyond that, when in reality the interface could be running well beyond
that rate.  Using SNMPv2's 64-bit counters will overcome this.



On Thu, 30 May 2002, Richard Cotrina wrote:

> 
> Hello :
> 
> I am using mrtg to monitor the traffic going and coming through a STM-1 POS
> interfaz in my Cisco Router. It was working fine until the values of
> incoming traffic started to be above 100Mbps.
> 
> The matter is that suddenly and without giving any errors in the log file,
> the values displayed (also the graph) for the incoming traffic fall down to
> very small values (a few kbps) but, in the orther hand, this does not happen
> with the outgoing traffic, that remains displayed in its correct values (
> it's usually not higher than 50Mbps ). This condition returns to normality
> without any reason, and also when the traffic is lower than 100Mbps.
> 
> The result of this, is that my mrtg graphs shows an apparently loss of
> traffic, but it is not caused by any traffic interruption, because the
> outgoing traffic is displayed continously.
> 
> I was using mrtg 2.9.17 when it started to happen, then I upgraded to
> 2.9.18pre11 and got the same results.
> 
> Does anyone have an idea on what is going on ?  As i mentioned before, the
> logs did not give me any error messages.
> Do I need a special option in the mrtg config file to handle high values of
> traffic correectly ?
> 
> Thanks in advance for any help,
> 
> Richard Cotrina
> 
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Tony Bourke				tony at vegan.net


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