[mrtg] Re: MRTG problem for OC3

[tony bourke]

I know this has been mentioned before, but here is the math to show why
MRTG or any SNMP program at 5 minute intervals cannot measure more than
114 Mbps of data:

One limitation to keep in mind is that the default configuration file
generated by MRTG cannot accurately measure bandwidth above about 114 Mbps
(Megabits per second). 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. To get around this, you must
change the configuration file to pole a special 64-bit interface counter,
or poll MRTG more frequently than 5 minutes.


Tony


On Tue, 24 Oct 2000, Carmen Chow wrote:

> 
> Dear all,
> 
> I try to perform MRTG for a OC3 (POS intf).  The graphs is nice when the
> traffic is below 100M.  However, it never growth over 120M even I've
> modified the interval to 2 mins.
> 
> The peak of the graph drop back to 20-30M once the peak is over 120M.
> Did anyone have encountered this problem and pls give me some hints to
> solve it.  Thanks.
> 
> Regards,
> Carmen Chow
> 
> 
> 
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Tony Bourke				tony at vegan.net



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