[mrtg] How mrtg counts used bandwith?

Steve Shipway s.shipway at auckland.ac.nz
Sun Nov 30 02:47:14 CET 2008

This is a common question.

In this case (IE, if the throughput is >100Mbps) you should instead use SNMPv2.  This will cause MRTG to use the Hc counters, which are64bit and so do not suffer from this problem.


From: mrtg-bounces at lists.oetiker.ch [mrtg-bounces at lists.oetiker.ch] On Behalf Of Alexey V. Panfilov [mrtg at subnets.ru]
Sent: Saturday, 29 November 2008 1:08 a.m.
To: mrtg at lists.oetiker.ch
Subject: [mrtg] How mrtg counts used bandwith?

Hi everyone!

I've the question: how mrtg counts used bandwidth of high-loaded ports?

AFAIK, mrtg uses ifInOctets.PORT_NUM and ifOutOctets.PORT_NUM OIDs.
For example, I've Cisco3560G and I got next result:
IF-MIB::ifOutOctets.10128 = Counter32: 1751739669
but Counter32 can have maximum value of 2^32-1.
So, if I've traffic around 1Gbit/sec on this port Counter will reset to
zero around 30 sec, but mrtg collects data every 5 min...

Please, explain me mrtg's algorithm...

P.S. Sorry for bad english

Best regards, Alexey V. Panfilov
ICQ: 1053180
Jabber: lehis at jabber.ru
mailto: ports at subnets.ru
www: http://www.subnets.ru

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