[mrtg] MaxBytes Question

[Steve Shipway]

When you're dealing with really big numbers, SNMPv2 and its 16bit numbers cannot cope, and you need to use SNMPv2 and its 64bit counters.

If you're using an interface with a bandwidth over 100Mbps (such as in this case), then you MUST use SNMPv2 to be certain of avoiding 'counter wrap' and get reliable graphs.  Also, interface speed over 100Mbps doesnt report correctly to cfgmaker unless you're using SNMPv2.

You can specify --snmp-options=":::::2" to cfgmaker to ensure that it uses SNMPv2 (and make sure your device has it enabled in its ACLs if necessary).

Normally, you don't need to bother about AbsMax unless you have a virtual circuit that can burst above its normal limit.
AbsMax defaults to the value of MaxBytes, and is the highest value deemed sane for this metric - any value higher is discarded.
MaxBytes is used for determining percentages, and the Maximum line on graphs.
Usually, they are both the same, but some ATM circuits and other cases allow usage >100% in some cases.

In your case, the Fibre interface is 1Gbps (this is AbsMax).  However, your provider is rate-limiting you to 100Mbps (this is MaxBytes).  It may be that the provider allows you to burst over this limit if they have free capacity.

If you do not have UnScaled set, then the graph is always scaled to show the MaxBytes line.  If you have a 1Gbps interface, this can result in a very boring graph as the actual data is a tiny green pixel at the bottom.  By setting UnScaled[_]:dwmy you can have the graph autoscale to show the data to its best effect, rather than the MaxBytes line.
Some MRTG/RRD frontends, such as Routers2, default to UnScaled mode for network interfaces; the native-mode MRTG will default to Scaled mode.

HTH

Steve


Steve Shipway
University of Auckland ITS
UNIX Systems Design Lead
s.shipway at auckland.ac.nz<mailto:s.shipway at auckland.ac.nz>
Ph: +64 9 373 7599 ext 86487

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