I'm afraid MRTG is not designed or even capable of tracking a cumulative counter, in the manner you're suggesting. While it does query the absolute values of the port traffic which each poll, it uses the difference between the two values to calculate the bounded rate, which it graphs. Furthermore, since MRTG log files do not grow in size over time, they must decrease in accuracy: The absolute values it receives from its polling and the deltas it calculates begin to have their precision fall off; this would not provide you with an accurate accounting over time. Since most low-end consumer devices only implement SNMP V1.0, its counters are only 32-bits: they roll over after 65536^2 octets, or about 4.3 Gigabytes of data. This too is a source of problems. MRTG has code to help it maintain a reasonable delta between rollovers, and device/port resets, but it does not, and cannot keep an absolute count of all packets sent.
At any given time,
snmpget can be used to poll the absolute numbers on an interface or group of interfaces. In the following example, I have determined (by perusing the output of an
snmpwalk) that my dslmodem's WiFi interface is number 9, and so I can pull its current absolute stats in this way:
[mini-nevie:~] nevinwilliams% snmpget -c public -v1 192.168.2.1 .sysUpTimeInstance .ifLastChange.9 .ifInOctets.9 .ifOutOctets.9
DISMAN-EVENT-MIB::sysUpTimeInstance = Timeticks: (187184300) 21 days, 15:57:23.00
IF-MIB::ifLastChange.9 = Timeticks: (2500757) 6:56:47.57
IF-MIB::ifInOctets.9 = Counter32: 824681152
IF-MIB::ifOutOctets.9 = Counter32: 317952735
Which tells me that (though I'm not sure if it's 21 days, or 7 hours; I think the former, as it's not used much) that my WiFi interface passed 1.1 gigabytes of data. (I have since rebooted my DSL modem)
Another way of seeing the absolute counters is with the command
[mini-nevie:~] nevinwilliams% snmpstatus -c public 192.168.2.1 [UDP:
[192.168.2.1]:161->[0.0.0.0]:0]=>[Siemens Subscriber Networks
6520-Series (E752)] Up: 0:25:25.00 Interfaces: 22, Recv/Trans packets:
16230/28039 | IP: 5530/1696 5 interfaces are down!
which, as you can see, provides a much more condensed format. However, such data simply does not graph well.