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Big discrepancy on interface counters?

I have a Cisco 877W connected via DSL to my ISP (via interface ATM0).

It's been up a bit more than 8 weeks -

router1#show ver | inc uptime

router1 uptime is 8 weeks, 2 days, 7 hours, 13 minutes

The interface counters haven't been cleared:

router1#show int atm0 | inc counters

  Last clearing of "show interface" counters never

and the counters read like this:

router1#show int atm0 | inc put,   

     97228417 packets input, 3889127918 bytes, 0 no buffer

     79091609 packets output, 1481177856 bytes, 0 underruns

Which if I add them up (I'm assuming, and waiting for confirmation, that my traffic allowance from my ISP is upload and download bytes added together) suggest that only a little under 5Gb has gone through that interface since the router was powered up. That's definitely wrong - I've downloaded a couple of Fedora installation ISOs in the last week that add up to that on their own. My ISP, on the other hand, suggests my usage in the last eight weeks is around 98Gb. That sounds equally wrong (much too high) to me, which is why I was looking at the interface counters in the first place.

This has me curious... do interface counters roll over to zero when they overflow a 32 bit integer or something strange like that? How high can they go? Not relevant to the 877 obviously but I can imagine counters on a 10Gig link rolling over many times a day if they were 32 bit.

Is there any way to see the true numbers now or is it too late? Next month I'll clear them daily and keep notes!

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