Throughput Issue - Packet Loss

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I have a 2821 router connected to a 100Mb Ethernet WAN Circuit (Tiered
to 30Mb). The far end of my Ethernet is a 7600.

On both ends I had a QoS policy prioritising a couple of apps. I noticed
  a small amount of packet loss, 3 pings every 1000 or so. Having
removed the QoS at both ends it made no difference through the day,
however, on an evening the circuit seems to run clear.

In peak business hrs the 2821 shows upto approx 4000- 5000pps rx & tx on
it's WAN port (no more than 127/255). The processor shows no more than
10% utilisation. I can't figure out why we may be getting packet loss.

We called the carrier and they removed their rate limiting on the
circuit, they see no interface errors, drops etc on their switches
connected to our routers. Our router likewise show no errors on the WAN
interface between us and the NTU's at either end.

The last mile is provided by another carrier. They apparently provide a
100mb fibre to our carrier and have no active kit in the middle. Our
carrier was the one rate limiting.

I am stumped as to what this could be. Is it feasible that although the
2800 shows low utilisation, the processor / box could still be dropping
packets before making it to the output queue. Furthermore, that these
'processor drops' would not record at the interface level ??



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