[c-nsp] Output Drops

[Peder at NetworkOblivion]

I've got a problem with output drops and I can't quite figure out what 
the issue is.  I am running 12.2.29 on a 7505.  Here is the config for 
the relevant interface:

interface Serial1/0/0:13
  ip address x.x.x.1 255.255.255.252
  service-policy output CUSTOMER-MAP
  encapsulation ppp

class-map match-all CUSTOMER
   match access-group name ToCustomer
   match ip precedence 5

policy-map CUSTOMER-MAP
   class CUSTOMER
     priority 512
   class class-default
     fair-queue

Basically, we are matching IP Prec 5 and a specific IP on the 
access-list for priority of voice.  The customer is complaining about 
quality issues every now and then.  When I check the inteface, I am 
seeing output drops even though there is a small amount of traffic.  The 
output below shows 40 packets dropped in 7 minutes.  I have a graph of 
utilization that updates every 30 seconds and it is never above 500kbps. 
  The 5-min average is ~115,000 bps.


7505#sh int Serial1/0/0:13
Serial1/0/0:13 is up, line protocol is up
   Hardware is cyBus T3
   Description: Blah
   Internet address is x.x.x.1/30
   MTU 1500 bytes, BW 1536 Kbit, DLY 20000 usec,
      reliability 255/255, txload 18/255, rxload 7/255
   Encapsulation PPP, crc 16, loopback not set
   Keepalive set (10 sec)
   LCP Open
   Listen: CDPCP
   Open: IPCP
   Last input 00:00:00, output 00:00:01, output hang never
   Last clearing of "show interface" counters 00:07:03
   Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 40
   Queueing strategy: VIP-based fair queuing
   Output queue: 0/40 (size/max)
   5 minute input rate 43000 bits/sec, 58 packets/sec
   5 minute output rate 114000 bits/sec, 62 packets/sec
      27965 packets input, 3012703 bytes, 0 no buffer
      Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
      0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
      29856 packets output, 9627702 bytes, 0 underruns
      0 output errors, 0 collisions, 0 interface resets
      0 output buffer failures, 0 output buffers swapped out
      0 carrier transitions no alarm present
   Timeslot(s) Used: 1-24, Transmitter delay is 0 flags
   non-inverted data


7505#sh pol int Serial1/0/0:13
  Serial1/0/0:13

   Service-policy output: CUSTOMER-MAP

     queue stats for all priority classes:
       queue size 0, queue limit 128
       packets output 34124, packet drops 0
       tail/random drops 0, no buffer drops 0, other drops 0

     Class-map: CUSTOMER (match-all)
       34124 packets, 2183936 bytes
       5 minute offered rate 48000 bps, drop rate 0 bps
       Match: access-group name ToCustomer
       Match: ip precedence 5
       Priority: kbps 512, burst bytes 12800, b/w exceed drops: 0

     Class-map: class-default (match-any)
       11127 packets, 10132207 bytes
       5 minute offered rate 88000 bps, drop rate 0 bps
       Match: any
       queue size 0, queue limit 256
       packets output 11171, packet drops 0
       tail/random drops 0, no buffer drops 0, other drops 0
       Fair-queue: per-flow queue limit 64

The policy doesn't show any drops due to bandwidth and the interface 
isn't going down, so what is causing the drops?  Is there some other 
commands I can use to try and figure that out?  Or does this sound like 
a bug?  We've been running this version for ~ 40 weeks and they just 
started complaining in the last 2-3 weeks.

Thanks.

Peder