[c-nsp] Unexplainable Output drops on serial interfaces

[Todd Shipway]

Hi,

We have multiple T1 interfaces across different cards and different type
of cards in a 7513.  Many interfaces are showing output drops and I
can't pinpoint why.  The interfaces are spread throughout the system and
I can't pinpoint a single point that could be causing the drops.  Below
is 2 interfaces showing the drops.

The bandwidth is very low when the drops are occurring, these 2
interfaces are part of a multilink interface which shows no drops.  Any
ideas as to what I should look for that could be causing this?

Serial9/0/0:12 is up, line protocol is up 
  Hardware is cyBus T3
  Description: Bonded T1
  MTU 1500 bytes, BW 1540 Kbit, DLY 20000 usec, 
     reliability 255/255, txload 11/255, rxload 5/255
  Encapsulation PPP, LCP Open, multilink Open
  Link is a member of Multilink bundle Multilink66, crc 16, loopback not
set
  Keepalive set (10 sec)
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters 6d03h
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops:
4050
  Queueing strategy: fifo
  Output queue: 0/100 (size/max)
  5 minute input rate 33000 bits/sec, 19 packets/sec
  5 minute output rate 68000 bits/sec, 18 packets/sec
     3121597 packets input, 667733893 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
     2929740 packets output, 1363133541 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

Serial9/0/0:11 is up, line protocol is up 
  Hardware is cyBus T3
  Description: Bonded T1
  MTU 1500 bytes, BW 1540 Kbit, DLY 20000 usec, 
     reliability 255/255, txload 10/255, rxload 5/255
  Encapsulation PPP, LCP Open, multilink Open
  Link is a member of Multilink bundle Multilink66, crc 16, loopback not
set
  Keepalive set (10 sec)
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters 6d03h
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops:
4085
  Queueing strategy: fifo
  Output queue: 0/100 (size/max)
  5 minute input rate 31000 bits/sec, 19 packets/sec
  5 minute output rate 65000 bits/sec, 17 packets/sec
     3122847 packets input, 668516220 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
     2930011 packets output, 1364226428 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

Multilink interface for the T1's above:
Multilink66 is up, line protocol is up 
  Hardware is multilink group interface
  Description: Bonded T1
  Internet address is 10.10.56.1/30
  MTU 1500 bytes, BW 3080 Kbit, DLY 100000 usec, 
     reliability 255/255, txload 11/255, rxload 4/255
  Encapsulation PPP, LCP Open, multilink Open
  Open: IPCP, loopback not set
  Keepalive set (10 sec)
  DTR is pulsed for 2 seconds on reset
  Last input 00:00:30, output never, output hang never
  Last clearing of "show interface" counters 1w0d
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: Class-based queueing
  Output queue: 0/40 (size/max)
  5 minute input rate 58000 bits/sec, 47 packets/sec
  5 minute output rate 139000 bits/sec, 49 packets/sec
     7075294 packets input, 1705142684 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
     7483773 packets output, 3524642362 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 output buffer failures, 0 output buffers swapped out
     0 carrier transitions

Any help would be appreciated.  As I said earlier, this scenario is
happening on multiple interfaces throughout the system.  A Channelized
DS3 card has been swapped out as a test as well as the output queue
raised from 40 to 100 with no change in drops.

-Todd

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