[c-nsp] Output drops mysteriously appear/disappear on 3750X

[Eric Van Tol]

Hi all,
I've got a pair of stacked 3750X switches running 15.0(1)SE3 and I'm noticing something funky.  On a pair of mirror destination ports, I'm seeing output drops appear and disappear in the CLI output of 'show interface' at random intervals.  What I mean by this is, after clearing counters on all interfaces, I can do a 'show int gi1/0/16' and see 'output drops 0' re-run the command one second later and see 'output drops 39428200', then re-run the command another second later and see 'output drops 0' again.  I see it on four monitor session destination ports.  The problem started somewhat randomly one day last week, after having working fine for two days prior after being put into production use.

Anyone ever seen this before?  I'm pretty sure it's not just a cosmetic issue, as the analyzer attached to these ports is seeing "packet loss" on the traffic its analyzing, but I don't believe the source ports are actually seeing any loss.

Output is below - take note of the 'Last clearing of counters' times.

-evt

~~~~~~~~~~~~~~
vsw1-1213c.ss.sls.md#sh int gi2/0/17
GigabitEthernet2/0/17 is up, line protocol is down (monitoring)
  Hardware is Gigabit Ethernet, address is d48c.b549.4511 (bia d48c.b549.4511)
  Description: VL Mirror Port
  MTU 9198 bytes, BW 1000000 Kbit/sec, DLY 10 usec, 
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
  input flow-control is off, output flow-control is unsupported 
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input never, output never, output hang never
  Last clearing of "show interface" counters 01:23:22
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  30 second input rate 0 bits/sec, 0 packets/sec
  30 second output rate 3868000 bits/sec, 2243 packets/sec
     0 packets input, 0 bytes, 0 no buffer
     Received 0 broadcasts (0 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 0 multicast, 0 pause input
     0 input packets with dribble condition detected
     24027518 packets output, 5217538085 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out
vsw1-1213c.ss.sls.md#sh int gi2/0/17
GigabitEthernet2/0/17 is up, line protocol is down (monitoring)
  Hardware is Gigabit Ethernet, address is d48c.b549.4511 (bia d48c.b549.4511)
  Description: VL Mirror Port
  MTU 9198 bytes, BW 1000000 Kbit/sec, DLY 10 usec, 
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
  input flow-control is off, output flow-control is unsupported 
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input never, output never, output hang never
  Last clearing of "show interface" counters 01:23:23
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 39428034
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  30 second input rate 0 bits/sec, 0 packets/sec
  30 second output rate 3868000 bits/sec, 2243 packets/sec
     0 packets input, 0 bytes, 0 no buffer
     Received 0 broadcasts (0 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 0 multicast, 0 pause input
     0 input packets with dribble condition detected
     24027518 packets output, 5217538085 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out
vsw1-1213c.ss.sls.md#sh int gi2/0/17
GigabitEthernet2/0/17 is up, line protocol is down (monitoring)
  Hardware is Gigabit Ethernet, address is d48c.b549.4511 (bia d48c.b549.4511)
  Description: VL Mirror Port
  MTU 9198 bytes, BW 1000000 Kbit/sec, DLY 10 usec, 
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
  input flow-control is off, output flow-control is unsupported 
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input never, output never, output hang never
  Last clearing of "show interface" counters 01:23:23
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  30 second input rate 0 bits/sec, 0 packets/sec
  30 second output rate 3868000 bits/sec, 2243 packets/sec
     0 packets input, 0 bytes, 0 no buffer
     Received 0 broadcasts (0 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 0 multicast, 0 pause input
     0 input packets with dribble condition detected
     24048581 packets output, 5222039119 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out