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SNMP interface packet counters not counting all packets?

Hello,

We had some problems with packet loss on our WAN due to buffers which were too small on the providers equipments, which is why I setup a script to poll the packets in and out interface counters and calculate the difference which gives me the packet loss (approximatly, since some packets are on their way). However, now things have settled down but I see a constant packet loss which I can not explain. Do really all packets get counted by the interface counters? If so, and since the the ethernet over SDH we have really is a point to point link, where does this constant loss of about 6 packets per minute come from?

The setup is as follow:

routerA gi0/1

    |

    | (fiber 1Gbps)

    |

[WAN provider, with Ethernet over SDH,

bandwidth is 150Mbps]

    |

    | (copper 1Gbps)

    |

routerB gi0/0

Trafic between routerA and routerB is encrypted. The interface counters show a few drops due to the QoS, but negligeable compared to the differences in counters (approximatly 9000 packets per day). The high amplitudes are due to higher bandwidth usage during the day.

Also, ip sla is configured on routerA to routerB, but this should not matter?

routerA: C3900e Software (C3900e-UNIVERSALK9-M), Version 15.1(4)M1

routerB: 3800 Software (C3845-ADVIPSERVICESK9-M), Version 12.4(13b)

routerA# sh run int gi 0/1

interface GigabitEthernet0/1

ip address 10.0.0.17 255.255.255.240

duplex auto

speed auto

no cdp enable

service-policy output SHAPE_WAN

end

routerA# sh int gi 0/1

GigabitEthernet0/1 is up, line protocol is up

  Hardware is iGbE, address is 5475.feed.beef (bia 5475.feed.beef)

  Description:

  Internet address is 10.0.0.17/28

  MTU 1500 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, 1Gbps, media type is SX

  output flow-control is XON, input flow-control is XON

  ARP type: ARPA, ARP Timeout 04:00:00

  Last input 00:00:18, output 00:00:00, output hang never

  Last clearing of "show interface" counters 4w0d

  Input queue: 0/75/0/45 (size/max/drops/flushes); Total output drops: 1136

  Queueing strategy: Class-based queueing

  Output queue: 0/1000/0 (size/max total/drops)

  5 minute input rate 526000 bits/sec, 421 packets/sec

  5 minute output rate 5578000 bits/sec, 667 packets/sec

     751459832 packets input, 1231220764 bytes, 0 no buffer

     Received 0 broadcasts (0 IP 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

     925583241 packets output, 798108913 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

routerB# sh run int gi 0/0

interface GigabitEthernet0/0

ip address 10.0.0.30 255.255.255.240

ip access-group secure_outside in

duplex auto

speed auto

media-type rj45

no cdp enable

service-policy output SHAPE_WAN

end

routerB# GigabitEthernet0/0 is up, line protocol is up

  Hardware is BCM1125 Internal MAC, address is 001a.feed.beee (bia 001a.feed.beee)

  Description:

  Internet address is 10.0.0.30/28

  MTU 1500 bytes, BW 1000000 Kbit, 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 RJ45

  output flow-control is XON, input flow-control is XON

  ARP type: ARPA, ARP Timeout 04:00:00

  Last input 00:00:00, output 00:00:00, output hang never

  Last clearing of "show interface" counters 4w0d

  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

  Queueing strategy: fifo

  Output queue: 0/40 (size/max)

  5 minute input rate 5207000 bits/sec, 632 packets/sec

  5 minute output rate 520000 bits/sec, 418 packets/sec

     925357168 packets input, 803914210 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 watchdog, 23174 multicast, 0 pause input

     0 input packets with dribble condition detected

     751736467 packets output, 1250200147 bytes, 0 underruns

     0 output errors, 0 collisions, 0 interface resets

     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

Thanks in advance for any help.

    Thomas

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