[c-nsp] understanding interface traffic counters of Cisco router and Cisco switch

[Sergey Nikitin]

Hi,

Most likely this is because of 802.1Q tag (4 bytes) added to the counter 
on a switch interface (and obviously you don't see this tag on a router 
interface). For example, interfaces Fa3/0 and Fa0/24:
773476480 - 771435576 = 2040904
2040904 / 510226 = 4

HTH

Martin T wrote:
> I made a following setup:
> 
> http://img828.imageshack.us/img828/5736/interfacestrafficcounte.png
> 
> ..and executed "iperf -s -u -fm" in "ubuntu" machine and "iperf -c
> 10.10.11.2 -fm -u -d -b 10m -t600" in "PE860" machine. Before the test
> I cleared all interface counters. Iperf results were following:
> 
> root at PE860:~# iperf -c 10.10.11.2 -fm -u -d -b 10m -t600
> ------------------------------------------------------------
> Server listening on UDP port 5001
> Receiving 1470 byte datagrams
> UDP buffer size: 0.12 MByte (default)
> ------------------------------------------------------------
> ------------------------------------------------------------
> Client connecting to 10.10.11.2, UDP port 5001
> Sending 1470 byte datagrams
> UDP buffer size: 0.12 MByte (default)
> ------------------------------------------------------------
> [  3] local 10.10.10.2 port 44911 connected with 10.10.11.2 port 5001
> [  4] local 10.10.10.2 port 5001 connected with 10.10.11.2 port 49469
> [ ID] Interval       Transfer     Bandwidth       Jitter   Lost/Total Datagrams
> [  4]  0.0-600.0 sec    715 MBytes  10.0 Mbits/sec  0.008 ms    0/510205 (0%)
> [  4]  0.0-600.0 sec  1 datagrams received out-of-order
> [  3]  0.0-600.0 sec    715 MBytes  10.0 Mbits/sec
> [  3] Sent 510206 datagrams
> [  3] Server Report:
> [  3]  0.0-600.0 sec    715 MBytes  10.0 Mbits/sec  0.026 ms
> 2/510205 (0.00039%)
> [  3]  0.0-600.0 sec  1 datagrams received out-of-order
> root at PE860:~#
> 
> 
> For some reason, the interface counters in switch(Fa0/1, Fa0/2, Fa0/23
> and Fa0/24):
> 
> Cisco2950#show interfaces Fa0/1 | i packets input|packets output
>      510227 packets input, 773472188 bytes, 0 no buffer
>      510236 packets output, 773484380 bytes, 0 underruns
> Cisco2950#show interfaces Fa0/2 | i packets input|packets output
>      510225 packets input, 773476416 bytes, 0 no buffer
>      510223 packets output, 773471932 bytes, 0 underruns
> Cisco2950#show interfaces Fa0/23 | i packets input|packets output
>      510230 packets input, 773476736 bytes, 0 no buffer
>      510233 packets output, 773479832 bytes, 0 underruns
> Cisco2950#show interfaces Fa0/24 | i packets input|packets output
>      510222 packets input, 773471868 bytes, 0 no buffer
>      510226 packets output, 773476480 bytes, 0 underruns
> Cisco2950#
> 
> ..show little bit different results than router counters:
> 
> C3640#show interfaces Fa2/0 | i packets input|packets output
>      510228 packets input, 771431340 bytes
>      510230 packets output, 771435816 bytes, 0 underruns
> C3640#show interfaces Fa3/0 | i packets input|packets output
>      510226 packets input, 771435576 bytes
>      510222 packets output, 771430980 bytes, 0 underruns
> C3640#
> 
> I tried this test multiple times with different bandwidth(-b) values
> and always the difference between router counters and switch counters
> were ~0.3%. If the difference were 1.2% - 1.3% then it would make
> sense because probably in this case router counts only up to IP
> header, but switch includes L2 header as well, but what might cause
> this 0.3% difference?
> 
> 
> regards,
> martin
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