[mrtg] Re: Different traffic on each side of a link

Alex van den Bogaerdt alex at slot.hollandcasino.nl
Tue Apr 9 13:08:25 MEST 2002

=?iso-8859-1?Q?Nicolai_Str=F8m_Gylling?= wrote:

> None of those apply, since we observe the behaviour on two different setups on different switches and NIC's.

NIC != SNMP.  If the server doesn't count properly (linux had such a
problem a while ago) changing NICs doesn't help.

> What settings in mrtg.cfg could discard valid data?

Due to various circumstances the rate as seen by MRTG can exceed
the line speed.  Not much, but it can.  If you discard that then
obviously it isn't counted.  If the rate is just above 10 Mbps,
so even a small error, you are discarding 3,000,000,000 bits which
is 375,000,000 bytes.

> Well, if the switch is smart, it knows what port the traffic is going to

and thus, if the switch is smart, it won't count the traffic.

You seem to insist that the switch is the problem.  Why not the server?

Perhaps you're not monitoring ip traffic, you're monitoring http
(or whatever) traffic in stead.

Or perhaps the switch connects to an internal network, with a higher
speed, and MRTG discards all traffic when the rates go above the lower.
If the server side does 15Mbps (say 7 Mbps to your switch and 8Mbps
to another server) you are counting 7 Mbps on the switch end and are
discarding the 15 Mbps on the server side...

Maybe you want to deploy a network sniffer.  Do the following:

1) Start a capture session
2) query the ifInOctets and ifOutOctets counters on both ends
3) wait some time, for instance a minute
4) query the ifInOctets and ifOutOctets counters on both ends again
5) stop the capture

In the capture you will see the device being polled so you know what
is counted from that moment on.  You can thus edit the capture file
and strip anything between (1) and (2) as well as between (4) and (5).

Now you can analize the data in your capture file and compare with
delta(ifInOctets) and delta(ifOutOctets).

 / alex at slot.hollandcasino.nl                  alex at ergens.op.het.net \
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