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

[Alex van den Bogaerdt]

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

> We are using MRTG/RRDTool to monitor and bill the traffic our costumers use. We are monitoring all customers in our end of the link, ex. a port in a switch.

Quite normal I'd say.

> Due to some problems we have done some monitoring in both ends of a link, and discovered that the trafficamount is not the same. In our testcases in was between an ethernet NIC in a server and a port in a switch (HP ProCurve 2524).
> 
> The trafficamount on the switchport are considerably higher than on the ethernet port in the server. 

Are you sure you don't mistake bits for bytes?

> What can cause the different readings from each end of the link? 

- bits vs. bytes
- faulty snmp on the server
- faulty snmp on the switch
- not seeing all traffic on the server
- seeing more on the switch than just traffic for this link
- faulty settings in your mrtg.cfg file, discarding valid data
  (MaxBytes could be too low, set it 10% above the link speed just
  to make sure).

> Will broadcast-traffic be answered by the switch, and also appear on the switch-port as traffic?

So?  If that's the case it should also be visible on the server.

> Is there a way to differentiate between the different kinds of traffic(broadcast vs. normal traffic), and thereby measure the same traffic-amount in each end of the link? 

Broadcast traffic *is* normal traffic.  And what broadcasts do you mean?
Ethernet broadcasts or ip broadcasts?  Or maybe even netbeui?  I don't
think a switch can differentiate between ip-broadcasts and other ip. The
other two may be possible, you will have to ask HP.


Tip: If possible, create some traffic for more than 10 minutes.  Check
the results.

HTH,
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