[mrtg] Re: mrtg traffic thru the switch?

Jay Hennigan jay at west.net
Thu Jan 29 21:34:09 MET 2004

On Thu, 29 Jan 2004, Monty Swiryn wrote:

> Hi,
> I am trying to get a strict definition about exactly what mrtg measures.

Short answer:

It depends.

Long answer:

MRTG measures whatever numbers are given to it by an SNMP poll, script,
or other input method.

> Here's the question:
> Does MRTG measure all traffic that simply hits the "IN" (or "OUT") side of
> a switch,
> OR, does it actually measure traffic that goes THROUGH the switch?
> In other words, does mrtg know the difference between (a) traffic that hits
> one of the switch interfaces and goes thru to the other interface, and (b)
> traffic that hits one of the switch interfaces and does not go thru to the
> other interface?

The question is properly directed to the switch vendor rather than to an
MRTG forum.  In the case of Cisco switches and the interface counters,
it is all traffic entering and leaving that interface regardless of the
protocol or ultimate destination or source.  If the switch throws the
traffic on the floor or if the traffic is sourced by or destined for the
switch itself, it still counts.  The packet is received on the interface,
counted, and then, delivered, processed by the switch itself, or thrown
on the floor.  For example, the interface communicating with the MRTG box
has SNMP traffic to and from the switch itself.  It doesn't pass through
to any other interface.  This traffic is counted.

Jay Hennigan - CCIE #7880 - Network Administration - jay at west.net
WestNet:  Connecting you to the planet.  805 884-6323      WB6RDV
NetLojix Communications, Inc.  -  http://www.netlojix.com/

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