[mrtg] Re: Lan monitoring

Henrik Berg Nielsen n99hbn at nat.sdu.dk
Sat Aug 24 08:35:21 MEST 2002

----- Original Message -----
From: "Peter Glanville" <peter_glanville at cuk.canon.co.uk>
To: "Henrik Berg Nielsen" <n99hbn at nat.sdu.dk>
Sent: Friday, August 23, 2002 12:55 AM
Subject: Re: [mrtg] Re: Lan monitoring

> Henrik,
> Traffic from one PC to the other will not run THROUGH the router if both
> PCs are on the same subnet. The traffic will go directly, through the Hub.
> If you are using a Hub, then the LAN port on the Router SHOULD see all
> traffic, but it would all be seen as bytes IN, regardless of which PC sent
> it, and which way it is travelling.
> You indicate that you monitor the WAN port and tried the LAN port. Do they
> produce equal (but opposite) graphs? (Bytes In to the WAN port should
> bytes out of the LAN port).
> If the two are equal and opposite, you must have a switch.
> If the two are different, the difference is the traffic in your LAN.
> OID maths will allow LANport - WANport (=LAN traffic) in MRTG. [Hang on,
> that needs to be LAN minus the reversed WAN port, ie Local = LAN In minus
> WAN Out]
> You might consider monitoring the PC, try the snmp4tpc website. This would
> also allow you to tell which PC is sending and which receiving, as the
> Router LAN port will see both as Inbound.
> Regards
> Peter

It is indeed a hub, it says so on top of the device, and I trust that :)

Both PC's are on the same subnet, so I think you hit it on the head with the
above comment. However, the two graps are identical, but the IN and OUT are

Realizing this is there any way around it for me to monitor the local
traffic between the two machines - I don't care to know which way the data
runs just the total bandwidth usage? Or is the only sulution for me to buy a
switch and use that instead or to set up a third machine acting as a gateway
(and perhaps firewall) for the two other machines?

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