[mrtg] Re: CableModem Interfaces

Christopher A. Congdon ccongdon at congdonweb.com
Wed Jun 8 16:42:22 MEST 2005

> Some questions/observations:
> 1) IF3 and IF4 just seem to be splitting up the data that I can also
> on
>    IF1.  Is that correct or am I missing some subtelty as to whay it
> split
>    that way?

I'm not sure why this is. It could have something to do with the way the
cable modems work. I know most cable modem operators have the ability to
limit traffic coming in or going out. In reflection of the fact that the
cable modem we have in the office (on a commercial account) can be set
to 2Mb up/2Mb Down, yet a residential user on the same network gets 2 Mb
Down/128Kb Up... I think maybe they decided that bandwidth shaping was
easier to control if inbound and outbound traffic was split onto two
separate 'interfaces'.

> 2) IF1 is the same data, just in reverse, of what I am seeing on the
> server's
>    eth0.  Again, is that correct or am I missing something?

Depends on your network layout. Does all of your internet traffic go
through the server? If so, then yes, the cable's IF1 is going to be a
mirror image of your server's ETH0, but only if the cable modem is the
only thing attached to your server's ETH0.

> 3) If the above are correct, if I want to graph my internet traffic,
all I
>    really need to grapg is my server's eth0.  I don't see any
> to
>    graphing the interfaces on the cablemode.  Is this correct too?

If all of your traffic goes through the server, then there is a
possibility you don't need to graph the cable modem. Question however...
Does your server have more than one NIC? If ETH0 is connected ONLY to
your cable modem and serves no other traffic on your network, then your
server's ETH0 graphs should match the combination of your cable modem's
> Thanks!
> ...Jake

No problem, glad to be of assistance.

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