[mrtg] Re: Two copies of MRTG disagree with each other about usage

Zach White zwhite-mrtg at darkstar.frop.org
Thu Jul 21 02:32:56 MEST 2005

On Thu, Jul 21, 2005 at 11:49:31AM +1200, Craig Humphrey wrote:
> Hi Zach,
> At first glance I'd say that your ISP is measuring in MegaBits per
> second and you're measuring in MegaBytes per second, the ratio seems
> about right.

I should have considered that possibility, actually. And as it turns out,
that's exactly the problem. I've switched my config over and things seem
to match up properly (after removing *.png and rerunning mrtg.)

> Why have the ISP coming in via a switch, can't you plug it straight into
> the Firewall? Then you don't have to VLAN off that part of the switch.
> If you still need MRTG stats, you/they should be able to get them from
> the firewall itself (though of course your side will probably still be
> plugged into a switch somewhere...)
> Also, I seem to recall people pointing out that Cisco interface stats
> for VLANs is fairly unreliable, but that could be just hearsay.

Well, I wasn't part of that decision, it's a network I've inherited. I
think the reasoning was to have SNMP only on the switch but still poll
for aggregate data. It also allows me to do things like speed limiting
and mirroring to another port for IDS purposes. All in all, an unusual
config but one that I think I like.

I've also noticed the unreliable stats for the VLAN. I have MRTG polling
all ports plus the VLAN for data, and the graph for the VLAN does
not match up at all with the graph for a port on the VLAN. Lots of
flat lines that indicate missing packets. It's not a huge deal for me
since I only have 2 ports in the VLAN, and the data coming from the
port now matches my ISP.

Thanks for the help, I'm a happy camper now.


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