[mrtg] Re: How do you monitor virtual interface traffic?

Daniel J McDonald dan.mcdonald at austinenergy.com
Mon Apr 14 18:11:36 MEST 2003

On Mon, 2003-04-14 at 10:22, Brenda Washington wrote:
> I want to monitor virtual interface traffic on a Cisco router using
> mrtg, but I'm not sure how to go about setting up my mrtg.cfg file to
> see these virtual interfaces. These virtual interfaces on the router
> (6513 is used as a layer 3 device) are named vlan1, vlan2, etc.,.  

They show up as normal interfaces under cfgmaker, but they don't have
very much traffic associated with them, since the counters don't include
cut-through traffic.

The real question becomes, then, why bother to monitor it?  If you want
to observe resource exhaustion, that won't tell you, since the effective
max-bytes for a virtual interface is the backplane speed of the switch
(32GB/sup1A or SupII with out fabric, or 256GB with fabric, or I forget
how much with the new Sup III...) Counting just the first packet of each
flow isn't going to get you there any time soon...

If you are monitoring it because it is a pretty number to put on a
graph, just tell management to forget it.  It's a meaningless number.

If you are trying to do billing, then I'd look at using netflow
accounting, but practice with it first...

Daniel J McDonald, CCIE 2495, CNX
Austin Energy

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