[c-nsp] Graphing SVI throughput on 3550s

[sthaug at nethelp.no]

> I've got mrtg/rrdtool graphing some SVIs on a 3550 switch.  The traffic 
> throughput figures are coming out wrong, and even a "show int Vlan8" 
> gives the wrong output:
> 
>    ...
>    1 minute input rate 188000 bits/sec, 234 packets/sec
>    1 minute output rate 186000 bits/sec, 233 packets/sec
>    ...
> 
> Why is this wrong?  I'm downloading an ISO image from outside that 
> network at about 800Kb/sec right now :)

Known problem. Won't be fixed.

> What do the counters represent?  More importantly, which OIDs must I 
> graph to see the true throughput of the SVI?

The counters probaby represent traffic going to/from the CPU of the
switch, not traffic forwarded by the switch hardware. The 3550 (and
for that matter the 3750 and probably also the 3560) do not have the
necessary hardware resources to give you per-SVI counters. You *can*
get reasonably reliable counters for the physical ports, though.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no