[mrtg] Re: traffic on cisco switch VLAN interface

[Greg.Volk at edwardjones.com]

}
}What does the traffic on vlan interface shown by mrtg on cisco switches
}means? Since VLAN is only a logical interface, it should not have any
}traffic on it.
It depends on which cisco product are you trying to use to count traffic
with the vlan interface.

IMHO, ideally a vlan interface would show _all_ the bytes in/out
on that vlan, irrelevant of physical port (switchport mode) 
configuration. 
In my experience, this is not the case. Some of the cisco switches 
correctly count traffic per-vlan on the vlan interface counters, and 
some do not. I suspect that this has to do with flow switching where
the first byte of each flow makes it up to the supervisor, but then
everything after that happens in hardware.

For the record, it appears that a 6509 running Native IOS 12.1.11bE4
counts per-vlan traffic stats correctly. 
However, a 2900xl-m running 12.0.5WC5 does not.

As a general rule, I do not trust the vlan interfaces for accurate
traffic stats. Nor do I trust them as port mirror/monitor/span sources.



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