[mrtg] Re: traffic on cisco switch VLAN interface

Rich Adamson radamson at routers.com
Tue Dec 2 16:17:07 MET 2003

> }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.

Having spent a fair number of years doing performance work with corp
clients, I can also add the majority of switch vendors (not just cisco) do 
not produce vlan stats that are anywhere near accurate. Most are off by hugh
factors (sometimes as much as 2x or more) implying programming errors.
Any switch with near accurate stats is generally an exception, not the rule.

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