[mrtg] Re: Cisco Switch: Interfaces don't add up right

Shane Presley shane.presley at gmail.com
Thu Apr 7 20:16:29 MEST 2005

Thanks Bill and all.  I think the 64 bit counters are working.  It
works in testing, and I'll confirm that it works tonight at midnight
when our backups happen.

And thanks for clearing up the uplink example.  I think I just did a
bad job of explaining our setup.  But you had it correct, all three
ports would be going out on the gig uplink.


On Apr 7, 2005 12:00 PM, Bill Wichers <billw at waveform.net> wrote:
> On a switch, if you have (for example) 3 ports bringing in 11 Mb/s and one
> port that it's *all* going out on (maybe an uplink), then you should see
> 33 Mb/s of outbound traffic on the one port going out. That is 11+11+11 =
> 33. Assuming the uplink's not saturated or dropping packets for some
> reason, the traffic will sum if it's all going out the same interface.
> A hub basically copies all inbound data on any port to all outbound ports
> simultaneously. A switch sorts the traffic based on hardware address (or
> more for L3-L7 switches), and only sends the traffic out the port the
> device the packet is destined to is connected to.

> For the original poster, your question sounds like a counter wrap problem.
> If you're running constant traffic out, then it's likely the counters wrap
> in about the same spot for each of MRTG's samples, so you'll see a
> constant number in the graphs that will be too low. Try using 64 bit
> counters on the gigabit ports only. The wrapping problem shows up at just
> over 114 Mb/s, which is due to the maximum number of bytes a 32 bit
> counter can keep track of in a 5 minute period.
>      -Bill

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