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

Bill Wichers billw at waveform.net
Thu Apr 7 18:00:06 MEST 2005

> Umm, because it's a switch and not a hub?  Remember the way switching
> technology operates.  It tries to preserve the bandwidth and keep each
> connection "private" and not share it like a hub would. Therefore you do
> not "add up" the total data going in, because the switch, if at all
> possible, is going to try to preserve the speed for each connection.

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.

> Server 1 - Media Server = 11Mbps

Mb/s = MegaBITS per second
MB/s = MegaBYTES per second

There is a difference of 8 times between the two.

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.


Waveform Technology
UNIX Systems Administrator

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