[mrtg] Odd Behavior with mrtg + rrdtool

[Jonathan Williams]

Hello again,

I've been playing with my setup for mrtg and rrdtool. I have things
polling the OIDs I want and I'm able to generate webpages with the
graphs I want to show. However, I'm getting some odd behavior, and I'm
not quite sure where it is coming from.

My setup is as follows:

A traffic generator is set to produce unidirectional traffic at a
constant rate of 11.6Mbps, fed in to eth0 on my router, and looped back
to the generator on eth1. I am polling ifInOctets and ifOutOctets on
both interfaces on 2 second intervals through the management port of the
router.

The graph I would expect to see would be a flat line for ifInOctets on
eth0 at 11.6Mbps, and a correlated flat line at 11.6Mbps for ifOutOctets
on eth1.

Predominately, this is what I see. However, once every 3-4 minutes there
is a dip in the graph. The dip generally spans 1 or 2 intervals in
length, and is on the order of magnitude of approximately 1/2 to 1/10th
of the expected value. I have verified that this is not a dip in the
actual traffic by querying the hardware directly on the router and
observing that across the dips in the graph, the delta between the
direct queries remains constant.

Doing an SNMP traffic capture in wireshark, I observed that mrtg appears
to be sending requests on approximately 2 second intervals, as expected
- except for when the dips in the graphs are appearing. In these cases,
mrtg sends a request once every second.

When setting mrtg to query once every second, the 'duplicate' requests
appeared more frequently, and were only separated by around 10ms. This
resulted in significantly more dips in the graph, each corresponding to
the extra SNMP-GET request from mrtg.

Based on my understanding of rrdtool, multiple data entries occuring
inside a single 'step' should not be causing this to happen. Is it
possible that MRTG is sending out these extra requests, but not handling
them until the specified interval length? Any suggestions are
appreciated!

J.Williams