[mrtg] similar graphs

Steve Shipway s.shipway at auckland.ac.nz
Thu Mar 17 21:05:25 CET 2011

This could be caused by something sending out a large number of broadcast or multicast packets, or packets to an unknown MAC address, which the switch would sent to all ports as it doesnt know on which port the true destination is located.  You would see spikes on all the switch ports for the VLAN.  Possibly you have something doing massive amounts of broadcasts, or something that is trying to scan the subnet by pinging all IPs resulting in many ARP requests going out?


Steve Shipway
University of Auckland ITS
UNIX Systems Design Lead
s.shipway at auckland.ac.nz
Ph: +64 9 373 7599 ext 86487

From: mrtg-bounces+s.shipway=auckland.ac.nz at lists.oetiker.ch [mrtg-bounces+s.shipway=auckland.ac.nz at lists.oetiker.ch] on behalf of Paul Macdonald [paul at ifdnrg.com]
Sent: Friday, 18 March 2011 3:18 a.m.
To: mrtg at lists.oetiker.ch
Subject: [mrtg] similar graphs


I am using mrtg for simple bandwidth monitoring on our cisco switches.

The graphs on one switch appear to have very very similar peaks across 4
or 5 ports, although the totals and the axis are quite differnet.

Each of the servers on each of the similar ports performs quite
different tasks so its very unlikely that they would have peaks at the
same times.

Has anyone encountered anything similar, should i suspect hardware
problems on the switch?

Typical mrtg targets are of the form :   (64 bit counters).

ifHCInOctets.10111&ifHCOutOctets.10111:XXXXXXXX at x.x.x.x:::::2

If i run some large transfers through any of the ports these show up
normally on the appropriate graphs, and the similarity disappears as

under normal usage the graphed load is as expected but the similarities
across these ( 4 out of 24 ) ports seem very mysterious and unexplained.

I plan to swap the feeds at some point to see if the behaviours follows,
but have not done this yet..



Paul Macdonald
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