[mrtg] Strange phantom spikes in graphs

[Steve Shipway]

> I just recently set up MRTG monitoring of one such installation a few
> days ago, and right away I began noticing strange traffic spikes
> appearing on the WAN and trunk router interfaces: sharp, narrow
> spikes of 60Mbps or more once or twice per day, 

Possible causes -

1) You are running MRTG inside a Virtual Machine (Xen, VMware, etc).  These cause clock skew and mess up calculations.  Don't do it.
2) Try running using SNMPv2 and see if this helps (although your traffic does not seem to be high enough for this to be the cause)
3) There really IS this traffic pattern.  Possibly there is a periodic broadcast storm, or ping flood, or huge SNMP traffic to the device, or something.  Unlikely but may be worth checking.
4) There is a fault in your router.  Unlikely.
5) Something else?

Steve