On Thu, 2009-02-12 at 08:03 -0800, pvh wrote: > Thanks for your reply Dan. > > I don't really understand how I could be experiencing counter roll-overs? > This would be an issue when a 32 bit counter wraps around in less than 5 > minutes, which cannot be happening on my fast ethernet interfaces (which btw > don't even go above 10% utilization).
No, it occurs when any of the interfaces rolls over once, because the aggregate traffic can mask the issue. > surely MRTG (well, Perl > I guess) is capable of handling this properly and is not just adding up my > 8x32 bit counters into a single 32bit counter? Nope. all it does is add them up, it does not add up the distilled numbers. What if you wanted to graph an OID as both a counter and a gauge in different instances? > Besides, wouldn't counter wrap result in _lower_ values than expected? No, merely random. What if two of the devices roll over at the same time? Then what? > Anyway if that's what happening then I don't understand the point of being > able to add/subtract/divide targets in MRTG if it doesn't work? It doesn't work reliably with counters. It works reasonably well with absolute and guage types. For low-bandwidth connections where there are only a couple of interfaces, it can probably survive a single roll-over. For 8 high-speed interfaces where multiple single rollovers are likely, it's completely unreliable. Patches welcome. -- Daniel J McDonald, CCIE #2495, CISSP #78281, CNX Austin Energy http://www.austinenergy.com
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