[mrtg] Re: 32 versus 64 bit counters

[Brander, Eric]

Ok, doing an SNMP walk on my 3Com 4005 produced a lot of "stuff".  However,
after searching through waves of meaningless (to me) data I found something
this:

~snip~
.iso.3.6.1.2.1.31.1.1.1.7.1 = Counter64: 4410839262
.iso.3.6.1.2.1.31.1.1.1.7.2 = Counter64: 4334336234
.iso.3.6.1.2.1.31.1.1.1.7.3 = Counter64: 41569266
.iso.3.6.1.2.1.31.1.1.1.7.4 = Counter64: 39891468
.iso.3.6.1.2.1.31.1.1.1.7.5 = Counter64: 8594911450
.iso.3.6.1.2.1.31.1.1.1.7.6 = Counter64: 60136207979
.iso.3.6.1.2.1.31.1.1.1.7.7 = Counter64: 12951696266
.iso.3.6.1.2.1.31.1.1.1.7.8 = Counter64: 953059
.iso.3.6.1.2.1.31.1.1.1.7.9 = Counter64: 22984343765
~snip

Ok, so, I have 64-bit counters it seems... and lots of them.  Can I use
these OIDs to graph?  Also, how do I find out which OID means input, which
output, errors, etc?
 -----Original Message-----
From: 	Alex van den Bogaerdt [mailto:alex at ergens.op.het.net] 
Sent:	Monday, September 16, 2002 10:02 AM
To:	'mrtg at list.ee.ethz.ch'
Subject:	[mrtg] Re: 32 versus 64 bit counters


On Mon, Sep 16, 2002 at 06:14:47AM -0700, Charles Milhans wrote:
> I am sorry for this newby type of question.
> 
> How do you determine what the polling interval should be when using 32bit
> and/or 64 bit counters and monitoring speeds of 2 Gbps?  I am trying to
> monitor fibre channel switches that use both 1 Gbps and 2 Gbps speeds.  I
> understand that the counters go from 0 to max value and then go back to 0.


MRTG has problems with counter wrapping:

1) It will not detect that a counter was incremented with 2^32 + some more
2) It will not use a decremented counter
         (or at least: That used to be the case)
3) It will not detect that a counter has wrapped more than once

If the counter increases with "2^32+x", SNMP reports only "x".
(the #1 case from above)

If the counter is decreased, this is an indication that the counter
wrapped  -or-  that the counter was reset.  MRTG discards the update.
(the #2 case from above)


If #1 happens a lot, your graph will show way too low values.
If #2 happens a lot, your graph will show repeated values.
If #3 happens, you shouldn't be using the 32 bit counters at all

Problem #1 occurs at 2^32 / number_of_seconds.  For a normal
setup, this will be at 14+ MBps or 114+ Mbps.

The other way around: 2^32/rate = number_of_seconds.
This means the problem will occur for a 2 Gbps interface every
17.something seconds.

Counter wrapping with a 64 bit counter on a fully utilized 2 Gbps
interface will happen every 73,786,976,294 seconds.  Let me know what
happens when you encounter this :)

HTH
Alex

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