[mrtg] Error in sum interfaces

[Koelstra, J. (Jan)]

The counter rollover occurs when the snmp counter has reached its maximum. By default SNMP uses 32bit counters. 
In a 32bit counter the maximum number wich can be stored is 2^32-1. After this the counter resets to 0 and starts counting up to ist maximum again. This reset to 0 is called the counter rollover.
MRTG has an build in mechanism that deals with counter rollovers: When the current value of the counter is lower then the previous value there must have been a counter rollover. Because there is no way to detect if there has been 1 or more counter rollover MRTG assumes there has been 1 rollover, and in the calculations the counter maximum is added to the current value. 
 
Counter rollovers occur frequently: When polling your target once per 5 minutes the rate per second is:
(current poll value - previous poll value) / 300 seconds.
When doing some calculalations you will find the counter rollover occurs every 5 minutes with a bandwith usage of about 114Mbit/s.
 
When graphing multiple target from one targetline there is a chance for a counter rollover in each individual target. This is where MRTG fails to graph the rate correctly: It can not know how many counter rollovers have occured in the last polling interval. 
What makes things even worse: When one counter rollover occured on one of the targets, and the other targets together processed enough data to compensate this rollover it is not even possible to detect the only one rollover! 
When using 64bit counters the limits are way higher, but the basic problem will be the same.
 
When storing the data in rrd format, the data for each individual target is stored after the counter rollover calculation. Now you can retrieve and summarize the data in a more reliable way.
 
HTH,
 
Jan.
 
 

	-----Original Message-----
	From: NAP [mailto:nap at etecsa.cu] 
	Sent: Friday, September 07, 2007 3:13 PM
	To: Koelstra, J. (Jan)
	Subject: RE: [mrtg] Error in sum interfaces
	
	

	My sum is with 5 interfaces

	this my Target.  My problem will be here?

	Only show 3 interfaces

	 

	Target[myrouter]: /x.x.x.x:public at a.b.c.d:::::2:+/y.y.y.y:public at a.b.c.e:::::2:+/z.z.z.z:public at a.b.c.f:::::2:

	 

	I'm beginner in mrtg and don't understand what do you say with counters rollovers and rrd for backend it

	 

	Tell me about my Target, and what I could do?

	 

	So Thanks. 

	 

	 

	
  _____  


	De: Koelstra, J. (Jan) [mailto:JKoelstra at MINSZW.NL] 
	Enviado el: viernes, 07 de septiembre de 2007 4:33
	Para: NAP; mrtg at lists.oetiker.ch
	Asunto: RE: [mrtg] Error in sum interfaces

	 

	How do you create the graph with the sum?

	When using a cfg file with Target: oidx at a.b.c.d + oidy at a.b.c.d + ... you will almost certain run into problems with counter rollovers.

	Without rrd for backend it isn't possible to create summarized graphs of (bandwith-)rates that are 100% accurate. 

	 

	HTH,

	 

	Jan.

	 

	 

		-----Original Message-----
		From: mrtg-bounces at lists.oetiker.ch [mailto:mrtg-bounces at lists.oetiker.ch] On Behalf Of NAP
		Sent: Wednesday, September 05, 2007 7:52 PM
		To: mrtg at lists.oetiker.ch
		Subject: [mrtg] Error in sum interfaces

		We have problem with interface sums. The values in the independent graphics don't correspond to the total value that we graphic.

		The file .cfg of independent interfaces and the sum graphic is the same.

		What will be the problem?

		We need help

		 

		Thanks, 

		 

		 

		 

		 

		 

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Dit bericht kan informatie bevatten die niet voor u is bestemd. Indien u niet de geadresseerde bent of dit bericht abusievelijk aan u is toegezonden, wordt u verzocht dat aan de afzender te melden en het bericht te verwijderen. De staat aanvaardt geen aansprakelijkheid voor schade, van welke aard ook, die verband houdt met risico's verbonden aan het elektronisch verzenden van berichten.
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This message may contain information that is not intended for you. If you are not the addressee or if this message was sent to you by mistake, you are requested to inform the sender and delete the message. The state accepts no liability for damage of any kind resulting from the risks inherent in the electronic transmission of messages.
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