[mrtg] OIDs changing.

Koelstra, J. (Jan) JKoelstra at MINSZW.NL
Mon Sep 3 10:27:22 CEST 2007

Nothing to be amazed of:
According to the SNMP definitions (RFC 1155) the instances have to be unique and must be accompanied by a description. 
They also have to be flexible to appear and disapear (like the statistics for a specific user in a OS, the definition of a new sub-interface or the statistics for a specific vpn tunnel etc.) 
Nothing is said about the index number staying the same all the time. 
Several vendors (Cisco, Microsoft) rebuild their indexes when starting the SNMP process. So the indexnumber can change after a reboot, especially after a some reconfiguration after the previous boot.
To avoid problems with broken graphs MRTG offers some methods to identify the right oid. See http://oss.oetiker.ch/mrtg/doc/mrtg-reference.en.html#IInterface_by_IP for details.

	-----Original Message-----
	From: mrtg-bounces at lists.oetiker.ch [mailto:mrtg-bounces at lists.oetiker.ch] On Behalf Of gaurav
	Sent: Friday, August 31, 2007 9:11 PM
	To: mrtg at lists.oetiker.ch
	Subject: [mrtg] OIDs changing.
	Dear all,
	Recently I was amazed to see few things in the devices I was using MRTG for.
	First, I have two cisco uBR 7200 with same IOS version and same model but the OIDs in them for same datas to are to be retrieved are different in the leaf OIDs whereas the branch OID is the same.
	i mean if was for one than was for the other..
	Second thing is one of the device got automatically rebooted because of some power failure lately, and i was unable to see MRTG graphs for few of the datas polled. Upon further diagnose i found that the leaf OIDs have increased with two 
	i mean if was the OID that was polled than after the reboot it wasnt existing and was the one that was providing the necessary values.
	Has any one came across this type of situations before...please do share..
	Does the OIDs get changed so abnormally or is it because of some faults in the device.


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