[mrtg] unknaszero alternatives

Tim Chambers tim.chambers at sydney.edu.au
Wed Oct 19 02:28:54 CEST 2011


I'm an end user of MRTG, not an administrator of MRTG and new to this list.
I noticed in the MRTG docs the assumption of using the last value if the 
SNMP response is not received. This is probably OK for the occasional 
dropped packet but not so good if it's indicative of a device/link failure.

My suggestion is a few options to compromise which rely on recording in the 
log if the entry is a result of a success/failure, good/bad, real/estimate 
(true/false for SNMP OK) or whatever you want to call it.

1) The first 1 or 2 times it uses the values of previous values.
2) Is it possible to check the status of a router or link to/through router in a 
way that uses TCP to verify if it is working or non-existent?
3) 3rd time it happens in a row (or whatever the admin uses as a threshold) 
it starts assuming the 0 values. If it can verify the router/link is still working 
then it might assume the link is near it's past/recent max. If the router/link 
can be verified to be lost then it's better to show 0.
4) Is it possible to change past assumed entries to be an average of the 
value before and after the temporary loss of a few responses when the 
SNMP response returns ? 

The basic fail on 3rd attempt would be the easiest to implement and with 
some impact to performance. The other options would require more code & 
IO and would slow the update process.

What do other admins, users and potential MRTG programmers think of my 
suggestions ?
Do you have any alternative suggestions ?
Is it worth spending time and resources to expand the MRTG features ?

Tim Chambers

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