MRTG graphs - strangeness

[normski]

Hi

I use mrtg to plot traffic usage on our switches and until recently its been OK. However the graphs have been displaying strange behaviour. Our traffic usage has been increasing and peak traffic can now be over 110mbits per second on our link to the outside world. The trouble is when the traffic goes over the 110 mbits per sec mark or their about I get large gaps appearing in the mrtg graphs (see attached file). I thought that this was due to an outage but Im not getting any calls about it and when I check the ipfix flows no gaps appear. And its not just mrtg that displays this blank spot. We run snmpc on the same server that we run mrtg. The graphs that snmpc produces the same behaviour - big gaps. The core switches that are being queried are 8600s running 4.1.8.3. MRTG & snmpc are running on a server with windows 2003 on it.

Is it the server or the core switches?

Any help appreciated.

Norman

[Flintstone]

Hi normski,

It looks like that either the 8600s are not responding to SNMP or that the server running MRTG or SNMPc is playing up?  So you will need to use another server to test where the problem lies?  It does sound like that the problem is with the server, but you will need to prove it?

CheerZ and good luck

[Michael McNamara]

Well this could be from a few different issues....

1) What do you have the MaxBytes option set to in MRTG for that interface? If the router/switch returns a value larger than the MaxBytes setting MRTG will discard the value.

2) You might be wrapping the ifInOctets and ifOutOctets counters since they are only 32bit integers. Are you using the HC counters and polling with SNMP v2?

I doubt it's the switch... most likely your MRTG installation is having issues with the large numbers that are being returned.

Good Luck!

[Stephane P]

Hi,

Recently I had the same problem with cacti
Use SNMPv2 and 64bit counters, this has solved my problem

bye

[normski]

Hi

Yep it needs snmpv2. 

You can supply an option to cfgmaker to tell it to use snmp ver 2  "--snmp-options=:::::2". So I did this but the file created had the nohc option for each interface eg "nohc[192.168.255.1_00-13-0a-9d-20-70]: yes". What this means is that cfgmaker had tried to get the HC counters but couldnt so defaulted to snmp ver 1.
So I did more digging and found this http://oss.oetiker.ch/mrtg-trac/ticket/33. which applies to the version I was running. I Installed the latest version of mrtg and its all working as expcted.

Thanks

Normski

[Michael McNamara]

I'm happy to hear that we were able to help!