[mrtg] MRTG'ing Gigabit speeds

[John Haggerty]

Hi,

I have been having trouble configuring MRTG to monitor traffic correctly 
from servers with Gigabit cards.

I'm running MRTG, version mrtg-2.10.15-1 on a Scientific Linux 4.1 (free 
rebuild of Red Hat Enterprise Linux 4 Update 1).

The machines I'm trying to monitor are SL 3.0.2 (RHEL 3 U 2), which have 
net-snmp-5.0.9-2.30E.3.

cfgmaker --output=/etc/mrtg/phnxbox.cfg --snmp-options=:::::3 --ifref=ip 
--global "workdir: /var/www/mrtg/" phnxbox0 phnxbox1 phnxbox2 phnxbox3 
phnxbox4 phnxbox5

First hint of a problem was that MaxBytes came to 12.5 Mbyte/sec, so 
snmp doesn't seem to get quite the right idea about the card.  Googling 
told me that you could override that by adding to /etc/snmp/snmpd.conf:

 > #               ifName Type     Speed
 > interface       eth0     6      1000000000
 > interface       eth1     6      1000000000

Fair enough.

But I think I can clearly see a problem with the counters rolling over 
32 bits, so that my tranfer rate is misreported by 0xffffffff/(1E6*5*60) 
Mbyte/sec, about 14 Mbyte/sec, so that when I have an 18 Mbyte/sec 
transfer going, I see it as 4 Mbyte/sec.

I tried snmp 2, but then

 > --base: snmpget public at phnxbox5:::::2:v4only for ifHighSpeed.1 ->  Mb/s
 > --base: snmpget public at phnxbox5:::::2:v4only for ifHCInOctets.1 -> 
--base: check for HighspeedCounters failed ... Dropping back to V1
 > --base: snmpget public at phnxbox5:::::2:v4only for ifHighSpeed.2 -> Mb/s
 > --base: snmpget public at phnxbox5:::::2:v4only for ifHCInOctets.2 -> 
--base: check for HighspeedCounters failed ... Dropping back to V1
 > --base: snmpget public at phnxbox5:::::2:v4only for ifHighSpeed.3 ->  Mb/s
 > --base: snmpget public at phnxbox5:::::2:v4only for ifHCInOctets.3 -> 
--base: check for HighspeedCounters failed ... Dropping back to V1

and snmp 1 surely has this issue.  (I'm not completely sure what's 
happening with snmp 3, even though it seems to work, because I can't 
figure out how to snmpwalk -v 3.)

The only thing that did any good was to reduce the polling time to 1 
minute in the crontab file, /etc/cron.d/mrtg, but then you can't specify 
the Interval: 1 in the .cfg file without an error, even though at least 
for some values, it looks about right, although I didn't test it to the 
highest speeds.

-- 
John Haggerty
email: haggerty at bnl.gov
voice/fax: 631 344 2286/4592
http://www.phenix.bnl.gov/~haggerty

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