[mrtg] [Fwd: Re: Aggregating ports via mrtg]

Dave Williams dave_williams at eli.net
Wed May 30 20:16:48 MEST 2001

A few thoughts about doing this:

1) There were several discussions last week that should be in the
archive about SNMPV1 32 bit counters overflowing when sampling
interfaces faster than 114Mbps using 5 minute sampling intervals.  So if
you sum up more than two heavily used 100M ethernet ports (servers
perhaps) you will not get accurate results.  You will have to sample the
SNMPV2 Mib values (different OID instances) which will cause MRTG to use
64bit counters.

2) I've always had problems when doing calculations that perform
division.  Rateup apparently won't work with MRTG log files that are not
integer values.  Perhaps you could write a script to do the bandwidth
utilization calculation rounding (rounding results, etc.).  RRDTool (I
have not used this yet) might be better doing this type of calculation.

3) Cisco switches have a global utilization statistic that I would think
is reporting the backplane usage.  Perhaps you can avoid the complicated
calculation altogether by looking a little deeper into the various mibs
available for your switch.  Using an existing Mib switch utilization
statistic would save a lot of SNMP Gets (a couple per port to get In/Out
values) too which should lighten the load on the switch as well.

4) Utilization might not be the key performance statistic (memory buffer
pools might be a better indication of how well the switch is doing).

Hope this helps...

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