port utilization

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Jul 8th, 2007
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For some uplink ports we get DFM alerts on utilization above 40% (as configured). However we monitor those same ports with MRTG, and we never get utilization above 10% ? Is there a way we can ?manually? monitor port usage so that we can determine who?s right: DFM or MRTG

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Joe Clarke Sun, 07/08/2007 - 13:30
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DFM uses the following formula when determining if a HighUtilization event should be triggered:


CurrentUtilization > UtilizationThreshold


Where UtilizationThreshold is your threshold (40% in this case) and CurrentUtilization is derived by:


CurrentUtilization = ((TrafficRate * 8) / MaxSpeed * 100;


Here, TrafficRate is derived by:


TrafficRate =

ifInOctetsRate + ifOutOctetsRate (HALFDUPLEX)

max(ifInOctetsRate, ifOutOctetsRate) (FULLDUPLEX)


ifInOctetsRate and ifOutOctetsRate are derived by:


ifInOctetsRate = rate(ifInOctets, PollingInterval)

ifOutOctetsRate = rate(ifOutOctets, PollingInterval)


Note: for some interfaces >= 100 Mbps ifHCInOctets and ifHCOutOctets may be used instead.


The PollingInterval is configured in DFM. By default, this is four minutes (it is five minutes for MRTG). So, the rate is calculated by measuring the rate of change of those SNMP objects over that interval.


The MaxSpeed above is the ifSpeed, or ifHighSpeed for interfaces > 4 Gbps.


When you get a HighUtilization event in DFM, you should see some of these figures filled in. Of course, it does help to see the whole picture.

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