Calculate bandwidth using SNMP index

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Jan 19th, 2009
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Hi


Can you tell me how to calculate bandwidth using snmp index ifInoctets and ifOutOctets.


I can see the output of this indexes using snmpwalk command at different time interval.


But how do I derive the utilization when it reaches 80%.


Archana.






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Giuseppe Larosa Tue, 01/20/2009 - 01:50
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Hello Archana,

to measure link usage you need to perform the difference between two readings divided for the time interval between the readings and by 8 (bits/byte)


[Ifinoctects (readingN+1) - Ifinoctects (readingN)] /8* (time_readingN+1 -time_readingN)


so now you now the speed and you need to compare to link speed to see when it reaches 80%.


Be aware that being these 32bits counters on high speed links they can wrap:

when the counter reaches 2^32-1 next read will be a lower number then the previous.


There are different ways to deal with this.

One is to use if available the HC versions of counters (64 bits counters)

Another is to read other mibs that track if wrapping has happened


Hope to help

Giuseppe


marchanamendon Thu, 01/29/2009 - 23:11
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Hello


Since this parameters are new for me and trying to find the formula,it would be great if you can tell me how do I know what is the bandwidth usage and formula to calculate.


I have collected the MIB value for serial interface of router for 1 min time interval.


At 12.25


IF-MIB::ifInOctets.7 = Counter32: 1427402450

IF-MIB::ifOutOctets.7 = Counter32: 4040163899


At 12.26


IF-MIB::ifInOctets.7 = Counter32: 1427697887

IF-MIB::ifOutOctets.7 = Counter32: 4041427307


ifspeed value for serial interface is this.


IF-MIB::ifSpeed.6 = Gauge32: 1544000


With these value how can I derive the percentage of bandwidth usage for the time period of 1 min.


It would great if you can work it out and explain me.


Thanks,

Archana.

Giuseppe Larosa Fri, 01/30/2009 - 03:31
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Hello Archana,

providing an example is a good idea.


time= 60 seconds


Diff. Inoctets = 1427697887 - 1427402450 = 295437 octets received in 60 seconds


1 octet = 8 bits


average bit rate = Diff.InOcoctets*8/60 =

39391,6 bps


so the usage is :

39391,6 / 1544000 = 2,55 % usage of BW inbound direction


you can perform a similar calculation for outbound direction


Hope to help

Giuseppe



marchanamendon Fri, 01/30/2009 - 03:48
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Hello Giuseppe,


Its really helpful with the way you have explained.


But I have one more doubt,the 2.55% which gives the utilization for an inbound direction.


How do I calculate the total bandwidth utilization for the link?


Thanks,

Archana.


Giuseppe Larosa Fri, 01/30/2009 - 04:58
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Hello Archana,

from a technical point of view I would keep separated the two directions because it is really normal that usage in the two directions can be quite different.


However, if you want to associate a usage percentage to both direction you can use:


overall_usage= (usage_inbound+usage_outbound)/2


Hope to help

Giuseppe


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