Calculation of load interval on switch and router ports

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May 26th, 2009
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Hi,

it would be fine if somebody could explain to me how the load interval is calculated in detail on switch and router ports.

I found the following information concerning this:

"Load data is gathered every 5 seconds. This data is used for a weighted average calculation in which more-recent load data has more weight in the computation than older load data".


Best Regards,

Thorsten Steffen


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Giuseppe Larosa Tue, 05/26/2009 - 04:49
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Hello Thorsten,

an exponentially weighted average is calculated with a formula like:


Current_Average_Load (N) = exp (1/T)* Current_Read + (1-exp(1/T)* Current_Average_Load (N-1)


the value at interval N is a function of current read and of previous read.

The greater is T more is the weight of previous values.

An exponential average converges in 4 T intervals:

if at istant t= 0 a constant bit rate of 2000000 bps starts on the interface it takes up to 4 times load interval parameter to have a value=

2000000 bps in the hyphotesis that before there was no traffic on it.


For this reason it is recommended to use the lowest value 30 seconds for the load interval.

With default 5 minutes it takes 20 minutes to read the real rate.


Hope to help

Giuseppe


thorsten.steffen Tue, 05/26/2009 - 06:19
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Hello Giuseppe,

am I right that a new computation is done every 5s when new load data is gathered; so every 5 seconds I get a new output of the bit rates shown with "show interface"?


Thanks,

Thorsten

Giuseppe Larosa Tue, 05/26/2009 - 06:41
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Hello Thorsten,

the load-interval influences the T value.


I'm not sure if sampling happens every 5 seconds.

My understanding as I've explained is different.




http://www.cisco.com/en/US/docs/ios/interface/command/reference/ir_l1.html#wp1014158


Says:


"Load data is gathered every five seconds. This data is used for a weighted-average calculation in which recent load data has more weight in the computation than older load data. If the load interval is set to 30 seconds, the average is computed for the last 30 seconds of load data. "


so it is true samples are taken every 5 seconds but load interval is what counts.



Hope to help

Giuseppe


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