Friday, November 13, 2009

BGP - Route Dampening .. Exponential Decay / Half life.!!

In BGP Dampening, Suppose they ask that " Route should be reused after 5 minutes "and all other parameters are default
than as per formula for BGP dampening


Max penalty = reuse-limit *2^(maximum suppress time/half time)

Suppose it flapped twice than max penalty will be 2000
2000 = 750 * 2 ^( 5mins / half life)
2000/750 = 2 ^ ( 5 / half life) { simplify 2000/750 = 8/3 }
8/3 = 2 ^ ( 5 / half life)
Take logarithm on both sides
Log (8/3) = Log 2 ^ 5/halflife)

{ Logarithms formulas Log x/y = (Log x - Log y) and
Log x^a = a.Log x }

after applying the log formula
Log 8 - Log 3 = (5xLog 2)/halflife
0.9030 - 0.4771 = 5x0.3010/halflife
0.4259 = 1.505/halflife
0.4259/1.505 = Halflife
3.53 = Halflife
Round it off

Half life = 4 ... Configuring Half life as 4 will ensure that suppressed prefix will be advertised after 5 mins.

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