BGP dampening formula

Hello,

 

Can anyone explain from where did we get this formula -P(t) = P(0)/2^(t/Half_Life) ?

 

As per Cisco documentation, the formula is:

Max-Penalty= reuse-limit *2^(max-suppress-time/half-life)

 

Seems like the Workbook is replacing Max-penalty with P(0) (Suppress-penalty), which are two different values, and the difference can be seen with the show ip bgp dampening parameters...

 

 

Am I missing something?

 

 

Comments

  • The P(t) = P(0)/2^(t/Half_Life) is the equation for exponential decay.

    Pt=P0^(t/half life)

    http://en.wikipedia.org/wiki/Exponential_decay#Mean_lifetime

    The formula you are referring to from the Cisco documentation is for the maximum penalty.

    Max-Penalty= reuse-limit *2^(max-suppress-time/half-life)

    Taking it a step further, use a Half Life Calculator (you can find one on Google) to help you understand the formula and how the solution guide calculated the half-life of 4.

    Here is the breakdown:

    P(T) = 750 =  is the quantity that still remains after a time [Default reuse limit]

    P(0) = 2000 = the initial quantity  [Default Suppress Limit]

    Time = 5  [The value from the question “the advertisement should resume in 5 minutes.”]

    If you enter them in the half-life calculator you get 3.5334752630571

    As part of the solution, they round up to 4.

    Next, you can calculate the Max-Penalty based on the formula below:

    “Max-Penalty= reuse-limit *2^(max-suppress-time/half-life)”

    Max-Penatly =  750 * 2^( max-suppress-time /4)

    Max-Suppress-time the default is 4 times the half-life = 16

    Max-Penatly =  750 * 2^( 16 /4)

    Max-Suppress penalty = 12000

    Solution Guide Answer:

    bgp dampening 4 750 2000 16

     4  is half life

    750 is default reuse limit

    2000 is default suppress limit

    Max-Suppress default is 4 times the half-life, which is 16

  • Can anyone explain from where did we get this formula -P(t) = P(0)/2^(t/Half_Life) ?

    See the following post - http://ieoc.com/forums/p/17521/152933.aspx I think this will cover your questions!

     

Sign In or Register to comment.