BGP route dampening penalty half-life
Route dampening is used to prevent a BGP router from installing a flapping BGP route, and to prevent a router from advertising such routes to other BGP routers.
This decreases the CPU load of routers and increases network stability.
One of the attributes used for BGP route dampening is the penalty, which has a default value of 1000 and will change depending upon the status of a particular route.
The value of the penalty is halved every 15 minutes, which is where the term "half-life" comes from. However this halving does not occur “all at once” every 15 minutes, but is reduced gradually over time.
The output from a debug confirms this:
EvD: accum. penalty decayed to 526 after 60 second(s) EvD: accum. penalty decayed to 502 after 60 second(s) EvD: accum. penalty decayed to 479 after 60 second(s)
From this you see that after 60 seconds the penalty has been reduced by 24. It is later reduced by 23 in the next 60 seconds. The rate of reduction every 60 seconds is such that by the time 15 minutes have elapsed, the value will reach 50% of what it was 15 minutes ago.
It is important to note that BGP route dampening is considered obsolete today.