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BGP Damping confusion ??????


i have study about the Bgp damping fron cisco site and different Bgp documents. but still confusion ,,, can you tell me about it in simple wordss

What is the purpose of bgp damping ????

what is penalty ????

and suppress limit of prefix????

Why we want to suppress themm????

I think to much Question !!!!!!!!!


Hall of Fame Super Silver

Re: BGP Damping confusion ??????


The explanation in simple terms is this: in Internet routing with BGP it is important that routes be stable. (In routing within the Enterprise stability is good, but in the Internet it is especially important.) Whenever a route changes (route is added to the table or route is withdrawn from the table) the Internet routers must update their routing tables and converge. The feature of dampening was added to BGP so that if a particular prefix becomes unstable (is added and is withdrawn several times within a particular amount of time) that prefix will be dampened. Dampening is supressing the prefix. If the prefix is supressed for an amount of time by the BGP neighbor who originally received the prefix then the other Internet routers will not have to converge as much. So it enhances stability in the Internet routing table.



Re: BGP Damping confusion ??????


let me start with the good news: RIPE declared BGP route flap dampening as obsolete and does discourage to use it (document RIPE-378).

BGP route flap dampening was meant to lower CPU load on BGP routers by implementing a penalty system to suppress instable pathes. So it was a sort of educational program: get stable or be suppressed!

Each time a path flaps (i.e. is removed from BGP and reannounced) it gets 1000 points penalty. Once you are above 2000 (default value) your path is suppressed for a configurable amount of time. There is also a timer, which specifies the amount of time to reduce the penalty by half. Below reuse limit your penalty points are set to 0.

So the result was:

1st flap - nothing (penalty 1000)

2nd flap - nothing (penalty <2000 because penalty was already "decaying")

3rd flap - suppress (penalty >2000) if the time between flaps was short compared to half time.

Lesson to learn: a flap or two every now and then does not harm you. Frequent flapping takes you out of business. So make sure your BGP implementation is stable!

With current BGP implementations it turns out to be more harmful than helpful. All details, pro and cons are clearly described in RIPE-378 ( and contained citations.

Hope this helps! Please rate all posts.

Regards, Martin