BGP - dampening

[balaji_170]

hi ,

can any body give clear picture about BGP dampenening

what i understand about this chapter - i mentioned below

1) BGP dampening  - help to prevent flap network propagate to neighbor

dampening parameters

  1) penatly - one time network flap (up or down) - 1000

  2) Half life time - penatly will come to half vale -   15

  3) supress - penatly value cross above value - ( dampening status)

R3(config-router)#bgp dampening 15 1000 3000 60

<1-45>     Half-life time for the penalty

<1-20000>  Value to start reusing a route

<1-20000>  Value to start suppressing a route

<1-255>  Maximum duration to suppress a stable route

But unable understand

  1) history in BGP table

  2)  Maximum duration to suppress a stable route

any site or video explain concept in depth let me know

[Akula Jyoti Lakshmi]

BGP Route Dampening is a way to suppress flapping routes so that they are "suppressed" instead of being advertised.  .An unstable network can cause BGP routes to flap, which can cause other BGP routers in the network to constantly reconverge.  This wastes valuable CPU cycles and can cause severe problems in the network.  Hence , as a best practice, ISPs use route dampening regularly.

Penalty - An incremented      numeric value that is assigned to a route each time it flaps.  A      route is penalized 1000 every time it flaps.  A route is considered      to have flapped when we receive a WITHDRAW and then an UPDATE for a      route.  If we drop and regain our adjacency with a BGP peer we do not      assign a penalty to the routes learned from that peer.

Max suppress limit - The      longest amount of time that a route can be suppressed.  The default      is 4x the half life time but can range from 1 to 255.

History entry - An entry used      to store flap information.  For the purposes of monitoring and      calculating the level of oscillation of a route, it is important to store      oscillation information in the router.  When the route stabilizes,      the history entry will become useless and will be flushed from the router.

[balaji_170]

Hi Jyothi,

  thanks for you reply

i had some doubt while doing the lab

Router

       R1  -----  R2 ----- R3  

configuartion

R2 - route dampening enabled

bgp dampening 10 1500 3000 40 

Half life time - 10 min

Reuse          - 1500

supress        - 3000

max- supress - 40 min

R1 - network 10.0.0.0/16 - avg 5 times oscillating (up and down) every minute

frist penatly - R2 ( history table)

four pentaly with  2 mins - R2 ( keep route 10.0/16 - suppress to dampening)

and if you observe

after 13 times network (10.0/16) ( Up and down)  only - Flap values increment with 1 value

i have confusion why after 13 times ( flap value +1  and 1000+ penatly)

Suppress

    if  max supress time expired (40 mins) - whether  route flushed out from own table and its should not poplulated to neighbors ( i am right )

if any mistake - correct me

[Akula Jyoti Lakshmi]

to  answer  your  1st  question,

Aroute  is  considered  to  have  flappedwhen we  receive  a WITHDRAW  and  then  an  UPDATE  for  a  route.

If  we  DROP  and  REGAIN  our  adjacency  with  a  BGP  peer  we  do  not  assign  a penalty to  routes  learned  from peer.

2nd  Question  :

Here  I see 

Reuse  (1500)  >  Penality (1000)

If  penalty  is  less  than  the  reuse  limit  ,  a supressed  route  i.e  up will no longer  be  supressed.

The  default  value  of  Reuse  is  750  and  can range  from  1  to  20000

~Jyoti