Re: BGP Dampening?

This is the explanation of BGP dampening from BGP
command-reference handbook

A route flap occurs when a prefix transitions from the up to the down state.
When the prefix goes from up to down, BGP must send a WITHDRAWN message.
When the prefix goes from down to up, BGP sends an UPDATE message. If the
prefix is constantly flapping, this can cause high CPU utilization while the
BGP routes are converging. Additionally, if you are redistributing BGP into
your IGP, the flapping route can cause instability in the IGP. Dampening is
a method to control the effect of a flapping route.

Assume that BGP route dampening has been applied to a particular route.
Remember that a route flap is when a prefix goes from the up to the down
state. When a route flaps, a penalty of 1000 is applied to the route. This
is a fixed parameter that cannot be changed. At time t1, the flapping route
incurs a penalty of 1000. Because the penalty is less than the suppress
limit, which has been configured to be 1500, the flapping route is not
dampened. Because the route has not been dampened, BGP sends a WITHDRAWN
message when the route goes from up to down and then an UPDATE message when
the route goes from down to up. When route dampening is enabled, BGP must
maintain a history of flapping routes. The penalty associated with the route
decreases over time. The rate of decrease in the penalty is a function of
the half-life. The half-life is the amount of time for the penalty to
decrease to half of its current value. If the half-life is 15 minutes, the
penalty decreases to 500 in 7.5 minutes. At time t2, the penalty decreases
to 600, and another route flap occurs. The route is again penalized, with an
additional value of 1000 added to the current value of the penalty. The
total penalty associated with the route is now 1600. When the total penalty
of a route becomes greater than the suppress limit, the route is dampened. A
dampened route means that BGP no longer sends UPDATE messages when the
prefix goes from the down to the up state. The flapping prefix remains in
the WITHDRAWN state. The route remains in the WITHDRAWN state until its
penalty decreases below the reuse limit. When the penalty decreases below
the reuse limit, BGP sends an UPDATE message for the route. Routes learned
via IBGP are not dampened

On Wed, Jul 22, 2009 at 9:14 PM, Joe Astorino <jastorino_at_ipexpert.com>wrote:

> I don't have it in front of me because I am out of town, but the BGP
> command-reference handbook from Cisco is pretty good. It gives pretty
> straight forward examples on just about everything. I have it at home and
> now I am curious as to what it says on this topic.
>
> On Wed, Jul 22, 2009 at 9:40 PM, Tony Varriale
> <tvarriale_at_flamboyaninc.com>wrote:
>
> > The Halabi book isn't really the bible anymore. In fact, it would be
> very
> > difficult for me to recommend it to anyone.
> >
> > tv
> >
> > -----Original Message-----
> > From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
> > Gary
> > Duncanson
> > Sent: Wednesday, July 22, 2009 5:57 PM
> > To: Joe Astorino
> > Cc: Ramanpreet Singh; ccielab_at_groupstudy.com
> > Subject: Re: BGP Dampening?
> >
> > This makes sense now.
> >
> > There's nothing in Halibi the 'bible' on BGP so there you go. I just
> > checked
> >
> > at least in the index for BGP dampening but it must be covered in the
> > fundamentals of the book.
> >
> > I suppose decay time is 'half life' in which case check out page 293 of
> the
> > book. Semantics :)
> >
> > Granualar stuff this! DocCD all the way.
> > ----- Original Message -----
> > From: "Joe Astorino" <jastorino_at_ipexpert.com>
> > To: "Ramanpreet Singh" <sikandar.raman_at_gmail.com>
> > Cc: "CCIE Groupstudy" <ccielab_at_groupstudy.com>
> > Sent: Wednesday, July 22, 2009 11:37 PM
> > Subject: Re: BGP Dampening?
> >
> >
> > > The half life time is the time in which the penalty will be half of
> what
> > > it
> > > is currently
> > > The Max suppress penaltly is just the max time the route can be
> > > suppressed.
> > >
> > > Check out the routing config guide under BGP for a much more in depth
> > > explanation
> > >
> > > On Wed, Jul 22, 2009 at 5:57 PM, Ramanpreet Singh
> > > <sikandar.raman_at_gmail.com>wrote:
> > >
> > >> Can anyone help me understanding Decay time and Max suppress penalty?
> > >>
> > >> I am familiar with other parameters but I am not able to understand
> how
> > >> Decay time and Max suppress penalty is calculated. Doc CD only
> explains
> > >> the
> > >> configurable parameters but not decay time and Max suppress penalty.
> > >>
> > >> Thanks!
> > >>
> > >>
> > >> Blogs and organic groups at http://www.ccie.net
> > >>
> > >>
> _______________________________________________________________________
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> > >>
> > >>
> > >>
> > >>
> > >>
> > >>
> > >>
> > >>
> > >
> > >
> > > --
> > > Regards,
> > >
> > > Joe Astorino - CCIE #24347 R&S
> > > Technical Instructor - IPexpert, Inc.
> > > Cell: +1.586.212.6107
> > > Fax: +1.810.454.0130
> > > Mailto: jastorino_at_ipexpert.com
> > >
> > >
> > > Blogs and organic groups at http://www.ccie.net
> > >
> > > _______________________________________________________________________
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> >
> >
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> > _______________________________________________________________________
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> >
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> >
> > _______________________________________________________________________
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> >
> >
> >
> >
> >
> >
> >
> >
>
>
> --
> Regards,
>
> Joe Astorino - CCIE #24347 R&S
> Technical Instructor - IPexpert, Inc.
> Cell: +1.586.212.6107
> Fax: +1.810.454.0130
> Mailto: jastorino_at_ipexpert.com
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html

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Received on Wed Jul 22 2009 - 21:34:10 ART