Subject: RE: I-BGP Oscillations

From: "Abarbanel, Benjamin" <[email-masked]>
Date: Thu, 6 Jun 2002 15:31:43 -0400

Pedro:
  Why not, IBGP Dampening is what I have been thinking aboute since this
problem was first exposed to IETF. I have started a draft but never finished
it, got talked out of it by one other gentlemen who wrote the original draft
on route oscillations.

Maybe its time to get serious on this.

Ben

-----Original Message-----
From: Pedro Roque Marques [mailto:[email-masked]]
Sent: Thursday, June 06, 2002 3:28 PM
To: Anindya Basu
Cc: [email-masked]
Subject: I-BGP Oscillations

Anindya Basu writes:

> One way to address this issue would be to *only* send out extra
> routes when necessary. In other words, if an oscillation involving
> I-BGP routes is detected, extra information for only that route
> could be sent out.

If we come up with a decent detection mechanism it would be so much
simpler just to reduce the rate of oscilation.

Regardless a decent detection mechanism needs to be proposed...

'Decency' criteria includes state that one needs to hold, etc...

In other words, the ammount of state required to detect such events
shouldn't be greater than the state necessary to advertise the best
external MED winner paths from all mesh participants.

Note that the answer 'ibgp dampening' that is certain to pop up in
peoples minds by association is a bit misleading. When you start
thinking about it is quite a different beast... it is not suficient to
say that you just apply the procedures applied to ebgp dampening since
the problem is quite different.

  Pedro.

---

From: Alvaro Retana <[email-masked]>
Date: Thu, 6 Jun 2002 21:54:38 -0400 (EDT)

On Thu, 6 Jun 2002, Abarbanel, Benjamin wrote:

->   Why not, IBGP Dampening is what I have been thinking aboute since
-> this problem was first exposed to IETF....

The problem with dampening is that you're just hiding the churn (making it
slower), not fixing it. :-(

-> From: Pedro Roque Marques [mailto:[email-masked]]
...
-> > One way to address this issue would be to *only* send out extra
-> > routes when necessary. In other words, if an oscillation involving
-> > I-BGP routes is detected, extra information for only that route
-> > could be sent out.
->
-> If we come up with a decent detection mechanism it would be so much
-> simpler just to reduce the rate of oscilation.

"reducing the rate of oscillation" sounds to me like dampening. =:-)
Again, that doesn't solve the problem, it just masks it, potentially
making the periods of 'incorrect' routing longer (as well as the periods
of correct routing, of course)...

-> Note that the answer 'ibgp dampening' that is certain to pop up in
-> peoples minds by association is a bit misleading.

I agree that iBGP dampening is not the answer.  However, a
"dampening-like" (no penalties) mechanism to detect the oscillation and
then act on it, would work.  Obviously, the oscillation characteristics of
a MED churn are not the same as the ones of "normal" flapping.

We had run some numbers a while back (maybe a year and a half ago) on data
from backbones experiencing the MED churn, and found that only about 1-2%
of the routes were **candidates** to oscillate.  IMHO, the state and extra
information is not significant. ;-)

Alvaro.

---

From: Enke Chen <[email-masked]>
Date: Fri, 07 Jun 2002 10:24:19 -0700

> Date: Thu, 6 Jun 2002 21:54:38 -0400 (EDT)
> From: Alvaro Retana <[email-masked]>
> To: "'Pedro Roque Marques'" <[email-masked]>,
> 	"Abarbanel, Benjamin" <[email-masked]>
> Cc: Anindya Basu <[email-masked]>, <[email-masked]>
> Subject: RE: I-BGP Oscillations
> In-Reply-To: <[email-masked]>
> Message-ID: <[email-masked]>
> MIME-Version: 1.0

[snip]

> a MED churn are not the same as the ones of "normal" flapping.

Regarding a "dampening-like" mechanism (but no dampening) for IBGP route
oscillation detection, I would like to point out that not only is such
a mechanism a good idea, it is also feasible to implement. In fact, the
mechanism has been implemented and in production release on Redback SE800
for a while. The command "sho bgp route flap-statistics internal" displays
the IBGP routes that are flapping, and their frequencies. Then one can
look closely at the ones with the high flap frequencies to see if they
are indeed experiencing the "persistent route oscillation".

-- Enke