Subject: RE:

From: Pedro Roque Marques <[email-masked]>
Date: Thu, 6 Jun 2002 12:35:16 -0700 (PDT)

Abarbanel, Benjamin writes:

> I have a third solution to propose to the list. Why not dampen the
> MED from the BGP Route Selection Criteria during conditions when a
> route is oscillating

As i mentioned in my previous e-mail to the list, the devil is in the
details...
Someone needs to specify the behaviour and do an analysis to see if it
is feasible... without some detailed analysis i don't think it is
possible to know if that is a feasible solution.

Keep in mind the following scenario:

You have N peering points with AS 1 in your ibgp mesh. Each of those N
points receives a route with an MED. When the best external route
changes, all of those N peering points advertise their external route
to the mesh and then withraw it as they converge to the second best
MED.

In this case are you going to hold dampening history for all these
paths ? Are you going to apply penalties to individual paths (as in
the ebgp case) or routes... don't forget that you need to dampen
things in a coverged state (i.e. no routing loops in the mesh).

  Pedro.

---

From: "Abarbanel, Benjamin" <[email-masked]>
Date: Thu, 6 Jun 2002 15:26:47 -0400

I have a third solution to propose to the list. Why not dampen the MED from
the BGP Route Selection Criteria during conditions when a route is
oscillating, thus temporarily removing the route from a possible oscillation
state and stoping it. This solution only imposes additional internal
resources on a given router and does not introduce scalability issues with
external routers. In addition, the oscillation is not critical to the
topology configured but to the behavior of the route.

In other words, I propose we treat the route oscillation not try to cure it,
since its source is topologically unpredictable and the existing BGP
protocol design does not warrant redefining its basic rules of operation.

Ben

-----Original Message-----
From: Pedro Roque Marques [mailto:[email-masked]]
Sent: Thursday, June 06, 2002 2:56 PM
To: f b shepherd
Cc: [email-masked]
Subject: 

f b shepherd writes:

> ==> It seems to me impossible to adhere to only propogating
> best-path and yet to eliminate all oscillations which were
> considering (or maybe even to only eleiminate types I and II churn).
> At least not without some other radical change to the protocol.

There is a simple answer, which is to address the current MED
behaviour. MEDs as current specified make it so that there isn't
possible to establish a global ordering between BGP paths. The
protocol when originally designed implicit required such property.

Given a global ordering of paths within an ibgp mesh no persistent
osciallation will occur and the protocol as originally specified will
be stable.

In order to achieve this, of course, one needs to encounter an
solution to the problem that MEDs are supposed to solve.

One possible solution is to mandate 'always-compare-med' behaviour
which will make paths have a global ordering. I remember there was
strong reaction against this from the operational folks but i'm not
quite sure what the arguments where (would apreciate if somebody could
refresh my memory).

Or we can attempt to specify a different attribute/mechanism that
achieves the desired effects.

  Pedro.