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Re: [Idr] draft-jakma-mrai-dep-00 (fwd)

Olivier Bonaventure <Olivier.Bonaventure@uclouvain.be> Wed, 26 November 2008 07:59 UTC

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Date: Wed, 26 Nov 2008 08:59:04 +0100
From: Olivier Bonaventure <Olivier.Bonaventure@uclouvain.be>
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To: Pedro Marques <roque@juniper.net>
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Cc: Yakov Rekhter <yakov@juniper.net>, idr@ietf.org
Subject: Re: [Idr] draft-jakma-mrai-dep-00 (fwd)
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Pedro,
> 
> In general my recommendation for MRAI values would be 0 for eBGP and 0
> for iBGP. As the draft correctly explains calculating a value that best
> fits the network is a very tricky business.

I would not recommend using the same MRAI timer for iBGP and eBGP. In a
large AS using route reflectors or confederations, we are often in a
situation where some routers of the AS learned an alternate path but did
not distribute it widely due to the alternate paths being hidden by
border routers (e.g. different local-pref or longer AS-Path), router
reflectors or at the borders of confederations. See e.g.
http://inl.info.ucl.ac.be/publications/quantifying-bgp-routes-diversity-insi
but discussions with operators showed that this is true for other networks.

In such large ASes, when a link fails iBGP should converge first in
order to allow all routers to learn the alternate path and advertise the
alternate path over eBGP sessions. This can be achieved, at least
partially, by using a longer MRAI timer for eBGP compared to iBGP.
As you note, the situation is different in BGP/MPLS VPNs environments or
when draft-walton-bgp-add-paths-06.txt is used.

I would not personally recommend to exempt withdraws from the MRAI on
eBGP sessions in large ASes.


Olivier

-- 
http://inl.info.ucl.ac.be , UCLouvain, Belgium
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