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Re: AS number consolidation
From: "Kevin Oberman" <oberman () es net>
Date: Mon, 02 Jun 2003 08:08:49 -0700

I just spoke with a couple of the people involved in the SBC transition
and they are thinking about putting a presentation for a NANOG after
they transition the former Ameritech ASes. 

This is a truly massive undertaking merging around 100 ASes from coast
to coast and re-aggregating all of the routing and, from my viewpoint,
have done an amazing job. I hope they do the presentation.

R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman () es net                       Phone: +1 510 486-8634

Date: Fri, 30 May 2003 09:03:54 -0500 (CDT)
From: Daniel Golding <dgold () FDFNet Net>
Sender: owner-nanog () merit edu

SBC just did one of the largest AS merges ever accomplished. Maybe one of
their engineers will give a brief report? One item to always look out for,
if you have BGP-speaking customers is the particular implementation of
"local-as" on your favorite brand of router. There are some variations...

- Some include the "imposter AS" in the AS path
- Some have a keyword for stripping out the "imposter AS"

The vendor documentation on this feature has been historically weak,
necessitating some lab work.

This can be an issue, as you could be accidentally prepending route
announcements learned from your downstreams by having both the new and old
AS in the path.

Daniel Golding

On Fri, 30 May 2003, David Luyer wrote:

Does anyone know of case studies of companies collapsing
multiple ASes
into one on their network? I have the Allegiance Telecom
presentation from
NANOG 27 but I would like to hear how other people have done
it as well.

We have to date collapsed 6 AS numbers into 1.

Approach was relatively simple - but it's been at
least a couple of years since the last merge so I'm
a little light on exact memory/detail:

  1.  Duplicate RADB (or other) entries across to new

  2.  Merge interior routing protocols across the ASen
      which are about to be merged.

  3.  Gradually grow the largest AS adding a router at
      a time (notify the BGP peers on the router, set a
      time, make the change).

To me this is one of the things where you can go over the
top in planning (I know an ISP who have been planning an
AS merge for years) or you can 'just do it'.

The second hardest thing, if you bother to do it, is
"cleaning" the old AS (in terms of RADB, other registries,
route filters in peers, etc) before returning it to the

The hardest thing is convincing the registry to take them
back... (you'd think that'd be easy, but it took by far
the longest of the whole job).


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