[107615] in North American Network Operators' Group
Re: why not AS number based prefixes aggregation
daemon@ATHENA.MIT.EDU (Owen DeLong)
Mon Sep 8 11:58:34 2008
From: Owen DeLong <owen@delong.com>
To: Scott Brim <swb@employees.org>
In-Reply-To: <20080908134627.GA4108@cisco.com>
Date: Mon, 8 Sep 2008 08:58:15 -0700
Cc: NANOG list <nanog@nanog.org>
Errors-To: nanog-bounces@nanog.org
On Sep 8, 2008, at 6:46 AM, Scott Brim wrote:
> Excerpts from yangyang. wang on Mon, Sep 08, 2008 09:20:38PM +0800:
>> Hi, everyone:
>>
>> For routing scalability issues, I have a question: why not
>> deploy AS
>> number based routing scheme? BGP is path vector protocol and the
>> shortest
>> paths are calculated based on traversed AS numbers. The prefixes in
>> the same
>> AS almost have the same AS_PATH associated, and aggregating prefixes
>> according to AS will shrink BGP routing table significantly. I
>> don't know
>> what comments the ISPs make on this kind of routing scheme.
>>
>>
>> -yang
>
> It might be the right level of granularity for policy but is too
> coarse for routing. You want to be able to route on prefixes (even if
> not everyone does it) for flexibility/TE. Also, ASNs are not
> aggregatable so we can't use them to represent a large number of
> independently routed networks.
An AS is a collection of networks with a common
routing policy. If you are subdividing into multiple routing policies
for
TE purposes, you, theoretically, need separate ASNs for each routing
policy.
BTW, if you have multiple unique routing policies, it is perfectly valid
to justify multiple ASNs on that basis.
Owen
