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Re: why not AS number based prefixes aggregation
- From: Owen DeLong
- Date: Mon Sep 08 11:58:31 2008
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
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