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Re: Heads up: Long AS-sets announced in the next few days
- From: Lorenzo Colitti
- Date: Thu Mar 03 19:50:38 2005
David Schwartz wrote:
We don't know the effectiveness of the technique. That depends on the
topology of the Internet, where you run the announcements from, etc.
etc. We do know the effect that the announcements will have on BGP
routers, i.e., cause them to ignore the path if they are in the AS-set,
and accept them otherwise (modulo policy, max aspath length, etc. etc.,
They are experimental in that yes, we are experimenting with a new
technique for topology discovery which to our knowledge has not been
So you do not know what affect your announcements will have.
Ok, so if everyone announcing an AS-set is announcing "routes that
falsely claim to have passed through another autonymous system", and you
are saying this shouldn't be done, then why aren't you complaining with
everyone who is announcing an AS-set?
So, given these considerations, is everyone announcing an AS-set
announcing "routes that falsely claim to have passed through another
Yes. From RFC1771:
You quote Section 5.1.2, but you don't mention that if you follow
Section 5.1.2 to the letter there is no way that an AS-path may contain
an AS-set. To summarize the whole of section 5.1.2, the various cases are:
[Quote of section 5.1.2 almost in its entirety]
So you are violating RFC1771, plain and simple. To then go and cite one
small section of RFC1771 in your defense is hypocritical.
Propagating a route learned from an UPDATE message:
a) To another router in same AS: don't modify AS-path
b) To a neighboring AS:
1. Path starts with AS_SEQUENCE: prepend own ASn
2. Path starts with AS_SET: prepend new AS_SEQUENCE with own AS in it
Originating a route:
a) To neighboring AS: announce own ASn as only element in path
b) To another router in same AS: announce empty AS-path
If you follow this to the letter, you must rule out both prepending "(In
this case, the AS number of the originating speaker's autonomous system
will be the only entry in the AS_PATH attribute)" and any form of
AS-set, since there is no way, following these rules, that an AS-set may
enter the AS-path in the first place.
If we are violating this section, then everyone else announcing an
AS-set, and - at least the way I read it - anyone doing prepending, is
doing so too. But nobody is suggesting that these things shouldn't be done.