AS Path Loops in practice ?

Stephen J. Wilcox steve at
Fri Dec 12 11:12:14 UTC 2003

Ok I've had a few responses on and off list on this but arent really getting 
this idea, as I said tho I've not done this so perhaps thats where I'm missing 
the crucial points..

> 1) It would only remove the "must default" clause if the provider either
> stripped (or overrode) the local-as, or if all of the private ASNs were
> unique.  That is a big headache.

You must strip the local-as, this is one command that can sit in a peer group on 
my ciscos so not a big deal.

> 2) Private ASNs are not, per RFC1918, supposed to be connected to the
> Internet, in much the same way that private IP space is not supposed to be
> connected to the Internet.  This can also be solved by stripping/overriding.

> 3) One advantage of using a public, albeit common, customer ASN is that if a
> customer has RIR-allocated space, those IPs will make it onto the global
> table, and will not suffer the filtering which may be present for the
> provider's own routes.

Ok this seems to be a difference, altho not sure why the custs IPs should need 
to do anything different from the providers IPs as presumably both need to be 
reachable from everywhere?


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