AS Path Loops in practice ?

Fri Dec 12 04:44:05 UTC 2003

--- "Stephen J. Wilcox" <steve at telecomplete.co.uk>
wrote:
> 
> > Most (all) large ISP's have a "customer ASN". 
> This allows a customer
> > to connect in multiple places, run BGP, and get
> something approximating
> > real redundancy to that carrier.  However, rather
> than allocate one
> > ASN to each customer, all customers use the same
> "customer ASN".
> > Yes, that means they must default to the provider
> (and/or have the
> > provider provide a default route) to reach the
> other customers using
> > this technique.
> 
> Perhaps I'm missing something having not done this
> myself but why arent the 
> customers just using private ASNs? That would also
> remove the 'must default' 
> clause.
> 
> Steve

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.

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.

=====
David Barak
-fully RFC 1925 compliant-

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