AS Path Loops in practice ?

Leo Bicknell bicknell at
Tue Dec 9 14:59:41 UTC 2003

In a message written on Tue, Dec 09, 2003 at 03:13:12PM +0100, Niels Bakker wrote:
> One cannot have discontiguous networks in the same ASN.  It's pretty
> simple: two routing policies, two ASes, two ASNs.  Unfortunately you
> weren't able to convince your customer of this.

This is simply not true, and trying to push it as an absolute
elimintes a very good way of configuring customers and conserving

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.

I believe there are a number of providers with hundreds, or perhaps
even thousands of customers using this feature, saving lots of ASN's
and causing no problems.  Externally this looks fairly neat, there is
a single ASN behind the backbone AS with all the customers in it.

To make this work you do need to enforce some restrictions.  First,
the customer ASN must never be connected to another network (this
is for multiple connections to a single provider only) to prevent
the wonky AS path issues, and second the customer must have a default
via some mechanism.

       Leo Bicknell - bicknell at - CCIE 3440
        PGP keys at
Read TMBG List - tmbg-list-request at,
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