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Re: AS Path Loops in practice ?

  • From: Niels Bakker
  • Date: Tue Dec 09 13:00:51 2003

* [email protected] (Leo Bicknell) [Tue 09 Dec 2003, 16:00 CET]:
> 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
> eliminates a very good way of configuring customers and conserving
> resources.

Why don't we hand out duplicate IP addresses too, while we're at it?
Let RIPE, ARIN and everybody all assign from 67/8 and force all HTTP
traffic through a proxy in globally unique address space run by each
ISP that receives such an allocation. Right now, every ISP that uses
the trick described earlier has to configure dangerous stuff anyway.

[stretching analogies way beyond their breaking points for fun and profit!]

> 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.

*shrug* then they're all using that "customer ASN" against the letter
and the spirit of what an AS used to stand for.  Private ASNs were
invented for this application, and you can strip them from your
announcements too.

For the outside world your "customer ASN" has exactly one routing
policy: Send it to the owner.  Traffic for those prefixes has no other
way of reaching its destination, so it doesn't break the system too
badly.  Of course, it's a different story when those customers start to
multihome with the "customer ASN..."

Anycast is another example (thanks, Joe Abley - I stupidly deleted your
CC'd mail before checking whether it had come in over NANOG as well) of
a situation that falls outside the original definition of an AS - is it
the same network existing in multiple places at the same time? - so one
possibility could be to amend the various standards to current practices.

	-- Niels.

                   the generation that used acid to escape reality
                      is now using antacid to deal with reality