North American Network Operators Group

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Re: AS Path Loops in practice ?

  • From: Jeff Aitken
  • Date: Thu Dec 11 22:43:50 2003

On Thu, Dec 11, 2003 at 11:07:03PM +0000, Stephen J. Wilcox wrote:
> 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.

What if you have more customers than there are private ASNs?  Think
about things like 2547-style VPNs, etc.

What if you want to propogate those customers' BGP announcements to
the world?  Which hardware vendors support a "strip-private-ASN"
feature?  Did they always do so?

If every such customer uses a private ASN, every other default-free
customer must accept routes from the ISP that contain private ASNs in
the as-path.  Which of your default-free customers might be filtering
those prefixes?

It makes it a little more difficult for the ISP to filter prefixes
with private ASNs in the path; those from some customers must be
honored; those from other customers and from peers should be dropped.
The ones that were supposed to be honored should be passed along to
other BGP-speaking customers but not to peers.  This is obviously not
an insurmountable problem, but it does add a lot of config complexity.

Private-ASN collisions (i.e., when one customer uses one ASN to talk 
to the ISP and another ASN internally which the provider assigns to
a different customer) will cause problems.

You WILL hear this from a customer: "I want to use ASN X for this
purpose because that's what my consultant said."  

Repeat, but s/a customer/another customer/.