AS Path Loops in practice ?
jaitken at aitken.com
Fri Dec 12 03:41:19 UTC 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'
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
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/.
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