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Re: AS Path Loops in practice ?
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/. Etc. --Jeff