> The AS_PATH has always been intended to represent the ASs that > propagated the update. > > The AS_PATH can be used to detect loops ONLY because it does represent > the ASs that propagated the update.
Sorry --but I just gave 5 examples of where the AS Path is intentionally modified without causing loops. The point is the loopfreeness, not the path the update took. Let me ask this in a different way. Suppose you have the following: A---B +-C-+ Now, for whatever reason, and with C's full knowledge and consent, B decides to advertise C's routes to A without putting itself in the AS Path. Is this possible? Yes, it is --using current commercial implementations of BGP. Is it wrong? Well, it doesn't cause a loop, does it? As for policy --isn't that up to the contract between B and C? Why should BGP enforce the contract terms B and C are able to write between themselves? And yes, this situation does exist in the real world --I just happen to be helping a customer set this up in a private environment. :-) Russ
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