[Fwd: bgp route oscillation]
Jeff Pickering <jeffp@caspiannetworks.com> Mon, 12 August 2002 15:27 UTC
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Date: Mon, 12 Aug 2002 10:27:39 -0700
From: Jeff Pickering <jeffp@caspiannetworks.com>
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Subject: [Fwd: bgp route oscillation]
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--- Begin Message ---Just a question on draft-walton as an oscillation solution: Using the example 2.1 in RFC345, we have in router Ra 3 routes in step 3): AS-Path MED IGP-cost x 6 100 0 13 y 6 100 1 4 z 10 100 10 5 Oscillations in this example occur because the preference rules result in a non-deterministic choice, more specifically because the rules result in choices between more than 2 routes that are non-transitive. In the example above: x beats y y beats z z beats x The proposal in draft-walton suggests avoiding oscillation by enabling Ra to always have knowledge of all three routes. However, this seems insufficient since x can be chosen best one time and z chosen another. To prevent oscillation, it would seem an additional requirement to always choose the same route, given the same set of choices. If this requirement is applied to the rfc3345 example, and the draft-walton solution applied, Ra could always choose route z as best and prevent oscilliation. It could also choose x as best with the same result. But this leads me to question the usefullness of the neighbor-as-best-path information. For the example in question, if we required Ra to make consistent choices, and dont use the proposed new info, in step 1) we say y beats z. Step 2 remains unchanged. In step 3), we force consistency and say y still beats z. x then beats y so we send send a withdraw to Rd. This breaks the loop and there is no oscillation. So, it would seem that the draft-walton solution is neither necessary nor sufficient unless of course I am missing something, which seems likely. Can anyone enlighten me? Jeff--- End Message ---
- [Fwd: bgp route oscillation] Jeff Pickering