Re: [Idr] 2 Week WG LC on draft-ietf-idr-as-migration (4/9 to 4/23/2105
Jeffrey Haas <jhaas@pfrc.org> Tue, 21 April 2015 20:41 UTC
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Date: Tue, 21 Apr 2015 16:41:36 -0400
From: Jeffrey Haas <jhaas@pfrc.org>
To: "Alvaro Retana (aretana)" <aretana@cisco.com>
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Cc: idr wg <idr@ietf.org>, "amante@apple.com" <amante@apple.com>, Susan Hares <shares@ndzh.com>
Subject: Re: [Idr] 2 Week WG LC on draft-ietf-idr-as-migration (4/9 to 4/23/2105
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Alvaro, On Mon, Apr 13, 2015 at 08:50:40PM +0000, Alvaro Retana (aretana) wrote: > I do have one item that I think may be above ?minor?: when using ?Local AS?, when should the ASN be prepended? When sending eBGP-received UPDATEs to iBGP neighbors (as it is specified currently), OR, then receiving those eBGP UPDATEs? If it is not done when receiving, then the AS_PATH will be inconsistent in the AS (the border router will have a shorter AS_PATH). I think this can lead to inconsistent route selection (given that the attributes are inconsistent inside the AS), but that of course may be mitigated by the fact that the application of Local AS may be towards stub networks in many (but not all) cases. I would really like to hear others? (beyond the authors) opinions. [My comments #3 and #7 in the Minor section below are related to this.] Obviously there's a long standing history with both variations on the feature along with a number of related but poorly (if ever) documented behaviors to worry about. A few here: RFC 4271, 5.1.2 notes we don't modify the AS_PATH on advertisement to an internal peer. Obviously implementations do that, semantic quibbling about doing it as part of modification of a route in adj-rib-in/loc-rib vs. adj-rib-out aside. Mostly a note of historical significance rather than saying "we shouldn't do that". Somewhat more relevant is in section 9.1.2: : If the AS_PATH attribute of a BGP route contains an AS loop, the BGP : route should be excluded from the Phase 2 decision function. AS loop : detection is done by scanning the full AS path (as specified in the : AS_PATH attribute), and checking that the autonomous system number of : the local system does not appear in the AS path. Operations of a BGP : speaker that is configured to accept routes with its own autonomous : system number in the AS path are outside the scope of this document. That last sentence was provided to provide the little documentation implementations had at the time on local-as. What is not stated is the somewhat problematic issue of the above paragraph. The issue has to do with where an implementation does its loop detection. Some implementations do loop detection at every BGP speaker. Some do them only at an ebgp edge. Some bias this behavior based on local-as being enabled or not. How you do your loop detection and when heavily biases your choice of implementation. In implementations that only requires local-as at your ebgp edges but do loop detection at route reflectors/external confederation routers, adding your local-as as part of import is a problem. So, consistency isn't only a big deal as part of when/where you do your local-as prepend, it's also a matter of where you do loop detection. If implementations in the absence of some "outside the scope" implementation were to pedantically implement the base BGP RFC, the migration mechanisms would be a bit biased to a given solution. -- Jeff
- Re: [Idr] 2 Week WG LC on draft-ietf-idr-as-migra… Russ White
- [Idr] 2 Week WG LC on draft-ietf-idr-as-migration… Susan Hares
- Re: [Idr] 2 Week WG LC on draft-ietf-idr-as-migra… Jon Mitchell
- Re: [Idr] 2 Week WG LC on draft-ietf-idr-as-migra… Robert Raszuk
- Re: [Idr] 2 Week WG LC on draft-ietf-idr-as-migra… Alvaro Retana (aretana)
- Re: [Idr] 2 Week WG LC on draft-ietf-idr-as-migra… George, Wes
- Re: [Idr] 2 Week WG LC on draft-ietf-idr-as-migra… Alvaro Retana (aretana)
- Re: [Idr] 2 Week WG LC on draft-ietf-idr-as-migra… Jeffrey Haas
- Re: [Idr] 2 Week WG LC on draft-ietf-idr-as-migra… George, Wes
- Re: [Idr] 2 Week WG LC on draft-ietf-idr-as-migra… George, Wes
- Re: [Idr] 2 Week WG LC on draft-ietf-idr-as-migra… Alvaro Retana (aretana)
- Re: [Idr] 2 Week WG LC on draft-ietf-idr-as-migra… George, Wes
- Re: [Idr] 2 Week WG LC on draft-ietf-idr-as-migra… Alvaro Retana (aretana)