Border Gateway Protocol (BGP) Persistent Route Oscillation Condition
RFC 3345
Document | Type | RFC - Informational (August 2002; Errata) | |
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Authors | Alvaro Retana , Daniel Walton , Danny McPherson , Vijay Gill | ||
Last updated | 2020-01-21 | ||
Replaces | draft-mcpherson-bgp-route-oscillation | ||
Stream | IETF | ||
Formats | plain text html pdf htmlized with errata bibtex | ||
Stream | WG state | (None) | |
Document shepherd | No shepherd assigned | ||
IESG | IESG state | RFC 3345 (Informational) | |
Action Holders |
(None)
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Consensus Boilerplate | Unknown | ||
Telechat date | |||
Responsible AD | Bill Fenner | ||
IESG note | Responsible: RFC Editor | ||
Send notices to | <yakov@juniper.net> |
Network Working Group D. McPherson Request for Comments: 3345 TCB Category: Informational V. Gill AOL Time Warner, Inc. D. Walton A. Retana Cisco Systems, Inc. August 2002 Border Gateway Protocol (BGP) Persistent Route Oscillation Condition Status of this Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract In particular configurations, the BGP scaling mechanisms defined in "BGP Route Reflection - An Alternative to Full Mesh IBGP" and "Autonomous System Confederations for BGP" will introduce persistent BGP route oscillation. This document discusses the two types of persistent route oscillation that have been identified, describes when these conditions will occur, and provides some network design guidelines to avoid introducing such occurrences. 1. Introduction The Border Gateway Protocol (BGP) is an inter-Autonomous System routing protocol. The primary function of a BGP speaking system is to exchange network reachability information with other BGP systems. In particular configurations, the BGP [1] scaling mechanisms defined in "BGP Route Reflection - An Alternative to Full Mesh IBGP" [2] and "Autonomous System Confederations for BGP" [3] will introduce persistent BGP route oscillation. The problem is inherent in the way BGP works: locally defined routing policies may conflict globally, and certain types of conflicts can cause persistent oscillation of the protocol. Given current practices, we happen to see the problem manifest itself in the context of MED + route reflectors or confederations. McPherson, et al. Informational [Page 1] RFC 3345 BGP Persistent Route Oscillation Condition August 2002 The current specification of BGP-4 [4] states that the MULTI_EXIT_DISC is only comparable between routes learned from the same neighboring AS. This limitation is consistent with the description of the attribute: "The MULTI_EXIT_DISC attribute may be used on external (inter-AS) links to discriminate among multiple exit or entry points to the same neighboring AS." [1,4] In a full mesh iBGP network, all the internal routers have complete visibility of the available exit points into a neighboring AS. The comparison of the MULTI_EXIT_DISC for only some paths is not a problem. Because of the scalability implications of a full mesh iBGP network, two alternatives have been standardized: route reflectors [2] and AS confederations [3]. Both alternatives describe methods by which route distribution may be achieved without a full iBGP mesh in an AS. The route reflector alternative defines the ability to re-advertise (reflect) iBGP-learned routes to other iBGP peers once the best path is selected [2]. AS Confederations specify the operation of a collection of autonomous systems under a common administration as a single entity (i.e. from the outside, the internal topology and the existence of separate autonomous systems are not visible). In both cases, the reduction of the iBGP full mesh results in the fact that not all the BGP speakers in the AS have complete visibility of the available exit points into a neighboring AS. In fact, the visibility may be partial and inconsistent depending on the location (and function) of the router in the AS. In certain topologies involving either route reflectors or confederations (detailed description later in this document), the partial visibility of the available exit points into a neighboring AS may result in an inconsistent best path selection decision as the routers don't have all the relevant information. If the inconsistencies span more than one peering router, they may result in a persistent route oscillation. The best path selection rules applied in this document are consistent with the current specification [4]. The persistent route oscillation behavior is deterministic and can be avoided by employing some rudimentary BGP network design principles until protocol enhancements resolve the problem. In the following sections a taxonomy of the types of oscillations is presented and a description of the set of conditions that will trigger route oscillations is given. We continue by providing several network design alternatives that remove the potential of thisShow full document text