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Versions: (draft-marques-idr-best-external) 00 01 02 03 04 05

Network Working Group                                         P. Marques
Internet-Draft                                               R. Fernando
Intended status: Standards Track                        Juniper Networks
Expires: November 15, 2009                                       E. Chen
                                                            P. Mohapatra
                                                           Cisco Systems
                                                            May 14, 2009


            Advertisement of the best external route in BGP
                  draft-ietf-idr-best-external-00.txt

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   Copyright (c) 2009 IETF Trust and the persons identified as the



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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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Abstract

   The base BGP specifications prevent a BGP speaker from advertising
   any route that is not the best route for a BGP destination.  This
   document specifies a modification of this rule.  Routes are divided
   into two categories, "external" and "internal".  A specification is
   provided for choosing a "best external route" (for a particular value
   of the Network Layer Reachability Information).  A BGP speaker is
   then allowed to advertise its "best external route" to its internal
   BGP peers, even if that is not the best route for the destination.
   The document explains why advertising the best external route can
   improve convergence time without causing routing loops.  Additional
   benefits include reduction of inter-domain churn and avoidance of
   permanent route oscillation.  The document also generalizes the
   notions of "internal" and "external" so that they can be applied to
   Route Reflector Clusters and Autonomous System Confederations.



























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  5
   2.  Algorithm for selection of best external route . . . . . . . .  5
   3.  Advertisement Rules  . . . . . . . . . . . . . . . . . . . . .  6
   4.  Consistency between routing and forwarding . . . . . . . . . .  6
   5.  Applications . . . . . . . . . . . . . . . . . . . . . . . . .  8
     5.1.  Fast Connectivity Restoration  . . . . . . . . . . . . . .  8
     5.2.  Inter-Domain Churn Reduction . . . . . . . . . . . . . . .  9
     5.3.  Reducing Persistent IBGP oscillation . . . . . . . . . . .  9
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   9.  Normative References . . . . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10



































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1.  Introduction

   The base BGP specifications prevent a BGP speaker from advertising
   any route that is not the best route for a BGP destination.  This
   document specifies a modification of this rule.  Routes are divided
   into two categories, "external" and "internal".  A specification is
   provided for choosing a "best external route" (for a particular value
   of the Network Layer Reachability Information).  A BGP speaker is
   then allowed to advertise its "best external route" to its internal
   BGP peers, even if that is not the best route for the destination.
   The document explains why advertising the best external route can
   improve convergence time without causing routing loops.  Additional
   benefits include reduction of inter-domain churn and avoidance of
   permanent route oscillation.

   The document also generalizes the notions of "internal" and
   "external" so that they can be applied to Route Reflector Clusters
   [RFC4456] and Autonomous System Confederations [RFC5065].  More
   specifically, two routers in the same route reflector cluster having
   an IBGP session between them are defined to be "internal" peers,
   whereas two routers in different clusters having an IBGP session are
   defined to be "external" peers.  Similarly, two routers in the same
   member AS of a confederation having an IBGP session between them are
   "internal" peers, whereas two routers in different member ASs of a
   confederation having a confed EBGP session between them are defined
   to be "external" peers.  The definition of "best external route"
   ensues from this definition in that it is the most preferred route
   among those received from the "external" neighbors.

   Advertising the best external route, when different from the best
   route, presents additional information into an IBGP mesh which may be
   of value for several purposes including:

   o  Faster restoration of connectivity, by providing additional paths,
      that may be used to fail over in case the primary path becomes
      invalid or is withdrawn.

   o  Reducing inter-domain churn and traffic blackholing due to the
      readily available alternate path.

   o  Reducing the potential for situations of permanent IBGP route
      oscillation, as discussed in some scenarios [RFC3345].

   o  Improving selection of lower MED routes from the same neighboring
      AS.

   This document defines procedures to select the best external route
   for each destination.  It also describes how above benefits are



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   realized with best external route announcement with the help of
   certain scenarios.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].


2.  Algorithm for selection of best external route

   Given that the intent in advertising an external route, when the best
   route for the same destination is an internal route, is to provide
   additional information into the IBGP mesh into which a route is
   participating, it is desirable to take into account the routes
   received from internal neighbors in the selection process.

   We propose a route selection algorithm that selects a total order
   between routes and which selects the same best route as the one
   currently specified [RFC4271].

   In order to achieve this, we need to introduce the concept of route
   group.  For a given NLRI, suppose the BGP decision process has run
   through all the steps prior to the MED comparison step (as defined in
   section 9.1.2.2 of [RFC4271].  Look at the set of routes that are
   still under consideration at that time.  Now partition this set into
   a number of disjoint route groups, where two routes are in the same
   group if and only if the neighbor AS of each route is the same.

   Routes are ordered within a group via MED or subsequent route
   selection rules.

   The order of all routes for the same destination is determined by the
   order of the best route in each group.

   As an example, the following set of received routes:


                     Path    AS    MED   rtr_id
                     a       1      10       10
                     b       2       5        1
                     c       1       5        5
                     d       2      20       20
                     e       2      30       30
                     f       3      10       20





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                      Figure 1: Path Attribute Table

   Would yield the following order (from the most to the least
   preferred):

   b < d < e < c < a < f

   In this example, comparison of the best route within each group
   provides the sequence (b < c < f).  The remaining routes are ordered
   in relation to their respective group best.

   The first route in the above ordering is indeed the best route for a
   given destination.  Eliminating the best route and executing the
   above steps leads us to a new total order of the routes.  The route
   to be advertised to a particular domain is selected by choosing the
   most preferred route that is external to that particular domain in
   the above order.  Note that whenever the overall best route is
   external it will automatically be selected by this algorithm.


3.  Advertisement Rules

   1.  In an AS domain, if a router has installed an internal route as
       best, it should advertise its "best external route" (as defined
       in the draft) to its internal neighbors.

   2.  In a Cluster domain, if a router (route reflector) has installed
       an external route as best, it should advertise its "best internal
       route" to its external neighbors.  (Advertising to internal
       neighbors is unchanged.)  Similarly, if the route reflector has
       installed an internal route as best, it should advertise its
       "best external route" to its internal (client) peers.  In order
       for the reflector to be able to advertise the best external route
       into the cluster, it is necessary that client-to-client
       reflection be disabled, since its advertisement may otherwise
       contain the best route within the cluster domain.

   3.  In a Confederation Member domain, if a router (confederation
       border router) has installed an internal route as best, it
       advertises its best external route to its internal neighbors.
       However, if it has installed an external route as best, it
       advertises its best internal route to its external neighbors.


4.  Consistency between routing and forwarding

   The BGP protocol, as defined in [RFC1771], specifies that a BGP
   speaker shall advertise to its internal peers the route with the



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   highest degree of preference among routes to the same destination
   received from external neighbors.

   This section discusses problems present with the approach described
   in [RFC1771] and the next section offers an alternative algorithm to
   select a best external route which can be advertised to an IBGP mesh.

   The internal update advertisement rules contained in the original
   BGP-4 specification [RFC1771] can lead to situations where traffic is
   forwarded through a route other than the route advertised by BGP.

   Inconsistencies between forwarding and routing are highly
   undesirable.  Service providers use BGP with the dual objective of
   learning reachability information and expressing policy over network
   resources.  The latter assumes that forwarding follows routing
   information.

   Consider the Autonomous system presented in figure 1, where r1 ... r4
   are members of a single IBGP mesh and routes a, b, and c are received
   from external peers.


                              AS 1 (c)
                                |
                              +----+           +----+
                              | r1 |...........| r2 |
                              +----+           +----+
                                .
                                .
                                .
                                .
                                .
                                .
                              +----+           +----+
                              | r3 |...........| r4 | --- ebgp --- AS X
                              +----+           +----+
                             /      \
                            /        \
                         AS 1 (a)  AS 2 (b)



                    Figure 2: Inconsistency in Routing








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                          Path    AS    MED   rtr_id
                          a       1      10        1
                          b       2       5       10
                          c       1       5        5

                    Figure 3: Path Attribute Table - 2

   Following the rules as specified in [RFC1771], router r3 will select
   path (b) received from AS 2 as its overall best to install in the
   Loc-Rib, since path (b) is preferable to path (c), the lowest MED
   route from AS 1.  However for the purposes of Internal Update route
   selection, it will ignore the presence of path (c), and elect (a) as
   its advertisement, via the router-id tie-breaking rule.

   In this scenario, router r4 will receive (c) from r1 and (a) from r3.
   It will pick the lowest MED route (c) and advertise it out via ebgp
   to AS X. However at this point routing is inconsistent with
   forwarding as traffic received from AS X will be forwarded towards AS
   2, while the ebgp advertisement is being made for an AS 1 path.

   Routing policies are typically specified in terms of neighboring
   ASes.  In the situation above, assuming that AS 1 is network for
   which this AS provides transit services while AS 2 and AS X are peer
   networks, one can easily see how the inconsistency between routing
   and forwarding would lead to transit being inadvertently provided
   between AS X and AS 2.  This could lead to persistent forwarding
   loops.

   Inconsistency between routing and forwarding may happen, whenever a
   bgp speaker chooses to advertise an external route into IBGP that is
   different from the overall best route and its overall best is
   external.


5.  Applications

5.1.  Fast Connectivity Restoration

   When two exits are available to reach a particular destination and
   one is preferred over the other, the availability of an alternate
   path provides fast connectivity restoration when the primary path
   fails.

   Restoration can be quick since the alternate path is already at hand.
   The border router could precompute the backup route and preinstall it
   in FIB ready to be switched when the primary goes away.  Note that
   this requires the border router that's the backup to also preinstall
   the secondary path and switch to it on failure.



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5.2.  Inter-Domain Churn Reduction

   Within an AS, the non availability of backup best leads to a border
   router sending a withdraw upstream when the primary fails.  This
   leads to inter-domain churn and packet loss for the time the network
   takes to converge to the alternate path.  Having the alternate path
   will reduces the churn and eliminates packet loss.

5.3.  Reducing Persistent IBGP oscillation

   Advertising the best external route, according to the algorithm
   described in this document will reduce the possibility of route
   oscillation by introducing additional information into the IBGP
   system.

   For a permanent oscillation condition to occur, it is necessary that
   a circular dependency between paths occurs such that the selection of
   a new best path by a router, in response to a received IBGP
   advertisement, causes the withdrawal of information that another
   router depends on in order to generate the original event.

   In vanilla BGP, when only the best overall route is advertised, as in
   most implementations, oscillation can occur whenever there are 2 or
   clusters/sub-ASes such that at least one cluster has more than one
   path that can potentially contribute to the dependency.


6.  Acknowledgments

   This document greatly benefits from the comments of Yakov Rekhter,
   John Scudder, Eric Rosen, and Jenny Yuan.


7.  IANA Considerations

   This document has no actions for IANA.


8.  Security Considerations

   There are no additional security risks introduced by this design.


9.  Normative References

   [RFC1771]  Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
              (BGP-4)", RFC 1771, March 1995.




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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3345]  McPherson, D., Gill, V., Walton, D., and A. Retana,
              "Border Gateway Protocol (BGP) Persistent Route
              Oscillation Condition", RFC 3345, August 2002.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, April 2006.

   [RFC5065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 5065, August 2007.


Authors' Addresses

   Pedro Marques
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA  94089
   USA

   Phone:
   Email: roque@juniper.net


   Rex Fernando
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA  94089
   USA

   Phone:
   Email: rex@juniper.net













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   Enke Chen
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   Phone:
   Email: enkechen@cisco.com


   Pradosh Mohapatra
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   Phone:
   Email: pmohapat@cisco.com

































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