[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]

Versions: (draft-raszuk-bgp-optimal-route-reflection) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21

IDR Working Group                                         R. Raszuk, Ed.
Internet-Draft                                              Bloomberg LP
Intended status: Standards Track                               C. Cassar
Expires: December 18, 2020                                         Tesla
                                                                 E. Aman
                                                           Telia Company
                                                        B. Decraene, Ed.
                                                                  Orange
                                                                 K. Wang
                                                        Juniper Networks
                                                           June 16, 2020


                 BGP Optimal Route Reflection (BGP-ORR)
             draft-ietf-idr-bgp-optimal-route-reflection-21

Abstract

   This document defines an extension to BGP route reflectors.  On route
   reflectors, BGP route selection is modified in order to choose the
   best path for their clients standpoint, rather than from the route
   reflectors standpoint.  Multiple type of granularity are proposed,
   from a per client BGP route selection or to a per peer group,
   depending on the scaling and precision requirements on route
   selection.  This solution is particularly applicable in deployments
   using centralized route reflectors, where choosing the best route
   based on the Route Reflector IGP location is suboptimal.  This
   facilitates, for example, best exit point policy (hot potato
   routing).

   The solution relies upon all route reflectors learning all paths
   which are eligible for consideration.  Best path selection is
   performed in each route reflector based on the IGP cost from a
   selected location in the link state IGP.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any




Raszuk, et al.          Expires December 18, 2020               [Page 1]


Internet-Draft        bgp-optimal-route-reflection             June 2020


   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 18, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Definitions of Terms Used in This Memo  . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Modifications to BGP Best Path selection  . . . . . . . . . .   5
     3.1.  IGP Based Best Path Selection from a different SPT root .   6
       3.1.1.  Restriction when BGP next hop is BGP prefix . . . . .   7
     3.2.  Best Path Selections granularity  . . . . . . . . . . . .   7
   4.  Solution Interactions . . . . . . . . . . . . . . . . . . . .   8
     4.1.  IGP and policy based optimal route refresh  . . . . . . .   8
     4.2.  Add-paths plus IGP and policy optimal route refresh . . .   8
     4.3.  Likely Deployments and need for backup  . . . . . . . . .   8
   5.  CPU and Memory Scalability  . . . . . . . . . . . . . . . . .   9
   6.  Advantages and Deployment Considerations  . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  11
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     11.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Appendix A.  Appendix: alternative solutions with limited
                applicability  . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15







Raszuk, et al.          Expires December 18, 2020               [Page 2]


Internet-Draft        bgp-optimal-route-reflection             June 2020


1.  Definitions of Terms Used in This Memo

   NLRI -   Network Layer Reachability Information.

   RIB -   Routing Information Base.

   AS -   Autonomous System number.

   VRF -   Virtual Routing and Forwarding instance.

   PE -   Provider Edge router

   RR -   Route Reflector

   POP -   Point Of Presence

   L3VPN -   Layer 3 Virtual Private Networks [RFC4364]

   6PE -   IPv6 Provider Edge Router

   IGP -   Interior Gateway Protocol

   SPT -   Shortest Path Tree

   best path -   the route chosen by the decision process detailed in
      [RFC 4271] section 9.1.2 and its subsections

   best path computation -   the decision process detailed in [RFC 4271]
      section 9.1.2 and its subsections

   best path algorithm -   the decision process detailed in [RFC 4271]
      section 9.1.2 and its subsections

   best path selection -   the decision process detailed in [RFC 4271]
      section 9.1.2 and its subsections

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Introduction

   There are three types of BGP deployments within Autonomous Systems
   today: full mesh, confederations and route reflection.  BGP route
   reflection [RFC4456] is the most popular way to distribute BGP routes
   between BGP speakers belonging to the same Autonomous System.



Raszuk, et al.          Expires December 18, 2020               [Page 3]


Internet-Draft        bgp-optimal-route-reflection             June 2020


   However, in some situations, this method suffers from non-optimal
   path selection.

   [RFC4456] asserts that, because the Interior Gateway Protocol (IGP)
   cost to a given point in the network will vary across routers, "the
   route reflection approach may not yield the same route selection
   result as that of the full IBGP mesh approach."  One practical
   implication of this assertion is that the deployment of route
   reflection may thwart the ability to achieve hot potato routing.  Hot
   potato routing attempts to direct traffic to the best AS exit point
   in cases where no higher priority policy dictates otherwise.  As a
   consequence of the route reflection method, the choice of exit point
   for a route reflector and its clients will be the exit point best for
   the route reflector - not necessarily the one best for the route
   reflector clients.

   Section 11 of [RFC4456] describes a deployment approach and a set of
   constraints which, if satisfied, would result in the deployment of
   route reflection yielding the same results as the iBGP full mesh
   approach.  This deployment approach makes route reflection compatible
   with the application of hot potato routing policy.  In accordance
   with these design rules, route reflectors have traditionally often
   been deployed in the forwarding path and carefully placed on the POP
   to core boundaries.

   The evolving model of intra-domain network design has enabled
   deployments of route reflectors outside of the forwarding path.
   Initially this model was only employed for new address families, e.g.
   L3VPNs and L2VPNs, however it has been gradually extended to other
   BGP address families including IPv4 and IPv6 Internet using either
   native routing or 6PE.  In such environments, hot potato routing
   policy remains desirable.

   Route reflectors outside of the forwarding path can be placed on the
   POP to core boundaries, but they are often placed in arbitrary
   locations in the core of large networks.

   Such deployments suffer from a critical drawback in the context of
   best path selection: A route reflector with knowledge of multiple
   paths for a given prefix will typically pick its best path and only
   advertise that best path to its clients.  If the best path for a
   prefix is selected on the basis of an IGP tie break, the path
   advertised will be the exit point closest to the route reflector.
   However, the clients are in a different place in the network topology
   than the route reflector.  In networks where the route reflectors are
   not in the forwarding path, this difference will be even more acute.





Raszuk, et al.          Expires December 18, 2020               [Page 4]


Internet-Draft        bgp-optimal-route-reflection             June 2020


   In addition, there are deployment scenarios where service providers
   want to have more control in choosing the exit points for clients
   based on other factors, such as traffic type, traffic load, etc.
   This further complicates the issue and makes it less likely for the
   route reflector to select the best path from the client's
   perspective.  It follows that the best path chosen by the route
   reflector is not necessarily the same as the path which would have
   been chosen by the client if the client had considered the same set
   of candidate paths as the route reflector.

3.  Modifications to BGP Best Path selection

   The core of this solution is the ability for an operator to specify
   on a per route reflector basis, or per peer/update group basis, or
   per peer basis the IGP location of the route reflector.  This core
   ability enables the route reflector to send to a given group of
   clients routes with shortest distance to the next hops from the
   position of the selected IGP location.  This core ability provides
   for freedom of route reflector physical location, and allows
   transient or permanent migration of this network control plane
   function to an arbitrary location.

   The choice of specific granularity (route reflector basis, peer/
   update group basis, or peer peer basis) is configured by the network
   operator.  An implementation is considered compliant with the
   document if it supports at least one listed grouping of IGP location.

   For purposes of route selection, the perspective of a client can
   differ from that of a route reflector or another client in two
   distinct ways:

      it can, and usually will, have a different position in the IGP
      topology, and

      it can have a different routing policy.

   These factors correspond to the issues described earlier.

   This document defines, on BGP Route Reflectors [RFC4456], two changes
   to the BGP Best Path selection algorithm:

      The first change is related to the IGP cost to the BGP Next Hop,
      which is done in the step e) in the BGP decision process.  The
      change consists in using the IGP cost from a different source than
      the route reflector itself.






Raszuk, et al.          Expires December 18, 2020               [Page 5]


Internet-Draft        bgp-optimal-route-reflection             June 2020


      The second change is the granularity of the BGP decision process,
      to allow for running multiple decisions process using different
      perspective or policies.

   A route reflector can implement either or both of the modifications
   in order to allow it to choose the best path for its clients that the
   clients themselves would have chosen given the same set of candidate
   paths.

   Both modifications rely upon all route reflectors learning all paths
   that are eligible for consideration.  In order to satisfy this
   requirement, path diversity enhancing mechanisms such as add-path may
   need to be deployed between route reflectors.

   A significant advantage of these approaches is that the route
   reflector clients do not need to run new software or hardware.

3.1.  IGP Based Best Path Selection from a different SPT root

   In this approach, optimal refers to the decision made during best
   path selection at the IGP metric to BGP next hop comparison step.
   This approach does not apply to path selection preference based on
   other policy steps and provisions.

   In addition to the change specified in [RFC4456] section 9, the BGP
   Decision Process Tie Breaking rules ([RFC4271] Sect.  9.1.2.2) are
   modified as follows.

   The below text in step e)

      e) Remove from consideration any routes with less-preferred
      interior cost.  The interior cost of a route is determined by
      calculating the metric to the NEXT_HOP for the route using the
      Routing Table.

   ...is replaced by this new text:

      e) Remove from consideration any routes with less-preferred
      interior cost.  The interior cost of a route is determined by
      calculating the metric from the selected IGP location to the
      NEXT_HOP for the route using the shortest IGP path tree rooted on
      the selected IGP location.

   This extension requires the knowledge of the IGP topology in order to
   be able to compute the shortest path tree rooted on any location and
   in particular on the selected IGP locations.  This knowledge can be
   gained with the use of the link state IGP such as IS-IS [ISO10589] or
   OSPF [RFC2328] [RFC5340] or via BGP-LS [RFC7752].  If an IGP is used,



Raszuk, et al.          Expires December 18, 2020               [Page 6]


Internet-Draft        bgp-optimal-route-reflection             June 2020


   the selected IGP location MUST to be within the area/level of the
   IGP.

   The configuration of the IGP location is outside of the scope of this
   document.  The operator may configure it manually, implementation may
   automate it based on heuristics, or it can be computed centrally and
   configured by an external system.

   This solution does not require any change (BGP or IGP) on the
   clients, as all required changes are limited to the route reflector.

   This solution applies to NLRIs of all address families, that can be
   route reflected.

3.1.1.  Restriction when BGP next hop is BGP prefix

   In situations where the BGP next hop is a BGP prefix itself the IGP
   metric of a route used for its resolution SHOULD be the final IGP
   cost to reach such next hop.  Implementations which can not inform
   BGP of the final IGP metric to a recursive next hop SHOULD treat such
   paths as least preferred during next hop metric comparison.  However
   such paths SHOULD still be considered valid for best path selection.

3.2.  Best Path Selections granularity

   BGP Route Reflector as per [RFC4456] runs the usual single Best Path
   Selection used to compute the node's routing table.  This may be
   suboptimal or even not usuable when the Route Reflector clients has
   significantly different IGP locations or BGP policies.  In some
   cases, there is a need to compute the Best Path Selection with an
   increased granularity, such as per peer/update group or per client
   basis.

   This requires running multiple best path selections or multiple
   subset of the best path selection.  If the required routing
   optimization is limited to the IGP cost to the BGP Next-Hop, which is
   typical if the goal is hot potato routing or a routing (more) similar
   to the one resulting from an iBGP full mesh between clients, only the
   step e) as defined [RFC4271] Sect.  9.1.2.2, needs to be duplicated
   on a per granularity basis.  If the routing routing optimization
   requires the use of different BGP policy for each element (e.g.
   peer), the a larger part of the decision process needs to be
   duplicated, up to the whole decision process as defined in section
   9.1 of [RFC4271].  This is for example the case when there is a need
   to use different policies to compute different degree of preference
   during Pahse 1.  This nedded for use cases involved traffic
   engineering perspective, or dedicating certain exit points for
   certain clients points.



Raszuk, et al.          Expires December 18, 2020               [Page 7]


Internet-Draft        bgp-optimal-route-reflection             June 2020


   In the latter case, the user MAY specify and apply a general policy
   on the route reflector to select a subset of exit points as the
   candidate exit points for its clients.  For a given client, the
   policy SHOULD also allow the operator to select different candidate
   exit points for different address families.  Regular path selection,
   including client's perspective IGP based best path selection stated
   above, will be applied to the candidate paths to select the final
   paths to advertise to the clients.

4.  Solution Interactions

4.1.  IGP and policy based optimal route refresh

   Depending on the actual deployment scenarios, service providers may
   configure IGP based optimal route reflection or policy based optimal
   route reflection.  It is also possible to configure both approaches
   together.  In cases where both are configured together, policy based
   optimal route reflection MUST be applied first to select the
   candidate paths, then IGP based optimal route reflection can be
   applied on top of the candidate paths to select the final path to
   advertise to the client.

   The expected use case for optimal route reflection is to avoid
   reflecting all paths to the client because the client either: does
   not support add-paths or does not have the capacity to process all of
   the paths.  Typically the route reflector would just reflect a single
   optimal route to the client.  However, the solutions MUST NOT prevent
   reflecting more than one optimal path to the client as path diversity
   may be desirable for load balancing or fast restoration.  In cases
   where add-path and optimal route reflection are configured together,
   the route reflector MUST reflect n optimal paths to a client, where n
   is the add-path count.

4.2.  Add-paths plus IGP and policy optimal route refresh

   The most complicated scenario is where add-path is configured
   together with both IGP based and policy based optimal route
   reflection.  In this scenario, the policy based optimal route
   reflection MUST be applied first to select the candidate paths (from
   add-path).  Subsequently, IGP based optimal route reflection will be
   applied on top of the candidate paths to select the best n paths to
   advertise to the client.

4.3.  Likely Deployments and need for backup

   With IGP based optimal route reflection, even though the IGP location
   could be specified on a per route reflector basis or per peer/update
   group basis or per peer basis, in reality, it's most likely to be



Raszuk, et al.          Expires December 18, 2020               [Page 8]


Internet-Draft        bgp-optimal-route-reflection             June 2020


   specified per peer/update group basis.  All clients with the same or
   similar IGP location can be grouped into the same peer/update group.
   An IGP location is then specified for the peer/update group.  The
   location is usually specified as the location of one of the clients
   from the peer group or an ABR to the area where clients are located.
   Also, one or more backup locations SHOULD be allowed to be specified
   for redundancy.  Implementations may wish to take advantage of peer
   group mechanisms in order to provide for better scalability of
   optimal route reflector client groups with similar properties.

5.  CPU and Memory Scalability

   For IGP based optimal route reflection, determining the shortest path
   and associated cost between any two arbitrary points in a network
   based on the IGP topology learned by a router is expected to add some
   extra cost in terms of CPU resources.  However, current SPF tree
   generation code is implemented efficiently in a number of
   implementations, and therefore this is not expected to be a major
   drawback.  The number of SPTs computed is expected to be of the order
   of the number of clients of a route reflector whenever a topology
   change is detected.  Advanced optimizations like partial and
   incremental SPF may also be exploited.  The number of SPTs computed
   is expected to be higher but comparable to some existing deployed
   features such as (Remote) Loop Free Alternate which computes a (r)SPT
   per IGP neighbor.

   For policy based optimal route reflection, there will be some
   overhead to apply the policy to select the candidate paths.  This
   overhead is comparable to existing BGP export policies and therefore
   should be manageable.

   By the nature of route reflection, the number of clients can be split
   arbitrarily by the deployment of more route reflectors for a given
   number of clients.  While this is not expected to be necessary in
   existing networks with best in class route reflectors available
   today, this avenue to scaling up the route reflection infrastructure
   is available.

   If we consider the overall network wide cost/benefit factor, the only
   alternative to achieve the same level of optimality would require
   significantly increasing state on the edges of the network.  This
   will consume CPU and memory resources on all BGP speakers in the
   network.  Building this client perspective into the route reflectors
   seems appropriate.







Raszuk, et al.          Expires December 18, 2020               [Page 9]


Internet-Draft        bgp-optimal-route-reflection             June 2020


6.  Advantages and Deployment Considerations

   The solutions described provide a model for integrating the client
   perspective into the best path computation for route reflectors.
   More specifically, the choice of BGP path factors in either the IGP
   cost between the client and the nexthop (rather than the IGP cost
   from the route reflector to the nexthop) or other user configured
   policies.

   Implementations considered compliant with this document allow the
   configuration of a logical location from which the best path will be
   computed, on the basis of either a peer, a peer group, or an entire
   routing instance.

   These solutions can be deployed in traditional hop-by-hop forwarding
   networks as well as in end-to-end tunneled environments.  In networks
   where there are multiple route reflectors and hop-by-hop forwarding
   without encapsulation, such optimizations SHOULD be enabled in a
   consistent way on all route reflectors.  Otherwise, clients may
   receive an inconsistent view of the network, in turn leading to
   intra-domain forwarding loops.

   With this approach, an ISP can effect a hot potato routing policy
   even if route reflection has been moved out of the forwarding plane,
   and hop-by-hop switching has been replaced by end-to-end MPLS or IP
   encapsulation.

   As per above, these approaches reduce the amount of state which needs
   to be pushed to the edge of the network in order to perform hot
   potato routing.  The memory and CPU resources required at the edge of
   the network to provide hot potato routing using these approaches is
   lower than what would be required to achieve the same level of
   optimality by pushing and retaining all available paths (potentially
   10s) per each prefix at the edge.

   The solutions above allow for a fast and safe transition to a BGP
   control plane using centralized route reflection, without
   compromising an operator's closest exit operational principle.  This
   enables edge-to-edge LSP/IP encapsulation for traffic to IPv4 and
   IPv6 prefixes.

   Regarding the client's IGP best-path selection, it should be self
   evident that this solution does not interfere with policies enforced
   above IGP tie breaking in the BGP best path algorithm.







Raszuk, et al.          Expires December 18, 2020              [Page 10]


Internet-Draft        bgp-optimal-route-reflection             June 2020


7.  Security Considerations

   Similarly to [RFC4456], this extension to BGP does not change the
   underlying security issues inherent in the existing IBGP [RFC4456].

   It however enables the deployment of base BGP Route Reflection as
   described in [RFC4456] to be possible using virtual compute
   environments without any negative consequence on the BGP routing path
   optimality.

   This document does not introduce requirements for any new protection
   measures, but it also does not relax best operational practices for
   keeping the IGP network stable or to pace rate of policy based IGP
   cost to next hops such that it does not have any substantial effect
   on BGP path changes and their propagation to route reflection
   clients.

8.  IANA Considerations

   This document does not request any IANA allocations.

9.  Acknowledgments

   Authors would like to thank Keyur Patel, Eric Rosen, Clarence
   Filsfils, Uli Bornhauser, Russ White, Jakob Heitz, Mike Shand, Jon
   Mitchell, John Scudder, Jeff Haas, Martin Djernaes, Daniele
   Ceccarelli, Kieran Milne, Job Snijders and Randy Bush for their
   valuable input.

10.  Contributors

   Following persons substantially contributed to the current format of
   the document:

   Stephane Litkowski
   Orange
   9 rue du chene germain
   Cesson Sevigne, 35512
   France

   stephane.litkowski@orange.com










Raszuk, et al.          Expires December 18, 2020              [Page 11]


Internet-Draft        bgp-optimal-route-reflection             June 2020


   Adam Chappell
   Interoute Communications
   31st Floor
   25 Canada Square
   London, E14 5LQ
   United Kingdom

   adam.chappell@interoute.com

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
              February 2006, <https://www.rfc-editor.org/info/rfc4360>.

   [RFC5492]  Scudder, J. and R. Chandra, "Capabilities Advertisement
              with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
              2009, <https://www.rfc-editor.org/info/rfc5492>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

11.2.  Informative References

   [ISO10589]
              International Organization for Standardization,
              "Intermediate system to Intermediate system intra-domain
              routeing information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473)", ISO/
              IEC 10589:2002, Second Edition, Nov 2002.

   [RFC1997]  Chandra, R., Traina, P., and T. Li, "BGP Communities
              Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
              <https://www.rfc-editor.org/info/rfc1997>.



Raszuk, et al.          Expires December 18, 2020              [Page 12]


Internet-Draft        bgp-optimal-route-reflection             June 2020


   [RFC1998]  Chen, E. and T. Bates, "An Application of the BGP
              Community Attribute in Multi-home Routing", RFC 1998,
              DOI 10.17487/RFC1998, August 1996,
              <https://www.rfc-editor.org/info/rfc1998>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC4384]  Meyer, D., "BGP Communities for Data Collection", BCP 114,
              RFC 4384, DOI 10.17487/RFC4384, February 2006,
              <https://www.rfc-editor.org/info/rfc4384>.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
              <https://www.rfc-editor.org/info/rfc4456>.

   [RFC4893]  Vohra, Q. and E. Chen, "BGP Support for Four-octet AS
              Number Space", RFC 4893, DOI 10.17487/RFC4893, May 2007,
              <https://www.rfc-editor.org/info/rfc4893>.

   [RFC5283]  Decraene, B., Le Roux, JL., and I. Minei, "LDP Extension
              for Inter-Area Label Switched Paths (LSPs)", RFC 5283,
              DOI 10.17487/RFC5283, July 2008,
              <https://www.rfc-editor.org/info/rfc5283>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [RFC5668]  Rekhter, Y., Sangli, S., and D. Tappan, "4-Octet AS
              Specific BGP Extended Community", RFC 5668,
              DOI 10.17487/RFC5668, October 2009,
              <https://www.rfc-editor.org/info/rfc5668>.

   [RFC5714]  Shand, M. and S. Bryant, "IP Fast Reroute Framework",
              RFC 5714, DOI 10.17487/RFC5714, January 2010,
              <https://www.rfc-editor.org/info/rfc5714>.

   [RFC6774]  Raszuk, R., Ed., Fernando, R., Patel, K., McPherson, D.,
              and K. Kumaki, "Distribution of Diverse BGP Paths",
              RFC 6774, DOI 10.17487/RFC6774, November 2012,
              <https://www.rfc-editor.org/info/rfc6774>.







Raszuk, et al.          Expires December 18, 2020              [Page 13]


Internet-Draft        bgp-optimal-route-reflection             June 2020


   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [RFC7911]  Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", RFC 7911,
              DOI 10.17487/RFC7911, July 2016,
              <https://www.rfc-editor.org/info/rfc7911>.

Appendix A.  Appendix: alternative solutions with limited applicability

   One possible valid solution or workaround to the best path selection
   problem requires sending all domain external paths from the route
   reflector to all its clients.  This approach suffers the significant
   drawback of pushing a large amount of BGP state and churn to all edge
   routers.  Many networks receive full Internet routing information in
   a large number of locations.  This could easily result in tens of
   paths for each prefix that would need to be distributed to clients.

   Notwithstanding this drawback, there are a number of reasons for
   sending more than just the single best path to the clients.  Improved
   path diversity at the edge is a requirement for fast connectivity
   restoration, and a requirement for effective BGP level load
   balancing.

   In practical terms, add/diverse path deployments [RFC7911] [RFC6774]
   are expected to result in the distribution of 2, 3, or n (where n is
   a small number) good paths rather than all domain external paths.
   When the route reflector chooses one set of n paths and distributes
   them to all its route reflector clients, those n paths may not be the
   right n paths for all clients.  In the context of the problem
   described above, those n paths will not necessarily include the
   closest exit point out of the network for each route reflector
   client.  The mechanisms proposed in this document are likely to be
   complementary to mechanisms aimed at improving path diversity.

   Another possibility to optimize exit point selection is the
   implementation of distributed route reflector functionality at key
   IGP locations in order to ensure that these locations see their
   viewpoints respected in exit selection.  Typically, however, this
   requires the installation of physical nodes to implement the
   reflection, and if exit policy subsequently changes, the reflector
   placement and position can become inappropriate.

   To counter the burden of physical installation, it is possible to
   build a logical overlay of tunnels with appropriate IGP metrics in



Raszuk, et al.          Expires December 18, 2020              [Page 14]


Internet-Draft        bgp-optimal-route-reflection             June 2020


   order to simulate closeness to key locations required to implement
   exit policy.  There is significant complexity overhead in this
   approach, however, enough so to typically make it undesirable.

   Trends in control plane decoupling are causing a shift from
   traditional routers to compute virtualization platforms, or even
   third-party cloud platforms.  As a result, without this proposal,
   operators are left with a difficult choice for the distribution and
   reflection of address families with significant exit diversity:

   o  centralized path selection, and tolerate the associated suboptimal
      paths, or

   o  defer selection to end clients, but lose potential route scale
      capacity

   The latter can be a viable option, but it is clearly a decision that
   needs to be made on an application and address family basis, with
   strong consideration for the number of available paths per prefix
   (which may even vary per prefix range, depending on peering policy,
   e.g. consider bilateral peerings versus onward transit arrangements)

Authors' Addresses

   Robert Raszuk (editor)
   Bloomberg LP
   731 Lexington Ave
   New York City, NY  10022
   USA

   Email: robert@raszuk.net


   Christian Cassar
   Tesla
   43 Avro Way
   Weybridge  KT13 0XY
   UK

   Email: ccassar@tesla.com


   Erik Aman
   Telia Company
   Solna SE-169 94
   Sweden

   Email: erik.aman@teliacompany.com



Raszuk, et al.          Expires December 18, 2020              [Page 15]


Internet-Draft        bgp-optimal-route-reflection             June 2020


   Bruno Decraene (editor)
   Orange

   Email: bruno.decraene@orange.com


   Kevin Wang
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   USA

   Email: kfwang@juniper.net






































Raszuk, et al.          Expires December 18, 2020              [Page 16]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/