Internet Engineering Task Force (IETF)                    R. Raszuk, Ed.
Request for Comments: 9107                       NTT Network Innovations
Category: Standards Track                               B. Decraene, Ed.
ISSN: 2070-1721                                                   Orange
                                                               C. Cassar

                                                                 E. Åman

                                                                 K. Wang
                                                        Juniper Networks
                                                             August 2021


                 BGP Optimal Route Reflection (BGP ORR)

Abstract

   This document defines an extension to BGP route reflectors.  On route
   reflectors, BGP route selection is modified in order to choose the
   best route from the standpoint of their clients, rather than from the
   standpoint of the route reflectors themselves.  Depending on the
   scaling and precision requirements, route selection can be specific
   for one client, common for a set of clients, or common for all
   clients of a route reflector.  This solution is particularly
   applicable in deployments using centralized route reflectors, where
   choosing the best route based on the route reflector's IGP location
   is suboptimal.  This facilitates, for example, a "best exit point"
   policy ("hot potato routing").

   The solution relies upon all route reflectors learning all paths that
   are eligible for consideration.  BGP route selection is performed in
   the route reflectors based on the IGP cost from configured locations
   in the link-state IGP.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9107.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Modifications to BGP Route Selection
     3.1.  Route Selection from a Different IGP Location
       3.1.1.  Restriction when the BGP Next Hop Is a BGP Route
     3.2.  Multiple Route Selections
   4.  Deployment Considerations
   5.  Security Considerations
   6.  IANA Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgments
   Contributors
   Authors' Addresses

1.  Introduction

   There are three types of BGP deployments within Autonomous Systems
   (ASes) 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 AS.  However, in
   some situations, this method suffers from non-optimal path selection.

   [RFC4456] asserts that, because the 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."  ("IBGP" stands for "Internal BGP".)  One practical
   implication of this fact 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 closest 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 that is
   optimal for the route reflector -- not necessarily the one that is
   optimal for its clients.

   Section 11 of [RFC4456] describes a deployment approach and a set of
   constraints that, 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 a hot potato routing policy.  In accordance
   with these design rules, route reflectors have often been deployed in
   the forwarding path and carefully placed on the boundaries between
   the Point of Presence (POP) and the network core.

   The evolving model of intra-domain network design has enabled
   deployments of route reflectors outside the forwarding path.
   Initially, this model was only employed for new services, e.g., IP
   VPNs [RFC4364]; however, it has been gradually extended to other BGP
   services, including the IPv4 and IPv6 Internet.  In such
   environments, a hot potato routing policy remains desirable.

   Route reflectors outside the forwarding path can be placed on the
   boundaries between the POP and the network core, 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
   BGP route selection: a route reflector with knowledge of multiple
   paths for a given route will typically pick its best path and only
   advertise that best path to its clients.  If the best path for a
   route 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.

   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 that would have
   been chosen by the client if the client had considered the same set
   of candidate paths as the route reflector.

2.  Terminology

   This memo makes use of the terms defined in [RFC4271] and [RFC4456].

   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.

3.  Modifications to BGP Route Selection

   The core of this solution is the ability for an operator to specify
   the IGP location for which the route reflector calculates interior
   cost to the next hop.  The IGP location is defined as a node in the
   IGP topology, it is identified by an IP address of this node (e.g., a
   loopback address), and it may be configured on a per-route-reflector
   basis, per set of clients, or on a per-client basis.  Such
   configuration will allow the route reflector to select and distribute
   to a given set of clients routes with the shortest distance to the
   next hops from the position of the selected IGP location.  This
   provides for freedom related to the route reflector's physical
   location and allows transient or permanent migration of this network
   control plane function to an arbitrary location with no impact on IP
   transit.

   The choice of specific granularity (route reflector, set of clients,
   or client) is configured by the network operator.  An implementation
   is considered compliant with this document if it supports at least
   one such grouping category.

   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 has a different position in the IGP topology.

   *  It can have a different routing policy.

   These factors correspond to the issues described earlier.

   This document defines, for BGP route reflectors [RFC4456], two
   changes to the BGP route selection algorithm:

   *  The first change, introduced in Section 3.1, is related to the IGP
      cost to the BGP next hop in the BGP Decision Process.  The change
      consists of using the IGP cost from a different IGP location than
      the route reflector itself.

   *  The second change, introduced in Section 3.2, is to extend the
      granularity of the BGP Decision Process, to allow for running
      multiple Decision Processes using different perspectives 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.

   A significant advantage of these approaches is that the route
   reflector's clients do not need to be modified.

3.1.  Route Selection from a Different IGP Location

   In this approach, "optimal" refers to the decision where the interior
   cost of a route is determined during step e) of Section 9.1.2.2
   ("Breaking Ties (Phase 2)") of [RFC4271].  It does not apply to path
   selection preference based on other policy steps and provisions.

   In addition to the change specified in Section 9 of [RFC4456], the
   text in step e) in Section 9.1.2.2 of [RFC4271] is modified as
   follows.

   RFC 4271 reads:

   |  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.

   This document modifies this text to read:

   |  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
   |      at the selected IGP location.

   In order to be able to compute the shortest path tree rooted at the
   selected IGP locations, knowledge of the IGP topology for the area/
   level that includes each of those locations is needed.  This
   knowledge can be gained with the use of the link-state IGP, such as
   IS-IS [ISO10589] or OSPF [RFC2328] [RFC5340], or via the Border
   Gateway Protocol - Link State (BGP-LS) [RFC7752].  When specifying
   the logical location of a route reflector for a group of clients, one
   or more backup IGP locations SHOULD be allowed to be specified for
   redundancy.  Further deployment considerations are discussed in
   Section 4.

3.1.1.  Restriction when the BGP Next Hop Is a BGP Route

   In situations where the BGP next hop is a BGP route itself, the IGP
   metric of a route used for its resolution SHOULD be the final IGP
   cost to reach such a next hop.  Implementations that cannot inform
   BGP of the final IGP metric to a recursive next hop MUST treat such
   paths as least preferred during next-hop metric comparisons.
   However, such paths MUST still be considered valid for BGP Phase 2
   route selection.

3.2.  Multiple Route Selections

   A BGP route reflector as per [RFC4456] runs a single BGP Decision
   Process.  BGP Optimal Route Reflection (BGP ORR) may require multiple
   BGP Decision Processes or subsets of the Decision Process in order to
   consider different IGP locations or BGP policies for different sets
   of clients.  This is very similar to what is defined in [RFC7947],
   Section 2.3.2.1.

   If the required routing optimization is limited to the IGP cost to
   the BGP next hop, only step e) and subsequent steps as defined in
   [RFC4271], Section 9.1.2.2 need to be run multiple times.

   If the routing optimization requires the use of different BGP
   policies for different sets of clients, a larger part of the Decision
   Process needs to be run multiple times, 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 degrees of preference during Phase 1.  This is
   needed for use cases involving traffic engineering or dedicating
   certain exit points for certain clients.  In the latter case, the
   user may specify and apply a general policy on the route reflector
   for a set of clients.  Regular path selection, including IGP
   perspectives for a set of clients as per Section 3.1, is then applied
   to the candidate paths to select the final paths to advertise to the
   clients.

4.  Deployment Considerations

   BGP ORR provides a model for integrating the client's perspective
   into the BGP route selection Decision Process for route reflectors.
   More specifically, the choice of BGP path takes into account either
   the IGP cost between the client and the next hop (rather than the IGP
   cost from the route reflector to the next hop) or other user-
   configured policies.

   The achievement of optimal routing between clients of different
   clusters relies upon all route reflectors learning all paths that are
   eligible for consideration.  In order to satisfy this requirement,
   BGP ADD-PATH [RFC7911] needs to be deployed between route reflectors.

   This solution can be deployed in hop-by-hop forwarding networks as
   well as in end-to-end tunneled environments.  To avoid routing loops
   in networks with multiple route reflectors and hop-by-hop forwarding
   without encapsulation, it is essential that the network topology be
   carefully considered in designing a route reflection topology (see
   also Section 11 of [RFC4456]).

   As discussed in Section 11 of [RFC4456], the IGP locations of BGP
   route reflectors are important and have routing implications.  This
   equally applies to the choice of the IGP locations configured on
   optimal route reflectors.  If a backup location is provided, it is
   used when the primary IGP location disappears from the IGP (i.e.,
   fails).  Just like the failure of a route reflector [RFC4456], it may
   result in changing the paths selected and advertised to the clients,
   and in general, the post-failure paths are expected to be less
   optimal.  This is dependent on the IGP topologies and the IGP
   distance between the primary and backup IGP locations: the smaller
   the distance, the smaller the potential impact.

   After selecting N suitable IGP locations, an operator can choose to
   enable route selection for all of them on all or on a subset of their
   route reflectors.  The operator may alternatively deploy single or
   multiple (backup case) route reflectors for each IGP location or
   create any design in between.  This choice may depend on the
   operational model (centralized vs. per region), an acceptable blast
   radius in the case of failure, an acceptable number of IBGP sessions
   for the mesh between the route reflectors, performance, and
   configuration granularity of the equipment.

   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 forwarding has been replaced by end-to-end MPLS or IP
   encapsulation.  Compared with a deployment of ADD-PATH on all
   routers, BGP ORR reduces the amount of state that needs to be pushed
   to the edge of the network in order to perform hot potato routing.

   Modifying the IGP location of BGP ORR does not interfere with
   policies enforced before IGP tie-breaking (step e) of [RFC4271],
   Section 9.1.2.2) in the BGP Decision Process.

   Calculating routes for different IGP locations requires multiple
   Shortest Path First (SPF) calculations and multiple (subsets of) BGP
   Decision Processes.  This scenario calls for more computing
   resources.  This document allows for different granularity, such as
   one Decision Process per route reflector, per set of clients, or per
   client.  A more fine-grained granularity may translate into more
   optimal hot potato routing at the cost of more computing power.
   Choosing to configure an IGP location per client has the highest
   precision, as each client can be associated with their ideal (own)
   IGP location.  However, doing so may have an impact on performance
   (as explained above).  Using an IGP location per set of clients
   implies a loss of precision but reduces the impact on the performance
   of the route reflector.  Similarly, if an IGP location is selected
   for the whole routing instance, the lowest precision is achieved, but
   the impact on performance is minimal.  In the last mode of operation
   (where an IGP location is selected for the whole routing instance),
   both precision and performance metrics are equal to route reflection
   as described in [RFC4456].  The ability to run fine-grained
   computations depends on the platform/hardware deployed, the number of
   clients, the number of BGP routes, and the size of the IGP topology.
   In essence, sizing considerations are similar to the deployments of
   BGP route reflectors.

5.  Security Considerations

   The extension specified in this document provides a new metric value
   using additional information for computing routes for BGP route
   reflectors.  While any improperly used metric value could impact the
   resiliency of the network, this extension does not change the
   underlying security issues inherent in the existing IBGP per
   [RFC4456].

   This document does not introduce requirements for any new protection
   measures.

6.  IANA Considerations

   This document has no IANA actions.

7.  References

7.1.  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>.

   [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>.

   [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>.

   [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>.

7.2.  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, November 2002.

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

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [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>.

   [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>.

   [RFC7947]  Jasinska, E., Hilliard, N., Raszuk, R., and N. Bakker,
              "Internet Exchange BGP Route Server", RFC 7947,
              DOI 10.17487/RFC7947, September 2016,
              <https://www.rfc-editor.org/info/rfc7947>.

Acknowledgments

   The 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 Djernæs, Daniele
   Ceccarelli, Kieran Milne, Job Snijders, Randy Bush, Alvaro Retana,
   Francesca Palombini, Benjamin Kaduk, Zaheduzzaman Sarker, Lars
   Eggert, Murray Kucherawy, Tom Petch, and Nick Hilliard for their
   valuable input.

Contributors

   The following persons contributed substantially to the current format
   of the document:

   Stephane Litkowski
   Cisco Systems

   Email: slitkows.ietf@gmail.com


   Adam Chappell
   GTT Communications, Inc.
   Aspira Business Centre
   Bucharova 2928/14a
   158 00 Prague 13 Stodůlky
   Czech Republic

   Email: adam.chappell@gtt.net


Authors' Addresses

   Robert Raszuk (editor)
   NTT Network Innovations

   Email: robert@raszuk.net


   Bruno Decraene (editor)
   Orange

   Email: bruno.decraene@orange.com


   Christian Cassar

   Email: cassar.christian@gmail.com


   Erik Åman

   Email: erik.aman@aman.se


   Kevin Wang
   Juniper Networks
   10 Technology Park Drive
   Westford, MA 01886
   United States of America

   Email: kfwang@juniper.net