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, . [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, . [RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended Communities Attribute", RFC 4360, DOI 10.17487/RFC4360, February 2006, . [RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February 2009, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 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, . 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, . [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, DOI 10.17487/RFC2328, April 1998, . [RFC4384] Meyer, D., "BGP Communities for Data Collection", BCP 114, RFC 4384, DOI 10.17487/RFC4384, February 2006, . [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, . [RFC4893] Vohra, Q. and E. Chen, "BGP Support for Four-octet AS Number Space", RFC 4893, DOI 10.17487/RFC4893, May 2007, . [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, . [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, . [RFC5668] Rekhter, Y., Sangli, S., and D. Tappan, "4-Octet AS Specific BGP Extended Community", RFC 5668, DOI 10.17487/RFC5668, October 2009, . [RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC 5714, DOI 10.17487/RFC5714, January 2010, . [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, . 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, . [RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder, "Advertisement of Multiple Paths in BGP", RFC 7911, DOI 10.17487/RFC7911, July 2016, . 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]