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rfc:rfc6774

Internet Engineering Task Force (IETF) R. Raszuk, Ed. Request for Comments: 6774 NTT MCL Category: Informational R. Fernando ISSN: 2070-1721 K. Patel

                                                         Cisco Systems
                                                          D. McPherson
                                                              Verisign
                                                             K. Kumaki
                                                      KDDI Corporation
                                                         November 2012
                 Distribution of Diverse BGP Paths

Abstract

 The BGP4 protocol specifies the selection and propagation of a single
 best path for each prefix.  As defined and widely deployed today, BGP
 has no mechanisms to distribute alternate paths that are not
 considered best path between its speakers.  This behavior results in
 a number of disadvantages for new applications and services.
 The main objective of this document is to observe that by simply
 adding a new session between a route reflector and its client, the
 Nth best path can be distributed.  This document also compares
 existing solutions and proposed ideas that enable distribution of
 more paths than just the best path.
 This proposal does not specify any changes to the BGP protocol
 definition.  It does not require a software upgrade of provider edge
 (PE) routers acting as route reflector clients.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 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).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6774.

Raszuk, et al. Informational [Page 1] RFC 6774 Diverse-BGP-Path Distribution November 2012

Copyright Notice

 Copyright (c) 2012 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
 (http://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. Introduction ....................................................2
 2. History .........................................................3
    2.1. BGP Add-Paths Proposal .....................................3
 3. Goals ...........................................................5
 4. Multi-Plane Route Reflection ....................................6
    4.1. Co-located Best- and Backup-Path RRs .......................8
    4.2. Randomly Located Best- and Backup-Path RRs ................10
    4.3. Multi-Plane Route Servers for Internet Exchanges ..........12
 5. Discussion on Current Models of IBGP Route Distribution ........13
    5.1. Full Mesh .................................................13
    5.2. Confederations ............................................14
    5.3. Route Reflectors ..........................................15
 6. Deployment Considerations ......................................15
 7. Summary of Benefits ............................................17
 8. Applications ...................................................18
 9. Security Considerations ........................................19
 10. Contributors ..................................................19
 11. Acknowledgments ...............................................20
 12. References ....................................................20
     12.1. Normative References ....................................20
     12.2. Informative References ..................................20

1. Introduction

 The current BGP4 protocol specification [RFC4271] allows for the
 selection and propagation of only one best path for each prefix.  As
 defined today, the BGP protocol has no mechanism to distribute paths
 other than best path between its speakers.  This behavior results in
 a number of problems in the deployment of new applications and
 services.

Raszuk, et al. Informational [Page 2] RFC 6774 Diverse-BGP-Path Distribution November 2012

 This document presents a mechanism for solving the problem based on
 the conceptual creation of parallel route-reflector planes.  It also
 compares existing solutions and proposes ideas that enable
 distribution of more paths than just the best path.  The parallel
 route-reflector planes solution brings very significant benefits at a
 negligible capex and opex deployment price as compared to the
 alternative techniques (full BGP mesh or add-paths [ADD-PATHS]) and
 is being considered by a number of network operators for deployment
 in their networks.
 This proposal does not specify any changes to the BGP protocol
 definition.  It does not require upgrades to provider edge or core
 routers, nor does it need network-wide upgrades.  The only upgrade
 required is the new functionality on the new or current route
 reflectors.

2. History

 The need to disseminate more paths than just the best path is
 primarily driven by three issues.  The first is the problem of BGP
 oscillations [RFC3345].  The second is the desire for faster
 reachability restoration in the event of failure of the network link
 or network element.  The third is a need to enhance BGP load-
 balancing capabilities.  These issues have led to the proposal of BGP
 add-paths [ADD-PATHS].

2.1. BGP Add-Paths Proposal

 As it has been proven that distribution of only the best path of a
 route is not sufficient to meet the needs of the continuously growing
 number of services carried over BGP, the add-paths proposal was
 submitted in 2002 to enable BGP to distribute more than one path.
 This is achieved by including an additional four-octet value called
 the "Path Identifier" as a part of the Network Layer Reachability
 Information (NLRI).
 The implication of this change on a BGP implementation is that it
 must now maintain a per-path, instead of per-prefix, peer
 advertisement state to track to which of the peers a given path was
 advertised.  This new requirement comes with its own memory and
 processing cost.
 An important observation is that distribution of more than one best
 path by the Autonomous System Border Routers (ASBRs) with multiple
 External BGP (EBGP) peers attached where no "next-hop self" is set
 may result in inconsistent best-path selection within the autonomous
 system.  Therefore, it is also required to attach the possible
 tiebreakers in the form of a new attribute and propagate those within

Raszuk, et al. Informational [Page 3] RFC 6774 Diverse-BGP-Path Distribution November 2012

 the domain.  The example of such an attribute for the purpose of fast
 connectivity restoration to address that very case of ASBR injecting
 multiple external paths into the Internal BGP (IBGP) mesh has been
 presented and discussed in "Advertisement of Multiple Paths in BGP"
 [ADD-PATHS].  Based on the additionally propagated information, best-
 path selection is recommended to be modified to make sure that best-
 and backup-path selection within the domain stays consistent.  More
 discussion on this particular point is contained in Section 6,
 "Deployment Considerations".  In the proposed solution in this
 document, we observe that to address most of the applications, just
 use of the best external advertisement is required.  For ASBRs that
 are peering to multiple upstream domains, setting "next-hop self" is
 recommended.
 The add-paths protocol extensions have to be implemented by all the
 routers within an Autonomous System (AS) in order for the system to
 work correctly.  Analyzing the benefits or risks associated with
 partial add-paths deployments remains quite a topic for research.
 The risk becomes even greater in networks not using some form of
 edge-to-edge encapsulation.
 The required code modifications can offer the foundation for
 enhancements, such as the "Fast Connectivity Restoration Using BGP
 Add-path" [FAST-CONN].  The deployment of such technology in an
 entire service-provider network requires software, and perhaps
 sometimes, in the case of End-of-Engineering or End-of-Life
 equipment, even hardware upgrades.  Such an operation may or may not
 be economically feasible.  Even if add-path functionality was
 available today on all commercial routing equipment and across all
 vendors, experience indicates that it may easily take years to
 achieve 100% deployment coverage within any medium or large global
 network.
 While it needs to be clearly acknowledged that the add-path mechanism
 provides the most general way to address the problem of distributing
 many paths between BGP speakers, this document provides a solution
 that is much easier to deploy and requires no modification to the BGP
 protocol where only a few additional paths may be required.  The
 alternative method presented is capable of addressing critical
 service-provider requirements for disseminating more than a single
 path across an AS with a significantly lower deployment cost.  That,
 in light of the number of general network scaling concerns documented
 in RFC 4984 [RFC4984], "Report from the IAB Workshop on Routing and
 Addressing", may provide a significant advantage.

Raszuk, et al. Informational [Page 4] RFC 6774 Diverse-BGP-Path Distribution November 2012

3. Goals

 The proposal described in this document is not intended to compete
 with add-paths.  It provides an interim solution until add-paths are
 standardized and implemented and until support for that function can
 be deployed across the network.
 It is presented to network operators as a possible choice and
 provides those operators who need additional paths today an
 alternative from the need to transition to a full mesh.  The Nth best
 path describes a set of N paths with different BGP next hops with no
 implication of ordering or preference among said N paths.
 It is intended as a way to buy more time, allowing for a smoother and
 gradual migration where router upgrades will be required for,
 perhaps, different reasons.  It will also allow the time required so
 that standard RP/RE memory size can easily accommodate the associated
 overhead with other techniques without any compromises.

Raszuk, et al. Informational [Page 5] RFC 6774 Diverse-BGP-Path Distribution November 2012

4. Multi-Plane Route Reflection

 The idea contained in the proposal assumes the use of route
 reflection within the network.
 Let's observe today's picture of a simple route-reflected domain:
                                  ASBR3
                                   ***
                                  *   *
                     +------------*   *-----------+
                     | AS1        *   *           |
                     |             ***            |
                     |                            |
                     |                            |
                     |                            |
                     | RR1         ***        RR2 |
                     | ***        *   *       *** |
                     |*   *       * P *      *   *|
                     |*   *       *   *      *   *|
                     | ***         ***        *** |
                     |                            |
                     |            IBGP            |
                     |                            |
                     |                            |
                     |      ***           ***     |
                     |     *   *         *   *    |
                     +-----*   *---------*   *----+
                           *   *         *   *
                            ***           ***
                           ASBR1         ASBR2
                                   EBGP
                   Figure 1: Simple route reflection
 Abbreviations used:
    RR - Route Reflector
    P - Core router
 Figure 1 shows an AS that is connected via EBGP peering at ASBR1 and
 ASBR2 to an upstream AS or set of ASes.  For a given destination "D",
 ASBR1 and ASBR2 may have an external path P1 and P2, respectively.
 The AS network uses two route reflectors, RR1 and RR2, for redundancy
 reasons.  The route reflectors propagate the single BGP best path for
 each route to all clients.  All ASBRs are clients of RR1 and RR2.

Raszuk, et al. Informational [Page 6] RFC 6774 Diverse-BGP-Path Distribution November 2012

 Following are the possible cases of the path information that ASBR3
 may receive from route reflectors RR1 and RR2:
 1.  When the best-path tiebreaker is the IGP distance: When paths P1
     and P2 are considered to be equally good best-path candidates,
     the selection will depend on the distance of the path's next hops
     from the route reflector making the decision.  Depending on the
     positioning of the route reflectors in the IGP topology, they may
     choose the same best path or a different one.  In such a case,
     ASBR3 may receive either the same path or different paths from
     each of the route reflectors.
 2.  When the best-path tiebreaker is MULTI_EXIT_DISC (MED) or
     LOCAL_PREF: In this case, only one path from the preferred exit
     point ASBR will be available to RRs since the other peering ASBR
     will consider the IBGP path as best and will not announce (or if
     already announced will withdraw) its own external path.  The
     exception here is the use of the BGP Best-External proposal
     [EXT-PATH], which will allow a stated ASBR to still propagate to
     the RRs on its own external path.  Unfortunately, RRs will not be
     able to distribute it any further to other clients, as only the
     overall best path will be reflected.
 There is no requirement of path ordering.  The "Nth best path" really
 describes set of N paths with different BGP next hops.
 The proposed solution is based on the use of additional route
 reflectors or new functionality enabled on the existing route
 reflectors that, instead of distributing the best path for each
 route, will distribute an alternative path other than best.  The
 best-path (main) reflector plane distributes the best path for each
 route as it does today.  The second plane distributes the second best
 path for each route, and so on.  Distribution of N paths for each
 route can be achieved by using N reflector planes.
 As diverse-path functionality may be enabled on a per-peer basis, one
 of the deployment models can be realized to continue advertisement of
 the overall best path from both route reflectors, while in addition a
 new session can be provisioned to get an additional path.  This will
 allow the uninterrupted use of the best path, even if one of the RRs
 goes down, provided that the overall best path is still a valid one.
 Each plane of the route reflectors is a logical entity and may or may
 not be co-located with the existing best-path route reflectors.
 Adding a route-reflector plane to a network may be as easy as
 enabling a logical router partition, new BGP process, or just a new
 configuration knob on an existing route reflector and configuring an
 additional IBGP session from the current clients if required.  There

Raszuk, et al. Informational [Page 7] RFC 6774 Diverse-BGP-Path Distribution November 2012

 are no code changes required on the route-reflector clients for this
 mechanism to work.  It is easy to observe that the installation of
 one or more additional route-reflector control planes is much cheaper
 and is easier than upgrading hundreds of route-reflector clients in
 the entire network to support different BGP protocol encoding.
 Diverse-path route reflectors need the new ability to calculate and
 propagate the Nth best path instead of the overall best path.  An
 implementation is encouraged to enable this new functionality on a
 per-neighbor basis.
 While this is an implementation detail, the code to calculate the Nth
 best path is also required by other BGP solutions.  For example, in
 the application of fast connectivity restoration, BGP must calculate
 a backup path for installation into the Routing Information Base
 (RIB) and Forwarding Information Base (FIB) ahead of the actual
 failure.
 To address the problem of external paths not being available to route
 reflectors due to LOCAL_PREF or MED factors, it is recommended that
 ASBRs enable [EXT-PATH] functionality in order to always inject their
 external paths to the route reflectors.

4.1. Co-located Best- and Backup-Path RRs

 To simplify the description, let's assume that we only use two route-
 reflector planes (N=2).  When co-located, the additional second-best-
 path reflectors are connected to the network at the same points from
 the perspective of the IGP as the existing best-path RRs.  Let's also
 assume that best-external functionality is enabled on all ASBRs.

Raszuk, et al. Informational [Page 8] RFC 6774 Diverse-BGP-Path Distribution November 2012

                                  ASBR3
                                   ***
                                  *   *
                     +------------*   *-----------+
                     | AS1        *   *           |
                     |             ***            |
                     |                            |
                     | RR1                    RR2 |
                     | ***                    *** |
                     |*   *        ***       *   *|
                     |*   *       *   *      *   *|
                     | ***        * P *       *** |
                     |*   *       *   *      *   *|
                     |*   *        ***       *   *|
                     | ***                    *** |
                     | RR1'       IBGP        RR2'|
                     |                            |
                     |                            |
                     |      ***           ***     |
                     |     *   *         *   *    |
                     +-----*   *---------*   *----+
                           *   *         *   *
                            ***           ***
                           ASBR1         ASBR2
                                   EBGP
            Figure 2: Co-located Second-Best-Path RR Plane
 The following is a list of configuration changes required to enable
 the second-best-path route-reflector plane:
 1.  Unless the same RR1/RR2 platform is being used, adding RR1' and
     RR2' either as the logical or physical new control-plane RRs in
     the same IGP points as RR1 and RR2, respectively.
 2.  Enabling best-external functionality on ASBRs.
 3.  Enabling RR1' and RR2' for second plane route reflection.
     Alternatively, instructing existing RR1 and RR2 to calculate the
     second-best path also.
 4.  Unless one of the existing RRs is set to advertise only diverse
     path to its current clients, configuring new ASBRs-RR' IBGP
     sessions.

Raszuk, et al. Informational [Page 9] RFC 6774 Diverse-BGP-Path Distribution November 2012

 The expected behavior is that under any BGP condition, the ASBR3 and
 P routers will receive both paths P1 and P2 for destination D.  The
 availability of both paths will allow them to implement a number of
 new services as listed in Section 8 ("Applications").
 As an alternative to fully meshing all RRs and RRs', an operator that
 has a large number of reflectors deployed today may choose to peer
 newly introduced RRs' to a hierarchical RR', which would be an IBGP
 interconnect point within the second plane as well as between planes.
 One deployment model of this scenario can be achieved by simply
 upgrading the existing route reflectors without deploying any new
 logical or physical platforms.  Such an upgrade would allow route
 reflectors to service both peers that have upgraded to add-paths, as
 well as those peers that cannot be immediately upgraded while at the
 same time allowing distribution of more than a single best path.  The
 obvious protocol benefit of using existing RRs to distribute towards
 their clients' best and diverse BGP paths over different IBGP
 sessions is the automatic assurance that such a client would always
 get different paths with their next hop being different.
 The way to accomplish this would be to create a separate IBGP session
 for each Nth BGP path.  Such a session should be preferably
 terminated at a different loopback address of the route reflector.
 At the BGP OPEN stage of each such session, a different bgp_router_id
 may be used.  Correspondingly, the route reflector should also allow
 its clients to use the same bgp_router_id on each such session.

4.2. Randomly Located Best- and Backup-Path RRs

 Now let's consider a deployment case in which an operator wishes to
 enable a second RR' plane using only a single additional router in a
 different network location from his current route reflectors.  This
 model would be of particular use in networks in which some form of
 end-to-end encapsulation (IP or MPLS) is enabled between provider-
 edge routers.
 Note that this model of operation assumes that the present best-path
 route reflectors are only control-plane devices.  If the route
 reflector is in the data-forwarding path, then the implementation
 must be able to clearly separate the Nth best-path selection from the
 selection of the paths to be used for data forwarding.  The basic
 premise of this mode of deployment assumes that all reflector planes
 have the same information to choose from, which includes the same set
 of BGP paths.  It also requires the ability to ignore the step of
 comparison of the IGP metric to reach the BGP next hop during best-
 path calculation.

Raszuk, et al. Informational [Page 10] RFC 6774 Diverse-BGP-Path Distribution November 2012

                                  ASBR3
                                   ***
                                  *   *
                     +------------*   *-----------+
                     | AS1        *   *           |
                     | IBGP        ***            |
                     |                            |
                     |             ***            |
                     |            *   *           |
                     | RR1        * P *       RR2 |
                     | ***        *   *       *** |
                     |*   *        ***       *   *|
                     |*   *                  *   *|
                     | ***         RR'        *** |
                     |             ***            |
                     |            *   *           |
                     |            *   *           |
                     |             ***            |
                     |      ***           ***     |
                     |     *   *         *   *    |
                     +-----*   *---------*   *----+
                           *   *         *   *
                            ***           ***
                           ASBR1         ASBR2
                                   EBGP
    Figure 3: Experimental Deployment of Second-Best-Path RR Plane
 The following is a list of configuration changes required to enable
 the second-best-path route reflector RR' as a single platform or to
 enable one of the existing control-plane RRs for diverse-path
 functionality:
 1.  If needed, adding RR' logical or physical as a new route
     reflector anywhere in the network.
 2.  Enabling best-external functionality on ASBRs.
 3.  Disabling IGP metric check in BGP best path on all route
     reflectors.
 4.  Enabling RR' or any of the existing RR for second plane path
     calculation.
 5.  If required, fully meshing newly added RRs' with all the other
     reflectors in both planes.  This condition does not apply if the
     newly added RR'(s) already have peering to all ASBRs/PEs.

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 6.  Configure new BGP sessions between ASBRs and RRs (unless one of
     the existing RRs is set to advertise only diverse path to its
     current clients).
 In this scenario, the operator has the flexibility to introduce the
 new additional route-reflector functionality on any existing or new
 hardware in the network.  Any existing routers that are not already
 members of the best-path route-reflector plane can be easily
 configured to serve the second plane either by using a
 logical/virtual router partition or by having their BGP
 implementation compliant to this specification.
 Even if the IGP metric is not taken into consideration when comparing
 paths during the best-path calculation, an implementation still has
 to consider paths with unreachable next hops invalid.  It is worth
 pointing out that some implementations today already allow for
 configuration that results in no IGP metric comparison during the
 best-path calculation.
 The additional planes of route reflectors do not need to be fully
 redundant as the primary plane does.  If we are preparing for a
 single network failure event, a failure of a non-backed-up Nth best-
 path route reflector would not result in a connectivity outage of the
 actual data plane.  The reason is that this would, at most, affect
 the presence of a backup path (not an active one) on the same parts
 of the network.  If the operator chooses to create the Nth best-path
 plane redundantly by installing not one, but two or more route
 reflectors serving each additional plane, the additional robustness
 will be achieved.
 As a result of this solution, ASBR3 and other ASBRs peering to RR'
 will be receiving the second best path.
 Similarly to Section 4.1, as an alternative to fully meshing all RRs
 and diverse path RRs', operators may choose to peer newly introduced
 RRs' to a hierarchical RR', which would be an IBGP interconnect point
 between planes.
 It is recommended that an implementation advertise the overall best
 path over the Nth diverse-path session if there is no other BGP path
 with a different next hop present.  This is equivalent to today's
 case where the client is connected to more than one RR.

4.3. Multi-Plane Route Servers for Internet Exchanges

 Another group of devices in which the proposed multi-plane
 architecture may be of particular applicability is the EBGP route
 servers used at many Internet exchange points.

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 In such cases, hundreds of ISPs are interconnected on a common LAN.
 Instead of having hundreds of direct EBGP sessions on each exchange
 client, a single peering is created to the transparent route server.
 The route server can only propagate a single best path.  Mandating
 the upgrade for hundreds of different service providers in order to
 implement add-path may be much more difficult as compared to asking
 them to provision one new EBGP session to an Nth best path route
 server plane.  This allows the distribution of more than the single
 best BGP path from a given route server to such an Internet exchange
 point (IX) peer.
 The solution proposed in this document fits very well with the
 requirement of having broader EBGP path diversity among the members
 of any Internet exchange point.

5. Discussion on Current Models of IBGP Route Distribution

 In today's networks, BGP4 operates as specified in [RFC4271].
 There are a number of technology choices for intra-AS BGP route
 distribution:
 1.  Full mesh
 2.  Confederations
 3.  Route reflectors

5.1. Full Mesh

 A full mesh, the most basic IBGP architecture, exists when all BGP
 speaking routers within the AS peer directly with all other BGP
 speaking routers within the AS, irrespective of where a given router
 resides within the AS (e.g., P router, PE router, etc.).
 While this is the simplest intra-domain path-distribution method,
 historically, there have been a number of challenges in realizing
 such an IBGP full mesh in a large-scale network.  While some of these
 challenges are no longer applicable, the following (as well as
 others) may still apply:
 1.  Number of TCP sessions: The number of IBGP sessions on a single
     router in a full-mesh topology of a large-scale service provider
     can easily reach hundreds.  Such numbers could be a concern on
     hardware and software used in the late 70s, 80s, and 90s.  Today,
     customer requirements for the number of BGP sessions per box are
     reaching thousands.  This is already an order of magnitude more
     than the potential number of IBGP sessions.  Advancements in the

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     hardware and software used in production routers means that
     running a full mesh of IBGP sessions should not be dismissed due
     to the resulting number of TCP sessions alone.
 2.  Provisioning: When operating and troubleshooting large networks,
     one of the topmost requirements is to keep the design as simple
     as possible.  When the autonomous system's network is composed of
     hundreds of nodes, it becomes very difficult to manually
     provision a full mesh of IBGP sessions.  Adding or removing a
     router requires reconfiguration of all other routers in the AS.
     While this is a real concern today, there is already work in
     progress in the IETF to define IBGP peering automation through an
     IBGP Auto Discovery mechanism [AUTO-MESH].
 3.  Number of paths: Another concern when deploying a full IBGP mesh
     is the number of BGP paths for each route that have to be stored
     at every node.  This number is very tightly related to the number
     of external peerings of an AS, the use of LOCAL_PREF or MED
     techniques, and the presence of best-external [EXT-PATH]
     advertisement configuration.  If we make a rough assumption that
     the BGP4-path data structure consumes about 80-100 bytes, the
     resulting control-plane memory requirement for 500,000 IPv4
     routes with one additional external path is 38-48 MB, while for 1
     million IPv4 routes, it grows linearly to 76-95 MB.  It is not
     possible to reach a general conclusion if this condition is
     negligible or if it is a show stopper for a full-mesh deployment
     without direct reference to a given network.
 To summarize, a full-mesh IBGP peering can offer natural
 dissemination of multiple external paths among BGP speakers.  When
 realized with the help of IBGP Auto Discovery peering automation,
 this seems like a viable deployment, especially in medium- and small-
 scale networks.

5.2. Confederations

 For the purpose of this document, let's observe that confederations
 [RFC5065] can be viewed as a hierarchical full-mesh model.
 Within each sub-AS, BGP speakers are fully meshed, and as discussed
 in Section 2.1, all full-mesh characteristics (number of TCP
 sessions, provisioning, and potential concern over number of paths
 still apply in the sub-AS scale).

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 In addition to the direct peering of all BGP speakers within each
 sub-AS, all sub-AS border routers must also be fully meshed with each
 other.  Sub-AS border routers configured with best-external
 functionality can inject additional (diverse) paths within a sub-AS.
 To summarize, it is technically sound to use confederations with the
 combination of best-external to achieve distribution of more than a
 single best path per route in a large autonomous systems.
 In topologies where route reflectors are deployed within the
 confederation sub-ASes, the technique described here applies.

5.3. Route Reflectors

 The main motivation behind the use of route reflectors [RFC4456] is
 the avoidance of the full-mesh session management problem described
 above.  Route reflectors, for good or for bad, are the most common
 solution today for interconnecting BGP speakers within an internal
 routing domain.
 Route-reflector peerings follow the advertisement rules defined by
 the BGP4 protocol.  As a result, only a single best path per prefix
 is sent to client BGP peers.  This is the main reason many current
 networks are exposed to a phenomenon called BGP path starvation,
 which essentially results in the inability to deliver a number of
 applications discussed later.
 When interconnecting BGP speakers between domains, the route
 reflection equivalent is popularly called the "Route Server" and is
 globally deployed today in many Internet exchange points.

6. Deployment Considerations

 Distribution of the diverse-BGP-paths proposal allows the
 dissemination of more paths than just the best path to the route-
 reflector or route-server clients of today's BGP4 implementations.
 As a deployment recommendation, it needs to be mentioned that fast
 connectivity restoration as well as a majority of intra-domain BGP-
 level load balancing needs can be accommodated with only two paths
 (overall best and second best).  Therefore, as a deployment
 recommendation, this document suggests use of N=2 with diverse-path.
 From the client's point of view, receiving additional paths via
 separate IBGP sessions terminated at the new route-reflector plane is
 functionally equivalent to constructing a full-mesh peering without
 the problems such a full mesh would come with, as discussed in
 earlier section.

Raszuk, et al. Informational [Page 15] RFC 6774 Diverse-BGP-Path Distribution November 2012

 By precisely defining the number of reflector planes, network
 operators have full control over the number of redundant paths in the
 network.  This number can be defined to address the needs of the
 service(s) being deployed.
 The Nth-plane route reflectors should act as control-plane network
 entities.  While they can be provisioned on the current production
 routers, selected Nth-best BGP paths should not be used directly in
 the date plane with the exception of such paths being BGP multipath
 eligible and such functionality is enabled.  Regarding RRs being in
 the data plane unless multipath is enabled, the second best path is
 expected to be a backup path and should be installed as such into the
 local RIB/FIB.
 The use of the term "planes" in this document is more of a conceptual
 nature.  In practice, all paths are still kept in the single table
 where normal best path is calculated.  This means that tools like the
 looking glass should not observe any changes or impact when
 diverse-path has been enabled.
 The proposed architecture deployed along with the BGP best-external
 functionality covers all three cases where the classic BGP route-
 reflection paradigm would fail to distribute alternate (diverse)
 paths.  These are
 1.  ASBRs advertising their single best-external paths with no
     LOCAL_PREF or MED present.
 2.  ASBRs advertising their single best-external paths with
     LOCAL_PREF or MED present and with BGP best-external
     functionality enabled.
 3.  ASBRs with multiple external paths.
 This section focuses on discussion of case 3 above in more detail.
 This describes the scenario of a single ASBR connected to multiple
 EBGP peers.  In practice, this peering scenario is quite common.  It
 is mostly due to the geographic location of EBGP peers and the
 diversity of those peers (for example, peering to multiple tier-1
 ISPs, etc.).  It is not designed for failure-recovery scenarios, as
 single failure of the ASBR would simultaneously result in loss of
 connectivity to all of the peers.  In most medium and large
 geographically distributed networks, there is always another ASBR or
 multiple ASBRs providing peering backups, typically in other
 geographically diverse locations in the network.

Raszuk, et al. Informational [Page 16] RFC 6774 Diverse-BGP-Path Distribution November 2012

 When an operator uses ASBRs with multiple peerings, setting next-hop
 self will effectively allow local repair of the atomic failure of any
 external peer without any compromise to the data plane.
 Traditionally, the most common reason for not setting next-hop self
 is the associated drawback of losing the ability to signal the
 external failures of peering ASBRs or links to those ASBRs by fast
 IGP flooding.  Such a potential drawback can be easily avoided by
 using a different peering address from the address used for next-hop
 mapping and removing the next-hop from the IGP at the last possible
 BGP path failure.
 Herein, one may correctly observe that in the case of setting next-
 hop self on an ASBR, attributes of other external paths such that the
 ASBR is peering with may be different from the attributes of its best
 external path.  Therefore, not injecting all of those external paths
 with their corresponding attributes cannot be compared to equivalent
 paths for the same prefix coming from different ASBRs.
 While such observation, in principle, is correct, one should put
 things in perspective of the overall goal, which is to provide data-
 plane connectivity upon a single failure with minimal
 interruption/packet loss.  During such transient conditions, using
 even potentially suboptimal exit points is reasonable, so long as
 forwarding information loops are not introduced.  In the mean time,
 the BGP control plane will on its own re-advertise the newly elected
 best external path, and route-reflector planes will calculate their
 Nth best paths and propagate them to its clients.  The result is that
 after seconds, even if potential suboptimality were encountered, it
 will be quickly and naturally healed.

7. Summary of Benefits

 Distribution of the diverse-BGP-paths proposal provides the following
 benefits when compared to the alternatives:
 1.  No modifications to the BGP4 protocol.
 2.  No requirement for upgrades to edge and core routers (as required
     in [ADD-PATHS]).  It is backward compatible with the existing BGP
     deployments.
 3.  Can be easily enabled by the introduction of a new route
     reflector, a route server plane dedicated to the selection and
     distribution of Nth best-path, or just by new configuration of
     the upgraded current route reflector(s).

Raszuk, et al. Informational [Page 17] RFC 6774 Diverse-BGP-Path Distribution November 2012

 4.  Does not require major modification to BGP implementations in the
     entire network, which would result in an unnecessary increase of
     memory and CPU consumption due to the shift from today's per-
     prefix to a per-path advertisement state tracking.
 5.  Can be safely deployed gradually on an RR cluster basis.
 6.  The proposed solution is equally applicable to any BGP address
     family as described in "Multiprotocol Extensions for BGP-4"
     [RFC4760].  In particular, it can be used "as is" without any
     modifications to both IPv4 and IPv6 address families.

8. Applications

 This section lists the most common applications that require the
 presence of redundant BGP paths:
 1.  Fast connectivity restoration in which backup paths with
     alternate exit points would be pre-installed as well as
     pre-resolved in the FIB of routers.  This allows for a local
     action upon reception of a critical event notification of
     network/node failure.  This failure recovery mechanism that is
     based on the presence of backup paths is also suitable for
     gracefully addressing scheduled maintenance requirements as
     described in [BGP-SHUTDOWN].
 2.  Multi-path load balancing for both IBGP and EBGP.
 3.  BGP control-plane churn reduction for both intra-domain and
     inter-domain.
 An important point to observe is that all of the above intra-domain
 applications are based on the use of reflector planes but are also
 applicable in the inter-domain Internet exchange point examples.  As
 discussed in Section 4.3, an Internet exchange can conceptually
 deploy shadow route server planes, each responsible for distribution
 of an Nth best path to its EBGP peers.  In practice, it may just be
 equal to a new short configuration and establishment of new BGP
 sessions to IX peers.

Raszuk, et al. Informational [Page 18] RFC 6774 Diverse-BGP-Path Distribution November 2012

9. Security Considerations

 The new mechanism for diverse BGP path dissemination proposed in this
 document does not introduce any new security concerns as compared to
 the base BGP4 specification [RFC4271] and especially when compared
 against full-IBGP-mesh topology.
 In addition, the authors observe that all BGP security issues as
 described in [RFC4272] apply to the additional BGP session or
 sessions as recommended by this specification.  Therefore, all
 recommended mitigation techniques to BGP security are applicable
 here.

10. Contributors

 The following people contributed significantly to the content of the
 document:
 Selma Yilmaz
 Cisco Systems
 170 West Tasman Drive
 San Jose, CA 95134
 US
 Email: seyilmaz@cisco.com
 Satish Mynam
 Juniper Networks
 1194 N. Mathilda Ave
 Sunnyvale, CA 94089
 US
 Email: smynam@juniper.net
 Isidor Kouvelas
 Cisco Systems
 170 West Tasman Drive
 San Jose, CA 95134
 US
 Email: kouvelas@cisco.com

Raszuk, et al. Informational [Page 19] RFC 6774 Diverse-BGP-Path Distribution November 2012

11. Acknowledgments

 The authors would like to thank Bruno Decraene, Bart Peirens, Eric
 Rosen, Jim Uttaro, Renwei Li, Wes George, and Adrian Farrel for their
 valuable input.
 The authors would also like to express a special thank you to a
 number of operators who helped optimize the provided solution to be
 as close as possible to their daily operational practices.  In
 particular, many thanks to Ted Seely, Shane Amante, Benson
 Schliesser, and Seiichi Kawamura.

12. References

12.1. Normative References

 [RFC4271]   Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "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.
 [RFC4760]   Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
             "Multiprotocol Extensions for BGP-4", RFC 4760, January
             2007.
 [RFC5226]   Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 5226,
             May 2008.

12.2. Informative References

 [ADD-PATHS] Walton, D., Chen, E., Retana, A., and J. Scudder,
             "Advertisement of Multiple Paths in BGP", Work in
             Progress, June 2012.
 [AUTO-MESH] Raszuk, R., "IBGP Auto Mesh", Work in Progress, January
             2004.
 [BGP-SHUTDOWN]
             Decraene, B., Francois, P., Pelsser, C., Ahmad, Z., and
             A.  Armengol, "Requirements for the Graceful Shutdown of
             BGP Sessions", Work in Progress, September 2009.

Raszuk, et al. Informational [Page 20] RFC 6774 Diverse-BGP-Path Distribution November 2012

 [EXT-PATH]  Marques, P., Fernando, R., Chen, E., Mohapatra, P., and
             H. Gredler, "Advertisement of the Best External Route in
             BGP", Work in Progress, January 2012.
 [FAST-CONN] Mohapatra, P., Fernando, R., Filsfils, C., and R. Raszuk,
             "Fast Connectivity Restoration Using BGP Add-path", Work
             in Progress), October 2011.
 [RFC3345]   McPherson, D., Gill, V., Walton, D., and A. Retana,
             "Border Gateway Protocol (BGP) Persistent Route
             Oscillation Condition", RFC 3345, August 2002.
 [RFC4272]   Murphy, S., "BGP Security Vulnerabilities Analysis", RFC
             4272, January 2006.
 [RFC5065]   Traina, P., McPherson, D., and J. Scudder, "Autonomous
             System Confederations for BGP", RFC 5065, August 2007.

Raszuk, et al. Informational [Page 21] RFC 6774 Diverse-BGP-Path Distribution November 2012

Authors' Addresses

 Robert Raszuk (editor)
 NTT MCL
 101 S Ellsworth Avenue Suite 350
 San Mateo, CA 94401
 United States
 EMail: robert@raszuk.net
 Rex Fernando
 Cisco Systems
 170 West Tasman Drive
 San Jose, CA 95134
 United States
 EMail: rex@cisco.com
 Keyur Patel
 Cisco Systems
 170 West Tasman Drive
 San Jose, CA 95134
 United States
 EMail: keyupate@cisco.com
 Danny McPherson
 Verisign, Inc.
 12061 Bluemont Way
 Reston, VA  20190
 United States
 EMail: dmcpherson@verisign.com
 Kenji Kumaki
 KDDI Corporation
 Garden Air Tower
 Iidabashi, Chiyoda-ku, Tokyo 102-8460
 Japan
 EMail: ke-kumaki@kddi.com

Raszuk, et al. Informational [Page 22]

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