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rfc:rfc6198

Internet Engineering Task Force (IETF) B. Decraene Request for Comments: 6198 France Telecom Category: Informational P. Francois ISSN: 2070-1721 UCL

                                                            C. Pelsser
                                                                   IIJ
                                                              Z. Ahmad
                                              Orange Business Services
                                                A.J. Elizondo Armengol
                                                        Telefonica I+D
                                                             T. Takeda
                                                                   NTT
                                                            April 2011
       Requirements for the Graceful Shutdown of BGP Sessions

Abstract

 The Border Gateway Protocol (BGP) is heavily used in Service Provider
 networks for both Internet and BGP/MPLS VPN services.  For resiliency
 purposes, redundant routers and BGP sessions can be deployed to
 reduce the consequences of an Autonomous System Border Router (ASBR)
 or BGP session breakdown on customers' or peers' traffic.  However,
 simply taking down or even bringing up a BGP session for maintenance
 purposes may still induce connectivity losses during the BGP
 convergence.  This is no longer satisfactory for new applications
 (e.g., voice over IP, online gaming, VPN).  Therefore, a solution is
 required for the graceful shutdown of a (set of) BGP session(s) in
 order to limit the amount of traffic loss during a planned shutdown.
 This document expresses requirements for such a solution.

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/rfc6198.

Decraene, et al. Informational [Page 1] RFC 6198 Reqs for Graceful BGP Shutdown April 2011

Copyright Notice

 Copyright (c) 2011 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. Conventions Used in This Document ...............................3
 3. Problem Statement ...............................................4
    3.1. Example of Undesirable BGP Routing Behavior ................4
    3.2. Causes of Packet Loss ......................................5
 4. Terminology .....................................................6
 5. Goals and Requirements ..........................................7
 6. Security Considerations ........................................10
 7. References .....................................................10
    7.1. Normative References ......................................10
    7.2. Informative References ....................................10
 Acknowledgments ...................................................11
 Appendix A. Reference BGP Topologies ..............................12
    A.1. EBGP Topologies ...........................................12
    A.2. IBGP Topologies ...........................................15
    A.3. Routing Decisions .........................................19

1. Introduction

 The Border Gateway Protocol (BGP) [RFC4271] is heavily used in
 Service Provider networks for both Internet and BGP/MPLS VPN services
 [RFC4364].  For resiliency purposes, redundant routers and BGP
 sessions can be deployed to reduce the consequences of an Autonomous
 System Border Router (ASBR) or BGP session breakdown on customers' or
 peers' traffic.
 We place ourselves in the context where a Service Provider performs a
 maintenance operation and needs to shut down one or multiple BGP
 peering link(s) or a whole ASBR.  If an alternate path is available
 within the Autonomous System (AS), the requirement is to avoid or
 reduce customer or peer traffic loss during the BGP convergence.

Decraene, et al. Informational [Page 2] RFC 6198 Reqs for Graceful BGP Shutdown April 2011

 Indeed, as an alternate path is available in the AS, it should be
 made possible to reroute the customer or peer traffic on this backup
 path before the BGP session(s) is/are torn down, the nominal path
 withdrawn, and the forwarding stopped.
 The requirements also cover the subsequent re-establishment of the
 BGP session as even this "UP" case can currently trigger route loss,
 and thus traffic loss, at some routers.
 BGP [RFC4271] and MP-BGP [RFC4760] do not currently have a mechanism
 to gracefully migrate traffic from one BGP next-hop to another
 without interrupting the flow of traffic.  When a BGP session is
 taken down, BGP behaves as if there were a sudden link or router
 failure and withdraws the prefixes learned over that session, which
 may trigger traffic loss.  While still being advertised as reachable,
 there is no mechanism to advertise to its BGP peers that the prefix
 will soon be unreachable.  When applicable, such mechanism would
 reduce or prevent traffic loss.  It would typically be applicable in
 case of a maintenance operation requiring the shutdown of a
 forwarding resource.  Typical examples would be a link or line card
 maintenance, replacement, or upgrade.  It may also be applicable for
 a software upgrade, as it may involve a firmware reset on the line
 cards and hence forwarding interruption.
 The introduction of route reflectors (RRs) as per [RFC4456] to solve
 scalability issues bound to Internal BGP (IBGP) full-meshes has
 worsened the duration of routing convergence as some route reflectors
 may hide the backup path.  Thus, depending on RR topology, more IBGP
 hops may be involved in the IBGP convergence.
 Note that these planned maintenance operations cannot be addressed by
 Graceful Restart (GR) extensions [RFC4724] as GR only applies when
 the forwarding is preserved during the control plane restart.  On the
 contrary, graceful shutdown applies when the forwarding is
 interrupted.
 Also, note that some protocols are already considering such a
 graceful shutdown procedure (e.g., GMPLS in [RFC5817]).
 A metric of success is the degree to which such a mechanism
 eliminates traffic loss during maintenance operations.

2. Conventions Used in This Document

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

Decraene, et al. Informational [Page 3] RFC 6198 Reqs for Graceful BGP Shutdown April 2011

3. Problem Statement

 As per [RFC4271], when one (or many) BGP session(s) are shut down, a
 BGP NOTIFICATION message is sent to the peer and the session is then
 closed.  A protocol convergence is then triggered both by the local
 router and by the peer.  Alternate paths to the destination are
 selected, if known.  If those alternate paths are not known prior to
 the BGP session shutdown, additional BGP convergence steps are
 required in each AS to search for an alternate path.
 This behavior is not satisfactory in a maintenance situation because
 the traffic that was directed towards the removed next-hops may be
 lost until the end of the BGP convergence.  As it is a planned
 operation, a make-before-break solution should be made possible.
 As maintenance operations are frequent in large networks [Reliable],
 the global availability of the network is significantly impaired by
 this BGP maintenance issue.

3.1. Example of Undesirable BGP Routing Behavior

 To illustrate these problems, let us consider the following simple
 example where one customer router "CUST" is dual-attached to two
 Service Providers' routers, "ASBR1" and "ASBR2".
 ASBR1 and ASBR2 are in the same AS and are owned by the same Service
 Provider.  Both are IBGP clients of the route reflector R1.
                      '
                AS1   '      AS2
                      '
                /-----------ASBR1---
               /                     \
              /                       \
          CUST                         R1
              \                       /
       Z/z     \                     /
                \-----------ASBR2---
                      '
                AS1   '      AS2
                      '
       Figure 1. Dual-Attached Customer

Decraene, et al. Informational [Page 4] RFC 6198 Reqs for Graceful BGP Shutdown April 2011

 Before the maintenance, packets for destination Z/z use the ASBR1-
 CUST link because R1 selects ASBR1's route based on the IGP cost.
 Let's assume the Service Provider wants to shut down the ASBR1-CUST
 link for maintenance purposes.  Currently, when the shutdown is
 performed on ASBR1, the following steps are performed:
    1. ASBR1 withdraws its prefix Z/z to its route reflector, R1.
    2. R1 runs its decision process, selects the route from ASBR2, and
       advertises the new path to ASBR1.
    3. ASBR1 runs its decision process and recovers the reachability
       of Z/z.
 Traffic is lost at step 1 when ASBR1 looses its route until step 3
 when it discovers a new path.
 Note that this is a simplified description for illustrative purposes.
 In a bigger AS, multiple steps of BGP convergence may be required to
 find and select the best alternate path (e.g., ASBR1 may be chosen
 based on a higher LOCAL_PREF, hierarchical route reflectors may be
 used, etc.).  When multiple BGP routers are involved and plenty of
 prefixes are affected, the recovery process can take longer than
 application requirements.

3.2. Causes of Packet Loss

 The loss of packets during maintenance has two main causes:
  1. lack of an alternate path on some routers, and
  1. transient routing inconsistency.
 Some routers may lack an alternate path because another router is
 hiding the backup path.  This router can be:
  1. a route reflector only propagating its best path.
  1. the backup ASBR not advertising the backup path because it

prefers the nominal path.

 This lack of knowledge regarding the alternate path is the first
 target of this requirements document.
 Transient routing inconsistencies happen during IBGP convergence
 because routers do not simultaneously update their Routing
 Information Bases (RIBs) and hence do not simultaneously update their

Decraene, et al. Informational [Page 5] RFC 6198 Reqs for Graceful BGP Shutdown April 2011

 Forwarding Information Bases (FIBs) entries.  This can lead to
 forwarding loops, which result in both link congestion and packet
 drops.  The duration of these transient micro-loops is dependent on
 the IBGP topology (e.g., number of route reflectors between ingress
 and egress ASBR), implementation differences among router platforms
 (which result in differences in the time taken to update specific
 prefix in the FIB), and forwarding mode (hop-by-hop IP forwarding
 versus tunneling).
 Note that when an IP lookup is only performed on entry to the AS, for
 example, prior to entry into a tunnel across the AS, micro-loops will
 not occur.  An example of this is when BGP is being used as the
 routing protocol for MPLS VPN as defined in [RFC4364].
 Note that [RFC5715] defines a framework for loop-free convergence.
 It has been written in the context of IP fast reroute for link state
 IGP [RFC5714], but some concepts are also of interest for BGP
 convergence.

4. Terminology

 g-shut: Graceful shutdown.  A method for explicitly notifying the BGP
 routers that a BGP session (and hence the prefixes learned over that
 session) is going to be disabled.
 g-noshut: Graceful no shutdown.  A method for explicitly notifying
 the BGP routers that a BGP session (and hence the prefixes learned
 over that session) is going to be enabled.
 g-shut initiator: the router on which the session(s) shutdown(s) is
 (are) performed for maintenance.
 g-shut neighbor: a router that peers with the g-shut initiator via
 (one of) the session(s) undergoing maintenance.
 affected prefixes: a prefix initially reached via the peering link(s)
 undergoing maintenance.
 affected router: a router reaching an affected prefix via a peering
 link undergoing maintenance.
 initiator AS: the autonomous system of the g-shut initiator router.
 neighbor AS(es): the autonomous system(s) of the g-shut neighbor
 router(s).

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5. Goals and Requirements

 Currently, when a BGP session of the router under maintenance is shut
 down, the router removes the routes and then triggers the BGP
 convergence on its BGP peers by withdrawing its route.
 The goal of BGP graceful shutdown of a (set of) BGP session(s) is to
 minimize traffic loss during a planned shutdown.  Ideally, a solution
 should reduce this traffic loss to zero.
 Another goal is to minimize and, preferably, to eliminate packet loss
 when the BGP session is re-established following the maintenance.
 As the event is known in advance, a make-before-break solution can be
 used in order to initiate the BGP convergence, find and install the
 alternate paths before the nominal paths are removed.  As a result,
 before the nominal BGP session is shut down, all affected routers
 learn and use the alternate paths.  Those alternate paths are
 computed by BGP, taking into account the known status of the network,
 which includes known failures that the network is processing
 concurrently with the BGP session graceful shutdown and possibly
 other known graceful shutdowns under way.  Therefore, multiple BGP
 graceful shutdowns overlapping within a short time frame are
 gracefully handled.  Indeed, a given graceful shutdown takes into
 account all previous ones.
 As a result, provided an alternate path with enough remaining
 capacity is available, the packets are rerouted before the BGP
 session termination and fewer packets (possibly none) are lost during
 the BGP convergence process since, at any time, all routers have a
 valid path.
 From the above goals, we can derive the following requirements:
 a)   A mechanism to advertise the maintenance action to all affected
      routers is REQUIRED.  Such a mechanism may be either implicit or
      explicit.  Note that affected routers can be located both in the
      local AS and in neighboring ASes.  Note also that the
      maintenance action can either be the shutdown of a BGP session
      or the establishment of a BGP session.
      The mechanism SHOULD allow BGP routers to minimize and,
      preferably, eliminate packet loss when a path is removed or
      advertised.  In particular, it SHOULD be ensured that the old
      path is not removed from the routing tables of the affected
      routers before the new path is known.

Decraene, et al. Informational [Page 7] RFC 6198 Reqs for Graceful BGP Shutdown April 2011

      The solution mechanism MUST significantly reduce and, ideally,
      eliminate packet loss.  A trade-off may be made between the
      degree of packet loss and the simplicity of the solution.
 b)   An Internet-wide convergence is OPTIONAL.  However, if the
      initiator AS and the neighbor AS(es) have a backup path, they
      SHOULD be able to gracefully converge before the nominal path is
      shut down.
 c)   The proposed solution SHOULD be applicable to any kind of BGP
      sessions (External BGP (EBGP), IBGP, IBGP route reflector
      client, EBGP confederations, EBGP multi hop, MultiProtocol BGP
      extension, etc.) and any address family.  If a BGP
      implementation allows the closing or enabling of a subset of
      Address Family Identifiers (AFIs) carried in an MP-BGP session,
      this mechanism MAY be applicable to this subset of AFIs.
      Depending on the kind of session, there may be some variations
      in the proposed solution in order to fulfill the requirements.
      The following cases should be handled in priority:
  1. The shutdown of an inter-AS link and therefore the shutdown of

an EBGP session;

  1. The shutdown of an ASBR and therefore the shutdown of all its

BGP sessions.

      Service Providers and platforms implementing a graceful shutdown
      solution should note that in BGP/MPLS VPN as per [RFC4364], the
      Provider Edge - Customer Edge (PE-CE) routing can be performed
      by protocols other than BGP (e.g., static routes, RIPv2, OSPF,
      IS-IS).  This is out of scope of this document.
 d)   The proposed solution SHOULD NOT change the BGP convergence
      behavior for the ASes exterior to the maintenance process,
      namely, ASes other than the initiator AS and its neighbor
      AS(es).
 e)   An incremental deployment on a per-AS or per-BGP session basis
      MUST be made possible.  In case of partial deployment, the
      proposed solution SHOULD incrementally improve the maintenance
      process.  It should be noted that in an inter-domain relation,
      one AS may have more incentive to use graceful shutdown than the
      other.  Similarly, in a BGP/MPLS VPN environment, it's much
      easier to upgrade the PE routers than the CE ones, mainly
      because there is at least an order of magnitude more CE and CE
      locations than PE and PE locations.  As a consequence, when

Decraene, et al. Informational [Page 8] RFC 6198 Reqs for Graceful BGP Shutdown April 2011

      splitting the cost of the solution between the g-shut initiator
      and the g-shut neighbor, the solution SHOULD favor a low-cost
      solution on the neighbor AS side in order to reduce the impact
      on the g-shut neighbor.  Impact should be understood as a
      generic term that includes first hardware, then software, then
      configuration upgrade.
 f)   Redistribution or advertisement of (static) IP routes into BGP
      SHOULD also be covered.
 g)   The proposed solution MAY be designed in order to avoid
      transient forwarding loops.  Indeed, forwarding loops increase
      packet transit-delay and may lead to link saturation.
 h)   The specific procedure SHOULD end when the BGP session is closed
      following the g-shut and once the BGP session is gracefully
      opened following the g-noshut.  In the end, once the planned
      maintenance is finished, the nominal BGP routing MUST be re-
      established.  The duration of the g-shut procedure, and hence
      the time before the BGP session is safely closed, SHOULD be
      discussed by the solution document.  Examples of possible
      solutions are the use of a pre-configured timer, the use of a
      message to signal the end of the BGP convergence, or the
      monitoring of the traffic on the g-shut interface.
 i)   The solution SHOULD be simple and simple to operate.  Hence, it
      MAY only cover a subset of the cases.  As a consequence, most of
      the above requirements are expressed as "SHOULD" rather than
      "MUST".
      The metrics to evaluate and compare the proposed solutions are:
  1. The duration of the remaining loss of connectivity when the

BGP session is brought down or up;

  1. The applicability to a wide range of BGP and network

topologies;

  1. The simplicity;
  1. The duration of transient forwarding loops;
  1. The additional load introduced in BGP (e.g., BGP messages sent

to peer routers, peer ASes, the Internet).

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6. Security Considerations

 At the requirements stage, this graceful shutdown mechanism is not
 expected to affect the security of the BGP protocol, especially if it
 can be kept simple.  No new sessions are required and the additional
 ability to signal the graceful shutdown is not expected to bring
 additional attack vectors, as BGP neighbors already have the ability
 to send incorrect or misleading information or even shut down the
 session.
 Security considerations MUST be addressed by the proposed solutions.
 In particular, they SHOULD address the issues of bogus g-shut
 messages and how they would affect the network(s), as well as the
 impact of hiding a g-shut message so that g-shut is not performed.
 The solution SHOULD NOT increase the ability of one AS to selectively
 influence routing decision in the peer AS (inbound Traffic
 Engineering) outside of the case of the BGP session shutdown.
 Otherwise, the peer AS SHOULD have means to detect such behavior.

7. References

7.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
            Border Gateway Protocol 4 (BGP-4)", RFC 4271, January
            2006.
 [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
            "Multiprotocol Extensions for BGP-4", RFC 4760, January
            2007.
 [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
            Reflection: An Alternative to Full Mesh Internal BGP
            (IBGP)", RFC 4456, April 2006.
 [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
            Networks (VPNs)", RFC 4364, February 2006.

7.2. Informative References

 [RFC5817]  Ali, Z., Vasseur, JP., Zamfir, A., and J. Newton,
            "Graceful Shutdown in MPLS and Generalized MPLS Traffic
            Engineering Networks", RFC 5817, April 2010.

Decraene, et al. Informational [Page 10] RFC 6198 Reqs for Graceful BGP Shutdown April 2011

 [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
            Convergence", RFC 5715, January 2010.
 [RFC5714]  Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
            5714, January 2010.
 [RFC4724]  Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
            Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
            January 2007.
 [Reliable] Network Strategy Partners, LLC. "Reliable IP Nodes: A
            prerequisite to profitable IP services", November 2002.
            http://www.nspllc.com/NewPages/Reliable_IP_Nodes.pdf

Acknowledgments

 The authors would like to thank Nicolas Dubois, Benoit Fondeviole,
 Christian Jacquenet, Olivier Bonaventure, Steve Uhlig, Xavier Vinet,
 Vincent Gillet, Jean-Louis le Roux, Pierre Alain Coste, and Ronald
 Bonica for their useful discussions on this subject, review, and
 comments.
 This document has been partly sponsored by the European project IST
 AGAVE.

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Appendix A. Reference BGP Topologies

 This section describes some frequent BGP topologies used both within
 the AS (IBGP) and between ASes (EBGP).  Solutions should be
 applicable to the following topologies and their combinations.

A.1. EBGP Topologies

 This section describes some frequent BGP topologies used between
 ASes.  In each figure, a line represents a BGP session.

A.1.1. One ASBR in AS1 Connected to Two ASBRs in the Neighboring AS2

 In this topology, we have an asymmetric protection scheme between AS1
 and AS2:
  1. On the AS2 side, two different routers are used to connect to

AS1.

  1. On the AS1 side, one single router with two BGP sessions is

used.

                  '
            AS1   '      AS2
                  '
            /----------- ASBR2.1
           /      '
          /       '
       ASBR1.1    '
          \       '
           \      '
            \----------- ASBR2.2
                  '
                  '
        AS1       '      AS2
                  '
       Figure 2. EBGP Topology with Redundant ASBR in One of the ASes
 BGP graceful shutdown is expected to be applicable for the
 maintenance of:
  1. one of the routers of AS2;
  1. one link between AS1 and AS2, performed either on an AS1 or AS2

router.

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 Note that in the case of maintenance of the whole router, all its BGP
 sessions need to be gracefully shutdown at the beginning of the
 maintenance and gracefully brought up at the end of the maintenance.

A.1.2. Two ASBRs in AS1 Connected to Two ASBRs in AS2

 In this topology, we have a symmetric protection scheme between AS1
 and AS2: on both sides, two different routers are used to connect AS1
 to AS2.
                    '
              AS1   '      AS2
                    '
       ASBR1.1----------- ASBR2.1
                    '
                    '
                    '
                    '
                    '
       ASBR1.2----------- ASBR2.2
                    '
          AS1       '      AS2
                    '
       Figure 3. EBGP Topology with Redundant ASBRs in Both ASes
 BGP graceful shutdown is expected to be applicable for the
 maintenance of:
  1. any of the ASBR routers (in AS1 or AS2);
  1. one link between AS1 and AS2, performed either on an AS1 or AS2

router.

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A.1.3. Two ASBRs in AS2 Each Connected to Two Different ASes

 In this topology, at least three ASes are involved.
                      '
                AS1   '      AS2
                      '
         ASBR1.1----------- ASBR2.1
            |         '
            |         '
       '''''|''''''''''
            |         '
            |         '
         ASBR3.1----------- ASBR2.2
                      '
            AS3       '      AS2
       Figure 4. EBGP Topology of a Dual-Homed Customer
 As the requirement expressed in Section 5 is to advertise the
 maintenance only within the initiator and neighbor ASes, not
 Internet-wide, BGP graceful shutdown solutions may not be applicable
 to this topology.  Depending on which routes are exchanged between
 these ASes, some protection for some of the traffic may be possible.
 For instance, if ASBR2.2 performs a maintenance affecting ASBR3.1,
 then ASBR3.1 will be notified.  However, ASBR1.1 may not be notified
 of the maintenance of the EBGP session between ASBR3.1 and ASBR2.2.

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A.2. IBGP Topologies

 This section describes some frequent BGP topologies used within an
 AS.  In each figure, a line represents a BGP session.

A.2.1. IBGP Full-Mesh

 In this topology, we have a full-mesh of IBGP sessions:
          P1 ----- P2
          | \    / |
          |  \  /  |
          |   \/   |     AS1
          |   /\   |
          |  /  \  |
          | /    \ |
        ASBR1.1--ASBR1.2
           \       /
            \     /
       ''''''\'''/''''''''''''
              \ /      AS2
             ASBR2.1
       Figure 5. IBGP Full-Mesh
 When the session between ASBR1.1 and ASBR2.1 is gracefully shut down,
 it is required that all affected routers of AS1 reroute traffic to
 ASBR1.2 before the session between ASBR1.1 and ASBR2.1 is shut down.
 Similarly, when the session between ASBR1.1 and ASBR2.1 is gracefully
 brought up, all affected routers of AS1 preferring ASBR1.1 over
 ASBR1.2 need to reroute traffic to ASBR1.1 before the less preferred
 path through ASBR1.2 is possibly withdrawn.

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A.2.2. Route Reflector

 In this topology, route reflectors are used to limit the number of
 IBGP sessions.  There is a single level of route reflectors and the
 route reflectors are fully meshed.
          P1 (RR)-- P2 (RR)
          | \      / |
          |  \    /  |
          |   \  /   |     AS1
          |    \/    |
          |    /\    |
          |   /  \   |
          |  /    \  |
          | /      \ |
        ASBR1.1    ASBR1.2
           \          /
            \        /
       ''''''\''''''/''''''''''''
              \    /
               \  /         AS2
              ASBR2.1
       Figure 6. Route Reflector
 When the session between ASBR1.1 and ASBR2.1 is gracefully shut down,
 all BGP routers of AS1 need to reroute traffic to ASBR1.2 before the
 session between ASBR1.1 and ASBR2.1 is shut down.
 Similarly, when the session between ASBR1.1 and ASBR2.1 is gracefully
 brought up, all affected routers of AS1 preferring ASBR1.1 over
 ASBR1.2 need to reroute traffic to ASBR1.1 before the less preferred
 path through ASBR1.2 is possibly withdrawn.

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A.2.3. Hierarchical Route Reflector

 In this topology, hierarchical route reflectors are used to limit the
 number of IBGP sessions.  There could be more than two levels of
 route reflectors and the top-level route reflectors are fully meshed.
       P1 (RR) --------  P2 (RR)
          |               |
          |               |
          |               |   AS1
          |               |
          |               |
        P3 (RR)          P4 (RR)
          |               |
          |               |
          |               |   AS1
          |               |
          |               |
        ASBR1.1         ASBR1.2
           \             /
            \           /
       ''''''\'''''''''/''''''''''''
              \       /
               \     /        AS2
               ASBR2.1
       Figure 7. Hierarchical Route Reflector
 When the session between ASBR1.1 and ASBR2.1 is gracefully shut down,
 all BGP routers of AS1 need to reroute traffic to ASBR1.2 before the
 session between ASBR1.1 and ASBR2.1 is shut down.
 Similarly, when the session between ASBR1.1 and ASBR2.1 is gracefully
 brought up, all affected routers of AS1 preferring ASBR1.1 over
 ASBR1.2 need to reroute traffic to ASBR1.1 before the less preferred
 path through ASBR1.2 is possibly withdrawn.

A.2.4. Confederations

 In this topology, a confederation of ASes is used to limit the number
 of IBGP sessions.  Moreover, RRs may be present in the member ASes of
 the confederation.

Decraene, et al. Informational [Page 17] RFC 6198 Reqs for Graceful BGP Shutdown April 2011

 Confederations may be run with different sub-options.  Regarding the
 IGP, each member AS can run its own IGP or they can all share the
 same IGP.  Regarding BGP, LOCAL_PREF may or may not cross the member
 AS boundaries.
 A solution should support the graceful shutdown and graceful bringing
 up of EBGP sessions between member ASes in the confederation in
 addition to the graceful shutdown and graceful bringing up of EBGP
 sessions between a member-AS and an AS outside of the confederation.
       ASBR1C.1 ---------- ASBR1C.2
          |                   |
          |                   |
          |       AS1C        |
          |                   |
          |                   |
       """|"""""""""""""""""""|"""
          |        "          |
        ASBR1A.2   "        ASBR1B.2
          |        "          |
          |        "          |
          |  AS1A  "   AS1B   |             AS1
          |        "          |
          |        "          |
        ASBR1A.1   "         ASBR1B.1
           \       "         /
            \      "        /
       ''''''\'''''''''''''/''''''''''''
              \           /
               \         /                   AS2
                 ASBR2.1
       Figure 8. Confederation
 In the above figure, member ASes AS1A, AS1B, and AS1C belong to a
 confederation of ASes in AS1.  AS1A and AS1B are connected to AS2.
 In normal operation, for the traffic toward AS2:
  1. AS1A sends the traffic directly to AS2 through ASBR1A.1.
  1. AS1B sends the traffic directly to AS2 through ASBR1B.1.
  1. AS1C load balances the traffic between AS1A and AS1B.
 When the session between ASBR1A.1 and ASBR2.1 is gracefully shut
 down, all BGP routers of AS1 need to reroute traffic to ASBR1B.1
 before the session between ASBR1A.1 and ASBR2.1 is shut down.

Decraene, et al. Informational [Page 18] RFC 6198 Reqs for Graceful BGP Shutdown April 2011

 Similarly, when the session between ASBR1A.1 and ASBR2.1 is
 gracefully brought up, all affected routers of AS1 preferring
 ASBR1A.1 over ASBR1B.1 need to reroute traffic to ASBR1A.1 before the
 less preferred path through ASBR1B.1 is possibly withdrawn.

A.3. Routing Decisions

 Here we describe some routing engineering choices that are frequently
 used in ASes and that should be supported by the solution.

A.3.1. Hot Potato (IGP Cost)

 The ingress router selects the nominal egress ASBR (AS exit point)
 based on the IGP cost to reach the BGP next-hop.

A.3.2. Cold Potato (BGP LOCAL_PREF)

 The ingress router selects the nominal egress ASBR based on the BGP
 LOCAL_PREF value set and advertised by the exit point.

A.3.3. Cold Potato (BGP Preference Set on Ingress)

 The ingress router selects the nominal egress ASBR based on
 preconfigured policy information.  (Typically, this is done by
 locally setting the BGP LOCAL_PREF based on the BGP communities
 attached on the routes).
 As per [RFC4271], note that if tunnels are not used to forward
 packets between the ingress and egress ASBR; this can lead to
 persistent forwarding loops.

Decraene, et al. Informational [Page 19] RFC 6198 Reqs for Graceful BGP Shutdown April 2011

Authors' Addresses

 Bruno Decraene
 France Telecom
 38-40 rue du General Leclerc
 92794 Issy Moulineaux cedex 9
 France
 EMail: bruno.decraene@orange-ftgroup.com
 Pierre Francois
 Universite catholique de Louvain
 Place Ste Barbe, 2
 Louvain-la-Neuve  1348
 BE
 EMail: francois@info.ucl.ac.be
 Cristel Pelsser
 Internet Initiative Japan
 Jinbocho Mitsui Building
 1-105 Kanda jinbo-cho
 Chiyoda-ku, Tokyo 101-0051
 Japan
 EMail: cristel@iij.ad.jp
 Zubair Ahmad
 Orange Business Services
 13775 McLearen Road, Oak Hill VA 20171
 USA
 EMail: zubair.ahmad@orange-ftgroup.com
 Antonio Jose Elizondo Armengol
 Division de Analisis Tecnologicos
 Technology Analysis Division
 Telefonica I+D
 C/ Emilio Vargas 6
 28043, Madrid
 EMail: ajea@tid.es
 Tomonori Takeda
 NTT Corporation
 9-11, Midori-Cho 3 Chrome
 Musashino-Shi, Tokyo 180-8585
 Japan
 EMail: takeda.tomonori@lab.ntt.co.jp

Decraene, et al. Informational [Page 20]

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