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Network Working Group                                       P. Mohapatra
Internet-Draft                                               R. Fernando
Intended status: Standards Track                             C. Filsfils
Expires: July 26, 2013                                     Cisco Systems
                                                               R. Raszuk
                                                            NTT MCL Inc.
                                                        January 22, 2013


            Fast Connectivity Restoration Using BGP Add-path
                draft-pmohapat-idr-fast-conn-restore-03

Abstract

   A BGP route defines an association of an address prefix with an "exit
   point" from the current Autonomous System (AS).  If the exit point
   becomes unreachable due to a failure, the route becomes invalid.
   This usually triggers an exchange of BGP control messages after which
   a new BGP route for the given prefix is installed.  However,
   connectivity can be restored more quickly if the router maintains
   precomputed BGP backup routes.  It can then switch to a backup route
   immediately upon learning that an exit point is unreachable, without
   needing to wait for the BGP control messages exchange.  This document
   specifies the procedures to be used by BGP to maintain and distribute
   the precomputed backup routes.  Maintaining these additional routes
   is also useful in promoting load balancing, performing maintenance
   without causing traffic loss, and in reducing churn in the BGP
   control plane.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 July 26, 2013.

Copyright Notice




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   Copyright (c) 2013 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
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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.



























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  5
   2.  Basic Idea . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Design Considerations  . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Ensuring Loop-Free Path Selection in an AS . . . . . . . .  6
       3.1.1.  Border routers announcing single path  . . . . . . . .  6
       3.1.2.  Border routers announcing multiple paths . . . . . . .  7
       3.1.3.  Confederations . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Keeping Path Attributes Independent of Decision Process  .  8
   4.  Edge_Discriminator attribute . . . . . . . . . . . . . . . . .  8
   5.  Calculation of Best and Backup Paths . . . . . . . . . . . . .  9
   6.  Advertising Multiple Paths . . . . . . . . . . . . . . . . . . 13
   7.  Deployment Considerations  . . . . . . . . . . . . . . . . . . 13
   8.  Applications . . . . . . . . . . . . . . . . . . . . . . . . . 14
     8.1.  Fast Connectivity Restoration  . . . . . . . . . . . . . . 14
     8.2.  Load Balancing . . . . . . . . . . . . . . . . . . . . . . 14
     8.3.  Churn Reduction  . . . . . . . . . . . . . . . . . . . . . 15
       8.3.1.  Inter-domain Churn Reduction . . . . . . . . . . . . . 15
       8.3.2.  Intra-Domain Churn Reduction . . . . . . . . . . . . . 15
     8.4.  Graceful Maintenance . . . . . . . . . . . . . . . . . . . 17
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     12.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18






















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1.  Introduction

   Within an autonomous system, the availability of multiple routes to a
   given destination, where each of the routes has a different "exit
   point" from the local AS provides the following benefits:

   o  Fault tolerance: Knowledge of multiple "exit points" leads to
      reduction in restoration time after failure.  For instance, a
      border router on receiving multiple paths to the same destination
      could decide to precompute a backup path and have it ready so that
      when the primary path becomes invalid, it could use the backup to
      quickly restore connectivity.  Currently the restoration time is
      dependent on BGP protocol re-convergence that includes a set of
      withdraw and advertisement messages in the network before a new
      best path can be learnt.

   o  Load balancing: The availability of multiple paths to reach the
      same destination enables load balancing of traffic provided the
      paths for the given destination satisfy certain constraints.

   o  Churn reduction: The advertisement of multiple routes, in certain
      scenarios (Section 8.3.2), could lead to less churn in the network
      upon a failure, since the presence of multiple paths helps contain
      the failure to the local AS where the failure occurs.

   o  Graceful maintenance: The availability of alternate exit points
      allows one to bring down a router for maintenance without causing
      significant traffic loss.

   Unfortunately, the border routers in an AS do not receive multiple
   paths for all prefixes.  The reason is three-fold:

   o  The current BGP specification [RFC4271] specifies routers to
      advertise only the best path for a destination to speakers.  The
      availability of multiple paths requires simultaneous distribution
      of multiple routes for a given prefix by a BGP speaker.  We refer
      to this property of the network as "path diversity".

   o  When a router selects an IBGP learnt path as best, it does not
      announce any path for that prefix to IBGP though it may have EBGP
      learnt paths available.  This loss of information leads to added
      churn and increases convergence time if the preferred path goes
      away.  A mechanism to advertise the best-external path to IBGP is
      proposed in [I-D.ietf-idr-best-external].

   o  Most service providers deploy one of the scaling techniques like
      route reflectors [RFC4456] or confederations[RFC5065] inside the
      AS and avoid iBGP full mesh.  Thus even when multiple paths exist,



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      the aggregation points (route reflectors or confederation border
      routers) advertise only the best path (as per the BGP base
      protocol).

   As an effect of this behavior, the ingress border routers to an AS do
   not receive additional paths necessary to provide the benefits cited
   above: e.g. perform a local recovery during network failures or
   achieve load balancing in steady state across multiple exit points.

   The mechanism to extend BGP to allow a given BGP speaker to advertise
   multiple paths simultaneously for a destination is defined in
   [I-D.ietf-idr-add-paths].  The current draft describes the use of
   this generic technique and certain additional procedures and
   implementation guidelines to enable the above applications.

   More specifically, this document describes extensions to BGP decision
   process to select backup paths in a manner that ensures the important
   property of consistent route selection within an AS.  It also
   introduces a new BGP attribute, Edge_Discriminator, that border
   routers should use to advertise multiple EBGP learnt paths for a
   given destination.  To aid with better description of the
   applications, the draft illustrates certain use case scenarios for
   each.

   One implication of multiple path advertisement is the associated
   cost, namely the performance overhead of processing and memory
   overhead of storing additional paths.  It is anticipated that the
   benefits listed above outweigh the cost in most scenarios.  Be that
   as it may, it is also expected that there will be configuration knobs
   provided to limit the number of additional paths propagated within an
   AS.

1.1.  Requirements Language

   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].


2.  Basic Idea

   This document proposes two main additions to the BGP procedures:

   1.  The decision process is modified to determine backup paths along
       with the best path selection when multiple paths for a
       destination are available.





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   2.  In addition to using these backup paths for fast connectivity
       restoration locally, BGP speakers also advertise these paths to
       IBGP to increase the overall path diversity.

       As alluded to in Section 1, BGP speakers that are the aggregation
       points (router reflectors or confederation border routers) need
       to announce backup paths to increase the path diversity at the
       ingress routers of an IBGP network (see Figure 2).  It may also
       be useful, in certain cases, for the border routers to advertise
       multiple paths received via EBGP for a destination when it is
       redundantly connected and is transparently passing the NEXT_HOP
       field unchanged instead of setting it to self (see Figure 4).  To
       this end, the draft defines a new attribute, Edge_Discriminator,
       that the border routers should advertise to ensure path selection
       consistency.

   The following sections elaborate on these points.


3.  Design Considerations

3.1.  Ensuring Loop-Free Path Selection in an AS

   It is critical that BGP speakers within an AS have an eventual
   consistent routing view of destinations and do not make conflicting
   decisions regarding best path selection that would otherwise cause
   forwarding loops.  The current BGP protocol ensures this property by
   defining a decision process that takes the attributes of paths as
   input and determines a degree of preference of the paths by applying
   a constant function.  A consistent view of attributes is disseminated
   through IBGP.  Thus each BGP speaker within the AS determines the
   same degree of preference of the paths after applying the constant
   function independently.  (The one exception is where IGP metric plays
   the tie breaking role.  In this case, different routers may choose
   different next hops that are closer to them; but loop freedom is
   guaranteed.).

   When the above mechanism is extended to select backup paths for the
   applications cited in this document, it is equally important to
   maintain the same consistency property for the backup paths, i.e.
   there should be no loops created when routers use the backup path in
   forwarding.  The rest of the document goes into the details of this
   for various scenarios.

3.1.1.  Border routers announcing single path

   In scenarios where all border routers advertise a single external
   path (their best path or best-external path) into IBGP, a consistent



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   routing view of best path and backup paths can be created across the
   AS with the current BGP selection rules.

3.1.2.  Border routers announcing multiple paths

   There are scenarios where border routers need to advertise the best
   and backup EBGP learnt paths with NEXT_HOP unchanged to IBGP.  If the
   border router sets next hop to self, the paths become
   indistinguishable and hence advertisement of only the best path is
   sufficient.  An example scenario is depicted in Figure 4.

   By using the add-path ([I-D.ietf-idr-add-paths]) extensions, the
   border routers could advertise multiple such EBGP-learnt paths.  But
   doing so can potentially create an inconsistency between the paths
   that the sending and receiving routers select for forwarding.  In
   other words, the routers in the IBGP mesh can make independent and
   separate decisions on the route selection since some of the values
   that play a role in the tie breaking steps of the decision process at
   the sender are not available to the rest of the BGP speakers of the
   AS.  These are mainly (1) the interior cost, i.e. the metric to reach
   the external next hop, (2) BGP identifier of the peer, (3) the peer
   IP address.  Due to this reduction in information, there can be
   inconsistency in the routing view within an AS.

   Additionally, [RFC5004] proposes an extension to avoid best path
   transitions at the border router between external paths based on a
   temporal order of receiving the paths.  This can also create an
   inconsistency across the BGP speakers in the path selection.

   This document proposes two modifications to ensure consistency:

   a.  Border routers SHOULD not apply the modification to the selection
       rules as proposed in [RFC5004] to avoid best path transitions for
       parallel EBGP connection scenario where the border router wishes
       to transitively transmit the NEXT_HOP value unchanged.

   b.  To overcome the "information reduction" problem described above,
       the document specifies an attribute called "Edge_Discriminator
       attribute" that encodes the properties of each path advertised
       that would otherwise not be included using the normal attributes
       in a BGP UPDATE message (see Section 4).

3.1.3.  Confederations

   When an AS employs confederations and the confederation border
   routers advertise multiple paths, there is no way to distinguish the
   originator (the actual egress border router originating the prefix to
   the AS).  To ensure consistent path selection, the confederation



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   border routers should create the ORIGINATOR_ID attribute as described
   in [RFC4456] that carries the BGP identifier of the originator of the
   route to the local AS.

3.2.  Keeping Path Attributes Independent of Decision Process

   In addition to providing consistency in path selection, the solution
   should satisfy the following important property: the attributes
   associated with a particular path should be invariant when a
   different path is advertised or withdrawn.  Other things being equal,
   it is best to avoid the potential churn introduced by the feedback
   loops that would occur if path attributes were changed at the sender
   as a result of running the decision process.  Thus we do not use any
   attributes with semantics like "this is my second best path", "this
   is my third best path", etc.  This requirement precludes use of
   marking or other means of indicating path ordering from sender's
   perspective since a change in the ordering requires re-advertising
   most of the paths.


4.  Edge_Discriminator attribute

   Edge_Discriminator attribute is an optional non-transitive attribute
   that is composed of a set of Type-Length-Value (TLVs) encodings.  The
   type code of the attribute is to be assigned by IANA.  Each TLV
   contains an attribute of the path from the border router that is not
   otherwise sent as part of the UPDATE message.  The TLV is structured
   as follows:


          0                   1
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |   Attr Type   |   Length      |
         |   (1 octet)   |  (1 octet)    |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                               |
         |             Value             |
         |                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


               Figure 1: Edge_Discriminator attribute format

   a.  Attr Type (1 octet): It identifies the type of the attribute that
       is encoded by the border router.  Unknown types are to be ignored
       and skipped upon receipt.  This document defines the following
       types:



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       *  Interior Cost: Attr Type = 1

       *  peer BGP Identifier: Attr Type = 2

       *  IPv4 Peer Address: Attr Type = 3

       *  IPv6 Peer Address: Attr Type = 4

   b.  Length (1 octet): the total number of octets of the Value field.

   c.  Value (variable): The value field encodes the attribute of the
       corresponding type.  For "Interior Cost" type, it encodes the
       four octet metric value.  For "BGP Identifier" type, it encodes
       the four-octet router identifier of the neighbor for the path.
       For "IPv4 Peering Address" type, the 4 byte BGP IPv4 peering
       address is encoded.  For "IPv6 Peering Address" type, the 16 byte
       BGP IPv6 peering address is encoded.

   A brief description of how a BGP speaker constructs the attribute is
   provided in Section 6.


5.  Calculation of Best and Backup Paths




























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                        /----------------------------------------\
                        |               +----+             IBGP  |
                                        | r1 |
                        |               +----+                   |
                                           .
                        |                   .                    |
                                             .
                        |                  +----+                |
                                           | RR |
                        |                . +----+ .              |
                                        .          .
                        |              .            .            |
                                   +----+           +----+
                        |          | r3 |           | r4 |       |
                                   +----+           +----+
                        |            |                 |         |
                        \            |  P1             | P2      /
                         ----------------------------------------
                                     |                 |
                                EBGP |                 | EBGP
                                       ...............
                                      /               \
                                        Destination a
                                      \               /
                                       ...............


                        Figure 2: Basic RR topology























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                        /----------------------------------------\
                        |                                        |
                                +----+           +----+
                        |       | r1 |...........| r2 |          |
                                +----+           +----+
                        |          .                .            |
                                   .   AS 65502     .
                        |          .                .            |
                                   .    +----+      .
                        |          .....|CBR2|.......            |
                                        +----+
                        |                 |                      |
                                          | CONFED EBGP
                        |               +----+                   |
                                   .....|CBR1|.......
                        |          .    +----+      .            |
                                   .                .
                        |          .   AS 65501     .            |
                                   .                .
                        |       +----+           +----+          |
                                | r3 |...........| r4 |
                        |       +----+           +----+          |
                                   |P1              |P2
                        |          |                |            |
                         ----------------------------------------
                                   |                |
                              EBGP |                | EBGP
                                     ...............
                                    /               \
                                      Destination a
                                    \               /
                                     ...............


                     Figure 3: Confederation topology
















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                        /----------------------------------------\
                        |               +----+             IBGP  |
                                        | r1 |
                        |               +----+                   |
                                           .
                        |                   .                    |
                                             .
                        |                  +----+                |
                                           | RR |
                        |                . +----+ .              |
                                        .          .
                        |              .            .            |
                                   +----+           +----+
                        |          | r3 |           | r4 |       |
                                   +----+           +----+
                        |           |  |                         |
                        \         P1|  |P2                       /
                         ----------------------------------------
                                    |  |
                                EBGP|  |EBGP

                              ...............
                             /               \
                               Destination a
                             \               /
                              ...............


             Figure 4: Border router with parallel eBGP links

   The decision process as described in [RFC4271] is followed to
   determine the overall best path for a destination.  In addition, the
   following rule SHOULD be inserted into the tie breaking rules of the
   BGP decision process after step f) (Sect. 9.1.2.2: [RFC4271]) and
   after the CLUSTER_LIST length check step (Sect. 9: [RFC4456]): a BGP
   speaker SHOULD apply the tie breaking steps (steps (e), (f), and (g)
   as defined in [RFC4271]) with the values encoded in the
   Edge_Discriminator attribute.

   Note that the above step effectively compares multiple paths that are
   advertised by the same egress border router (since the BGP Identifier
   comparison step earlier would have eliminated paths from different
   egress border routers).

   Consider the network in Figure 4. r3 learns two paths P1 and P2 for
   destination a and wishes to advertise both to the iBGP mesh with
   NEXT_HOP value unchanged.  We need to ensure that both r3 and the
   other ingress routers in the network (r1, r4) make a consistent route



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   selection for the best and the backup paths for destination a.  The
   current tie breaking rules [step f) comparison of router ID or
   ORIGINATOR_ID and step g) comparision of peering ID] are insufficient
   since at the ingress routers, both the paths will be received with
   same values for each of the above parameters.  Hence an additional
   tie breaking rule comparing the original values that the border
   router itself used to tie break the paths is required.

   Once the best path is chosen, eliminate that path and all paths that
   have the same BGP Identifier or NEXT_HOP as the choosen best path.
   Note that as specified in [RFC4456], if the path carries the
   ORIGINATOR_ID attribute, that should be treated as the BGP
   Identifier.  Then rerun the best path procedure to choose the backup
   path.  The Tie Breaking rules of the BGP decision process for second
   best path selection are also modified as described above.

   This mechanism can be recursively used to calculate multiple backup
   paths if desired.


6.  Advertising Multiple Paths

   The technique outlined in [I-D.ietf-idr-add-paths] is used to
   advertise best and backup paths selected with the rules described in
   Section 5.  For the purposes of the applications cited in this
   document, the "Path Identifier" is always treated as an opaque value
   with no semantics.

   When an egress border router chooses to advertise multiple paths
   learnt via EBGP to IBGP, it SHOULD include the Edge_Discriminator
   attribute as defined in Section 4 for each of the paths.  The
   attribute is constructed by encoding the following properties of the
   path in TLV format:

   o  The interior cost to reach the NEXT_HOP of the path, encoded with
      type 1.

   o  The BGP identifier of the EBGP peer from which it received the
      path, encoded with type 2.

   o  The peer address of the EBGP peer from which it received the path,
      encoded either with type 3 or 4.


7.  Deployment Considerations

   To ensure consistency in path selection process across all the
   routers in an AS, the deployment considerations from the individual



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   scaling technology employed in the network should be inherited/
   applied.  For example, as specified in [RFC4456], the intra-cluster
   IGP metric values should be better than the inter-cluster IGP metric
   values.  Similar considerations as specified in [RFC5065] should be
   designed.


8.  Applications

8.1.  Fast Connectivity Restoration

   Consider the network in Figure 2.  All 4 routers indicated are part
   of a single AS. r3 and r4 are the border routers.  Suppose r3 and r4
   receive paths P1 and P2 for the same prefix.  Also assume that P1 is
   the preferred exit.

   There are two scenarios to consider:

   case 1:  P1 is the preferred exit for all routers within the AS
      (including r4).  In this case, if r4 follows [RFC4271], r4
      withdraws P2 from the IBGP cloud.

   case 2:  P2 is preferred exit by r4.  In this case, if RR follows
      [RFC4271], RR gets both paths, chooses one and sends it to r1.

   In both the cases above, 'r1' holds only a single path and only after
   a failure that makes P1 unavailable, it receives the alternate path
   (P2).

   However, if both paths were available to 'r1' and all other border
   routers in the network, then they could precompute backup paths and
   keep them ready to restore connectivity upon being notified of a
   failure.  The failure notification could be triggered due to a link
   failure between 'r3' and its EBGP neighbor.  This failure could be
   propagated to other routers in r3's AS either via IGP or BGP,
   resulting in invalidating on all these routers their primary paths
   that were advertised by that neighbor to r3 (and that r3 subsequently
   re-advertised into IBGP).  Once these paths are invalidated, all
   these routers could switch to the precomputed backup paths, without
   waiting for any additional BGP advertisements.

8.2.  Load Balancing

   In the above network, not only can the additional path be used as a
   standby best, but can also be used in steady state to load balance
   traffic across the two exit points.





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8.3.  Churn Reduction

   There are two aspects to reducing churn - Inter-domain and Intra-
   domain.

8.3.1.  Inter-domain Churn Reduction

   Consider the network diagram in Figure 5.



                                           +----+
                                           | r5 |
                                           +----+
                                              | EBGP
                                        -------------
                                              |
                                           +----+
                                           | r1 |
                                           +----+
                                        .          .
                                       .            .
                                   +----+           +----+
                                   | r3 |           | r4 |
                                   +----+           +----+
                                     | P1              | P2


                                 Figure 5

   'r5' is an EBGP peer of 'r1'.  Today, if path P1 goes away, due to
   the non-availability of other paths, 'r1' sends a withdraw to r5 thus
   triggering a churn in the Internet.  This could be significant if
   there are multiple prefixes involved.  On the other hand, if r1 had
   an alternate path (with identical attributes), then this churn could
   be entirely avoided by r1 performing a local repair.

8.3.2.  Intra-Domain Churn Reduction

   Since advertising multiple paths in general increases the path
   diversity at the border routers, some of the control plane churn in
   terms of a stream of advertisements, withdraws, and re-advertisements
   can be reduced, thus improving the stability of the network.








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                                   AS 2
                                     |
                                     |
                                   +----+           +----+
                                   | r1 |           | r2 |
                                   +----+           +----+
                                        .          .
                                         .        .
                                          .      .
                                           +----+
                                           | RR |
                                         . +----+ .
                                        .          .
                                       .            .
                                   +----+           +----+
                                   | r3 |           | r4 |
                                   +----+           +----+
                                         \         /
                                          \       / eBGP dual-homing
                                           AS 1(a)


                                 Figure 6

   Assuming router r3's path is the best path in the AS, RR advertises
   the corresponding route information to the iBGP network.  If r3 goes
   down (or the peering link [r3, AS1] fails and r3 didn't change the
   next hop to itself), the following will be sequence of updates from
   router r1 to AS 2:

   o  Initial update for all prefixes when r1 chooses best path,

   o  Withdraws for all prefixes when r1 detects failure,

   o  Re-advertisement of all prefixes when the RR chooses router r4's
      path as the new best path and advertises to r1.

   With both the paths advertised and received on router r1, the
   sequence of updates reduces to:

   o  Initial update for all prefixes when r1 chooses best path,

   o  Re-advertisement of all prefixes when r1 detects failure and
      chooses router r4's path as the new best path







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8.4.  Graceful Maintenance

   [RFC6198] defines requirements for graceful maintenance of routers in
   a service provider network.  Current BGP operations treat this as a
   sudden link or node failure and try to reconverge that can take in
   the order of seconds or minutes.

   With the procedures defined in this document, since alternate paths
   are available at the ingress routers, taking down egress routers from
   the network does not result in a network-wide reconvergence event.


9.  Acknowledgements

   The authors would like to thank Enke Chen for the many discussions
   resulting in this work.  In addition, the authors would also like to
   acknowledge valuable review and suggestions from Eric Rosen, Yakov
   Rekhter, and John Scudder on this document.


10.  IANA Considerations

   This document defines a new BGP optional non-transitive attribute
   type, called Edge_Discriminator attribute.  The attribute type is to
   be assigned by IANA.

   This document introduces Attr TLVs within the above attribute.  The
   type space for these should be set up by IANA as a registry of
   1-octet attr types.  These should be assigned on a first-come-first-
   serve basis.

   This document defines the following attr types that should be
   assigned in the registry:

          Attr                                     Type
          ---------------                         -----
          Interior Cost                             1
          Peer BGP Identifier                       2
          IPv4 Peer Address                         3
          IPv6 Peer Address                         4


11.  Security Considerations

   There are no additional security risks introduced by this design.


12.  References



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12.1.  Normative References

   [I-D.ietf-idr-add-paths]
              Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP",
              draft-ietf-idr-add-paths-08 (work in progress),
              December 2012.

   [I-D.ietf-idr-best-external]
              Marques, P., Fernando, R., Chen, E., Mohapatra, P., and H.
              Gredler, "Advertisement of the best external route in
              BGP", draft-ietf-idr-best-external-05 (work in progress),
              January 2012.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "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.

   [RFC5065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 5065, August 2007.

   [RFC6198]  Decraene, B., Francois, P., Pelsser, C., Ahmad, Z.,
              Elizondo Armengol, A., and T. Takeda, "Requirements for
              the Graceful Shutdown of BGP Sessions", RFC 6198,
              April 2011.

12.2.  Informative References

   [RFC5004]  Chen, E. and S. Sangli, "Avoid BGP Best Path Transitions
              from One External to Another", RFC 5004, September 2007.


Authors' Addresses

   Pradosh Mohapatra
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   Email: pmohapat@cisco.com




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   Rex Fernando
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   Email: rex@cisco.com


   Clarence Filsfils
   Cisco Systems
   Brussels,
   Belgium

   Email: cfilsfil@cisco.com


   Robert Raszuk
   NTT MCL Inc.
   101 S Ellsworth Avenue Suite 350
   San Mateo, CA  94401
   USA

   Email: robert@raszuk.net



























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