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Versions: 00 01

Network Working Group                                    Pierre Francois
Internet-Draft                          Universite catholique de Louvain
Intended status: Informational                            Bruno Decraene
Expires: September 7, 2009                                France Telecom
                                                         Cristel Pelsser
                                                         NTT Corporation
                                                       Clarence Filsfils
                                                           Cisco Systems
                                                           March 6, 2009


                     Graceful BGP session shutdown
                      draft-francois-bgp-gshut-01

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Copyright Notice



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   Copyright (c) 2009 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 in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   This draft describes operational procedures aimed at reducing the
   amount of traffic lost during planned maintenances of routers,
   involving the shutdown of BGP peering sessions.





































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Packet loss upon manual eBGP session shutdown  . . . . . . . .  5
   4.  Practices to avoid packet losses . . . . . . . . . . . . . . .  5
     4.1.  Improving availability of alternate paths  . . . . . . . .  6
     4.2.  Graceful shutdown procedures for eBGP sessions . . . . . .  6
       4.2.1.  Outbound traffic . . . . . . . . . . . . . . . . . . .  6
       4.2.2.  Inbound traffic  . . . . . . . . . . . . . . . . . . .  7
     4.3.  Graceful shutdown procedures for iBGP sessions . . . . . .  9
   5.  Forwarding modes and forwarding loops  . . . . . . . . . . . .  9
   6.  Dealing with Internet policies . . . . . . . . . . . . . . . . 10
   7.  Effect of the g-shut procedure on the convergence  . . . . . . 10
     7.1.  Maintenance of an eBGP session . . . . . . . . . . . . . . 10
       7.1.1.  Propagation on the other eBGP sessions of the
               g-shut initiator . . . . . . . . . . . . . . . . . . . 10
       7.1.2.  Propagation on the other iBGP sessions of the
               g-shut initiator . . . . . . . . . . . . . . . . . . . 11
       7.1.3.  Propagation of updates in an iBGP full-mesh  . . . . . 11
       7.1.4.  Propagation of updates from iBGP to iBGP in a RR
               hierarchy  . . . . . . . . . . . . . . . . . . . . . . 11
     7.2.  Maintenance of an iBGP session . . . . . . . . . . . . . . 12
     7.3.  Applicability of the g-shut procedure  . . . . . . . . . . 13
     7.4.  Summary of operations  . . . . . . . . . . . . . . . . . . 13
       7.4.1.  Pre-configuration  . . . . . . . . . . . . . . . . . . 13
       7.4.2.  Operations at maintenance time . . . . . . . . . . . . 13
   8.  Link Up cases  . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Unreachability local to the ASBR . . . . . . . . . . . . . 13
     8.2.  iBGP convergence . . . . . . . . . . . . . . . . . . . . . 14
   9.  Alternative techniques with limited applicability  . . . . . . 15
     9.1.  In-filter reconfiguration  . . . . . . . . . . . . . . . . 15
     9.2.  Multi Exit Discriminator tweaking  . . . . . . . . . . . . 16
     9.3.  IGP distance Poisoning . . . . . . . . . . . . . . . . . . 16
   10. IANA considerations  . . . . . . . . . . . . . . . . . . . . . 16
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17












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1.  Introduction

   Routing changes in BGP can be caused by planned, manual, maintenance
   operations.  This document discusses operational procedures to be
   applied in order to reduce or eliminate losses of packets during the
   maintenance.  These losses come from the transient lack of
   reachability during the BGP convergence following the shutdown of an
   eBGP peering session between two Autonomous System Border Routers
   (ASBR).

   This document presents procedures for the cases where the forwarding
   plane is impacted by the maintenance, hence when the use of Graceful
   Restart does not apply.

   The procedures described in this document can be applied to reduce or
   avoid packet loss for outbound and inbound traffic flows initially
   forwarded along the peering link to be shut down.  These procedures
   allow routers to keep using old paths until alternate ones are
   learned, ensuring that routers always have a valid route available
   during the convergence process.

   The goal of the document is to meet the requirements described in
   [REQS] at best, without changing the BGP protocol or BGP
   implementations.

   Still, it explains why reserving a community value for the purpose of
   BGP session graceful shutdown would reduce the management overhead
   bound with the solution.  It would also allow vendors to provide an
   automatic graceful shutdown mechanism that does not require any
   configuration at maintenance time.


2.  Terminology

   g-shut initiator : a router on which the session shutdown is
   performed for the maintenance.

   g-shut neighbor : a router that peers with the g-shut initiator via
   (one of) the session(s) to be shut down.

   Note that for the link-up case, we will refer to these nodes as g-no-
   shut initiator, and g-no-shut neighbor.

   Initiator AS : the Autonomous System of the g-shut initiator.

   Neighbor AS : the Autonomous System of the g-shut neighbor.

   Affected path / Nominal / pre-convergence path : a BGP path via the



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   peering link(s) undergoing the maintenance.  This path will no longer
   exist after the shutdown.

   Affected prefix : a prefix initially reached via an affected path.

   Affected router : a router having an affected prefix.

   Backup / alternate / post-convergence path : a path toward an
   affected prefix that will be selected as the best path by an affected
   router, when the link is shut down and the BGP convergence is
   completed.

   Transient alternate path : a path towards an affected prefix that may
   be transiently selected as best by an affected router during the
   convergence process but that is not a post-convergence path.

   Loss of Connectivity (LoC) : the state when a router has no path
   towards an affected prefix.


3.  Packet loss upon manual eBGP session shutdown

   Packets can be lost during a manual shutdown of an eBGP session for
   two reasons.

   First, routers involved in the convergence process can transiently
   lack of paths towards an affected prefix, and drop traffic destined
   to this prefix.  This is because alternate paths can be hidden by
   nodes of an AS.  This happens when the paths are not selected as best
   by the ASBR that receive them on an eBGP session, or by Route
   Reflectors that do not propagate them further in the iBGP topology
   because they do not select them as best.

   Second, within the AS, routers' FIB can be transiently inconsistent
   during the BGP convergence and packets towards affected prefixes can
   loop and be dropped.  Note that these loops only happen when ASBR-to-
   ASBR encapsulation is not used within the AS.

   This document only addresses the first reason.


4.  Practices to avoid packet losses

   This section describes means for an ISP to reduce the transient loss
   of packets upon a manual shutdown of a BGP session.






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4.1.  Improving availability of alternate paths

   All solutions that increase the availability of alternate BGP paths
   in routers performing packet lookups in BGP tables [BestExternal]
   [AddPath] help in reducing the LoC bound with manual shutdown of eBGP
   sessions.

   One solution increasing diversity in such a way that, at any single
   step of the convergence process following the eBGP session shutdown,
   a BGP router does not receive a message withdrawing the only path it
   currently knows for a given NLRI, allows for a simplified g-shut
   procedure.  This simplified procedure would only tackle potential LoC
   for the inbound traffic.

   Using advertise-best-external [BestExternal] on ASBRs and RRs helps
   in avoiding lack of alternate paths in route reflectors upon a
   convergence.  Hence it reduces the LoC duration for the outbound
   traffic of the ISP upon an eBGP Session shutdown by reducing the iBGP
   path hunting.

   Still it does not ensure that BGP routers will always have at least
   one path towards affected prefixes during the convergence following
   the event.  This property may be verified in future revisions of
   [BestExternal], notably of its Section 4, hence the current proposal
   will be updated accordingly.

   Increasing diversity with [AddPath] might lead to the respect of this
   property, depending on the path propagation decision process that
   add-path compliant routers would use.

   Note that the LoC for the inbound traffic of the maintained router,
   induced by a lack of alternate path propagation within the iBGP
   topology of a neighboring AS is not under the control of the operator
   performing the maintenance, hence the procedure described in
   Section 4.2.2 should be applied upon the maintenance, even if not
   required for the outbound traffic.

4.2.  Graceful shutdown procedures for eBGP sessions

   This section aims at describing a procedure to be applied to reduce
   the LoC with readily available BGP features, and without assuming a
   particular iBGP design in the Initiator and Neighbor ASes.

4.2.1.  Outbound traffic

   This section discusses a mean to render the affected paths less
   desirable by the BGP decision process of affected routers, still
   allowing these to be used during the convergence while alternate



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   paths are propagated to the affected routers.

   A decrease of the local-pref value of the affected paths can be
   issued in order to render the affected paths less preferable, at the
   highest possible level of the BGP Decision Process.

   This operation can be performed by reconfiguring the out-filters
   associated with the iBGP sessions established by the g-shut
   initiator.

   The modification of the filters MUST supplant any other rule
   affecting the local-pref value of the old paths.

   Compared to using an in-filter of the eBGP session to be shut down,
   the modification of the out-filters will not let the g-shut initiator
   switch to another path, as the input to the BGP decision process of
   that router does not change.  As a consequence, the g-shut initiator
   will not send a withdraw message over its iBGP sessions when it
   receives an alternate path over an iBGP session.  It will however
   modify the local-pref of the affected paths so that upstream routers
   will switch to alternate ones.

   When the actual shutdown of the session is performed, the g-shut
   initiator will itself switch to the alternate paths.

4.2.2.  Inbound traffic

   The solution described for the outbound traffic can be applied at the
   neighbor AS.  This can be done either "manually" or by using a
   community value dedicated to this task.

4.2.2.1.  Phone call

   The operator performing the maintenance of the eBGP session can
   contact the operator at the other side of the peering link, and let
   him apply the procedure described above for its own outbound traffic.

4.2.2.2.  Community tagging

   A community value (referred to as GSHUT community in this document)
   can be agreed upon by neighboring ASes.  A path tagged with this
   community must be considered as soon to be affected by a maintenance
   operation.

4.2.2.2.1.  Pre-Configuration

   A g-shut neighbor is pre-configured to set a low local-pref value for
   the paths received over eBGP sessions which are tagged with the GSHUT



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   community.

   This rule must supplant any other rule affecting the local-pref value
   of the paths.

   This local-pref reconfiguration SHOULD be performed at the out-
   filters of the iBGP sessions of the g-shut neighbor.  That is, the
   g-shut neighbor does not take into account this low local-pref in its
   own BGP best path selection.  As described in Section 4.2.1 this
   avoids sending the withdraw messages that can lead to LoC.

4.2.2.2.2.  Operational action upon maintenance

   Upon the manual shutdown, the output filter associated with the
   maintained eBGP session will be modified on the g-shut initiator so
   as to tag all the paths advertised over the session with the GSHUT
   community.

4.2.2.2.3.  Transitivity of the community

   If the GSHUT community is an extended community, it SHOULD be set non
   transitive.

   If a normal community is used, this community SHOULD be removed from
   the path by the ASBR of the peer receiving it.  If not, the GSHUT
   community MAY be removed from the path by all the ASBRs of the
   neighboring AS, before propagating the path to other peers.

   Not propagating the community further in the Internet reduces the
   amount of BGP churn and avoids rerouting in distant ASes that would
   also recognize this community value.  In other words, it helps
   concealing the convergence at the maintenance location.

   There are cases where an interdomain exploration is to be performed
   to recover the reachability, e.g., in the case of a shutdown in
   confederations where the alternate paths will be found in another AS
   of the confederation.  In such scenarios, the community value SHOULD
   be allowed to transit through the confederation but MAY be removed
   from the paths advertised outside of the confederation.

   When the local-pref value of a path is conserved upon its propagation
   from one AS of the confederation to the other, there is no need to
   have the GSHUT community be propagated throughout that confederation.

4.2.2.2.4.  Easing the configuration for G-SHUT

   From a configuration burden viewpoint, it would be much easier to
   reserve a value for the GSHUT community.



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   First, on the g-shut initiator, an operator would have a single
   configuration rule to be applied at the maintenance time, which would
   not depend on the identity of its peer.  This would make the
   maintenance operations less error prone.

   Second, on the g-shut neighbor, a simple filter related to g-shut can
   be applied to all iBGP sessions.  Additionnaly, this filter doesn't
   need to be updated each time neighboring ASes are added or removed.

4.3.  Graceful shutdown procedures for iBGP sessions

   If the iBGP topology is viable after the maintenance of the session,
   i.e, if all BGP speakers of the AS have an iBGP signaling path for
   all prefixes advertised on this g-shut iBGP session, then the
   shutdown of an iBGP session does not lead to transient
   unreachability.

   However, in the case of a shutdown of a router, a reconfiguration of
   the out-filters of the g-shut initiator should be performed to set a
   low local-pref value for the paths originated by the g-shut initiator
   (e.g, BGP aggregates redistributed from other protocols, including
   static routes).

   This behavior is equivalent to the recommended behavior for paths
   "redistributed" from eBGP sessions to iBGP sessions in the case of
   the shutdown of an ASBR.


5.  Forwarding modes and forwarding loops

   If the AS applying the solution does not rely on encapsulation to
   forward packets from the Ingress Border Router to the Egress Border
   Router, then transient forwarding loops and consequent packet losses
   can occur during the convergence process, even if the procedure
   described above is applied.  Hence if zero LoC is required,
   encapsulation is required between ASBRs of the AS.

   Using the out-filter reconfiguration avoids the forwarding loops
   between the g-shut initiator and its directly connected upstream
   neighbors.  Indeed, when this reconfiguration is applied, the g-shut
   initiator keeps using its own external path and lets the upstream
   routers converge to the alternate ones.  During this phase, no
   forwarding loops can occur between the g-shut initiator and its
   upstream neighbors as the g-shut initiator keeps using the affected
   paths via its eBGP peering links.  When all the upstream routers have
   switched to alternate paths, the transition performed by the g-shut
   initiator when the session is actually shut down, will be loopfree.
   Transient forwarding loops between other routers will not be avoided



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   with this procedure.


6.  Dealing with Internet policies

   A side gain of the maintenance solution is that it can also reduce
   the churn implied by a shutdown of an eBGP session.

   For this, it is recommended to apply the filters modifying the local-
   pref value of the paths to values strictly lower but as close as
   possible to the local-pref values of the post-convergence paths.

   For example, if an eBGP link is shut down between a provider and one
   of its customers, and another link with this customer remains active,
   then the value of the local-pref of the old paths SHOULD be decreased
   to the smallest possible value of the 'customer' local_pref range,
   minus 1.  Thus, routers will not transiently switch to paths received
   from shared-cost peers or providers, which could lead to the
   propagation of withdraw messages over eBGP sessions with shared-cost
   peers and providers.

   Proceeding like this reduces both BGP churn and traffic shifting as
   routers will less likely switch to transient paths.

   In the above example, it also prevents transient unreachabilities in
   the neighboring AS that are due to the sending of "abrupt" withdraw
   messages to shared-cost peers and providers.


7.  Effect of the g-shut procedure on the convergence

   This section describes the effect of applying the solution.

7.1.  Maintenance of an eBGP session

   This section describes the effect of applying the solution for the
   shutdown of an eBGP session.

7.1.1.  Propagation on the other eBGP sessions of the g-shut initiator

   Nothing is propagated on the other eBGP sessions when the out-filters
   reconfiguration step is applied.  The reconfiguration is indeed only
   defined for its iBGP sessions.

   The reconfiguration of the iBGP out-filters will trigger the
   reception of alternate paths at the g-shut initiator.  As the eBGP
   in-filters have not been modified at that step, the old paths are
   still preferred by the g-shut initiator.



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7.1.2.  Propagation on the other iBGP sessions of the g-shut initiator

   During the out-filter reconfiguration, path updates are propagated
   with a reduced local-pref value for the affected paths.  As a
   consequence, Route Reflectors and distant ASBRs select and propagate
   alternate paths through the iBGP topology as they no longer select
   the old paths as best.

   When the shut-down is performed, for each affected prefix, the g-shut
   initiator propagates on its iBGP sessions:

   .  The alternate path, if the best path was received over an eBGP
   sessions.

   .  A withdraw, if the best path was received over an iBGP sessions.

7.1.3.  Propagation of updates in an iBGP full-mesh

   No transient LoC can occur if a reconfiguration of the iBGP out-
   filters on the g-shut initiator is performed.

7.1.4.  Propagation of updates from iBGP to iBGP in a RR hierarchy

   Upon the reception of the update of a primary path with a lower
   local-pref value from a client, a Route Reflector RR1 will either
   propagate the update, or select an alternate path, depending on the
   fact that the updated primary path is still the best one w.r.t. the
   state of the Adj-Rib-In of RR1.

   If the updated primary path is still the best, then the RR will
   propagate an update for this path to the iBGP neighbors to which it
   previously advertised the path.  Hence it cannot cause transient lack
   of path in the Adj-Rib-In of its iBGP neighbors.

   If an alternate path is picked, and this path was also originated by
   a client of RR1, an update will also be propagated to the same
   neighbors as the one to which the primary path was initially
   propagated.  Hence it cannot cause transient lack of path in the Adj-
   Rib-In of its iBGP neighbors.

   If an alternate path is picked, and this path was received from a
   member of its Route-Reflector iBGP full-mesh, then a withdraw message
   is sent.  As the alternate path has been sent over each session of
   the iBGP full-mesh, the propagation of a withdraw for the primary
   path of RR1 is done to routers that are expected to know the
   alternate path picked by RR1.

   The following example describes a situation where some corner case



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   timings could lead to transient unreachability from some members of
   the iBGP full-mesh.

        1.  A Route Reflector RR1 only knew about the primary path upon
        the shutdown.

        2.  A member of its RR full-mesh, RR2, propagates an update of
        the old path with a lower local-pref.

        3.  Another member of its RR full-mesh, RR3 processes the
        update, selects an alternate path, and propagates an update in
        the mesh.

        4.  RR2 receives the alternate path, selects it as best, and
        hence withdraws the updated old path on the iBGP sessions of the
        mesh.

        5.  If for any reason, RR1 receives and processes the withdraw
        generated in step 4 before processing the update generated in
        step 3, RR1 transiently suffers from unreachability for the
        affected prefix.

   In such corner cases, the solution improves the iBGP convergence
   behavior/LoC but does not ensure 0 packet loss, as we cannot define a
   simple solution relying only on a reconfiguration of the filters of
   the g-shut initiator.  Improving the availability of alternate paths
   in Route Reflectors, using [BestExternal], or [AddPath], seems to be
   the most pragmatic solution to these corner cases.

   The use of [BestExternal] in the iBGP full-mesh between RRs can solve
   these corner cases by ensuring that within an AS, the advertisement
   of a new path is not translated into the withdraw of a former path.

   Indeed, "best-external" ensures that an ASBR does not withdraw a
   previously advertised (eBGP) path when it receives an additional,
   preferred path over an iBGP session.  Also, "best-intra-cluster"
   ensures that a RR does not withdraw a previously advertised (iBGP)
   path to its non clients (e.g. other RRs in a mesh of RR) when it
   receives a new, preferred path over an iBGP session.

7.2.  Maintenance of an iBGP session

   If the shutdown does not temper with the viability of the iBGP
   topology, the described procedure is sufficient to avoid LoC.







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7.3.  Applicability of the g-shut procedure

   The applicability of the procedure described in this draft to the
   cases presented in [REQS] can be shown by combining the effects
   described in this section.  A complete case by case analysis will be
   provided in the next versions of the draft.

7.4.  Summary of operations

   This section summarizes the configurations and actions to be
   performed to support the g-shut procedure for eBGP peering links.

7.4.1.  Pre-configuration

   On each ASBR supporting the g-shut procedure, set-up an out-filter
   applied on all iBGP sessions of the ASBR, that :

   . sets the local-pref of the paths tagged with the g-shut community
   to a low value

   . removes the g-shut community from the path.

7.4.2.  Operations at maintenance time

   On the g-shut initiator :

   .  Apply an in-filter on the maintained BGP session to tag the paths
   received over the session with the g-shut community.

   .  Apply an out-filter on the maintained BGP session to tag the paths
   propagated over the session with the g-shut community.

   .  Wait for convergence to happen.

   .  Perform a BGP session shutdown.


8.  Link Up cases

   We identify two potential causes for transient packet losses upon an
   eBGP link up event.  The first one is local to the g-shut initiator,
   the second one is due to the BGP convergence following the injection
   of new best paths within the iBGP topology.

8.1.  Unreachability local to the ASBR

   An ASBR that selects as best a path received over a newly brought up
   eBGP session may transiently drop traffic.  This can typically happen



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   when the nexthop attribute differs from the IP address of the eBGP
   peer, and the receiving ASBR has not yet resolved the MAC address
   associated with the IP address of that "third party" nexthop.

   A BGP speaker implementation could avoid such losses by ensuring that
   "third party" nexthops are resolved before installing paths using
   these in the RIB.

   If the link up event corresponds to an eBGP session that is being
   manually brought up, over an already up multi-access link, then the
   operator can ping third party nexthops that are expected to be used
   before actually bringing the session up, or ping directed broadcast
   the subnet IP address of the link.  By proceeding like this, the MAC
   addresses associated with these third party nexthops will be resolved
   by the g-no-shut initiator.

8.2.  iBGP convergence

   Similar corner cases as described in Section 7.1.4 for the link down
   case, can occur during an eBGP link up event.

   A typical example for such transient unreachability for a given
   prefix is the following :

        1.  A Route Reflector, RR1, is initially advertising the current
        best path to the members of its iBGP RR full-mesh.  It
        propagated that path within its RR full-mesh.  Another route
        reflector of the full-mesh, RR2, knows only that path towards
        the prefix.
        2.  A third Route Reflector of the RR full-mesh, RR3 receives a
        new best path orginated by the "g-no-shut" initiator, being one
        of its RR clients.  RR3 selects it as best, and propagates an
        UPDATE within its RR full-mesh, i.e., to RR1 and RR2.
        3.  RR1 receives that path, reruns its decision process, and
        picks this new path as best.  As a result, RR1 withdraws its
        previously announced best-path on the iBGP sessions of its RR
        full-mesh.
        4.  If, for any reason, RR3 processes the withdraw generated in
        step 3, before processing the update generated in step 2, RR3
        transiently suffers from unreachability for the affected prefix.

   The use of [BestExternal] among the RR of the iBGP full-mesh can
   solve these corner cases by ensuring that within an AS, the
   advertisement of a new route is not translated into the withdraw of a
   former route.

   Indeed, "best-external" ensures that an ASBR does not withdraw a
   previously advertised (eBGP) path when it receives an additional,



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   preferred path over an iBGP session.  Also, "best-intra-cluster"
   ensures that a RR does not withdraw a previously advertised (iBGP)
   path to its non clients (e.g. other RRs in a mesh of RR) when it
   receives a new, preferred path over an iBGP session.


9.  Alternative techniques with limited applicability

   A few alternative techniques have been considered to provide g-shut
   capabilities but have been rejected due to their limited
   applicability.  This section describe them for possible reference.

9.1.  In-filter reconfiguration

   An In-filter reconfiguration on the eBGP session undergoing the
   maintenance could be performed instead of out-filter reconfigurations
   on the iBGP sessions of the g-shut initiator.

   Upon the application of the maintenance procedure, if the g-shut
   initiator has an alternate path in its Adj-Rib-In, it will switch to
   it directly.

   If this new path was advertised by an eBGP neighbor of the g-shut
   initiator, the g-shut initiator will send a BGP Path Update message
   advertising the new path over its iBGP and eBGP sessions.

   If this new path was received over an iBGP session, the g-shut
   initiator will select that path and withdraw the previously
   advertised path over its non-client iBGP sessions.  There can be iBGP
   topologies where the iBGP peers of the g-shut initiator do not know
   an alternate path, and hence may drop traffic.

   Also, applying an In-filter reconfiguration on the eBGP session
   undergoing the maintenance may lead to transient LoC, in full-mesh
   iBGP topologies if

        a.  An ASBR of the initiator AS, ASBR1 did not initially select
        its own external path as best, and

        b.  An ASBR of the initiator AS, ASBR2 advertises a new path
        along its iBGP sessions upon the reception of ASBR1's update
        following the in-filter reconfiguration on the g-shut initiator,
        and

        c.  ASBR1 receives the update message, runs its Decision Process
        and hence withdraws its external path after having selected
        ASBR2's path as best, and




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        d.  An impacted router of the AS processes the withdraw of ASBR1
        before processing the update from ASBR2.

   Applying a reconfiguration of the out-filters prevents such transient
   unreachabilities.

   Indeed, when the g-shut initiator propagates an update of the old
   path first, the withdraw from ASBR2 does not trigger unreachability
   in other nodes, as the old path is still available.  Indeed, even
   though it receives alternate paths, the g-shut initiator keeps using
   its old path as best as the in-filter of the maintained eBGP session
   has not been modified yet.

   Applying the out-filter reconfiguration also prevents packet loops
   between the g-shut initiator and its direct neighbors when
   encapsulation is not used between the ASBRs of the AS.

9.2.  Multi Exit Discriminator tweaking

   The MED attribute of the paths to be avoided can be increased so as
   to force the routers in the neighboring AS to select other paths.

   The solution only works if the alternate paths are as good as the
   initial ones with respect to the Local-Pref value and the AS Path
   Length value.  In the other cases, increasing the MED value will not
   have an impact on the decision process of the routers in the
   neighboring AS.

9.3.  IGP distance Poisoning

   The distance to the BGP nexthop corresponding to the maintained
   session can be increased in the IGP so that the old paths will be
   less preferred during the application of the IGP distance tie-break
   rule.  However, this solution only works for the paths whose
   alternates are as good as the old paths with respect to their Local-
   Pref value, their AS Path length, and their MED value.

   Also, this poisoning cannot be applied when nexthop self is used as
   there is no nexthop specific to the maintained session to poison in
   the IGP.


10.  IANA considerations

   Applying the g-shut procedure is rendered much easier with a reserved
   g-shut community value.  Hence this draft suggests to reserve a
   community value, e.g., 0xFFFF0000, for this purpose.



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11.  Security Considerations

   By providing the g-shut service to a neighboring AS, an ISP provides
   means to this neighbor to lower the local-pref value assigned to the
   paths received from this neighbor.

   The neighbor could abuse the technique and do inbound traffic
   engineering by declaring some prefixes as undergoing a maintenance so
   as to switch traffic to another peering link.

   If this behavior is not tolerated by the ISP, it SHOULD monitor the
   use of the g-shut community by this neighbor.


12.  Acknowledgments

   The authors wish to thank Olivier Bonaventure and Pradosh Mohapatra
   for their useful comments on this work.


13.  References

   [AddPath]  D. Walton, A. Retana, and E. Chen, "Advertisement of
              Multiple Paths in BGP", draft-walton-bgp-add-paths-06.txt
              (work in progress).

   [BestExternal]
              Marques, P., Fernando, R., Chen, E., and P. Mohapatra,
              "Advertisement of the best-external route to IBGP",
               draft-marques-idr-best-external-00.txt, July 2008.

   [REQS]     Decraene, B., Francois, P., Pelsser, C., and Z. Ahmad,
              "Requirements for the graceful shutdown of BGP sessions",
               draft-decraene-bgp-graceful-shutdown-requirements-00.txt
              , December 2007.


Authors' Addresses

   Pierre Francois
   Universite catholique de Louvain
   Place Ste Barbe, 2
   Louvain-la-Neuve  1348
   BE

   Email: pierre.francois@uclouvain.be
   URI:   http://inl.info.ucl.ac.be/pfr




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   Bruno Decraene
   France Telecom
   38-40 rue du General Leclerc
   92794 Issi Moulineaux cedex 9
   FR

   Email: bruno.decraene@orange-ftgroup.com


   Cristel Pelsser
   NTT Corporation
   9-11, Midori-Cho 3 Chrome
   Musashino-Shi, Tokyo  180-8585
   JP

   Email: pelsser.cristel@lab.ntt.co.jp


   Clarence Filsfils
   Cisco Systems
   De kleetlaan 6a
   Diegem  1831
   BE

   Email: cfilsfil@cisco.com


























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