





Internet Engineering Task Force (IETF)                          S. Hegde
Request for Comments: 8379                        Juniper Networks, Inc.
Category: Standards Track                                      P. Sarkar
ISSN: 2070-1721                                             Arrcus, Inc.
                                                              H. Gredler
                                                            RtBrick Inc.
                                                              M. Nanduri
                                                        ebay Corporation
                                                                L. Jalil
                                                                 Verizon
                                                                May 2018


                      OSPF Graceful Link Shutdown

Abstract

   When a link is being prepared to be taken out of service, the traffic
   needs to be diverted from both ends of the link.  Increasing the
   metric to the highest value on one side of the link is not sufficient
   to divert the traffic flowing in the other direction.

   It is useful for the routers in an OSPFv2 or OSPFv3 routing domain to
   be able to advertise a link as being in a graceful-shutdown state to
   indicate impending maintenance activity on the link.  This
   information can be used by the network devices to reroute the traffic
   effectively.

   This document describes the protocol extensions to disseminate
   graceful-link-shutdown information in OSPFv2 and OSPFv3.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8379.







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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Flooding Scope  . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Protocol Extensions . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  OSPFv2 Graceful-Link-Shutdown Sub-TLV . . . . . . . . . .   4
     4.2.  Remote IPv4 Address Sub-TLV . . . . . . . . . . . . . . .   4
     4.3.  Local/Remote Interface ID Sub-TLV . . . . . . . . . . . .   5
     4.4.  OSPFv3 Graceful-Link-Shutdown Sub-TLV . . . . . . . . . .   6
     4.5.  BGP-LS Graceful-Link-Shutdown TLV . . . . . . . . . . . .   6
     4.6.  Distinguishing Parallel Links . . . . . . . . . . . . . .   7
   5.  Elements of Procedure . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Point-to-Point Links  . . . . . . . . . . . . . . . . . .   8
     5.2.  Broadcast/NBMA Links  . . . . . . . . . . . . . . . . . .   9
     5.3.  Point-to-Multipoint Links . . . . . . . . . . . . . . . .  10
     5.4.  Unnumbered Interfaces . . . . . . . . . . . . . . . . . .  10
     5.5.  Hybrid Broadcast and P2MP Interfaces  . . . . . . . . . .  10
   6.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .  10
   7.  Applications  . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Overlay Network . . . . . . . . . . . . . . . . . . . . .  11
     7.2.  Controller-Based Deployments  . . . . . . . . . . . . . .  12
     7.3.  L3VPN Services and Sham Links . . . . . . . . . . . . . .  13
     7.4.  Hub and Spoke Deployment  . . . . . . . . . . . . . . . .  13
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     10.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17




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1.  Introduction

   This document describes a mechanism for gracefully taking a link out
   of service while allowing it to be used if no other path is
   available.  It also provides a mechanism to divert the traffic from
   both directions of the link.

   Many OSPFv2 or OSPFv3 deployments run on overlay networks provisioned
   by means of pseudowires or L2 circuits.  Prior to devices in the
   underlying network going offline for maintenance, it is useful to
   divert the traffic away from the node before maintenance is actually
   performed.  Since the nodes in the underlying network are not visible
   to OSPF, the existing stub-router mechanism described in [RFC6987]
   cannot be used.  In a service provider's network, there may be many
   CE-to-CE connections that run over a single PE.  It is cumbersome to
   change the metric on every CE-to-CE connection in both directions.
   This document provides a mechanism to change the metric of the link
   on the remote side and also use the link as a last-resort link if no
   alternate paths are available.  An application specific to this use
   case is described in detail in Section 7.1.

   This document provides mechanisms to advertise graceful-link-shutdown
   state in the flexible encodings provided by "OSPFv2 Prefix/Link
   Attribute Advertisement" [RFC7684] and the E-Router-LSA [RFC8362] for
   OSPFv3.  Throughout this document, OSPF is used when the text applies
   to both OSPFv2 and OSPFv3.  OSPFv2 or OSPFv3 is used when the text is
   specific to one version of the OSPF protocol.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Motivation

   The motivation of this document is to reduce manual intervention
   during maintenance activities.  The following objectives help to
   accomplish this in a range of deployment scenarios.

   1.  Advertise impending maintenance activity so that traffic from
       both directions can be diverted away from the link.

   2.  Allow the solution to be backward compatible so that nodes that
       do not understand the new advertisement do not cause routing
       loops.



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   3.  Advertise the maintenance activity to other nodes in the network
       so that Label Switched Path (LSP) ingress routers/controllers can
       learn about the impending maintenance activity and apply specific
       policies to reroute the LSPs for deployments based on Traffic
       Engineering (TE).

   4.  Allow the link to be used as a last-resort link to prevent
       traffic disruption when alternate paths are not available.

3.  Flooding Scope

   The graceful-link-shutdown information is flooded in an area-scoped
   Extended Link Opaque LSA [RFC7684] for OSPFv2 and in an E-Router-LSA
   for OSPFv3 [RFC8362].  The Graceful-Link-Shutdown sub-TLV MAY be
   processed by the head-end nodes or the controller as described in the
   Section 7.  The procedures for processing the Graceful-Link-Shutdown
   sub-TLV are described in Section 5.

4.  Protocol Extensions

4.1.  OSPFv2 Graceful-Link-Shutdown Sub-TLV

   The Graceful-Link-Shutdown sub-TLV identifies the link as being
   gracefully shutdown.  It is advertised in the Extended Link TLV of
   the Extended Link Opaque LSA as defined in [RFC7684].

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Type             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 1: Graceful-Link-Shutdown Sub-TLV for OSPFv2

   Type: 7

   Length: 0

4.2.  Remote IPv4 Address Sub-TLV

   This sub-TLV specifies the IPv4 address of the remote endpoint on the
   link.  It is advertised in the Extended Link TLV as defined in
   [RFC7684].  This sub-TLV is optional and MAY be advertised in an
   area-scoped Extended Link Opaque LSA to identify the link when there
   are multiple parallel links between two nodes.






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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Type             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Remote IPv4 Address                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 2: Remote IPv4 Address Sub-TLV

   Type: 8

   Length: 4

   Value: Remote IPv4 address.  The remote IPv4 address is used to
   identify a particular link on the remote side when there are multiple
   parallel links between two nodes.

4.3.  Local/Remote Interface ID Sub-TLV

   This sub-TLV specifies Local and Remote Interface IDs.  It is
   advertised in the Extended Link TLV as defined in [RFC7684].  This
   sub-TLV is optional and MAY be advertised in an area-scoped Extended
   Link Opaque LSA to identify the link when there are multiple parallel
   unnumbered links between two nodes.  The Local Interface ID is
   generally readily available.  One of the mechanisms to obtain the
   Remote Interface ID is described in [RFC4203].

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Type             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Local Interface ID                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Remote Interface ID                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: Local/Remote Interface ID Sub-TLV

   Type: 9

   Length: 8

   Value: 4 octets of the Local Interface ID followed by 4 octets of the
   Remote Interface ID.





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4.4.  OSPFv3 Graceful-Link-Shutdown Sub-TLV

   The Graceful-Link-Shutdown sub-TLV is carried in the Router-Link TLV
   as defined in [RFC8362] for OSPFv3.  The Router-Link TLV contains the
   Neighbor Interface ID and can uniquely identify the link on the
   remote node.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Type             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 4: Graceful-Link-Shutdown Sub-TLV for OSPFv3

   Type: 8

   Length: 0

4.5.  BGP-LS Graceful-Link-Shutdown TLV

   BGP-LS as defined in [RFC7752] is a mechanism that distributes
   network information to the external entities using the BGP routing
   protocol.  Graceful link shutdown is important link information that
   the external entities can use for various use cases as defined in
   Section 7.  BGP Link Network Layer Reachability Information (NLRI) is
   used to carry the link information.  A new TLV called "Graceful-Link-
   Shutdown" is defined to describe the link attribute corresponding to
   graceful-link-shutdown state.  The TLV format is as described in
   Section 3.1 of [RFC7752].  There is no Value field, and the Length
   field is set to zero for this TLV.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Type             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 5: Graceful-Link-Shutdown TLV for BGP-LS

   Type: 1121

   Length: 0








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4.6.  Distinguishing Parallel Links

                    ++++++++++I.w            I.y+++++++++++
                    |Router A|------------------|Router B |
                    |        |------------------|         |
                    ++++++++++I.x            I.z+++++++++++

                         Figure 6: Parallel Links

   Consider two routers, A and B, connected with two parallel
   point-to-point interfaces.  I.w and I.x represent the interface
   address on Router A's side, and I.y and I.z represent interface
   addresses on Router B's side.  The Extended Link Opaque LSA as
   defined in [RFC7684] describes links using Link Type, Link ID, and
   Link Data.  For example, a link with the address I.w is described as
   below on Router A.

      Link Type = Point-to-point

      Link ID = Router ID of B

      Link Data = I.w

   A third node (controller or head-end) in the network cannot
   distinguish the interface on Router B, which is connected to this
   particular Interface on Router A based on the link information
   described above.  The interface with address I.y or I.z could be
   chosen due to this ambiguity.  In such cases, a Remote IPv4 Address
   sub-TLV should be originated and added to the Extended Link TLV.  The
   use cases as described in Section 7 require controller or head-end
   nodes to interpret the graceful-link-shutdown information and hence
   the need for the Remote IPv4 Address sub-TLV.  I.y is carried in the
   Extended Link TLV, which unambiguously identifies the interface on
   the remote side.  The OSPFv3 Router-Link TLV as described in
   [RFC8362] contains an Interface ID and a neighbor's Interface ID,
   which can uniquely identify connecting the interface on the remote
   side; hence, OSPFv3 does not require a separate remote IPv6 address
   to be advertised along with the OSPFv3 Graceful-Link-Shutdown
   sub-TLV.












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5.  Elements of Procedure

   As defined in [RFC7684], every link on the node will have a separate
   Extended Link Opaque LSA.  The node that has the link to be taken out
   of service MUST advertise the Graceful-Link-Shutdown sub-TLV in the
   Extended Link TLV of the Extended Link Opaque LSA for OSPFv2, as
   defined in [RFC7684], and in the Router-Link TLV of E-Router-LSA for
   OSPFv3.  The Graceful-Link-Shutdown sub-TLV indicates that the link
   identified by the sub-TLV is subjected to maintenance.

   For the purposes of changing the metric OSPFv2 and OSPFv3 Router-LSAs
   need to be reoriginated.  To change the Traffic Engineering metric,
   TE Opaque LSAs in OSPFv2 [RFC3630] and Intra-area-TE-LSAs in OSPFv3
   [RFC5329] need to be reoriginated.

   The graceful-link-shutdown information is advertised as a property of
   the link and is flooded through the area.  This information can be
   used by ingress routers or controllers to take special actions.  An
   application specific to this use case is described in Section 7.2.

   When a link is ready to carry traffic, the Graceful-Link-Shutdown
   sub-TLV MUST be removed from the Extended Link TLV/Router-Link TLV,
   and the corresponding LSAs MUST be readvertised.  Similarly, the
   metric MUST be set to original values, and the corresponding LSAs
   MUST be readvertised.

   The procedures described in this document may be used to divert the
   traffic away from the link in scenarios other than link-shutdown or
   link-replacement activity.

   The precise action taken by the remote node at the other end of the
   link identified for graceful-shutdown depends on the link type.

5.1.  Point-to-Point Links

   The node that has the link to be taken out of service MUST set the
   metric of the link to MaxLinkMetric (0xffff) and reoriginate its
   Router-LSA.  The Traffic Engineering metric of the link SHOULD be set
   to (0xffffffff), and the node SHOULD reoriginate the corresponding TE
   Link Opaque LSAs.  When a Graceful-Link-Shutdown sub-TLV is received
   for a point-to-point link, the remote node MUST identify the local
   link that corresponds to the graceful-shutdown link and set its
   metric to MaxLinkMetric (0xffff), and the remote node MUST
   reoriginate its Router-LSA with the changed metric.  When TE is
   enabled, the Traffic Engineering metric of the link SHOULD be set to
   (0xffffffff) and follow the procedures in [RFC5817].  Similarly, the





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   remote node SHOULD set the Traffic Engineering metric of the link to
   0xffffffff and SHOULD reoriginate the TE Link Opaque LSA for the link
   with the new value.

   The Extended Link Opaque LSAs and the Extended Link TLV are not
   scoped for multi-topology [RFC4915].  In multi-topology deployments
   [RFC4915], the Graceful-Link-Shutdown sub-TLV advertised in an
   Extended Link Opaque LSA corresponds to all the topologies that
   include the link.  The receiver node SHOULD change the metric in the
   reverse direction for all the topologies that include the remote link
   and reoriginate the Router-LSA as defined in [RFC4915].

   When the originator of the Graceful-Link-Shutdown sub-TLV purges the
   Extended Link Opaque LSA or reoriginates it without the
   Graceful-Link-Shutdown sub-TLV, the remote node must reoriginate the
   appropriate LSAs with the metric and TE metric values set to their
   original values.

5.2.  Broadcast/NBMA Links

   Broadcast or Non-Broadcast Multi-Access (NBMA) networks in OSPF are
   represented by a star topology where the Designated Router (DR) is
   the central point to which all other routers on the broadcast or NBMA
   network logically connect.  As a result, routers on the broadcast or
   NBMA network advertise only their adjacency to the DR.  Routers that
   do not act as DRs do not form or advertise adjacencies with each
   other.  For the broadcast links, the MaxLinkMetric on the remote link
   cannot be changed since all the neighbors are on same link.  Setting
   the link cost to MaxLinkMetric would impact all paths that traverse
   any of the neighbors connected on that broadcast link.

   The node that has the link to be taken out of service MUST set the
   metric of the link to MaxLinkMetric (0xffff) and reoriginate the
   Router-LSA.  The Traffic Engineering metric of the link SHOULD be set
   to (0xffffffff), and the node SHOULD reoriginate the corresponding TE
   Link Opaque LSAs.  For a broadcast link, the two-part metric as
   described in [RFC8042] is used.  The node originating the
   Graceful-Link-Shutdown sub-TLV MUST set the metric in the
   Network-to-Router Metric sub-TLV to MaxLinkMetric (0xffff) for OSPFv2
   and OSPFv3 and reoriginate the corresponding LSAs.  The nodes that
   receive the two-part metric should follow the procedures described in
   [RFC8042].  The backward-compatibility procedures described in
   [RFC8042] should be followed to ensure loop-free routing.








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5.3.  Point-to-Multipoint Links

   Operation for the point-to-multipoint (P2MP) links is similar to the
   point-to-point links.  When a Graceful-Link-Shutdown sub-TLV is
   received for a point-to-multipoint link, the remote node MUST
   identify the neighbor that corresponds to the graceful-shutdown link
   and set its metric to MaxLinkMetric (0xffff).  The remote node MUST
   reoriginate the Router-LSA with the changed metric for the
   corresponding neighbor.

5.4.  Unnumbered Interfaces

   Unnumbered interfaces do not have a unique IP address and borrow
   their address from other interfaces.  [RFC2328] describes procedures
   to handle unnumbered interfaces in the context of the Router-LSA.  We
   apply a similar procedure to the Extended Link TLV advertising the
   Graceful-Link-Shutdown sub-TLV in order to handle unnumbered
   interfaces.  The Link-Data field in the Extended Link TLV includes
   the Local Interface ID instead of the IP address.  The Local/Remote
   Interface ID sub-TLV MUST be advertised when there are multiple
   parallel unnumbered interfaces between two nodes.  One of the
   mechanisms to obtain the Interface ID of the remote side is defined
   in [RFC4203].

5.5.  Hybrid Broadcast and P2MP Interfaces

   Hybrid Broadcast and P2MP interfaces represent a broadcast network
   modeled as P2MP interfaces.  [RFC6845] describes procedures to handle
   these interfaces.  Operation for the Hybrid interfaces is similar to
   operation for the P2MP interfaces.  When a Graceful-Link-Shutdown
   sub-TLV is received for a hybrid link, the remote node MUST identify
   the neighbor that corresponds to the graceful-shutdown link and set
   its metric to MaxLinkMetric (0xffff).  All the remote nodes connected
   to the originator MUST reoriginate the Router-LSA with the changed
   metric for the neighbor.

6.  Backward Compatibility

   The mechanisms described in the document are fully backward
   compatible.  It is required that the node adverting the
   Graceful-Link-Shutdown sub-TLV as well as the node at the remote end
   of the graceful-shutdown link support the extensions described herein
   for the traffic to be diverted from the graceful-shutdown link.  If
   the remote node doesn't support the capability, it will still use the
   graceful-shutdown link, but there are no other adverse effects.  In
   the case of broadcast links using two-part metrics, the backward-
   compatibility procedures as described in [RFC8042] are applicable.




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7.  Applications

7.1.  Overlay Network

   Many service providers offer L2 services to a customer connecting
   different locations.  The customer's IGP protocol creates a seamless
   private network (overlay network) across the locations for the
   customer.  Service providers want to offer graceful-shutdown
   functionality when the PE device is taken out for maintenance.  There
   can be large number of customers attached to a PE node, and the
   remote endpoints for these L2 attachment circuits are spread across
   the service provider's network.  Changing the metric for all
   corresponding L2 circuits in both directions is a tedious and error-
   prone process.  The graceful-link-shutdown feature simplifies the
   process by increasing the metric on the CE-CE overlay link so that
   traffic in both directions is diverted away from the PE undergoing
   maintenance.  The graceful-link-shutdown feature allows the link to
   be used as a last-resort link so that traffic is not disrupted when
   alternate paths are not available.

                     ------PE3---------------PE4------CE3
                   /                           \
                 /                               \
              CE1---------PE1----------PE2---------CE2
                                       \
                                        \
                                         ------CE4

   CE: Customer Edge
   PE: Provider Edge

                         Figure 7: Overlay Network

   In the example shown in Figure 7, when the PE1 node is going out of
   service for maintenance, a service provider sets the PE1 to stub-
   router state and communicates the pending maintenance action to the
   overlay customer networks.  The mechanisms used to communicate
   between PE1 and CE1 is outside the scope of this document.  CE1 sets
   the graceful-link-shutdown state on its links connecting CE3, CE2,
   and CE4, changes the metric to MaxLinkMetric, and reoriginates the
   corresponding LSA.  The remote end of the link at CE3, CE2, and CE4
   also set the metric on the link to MaxLinkMetric, and the traffic
   from both directions gets diverted away from PE1.








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7.2.  Controller-Based Deployments

   In controller-based deployments where the controller participates in
   the IGP protocol, the controller can also receive the
   graceful-link-shutdown information as a warning that link maintenance
   is imminent.  Using this information, the controller can find
   alternate paths for traffic that uses the affected link.  The
   controller can apply various policies and reroute the LSPs away from
   the link undergoing maintenance.  If there are no alternate paths
   satisfying the constraints, the controller might temporarily relax
   those constraints and put the service on a different path.
   Increasing the link metric alone does not specify the maintenance
   activity as the metric could increase in events such as LDP-IGP
   synchronization.  An explicit indication from the router using the
   Graceful-Link-Shutdown sub-TLV is needed to inform the controller or
   head-end routers.

                              _____________
                             |             |
               --------------| Controller  |--------------
               |             |____________ |             |
               |                                         |
               |--------- Primary Path ------------------|
               PE1---------P1----------------P2---------PE2
                           |                  |
                           |                  |
                           |________P3________|

                              Alternate Path

              Figure 8: Controller-Based Traffic Engineering

   In the above example, the PE1->PE2 LSP is set up to satisfy a
   constraint of 10 Gbps bandwidth on each link.  The links P1->P3 and
   P3->P2 have only 1 Gbps capacity, and there is no alternate path
   satisfying the bandwidth constraint of 10 Gbps.  When the P1->P2 link
   is being prepared for maintenance, the controller receives the
   graceful-link-shutdown information, as there is no alternate path
   available that satisfies the constraints, and the controller chooses
   a path that is less optimal and temporarily sets up an alternate path
   via P1->P3->P2.  Once the traffic is diverted, the P1->P2 link can be
   taken out of service for maintenance/upgrade.









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7.3.  L3VPN Services and Sham Links

   Many service providers offer Layer 3 Virtual Private Network (L3VPN)
   services to customers, and CE-PE links run OSPF [RFC4577].  When the
   PE is taken out of service for maintenance, all the links on the PE
   can be set to graceful-link-shutdown state, which will guarantee that
   the traffic to/from dual-homed CEs gets diverted.  The interaction
   between OSPF and BGP is outside the scope of this document.  A
   mechanism based on [RFC6987] with summaries and externals that are
   advertised with high metrics could also be used to achieve the same
   functionality when implementations support high metrics advertisement
   for summaries and externals.

   Another useful use case is when ISPs provide sham-link services to
   customers [RFC4577].  When the PE goes out of service for
   maintenance, all sham links on the PE can be set to graceful-link-
   shutdown state, and traffic can be diverted from both ends without
   having to touch the configurations on the remote end of the sham
   links.

7.4.  Hub and Spoke Deployment

   OSPF is largely deployed in Hub and Spoke deployments with a large
   number of Spokes connecting to the Hub.  It is a general practice to
   deploy multiple Hubs with all Spokes connecting to these Hubs to
   achieve redundancy.  The mechanism defined in [RFC6987] can be used
   to divert the Spoke-to-Spoke traffic from the overloaded Hub router.
   The traffic that flows from Spokes via the Hub into an external
   network may not be diverted in certain scenarios.  When a Hub node
   goes down for maintenance, all links on the Hub can be set to
   graceful-link-shutdown state, and traffic gets diverted from the
   Spoke sites as well without having to make configuration changes on
   the Spokes.

8.  Security Considerations

   This document utilizes the OSPF packets and LSAs described in
   [RFC2328] , [RFC3630], [RFC5329], and [RFC5340].  The authentication
   procedures described in [RFC2328] for OSPFv2 and [RFC4552] for OSPFv3
   are applicable to this document as well.  This document does not
   introduce any further security issues other than those discussed in
   [RFC2328] and [RFC5340].









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9.  IANA Considerations

   IANA has registered the following in the "OSPFv2 Extended Link TLV
   Sub-TLVs" registry:

      7 - Graceful-Link-Shutdown Sub-TLV

      8 - Remote IPv4 Address Sub-TLV

      9 - Local/Remote Interface ID Sub-TLV

   IANA has registered the following value in the "OSPFv3 Extended-LSA
   Sub-TLVs" registry:

      8 - Graceful-Link-Shutdown sub-TLV

   IANA has registered the following value in the "BGP-LS Node
   Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs"
   registry [RFC7752]":

      1121 - Graceful-Link-Shutdown TLV

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              DOI 10.17487/RFC3630, September 2003,
              <https://www.rfc-editor.org/info/rfc3630>.

   [RFC5329]  Ishiguro, K., Manral, V., Davey, A., and A. Lindem, Ed.,
              "Traffic Engineering Extensions to OSPF Version 3",
              RFC 5329, DOI 10.17487/RFC5329, September 2008,
              <https://www.rfc-editor.org/info/rfc5329>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.



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   [RFC5817]  Ali, Z., Vasseur, JP., Zamfir, A., and J. Newton,
              "Graceful Shutdown in MPLS and Generalized MPLS Traffic
              Engineering Networks", RFC 5817, DOI 10.17487/RFC5817,
              April 2010, <https://www.rfc-editor.org/info/rfc5817>.

   [RFC6845]  Sheth, N., Wang, L., and J. Zhang, "OSPF Hybrid Broadcast
              and Point-to-Multipoint Interface Type", RFC 6845,
              DOI 10.17487/RFC6845, January 2013,
              <https://www.rfc-editor.org/info/rfc6845>.

   [RFC6987]  Retana, A., Nguyen, L., Zinin, A., White, R., and D.
              McPherson, "OSPF Stub Router Advertisement", RFC 6987,
              DOI 10.17487/RFC6987, September 2013,
              <https://www.rfc-editor.org/info/rfc6987>.

   [RFC7684]  Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
              Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
              Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
              2015, <https://www.rfc-editor.org/info/rfc7684>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [RFC8042]  Zhang, Z., Wang, L., and A. Lindem, "OSPF Two-Part
              Metric", RFC 8042, DOI 10.17487/RFC8042, December 2016,
              <https://www.rfc-editor.org/info/rfc8042>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8362]  Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
              F. Baker, "OSPFv3 Link State Advertisement (LSA)
              Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
              2018, <https://www.rfc-editor.org/info/rfc8362>.













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10.2.  Informative References

   [RFC4203]  Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
              Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
              <https://www.rfc-editor.org/info/rfc4203>.

   [RFC4552]  Gupta, M. and N. Melam, "Authentication/Confidentiality
              for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
              <https://www.rfc-editor.org/info/rfc4552>.

   [RFC4577]  Rosen, E., Psenak, P., and P. Pillay-Esnault, "OSPF as the
              Provider/Customer Edge Protocol for BGP/MPLS IP Virtual
              Private Networks (VPNs)", RFC 4577, DOI 10.17487/RFC4577,
              June 2006, <https://www.rfc-editor.org/info/rfc4577>.

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, DOI 10.17487/RFC4915, June 2007,
              <https://www.rfc-editor.org/info/rfc4915>.

Acknowledgements

   Thanks to Chris Bowers for valuable input and edits to the document.
   Thanks to Jeffrey Zhang, Acee Lindem, and Ketan Talaulikar for their
   input.  Thanks to Karsten Thomann for careful review and input on the
   applications where graceful link shutdown is useful.

   Thanks to Alia Atlas, Deborah Brungard, Alvaro Retana, Andrew G.
   Malis, and Tim Chown for their valuable input.





















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Authors' Addresses

   Shraddha Hegde
   Juniper Networks, Inc.
   Embassy Business Park
   Bangalore, KA  560093
   India

   Email: shraddha@juniper.net


   Pushpasis Sarkar
   Arrcus, Inc.

   Email: pushpasis.ietf@gmail.com


   Hannes Gredler
   RtBrick Inc.

   Email: hannes@rtbrick.com


   Mohan Nanduri
   ebay Corporation
   2025 Hamilton Avenue
   San Jose, CA  98052
   United States of America

   Email: mnanduri@ebay.com


   Luay Jalil
   Verizon

   Email: luay.jalil@verizon.com















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