    Internet Engineering Task Force (IETF)                           N. Shen
    Request for Comments: 8500                                 Cisco Systems
    Category: Standards Track                                      S. Amante
    ISSN: 2070-1721                                               Apple Inc.
                                                              M. Abrahamsson
                                                            T-Systems Nordic
                                                               February 2019

                       IS-IS Routing with Reverse Metric

    Abstract

       This document describes a mechanism to allow IS-IS routing to quickly
       and accurately shift traffic away from either a point-to-point or
       multi-access LAN interface during network maintenance or other
       operational events.  This is accomplished by signaling adjacent IS-IS
       neighbors with a higher reverse metric, i.e., the metric towards the
       signaling IS-IS router.

    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/rfc8500.

    Copyright Notice

       Copyright (c) 2019 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.

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    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
         1.1.  Node and Link Isolation . . . . . . . . . . . . . . . . .   2
         1.2.  Distributed Forwarding Planes . . . . . . . . . . . . . .   3
         1.3.  Spine-Leaf Applications . . . . . . . . . . . . . . . . .   3
         1.4.  LDP IGP Synchronization . . . . . . . . . . . . . . . . .   3
         1.5.  IS-IS Reverse Metric  . . . . . . . . . . . . . . . . . .   3
         1.6.  Specification of Requirements . . . . . . . . . . . . . .   4
       2.  IS-IS Reverse Metric TLV  . . . . . . . . . . . . . . . . . .   4
       3.  Elements of Procedure . . . . . . . . . . . . . . . . . . . .   6
         3.1.  Processing Changes to Default Metric  . . . . . . . . . .   6
         3.2.  Multi-Topology IS-IS Support on Point-to-Point Links  . .   7
         3.3.  Multi-access LAN Procedures . . . . . . . . . . . . . . .   7
         3.4.  LDP/IGP Synchronization on LANs . . . . . . . . . . . . .   8
         3.5.  Operational Guidelines  . . . . . . . . . . . . . . . . .   9
       4.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
       5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
       6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
         6.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
         6.2.  Informative References  . . . . . . . . . . . . . . . . .  11
       Appendix A.  Node Isolation Challenges  . . . . . . . . . . . . .  13
       Appendix B.  Link Isolation Challenges  . . . . . . . . . . . . .  13
       Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  15
       Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

    1.  Introduction

       The IS-IS [ISO10589] routing protocol has been widely used in
       Internet Service Provider IP/MPLS networks.  Operational experience
       with the protocol combined with ever increasing requirements for
       lossless operations have demonstrated some operational issues.  This
       document describes the issues and a mechanism for mitigating them.

       This document defines the IS-IS "Reverse Metric" mechanism that
       allows an IS-IS node to send a Reverse Metric TLV through the IS-IS
       Hello (IIH) PDU to the neighbor or pseudonode to adjust the routing
       metric on the inbound direction.

    1.1.  Node and Link Isolation

       The IS-IS routing mechanism has the overload bit, which can be used
       by operators to perform disruptive maintenance on the router.  But in
       many operational maintenance cases, it is not necessary to divert all
       the traffic away from this node.  It is necessary to avoid only a
       single link during the maintenance.  More detailed descriptions of
       the challenges can be found in Appendices A and B of this document.

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    1.2.  Distributed Forwarding Planes

       In a distributed forwarding platform, different forwarding line cards
       may have interfaces and IS-IS connections to neighbor routers.  If
       one of the line card's software resets, it may take some time for the
       forwarding entries to be fully populated on the line card, in
       particular if the router is a PE (Provider Edge) router in an ISP's
       MPLS VPN.  An IS-IS adjacency may be established with a neighbor
       router long before the entire BGP VPN prefixes are downloaded to the
       forwarding table.  It is important to signal to the adjacent IS-IS
       routers to raise metric values and not to use the corresponding IS-IS
       adjacency inbound to this router if possible.  Temporarily signaling
       the 'Reverse Metric' over this link to discourage the traffic via the
       corresponding line card will help to reduce the traffic loss in the
       network.  In the meantime, the remote PE routers will select a
       different set of PE routers for the BGP best path calculation or use
       a different link towards the same PE router on which a line card is
       resetting.

    1.3.  Spine-Leaf Applications

       In the IS-IS Spine-Leaf extension [IS-IS-SL-EXT], the leaf nodes will
       perform equal-cost or unequal-cost load sharing towards all the spine
       nodes.  In certain operational cases, for instance, when one of the
       backbone links on a spine node is congested, a spine node can push a
       higher metric towards the connected leaf nodes to reduce the transit
       traffic through the corresponding spine node or link.

    1.4.  LDP IGP Synchronization

       In [RFC5443], a mechanism is described to achieve LDP IGP
       synchronization by using the maximum link metric value on the
       interface.  But in the case of a new IS-IS node joining the broadcast
       network (LAN), it is not optimal to change all the nodes on the LAN
       to the maximum link metric value, as described in [RFC6138].  In this
       case, the Reverse Metric can be used to discourage both outbound and
       inbound traffic without affecting the traffic of other IS-IS nodes on
       the LAN.

    1.5.  IS-IS Reverse Metric

       This document uses the routing protocol itself as the transport
       mechanism to allow one IS-IS router to advertise a "reverse metric"
       in an IS-IS Hello (IIH) PDU to an adjacent node on a point-to-point
       or multi-access LAN link.  This would allow the provisioning to be
       performed only on a single node, setting a "reverse metric" on a link
       and having traffic bidirectionally shift away from that link
       gracefully to alternate viable paths.

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       This Reverse Metric mechanism is used for both point-to-point and
       multi-access LAN links.  Unlike the point-to-point links, the IS-IS
       protocol currently does not have a way to influence the traffic
       towards a particular node on LAN links.  This mechanism provides
       IS-IS routing with the capability of altering traffic in both
       directions on either a point-to-point link or a multi-access link of
       an IS-IS node.

       The metric value in the Reverse Metric TLV and the Traffic
       Engineering metric in the sub-TLV being advertised are offsets or
       relative metrics to be added to the existing local link and Traffic
       Engineering metric values of the receiver; the accumulated metric
       value is bounded as described in Section 2.

    1.6.  Specification of Requirements

       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.  IS-IS Reverse Metric TLV

       The Reverse Metric TLV is a new TLV to be used inside an IS-IS Hello
       PDU.  This TLV is used to support the IS-IS Reverse Metric mechanism
       that allows a "reverse metric" to be sent to the IS-IS neighbor.

           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    |    Flags      |     Metric
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Metric  (Continued)       | sub-TLV Len   |Optional sub-TLV
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 1: Reverse Metric TLV

       The Value part of the Reverse Metric TLV is composed of a 3 octet
       field containing an IS-IS Metric value, a 1 octet field of Flags, and
       a 1 octet Reverse Metric sub-TLV length field representing the length
       of a variable number of sub-TLVs.  If the "sub-TLV Len" is non-zero,
       then the Value field MUST also contain one or more sub-TLVs.

       The Reverse Metric TLV MAY be present in any IS-IS Hello PDU.  A
       sender MUST only transmit a single Reverse Metric TLV in an IS-IS
       Hello PDU.  If a received IS-IS Hello PDU contains more than one

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       Reverse Metric TLV, an implementation MUST ignore all the Reverse
       Metric TLVs.

          TYPE: 16
          LENGTH: variable (5 - 255 octets)
          VALUE:

             Flags (1 octet)
             Metric (3 octets)
             sub-TLV length (1 octet)
             sub-TLV data (0 - 250 octets)

              0 1 2 3 4 5 6 7
             +-+-+-+-+-+-+-+-+
             |  Reserved |U|W|
             +-+-+-+-+-+-+-+-+

                                  Figure 2: Flags

       The Metric field contains a 24-bit unsigned integer.  This value is a
       metric offset that a neighbor SHOULD add to the existing configured
       Default Metric for the IS-IS link [ISO10589].  Refer to "Elements of
       Procedure" in Section 3 of this document for details on how an IS-IS
       router should process the Metric field in a Reverse Metric TLV.

       The Metric field, in the Reverse Metric TLV, is a "reverse offset
       metric" that will either be in the range of 0 - 63 when a "narrow"
       IS-IS metric is used (IS Neighbors TLV / Pseudonode LSP) [RFC1195] or
       in the range of 0 - (2^24 - 2) when a "wide" Traffic Engineering
       metric value is used (Extended IS Reachability TLV) [RFC5305]
       [RFC5817].  As described below, when the U bit is set, the
       accumulated value of the wide metric is in the range of
       0 - (2^24 - 1), with the (2^24 - 1) metric value as non-reachable in
       IS-IS routing.  The IS-IS metric value of (2^24 - 2) serves as the
       link of last resort.

       There are currently only two Flag bits defined.

       W bit (0x01): The "Whole LAN" bit is only used in the context of
       multi-access LANs.  When a Reverse Metric TLV is transmitted from a
       node to the Designated Intermediate System (DIS), if the "Whole LAN"
       bit is set (1), then a DIS SHOULD add the received Metric value in
       the Reverse Metric TLV to each node's existing Default Metric in the
       Pseudonode LSP.  If the "Whole LAN" bit is not set (0), then a DIS
       SHOULD add the received Metric value in the Reverse Metric TLV to the
       existing "default metric" in the Pseudonode LSP for the single node
       from whom the Reverse Metric TLV was received.  Please refer to
       "Multi-access LAN Procedures", in Section 3.3, for additional

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       details.  The W bit MUST be clear when a Reverse Metric TLV is
       transmitted in an IIH PDU on a point-to-point link and MUST be
       ignored when received on a point-to-point link.

       U bit (0x02): The "Unreachable" bit specifies that the metric
       calculated by the addition of the reverse metric to the "default
       metric" is limited to the maximum value of (2^24-1).  This "U" bit
       applies to both the default metric in the Extended IS Reachability
       TLV and the Traffic Engineering Default Metric sub-TLV of the link.
       This is only relevant to the IS-IS "wide" metric mode.

       The Reserved bits of Flags field MUST be set to zero and MUST be
       ignored when received.

       The Reverse Metric TLV MAY include sub-TLVs when an IS-IS router
       wishes to signal additional information to its neighbor.  In this
       document, the Reverse Metric Traffic Engineering Metric sub-TLV, with
       Type 18, is defined.  This Traffic Engineering Metric contains a
       24-bit unsigned integer.  This sub-TLV is optional; if it appears
       more than once, then the entire Reverse Metric TLV MUST be ignored.
       Upon receiving this Traffic Engineering METRIC sub-TLV in a Reverse
       Metric TLV, a node SHOULD add the received Traffic Engineering Metric
       offset value to its existing configured Traffic Engineering Default
       Metric within its Extended IS Reachability TLV.  The use of other
       sub-TLVs is outside the scope of this document.  The "sub-TLV Len"
       value MUST be set to zero when an IS-IS router does not have Traffic
       Engineering sub-TLVs that it wishes to send to its IS-IS neighbor.

    3.  Elements of Procedure

    3.1.  Processing Changes to Default Metric

       It is important to use the same IS-IS metric type on both ends of the
       link and in the entire IS-IS area or level.  On the receiving side of
       the 'reverse-metric' TLV, the accumulated value of the configured
       metric and the reverse-metric needs to be limited to 63 in "narrow"
       metric mode and to (2^24 - 2) in "wide" metric mode.  This applies to
       both the Default Metric of Extended IS Reachability TLV and the
       Traffic Engineering Default Metric sub-TLV in LSP or Pseudonode LSP
       for the "wide" metric mode case.  If the "U" bit is present in the
       flags, the accumulated metric value is to be limited to (2^24 - 1)
       for both the normal link metric and Traffic Engineering metric in
       IS-IS "wide" metric mode.

       If an IS-IS router is configured to originate a Traffic Engineering
       Default Metric sub-TLV for a link but receives a Reverse Metric TLV
       from its neighbor that does not contain a Traffic Engineering Default

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       Metric sub-TLV, then the IS-IS router MUST NOT change the value of
       its Traffic Engineering Default Metric sub-TLV for that link.

    3.2.  Multi-Topology IS-IS Support on Point-to-Point Links

       The Reverse Metric TLV is applicable to Multi-topology IS-IS (M-ISIS)
       [RFC5120].  On point-to-point links, if an IS-IS router is configured
       for M-ISIS, it MUST send only a single Reverse Metric TLV in IIH PDUs
       toward its neighbor(s) on the designated link.  When an M-ISIS router
       receives a Reverse Metric TLV, it MUST add the received Metric value
       to its Default Metric of the link in all Extended IS Reachability
       TLVs for all topologies.  If an M-ISIS router receives a Reverse
       Metric TLV with a Traffic Engineering Default Metric sub-TLV, then
       the M-ISIS router MUST add the received Traffic Engineering Default
       Metric value to each of its Default Metric sub-TLVs in all of its MT
       Intermediate Systems TLVs.  If an M-ISIS router is configured to
       advertise Traffic Engineering Default Metric sub-TLVs for one or more
       topologies but does not receive a Traffic Engineering Default Metric
       sub-TLV in a Reverse Metric TLV, then the M-ISIS router MUST NOT
       change the value in each of the Traffic Engineering Default Metric
       sub-TLVs for all topologies.

    3.3.  Multi-access LAN Procedures

       On a Multi-access LAN, only the DIS SHOULD act upon information
       contained in a received Reverse Metric TLV.  All non-DIS nodes MUST
       silently ignore a received Reverse Metric TLV.  The decision process
       of the routers on the LAN MUST follow the procedure in
       Section 7.2.8.2 of [ISO10589], and use the "Two-way connectivity
       check" during the topology and route calculation.

       The Reverse Metric Traffic Engineering sub-TLV also applies to the
       DIS.  If a DIS is configured to apply Traffic Engineering over a link
       and it receives Traffic Engineering Metric sub-TLV in a Reverse
       Metric TLV, it should update the Traffic Engineering Default Metric
       sub-TLV value of the corresponding Extended IS Reachability TLV or
       insert a new one if not present.

       In the case of multi-access LANs, the "W" Flags bit is used to signal
       from a non-DIS to the DIS whether or not to change the metric and,
       optionally, Traffic Engineering parameters for all nodes in the
       Pseudonode LSP or solely the node on the LAN originating the Reverse
       Metric TLV.

       A non-DIS node, e.g., Router B, attached to a multi-access LAN will
       send the DIS a Reverse Metric TLV with the W bit clear when Router B
       wishes the DIS to add the Metric value to the Default Metric
       contained in the Pseudonode LSP specific to just Router B.  Other

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       non-DIS nodes, e.g., Routers C and D, may simultaneously send a
       Reverse Metric TLV with the W bit clear to request the DIS to add
       their own Metric value to their Default Metric contained in the
       Pseudonode LSP.

       As long as at least one IS-IS node on the LAN sending the signal to
       DIS with the W bit set, the DIS would add the metric value in the
       Reverse Metric TLV to all neighbor adjacencies in the Pseudonode LSP,
       regardless if some of the nodes on the LAN advertise the Reverse
       Metric TLV without the W bit set.  The DIS MUST use the reverse
       metric of the highest source MAC address Non-DIS advertising the
       Reverse Metric TLV with the W bit set.

       Local provisioning on the DIS to adjust the Default Metric(s) is
       another way to insert Reverse Metric in the Pseudonode LSP towards an
       IS-IS node on a LAN.  In the case where a Reverse Metric TLV is also
       used in the IS-IS Hello PDU of the node, the local provisioning MUST
       take precedence over received Reverse Metric TLVs.  For instance,
       local policy on the DIS may be provisioned to ignore the W bit
       signaling on a LAN.

       Multi-topology IS-IS [RFC5120] specifies there is no change to
       construction of the Pseudonode LSP regardless of the Multi-topology
       (MT) capabilities of a multi-access LAN.  If any MT capable node on
       the LAN advertises the Reverse Metric TLV to the DIS, the DIS should
       update, as appropriate, the Default Metric contained in the
       Pseudonode LSP.  If the DIS updates the Default Metric and floods a
       new Pseudonode LSP, those default metric values will be applied to
       all topologies during Multi-topology Shortest Path First
       calculations.

    3.4.  LDP/IGP Synchronization on LANs

       As described in [RFC6138], when a new IS-IS node joins a broadcast
       network, it is unnecessary and sometimes even harmful for all IS-IS
       nodes on the LAN to advertise the maximum link metric.  [RFC6138]
       proposes a solution to have the new node not advertise its adjacency
       towards the pseudonode when it is not in a "cut-edge" position.

       With the introduction of Reverse Metric in this document, a simpler
       alternative solution to the above mentioned problem can be used.  The
       Reverse Metric allows the new node on the LAN to advertise its
       inbound metric value to be the maximum, and this puts the link of
       this new node in the last resort position without impacting the other
       IS-IS nodes on the same LAN.

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       Specifically, when IS-IS adjacencies are being established by the new
       node on the LAN, besides setting the maximum link metric value
       (2^24 - 2) on the interface of the LAN for LDP IGP synchronization as
       described in [RFC5443], it SHOULD advertise the maximum metric offset
       value in the Reverse Metric TLV in its IIH PDU sent on the LAN.  It
       SHOULD continue this advertisement until it completes all the LDP
       label binding exchanges with all the neighbors over this LAN, either
       by receiving the LDP End-of-LIB [RFC5919] for all the sessions or by
       exceeding the provisioned timeout value for the node LDP/IGP
       synchronization.

    3.5.  Operational Guidelines

       For the use case in Section 1.1, a router SHOULD limit the period of
       advertising a Reverse Metric TLV towards a neighbor only for the
       duration of a network maintenance window.

       The use of a Reverse Metric does not alter IS-IS metric parameters
       stored in a router's persistent provisioning database.

       If routers that receive a Reverse Metric TLV send a syslog message or
       SNMP trap, this will assist in rapidly identifying the node in the
       network that is advertising an IS-IS metric or Traffic Engineering
       parameters different from that which is configured locally on the
       device.

       When the link Traffic Engineering metric is raised to (2^24 - 1)
       [RFC5817], either due to the Reverse Metric mechanism or by explicit
       user configuration, this SHOULD immediately trigger the CSPF
       (Constrained Shortest Path First) recalculation to move the Traffic
       Engineering traffic away from that link.  It is RECOMMENDED also that
       the CSPF does the immediate CSPF recalculation when the Traffic
       Engineering metric is raised to (2^24 - 2) to be the last resort
       link.

       It is advisable that implementations provide a configuration
       capability to disable any IS-IS metric changes by a Reverse Metric
       mechanism through neighbors' Hello PDUs.

       If an implementation enables this mechanism by default, it is
       RECOMMENDED that it be disabled by the operators when not explicitly
       using it.

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    4.  Security Considerations

       Security concerns for IS-IS are addressed in [ISO10589], [RFC5304],
       [RFC5310], and with various deployment and operational security
       considerations in [RFC7645].  The enhancement in this document makes
       it possible for one IS-IS router to manipulate the IS-IS Default
       Metric and, optionally, Traffic Engineering parameters of adjacent
       IS-IS neighbors on point-to-point or LAN interfaces.  Although IS-IS
       routers within a single Autonomous System nearly always are under the
       control of a single administrative authority, it is highly
       recommended that operators configure authentication of IS-IS PDUs to
       mitigate use of the Reverse Metric TLV as a potential attack vector.

    5.  IANA Considerations

       IANA has allocated IS-IS TLV Codepoint 16 for the Reverse Metric TLV.
       This new TLV has the following attributes: IIH = y, LSP = n, SNP = n,
       Purge = n.

       This document also introduces a new registry for sub-TLVs of the
       Reverse Metric TLV.  The registration policy is Expert Review as
       defined in [RFC8126].  This registry is part of the "IS-IS TLV
       Codepoints" registry.  The name of the registry is "Sub-TLVs for TLV
       16 (Reverse Metric TLV)".  The defined values are:

          0:       Reserved
          1-17:    Unassigned
          18:      Traffic Engineering Metric as specified in this document
                   (Section 2)
          19-255:  Unassigned

    6.  References

    6.1.  Normative References

       [ISO10589] ISO, "Information technology -- Telecommunications and
                  information exchange between systems -- Intermediate
                  System to Intermediate System intra-domain routeing
                  information exchange protocol for use in conjunction with
                  the protocol for providing the connectionless-mode network
                  service (ISO 8473)", ISO/IEC 10589:2002, Second Edition,
                  November 2002.

       [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
                  dual environments", RFC 1195, DOI 10.17487/RFC1195,
                  December 1990, <https://www.rfc-editor.org/info/rfc1195>.

    Shen, et al.                 Standards Track                   [Page 10]

    RFC 8500                  IS-IS Reverse Metric             February 2019

       [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>.

       [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
                  Topology (MT) Routing in Intermediate System to
                  Intermediate Systems (IS-ISs)", RFC 5120,
                  DOI 10.17487/RFC5120, February 2008,
                  <https://www.rfc-editor.org/info/rfc5120>.

       [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
                  Engineering", RFC 5305, DOI 10.17487/RFC5305, October
                  2008, <https://www.rfc-editor.org/info/rfc5305>.

       [RFC5443]  Jork, M., Atlas, A., and L. Fang, "LDP IGP
                  Synchronization", RFC 5443, DOI 10.17487/RFC5443, March
                  2009, <https://www.rfc-editor.org/info/rfc5443>.

       [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
                  Writing an IANA Considerations Section in RFCs", BCP 26,
                  RFC 8126, DOI 10.17487/RFC8126, June 2017,
                  <https://www.rfc-editor.org/info/rfc8126>.

       [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>.

    6.2.  Informative References

       [IS-IS-SL-EXT]
                  Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS
                  Routing for Spine-Leaf Topology", Work in Progress,
                  draft-ietf-lsr-isis-spine-leaf-ext-00, December 2018.

       [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
                  Authentication", RFC 5304, DOI 10.17487/RFC5304, October
                  2008, <https://www.rfc-editor.org/info/rfc5304>.

       [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
                  and M. Fanto, "IS-IS Generic Cryptographic
                  Authentication", RFC 5310, DOI 10.17487/RFC5310, February
                  2009, <https://www.rfc-editor.org/info/rfc5310>.

       [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>.

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       [RFC5919]  Asati, R., Mohapatra, P., Chen, E., and B. Thomas,
                  "Signaling LDP Label Advertisement Completion", RFC 5919,
                  DOI 10.17487/RFC5919, August 2010,
                  <https://www.rfc-editor.org/info/rfc5919>.

       [RFC6138]  Kini, S., Ed. and W. Lu, Ed., "LDP IGP Synchronization for
                  Broadcast Networks", RFC 6138, DOI 10.17487/RFC6138,
                  February 2011, <https://www.rfc-editor.org/info/rfc6138>.

       [RFC7645]  Chunduri, U., Tian, A., and W. Lu, "The Keying and
                  Authentication for Routing Protocol (KARP) IS-IS Security
                  Analysis", RFC 7645, DOI 10.17487/RFC7645, September 2015,
                  <https://www.rfc-editor.org/info/rfc7645>.

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    Appendix A.  Node Isolation Challenges

       On rare occasions, it is necessary for an operator to perform
       disruptive network maintenance on an entire IS-IS router node, i.e.,
       major software upgrades, power/cooling augments, etc.  In these
       cases, an operator will set the IS-IS Overload Bit (OL bit) within
       the Link State Protocol Data Units (LSPs) of the IS-IS router about
       to undergo maintenance.  The IS-IS router immediately floods its
       updated LSPs to all IS-IS routers in the IS-IS domain.  Upon receipt
       of the updated LSPs, all IS-IS routers recalculate their Shortest
       Path First (SPF) tree excluding IS-IS routers whose LSPs have the OL
       bit set.  This effectively removes the IS-IS router about to undergo
       maintenance from the topology, thus preventing it from receiving any
       transit traffic during the maintenance period.

       After the maintenance activity has completed, the operator resets the
       IS-IS Overload Bit within the LSPs of the original IS-IS router
       causing it to flood updated IS-IS LSPs throughout the IS-IS domain.
       All IS-IS routers recalculate their SPF tree and now include the
       original IS-IS router in their topology calculations, allowing it to
       be used for transit traffic again.

       Isolating an entire IS-IS router from the topology can be especially
       disruptive due to the displacement of a large volume of traffic
       through an entire IS-IS router to other suboptimal paths (e.g., those
       with significantly larger delay).  Thus, in the majority of network
       maintenance scenarios, where only a single link or LAN needs to be
       augmented to increase its physical capacity, or is experiencing an
       intermittent failure, it is much more common and desirable to
       gracefully remove just the targeted link or LAN from service
       temporarily, so that the least amount of user-data traffic is
       affected during the link-specific network maintenance.

    Appendix B.  Link Isolation Challenges

       Before network maintenance events are performed on individual
       physical links or LANs, operators substantially increase the IS-IS
       metric simultaneously on both devices attached to the same link or
       LAN.  In doing so, the devices generate new Link State Protocol Data
       Units (LSPs) that are flooded throughout the network and cause all
       routers to gradually shift traffic onto alternate paths with very
       little or no disruption to in-flight communications by applications
       or end users.  When performed successfully, this allows the operator
       to confidently perform disruptive augmentation, fault diagnosis, or
       repairs on a link without disturbing ongoing communications in the
       network.

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       There are a number of challenges with the above solution.  First, it
       is quite common to have routers with several hundred interfaces and
       individual interfaces that move anywhere from several hundred
       gigabits/second to terabits/second of traffic.  Thus, it is
       imperative that operators accurately identify the same point-to-point
       link on two separate devices in order to increase (and afterward
       decrease) the IS-IS metric appropriately.  Second, the aforementioned
       solution is very time-consuming and even more error-prone to perform
       when it's necessary to temporarily remove a multi-access LAN from the
       network topology.  Specifically, the operator needs to configure ALL
       devices that have interfaces attached to the multi-access LAN with an
       appropriately high IS-IS metric (and then decrease the IS-IS metric
       to its original value afterward).  Finally, with respect to multi-
       access LANs, there is currently no method to bidirectionally isolate
       only a single node's interface on the LAN when performing more fine-
       grained diagnoses and repairs to the multi-access LAN.

       In theory, use of a Network Management System (NMS) could improve the
       accuracy of identifying the appropriate subset of routers attached to
       either a point-to-point link or a multi-access LAN.  It could also
       signal to those devices, using a network management protocol, to
       adjust the IS-IS metrics on the pertinent set of interfaces.  The
       reality is that NMSs are, to a very large extent, not used within
       Service Provider's networks for a variety of reasons.  In particular,
       NMSs do not interoperate very well across different vendors or even
       separate platform families within the same vendor.

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    Acknowledgments

       The authors would like to thank Mike Shand, Dave Katz, Guan Deng,
       Ilya Varlashkin, Jay Chen, Les Ginsberg, Peter Ashwood-Smith, Uma
       Chunduri, Alexander Okonnikov, Jonathan Harrison, Dave Ward, Himanshu
       Shah, Wes George, Danny McPherson, Ed Crabbe, Russ White, Robert
       Raszuk, Tom Petch, Stewart Bryant, and Acee Lindem for their comments
       and contributions.

    Contributors

       Tony Li

       Email: tony.li@tony.li

    Authors' Addresses

       Naiming Shen
       Cisco Systems
       560 McCarthy Blvd.
       Milpitas, CA  95035
       United States of America

       Email: naiming@cisco.com

       Shane Amante
       Apple Inc.
       One Apple Park Way
       Cupertino, CA  95014
       United States of America

       Email: amante@apple.com

       Mikael Abrahamsson
       T-Systems Nordic
       Kistagangen 26
       Stockholm
       Sweden

       Email: Mikael.Abrahamsson@t-systems.se

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