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rfc:rfc8500

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.

Shen, et al. Standards Track [Page 1] RFC 8500 IS-IS Reverse Metric February 2019

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.

Shen, et al. Standards Track [Page 2] RFC 8500 IS-IS Reverse Metric February 2019

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.

Shen, et al. Standards Track [Page 3] RFC 8500 IS-IS Reverse Metric February 2019

 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

Shen, et al. Standards Track [Page 4] RFC 8500 IS-IS Reverse Metric February 2019

 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

Shen, et al. Standards Track [Page 5] RFC 8500 IS-IS Reverse Metric February 2019

 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

Shen, et al. Standards Track [Page 6] RFC 8500 IS-IS Reverse Metric February 2019

 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

Shen, et al. Standards Track [Page 7] RFC 8500 IS-IS Reverse Metric February 2019

 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.

Shen, et al. Standards Track [Page 8] RFC 8500 IS-IS Reverse Metric February 2019

 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.

Shen, et al. Standards Track [Page 9] RFC 8500 IS-IS Reverse Metric February 2019

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>.

Shen, et al. Standards Track [Page 11] RFC 8500 IS-IS Reverse Metric February 2019

 [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>.

Shen, et al. Standards Track [Page 12] RFC 8500 IS-IS Reverse Metric February 2019

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.

Shen, et al. Standards Track [Page 13] RFC 8500 IS-IS Reverse Metric February 2019

 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.

Shen, et al. Standards Track [Page 14] RFC 8500 IS-IS Reverse Metric February 2019

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

Shen, et al. Standards Track [Page 15]

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