faqs.org - Internet FAQ Archives

RFC 8500 - IS-IS Routing with Reverse Metric

Or Display the document by number

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


   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

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.

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.

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

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.

   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",
   "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

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

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

         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

   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

   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 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

   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

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.

   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

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

   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

   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.

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>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [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,

   [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

              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>.

   [RFC5919]  Asati, R., Mohapatra, P., Chen, E., and B. Thomas,
              "Signaling LDP Label Advertisement Completion", RFC 5919,
              DOI 10.17487/RFC5919, August 2010,

   [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,

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

   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.


   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.


   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

   Email: Mikael.Abrahamsson@t-systems.se


User Contributions:

Comment about this RFC, ask questions, or add new information about this topic: