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Versions: (draft-amante-isis-reverse-metric) 00 01 02 03 04 05 06 07 08 09 10 11 12 13

Networking Working Group                                         N. Shen
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                               S. Amante
Expires: July 12, 2018                                       Apple, Inc.
                                                          M. Abrahamsson
                                                        T-Systems Nordic
                                                         January 8, 2018


                   IS-IS Routing with Reverse Metric
                   draft-ietf-isis-reverse-metric-08

Abstract

   This document describes the mechanism to allow IS-IS routing to
   quickly and accurately shift traffic away from either a point-to-
   point or multi-access LAN interface by signaling to an adjacent IS-IS
   neighbor with the metric towards itself during network maintenance or
   other operational events.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on July 12, 2018.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   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.  Processing Changes to Default Metric for Multi-Topology
           IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Multi-Access LAN Procedures . . . . . . . . . . . . . . .   7
     3.4.  Point-To-Point Link Procedures  . . . . . . . . . . . . .   8
     3.5.  LDP/IGP Synchronization on LANs . . . . . . . . . . . . .   8
     3.6.  Operational Guidelines  . . . . . . . . . . . . . . . . .   9
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Node Isolation Challenges  . . . . . . . . . . . . .  11
   Appendix B.  Link Isolation Challenges  . . . . . . . . . . . . .  12
   Appendix C.  Contributors' Addresses  . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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 new mechanism for improving it.

1.1.  Node and Link Isolation

   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 displace
   all the traffic away from this node.  It is useful to augment only a
   single link or LAN for the maintenance.  More detailed descriptions



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   of the challenges can be found in Appendix A and Appendix 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 this line-card, in
   particular if the router is a PE (Provider Edge) router in ISP's MPLS
   VPN.  The 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 network not to
   use this particular IS-IS adjacency inbound to this router if
   possible.  Temporarily pushing out the 'Reverse Metric' over this
   link to discourage the traffic into this line-card will help to
   reduce the traffic loss in the network.  At 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 another line-card is recovering.

1.3.  Spine-Leaf Applications

   In the IS-IS Spine-Leaf extension [I-D.shen-isis-spine-leaf-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, this spine node can push a higher metric towards the
   connected leaf nodes to reduce the transit traffic through this spine
   node or link.

1.4.  LDP IGP Synchronization

   In the [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].  This
   Reverse Metric can be used in this case to discourage both outbound
   and inbound traffic without affecting the traffic of other existing
   IS-IS nodes on the LAN.

1.5.  IS-IS Reverse Metric

   This document proposes that the routing protocol itself be 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



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   to be performed only on a single node, set a "reverse metric" on a
   link and have traffic bidirectionally shift away from that link
   gracefully to alternate, viable paths.

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

   The metric value in the "reverse metric" TLV and the TE metric in the
   sub-TLV being advertised is an offset or relative metric to be added
   on top of the existing local link and TE metric value of the
   receiver.

1.6.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  IS-IS Reverse Metric TLV

   The Reverse Metric TLV is composed of a 1 octet field of Flags, a 3
   octet field containing an IS-IS Metric, and a 1 octet Traffic
   Engineering (TE) sub-TLV length field representing the length of a
   variable number of Extended Intermediate System (IS) Reachability
   sub-TLVs.  If the "sub-TLV len" is non-zero, then the Value field
   MUST also contain data of 1 or more Extended IS Reachability sub-
   TLVs.

   The Reverse Metric TLV is optional.  The Reverse Metric TLV may be
   present in any IS-IS Hello PDU.  A sender MUST only transmit a single
   Reverse Metric TLV in a IS-IS Hello PDU.  If a received IS-IS Hello
   PDU contains more than one Reverse Metric TLV, an implementation
   SHOULD ignore all the Reverse Metric TLVs in this error condition.

       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 Offset
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          Metric Offset (Continue)     | sub-TLV Len   |Optional sub-TLV
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Reverse Metric TLV




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      TYPE: TBD (be replaced by the value that IANA allocates)
      LENGTH: variable (5 - 255 octets)
      VALUE:

         Flags (1 octet)
         Metric Offset (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 1: Flags

   The Metric Offset field contains a 24-bit unsigned integer of an IS-
   IS metric that a neighbor SHOULD add to the existing, configured
   "default metric" of the IS-IS link.  Refer to "Elements of
   Procedure", in Section 3 for details on how an IS-IS router should
   process the Metric Offset field in a Reverse Metric TLV.

   There is 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
   (non-DIS) node to the DIS, if the "Whole LAN" bit is set (1), then a
   DIS SHOULD add the received Metric Offset 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 Offset 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 unset when a Reverse Metric TLV is
   transmitted in a IIH PDU onto a point-to-point link to a neighbor,
   and the W bit MUST be ignored upon receiving on a point-to-point
   link.

   U bit (0x02): The "Unreachable" bit is used by the IS-IS node to
   request the neighbor for setting the accumulated metric value to be
   limited to (2^24-1).  This "U" bit applies to both the default metric
   of Extended IS Reachability TLV and the TE default-metric sub-TLV of
   the link.  This is only relevant to the IS-IS "wide" metric mode.

   The "sub-TLV Len" value is non-zero when an IS-IS router wishes to
   signal that its neighbor alter parameters contained in the neighbor's
   Traffic Engineering "Extended IS Reachability TLV", as defined in



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   [RFC5305].  This document defines that only the "Traffic Engineering
   Default Metric" sub-TLV, sub-TLV Type 18, may be sent toward
   neighbors in the Reverse Metric TLV, because that is used in
   Constrained Shortest Path First (CSPF) computations.  Upon receiving
   this TE sub-TLV in a Reverse Metric TLV, a node SHOULD add the
   received TE default metric to its existing, configured TE default
   metric within its Extended IS Reachability TLV.  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 TE sub-TLVs
   that it wishes to send to its IS-IS neighbor.

3.  Elements of Procedure

3.1.  Processing Changes to Default Metric

   The Metric Offset field, in the Reverse Metric TLV, is a "default
   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].  It is important to use the same IS-IS metric mode in both
   ends of the link.  On the receiving side of the 'reverse-metric' TLV,
   the accumulated value of 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 TE default-metric sub-TLV in LSP
   or Pseudonode LSP with the "wide" metric mode case.  If the "U" bit
   is present in the flag, the accumulated metric value is to be limited
   to (2^24 - 1) instead, and this applies to both the normal link
   metric and TE metric in IS-IS "wide" metric mode.

   If an IS-IS router is configured to originate a TE Default Metric
   sub-TLV for a link, but receives a Reverse Metric TLV from its
   neighbor that does not contain a TE Default Metric sub-TLV, then the
   IS-IS router MUST NOT change the value of its TE Default Metric sub-
   TLV for that link.

   Routers MUST scan the Metric Offset value and TE sub-TLVs in all
   subsequently received Reverse Metric TLVs.  If changes are observed
   by a receiver of the Reverse Metric TLV in the Metric Offset value or
   TE Default Metric sub-TLV value, the receiving router MUST update its
   advertised IS-IS default metric or Traffic Engineering parameters in
   the appropriate TLVs, recompute its SPF tree and flood new LSPs to
   other IS-IS routers.







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3.2.  Processing Changes to Default Metric for Multi-Topology IS-IS

   The Reverse Metric TLV is applicable to Multi-Topology IS-IS (M-ISIS)
   [RFC5120] capable 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 that is
   about to undergo maintenance.  When an M-ISIS router receives a
   Reverse Metric TLV it MUST add the received Metric Offset value to
   its default metric in all Extended IS Reachability TLVs for all
   topologies.  If an M-ISIS router receives a Reverse Metric TLV with a
   TE Default Metric sub-TLV, then the M-ISIS router MUST add the
   received TE Default Metric value to each of its TE Default Metric
   sub-TLVs in all of its MT Intermediate Systems TLVs.  If an M-ISIS
   router is configured to advertise TE Default Metric sub-TLVs for one
   or more topologies, but does not receive a TE Default Metric sub-TLV
   in a Reverse Metric TLV, then the M-ISIS router MUST add the value in
   Metric Offset field of the Reverse Metric TLV to each of the TE
   Default Metric sub-TLVs for all topologies.  The M-ISIS should flood
   its newly updated MT IS TLVs and recompute its SPF/CSPF accordingly.

   Multi-Topology IS-IS [RFC5120] specifies there is no change to
   construction of the Pseudonode LSP, regardless of the Multi-Topology
   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 act
   according to the "Multi-Access LAN Procedures" in Section 3.3 to
   update, as appropriate, the default metric contained in the
   Pseudonode LSP.  If the DIS updates the default metric in and floods
   a new Pseudonode LSP, those default metric values will be applied to
   all topologies during Multi-Topology SPF calculations.

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

   In the case of multi-access LANs, the "W" Flags bit is used to signal
   from a non-DIS to the DIS whether to change the metric and optionally




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   Traffic Engineering parameters for all nodes in the Pseudonode LSP or
   a single node on the LAN, (the originator of the Reverse Metric TLV).

   A non-DIS node, e.g.: Router B, attached to a multi-access LAN will
   send a Reverse Metric TLV with the W bit set to 0 to the DIS, when
   Router B wishes the DIS to add the Metric Offset value to the default
   metric contained in the Pseudonode LSP specific to just Router B.
   Other non-DIS nodes, i.e.: Routers C and D, may simultaneously send a
   Reverse Metric TLV with the W bit set to 0 to request the DIS add
   their own Metric Offset value to their default metric contained in
   the Pseudonode LSP.  When the DIS receives a properly formatted
   Reverse Metric TLV with the W bit set to 0, the DIS MUST only add the
   default metric contained in its Pseudonode LSP for the specific
   neighbor that sent the Reverse Metric TLV.

   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 send the Reverse Metric
   TLV without the W bit set.  The DIS MUST use the metric of the
   highest source MAC address of the node sending the TLV with the W bit
   set.  The DIS MUST use the metric value towards the nodes which
   explicitly send the Reverse Metric TLV.

   Local provisioning on the DIS to adjust the default metric(s)
   contained in the Pseudonode LSP MUST take precedence over received
   Reverse Metric TLVs.  For instance, local policy of the DIS may be
   provisioned to ignore the W bit signaling on a LAN.

3.4.  Point-To-Point Link Procedures

   On a point-to-point link, there is already a "configured" IS-IS
   interface metric to be applied over the link towards the IS-IS
   neighbor.

   When IS-IS receives the IIH PDU with the "Reverse Metric" on a point-
   to-point link and if the local policy allows the supporting of
   "Reverse Metric", it MUST add the metric value in the "Metric" field
   of the TLV to the locally configured interface metric value to be the
   metric for this IS-IS adjacency.

3.5.  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 to put IS-IS
   maximum link metric on all the nodes.  [RFC6138] proposes a solution
   to have the new node not advertising the adjacency towards the
   pseudo-node when it is not in a "cut-edge" position.



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   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 have the 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 the LDP IGP synchronization as
   described in [RFC5443], it SHOULD advertise the maximum metric offset
   value in the Reverse Metric TLV in its IIH PDU to 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 on the node.

3.6.  Operational Guidelines

   A router MUST advertise a Reverse Metric TLV toward a neighbor only
   for the period during which it wants a neighbor to temporarily update
   its IS-IS metric or TE parameters towards it.

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

   Routers that receive a Reverse Metric TLV MAY send a syslog message
   or SNMP trap, in order to assist in rapidly identifying the node in
   the network that is asserting an IS-IS metric or Traffic Engineering
   parameters different from that which is configured locally on the
   device.

   It is RECOMMENDED that implementations provide a capability to
   disable any changes to a node's, or individual interfaces of the
   node, default metric or Traffic Engineering parameters based upon
   receiving properly formatted Reverse Metric TLVs.

4.  Security Considerations

   The enhancement in this document makes it possible for one IS-IS
   router to manipulate the IS-IS default metric or optionally Traffic
   Engineering parameters of adjacent IS-IS neighbors.  Although IS-IS
   routers within a single Autonomous System nearly always reside 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,
   particularly on multi-access LANs.




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5.  IANA Considerations

   This document requests that IANA allocate from the IS-IS TLV
   Codepoints Registry a new TLV, referred to as the "Reverse Metric"
   TLV, possibly from the "Unassigned" range of 244-250, with the
   following attributes: IIH = y, LSP = n, SNP = n, Purge = n.

6.  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
   Razsuk and Tom Petch for their comments and contributions.

   This document was produced using Marshall Rose's xml2rfc tool.

7.  References

7.1.  Normative References

   [I-D.shen-isis-spine-leaf-ext]
              Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS
              Routing for Spine-Leaf Topology", draft-shen-isis-spine-
              leaf-ext-03 (work in progress), March 2017.

   [ISO10589]
              ISO, "Intermediate system to Intermediate system routeing
              information exchange protocol for use in conjunction with
              the Protocol for providing the Connectionless-mode Network
              Service (ISO 8473)", ISO/IEC 10589:2002.

   [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, <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>.





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   [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>.

7.2.  Informative References

   [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>.

   [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, <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>.

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 the
   updated LSPs to all IS-IS routers throughout 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
   forwarding any transit traffic during the maintenance period.

   After the maintenance activity is 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



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   through an entire IS-IS router to other, sub-optimal paths, (i.e.:
   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 while intrusive augment, diagnostic and/or replacement
   procedures are being executed.

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, to 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.

   The challenge with the above solution are as follows.  First, it is
   quite common to have routers with several hundred interfaces onboard
   and individual interfaces that are transferring 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 its necessary to temporarily remove a multi-access LAN from the
   network topology.  Specifically, the operator needs to configure ALL
   devices's 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 performed more
   fine-grained diagnosis 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 as well as
   signaling from the NMS to those devices, using a network management
   protocol, to adjust the IS-IS metrics on the pertinent set of
   interfaces.  The reality is that NMS are, to a very large extent, not
   used within Service Provider's networks for a variety of reasons.  In



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   particular, NMS do not interoperate very well across different
   vendors or even separate platform families within the same vendor.

   The risks of misidentifying one side of a point-to-point link or one
   or more interfaces attached to a multi-access LAN and subsequently
   increasing its IS-IS metric are potentially increased latency, jitter
   or packet loss.  This is unacceptable given the necessary performance
   requirements for a variety of applications, the customer perception
   for near lossless operations and the associated, demanding Service
   Level Agreement's (SLAs) for all network services.

Appendix C.  Contributors' Addresses

   Tony Li

   Email: tony.li@tony.li

Authors' Addresses

   Naiming Shen
   Cisco Systems
   560 McCarthy Blvd.
   Milpitas, CA  95035
   USA

   Email: naiming@cisco.com


   Shane Amante
   Apple, Inc.
   1 Infinite Loop
   Cupertino, CA  95014
   USA

   Email: samante@apple.com


   Mikael Abrahamsson
   T-Systems Nordic
   Kistagangen 26
   Stockholm
   SE

   Email: Mikael.Abrahamsson@t-systems.se







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