IS-IS Routing with Reverse Metric
draft-ietf-isis-reverse-metric-17

Networking Working Group                                         N. Shen
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                               S. Amante
Expires: February 26, 2017                                   Apple, Inc.
                                                          M. Abrahamsson
                                                        T-Systems Nordic
                                                         August 25, 2016


                   IS-IS Routing with Reverse Metric
                   draft-ietf-isis-reverse-metric-04

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 26, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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.  Mobility Cases  . . . . . . . . . . . . . . . . . . . . .   3
     1.4.  Spine-Leaf Applications . . . . . . . . . . . . . . . . .   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.  Operational Guidelines  . . . . . . . . . . . . . . . . .   8
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Node Isolation Challenges  . . . . . . . . . . . . .  10
   Appendix B.  Link Isolation Challenges  . . . . . . . . . . . . .  11
   Appendix C.  Use of Reverse Metric for LDP/IGP Synchronization on
                LAN's  . . . . . . . . . . . . . . . . . . . . . . .  12
   Appendix D.  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.  The IS-
   IS adjacency may be established with a neighbor router long before
   the entire BGP 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.

1.3.  Mobility Cases

   When the IS-IS is run on some mobile devices, either in point-to-
   point links or in broadcast networks, it is important to have the
   routing metric to influence the traffic in both directions.  When a
   node is moving farther away, it not only needs to raise the cost for
   traffic from this router to the network, but also it is important to
   raise the cost for the traffic from the network towards the router.
   When a node is moving closer, it can lower the cost on both metrics.

1.4.  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.

1.5.  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
   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.





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   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 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 a multi-
   access link of a node.

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-TLV's.  If the 'S' bit in the Flags field is set to 1, then the
   Value field MUST also contain data of 1 or more Extended IS
   Reachability sub-TLV's.

   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.

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

                            Reverse Metric TLV

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

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






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

                              Figure 1: Flags

   The Metric field contains a 24-bit unsigned integer of an IS-IS
   metric that a neighbor SHOULD add to the existing, configured
   "default metric" contained within its IS Neighbors TLV, Extended IS
   Reachability TLV's for point-to-point links, or Pseudonode LSP by the
   Designated Intermediate System (DIS) for multi-access LAN's, back
   toward the router and the link that originated this Reverse Metric
   TLV.  Refer to "Elements of Procedure", in Section 3 for details on
   how an IS-IS router should process the Metric 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 LAN's.  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 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 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.

   S bit (0x02): The "TE sub-TLV" bit MUST be set 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 [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-
   TLV's is outside the scope of this document.  The S bit MUST NOT be
   set when an IS-IS router does not have TE sub-TLV's that it wishes to
   send to its IS-IS neighbor.





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3.  Elements of Procedure

3.1.  Processing Changes to Default Metric

   The Metric 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].  It is
   RECOMMENDED that implementations, by default, place the appropriate
   maximum default metric value, 63 or (2^24 - 2), in the Metric field
   and TE Default Metric sub-TLV of the Reverse Metric TLV, since the
   most common use is to indicate the link of the router is overloaded
   and to remove the link from the topology, except for use as a last-
   resort path.

   In order to ensure that an individual TE link is used as a link of
   last resort during SPF computation, its metric MUST NOT be greater
   than or equal to (2^24 - 1) [RFC5305].  Therefore, a receiver of a
   Reverse Metric TLV MUST use the numerically smallest value of either
   the sum of its existing default metric and the Metric value in the
   Reverse Metric TLV or (2^24 - 2), as the default metric when updating
   its Extended IS Reachability TLV and TE default-metric sub-TLV's that
   it will then flood throughout the IS-IS domain, using normal IS-IS
   procedures.  Likewise, originators of a Pseudonode LSP or IS
   Neighbors TLV MUST use the numerically smallest value of either the
   sum of its existing default metric and the Metric value it receives
   in a Reverse Metric TLV or 63 when updating the corresponding
   Pseudonode LSP or IS Neighbor TLV before they are flooded.  This also
   applies when an IS-IS router is only configured or capable of sending
   a "narrow" IS-IS default metric, in the range of 0 - 63, but receives
   a "wide" Metric value in a Reverse Metric TLV, in the range of 64 -
   (2^24 - 2).  In this case, the receiving router MUST use the maximum
   "narrow" IS-IS default metric, 63, as its IS-IS default metric value
   in its updated IS Neighbor TLV or Pseudonode LSP that it floods.

   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 add the value in the Metric field of the Reverse
   Metric TLV to its own TE Default Metric sub-TLV for that link.  The
   IS-IS router should then flood the updated Extended IS Reachability
   TLV, including its updated TE Default Metric sub-TLV, using normal
   IS-IS procedures.

   Routers MUST scan the Metric value and TE sub-TLV's in all
   subsequently received Reverse Metric TLV's.  If changes are observed
   by a receiver of the Reverse Metric TLV in the Metric value or TE



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   Default Metric sub-TLV value, the receiving router MUST update its
   advertised IS-IS default metric or Traffic Engineering parameters in
   the appropriate TLV's, recompute its SPF tree and flood new LSP's to
   other IS-IS routers.

   If the router does not understand the Reverse Metric TLV or is
   explicitly configured to ignore received Reverse Metric TLV's, then
   it MUST NOT update the default metric in its IS Neighbors TLV,
   Extended IS Reachability TLV, TE Default Metric sub-TLV, Multi-
   Topology Intermediate Systems TLV, or Pseudonode LSP, nor execute
   other procedures that would result from acting on a Reverse Metric
   TLV, such as recomputing its SPF tree.

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 PDU's 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 value to its
   default metric in all Extended IS Reachability TLV's 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-TLV's in all of its MT Intermediate Systems TLV's.  If an M-ISIS
   router is configured to advertise TE Default Metric sub-TLV's 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 field of the Reverse Metric TLV to each of the TE Default
   Metric sub-TLV's for all topologies.  The M-ISIS should flood its
   newly updated MT IS TLV's 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.



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   In the case of multi-access LAN's, the "W" Flags bit is used to
   signal from a non-DIS to the DIS whether to change the metric and
   optionally 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 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 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 TLV's.  For instance, local policy of the DIS may be
   provisioned to ignore the W bit signaling on a LAN.

3.4.  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.

   During the period when a Reverse Metric TLV is used, IS-IS routers
   that are generating and receiving a Reverse Metric TLV MUST NOT
   change their existing IS-IS metric or Traffic Engineering parameters
   in their persistent provisioning database, since those parameters are
   carefully derived from off-line capacity planning tools and are
   difficult to restore to their original values.

   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



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   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 TLV's.

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 PDU's to
   mitigate use of the Reverse Metric TLV as a potential attack vector,
   particularly on multi-access LAN's.

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, 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,
   Jonathan Harrison, Dave Ward, Himanshu Shah, Wes George, Danny
   McPherson, Ed Crabbe, Russ White and Robert Razsuk for their
   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. and S. Thyamagundalu, "IS-IS Routing for Spine-
              Leaf Topology", draft-shen-isis-spine-leaf-ext-01 (work in
              progress), April 2016.







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   [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, <http://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,
              <http://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,
              <http://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, <http://www.rfc-editor.org/info/rfc5305>.

7.2.  Informative References

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

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 (LSP's) of the IS-IS router about
   to undergo maintenance.  The IS-IS router immediately floods the
   updated LSP's to all IS-IS routers throughout the IS-IS domain.  Upon
   receipt of the updated LSP's, all IS-IS routers recalculate their
   Shortest Path First (SPF) tree excluding IS-IS routers whose LSP's
   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.





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   After the maintenance activity is completed, the operator resets the
   IS-IS Overload Bit within the LSP's of the original IS-IS router
   causing it to flood updated IS-IS LSP's 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, 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 LAN's, 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 (LSP's) 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 LAN's, there is currently no method to bidirectionally




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   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
   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 (SLA's) for all network services.

Appendix C.  Use of Reverse Metric for LDP/IGP Synchronization on LAN's

   This document primarily outlines the use of IS-IS Reverse Metric TLV
   for networks that use IP forwarding.  However, it is also critical to
   consider application of the IS-IS Reverse Metric TLV to networks that
   use MPLS forwarding, specifically networks that use IS-IS as the IGP
   and LDP for signaling MPLS labels used for forwarding.  In these
   networks, it is often the case that IS-IS will become operational and
   determine the shortest path through a link or LAN prior to LDP
   becoming operational (forming an adjacency with a LDP neighbor and
   exchanging LDP labels), which results in temporary blackholing for
   data traffic reliant on MPLS forwarding.

   This scenario should be avoided in MPLS networks where IS-IS is the
   IGP and LDP signaling is used to exchange tunnel labels over a LAN.
   In these cases, it is recommended that the IS-IS Reverse Metric TLV
   be utilized when IS-IS and LDP adjacencies are in the process of
   becoming established among one, or several, routers attached to a
   common multi-access LAN.

   Specifically, when an IS-IS adjacency is being established from a
   non-DIS node, the non-DIS should transmit a IS-IS Reverse Metric TLV
   toward the DIS with the W-bit not set (0), as per
   "Elements of Procedure" in Section 3 of this document, until the non-
   DIS router either: a) completes transmission of a LDP End-of-LIB
   marker [RFC5919] toward the DIS; or, b) expiration of a local (pre-
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   operational to the DIS.  At this point, the non-DIS router should
   cease advertisement of the IS-IS Reverse Metric TLV, which should
   cause the (re-)advertisement of normal default metric(s) to itself in
   the Pseudonode LSP.

Appendix D.  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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