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Versions: 00 01 02 draft-ietf-isis-reverse-metric

IS-IS Working Group                                              N. Shen
Internet-Draft                                                     T. Li
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: September 14, 2011                                    S. Amante
                                                  Level 3 Communications
                                                          M. Abrahamsson
                                                                   Tele2
                                                          March 13, 2011


        IS-IS Reverse Metric TLV for Network Maintenance Events
                  draft-amante-isis-reverse-metric-02

Abstract

   This document describes an improved IS-IS neighbor management scheme
   which can be used to enhance network performance by allowing
   operators to quickly and accurately shift traffic away from a point-
   to-point or multi-access LAN interface by allowing one IS-IS router
   to signal to a second, adjacent IS-IS neighbor to adjust its IS-IS
   metric that should be used to temporarily reach the first IS-IS
   router during network maintenance 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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 14, 2011.

Copyright Notice

   Copyright (c) 2011 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
   (http://trustee.ietf.org/license-info) in effect on the date of



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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Node Isolation Challenges  . . . . . . . . . . . . . . . .  3
     1.2.  Link Isolation Challenges  . . . . . . . . . . . . . . . .  3
     1.3.  IS-IS Reverse Metric . . . . . . . . . . . . . . . . . . .  4
     1.4.  Specification of Requirements  . . . . . . . . . . . . . .  5

   2.  IS-IS Reverse Metric TLV . . . . . . . . . . . . . . . . . . .  5

   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 . . . . . . . . . . . . . . . . . . .  8
     3.3.  Multi-Access LAN Procedures  . . . . . . . . . . . . . . .  8
     3.4.  Order of Operations  . . . . . . . . . . . . . . . . . . . 10
     3.5.  Operational Guidelines . . . . . . . . . . . . . . . . . . 10

   4.  Reverse Metric TLV Example Use Cases . . . . . . . . . . . . . 11

   5.  Operational Considerations . . . . . . . . . . . . . . . . . . 11

   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12

   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12

   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12

   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 13

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13










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1.  Introduction

   The IS-IS [ISO 10589] 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 one issue and a new mechanism for improving it.

1.1.  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.

   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.

1.2.  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



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   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
   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.

1.3.  IS-IS Reverse Metric

   This document proposes that the routing protocol itself be the
   transport mechanism to allow one IS-IS router to advertise to an
   adjacent node on a point-to-point or multi-access LAN link a "reverse



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   metric" in a IS-IS Hello (IIH) PDU.  This would allow an operator to
   only configure a single router, set a "reverse metric" on a link and
   have traffic bidirectionally shift away from that link gracefully to
   alternate, viable paths.

1.4.  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 1 octet for the Type, 1 octet
   that specifies the number of bytes in the Value field and a variable-
   length Value field.  The Value field starts with a 1 octet field of
   Flags followed by a 3 octet field containing an IS-IS Metric and,
   lastly, 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.

      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)

   Flags

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

                              Figure 1: Flags

   The Reverse Metric TLV Type is TBD.  Please refer to IANA
   Considerations, in Section 7, for more details.




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   The Metric field contains a 24-bit unsigned integer of an IS-IS
   metric a neighbor SHOULD add to the existing, configured "default
   metric" contained within its IS Neighbors TLV or 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 that originated this Reverse Metric TLV.  Refer to
   "Elements of Procedure", below in Section 3, for details of 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 (0) when a Reverse Metric TLV is transmitted in a IIH PDU
   onto a point-to-point link to an IS-IS neighbor.

   S bit (0x02): The "TE sub-TLV" bit MUST be set (1) 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 receipt
   of 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 (0) when an IS-IS router does not have TE
   sub-TLV's that it wishes to send to its IS-IS neighbor.


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



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   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 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
   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, according to the recommendations outlined in
   Section 3.4, Order of Operations, below.

   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,



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   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.

   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



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   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.

   It is possible for one node, Router A, to signal to the DIS with the
   W bit set to 1, in which case the DIS would add the Metric value in
   the Reverse Metric TLV to all neighbor adjacencies in the Pseudonode
   LSP and transmit a new Pseudonode LSP to all nodes in the IS-IS
   domain.  Later, a second node on the LAN, Router B, could signal to
   the DIS with the W bit also set to 1.  In this case, the DIS MUST use
   the highest source MAC address from IIH PDU's containing Reverse
   Metric TLV's it receives as the tie-breaker to determine the sole
   Reverse Metric TLV used as the source for the Metric value that will
   be added to the default metric for all nodes in the Pseudonode LSP.
   If the source MAC address was highest in IIH PDU's containing a
   Reverse Metric TLV received from Router B, then the DIS MUST add the
   Metric value to the default metric of all neighbors in its Pseudonode
   LSP and flood the LSP to all nodes in the IS-IS domain.  On the other
   hand, if the DIS determines that Router A's IIH PDU's, containing
   Reverse Metric TLV's, have the highest source MAC address, then the
   DIS will ignore Router B's Reverse Metric TLV and continue to use the
   Metric value found in Router A's Reverse Metric TLV to add to the
   default metric of all neighbors in the Pseudonode LSP.  When this
   occurs, the DIS MAY send a single syslog message or SNMP trap
   indicating that it has received a Reverse Metric TLV from a neighbor,
   but is ignoring it due to it being received from a neighbor with a
   lower MAC address.

   Another scenario is that one node, Router A, may signal the DIS with
   the W bit set to 1.  The DIS would add the Metric value to the
   default metric for all neighbors in the Pseudonode LSP and flood the
   LSP.  Later, a second node on the LAN, Router B, could signal the DIS
   with the W bit set to 0, which indicates to the DIS that Router B is
   requesting the DIS only add the Metric value in the Reverse Metric
   TLV from Router B to the default metric for Router B in the
   Pseudonode LSP.  The DIS MUST honor a neighbor's Reverse Metric TLV
   to update its individual default metric in the Pseudonode LSP even if
   the DIS receives prior or later requests to assert a Whole LAN metric
   from other nodes on the same LAN.

   In all cases above, the DIS is MUST use 0 as the base default-metric



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   value for each neighbor contained in the Pseudonode LSP to which the
   DIS will add the Metric value in the Reverse Metric TLV(s) it
   receives from neighbors on the LAN.

   Local configuration on the DIS to adjust the default metric(s)
   contained in the Pseudonode LSP, as documented in
   [I-D.shen-isis-oper-enhance] MUST take precedence over received
   Reverse Metric TLV's.

3.4.  Order of Operations

   When an IS-IS router starts or stops generating a Reverse Metric TLV,
   it will go through a process of updating its own IS-IS metric and
   optionally Traffic Engineering parameters in its IS Neighbors TLV,
   Extended IS Reachbaility TLV or Pseudonode LSP, flooding updated
   LSP's (using normal IS-IS mechanisms), recompute its SPF/CSPF tree
   plus corresponding metrics to IP prefixes, update its FIB and begin
   advertising the Reverse Metric TLV in IIH PDU's toward its
   corresponding neighbor(s) on the appropriate link or LAN.  Likewise,
   when IS-IS neighbor(s) start or stop receiving a Reverse Metric TLV,
   they will go through a similar process.  It is critical that devices
   which implement the Reverse Metric TLV conduct this process in a
   deterministic order that minimizes the possibilities to generate
   temporary micro forwarding loops during a metric increase and
   decrease.

3.5.  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.

   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 stored (e.g.: hard disk, etc.) configurations, 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
   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



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   receipt of properly formatted Reverse Metric TLV's.


4.  Reverse Metric TLV Example Use Cases

   The following is a brief example illustrating one use case of the
   Reverse Metric TLV.  In order to isolate a point-to-point link from
   the IS-IS network, an operator would configure one router, Router A,
   attached to a point-to-point link with a "Reverse Metric".  This
   should not affect the configuration of the existing IS-IS default
   metric previously configured on the router's interface.  Assuming
   Router A is using IS-IS Extensions for Traffic Engineering [RFC5305],
   this should trigger Router A to update its Traffic Engineering
   Default Metric sub-TLV in its own Extended IS Reachability TLV,
   recompute its SPF tree and corresponding metrics to IP prefixes in
   the IS-IS domain and begin the process of flooding a new LSP
   throughout the network.  Router A would also begin transmitting a
   Reverse Metric TLV, with an appropriate Metric value, in an IIH PDU,
   to its adjacent neighbor, Router B. Upon receipt of the Reverse
   Metric TLV, Router B would add the received Metric or TE default
   metric sub-TLV value to its own Traffic Engineering Default Metric
   sub-TLV, recalculate its SPF tree and associated route topology as
   well as start flooding a new LSP containing the updated Extended IS
   Reachability TLV throughout the network.  As nodes in the network
   receive the associated LSP's from Router A and B and recalculate a
   new SPF tree, and route topology, traffic should gracefully shift
   onto alternate paths away from the A-B link; ultimately, after all
   nodes in the network recompute their SPF tree link A-B should only be
   used as a link of last-resort.  The operator can inspect traffic
   counters on the A-B interface to determine if the link was
   successfully isolated from the topology and proceed with necessary
   fault diagnosis or maintenance of the associated link.

   When the maintenance activity is complete, the operator would remove
   the reverse metric configuration from Router A, which would cease
   advertisement of the Reverse Metric TLV in IIH PDU's to Router B.
   Both routers would revert to their originally configured IS-IS
   metric, recompute new SPF trees and corresponding metrics to IP
   prefixes and originate new LSP's.  As the new LSP's are received and
   SPF is recalculated by nodes in the IS-IS domain, traffic should
   gradually shift back onto link A-B.


5.  Operational Considerations

   Since the Reverse Metric TLV may not be recognized by adjacent IS-IS
   neighbors, operators should inspect input and output traffic
   throughput counters on the local router to ensure that traffic has



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   bidirectionally shifted away from a link before starting any
   maintenance activities.


6.  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.


7.  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.


8.  Acknowledgements

   The authors would like to thank Mike Shand, Dave Katz, Guan Deng,
   Ilya Varlashkin, Jay Chen, Les Ginsberg and Peter Ashwood-Smith,
   Jonathan Harrison, Dave Ward, Himanshu Shah and Wes George for their
   contributions.


9.  References

9.1.  Normative References

   [ISO 10589]
              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, December 1990.

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




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   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120, February 2008.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, October 2008.

9.2.  Informative References

   [I-D.shen-isis-oper-enhance]
              Shen, N., Li, T., Amante, S., and M. Abrahamsson, "IS-IS
              Operational Enhancements for Network Maintenance Events",
              draft-shen-isis-oper-enhance-00 (work in progress),
              October 2010.


Authors' Addresses

   Naiming Shen
   Cisco Systems, Inc.
   225 West Tasman Drive
   San Jose, CA  95134
   USA

   Email: naiming@cisco.com


   Tony Li
   Cisco Systems, Inc.
   225 West Tasman Drive
   San Jose, CA  95134
   USA

   Email: tli@cisco.com


   Shane Amante
   Level 3 Communications
   1025 Eldorado Blvd
   Broomfield, CO  80021
   USA

   Email: shane@level3.net








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   Mikael Abrahamsson
   Tele2

   Email: swmike@swm.pp.se















































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