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Versions: 00                                                            
IS-IS Working Group                                              N. Shen
Internet-Draft                                                     T. Li
Intended status: Informational                       Cisco Systems, Inc.
Expires: April 3, 2011                                         S. Amante
                                                  Level 3 Communications
                                                          M. Abrahamsson
                                                      September 30, 2010

     IS-IS Operational Enhancements for Network Maintenance Events


   This document describes an improved IS-IS neighbor management scheme
   which can be used to enhance operational experience in terms of
   convergence speed and finer control of neighbor cost over a LAN.

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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   This Internet-Draft will expire on April 3, 2011.

Copyright Notice

   Copyright (c) 2010 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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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Interface Shutdown Black Hole . . . . . . . . . . . . . . . 3
     1.2.  LAN of Last Resort  . . . . . . . . . . . . . . . . . . . . 3
     1.3.  Specification of Requirements . . . . . . . . . . . . . . . 3

   2.  Sending Hellos with Fast Exit Notification  . . . . . . . . . . 3

   3.  Pseudonodes with Non-zero Metrics . . . . . . . . . . . . . . . 4
     3.1.  Operational Considerations  . . . . . . . . . . . . . . . . 5

   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5

   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5

   6.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5

   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6

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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 those issues and some mechanisms for dealing with
   those issues.  These mechanisms do involve implementation support,
   but do not require protocol changes.

1.1.  Interface Shutdown Black Hole

   One of these operationally problematic issues occurs when IS-IS is
   disabled on only one side of a link.  This can result in a
   significant delay before neighbor(s) on the other end of the same
   link notice this change.  In turn, this can result in several seconds
   during which traffic is blackholed, until the IS-IS neighbor(s) time
   out the adjacency and IS-IS reconverges.

1.2.  LAN of Last Resort

   Another issue stems from a situation when operators want to
   temporarily make an interface a "last resort" link for transit
   traffic.  This is a straightforward, through cumbersome, operation to
   perform on a point-to-point link.  Each device on the link is
   reconfigured to use very high metric.  This causes traffic to divert
   to other links in the network.  This same operation is more difficult
   on a multi-access LAN.  There, the operator would have to increase
   the metric on each and every interface attached to the LAN, requiring
   the reconfiguration of a number of systems.

1.3.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Sending Hellos with Fast Exit Notification

   When an operator shuts down IS-IS on an interface, as described in
   Section 1.1, there is a significant interval before the change is
   noticed by all adjacencies and traffic is subsequently re-routed
   around this link.  This delay is unnecessary, as neighbors should not
   have to wait for the adjacency to timeout, particularly when there
   exist alternate, viable, paths to downstream neighbors.  This delay
   can be eliminated by carefully removing the adjacency between
   neighbors prior to actually disabling IS-IS on the interface.

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   An IS-IS adjacency uses the 3-way handshake protocol as defined in
   [ISO 10589] for multi-access LANs and [RFC3373] for point-to-point
   links.  In both cases, the IS to IS Hello (IIH) message used to
   establish and maintain the adjacency carries the system identifier of
   the adjacent systems.  The receiving system expects to see its own
   system identifier listed.  If not, then it must drop the adjacency.

   An implementation that wishes to avoid the issue in Section 1.1 can
   do so by sending out a final IIH that includes no neighboring system
   IDs.  When this is received, it should cause all neighbors to drop
   their adjacencies with the router that sent the IIH.  This will also
   cause the systems to update their Link State Protocol Data Units
   (LSPs), flood them and reconverge to new paths.  The technique is
   known as Fast Exit Notification.

   This approach is not guaranteed.  If the final IIH is lost on the
   link, then the neighboring systems will have to wait to time out the
   adjacency.  Since this is unlikely, it is still a useful
   optimization.  Implementations that require an even higher degree of
   assurance can retransmit the final IIH, possibly multiple times.

3.  Pseudonodes with Non-zero Metrics

   If an operator wishes to reconfigure a multi-access LAN so that it is
   only used as a resource of the last resort, then with current
   mechanisms, the operator must reconfigure each node on the LAN to
   give the LAN a high metric, as described in Section 1.2.  It would be
   much easier for the operator if they could make a single
   configuration change that would cause IS-IS to treat the multi-access
   LAN as a link of last resort.

   [ISO 10589] defines the pseudonode LSP as having a metric of zero.
   This implies that during the Shortest Path First (SPF) calculation,
   the metric for traversing the LAN is solely based on the metric set
   by the IS used to access the LAN.  Thereby, the pseudonode does not
   contribute to the cost of traversing the LAN.

   However, from the point of view of the SPF calculation, the metric in
   the pseudonode LSP does not have to be zero.  Instead, the metric in
   a pseudonode LSP could be treated just like a normal LSP and have
   non-zero metrics to some or all of the systems on the LAN.  This can
   then be used to simplify the operation for turning a LAN into a link
   of last resort.  This could be done by having the Designated
   Intermediate System (DIS) change all of the metrics within the
   pseudonode LSP to a high value.  This would effectively make the LAN
   look very 'expensive' and cause SPF calculations to converge to
   alternate links, if at all possible.

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   Because this change to the usage of the pseudonode LSP is in direct
   contradiction to the existing IS-IS specification, extreme caution is
   necessary.  Implementations that would not interpret a non-zero
   pseudonode metric correctly might cause forwarding loops.  As of this
   writing, we are actively surveying existing known implementations to
   determine if setting a non-zero metric in a pseudonode LSP will be
   interpreted properly.

   This technique can also be used to divert traffic away from a subset
   of the nodes on the LAN.  If the DIS increases the metric from the
   pseudonode to a subset of the systems on the LAN, then traffic will
   avoid exiting the LAN via that subset of systems.

3.1.  Operational Considerations

   A further simplification is to allow any system to temporarily become
   the DIS, when it is directed to, and set a non-zero metric in the
   pseudonode.  This is beneficial because the operator would otherwise
   first have to determine the current DIS, access that system and
   reconfigure it.  If an implementation wishes to support this, then it
   can provide an operation that both changes its priority on the LAN so
   that a node first becomes DIS and then generates a new pseudonode LSP
   with the non-zero metric.

   If there is a concern that the DIS may change, it is prudent to
   define another node on the same LAN with the second highest priority
   for becoming DIS.  This node can be configured to also set the metric
   in its pseudonode LSP appropriately if it becomes the new DIS.

4.  Security Considerations

   This document raises no new security issues for IS-IS.

5.  Acknowledgements

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

6.  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

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              Service (ISO 8473)", ISO/IEC 10589:2002.

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

   [RFC3373]  Katz, D. and R. Saluja, "Three-Way Handshake for
              Intermediate System to Intermediate System (IS-IS) Point-
              to-Point Adjacencies", RFC 3373, September 2002.

Authors' Addresses

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

   Email: naiming@cisco.com

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

   Email: tli@cisco.com

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

   Email: shane@level3.net

   Mikael Abrahamsson

   Email: swmike@swm.pp.se

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