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Graceful OSPF Restart
RFC 3623

Network Working Group                                             J. Moy
Request for Comments: 3623                             Sycamore Networks
Category: Standards Track                              P. Pillay-Esnault
                                                        Juniper Networks
                                                               A. Lindem
                                                        Redback Networks
                                                           November 2003

                         Graceful OSPF Restart

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This memo documents an enhancement to the OSPF routing protocol,
   whereby an OSPF router can stay on the forwarding path even as its
   OSPF software is restarted.  This is called "graceful restart" or
   "non-stop forwarding".  A restarting router may not be capable of
   adjusting its forwarding in a timely manner when the network topology
   changes.  In order to avoid the possible resulting routing loops, the
   procedure in this memo automatically reverts to a normal OSPF restart
   when such a topology change is detected, or when one or more of the
   restarting router's neighbors do not support the enhancements in this
   memo.  Proper network operation during a graceful restart makes
   assumptions upon the operating environment of the restarting router;
   these assumptions are also documented.

Moy, et al.                 Standards Track                     [Page 1]
RFC 3623                 Graceful OSPF Restart             November 2003

Table of Contents

   1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Operation of Restarting Router . . . . . . . . . . . . . . . .  3
       2.1.  Entering Graceful Restart. . . . . . . . . . . . . . . .  4
       2.2.  When to Exit Graceful Restart. . . . . . . . . . . . . .  5
       2.3.  Actions on Exiting Graceful Restart. . . . . . . . . . .  6
   3.  Operation of Helper Neighbor . . . . . . . . . . . . . . . . .  7
       3.1.  Entering Helper Mode . . . . . . . . . . . . . . . . . .  7
       3.2.  Exiting Helper Mode. . . . . . . . . . . . . . . . . . .  8
   4.  Backward Compatibility . . . . . . . . . . . . . . . . . . . .  9
   5.  Unplanned Outages. . . . . . . . . . . . . . . . . . . . . . . 10
   6.  Interaction with Traffic Engineering . . . . . . . . . . . . . 11
   7.  Possible Future Work . . . . . . . . . . . . . . . . . . . . . 11
   8.  Intellectual Property Rights Notice. . . . . . . . . . . . . . 11
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
       9.1.  Normative References . . . . . . . . . . . . . . . . . . 11
       9.2.  Informative References . . . . . . . . . . . . . . . . . 11
   A.  Grace-LSA Format . . . . . . . . . . . . . . . . . . . . . . . 13
   B.  Configurable Parameters. . . . . . . . . . . . . . . . . . . . 15
   Security Considerations. . . . . . . . . . . . . . . . . . . . . . 16
   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 18

1.  Overview

   Today many Internet routers implement a separation of control and
   forwarding functions.  Certain processors are dedicated to control
   and management tasks such as OSPF routing, while other processors
   perform the data forwarding tasks.  This separation creates the
   possibility of maintaining a router's data forwarding capability
   while the router's control software is restarted/reloaded.  We call
   such a possibility "graceful restart" or "non-stop forwarding".

   The OSPF protocol presents a problem to graceful restart whereby,
   under normal operation, OSPF intentionally routes around a restarting
   router while it rebuilds its link-state database.  OSPF avoids the
   restarting router to minimize the possibility of routing loops and/or
   black holes caused by lack of database synchronization.  Avoidance is
   accomplished by having the router's neighbors reissue their LSAs,
   omitting links to the restarting router.

   However, if (a) the network topology remains stable and (b) the
   restarting router is able to keep its forwarding table(s) across the
   restart, it would be safe to keep the restarting router on the

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