RFC 3623 (rfc3623) - Page 2 of 18

Graceful OSPF Restart

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
   forwarding path.  This memo documents an enhancement to OSPF that
   makes such graceful restart possible, and automatically reverts back



Moy, et al.                 Standards Track