RFC 3623 (rfc3623) - Page 3 of 18

Graceful OSPF Restart

RFC 3623                 Graceful OSPF Restart             November 2003


   to a standard OSPF restart for safety when network topology changes
   are detected.

   In a nutshell, the OSPF enhancements for graceful restart are as
   follows:

   -  The router attempting a graceful restart originates link-local
      Opaque-LSAs, herein called Grace-LSAs, announcing its intention to
      perform a graceful restart within a specified amount of time or
      "grace period".

   -  During the grace period, its neighbors continue to announce the
      restarting router in their LSAs as if it were fully adjacent
      (i.e., OSPF neighbor state Full), but only if the network topology
      remains static (i.e., the contents of the LSAs in the link-state
      database having LS types 1-5,7 remain unchanged and periodic
      refreshes are allowed).

   There are two roles being played by OSPF routers during graceful
   restart.  First there is the router that is being restarted.  The
   operation of this router during graceful restart, including how the
   router enters and exits graceful restart, is the subject of Section
   2.  Then there are the router's neighbors, which must cooperate in
   order for the restart to be graceful.  During graceful restart, we
   say that the neighbors are running in "helper mode".  Section 3
   covers the responsibilities of a router running in helper mode,
   including entering and exiting helper mode.

2.  Operation of Restarting Router

   After the router restarts/reloads, it must change its OSPF processing
   somewhat until it re-establishes full adjacencies with all its former
   fully-adjacent neighbors.  This time period, between the
   restart/reload and the reestablishment of adjacencies, is called
   "graceful restart".  During graceful restart:

      1) The restarting router does not originate LSAs with LS types 1-
         5,7.  Instead, the restarting router wants the other routers in
         the OSPF domain to calculate routes using the LSAs that it
         originated prior to its restart.   During this time, the
         restarting router does not modify or flush received self-
         originated LSAs, (see Section 13.4 of [1]). Instead they are
         accepted as valid.  In particular, the grace-LSAs that the
         restarting router originated before the restart are left in
         place.  Received self-originated LSAs will be dealt with when
         the router exits graceful restart (see Section 2.3).





Moy, et al.                 Standards Track