RFC 3623 (rfc3623) - Page 4 of 18

Graceful OSPF Restart

RFC 3623                 Graceful OSPF Restart             November 2003


      2) The restarting router runs its OSPF routing calculations, as
         specified in Section 16 of [1].  This is necessary to return
         any OSPF virtual links to operation.  However, the restarting
         router does *not* install OSPF routes into the system's
         forwarding table(s) and relies on the forwarding entries that
         it installed prior to the restart.

      3) If the restarting router determines that it was the Designated
         Router on a given segment prior to the restart, it elects
         itself as the Designated Router again.  The restarting router
         knows that it was the Designated Router if, while the
         associated interface is in Waiting state, a Hello packet is
         received from a neighbor listing the router as the Designated
         Router.

   Otherwise, the restarting router operates the same as any other OSPF
   router.  It discovers neighbors using OSPF's Hello protocol, elects
   Designated and Backup Designated Routers, performs the Database
   Exchange procedure to initially synchronize link-state databases with
   its neighbors, and maintains this synchronization through flooding.

   The processes of entering graceful restart, and of exiting graceful
   restart (either successfully or not) are covered in the following
   sections.

2.1.  Entering Graceful Restart

   The router (call it Router X) is informed of the desire for its
   graceful restart when an appropriate command is issued by the network
   operator.  The network operator may also specify the length of the
   grace period, or the necessary grace period may be calculated by the
   router's OSPF software.  In order to avoid the restarting router's
   LSAs from aging out, the grace period should not exceed LSRefreshTime
   (1800 second) [1].

   In preparation for the graceful restart, Router X must perform the
   following actions before its software is restarted/reloaded:

      (Note that common OSPF shutdown procedures are *not* performed,
      since we want the other OSPF routers to act as if Router X remains
      in continuous service.  For example, Router X does not flush its
      locally originated LSAs, since we want them to remain in other
      routers' link-state databases throughout the restart period.)

      1) Router X must ensure that its forwarding table(s) is/are up-
         to-date and will remain in place across the restart.





Moy, et al.                 Standards Track