Modes Of Graceful Restart Operation; Restart Mode; Helper Mode - Cisco NCS 5500 Series Configuration Manuals

Modes of Graceful Restart Operation

It is recommended to use passive configuration on interfaces that are connecting LAN segments with hosts
to the rest of the network, but are not meant to be transit links between routers.
Modes of Graceful Restart Operation
The operational modes that a router can be in for this feature are restart mode and helper moderestart mode,
helper mode, and protocol shutdown mode. Restart mode occurs when the OSPFv3 process is doing a graceful
restart. Helper mode refers to the neighbor routers that continue to forward traffic on established OSPFv3
routes while OSPFv3 is restarting on a neighboring router.

Restart Mode

When the OSPFv3 process starts up, it determines whether it must attempt a graceful restart. The determination
is based on whether graceful restart was previously enabled. (OSPFv3 does not attempt a graceful restart upon
the first-time startup of the router.) When OSPFv3 graceful restart is enabled, it changes the purge timer in
the RIB to a nonzero value.
During a graceful restart, the router does not populate OSPFv3 routes in the RIB. It tries to bring up full
adjacencies with the fully adjacent neighbors that OSPFv3 had before the restart. Eventually, the OSPFv3
process indicates to the RIB that it has converged, either for the purpose of terminating the graceful restart
(for any reason) or because it has completed the graceful restart.
If OSPFv3 attempts a restart too soon after the most recent restart, the OSPFv3 process is most likely crashing
repeatedly, so the new graceful restart stops running. To control the period between allowable graceful restarts,
use the graceful-restart interval command. When OSFPv3 starts a graceful restart with the first interface
that comes up, a timer starts running to limit the duration (or lifetime) of the graceful restart. You can configure
this period with the graceful-restart lifetime command. On each interface that comes up, a grace LSA (Type
11) is flooded to indicate to the neighboring routers that this router is attempting graceful restart. The neighbors
enter into helper mode. The designated router and backup designated router check of the hello packet received
from the restarting neighbor is bypassed, because it might not be valid.

Helper Mode

Helper mode is enabled by default. When a (helper) router receives a grace LSA (Type 11) from a router that
is attempting a graceful restart, the following events occur:
• If helper mode has been disabled through the graceful-restart helper disable command, the router drops
• If helper mode is enabled, the router enters helper mode if all of the following conditions are met:
• Upon entering helper mode, a router performs its helper function for a specific period of time. This time
Routing Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.3.x
110
the LSA packet.
• The local router itself is not attempting a graceful restart.
• The local (helping) router has full adjacency with the sending neighbor.
• The value of lsage (link state age) in the received LSA is less than the requested grace period.
• The sender of the grace LSA is the same as the originator of the grace LSA.
period is the lifetime value from the router that is in restart mode—minus the value of lsage in the received
grace LSA. If the graceful restart succeeds in time, the helper's timer is stopped before it expires. If the
helper's timer does expire, the adjacency to the restarting router is brought down, and normal OSPFv3
functionality resumes.
Implementing OSPF