Cisco Cisco IOS Software Release 12.4(6)T

A key element of NSF is packet forwarding. The OSPF protocol depends on CEF to continue forwarding 
packets during switchover while the routing protocols rebuild the Routing Information Base (RIB) 
tables. Once OSPF has converged, CEF updates the Forwarding Information Base (FIB) table and 
removes stale route entries. CEF then updates the line cards with the new FIB information. CEF 
maintains the FIB and uses the FIB information that was current at the time of a switchover to continue 
forwarding packets during the switchover. This feature reduces traffic interruption during the 
switchover.

During normal NSF operation, CEF on the active RP synchronizes its current FIB and adjacency 
databases with the FIB and adjacency databases on the standby RP. Upon switchover of the active RP, 
the standby RP initially has FIB and adjacency databases that are mirror images of those that were 
current on the active RP. For platforms with intelligent line cards, the line cards maintain the current 
forwarding information over a switchover; for platforms with forwarding engines, CEF keeps the 
forwarding engine on the standby RP current with changes that are sent to it by CEF on the active RP. 
In this way, the line cards or forwarding engines can continue forwarding after a switchover as soon as 
the interfaces and a data path are available.

As the OSPF routing protocol starts to repopulate the RIB on a prefix-by-prefix basis, the updates in turn 
cause prefix-by-prefix updates that CEF uses to update the FIB and adjacency databases. Existing and 
new entries receive the new version number, indicating that they have been refreshed. The forwarding 
information is updated on the line cards or forwarding engines during convergence. The RP signals when 
the RIB has converged. The software removes all FIB and adjacency entries that have an epoch older 
than the current switchover epoch. The FIB now represents the newest routing protocol forwarding 
information.

The OSPF routing protocol runs on only the active RP, and OSPF receives routing updates from OSPF 
neighbor routers. The OSPF routing protocol does not run on the standby RP. Following a switchover, 
OSPF requests that the NSF-aware neighbor devices send state information to help rebuild the routing 
tables.

For NSF operation, OSPF depends on CEF to continue forwarding packets while OSPF rebuilds the 
routing information.

When a neighbor router that is on the same network segment as the restarting router receives a 
grace-LSA from the restarting router, the neighbor enters helper mode as long as the following criteria 
are met:

The neighbor must have a full adjacency with the restarting router over the associated network 
segment.

There have been no changes to the link-state database since the restarting router began restarting.

The grace period has not yet expired.

Local policy allows the neighbor router to act as a helper router.

The neighbor router must not be in its own graceful restart process.

Helper mode for this router has not been disabled by the network administrator.