A key element of NSF is packet forwarding. In a Cisco networking device, packet forwarding is provided by CEF. CEF maintains
the FIB, and uses the FIB information that was current at the time of the switchover to continue forwarding packets during
a 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 will
maintain the current forwarding information over a switchover; for platforms with forwarding engines, CEF will keep 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 will be able to continue forwarding after a switchover as soon as the interfaces and a data path are available.
As the routing protocols start to repopulate the RIB on a prefix-by-prefix basis, the updates in turn cause prefix-by-prefix
updates for CEF, which it uses to update the FIB and adjacency databases. Existing and new entries will receive the new version
(“epoch”) number, indicating that they have been refreshed. The forwarding information is updated on the line cards or forwarding
engine 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 routing protocols run only on the active RP, and they receive routing updates from their neighbor routers. Routing protocols
do not run on the standby RP. Following a switchover, the routing protocols request that the NSF-aware neighbor devices send
state information to help rebuild the routing tables.
For NSF operation, the routing protocols depend on CEF to continue forwarding packets while the routing protocols rebuild
the routing information.