In the following content, the term Route Processor (RP) is used to describe the route processing engine on all networking
devices, regardless of the platform designation, unless otherwise noted.
NSF works with the SSO feature in Cisco software to minimize the amount of time a network is unavailable to its users following
a switchover. The main objective of NSF is to continue forwarding IP packets following an RP switchover.
Usually, when a networking device restarts, all routing peers of that device detect that the device went down and then came
back up. This transition results in what is called a routing flap, which could spread across multiple routing domains. Routing
flaps caused by routing restarts create routing instabilities, which are detrimental to the overall network performance. NSF
helps to suppress routing flaps in SSO-enabled devices, thus reducing network instability.
NSF allows for the forwarding of data packets to continue along known routes while the routing protocol information is being
restored following a switchover. With NSF, peer networking devices do not experience routing flaps. Data traffic is forwarded
through intelligent line cards or dual forwarding processors (FPs) while the standby RP assumes control from the failed active
RP during a switchover. The ability of line cards and FPs to remain up through a switchover and to be kept current with the
Forwarding Information Base (FIB) on the active RP is key to NSF operation.
The NSF feature provides the following benefits:
Improved network availability—NSF continues forwarding network traffic and application state information so that user session
information is maintained after a switchover.
Overall network stability—Network stability may be improved with the reduction in the number of route flaps that had been
created when devices in the network failed and lost their routing tables.
Neighboring devices do not detect link flapping—Because the interfaces remain up across a switchover, neighboring devices
do not detect a link flap (that is, the link does not go down and come back up).
Prevention of routing flaps—Because SSO continues forwarding network traffic in the event of a switchover, routing flaps
No loss of user sessions—User sessions established prior to the switchover are maintained.
NSF always runs together with SSO. SSO supported protocols and applications must be high-availability (HA)-aware. A feature
or protocol is HA-aware if it maintains, either partially or completely, undisturbed operation during an RP switchover. For
some HA-aware protocols and applications, state information is synchronized from the active to the standby processor.