Cisco Cisco IOS Software Release 12.0(22)S

This document describes the Cisco Nonstop Forwarding (NSF) feature in Cisco IOS Release 12.0(22)S. 
It includes the following sections:

Throughout this document, 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. For 
example, on the Cisco 10000 series Internet router the RP is referred to as the Performance Routing 
Engine (PRE), on the Cisco 12000 series Internet router the RP is referred to as the Gigabit Route 
Processor (GRP), and on the Cisco 7500 series router the RP is referred to as the Route Switch 
Processor (RSP).

Cisco NSF works with the Stateful Switchover (SSO) feature in Cisco IOS software. SSO is a 
prerequisite of Cisco NSF. NSF works with SSO to minimize the amount of time a network is unavailable 
to its users following a switchover. The main objective of Cisco NSF is to continue forwarding IP packets 
following a route processor (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. Cisco NSF helps to suppress routing flaps in 
SSO-enabled devices, thus reducing network instability.

12.0(22)S

This feature was introduced.