In a control plane failure or a forced switchover event, the router continues to forward packets using the existing stale forwarding information.
Nonstop forwarding is used to notify peer routers to continue forwarding and receiving packets, even if the route processor (control plane) is not working or is in a switch-over state. Nonstop forwarding requires clean control plane and data plane separation and usually the forwarding information is distributed to the line cards.
This method of availability has both advantages and disadvantages. Nonstop forwarding continues to forward packets using the existing stale forwarding information during a failure. This may cause routing loops and black holes; surrounding routers must adhere to separate extension standards for each protocol. Each vendor must support protocol extensions for router interoperability.