Page "Cisco IOS" Paragraph 33

In all versions of Cisco IOS, packet routing and forwarding ( switching ) are distinct functions.

Routing and other protocols run as Cisco IOS processes and contribute to the Routing Information Base ( RIB ).

This is processed to generate the final IP forwarding table ( FIB, Forwarding Information Base ), which is used by the forwarding function of the router.

On router platforms with software-only forwarding ( e. g., Cisco 7200 ) most traffic handling, including access control list filtering and forwarding, is done at interrupt level using Cisco Express Forwarding ( CEF ) or dCEF ( Distributed CEF ).

This means IOS does not have to do a process context switch to forward a packet.

Routing functions such as OSPF or BGP run at the process level.

In routers with hardware-based forwarding, such as the Cisco 12000 series, IOS computes the FIB in software and loads it into the forwarding hardware ( such as an ASIC or network processor ), which performs the actual packet forwarding function.