With the low cost of general purpose central processing units, the price of software based routers is become more affordable at the expensive of total packet throughput.
When a route processor (RP) is equipped with a forwarding engine so it can make all of the packet switching decisions, this is known as a centralised forwarding architecture.
If line cards are equipped with forwarding engines so they can make packet forwarding decisions without the intervention of the route processor, this is known as distributed forwarding architecture.
In a centralised forwarding architecture, when a packet is received on the ingress line card it is transmitted to forwarding engine of the route processor. The forwarding engine examines the packets headers and sends out the packet via a port via the egress line card.