You can understand the function of the Packet Forwarding Engine by
following the flow of a packet through the router: first into a PIC,
then through the switching fabric, and finally out another PIC for
transmission on a network link. Generally, the data flows through
the Packet Forwarding Engine as follows:
Packets enter the router through incoming PIC interfaces,
which contain controllers that perform media-specific processing.
The PICs pass the packets to the FPCs, where they are
divided into cells and are distributed to the router’s buffer
memory.
The Packet Forwarding Engine performs route lookups, forwards
the notification to the destination port, reassembles the cells into
packets, and sends them to the destination port on the outgoing PIC.
The PIC performs encapsulation and other media-specific
processing, and sends the packets out into the network.