Switching via memory
Traditional computers with switching between input and output being doen under direct control of the CPU input and output ports functioned as traditional I/O devices in a traditional operating systems. Aninput port with an arriving packet first signalled the routing processor. The packet was then copied from the input port into processor memory. The routing processor then extract the destination address from the heard looked up the appropriate output port in the forwarding table and the packet to the output buffers. Note that if memory bandwidth is B packets per second can be written into or read from memory them the overall forwarding throughput ( the total rate at which packets are transferred from input ports to output ports ) must be less than B/2.