ce had to be adopted and implemented. Instead of sending packets Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. SWITCH FABRICS 272 NETWORK PROCESSORS back and forth between the routing CPU and the line cards, packets would now be temporarily stored the line card upon arrival and only some information from their header would need to be escalated for study and action. A centrally located CPU would receive only that information (as opposed to all the packets in their entirety) and decide upon the forwarding fate of each packet. The forwarding instructions would then be communicated back to the requesting line card, which would possess the local intelligence to forward the packet from its local buffers directly to the correct destination. Designers realized that there was (i) work that needed to be performed on the packet content and (ii) work that needed to be done regarding the packet and its destination. The principle of slow and fast path processing planes was born. Line cards connect to the switch fabric over a backplane that carries data traffic to and from cards as well as control and arbitration signaling. The control function can be implemented on the switch fabric card, or it can be distributed on the line cards. In some cases, it can also be implemented on a separate control card. In order to improve this original packet switch/router architecture, designers started to come up with evolutionary thinking. When the bus became the bottleneck despite the decreased bus traffic, the concept of the switch fabric was developed. This component would switch traffic from input ports to output ports much faster than could ever be imagined at that time. However, when the I/O bus was not the problem anymore, the line cards became the problem. Line cards soon evolved from their original limited status. Instead of being dumb I/O adapters, they found