objective. Both approaches have pros and cons. In previous chapters when we discussed the search and classification processes in relation to packet routing, we discussed the mechanisms through which input packets will be ultimately routed to the appropriate output ports by a switch/router. However, a packet switch must be able to perform an equally important role. The resolution of any potential output-port contention must be carried out cor- rectly, which involves some sort of buffering. One design approach is to physically separate routing decisions from buffering management in order to avoid contention at the output. In that case, a switch consists of an internal fabric (crossbar) routing network that can connect any input to any output and a set of buffers either at the input or out- put ports of this fabric or internal routing network. Datagram has been used as a generic term for either an IP packet or ATM cell. The difference between a datagram switch/router and an ATM switch in our discussion is that the former can switch packets of variable length, whereas the latter's load will always be of fixed length. In ATM switches, the port-mapping function will decide what to do with a specific cell based on the VCI that it con- tains. However, in packet switches/routers, the hardware must first parse the entire destination address before it can decide what to do with the packet or datagram. In addition, packet size also affects performance in a switching network to a certain extent. Chap- ter 13 discusses this addressing, parsing, and routing decision context in more detail. Packets at the input can now be broken by the fabric into smaller frames that are appropriately formatted and tagged and then routed over the fabric core at the output of which they will be stripped of their tags and auto- matically reassembled in orderly queues at the intended output ports. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights rese