【正文】 the concepts behind LAN switching mon to all switch vendors. The chapter begins by looking at how data are received by a switch, followed by mechanisms used to switch data as efficiently as possible, and concludes with forwarding data toward their destinations. These concepts are not specific to Cisco and are valid when examining the capabilities of any LAN switch. 1. Receiving Data— Switching Modes The first step in LAN switching is receiving the frame or packet, depending on the capabilities of the switch, from the transmitting device or host. Switches making forwarding decisions only at Layer 2 of the OSI model refer to data as frames, while switches making forwarding decisions at Layer 3 and above refer to data as packets. This chapter39。s examination of switching begins from a Layer 2 point of view. Depending on the model, varying amounts of each frame are stored and examined before being switched. Three types of switching modes have been supported on Catalyst switches: ?Cut through ?Fragment free ?Store and forward These three switching modes differ in how much of the frame is received and examined by the switch before a forwarding decision is made. The next sections describe each mode in detail. CutThrough Mode Switches operating in cutthrough mode receive and examine only the first 6 bytes of a frame. These first 6 bytes represent the destination MAC address of the frame, which is sufficient information to make a forwarding decision. Although cutthrough switching offers the least latency when transmitting frames, it is susceptible to transmitting fragments created via Ether collisions, runts (frames less than 64 bytes), or damaged frames. FragmentFree Mode Switches operating in fragmentfree mode receive and examine the first 64 bytes of frame. Fragment free is referred to as fast forward mode in some Cisco Catalyst documentation. Why examine 64 bytes? In a properly designed Ether work, collision fragments must be detected in the first 64 bytes. StoreandForward Mode Switches operating in storeandforward mode receive and examine the entire frame, resulting in the most errorfree type of switching. As switches utilizing faster processor and applicationspecific integrated circuits (ASICs) were introduced, the need to support cutthrough and fragmentfree switching was no longer necessary. As a result, all new 中英文資料 2 Cisco Catalyst switches utilize storeandforward switching. Figure21 pares each of the switching modes. Modes 2. Switching Data Regardless of how many bytes of each frame are examined by the switch, the frame must eventually be switched from the input or ingress port to one or more output or egress ports. A switch fabric is a general term for the munication channels used by the switch to transport frames, carry forwarding decision information, and relay management information throughout the switch. A parison could be made between the switching fabric in a Catalyst switch and a transmission on an automobile. In an automobile, the transmission is responsible for relaying power from the engine to the wheels of the car. In a Catalyst switch, the switch fabric is responsible for relaying frames from an input or ingress port to one or more output or egress ports. Regardless of model, whenever a new switching platform is introduced, the documentation will generally refer to the transmission as the switching fabric. Although a variety of techniques have been used to implement switching fabrics on Cisco Catalyst platforms, two major architectures of switch fabrics are mon: ?Shared bus ?Crossbar Shared Bus Switching In a shared bus architecture, all line modules in the switch share one data path. A central arbiter determines how and when to grant requests for access to the bus from each line card. Various methods of achieving fairness can be used by the arbiter depending on the configuration of the switch. A shared bus architecture is much like multiple lines at an airport ticket counter, with only one ticketing agent processing customers at any given time. Figure22illustrates a roundrobin servicing of frames as they enter a switch. Roundrobin is the simplest method of servicing frames in the order in which they are received. Current Catalyst switching platforms such as the Catalyst 6500 support a variety of quality of service (QoS) features to provide priority service to specified traffic flows. Figure 22. RoundRobin Service Order The following list and Figure 23 illustrate the basic concept of moving frames from the received port or ingress, to the transmit port(s) or egress using a shared bus architecture: Frame received from Host1— The ingress port on the switch receives the entire frame from Host1 and stores it in a receive buffer. The port checks the frame39。s local arbiter w