局域网交换机体系结构外文中英文翻译.doc

中文5030字 英文资料翻译 LAN Switch Architecture This chapter introduces many of the concepts behind LAN switching common 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 chapters 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. 1.1 Cut-Through Mode Switches operating in cut-through 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 cut-through switching offers the least latency when transmitting frames, it is susceptible to transmitting fragments created via Ethernet collisions, runts (frames less than 64 bytes), or damaged frames. 1.2 Fragment-Free Mode Switches operating in fragment-free 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 designe