【正文】 每臺(tái) Catalyst 交換機(jī)都會(huì)為每個(gè)分組創(chuàng)建報(bào)頭或者標(biāo)簽，并且以該報(bào)頭或者標(biāo)簽作為轉(zhuǎn)發(fā)決策的基礎(chǔ)。 Catalyst 交換機(jī)采用多種技術(shù)來(lái)避免線(xiàn)端阻塞，例如采用每端口緩沖。 如圖 27 所示，流量發(fā)生器能夠產(chǎn)生 擁塞現(xiàn)象。為了確定交換矩陣的總連接帶寬，請(qǐng)查看各種平臺(tái)和線(xiàn)路卡的技術(shù)指標(biāo)。 4. 過(guò)度預(yù)定交換矩陣 交換機(jī)制造商喜 歡使用術(shù)語(yǔ)無(wú)阻塞米表明連接到交換矩陣的交換端口能夠達(dá)到線(xiàn)速。 Catalyst 1200 能夠支持以太網(wǎng)和 FDDI，并且具有 4MB 的共享分組 DRAM（ Dynamic RandomAccess Memory，動(dòng)態(tài)隨機(jī)訪(fǎng)問(wèn)內(nèi)存）。 通過(guò)使用 168KB 的發(fā)送緩沖區(qū)，每個(gè)端口最多能夠創(chuàng)建 2500 個(gè) 64 字節(jié)的緩沖 區(qū)。 如果發(fā)往端口的流量超過(guò)了它所能發(fā)送的流量 ,那么就需要使用緩沖。在 Cisco Catalyst 5500 平臺(tái)中，吉比特以太網(wǎng)線(xiàn)路卡要求訪(fǎng)問(wèn)所有 3 條數(shù)據(jù)總線(xiàn)。 2在這個(gè)示例中，假定交換機(jī)知道主機(jī) 2 的位置，并且僅在連接到主機(jī) 2 的端口發(fā)送幀。如果幀存在缺陷（例如殘幀、碎片、無(wú)效 CRC 或者巨型幀），那么端口將丟棄該幀，并且將增加相關(guān)計(jì)數(shù)器的數(shù)值。根據(jù)交換機(jī)配置的情況，仲裁器能夠使用多種公平方法。 中英文資料 8 圖 21 比較各種交換模式之間的區(qū)別。 直通模式 如果交換機(jī)工作在直通模式，那么它將只接收和檢查幀的的前 6 個(gè)字節(jié)。 1. 數(shù)據(jù)接收 交換模式 在局域網(wǎng)交換中，根據(jù)交換機(jī)功能的不同，第一步就是從發(fā)送設(shè)備或主機(jī)接收幀或分組。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。隨后，本章介紹保證高效數(shù)據(jù)交換的一些機(jī)制。 Catalyst 交換機(jī)攴持下述三種交換模式 : ?