【正文】 . In Figure 27, a congestion scenario is created using a traffic generator. Port 1 on the traffic generator is connected to Port 1 on the switch, generating traffic at a 50 percent rate, destined for both Ports 3 and 4. Port 2 on the traffic generator is connected to Port 2 on the switch, generating traffic at a 100 percent rate, destined for only Port 4. This situation creates congestion for traffic destined to be forwarded by Port 4 on the switch because traffic equal to 150 percent of the forwarding capabilities of that port is being sent. Without proper buffering and forwarding algorithms, traffic destined to be transmitted by Port 3 on the switch may have to wait until the congestion on Port 4 clears. Figure 27. HeadofLine Blocking Headofline blocking can also be experienced with crossbar switch fabrics because many, if not all, line cards have highspeed connections into the switch fabric. Multiple line cards may attempt to create a connection to a line card that is already busy and must wait for the receiving line card to bee free before transmitting. In this case, data destined for a different line card that is not busy is blocked by the frames at the head of the line. 中英文資料 6 Catalyst switches use a number of techniques to prevent headofline blocking。 one important example is the use of per port buffering. Each port maintains a small ingress buffer and a larger egress buffer. Larger output buffers (64 Kb to 512 k shared) allow frames to be queued for transmit during periods of congestion. During normal operations, only a small input queue is necessary because the switching bus is servicing frames at a very high speed. In addition to queuing during congestion, many models of Catalyst switches are capable of separating frames into different input and output queues, providing preferential treatment or priority queuing for sensitive traffic such as voice. Chapter 8 will discuss queuing in greater detail. 6. Forwarding Data Regardless of the type of switch fabric, a decision on which ports should forward a frame and which should flush or discard the frame must occur. This decision can be made using only the information found at Layer 2 (source/destination MAC address), or on other factors such as Layer 3 (IP) and Layer 4 (Port). Each switching platform supports various types of ASICs responsible for making the intelligent switching decisions. Each Catalyst switch creates a header or label for each packet, and forwarding decisions are based on this header or label. Chapter 3 will include a more detailed discussion of how various platforms make forwarding decisions and ultimately forward data. 7. Summary Although a wide variety of different approaches exist to optimize the switching of data, many of the core concepts are closely related. The Cisco Catalyst line of switches focuses on the use of shared bus, crossbar switching, and binations of the two depending on the platform to achieve very highspeed switching solutions. Highspeed switching ASICs use shared and per port buffers to reduce congestion and prevent headofline blocking. 中英文資料 7 中文翻譯： 局域網(wǎng)交換機(jī)體系結(jié)構(gòu) 本章將介紹所有交換機(jī)生產(chǎn)廠商都遵守的局域網(wǎng)交換技術(shù)的一些基本概念。本章首先介紹交換機(jī)如何接受數(shù)據(jù)。隨后，本章介紹保證高效數(shù)據(jù)交換的一些機(jī)制。最后，本章介紹如何將數(shù)據(jù)轉(zhuǎn)發(fā)給目標(biāo)。這些概念并非 Cisco 交換機(jī)所特有的，而是在查看局域網(wǎng)交換機(jī)功能的時(shí)候，對(duì)所有交換機(jī)產(chǎn)品都適用的。 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。對(duì)于僅在 OSI 模型的第 2 層進(jìn)行轉(zhuǎn)發(fā)決策的交換機(jī)，它們將數(shù)據(jù)看作幀。而對(duì)于在 OSI模型的第 3 層 或者更高層進(jìn)行轉(zhuǎn)發(fā)決策的交換機(jī)，它們將數(shù)據(jù)看作分組。本章首先從第2 層的角度來研究交換機(jī)。根據(jù)具體型號(hào)的不同，交換機(jī)在數(shù)據(jù)交換之前所存儲(chǔ)和檢查的楨數(shù)目也存在一定差異。 Catalyst 交換機(jī)攴持下述三種交換模式 : ? 直通模式； ? 碎片隔離模式； ? 存儲(chǔ)轉(zhuǎn)發(fā)模式。 上述 3 種交換模式的區(qū)別在于交換機(jī)在制定轉(zhuǎn)發(fā)決策之前所接收和檢查的幀數(shù)目。下面將詳細(xì)討論每種交換模式。 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)。這 6 個(gè)字節(jié)代表了幀的日標(biāo) MAC 地址 ,交換機(jī)利用這些信息足以做 出轉(zhuǎn)發(fā)決策。盡管直通交換能夠在數(shù)據(jù)傳送的時(shí)候提供最低的延遲 ,但卻容易傳送以太網(wǎng)碰撞所產(chǎn)生的碎片、殘幀 (runt)或受損幀。 碎片隔離模式 如呆交換機(jī)工作在碎片隔離模式 ,那么它將接收和檢查全幀的前 64 個(gè)字節(jié)。在某些 Cisco Catalyst 交換機(jī)的文檔中 ,碎片隔離又稱為“快速轉(zhuǎn)發(fā)”模式。為什么交換機(jī)檢查幀的前64 個(gè)字節(jié)呢 ?因?yàn)樵谠O(shè)計(jì)良好的以太網(wǎng)網(wǎng)絡(luò)中 ,碰撞碎片必須在前 64 字節(jié)中檢測(cè)出來。 存儲(chǔ)轉(zhuǎn)發(fā)模式 如果交換機(jī)工作在存儲(chǔ)轉(zhuǎn)發(fā)模式 ,那么它將接收和檢查整幀 ,因此它是錯(cuò)誤率最低的交換模式。 由于 采用速度更快的處理器和 ASIC（ ApplicationSpecific Integrated Circuit，專用集成電路），交換機(jī)不必支持直通交換機(jī)和碎片隔離交換，因此，所有新型的 Cisco Catalyst 交換機(jī)都采用存儲(chǔ)轉(zhuǎn)發(fā)交換。 中英文資料 8 圖 21 比較各種交換模式之間的區(qū)別。 圖 21 交換模式 2. 數(shù)據(jù)交換 無論交換機(jī)需要檢查幀的多少字節(jié)，幀最終都將由輸入或入站端口交換到單個(gè)或多個(gè)輸出和出站端口。交換矩陣 (switch fabric)是交換機(jī)通信信道的一個(gè)常用術(shù)語 ,它可以在交換機(jī)內(nèi)部傳送幀、承載轉(zhuǎn)發(fā)決策 信慮、和轉(zhuǎn)送管理信息。 Catalyst 交換機(jī)中的交換矩陣可以看作汽車中的傳動(dòng)裝置，在汽車中 ,傳動(dòng)裝置負(fù)責(zé)將引擎的動(dòng)力傳遞給汽車輪子；在Catalyst 交換機(jī)中，交換矩陣負(fù)責(zé)將輸入或入站端口的幀轉(zhuǎn)送給單個(gè)或多個(gè)輸出和出站端口。無論具體型號(hào)如何，也無論何時(shí)產(chǎn)生新的交換平臺(tái) ,所有文檔都會(huì)將“傳動(dòng)裝置”作為交換矩陣。 盡管 Cisco Catalyst 平臺(tái)已經(jīng)采用多種技術(shù)來實(shí)現(xiàn)交換矩陣，但以下兩種體系結(jié)構(gòu)的交換矩陣最為常見： ? 共享總線； ? 交叉矩陣。 共享總線交換 在共享總線的體系結(jié)構(gòu)中，交換機(jī)的所有線 路模塊都共享 1 個(gè)數(shù)據(jù)通路。中央仲裁器決定何時(shí)授予各線路卡訪問總線的請(qǐng)求。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。共享總線體系結(jié)構(gòu)非常類似于機(jī)場票務(wù)柜臺(tái)前的多個(gè)隊(duì)列，但任何時(shí)候僅有 1個(gè)票務(wù)代理處理客戶請(qǐng)求。 圖 22 舉例說明幀進(jìn)入交換機(jī)時(shí)的循環(huán)服務(wù)過程。如果希望根據(jù)幀的接收順序進(jìn)行服務(wù)，那么循環(huán)是最簡單的方法。為了能夠給特定通信流量提供優(yōu)先級(jí)服務(wù)，當(dāng)前的 Catalyst交換平臺(tái)（例如 Catalyst 6500）能夠支持各種各樣的 QoS（ Quanlity Of Service，服務(wù)質(zhì)量）特性。 圖 22 循環(huán)