1、中英文资料 1 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 wi
2、th 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, fro
3、m 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. Depe
4、nding 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 b
5、y 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 suffi
6、cient 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 operat
7、ing 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 designed Ethernet network, collision fragments must be detected in the first 64 bytes. 1.3 Store-and-
8、Forward Mode Switches operating in store-and-forward mode receive and examine the entire frame, resulting in the most error-free type of switching. As switches utilizing faster processor and application-specific integrated circuits (ASICs) were introduced, the need to support cut-through and fragmen
9、t-free switching was no longer necessary. As a result, all new 中英文资料 2 Cisco Catalyst switches utilize store-and-forward switching. Figure2-1 compares each of the switching modes. Figure2-1.Switching Modes 2. Switching Data Regardless of how many bytes of each frame are examined by the switch, the f
10、rame 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 communication channels used by the switch to transport frames, carry forwarding decision information, and relay management information throughout the switc
11、h. A comparison 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 fram
12、es 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 fab
13、rics on Cisco Catalyst platforms, two major architectures of switch fabrics are common: Shared bus Crossbar 2.1 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 fro
14、m 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. Figure2-2illustra
15、tes a round-robin servicing of frames as they enter a switch. Round-robin 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 serv
16、ice to specified traffic flows. Figure 2-2. Round-Robin Service Order The following list and Figure 2-3 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 th
17、e switch receives the entire frame from Host1 and stores it in a receive buffer. The port checks the frames Frame Check Sequence (FCS) for errors. If the frame is defective (runt, fragment, invalid CRC, or Giant), the port discards the frame and increments the appropriate counter. Requesting access
18、to the data bus A header containing information necessary to make a forwarding decision is added to the frame. The line card then requests access or permission to transmit the frame onto the data bus. Frame transmitted onto the data bus After the central arbiter grants access, the frame is transmitt
19、ed onto the data bus. Frame is received by all ports In a shared bus architecture, every frame transmitted is received by all 中英文资料 3 ports simultaneously. In addition, the frame is received by the hardware necessary to make a forwarding decision. Switch determines which port(s) should transmit the
20、frame The information added to the frame in step 2 is used to determine which ports should transmit the frame. In some cases, frames with either an unknown destination MAC address or a broadcast frame, the switch will transmit the frame out all ports except the one on which the frame was received. P
21、ort(s) instructed to transmit, remaining ports discard the frame Based on the decision in step 5, a certain port or ports is told to transmit the frame while the rest are told to discard or flush the frame. Egress port transmits the frame to Host2 In this example, it is assumed that the location of
22、Host2 is known to the switch and only the port connecting to Host2 transmits the frame. One advantage of a shared bus architecture is every port except the ingress port receives a copy of the frame automatically, easily enabling multicast and broadcast traffic without the need to replicate the frame
23、s for each port. This example is greatly simplified and will be discussed in detail for Catalyst platforms that utilize a shared bus architecture in Chapter 3, Catalyst Switching Architecture. Figure 2-3. Frame Flow in a Shared Bus 2.2 Crossbar Switching In the shared bus architecture example, the s
24、peed of the shared data bus determines much of the overall traffic handling capacity of the switch. Because the bus is shared, line cards must wait their turns to communicate, and this limits overall bandwidth. A solution to the limitations imposed by the shared bus architecture is the implementatio
25、n of a crossbar switch fabric, as shown in Figure 2-4. The term crossbar means different things on different switch platforms, but essentially indicates multiple data channels or paths between line cards that can be used simultaneously. In the case of the Cisco Catalyst 5500 series, one of the first
26、 crossbar architectures advertised by Cisco, three individual 1.2-Gbps data buses are implemented. Newer Catalyst 5500 series line cards have the necessary connector pins to connect to all three buses simultaneously, taking advantage of 3.6 Gbps of aggregate bandwidth. Legacy line cards from the Cat
27、alyst 5000 are still compatible with the Catalyst 5500 series by connecting to only one of the three data buses. Access to all three buses is required by Gigabit Ethernet cards on the Catalyst 5500 platform. A crossbar fabric on the Catalyst 6500 series is enabled with the Switch Fabric Module (SFM)
28、 and Switch Fabric Module 2 (SFM2). The SFM provides 128 Gbps of bandwidth (256 Gbps full duplex) to line cards via 16 individual 8-Gbps connections to the crossbar switch fabric. The SFM2 was introduced to support the Catalyst 6513 13-slot chassis and includes architecture optimizations over the SF
29、M. Figure 2-4. Crossbar Switch Fabric 3. Buffering Data Frames must wait their turn for the central arbiter before being transmitted in shared bus architectures. Frames can also potentially be delayed when congestion occurs in a crossbar switch fabric. As a result, frames must be buffered until transmitted. Without an effective buffering scheme, frames are more likely to be dropped anytime traffic oversubscription or congestion occurs.
