【正文】 witching modes have been supported on Catalyst switches:?Cut through?Fragment free?Store and forwardThese 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 ModeSwitches 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 Ethernet collisions, runts (frames less than 64 bytes), or damaged frames. FragmentFree ModeSwitches 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 Ethernet network, collision fragments must be detected in the first 64 bytes. StoreandForward ModeSwitches 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 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 SwitchingIn 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 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 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 transmitted onto the data bus.Frame is received by all ports— In a shared bus architecture, every frame transmitted is received by all 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 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.Port(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 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 frames 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 23. Frame Flow in a Shared Bus Crossbar SwitchingIn the shared bus architecture example, the speed of the shared data bus determines much of the overall traffic handling capacity of the switch. Because the b