the switch. Latency is directly related to the configured switching process and volume of traffic. Latency is measured in fractions of a second. With networking devices operating at incredibly high speeds, every additional nanosecond of latency adversely affects network performance.
Content 4.2 Introduction to LAN Switching 4.2.7 Layer 2 and layer 3 switching Switching is the process of receiving an incoming frame on one interface and delivering that frame out another interface. Routers use Layer 3 switching to route a packet. Switches use Layer 2 switching to forward frames.The difference between Layer 2 and Layer 3 switching is the type of information inside the frame that is used to determine the correct output interface. Layer 2 switching is based on MAC address information. Layer 3 switching is based on network layer addresses or IP addresses. Layer 2 switching looks at a destination MAC address in the frame header and forwards the frame to the appropriate interface or port based on the MAC address in the switching table. The switching table is contained in Content Addressable Memory (CAM). If the Layer 2 switch does not know where to send the frame, it broadcasts the frame out all ports to the network. When a reply is returned, the switch records the new address in the CAM. Layer 3 switching is a function of the network layer. The Layer 3 header information is examined and the packet is forwarded based on the IP address. Traffic flow in a switched or flat network is inherently different from the traffic flow in a routed or hierarchical network. Hierarchical networks offer more flexible traffic flow than flat networks.
Content 4.2 Introduction to LAN Switching 4.2.8 Symmetric and asymmetric switching LAN switching may be classified as symmetric or asymmetric based on the way in which bandwidth is allocated to the switch ports. A symmetric switch provides switched connections between ports with the same bandwidth. An asymmetric LAN switch provides switched connections between ports of unlike bandwidth, such as a combination of 10 Mbps and 100 Mbps ports. Asymmetric switching enables more bandwidth to be dedicated to the server switch port in order to prevent a bottleneck. This allows smoother traffic flows where multiple clients are communicating with a server at the same time. Memory buffering is required on an asymmetric switch. The use of buffers keeps the frames contiguous between different data rate ports.
Content 4.2 Introduction to LAN Switching 4.2.9 Memory buffering An Ethernet switch may use a buffering technique to store and forward frames. Buffering may also be used when the destination port is busy. The area of memory where the switch stores the data is called the memory buffer. This memory buffer can use two methods for forwarding frames, port-based memory buffering and shared memory buffering. In port-based memory buffering frames are stored in queues that are linked to specific incoming ports. A frame is transmitted to the outgoing port only when all the frames ahead of it in the queue have been successfully transmitted. It is possible for a single frame to delay the transmission of all the frames in memory because of a busy destination port. This delay occurs even if the other frames could be transmitted to open destination ports. Shared memory buffering deposits all frames into a common memory buffer which all the ports on the switch share. The amount of buffer memory required by a port is dynamically allocated. The frames in the buffer are linked dynamically to the transmit port. This allows the packet to be received on one port and then transmitted on another port, without moving it to a different queue. The switch keeps a map of frame to port links showing where a packet needs to be transmitted. The map link is cleared after the frame has been successfully transmitted. The memory buffer is shared. The number of frames stored in the buffer is restricted by the size of the entire memory buffer, and not limited to a single port buffer. This permits larger frames to be transmitted with fewer dropped frames. This is important to asynchronous switching, where frames are being exchanged between different rate ports.
Content 4.2 Introduction to LAN Switching 4.2.10 Two switching methods The following two switching modes are available to forward frames: The following are two forms of cut-through switching: The latency of each switching mode depends on how the switch forwards the frames. To accomplish faster frame forwarding, the switch reduces the time for error checking. However, reducing the error checking time can lead to a higher number of retransmissions.
Content 4.3 Switch Operation 4.3.1 Functions of Ethernet switches A switch is a network device that selects a path or circuit for sending a frame to its destination. Both switches and bridges operate at Layer 2 of the OSI model. Switches are sometimes called multiport bridges or switching hubs. Switches make decisions based on MAC addresses and therefore, are Layer 2 devices. In contrast, hubs regenerate the Layer 1 signals out of all ports without making any decisions. Since a switch has the capacity to make path selection decisions, the LAN becomes much more efficient. Usually, in an Ethernet network the workstations are connected directly to the switch. Switches learn which hosts are connected to a port by reading the source MAC address in frames. The switch opens a virtual circuit between the source and destination nodes only. This confines communication to those two ports without affecting traffic on other ports. In contrast, a hub forwards data out all of its ports so that all hosts see the data and must process it, even if that data is not intended for it. High-performance LANs are usually fully switched.