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06.5 Switch Operation
9/11/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/6/pages/5 1/2 Switch Operation Section 6: Understanding Ethernet and Switch Operation Take a look at how switches work. When a PC wants to send data to another device, it sends the data to the switch that it is directly attached to. Switches are Layer 2 devices and are only concerned with Layer 2 addresses, which are MAC addresses. At Layer 2, the data is referred to as frames. When the switch receives the frame, it looks at the destination MAC address. It then tries to determine which port the device with that MAC address is attached to. It does this by looking at a table in its memory that stores all of the MAC addresses that it knows about. This table is called the CAM table. If the switch does not know which port the destination device is attached to, it sends the frame out all the ports. If the destination device is accessible via any of the ports, the frame reaches the destination. When the destination device replies, the switch learns which port it is attached to and adds that to the CAM table. Ethernet switches selectively forward individual frames from a receiving port to the port where the destination node is connected. This selective forwarding process can be thought of as establishing a momentary pointtopoint connection between the transmitting and receiving nodes. The connection is made only long enough to forward a single frame. During this instant, the two nodes have a fullbandwidth connection between them and represent a logical pointtopoint connection. The video uses an example to explain the process. Storeandforward and cutthrough are two switching methods, both of which base their forwarding decisions on destination MAC addresses. Both methods also learn MAC addresses, as shown in the video. What differentiates the two methods is the point at which the switch starts forwarding a frame. A storeandforward switch makes a forwarding decision on a frame after it has received the entire frame and checked its integrity. It drops the frame if the integrity check proves the frame to be invalid. A cutthrough switch engages in the forwarding process soon after it has examined the destination MAC address of the incoming frame. A storeandforward switch drops invalid frames, but cutthrough devices forward them. The purpose of the cutthrough method is to enable low switching latency, which enables upperlayer applications to get their job done as quickly as possible. 9/11/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/6/pages/5 2/2 Some switches can operate in either cutthrough or storeandforward mode. Other switches include adaptive cutthrough switching technology with automatic error packet detection and elimination. These capabilities fulfill user requirements for lowlatency switching applications while maintaining the data integrity of storeandforward devices. Content Unavailable This content is unavailable in this version of the course. Up Next: Duplex Communication