the network. If two stations establish a session that uses a significant level of bandwidth, the network performance of all other stations attached to the hub is degraded.

To reduce degradation, the switch treats each interface as an individual segment. When stations on different interfaces need to communicate, the switch forwards frames at wire speed from one interface to the other, to ensure that each session receives full bandwidth.

To efficiently switch frames between interfaces, the switch maintains an address table. When a frame enters the switch, it associates the MAC address of the sending station with the interface on which it was received. The main features of Ethernet switches are:

Isolate traffic among segments
Achieve greater amount of bandwidth per user by creating smaller collision domains

The first feature, isolate traffic among segments, is known as microsegmentation. Microsegmentation is the name given to the smaller units into which the networks are divided by use of Ethernet switches. Each segment uses the CSMA/CD access method to maintain data traffic flow among the users on that segment. Such segmentation allows multiple users to send information at the same time on the different segments without slowing down the network. By using the segments in the network fewer users and/or devices are sharing the same bandwidth when communicating with one another. Each segment has its own collision domain. Ethernet switches filter the traffic by redirecting the datagrams to the correct port or ports, which are based on Layer 2 MAC addresses. The second function of an Ethernet switch is to ensure each user has more bandwidth by creating smaller collision domains. Both Ethernet and Fast Ethernet switches allow the segmentation of a LAN, thus creating smaller collision domains. Each segment becomes a dedicated network link, like a highway lane functioning at up to 100 Mbps. Popular servers can then be placed on individual 100-Mbps links. Often in networks of today, a Fast Ethernet switch will act as the backbone of the LAN, with Ethernet hubs, Ethernet switches, or Fast Ethernet hubs providing the desktop connections in workgroups. As demanding new applications such as desktop multimedia or videoconferencing become more popular, certain individual desktop computers will have dedicated 100-Mbps links to the network. Web Links Switched Ethernet http://www.maznets.com/tech/ switched.htm

Content 4.3 Switch Operation 4.3.2 Frame transmission modes There are three main frame transmission modes:

Fast-forward – With this transmission mode, the switch reads the destination address before receiving the entire frame. The frame is then forwarded before the entire frame arrives. This mode decreases the latency of the transmission but has poor LAN switching error detection. Fast-forward is the term used to indicate a switch is in cut-through mode.
Store-and-forward – The entire frame is received before any forwarding takes place. The destination and source addresses are read and filters are applied before the frame is forwarded. Latency occurs while the frame is being received. Latency is greater with larger frames because the entire frame must be received before the switching process begins. The switch has time available to check for errors, which allows more error detection.
Fragment-free – This mode of switching reads the first 64 bytes of an Ethernet frame and then begins forwarding it to the appropriate port or ports. Fragment-free is a term used to indicate the switch is using modified cut-through switching.

Another transmission mode is a combination of cut-through and store-and-forward. This hybrid mode is called adaptive cut-through. In this mode, the switch uses cut-through until it detects a given number of errors. Once the error threshold is reached, the switch changes to store-and-forward mode. Interactive Media Activity Drag and Drop: Switching Method Trigger Points When the student has completed this activity, the student will be able to understand the different methods of switching. Web Links LAN Switching and VLANs http://www.cisco.com/univercd/cc/
td/doc/cisintwk/ito_doc/lanswtch.htm

Content 4.3 Switch Operation 4.3.3 How switches and bridges learn addresses Bridges and switches only forward frames, which need to travel from one LAN segment to another. To accomplish this task, they must learn which devices are connected to which LAN segment. A bridge is considered an intelligent device because it can make decisions based on MAC addresses. To do this, a bridge refers to an address table. When a bridge is turned on, broadcast messages are transmitted asking all the stations on the local segment of the network to respond. As the stations return the broadcast message, the bridge builds a table of local addresses. This process is called learning. Bridges and switches learn in the following ways:

Reading the source MAC address of each received frame or datagram
Recording the port on which the MAC address was received.

In this way, the bridge or switch learns which addresses belong to the devices connected to each port. The learned addresses and associated port or interface are stored in the addressing table. The bridge examines the destination address of all received frames. The bridge then scans the address table searching for the destination address. CAM is used in switch applications:

To take out and process the address information from incoming data packets
To compare the destination address with a table of addresses stored within it

The CAM stores host MAC addresses and associated port numbers. The CAM compares the received destination MAC address against the CAM table contents. If the comparison yields a match, the port is provided, and routing control forwards the packet to the correct port and address. An Ethernet switch can learn the address of each device on the network by reading the source address of each frame transmitted and noting the port where the frame entered the switch. The switch then adds this information to its forwarding database. Addresses are learned dynamically. This means that as new addresses are read, they are learned and stored in CAM. When a source address is not found in CAM, it is learned and stored for future use. Each time an address is stored, it is time stamped. This allows for addresses to be stored for a set period of time. Each time an address is referenced or found in CAM, it receives a new time stamp. Addresses that are not referenced during a set period of time are removed from the list. By removing aged or old addresses, CAM maintains an accurate and functional forwarding database. The processes followed by the CAM are as follows:

If the address is not found, the bridge forwards the frame out all ports except the port on which it was received. This process is called flooding. The address may also have been deleted by the bridge because the bridge software was recently restarted, ran short of address entries in the address table, or deleted the address because it was too old. Since the bridge does not know which port to use to forward the frame, it will send it to out all ports, except the one from which it was received. It is clearly unnecessary to send it back to the same cable segment from which it was received, since any other computer or bridges on this cable must already have received the packet.
If the address is found in an address table and the address is associated with the port on which it was received, the frame is discarded. It must already have been received by the destination.
If the address is found in an address table and the address is not associated with the port on which it was received, the bridge forwards the frame to the port associated with the address.

Web Links LAN Switching and VLANs http://www.cisco.com/univercd/cc/
td/doc/cisintwk/ito_doc/lanswtch.htm

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