connected to each switch port. When two connected hosts want to communicate with each other, the switch looks up the switching table and establishes a virtual connection between the ports. The virtual circuit is maintained until the session is terminated. In Figure , Host B and Host C want to communicate with each other. The switch creates the virtual connection which is referred to as a microsegment. The microsegment behaves as if the network has only two hosts, one host sending and one receiving providing maximum utilization of the available bandwidth. Switches reduce collisions and increase bandwidth on network segments because they provide dedicated bandwidth to each network segment.
Content 4.3 Switch Operation 4.3.9 Switches and broadcast domains Communication in a network occurs in three ways. The most common way of communication is by unicast transmissions. In a unicast transmission, one transmitter tries to reach one receiver.Another way to communicate is known as a multicast transmission. Multicast transmission occurs when one transmitter tries to reach only a subset, or a group, of the entire segment. The final way to communicate is by broadcasting. Broadcasting is when one transmitter tries to reach all the receivers in the network. The server station sends out one message and everyone on that segment receives the message. When a device wants to send out a Layer 2 broadcast, the destination MAC address in the frame is set to all ones. A MAC address of all ones is FF:FF:FF:FF:FF:FF in hexadecimal. By setting the destination to this value, all the devices will accept and process the broadcasted frame. The broadcast domain at Layer 2 in referred to as the MAC broadcast domain. The MAC broadcast domain consists of all devices on the LAN that receive frame broadcasts by a host to all other machines on the LAN. A switch is a Layer 2 device. When a switch receives a broadcast, it forwards it to each port on the switch except the incoming port. Each attached device must process the broadcast frame. This leads to reduced network efficiency, because available bandwidth is used for broadcasting purposes. When two switches are connected, the broadcast domain is increased. In this example a broadcast frame is forwarded to all connected ports on Switch 1. Switch 1 is connected to Switch 2. The frame is propagated to all devices connected to Switch 2. The overall result is a reduction in available bandwidth. This happens because all devices in the broadcast domain must receive and process the broadcast frame. Routers are Layer 3 devices. Routers do not propagate broadcasts. Routers are used to segment both collision and broadcast domains.
Content 4.3 Switch Operation 4.3.10 Communication between switches and workstation When a workstation connects to a LAN, it is unconcerned about the other devices that are connected to the LAN media. The workstation simply transmits data frames using a NIC to the network medium. The workstation could be attached directly to another workstation, using a crossover cable or attached to a network device, such as a hub, switch, or router, using a straight-through cable. Switches are Layer 2 devices that use intelligence to learn the MAC addresses of the devices that are attached to the ports of the switch. This data is entered into a switching table. Once the table is complete, the switch can read the destination MAC address of an incoming data frame on a port and immediately forward it. Until a device transmits, the switch does not know its MAC address. Switches provide significant scalability on a network and may be directly connected. Figure illustrates one scenario of frame transmission utilizing a multi-switch network.
Content Summary An understanding of the following key points should have been achieved: