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What is a switch

Source: Shangpin China | Type: website encyclopedia | Time: June 24, 2014
Switch, which is the most widely used network device in LAN.

1. Switch working level and function

The switch works on the data link layer of the OSI reference model. Different models, brands and the number of access ports are different. Figure 3-9 shows a switch with 16 interfaces


Switch


There are two main functions:

(1) Fast switching function: the sending node and the receiving node are connected through the switch, and the data frame can be quickly switched from the source MAC address to the destination MAC address directly.

(2) Store and forward function: send the data frame to the internal buffer of the switch before forwarding. This kind of switch with store and forward type can check the error of data frames and filter out problematic data frames.

2. Operating principle of switch

The switch not only has a built-in MAC address exchange table, but also has a high bandwidth Beijing website production See Fig. 3-10 for transmission bus and built-in switching matrix. All ports of the switch are directly attached to the back bus.
Operating principle of switch
When a port of the switch receives a data frame, the processing port will look up in the built-in "MAC Address Exchange Table" to determine the port where the destination MAC is located, and then quickly exchange the data to the destination port through the built-in exchange matrix; if the destination MAC is in the "MAC Address Exchange Table" If the address does not exist in or is broadcast address, the switch sends the data frame to all ports except this receiving port in broadcast mode.

In essence, a switch is a multi port bridge. Each port contains a MAC entity, but it is no longer used. In other words, the switch can have no MAC address. When each port is directly connected to the host, data exchange can be conducted between the hosts in full duplex mode.

As shown in Figure 3-11, it is a network connected by switches. The upper terminal receiving equipment is the switch, and the lower terminal receiving equipment is the hub. Host A sends data frames to host B. Switch port El receives the data frames sent by A, queries the MAC address exchange table, and learns that B is on port E2. Then the switch directly exchanges the data frames to port E2 through the built-in exchange matrix.
Network built by switches

It can be seen from the above that each port of the switch can be regarded as an independent network segment, and the nodes connected to it can enjoy all the bandwidth of the port without competing with other devices. In other words, one port of the switch is a conflict domain. For example, hosts X, Y, Z share port E3 through the hub (see MAC exchange table) and are located in a conflict domain. However, the switch does not block broadcast, and all nodes connected by the switch are still in the same broadcast domain.

[Q&A] Each port of the switch is in a conflict domain. How much can a 16 port switch conduct concurrent communication at the same time? Answer: There can be 8 pairs of concurrent communication.

3. Generation of exchange attenuation

The MAC address exchange table of the switch is the same as the bridge table. It is empty at the beginning and is established through the "self-learning" process. When the switch receives a data frame, it will automatically record the source MAC address of the data frame and the corresponding port mapping one by one; When the destination MAC of the data frame cannot be determined (that is, it is unknown which port the destination MAC is connected to), the flooding method is used to broadcast to other ports other than the port receiving the data frame, and finally a complete exchange table is generated.
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