What is a switch? What are the applications of switches?

A switch is also a hub device in the network. Like a hub, it can connect network nodes with itself as the center, and can regenerate and amplify the received information to increase the transmission distance of the network; but it is different from a hub. ——A switch is a switching device.

As a switching device, each port of the switch can provide dedicated bandwidth to the node connected to it, allowing each node to occupy the channel exclusively. The relationship between switch ports, nodes, and switch capacities is as follows.

1. The switch mainly has the following characteristics:

1) Exclusive bandwidth. If a switch with a port rate of 100Mbit/s is connected to N computers at the same time, the total bandwidth of the network is N x 100Mbit/s. In other words, the network bandwidth of a switched Ethernet network built with switches will not decrease as the number of nodes increases, and network performance will not decrease as the load increases.

2) Multiple pairs of nodes can communicate in parallel. The switch allows multiple pairs of devices connected to itself to establish communication links at the same time for data exchange.

3) The port rate can be flexibly configured. The switch allows each node to flexibly configure the port speed according to its own needs, and the switch not only supports ports with a certain speed, but also supports port adaptive configuration.

4) Easy to manage. The switch supports the construction of virtual LAN (VLAN), and uses software to divide and manage devices in the network through logical work groups.

In addition, switches are compatible with networks built using hubs and can replace hubs when transitioning from shared LAN to switched Ethernet to achieve seamless network connectivity.

2. Working principle of the switch:

1) When the switch receives data, it will first check the MAC address contained in the data, and then forward the data out of the port where the destination host is located. The switch can realize this function because the switch has a MAC address table in its memory, which records the corresponding information between all MAC addresses in the network and each port of the switch. When a data frame needs to be forwarded through the switch, the switch obtains the port corresponding to the destination device based on the internally stored MAC address table, and forwards the data through the found port, as shown in the figure below.

2) Assume that device pc2 sends a data frame to host pc3. After receiving the data frame, the switch will first find out that the destination MAC address is 00-0B-2F-4B-60-57, and then query the MAC address table to find the destination MAC. The port number E0/5 of the address connection forwards data out of port E0/5.

3) The MAC address table in the switch is initially empty. After the switch is put into use, it will take certain measures to build and improve the MAC address table. This process mainly includes four important concepts: learning, forwarding, flooding and updating.

①Learning: When the device pc1 connected to port E0/1 wants to send a data frame to another device, the switch will first check the source MAC address (00-0B-2F-4B-60-26) in the data frame to determine the MAC Whether there is a relevant record in the address table, if so, update the record (00-0B-2F-4B-60-26, E0/1), otherwise add a new record.

②Forwarding: The switch checks the destination MAC address in the data frame, queries the MAC address table for records related to the destination MAC address, and if the corresponding record is found, forwards the data frame to the port corresponding to the record.

③ Flooding: If there is no record related to the destination MAC address in the MAC address table, the switch cannot obtain the port connected to the destination host for a while. At this time, the switch will send data frames to all ports except the source port (this is flooding ), and after the corresponding destination port replies, the switch records the source MAC address and corresponding port of the response data frame to facilitate subsequent forwarding.

④Update: To ensure the accuracy of the MAC address table, the switch internally updates the table at regular intervals.

3. Data exchange mode: The data exchange mode of the switch is divided into two types: direct exchange and storage exchange. Direct switching means that after the switch receives the data frame, it immediately obtains the destination address in the frame, obtains the destination port number through the MAC address table, and forwards the data frame. This data exchange method has high efficiency and small delay, but it has the following shortcomings:

1) Lower reliability. Data may be damaged due to collisions during transmission, but the direct exchange method does not check the integrity and correctness of the data frame and directly forwards the data, which cannot guarantee the reliability of the data frame transmission.

2) Ports with different speeds cannot be connected directly. Since there is no buffering, input/output ports with different speeds cannot be connected directly.

3) Difficulty in implementation. As the number of switch ports increases, the complexity of the switching matrix also increases, making implementation more difficult.

Store and forward is a widely used data exchange method. Using this method, after receiving the data frame, the switch will store and verify the data frame. If the verification result shows that the data is correct, the destination MAC address will be taken out and mapped Table to find the corresponding port for forwarding. Compared with direct switching, the store-and-forward method has a larger delay, but it has error detection capabilities and can support data exchange between ports at different rates.