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What is the switch used for

The main functions of the switch include physical addressing, network topology, error checking, frame sequence and flow control. The switch also has some new features, such as support for VLAN (virtual LAN), support for link convergence, and even some firewall functions.


The Ethernet switch understands the MAC address of each port connected to the device, and maps the address to the corresponding port in the MAC address table in the switch cache.

Forwarding / filtering:

When the destination address of a data frame is mapped in the MAC address table, it is forwarded to the port of the connection destination node instead of all ports (such as the data frame is forwarded to all ports for broadcast / multicast frames).

Elimination circuit:

When a switch includes a redundant loop, the Ethernet switch avoids the generation of the loop through the spanning tree protocol while allowing backup paths.

In addition to the ability to connect the same type of network, switches can also be interconnected between different types of networks, such as Ethernet and fast Ethernet. Many switches are now able to provide high-speed connection ports, such as fast Ethernet or FDDI, to connect other switches in the network or to provide additional bandwidth for key servers with large bandwidth occupancy.

In general, each port of a switch is used to connect a separate network segment, but sometimes, in order to provide faster access speed, we can connect some important network computers directly to the port of the switch. In this way, the key servers and important users of the network will have faster access speed and support greater information traffic.

Finally, briefly summarize the basic functions of the switch:

1. like a hub, the switch provides a large number of ports for cable connections, which allows star topology routing.

2. like repeater, hub and bridge, when it repeats frames, the exchange opportunity regenerates an undistorted square electrical signal.

3. like bridges, switches use the same forwarding or filtering logic on each port.

4. like a bridge, a switch divides a LAN into a plurality of conflicting domains, and each conflict domain has an independent broadband, which greatly improves the bandwidth of the LAN.

5., in addition to the functions of bridges, hubs, and repeater, switches also provide more advanced functions, such as virtual LAN (VLAN) and higher performance.

The traditional switch developed from the bridge belongs to the OSI second layer, that is, the data link layer device. It addresses according to the MAC address and selects route through the station table. The establishment and maintenance of the station table is carried out automatically by the switch. Routers belong to the third layer of OSI, that is, network layer devices. They are addressed according to IP addresses and are generated through routing table routing protocols. The biggest advantage of the switch is fast. Because the switch only has to recognize the MAC address in the frame, the algorithm is simple and convenient for ASIC to produce the forwarding port according to the MAC address, so the forwarding speed is very high. But the switch's working mechanism also brings some problems.

1.The  loop: according to the learning of switch address and the establishment algorithm of station table, there is no loop between switches. Once there is a loop, the spanning tree algorithm must be launched to block the port that generates the loop. The router routing protocol does not have this problem. There are multiple paths between routers to balance load and improve reliability.

2. load concentration: there is only one access between switches, so that information can be concentrated on a communication link and can not be dynamically allocated to balance load. The routing protocol algorithm of the router can avoid this. The OSPF routing protocol algorithm can not only generate multiple routes, but also select the best routes for different network applications.

3. broadcast control: switches can only narrow the collision domain, but not narrow the broadcast domain. The whole switched network is a large broadcast domain, and broadcast messages are dispersed to the entire switched network. The router can isolate the broadcast domain, and broadcast packets cannot be broadcast through routers.

4. subnet Division: switches can only identify MAC addresses. MAC address is physical address, and uses a flat address structure, so it can not be divided into subnets according to the MAC address. The router identifies the IP address, and the IP address is assigned by the network administrator. It is a logical address and the IP address has a hierarchical structure. It is divided into network numbers and host numbers. It can be used to divide subnets very conveniently. The main function of the router is to connect different networks.

5. security problem: Although the switch can also filter the frame according to the source MAC address, the destination MAC address and the contents in other frames, the router filters the message according to the source IP address, the destination IP address, the TCP port address and so on.

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