How does the switch work

1. Overview  Switching technology is a switching product with simplification, low price, high performance and high port density, reflecting the operation of the complex switching technology of bridging technology in the second layer of the OSI reference model. Like the bridge, the switch makes decision information forwarding relatively simply by the MAC address in each packet. This forwarding decision generally does not consider deeper other information hidden in the package. Unlike bridges, the switch forwarding delay is very small, and the operation is close to the performance of a single LAN, far exceeding the performance of forwarding between ordinary bridged Internet networks. Switching technology allows shared and dedicated LAN segments to adjust bandwidth to alleviate bottlenecks in the flow of information between LANs. There are now exchange products for Ethernet, Fast Ethernet, FDDI and ATM technologies. The switches economically divide the network into small conflict domains, providing higher bandwidth for each workstation. The transparency of the protocol allows the switch to be installed directly in a multi-protocol network with simple software configuration; the switch uses existing cables, repeaters, hubs and workstation network cards, and does not require high-level hardware upgrades; the switch is transparent to workstations, which is cheap in management and simplifies the increase in network nodes, movement and network changes.  Using specially designed integrated circuits allows switches to forward information in parallel at line rates across all ports, providing much higher operating performance than traditional bridges. Application-specific integrated circuit technology enables the switch to operate with the above performance with more ports, which is less expensive than conventional bridges.  2 and 3 exchange technologies  1. Port switching technology first appeared in slot-type hubs. The backplane of this type of hub is usually divided into multiple Ethernet segments (each network segment is a broadcast domain), without bridges or routing connections, and the networks are not connected. After inserting the main module, it is usually assigned to the network segment of a certain backplane. Port switching is used to allocate and balance the ports of the Ethernet module among multiple network segments of the backplane. Depending on the degree of support, port switching can also be subdivided into: Module switching: Migrate the entire module through network segments. Port group switching: Usually, the ports on the module are divided into several groups, and each group of ports allows network segment migration. Port-level switching: supports migration of each port between different network segments. This switching technology is based on the first layer of OSI and has the advantages of flexibility and load balancing. If configured properly, it can also be done to a certain extent, but the characteristics of the shared transmission medium have not been changed, and it cannot be called a true exchange. 2. Frame switching Frame switching is the most widely used local area network switching technology at present. It provides a mechanism for parallel transmission by microsegmenting traditional transmission media to reduce conflict domains and obtain high bandwidth. Generally speaking, the implementation technology of each company's product will be different, but the processing methods of network frames are generally as follows: Direct switching: provides line-speed processing capability, the switch only reads out the first 14 bytes of the network frame, and then transmits the network frame to the corresponding port. Store and forwarding: Verify and control the reading of network frames. The former method has very fast switching speeds, but lacks more advanced control of network frames, lacks intelligence and security, and cannot support switching of ports with different rates. Therefore, various manufacturers focus on the latter technology. Some manufacturers even decompose network frames and decompose frames into fixed-sized cells. The cell processing is very easy to implement with hardware, and the processing speed is fast. At the same time, it can complete advanced control functions (such as LET hubs of MADGE, the United States) such as priority control.  3. Cell exchange ATM technology represents the future direction of the development of network and communication technology, and is also a "good medicine" to solve many problems in current network communication. ATM uses cell exchange with a fixed length of 53 bytes. Because of the fixed length, it is easy to implement with hardware. ATMs use dedicated non-differential connections and operate in parallel. Multiple nodes can be established simultaneously through one switch, but they will not affect the communication capabilities between each node. ATM also allows multiple virtual links to be established in the source node, the target node and the node to ensure sufficient bandwidth and fault tolerance. ATM uses statistical time division circuits for multiplexing, which can greatly improve the utilization rate of the channel. The bandwidth of the ATM can reach 25M, 155M, 622M or even several Gb transmission capabilities. 3. Types and selection of LAN switches. LAN switches can be divided into: large network switches; token ring switches; FDDI switches; ATM switches; fast Ethernet switches, etc. If divided according to the switch application field, it can be divided into: desktop switch; workgroup switch; backbone switch; enterprise switch; segmented switch; port switch; network switch, etc.   LAN switches are the core equipment that constitutes the network system. For users, the most important indicators of LAN switches are port configuration, data exchange capability, packet switching speed and other factors. Therefore, when selecting a switch, the following things should be paid attention to: (1) The number of switch ports; (2) The type of switch ports; (3) The expansion capability of the system; (4) The connection means of the backbone; (5) The total switching capability of the switch; (6) Whether routing capability is required; (7) Whether hot switching capability is required; (8) Whether fault tolerance is required; (9) Whether it can be compatible with existing equipment and smoothly connected; (10) Network management capability. 4. Several notable issues in switch applications 1. Bottleneck problems in the switch network The switch itself can achieve very high processing speed, and users often believe in the Gbps-level high-speed backplane advertised by manufacturers. In fact, this is a misunderstanding. The network used by the workstation or server connected to the network is a large network, which follows the CSMA/CD media access rules. In the current client/server mode network, multiple workstations will access the server simultaneously, so it is very easy to form a server bottleneck. Some manufacturers have taken this into consideration and designed one or more high-speed ports in the switch (such as 3COM Linkswitch1000 can be configured with one or two 100Mbps ports) to facilitate users to connect to the server or high-speed backbone network. Users can also eliminate bottlenecks by designing multiple servers (for business division) or adding multiple network cards. The switch can also support spanning tree algorithms to facilitate redundant connections in fault-tolerant user architecture. 2. Broadcast frames in the network  The network operating systems that are currently widely used include Netware, Windows NT, etc., and the Lan Server server provides services to clients by sending network broadcast frames. The existence of broadcast packets in this type of LAN will greatly reduce the efficiency of the switch. At this time, the virtual network function of the switch (not every switch supports virtual network) can be used to limit broadcast packets to a certain range. The ports of each switch support a certain number of MAC addresses, so that the switch can "memorize" the situation of a group of connection sites on this port. The number of MACs supported by different switch ports provided by manufacturers is also different. Users must pay attention to the number of connection endpoints of the switch port when using it. If the number of MACs given by the manufacturer exceeds the number of MACs given by the manufacturer, when the switch receives a network frame, only the MAC address of its destination station does not exist in the MAC address table of the switch port, then the frame will be broadcasted to each port of the switch.  3. Division of virtual networks Virtual networks are an important function of switches. There are usually three forms of implementation of virtual networks: (1) Static port allocation The division of static virtual networks is usually the network management personnel use network management software or directly set up the switch ports to make them directly subordinate to a virtual network. These ports remain these slave properties unless the network administrator resets it. Although this method is troublesome, it is safer and easy to configure and maintain. (2) Dynamic Virtual Network Ports that support dynamic virtual networks can be automatically determined with the help of intelligent management software. A port determines the slave of a virtual network by using the MAC address, logical address or protocol type of the network packet. When a network node is just connected to the network, the switch port has not been allocated, so the switch dynamically divides the port into a virtual network by reading the network node's MAC address. In this way, once the network administrator is configured, the user's computer can flexibly change the switch port without changing the slave attributes of the user's virtual network. Moreover, if an undefined MAC address appears in the network, the network administrator can be alerted. (3) Multi-virtual network port configuration This configuration supports one user or one port to access multiple virtual networks at the same time. This allows a network server to be configured into multiple business departments (each service is set as a virtual network) to be accessed simultaneously, and the resources of multiple virtual networks can be accessed simultaneously. It also allows connections between multiple virtual networks to be completed with only one routing port. But this will bring about safety risks. Industry regulations for virtual networks are being formulated, so there is no interoperability of the products of various companies. Cisco has developed the Inter-Switch Link (ISL) virtual network protocol, which supports virtual networks across backbone networks (ATM, FDDI, Fast Ethernet). But the agreement was accused of lack of security considerations. Traditional computer networks use a large number of shared Hubs, and good results can be obtained by flexible access to computer ports. 4. Application of high-speed LAN technology Although fast Ethernet technology maintains good compatibility with traditional large networks in some aspects, 100BASE-TX, 100BASAE-T4 and 100BASE-FX have relatively large restrictions on transmission distance and casing. These limitations can be broken through with a 100Mbps switch. At the same time, only the switch port can support duplex high-speed transmission. Currently, CDDI/FDDI switching technology has also appeared. In addition, the price of CDDI/FDDI ports is also on a downward trend, and it also has relatively large advantages in transmission distance and security. Therefore, it is a better choice for large network backbones. 3COM's main exchange products include the Linkswitch series and the LANplex series; BAY's main exchange products include LattisSwitch2800, BAY stack workgroup, System3O00/5000 (some optional switching modules are provided); Cisco's main exchange products include Catalyst 1000/2000/3000/5000 series. The product forms of the three companies seem to be similar, and the prices of the products are relatively close. In addition to considering the specific needs of the network environment in the design (emphasizing the reasonable matching of ports), it also needs to be considered overall, such as network management, network applications, etc. With the development and maturity of ATM technology and the intensification of market competition, the price of frame switches will fall further, and it will become an important solution for working group networks. Article entry: aaadxmm     Editor in charge: aaadxmm

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