How switches work

How switches work

I. Overview

In 1993, LAN switching equipment emerged. In 1994, the exchange network technology boom began in China. In fact, the exchange technology is a simplified, low-cost, high-performance, and high-port intensive exchange product, reflecting the second-layer operations of the complex exchange technology in the OSI reference model. Like the bridge adapter, the switch simply decides to forward information based on the MAC address in each package. This type of forwarding decision generally does not consider the deeper information hidden in the package. The difference with the bridge adapter is that the forwarding delay of the switch is very small, and the operation is similar to the performance of a single LAN, far exceeding the forwarding performance between common Bridging networks.

The exchange technology allows the shared and dedicated LAN segments to adjust the bandwidth to reduce the bottleneck of information flow between the LAN. Now there are Ethernet, fast Ethernet, FDDI and ATM exchange products.

Similar to traditional bridges, vswitches provide many network interconnection functions. Vswitches can economically divide networks into small conflicting domains to provide higher bandwidth for each workstation. Protocol transparency allows the switch to be directly installed in a multi-protocol network with simple software configuration. The switch uses the NIC of the existing cable, repeater, hub, and workstation, you do not need to perform high-level hardware upgrades. vswitches are transparent to workstations. This reduces management costs and simplifies the operations of adding, moving, and changing network nodes.

Using a specially designed integrated circuit, the switch can forward information on all ports at a line rate in parallel, providing much higher operation performance than the traditional bridge. For example, theoretically, a single Ethernet port can provide a transmission rate of 14880bps for data packets containing 64 octal packets. This means that a "line rate" Ethernet switch with 12 ports that support six parallel data streams must provide an overall throughput of BPS (6-channel information flow x14880bps/channel information flow ). The dedicated integrated circuit technology enables the switch to run with more ports. The port cost is lower than that of the traditional bridge.

Ii. Three exchange technologies

1. Port Switching

Port switching technology first appeared in a slot hub. The backplane of these hubs is usually divided into multiple Ethernet segments (each segment is a broadcast domain) without a bridge or route connection, networks do not communicate with each other. After the master module is inserted, it is usually allocated to the network segment of a backplane. Port switching is used to allocate and balance the Ethernet module ports among multiple network segments of the backplane. Based on the degree of support, port switching can also be subdivided:

· Module switching: migrate the entire module to the network segment.

· Port group switch: ports on the module are usually divided into several groups, and each group of ports allows network segment migration.

· Port-level switch: each port can be migrated between different network segments. This switching technology is based on the first layer of OSI and has the advantages of flexibility and load balancing capabilities. If properly configured, the customer can still make a certain degree of errors, but it does not change the characteristics of the shared transmission media, rather than being called a real exchange.

2. Frame Switching

Frame switching is currently the most widely used LAN switching technology. It provides a parallel transmission mechanism to reduce conflicting domains and obtain high bandwidth by performing differential segments on Traditional transmission media. Generally, the implementation technologies of each company's products are different, but there are generally the following processing methods for network frames:

· Pass-through switching: Provides line rate processing capabilities. The switch only reads the first 14 bytes of the network frame and then transfers the network frame to the corresponding port.

· Storage forwarding: checks and controls the reading of network frames.

The exchange speed of the previous method is very fast, but it lacks more advanced control over network frames, lacks Intelligence and Security, and cannot support port exchange with different rates. Therefore, each vendor focuses on the latter technology.

Some manufacturers even break down network frames and break down frames into fixed cells. This cell processing is easy to implement by hardware, and the processing speed is fast, at the same time, it can complete advanced control functions (such as the let hub of Madge) such as priority control.

3. Cell Exchange

The ATM technology represents the future direction of the development of network and communication technologies. It is also a "good medicine" to solve many problems in the current network communication. The ATM uses a fixed length of 53 bytes of Cell Exchange. Because the length is fixed, it is easy to implement with hardware. ATM uses dedicated non-differential connections and runs in parallel. multiple nodes can be established through a switch at the same time, but the communication capability between each node is not affected. ATM also allows multiple virtual links at the source node, target node, and node to ensure sufficient bandwidth and fault tolerance. The statistical time-division circuit is used in ATM, which can greatly improve the channel utilization. The bandwidth of an ATM can reach 25 m, 155 m, m, or even several GB.

Iii. LAN switch types and selection

LAN switches can be divided:

· Large network switches;

· A wildcard ring switch;

· FDDI switch;

· ATM switch;

· Fast Ethernet switch.

Divided by the switch application field, it can be divided:

· Desktop switches;

· Workgroup switches;

· Trunk switch;

· Enterprise switches;

· Segmented switch;

· Port switch;

· Network switches.

LAN switches are the core devices that make up the network system. For users, the main indicators of LAN switches are port configuration, data exchange capability, Packet Exchange speed, and other factors. Note the following when selecting a vswitch:

(1) Number of switch ports;

(2) Switch Port type;

(3) system expansion capability;

(4) trunk line connection means;

(5) Total switch switching capability;

(6) Whether route selection is required;

(7) Whether hot switching is required;

(8) whether fault tolerance is required;

(9) compatibility with existing devices and smooth connection;

(10) network management capability.

4. Several notable problems in Switch Applications

1. bottlenecks in vswitch Networks

The processing speed of the switch itself can reach a very high level. Users are often superstitious about the Gbps-level high-speed backplane. In fact, this is a misunderstanding. The network used by workstations or servers connected to the access network is a large network, which follows the CSMA/CD media access rules. In the current customer/Server mode, multiple workstations access the server at the same time, so it is very easy to form a server bottleneck. Some vendors have designed one or more high-speed ports in the vswitch (for example, one or two Mbps ports can be configured for the linkswitch1000 of 3Com) to facilitate the user to connect to the server or high-speed backbone network. You can also design multiple servers (for Business Division) or append multiple NICs to eliminate the bottleneck. The switch also supports Spanning TreeAlgorithmTo facilitate fault-tolerant Redundant connections in your architecture.

2. Broadcast frames in the Network

Currently, Netware and Windows NT are widely used network operating systems, while LAN server servers provide services to clients by sending network broadcast frames. The existence of broadcast packets in these LAN greatly reduces the efficiency of the switch. In this case, you can use the virtual network function of the switch (not all vswitches support the virtual network) to limit the broadcast packets to a certain range.

The ports of each vswitch support a certain number of MAC addresses, so that the vswitch can "remember" The status of a group of connected sites on this port, the number of MAC ports supported by different vswitch ports provided by the manufacturer varies. You must pay attention to the number of connection endpoints of the vswitch ports during use. If the number of MAC addresses exceeds the number of MAC addresses specified by the manufacturer, when the switch receives a network frame, only the MAC address of the target station does not exist in the MAC address table of the switch port, the frame is broadcast to each port of the vswitch.

3. Virtual Network Division

A virtual network is an important function of a vswitch. Generally, there are three ways to implement a virtual network:

(1) Static port allocation

Static virtual networks are usually divided by network administrators using network management software or directly setting the port of a switch so that they can directly subordinate to a virtual network. These ports keep these properties until the Administrator resets them. Although this method is troublesome, it is safe and easy to configure and maintain.

(2) Dynamic Virtual Network

Supports Dynamic Virtual Network ports, which can be automatically determined by intelligent management software. The port is determined by using the MAC address, logical address, or protocol type of the Network Package. When a network node just connects to the access network, the switch port is not allocated, so the switch dynamically switches the port into a virtual network by reading the MAC address of the network node. In this way, once the network administrator is configured, the user's computer can flexibly change the switch port, without changing the user's Virtual Network's conformances. If an undefined MAC address appears in the network, you can send an alarm to the network administrator.

(3) Configure multiple virtual network ports

This configuration allows one user or one port to access multiple virtual networks at the same time. In this way, you can configure a network server as multiple business departments (each service is set as a virtual network) and access resources of multiple virtual networks at the same time, it also allows connections between multiple virtual networks to be completed through only one routing port. But this will bring security risks. The specifications of the virtual network industry are being formulated, so the products of various companies are not interoperable. Cisco has developed the Inter-switch link (ISL) virtual network protocol, which supports virtual networks across Backbone Networks (ATM, FDDI, Fast Ethernet. However, the Protocol is accused of lack of security considerations. A large number of shared hubs are used in traditional computer networks, which can be achieved through flexible access to computer ports.

4. Application of High-speed LAN technology

Although the Fast Ethernet technology maintains good compatibility with the traditional large network in some aspects, 100base-tx, 100basae-t4, and 100base-fx impose great limitations on the transmission distance and cascade. A Mbps switch can break these limitations. At the same time, only the switch port can support duplex high-speed transmission.

Currently, cddi/FDDI exchange technology has emerged. In addition, the port price of cddi/FDDI has also declined, and it also has great advantages in terms of transmission distance and security, therefore, it is a good choice for large network backbone.

3Com's main switching products include linkswitch series and lanplex series; Bay's main switching products include lattisswitch2800, Bay stack Workgroup, system3o00/5000 (some optional switching modules are provided ); cisco's main switch products include the catalyst 1000/2000/3000/5000 Series.

The product forms of the three companies seem to have similarities, and the prices of the products are similar. In addition to the specific needs of the network environment in the design (the port combination is emphasized to be reasonable, it also needs to be considered as a whole, such as network management and network applications. 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 networking.