Introduction to hardware in the Network

Addressing functions of OSI hierarchical interconnection devices
Physical Layer relay and hub copy BITs between cable segments without IP addresses
Link Layer bridges store forwarding frame MAC addresses between LANs
Network Layer routers forward group network addresses between different networks
Gateways at or above the transport layer provide interconnection interfaces between different systems

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REPEATER)
A repeater is a connection device on the network physical layer. It is applicable to the interconnection of two identical networks. The main function is to resend or forward data signals to expand the network transmission distance. A repeater is a physical layer device of the OSI model, a network device for signal regeneration and restoration.
Due to the influence of transmission line noise, the digital or analog signal carrying information can only transmit a limited distance. The repeater function is to regenerate and send the received signal, thus increasing the signal transmission distance. It is the simplest network interconnection device that connects two or more network segments of the same network. For example, Ethernet often uses a repeater to extend the cable length of the bus. The maximum length of each Ethernet segment of the standard fine cable is 185, and the maximum length is 5. Therefore, after a repeater is added, the maximum length of the network cable can be increased to 925 meters. Generally, the network segments at both ends of the repeater are network segments rather than subnets.

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HUB)
The main function of a hub is to rebuild and enlarge the received signal to expand the transmission distance of the network. At the same time, all nodes are concentrated on the nodes centered on it. It works at the first layer of the OSI (Open System Interconnection Reference Model) reference model, that is, the "Physical Layer ". The Hub, like the network adapter and network cable, is a basic device in the LAN and uses CSMA/CD (a detection protocol) for access.

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Bridge
The device connected to the data link layer is a bridge. In the network connection, it acts as a data receiving, address filtering, and data forwarding device. It is used to exchange data between multiple network systems. It not only expands the distance or range of the network, but also improves the performance, reliability, and security of the network.
After network 1 and Network 2 are connected through a bridge, the bridge receives the packet sent by network 1 and checks the address in the packet. If the address belongs to network 1, it will discard it. On the contrary, if it is the address of Network 2, it will continue to be sent to Network 2. in this way, the bridge isolation information can be used to divide the same network number into multiple network segments (belonging to the same network number) and isolate the security network segments to prevent unauthorized access by users in other network segments. Due to the segmentation of the network, each network segment is relatively independent (belongs to the same network number), the failure of one network segment does not affect the operation of the other network segment

A bridge can be a specialized hardware device or a bridge software installed on a computer. Multiple network adapters (NICS) are installed on the computer ).

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Modem MODEL
Modem is short for Modulator and Demodulator. It can translate computer digital signals into pulse signals that can be transmitted along common telephone lines, these pulse signals can be received by another modem at the other end of the line and translated into computer-understandable languages.
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Vro
It is a device that connects various LAN and WAN devices on the Internet. It will automatically select and set routes based on the channel conditions, and send signals in the order of the best path, the main difference between routing and switching is that switching occurs at the Layer 2 (data link layer) of the OSI reference model, while routing occurs at the Layer 3 (Network Layer. This difference determines that different control information needs to be used in the process of routing and exchange of mobile information. Therefore, the two methods to implement their respective functions are different.
Workflow:
(1) workstation A sends the address 12.0.0.5 of workstation B together with the data information to router 1 in the form of data packets. (2) After receiving the data packet from workstation A, vro1 1 first extracts the address 12.0.0.5 from the packet header and calculates the optimal path to workstation B Based on the path table: r1-> R2-> R5-> B, and send the data packet to vro2 2.
(3) vro2 2 repeats vro1 1 and forwards data packets to vro5 5.
(4) vro5 5 extracts the destination address and finds that 12.0.0.5 is on the network segment connected to the vro. Therefore, the packet is directly sent to workstation B.
(5) workstation B receives A packet from workstation A, and the communication process ends.
In fact, in addition to the above-mentioned routing selection, the router also has the network traffic control function. Some routers only support a single protocol, but most routers support transmission of multiple protocols, that is, multi-protocol routers. Because each protocol has its own rules, to complete multiple Protocol algorithms in a router, it is bound to reduce the performance of the router. Therefore, we believe that the performance of vrouters supporting multiple protocols is relatively low. When purchasing a vro, You need to select the desired network protocol vro based on your actual situation.
Function:
1. Protocol Conversion
2. Route Selection
3. Route Selection for multiple protocols
4. Traffic Control
5. Filtering and isolation
6. Segmentation and assembly
7. Network Management
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Vswitch
Function:
1. Physical addressing
2. Network Topology
3. Error Check
4. Traffic Control

L2 Switch
The layer-2 switching technology is mature. The layer-2 switch is a data link layer device that can identify the MAC address information in the data packet and forward it according to the MAC address, the MAC addresses and corresponding ports are recorded in an internal address table.
The specific workflow is as follows:
(1) When the switch receives a packet from a port, it first reads the source MAC address from the packet header, so that it knows the port on which the machine with the source MAC address is connected;
(2) read the target MAC address in the header and find the corresponding port in the address table;
(3) If the table contains a port corresponding to the destination MAC address, copy the data packet directly to the port;
(4) If no corresponding port is found in the table, the packet is broadcast to all ports. When the target machine responds to the source machine, the switch can learn which port the target MAC address corresponds, the next time you transmit data, you no longer need to broadcast all ports.
In this process, you can learn the MAC address information of the entire network,
The second-layer switch creates and maintains its own address table.

The working principle of a L2 Switch can be inferred from the following three points:
(1) because the switch exchanges data on most ports at the same time, a wide switching bus bandwidth is required,
If a L2 Switch has N ports, and the bandwidth of each port is M, and the bandwidth of the switch bus exceeds N × M, the switch can realize line rate switching;
(2) write the MAC address of the machine connected by the Learning port to the address table,
The size of the address table (generally two Representation Methods: one is beffer ram and the other is the value of the MAC table ),
The address table size affects the access capacity of the vswitch;
(3) Another layer-2 switch generally contains an ASIC dedicated for processing data packet forwarding.
(Application specific Integrated Circuit) chip, so the forwarding speed can be very fast.
Because different manufacturers use different ASIC, the product performance is directly affected.
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L3 Switch
Layer-3 switches use layer-3 Switching Technology
To put it simply, layer-3 switching technology is layer-2 switching technology + layer-3 forwarding technology. It solves the network bottleneck caused by low speed and complexity of traditional routers.

What is layer-3 switching?
Layer-3 switching (also known as multi-layer switching technology or IP address switching technology) is proposed in contrast to the traditional concept of switching. As we all know, the traditional exchange technology is implemented at the Layer 2-data link layer in the OSI network standard model, the layer-3 switching technology implements high-speed packet forwarding in the layer-3 network model. To put it simply, layer-3 switching technology is layer-2 switching technology + layer-3 forwarding technology.

The emergence of layer-3 switching technology solves the problem that the subnet must be managed by routers after the network segments are divided in the LAN, and solves the network bottleneck caused by the low speed and complexity of traditional routers.

Layer-3 Switching Principle
A device with layer-3 switching is a layer-2 switch with layer-3 routing, but it is an organic combination of the two, it is not simply to overlay the hardware and software of a router device on a LAN switch.

The principle is: Assume that two sites A and B that use the IP protocol communicate with each other through the layer-3 Switch. When sending Site A, compare its IP address with the IP address of site B, determine whether Site B is in the same subnet as itself. If the destination site B and the destination site A are in the same subnet, Layer 2 Forwarding is performed. If two sites are not in the same subnet, if Station A needs to communicate with Station B, station A must send an ARP (Address Resolution) packet to the "Default Gateway, the IP address of the "Default Gateway" is actually a layer-3 Switch module. When station A broadcasted an ARP request to the IP address of the "Default Gateway", if the layer-3 Switching Module knew the MAC address of Station B during the previous communication, then the MAC address of B is returned to the sending Site. Otherwise, the layer-3 Switch module broadcasts an ARP request to Site B based on the route information. Site B receives the ARP request and then replies to the layer-3 Switch module with its MAC address, the layer-3 Switching Module saves the address and sends it back to the sender Site A. It also sends the MAC address of Site B to the MAC address table of the layer-2 switching engine. After that, when A sends all the packets to B, the information can be exchanged at A high speed. Because layer-3 processing is only required in the routing process, most of the data is forwarded through layer-2 switch, so the speed of layer-3 switch is very fast, close to the speed of layer-2 switch, and the price is much lower than that of the same router.

L3 Switch Type
Layer-3 switches can be divided into pure hardware and pure software based on their data processing.

(1) Hardware-only layer-3 technology is relatively complex, costly, but fast, with good performance and strong load capabilities. The principle is to use an ASIC chip and hardware to search and refresh route tables.

Principle of hardware-only layer-3 Switch

When the data is received by the port interface chip, first find the target MAC address in the layer-2 switching chip. If yes, perform layer-2 forwarding; otherwise, send the data to the layer-3 engine. In the layer-3 engine, the ASIC chip searches for the corresponding route table information, compares to the destination IP address of the data, and then sends the ARP packet to the destination host to obtain the MAC address of the host, send the MAC address to a layer-2 chip and the layer-2 Chip forwards the packet.

(2) The software-based layer-3 switch technology is relatively simple, but slow, not suitable for cooperation. The principle is to use the CPU software to find the route table.

Principle of layer-3 Software Switch

When the data is received by the port interface chip, first find the target MAC address in the layer-2 switching chip. If the data is found, perform layer-2 forwarding. Otherwise, the data is sent to the CPU. The CPU looks for the corresponding route table information, which is opposite to the destination IP address of the Data. Then, it sends the ARP packet to the destination host to obtain the MAC address of the host and sends the MAC address to the L2 chip, the second-Layer Chip forwards the packet. Because low-cost CPU processing is slow, the processing speed of such a layer-3 switch is slow.

Market product selection
In recent years, the construction of Broadband IP networks has become a hot topic. Next we will introduce some layer-3 switches that are suitable for the access layer or small and medium size convergence layer. Mainstream third-tier switches on the market include Cisco Catalyst 2948G-L3, Extreme Summit24, AlliedTelesyn Rapier24, and so on. These three-tier switch products have their own characteristics, covers most of the application features of layer-3 switches. Of course, when selecting a layer-3 switch, you can determine and select the products of the above products or other manufacturers based on your own needs, for example, Passport/Acceler series of beidian network, SSR series of original Cabletron (after Cabletron Release 4, most of SSR L3 switches have been incorporated into Riverstone), and Cajun of Avaya
M series and 3Com Superstack3 4005 series. In addition, Chinese network manufacturers such as shenzhoudigital network, TCL network, Shanghai Radio and Television yingzhi, Ziguang network, and shouxin have launched three-layer switch products. The following describes three of these products, so that you can fully understand the layer-3 Switch and select an appropriate model based on your situation.

The Cisco Catalyst 2948G-L3 switch provides a complete solution in combination with industry-standard IOS. In versions 12.0 (10) and above, it fully supports the IOS Access Control List ACL, and works with the core Catalyst 6000, end-to-End comprehensive broadband MAN construction can be completed (Catalyst 6000 uses the MSFC module to complete its multi-layer switching service, and has stopped using the RSM Routing Switching Module, IOS version 6.1 and above fully support ACL ).

Extreme's three-tier switch product solution provides unique Ethernet bandwidth distribution capabilities, with a cut unit of 200 kbps or kbps. Service providers can charge fees based on bandwidth usage, fixed delay transmission of audio and video.

The three-tier switch of AlliedTelesyn Rapier24 provides PPPoE features, enrich and improve the user authentication and billing methods, and is suitable for multiple access networks, flexible applications, and easy to choose services, at the same time, it protects the existing investment of users. In addition, it can be used with NAT (Network Address Translation) and DHCP Server functions, which is favored by many service providers.

In short, three-tier switches have been widely used since the concept was proposed. Although they have only been used for a few years, their extended functions have been enriched with practical applications. With the development of ASIC hardware chip technology and the promotion of practical application, layer-3 switching technology and products will be further developed.

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Layer-4 Switch
Both layer-2 and layer-3 switches are end-to-end exchange processes based on port addresses. Although this MAC address and IP address-based switch technology can greatly improve the data transmission rate between nodes, however, it is impossible to determine or dynamically limit the port switching process and data traffic based on the application requirements of the port host, that is, there is a lack of layer-4 intelligent application switching requirements. The layer-4 switch not only performs end-to-end switching, but also determines or limits the switching traffic based on the Application Characteristics of the port host. To put it simply, the layer-4 switch is based on the packet exchange process at the transport layer. It is a new type of LAN switch based on the Application exchange requirements at the TCP/IP application layer.

Key technologies supported by layer-4 vswitches

1. packet filtering/Security Control
2. Service Quality
3. Server Load balancer
4. Backup host connection
5. Statistics

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Gateway
Also known as the inter-network connector and Protocol converter. The gateway is the most complex network interconnection device on the transport layer to achieve network interconnection. It is only used for different network interconnection between two high-level protocols. Gateways can be used for both Wan and LAN interconnection. A gateway is a computer system or device that acts as a conversion task. The gateway is a translator between two systems that use different communication protocols, data formats, languages, and even completely different architectures. Unlike the Net Bridge, the gateway repacks the received information to meet the needs of the target system. The gateway can also provide filtering and security functions. Most gateways run on the top layer of the OSI Layer 7 protocol-Application Layer