Switches in the local network. Choosing the Right LAN Device Choosing a Switch

If earlier the network cable through which the data was transferred was simply connected directly to the computer, now the situation has changed. In one residential apartment, in an office or a large company, it often becomes necessary to create a computer network.

For this, devices that are included in the category "computer equipment" are used. These devices include a switch that allows. So what is a switch, and how to use it to build a computer network?

What are switch devices for?

Literally translated from in English, the computer term "switch" refers to a device that is used to create local network through the combination of several computers. A synonym for the word switch is a switch or a switch.

A switch is a kind of bridge with many ports through which packet data is transmitted to specific recipients. The switch helps to optimize the network, reduces the load in it, increases the level of security, fixes individual MAC addresses, which allows you to quickly and efficiently transfer data.

Such switches were able to replace the hubs that were previously used to build computer networks. A switch is a smart device that can process the received information about connected devices, and then redirect the data to a specific address. As a result, network performance is increased several times and the Internet speeds up.

Equipment types

Switch devices are divided into different types according to the following criteria:

Port type.
Number of ports.
Port speeds are 10 Mbps, 100 Mbps and 1000 Sbps.
Managed and unmanaged devices.
Manufacturers.
Functions.
Specifications.

By the number of ports, switch switches are divided into:

8-port.
16 ports.
24 ports.
48 ports.

For a home and a small office, a switch with 8 or 16 ports that operate at a speed of 100 Mbps is suitable.

For large enterprises, companies and firms, ports with a speed of 1000 Mbps are needed. Such devices are needed to connect servers and large communication equipment.

Unmanaged switches are the simplest of equipment. Complex switches are managed at the network or third layer of the OSI model - Layer 3 Switch.

Management is also carried out through methods such as:

Web interface.
Command line interface.
SNMP and RMON protocols.

Complex or managed switches allow VLAN, QoS, mirroring, and aggregation features. Also, such switches are combined into one device, which is called a stack. It is intended to increase the number of ports. Other ports are used for stacking.

What do providers use?

When creating a computer network, provider companies create one of its levels:

Access level.
Aggregation level.
kernel level.

Levels are needed in order to make it easier to handle the network: scale, configure, introduce redundancy, design the network.

At the access level of the switch device, end users must be connected to a 100 Mbps port. Other requirements for the device include:

Connection via SFP to the aggregation level switch, where information is transferred at a speed of 1 gigabyte per second.
Support VLAN, acl, port security.
Support for security features.

According to this scheme, three levels of the network are created from the Internet provider. First, a network is being formed at the level of a residential building (multi-storey, private).

Then the network is “scattered” over the microdistrict, when several residential buildings, offices, and companies are connected to the network. At the last stage, a core level network is created, when entire microdistricts are connected to the network.

The formation of a network at Internet providers occurs using Ethernet technology, which allows you to connect subscribers to the network.

How does a switch work?

Thus, when a data packet is received on a particular port of the equipment, which is intended for only one PC, then the information is transmitted addressfully to the specified port. When the MAC address has not yet been determined, the information is passed on to the remaining interfaces. Traffic localization occurs during the operation of the switch device, when the MAC table is filled with the necessary addresses.

Features of setting device parameters

Making appropriate changes to the parameters of the switch device is the same for each model. Setting up the equipment requires performing step-by-step actions:

Create two VLAN ports - for clients and for managing switches. VLANs must be designated in the settings as switch ports.
Configure port security to not receive more than one MAC address per port. This will avoid sending information to another port. Sometimes there may be a merge of the home network's Broadcast domain with the ISP's domain.
Disable STP on the client port so that other users cannot pollute the provider's network with various BPDUs.
Set the loopback detection parameter. This will allow invalid, defective network cards to be rejected, and not interfere with the work of users who are connected to the port.
Create and configure the acl parameter to prevent non-PPPoE packets from passing through to the user's network. To do this, in the settings you need to block such unnecessary protocols as DCHP, ARP, IP. These protocols are designed to allow users to communicate directly, bypassing the PPPoE protocols.
Create an acl that denies PPPoE RADO packets coming from client ports.
Enable Storm Control, which will allow you to deal with multicast and broadcast floods. This setting should block non-PPPoE traffic.

If something goes wrong, then it is worth checking PPPoE, which can be attacked by viruses or fake data packets. Due to inexperience and ignorance, users may incorrectly configure the last parameter, and then you need to contact the operator of the Internet service provider for help.

How to connect a switch?

Creating a local network from computers or laptops requires the use of a network switch - a switch. Before setting up the equipment and creating the desired network configuration, the process of physically deploying the network takes place. This means that a connection is being created between the switch and the computer. To do this, use a network cable.

Connections between network nodes are made using a patch cord - a special type of network communication cable made on the basis of a twisted pair. It is recommended to purchase a network cable from a specialized store so that the connection process goes smoothly.

There are two ways to set up a switch:

Through the console port, which is designed to make the primary switch settings.
Via the universal Ethernet port.

The choice of connection method depends on the interface of the equipment. Connecting through the console port does not consume any switch bandwidth. This is one of the advantages this method connections.

It is necessary to launch the VT 100 terminal emulator, then select the connection parameters in accordance with the notation in the documentation. When the connection is made, the user or employee of the Internet company enters a login and password.

To connect via the Ethernet port, you will need an IP address, which is indicated in the documents for the device or requested from the provider.

When the settings are made and using the switch created computer network, users from their PCs or laptops should be able to access the Internet without any problems.

When choosing a device to create a network, you need to consider how many computers will be connected to it, what is the speed of the ports, how they work. Modern providers use Ethernet technology to connect, which allows you to get a high-speed network using a single cable.

03/18/1997 Dmitry Ganzha

Switches are central to today's local area networks. TYPES OF SWITCHING SWITCHING HUB RISC AND ASIC PACKET PROCESSING TECHNIQUES HIGH-END SWITCH ARCHITECTURE CONSTRUCTION OF VIRTUAL NETWORKS LAYER 3 SWITCHING CONCLUSION Switching is one of the most popular modern technologies.

Switches are central to today's local area networks.

Switching is one of the most popular modern technologies. Switches are pushing bridges and routers to the periphery of local networks, leaving behind them the role of organizing communication through global network. Such popularity of switches is primarily due to the fact that they allow, due to micro-segmentation, to increase network performance compared to shared networks with the same nominal bandwidth. In addition to dividing the network into small segments, switches make it possible to organize connected devices into logical networks and easily regroup them when necessary; in other words, they allow you to create virtual networks.

What is a switch? According to the IDC definition, "a switch is a device that is structurally made in the form of a hub and acts as a high-speed multiport bridge; the built-in switching mechanism allows segmenting the local network and allocating bandwidth to end stations in the network" (see M. Kulgin's article "Build a network, plant a tree..." in the February issue LAN). However, this definition applies primarily to frame switches.

TYPES OF SWITCHING

Switching is commonly understood to mean four different technologies - configuration switching, frame switching, cell switching, and frame-to-cell conversion.

Configuration switching is also known as port switching, in which a particular port on a smart hub module is assigned to one of the internal Ethernet (or Token Ring) segments. This assignment is made remotely through programmatic network management when users and resources are connected or moved around the network. Unlike other switching technologies, this method does not improve the performance of a shared LAN.

Frame switching, or LAN switching, uses standard Ethernet (or Token Ring) frame formats. Each frame is processed by the nearest switch and transmitted further through the network directly to the recipient. As a result, the network turns into a set of high-speed direct channels operating in parallel. How frames are switched inside the switch, we will consider below using the example of a switching hub.

Cell switching is used in ATM. The use of small fixed-length cells makes it possible to create low-cost, high-speed switching structures at the hardware level. Both frame switches and cell switches can support multiple independent workgroups regardless of their physical connection (see "Building Virtual Networks" section).

Frame-to-cell conversion allows, for example, a station with an Ethernet card to communicate directly with devices on an ATM network. This technology is used in LAN emulation.

In this lesson, we will be primarily interested in frame switching.

SWITCHING HUBS

The first switching hub called EtherSwictch was introduced by Kalpana. This hub allowed to reduce network competition by reducing the number of nodes in a logical segment using micro-segmentation technology. Essentially, the number of stations in one segment was reduced to two: the station initiating the request and the station responding to the request. No other station sees the information transmitted between them. Packets are transmitted as if through a bridge, but without the delay inherent in the bridge.

In a switched Ethernet network, each member of a group of several users can be simultaneously guaranteed throughput 10 Mbps. To understand how such a hub works, the analogy with the usual old telephone exchange, in which the participants in the dialogue are connected by coaxial cable. When a subscriber called the "eternal" 07 and asked to be connected to such and such a number, the operator first of all checked whether the line was available; if so, he connected the participants directly with a piece of cable. No one else (with the exception of the secret services, of course) could hear their conversation. After the call ended, the operator disconnected the cable from both ports and waited for the next call.

Switching hubs operate in a similar way (see Figure 1): they pass packets from an ingress port to an egress port through the switching fabric. When a packet hits an ingress port, the switch reads its MAC address (i.e., layer 2 address) and it is immediately forwarded to the port associated with that address. If the port is busy, then the packet is queued. Essentially, a queue is a buffer on an input port where packets wait for the right port to become free. However, the buffering methods are somewhat different.

Picture 1.
Switching hubs function similarly to the old telephone switches: they connect an input port directly to an output port through a switching matrix.

PACKET HANDLING METHODS

In end-to-end switching (also called on-the-fly switching and bufferless switching), the switch only reads the address of an incoming packet. The packet is transmitted further regardless of the absence or presence of errors in it. This can significantly reduce the packet processing time, since only the first few bytes are read. Therefore, it is up to the receiving end to identify defective packets and request their retransmission. However, modern cable systems are sufficiently reliable that the need for retransmission on many networks is minimal. However, no one is immune from errors in case of cable damage, malfunction network board or interference from an external electromagnetic source.

When switching with intermediate buffering, the switch, when receiving a packet, does not forward it further until it has read it completely, or in any case, it has read all the information it needs. It not only determines the recipient's address, but also checks the checksum, that is, it can cut off defective packets. This allows you to isolate the error-producing segment. Thus, buffered switching emphasizes reliability over speed.

In addition to the above two, some switches use a hybrid method. Under normal conditions, they perform end-to-end switching, but at the same time they monitor the number of errors by checking the checksums. If the number of errors reaches a predetermined threshold value, they go into switching mode with intermediate buffering. When the number of errors is reduced to an acceptable level, they return to the end-to-end switching mode. This type of switching is called threshold or adaptive switching.

RISC AND ASIC

Often, buffered switches are implemented using standard RISC processors. One advantage of this approach is that it is relatively cheap compared to ASIC switches, but it is not very good for specialized applications. Switching in such devices is carried out using software, so their functionality can be changed by upgrading the installed software. The disadvantage is that they are slower than ASIC-based switches.

ASIC switches are designed to perform specialized tasks: all their functionality is "hardwired" into the hardware. There is a drawback to this approach: when modernization is necessary, the manufacturer is forced to rework the circuit. ASICs typically provide end-to-end switching. The ASIC switch fabric creates dedicated physical paths between an input port and an output port, as shown in .

HIGH-END SWITCH ARCHITECTURE

High-end switches are typically modular in design and can perform both packet switching and cell switching. The modules of such a switch carry out switching between networks different types, including Ethernet, Fast Ethernet, Token Ring, FDDI and ATM. The main switching mechanism in such devices is the ATM switching structure. We will consider the architecture of such devices using the Centillion 100 from Bay Networks as an example.

Switching is carried out using the following three hardware components (see Figure 2):

ATM backplane for ultra-high-speed cell transfer between modules;

CellManager ASIC on each module to control the transmission of cells across the backplane;

SAR ASIC on each module to convert frames to cells and vice versa.

(1x1)

Figure 2.
Cell switching is increasingly used in high-end switches due to its high speed and ease of migration to ATM.

Each switch module has I/O ports, buffer memory, and a CellManager ASIC. In addition, each LAN module also has a RISC processor to perform frame switching between local ports and a packetizer/decomposer to convert frames and cells to each other. All modules can independently switch between their ports, so that only traffic destined for other modules is sent through the backplane.

Each module maintains its own table of addresses, and the main control processor combines them into one common table, so that a single module can see the network as a whole. If, for example, the Ethernet module receives a packet, it determines to whom the packet is addressed. If the address is in the local address table, then the RISC processor switches the packet between the local ports. If the destination is on another module, then the assembler/disassembler converts the packet into cells. The CellManager specifies a destination mask to identify the module(s) and port(s) to which the cell payload is destined. Any module whose card mask bit is set in the destination mask copies the cell to local memory and transmits data to the appropriate output port in accordance with the set port mask bits.

BUILDING VIRTUAL NETWORKS

In addition to improving performance, switches allow you to create virtual networks. One of the methods for creating a virtual network is to create a broadcast domain by logically connecting ports within the physical infrastructure of a communication device (this can be either an intelligent hub - configuration switching, or a switch - frame switching). For example, odd-numbered ports on an eight-port device are assigned to one virtual network, and even-numbered ports are assigned to another. As a result, a station in one virtual network is isolated from stations in another. The disadvantage of this virtual networking method is that all stations connected to the same port must belong to the same virtual network.

Another method for creating a virtual network is based on the MAC addresses of connected devices. With this method of organizing a virtual network, any employee can connect, for example, his laptop to any port of the switch, and it will automatically determine if its user belongs to a particular virtual network based on the MAC address. This method also allows users connected to the same switch port to belong to different virtual networks. For more information about virtual networks, see A. Avduevsky's article "Such real virtual networks" in the March issue of LAN this year.

THIRD LEVEL SWITCHING

For all their advantages, switches have one significant drawback: they are unable to protect the network from broadcast packet avalanches, and this leads to unproductive network load and increased response time. Routers can monitor and filter unnecessary broadcast traffic, but they are an order of magnitude slower. So, according to the Case Technologies documentation, the typical performance of a router is 10,000 packets per second, and this cannot be compared with that of a switch - 600,000 packets per second.

As a result, many manufacturers began to build routing functions into switches. To prevent the switch from slowing down significantly, apply various methods: For example, both Layer 2 switching and Layer 3 switching are implemented directly in hardware (in ASICs). Various manufacturers This technology is called differently, but the goal is the same: the routing switch must perform the functions of the third layer at the same speed as the functions of the second layer. An important factor is the price of such a device per port: it should also be low, like the switches (see the article by Nick Lippis in the next issue of LAN magazine).

CONCLUSION

Switches are both structurally and functionally very diverse; in one small article it is impossible to cover all their aspects. In the next tutorial, we'll take a closer look at ATM switches.

Dmitry Ganzha is the executive editor of LAN. He can be contacted at: [email protected].

Switches in the local network

How to choose a switch with the existing diversity? The functionality of modern models is very different. You can purchase both the simplest unmanaged switch and the multifunctional managed switch, which is not much different from a full-fledged router. An example of the latter is the Mikrotik CRS125-24G-1S-2HND-IN from the new Cloud Router Switch line. Accordingly, the price of such models will be much higher.

Therefore, when choosing a switch, first of all, you need to decide which of the functions and parameters of modern switches you need, and which ones you should not overpay for. But first, a little theory.

Types of switches

However, if previously managed switches differed from unmanaged ones, including a wider range of functions, now the difference can only be in the possibility or impossibility remote control device. For the rest - even in the most simple models manufacturers add additional functionality, often increasing their cost.

Therefore, on this moment the classification of switches by levels is more informative.

Switch layers

In order to choose a switch that best suits our needs, we need to know its level. This setting is determined based on which OSI (data transfer) networking model the device is using.

Devices first level using physical data transmission have almost disappeared from the market. If someone else remembers hubs, then this is just an example of the physical level, when information is transmitted in a continuous stream.
Level 2. It includes almost all unmanaged switches. The so-called canal network model. Devices divide incoming information into separate packets (frames, frames), check them and send them to a specific recipient device. The basis for distributing information in second-level switches is MAC addresses. Of these, the switch makes up an addressing table, remembering which port corresponds to which MAC address. They don't understand IP addresses.

Level 3. By choosing such a switch, you get a device that already works with IP addresses. It also supports many other data manipulation features: converting logical addresses to physical ones, network protocols IPv4, IPv6, IPX, etc., pptp, pppoe, vpn connections, and others. On the third, network level of data transfer, almost all routers and the most "advanced" part of the switches work.

Level 4. The OSI networking model that is used here is called transport. Not even all routers are available with support for this model. The distribution of traffic occurs at an intelligent level - the device is able to work with applications and, based on the headers of data packets, send them to the desired address. In addition, transport layer protocols, such as TCP, guarantee the reliability of packet delivery, the preservation of a certain sequence of their transmission, and are able to optimize traffic.

Choose a switch - read the characteristics

How to choose a switch by parameters and functions? Consider what is meant by some of the commonly used designations in the specifications. The basic parameters include:

Number of ports. Their number varies from 5 to 48. When choosing a switch, it is better to provide a margin for further network expansion.

Basic baud rate. Most often, we see the designation 10/100/1000 Mbps - the speeds that each port of the device supports. That is, the selected switch can operate at 10 Mbps, 100 Mbps, or 1000 Mbps. There are quite a few models that are equipped with both gigabit and 10/100 Mb / s ports. Most modern switches work according to the IEEE 802.3 Nway standard, automatically detecting the port speed.

bandwidth and internal bandwidth. The first value, also called the switching matrix, is the maximum amount of traffic that can be passed through the switch per unit of time. It is calculated very simply: number of ports x port speed x 2 (duplex). For example, an 8-port gigabit switch has a throughput of 16 Gbps.
The internal bandwidth is usually indicated by the manufacturer and is only needed for comparison with the previous value. If the declared internal bandwidth is less than the maximum, the device will not cope well with heavy loads, slow down and freeze.

Auto MDI/MDI-X detection. This is auto-detection and support for both standards, according to which the twisted pair was crimped, without the need for manual connection control.

Expansion slots. Connectivity additional interfaces, for example, optical.

MAC address table size. To select a switch, it is important to calculate in advance the size of the table you need, preferably taking into account future network expansion. If there are not enough records in the table, the switch will write new ones over the old ones, and this will slow down data transfer.

Form Factor. The switches are available in two housing styles: desktop/wall-mounted and rack-mounted. In the latter case, the standard size of the device is 19 inches. Dedicated rack ears can be removed.

We select a switch with the functions we need to work with traffic

flow control ( flow control, IEEE 802.3x protocol). It provides for the coordination of sending and receiving data between the sending device and the switch at high loads, in order to avoid packet loss. The feature is supported by almost every switch.

jumbo frame- extended packages. It is used for speeds from 1 Gbit / s and higher, allows you to speed up data transfer by reducing the number of packets and the time for their processing. The function is available in almost every switch.

Full-duplex and Half-duplex modes. Almost all modern switches support auto-negotiation between half duplex and full duplex (data transfer only in one direction, data transfer in both directions at the same time) to avoid network problems.

Traffic prioritization (IEEE 802.1p standard)- the device is able to detect more important packets (for example, VoIP) and send them first. When choosing a switch for a network where a significant part of the traffic will be audio or video, you should pay attention to this feature.

Support VLAN(standard IEEE 802.1q). VLAN is a convenient tool for distinguishing between individual sections: the internal network of the enterprise and the public network for customers, various departments, etc.

To ensure security within the network, control or check the performance of network equipment, mirroring (traffic duplication) can be used. For example, all incoming information is sent to one port for verification or recording by certain software.

Port Forwarding. You may need this feature to deploy a server with Internet access, or for online games.

Loop protection - STP and LBD functions. Especially important when choosing unmanaged switches. It is almost impossible to detect the resulting loop in them - a looped section of the network, the cause of many glitches and freezes. LoopBack Detection automatically blocks the port where a loop has occurred. The STP protocol (IEEE 802.1d) and its more advanced descendants - IEEE 802.1w, IEEE 802.1s - act a little differently, optimizing the network for a tree structure. Initially, the structure provides for spare, looped branches. By default, they are disabled, and the switch starts them only when there is a loss of communication on some main line.

Link Aggregation (IEEE 802.3ad). Increases the bandwidth of the channel by combining several physical ports into one logical one. The maximum throughput according to the standard is 8 Gbps.

Stacking. Each manufacturer uses their own stacking designs, but in general this feature refers to the virtual combination of multiple switches into a single logical device. The goal of stacking is to get large quantity ports than is possible when using a physical switch.

Switch functions for monitoring and troubleshooting

Many switches detect a cable connection failure, usually when the device is turned on, as well as the type of failure - wire breakage, short circuit and so on. For example, D-Link has special indicators on the case:

Protection against virus traffic (Safeguard Engine). The technique allows to increase the stability of work and protect the central processor from being overloaded by the "garbage" traffic of virus programs.

Power features

Energy saving.How to choose a switch that will save you electricity? pay attentione for power saving features. Some manufacturers, such as D-Link, offer power throttling switches. For example, a smart switch monitors devices connected to it, and if one of them is not working at the moment, the corresponding port is put into "sleep mode".

Power over Ethernet (PoE, IEEE 802.af standard). A switch using this technology can power devices connected to it over twisted pair.

Built-in lightning protection. Very desired function, however, it must be remembered that such switches must be grounded, otherwise the protection will not work.

website

Switch one of critical devices used in building a local network. In this article, we will talk about what kind of switches are and dwell on the important characteristics that you need to consider when choosing a LAN switch.

To begin with, let's consider a general block diagram in order to understand what place the switch occupies in the local network of an enterprise.

The figure above shows the most common structural scheme small local area network. As a rule, access switches are used in such local networks.

Access switches are directly connected to end users, giving them access to local network resources.

However, in large local area networks, switches perform the following functions:

Network access level. As mentioned above, access switches provide connection points for end user devices. In large local networks, access switch frames do not interact with each other, but are transmitted through distribution switches.

Distribution level. Switches of this layer forward traffic between access switches, but do not interact with end users.

System kernel level. Devices of this type combine data transmission channels from distribution level switches in large territorial local networks and provide a very high speed of switching data flows.

Switches are:

Unmanaged switches. These are ordinary stand-alone devices on a local network that manage data transfer on their own and do not have the ability to additional settings. In view of the ease of installation and low price, they are widely used for installation at home and small businesses.

Managed Switches. More advanced and expensive devices. They allow the network administrator to independently configure them for the given tasks.

Managed switches can be configured in one of the following ways:

Through the console port Via WEB interface

Through Telnet Via SNMP protocol

Through SSH

Switch layers

All switches can be divided into model levels OSI . The higher this level, the more capabilities the switch has, however, its cost will be much higher.

Layer 1 switches. TO given level include hubs, repeaters and other devices operating at the physical level. These devices were at the dawn of the development of the Internet and are not currently used on the local network. Having received a signal, a device of this type simply transmits it further, to all ports except the sender port

Layer 2 switches (layaer2). This level includes unmanaged and part of managed switches ( switch ) working at the data link layer of the model OSI . Layer 2 switches work with frames - frames: a stream of data divided into portions. Having received the frame, the layer 2 switch subtracts the sender's address from the frame and enters it into its table MAC addresses, matching this address to the port on which it received this frame. Thanks to this approach, Layer 2 switches forward data only to the destination port, without creating excess traffic on other ports. Layer 2 switches do not understand IP addresses located at the third network layer of the model OSI and work only at the data link layer.

Layer 2 switches support the most common protocols such as:

IEEE 802.1 q or VLAN virtual local area networks. This protocol allows you to create separate logical networks within the same physical network.

For example, devices connected to the same switch, but located in different VLAN will not see each other and will be able to transmit data only in their own broadcast domain (devices from the same VLAN). Between themselves, the computers in the figure above will be able to transfer data using a device operating at the third level with IP addresses: router.

IEEE 802.1p (Priority tags ). This protocol is initially present in the protocol IEEE 802.1 q and is a 3-bit field from 0 to 7. This protocol allows you to mark and sort all traffic in order of importance by setting priorities (maximum priority 7). Frames with higher priority will be forwarded first.

IEEE 802.1d spanning tree protocol (STP).This protocol builds a local network in a tree structure to avoid network loopbacks and prevent the formation of a network storm.

Let's say the installation of the local network is made in the form of a ring to increase the fault tolerance of the system. The switch with the highest priority on the network is selected as the Root.In the example above, SW3 is the root. Without delving into protocol execution algorithms, switches calculate the path with the maximum cost and block it. For example, in our case, the shortest path from SW3 to SW1 and SW2 will be through their own dedicated interfaces (DP) Fa 0/1 and Fa 0/2 . In this case, the default path cost for the 100 Mbps interface will be 19. The Fa 0/1 interface of the LAN switch SW1 is blocked because the total path cost will be the sum of two hops between 100 Mbps interfaces 19+19=38.

If the working route is corrupted, the switches will perform path recalculation and unblock the port.

IEEE 802.1w Rapid spanning tree protocol (RSTP).Enhanced 802.1 d , which has higher stability and lower link recovery time.

IEEE 802.1s Multiple spanning tree protocol.The latest version, taking into account all the shortcomings of the protocols STP and RSTP.

IEEE 802.3ad Link aggregation for parallel link.This protocol allows you to combine ports into groups. The total speed of this aggregation port will be the sum of the speeds of each port in it.The maximum speed is defined by the IEEE 802.3ad standard and is 8 Gbps.

Layer 3 switches (layer3). These devices are also called multiswitches since they combine the capabilities of switches working at the second level and routers working with IP packages at the third level.Layer 3 switches fully support all the features and standards of Layer 2 switches. They can work with network devices by IP addresses. Layer 3 switch supports the establishment of various connections: l 2 tp , pptp, pppoe, vpn etc.

Layer 4 switches (Layer 4) . L4 level devices operating at the transport layer of the model OSI . Responsible for ensuring the reliability of data transmission. These switches can, based on the information from the packet headers, understand the ownership of the traffic different applications and make decisions about redirecting such traffic based on this information. The name of such devices has not settled down, sometimes they are called intelligent switches, or L4 switches.

Key Features of Switches

Number of ports. Currently, there are switches with the number of ports from 5 to 48. This parameter determines the number of network devices that can be connected to this switch.

For example, when building a small local network of 15 computers, we need a switch with 16 ports: 15 for connecting end devices and one for installing and connecting a router to access the Internet.

Transfer rate. This is the speed at which each switch port operates. Typically, speeds are indicated as follows: 10/100/1000 Mbps. The port speed is determined during auto-negotiation with the end device. On managed switches, this setting can be configured manually.

For example : Client device A PC with a 1Gbps NIC is connected to a switch port at 10/100Mbps c . As a result of auto-negotiation, devices agree to use the highest possible speed of 100 Mbps.

Auto negotiation port between Full - duplex and half - duplex. Full-duplex: data is transmitted simultaneously in two directions. half-duplex data transfer is carried out first in one direction, then in the other direction sequentially.

Switching matrix internal bandwidth. This parameter shows how overall speed the switch can process data from all ports.

For example: in the local network there is a switch that has 5 ports operating at a speed of 10/100 Mbps. IN technical specifications switching matrix parameter is 1 Gbit/ c . This means that each port is in Full-duplex can operate at 200Mbps c (100 Mbps downlink and 100 Mbps downlink). Let the parameter of this switching matrix be less than the specified one. This means that at the time of peak loads, the ports will not be able to operate at the advertised speed of 100 Mbps.

Auto MDI / MDI-X cable type negotiation. This function allows you to determine which of the two methods was used to crimp the twisted pair EIA/TIA-568A or EIA/TIA-568B. When installing local networks, the EIA / TIA-568B scheme is most widely used.

Stacking - this is the combination of several switches into one single logical device. Different switch manufacturers use different stacking technologies, such as c isco uses Stack Wise stacking technology with a 32Gbps switch bus and Stack Wise Plus with a 64Gbps switch bus.

For example, this technology is relevant in large local networks, where it is required to connect more than 48 ports on the basis of one device.

Mounting for 19” rack. At home and small local networks, switches are often installed on flat surfaces or mounted on a wall, however, the presence of so-called "ears" is necessary in larger local networks where active equipment is located in server cabinets.

MAC table sizeaddresses . Switch (switch) is a device operating at the 2nd level of the model OSI . Unlike the hub, which simply redirects the received frame to all ports except the sender port, the switch learns: remembers MAC the address of the sender's device, entering it, the port number and the lifetime of the entry in the table. Using this table the switch forwards the frame not to all ports, but only to the destination port. If the number of network devices in the local network is significant and the size of the table is full, the switch starts to overwrite older entries in the table and writes new ones, which significantly reduces the speed of the switch.

jumbo frame . This feature allows the switch to operate with a larger packet size than that specified by the Ethernet standard. After each packet is received, some time is spent processing it. When using an increased packet size using Jumbo Frame technology, you can save on packet processing time in networks where data transfer rates of 1 Gb / s and higher are used. There is no big gain at lower speed

Switching modes.In order to understand the principle of operation of switching modes, first consider the structure of the frame transmitted at the data link layers between a network device and a switch in a local network:

As you can see from the picture:

First comes the preamble signaling the start of the frame transmission,
Then MAC destination address ( DA) and MAC sender's address ( SA)
Third level identifier: IPv 4 or IPv 6 in use
And in the end check sum FCS: A 4 byte CRC value used to detect transmission errors. Calculated by the sending party, and placed in the FCS field. The receiving side calculates this value on its own and compares it with the received value.

Now consider the switching modes:

Store-and-forward. This mode switching saves the entire frame to the buffer and checks the field FCS , which is located at the very end of the frame, and if the checksum of this field does not match, discards the entire frame. As a result, the likelihood of network congestion is reduced, since it is possible to discard frames with an error and delay the transmission time of the packet. This technology present in more expensive switches.

Cut through . More simple technology. In this case, frames can be processed faster because they are not buffered completely. For analysis, the data is stored in the buffer from the beginning of the frame to MAC address destination (DA) inclusive. The switch reads this MAC address and forwards it to the destination. The disadvantage of this technology is that the switch, in this case, forwards both dwarf, less than 512 bit intervals, and damaged packets, increasing the load on the local network.

PoE support

Power over ethernet technology allows you to power network device over the same cable. This decision allows you to reduce the cost of additional installation of supply lines.

The following PoE standards exist:

PoE 802.3af supports equipment up to 15.4W

PoE 802.3at supports equipment up to 30W

Passive PoE

PoE 802.3 af/at have intelligent control circuits for supplying voltage to the device: before applying power to the PoE device, the af/at standard source coordinates with it in order to avoid damage to the device. Passiv PoE is much cheaper than the first two standards, power is directly supplied to the device via free pairs network cable without any agreement.

Characteristics of the standards

The PoE 802.3af standard is supported by most low cost IP cameras, IP phones and access points.

The PoE 802.3at standard is present in more expensive models of IP surveillance cameras, where it is not possible to keep within 15.4 watts. In this case, both the IP video camera and the PoE source (switch) must support this standard.

Expansion slots. Switches may have additional expansion slots. The most common are SFP modules (Small Form-factor Pluggable). Modular, compact transceivers used for data transmission in the telecommunications environment.

SFP modules are inserted into a free SFP port of a router, switch, multiplexer or media converter. While there are Ethernet SFP modules, the most commonfiber optic modules are used to connect the main channel when transmitting data over long distances, inaccessible to the Ethernet standard. SFP modules are selected depending on the distance, data transfer rate. The most common are dual-fiber SFP modules that use one fiber for receiving and another for transmitting data. However, WDM technology allows you to transmit data at different wavelengths over a single optical cable.

SFP modules are:

SX - 850 nm is used with multi-mode optical cable at a distance of up to 550m
LX - 1310 nm is used with both types of optical cable (SM and MM) at a distance of up to 10 km
BX - 1310/1550 nm is used with both types of optical cable (SM and MM) at a distance of up to 10 km
XD - 1550nm is used with single mode cable up to 40km, ZX up to 80km, EZ or EZX up to 120km and DWDM

The SFP standard itself provides for data transfer at a speed of 1 Gb / s, or at a speed of 100 Mb / s. For faster data transfer, SFP+ modules have been developed:

SFP+ data transfer at 10 Gbps
XFP data transfer at 10 Gbps
QSFP+ data transfer at 40 Gbps
CFP data transfer at 100 Gbps

However, at higher speeds, the signals are processed at high frequencies. This requires a larger heat sink and, accordingly, large dimensions. Therefore, in fact, the SFP form factor has been preserved only in SFP + modules.

Conclusion

Many readers have probably come across unmanaged switches and budget managed Layer 2 switches in small local area networks. However, the choice of switches for building larger and more technically complex local networks is best left to professionals.

When installing local networks, Safe Kuban uses switches of the following brands:

Professional Solution:

Cisco

Qtech

Budget solution

D-Link

Tp Link

Tenda

Bezopasnaya Kuban performs installation, commissioning and maintenance of local networks in Krasnodar and the South of Russia.