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A switch or switch is a digital device logical interconnection of computer networks operating at Layer 2 ( link layer data) of the OSI model . Its function is to interconnect two or more network segments, similar to bridges (bridges), passing data from one segment to another in accordance with the MAC address destination for frames on the network.

switches are used when you want to connect multiple networks, merging them into one. Like bridges, as they work as a filter on the network, improve performance and safety of LANs (Local Area Network, Local Area Network). Introduction to the operation of switches

The switches have the ability to learn and storing the network addresses of Level 2 (MAC addresses ) of the devices reachable through each of its ports. For example, a computer connected directly to a port on a switch causes the switch stores the MAC address. This allows, unlike hubs or hubs, the information for a device to go from the source port to destination port. In the case of connecting two switches or a switch and a hub, each switch will learn MAC addresses of devices accessible through its ports, so the interconnection port of the MAC store the other switch devices.

Loops network traffic and flooding

As previously commented, one of the critical points of these teams are the loops (cycles) consisting enable two different paths from one computer to another through a set of switches. Loops occur because the switch that detects that a device is accessible through two ports on both the frame issue. To bring this plot to the next switch, this re-send the frame through the ports so as to achieve the team. This process causes each frame to multiply exponentially, to produce the so-called network flooding, thus causing failure or fall of communications. Classification
Switches

according to method of addressing the frames used:

switches Store-and-Forward save each frame into a buffer before the exchange of information to the output port. While the plot is in the buffer, the switch calculates the CRC and measures the size of it. If the CRC fails, or the size is very small or very large (an Ethernet frame is between 64 bytes and 1518 bytes) the frame is discarded. If everything is in order is routed to the output port.
This method ensures error-free operations and increases the confidence of the network. But time used to store and check each time frame adds a significant delay to process them. The delay or total delay is proportional to frame size: the larger the plot, the greater the delay. Cut-Through

The Cut-Through Switches are designed to reduce this latency. These switches minimize the delay reading only the first 6 bytes of raster data, which contains the destination MAC address, and immediately routed.
The problem with this type of switch is that it detects collisions caused by corrupted frames (called runts) or CRC errors. The greater the number of collisions on the network, the higher the bandwidth consumed when routing frames corrupt.
A second type of cut-through switch, known as free fragment, was designed to eliminate this problem. The switch always reads the first 64 bytes of each frame, ensuring that you have at least the minimum size, and runts avoiding routing through the network. Adaptive Cut-Through

switches that process adaptive frame mode support both store-and-forward and cut-through. Either mode can be activated by the network administrator, or the switch may be smart enough to choose between the two methods, based on the number of frames with error that pass through the ports.
When the number of corrupt frames reaches a certain level, the switch can switch from cut-through to store-and-forward, returning to previous mode when the network returns to normal.
cut-through switches are used in small workgroups and small departments. In these applications requires a good amount of work or throughput because the network are potential errors in the segment level, without impacting the corporate network.

Switches store-and-forward are used in corporate networks, where necessary error handling.
Depending on how segmentation of the sub-networks:
Switches Layer 2 or Layer 2 switches are the switches
traditional that function as multi-port bridges. Its main purpose is to divide a LAN into multiple collision domains, or in cases of ring networks, the LAN segment in different rings. Shipping based its decision on the destination MAC address contained in each frame.
Layer 2 switches enable multiple simultaneous transmissions without interfering with other sub-networks. Layer 2 switches fail, however, filter broadcasts or broadcasts, multicasts (in the case where more than one sub-network containing the stations belonging to the multicast group of destination), or frames whose destination has not yet been included in routing table.
Switches Layer 3 or Layer 3 switches are the switches
that in addition to the traditional functions of layer 2, incorporate some features of routing or routing, such as determining the path based on information from network layer (layer 3 OSI model ), validation of the integrity of the wiring layer 3 by checksum and support to traditional routing protocols (RIP, OSPF, etc)
Layer 3 switches also support the definition of virtual networks ( VLAN 's), and as models enable communication between different VLANs without the need for an external router.
By allowing the union of segments of different domains or broadcast media, layer 3 switches are particularly recommended for the segmentation of networks LAN very large, where the simple use of Layer 2 switches cause a loss performance and efficiency of the LAN due to excessive amount of broadcasts.
can say that the typical implementation of a Layer 3 switch is more scalable than a router, because the latter uses the techniques of routing and routing level 3 to level 2 as supplements, while the switches routing function overlap routing above, applying the first where necessary.
Within Layer 3 Switches are:
Packet-by-Packet (Packet by Packet)
Basically, a packet switch By Packet is a special case of switch Store-and-Forward then, like them, store and review the package, calculating the CRC and decoding the header of the network layer to define its path through the routing protocol adopted.
Layer-3
A Cut-through switch Cut-Through Layer 3 (not to be confused with Cut-Through switch), looks at a few fields, determines the destination address (through information from headers or headers of Layer 2 3), and from that moment a connection point to point (at 2) for high packet transfer rate.
Each manufacturer has its own design to enable the correct identification of data flows. As an example, we have the "IP Switching" of Ipsilon, the "Virtual Networking SecureFast Cabletron", the "Fast IP", 3Com.
The only project adopted as a de facto standard, implemented by different manufacturers, is the MPOA (Multi Protocol Over ATM). The MPOA, at the expense of its proven efficiency, is complex and expensive to implement, and limited in ATM backbones.
addition, a Layer 3 switch Cut-Through, from time-to-peer connection is established, you can run in the "Store-and-Forward" or "Cut-Through"
Switches Layer 4 or Layer 4 Switches
are in the market recently and there is a dispute regarding the proper classification of these teams. They are often called Layer 3 + (Layer 3 Plus).
basically incorporate the functionality of a Layer 3 switch's ability to implement policies and filters information from layer 4 or higher, and TCP / UDP, SNMP, FTP, switches etc.Los Store-and-Forward each frame stored in a buffer before the exchange of information to the output port. While the plot is in the buffer, the switch calculates the CRC and measures the size of it. If the CRC fails, or size is very small or very large (an Ethernet frame is between 64 bytes and 1518 bytes) the frame is discarded. If everything is in order is routed to the output port.
This method ensures error-free operations and increases the confidence of the network. But the time used to store and check each time frame adds a significant delay to process them. The delay or total delay is proportional to frame size: the larger the plot, the greater the delay. Cut-Through

The Cut-Through Switches are designed to reduce this latency. These switches minimize the delay reading only the first 6 bytes of raster data, which contains the destination MAC address, and immediately routing.
The problem with this type of switch is that it detects collisions caused by corrupted frames (called runts) or CRC errors. The greater the number of collisions on the network, the higher the bandwidth consumed when routing frames corrupt.
A second type of cut-through switch, known as free fragment, was designed to eliminate this problem. The switch always reads the first 64 bytes of each frame, ensuring that you have at least the minimum size, and runts avoiding routing through the network.
Adaptive Cut-Through Switches
processing adaptive frame mode support both store-and-forward and cut-through. Either mode can be activated by the network administrator, or the switch may be smart enough to choose between the two methods, based on the number of frames with error that pass through the ports.
When the number of corrupt frames reaches a certain level, the switch may change from cut-through to store-and-forward, returning to previous mode when the network returns to normal.
cut-through switches are used in small workgroups and small departments. In these applications requires a good amount of work or throughput because the network are potential errors in the segment level, without impacting the corporate network.
Switches store-and-forward are used in corporate networks, where necessary error handling.
Depending on how the segmentation of the sub-networks:
Switches Layer 2 or Layer 2 Switches
are the traditional switches, which function as multi-port bridges. Its main purpose is to divide a LAN into multiple collision domains, or in cases of ring networks, the LAN segment in different rings. Shipping based its decision on the destination MAC address contained in each frame.
Layer 2 switches enable multiple simultaneous transmissions without interfering with other sub-networks. Layer 2 switches fail, however, filter broadcasts or broadcasts, multicasts (in the case where more a sub-network containing the stations belonging to the multicast group of destination), or frames whose destination has not yet been included in the routing table.
Switches Layer 3 or Layer 3 Switches
are switches that, in addition to the traditional functions of layer 2, incorporate some features of routing or routing, such as determining the path based on information network layer (layer 3 OSI model ), validation of the integrity of the wiring layer 3 by checksum and support to traditional routing protocols (RIP, OSPF, etc)
Layer 3 switches also support the definition of virtual networks (VLAN 's), and as models enable communication between different VLANs without the need for an external router.
By allowing the union of segments of different domains or broadcast media, layer 3 switches are particularly recommended for the segmentation of networks LAN very large, where the simple use of Layer 2 switches cause a loss performance and efficiency of the LAN due to excessive amount of broadcasts. Arguably
implementation A typical layer 3 switch is more scalable than a router, because the latter uses the techniques of routing and routing level 3 to level 2 as supplements, while the switches routing function superimposed over the routing, using the first where necessary.
Within Layer 3 Switches are:
Packet-by-Packet (Packet by Packet)
Basically, a packet switch By Packet is a special case of switch Store-and-Forward then, like them, store and review the package, calculating the CRC and decoding the header of the network layer to define its path through the routing protocol adopted.
Layer-3 Cut-through switch Layer
A 3 Cut-Through (not to be confused with Cut-Through switch), looks at a few fields, determines the destination address (through information from headers or headers of Layer 2 and 3), and from that moment on, set a point-to-point (level 2) for high packet transfer rate.
Each manufacturer has its own design to enable the correct identification of data flows. As an example, we have the "IP Switching" of Ipsilon, the "Virtual Networking SecureFast Cabletron", the "Fast IP", 3Com.
The only project adopted as a de facto standard, implemented by different manufacturers, is the MPOA (Multi Protocol Over ATM). The MPOA in detriment of its proven efficiency, is complex and expensive to implement, and limited in ATM backbones.
addition, a Layer 3 switch Cut-Through, from time-to-peer connection is established, will operate in the "Store-and-Forward" or "Cut-Through"
or Layer 4 Switches Layer 4 Switches
're in the market recently and there is a dispute regarding the proper classification of these teams. They are often called Layer 3 + (Layer 3 Plus).
basically incorporate the functionality of a Layer 3 switch's ability to implement policies and filters information from layer 4 or higher, such as ports TCP / UDP, SNMP, FTP, etc..