Switch

Switch 

Switch is a network control (or network connecting/ network equipment) device. It is the device which is used to interconnect computers, workstations, servers and other networking devices to form IT network. It connects communicating devices using separate cables. One end of the cable is connected to communicating device and other end is connected to switch. So it provides a central point of connections for cables from other devices to create network (LAN). It is also used to connect two or more LANs (multiple LANs) to facilitate network.

Switch operates at Physical layer as well as Data Link Layer of OSI model. It handles network data which is in the form of frame.  

Switch looks like Hub, it is also used to connect all devices on a network so that they can communicate with each other. But switch and hub has some functional differences in hub and switch. When data comes at one port of hub, it duplicate that data and send on all other ports except the received port. It means Hub broadcast data to each and every connected node.

But switch is the intelligence device which can learn MAC addresses and forward packet to exact destination. Switch maintains record in switching table. The switch table record contains the MAC addresses and physical port providing access connection to system which MAC is recorded.

Initially switching table is blank, there is no entry of MAC address and physical port number. Record is inserted when switch is power on and connected nodes start communicating. 

First time when a frame is received on switch (when switching table is blank) switch retrieves the source and destination MAC addresses which are contained within frame. Switch learn the MAC address and make entry in table about the source systems MAC address and connected port. But switch don't know the destination system is connected to which port, it will do flooding. Means it will send the incoming frame on all other port, in short it broadcast the frame. In this fashion switch learn MAC addresses and from second time onwards it will do unicast. Means it will send the received frame to appropriate port which give access to destination.

Switches are similar to bridges in that both route on frame addresses. The main difference is that a switch is most often used to connect individual computers and bridge is used to connect LAN segments.

Switch provides bridging functionality with greater efficiency. It acts as a multi port bridge to connect network devices or segments in LAN.

Switches are capable of inspecting incoming data packets, determine source and destination of device of each packet and forward them appropriately.

Since each switch port usually goes to a single computer, switches must have space for many more line cards than do bridges intended to connect only LANs. Each line card provides buffer space for frames arriving on its ports. Since each port is its own collision domain, switches never lose frames to collisions. However, if frames come in faster than they can be retransmitted, the switch may run out of buffer space and have to start discarding frames.

Most switches are active, that is they electrically amplify the signal as it moves from one device to another.

Switches are usually configured with 8 port, 12 port, or 24 port and come with RJ-45 port interface

Switches are often used in implementation of star or tree topology.

Switch is also called as layer 2 device which means that the device that operates at layer 2 (Physical Layer and Data Link Layer) of OSI model.

There are two types of switches that are Layer 2 switch and Layer 3 switch. Layer-2 devices build hardware address tables, which will contain minimum two things. One is Hardware addresses (Physical address or MAC address) of host devices which are connected to switch. Second The port to which hardware address is associated

Using this above information, Layer-2 devices will make intelligent forwarding decisions based on contents and context of frame (Data-Link Layer) headers.

A frame can then be forwarded out only the appropriate destination port which gives access to destination, instead of all ports. There is one to one (unicast) communication possible using switch instead one to many (broadcast).

Layer-2 forwarding was originally referred to as bridging. Bridging is a largely deprecated term (mostly for marketing purposes), and Layer-2 forwarding is now commonly referred to as switching.

There are few technological differences in between switching and bridging. Remarkable difference is switches usually have more ports as compare to bridge (switches having a higher port-density), and second difference is, switches can make forwarding decisions at very high speed (wire speed). Switches use specialized hardware circuits to make forwarding decision faster, these hardware circuits called as ASICs (Application-Specific Integrated Circuits). Otherwise, bridges and switches are nearly identical in function.

Ethernet switches build MAC-address tables through a dynamic learning process. A switch behaves much like a hub when first powered on. The switch will flood every frame, including unicasts, out every port but the originating port. The switch will then build the MAC-address table by examining the source MAC address of each frame. Switches always learn from the source MAC address.

However, as the MAC-address table becomes populated, the flooding of frames will decrease, allowing the switch to perform more efficient forwarding decisions.

While hubs were limited to half-duplex communication, switches can operate in full duplex. Each individual port on a switch belongs to its own collision domain. Thus, switches create more collision domains, which results in fewer collisions.

Like hubs though, switches belong to only one broadcast domain. A Layer-2 switch will forward both broadcasts and multicasts out every port but the originating port. Only Layer-3 devices separate broadcast domains.

Because of this, Layer-2 switches are poorly suited for large, scalable networks. The Layer-2 header provides no mechanism to differentiate one network from another, only one host from another. This poses significant difficulties. If only hardware addressing existed, all devices would technically be on the same network.

There are three things that switches do that hubs do not that includes first is Hardware address learning, second Intelligent forwarding of frames and third is Loop avoidance.

 

Layer-2 Forwarding Methods

Switches support three methods of forwarding frames. Each method copies all or part of the frame into memory, providing different levels of latency and reliability. Latency is delay - less latency results in quicker forwarding.

The Store-and-Forward method copies the entire frame into memory, and performs a Cycle Redundancy Check (CRC) to completely ensure the integrity of the frame. However, this level of error-checking introduces the highest latency of any of the switching methods.

The Cut-Through (Real Time) method copies only enough of a frame’s header to determine its destination address. This is generally the first 6 bytes following the preamble. This method allows frames to be transferred at wire speed, and has the least latency of any of the three methods. No error checking is attempted when using the cut-through method.

The Fragment-Free (Modified Cut-Through) method copies only the first 64 bytes of a frame for error-checking purposes. Most collisions or corruption occur in the first 64 bytes of a frame.

Network Equipment Manufacturers 

• Cisco
• Juniper
• Dlink
• Digisol
• Netgear