The Use of Network Switches in a Network

A network switch is a computer networking device that connects network segments. Switchescommonly refer to a multi-port network bridge that process and route data at the data link layer (layer 2) of the OSI model. Switches that additionally process data at the network layer (Layer 3) and above are often referred to as Layer 3 cisco 3750 series switch or multilayer switches.

The network switch plays an integral part in most modern Ethernet local area networks (LANs). Mid-to-large sized LANs contain a number of linked managed switches. Small office/home office (SOHO) applications typically use a single switch, or an all-purpose converged device such as a gateway to access small office/home broadband services such as DSL or cable internet. In most of these cases, the end-user device contains a router and components that interface to the particular physical broadband technology. User devices may also include a telephone interface for VoIP.

Functions and Roles of Switches in Networks

An Ethernet switch operates at the data link layer of the OSI model to create a separate collision domain for each switch port. With 4 computers (e.g., A, B, C, and D) on 4 switch ports, A and B can transfer data back and forth, while C and D also do so simultaneously, and the two conversations will not interfere with one another. In the case of a hub, they would all share the bandwidth and run in half duplex, resulting in collisions, which would then necessitate retransmissions. Using a switch is called micro segmentation. This allows computers to have dedicated bandwidth on point-to-point connections to the network and to therefore run in full duplex without collisions.

Switches may operate at one or more layers of the OSI model, including data link, network, or transport (i.e., end-to-end). A device that operates simultaneously at more than one of these layers is known as a multilayer switch.

In switches intended for commercial use, built-in or modular interfaces make it possible to connect different types of networks, including Ethernet, Fibre Channel, ATM, ITU-T G.hn and 802.11. This connectivity can be at any of the layers mentioned. While Layer 2 functionality is adequate for bandwidth-shifting within one technology, interconnecting technologies such as Ethernet and token ring are easier at Layer 3.

Interconnection of different Layer 3 networks is done by routers. If there are any features that characterize "Layer-3 cisco 3560X switch" as opposed to general-purpose routers, it tends to be that they are optimized, in larger switches, for high-density Ethernet connectivity.

In some service provider and other environments where there is a need for a great deal of analysis of network performance and security, switches may be connected between WAN routers as places for analytic modules. Some vendors provide firewall,network intrusion detection,[4] and performance analysis modules that can plug into switch ports. Some of these functions may be on combined modules.

In other cases, the switch is used to create a mirror image of data that can go to an external device. Since most switch port mirroring provides only one mirrored stream, network hubs can be useful for fanning out data to several read-only analyzers, such as intrusion detection systems and packet sniffers.

NOTES

LAYER 2 NETWORKS

 A switch works with MAC addresses cisco 3560G switch at layer 2 of the OSI 7 layer model. Devices used on a Layer 2 Ethernet network include network interface cards, hubs, bridges and switches. Ethernet devices use 'burned in' MAC addresses to identify each host. These addresses are attached to network interface cards and cannot be changed. The address of a computer can only be changed if the network interface card is changed. Layer 2 networks are connected together using hubs, bridges and switches. Hubs take each frame that is received and send duplicate frames simultaneously out all other ports. Bridges forward broadcasts between two connected networks until they have located all hosts. Switches forward broadcast traffic only when it doesn't know where a host is. None of these devices have to disassemble or reassemble data, only forward it. Because nothing is done to the data along the way, layer 2 networks are often considered to be 'faster' than a layer 3 network.

LAYER 3 NETWORKS

A Layer 3 switch is a high-performance device for network routing. A router works with IP addresses at layer 3 of the model. Layer 3 networks are built to run on on layer 2 networks. In an IP layer 3 network, the IP portion of the datagram has to be read. This requires stripping off the datalink layer frame information.cisco 3560E switch Once the protocol frame information is stripped, the IP datagram has to be reassembled. Once the IP datagram is reassembled, the hop count has to be decremented, the header checksum has to be recalculated, a lookup for routing must be made, and only then can the IP datagram be chopped back up and inserted into frames and transmitted to the next hop. All of this takes extra time.

Layer 2 networks forward all traffic, especially ARP and DHCP broadcasts. Anything transmitted by one device is forwarded to all devices. When the network gets large enough, the broadcast traffic begins to create congestion and decreases network efficiency.

By contrast, layer 3 devices restrict broadcast traffic such as ARP and DHCP broadcasts to the local network. This reduces overall traffic levels by allowing administrators to chop networks into smaller parts and restrict broadcasts to only that sub-network. This means there is a limit to the size of a layer 2 network. A layer 3 network, prperly configured, can grow to any size.