TECHNOLOGY OPTIONS
Switches are devices that filter and forward packets between LAN segments. Switches operate at the data link layer (layer 2) and sometimes the network layer (layer 3) of the OSI Model and therefore support any packet protocol. LANs that use switches to join segments are called switched LANs or, in the case of Ethernet networks, switched Ethernet LANs.

Switches have multiple ports, each of which can support an entire Ethernet, FDDI or Token Ring segment. With a different segment connected to each of the ports, it can switch packets between them as needed. In effect, a switch acts like a very fast multi-port bridge because packets are filtered based on the destination address. Switches are starting to replace hubs and routers in many installations.

Switching technology is increasing the efficiency and speed of networks. This technology is making current systems more powerful, while at the same time facilitating the migration to faster networks. Switching directs network traffic in a very efficient manner. It sends information directly from the port of origin to only its destination port. Switching increases network performance, enhances flexibility and eases moves, adds and changes. Switching establishes a direct line of communication between two ports and maintains multiple simultaneous links between various ports. It proficiently manages network traffic by reducing media sharing, traffic is contained to the segment for which it is destined, be it a server, power user or workgroup.

Switches come in different categories:
l Unmanaged switch: These switches come in the 4 to 24 port varieties. These switches allow simultaneous transmission of multiple packets via an internal high-speed data channel. The learning function in the switch stores the address and corresponding port number of each incoming and outgoing packet in a routing table. This information is subsequently used to filter packets whose destination address is on the same segment as the source address. Unmanaged switches are inexpensive, but lack features for management. These are comparable to an unmanaged hub, except they have the speed of a switch.

Some features of an unmanaged switch are

Automatic detection of MDI-X and MDI crossover function

Conformation to IEEE 802.3 10BASE-T and IEEE 802.3u 100BASE-TX specifications.

Store-and-forward scheme to forward packets

Frame filtering and forwarding function for each port.

Automatic MAC address learning and aging function

Automatic local traffic filtering

Auto-negotiation on duplex mode

l Workgroup switch: Similar to unmanaged switch, except provide management of the unit. Sometimes they also provide Gigabit ports to uplink to larger backbone switches.

l Stackable switch: They usually have proprietary cables to interconnect them together. It allows a stack of switches to only use one IP-address for management. Some use Gigabit links to interconnect them and to uplink them to backbone switches.

Features of a stackable switch include

Fault tolerance so that if one switch fails, the other switches in the stack can continue to operate

Port redundancy so that if one port fails, a backup port can be automatically substituted

Hardware and software to let the user manage the switches using the Simple Network Management Protocol (SNMP)

l Chassis Switch, Backbone Switch or Core Switch: Usually support Layer 3 switching, along with Layer 2 switching and many high level protocols. The Chassis have blades similar to high-end routers. So one can mix and match different interfaces for connecting different types of networks together.

Switches are also classified according to the functionalities of the OSI model.

l Layer 2 Switches (Data-link Layer) operate using physical network addresses. Physical addresses, also known as link-layer, hardware, or MAC-layer addresses, identify individual devices. Most hardware devices are permanently assigned this number during the manufacturing process. Switches operating at Layer 2 are very fast because they are just sorting physical addresses, but they usually are not very smart—i.e., they do not look at the data packet very closely to learn anything more about where it is headed.

l Layer 3 Switching (Network Layer) attempts to reduce the performance bottlenecks associated with traditional routers. Layer 3 switches use network or IP addresses that identify locations on the network. They read network addresses more closely than Layer 2 switches—they identify network locations as well as the physical device. A location can be a LAN workstation, a location in a computer’s memory, or even a different packet of data traveling through a network. Switches operating at Layer 3 are smarter than Layer 2 devices and incorporate routing functions to actively calculate the best way to send a packet to its destination. But although they are smarter, they may not be as fast if their algorithms, fabric, and processor do not support high speeds.

l Layer 4 (Transport Layer) of the OSI Model coordinates communications between systems. Layer 4 switches are capable of identifying which application protocols (HTTP, SMTP, FTP, and so forth) are included with each packet, and they use this information to hand off the packet to the appropriate higher-layer software. Layer 4 switches make packet-forwarding decisions based not only on the MAC address and IP address, but also on the application to which a
packet belongs. Because Layer 4 devices enable one to establish priorities for network traffic based on application, one can assign a high priority to packets belonging to vital in-house applications such as Smartstream, with different forwarding rules for low-priority packets such as generic HTTP-based Internet traffic. Layer 4 switches also  provide an effective wire-speed
security shield for your network because any company- or industry-specific protocols can be confined to only authorized switched ports or users. This security feature is often reinforced with traffic filtering and forwarding features.

l Layer 5 Switches: This aims to use session level information in addition to layers 2,3, and 4 information to route traffic in the network. The system consists of a switch core to which a number of custom- built intelligent port controllers are attached. In addition, it is equipped with a processor complex. The job of the port controllers is to identify the packets that require layer 5 processing and forward them to the processor. The port controllers process the rest of the packets. As the CPU processes only a very small fraction of the packets, it achieves very high speeds while delivering useful layer 5 functionality. In fact application level proxies, which are functionally equivalent to the L5 switch, have been around for years. L5 combines the functionalities of an application layer proxy and the data handling capabilities of a switch into a single system. Though it can be used anywhere in the network the L5 switch is most useful as a front-end to a server cluster. It makes it possible to partition the URL space among the server nodes thus improving the performance of the server cluster.