All Topics  
Switching

 

 
   Email Print
   Bookmark   Link
 

 

Switching
 
 
  Topics Switching Topic Home

LAN switching is a form of packet switching
Packet switching

Packet switching is a network communications method that groups all transmitted data, irrespective of content, type, or structure into suitably-sized blocks, called packets....
 used in local area network
Local area network

A local area network is a computer network covering a small physical area, like a home, office, or small group of buildings, such as a school, or an airport....
s. Switching technologies are crucial to network design, as they allow traffic to be sent only where it is needed in most cases, using fast, hardware-based methods.

History
Prices on OSI model
OSI model

The Open Systems Interconnection Reference Model is an abstract description for layered communications and computer network protocol design. It was developed as part of the Open Systems Interconnection initiative....
 layer 2 switching equipment dropped dramatically around the turn of the 21st century, making it easier to justify the cost of buying switches for an entire local area network
Local area network

A local area network is a computer network covering a small physical area, like a home, office, or small group of buildings, such as a school, or an airport....
.






Discussion
Ask a question about 'Switching'
Start a new discussion about 'Switching'
Answer questions from other users
Full Discussion Forum



Encyclopedia


LAN switching is a form of packet switching
Packet switching

Packet switching is a network communications method that groups all transmitted data, irrespective of content, type, or structure into suitably-sized blocks, called packets....
 used in local area network
Local area network

A local area network is a computer network covering a small physical area, like a home, office, or small group of buildings, such as a school, or an airport....
s. Switching technologies are crucial to network design, as they allow traffic to be sent only where it is needed in most cases, using fast, hardware-based methods.

History


Prices on OSI model
OSI model

The Open Systems Interconnection Reference Model is an abstract description for layered communications and computer network protocol design. It was developed as part of the Open Systems Interconnection initiative....
 layer 2 switching equipment dropped dramatically around the turn of the 21st century, making it easier to justify the cost of buying switches for an entire local area network
Local area network

A local area network is a computer network covering a small physical area, like a home, office, or small group of buildings, such as a school, or an airport....
. This doesn't mean that every business can afford switch ports for all users, but it does allow for a cost-effective upgrade method.

Layer 2 switching

Layer 2 switching is hardware based, which means it uses the media access control address (MAC address) from the host's network interface cards
Network card

A network card, network adapter, network interface controller , network interface card, or LAN adapter is a computer hardware component designed to allow computers to communicate over a computer network....
 (NICs) to decide where to forward frames. Switches use application-specific integrated circuit
Application-specific integrated circuit

An application-specific integrated circuit is an integrated circuit customized for a particular use, rather than intended for general-purpose use....
s (ASICs) to build and maintain filter tables (also known as MAC address tables). One way to think of a layer 2 switch is as a multiport bridge
Network bridge

A network bridge connects multiple network segments at the data link layer of the OSI model, and the term layer 2 switch is very often used interchangeably with bridge....
.

Layer 2 switching provides the following
  • Hardware-based bridging (MAC)
  • Wire speed
  • High speed
  • Low latency
  • Low cost


Layer 2 switching is highly efficient because there is no modification to the data packet, only to the frame encapsulation of the packet, and only when the data packet is passing through dissimilar media (such as from Ethernet to FDDI). Layer 2 switching is used for workgroup connectivity and network segmentation (breaking up collision domains). This allows a flatter network design with more network segments than traditional 10BaseT shared networks. Layer 2 switching has helped develop new components in the network infrastructure
  • Server farms — Servers are no longer distributed to physical locations because virtual LANs can be created to create broadcast domains in a switched internetwork. This means that all servers can be placed in a central location, yet a certain server can still be part of a workgroup in a remote branch, for example.
  • Intranets — Allows organization-wide client/server communications based on a Web technology.


These new technologies allow more data to flow off from local subnets and onto a routed network, where a router's performance can become the bottleneck.

Limitations

Layer 2 switches have the same limitations as bridge networks. Remember that bridges are good if a network is designed by the 80/20 rule
Pareto principle

The Pareto principle states that, for many events, roughly 80% of the effects come from 20% of the causes.Business management thinker Dr. Joseph Moses Juran suggested the principle and named it after Italian economist Vilfredo Pareto, who observed that 80% of the land in Italy was owned by 20% of the population....
: users spend 80 percent of their time on their local segment.

Bridged networks break up collision domains, but the network remains one large broadcast domain. Similarly, layer 2 switches (bridges) cannot break up broadcast domains, which can cause performance issues and limits the size of your network. Broadcast and multicasts, along with the slow convergence of spanning tree, can cause major problems as the network grows. Because of these problems, layer 2 switches cannot completely replace routers in the internetwork.

Layer 3 switching

The only difference between a layer 3
Network layer

The Network Layer is Layer 3 in the OSI model of computer networking. The Network Layer responds to service requests from the Transport Layer and issues service requests to the Data Link Layer....
 switch and a router
Router

A router is a Computer network device whose software and hardware are usually tailored to the tasks of routing and forwarding information. For example, on the Internet, information is directed to various paths by routers....
 is the way the administrator creates the physical implementation. Also, traditional routers use microprocessors to make forwarding decisions, and the switch performs only hardware-based packet switching. However, some traditional routers can have other hardware functions as well in some of the higher-end models. Layer 3 switches can be placed anywhere in the network because they handle high-performance LAN traffic and can cost-effectively replace routers. Layer 3 switching is all hardware-based packet forwarding, and all packet forwarding is handled by hardware ASICs. Layer 3 switches really are no different functionally than a traditional router and perform the same functions, which are listed here
  • Determine paths based on logical address
    Logical address

    In Data Networks:In computer networks, a logical address refers to a network layer address such as an IP address, an X.25/X.121 or IPX address....
    ing
  • Run layer 3 checksum
    Checksum

    A checksum or hash sum is a fixed-size data computed from an arbitrary block of digital data for the purpose of error detection that may have been introduced during its telecommunications or computer storage....
    s (on header only)
  • Use Time to Live
    Time to live

    Time to live is a limit on the period of time or number of iterations or transmissions in computer and computer network technology that a unit of data can experience before it should be discarded....
     (TTL)
  • Process and respond to any option information
  • Update Simple Network Management Protocol
    Simple Network Management Protocol

    Simple Network Management Protocol is used in network management systems to Network monitoring network-attached devices for conditions that warrant administrative attention....
     (SNMP) managers with Management Information Base
    Management information base

    A management information base stems from the Open Systems Interconnection/ISO Network management model and is a type of database used to manage the devices in a communications network....
     (MIB) information
  • Provide Security


The benefits of layer 3 switching include the following
  • Hardware-based packet forwarding
  • High-performance packet switching
  • High-speed scalability
  • Low latency
    Lag

    In computing and especially computer networks, lag is a term used where the computer freezes and then continues some time later when an action is performed, for example clicking a mouse button....
  • Lower per-port cost
  • Flow accounting
  • Security
  • Quality of service
    Quality of service

    In the field of computer networking and other packet-switched telecommunication networks, the Traffic engineering term quality of service refers to resource reservation control mechanisms rather than the achieved service quality....
     (QoS)


Layer 4 switching

Layer 4 switching is considered a hardware-based layer 3 switching technology that can also consider the application used (for example, Telnet or FTP).

Layer 4 switching provides additional routing above layer 3 by using the port numbers found in the Transport layer header to make routing decisions.

These port numbers are found in Request for Comments
Request for Comments

In computer network engineering, a request for comments is a memorandum published by the Internet Engineering Task Force describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems....
 (RFC) 1700 and reference the upper-layer protocol, program, or application.

Layer 4 information has been used to help make routing decisions for quite a while. For example, extended access lists can filter packets based on layer 4 port numbers. Another example is accounting information gathered by NetFlow switching in Cisco's higher-end routers.

The largest benefit of layer 4 switching is that the network administrator can configure a layer 4 switch to prioritize data traffic by application, which means a QoS can be defined for each user.

For example, a number of users can be defined as a Video group and be assigned more priority, or band-width, based on the need for video conferencing.

However, because users can be part of many groups and run many applications, the layer 4 switches must be able to provide a huge filter table or response time would suffer. This filter table must be much larger than any layer 2 or 3 switch. A layer 2 switch might have a filter table only as large as the number of users connected to the network and may be even less if some hubs are used within the switched fabric. However, a layer 4 switch might have five or six entries for each and every device connected to the network. If the layer 4 switch does not have a filter table that includes all the information, the switch will not be able to produce wire-speed results.

Multi-layer switching (MLS)

Multi-layer switching combines layer 2, 3, and 4 switching technologies and provides high-speed scalability with low latency. It accomplishes this high combination of high-speed scalability with low latency by using huge filter tables based on the criteria designed by the network administrator.

Multi-layer switching can move traffic at wire speed and also provide layer 3 routing, which can remove the bottleneck from the network routers. This technology is based on the idea of "route once, switch many".

Multi-layer switching can make routing/switching decisions based on the following
  • MAC source/destination address in a Data Link frame
  • IP source/destination address in the Network layer header
  • Protocol field in the Network layer header
  • Port source/destination numbers in the Transport layer header


There is no performance difference between a layer 3 and a layer 4 switch because the routing/switching is all hardware based.

See also

  • Network switch
    Network switch

    A network switch is a computer networking device that connects computer network Network segment.The term commonly refers to a Network bridge that processes and routes data at the Data link layer of the OSI model....
  • Content switch
  • Residential gateway
    Residential gateway

    A residential gateway is a home networking device. The term is generally used to cover any networking appliance used in homes. The term however is misleading....


External links

  • about routing and switching