Switching
Encyclopedia
LAN switching is a form of packet switching
used in local area network
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.
switching is hardware based, which means it uses the media access control address (MAC address) from the host's network interface cards
(NICs) to decide where to forward frames. Switches use application-specific integrated circuit
(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.
Layer 2 switching provides the following
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 domain
s). 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
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.
. Bridges are good if a network is designed by the 80/20 rule
: 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.
switch and router 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
The benefits of layer 3 switching include the following
Switching algorithm is relatively simple. it is the same for most of the routing protocols. in most cases a host determines that it must send a packet to another host. Having acquired a routers address by some means, the source host send a packet address specifically to a routers physical(MAC_) address, this time with the protocol(network layer)address of the destination host.
As it examines the packets destinations protocole address, the router determines that it either knows or does not know how to forward packet to the next hop. If the router does not know how to do it, it typically drops the packet. if it knows how to forward packet, however, it changes the destinations physical address to that of the next hop n transmittess the packet.
The next hop may be the destination or the next router, which executes the same switching process. As the packet moves through the internetwork, its physical address changes, but its protocol address remains same.
IEEE has developed the hierarchical terminology that is useful in describing this process. The network devices without capability to forward packets between subnetworks are called end system(ES), whereas network devices with this capabilities are called intermediate systems(IS). IS are further divided into those that can communicate within routing domain(Intradomain ES) and those that communicate both within and between routing domains(Interdomains IS) a routing domain is generally considered as portion of an internetwork under common administrative authority that is regulated by a particular set of administrative guidlines. Routing domains are also called as autonomous systems.
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
(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 open standards using sFlow
provided by companies like Arista Networks
or proprietary solutions like 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.
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
There is no performance difference between a layer 3 and a layer 4 switch because the routing/switching is all hardware based.
Packet switching
Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...
used in local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
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.
Layer 2 switching
Layer 2Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
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 interface controller is a computer hardware component that connects a computer to 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. For example, a chip designed solely to run a cell phone is an ASIC...
(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.
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 domain
Collision domain
A collision domain is a section of a network where data packets can collide with one another when being sent on a shared medium or through repeaters, in particular, when using early versions of Ethernet. A network collision occurs when more than one device attempts to send a packet on a network...
s). 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 farmServer farmA server farm or server cluster is a collection of computer servers usually maintained by an enterprise to accomplish server needs far beyond the capability of one machine. Server farms often have backup servers, which can take over the function of primary servers in the event of a primary server...
s — 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. - IntranetIntranetAn intranet is a computer network that uses Internet Protocol technology to securely share any part of an organization's information or network operating system within that organization. The term is used in contrast to internet, a network between organizations, and instead refers to a network...
s — 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 networksBridging (networking)
Bridging is a forwarding technique used in packet-switched computer networks. Unlike routing, bridging makes no assumptions about where in a network a particular address is located. Instead, it depends on flooding and examination of source addresses in received packet headers to locate unknown...
. 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 consultant Joseph M...
: 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
Broadcast domain
A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments....
. 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 3Network layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
switch and router 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
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. For example, a chip designed solely to run a cell phone is an ASIC...
. 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 addressLogical addressIn computing, a logical address is the address at which an item appears to reside from the perspective of an executing application program....
ing - Run layer 3 checksumChecksumA checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and...
s (on header only) - Use Time to LiveTime to liveTime to live is a mechanism that limits the lifespan of data in a computer or network. TTL may be implemented as a counter or timestamp attached to or embedded in the data. Once the prescribed event count or timespan has elapsed, data is discarded. In computer networking, TTL prevents a data...
(TTL) - Process and respond to any option information
- Update Simple Network Management ProtocolSimple Network Management ProtocolSimple Network Management Protocol is an "Internet-standard protocol for managing devices on IP networks. Devices that typically support SNMP include routers, switches, servers, workstations, printers, modem racks, and more." It is used mostly in network management systems to monitor...
(SNMP) managers with Management Information BaseManagement information baseA management information base is a virtual database used for managing the entities in a communications network. Most often associated with the Simple Network Management Protocol , the term is also used more generically in contexts such as in OSI/ISO Network management model...
(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 latencyLagLag is a common word meaning to fail to keep up or to fall behind. In real-time applications, the term is used when the application fails to respond in a timely fashion to inputs...
- Lower per-port cost
- Flow accounting
- Security
- Quality of serviceQuality of serviceThe quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
(QoS)
- SWITCHING:
Switching algorithm is relatively simple. it is the same for most of the routing protocols. in most cases a host determines that it must send a packet to another host. Having acquired a routers address by some means, the source host send a packet address specifically to a routers physical(MAC_) address, this time with the protocol(network layer)address of the destination host.
As it examines the packets destinations protocole address, the router determines that it either knows or does not know how to forward packet to the next hop. If the router does not know how to do it, it typically drops the packet. if it knows how to forward packet, however, it changes the destinations physical address to that of the next hop n transmittess the packet.
The next hop may be the destination or the next router, which executes the same switching process. As the packet moves through the internetwork, its physical address changes, but its protocol address remains same.
IEEE has developed the hierarchical terminology that is useful in describing this process. The network devices without capability to forward packets between subnetworks are called end system(ES), whereas network devices with this capabilities are called intermediate systems(IS). IS are further divided into those that can communicate within routing domain(Intradomain ES) and those that communicate both within and between routing domains(Interdomains IS) a routing domain is generally considered as portion of an internetwork under common administrative authority that is regulated by a particular set of administrative guidlines. Routing domains are also called as autonomous systems.
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.Through the Internet Society, engineers and...
(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 open standards using sFlow
SFlow
sFlow is a technology for monitoring network, wireless andhost devices.The sFlow.org consortium is the authoritative source for the sFlow protocol specifications: previous version of sFlow, including RFC 3176, have been deprecated.- Operation :...
provided by companies like Arista Networks
Arista Networks
Arista Networks is a computer networking company headquartered in Santa Clara, California, USA. The company designs and sells network switches for datacenter, high-performance computing and high-frequency trading environments...
or proprietary solutions like 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.
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.
External links
- Arista Networks industries leading 10G data center switching products
- Cisco IT Case Studies about routing and switching
- Is Switching a Commodity?" Fact or Fiction with Jennifer Geisler (Flash software required)