A multilayer switch (MLS) is a

computer networking device Networking hardware, also known as network equipment or computer networking devices, are electronic devices which are required for communication and interaction between devices on a computer network. Specifically, they mediate data transmission in ...

that switches on

OSI layer 2 The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...

like an ordinary

network switch A network switch (also called switching hub, bridging hub, and, by the IEEE, MAC bridge) is networking hardware that connects devices on a computer network by using packet switching to receive and forward data to the destination device. A netw ...

and provides extra functions on higher

OSI layers The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...

. The MLS was invented by engineers at

Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and Harlan Anderson in 1957. Olsen was president un ...

. Switching technologies are crucial to

network design Network planning and design is an iterative process, encompassing topological design, network-synthesis, and network-realization, and is aimed at ensuring that a new telecommunications network or service meets the needs of the subscriber and op ...

, as they allow traffic to be sent only where it is needed in most cases, using fast, hardware-based methods. Switching uses different kinds of network switches. A standard switch is known as a ''layer 2 switch'' and is commonly found in nearly any LAN. ''Layer 3'' or ''layer 4'' switches require advanced technology (see managed switch) and are more expensive and thus are usually only found in larger LANs or in special network environments.

Multilayer switch

Multi-layer switching combines layer 2, 3 and 4 switching technologies and provides high-speed scalability with low latency. Multi-layer switching can move traffic at wire speed and also provide layer 3 routing. There is no performance difference between forwarding at different layers because the routing and switching are all hardware-based routing decisions are made by specialized

application-specific integrated circuit An application-specific integrated circuit (ASIC ) is an integrated circuit (IC) chip customized for a particular use, rather than intended for general-purpose use, such as a chip designed to run in a digital voice recorder or a high-effici ...

s (ASICs) with the help of

content-addressable memory Content-addressable memory (CAM) is a special type of computer memory used in certain very-high-speed searching applications. It is also known as associative memory or associative storage and compares input search data against a table of stored d ...

. Multi-layer switching can make routing and switching decisions based on the following *

MAC address A media access control address (MAC address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment. This use is common in most IEEE 802 networking tec ...

in a data link frame * Protocol field in the data link frame * IP address in the

network layer In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers. Functions The network layer provides the means of tran ...

header * Protocol field in the network layer header * Port numbers in the

transport layer In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet protocol suite and the OSI model. The protocols of this layer provide end-to-end ...

header MLSs implement QoS in hardware. A multilayer switch can prioritize

packet Packet may refer to: * A small container or pouch ** Packet (container), a small single use container ** Cigarette packet ** Sugar packet * Network packet, a formatted unit of data carried by a packet-mode computer network * Packet radio, a fo ...

s by the 6 bit

differentiated services code point Differentiated services or DiffServ is a computer networking architecture that specifies a mechanism for classifying and managing network traffic and providing quality of service (QoS) on modern IP networks. DiffServ can, for example, be used t ...

(DSCP). These 6 bits were originally used for type of service. The following 4 mappings are normally available in an MLS: * From OSI layer 2, 3 or 4 to IP DSCP (for IP packets) or IEEE 802.1p * From IEEE 802.1p to IP DSCP * From IP DSCP to IEEE 802.1p * From VLAN IEEE 802.1p to port egress queue. MLSs are also able to route IP traffic between

VLAN A virtual local area network (VLAN) is any broadcast domain that is partitioned and isolated in a computer network at the data link layer ( OSI layer 2).IEEE 802.1Q-2011, ''1.4 VLAN aims and benefits'' In this context, virtual, refers to a ph ...

s like a common router. The routing is normally as quick as switching (at wire speed).

Layer-2 switching

Layer-2 The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...

switching uses the

of the host's network interface cards (NICs) to decide where to forward frames. Layer 2 switching is hardware-based, which means switches use ASICs to build and maintain the

Forwarding information base A forwarding information base (FIB), also known as a forwarding table or MAC table, is most commonly used in network bridging, routing, and similar functions to find the proper output network interface controller to which the input interface shou ...

and to perform

packet forwarding Packet forwarding is the relaying of packets from one network segment to another by nodes in a computer network. The network layer in the OSI model is responsible for packet forwarding. Models The simplest forwarding model unicastinginvolve ...

at wire speed. One way to think of a layer-2 switch is as a multiport

bridge A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or rail) without blocking the way underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually somethi ...

. Layer-2 switching is highly efficient because there is no modification to the frame required. Encapsulation of the packet changes only when the data packet passes through dissimilar media (such as from Ethernet to FDDI). Layer-2 switching is used for workgroup connectivity and network segmentation (breaking up

collision domain In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great fo ...

s). This allows a flatter network design with more network segments than traditional networks joined by

repeater hub In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction. Som ...

s and routers. Layer-2 switches have the same limitations as bridges. Bridges break up collision domains, but the network remains one large

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. In ...

which can cause performance issues and limits the size of a 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. Bridges are good if a network is designed by the 80/20 rule: users spend 80 percent of their time on their local segment.

Layer-3 switching

A layer-3 switch can perform some or all of the functions normally performed by a router. Most network switches, however, are limited to supporting a single type of physical network, typically Ethernet, whereas a router may support different kinds of physical networks on different ports. Layer-3 switching is solely based on (destination)

IP address An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...

stored in the header of

IP datagram A datagram is a basic transfer unit associated with a packet-switched network. Datagrams are typically structured in header and payload sections. Datagrams provide a connectionless communication service across a packet-switched network. The deli ...

(layer-4 switching may use other information in the header). The difference between a layer-3 switch and a router is the way the device is making the routing decision. Traditionally, routers use microprocessors to make forwarding decisions in software, while the switch performs only hardware-based packet switching (by specialized ASICs with the help of content-addressable memory). However, many routers now also have advanced hardware functions to assist with forwarding. The main advantage of layer-3 switches is the potential for lower network latency as a packet can be routed without making extra network hops to a router. For example, connecting two distinct segments (e.g. VLANs) with a router to a standard layer-2 switch requires passing the frame to the switch (first L2 hop), then to the router (second L2 hop) where the packet inside the frame is routed (L3 hop) and then passed back to the switch (third L2 hop). A layer-3 switch accomplishes the same task without the need for a router (and therefore additional hops) by making the routing decision itself, i.e. the packet is routed to another subnet and switched to the destination network port simultaneously. Because many layer-3 switches offer the same functionality as traditional routers they can be used as cheaper, lower latency replacements in some networks. Layer 3 switches can perform the following actions that can also be performed by routers: * determine paths based on

logical address In computing, a logical address is the address at which an item ( memory cell, storage element, network host) appears to reside from the perspective of an executing application program. A logical address may be different from the physical addr ...

ing * check and recompute layer-3 header

checksum A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify data ...

s * examine and update

time to live Time to live (TTL) or hop limit is a mechanism which limits the lifespan or lifetime 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 times ...

(TTL) field * process and respond to any option information * update

Simple Network Management Protocol Simple Network Management Protocol (SNMP) is an Internet Standard protocol for collecting and organizing information about managed devices on IP networks and for modifying that information to change device behaviour. Devices that typically ...

(SNMP) managers with

Management Information Base A management information base (MIB) is a database used for managing the entities in a communication network. Most often associated with the Simple Network Management Protocol (SNMP), the term is also used more generically in contexts such as in ...

(MIB) information The benefits of layer 3 switching include the following: * fast hardware-based packet forwarding with low latency * lower per-port cost compared to pure routers * flow accounting *

Quality of service Quality of service (QoS) is the description or measurement of the overall performance of a service, such as a telephony or computer network, or a cloud computing service, particularly the performance seen by the users of the network. To quantitat ...

(QoS) IEEE has developed hierarchical terminology that is useful in describing forwarding and switching processes. Network devices without the capability to forward packets between subnetworks are called end systems (ESs, singular ES), whereas network devices with these capabilities are called intermediate systems (ISs). ISs are further divided into those that communicate only within their routing domain (intradomain IS) and those that communicate both within and between routing domains (interdomains IS). A routing domain is generally considered as a portion of an internetwork under common administrative authority and is regulated by a particular set of administrative guidelines. Routing domains are also called autonomous systems. A common layer-3 capability is an awareness of

IP multicast IP multicast is a method of sending Internet Protocol (IP) datagrams to a group of interested receivers in a single transmission. It is the IP-specific form of multicast and is used for streaming media and other network applications. It uses spec ...

through

IGMP snooping IGMP snooping is the process of listening to Internet Group Management Protocol (IGMP) network traffic to control delivery of IP multicasts. Network switches with IGMP snooping listen in on the IGMP conversation between hosts and routers and m ...

. With this awareness, a layer-3 switch can increase efficiency by delivering the traffic of a multicast group only to ports where the attached device has signaled that it wants to listen to that group. Layer-3 switches typically support

IP routing IP routing is the application of routing methodologies to IP networks. This involves not only protocols and technologies but includes the policies of the worldwide organization and configuration of Internet infrastructure. In each IP network nod ...

between VLANs configured on the switch. Some layer-3 switches support the

routing protocol A routing protocol specifies how routers communicate with each other to distribute information that enables them to select routes between nodes on a computer network. Routers perform the traffic directing functions on the Internet; data packets ...

s that routers use to exchange information about routes between networks.

Layer 4 switching

Layer 4 switching means hardware-based layer 3 switching technology that can also consider the type of network traffic (for example, distinguishing between UDP and TCP). Layer 4 switching provides additional datagram inspection by reading the

port number In computer networking, a port is a number assigned to uniquely identify a connection endpoint and to direct data to a specific service. At the software level, within an operating system, a port is a logical construct that identifies a specific ...

s found in the transport layer header to make routing decisions (i.e. ports used by

HTTP The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...

FTP The File Transfer Protocol (FTP) is a standard communication protocol used for the transfer of computer files from a server to a client on a computer network. FTP is built on a client–server model architecture using separate control and data ...

and

VoIP Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet t ...

). These port numbers are found in RFC 1700 and reference the upper-layer protocol, program, or application. Using layer-4 switching, the network administrator can configure a layer-4 switch to prioritize data traffic by application. Layer-4 information can also be used to help make routing decisions. 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, short for "sampled flow", is an industry standard for packet export at Layer 2 of the OSI model. sFlow was originally developed by InMon Corp. It provides a means for exporting truncated packets, together with interface counters for the purp ...

. A layer-4 switch can use information in the transport-layer protocols to make forwarding decisions. Principally this refers to an ability to use source and destination port numbers in TCP and UDP communications to allow, block and prioritize communications.

Layer 4–7 switch, web switch, or content switch

Some switches can use packet information up to OSI layer 7; these may be called layer 4–7 switches, , , web switches or application switches. Content switches are typically used for load balancing among groups of servers. Load balancing can be performed on

HTTPS Hypertext Transfer Protocol Secure (HTTPS) is an extension of the Hypertext Transfer Protocol (HTTP). It is used for secure communication over a computer network, and is widely used on the Internet. In HTTPS, the communication protocol is enc ...

VPN A virtual private network (VPN) extends a private network across a public network and enables users to send and receive data across shared or public networks as if their computing devices were directly connected to the private network. The be ...

, or any TCP/IP traffic using a specific port. Load balancing often involves destination network address translation so that the client of the load-balanced service is not fully aware of which server is handling its requests. Some layer 4–7 switches can perform Network address translation (NAT) at wire speed. Content switches can often be used to perform standard operations such as SSL encryption and decryption to reduce the load on the servers receiving the traffic, or to centralize the management of

digital certificate In cryptography, a public key certificate, also known as a digital certificate or identity certificate, is an electronic document used to prove the validity of a public key. The certificate includes information about the key, information about the ...

s. Layer 7 switching is a technology used in a

content delivery network A content delivery network, or content distribution network (CDN), is a geographically distributed network of proxy servers and their data centers. The goal is to provide high availability and performance by distributing the service spatially rel ...

(CDN). Some applications require that repeated requests from a client are directed at the same application server. Since the client isn't generally aware of which server it spoke to earlier, content switches define a notion of stickiness. For example, requests from the same source IP address are directed to the same application server each time. Stickiness can also be based on SSL IDs, and some content switches can use

cookies A cookie is a baked or cooked snack or dessert that is typically small, flat and sweet. It usually contains flour, sugar, egg, and some type of oil, fat, or butter. It may include other ingredients such as raisins, oats, chocolate chi ...

to provide this functionality.

Layer 4 load balancer

The router operates on the

and makes decisions on where to send the packets. Modern load balancing routers can use different rules to make decisions on where to route traffic. This can be based on least load, or fastest

response time Response time may refer to: *The time lag between an electronic input and the output signal which depends upon the value of passive components used. * Responsiveness, how quickly an interactive system responds to user input * Response time (biolog ...

s, or simply balancing requests out to multiple destinations providing the same services. This is also a redundancy method, so if one machine is not up, the router will not send traffic to it. The router may also have NAT capability with port and transaction awareness and performs a form of port translation for sending incoming packets to one or more machines that are hidden behind a single IP address.

Layer 7

Layer-7 switches may distribute the load based on

uniform resource locator A Uniform Resource Locator (URL), colloquially termed as a web address, is a reference to a web resource that specifies its location on a computer network and a mechanism for retrieving it. A URL is a specific type of Uniform Resource Identifi ...

s (URLs), or by using some installation-specific technique to recognize application-level transactions. A layer-7 switch may include a

web cache A Web cache (or HTTP cache) is a system for optimizing the World Wide Web. It is implemented both client-side and server-side. The caching of multimedias and other files can result in less overall delay when browsing the Web. Parts of the sys ...

and participate in a CDN.

References

{{reflist

External links

What is the difference between a Layer-3 switch and a router?Multilayer Switching
Networking hardware