Network switch

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Avaya ERS 2550T-PWR 50-port network switch

A network switch or switching hub is a computer networking device that links network segments or network devices. The term commonly refers to a multi-port network bridge that processes and routes 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 called layer-3 switches or multilayer switches.

Switches exist for various types of networks including Fibre Channel, Asynchronous Transfer Mode, InfiniBand, Ethernet and others. The first Ethernet switch was introduced by Kalpana in 1990.[1]

Function[edit]

A switch is a telecommunication device that receives a message from any device connected to it and then transmits the message only to the device for which the message was meant. This makes the switch a more intelligent device than a hub (which receives a message and then transmits it to all the other devices on its network). 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 residential 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.

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, any pair (e.g. A and B) can transfer data back and forth while the other pair (e.g. C and D) also do so simultaneously, and the two conversations will not interfere with one another. In full duplex mode, these pairs can also overlap (e.g. A transmits to B, simultaneously B to C, and so on). In the case of a repeater hub, they would all share the bandwidth and run in half duplex, resulting in collisions, which would then necessitate retransmissions.

Microsegmentation[edit]

Using a bridge or a switch (or a router) to split a larger collision domain into smaller ones in order to reduce collision probability and improve overall throughput is called segmentation. In the extreme of microsegmentation, each device is located on a dedicated switch port. In contrast to an Ethernet hub, there is a separate collision domain on each of the switch ports. This allows computers to have dedicated bandwidth on point-to-point connections to the network and also to run in full duplex without collisions. Full duplex mode has only one transmitter and one receiver per 'collision domain', making collisions impossible.

Role of switches in a network[edit]

Switches may operate at one or more layers of the OSI model, including data link and network. 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 is easier at layer 3.

Devices that interconnect at layer 3 are traditionally called routers, so layer-3 switches can also be regarded as (relatively primitive) routers.

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,[2][3] network intrusion detection,[4] and performance analysis modules that can plug into switch ports. Some of these functions may be on combined modules.[5]

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.

Layer-specific functionality[edit]

A modular network switch with three network modules (a total of 24 Ethernet and 14 Fast Ethernet ports) and one power supply.

While switches may learn about topologies at many layers, and forward at one or more layers, they do tend to have common features. Other than for high-performance applications, modern commercial switches use primarily Ethernet interfaces.

At any layer, a modern switch may implement power over Ethernet (PoE), which avoids the need for attached devices, such as a VoIP phone or wireless access point, to have a separate power supply. Since switches can have redundant power circuits connected to uninterruptible power supplies, the connected device can continue operating even when regular office power fails.

Layer 1 (Hubs versus higher-layer switches)[edit]

A network hub, or repeater, is a simple network device. Repeater hubs do not manage any of the traffic that comes through them. Any packet entering a port is flooded out or "repeated" on every other port, except for the port of entry. Since every packet is repeated on every other port, packet collisions affect the entire network, limiting its capacity.

A switch creates the – originally mandatory – Layer 1 end-to-end connection only virtually. Its bridge function selects which packets are forwarded to which port(s) on the basis of information taken from layer 2 (or higher), removing the requirement that every node be presented with all data. The connection lines are not "switched" literally, it only appears like this on the packet level. "Bridging hub", "switching hub", or "multiport bridge" would be more appropriate terms.

There are specialized applications where a hub can be useful, such as copying traffic to multiple network sensors. High end switches have a feature which does the same thing called port mirroring.

By the early 2000s, there was little price difference between a hub and a low-end switch.[6]

Layer 2[edit]

A network bridge, operating at the data link layer, may interconnect a small number of devices in a home or the office. This is a trivial case of bridging, in which the bridge learns the MAC address of each connected device.

Single bridges also can provide extremely high performance in specialized applications such as storage area networks.

Classic bridges may also interconnect using a spanning tree protocol that disables links so that the resulting local area network is a tree without loops. In contrast to routers, spanning tree bridges must have topologies with only one active path between two points. The older IEEE 802.1D spanning tree protocol could be quite slow, with forwarding stopping for 30 seconds while the spanning tree reconverged. A Rapid Spanning Tree Protocol was introduced as IEEE 802.1w. The newest standard Shortest path bridging (IEEE 802.1aq) is the next logical progression and incorporates all the older Spanning Tree Protocols (IEEE 802.1D STP, IEEE 802.1w RSTP, IEEE 802.1s MSTP) that blocked traffic on all but one alternative path. IEEE 802.1aq (Shortest Path Bridging SPB) allows all paths to be active with multiple equal cost paths, provides much larger layer 2 topologies (up to 16 million compared to the 4096 VLANs limit),[7] faster convergence, and improves the use of the mesh topologies through increase bandwidth and redundancy between all devices by allowing traffic to load share across all paths of a mesh network.[8][9][10][11]

While layer 2 switch remains more of a marketing term than a technical term,[citation needed] the products that were introduced as "switches" tended to use microsegmentation and Full duplex to prevent collisions among devices connected to Ethernet. By using an internal forwarding plane much faster than any interface, they give the impression of simultaneous paths among multiple devices. 'Non-blocking' devices use a forwarding plane or equivalent method fast enough to allow full duplex traffic for each port simultaneously.

Once a bridge learns the addresses of its connected nodes, it forwards data link layer frames using a layer 2 forwarding method. There are four forwarding methods a bridge can use, of which the second through fourth method were performance-increasing methods when used on "switch" products with the same input and output port bandwidths:

  1. Store and forward: The switch buffers and verifies each frame before forwarding it.
  2. Cut through: The switch reads only up to the frame's hardware address before starting to forward it. Cut-through switches have to fall back to store and forward if the outgoing port is busy at the time the packet arrives. There is no error checking with this method.
  3. Fragment free: A method that attempts to retain the benefits of both store and forward and cut through. Fragment free checks the first 64 bytes of the frame, where addressing information is stored. According to Ethernet specifications, collisions should be detected during the first 64 bytes of the frame, so frames that are in error because of a collision will not be forwarded. This way the frame will always reach its intended destination. Error checking of the actual data in the packet is left for the end device.
  4. Adaptive switching: A method of automatically selecting between the other three modes.

While there are specialized applications, such as storage area networks, where the input and output interfaces are the same bandwidth, this is not always the case in general LAN applications. In LANs, a switch used for end user access typically concentrates lower bandwidth and uplinks into a higher bandwidth.

Layer 3[edit]

Within the confines of the Ethernet physical layer, a layer-3 switch can perform some or all of the functions normally performed by a router. The most common layer-3 capability is awareness of IP multicast through IGMP snooping. 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 4[edit]

While the exact meaning of the term layer-4 switch is vendor-dependent, it almost always starts with a capability for network address translation, but then adds some type of load distribution based on TCP sessions.[12]

The device may include a stateful firewall, a VPN concentrator, or be an IPSec security gateway.

Layer 7[edit]

Layer-7 switches may distribute loads based on Uniform Resource Locator URL or by some installation-specific technique to recognize application-level transactions. A layer-7 switch may include a web cache and participate in a content delivery network.[13]

Rack-mounted 24-port 3Com switch

Types of switches[edit]

Form factor[edit]

Configuration options[edit]

Typical switch management features[edit]

Linksys 48-port switch
HP Procurve rack-mounted switches mounted in a standard Telco Rack 19-inch rack with network cables

Traffic monitoring on a switched network[edit]

Unless port mirroring or other methods such as RMON, SMON or sFlow are implemented in a switch,[15] it is difficult to monitor traffic that is bridged using a switch because only the sending and receiving ports can see the traffic. These monitoring features are rarely present on consumer-grade switches.

Two popular methods that are specifically designed to allow a network analyst to monitor traffic are:

Another method to monitor may be to connect a layer-1 hub between the monitored device and its switch port. This will induce minor delay, but will provide multiple interfaces that can be used to monitor the individual switch port.

See also[edit]

References[edit]

  1. ^ Robert J. Kohlhepp (2000-10-02). "The 10 Most Important Products of the Decade". Network Computing. Retrieved 2008-02-25. 
  2. ^ Cisco Catalyst 6500 Series Firewall Services Module, Cisco Systems,2007
  3. ^ Switch 8800 Firewall Module, 3Com Corporation, 2006
  4. ^ Cisco Catalyst 6500 Series Intrusion Detection System (IDSM-2) Module, Cisco Systems,2007
  5. ^ Getting Started with Check Point Fire Wall-1, Checkpoint Software Technologies Ltd., n.d.
  6. ^ Matthew Glidden (October 2001). "Switches and Hubs". About This Particular Macintosh blog. Retrieved June 9, 2011. 
  7. ^ Shuang Yu. "IEEE APPROVES NEW IEEE 802.1aq™ SHORTEST PATH BRIDGING STANDARD". IEEE Standards Association. Retrieved 19 June 2012. "Using the IEEE’s next-generation VLAN, called a Service Interface Identifier (I-SID), it is capable of supporting 16 million unique services compared to the VLAN limit of four thousand." 
  8. ^ Peter Ashwood-Smith (24 Feb 2011). "Shortest Path Bridging IEEE 802.1aq Overview". Huawei. Retrieved 11 May 2012. 
  9. ^ Jim Duffy (11 May 2012). "Largest Illinois healthcare system uproots Cisco to build $40M private cloud". PC Advisor. Retrieved 11 May 2012. "Shortest Path Bridging will replace Spanning Tree in the Ethernet fabric." 
  10. ^ "IEEE Approves New IEEE 802.1aq Shortest Path Bridging Standard". Tech Power Up. 7 May 2012. Retrieved 11 May 2012. 
  11. ^ D. Fedyk, Ed.,; P. Ashwood-Smith, Ed.,; D. Allan, A. Bragg,; P. Unbehagen (April 2012). "IS-IS Extensions Supporting IEEE 802.1aq". IETF. Retrieved 12 May 2012. 
  12. ^ S. Sathaye (January 1999), The Ins and Outs of Layer 4+ Switching, NANOG 15, "It usually means one of two things: - 1. Layer 4 information is used to prioritize and queue traffic (routers have done this for years) - 2. Layer 4 information is used to direct application sessions to different servers (next generation load balancing)." 
  13. ^ How worried is too worried? Plus, a Global Crossing Story., NANOG mailing list archives, S. Gibbard,October 2001
  14. ^ Tech specs for a sample HP "web-managed" switch (archived page from web.archive.org)
  15. ^ Remote Network Monitoring Management Information Base, RFC 2819, S. Waldbusser,May 2000

External links[edit]