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Main article: "Multilayer switch
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 vs. higher-layer switches)[edit]

A "network hub, or a repeater, is a simple network device that does not manage any of the traffic coming through it. 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 overall capacity.

A network switch creates the layer 1 end-to-end connection only virtually, while originally it was mandatory. The bridging function of a switch uses information taken from layer 2 to select for each packet the particular port(s) it has to be forwarded to, removing the requirement that every node is presented with all traffic. As a result, the connection lines are not "switched" literally, instead they only appear that way on the packet level.

There are specialized applications in which a network hub can be useful, such as copying traffic to multiple network sensors. High-end network switches usually have a feature called "port mirroring that provides the same functionality.

By the early 2000s, there was little price difference between a hub and a low-end switch.[12]

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.

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),[13] faster convergence, and improves the use of the mesh topologies through increased bandwidth and redundancy between all devices by allowing traffic to load share across all paths of a mesh network.[14][15][16][17]

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; a frame is received in its entirety before it is forwarded.
  2. "Cut through: the switch starts forwarding after the frame's destination address is received. When the outgoing port is busy at the time, the switch falls back to store-and-forward operation. 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.[18][19]

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 signalled 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.[20]

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

Layer 7[edit]

Layer-7 switches may distribute the load based on "uniform resource locators (URLs), or by using 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 (CDN).[21]

Types of switches[edit]

A "rack-mounted 24-port "3Com switch

Form factors[edit]

Switches are available in many form factors, including: desktop units not mounted in an enclosure which are typically intended to be used in a home or office environment outside a wiring closet; rack-mounted switches for use in an "equipment rack; large "chassis units with swappable module cards; "DIN rail mounted for use in "industrial environments; and small installation switches, mounted into a cable duct, floor box or communications tower, as found, for example, in "FTTO Infrastructures.

Configuration options[edit]

Typical switch management features[edit]

A couple of managed "D-Link Gigabit Ethernet rackmount switches, connected to the Ethernet ports on a few "patch panels using "Category 6 "patch cables (all equipment is installed in a standard 19-inch rack)

Traffic monitoring on a switched network[edit]

Unless port mirroring or other methods such as "RMON, SMON or "sFlow are implemented in a switch,[23] 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]


  1. ^ IEEE 802.1D
  2. ^ "Hubs Versus Switches – Understand the Tradeoffs" (PDF). ccontrols.com. 2002. Retrieved 2013-12-10. 
  3. ^ Thayumanavan Sridhar (September 1998). "Layer 2 and Layer 3 Switch Evolution". cisco.com. The Internet Protocol Journal. Cisco Systems. Retrieved 2014-08-05. 
  4. ^ Robert J. Kohlhepp (2000-10-02). "The 10 Most Important Products of the Decade". Network Computing. Archived from the original on 2010-01-05. Retrieved 2008-02-25. 
  5. ^ "Cisco Networking Academy's Introduction to Basic Switching Concepts and Configuration". "Cisco Systems. 2014-03-31. Retrieved 2015-08-17. 
  6. ^ Joe Efferson; Ted Gary; Bob Nevins (February 2002). "Token-Ring to Ethernet Migration" (PDF). "IBM. p. 13. Retrieved 2015-08-11. 
  7. ^ Thayumanavan Sridhar (September 1998). "The Internet Protocol Journal - Volume 1, No. 2: Layer 2 and Layer 3 Switch Evolution". "Cisco Systems. Retrieved 2015-08-11. 
  8. ^ Cisco Catalyst 6500 Series Firewall Services Module, Cisco Systems,2007
  9. ^ Switch 8800 Firewall Module, 3Com Corporation, 2006
  10. ^ Cisco Catalyst 6500 Series Intrusion Detection System (IDSM-2) Module, Cisco Systems,2007
  11. ^ Getting Started with Check Point Fire Wall-1, Checkpoint Software Technologies Ltd., n.d.
  12. ^ Matthew Glidden (October 2001). "Switches and Hubs". About This Particular Macintosh blog. Retrieved June 9, 2011. 
  13. ^ 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. 
  14. ^ Peter Ashwood-Smith (24 Feb 2011). "Shortest Path Bridging IEEE 802.1aq Overview" (PDF). Huawei. Retrieved 11 May 2012. 
  15. ^ 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. 
  16. ^ "IEEE Approves New IEEE 802.1aq Shortest Path Bridging Standard". Tech Power Up. 7 May 2012. Retrieved 11 May 2012. 
  17. ^ 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. 
  18. ^ Dong, Jielin. Network Dictionary. Javvin Technologies Inc. p. 23. "ISBN "9781602670006. Retrieved 25 June 2016. 
  19. ^ "Cray makes its ethernet switches responsive to net conditions". IDG Network World Inc. 1 July 1996. Retrieved 25 June 2016. 
  20. ^ 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). 
  21. ^ How worried is too worried? Plus, a Global Crossing Story., NANOG mailing list archives, S. Gibbard,October 2001
  22. ^ Tech specs for a sample HP "web-managed" switch at the "Wayback Machine (archived December 13, 2007)
  23. ^ Remote Network Monitoring Management Information Base, RFC 2819, S. Waldbusser,May 2000

External links[edit]

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