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{{three other uses|a computer networking device|the kind of rotating cutting tool|wood router|the type of network router found in many homes|residential gateway|the software used in ]|routing (Electronic Design Automation)}}
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A '''router''' ({{pronEng|'rautər}} in the USA, {{pronEng|'ru:tər}} in the UK and ], or either pronunciation in Australia and Canada) is a ] whose software and hardware are usually tailored to the tasks of ] and ] information. Routers generally contain a specialized ] (e.g. ]'s ] or ] JUNOS and JUNOSe or ] XOS), ], ], ], and one or more ]s, as well as two or more network interfaces. High-end routers contain many processors and specialized ]s (ASIC) and do a great deal of ]. Chassis based systems like the ] ] or ] routing switch, (pictured right) have multiple ASICs on every module and allow for a wide variety of ], ], METRO, and ] port technologies or other connections that are customizable. Much simpler routers are used where cost is important and the demand is low, for example in providing a home internet service. With appropriate software (such as ], ], ] or ]), a standard PC can act as a router.

Routers connect two or more logical ], which do not necessarily map one-to-one to the physical interfaces of the router.<ref>,RFC 1812, F. Baker,June 1995</ref> The term '''layer 3 switch''' often is used interchangeably with router, but ] is really a general term without a rigorous technical definition. In marketing usage, it is generally optimized for Ethernet LAN interfaces and may not have other physical interface types.

Routers operate in two different planes <ref>,RFC 3654, H. Khosravi & T. Anderson,November 2003</ref>:
* ], in which the router learns the outgoing interface that is most appropriate for forwarding specific packets to specific destinations,
* ], which is responsible for the actual process of sending a packet received on a logical interface to an outbound logical interface.

== Control Plane ==
{{Main|Control Plane}}

]

Control Plane processing leads to the construction of what is variously called a ] or routing information base (RIB). The RIB may be used by the Forwarding Plane to look up the outbound interface for a given packet, or, depending on the router implementation, the Control Plane may populate a separate ] (FIB) with destination information. RIBs are optimized for efficient updating with control mechanisms such as ], while FIBs are optimized for the fastest possible lookup of the information needed to select the outbound interface.

The Control Plane constructs the routing table from knowledge of the up/down status of its local interfaces, from hard-coded ], and from exchanging ] information with other routers. It is not compulsory for a router to use routing protocols to function, if for example it was configured solely with static routes. The routing table stores the best routes to certain network destinations, the "routing metrics" associated with those routes, and the path to the next hop router.

Routers do maintain ] on the routes in the RIB/routing table, but this is quite distinct from not maintaining state on individual packets that have been forwarded.

== Forwarding Plane (a.k.a. Data Plane) ==
{{Main|Forwarding Plane}}

For the pure ] (IP) forwarding function, router design tries to minimize the ] information kept on individual packets. Once a packet is forwarded, the router should no longer retain statistical information about it. It is the sending and receiving endpoints that keeps information about such things as errored or missing packets.

Forwarding decisions can involve decisions at layers other than the IP internetwork layer or OSI layer 3. Again, the marketing term switch can be applied to devices that have these capabilities. A function that forwards based on data link layer, or OSI layer 2, information, is properly called a ]. Marketing literature may call it a layer 2 switch, but a switch has no precise definition.

Among the most important forwarding decisions is deciding what to do when congestion occurs, i.e., packets arrive at the router at a rate higher than the router can process. Three policies commonly used in the Internet are ], ], and ]. Tail drop is the simplest and most easily implemented; the router simply drops packets once the length of the queue exceeds the size of the buffers in the router. Random early detection (RED) probabilistically drops datagrams early when the queue exceeds a configured size. Weighted random early detection requires a weighted average queue size to exceed the configured size, so that short bursts will not trigger random drops.

==Types of routers==

Routers may provide connectivity inside enterprises, between enterprises and the Internet, and inside ] (ISP). The largest routers (for example the ] ] or Juniper T1600) interconnect ISPs, are used inside ISPs, or may be used in very large enterprise networks. The smallest routers provide connectivity for small and home offices.

===Routers for Internet connectivity and internal use===

Routers intended for ISP and major enterprise connectivity will almost invariably exchange routing information with the ]. RFC 4098<ref>,RFC 4098, H. Berkowitz ''et al.'',June 2005</ref> defines several types of BGP-speaking routers:
* Provider Edge Router: Placed at the edge of an ISP network, it speaks external BGP (eBGP) to a BGP speaker in another provider or large enterprise Autonomous System (]).
* Subscriber Edge Router: Located at the edge of the subscriber's network, it speaks eBGP to its provider's AS(s). It belongs to an end user (enterprise) organization.
* Inter-provider Border Router: Interconnecting ISPs, this is a BGP speaking router that maintains BGP sessions with other BGP speaking routers in other providers' ASes.
* Core router: A router that resides within the middle or backbone of the LAN network rather than at its periphery.
::Within an ISP: Internal to the provider's AS, such a router speaks internal BGP (iBGP) to that provider's edge routers, other intra-provider core routers, or the provider's inter-provider border routers.
::"Internet backbone:" The Internet does not have a clearly identifiable backbone, as did its predecessors. See ] (DFZ). Nevertheless, it is the major ISPs' routers that make up what many would consider the core. These ISPs operate all four types of the BGP-speaking routers described here. In ISP usage, a "core" router is internal to an ISP, and used to interconnect its edge and border routers. Core routers may also have specialized functions in ]s based on a combination of BGP and ] (MPLS)<ref>,RFC 2547, E. Rosen and Y. Rekhter,April 2004</ref>.

===Small Office Home Office (SOHO) connectivity===
{{Main|Residential gateway}}

Residential gateways (often called routers) are frequently used in homes to connect to a broadband service, such as IP over ] or ]. A home router may allow connectivity to an enterprise via a secure ].

While functionally similar to routers, residential gateways use ] in addition to routing. Instead of connecting local computers to the remote network directly, a residential gateway makes multiple local computers appear to be a single computer.

===Enterprise Routers===

All sizes of routers may be found inside enterprises. The most powerful routers tend to be found in ISPs but academic and research facilities, as well as large businesses, may also need large routers.

A three-layer model is in common use, not all of which need be present in smaller networks <ref>{{cite book
| last = Oppenheimer
| first = Priscilla
| authorlink =
| coauthors =
| title = Top-Down Network Design
| publisher = Cisco Press
| year = 2004
| location = Indianapolis
| isbn = 1587051524}}</ref>.
==== Access ====

Access routers,including SOHO, are located at customer sites such as branch offices that do not need hierarchical routing of their own. Typically, they are optimized for low cost.

==== Distribution ====

Distribution routers aggregate traffic from multiple access routers, either at the same site, or to collect the data streams from multiple sites to a major enterprise location. Distribution routers often are responsible for enforcing quality of service across a WAN, so they may have considerable memory, multiple WAN interfaces, and substantial processing intelligence.

They may also provide connectivity to groups of servers or to external networks. In the latter application, the router's functionality must be carefully considered as part of the overall security architecture. Separate from the router may be a ] or ] concentrator, or the router may include these and other security functions.

When an enterprise is primarily on one campus, there may not be a distinct distribution tier, other than perhaps off-campus access. In such cases, the access routers, connected to LANs, interconnect via core routers.

==== Core ====

In enterprises, ] may provide a "collapsed backbone" interconnecting the distribution tier routers from multiple buildings of a campus, or large enterprise locations. They tend to be optimized for high bandwidth.

When an enterprise is widely distributed with no central location(s), the function of core routing may be subsumed by the WAN service to which the enterprise subscribes, and the distribution routers become the highest tier.

==History==
] and the first IMP. Taken from http://www.lk.cs.ucla.edu/personal_history.html]]
] in 1987.]]
The very first device that had fundamentally the same functionality as a router does today, i.e a ], was the ] (IMP); IMPs were the devices that made up the ], the first ] network. The idea for a router (although they were called "gateways" at the time) initially came about through an international group of computer networking researchers called the International Network Working Group (INWG). Set up in 1972 as an informal group to consider the technical issues involved in connecting different networks, later that year it became a subcommittee of the ].
<ref>Davies, Shanks, Heart, Barker, Despres, Detwiler, and Riml, "Report of Subgroup 1 on Communication System", INWG Note #1.</ref>

These devices were different from most previous packet switches in two ways. First, they connected dissimilar kinds of networks, such as ]s and ]s. Second, they were ] devices, which had no role in assuring that traffic was delivered reliably, leaving that entirely to the ] (although this particular idea had been previously pioneered in the ] network).

The idea was explored in more detail, with the intention to produce real prototype system, as part of two contemporaneous programs. One was the initial ]-initiated program, which created the ] architecture of today.
<ref>Vinton Cerf, Robert Kahn, , IEEE Transactions on Communications, Volume 22, Issue 5, May 1974, pp. 637 - 648.</ref>
The other was a program at ] to explore new networking technologies, which produced the ] system, although due to corporate intellectual property concerns it received little attention outside Xerox until years later.
<ref>David Boggs, John Shoch, Edward Taft, Robert Metcalfe, , IEEE Transactions on Communications, Volume 28, Issue 4, April 1980, pp. 612- 624.</ref>

The earliest Xerox routers came into operation sometime after early 1974. The first true IP router was developed by Virginia Strazisar at ], as part of that DARPA-initiated effort, during 1975-1976. By the end of 1976, three ]-based routers were in service in the experimental prototype Internet.
<ref>Craig Partridge, S. Blumenthal, ; IEEE Annals of the History of Computing, Volume 28, Issue 1; January-March 2006.</ref>

The first multiprotocol routers were independently created by staff researchers at ] and ] in 1981; the Stanford router was done by ], and the MIT one by Noel Chiappa; both were also based on PDP-11s.
<ref>, Public Broadcasting Service, Accessed August 11, 2007.</ref>
<ref>, NetworkWorld, Accessed June 22, 2007.</ref>
<ref>David D. Clark, "M.I.T. Campus Network Implementation", CCNG-2, Campus Computer Network Group, M.I.T., Cambridge, 1982; pp. 26.</ref>
<ref>Pete Carey, "A Start-Up's True Tale: Often-told story of Cisco's launch leaves out the drama, intrigue", San Jose Mercury News, December 1, 2001.</ref>

As virtually all networking now uses IP at the network layer, multiprotocol routers are largely obsolete, although they were important in the early stages of the growth of computer networking, when several protocols other than TCP/IP were in widespread use. Routers that handle both IPv4 and IPv6 arguably are multiprotocol, but in a far less variable sense than a router that processed AppleTalk, DECnet, IP, and Xerox protocols.

In the original era of routing (from the mid-1970s through the 1980s), general-purpose ]s served as routers. Although general-purpose computers can perform routing, modern high-speed routers are highly specialized computers, generally with extra hardware added to accelerate both common routing functions such as packet forwarding and specialised functions such as ] encryption.

Still, there is substantial use of ] and ] machines, running open source routing code, for routing research and selected other applications. While ] operating system was independently designed, other major router operating systems, such as those from ] and ], are extensively modified but still have Unix ancestry.

==See also==
{{Wiktionarypar|router}}
{{Wikibooks|Computer Networks/Routing}}
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ] (NAT)
* ]
* ]
* ]
* ]
* ]


== References ==
{{Reflist}}

==External links==
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Revision as of 08:59, 8 November 2008

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