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A router is a device that determines the proper path for data to travel
between different networks, and forwards data packets to the next device along
this path. They connect networks together; a LAN to a WAN for example, to access
the Internet. Some units, like the Cisco 1800 (pictured), are available in both
wired and wireless models.
A more precise definition of a router is a computer networking device that
interconnects separate logical subnets. Routers are now available in many types,
though all are fundamentally doing the same job. A router is a computer whose
software and hardware are usually tailored to the tasks of routing and
forwarding, generally containing a specialized operating system (e.g. Cisco's
IOS or Juniper Networks JunOS or Extreme Networks XOS), RAM, NVRAM, flash
memory, and one or more processors. High-end routers contain many processors and
specialized Application-specific integrated circuits (ASIC) and do a great deal
of parallel processing. However, with the proper software (such as XORP or
Quagga), even commodity PCs can act as routers.
Routers connect with two or more logical subnets, which do not necessarily map
one-to-one to the physical interfaces of the router.
The term switch or layer 3 switch or network switch often is used
interchangeably with router, but switch is really a marketing term without a
rigorous technical definition (though a switch is commonly understood as a
network hub with switched ports, which might or might not also perform
additional routing functions).
Chassis systems like the Nortel MERS-8600 or ERS-8600 routing switch, allow for
a wide variety of LAN, MAN, METRO, and WAN port technologies or other
connections that are customizable.
Routers operate in two different planes :
* Control Plane, in which the router learns the outgoing interface that is most
appropriate for forwarding specific packets to specific destinations,
* Forwarding Plane, which is responsible for the actual process of sending a
packet received on a logical interface to an outbound logical interface.
Control Plane
Control Plane processing leads to the construction of what is variously called a
routing table 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 Forwarding Information Base (FIB) with destination information. RIBs
are optimized for efficient updating with control mechanisms such as routing
protocols, 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 static routes, and from
exchanging routing protocol 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 state 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
For the pure Internet Protocol (IP) forwarding function, router design tries to
minimize the state information kept on individual packets. Once a packet is
forwarded, the router should retain no more than statistical information about
it. It is the sending and receiving endpoint that keeps information on 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 bridge, or
layer 2 switch. A physical device called a router may also have the capability
to forward based on information at other layers, if it has software that can
make decisions at these other layers.
Types of routers
Routers may provide connectivity inside enterprises, between enterprises and the
Internet, and inside Internet Service Providers (ISP). The largest routers (for
example the Cisco CRS-1 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 (for example the Linksys
BEFSR41).
Routers for Internet connectivity and internal use
Routers intended for ISP and major enterprise connectivity will almost
invariably exchange routing information with the Border Gateway Protocol. RFC
4098 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 (AS).
* 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
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 default-free zone (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 virtual
private networks based on a combination of BGP and Multi-Protocol Label
Switching (MPLS) .
Small and Home Office (SOHO) connectivity
M Residential gateway
Residential gateways (often called routers) are frequently used in homes to
connect to a broadband service, such as IP over cable or DSL. A home router may
allow connectivity to an enterprise via a secure Virtual Private Network.
While functionally similar to routers, residential gateways use network address
translation instead of routing. Instead of connecting local computers to the
remote network directly, a residential gateway must make local computers appear
to be a single computer.
Enterprise Routers
All sizes of routers may be found inside enterprises. While the most powerful
routers tend to be found in ISPs, academic and research facilities, as well as
large businesses, may need large routers.
A three-layer model is in common use, not all of which need be present in
smaller networks .
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 Firewall or VPN 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, core routers 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
The very first device that had fundamentally the same functionality as a router
does today, i.e a packet switch, was the Interface Message Processor (IMP); IMPs
were the devices that made up the ARPANET, the first packet switching 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
International Federation for Information Processing.
These devices were different from most previous packet switches in two ways.
First, they connected dissimilar kinds of networks, such as serial lines and
local area networks. Second, they were connectionless devices, which had no role
in assuring that traffic was delivered reliably, leaving that entirely to the
hosts (although this particular idea had been previously pioneered in the
CYCLADES network).
The idea was explored in more detail, with the intention to produce an actual
prototype system, as part of two contemporaneous programs. One was the initial
DARPA-initiated program, which created the TCP/IP architecture of today. The
other was a program at Xerox PARC to explore new networking technologies, which
produced the PARC Universal Packet system, although due to corporate
intellectual property concerns it received little attention outside Xerox until
years later.
The earliest Xerox routers came into operation sometime after early 1974. The
first true IP router was developed by Virginia Strazisar at BBN, as part of that
DARPA-initiated effort, during 1975-1976. By the end of 1976, three PDP-11-based
routers were in service in the experimental prototype Internet.
The first multiprotocol routers were independently created by staff researchers
at MIT and Stanford in 1981; the Stanford router was done by William Yeager, and
the MIT one by Noel Chiappa; both were also based on PDP-11s.
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 mini-computers 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 IPsec
encryption.
Still, there is substantial use of Linux and Unix machines, running open source
routing code, for routing research and selected other applications. While
Cisco's operating system was independently designed, other major router
operating systems, such as those from Juniper Networks and Extreme Networks, are
extensively modified but still have Unix ancestry.
Other changes also improve reliability, such as redundant control processors
with stateful failover, and using storage having no moving parts for program
loading. As much reliability comes from operational techniques for running
critical routers as it does to the router design itself. It is the best common
practice, for example, to use redundant uninterruptible power supplies for all
critical network elements, with generator backup for the batteries or flywheels
of those power supplies.
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