A router (pronounced /’rautsr / in the USA and Australia, and pronounced /ruitar / in the UK) is a computer whose software and hardware are usually tailored to the tasks of routing and forwarding information. Routers generally contain a specialized operating system (e.g. Cisco’s LOS or Juniper Networks JUNOS and JUNOSe 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. Chassis based systems like the Nortel MERS-8600 or ERS-8600 routing switch, (pictured right) have multiple ASIC’s on every module and allow for a wide variety of LAN, MAN, METRO, and WAN 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 Untangle, Smooth Wall, XORP or Quagga), a standard PC can act as a router.

Routers connect two or more logical subnets, which do not necessarily map one-to-one to the physical interfaces of the router. [T The term layer 3 switch often is used interchangeably with router, but switch 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 [2]:

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

Routers are like intersections whereas switches are like streets. 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 (Fill) 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.

Forwarding Plane (a.k.a. Data Plane)

For the pine 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 no longer retain statistical information about it. It. is the sending and receiving end points that keep information about such things as error or miss.


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