thumb|right|Cisco 1800 Router 180px|thumb|right|Nortel ERS 8600 thumb|right|Cisco 7600 Routers A router ( in the USA and Australia, in Canada, the UK, and Ireland, these last two making a pronounced distinction with the tool used to rout wood) is a networking device whose software and hardware are usually tailored to the tasks of routing and forwarding information. For example, on the Internet, information is directed to various paths by routers.
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thumb|right|Cisco 1800 Router 180px|thumb|right|Nortel ERS 8600 thumb|right|Cisco 7600 Routers A router ( in the USA and Australia, in Canada, the UK, and Ireland, these last two making a pronounced distinction with the tool used to rout wood) is a networking device whose software and hardware are usually tailored to the tasks of routing and forwarding information. For example, on the Internet, information is directed to various paths by routers.
Routers connect two or more logical subnets, which do not necessarily map one-to-one to the physical interfaces of the router. The term "layer 3 switch" often is used interchangeably with router, but switch is 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. In comparison, a network hub does not do any routing, instead every packet it receives on one network line gets forwarded to all the other network lines.
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
Main: 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 (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" delay,distance,queue length 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.
