A network layer device.
A router is a networking device that forwards data packets between computer networks. Routers perform the traffic directing functions on the Internet. Data sent through the internet, such as a web page or email, is in the form of data packets. A packet is typically forwarded from one router to another router through the networks that constitute an internetwork until it reaches its destination node.
A router is connected to two or more data lines from different networks. When a data packet comes in on one of the lines, the router reads the network address information in the packet to determine the ultimate destination. Then, using information in its routing table or routing policy, it directs the packet to the next network on its journey.
Router builds up a routing table listing the preferred routes between any two systems on the interconnected networks.
A router has two types of network element components organized onto separate planes:
Control plane: A router maintains a routing table that lists which route should be used to forward a data packet, and through which physical interface connection. It does this using internal preconfigured directives, called static routes, or by learning routes dynamically using a routing protocol. Static and dynamic routes are stored in the routing table. The control-plane logic then strips non-essential directives from the table and builds a forwarding information base (FIB) to be used by the forwarding plane.
Forwarding plane: The router forwards data packets between incoming and outgoing interface connections. It forwards them to the correct network type using information that the packet header contains matched to entries in the FIB supplied by the control plane.
A router may have interfaces for different types of physical layer connections, such as copper cables, fiber optic, or wireless transmission. It can also support different network layer transmission standards. Each network interface is used to enable data packets to be forwarded from one transmission system to another. Routers may also be used to connect two or more logical groups of computer devices known as subnets, each with a different network prefix.
Routers are also often distinguished on the basis of the network in which they operate. A router in a local area network (LAN) of a single organisation is called an interior router. A router that is operated in the Internet backbone is described as exterior router. While a router that connects a LAN with the Internet or a wide area network (WAN) is called a border router, or gateway router.
Routers intended for ISP and major enterprise connectivity usually exchange routing information using the Border Gateway Protocol (BGP). RFC 4098 standard defines the types of BGP routers according to their functions:
Also called a provider edge router, is placed at the edge of an ISP network. The router uses External BGP to EBGP routers in other ISPs, or a large enterprise Autonomous System.
Subscriber edge router:
Also called a Customer Edge router, is located at the edge of the subscriber’s network, it also uses EBGP to its provider’s Autonomous System. It is typically used in an (enterprise) organization.
Inter-provider border router:
Interconnecting ISPs, is a BGP router that maintains BGP sessions with other BGP routers in ISP Autonomous Systems.
A core router resides within an Autonomous System as a back bone to carry traffic between edge routers.
Within an ISP:
In the ISP’s Autonomous System, a router uses internal BGP to communicate with other ISP edge routers, other intranet core routers, or the ISP’s intranet provider border routers.
The Internet no longer has a clearly identifiable backbone, unlike its predecessor networks. See default-free zone (DFZ). The major ISPs’ system routers make up what could be considered to be the current Internet backbone core. ISPs operate all four types of the BGP routers described here. An ISP “core” router is 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 protocols.
Routers are also used for port forwarding between private Internet-connected servers.
Voice/Data/Fax/Video Processing Routers:
Commonly referred to as access servers or gateways, these devices are used to route and process voice, data, video and fax traffic on the Internet. Since 2005, most long-distance phone calls have been processed as IP traffic (VOIP) through a voice gateway. Use of access server type routers expanded with the advent of the Internet, first with dial-up access and another resurgence with voice phone service.