What Is a Router?

   A router is an electronic device that interconnects two or more computer networks, and selectively interchanges packets of data between them. Each data packet contains address information that a router can use to determine if the source and destination are on the same network, or if the data packet must be transferred from one network to another. Where multiple routers are used in a large collection of interconnected networks, the routers exchange information about target system addresses, so that each router can build up a table showing the preferred paths between any two systems on the interconnected networks.

A router is a networking device whose software and hardware are customized to the tasks of routing and forwarding information. A router has two or more network interfaces, which may be to different physical types of network (such as copper cables, fiber, or wireless) or different network standards. Each network interface is a specialized device that converts electric signals from one form to another.

Routers connect two or more logical subnets, which do not share a common network address. The subnets in the router do not necessarily map one-to-one to the physical interfaces of the router. The term "layer 3 switching" is used often interchangeably with the term "routing". The term switching is generally used to refer to data forwarding between two network devices that share a common network address. This is also called layer 2 switching or LAN switching.

Conceptually, a router operates in two operational planes (or sub-systems):

• Control plane: where a router builds a table (called routing table) as how a packet should be forwarded through which interface, by using either statically configured statements (called static routes) or by exchanging information with other routers in the network through a dynamical routing protocol;
• Forwarding plane: where the router actually forwards traffic (called packets in IP) from ingress (incoming) interfaces to an egress (outgoing) interface that is appropriate for the destination address that the packet carries with it, by following rules derived from the routing table that has been built in the control plane.

Types of routers

Routers may provide connectivity inside enterprises, between enterprises and the Internet, and inside internet service providers (ISPs). 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.

Routers for Internet connectivity and internal use

Routers intended for ISP and major enterprise connectivity almost invariably exchange routing information using the Border Gateway Protocol (BGP). RFC 4098 defines several types of BGP-speaking routers according to the routers' functions:

• Edge router (ER): An ER is placed at the edge of an ISP network. The router speaks external BGP (EBGP) to a BGP speaker in another provider or large enterprise Autonomous System(AS). This type of router is also called PE (Provider Edge) routers.
• Subscriber edge router (SER): An SER is 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. This type of router is also called CE (Customer Edge) routers.
• 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 core router is one that resides within an AS as back bone to carry traffic between edge routers.

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, the major ISPs' routers 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).

Routers are also used for port forwarding for private servers.