A router is a device working at the third layer (network layer) of the OSI (Open System Interconnect) reference model, its main function is to makerouting decision among routes for data packets.

It is a device that forwards data packets along networks. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP’s network. Routers are located at gateways, the places where two or more networks connect.

It is an internetworking device that connects two computer networks. It makes use of an internet protocol and assumes that all of the attached devices on the networks use the same communications architecture and protocols.

On the Internet, a router is a device or, in some cases, software in a computer, that determines the next network point to which a packet should be forwarded toward its destination. The router is connected to at least two networks and decides which way to send each information packet based on its current understanding of the state of the networks it is connected to. A router is located at any gateway (where one network meets another), including each Internet point-of-presence. A router is often included as part of a network switch.

Routers use headers and forwarding tables to determine the best path for forwarding the packets, and they use protocols such as ICMP to communicate with each other and configure the best route between any two hosts.

Very little filtering of data is done through routers.

An edge router is a router that interfaces with an asynchronous transfer mode (ATM) network. A brouter is a network bridge combined with a router.

End systems are not usually directly attached to each other via a single communication link. Instead, they are indirectly connected to each other through intermediate switching devices known as routers. A router takes a chunk of information arriving on one of its incoming communication links and forwards that chunk of information on one of its outgoing communication links. In the jargon of computer networking, the chunk of information is called a packet. The path that the packet takes from the sending end system, through a series of communication links and routers, to the receiving end system is known as a route or path through the net work. Rather than provide a dedicated path between communicating end systems, the Internet uses a technique known as packet switching that allows multiple communicating end systems to share a path, or parts of a path, at the same time.

What’s inside a Router ?

The switching function of a router – the actual transfer of datagrams from a router’s incoming links to the appropriate outgoing links. In the network layer, the real work (that is, the reason the network layer exists in the first place) is the forwarding of datagrams. A key component of this forwarding process is the transfer of a datagram from a router’s incoming link to an outgoing link. A high-level view of a generic router architecture is shown. Four components of a router can be identified.

Input ports. The input port performs several functions. It performs the physical layer functionality of terminating an incoming physical link to a router. It performs the data link layer functionality needed to interoperate with the data link layer functionality needed to interoperate with the data link layer functionality on the other side of the incoming link. It also performs a lookup and forwarding function so that a packet forwarded into the switching fabric of the router emerges at the appropriate output port. Control packets are forwarded from the input port to the routing processor. In practice, multiple ports are often gathered together on a single line card within a router.

Switching fabric. The switching fabric connects the router’s input ports to its out-put ports. This switching fabric is completely contained within the router – a net-work inside of a network router!

Output ports. An output port stores the packets that have been forwarded to it through the switching fabric and then transmits the packets on the outgoing link. The output port thus performs the reverse data link and physical layer functionality as the input port. When a link is bi-directional, an output port to the link will typically be paired with the input port for that link, on the same line card.

Routing processor. The routing processor executes the routing protocols, maintains the routing information and forwarding tables, and performs network management functions within the router.

Working of routers

A router may create or maintain a table of the available routes and their conditions and use this information along with distance and cost algorithms to determine the best route for a given packet. Typically, a packet may travel through a number of network points with routers before arriving at its destination. Routing is a function associated with the Network layer (layer 3) in the standard model of network programming, the Open Systems Interconnection (OSI) model. A layer-3 switch is a switch that can perform routing functions.

Routers operate at the Network Layer rather than the Data Link Layer of the OSI model. See the Router and OSI diagram below. Routers interconnect networks that have the same communications architecture but possibly different lower level architectures. In other words, routers are protocol dependent. A router can usually be used instead of a bridge.

Previously, routers have been slower and more expensive than bridges; now, however, both price and performance of routers is approaching that of bridges. Note that, in order to be "routable," architectures must have a Network Layer. Not all do. The classic example is the DEC LAT protocol; it must be bridged.

Figure. Router and OSI

Routers can be used instead of bridges to erect a firewall between parts of the network. The protocols used for routing by the Network Layers of the various communication architectures do not involve the sending of broadcast packets. By using routers, as shown here, broadcast messages are kept behind a "firewall." When addressing and other problems occur, they are experienced by only a small group of users and are much easier to localize and resolve.

Fig. 109

Figure . Routers and Firewalls.

Routers operate at the Network Layer of the communications architecture and are transparent to the upper layers. Routers interconnect networks that have the same communications architectures. A multiprotocol router handles multiple communications architectures but does not convert protocols. Two half-routers or WAN routers connect two networks transparently across a point-to-point link. Routers use routing tables to route packets. TCP/IP routers use the EGP protocol to communicate routing information to core gateways on the Internet backbone.

Routing

In internetworking, the process of moving a packet of data from source to destination. Routing is usually performed by a dedicated device called a router. Routing is a key feature of the Internet because it enables messages to pass from one computer to another and eventually reach the target machine. Each intermediary computer performs routing by passing along the message to the next computer. Part of this process involves analyzing a routing table to determine the best path.

Routing is often confused with bridging, which performs a similar function. The principal difference between the two is that bridging occurs at a lower level and is therefore more of a hardware function whereas routing occurs at a higher level where the software component is more important. And because routing occurs at a higher level, it can perform more complex analysis to determine the optimal path for the packet.

The routers in an internet are responsible for receiving and forwarding packets through the interconnected set of networks. Each router makes routing decision based on knowledge of the topology and conditions of the internet. In a simple internet, a fixed routing scheme is possible. In more complex internets, a degree of dynamic cooperation is needed among the routers. In particular, the router must avoid portions of the network that are congested. To make such dynamic routing decisions, routers exchange routing information using a special routing protocol for that purpose. Information is needed about the status of the internet, in terms of which networks can be reached by which routes, and the delay characteristics of various routes.