Router

1. As in software, router is a system level function that directs a call to an application.

2. As in hardware, routers are the central switching offices of the Internet and corporate Intranets and WANs. Routers are bought by everybody - from backbone service providers to local ISPs, from corporations to universities. The main provider of routers in the world is Cisco. It has built its gigantic business on selling routers - from small ones, connecting a simple corporate LAN to the Internet, to corporate enterprise wide networks, to huge ones connecting the largest of the largest backbone service providers. A router is, in the strictest terms, an interface between two networks.

Routers are highly intelligent devices which connect like and unlike LANs (Local Area Networks). They connect to MANs (Metropolitan Area Networks) and WANs (Wide Area Networks), such as X.25, Frame Relay and ATM. Routers are protocol-sensitive, typically supporting multiple protocols. Routers most commonly operate at the bottom 3 layers of the OSI model, using the Physical, Link and Network Layers to provide addressing and switching. Routers also may operate at Layer 4, the Transport Layer, in order to ensure end-to-end reliability of data transfer.

Routers are much more capable devices than are bridges, which operate primarily at Layer 1, and switches, which operate primarily at Layer 2. Routers send their traffic based on a high level of intelligence inside themselves. This intelligence allows them to consider the network as a whole. How they route (also called routing considerations) might include destination address, packet priority level, least-cost route, minimum route delay, minimum route distance, route congestion level, and community of interest. Routers are unique in their ability to consider an enterprise network as comprising multiple physical and logical subnets (subnetworks). Thereby, they are quite capable of confining data traffic within a subnet, on the basis of privilege as defined in a policy-based routing table. In a traditional router topology, each router port defines a physical subnet, and each subnet is a broadcast domain. Within that domain, all connected devices share broadcast traffic; devices outside of that domain can neither see that traffic, nor can they respond to it. Contemporary routers have the ability to define subnets on a logical basis, based on logical address (e.g., MAC or IP address) information contained within the packet header, and acted upon through consultation with a programmed routing table. In addition to standalone routers developed specifically for that purpose, server-based routers can be implemented. Such routers are in the form of high-performance PCs with routing software. As software will perform less effectively and efficiently than firmware, such devices generally are considered to be less than desirable for large enterprise-wide application, although they do serve well in support of smaller remote offices and less-intensive applications. Routers also are self-learning, as they can communicate their existence and can learn of the existence of new routers, nodes and LAN segments. Routers constantly monitor the condition of the network, as a whole, in order to dynamically adapt to changes in network conditions.

Characteristics of routers can include: LAN extension, store and forward, support for multiple media, support for multiple LAN segments, support for disparate LAN protocols, filtering, encapsulation, accommodation of various and large packet sizes, high-speed internal buses (1+Gbps), self-learning, routing based on multiple factors, route length, number of hops, route congestion, traffic type, support for a community of interest (VLAN), redundancy, and network management via SNMP (Simple Network Management Protocol).

Router protocols include both bridging and routing protocols, as they perform both functions. These protocols fall into 3 categories:

1. Gateway protocols establish router-to-router connections between like routers. The gateway protocol passes routing information and keep alive packets during periods of idleness.

2. Serial Line Protocols provide for communications over serial or dial-up links connecting unlike routers. Examples include HDLC (High-level Data Link Control), SLIP (Serial Line Interface Protocol) and PPP (Point-to-Point Protocol).

3. Protocol Stack Routing and Bridging Protocols advise the router as to which packets should be routed and which should be bridged.