Routers are highly intelligent network devices that can support connectivity between both like and disparate LANs and can provide access to various WANs, such as Frame Relay, IP and ISDN. (Figure 12)
Routers typically operate at the bottom three layers of the OSI model using the Physical Layer, Data Link Layer, and Network Layer to provide connectivity, addressing, and switching. Routers also have the capability to operate at all seven layers of the OSI Reference Model, if so equipped.
In addition to supporting filtering and encapsulation, routers route traffic based on a high level of intelligence that enables them to consider the network as a whole. This is in stark contrast to bridges, hubs, and switches, which view the network simply on a link-by-link basis. (Note: Layer 3 switches cross the line into the routing domain, although at a relatively simple level.) Routing considerations might include destination address, payload type, packet priority level, least-cost route, minimum route delay, minimum route distance, and route congestion level. Routers also are self-learning, as they can communicate their existence to other devices and can learn of the existence of new routers, nodes, and LAN segments. Routers constantly monitor the condition of the network as a whole, thereby dynamically adapting to changes in the condition of the network from edge to edge. Routers are multiport devices with high-speed ports running at rates up to 155 Mbps or more and with high-speed internal buses that can be on the order of 1 Gbps in the aggregate. Additionally, routers typically provide some level of redundancy so they are less susceptible to catastrophic failure.
Routers are unique in their ability to route data based on programmable network policy. Policy-based routers can provide various levels of service based on factors such as the identification of the user, the terminal and the type of payload. From one edge of the network to the other, an edge router can select the most appropriate path through the various switches or routers positioned in the core. An important part of this process often is that of dividing the network into multiple subnets . Users associated with a subnet may be afforded access to only a limited subset of network resources in the form of sites, links, hosts, files, databases, and applications. In addition to being limited in terms of access to such a resource, users of another subnet may be prevented from receiving data from it. In effect, even the very existence of those resources is masked from view. Creation of such isolated subnets may serve for reasons of security or simply as a means of avoiding unnecessary congestion.
Routers are available that sense a network failure and reestablish the connection via an alternate means, which may include a totally different network and service.
|Figure 12: Cisco Router|