Routers
Routers
Routers are an increasingly common sight in any network environment, from a small home office that uses one to connect to an Internet service provider (ISP) to a corporate IT environment where racks of routers manage data communication with disparate remote sites. Routers make internetworking possible, and in view of this, they warrant detailed attention.
Routers are network devices that literally route data around the network. By examining data as it arrives, the router can determine the destination address for the data; then, by using tables of defined routes, the router determines the best way for the data to continue its journey. Unlike bridges and switches, which use the hardware-configured MAC address to determine the destination of the data, routers use the software-configured network address to make decisions. This approach makes routers more functional than bridges or switches, and it also makes them more complex because they have to work harder to determine the information.
Function of Routers
The basic requirement for a router is that it must have at least two network interfaces. If they are LAN interfaces, the router can manage and route the information between two LAN segments. More commonly, a router is used to provide connectivity across wide area network (WAN) links.
A router derives can route data it receives from one network onto another. When a router receives a packet of data, it reads the header of the packet to determine the destination address. Once it has determined the address, it looks in its routing table to determine whether it knows how to reach the destination and, if it does, it forwards the packet to the next hop on the route. The next hop might be the final destination, or it might be another router.
A routing tables play a very important role in the routing process. They are the means by which the router makes its decisions. For this reason, a routing table needs to be two things. It must be up-to-date, and it must be complete.
There are two ways that the router can get the information for the routing table —
1. Static routing
2. Dynamic routing
Static Routing
In environments that use static routing, routes and route information are entered into the routing tables manually. Not only can this be a time-consuming task, but also errors are more common. Additionally, when there is a change in the layout, or topology, of the network, statically configured routers must be manually updated with the changes. Again, this is a time consuming and potentially error-laden task. For these reasons, static routing is suited to only the smallest environments with perhaps just one or two routers. A far more practical solution, particularly in larger environments, is to use dynamic routing.
Dynamic Routing
In a dynamic routing environment, routers use special routing protocols to communicate. The purpose of these protocols is simple; they enable routers to pass on information about themselves to other routers so that other routers can build routing tables. There are two types of routing protocols used—the older distance vector protocols and the newer link state protocols.