The Role of Routing in Computer Networks
The primary business drivers for routing are to provide ubiquitous, secure, and ready application availability and web access for employees. This must be done with a total cost of ownership (TCO) that is affordable and justifiable in light of the company's objectives and initiatives. When properly used as part of a network foundation, routers can help optimize application availability, improve employee productivity, boost customer responsiveness and loyalty, and spur company competitiveness and time to market. Routers also can help overcome the limitations of geography between small-medium businesses and their customers, partners, and suppliers. They can mitigate business conduct restraints that are tied to physical office locations.
Routers carry out two basic functions—they select a path between networks, and they securely transmit information packets across that path toward an intended destination. In so doing, they draw on routing protocols and algorithms. These algorithms are designed to plot routes using such criteria as throughput, delay, simplicity, low overhead, reliability/stability, and flexibility. Tables of available routes and their conditions are created so that routers can use the most efficient paths possible for each transmission. This process is similar to maps created by auto clubs that show drivers where roadwork is under way so that they can avoid potential areas of congestion. When a packet is received at a router, the router opens it, looks at the network destination address, and then calculates the next hop in the best, or lowest-cost, route to the destination. A hop is measured by the passage of a packet through a router. For example, if a packet passes through three routers, it uses three hops to reach its destination. Achieving low cost with routing is especially important, because WAN bandwidth is expensive.
Routers can be hardware- or software-based. They are different from Layer 3 switches in that they support multiple protocols besides IP. They can connect different kinds of networks to form internetworks (and therefore are said to be medium-independent). Routers also differ from Layer 2 switches because they enable the building of very large networks, such as the Internet. This is in sharp contrast to the limited scalability of Layer 2 networks using L2 switches. Internetworking is about linking computing devices and workers through a maze of telecommunications lines. This ability to provide communications route diversity across various links is what ensures that business applications sustain an uptime level that is satisfactory to business executives.
To small-medium business end users, routers are merely connectivity devices or intermediaries. The simplest router configuration possible has only two interconnected networks, or interfaces—an Ethernet LAN connection and a WAN connection. With the exception of the IT staff that manages them, most network users communicate through routers rather than with routers. In fact, hackers sometimes try to direct traffic to routers to shut down a business. That is the principle behind denial of service (DoS) attacks. Security precautions must be taken to protect against this. Chapter 5, "Network Security Basics," discusses the appropriate precautions.
As mentioned previously, L2/L3 switches now dominate the LAN. Routers work with switches in a complementary manner to provide WAN access. Switches hand off traffic to routers at the network edge to access the Internet or send information across a WAN. The network edge is the demarcation between the company LAN and the WAN, or between a teleworker's home LAN and the WAN. When employees send e-mails or access the web, they are typically sending their messages and information requests locally using switches and across the Internet via a number of routers.