This section examines how routers work and introduces routing tables and routing protocols. Routers work at the OSI model network layer. The main functions of a router are first to determine the best path that each packet should take to get to its destination and second to send the packet on its way. Sending the packet out the appropriate interface, along the best path, is also called switching the packet because the packet is encapsulated in a new frame, with the appropriate framing information.
Therefore, a router's job is much like that of a worker at a post office. The postal worker looks at the address label on the letter (the network layer address on the packet), determines which way the letter (the packet) should be sent, and then sends it. The comparison between the post office and a router is illustrated in Figure 1-11.
Figure 1-11 A Router Behaves Much Like a Worker at a Post Office
A letter arrives at the post office.
The postal worker looks at the "To:" address.
The postal worker determines where the letter should go next, on its way to the final destination. For example:
Local: put on truck.
Cross-country: put on plane to next city.
A packet arrives at the router's interface.
The router looks at the "destination" address.
The router determines where the packet should go next, on its way to the final destination. For example:
On attached LAN: put on LAN for recipient.
On distant network: send across WAN to next router.
Deliver to final recipient.
NOTE This discussion of routers is concerned with the traditional role of routers in a network, at the OSI model network layer. Routers are now taking on more functions—for example, in QoS and security areas; these other functions are described in the relevant chapters throughout this book.
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