Routing is an OSI Layer 3 function. Routing is a hierarchical organizational scheme that allows individual addresses to be grouped together. These individual addresses are treated as a single unit until the destination address is needed for final delivery of the data. Routing is the process of finding the most efficient path from one device to another. The primary device that performs the routing process is the router.

The following are the two key functions of a router:

• Routers must maintain routing tables and make sure other routers know of changes in the network topology. This function is performed using a routing protocol to communicate network information with other routers.
• When packets arrive at an interface, the router must use the routing table to determine where to send them. The router switches the packets to the appropriate interface, adds the necessary framing information for the interface, and then transmits the frame.

A router is a network layer device that uses one or more routing metrics to determine the optimal path along which network traffic should be forwarded. Routing metrics are values used in determining the advantage of one route over another. Routing protocols use various combinations of metrics for determining the best path for data.

Routers interconnect network segments or entire networks. Routers pass data frames between networks based on Layer 3 information. Routers make logical decisions regarding the best path for the delivery of data. Routers then direct packets to the appropriate output port to be encapsulated for transmission. The encapsulation and de-encapsulation process occurs each time a packet transfers through a router. As shown in Figure 4, the process of sending data from one device to another involves the process of encapsulation and de-encapsulation. This process breaks up the data stream into segments, adds the appropriate headers and trailers then transmits the data. The de-encapsulation process is the opposite process, removing the headers and trailers, then recombining the data into a seamless stream.

This course focuses on the most common routable protocol, which is the Internet Protocol (IP). Other examples of routable protocols include IPX/SPX and AppleTalk. These protocols provide Layer 3 support. Non-routable protocols do not provide Layer 3 support. The most common non-routable protocol is NetBEUI. NetBEUI is a small, fast, and efficient protocol that is limited to frame delivery within one segment.

Web Links Routing Basics http://www.cisco.com/univercd/cc/ td/doc/cisintwk/ ito_doc/routing.htm