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| 3.1 |  | Routing |
| 3.1.1 |  | Routing fundamentals |

One of the primary jobs of a router is to determine the best path to a given destination. A router learns paths, or routes, from the static configuration entered by an administrator or dynamically from other routers, through routing protocols. Routers keep a routing table in RAM. A routing table is a list of the best known available routes. Routers use this table to make decisions about how to forward a packet. Issue the

show ip route command to view the TCP/IP routing table.

A routing table maps network prefixes to an outbound interface. When RTA receives a packet destined for 192.168.4.46, it looks for the prefix 192.168.4.0/24 in the routing table. RTA then forwards the packet out an interface, such as Ethernet0, based on the routing table entry. If RTA receives a packet destined for 10.3.21.5, it sends that packet out Serial0 (S0).

The first few lines in Figure list the possible codes that designate how the router learned the route. This table shows four routes for directly connected networks. They are labeled with a C in the routing table. RTA drops any packet destined for a network that is not listed in the routing table. To forward to other destinations, the routing table for RTA will need to include more routes. New routes can be added by using one of the following two methods:

- Static routing – An administrator manually defines routes to one or more destination networks.
- Dynamic routing – Routers follow rules defined by routing protocols to exchange routing information and independently select the best path.

Administratively defined routes are referred to as static routes because they do not change until a network administrator manually programs the changes. Routes learned from other routers are dynamic because they change automatically as neighboring routers update each other with new information. Each method has fundamental advantages and disadvantages.

The following sections describe how to configure both static and dynamic routing on a Cisco router.

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