Basic Router Concepts

What is a Router?

A router is a device that forwards traffic between networks based on network layer information in the data and on routing tables aintained by the router. In these routing tables, a router builds up a logical picture of the overall network by gathering and exchanging information with other routers in the network. Using this information, the router chooses the best path for forwarding network traffic. Routers vary in performance and scale, number of routing protocols supported, and types ofphysical WAN connection they support.

Routing Information Protocol

One of the protocols used by a router to build and maintain a picture of the network is the Routing Information Protocol (RIP). Using RIP, routers periodically update one another and check for changes to add to the routing table. RIP-2 supports subnet and multicast protocols. RIP is not required for most home applications. 

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Understanding Static and Default Routes

Static routes are useful in stub networks in which we want to control the routing behavior by manually configuring destination networks into the routing table:

Router(config)#ip route 10.0.0.0 255.0.0.0 192.168.2.5

A floating static route can be configured when redundant connections exist and you want to use the redundant link if the primary fails. This is configured by adding a higher administrative distance at the end of a static route:

Router(config)#ip route 10.0.0.0 255.0.0.0 192.168.2.9 2

A default route is a gateway of last resort for a router when there isn’t a specific match for an IP destination network in the routing table (such as packets destined for the Internet):

Router(config)#ip route 0.0.0.0 0.0.0.0 serial 0/0

With routing protocols, you can specify a default network, which is a network in the routing table that routing devices consider to be the gateway of last resort. Using their routing protocols, they determine the best path to the default network:

Router(config)#ip default-network 192.168.1.0

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Static Routes

Static routes are commonly used when you are routing from a network to a stub network. A stub network (sometimes called a leaf node) is a network accessed by a single route. Static routes can also be useful for specifying a “gateway of last resort” to which all packets with an unknown destination address are sent. Following is the syntax for configuring a static route:

RouterX(config)# ip route network [mask] {address | interface}[distance] [permanent]

Summary of Static Routing

Routing is the process by which items get from one location to another. In networking, a router is the device used to route traffic. Routers can forward packets over static routes or dynamic routes based on the router configuration.

■ Static routers use a route that a network administrator enters into the router manually. Dynamic routes use a router that a network routing protocol adjusts automatically for topology or traffic changes.

■ Unidirectional static routes must be configured to and from a stub network to allow communications to occur.

■ The ip route command can be used to configure default route forwarding.

■ The show ip route command verifies that static routing is properly configured. Static routes are signified in the command output by “S.”

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Static and Dynamic Route Comparison

Routers can forward packets over static routes or dynamic routes based on the router configuration. The two ways to tell the router where to forward packets to destination networks that are not directly connected are as follows:

■ Static route: The router learns routes when an administrator manually configures the static route. The administrator must manually update this static route entry whenever an internetwork topology change requires an update. Static routes are user-defined routes that specify the path that packets take when moving between a source and a destination. These administrator-defined routes allow very precise control over the

routing behavior of the IP internetwork.

■ Dynamic route: The router dynamically learns routes after an administrator configures a routing protocol that helps determine routes. Unlike the situation with static routes, after the network administrator enables dynamic routing, the routing process automatically updates route knowledge whenever new topology information is received. The router learns and maintains routes to the remote destinations by exchanging routing updates with other routers in the internetwork.

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