Content Overview Routing is nothing more than directions for getting from one network to another. These directions, also known as routes, can be dynamically given to the router by another router, or they can be statically assigned to the router by an administrator. This module introduces the concept of dynamic routing protocols, describes the classes of dynamic routing protocol, and gives examples of protocols in each class. A network administrator chooses a dynamic routing protocol based upon many considerations. The size of the network, the bandwidth of available links, the processing power of the network routers, the brands and models of routers on the network, and the protocols already in use on the network are all factors that must be considered in choosing a routing protocol. This module will provide more details about the differences between routing protocols that help network administrators make a choice. Students completing this module should be able to:
Content 6.1 Introduction to Static Routing 6.1.1 Introducing routing Routing is the process that a router uses to forward packets toward the destination network. A router makes decisions based upon the destination IP address of a packet. All devices along the way use the destination IP address to point the packet in the correct direction so that the packet eventually arrives at its destination. In order to make the correct decisions, routers must learn the direction to remote networks. When routers use dynamic routing, this information is learned from other routers. When static routing is used, a network administrator configures information about remote networks manually.Because static routes must be configured manually, any network topology changes require the network administrator to add and delete static routes to account for the changes. In a large network this manual maintenance of routing tables could require a tremendous amount of administrative time. On small networks with few possible changes, static routes require very little maintenance. Because of the extra administrative requirements, static routing does not have the scalability of dynamic routing. Even in large networks, static routes that are intended to accomplish a specific purpose are often configured in conjunction with a dynamic routing protocol. Web Links Introduction to Routing http://www.cisco.com/networkers/ nw99_pres/301.pdf
Content 6.1 Introduction to Static Routing 6.1.2 Static route operation Static route operations can be divided into these three parts: Since a static route is manually configured, the administrator must configure the static route on the router using the ip route command. The correct syntax for the ip route command is shown in Figure . In Figures and , the network administrator of the Hoboken router needs to configure a static route pointing to the 172.16.1.0/24 and 172.16.5.0/24 networks on the other routers. The administrator could enter either of two commands to accomplish this objective. The method in Figure specifies the outgoing interface. The method in Figure specifies the next-hop IP address of the adjacent router. Either of the commands will install a static route in the routing table of Hoboken. The only difference between the two is in the administrative distance assigned to the route by the router as it is placed in the routing table. The administrative distance is an optional parameter that gives a measure of the reliability of the route. A lower value for the administrative distance indicates the more reliable route. Thus, a route with a lower administrative distance will be installed before an identical route with a higher administrative distance. The default administrative distance when using next-hop address is 1, while the default administrative distance when using the outgoing interface is 0. If an administrative distance other than the default is desired, a value between 0 and 255 is entered after the next-hop or outgoing interface as follows: waycross(config)#ip route 172.16.3.0 255.255.255.0 172.16.4.1 130 If the router cannot reach the outgoing interface that is being used in the route, the route will not be installed in the routing table. This means if that interface is down, the route will not be placed in the routing table. Sometimes static routes are used for backup purposes. A static route can be configured on a router that will only be used when the dynamically learned route has failed. To use a static route in this manner, simply set the administrative distance higher than that of the dynamic routing protocol being used. Lab Activity e-Lab Activity: Static Route Operation In this lab, the student will learn how to create a static route. Lab Activity e-Lab Activity: Static Routes In this lab, the students will practice using static routes by troubleshooting a network connected in the fashion outlined in the topology map.
Content 6.1 Introduction to Static Routing 6.1.3 Configuring static routes This section lists the steps for configuring static routes and gives an example of a simple network for which static routes might be configured.Use the following steps to configure static routes:
  1. Determine all desired destination networks, their subnet masks, and their gateways. A gateway can be either a local interface or a next hop address that leads to the desired destination.
  2. Enter global configuration mode.
  3. Type the ip route command with a destination address and subnet mask followed by their corresponding gateway from Step one. Including an administrative distance is optional.
  4. Repeat Step three for as many destination networks as were defined in Step one.
  5. Exit global configuration mode.
  6. Save the active configuration to NVRAM by using the copy running-config startup-config command.
The example network is a simple three-router configuration. Hoboken must be configured so that it can reach the 172.16.1.0 network and the 172.16.5.0 network. Both of these networks have a subnet mask of 255.255.255.0. Packets that have a destination network of 172.16.1.0 need to be routed to Sterling and packets that have a destination address of 172.16.5.0 need to be routed to Waycross. Static routes can be configured to accomplish this task. Both static routes will first be configured to use a local interface as the gateway to the destination networks. Since the administrative distance was not specified, it will default to 0 when the route is installed in the routing table. Note that an administrative distance of 0 is the same as a directly connected network. The same two static routes can also be configured using a next-hop address as their gateway. The first route to the 172.16.1.0 network has a gateway of 172.16.2.1. The second route to the 172.16.5.0 network has a gateway of 172.16.4.2. Since the administrative distance was not specified, it defaults to 1. Lab Activity e-Lab Activity: Configuring Static Routes In this lab, the student will learn to configure some static routes. Web Links ip route Command http://www.cisco.com/en/US/products/sw/ iosswrel/ps1835/products_command_reference_ chapter09186a00800ca75a.html#1018067
Content 6.1 Introduction to Static Routing