PC and Industrial Networks

Basic IOS Configuration There are several methods available for configuring Cisco routers.

• over the network from a TFTP server. • through the menu interface provided at bootup. • from the menu interface provided by using the command setup. • from the IOS command-line interface.

The main reason for using the command-line interface instead of a menu driven interface is speed. Once you have invested the time to learn the command-line commands, you can perform many operations much more quickly than by using a menu. This is basically true of all command-line vs. menu interfaces. What makes it especially efficient to learn the command-line interface of the Cisco IOS is that it is standard across all Cisco routers.

Modes

The Cisco IOS command-line interface is organized around the idea of modes. You move in and out of several different modes while configuring a router, and which mode you are in determines what commands you can use. Each mode has a set of commands available in that mode, and some of these commands are only available in that mode. In any mode, typing a question mark will display a list of the commands available in that mode. Router> ?

Unprivileged and privileged modes

When you first connect to the router and provide the password, you enter EXEC mode, the first mode in which you can issue commands from the command-line. From here you can use such unprivileged commands as ping, and telnet. You can also use some of the show commands to obtain information about the system. In unprivileged mode you use commands like, show version to display the version of the IOS the router is running. Typing show ? will diplay all the show commands available in the mode you are presently in. Router> show ? You must enter privileged mode to configure the router. You do this by using the command enable. Privileged mode will usually be password protected. You have the option of not password protecting privileged mode, but it is HIGHLY recommended that you do. When you issue the command enable and provide the password, you will enter privileged mode. To help the user keep track of what mode they are in, the command-line prompt changes each time you enter a different mode. When you switch from unprivileged mode to privileged mode, the prompt changes from:

Router> to Router# Within privileged mode there are many sub-modes. Once you enter privileged mode the prompt ends with a pound sign (#). There are numerous modes you can enter only after entering privileged mode. Each of these modes has a prompt of the form: Router(arguments)# They still all end with the pound sign. Once you enter privileged mode, you have access to all the configuration information and options the IOS provides.

Configuring

You should familiarize yourself with the show commands before beginning to configure the router. Enter privileged mode by issuing the command enable, then issue several show commands to see what they display. Remember, the command show ? will display all the show commands available in the current mode. Try out the following commands: Router# show interfaces Router# show ip protocols Router# show ip route Router# show ip arp When you enter privileged mode by using the command enable, you are in the top-level mode of privileged mode. It is in this top-level mode that you can display most of the information about the router. You do this with the show commands. You can learn the configuration of interfaces and whether they are up or down. You can display what IP protocols are in use, such as dynamic routing protocols. You can view the route and ARP tables, and these are just a few of the more important options. As you configure the router, you will enter various sub-modes to set options, then return to the parent mode to display the results of your commands. You also return to the parent mode to enter other sub-modes. To return to the parent mode, you hit ctrl-z. This puts any commands you have just issued into effect, and returns you to parent mode.

Global configuration (config)

To configure any feature of the router, you must enter configuration mode. This is the first sub-mode of the parent mode. In the parent mode, you issue the command config. Router# config terminal Router(config)#

In configuration mode you can set options that apply system-wide, also referred to as "global configurations." For instance, it is a good idea to name your router so that you can easily identify it. You do this in configuration mode with the hostname command. Router(config)#hostname ExampleName ExampleName(config)# As demonstrated above, when you set the name of the host with the hostname command, the prompt immediately changes by replacing Router with ExampleName. (Note: It is a good idea to name your routers with an organized naming scheme.) Another useful command issued from config mode is the command to designate the DNS server to be used by the router: ExampleName(config)#ip name-server aa.bb.cc.dd ExampleName(config)#ctrl-Z ExampleName# This is also where you set the password for privileged mode. ExampleName(config)#enable secret examplepassword ExampleName(config)#ctrl-Z ExampleName# Until you hit ctrl-Z (or type exit until you reach parent mode) your command has not been put into affect. You can enter config mode, issue several different commands, then hit ctrl-Z to activate them all. Each time you hit ctrl-Z you return to parent mode and the prompt: ExampleName# Here you use show commands to verify the results of the commands you issued in config mode. To verify the results of the ip name-server command, issue the command show host.

Configuring interfaces

Cisco interface naming is straightforward. Individual interfaces are referred to by this convention: media type slot#/port# "Media type" refers to the type of media that the port is an interface for, such as Ethernet, Token Ring, FDDI, serial, etc. Slot numbers are only applicable for routers that provide slots into which you can install modules. These modules contain several ports for a given media. Port number refers to the port in reference to the other ports in that module. Numbering is left-to-right, and all numbering starts at 0, not at one. Therefore, to display the configuration of an interface that contains ports, you use the command:

ExampleName# show interface serial 0/0 If your router does not have slots, then the interface name consists only of: media type port# For example: ExampleName#show interface serial 0 Here is an example of configuring a serial port with an IP address: ExampleName#config ExampleName(config)#interface serial 1/1 ExampleName(config-if)#ip address 192.168.155.2 255.255.255.0 ExampleName(config-if)#no shutdown ExampleName(config-if)#ctrl-Z ExampleName# Then to verify configuration: ExampleName#show interface serial 1/1 Note the no shutdown command. An interface may be correctly configured and physically connected, yet be "administratively down." In this state it will not function. The command for causing an interface to be administratively down is shutdown. ExampleName(config)#interface serial 1/1 ExampleName(config-if)#shutdown ExampleName(config-if)#ctrl-Z ExampleName#show interface serial 1/1 In the Cisco IOS, the way to reverse or delete the results of any command is to simply put no in front of it. For instance, if we wanted to unassign the IP address we had assigned to interface serial 1/1: ExampleName(config)#interface serail 1/1 ExampleName(config-if)#no ip address 192.168.155.2 255.255.255.0 ExampleName(config-if)ctrl-Z ExampleName#show interface serial 1/1 Configuring most interfaces for LAN connections might consist only of assigning a network layer address and making sure the interface is not administratively shutdown. It is usually not necessary to stipulate data-link layer encapsulation. Note that it is often necessary to stipulate the appropriate data-link layer encapsulation for WAN connections. Serial interfaces default to using HDLC. You will need to look up the IOS command encapsulation for more details.

Saving your configuration

Once you have configured routing on the router, and you have configured individual interfaces, your router should be capable of routing traffic. Give it a few moments to talk to its neighbors, then issue the commands show ip route and show ip arp. There should now be entries in these tables learned from the routing protocol. If you turned the router off right now, and turned it on again, you would have to start configuration over again. Your running configuration is not saved to any permanent storage media. You can see this configuration with the command show running-config. ExampleName#show running-config You do want to save your successful running configuration. Issue the command copy running-config startup-config. ExampleName#copy running-config startup-config Your configuration is now saved to non-volatile RAM (NVRAM). Issue the command show startup-config. ExampleName#show startup-config Now any time you need to return your router to that configuration, issue the command copy startup-config running-config. ExampleName#copy startup-config running-config

3.3 Routing

IP routing is automatically enabled on Cisco routers. If it has been previously disabled on your router, you turn it back on in config mode with the command ip routing.

ExampleName(config)#ip routing ExampleName(config)#ctrl-Z

There are two main ways a router knows where to send packets. The administrator can assign static routes, or the router can learn routes by employing a dynamic routing protocol.

These days static routes are generally used in very simple networks or in particular cases that necessitate their use. To create a static route, the administrator tells the router operating system that any network traffic destined for a specified network layer address should be forwarded to a similiarly specified network layer address. In the Cisco IOS this is done with the ip route command.

ExampleName#config ExampleName(config)#ip route 172.16.0.0 255.255.255.0 192.168.150.1 ExampleName(config)#ctrl-Z ExampleName#show ip route

Two things to be said about this example. First, the packet destination address must include the subnet mask for that destination network. Second, the address it is to be forwarded to is the specified addres of the next router along the path to the destination. This is the most common way of setting up a static route, and the only one this document covers. Be aware, however, that there are other methods.

Dynamic routing protocols, running on connected routers, enable those routers to share routing information. This enables routers to learn the routes available to them. The advantage of this method is that routers are able to adjust to changes in network topologies. If a route is physically removed, or a neighbor router goes down, the routing protocol searches for a new route. Routing protocols can even dynamically choose between possible routes based on variables such as network congestion or network reliability.

There are many different routing protocols, and they all use different variables, known as "metrics," to decide upon appropriate routes. Unfortunately, a router needs to be running the same routing protocols as its neighbors. Many routers can, however, run mutliple protocols. Also, many protocols are designed to be able to pass routing information to other routing protocols. This is called "redistribution." The author has no experience with trying to make redistribution work. There is an IOS redistribute command you can research if you think this is something you need. This document's compagnion case study describes an alternative method to deal with different routing protocols in some circumstances.

Routing protocols are a complex topic and this document contains only this superficial description of them. There is much to learn about them, and there are many sources of information about them available. An excelent source of information on this topic is Cisco's website, http://www.cisco.com.

This document describes how to configure the Routing Information Protocol (RIP) on Cisco routers. From the command-line, we must explicitly tell the router which protocol to use, and what networks the protocol will route for.

ExampleName#config ExampleName(config)#router rip ExampleName(config-router)#network aa.bb.cc.dd ExampleName(config-router)#network ee.ff.gg.hh ExampleName(config-router)#ctrl-Z ExampleName#show ip protocols

Now when you issue the show ip protocols command, you should see an entry describing RIP configuration.

Saving your configuration

Once you have configured routing on the router, and you have configured individual interfaces, your router should be capable of routing traffic. Give it a few moments to talk to its neighbors, then issue the commands show ip route and show ip arp. There should now be entries in these tables learned from the routing protocol.

If you turned the router off right now, and turned it on again, you would have to start configuration over again. Your running configuration is not saved to any permanent storage media. You can see this configuration with the command show running-config.

ExampleName#show running-config

You do want to save your successful running configuration. Issue the command copy running-config startup-config.

ExampleName#copy running-config startup-config

Your configuration is now saved to non-volatile RAM (NVRAM). Issue the command show startup-config.

ExampleName#show startup-config

Now any time you need to return your router to that configuration, issue the command copy startup-config running-config.

ExampleName#copy startup-config running-config

3.5 Example configuration

4. Troubleshooting Inevitably, there will be problems. Usually, it will come in the form of a user notifying you that they can not reach a certain destination, or any destination at all. You will need to be able to check how the router is attempting to route traffic, and you must be able to track down the point of failure.

You are already familiar with the show commands, both specific commands and how to learn what other show commands are available. Some of the most basic, most useful commands you will use for troubleshooting are:

ExampleName#show interfaces ExampleName#show ip protocols ExampleName#show ip route ExampleName#show ip arp

4.1 Testing connectivity

It is very possible that the point of failure is not in your router configuration, or at your router at all. If you examine your router's configuration and operation and everything looks good, the problem might be be farther up the line. In fact, it may be the line itself, or it could be another router, which may or may not be under your administration.

One extremely useful and simple diagnostic tool is the ping command. Ping is an implementation of the IP Message Control Protocol (ICMP). Ping sends an ICMP echo request to a destination IP address. If the destination machine receives the request, it responds with an ICMP echo response. This is a very simple exchange that consists of:

Hello, are you alive?

Yes, I am.

ExampleName#ping xx.xx.xx.xx

If the ping test is successful, you know that the destination you are having difficulty reaching is alive and physically reachable.

If there are routers between your router and the destination you are having difficulty reaching, the problem might be at one of the other routers. Even if you ping a router and it responds, it might have other interfaces that are down, its routing table may be corrupted, or any number of other problems may exist.

To see where packets that leave your router for a particular destination go, and how far, use the trace command.

ExampleName#trace xx.xx.xx.xx

It may take a few minutes for this utility to finish, so give it some time. It will display a list of all the hops it makes on the way to the destination.

4.2 debug commands

There are several debug commands provided by the IOS. These commands are not covered here. Refer to the Cisco website for more information.

4.3 Hardware and physical connections

Do not overlook the possibility that the point of failure is a hardware or physical connection failure. Any number of things can go wrong, from board failures to cut cables to power failures. This document will not describe troubleshooting these problems, except for these simple things.

Check to see that the router is turned on. Also make sure that no cables are loose or damaged. Finally, make sure cables are plugged into the correct ports. Beyond this simple advice you will need to check other sources.

4.4 Out of your control

If the point of failure is farther up the line, the problem might lie with equipment not under your administration. Your only option might be to contact the equipment's administrator, notify them of your problem, and ask them for help. It is in your interest to be courteous and respectful. The other administrator has their own problems, their own workload and their own priorities. Their agenda might even directly conflict with yours, such as their intention to change dynamic routing protocols, etc. You must work with them, even if the situation is frustrating. Alienating someone with the power to block important routes to your network is not a good idea.