CCNA: Chapter 3-5 Gathering Information and Verifying Configuration

10. September 2016 Cisco, Study Guides 0
CCNA: Chapter  3-5 Gathering Information and Verifying Configuration

While correctly configuring an IOS device is important, you should also be able to gather information and verify configuration easily. For this purpose, the IOS has many commands. You have already learned about the ping and traceroute commands in the previous section. This section introduces you to a range of show commands using which you can gather a lot of information and verify configuration and operation of the device.

The most prominent of the show commands is the show running-config command that displays the current running config of the device. Remember that running-config is different from startup-config. The example below shows the running-config of a device.



In the above output notice the various configurations from previous sections highlighted. In a single output, you can verify the entire configuration of the router.

Similar to the show running-config command, the show startup-config command shows the configuration stored in the NVRAM. If the running configuration is saved, the output of both commands will be the same as shown below:

Remember the system information displayed during the boot process? You can see that information again using the show version command. An example of the output of this command is shown below:

Notice that most of the information from the boot process is repeated in the above output. This command can be used to find the IOS version, number and types of interfaces, memory sizes etc.

The next show command that you should be familiar with is the show interfaces command. This command shows information related to an interface as shown below:

Some important lines in the above outputs are highlighted. Let us look at them and see what they mean. The most important line in the above output is the first line:

This line shows that the interface is up and the line protocol is up, which means that the interface is connected properly. Things are not always this good. You can various some variations in the interface and line protocol status that indicate some problem. If the interface and line protocol both are shown as down, this indicates a problem with cabling or the interface and essentially a problem at the physical layer. If the interface is up but the line protocol is down, this indicates a problem at layer2 such as a framing or encapsulation problem. A third status that you can encounter is the administratively down status. This means that the shutdown command has been configured on the interface. You can use the no shut command in the interface configuration mode to bring it up.

The next three lines show the MAC address, IP Address, subnet mask and the MTU of the interface:

The next highlighted line shows the duplex and speed of the interface. As you can see, the interface is operating at 100Mb/s full duplex.

While troubleshooting, you might need to reset various counters such as count of packets input and output etc in the show interfaces output. This can be done using the clear counter command as shown below:

While the show interfaces command shows a lot of information, it is primarily geared towards layer 1 and layer 2 details. You can use the show ip interface command to look at IP related information as shown below:

While most of the above output is irrelevant to CCNA, you can see that the first four lines show the IP address, subnet mask, broadcast address and MTU of the interface. If you quickly want to see the IP address and status of all interfaces in the device, you can use the show ip interface brief command as shown below:

While show interfaces and show ip interfaces commands are useful to find a bunch of information, the show ip interface brief command is used to quickly find the status and address of every interface. Similarly, the show protocols command can be used to find this information as shown below:

The show protocols command is more useful if you have multiple layer 3 protocols running on the device.

When working with show commands, one useful feature that you can use is piping. Using pipes, you can search for specific lines in an entire output. Take a look at the example below:

In the above example, the output of show running-config is piped (using a pipe symbol!) and then the router is told to show lines form the output that include the word address. Similar to include, you can ask the router to exclude certain words. In the example below, the word unassigned is excluded from the show ip interface brief command to show only interface that have an IP address:

The last pipe option that you need to know about, is the begin option. This option filters the output and shows the output starting from the line that contains the given word. Take a look at the example below:

In the example above, the output of show running-config is filtered to show only lines after the line containing the words “line con”. Essentially you filtered the output to see the configuration of all the three lines.

Apart from the above discussed show commands, there are many that will be discussed throughout the book with relevant topics. For now, you should be comfortable using these commands and finding information.


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