What does the command config register 0x2142 provide?

Cisco IOS routers and switches use a configuration register for some specific settings. These are settings that are not stored in the startup or running configuration.

There are 16 bits (4 hexadecimal digits) reserved for the configuration register. The two most popular things we can do with the configuration register are:

• Changing the baud rate from 9600 to something else.
• Telling the router to ignore the NVRAM during startup so that we can do a password recovery.

Let’s take a look at the default setting of the configuration register:

R1#show version | include register
Configuration register is 0x2102

The default value is 0x2102, what does this mean? Let’s break it down:

Hexadecimal2102Binary0010000100000010Bit Number15 14 13 1211 10 9 87 6 5 43 2 1 0

The 0x part means that we are looking at hexadecimal characters. 2102 is the default setting that tells the router to use a console speed of 9600 baud and to load the Cisco IOS image from flash memory.

How do we know this? You will have to check the Cisco documentation.There you can find the most common settings but also an explanation of what each bit does. We can see that:

• bit 13 tells the router to boot the default ROM software if network boot fails.
• bit 8 disables break.
• bit 1 tells the router to boot the first system image in onboard flash memory.

Bit 5, 11 and 12 are used to change the baud rate. If you don’t touch these settings then the default will be 9600 baud.

If you want to do a password recovery then we will have to tell the router to ignore the content of the NVRAM (that’s where the startup config is). We do this by enabling bit 6:

Hexadecimal2142Binary0010000101000010Bit Number15 14 13 1211 10 9 87 6 5 43 2 1 0

We will get hexadecimal value 0x2142.

How do we configure this on a router or switch? You can do this in the running-configuration:

R1#configure terminal
R1(config)#config-register ?
<0x0-0xFFFF>  Config register number

R1(config)#config-register 0x2142

Just type the hexadecimal value you want to use.

Even though we configure this in the configuration mode, keep in mind these changes are not saved in the running-configuration but directly in the configuration register.

We can verify our work with the show version command:

R1#show version | include register
Configuration register is 0x2102 (will be 0x2142 at next reload)

Make sure you reload the router or switch. You can also change the configuration register from rommon:

📌   The configuration register setting of 0x2102 provides what function to a router?

📌   Which command will show you the IOS version running on your router?

📌   You need to gather the IP address of a remote switch that is located in Hawaii. What can you do to find the address?

📌   You copy a configuration from a network host to a router\'s RAM. The configuration looks correct, yet it is not working at all. What could the problem be?

📌   Which command loads a new version of the Cisco IOS into a router?

📌   You are troubleshooting a connectivity problem in your corporate network and want to isolate the problem. You suspect that a router on the route to an unreachable network is at fault. What IOS user exec command should you issue?

📌   You save the configuration on a router with the copy running-config startup-config command and reboot the router. The router, however, comes up with a blank configuration. What can the problem be?

📌   You are unsuccessful in telnetting into a remote device. What could the problem be? 1.IP addresses are incorrect. 2.Access control list is filtering Telnet. 3.There is a defective serial cable. 4.The VTY password is missing.

📌   What information is displayed by the show hosts command? 1.Temporary DNS entries 2.The names of the routers created using the hostname command 3.The IP addresses of workstations allowed to access the router 4.Permanent name-to-address ma

📌   You telnet to a router and make your necessary changes; now you want to end the Telnet session. What command do you type in?

📌   Which command will copy the IOS to a backup host on your network?

📌   You have your laptop directly connected into a router\'s Ethernet port. Which of the following are among the requirements for the copy flash tftp command to be successful? 1.TFTP server software must be running on the router. 2.TFTP serv

📌   If you want to have more than one Telnet session open at the same time, what keystroke combination would you use?

📌   You telnet into a remote device and type debug ip rip, but no output from the debug command is seen. What could the problem be?

📌   Which command displays the configuration register setting?

📌   A network administrator wants to upgrade the IOS of a router without removing the image currently installed. What command will display the amount of memory consumed by the current IOS image and indicate whether there is enough room available to hold

📌   What should the configuration register value be after you successfully complete the password recovery procedure and return the router to normal operation?

What is config register 0x2101?

What is the role of the configuration register?

Which command display the configuration register setting?

Which command will show you the IOS version running on your router?