| As a CCNA candidate, you most likely have some | | | | misconception about Cisco loopback interfaces that |
| background in PC hardware and workstation support. | | | | you want to get clear on now, though. You're probably |
| If so, you're already familiar with loopback interfaces, | | | | familiar with loopback interfaces on a PC, and may |
| particularly 127.0.0.1, the loopback address assigned to | | | | even know that the address range 127.0.0.0 is |
| a PC. | | | | reserved for loopback addressing. |
| When you're learning all about the different physical | | | | Note that this reserved address range does not apply |
| interfaces for your CCNA exam - serial, ethernet, and | | | | to loopbacks on Cisco devices, however. If you |
| BRI, among others - there's one logical interface you | | | | attempt to assign an address from this range to a |
| need to know about, and that is - you guessed it! - the | | | | Cisco loopback interface, you get this result: |
| loopback interface. | | | | R1#conf t |
| What isn't as immediately apparent is why we use | | | | Enter configuration commands, one per line. End with |
| loopback interfaces on routers and switches to begin | | | | CNTL/Z. |
| with. Many of the Cisco router features that can use | | | | R1(config)#interface loopback0 |
| loopbacks are intermediate and advanced features | | | | R1(config-if)#ip address 127.0.0.2 255.255.255.0 |
| that you'll learn about in your CCNP and CCIE studies, | | | | Not a valid host address - 127.0.0.2 |
| but these features all come back to one basic | | | | R1(config-if)#ip address 127.1.1.1 255.255.255.0 |
| concept: If the loopback interface on a router is down, | | | | Not a valid host address - 127.1.1.1 |
| that means the router is unavailable as a whole. | | | | The range 127.0.0.0 is reserved for host loopbacks |
| In contrast, a physical interface being down does not | | | | (such as PCs), not routers or switches. The most |
| mean the router itself is out of commission. A router's | | | | commonly used address from this range is 127.0.0.1 - if |
| ethernet port can go down, but the other physical | | | | you can't ping that on a workstation, that means you |
| interfaces on that router are still operational. Since a | | | | can't ping yourself, which means there's a problem with |
| loopback interface is logical, there's nothing physical | | | | the TCP/IP install itself. |
| that can go wrong with it. | | | | Keep these details in mind on the exam and in the |
| As I mentioned, you'll learn different Cisco router and | | | | workplace, and you're on your way to CCNA exam |
| switch features that utilize loopback interfaces as you | | | | success! |
| climb the Cisco certification ladder. There's one | | | | |