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