Cisco CCNA Exam Tutorial: Loopback Interfaces

As a CCNA nominee, you most presumable have some backdrop in PC hardware and workstation assist. If so, you're already familiar with loopback interfaces, especially 127.0.0.1, the loopback address assigned to a PC.

When you're studying always approximately the different physical interfaces as your CCNA examination - reg code, ethernet, and BRI, among others - there's one rational interface you need apt understand almost, and namely namely - you surmised it! - the loopback interface.

What isn't for quickly obvious is why we use loopback interfaces on routers and switches to begin with. Many of the Cisco router features that can use loopbacks are intermediate and progressive functions that you'll learn about in your CCNP and CCIE studies, but these features all come behind to one elementary concept: If the loopback interface on a router is down, that means the router is unavailable as a whole.

In contrast, a physical interface creature down does not mean the router itself is out of commission. A router's ethernet port tin go down, merely the additional physical interfaces on that router are still operational. Since a loopback interface is logical, there's nought physical that can go wrong with it.

As I said, you'll learn different Cisco router and alternate features that utilize loopback interfaces as you climb the Cisco appraisal stepladder. There's one misconception about Cisco loopback interfaces that you ambition to obtain explicit on immediately, whereas. You're probably versed with loopback interfaces on a PC, and may even know thatthe address range 127.0.0.0 is reserved for loopback addressing.

Note that this reserved address range does not apply to loopbacks on Cisco devices, although. If you venture to assign one address from this range to a Cisco loopback interface, you get this result:

R1#conf t

Enter configuration commands, an per line. End with CNTL/Z.

R1(config)#interface loopback0

R1(config-if)#ip address 127.0.0.2 255.255.255.0

Not a valid host address - 127.0.0.2

R1(config-if)#ip address 127.1.1.1 255.255.255.0

Not a valid host address - 127.1.1.1

The range 127.0.0.0 is reserved for host loopbacks (such as PCs), not routers or switches. The most commonly accustomed address from this scope is 127.0.0.1 - if you can't ping that on a workstation, that means you can't ping yourself, which means there's a problem with the TCP/IP install itself.

Keep these details in mind on the exam and in the workplace, and you're on your way to CCNA exam success!