As a CCNA candidate, you almost certainly have some background in PC hardware and workstation support. In that case, you’re already conversant in loopback interfaces, notably 127.0.0.1, the loopback address assigned to a PC.
If you’re studying all about the totally different bodily interfaces in your CCNA exam – serial, ethernet, and BRI, amongst others – there’s one logical interface it’s good to find out about, and that’s – you guessed it! – the loopback interface.
What is not as instantly obvious is why we use loopback interfaces on routers and switches to begin with. Most of the Cisco router features that may use loopbacks are intermediate and superior options that you’re going to learn about in your CCNP and CCIE research, however these options all come back to 1 fundamental concept: If the loopback interface on a router is down, which means the router is unavailable as a whole.
In distinction, a physical interface being down does not imply the router itself is out of commission. A router’s ethernet port can go down, however the different physical interfaces on that router are nonetheless operational. Since a loopback interface is logical, there’s nothing physical that may go mistaken with it.
As I mentioned, you may learn completely different Cisco router and change options that make the most of loopback interfaces as you climb the Cisco certification ladder. There’s one false impression about Cisco loopback interfaces that you just want to get clear on now, though. You’re most likely acquainted with loopback interfaces on a PC, and will even know that the address range 127.0.0.zero is reserved for loopback addressing.
Observe that this reserved tackle vary doesn’t apply to loopbacks on Cisco devices, however. For those who try to assign an address from this vary to a Cisco loopback interface, you get this outcome:
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R1conf t
Enter configuration commands, one per line. Finish with CNTL/Z.
R1(config)interface loopback0
R1(config-if)ip handle 127.0.0.2 255.255.255.0
Not a sound host address – 127.0.0.2
R1(config-if)ip handle 127.1.1.1 255.255.255.0
Not a legitimate host tackle – 127.1.1.1
The range 127.0.0.zero is reserved for host loopbacks (corresponding to PCs), not routers or switches. The most commonly used deal with from this range is 127.0.0.1 – if you can’t ping that on a workstation, which means you’ll be able to’t ping your self, which suggests there’s a problem with the TCP/IP set up itself.
Preserve these particulars in thoughts on the exam and within the office, and also you’re on your method to CCNA exam success!
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