As a CCNA candidate, you most likely have some background in PC hardware and workstation support. If so, you’re already familiar with loopback interfaces, particularly 127.0.0.1, the loopback address assigned to a PC.
When you’re learning all about the different physical interfaces for your CCNA exam – serial, ethernet, and BRI, among others – there’s one logical interface you need to know about, and that is – you guessed it! – the loopback interface.
What isn’t as immediately apparent 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 advanced features that you’ll learn about in your CCNP and CCIE studies, but these features all come back to one basic concept: If the loopback interface on a router is down, that means the router is unavailable as a whole.
In contrast, a physical interface being down does not mean the router itself is out of commission. A router’s ethernet port can go down, but the other physical interfaces on that router are still operational. Since a loopback interface is...
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